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Transcript of Schein Er 2015
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Accepted Manuscript
Progress Towards Self-Healing Polymers for Composite Structural Applications
Margaret Scheiner Tarik J Dickens Okenwa Okoli
PII S0032-3861(15)30364-5
DOI 101016jpolymer201511008
Reference JPOL 18245
To appear in Polymer
Received Date 14 August 2015
Accepted Date 2 November 2015
Please cite this article as Scheiner M Dickens TJ Okoli O Progress Towards Self-Healing Polymers
for Composite Structural ApplicationsPolymer (2015) doi 101016jpolymer201511008
This is a PDF file of an unedited manuscript that has been accepted for publication As a service to
our customers we are providing this early version of the manuscript The manuscript will undergo
copyediting typesetting and review of the resulting proof before it is published in its final form Please
note that during the production process errors may be discovered which could affect the content and alllegal disclaimers that apply to the journal pertain
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Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
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Progress Towards Self-Healing Polymers for Composite Structural Applications
Margaret Scheiner Tarik J Dickens Okenwa Okoli
Industrial amp Manufacturing Engineering FAMU-FSU College of Engineering
2525 Pottsdamer St Tallahassee Florida 32310
Abstract
Repair in composite materials is tending towards autonomic healing systems This is atechnological departure from the mechanical repair currently practiced in industry For
reinforced polymer matrix composites failure tends to occur in the matrix or matrix-
reinforcement interface The most common failure mode is the formation and propagation of
microcracks that reduce the materialrsquos structural capabilities Damage may be fixed throughtraditional bolted or bonded repair methods but such repair requires temporary decommission of
a part collection of repair materials and employee time and effort to enact the repair This
review describes methods of self-repair and healing for polymeric materials with a focus onstructural applications of these self-healing materials From intrinsically healing polymers to
self-healing-enabled polymer composites with dispersed agents or vascular networks this review
examines the chemistries and mechanisms which enable self-healing
Keywords self-healing polymer composites dispersed agents vascular networks
Contents1
Introduction 2
11
Traditional Composite Monitoring NDI to SHM 3
12
Composite Repair Practices 3
13
Self-Repair Healing Efficiency 4
2
Self-Healing Polymers 6
21 Covalent Bonding 7
22
Supramolecular Chemistry 12
3
Self-Healing Composites Dispersed Agents 15
31
Encapsulation 16
32
Remote Self-Healing 23
33
Shape Memory Assisted Self-Healing 24
4 Self-Healing Composites Vascular Networks 26
41
Design Considerations 29
42
Scaling to Bulk 32
5
Knowledge Assessment 32
6
Concluding remarks 35
References 38
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1 Introduction
Everything experiences wear and tear in everyday life The difference between biological structures and
mechanical structures is that biological beings automatically heal The process undertaken by a structure
to repair a damaged area without additional material is designated ldquoself-healingrdquo Self-healing may
involve the addition of energy (thermal electrical mechanical etc) This definition allows considerationof all healing processes while avoiding the problem of defining a lsquono thermal energy addedrsquo state Thus
healing can be categorized into two types (1) that which requires external intervention (ie such as a
temperature increase or application of ultraviolet radiation) and (2) that which does not require such
intervention typically referred to as lsquoautonomousrsquo healing [1]
Biomimetic self-healing synthetic materials imitate the procedures from natural organic systems
Observation of the various biological methods used by living things to heal has led to the creation of
synthetic materials capable of self-healing [2] An example of biological self-healing is after a child falls
and scrapes the skin off hisher knee blood wells up clots form and skin regrows Mimicking the process
as a whole is complex for there are clearly several disparate steps each of which involves highly
coordinated complex activities on the cellular and even molecular level [3 4] Rather than attempting to
copy the entire process engineers creating biomimetic systems can use the natural procedure to inspire
and to guide material development [5] continuing the development of smart materials (which are
responsive to external stimuli) [6] Ideally the self-healing process is repeatable that is the same sample
can successfully heal after repeated incidents of damage Self-healing parts should then have much longer
lifetimes than those formed from non-healing materials [7] With the SHM signals imitating the nervous
system of a composite part and with the ability of the part to self-heal concerns about composite part
failure causing plane crashes should be mitigated
Fiber-reinforced polymer composites (FRPCs) are used in a vast variety of applications in diverse
industries For example both military and civil aircraft include composite materials for their strength
benefits and weight savings [8] Boeing and Airbus have produced jetliners composed of 50 and 53
composite materials by weight for commercial flights [9] FRPCs are relatively cheap strong and
lightweight weight savings turn into better gas mileage meaning each flight the aircraft undertakes costs
less The biggest worry about heavy reliance on composite materials in commercial aircraft is part
maintenance repair and overhaul [10] For companies this translates to a trade-off of costs but for the
general consumer this translates to concern of part failure and aircraft crashes due to the use of these new
composite materials
Any material may eventually fail even under normal loading conditions The breadth of features which
affect the structural health of composites makes prediction of their mechanical properties more difficult
than for traditional materials [11-13] meaning physical inspection of parts is required to check fordamage FRPCs can suffer extreme internal damage from low-velocity impact and show little if any
external indication that damage has occurred [14 15] Thus non-destructive inspection (NDI) beyond
visible inspection is required to check for possible damage If a mechanical structure could self-heal
efficiently and reliably the repair technologies discussed in the previous section would no longer be
needed The question which arises is how can self-healing be enabled within mechanical systems
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11 Traditional Composite Monitoring NDI to SHM
Traditional NDI is costly and time-consuming meaning that frequent inspection is often limited to small
areas and critical damage can go unnoticed [15 16] To prevent possible aircraft crashes and other
catastrophic failures much research has been devoted to improving NDI Structural health monitoring
(SHM) could be considered an extension of NDI since it involves damage detection but in real-time
rather than just at individual inspection times [17] A SHM system incorporates sensors data transmissiondevices and external devices for data analysis or storage Such a system enables continuous real-time
updates on the integrity of the structure A significant portion of early SHM systems characterized
damage by analyzing vibrational changes but progress has been made regarding the use of fiber optic
sensors wireless data acquisition and microelectromechanical systems (MEMS) [18]
In the past decade much more research has been done regarding SHM The program for the 9 th
International Workshop on SHM [19] presents research both on the ldquotraditionalrdquo types of SHM used in
the first two Workshops and on the development of newer techniques such as using flying [20] or
climbing [21] robots to monitor civil engineering structures The most common techniques currently used
for SHM include acoustic emission and ultrasonic testing imaging methods and radiography and fiber
optic methods [8 22]
12 Composite Repair Practices
After damage has been recognized there remains the question of what to do about it Repair practices are
tailored to mend specific types of failure FRPC materials have several failure modes [23] Within a single
lamina the reinforcing fibers may break the matrix may crack or the interface between the two may fail
potentially leading to fiber pullout FRPC laminates may suffer failure within individual plies or between
plies (delamination) Highlighting the progress from self-healing polymers to self-healing composites
this report focuses solely on matrix failure While the shape memory composites and the vascular
composites discussed in this report could be considered a type of functionalized reinforcement it remains
difficult to repair the typical glass or carbon fibers used in composites today [24] Healing of interface
failure has been investigated and can be researched elsewhere [25-28] Following the theme of this article
the referenced repair practices are for addressing matrix failure rather than delamination interface failure
or fiber breakage
A fairly straightforward method to repair localized matrix damage is to add a patch on top of the damaged
area [29] Good patches are resistant to cyclic loading damage have a high immunity to corrosion and
easily shape to fit the structurersquos geometry [30] Material properties of the patch and the structural
material should be well-matched For example if the thermal expansion coefficients are significantly
different temperature changes will cause stress planes between the part and the patch and increase the
likelihood of patch failure [31] The adhesive is as important as the patching material for if the adhesive
fails the patch will de-bond and the damage will again be exposed [32] Patches may be bolted or bonded to the damaged structure Bolted repair is the current standard repair method for commercial composite
aircraft [33-35] Bonded repair is the method of choice in repairing damaged military composite aircraft
[36] Table 1 highlights some advantages and disadvantages of bolted and bonded repair particularly as it
pertains to composite aircraft
Table 1 Advantages and disadvantages of three repair types for composite aircraft
Repair Typical Advantages Disadvantages
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Method Repair
Material
Bolted Aluminum or
titanium [37]
Permanent damage tolerant [33]
existing tools and skills [35] can
made and disassembled in
uncontrolled environment effective
repair of composite delamination[38]
More efficient for highly loaded
structures [38]
Bolt holes lower load
carrying capability and alter
stress concentrations [38]
protruding patches reduce
aerodynamic properties whileflush patches require a large
number of fasteners [37]
Bonded Adhesive or
resin
Appropriate
compositepatch often
multi-layer
boron or
carbon fiber
prepreg
Permanent damage tolerant
improved finish (aerodynamic
aesthetic) long [33]
More efficient for lightly loadedstructures [38]
Requires controlled
environment and strict
cleaning pre-processing steps
high sensitivity to bondimperfections in thick
structures often over 24
hours of part downtime
necessary [38] processing
steps are highly dependent onpresence of moisture [37]
choice of an appropriate adhesive depends on many situational variables resin may include chopped glass
or carbon fibers [37]
A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]
This technique can be used in metals [40] and composite materials [41] This type of repair may be
achieved using the same material as the matrix or a different adhesive Ideally the injected material
should fill all voids within the matrix Filling all voids prevents high stress concentrations which would
lead to further crack growth [37 42] Of course a patch may be used in conjunction with injection
leading to significant recovery in tensile and bending strength [43] This type of repair may be used to
heal surface or internal damage but the damage location must first be known
13
Self-Repair Healing Efficiency
Healing efficiency of a material property Q is defined using Equation (1) [44]
983101
(1)
The subscripts refer to whether the material property is measured after healing (healed) or before damage
occurs (initial) A perfectly healed material would have R(Q) = 1 While reviewing the applicability of
materials based on their healing efficiency one may wish to keep in mind that skin scar tissue has a much
lower toughness (K) than does uninjured skin with R(K) asymp 02 [45] [46]
In many cases healing efficiency is defined in terms of the fracture toughness R(K) [47-49] but some
authors report healing efficiency in terms of the fracture stress or material strength R(σ) [50 51]
elongation or extensibility [52-54] peak load [44 55] or various moduli (eg R(Ersquo) [56 57]) Many
reports do not attempt to define a healing efficiency but only report that the material heals often with the
aid of optical images of damaged and healed samples Table 2 summarizes the type of healing efficiency
reported for an illustrative set of material systems The choice of which tests to do and thus what healing
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efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
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51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
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53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
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57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
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adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
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1525-1532
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copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
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70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
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142-149
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77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
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3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
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3) p 15-2381 Deng G et al Dynamic hydrogels with an environmental adaptive self-healing ability and dual
responsive solndashgel transitions ACS Macro Letters 2012 1(2) p 275-279
82 Ramachandran D F Liu and MW Urban Self-repairable copolymers that change color RSC
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Organic Coatings 2015 85 p 189-198
84 Liu Y-L and T-W Chuo Self-healing polymers based on thermally reversible Diels-Alder
chemistry Polymer Chemistry 2013 4(7) p 2194-2205
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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67(2) p 201-212
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
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Progress Towards Self-Healing Polymers for Composite Structural Applications
Margaret Scheiner Tarik J Dickens Okenwa Okoli
Industrial amp Manufacturing Engineering FAMU-FSU College of Engineering
2525 Pottsdamer St Tallahassee Florida 32310
Abstract
Repair in composite materials is tending towards autonomic healing systems This is atechnological departure from the mechanical repair currently practiced in industry For
reinforced polymer matrix composites failure tends to occur in the matrix or matrix-
reinforcement interface The most common failure mode is the formation and propagation of
microcracks that reduce the materialrsquos structural capabilities Damage may be fixed throughtraditional bolted or bonded repair methods but such repair requires temporary decommission of
a part collection of repair materials and employee time and effort to enact the repair This
review describes methods of self-repair and healing for polymeric materials with a focus onstructural applications of these self-healing materials From intrinsically healing polymers to
self-healing-enabled polymer composites with dispersed agents or vascular networks this review
examines the chemistries and mechanisms which enable self-healing
Keywords self-healing polymer composites dispersed agents vascular networks
Contents1
Introduction 2
11
Traditional Composite Monitoring NDI to SHM 3
12
Composite Repair Practices 3
13
Self-Repair Healing Efficiency 4
2
Self-Healing Polymers 6
21 Covalent Bonding 7
22
Supramolecular Chemistry 12
3
Self-Healing Composites Dispersed Agents 15
31
Encapsulation 16
32
Remote Self-Healing 23
33
Shape Memory Assisted Self-Healing 24
4 Self-Healing Composites Vascular Networks 26
41
Design Considerations 29
42
Scaling to Bulk 32
5
Knowledge Assessment 32
6
Concluding remarks 35
References 38
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1 Introduction
Everything experiences wear and tear in everyday life The difference between biological structures and
mechanical structures is that biological beings automatically heal The process undertaken by a structure
to repair a damaged area without additional material is designated ldquoself-healingrdquo Self-healing may
involve the addition of energy (thermal electrical mechanical etc) This definition allows considerationof all healing processes while avoiding the problem of defining a lsquono thermal energy addedrsquo state Thus
healing can be categorized into two types (1) that which requires external intervention (ie such as a
temperature increase or application of ultraviolet radiation) and (2) that which does not require such
intervention typically referred to as lsquoautonomousrsquo healing [1]
Biomimetic self-healing synthetic materials imitate the procedures from natural organic systems
Observation of the various biological methods used by living things to heal has led to the creation of
synthetic materials capable of self-healing [2] An example of biological self-healing is after a child falls
and scrapes the skin off hisher knee blood wells up clots form and skin regrows Mimicking the process
as a whole is complex for there are clearly several disparate steps each of which involves highly
coordinated complex activities on the cellular and even molecular level [3 4] Rather than attempting to
copy the entire process engineers creating biomimetic systems can use the natural procedure to inspire
and to guide material development [5] continuing the development of smart materials (which are
responsive to external stimuli) [6] Ideally the self-healing process is repeatable that is the same sample
can successfully heal after repeated incidents of damage Self-healing parts should then have much longer
lifetimes than those formed from non-healing materials [7] With the SHM signals imitating the nervous
system of a composite part and with the ability of the part to self-heal concerns about composite part
failure causing plane crashes should be mitigated
Fiber-reinforced polymer composites (FRPCs) are used in a vast variety of applications in diverse
industries For example both military and civil aircraft include composite materials for their strength
benefits and weight savings [8] Boeing and Airbus have produced jetliners composed of 50 and 53
composite materials by weight for commercial flights [9] FRPCs are relatively cheap strong and
lightweight weight savings turn into better gas mileage meaning each flight the aircraft undertakes costs
less The biggest worry about heavy reliance on composite materials in commercial aircraft is part
maintenance repair and overhaul [10] For companies this translates to a trade-off of costs but for the
general consumer this translates to concern of part failure and aircraft crashes due to the use of these new
composite materials
Any material may eventually fail even under normal loading conditions The breadth of features which
affect the structural health of composites makes prediction of their mechanical properties more difficult
than for traditional materials [11-13] meaning physical inspection of parts is required to check fordamage FRPCs can suffer extreme internal damage from low-velocity impact and show little if any
external indication that damage has occurred [14 15] Thus non-destructive inspection (NDI) beyond
visible inspection is required to check for possible damage If a mechanical structure could self-heal
efficiently and reliably the repair technologies discussed in the previous section would no longer be
needed The question which arises is how can self-healing be enabled within mechanical systems
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11 Traditional Composite Monitoring NDI to SHM
Traditional NDI is costly and time-consuming meaning that frequent inspection is often limited to small
areas and critical damage can go unnoticed [15 16] To prevent possible aircraft crashes and other
catastrophic failures much research has been devoted to improving NDI Structural health monitoring
(SHM) could be considered an extension of NDI since it involves damage detection but in real-time
rather than just at individual inspection times [17] A SHM system incorporates sensors data transmissiondevices and external devices for data analysis or storage Such a system enables continuous real-time
updates on the integrity of the structure A significant portion of early SHM systems characterized
damage by analyzing vibrational changes but progress has been made regarding the use of fiber optic
sensors wireless data acquisition and microelectromechanical systems (MEMS) [18]
In the past decade much more research has been done regarding SHM The program for the 9 th
International Workshop on SHM [19] presents research both on the ldquotraditionalrdquo types of SHM used in
the first two Workshops and on the development of newer techniques such as using flying [20] or
climbing [21] robots to monitor civil engineering structures The most common techniques currently used
for SHM include acoustic emission and ultrasonic testing imaging methods and radiography and fiber
optic methods [8 22]
12 Composite Repair Practices
After damage has been recognized there remains the question of what to do about it Repair practices are
tailored to mend specific types of failure FRPC materials have several failure modes [23] Within a single
lamina the reinforcing fibers may break the matrix may crack or the interface between the two may fail
potentially leading to fiber pullout FRPC laminates may suffer failure within individual plies or between
plies (delamination) Highlighting the progress from self-healing polymers to self-healing composites
this report focuses solely on matrix failure While the shape memory composites and the vascular
composites discussed in this report could be considered a type of functionalized reinforcement it remains
difficult to repair the typical glass or carbon fibers used in composites today [24] Healing of interface
failure has been investigated and can be researched elsewhere [25-28] Following the theme of this article
the referenced repair practices are for addressing matrix failure rather than delamination interface failure
or fiber breakage
A fairly straightforward method to repair localized matrix damage is to add a patch on top of the damaged
area [29] Good patches are resistant to cyclic loading damage have a high immunity to corrosion and
easily shape to fit the structurersquos geometry [30] Material properties of the patch and the structural
material should be well-matched For example if the thermal expansion coefficients are significantly
different temperature changes will cause stress planes between the part and the patch and increase the
likelihood of patch failure [31] The adhesive is as important as the patching material for if the adhesive
fails the patch will de-bond and the damage will again be exposed [32] Patches may be bolted or bonded to the damaged structure Bolted repair is the current standard repair method for commercial composite
aircraft [33-35] Bonded repair is the method of choice in repairing damaged military composite aircraft
[36] Table 1 highlights some advantages and disadvantages of bolted and bonded repair particularly as it
pertains to composite aircraft
Table 1 Advantages and disadvantages of three repair types for composite aircraft
Repair Typical Advantages Disadvantages
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Method Repair
Material
Bolted Aluminum or
titanium [37]
Permanent damage tolerant [33]
existing tools and skills [35] can
made and disassembled in
uncontrolled environment effective
repair of composite delamination[38]
More efficient for highly loaded
structures [38]
Bolt holes lower load
carrying capability and alter
stress concentrations [38]
protruding patches reduce
aerodynamic properties whileflush patches require a large
number of fasteners [37]
Bonded Adhesive or
resin
Appropriate
compositepatch often
multi-layer
boron or
carbon fiber
prepreg
Permanent damage tolerant
improved finish (aerodynamic
aesthetic) long [33]
More efficient for lightly loadedstructures [38]
Requires controlled
environment and strict
cleaning pre-processing steps
high sensitivity to bondimperfections in thick
structures often over 24
hours of part downtime
necessary [38] processing
steps are highly dependent onpresence of moisture [37]
choice of an appropriate adhesive depends on many situational variables resin may include chopped glass
or carbon fibers [37]
A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]
This technique can be used in metals [40] and composite materials [41] This type of repair may be
achieved using the same material as the matrix or a different adhesive Ideally the injected material
should fill all voids within the matrix Filling all voids prevents high stress concentrations which would
lead to further crack growth [37 42] Of course a patch may be used in conjunction with injection
leading to significant recovery in tensile and bending strength [43] This type of repair may be used to
heal surface or internal damage but the damage location must first be known
13
Self-Repair Healing Efficiency
Healing efficiency of a material property Q is defined using Equation (1) [44]
983101
(1)
The subscripts refer to whether the material property is measured after healing (healed) or before damage
occurs (initial) A perfectly healed material would have R(Q) = 1 While reviewing the applicability of
materials based on their healing efficiency one may wish to keep in mind that skin scar tissue has a much
lower toughness (K) than does uninjured skin with R(K) asymp 02 [45] [46]
In many cases healing efficiency is defined in terms of the fracture toughness R(K) [47-49] but some
authors report healing efficiency in terms of the fracture stress or material strength R(σ) [50 51]
elongation or extensibility [52-54] peak load [44 55] or various moduli (eg R(Ersquo) [56 57]) Many
reports do not attempt to define a healing efficiency but only report that the material heals often with the
aid of optical images of damaged and healed samples Table 2 summarizes the type of healing efficiency
reported for an illustrative set of material systems The choice of which tests to do and thus what healing
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efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
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Care 2013 2(2) p 37-43
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
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Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
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53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
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57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
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1525-1532
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42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
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copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
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between neat dynamic covalent polymers at room temperature Chemical Communications
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70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
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142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
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74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
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77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
trisulphides Australian Journal of Chemistry 1963 16(6) p 1027-1037
78 McAllan DT et al The preparation and properties of sulfur compounds related to petroleum
I The dialkyl sulfides and disulfides Journal of the American Chemical Society 1951 73(8) p
3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
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3) p 15-2381 Deng G et al Dynamic hydrogels with an environmental adaptive self-healing ability and dual
responsive solndashgel transitions ACS Macro Letters 2012 1(2) p 275-279
82 Ramachandran D F Liu and MW Urban Self-repairable copolymers that change color RSC
Advances 2012 2(1) p 135-144
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Organic Coatings 2015 85 p 189-198
84 Liu Y-L and T-W Chuo Self-healing polymers based on thermally reversible Diels-Alder
chemistry Polymer Chemistry 2013 4(7) p 2194-2205
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86 Laita H S Boufi and A Gandini The application of the Diels-Alder reaction to polymers
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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67(2) p 201-212
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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3171-3177
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Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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Progress Towards Self-Healing Polymers for Composite Structural Applications
Margaret Scheiner Tarik J Dickens Okenwa Okoli
Industrial amp Manufacturing Engineering FAMU-FSU College of Engineering
2525 Pottsdamer St Tallahassee Florida 32310
Abstract
Repair in composite materials is tending towards autonomic healing systems This is atechnological departure from the mechanical repair currently practiced in industry For
reinforced polymer matrix composites failure tends to occur in the matrix or matrix-
reinforcement interface The most common failure mode is the formation and propagation of
microcracks that reduce the materialrsquos structural capabilities Damage may be fixed throughtraditional bolted or bonded repair methods but such repair requires temporary decommission of
a part collection of repair materials and employee time and effort to enact the repair This
review describes methods of self-repair and healing for polymeric materials with a focus onstructural applications of these self-healing materials From intrinsically healing polymers to
self-healing-enabled polymer composites with dispersed agents or vascular networks this review
examines the chemistries and mechanisms which enable self-healing
Keywords self-healing polymer composites dispersed agents vascular networks
Contents1
Introduction 2
11
Traditional Composite Monitoring NDI to SHM 3
12
Composite Repair Practices 3
13
Self-Repair Healing Efficiency 4
2
Self-Healing Polymers 6
21 Covalent Bonding 7
22
Supramolecular Chemistry 12
3
Self-Healing Composites Dispersed Agents 15
31
Encapsulation 16
32
Remote Self-Healing 23
33
Shape Memory Assisted Self-Healing 24
4 Self-Healing Composites Vascular Networks 26
41
Design Considerations 29
42
Scaling to Bulk 32
5
Knowledge Assessment 32
6
Concluding remarks 35
References 38
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1 Introduction
Everything experiences wear and tear in everyday life The difference between biological structures and
mechanical structures is that biological beings automatically heal The process undertaken by a structure
to repair a damaged area without additional material is designated ldquoself-healingrdquo Self-healing may
involve the addition of energy (thermal electrical mechanical etc) This definition allows considerationof all healing processes while avoiding the problem of defining a lsquono thermal energy addedrsquo state Thus
healing can be categorized into two types (1) that which requires external intervention (ie such as a
temperature increase or application of ultraviolet radiation) and (2) that which does not require such
intervention typically referred to as lsquoautonomousrsquo healing [1]
Biomimetic self-healing synthetic materials imitate the procedures from natural organic systems
Observation of the various biological methods used by living things to heal has led to the creation of
synthetic materials capable of self-healing [2] An example of biological self-healing is after a child falls
and scrapes the skin off hisher knee blood wells up clots form and skin regrows Mimicking the process
as a whole is complex for there are clearly several disparate steps each of which involves highly
coordinated complex activities on the cellular and even molecular level [3 4] Rather than attempting to
copy the entire process engineers creating biomimetic systems can use the natural procedure to inspire
and to guide material development [5] continuing the development of smart materials (which are
responsive to external stimuli) [6] Ideally the self-healing process is repeatable that is the same sample
can successfully heal after repeated incidents of damage Self-healing parts should then have much longer
lifetimes than those formed from non-healing materials [7] With the SHM signals imitating the nervous
system of a composite part and with the ability of the part to self-heal concerns about composite part
failure causing plane crashes should be mitigated
Fiber-reinforced polymer composites (FRPCs) are used in a vast variety of applications in diverse
industries For example both military and civil aircraft include composite materials for their strength
benefits and weight savings [8] Boeing and Airbus have produced jetliners composed of 50 and 53
composite materials by weight for commercial flights [9] FRPCs are relatively cheap strong and
lightweight weight savings turn into better gas mileage meaning each flight the aircraft undertakes costs
less The biggest worry about heavy reliance on composite materials in commercial aircraft is part
maintenance repair and overhaul [10] For companies this translates to a trade-off of costs but for the
general consumer this translates to concern of part failure and aircraft crashes due to the use of these new
composite materials
Any material may eventually fail even under normal loading conditions The breadth of features which
affect the structural health of composites makes prediction of their mechanical properties more difficult
than for traditional materials [11-13] meaning physical inspection of parts is required to check fordamage FRPCs can suffer extreme internal damage from low-velocity impact and show little if any
external indication that damage has occurred [14 15] Thus non-destructive inspection (NDI) beyond
visible inspection is required to check for possible damage If a mechanical structure could self-heal
efficiently and reliably the repair technologies discussed in the previous section would no longer be
needed The question which arises is how can self-healing be enabled within mechanical systems
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11 Traditional Composite Monitoring NDI to SHM
Traditional NDI is costly and time-consuming meaning that frequent inspection is often limited to small
areas and critical damage can go unnoticed [15 16] To prevent possible aircraft crashes and other
catastrophic failures much research has been devoted to improving NDI Structural health monitoring
(SHM) could be considered an extension of NDI since it involves damage detection but in real-time
rather than just at individual inspection times [17] A SHM system incorporates sensors data transmissiondevices and external devices for data analysis or storage Such a system enables continuous real-time
updates on the integrity of the structure A significant portion of early SHM systems characterized
damage by analyzing vibrational changes but progress has been made regarding the use of fiber optic
sensors wireless data acquisition and microelectromechanical systems (MEMS) [18]
In the past decade much more research has been done regarding SHM The program for the 9 th
International Workshop on SHM [19] presents research both on the ldquotraditionalrdquo types of SHM used in
the first two Workshops and on the development of newer techniques such as using flying [20] or
climbing [21] robots to monitor civil engineering structures The most common techniques currently used
for SHM include acoustic emission and ultrasonic testing imaging methods and radiography and fiber
optic methods [8 22]
12 Composite Repair Practices
After damage has been recognized there remains the question of what to do about it Repair practices are
tailored to mend specific types of failure FRPC materials have several failure modes [23] Within a single
lamina the reinforcing fibers may break the matrix may crack or the interface between the two may fail
potentially leading to fiber pullout FRPC laminates may suffer failure within individual plies or between
plies (delamination) Highlighting the progress from self-healing polymers to self-healing composites
this report focuses solely on matrix failure While the shape memory composites and the vascular
composites discussed in this report could be considered a type of functionalized reinforcement it remains
difficult to repair the typical glass or carbon fibers used in composites today [24] Healing of interface
failure has been investigated and can be researched elsewhere [25-28] Following the theme of this article
the referenced repair practices are for addressing matrix failure rather than delamination interface failure
or fiber breakage
A fairly straightforward method to repair localized matrix damage is to add a patch on top of the damaged
area [29] Good patches are resistant to cyclic loading damage have a high immunity to corrosion and
easily shape to fit the structurersquos geometry [30] Material properties of the patch and the structural
material should be well-matched For example if the thermal expansion coefficients are significantly
different temperature changes will cause stress planes between the part and the patch and increase the
likelihood of patch failure [31] The adhesive is as important as the patching material for if the adhesive
fails the patch will de-bond and the damage will again be exposed [32] Patches may be bolted or bonded to the damaged structure Bolted repair is the current standard repair method for commercial composite
aircraft [33-35] Bonded repair is the method of choice in repairing damaged military composite aircraft
[36] Table 1 highlights some advantages and disadvantages of bolted and bonded repair particularly as it
pertains to composite aircraft
Table 1 Advantages and disadvantages of three repair types for composite aircraft
Repair Typical Advantages Disadvantages
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Method Repair
Material
Bolted Aluminum or
titanium [37]
Permanent damage tolerant [33]
existing tools and skills [35] can
made and disassembled in
uncontrolled environment effective
repair of composite delamination[38]
More efficient for highly loaded
structures [38]
Bolt holes lower load
carrying capability and alter
stress concentrations [38]
protruding patches reduce
aerodynamic properties whileflush patches require a large
number of fasteners [37]
Bonded Adhesive or
resin
Appropriate
compositepatch often
multi-layer
boron or
carbon fiber
prepreg
Permanent damage tolerant
improved finish (aerodynamic
aesthetic) long [33]
More efficient for lightly loadedstructures [38]
Requires controlled
environment and strict
cleaning pre-processing steps
high sensitivity to bondimperfections in thick
structures often over 24
hours of part downtime
necessary [38] processing
steps are highly dependent onpresence of moisture [37]
choice of an appropriate adhesive depends on many situational variables resin may include chopped glass
or carbon fibers [37]
A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]
This technique can be used in metals [40] and composite materials [41] This type of repair may be
achieved using the same material as the matrix or a different adhesive Ideally the injected material
should fill all voids within the matrix Filling all voids prevents high stress concentrations which would
lead to further crack growth [37 42] Of course a patch may be used in conjunction with injection
leading to significant recovery in tensile and bending strength [43] This type of repair may be used to
heal surface or internal damage but the damage location must first be known
13
Self-Repair Healing Efficiency
Healing efficiency of a material property Q is defined using Equation (1) [44]
983101
(1)
The subscripts refer to whether the material property is measured after healing (healed) or before damage
occurs (initial) A perfectly healed material would have R(Q) = 1 While reviewing the applicability of
materials based on their healing efficiency one may wish to keep in mind that skin scar tissue has a much
lower toughness (K) than does uninjured skin with R(K) asymp 02 [45] [46]
In many cases healing efficiency is defined in terms of the fracture toughness R(K) [47-49] but some
authors report healing efficiency in terms of the fracture stress or material strength R(σ) [50 51]
elongation or extensibility [52-54] peak load [44 55] or various moduli (eg R(Ersquo) [56 57]) Many
reports do not attempt to define a healing efficiency but only report that the material heals often with the
aid of optical images of damaged and healed samples Table 2 summarizes the type of healing efficiency
reported for an illustrative set of material systems The choice of which tests to do and thus what healing
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efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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Care 2013 2(2) p 37-43
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
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51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
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53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
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57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
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adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
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1525-1532
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copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
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70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
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142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
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74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
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77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
trisulphides Australian Journal of Chemistry 1963 16(6) p 1027-1037
78 McAllan DT et al The preparation and properties of sulfur compounds related to petroleum
I The dialkyl sulfides and disulfides Journal of the American Chemical Society 1951 73(8) p
3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
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3) p 15-2381 Deng G et al Dynamic hydrogels with an environmental adaptive self-healing ability and dual
responsive solndashgel transitions ACS Macro Letters 2012 1(2) p 275-279
82 Ramachandran D F Liu and MW Urban Self-repairable copolymers that change color RSC
Advances 2012 2(1) p 135-144
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Organic Coatings 2015 85 p 189-198
84 Liu Y-L and T-W Chuo Self-healing polymers based on thermally reversible Diels-Alder
chemistry Polymer Chemistry 2013 4(7) p 2194-2205
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86 Laita H S Boufi and A Gandini The application of the Diels-Alder reaction to polymers
bearing furan moieties 1 Reactions with maleimides European polymer journal 1997 33(8) p1203-1211
87 Gousseacute C A Gandini and P Hodge Application of the Diels-Alder reaction to polymers
bearing furan moieties 2 Diels-Alder and retro-Diels-Alder reactions involving furan rings in
some styrene copolymers Macromolecules 1998 31(2) p 314-32188 Toncelli C et al Properties of reversible Diels-Alder furanmaleimide polymer networks as
function of crosslink density Macromolecular Chemistry and Physics 2012 213(2) p 157-165
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component intrinsic polymers Polymer 2015 69 p 321-329
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2015 In Press p In Press
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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Macromolecules 2002 35(21) p 7878-7882
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microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
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3171-3177
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241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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1 Introduction
Everything experiences wear and tear in everyday life The difference between biological structures and
mechanical structures is that biological beings automatically heal The process undertaken by a structure
to repair a damaged area without additional material is designated ldquoself-healingrdquo Self-healing may
involve the addition of energy (thermal electrical mechanical etc) This definition allows considerationof all healing processes while avoiding the problem of defining a lsquono thermal energy addedrsquo state Thus
healing can be categorized into two types (1) that which requires external intervention (ie such as a
temperature increase or application of ultraviolet radiation) and (2) that which does not require such
intervention typically referred to as lsquoautonomousrsquo healing [1]
Biomimetic self-healing synthetic materials imitate the procedures from natural organic systems
Observation of the various biological methods used by living things to heal has led to the creation of
synthetic materials capable of self-healing [2] An example of biological self-healing is after a child falls
and scrapes the skin off hisher knee blood wells up clots form and skin regrows Mimicking the process
as a whole is complex for there are clearly several disparate steps each of which involves highly
coordinated complex activities on the cellular and even molecular level [3 4] Rather than attempting to
copy the entire process engineers creating biomimetic systems can use the natural procedure to inspire
and to guide material development [5] continuing the development of smart materials (which are
responsive to external stimuli) [6] Ideally the self-healing process is repeatable that is the same sample
can successfully heal after repeated incidents of damage Self-healing parts should then have much longer
lifetimes than those formed from non-healing materials [7] With the SHM signals imitating the nervous
system of a composite part and with the ability of the part to self-heal concerns about composite part
failure causing plane crashes should be mitigated
Fiber-reinforced polymer composites (FRPCs) are used in a vast variety of applications in diverse
industries For example both military and civil aircraft include composite materials for their strength
benefits and weight savings [8] Boeing and Airbus have produced jetliners composed of 50 and 53
composite materials by weight for commercial flights [9] FRPCs are relatively cheap strong and
lightweight weight savings turn into better gas mileage meaning each flight the aircraft undertakes costs
less The biggest worry about heavy reliance on composite materials in commercial aircraft is part
maintenance repair and overhaul [10] For companies this translates to a trade-off of costs but for the
general consumer this translates to concern of part failure and aircraft crashes due to the use of these new
composite materials
Any material may eventually fail even under normal loading conditions The breadth of features which
affect the structural health of composites makes prediction of their mechanical properties more difficult
than for traditional materials [11-13] meaning physical inspection of parts is required to check fordamage FRPCs can suffer extreme internal damage from low-velocity impact and show little if any
external indication that damage has occurred [14 15] Thus non-destructive inspection (NDI) beyond
visible inspection is required to check for possible damage If a mechanical structure could self-heal
efficiently and reliably the repair technologies discussed in the previous section would no longer be
needed The question which arises is how can self-healing be enabled within mechanical systems
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11 Traditional Composite Monitoring NDI to SHM
Traditional NDI is costly and time-consuming meaning that frequent inspection is often limited to small
areas and critical damage can go unnoticed [15 16] To prevent possible aircraft crashes and other
catastrophic failures much research has been devoted to improving NDI Structural health monitoring
(SHM) could be considered an extension of NDI since it involves damage detection but in real-time
rather than just at individual inspection times [17] A SHM system incorporates sensors data transmissiondevices and external devices for data analysis or storage Such a system enables continuous real-time
updates on the integrity of the structure A significant portion of early SHM systems characterized
damage by analyzing vibrational changes but progress has been made regarding the use of fiber optic
sensors wireless data acquisition and microelectromechanical systems (MEMS) [18]
In the past decade much more research has been done regarding SHM The program for the 9 th
International Workshop on SHM [19] presents research both on the ldquotraditionalrdquo types of SHM used in
the first two Workshops and on the development of newer techniques such as using flying [20] or
climbing [21] robots to monitor civil engineering structures The most common techniques currently used
for SHM include acoustic emission and ultrasonic testing imaging methods and radiography and fiber
optic methods [8 22]
12 Composite Repair Practices
After damage has been recognized there remains the question of what to do about it Repair practices are
tailored to mend specific types of failure FRPC materials have several failure modes [23] Within a single
lamina the reinforcing fibers may break the matrix may crack or the interface between the two may fail
potentially leading to fiber pullout FRPC laminates may suffer failure within individual plies or between
plies (delamination) Highlighting the progress from self-healing polymers to self-healing composites
this report focuses solely on matrix failure While the shape memory composites and the vascular
composites discussed in this report could be considered a type of functionalized reinforcement it remains
difficult to repair the typical glass or carbon fibers used in composites today [24] Healing of interface
failure has been investigated and can be researched elsewhere [25-28] Following the theme of this article
the referenced repair practices are for addressing matrix failure rather than delamination interface failure
or fiber breakage
A fairly straightforward method to repair localized matrix damage is to add a patch on top of the damaged
area [29] Good patches are resistant to cyclic loading damage have a high immunity to corrosion and
easily shape to fit the structurersquos geometry [30] Material properties of the patch and the structural
material should be well-matched For example if the thermal expansion coefficients are significantly
different temperature changes will cause stress planes between the part and the patch and increase the
likelihood of patch failure [31] The adhesive is as important as the patching material for if the adhesive
fails the patch will de-bond and the damage will again be exposed [32] Patches may be bolted or bonded to the damaged structure Bolted repair is the current standard repair method for commercial composite
aircraft [33-35] Bonded repair is the method of choice in repairing damaged military composite aircraft
[36] Table 1 highlights some advantages and disadvantages of bolted and bonded repair particularly as it
pertains to composite aircraft
Table 1 Advantages and disadvantages of three repair types for composite aircraft
Repair Typical Advantages Disadvantages
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Method Repair
Material
Bolted Aluminum or
titanium [37]
Permanent damage tolerant [33]
existing tools and skills [35] can
made and disassembled in
uncontrolled environment effective
repair of composite delamination[38]
More efficient for highly loaded
structures [38]
Bolt holes lower load
carrying capability and alter
stress concentrations [38]
protruding patches reduce
aerodynamic properties whileflush patches require a large
number of fasteners [37]
Bonded Adhesive or
resin
Appropriate
compositepatch often
multi-layer
boron or
carbon fiber
prepreg
Permanent damage tolerant
improved finish (aerodynamic
aesthetic) long [33]
More efficient for lightly loadedstructures [38]
Requires controlled
environment and strict
cleaning pre-processing steps
high sensitivity to bondimperfections in thick
structures often over 24
hours of part downtime
necessary [38] processing
steps are highly dependent onpresence of moisture [37]
choice of an appropriate adhesive depends on many situational variables resin may include chopped glass
or carbon fibers [37]
A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]
This technique can be used in metals [40] and composite materials [41] This type of repair may be
achieved using the same material as the matrix or a different adhesive Ideally the injected material
should fill all voids within the matrix Filling all voids prevents high stress concentrations which would
lead to further crack growth [37 42] Of course a patch may be used in conjunction with injection
leading to significant recovery in tensile and bending strength [43] This type of repair may be used to
heal surface or internal damage but the damage location must first be known
13
Self-Repair Healing Efficiency
Healing efficiency of a material property Q is defined using Equation (1) [44]
983101
(1)
The subscripts refer to whether the material property is measured after healing (healed) or before damage
occurs (initial) A perfectly healed material would have R(Q) = 1 While reviewing the applicability of
materials based on their healing efficiency one may wish to keep in mind that skin scar tissue has a much
lower toughness (K) than does uninjured skin with R(K) asymp 02 [45] [46]
In many cases healing efficiency is defined in terms of the fracture toughness R(K) [47-49] but some
authors report healing efficiency in terms of the fracture stress or material strength R(σ) [50 51]
elongation or extensibility [52-54] peak load [44 55] or various moduli (eg R(Ersquo) [56 57]) Many
reports do not attempt to define a healing efficiency but only report that the material heals often with the
aid of optical images of damaged and healed samples Table 2 summarizes the type of healing efficiency
reported for an illustrative set of material systems The choice of which tests to do and thus what healing
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efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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of applied polymer science 2004 93(2) p 920-926
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2015 In Press p In Press
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10018
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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University of Illinois at Urbana Champaign Illinois USA p 290
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the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
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the Royal Society 2007 4(13) p 389-393
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Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
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coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
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Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
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193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
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healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
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2009 113(20) p 8688-8695
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healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
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2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
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challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
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demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
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Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
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215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
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shape memory polymer fibers Polymer 2013 54 p 5075-5086
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exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
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biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
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226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
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3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
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reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
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235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
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Science and Manufacturing 2011 42(6) p 639-648
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engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
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review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 554
ACCEPTED MANUSCRIPT
11 Traditional Composite Monitoring NDI to SHM
Traditional NDI is costly and time-consuming meaning that frequent inspection is often limited to small
areas and critical damage can go unnoticed [15 16] To prevent possible aircraft crashes and other
catastrophic failures much research has been devoted to improving NDI Structural health monitoring
(SHM) could be considered an extension of NDI since it involves damage detection but in real-time
rather than just at individual inspection times [17] A SHM system incorporates sensors data transmissiondevices and external devices for data analysis or storage Such a system enables continuous real-time
updates on the integrity of the structure A significant portion of early SHM systems characterized
damage by analyzing vibrational changes but progress has been made regarding the use of fiber optic
sensors wireless data acquisition and microelectromechanical systems (MEMS) [18]
In the past decade much more research has been done regarding SHM The program for the 9 th
International Workshop on SHM [19] presents research both on the ldquotraditionalrdquo types of SHM used in
the first two Workshops and on the development of newer techniques such as using flying [20] or
climbing [21] robots to monitor civil engineering structures The most common techniques currently used
for SHM include acoustic emission and ultrasonic testing imaging methods and radiography and fiber
optic methods [8 22]
12 Composite Repair Practices
After damage has been recognized there remains the question of what to do about it Repair practices are
tailored to mend specific types of failure FRPC materials have several failure modes [23] Within a single
lamina the reinforcing fibers may break the matrix may crack or the interface between the two may fail
potentially leading to fiber pullout FRPC laminates may suffer failure within individual plies or between
plies (delamination) Highlighting the progress from self-healing polymers to self-healing composites
this report focuses solely on matrix failure While the shape memory composites and the vascular
composites discussed in this report could be considered a type of functionalized reinforcement it remains
difficult to repair the typical glass or carbon fibers used in composites today [24] Healing of interface
failure has been investigated and can be researched elsewhere [25-28] Following the theme of this article
the referenced repair practices are for addressing matrix failure rather than delamination interface failure
or fiber breakage
A fairly straightforward method to repair localized matrix damage is to add a patch on top of the damaged
area [29] Good patches are resistant to cyclic loading damage have a high immunity to corrosion and
easily shape to fit the structurersquos geometry [30] Material properties of the patch and the structural
material should be well-matched For example if the thermal expansion coefficients are significantly
different temperature changes will cause stress planes between the part and the patch and increase the
likelihood of patch failure [31] The adhesive is as important as the patching material for if the adhesive
fails the patch will de-bond and the damage will again be exposed [32] Patches may be bolted or bonded to the damaged structure Bolted repair is the current standard repair method for commercial composite
aircraft [33-35] Bonded repair is the method of choice in repairing damaged military composite aircraft
[36] Table 1 highlights some advantages and disadvantages of bolted and bonded repair particularly as it
pertains to composite aircraft
Table 1 Advantages and disadvantages of three repair types for composite aircraft
Repair Typical Advantages Disadvantages
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Method Repair
Material
Bolted Aluminum or
titanium [37]
Permanent damage tolerant [33]
existing tools and skills [35] can
made and disassembled in
uncontrolled environment effective
repair of composite delamination[38]
More efficient for highly loaded
structures [38]
Bolt holes lower load
carrying capability and alter
stress concentrations [38]
protruding patches reduce
aerodynamic properties whileflush patches require a large
number of fasteners [37]
Bonded Adhesive or
resin
Appropriate
compositepatch often
multi-layer
boron or
carbon fiber
prepreg
Permanent damage tolerant
improved finish (aerodynamic
aesthetic) long [33]
More efficient for lightly loadedstructures [38]
Requires controlled
environment and strict
cleaning pre-processing steps
high sensitivity to bondimperfections in thick
structures often over 24
hours of part downtime
necessary [38] processing
steps are highly dependent onpresence of moisture [37]
choice of an appropriate adhesive depends on many situational variables resin may include chopped glass
or carbon fibers [37]
A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]
This technique can be used in metals [40] and composite materials [41] This type of repair may be
achieved using the same material as the matrix or a different adhesive Ideally the injected material
should fill all voids within the matrix Filling all voids prevents high stress concentrations which would
lead to further crack growth [37 42] Of course a patch may be used in conjunction with injection
leading to significant recovery in tensile and bending strength [43] This type of repair may be used to
heal surface or internal damage but the damage location must first be known
13
Self-Repair Healing Efficiency
Healing efficiency of a material property Q is defined using Equation (1) [44]
983101
(1)
The subscripts refer to whether the material property is measured after healing (healed) or before damage
occurs (initial) A perfectly healed material would have R(Q) = 1 While reviewing the applicability of
materials based on their healing efficiency one may wish to keep in mind that skin scar tissue has a much
lower toughness (K) than does uninjured skin with R(K) asymp 02 [45] [46]
In many cases healing efficiency is defined in terms of the fracture toughness R(K) [47-49] but some
authors report healing efficiency in terms of the fracture stress or material strength R(σ) [50 51]
elongation or extensibility [52-54] peak load [44 55] or various moduli (eg R(Ersquo) [56 57]) Many
reports do not attempt to define a healing efficiency but only report that the material heals often with the
aid of optical images of damaged and healed samples Table 2 summarizes the type of healing efficiency
reported for an illustrative set of material systems The choice of which tests to do and thus what healing
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efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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ACCEPTED MANUSCRIPT
structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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135 Burattini S et al A novel self-healing supramolecular polymer system Faraday Discussions2009 143 p 251-264
136 Burattini S et al A self-repairing supramolecular polymer system healability as a
consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
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ACCEPTED MANUSCRIPT
137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
compounds Macromolecular Rapid Communications 2009 30(6) p 459-463
138 Burattini S et al A supramolecular polymer based on tweezer-type π minusπ stacking interactions
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6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
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411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
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2007 14(5) p 391-397
142 Wool RP and KM OConnor A theory crack healing in polymers Journal of Applied Physics
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USA
149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
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2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
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156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
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159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
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nanocapsules Progress in Organic Coatings 2015 84 p 97-106
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Journal of Microencapsulation 2003 20(6) p 719-730
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formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
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168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
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dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Method Repair
Material
Bolted Aluminum or
titanium [37]
Permanent damage tolerant [33]
existing tools and skills [35] can
made and disassembled in
uncontrolled environment effective
repair of composite delamination[38]
More efficient for highly loaded
structures [38]
Bolt holes lower load
carrying capability and alter
stress concentrations [38]
protruding patches reduce
aerodynamic properties whileflush patches require a large
number of fasteners [37]
Bonded Adhesive or
resin
Appropriate
compositepatch often
multi-layer
boron or
carbon fiber
prepreg
Permanent damage tolerant
improved finish (aerodynamic
aesthetic) long [33]
More efficient for lightly loadedstructures [38]
Requires controlled
environment and strict
cleaning pre-processing steps
high sensitivity to bondimperfections in thick
structures often over 24
hours of part downtime
necessary [38] processing
steps are highly dependent onpresence of moisture [37]
choice of an appropriate adhesive depends on many situational variables resin may include chopped glass
or carbon fibers [37]
A specific type of bonded repair is to inject additional material into a damaged area and to cure it [39]
This technique can be used in metals [40] and composite materials [41] This type of repair may be
achieved using the same material as the matrix or a different adhesive Ideally the injected material
should fill all voids within the matrix Filling all voids prevents high stress concentrations which would
lead to further crack growth [37 42] Of course a patch may be used in conjunction with injection
leading to significant recovery in tensile and bending strength [43] This type of repair may be used to
heal surface or internal damage but the damage location must first be known
13
Self-Repair Healing Efficiency
Healing efficiency of a material property Q is defined using Equation (1) [44]
983101
(1)
The subscripts refer to whether the material property is measured after healing (healed) or before damage
occurs (initial) A perfectly healed material would have R(Q) = 1 While reviewing the applicability of
materials based on their healing efficiency one may wish to keep in mind that skin scar tissue has a much
lower toughness (K) than does uninjured skin with R(K) asymp 02 [45] [46]
In many cases healing efficiency is defined in terms of the fracture toughness R(K) [47-49] but some
authors report healing efficiency in terms of the fracture stress or material strength R(σ) [50 51]
elongation or extensibility [52-54] peak load [44 55] or various moduli (eg R(Ersquo) [56 57]) Many
reports do not attempt to define a healing efficiency but only report that the material heals often with the
aid of optical images of damaged and healed samples Table 2 summarizes the type of healing efficiency
reported for an illustrative set of material systems The choice of which tests to do and thus what healing
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ACCEPTED MANUSCRIPT
efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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ACCEPTED MANUSCRIPT
structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
efficiency to report changes between research groups though fracture toughness is most often reported
for epoxy systems
Table 2 Types of healing efficiencies reported in various material systems
Material property Q R(Q) [Ref]
estimated from figures
Matrix Material
Cohesive Recovery
(1 ndash Vt Vt0)
0-1 [58] Epoxy
Extensibility 04-09 [52]
1 [54]
045 [53]
Poly(styrene)
Poly(acrylamide stearyl methacrylate)
Poly(n-butyl acrylate) + poly(styrene)
block copolymer
Flexural Strength 055-093 [59] Epoxy
Fracture Load 107-148 [44]
009-024 [55]
Epoxy
Poly(dimethyl siloxane)
Fracture Stress (σ) 0-095 [50]
0-073 [51]
Poly(sulfide)s
Poly(vinyl alcohol)
Fracture Toughness (K) 07-12 [48]
084-097 [47] 03-09 [49]
Poly(dimethyl siloxane)
Epoxy
Tensile Modulus (E) 094 [56]
0-1125 [57]
Poly(n-butyl acrylate)
Poly(urethane)
Healing efficiency varies widely within any material system For example neat poly(dimethyl siloxane)
(PDMS) has R(K) = 002 but R(K) = 07-12 was reached by incorporating microcapsules with the
relevant resin and initiator for the PDMS system [48] Healing efficiency also varies widely between
material properties For example a poly(imide) system had a healing efficiency of 95 for elongation to
break but only 77 in terms of fracture toughness [60]
Healing efficiency is a good way to see how well a given material system recovers a given mechanical
property but it is not the entire story A fracture strength healing efficiency of 100 was reported for ahollow fiber-reinforced epoxy composite This value compares the healed composite to the pre-damage
healing-enabled composite [61] However the added constituents affect the virgin (pre-damage) strength
of the material [62] so the healing efficiency of 100 results in a material with only 87 of the strength
of the unmodified laminate [61]
Figure 1 shows the number of papers published per year containing the phrase ldquoself-healing polymerrdquo as
found via EngineeringVillage [63] Despite this interest research is still needed to understand the virgin
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ACCEPTED MANUSCRIPT
structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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ACCEPTED MANUSCRIPT
21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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ACCEPTED MANUSCRIPT
Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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ACCEPTED MANUSCRIPT
after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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ACCEPTED MANUSCRIPT
Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
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4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
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14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
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15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
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22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
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41 Raghavan J and RP Wool Interfaces in repair recycling joining and manufacturing of polymers and polymer composites Journal of Applied Polymer Science 1999 71(5) p 775-785
42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
43 Liu D CY Lee and X Lu Repairability of impact-induced damage in SMC composites
Journal of composite materials 1993 27(13) p 1257-1271
44 White SR et al Autonomic healing of polymer composites Nature 2001 409(6822) p 794-797
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45 Corr DT et al Biomechanical behavior of scar tissue and uninjured skin in a porcine model Wound Repair and Regeneration 2009 17(2) p 250-259
46 Corr DT and DA Hart Biomechanics of scar tissue and uninjured skin Advances in Wound
Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
Experimental Mechanics 2002 42(4) p 372-379
48 Keller MW SR White and NR Sottos A self ‐ healing poly(dimethyl siloxane) elastomer
Advanced Functional Materials 2007 17(14) p 2399-2404
49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
vascular materials Advanced Materials 2010 22(45) p 5159-5163
50 Canadell J H Goossens and B Klumperman Self-healing materials based on disulfide links
Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
Macro Letters 2012 1(11) p 1233-1236
52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
56 Amamoto Y et al Repeatable photoinduced self ‐ healing of covalently cross‐ linked polymers
through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
60 Burattini S et al A healable supramolecular polymer blend based on aromatic π minus π stacking
and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
61 Trask RS GJ Williams and IP Bond Bioinspired self-healing of advanced composite
structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
62 Williams G R Trask and I Bond A self-healing carbon fibre reinforced polymer for
aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
httpwwwengineeringvillagecom64 Goacutemez DG et al In-depth numerical analysis of the TDCB specimen for characterization of
self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
69 Ono T T Nobori and J-MP Lehn Dynamic polymer blendsmdashcomponent recombination
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
structural properties of self-healing materials if they are to replace current structural materials the
toughness [64] and failure strength (among other properties) must be adequate Healing efficiency only
describes how well the material heals it does not indicate how the healing-enabled material performs
structurally compared to the original material
Figure 1 Number of publications per year containing the phrase ldquoself-healing polymerrdquo where 2015
contains number of publications for 2015 through July Data from [63]
To determine which material system is the best for any given application one must have a broad
knowledge of all potentially relevant self-healing materials This review outlines self-healing in
polymeric composite materials with a biomimetic approach in mind Engineered self-healing materials
can be said to imitate various stages in the biological healing process of bleeding The specific steps are
(i) bleeding (ii) clotting and (iii) regrowth The following sections discuss in detail three types of self-
healing polymeric materials self-healing polymers (regrowth) self-healing composites with dispersed
agents (clotting) and self-healing composites with vascular networks (bleeding)
2
Self-Healing Polymers
The final step in healing of a flesh wound is regrowth of the skin and underlying tissue This level of
healing involves fusion of the failure surfaces Ideally the healed area would be indistinguishable from
undamaged areas In a polymer system regrowth is accomplished through mechanisms which reconnect
the broken polymer chains The presence of reactive groups such as -C=C -COOH -NH2 -OH -SH -Si-O -S-S and -C=O (where C is carbon O is oxygen H is hydrogen N is nitrogen and S is sulfur) [65]
free radicals and cyclic structures enable self-healing Types of fusion of failure surfaces within
polymeric materials can be divided into two major groups reactions involving molecular covalent bonds
and those involving supramolecular chemistry [66]
983092983095
983096983097
983089983089983090 983089983088983092
983089983093983095983089983095983096
983090983093983094
983091983089983093983091983091983094
983089983095983096
983088
983093983088
983089983088983088
983089983093983088
983090983088983088
983090983093983088
983091983088983088
983091983093983088
983090983088983088983094 983090983088983088983095 983090983088983088983096 983090983088983088983097 983090983088983089983088 983090983088983089983089 983090983088983089983090 983090983088983089983091 983090983088983089983092 983090983088983089983093983082
983118983157983149983138983141983154 983151983142 983120983157983138983148983145983139983137983156983145983151983150983155 983106983161 983129983141983137983154
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21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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ACCEPTED MANUSCRIPT
after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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ACCEPTED MANUSCRIPT
Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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ACCEPTED MANUSCRIPT
materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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ACCEPTED MANUSCRIPT
structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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ACCEPTED MANUSCRIPT
Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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stimuli-responsive nanocontainers in self-healing coatings ACS nano 2013 7(3) p 24700-
2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
Structures 1996 35(3) p 263-269
153 Motuku M UK Vaidya and GM Janowski Parametric studies on self-repairing approaches
for resin infused composites subjected to low velocity impact Smart Materials and Structures
1999 8(5) p 623-638
154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
nanotubendashpolymer composites Carbon 2006 44(9) p 1624-1652
158 Wu AS et al Sensing of damage and healing in three-dimensional braided composites with
vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
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2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
21 Covalent Bonding
Covalent bonds break and reform depending on the local environment In terms of self-healing this
means bonds will reform after damage if given favorable conditions Many polymeric materials exploit
dynamic reversible covalent bonding to enable self-healing Low molecular weight polymers tend to
have high mobility and thus are often self-healing to some extent However not all low molecular weight
polymers exhibit self-healing For example unmodified polystyrene has a relatively low molecularweight but does not exhibit self-healing properties However simple modifications of polystyrene do
enable self-repair [67] Though the specifics depend on the exact material of interest healing mechanisms
based on covalent bonding can be grouped into three major categories general chain exchange reactions
cycloaddition and free radical reactions
Chain exchange reactions involve the reorganization of bonds (generally between chains sometimes
within a single chain) An example chain exchange reaction is the (re)formation of links between
acylhydrazines grafted onto the ends of polyethylene oxide (PEO) photographs illustrating the healing
properties of PEO by Deng et al are shown in Figure 2 [68] Two PEO samples were created colored
(one with carbon black and the other with rhodamine) and broken A carbon black half was placed in
contact with a rhodamine half After seven hours at room temperature the two halves had fused into a
single entity with a strong enough bond to withstand being squeezed by tweezers Healing in PEO is
achieved at ambient conditions [69] via the room temperature formation of bonds between the
acylhydrazine ends [70] These networks self-heal at ambient conditions [69] The bond-shuffling
reactions of disulfide chains and silonate end groups are additional examples of chain exchange reactions
[65] Healing in these systems is quick usually complete within 24 hours even at room temperature [71]
Figure 3 consists of time-delayed optical micrographs of a self-healing thiol-functinonalized polymer
[72] A razor blade was used to create a 50 microm wide and 500 microm long cut in the gt 15 microm thick polymer
film Within the first minute the ends of the cut began to close The cut was barely visible after one hour
of healing and it was fully healed within 24 hours
Figure 2 Optical images of self-healing covalent PEO gels (a) broken gel containing carbon black (b)
broken gel containing rhodamine (c) bicolor gel (d) healed gel (e) squeezed healed gel [68]
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ACCEPTED MANUSCRIPT
Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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ACCEPTED MANUSCRIPT
Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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ACCEPTED MANUSCRIPT
after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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ACCEPTED MANUSCRIPT
Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
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53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
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54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
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57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
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and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
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structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
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aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
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self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Figure 3 Optical micrographs of thiol-functionalized polymer under ambient conditions [72]
Neighboring disulfide bonds can switch bond locations via either free radical or ionic intermediates [73]
Figure 4 depicts a disulfide chain exchange [74] Disulfide free radicals may be formed through heating
[75] oxidation [76] or photolysis [77] Bond cleavage resulting in ionic intermediates is known as ionic
scission and may occur under other various conditions [78]- [79] S-S bonds may also be broken through
a reduction reaction where two thiol (S-H) groups are formed [65] The S-S bonds will reform through an
oxidation reaction Disulfide bonds have been incorporated into low glass transition temperature (Tg)
polymer networks (poly(ethylene glycol [80]) and high Tg networks (poly(n-butyl acrylate) [72])
Figure 4 Disulfide chain exchange figure modified from [74]
Amamoto et al showed that thiuram disulfide units incorporated in a low Tg polyurethane enable room
temperature self-healing under visible light [57] Disulfide bonds also enable room -temperature
self-healing in rubbers with near 100 healing efficiency of failure stress [50] and cohesive recovery[58] A self-healing hydrogel was synthesized incorporating both acylhydrazone and disulfide bonds did
successfully heal but the fracture stress healing efficiency was only 50 [81] Part of the reason for this
low healing efficiency may be due to the concentration of reactive groups Figure 5 is a graph of recovery
of strength as a function of disulfide group concentration [50] Clearly higher concentrations of the
reactive group lead to higher strength recovery While a given material system may not initially seem to
have a high enough healing efficiency one may not be analyzing the highest efficiencies possible for that
material However the concentration of the active group cannot be increased indefinitely (up to the
physical limit of 100 ) without altering other material properties Consider for example if Amamoto et
alrsquos polyurethane material was altered to contain 100 disulfide groups it would no longer be
polyurethane and one should not expect it to maintain polyurethanersquos properties
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Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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ACCEPTED MANUSCRIPT
Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Figure 5 Recovery of strength as a function of disulfide group concentrations Figure modified from [50]
Some self-healing materials combine healing with sensing That is the material conveys the information
that damage has occurred A notable example of a self-healing polymer that also indicates damage has
occurred is the covalently bonded poly(methyl methacrylate n-butyl acrylate1 3-dihydro-1 3 3-
trimethylspiro[2H-indole-23rsquo-[3H]-naphth[2 1-b][1 4]-oxazine]-2-amino-2-methylacrylate) or
p(MMAnBASNO) copolymer shown in Figure 6(a) [82] When the material is scratched the damaged
area turns red as shown in Figure 6(b) Figure 6(c) shows the reverse color change and healing of the
wound after exposure to acidic vapors Healing will also occur under sunlight or increased temperature
Bailey et al have shown that self-healing polymers may have additional functionalities such as electrical
conductivity [83]
Figure 6 Optical images of p(MMA nBASNO) copolymer (a) pre-scratch (b) post-scratch (c) repaired
[82]
Cycloaddition is a specific type of chain exchange reaction where unsaturated molecules combine and
form a ring A common cycloaddition reaction is the Diels-Alder reaction reversible cross-linking via a
[4 + 2] cycloaddition The bracket notation indicates the number of electrons each molecule contributes
In the case of a Diels-Alder reaction one molecular contributes four electrons while the other contributes
two The Diels-Alder reaction has been harnessed to enable self-healing in a number of materials
including epoxies polyacrylates and polyamides [84] In these materials cracking or elevating the
temperature of the material breaks the bond between diene and dienophile [85] Lowering the temperature
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ACCEPTED MANUSCRIPT
after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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ACCEPTED MANUSCRIPT
Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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ACCEPTED MANUSCRIPT
Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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ACCEPTED MANUSCRIPT
materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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2015 In Press p In Press
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ACCEPTED MANUSCRIPT
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10018
127 Herbst F S Seiffert and WH Binder Dynamic supramolecular poly(isobutylene)s for self-healing materials Polymer Chemistry 2012 3(11) p 3084-3092
128 Banerjee S et al Photoinduced smart self-healing polymer sealant for photovoltaics ACS
applied materials amp interfaces 2015 7(3) p 2064-2072
129 Phadke A et al Rapid self-healing hydrogels Proceedings of the National Academy of
Sciences of the United States of America 2012 109(12) p 4383-4388
130 Cordier P et al Self-healing and thermoreversible rubber from supramolecular assembly Nature 2008 451(7181) p 977-980
131 Montarnal D et al Versatile one-pot synthesis of supramolecular plastics and self-healing
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134 Colquhoun HM and Z Zhu Recognition of polyimide sequence information by a molecular
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135 Burattini S et al A novel self-healing supramolecular polymer system Faraday Discussions2009 143 p 251-264
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consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
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137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
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6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
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411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
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142 Wool RP and KM OConnor A theory crack healing in polymers Journal of Applied Physics
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149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
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2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
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156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
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159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
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formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
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168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
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171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
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173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
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174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
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175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
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178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
after damage causes the covalent bonds to reform healing the crack [86 87] In-depth analysis of a furan
thermoset polymer (the diene) and maleimide (the dienophile) network shows that the concentration of
crosslinking groups increases ability to self-heal [88] similar to the healing-concentration relationship in
disulfides [50] Changing the reactive groups present in methacrylate polymers alters healing behavior
with an oxygen-containing linker reportedly showing better healing ability than polar co-monomers [89]
It has even been shown that nanoparticles may be used to introduce this type of healing capability intoother polymers [90 91]
In addition to the Diels-Alder reaction other cycloaddition reactions may be utilized to form self-healing
polymers The [2 + 2] cycloaddition of 111-tris-(cinnamoyloxymethyl)ethane (TCE) monomers forms
cyclobutane [92] When the C-C bond in the cyclobutane ring breaks there are only separate cinnamoyl
groups Under UV exposure (gt 280 nm) [2 + 2] cycloaddition heals the bond reforming the cyclobutane
ring This reaction is illustrated in Figure 7 [92] A similar [2 + 2] cycloaddition can be observed in
coumarin [93] Perfluorocyclobutane polymers break under stress forming trifluorovinyl ether
monomers [94] Further stress causes a [2 + 2] cycloaddition to reform the polymer network indicating
that stress-induced crosslinking may be a useful mechanism for self-healing Anthracene derivatives
polymerize under UV radiation via a [4 + 4] cycloaddition reaction [95] and could also be incorporated tosynthesized self-healing polymers
Figure 7 Self-healing via [2 + 2] cycloaddition within cinnamoyl groups[92]
While light-induced self-healing shows much promise for creating self-healing structural materials the
radiation requirement may cause problems [65] First obviously a light source is required preferably of
monochromatic radiation Secondly the radiation may have unintended effects side-reactions may occur
For example radiation may increase the local temperature which could negatively affect the overall
healing process
A number of chain exchange reactions involve free radical intermediates As already discussed the
cleavage and restructuring of disulfide bonds may or may not involve free radicals depending on how
the bonds break For most self-healing polymers such as polyurethane [96] the healing process requiresfree radical intermediates Free radicals are very reactive in liquid or gaseous phases but their mobility
(and thus reactivity) drop within solid networks For healing to occur cleaved chain ends with reactive
groups must move to meet each other and react- all before other reactions intercept the free radicals For
efficient self-healing it is imperative to avoid radical-oxygen interactions [97] If the free radicals interact
with oxygen they cannot interact with other chain ends and thus the material will not self-heal [1]
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ACCEPTED MANUSCRIPT
Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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ACCEPTED MANUSCRIPT
Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
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15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
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42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
43 Liu D CY Lee and X Lu Repairability of impact-induced damage in SMC composites
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45 Corr DT et al Biomechanical behavior of scar tissue and uninjured skin in a porcine model Wound Repair and Regeneration 2009 17(2) p 250-259
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Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
Experimental Mechanics 2002 42(4) p 372-379
48 Keller MW SR White and NR Sottos A self ‐ healing poly(dimethyl siloxane) elastomer
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
vascular materials Advanced Materials 2010 22(45) p 5159-5163
50 Canadell J H Goossens and B Klumperman Self-healing materials based on disulfide links
Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
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and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
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structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
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aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
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self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
69 Ono T T Nobori and J-MP Lehn Dynamic polymer blendsmdashcomponent recombination
between neat dynamic covalent polymers at room temperature Chemical Communications
2005(12) p 1522-1524
70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
71 Rekondo A et al Catalyst-free room-temperature self-healing elastomers based on aromatic
disulfide metathesis Materials Horizons 2014 1 p 237-240
72 Yoon JA et al Self-healing polymer films based on thiolndashdisulfide exchange reactions and
self-healing kinetics measured using atomic force microscopy Macromolecules 2011 45(1) p
142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
American Chemical Society 2003 125(22) p 6624-6625
74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
Macromolecules 2008 41(14) p 5197-5202
75 Dogadkin B et al Polymerization phenomena in the vulcanization process Rubber Chemistry
and Technology 1954 27(4) p 920-92476 Nelander B and S Sunner Cogwheel effect in dialkyl disulfides Journal of the American
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Self-healing polystyrene can be synthesized by incorporating alkoxyamine bonds (C-ON) to form
dynamic reversible crosslinks [98] Figure 8 shows the disassociation of the alkoxyamine group and
subsequent free radical formation [74] This material along with many others will only heal if damage
causes free radicals to form Damage which severs the C-C backbone does not result in reactive groups on
the chain ends and thus does not allow for self-repair
Figure 8 Chain exchange facilitated by alkoxyamine free radical [74]
Environmental conditions are quite important for free radical stability Temperature has a major effect on
free radical stability [99 100] but there are other considerations In polycarbonate chains the presence of
sodium carbonate (Na2CO3) facilitates chain end interactions [101] Better interactions between chain
ends means more chain reconnections and thus better network repair The pH of a system may also be
important The maximum strength of 34-dihydroxphenylalanine-functionalized poly(ethylene glycol)
(DOPA-functionalized PEG) polymer depends on the relationship of the pH of the system and the
polymerrsquos acid dissociation constant [102] The DOPA-functionalized PEG can easily be edited to modifythe dissociation constant allowing precise design of a pH-controlled material
Trithiocarbonates (TTCs)- compounds containing CS3- enable bond reshuffling via free radical
intermediates [65] Incorporation of crosslinking TTCs enables self-healing in poly(methyl methacrylate)
(PMMA) and polystyrene [103] The C-S bonds in TTC rupture and reform when stimulated by UV
radiation of the appropriate wavelength [56] Reversible addition-fragmentation chain-transfer (RAFT)
polymerization of n-butyl acrylate (BA) with a TCC crosslinking unit results in a self-healing material via
highly mobile free radicals [56] The poly(BA) material reliably self-heals under UV radiation even after
repeated damage Figure 9 shows photographs of poly(BA) (a) after damage and (b) after healing under
330 nm radiation for 24 hours [51]
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Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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ACCEPTED MANUSCRIPT
Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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ACCEPTED MANUSCRIPT
[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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ACCEPTED MANUSCRIPT
For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
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80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
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healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
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1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
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Technology 2005 65(15-16) p 2474-2480
8 Olawale DO et al Progress in triboluminescence-based smart optical sensor system Journalof Luminescence 2011 131(7) p 1407-1418
9 Michaels D Their New Materials in The Wall Street Journal 2013 Dow Jones amp Company
New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
2013 Le Mauricien Ltd Port Louis Republic of Mauritius
11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
12 Okoli OI and GF Smith Failure modes of fibre reinforced composites The effects of strain
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13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
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14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
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15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
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16 Kessler MR NR Sottos and SR White Self-healing structural composite materials
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17 Balageas D Introduction to Structural Health Monitoring in Structural Health Monitoring D
Balageas C-P Fritzen and A Guumlemes Editors 2006 ISTE Ltd United States18 Sohn H et al A Review of Structural Health Monitoring Literature 1996-2001 2004 Los
Alamos National Laboratory
19 Lonkar K and S Roy IWSHM 2013 Program 2013 [cited 2014 May 20] Available from
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20 Kuo C-H et al Unmanned robot system for structure health monitoring and non-destructive
building inspection current technologies overview and future improvements in 9th International
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21 Shin J-U et al Quadrotor-based wall-climbing robot for structural health monitoring in 9th
International Workshop on Structural Health Monitoring 2013 Stanford University Stanford
CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
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23 Mallick PK Fiber-Reinforced Composites Materials Manufacturing and Design 2nd edDekker Mechanical Engineering 1993 New York New York USA CRC Press
24 Zwaag Svd AM Grande and W Post Review of current strategies to induce self-healing
behaviour in fibre reinforced polymer based composites Materials science and Technology
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25 Peterson AM RE Jensen and GR Palmese Thermoreversible and remendable glassndash
polymer interface for fiber-reinforced composites Composites Science and Technology 201171 p 586-592
26 Jones AR et al Full recovery of fibermatrix interfacial bond strength using a
microencapsulated solvent-based healing system Composites Science and Technology 2013 79
p 1-727 Sanada K N Itaya and Y Shindo Self-healing of interfacial debonding in fiber-reinforced
polymers and effect of microstructure on strength recovery Open Mechanical Engineering
Journal 2008 2 p 97-103
28 Blaiszik B J et al Autonomic recovery of fibermatrix interfacial bond strength in a model
composite Advanced Functional Materials 2010 20(20) p 3547-3554
29 Liu X and G Wang Progressive failure analysis of bonded composite repairs Composite
Structures 2007 81(3) p 331340
30 Baker A Bonded composite repair of fatigue-cracked primary aircraft structure CompositeStructures 1999 47(1-4) p 431-443
31 Naboulsi S and S Mall Thermal effects on adhesively bonded composite repair of cracked
aluminum panels Theoretical and applied fracture mechanics 1997 26(1) p 1-12
32 Chaudhry Z et al Monitoring the integrity of composite patch structural repair via piezoelectric
actuatorssensors in AIAAASMEASCEAHSASC 36th Structures Structural Dynamics and
Materials Conference Adaptive Structures Forum 1997 New Oreleans LA USA AIAA Publishing
33 Hale J Boeing 787 from the Ground Up in QTR_04 - A Quarterly Publication 2006 Boeing
34 Mahdi S Composite Repair Analysis 2007 Airbus Spring 2007 CACRC Meeting
35 Hellard G Composites in Airbus- A Long Story of Innovations and Experiences 2008 Airbus
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36 Committee on Aging of US Air Force Aircraft NMAB Aging of US Air Force Aircraft 1997 Commission on Engineering and Technical Systems National Reseasrch Council
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38 Kelly LJ Introductory chapter in Bonded Repair of Aircraft Structures AA Baker and R
Jones Editors 1988 Martinus Nijhoff Publishers Boston MA USA p 1-18
39 Dittrich K S Kupczyk and HW Schroeder Repair of composite materials in Google Patents
GPaTMO (DPMA) Editor 1990 Dornier Luftfahrt GmbH Germany
40 Ur-Rehman A and PF Thomason The effect of artificial fatigue-crack closure on fatigue-crack
growth Fatigue amp Fracture of Engineering Materials amp Structures 1993 16(10) p 1081-1090
41 Raghavan J and RP Wool Interfaces in repair recycling joining and manufacturing of polymers and polymer composites Journal of Applied Polymer Science 1999 71(5) p 775-785
42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
43 Liu D CY Lee and X Lu Repairability of impact-induced damage in SMC composites
Journal of composite materials 1993 27(13) p 1257-1271
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277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Figure 9 Photographs of BA polymer (a) after damage and (b) after healing [56]
Four-membered rings form particularly stable free radicals [65] Four-membered rings also tend to have
low ring-opening activation energy oxetanes for example require just 10-40 kilojoules [104] roughly
equal to the amount of energy released by burning a single gram of coal Ghosh et al developed aself-healing heterogeneous network comprised of polyurethane oxetane (OXE) and chitosan (CHI) [96]
The OXE provides a four-membered ring and the CHI provides UV-sensitivity [105] The same research
group went on to develop an oxolane (OXO)-CHI-polyurethane network [106] OXO was chosen for its
structural similarities to OXE and its much lower activation energy [107] Both the OXE-CHI and the
OXO-CHI polyurethane networks self-healed under UV light but the OXO-containing network repaired
more slowly [106] The difference in repair times was attributed to a difference in ring strain
Materials which do not require external stimuli to initiate the healing process are of particular interest for
commercial applications Diarylbibenzofuranone (DABBF) has been used as the crosslinking agent in
several types of polymers [108] Chosen for its easily obtained state of thermodynamic equilibrium [109]
cleaved DABBF forms stable free radicals with high oxygen tolerance [110] Polymers incorporatingDABBF were found to self-repair at room temperature without any external stimuli with fracture stress
healing efficiency over 95 [108]
22 Supramolecular Chemistry
Supramolecular chemistry has been a focus of research efforts for over 50 years [111 112] Several
self-healing mechanisms depend on the structure of the overall network rather than the organization of
individual molecules Supramolecular interactions allow faster networks remodeling than do covalent
bonds Though both covalent bonds and supramolecular interactions are directional supramolecular
interactions tend to be more sensitive [113] Unfortunately supramolecular polymers networks tend to
have a lower Tg meaning the polymers are relatively soft and may not be useful in structural applications
Supramolecular chemistry of interest in self-healing materials can be categorized as hydrogen bondingπ -π stacking interactions and ionomer healing
Even though hydrogen bonds are generally weaker than covalent bonds significant strength can be
obtained due to the hydrogen bonding within certain materials [114 115] Alignment of multiple
hydrogen bonds in a row allows control over many material properties including viscosity and chain
length [116] Furthermore units with four hydrogen bonds tend to be more stable than those with just two
or three and may have increased strength [117 118]
a) b)
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Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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ACCEPTED MANUSCRIPT
Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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ACCEPTED MANUSCRIPT
Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
1 Urban MW Dynamic materials The chemistry of self-healing Nature chemistry 2012 4(2) p
80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
and molecular mechanisms Journal of International Medical Research 2009 37(5) p 1528-
1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositemdashPart II In situ self-healing Composites Science and
Technology 2005 65(15-16) p 2474-2480
8 Olawale DO et al Progress in triboluminescence-based smart optical sensor system Journalof Luminescence 2011 131(7) p 1407-1418
9 Michaels D Their New Materials in The Wall Street Journal 2013 Dow Jones amp Company
New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
2013 Le Mauricien Ltd Port Louis Republic of Mauritius
11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
12 Okoli OI and GF Smith Failure modes of fibre reinforced composites The effects of strain
rate and fibre content Journal of Materials Science 1998 33(22) p 5415-5422
13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
of carbon fiber reinforced polymer composite Composite Structures 2015 124 p 345-356
14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
composite sandwich panels Composites Part A Applied Science and Manufacturing 2001
32(9) p 1189-1196
15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
prediction Composites Part A Applied Science and Manufacturing 2002 33(3) p 315-321
16 Kessler MR NR Sottos and SR White Self-healing structural composite materials
Composites Part A Applied Science and Manufacturing 2003 34(8) p 743-753
17 Balageas D Introduction to Structural Health Monitoring in Structural Health Monitoring D
Balageas C-P Fritzen and A Guumlemes Editors 2006 ISTE Ltd United States18 Sohn H et al A Review of Structural Health Monitoring Literature 1996-2001 2004 Los
Alamos National Laboratory
19 Lonkar K and S Roy IWSHM 2013 Program 2013 [cited 2014 May 20] Available from
httpstructurestanfordeduworkshopprogramhtml
20 Kuo C-H et al Unmanned robot system for structure health monitoring and non-destructive
building inspection current technologies overview and future improvements in 9th International
Workshop on Structural Health Monitoring 2013 Stanford University Stanford CA
21 Shin J-U et al Quadrotor-based wall-climbing robot for structural health monitoring in 9th
International Workshop on Structural Health Monitoring 2013 Stanford University Stanford
CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
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213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Ureidopyrimidinone (UPy) is easy to prepare and has a high dimerization constant which aids in
constructing polymers with high degrees of polymerization [119] UPy is very stable partially due to its
quadruple hydrogen bonds [120] An example of UPyrsquos hydrogen bonding is shown in Figure 10 [121]
An investigation of a number of UPy and other supramolecular polymers revealed that a number of bulk
properties including melt viscosity are highly temperature dependent [122] The temperature dependent
properties of UPy can be combined with a thermally responsive polymer matrix to develop materials withthermo-regulated self-healing behavior [123] UPy has also been used within poly(ethylene-co-butylene)
reinforced with cellulose nanocrystals [124] resulting in a UV-sensitive self-healing composite
Figure 10 Chemical structure of a hydrogen bonded UPy-dimer [121]
A number of other self-healing materials have been created using the properties of hydrogen bonding
such as poly(isobutylene) (PIB) PIB exhibits extensive hydrogen bonding [125 126] Switching out the
hydrogen bonding moieties in PIB allows control over clustering behavior of the polymer and thus controlover its self-healing [127] Coumarin-functionalized PIB heals under sunlight and has been successfully
used to create a self-healing coat for photovoltaic devices [128] Incorporation of dangling polar
side-chains into acryloyl-6-aminocaproic acid precursors has led to the development of rapidly
self-healing hydrogels [129] Poly(vinyl alcohol) (PVA) hydrogels autonomously self-heal with ~72
fracture stress healing efficiency [51] The self-healing behavior of PVA gel can be seen in the
photographs in Figure 11 [51] Similar to the covalently healing PEO gels in Figure 2 [68] two separate
PVA blocks were formed and one was colored with rhodamine B [51] The blocks were cut and one half
of each was placed to form a bicolored gel After 12 hours at ambient conditions the bicolored gel healed
into a single unit The healed gel can be stretched up to 100 extension Figure 12 shows the fracture
stress of PVA samples healed under identical conditions after different amounts of separation time [51]
Longer separation time results in less fracture stress recovery The lower healing efficiency may be due toa decrease in concentration of reactive groups over time As demonstrated in other systems (see Figure 5)
[50] healing efficiency greatly depends on reactive group concentration As time passes these groups
react If the void volume is too large reactions may occur on a single side of the damaged area leading to
a partially healed state
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Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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ACCEPTED MANUSCRIPT
structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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ACCEPTED MANUSCRIPT
Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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ACCEPTED MANUSCRIPT
Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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ACCEPTED MANUSCRIPT
Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
1 Urban MW Dynamic materials The chemistry of self-healing Nature chemistry 2012 4(2) p
80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
and molecular mechanisms Journal of International Medical Research 2009 37(5) p 1528-
1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositemdashPart II In situ self-healing Composites Science and
Technology 2005 65(15-16) p 2474-2480
8 Olawale DO et al Progress in triboluminescence-based smart optical sensor system Journalof Luminescence 2011 131(7) p 1407-1418
9 Michaels D Their New Materials in The Wall Street Journal 2013 Dow Jones amp Company
New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
2013 Le Mauricien Ltd Port Louis Republic of Mauritius
11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
12 Okoli OI and GF Smith Failure modes of fibre reinforced composites The effects of strain
rate and fibre content Journal of Materials Science 1998 33(22) p 5415-5422
13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
of carbon fiber reinforced polymer composite Composite Structures 2015 124 p 345-356
14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
composite sandwich panels Composites Part A Applied Science and Manufacturing 2001
32(9) p 1189-1196
15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
prediction Composites Part A Applied Science and Manufacturing 2002 33(3) p 315-321
16 Kessler MR NR Sottos and SR White Self-healing structural composite materials
Composites Part A Applied Science and Manufacturing 2003 34(8) p 743-753
17 Balageas D Introduction to Structural Health Monitoring in Structural Health Monitoring D
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206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Figure 11 Optical images of PVA gel (a) two separate blocks (b) two halves of each original hydrogel (c)
bicolored gel (d) bent healed hydrogel (e) stretched healed hydrogel [51]
Figure 12 Fracture stress of various samples healed under identical conditions 0 1 or 24 hours after damage
[51]
Thermoreversible rubbers incorporating functional groups attached to carboxylic acids self-heal at room
temperature [130] The process for creating these rubbers is simple with just three steps required Slight
variations produce a wide variety of solid and viscoelastic rubbers [131] The healing in these rubbers is
activated by the damage event a promising characteristic for autonomy [132] Unfortunately exposure to
raised temperatures or moisture significantly decreases healing ability Above 110 degC irreversible cross-
linking prevents healing [133]
Heterogeneous systems are particularly interesting for the design of self-healing materials Clever
combination of a ldquohardrdquo backbone (high Tg materials like polystyrene) with ldquosoftrdquo brushes (low Tg
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ACCEPTED MANUSCRIPT
materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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ACCEPTED MANUSCRIPT
structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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ACCEPTED MANUSCRIPT
Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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ACCEPTED MANUSCRIPT
Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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ACCEPTED MANUSCRIPT
[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
1 Urban MW Dynamic materials The chemistry of self-healing Nature chemistry 2012 4(2) p
80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
and molecular mechanisms Journal of International Medical Research 2009 37(5) p 1528-
1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
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Technology 2005 65(15-16) p 2474-2480
8 Olawale DO et al Progress in triboluminescence-based smart optical sensor system Journalof Luminescence 2011 131(7) p 1407-1418
9 Michaels D Their New Materials in The Wall Street Journal 2013 Dow Jones amp Company
New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
2013 Le Mauricien Ltd Port Louis Republic of Mauritius
11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
12 Okoli OI and GF Smith Failure modes of fibre reinforced composites The effects of strain
rate and fibre content Journal of Materials Science 1998 33(22) p 5415-5422
13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
of carbon fiber reinforced polymer composite Composite Structures 2015 124 p 345-356
14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
composite sandwich panels Composites Part A Applied Science and Manufacturing 2001
32(9) p 1189-1196
15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
prediction Composites Part A Applied Science and Manufacturing 2002 33(3) p 315-321
16 Kessler MR NR Sottos and SR White Self-healing structural composite materials
Composites Part A Applied Science and Manufacturing 2003 34(8) p 743-753
17 Balageas D Introduction to Structural Health Monitoring in Structural Health Monitoring D
Balageas C-P Fritzen and A Guumlemes Editors 2006 ISTE Ltd United States18 Sohn H et al A Review of Structural Health Monitoring Literature 1996-2001 2004 Los
Alamos National Laboratory
19 Lonkar K and S Roy IWSHM 2013 Program 2013 [cited 2014 May 20] Available from
httpstructurestanfordeduworkshopprogramhtml
20 Kuo C-H et al Unmanned robot system for structure health monitoring and non-destructive
building inspection current technologies overview and future improvements in 9th International
Workshop on Structural Health Monitoring 2013 Stanford University Stanford CA
21 Shin J-U et al Quadrotor-based wall-climbing robot for structural health monitoring in 9th
International Workshop on Structural Health Monitoring 2013 Stanford University Stanford
CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
23 Mallick PK Fiber-Reinforced Composites Materials Manufacturing and Design 2nd edDekker Mechanical Engineering 1993 New York New York USA CRC Press
24 Zwaag Svd AM Grande and W Post Review of current strategies to induce self-healing
behaviour in fibre reinforced polymer based composites Materials science and Technology
2014 30(13a) p 1633-1641
25 Peterson AM RE Jensen and GR Palmese Thermoreversible and remendable glassndash
polymer interface for fiber-reinforced composites Composites Science and Technology 201171 p 586-592
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216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
materials like poly(n-butyl acrylate)) yields a self-healing polymer [53] The backbone provides strength
while the brushes facilitate hydrogen bonding Polystyrene (backbone) and polyacrylate amide (brushes)
form a spontaneously self-healing multiphase polymer [52] Similar hydrophobichydrophilic interactions
are utilized in certain self-healing hydrogels The copolymer of acrylamide and stearyl methacrylate
(C18) self-heals via reversible crosslinking zones [54] Healing in the C18-acrylamide gels seems to be
driven by free non-associated C18 blocks near the failure surface
Another type of supremolecular interaction which has been investigated is the stacking of π electron
orbitals such as that found between pyrenyl dertivatives and diimide residue in certain polymers [134]
A blend of folding ldquotweezer-typerdquo polyimide and linear polysiloxane as a backbone has been found to
self-heal at 100 degC [135] Similarly a polyimide-polyamide network heals with 100 tensile modulus
healing efficiency at 50 degC [136] These polyimide polymers are able to heal due to careful positioning of
phrenyl residues at the ends of the backbone chains in conjunction with the folding ability of the
polydiimide [137]
π-π stacking can be used in conjunction with hydrogen bonding in hybrid polymers Polyimide with
pyrenemethylurea-functionalized polybutadiene has a toughness healing efficiency of 77 [60]Similarly bis-pyrenyl-functionalized polyamide self-heals at 140 degC with 100 tensile modulus healing
efficiency [138] A drawback of these supramolecular polymeric networks is that they are necessarily
rather weaker than chemically bonded networks To develop a gel with a higher mechanical strength Xu
et al synthesized a number of self-healing nitrobenzoxadiazol-appended cholesterol derivatives [139]
With an appropriate gelator concentration the yield strength of such gels reaches 23 kPa an improvement
over other low-molecular mass gelators but on par with the yield strengths reports in Ref [129] (35 kPa)
and Ref [51] (200 kPa) The healing efficiency of Xu el alrsquos gels was not reported
An additional self-healing reaction which does not fit well into the above categories is that of the
ionomer poly(ethylene-co-methacrylic acid) In this material the healing of puncture wounds is
significantly different from the healing of sawing or cutting damage [140] This type of healing has beentermed an ionic interaction [65] but it has actually been determined that ionic components are
unnecessary for healing to occur [141] This type of self-healing occurs in two steps In the first step the
projectile impact disrupts the ionomeric network and friction between the projectile and the material
generates heat The heat is transferred to the polymer surrounding the damage area causing localized
melting In the second step the molten surfaces fuse together as would polymer chains with high mobility
[142] Ionic concentration may help the process along but too high a concentration actually reduces the
healing efficiency [141]
3
Self-Healing Composites Dispersed Agents
Before skin can regrow over a flesh wound the wound must close Many engineered materials mimic this
clotting step The healing agents may by liquid or solid In the previous section the healing agent was
simply the solid polymer matrix However many of the materials described in the preceding section have
low Tg toughness andor strength making them undesirable as structural materials This section
discusses the development of self-healing composite materials capable of holding the loads required of
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ACCEPTED MANUSCRIPT
structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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ACCEPTED MANUSCRIPT
Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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ACCEPTED MANUSCRIPT
Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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ACCEPTED MANUSCRIPT
[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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ACCEPTED MANUSCRIPT
For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
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80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
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healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
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1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
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6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
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New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
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11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
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13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
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14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
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15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
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Balageas C-P Fritzen and A Guumlemes Editors 2006 ISTE Ltd United States18 Sohn H et al A Review of Structural Health Monitoring Literature 1996-2001 2004 Los
Alamos National Laboratory
19 Lonkar K and S Roy IWSHM 2013 Program 2013 [cited 2014 May 20] Available from
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20 Kuo C-H et al Unmanned robot system for structure health monitoring and non-destructive
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21 Shin J-U et al Quadrotor-based wall-climbing robot for structural health monitoring in 9th
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CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
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25 Peterson AM RE Jensen and GR Palmese Thermoreversible and remendable glassndash
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26 Jones AR et al Full recovery of fibermatrix interfacial bond strength using a
microencapsulated solvent-based healing system Composites Science and Technology 2013 79
p 1-727 Sanada K N Itaya and Y Shindo Self-healing of interfacial debonding in fiber-reinforced
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Journal 2008 2 p 97-103
28 Blaiszik B J et al Autonomic recovery of fibermatrix interfacial bond strength in a model
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29 Liu X and G Wang Progressive failure analysis of bonded composite repairs Composite
Structures 2007 81(3) p 331340
30 Baker A Bonded composite repair of fatigue-cracked primary aircraft structure CompositeStructures 1999 47(1-4) p 431-443
31 Naboulsi S and S Mall Thermal effects on adhesively bonded composite repair of cracked
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32 Chaudhry Z et al Monitoring the integrity of composite patch structural repair via piezoelectric
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38 Kelly LJ Introductory chapter in Bonded Repair of Aircraft Structures AA Baker and R
Jones Editors 1988 Martinus Nijhoff Publishers Boston MA USA p 1-18
39 Dittrich K S Kupczyk and HW Schroeder Repair of composite materials in Google Patents
GPaTMO (DPMA) Editor 1990 Dornier Luftfahrt GmbH Germany
40 Ur-Rehman A and PF Thomason The effect of artificial fatigue-crack closure on fatigue-crack
growth Fatigue amp Fracture of Engineering Materials amp Structures 1993 16(10) p 1081-1090
41 Raghavan J and RP Wool Interfaces in repair recycling joining and manufacturing of polymers and polymer composites Journal of Applied Polymer Science 1999 71(5) p 775-785
42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
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glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
structural components The method of activating the healing agents is a major factor in the range of
achieved healing efficiencies As shown in Figure 13 the different components for certain material
systems may be (a) manually injected into the crack (b) incorporated within the material matrix or (c) a
combination of incorporation and injection [16] For an epoxy material system toughness healing
efficiency ranged from 38 to 99 depending on the method of incorporation [16] Up to 119 healing
efficiency has been reached by altering the epoxy chemistry [143 144] For true self -healing injection isnot a valid incorporation method Healing efficiencies reported in the following sections are for self-
healing specimens The healing agents in a self-healing composites are often liquids which must be
encapsulated to separate the healing agent from the matrix material as discussed in the next subsection
The subsequent subsections describe remote self-healing and shape memory assisted self-healing
techniques either of which could be used in conjunction with an appropriate encapsulation technique for
that material system
Figure 13 Three methods of inclusion for the healing agents and catalyst in a composite (a) injection (b)
incorporation) (c) a combination of injection amp incorporation Figure modified from [16]
31
EncapsulationThe idea of capturing crosslinking reactants andor catalysts within microcapsules was first presented for
use in the textile industry [145] The exploitation of encapsulation quickly expanded to include the
materials needed for polymer coatings [146] An encapsulated system which specified that the
microspheres rupture under light pressure was proposed in 1978 again for use in textiles [147] The idea
of enclosing reactants and implanting the capsules within another material was developed for use in
biological wound healing [148] and later in composite materials and coatings to enable self-healing and
protection [149] Encapsulation may be accomplished using hollow fibers [150] nanotubes [151] or
microspheres [44] Following the discussion of these types of encapsulation is a discussion on the various
materials which may be used in these systems specifically the catalyst and healing agents as well as the
concentration and dispersion of these materials
Dry proposed a self-repairing composite material based on incorporated hollow fibers [150] The size
shape and composition of the fibers can be altered as a particular application dictated The hollow fibers
are filled with a healing agent The invention was proposed for use in both cementitious and
fiber-reinforced polymer composites Hollow fiber encapsulation is often grouped with vascular systems
more fully discussed in the following section The key difference is that vascular systems are accessible
from outside the bulk material additional liquid healing agent can be added to the system at will
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Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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ACCEPTED MANUSCRIPT
[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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ACCEPTED MANUSCRIPT
caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
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Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
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Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
Macro Letters 2012 1(11) p 1233-1236
52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
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and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
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structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
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aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
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self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
69 Ono T T Nobori and J-MP Lehn Dynamic polymer blendsmdashcomponent recombination
between neat dynamic covalent polymers at room temperature Chemical Communications
2005(12) p 1522-1524
70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
71 Rekondo A et al Catalyst-free room-temperature self-healing elastomers based on aromatic
disulfide metathesis Materials Horizons 2014 1 p 237-240
72 Yoon JA et al Self-healing polymer films based on thiolndashdisulfide exchange reactions and
self-healing kinetics measured using atomic force microscopy Macromolecules 2011 45(1) p
142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
American Chemical Society 2003 125(22) p 6624-6625
74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
Macromolecules 2008 41(14) p 5197-5202
75 Dogadkin B et al Polymerization phenomena in the vulcanization process Rubber Chemistry
and Technology 1954 27(4) p 920-92476 Nelander B and S Sunner Cogwheel effect in dialkyl disulfides Journal of the American
Chemical Society 1972 94(10) p 3574-3577
77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Dry demonstrated that hollow glass pipette tubes filled with cyanoacrylate resin enable self-healing in a
reinforced polymer material [152] Motuku later showed that other materials could be used as the hollow
fiber or capillary [153] Copper and aluminum capillaries were found to be less useful for self-healing
than glass capillaries since glassrsquo brittleness means it breaks easily and allows the encapsulated resin to
flow out into the crack In both Dryrsquos and Motukursquos experiments the flow of the resin into the crack was
visually observed healing efficiency was not determined
Many features factor into the efficiency of these self-healing systems The viscosity of the healing agent
and the diameter of the hollow fibers determine how well the resin flows out into the crack Figure 14
shows a fractured resin-filled hollow glass fiber with minimal resin flow into the damage area due to a
poor viscosity-diameter match [154] Related to viscosity is of course the temperature of the system and
the time allowed for healing Additional factors to consider are related to the method of incorporation for
the resin-infused fibers [155] Fiber spacing and length the fraction of filled fibers versus simple fibers
for reinforcement weave and lamination pattern may all have an effect on both the healing efficiency of
the system and the virgin mechanical properties
Figure 14 Fractured resin-filled hollow glass fiber [154]
Nanotubes may also be used to encapsulate materials necessary for healing Carbon nanotubes (CNTs)
[156] are being widely embraced as reinforcement materials for polymer composites for their impressive
mechanical properties and potential for additional functionalities [157] such as electrical resistance-based
sensing [158] The question then arises since composites are already being fabricated containing CNTs
can the CNTs be further functionalized to enable self-healing From a molecular dynamics point of view
Lanzara et al proposed that CNTs may indeed be used as nanoreservoirs to contain healing materials
[159] Of course such a system will only be possible if the healing agent can be injected inside the CNTs
and only be effective if the CNTs actually rupture to release the encapsulated materials The research on
failure of CNTs is extensive [160] and complex [161] but as of yet they have not been utilized asnanoreservoirs despite being used as reinforcement [162] The major issue is getting the healing agent to
release upon damage since CNTs are very strong and thus may not rupture Concerns about the small
diameter of the nanotubes and resin viscosity are not as alarming for SiO2-polymer hybrid nanotubes
[151] and polyelectrolyte nanocapsules [163] have been successfully used as the capsules within
anti-corrosion coatings proving that nanoreservoirs are viable
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Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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10018
127 Herbst F S Seiffert and WH Binder Dynamic supramolecular poly(isobutylene)s for self-healing materials Polymer Chemistry 2012 3(11) p 3084-3092
128 Banerjee S et al Photoinduced smart self-healing polymer sealant for photovoltaics ACS
applied materials amp interfaces 2015 7(3) p 2064-2072
129 Phadke A et al Rapid self-healing hydrogels Proceedings of the National Academy of
Sciences of the United States of America 2012 109(12) p 4383-4388
130 Cordier P et al Self-healing and thermoreversible rubber from supramolecular assembly Nature 2008 451(7181) p 977-980
131 Montarnal D et al Versatile one-pot synthesis of supramolecular plastics and self-healing
rubbers Journal of the American Chemical Society 2009 131(23) p 7966-7967
132 Maes F et al Activation and deactivation of self-healing in supramolecular rubbers Soft
Matter 2012 8(5) p 1681-1687
133 Zhang R et al Heterogeneity segmental and hydrogen bond dynamics and aging ofsupramolecular self-healing rubber Macromolecules 2013 46(5) p 1841-1850
134 Colquhoun HM and Z Zhu Recognition of polyimide sequence information by a molecular
tweezer Angewandte Chemie 2004 43(38) p 5040-5045
135 Burattini S et al A novel self-healing supramolecular polymer system Faraday Discussions2009 143 p 251-264
136 Burattini S et al A self-repairing supramolecular polymer system healability as a
consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
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137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
compounds Macromolecular Rapid Communications 2009 30(6) p 459-463
138 Burattini S et al A supramolecular polymer based on tweezer-type π minusπ stacking interactions
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6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
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411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
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142 Wool RP and KM OConnor A theory crack healing in polymers Journal of Applied Physics
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146 Bulbenko GF EH Sorg and JP Gallagher One-part polythiol compositions containing
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147 Wolinski LE and PD Berezuk Thermoplastic polyurethane resin dissolved in an acrylic
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USA
149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
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151 Li GL et al Silicapolymer double-walled hybrid nanotubes synthesis and application as
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2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
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154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
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156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
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vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
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162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
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nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
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165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
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168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Nearly three decades after the initial encapsulation patent White et al presented a polymer composite
incorporating catalyst and a healing agent encapsulated within microspheres [44] such as the hollow
glass bubbles shown in Figure 15 [164] The key behind Whitersquos self-healing polymer is ring-opening
metathesis polymerization (ROMP) Bis(tricyclohexylphosphine) benzylidine ruthenium(IV) dichloride
(Grubbsrsquo catalyst) polymerizes dicyclopentadiene (DCPD) within minutes at room temperature To create
a self-healing composite the catalyst is dispersed throughout the resin matrix and DCPD is encapsulatedin-situ In-situ encapsulation is accomplished with urea-formaldehyde (UF) shells [165] Damage to the
composite causes the microcapsules break releasing the DCPD into the matrix where it reacts with the
catalyst Figure 16 illustrates the damage-to-healing process [44] Before any damage occurs there are
microcapsules and smaller catalyst particles dispersed throughout the matrix The microcapsules contain
liquid healing agent In Figure 16(a) crack initiation occurs and a crack starts propagating through the
matrix In Figure 16(b) the crack continues to grow and ruptures two microcapsules releasing healing
agent into the damaged area In Figure 16(c) the healing agent reacts with catalyst particles in the
damaged area The healing agent cures repairing the damage The encapsulation process has been well
documented [166] and proves to be useful in many industries including electronics packaging
automotive [167] and even sports [168] A numerical model describing the crack retardation and closure
in this type of composite has been developed [169] Either or both of the catalyst and healing agent may
be encapsulated [170]
Figure 15 SEM image of hollow glass bubbles used in encapsulation-based self-healing epoxy polymer [164]
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ACCEPTED MANUSCRIPT
Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
1 Urban MW Dynamic materials The chemistry of self-healing Nature chemistry 2012 4(2) p
80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
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193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Figure 16 Diagram of healing process in a microencapsulated system (a) crack initiation (b) healing agent
release (c) curing [44]
White et al [44] paved the way for encapsulation-based self-healing [171] A phenomenological cure
kinetics model shows DCPD should heal at 80degC with nearly twice the efficiency it would have at room
temperature [172] A number of experiments have been done to investigate the effect on material strength
and healing of different types of microcapsules Inclusion of 180 microm diameter UF shells has been found
to increase the virgin toughness up to 127 that of neat resin [173] Smaller diameters tend to lower the
failure load [174] A variety of materials may be used for the microcapsules initial microspheres were
made of UF but silica [173] and melamine-urea-formaldehyde [175] have also been used
Special interest has been given to employment of the catalyst Several models have been developed to
describe the curing behavior based on catalyst concentration [172] More recently it has been determined
that Grubbsrsquo catalyst exists in at least three polymorphs each with its own distinct crystal shape
dissolution kinetics and thermal stability [176] 2nd generation Grubbsrsquo catalyst was considered for use in
self-healing composites particularly for its thermal stability [177] Later the two forms were revisited and
1
st
generation Grubbsrsquo catalyst was favored since it was found to catalyze faster as well as have atendency to be more homogeneously distributed through the matrix [178] To avoid using the
ruthenium-based Grubbsrsquo catalyst tungsten(VI) chloride (WCl6) was identified as a potential catalyst
[179] WCl6 is cheaper is widely available and has a significantly higher melting point (275 degC) than
does Grubbsrsquo catalyst (153 degC) In an epoxy matrix a toughness healing efficiency of 20 when both
DCPD and WCl6 were embedded but an efficiency of 107 was reached when the WCl6 was embedded
and DCPD was injected into the crack [180] More recently scandium(III) triflate has been suggested as a
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ACCEPTED MANUSCRIPT
solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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ACCEPTED MANUSCRIPT
caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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ACCEPTED MANUSCRIPT
Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
43 Liu D CY Lee and X Lu Repairability of impact-induced damage in SMC composites
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Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
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Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
56 Amamoto Y et al Repeatable photoinduced self ‐ healing of covalently cross‐ linked polymers
through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
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and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
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structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
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aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
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self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
69 Ono T T Nobori and J-MP Lehn Dynamic polymer blendsmdashcomponent recombination
between neat dynamic covalent polymers at room temperature Chemical Communications
2005(12) p 1522-1524
70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
71 Rekondo A et al Catalyst-free room-temperature self-healing elastomers based on aromatic
disulfide metathesis Materials Horizons 2014 1 p 237-240
72 Yoon JA et al Self-healing polymer films based on thiolndashdisulfide exchange reactions and
self-healing kinetics measured using atomic force microscopy Macromolecules 2011 45(1) p
142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
American Chemical Society 2003 125(22) p 6624-6625
74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
Macromolecules 2008 41(14) p 5197-5202
75 Dogadkin B et al Polymerization phenomena in the vulcanization process Rubber Chemistry
and Technology 1954 27(4) p 920-92476 Nelander B and S Sunner Cogwheel effect in dialkyl disulfides Journal of the American
Chemical Society 1972 94(10) p 3574-3577
77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
trisulphides Australian Journal of Chemistry 1963 16(6) p 1027-1037
78 McAllan DT et al The preparation and properties of sulfur compounds related to petroleum
I The dialkyl sulfides and disulfides Journal of the American Chemical Society 1951 73(8) p
3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
thiol-disulfide systems Journal of the American Chemical Society 1957 79(17) p 4589-4593
80 Miyata K et al Freeze-dried formulations for in vivo gene delivery of PEGylated polyplex
micelles with disulfide crosslinked cores to the liver Journal of Controlled Release 2005 109(1-
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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67(2) p 201-212
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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solid phase alternative catalyst showing up to 86 healing efficiency when paired with (diglycidyl ether
bisphenol A)-(ethyl phenylacetate) as a healing agent [181]
The other healing agents involved in the healing reaction are of equal importance DCPD has two
stereoisomers with the form highly affecting healing mechanics [182] The exo-isomer is over an order of
magnitude more reactive than the endo-isomer [183] but has a lower healing efficiency because the fast
polymerization blocks the full release of the healing agent [182] The endo-iosmer has the added benefits
of being commercially available having a long shelf life and having a low viscosity [182] Blending
DCPD with 5-ethylidene-2-norbornene (ENB) resulted in a material with an accelerated cure reaction
requiring less catalyst [184] CuBr2-(2-methylimidazole)4 is a stable alternative to DCPD with higher
adhesion strength than the typical epoxy healing agent [185] A healing efficiency of 104 has been
reported for a system using epoxy with mercaptan as the hardener [74] DCPD can also be replaced with a
liquid phase diisocyanate which being reactive with water removes the need for any catalyst [186]
Hexamethylene diisocyanate has been found to be exceedingly useful as an anti-corrosion coating and
may find a use in bulk self-healing materials [187 188] Mixing a low-viscosity healing agent with a
diisocyanate may further improve healing ability [189] It is of course important to match the matrix
material the healing agent and whatever hardener or catalyst is required What healing agent is best inone matrix may not be ideal in a different matrix Figure 17 highlights this difference with the healing
efficiencies of three different epoxy matrices using three different healing agent mixtures [177] M1 is
EPON 828 cured with diethylenetriamine (DETA) M2 is EPON 828 containing Heloxy 71 as a
flexibilizer and cured with Ancamine K53 M3 is EPON 862 cured with EPICURE 3274 Healing agents
were DCPD either alone mixed with 5-norbornene-2-carboxylic acid (NCA) or mixed with 5-ethylidene-
2-norbornene (ENB) with the norbornene compounds included as adhesion promoters Furthermore self-
healing composite systems do not require an epoxy matrix For example poly(dimethyl siloxane)
(PDMS) and poly(diethoxy siloxane) (PDES) can be combined to form a chemically stable self-healing
material [55] This material holds the notable benefit of stability in humid or wet environments though
the fracture stress healing efficiency is rather low under 25 The PDMSPDES material has been
proposed for a self-healing coating for structural materials [170] Other matrix materials may be chosen
by careful consideration of polymers capable of self-healing like PDMS [190]
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Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Figure 17 Healed peak fracture load for samples using three different epoxy matrices and three different
encapsulated healing agents [177]
Concentration and size of both the healing agent and the catalyst need to be considered [191] As seen in
Figure 18 a poly(dimethyl siloxane) (PDMS) matrix with microencapsulated resin and initiator may havean average toughness healing efficiency anywhere between 2 and 115 based on the concentrations of
the resin capsules and the initiator capsules [48] The samples in Figure 18(a) were formed with 5 wt
initiator microcapsule concentration The samples in Figure 18(b) were formed with 10 wt resin
microcapsule concentration The effect of microcapsule concentration on healing efficiency is
additionally linked to the size of the microcapsules Figure 19(a) shows the toughness healing efficiency
in an epoxy network with UF-encapsulated DCPD changes dramatically based on microcapsule
concentration and size [173] Part of the jump in healing efficiency however is the effect of
microcapsules on the virgin toughness of a specimen Figure 19(b) shows the difference between virgin
and healed fracture toughness for the same material system as in Figure 19(a) with 180 microm diameter
capsules [47] Though the healing efficiency with 5 wt capsule concentration is greater than that with
15 wt capsule concentration the actual fracture toughness for the healed sample is (slightly) higher at
15 wt Tagliavia et al showed that the capsule wall thickness does not affect flexural strength of the
composite [192]
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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ACCEPTED MANUSCRIPT
[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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ACCEPTED MANUSCRIPT
caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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ACCEPTED MANUSCRIPT
Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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Care 2013 2(2) p 37-43
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49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
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Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
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52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
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53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
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57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
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12051-12058
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1525-1532
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42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
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67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
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between neat dynamic covalent polymers at room temperature Chemical Communications
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70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
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72 Yoon JA et al Self-healing polymer films based on thiolndashdisulfide exchange reactions and
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142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
American Chemical Society 2003 125(22) p 6624-6625
74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
Macromolecules 2008 41(14) p 5197-5202
75 Dogadkin B et al Polymerization phenomena in the vulcanization process Rubber Chemistry
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77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
trisulphides Australian Journal of Chemistry 1963 16(6) p 1027-1037
78 McAllan DT et al The preparation and properties of sulfur compounds related to petroleum
I The dialkyl sulfides and disulfides Journal of the American Chemical Society 1951 73(8) p
3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
thiol-disulfide systems Journal of the American Chemical Society 1957 79(17) p 4589-4593
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3) p 15-2381 Deng G et al Dynamic hydrogels with an environmental adaptive self-healing ability and dual
responsive solndashgel transitions ACS Macro Letters 2012 1(2) p 275-279
82 Ramachandran D F Liu and MW Urban Self-repairable copolymers that change color RSC
Advances 2012 2(1) p 135-144
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Organic Coatings 2015 85 p 189-198
84 Liu Y-L and T-W Chuo Self-healing polymers based on thermally reversible Diels-Alder
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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67(2) p 201-212
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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Figure 18 Toughness healing efficiency in a PDMS elastomer (a) as a function of resin capsule concentration
and (b) as a function of initiator capsule concentration [48]
Figure 19 (a) Toughness healing efficiency as a function of microcapsule concentration and microcapsule
diameter [173] (b) Fracture toughness of virgin and healed samples with 180 microm diameter capsules [47]
Additionally dispersion and method of incorporation of the additives must be considered Unlike
continuous fibers which can be woven into the reinforcing structure microcapsules must be dispersed
somehow during the resin infusion process Uniform distribution is difficult to obtain [193] Dispersion is
especially important in the case of nanocapsules to avoid clumping 70 nm silica (SiO2) particles coated
with poly(ethylene imine)poly(styrene sulfonate) show promise for use as protective coatings but will
form clumps if improper processing conditions are used [194] SiO2 has the added advantage that the
nanocapsules can be synthesized to be a desired size and with added amine functionality as desired [195]
SiO2-polymer hybrid nanotubes allow pH- temperature- or redox-dependent release depending on the
polymer graft [151] Finally the environmental conditions of the system during the healing process must
be stated by the material developer before use The healing efficiency of many systems depends on
temperature allowed during healing Figure 20 illustrates the temperature dependence of an epoxy system
a) b)
a) b)
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[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
References
1 Urban MW Dynamic materials The chemistry of self-healing Nature chemistry 2012 4(2) p
80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
and molecular mechanisms Journal of International Medical Research 2009 37(5) p 1528-
1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositemdashPart II In situ self-healing Composites Science and
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Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
[74] Note the time dependency follows a t14 relationship as expected for self-healing polymers [142]
Similar dependencies are to be expected in pH- or redox-dependent systems
Figure 20 Healing efficiency of an epoxymercaptan system as a function of time at different temperatures
[74]
32 Remote Self-Healing
Dispersed agents need not be encapsulated healing materials Remote self-healing- healing via polymer
flow induced by localized melting- has been realized using superparamagentic γ-Fe2O3 nanoparticles
embedded within a thermoplastic film [196] Applying an oscillating magnetic field excites the magneticmoment of the nanoparticles increasing the nanoparticle-polymer interface temperature The increased
temperature causes localized melting of the thermoplastic which then flows into the crack as seen in
Figure 21 [65 196] This material heals with up to 98 efficiency in terms of the Youngrsquos modulus and
strain at break and can be healed multiple times
Figure 21 Crack in a polymer matrix healed via localized melting as superparamagnetic nanoparticles
oscillate in a magnetic field Image from [65] based on [196]
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For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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ACCEPTED MANUSCRIPT
caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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ACCEPTED MANUSCRIPT
Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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ACCEPTED MANUSCRIPT
Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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ACCEPTED MANUSCRIPT
Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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applied materials amp interfaces 2015 7(3) p 2064-2072
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Sciences of the United States of America 2012 109(12) p 4383-4388
130 Cordier P et al Self-healing and thermoreversible rubber from supramolecular assembly Nature 2008 451(7181) p 977-980
131 Montarnal D et al Versatile one-pot synthesis of supramolecular plastics and self-healing
rubbers Journal of the American Chemical Society 2009 131(23) p 7966-7967
132 Maes F et al Activation and deactivation of self-healing in supramolecular rubbers Soft
Matter 2012 8(5) p 1681-1687
133 Zhang R et al Heterogeneity segmental and hydrogen bond dynamics and aging ofsupramolecular self-healing rubber Macromolecules 2013 46(5) p 1841-1850
134 Colquhoun HM and Z Zhu Recognition of polyimide sequence information by a molecular
tweezer Angewandte Chemie 2004 43(38) p 5040-5045
135 Burattini S et al A novel self-healing supramolecular polymer system Faraday Discussions2009 143 p 251-264
136 Burattini S et al A self-repairing supramolecular polymer system healability as a
consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
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137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
compounds Macromolecular Rapid Communications 2009 30(6) p 459-463
138 Burattini S et al A supramolecular polymer based on tweezer-type π minusπ stacking interactions
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6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
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411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
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2007 14(5) p 391-397
142 Wool RP and KM OConnor A theory crack healing in polymers Journal of Applied Physics
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polymers and composites Progress in Polymer Science 2015 In Press p In Press
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146 Bulbenko GF EH Sorg and JP Gallagher One-part polythiol compositions containing
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147 Wolinski LE and PD Berezuk Thermoplastic polyurethane resin dissolved in an acrylic
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USA
149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
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151 Li GL et al Silicapolymer double-walled hybrid nanotubes synthesis and application as
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2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
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154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
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vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
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162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
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163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
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168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
For some material systems healing may be achieved through a variety of stimuli For example graphene
layers cause localized heating upon the application of infrared light electricity or electromagnetic waves
Incorporation of graphene layers within a polyurethane matrix permits self-healing with a tensile strength
healing efficiency of 98 [197] As may be observed in Figure 22 the healing efficiency in this system
varies consistently with weight fraction above a certain threshold level [197] Interestingly this required
threshold changes based on which healing method is employed Results by Huang et al show thethreshold is (a) 1 wt graphene for infrared healing (b) 5 wt for electrical healing and (c) ~1 wt for
electromagnetic wave healing This system also heals reliably for multiple damage-healing cycles with
98 healing efficiency even after 20 cycles A drawback of this method is that localized temperature
increases will only cause melting (and thus healing) in thermoplastic polymers and not thermosets
limiting options for structural materials
Figure 22 Tensile strength healing efficiency of the few-graphene-polyurethane system showing clear
thresholds required for healing incited by (a) infrared light (b) electrical signals (c) electromagnetic waves
[197]
Elsewhere [198-201] light-responsive protective coatings have been implemented by combining the ideas
of remote self-healing and encapsulation Light-sensitive porous TiO2 coated in benzotriazole (a corrosion
inhibitor) and encapsulated within polyelectrolyte shells [200] undergoes a series of steps nearlyinstantaneously with a UV trigger UV irradiation causes photocatalytic processes at the TiO2 surface
effecting a localized pH change The pH change then causes the polyelectrolyte shell to open releasing
the inhibitor SiO2 particles encapsulated within polyelectrolyte may be used in a similar manner [199]
The requisite wavelength for the healing stimulus changes based on the nanoparticle substance Noble
metal nanoparticles convert incident radiation to heat with high efficiency [201] and may be of interest for
use in self-healing composites For example a blend of gold nanoparticles and zinc phthalocyanines heals
under laser pulse irradiation and could be incorporated to enable healing in a composite [198]
33 Shape Memory Assisted Self-Healing
A key aspect of healing is network remodeling the sides of the crack must close to accomplish healing
The dispersed agents composites discussed earlier in this section heal when extra parent material is
available to fill the crack and react so the area regains its mechanical properties Higher healing
efficiencies are reached when the healing agent fills the entire crack [202] A shape memory material
(SMM) has a lsquosetrsquo starting shape after the proper stimulus is applied it lsquoresetsrsquo to the original shape
[203] Metallic SMM wires incorporated within composite materials reduce crack size once activated
[204] permitting higher healing efficiencies with minimal healing agent [205] A schematic of this
process is shown in Figure 23 [206]
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ACCEPTED MANUSCRIPT
Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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ACCEPTED MANUSCRIPT
caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
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14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
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15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
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42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
43 Liu D CY Lee and X Lu Repairability of impact-induced damage in SMC composites
Journal of composite materials 1993 27(13) p 1257-1271
44 White SR et al Autonomic healing of polymer composites Nature 2001 409(6822) p 794-797
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45 Corr DT et al Biomechanical behavior of scar tissue and uninjured skin in a porcine model Wound Repair and Regeneration 2009 17(2) p 250-259
46 Corr DT and DA Hart Biomechanics of scar tissue and uninjured skin Advances in Wound
Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
Experimental Mechanics 2002 42(4) p 372-379
48 Keller MW SR White and NR Sottos A self ‐ healing poly(dimethyl siloxane) elastomer
Advanced Functional Materials 2007 17(14) p 2399-2404
49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
vascular materials Advanced Materials 2010 22(45) p 5159-5163
50 Canadell J H Goossens and B Klumperman Self-healing materials based on disulfide links
Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
Macro Letters 2012 1(11) p 1233-1236
52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
56 Amamoto Y et al Repeatable photoinduced self ‐ healing of covalently cross‐ linked polymers
through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
60 Burattini S et al A healable supramolecular polymer blend based on aromatic π minus π stacking
and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
61 Trask RS GJ Williams and IP Bond Bioinspired self-healing of advanced composite
structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
62 Williams G R Trask and I Bond A self-healing carbon fibre reinforced polymer for
aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
httpwwwengineeringvillagecom64 Goacutemez DG et al In-depth numerical analysis of the TDCB specimen for characterization of
self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
69 Ono T T Nobori and J-MP Lehn Dynamic polymer blendsmdashcomponent recombination
between neat dynamic covalent polymers at room temperature Chemical Communications
2005(12) p 1522-1524
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Figure 23 Illustration of SMM wires acting to close a crack [206]
SMMs respond to a wide variety of stimuli including temperature [207] magnetic [208] or electrical
[209] fields water [210] or other chemicals [211] and light [212]With so many options to work with
development of SMMs is a growing field and their unique properties may give materials many additional
functionalities [213] Composite SMMs are also being created such as an electroactive carbon
nanotube-reinforced polyurethane composite [214] Composite sandwich panels of carbon nanotube
reinforced polymer matrix layered with a polymeric SMM demonstrated reliable SMM-enabled healing of
repeated impact damage [215] With added components systems get more complex but good models
explain how the many constituents affect a compositersquos material properties A model of the
thermomechanical properties of self-healing SMM functionalized syntactic foam has been developed and
verified against uniaxial experiments [216]
The primary advantage of using SMMs in self-healing materials is that they can shrink the crack and
increase the healing efficiency for both manually injected [205] and microencapsulated [204] healing
agents However there are some major concerns with the design of SMM-enabled self-healing materials
For example improper alignment of the SMM within the composite may not result in crack shrinkage and
may even increase the crack size [65] Additionally incorporation of supplementary materials can be
expected to affect mechanical properties either beneficially or detrimentally depending on the overall
structure [204] Li and Zhang showed that healing efficiency increases as SMM fiber length increases but
non-linearly so careful study of these materials is necessary before their behaviors can be fully
understood [217] Finally some SMMs may not be useful in certain industries thermally activated
SMMs for example could not be used in an application where they are regularly exposed to temperature
cycles including their lsquoshape settingrsquo and lsquoshape resettingrsquo temperatures
Several shape memory-assisted self-healing composites have been fabricated which consist of only
thermoset and thermoplastic polymers and do not require any encapsulated healing agents 6
thermoplastic particles dispersed inside a shape memory polystyrene matrix recovers 65 of the peak
bending load when healed at 150 degC for just 20 minutes [218] Unfortunately healing efficiency in this
system decreases significantly as cycles of damage and healing occur with a sharp decline after the 4th
healing cycle seen in terms of peak bending load in Figure 24 [218] Thermoplastic linear poly(ε-
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caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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References
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80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
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1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
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Technology 2005 65(15-16) p 2474-2480
8 Olawale DO et al Progress in triboluminescence-based smart optical sensor system Journalof Luminescence 2011 131(7) p 1407-1418
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New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
2013 Le Mauricien Ltd Port Louis Republic of Mauritius
11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
12 Okoli OI and GF Smith Failure modes of fibre reinforced composites The effects of strain
rate and fibre content Journal of Materials Science 1998 33(22) p 5415-5422
13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
of carbon fiber reinforced polymer composite Composite Structures 2015 124 p 345-356
14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
composite sandwich panels Composites Part A Applied Science and Manufacturing 2001
32(9) p 1189-1196
15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
prediction Composites Part A Applied Science and Manufacturing 2002 33(3) p 315-321
16 Kessler MR NR Sottos and SR White Self-healing structural composite materials
Composites Part A Applied Science and Manufacturing 2003 34(8) p 743-753
17 Balageas D Introduction to Structural Health Monitoring in Structural Health Monitoring D
Balageas C-P Fritzen and A Guumlemes Editors 2006 ISTE Ltd United States18 Sohn H et al A Review of Structural Health Monitoring Literature 1996-2001 2004 Los
Alamos National Laboratory
19 Lonkar K and S Roy IWSHM 2013 Program 2013 [cited 2014 May 20] Available from
httpstructurestanfordeduworkshopprogramhtml
20 Kuo C-H et al Unmanned robot system for structure health monitoring and non-destructive
building inspection current technologies overview and future improvements in 9th International
Workshop on Structural Health Monitoring 2013 Stanford University Stanford CA
21 Shin J-U et al Quadrotor-based wall-climbing robot for structural health monitoring in 9th
International Workshop on Structural Health Monitoring 2013 Stanford University Stanford
CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
23 Mallick PK Fiber-Reinforced Composites Materials Manufacturing and Design 2nd edDekker Mechanical Engineering 1993 New York New York USA CRC Press
24 Zwaag Svd AM Grande and W Post Review of current strategies to induce self-healing
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Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
caprolactone) (l-PCL) embedded in thermoset end-functionalized poly(ε-caprolactone) (n-PCL) has a
peak load healing efficiency of 95 [219] Building off the l-n-PCL network Luo et al developed a
composite incorporating electro-spun PCL fibers distributed in a shape memory epoxy matrix [206]
Upon heating the epoxy matrix works to regain its original shape while the fibers simultaneously work to
fill in the crack
Figure 24 Decreasing trend in peak bending load as a function of healing cycle [218]
4 Self-Healing Composites Vascular Networks
In order for clotting to occur the required materials must gather at the damaged location A major
component of healing in biological systems is the flow of materials to the area of damage The human
circulatory system transports necessary oxygen nutrients and blood to every single cell in the body This
idea of distributed transport was presented as a method to enable self-healing in cement [220] and has
since been embraced in the development of self-healing polymer systems [153] The major identifying
characteristic of a vascular system is an interconnected hollow network which either can be refilled
manually or is connected to a reservoir of healing agents Pang et al investigated the effect of storage
time on healing efficiency [59] Identical samples were prepared then stored for various amounts of time
before damage The same methods for damaging healing and testing were then used for each sample
Figure 25 shows the flexural strength of these samples The overall trend indicates that a healing agent
that has passed its shelf-life does not heal effectively and may even further reduce the flexural strength ofthe structure After 9 weeks no healing is seen as the flexural strength is actually worse than that of the
damaged sample Connecting a vascular network to an external reservoir allows easy maintenance of the
healing agents so expired material can be switched out with new material Healing efficiencies as high as
95 have been reported in 60 microm hollow glass fiber-reinforced epoxy healed at room temperature for
24 hours [221] The use of UV fluorescent dyes included in the healing agent allow easier visual analysis
and very obviously highlights surface damage decreasing the time needed for part inspection [59]
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Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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ACCEPTED MANUSCRIPT
Figure 25 Flexural strength of (A) undamaged samples and (B-E) samples stored for various amounts of time
before damage and healing [59]
Many vascular networks are created by embedding hollow tubing within some matrix material [59 153
221] However a vascular network without tubing may be created by using a sacrificial material to form
the network After the part is created the sacrificial material is removed leaving a hollow network
throughout the part Such a tube-free microvascular network can be created in a part through layer-by-
layer techniques Direct-write assembly [222] has been used to create networks of fugitive ink within a
ductile matrix [223] The ink is readily removed with moderate heat under light vacuum The vascular
network is filled with a liquid healing agent A (a) schematic and (b) optical image of this set up is shown
in Figure 26 [223] Figure 26(b) shows bubbles in the coating caused by released healing agent Using the
same chemistry as [44] toughness healing efficiencies R(K) up to 70 were initially reported though
efficiencies drop to around 40 after repeated damage [223] Additional research has led to an increase
in R(K) to average values over 80 with a healing efficiency over 60 reported after 16 healing cycles
for an epoxy system using Epicure 3046 [224] For certain material systems healing efficiencies may
remain above 50 even after 25 damage-healing cycles as shown in Figure 27 [225]
A B C D E
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ACCEPTED MANUSCRIPT
Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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ACCEPTED MANUSCRIPT
41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Figure 26 (a) Schematic of microvascular substrate (b) optical impage of actual microvascular system after
damage [223]
Figure 27 Average healing efficiency of microcapsule (blue) single vascular network (red) and dual vascular
network (black) systems [225]
a)
b
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ACCEPTED MANUSCRIPT
41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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ACCEPTED MANUSCRIPT
Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
nanotubendashpolymer composites Carbon 2006 44(9) p 1624-1652
158 Wu AS et al Sensing of damage and healing in three-dimensional braided composites with
vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
quantum mechanical mechanisms and those of empirical potentials Chemical Physics Letters
2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 3154
ACCEPTED MANUSCRIPT
41 Design Considerations
The pressure within the vascular system needs to be high enough that healing agents are distributed
throughout the network [226] In animals the heart muscle pumps blood throughout arteries and veins In
very dense networks pumps may not be necessary as capillary forces serve to ensure flow [49] In such a
system healing agents mix within the crack through diffusion External pumps can be used to direct the
flow of the healing material to damaged areas [227] but such a system requires a computer or humanintervention to activate the pumping routine Such a highly pressurized flow may improve mixing and
thereby increase healing efficiencies External pumps have also been used in various pumping routines in
a sparse vascular network with different pumping routines resulting in different average healing
efficiencies as seen in Figure 28 [228] Increased toughness healing efficiencies are found for pressurized
networks versus systems at static pressure at least for the first eight healing cycles
Figure 28 Average healing efficiency versus healing cycle number for self-healing samples with identical
vascular networks using two different pumping routines or only static pressure [228]
The organization and architecture of the vascular network is important for mechanical properties flow
dynamics and crack propagation [226] It is well known that additives affect the mechanical properties of
composite materials- it is after all the entire reason for including reinforcement materials It is harder to
establish what the exact effect is especially as the effect depends on the additiversquos material
morphological properties and distribution as well as the matrix material and the properties of the
interface between them It has been shown that the volume fraction of microcapsules affects crack
patterns and propagation Figure 29 shows how crack propagation in (a) neat resin differs from that in (b)
resin with incorporated microspheres [229] Embedded capillaries are expected to show similar crack
propagation patterns particularly since resin pockets tend to form around vascules as seen in Figure 30
[230] Zainuddin et al have shown that sharp cracks form near the hollow glass fibers incorporated into
composites [231] It has not yet been determined if the effect on crack propagation within these
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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ACCEPTED MANUSCRIPT
Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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Corporation USA p 5
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monomer plus an additional acrylic monomer free radical catalyst in Google Patents USPTO
Editor 1978 Pratt amp Lamert Inc
148 Arnold PS Wound implant materials USPTO Editor 1995 Johnson amp Johnson Medical Inc
USA
149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
polymeric coatings Progress in Organic Coatings 2011 72 p 211-221150 Dry CM Self-repairing reinforced matrix materials USPTO Editor 1996 Carolyn M Dry
USA p 21
151 Li GL et al Silicapolymer double-walled hybrid nanotubes synthesis and application as
stimuli-responsive nanocontainers in self-healing coatings ACS nano 2013 7(3) p 24700-
2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
Structures 1996 35(3) p 263-269
153 Motuku M UK Vaidya and GM Janowski Parametric studies on self-repairing approaches
for resin infused composites subjected to low velocity impact Smart Materials and Structures
1999 8(5) p 623-638
154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
nanotubendashpolymer composites Carbon 2006 44(9) p 1624-1652
158 Wu AS et al Sensing of damage and healing in three-dimensional braided composites with
vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
quantum mechanical mechanisms and those of empirical potentials Chemical Physics Letters
2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
composites is detrimental Indeed it may even be beneficial biologically crack redirection within
cortical bone increases the bonersquos toughness [232]
Figure 29 SEM micrographes of fractures surfaces for (a) neat resin and (b) resin with 015 volume fraction
microspheres [229]
Figure 30 Optical micrograph of circular vascule (diameter of 200 microm) and the resin-rich pocket forming
around it within a fiber laminate [230]
Several network architectures have been proposed from a simple structure of uniplanar parallel hollow
fibers [233] to more complex uniplanar branched networks which mimic the tree-like appearance found in
lungs [234] Figure 31(a) shows a diagram of a straight vascular system [233] Figure 31(b) is a schematic
of a more complex branching network [234] Such uniplanar architectures are not effective for healing
delamination To avoid this issue three-dimensional vascular networks may be included in a composite
via vaporization of sacrificial fibers [235] similar to the direct-write assembly technique discussed earlier
[223 224] In these networks a fiber is woven through the composite layup In Esser-Kahnrsquos work thesacrificial fibers were made of polylactide (PLA) [235] After the composite was cured the PLA was
vaporized by heating the sample above 200 degC Figure 32 shows (a) a schematic (b) and an optical image
of a straight weave three-dimensional network [235]
a) b)
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ACCEPTED MANUSCRIPT
Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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2478
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Structures 1996 35(3) p 263-269
153 Motuku M UK Vaidya and GM Janowski Parametric studies on self-repairing approaches
for resin infused composites subjected to low velocity impact Smart Materials and Structures
1999 8(5) p 623-638
154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
nanotubendashpolymer composites Carbon 2006 44(9) p 1624-1652
158 Wu AS et al Sensing of damage and healing in three-dimensional braided composites with
vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
quantum mechanical mechanisms and those of empirical potentials Chemical Physics Letters
2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Figure 31 (a) Diagram of a straight vascular system modified from [233] (b) Schematic of multi-branched
vascular network [234]
Figure 32 (a) Schematic and (b) optical image of a straight-weave three-dimensional network Sacrificial
fibers (pink) are woven throughout a glass fiber mat [235]
Healing efficiencies of a herringbone three-dimensional network are 80-125 whereas a parallel network
using the same materials reports healing efficiencies of 35-80 as shown in Figure 33 [236]
Interestingly the highest efficiencies in this system were found after the second and third self-heal cycles
rather than the first healing cycle More work is needed to identify the major advantages and
disadvantages of various architectures paying particular attention to benefits versus complexity [237]
a) b)
a) b)
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Figure 33 Average healing efficiencies obtained using two patterns (parallel and herringbone) in a vascular
network [236]
42 Scaling to Bulk
Scaling the vascular system for bulk materials rather than just coatings involves several potentialcomplications that are present but less essential in coatings [238] Adequate fluid flow is dependent on
sufficient pressure within the network possibly requiring use of a pump Fluid supply cannot be
interrupted extremities of the vascular system may break to release healing agents but for continued
healing ability there must be an uninterrupted connection between the local network and the reservoir for
the healing material If the fluid circulates through the network there must be an uninterrupted path in
two directions possibly requiring a duplicated network Some complications are dependent on the local
environment For example the liquid agent must have a low enough viscosity to easily flow through and
out of the vascular network but viscosity will change depending on temperature However as vacuum-
assisted resin transfer molding has been successfully used to create vascular composites [239] the
outlook for this type of self-healing composite is promising
5 Knowledge Assessment
A number of self-healing materials have been termed ldquoautonomicrdquo ndash that is they heal automatically as
soon as damage occurs with no external energy added to the system Table 3 summarizes potentially
autonomic and non-autonomic self-healing material systems Materials in the ldquo(Potentially) Autonomicrdquo
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
column have been proven to heal autonomously at room temperature Materials listed in the ldquoNon-
Autonomicrdquo column with temperature as the activation either did not heal at room temperature or did not
have room temperature healing data reported For many applications the material will not be in a 20deg C
environment Work is needed to characterize the effect of temperature (and temperature cycles) on
healing efficiency for the variety of mechanisms Future work could include further developing fiber optic
damage monitoring methods which have the major advantage of providing in-situ and distributed sensing[240]
Table 3 Summary of (potentially) autonomic and non-autonomic self-healing systems
Type (Potentially) Autonomic Non-Autonomic
Material [Ref] Material Activation [Ref]
Polymer thiol-functionalized poly(n-
butyl acrylate) [72]
polyethylene oxide (PEO) pH [68]
pH gt 100degC [69]
ploy(ethylene-co-
methacrylic acid) [140141]
poly(dimethyl siloxane)
(PDMS)
90degC [190]
cyanoacrylateepoxy [152] thirum disulfide-functionalized
polyurethane
visible light [57]
diarylbibenzofuranone-
functionalized polymers
[108]
tris-
(cinnamoyloxymethyl)
ethane
gt280 nm radiation [92]
poly(isobutylene) [127] coumarin-functionalizedpolyurethane
254-350 nm radiation [93]
poly(vinyl alcohol) [51] perfluorocyclobutane
polymers
180degC [94]
thermoreversible rubbers
[130 131]
anthracene derivatives 366 nm radiation [95]
styrene-(n-butyl acrylate)copolymer [52 53]
methyl methacrylate n-butyl
acrylatespironapthoxazin
e) copolymer
acidic vapors sunlight orincreased temperature [82]
acrylamide-(stearyl
methacrylate) copolymer[54]
trithiocarbonate-
functionalized n-butylacrylate
220-390 nm radiation [56]
trithiocarbonate-
functionalized
poly(methyl
methacrylate)
submerged in anisole under
nitrogen atmosphere [103]
oxtane-chitosan 120 nm radiation [96]
oxolane-chitosan acidic solution [105]302 nm radiation [106]
UPy-functionalized
poply(ethylene-co-
butylene)
320-390 nm radiation [124]
bis-pyrenyl-
functionalized polyamide
140 deg C [138]
polyimide-polybutadiene 100degC [60]
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
References
1 Urban MW Dynamic materials The chemistry of self-healing Nature chemistry 2012 4(2) p
80-82
2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
enhance performance Bioinspiration amp Biomimetics 2007 2(1) p 1-93 Diegelmann RF and MS Evans Wound healing an overview of acute fibrotic and delayed
healing Frontiers in bioscience 2004 9 p 283-289
4 Velnar T T Bailey and V Smrkolj The wound healing process an overview of the cellular
and molecular mechanisms Journal of International Medical Research 2009 37(5) p 1528-
1542
5 Bar-Cohen Y Biomimeticsmdashusing nature to inspire human innovation Bioinspiration amp
Biomimetics 2006 1(1) p P1-P12
6 Theato P et al Stimuli responsive materials Chemical Society hellip 2013 42(17) p 7055-7056
7 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositemdashPart II In situ self-healing Composites Science and
Technology 2005 65(15-16) p 2474-2480
8 Olawale DO et al Progress in triboluminescence-based smart optical sensor system Journalof Luminescence 2011 131(7) p 1407-1418
9 Michaels D Their New Materials in The Wall Street Journal 2013 Dow Jones amp Company
New York City New York USA
10 Chady T Airbus versus Boeing - composite materials The skys the limit in Le Mauricien
2013 Le Mauricien Ltd Port Louis Republic of Mauritius
11 Baker AA R Jones and RJ Callinan Damage tolerance of graphiteepoxy composites
Composite Structures 1985 4(1) p 15-44
12 Okoli OI and GF Smith Failure modes of fibre reinforced composites The effects of strain
rate and fibre content Journal of Materials Science 1998 33(22) p 5415-5422
13 Li W et al Micro-mechanics of failure for fatigue strength prediction of bolted joint structures
of carbon fiber reinforced polymer composite Composite Structures 2015 124 p 345-356
14 Besant T GAO Davies and D Hitchings Finite element modelling of low velocity impact of
composite sandwich panels Composites Part A Applied Science and Manufacturing 2001
32(9) p 1189-1196
15 Okoli OI and A Abdul-Latif Failure in composite laminates overview of an attempt at
prediction Composites Part A Applied Science and Manufacturing 2002 33(3) p 315-321
16 Kessler MR NR Sottos and SR White Self-healing structural composite materials
Composites Part A Applied Science and Manufacturing 2003 34(8) p 743-753
17 Balageas D Introduction to Structural Health Monitoring in Structural Health Monitoring D
Balageas C-P Fritzen and A Guumlemes Editors 2006 ISTE Ltd United States18 Sohn H et al A Review of Structural Health Monitoring Literature 1996-2001 2004 Los
Alamos National Laboratory
19 Lonkar K and S Roy IWSHM 2013 Program 2013 [cited 2014 May 20] Available from
httpstructurestanfordeduworkshopprogramhtml
20 Kuo C-H et al Unmanned robot system for structure health monitoring and non-destructive
building inspection current technologies overview and future improvements in 9th International
Workshop on Structural Health Monitoring 2013 Stanford University Stanford CA
21 Shin J-U et al Quadrotor-based wall-climbing robot for structural health monitoring in 9th
International Workshop on Structural Health Monitoring 2013 Stanford University Stanford
CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
8182019 Schein Er 2015
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2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
To illustrate the property deficiency of current self-healing epoxy-based composites one may compare
the healing efficiency of those materials to a relevant structural property such as virgin fracture
toughness as seen in Figure 34 Figure 34 indicates achieved healing efficiencies versus virgin fracture
toughness for self-healing epoxy-based composites (data from Refs [16 47 74 173 180 182 204 205
228]) One should note that even the fiber-reinforced self-healing epoxy composites have virgin fracture
toughness under 3 MPa m12 roughly 10 that of typical carbon fiber-reinforced epoxy composites (25-
40 MPa m12 [241]) At present self-healing epoxies are not useful for commercial structural applications
polyimide-poluamide 80degC [136]
polydiimide-polysiloxane 115degC [135]
ueridopyrimidone (UPy) low temperature or humid
environment [123]
polymer
composite
epoxy + dicyclopentadiene
(DCPD) + Grubbsrsquo catalyst[44]
PDMS-poly(dimethyl
siloxane) (PDES)
50 degC [55 170]
5-ethylidene-2-norbornene
(as healing agent for a
polymer matrix) [175]
SiO2-polymer nanotubes
eg containing
benzotriazole (for anti-
corrosion)
pH redox reaction
temperature [151]
epoxy + DCPD + tungsten
chloride [180]
thermoplastic film +
superparamagnetic
nanoparticles
oscillating magnetic field
[196]
epoxy + (diglycidyl ether
bisphenol A)-(ethyl
phenylacetate) + scandium
triflate [181]
polyurethane + graphene
layers
infrared light electricity
electromagnetic waves [197]
epoxy + DCPD + 5-
ethylidene-2-norbornene
[184]
shape memory epoxy +
poly(ε-caprolactone)
fibers
80degC [206]
epoxy + CuBr2(2-
methylimidazole)4 [185]
linearnetwork poly(ε-
caprolactone)
80degC [219]
epoxy + mercaptan [74] shape memory
polystyrene + copolyester
150 degC [218]
poly(dimethyl siloaxane)
resin amp initiator [48]
polymer + isophorone
diisocyanate + water [186]
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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32(9) p 1189-1196
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prediction Composites Part A Applied Science and Manufacturing 2002 33(3) p 315-321
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CA
22 Chong KP NJ Carino and G Washer Health monitoring of civil infrastructures SmartMaterials and Structures 2003 12(3) p 483-493
8182019 Schein Er 2015
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25 Peterson AM RE Jensen and GR Palmese Thermoreversible and remendable glassndash
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p 1-727 Sanada K N Itaya and Y Shindo Self-healing of interfacial debonding in fiber-reinforced
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28 Blaiszik B J et al Autonomic recovery of fibermatrix interfacial bond strength in a model
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42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
43 Liu D CY Lee and X Lu Repairability of impact-induced damage in SMC composites
Journal of composite materials 1993 27(13) p 1257-1271
44 White SR et al Autonomic healing of polymer composites Nature 2001 409(6822) p 794-797
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45 Corr DT et al Biomechanical behavior of scar tissue and uninjured skin in a porcine model Wound Repair and Regeneration 2009 17(2) p 250-259
46 Corr DT and DA Hart Biomechanics of scar tissue and uninjured skin Advances in Wound
Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
Experimental Mechanics 2002 42(4) p 372-379
48 Keller MW SR White and NR Sottos A self ‐ healing poly(dimethyl siloxane) elastomer
Advanced Functional Materials 2007 17(14) p 2399-2404
49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
vascular materials Advanced Materials 2010 22(45) p 5159-5163
50 Canadell J H Goossens and B Klumperman Self-healing materials based on disulfide links
Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
Macro Letters 2012 1(11) p 1233-1236
52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
56 Amamoto Y et al Repeatable photoinduced self ‐ healing of covalently cross‐ linked polymers
through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
60 Burattini S et al A healable supramolecular polymer blend based on aromatic π minus π stacking
and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
61 Trask RS GJ Williams and IP Bond Bioinspired self-healing of advanced composite
structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
62 Williams G R Trask and I Bond A self-healing carbon fibre reinforced polymer for
aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
httpwwwengineeringvillagecom64 Goacutemez DG et al In-depth numerical analysis of the TDCB specimen for characterization of
self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
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Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
Figure 34 Visual summary of achieved healing efficiency versus virgin fracture toughness for epoxy systems
Data from [16 47 74 173 180 182 204 205 228]
The incorporation of microcapsules within a matrix is known to have an effect on the structural properties
of the material [174] However characterization of the effect of microcapsule size on failure strength or
failure toughness has yet to be performed Capsule diameter wall thickness and material are obvious
variables of interest Such characterization could combine analysis of the effect on structural properties
with analysis of the effect on healing efficiencies since samples must be broken before they can be
healed For additional analysis of healing efficiencies variables in an experimental design could include
healing temperature and time Furthermore while the healing of pure polymer systems has been described
with the reptation model [142] models for self-healing composite systems are sorely lacking
6 Concluding remarks
Though fiber-reinforced polymer composites are widely used in many industries failure prediction in
these materials is still being developed Without accurate and precise failure prediction parts andstructures must be physically inspected to check for damage As composite materials can suffer internal
damage without showing any external sign non-destructive inspection can be costly and time-consuming
This expense of inspection led to the idea to create self-healing structures structures formed of materials
which are able to repair damage without additional material To quantify the healing ability of these
engineered materials ldquohealing efficiencyrdquo for a given material property is defined as the ratio of healed
and virgin quantities Healing efficiency may be reported in terms of any measurable material property
including but not limited to fracture toughness fracture stress extensibility or various moduli The
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ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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pyrometry Polymer 2005 46(26) p 12109-12117
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substituted chitosan-polyurethane (OXE-CHI-PUR) networks Journal of Materials Chemistry
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2015 In Press p In Press
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ACCEPTED MANUSCRIPT
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Today 2004 7(4) p 34-39
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10018
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applied materials amp interfaces 2015 7(3) p 2064-2072
129 Phadke A et al Rapid self-healing hydrogels Proceedings of the National Academy of
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Matter 2012 8(5) p 1681-1687
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consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
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137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
compounds Macromolecular Rapid Communications 2009 30(6) p 459-463
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molecular design for healability and enhanced toughness Chemistry of Materials 2010 23(1) p
6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
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411
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2478
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ACCEPTED MANUSCRIPT
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formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
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2004 Acushnet Company USA
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Chemistry 2010 1(7) p 978-987
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containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
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Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
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178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 3854
ACCEPTED MANUSCRIPT
major drawback of using healing efficiency as a metric of ldquogoodnessrdquo of composite material systems is
that it does not take into account the effect of enabling self-repair specifically that added constituents
may weaken composite structures A material with 100 healing efficiency may sound like a perfect
option for a building material but it should not be used if its strength toughness or moduli are not high
enough for the given application For clever design of stronger tougher or stiffer materials one first
needs to understand existing materials With this aim in mind this paper summarized self-healingmaterials into three major sections and discussed several examples
Self-healing within bulk polymers may occur by a number of mechanisms Covalent bonds may break
upon damage and reform (heal) under favorable conditions Polyethylene oxide (PEO) for example heals
via a chain exchange reaction at room temperature [68-70] Disulfide bonds are particularly adept at
undergoing chain exchange reactions and have been used to enable healing in a number of materials [50
57 58 72 81] Cyclic groups may also enable healing and have been incorporated within several self-
healing materials [86 88 92-94] Cycloaddition occurs under material-specific conditions Damaged
perfluorocyclobutane polymers for example undergo cycloaddition and heal under stress [94] while
other materials require radiation to heal [92 93 95] Drawbacks of light-induced self-healing include (i) a
light source is necessary and (ii) radiation may have unintended side-effects Self-healing may also beaccomplished via free radical interactions [98 102 103 108] A major limitation of free radical healing is
the reactivity of the free radicals they may react with contaminants such as oxygen before reacting with
each other and thus not heal Supramolecular chemistry may also be harnessed to enable self-healing
including hydrogen bonding [51-54 123 124 127 132 133] π- π stacking interactions [60 135 136
138 139] and ionmeric healing [140 141] Some limitations of these materials are that healing efficiency
depends on reactive group concentration size of damaged area and time between the damage event and
initialization of healing [51] Furthermore cross-linking at higher temperatures reduces the healing ability
of certain materials including self-healing rubbers [133]
Self-healing may be enabled via dispersed agents within polymeric materials including structural
composites like fiber-reinforced epoxy Self-healing may be enabled by various dispersed agents
including encapsulation remote self-healing and shape memory assisted self-healing Encapsulation may
be accomplished using hollow fibers [150] nanotubes [151] or microspheres [44] The encapsulating
material may be glass [152] metal [153] or polymer [44 165] The viscosity of the healing agent must be
matched to the diameter of the capsule to obtain good flow [154] More work is needed to characterize the
effect on mechanical properties and healing efficiency of the capsulesrsquo size concentration and dispersion
Significant research may also be done on the healing materials different liquid healing agents hardeners
and catalysts may yield better healing properties In certain matrices the dispersed agents need not be
healing agents but rather materials which can be excited to induce localized melting [196 197]
Graphene is of particular interest as it has been shown to heal reliably for repeated damage cycles and for
several different stimuli [197] Unfortunately localized heating will only cause melt in thermoplasticpolymers and not thermosets so the choice of matrix materials is limited Dispersed shape memory
materials (SMMs) can be used to assist healing by reducing crack size and thereby increase healing
efficiency [204-206 218 219] An intriguing aspect of shape memory assisted healing is that SMMs
respond to a variety of stimuli Major limitations of using SMMs within self-healing materials are (i)
improper alignment of the SMM within the composite may increase crack size [65] (ii) the inclusion of
SMM will affect mechanical properties [204] and (iii) applications will be limited by the SMMrsquos mode
of activation
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ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 3954
ACCEPTED MANUSCRIPT
A third type of self-healing may be accomplished via vascular networks incorporated into a composite
These networks may be formed by embedding hollow tubing with a matrix [59 153 220 221] or by
incorporating a sacrificial material which is then removed [223-225] These material systems are capable
of repeatedly self-healing even after 25 damage cycles [225] but only so long as the incorporated healing
agents are relatively new [59] To avoid issues with shelf-life of healing agents within the vascular
system the network may be connected to an external reservoir and a pump system to allow for freshhealing materials to be flushed through the system as needed External pumps can be used to improve
mixing and healing efficiencies but utilizing pumps requires pumping routines to be developed for the
specific vascular network architecture being used [227 228] The network architecture will affect
composite microstructure [230] and may increase difficulty of manufacturing Network architecture is
also expected to affect mechanical properties flow dynamics and crack propagation as well as failure
modes of these composites [226 233 236 237] In addition to these considerations addition potential
complications must be addressed before vascular systems can be used in bulk structural materials
adequate fluid healing agent flow must be maintained necessitating pressure control within the network
as well as uninterrupted fluid supply
While a number of self-healing materials have been presented few are capable of autonomous healingand those that have been identified as potentially autonomous are typically only characterized at ambient
conditions (ie 20 degC) Work is needed to characterize the effect on healing efficiency varying
temperature and cyclic temperature may have Furthermore most of the self-healing materials presented
herein are not structurally capable A comparison of self-healing epoxy-based composites and typical
epoxy composites highlights this property deficiency fiber-reinforced self-healing epoxy composites
have virgin fracture toughness roughly 10 that of typical carbon fiber-reinforced epoxy composites [16
205 241] Perhaps the greatest limitation on commercialization of self-healing materials is that lack of
characterization of effect on mechanical properties of healing-enabling constituents such as microcapsuls
or vascular networks
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ACCEPTED MANUSCRIPT
References
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2 Trask RS HR Williams and IP Bond Self-healing polymer composites mimicking nature to
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microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
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Experimental Mechanics 2002 42(4) p 372-379
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Advanced Functional Materials 2007 17(14) p 2399-2404
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vascular materials Advanced Materials 2010 22(45) p 5159-5163
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Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
Macro Letters 2012 1(11) p 1233-1236
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chemistry 2012 4 p 467-472
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124(42) p 10713-10717
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18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
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disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
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adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
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p 1791-1799
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and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
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structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
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aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
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42(17) p 7446-7467
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European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
8182019 Schein Er 2015
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68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
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between neat dynamic covalent polymers at room temperature Chemical Communications
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component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
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self-healing kinetics measured using atomic force microscopy Macromolecules 2011 45(1) p
142-149
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American Chemical Society 2003 125(22) p 6624-6625
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Chemical Society 1972 94(10) p 3574-3577
77 Milligan B DE Rivett and WE Savige The photolysis of dialkyl sulphides disulphides and
trisulphides Australian Journal of Chemistry 1963 16(6) p 1027-1037
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I The dialkyl sulfides and disulfides Journal of the American Chemical Society 1951 73(8) p
3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
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micelles with disulfide crosslinked cores to the liver Journal of Controlled Release 2005 109(1-
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responsive solndashgel transitions ACS Macro Letters 2012 1(2) p 275-279
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Advances 2012 2(1) p 135-144
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chemistry Polymer Chemistry 2013 4(7) p 2194-2205
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bearing furan moieties 1 Reactions with maleimides European polymer journal 1997 33(8) p1203-1211
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bearing furan moieties 2 Diels-Alder and retro-Diels-Alder reactions involving furan rings in
some styrene copolymers Macromolecules 1998 31(2) p 314-32188 Toncelli C et al Properties of reversible Diels-Alder furanmaleimide polymer networks as
function of crosslink density Macromolecular Chemistry and Physics 2012 213(2) p 157-165
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component intrinsic polymers Polymer 2015 69 p 321-329
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90 Thakur VK and MR Kessler Self-healing polymer nanocomposite materials A review Polymer 2015 69 p 369-383
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2015 In Press p In Press
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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Macromolecules 2002 35(21) p 7878-7882
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microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
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3171-3177
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241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
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189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4154
ACCEPTED MANUSCRIPT
23 Mallick PK Fiber-Reinforced Composites Materials Manufacturing and Design 2nd edDekker Mechanical Engineering 1993 New York New York USA CRC Press
24 Zwaag Svd AM Grande and W Post Review of current strategies to induce self-healing
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2014 30(13a) p 1633-1641
25 Peterson AM RE Jensen and GR Palmese Thermoreversible and remendable glassndash
polymer interface for fiber-reinforced composites Composites Science and Technology 201171 p 586-592
26 Jones AR et al Full recovery of fibermatrix interfacial bond strength using a
microencapsulated solvent-based healing system Composites Science and Technology 2013 79
p 1-727 Sanada K N Itaya and Y Shindo Self-healing of interfacial debonding in fiber-reinforced
polymers and effect of microstructure on strength recovery Open Mechanical Engineering
Journal 2008 2 p 97-103
28 Blaiszik B J et al Autonomic recovery of fibermatrix interfacial bond strength in a model
composite Advanced Functional Materials 2010 20(20) p 3547-3554
29 Liu X and G Wang Progressive failure analysis of bonded composite repairs Composite
Structures 2007 81(3) p 331340
30 Baker A Bonded composite repair of fatigue-cracked primary aircraft structure CompositeStructures 1999 47(1-4) p 431-443
31 Naboulsi S and S Mall Thermal effects on adhesively bonded composite repair of cracked
aluminum panels Theoretical and applied fracture mechanics 1997 26(1) p 1-12
32 Chaudhry Z et al Monitoring the integrity of composite patch structural repair via piezoelectric
actuatorssensors in AIAAASMEASCEAHSASC 36th Structures Structural Dynamics and
Materials Conference Adaptive Structures Forum 1997 New Oreleans LA USA AIAA Publishing
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34 Mahdi S Composite Repair Analysis 2007 Airbus Spring 2007 CACRC Meeting
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Global Investor Forum
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Washington DC
37 Trabocco RE TM Donnellan and JG Williams Repair of composite aircraft in Bonded
repair of aircraft structures AA Baker and R Jones Editors 1988 Martinus Nijhoff
Publishers Boston MA USA
38 Kelly LJ Introductory chapter in Bonded Repair of Aircraft Structures AA Baker and R
Jones Editors 1988 Martinus Nijhoff Publishers Boston MA USA p 1-18
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GPaTMO (DPMA) Editor 1990 Dornier Luftfahrt GmbH Germany
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growth Fatigue amp Fracture of Engineering Materials amp Structures 1993 16(10) p 1081-1090
41 Raghavan J and RP Wool Interfaces in repair recycling joining and manufacturing of polymers and polymer composites Journal of Applied Polymer Science 1999 71(5) p 775-785
42 Brown EN SR White and NR Sottos Retardation and repair of fatigue cracks in a
microcapsule toughened epoxy compositendash Part I manual infiltration Composites Science and
Technology 2005 65(15-16) p 2466-2473
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Journal of composite materials 1993 27(13) p 1257-1271
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httpslidepdfcomreaderfullschein-er-2015 4254
ACCEPTED MANUSCRIPT
45 Corr DT et al Biomechanical behavior of scar tissue and uninjured skin in a porcine model Wound Repair and Regeneration 2009 17(2) p 250-259
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Care 2013 2(2) p 37-43
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Experimental Mechanics 2002 42(4) p 372-379
48 Keller MW SR White and NR Sottos A self ‐ healing poly(dimethyl siloxane) elastomer
Advanced Functional Materials 2007 17(14) p 2399-2404
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vascular materials Advanced Materials 2010 22(45) p 5159-5163
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Macromolecules 2011 44(8) p 2536-2541
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Macro Letters 2012 1(11) p 1233-1236
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chemistry 2012 4 p 467-472
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124(42) p 10713-10717
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18(8) p 997-1000
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through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
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disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
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adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
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p 1791-1799
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and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
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structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
62 Williams G R Trask and I Bond A self-healing carbon fibre reinforced polymer for
aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
httpwwwengineeringvillagecom64 Goacutemez DG et al In-depth numerical analysis of the TDCB specimen for characterization of
self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
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42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
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between neat dynamic covalent polymers at room temperature Chemical Communications
2005(12) p 1522-1524
70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
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disulfide metathesis Materials Horizons 2014 1 p 237-240
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self-healing kinetics measured using atomic force microscopy Macromolecules 2011 45(1) p
142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
American Chemical Society 2003 125(22) p 6624-6625
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Macromolecules 2008 41(14) p 5197-5202
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and Technology 1954 27(4) p 920-92476 Nelander B and S Sunner Cogwheel effect in dialkyl disulfides Journal of the American
Chemical Society 1972 94(10) p 3574-3577
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trisulphides Australian Journal of Chemistry 1963 16(6) p 1027-1037
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I The dialkyl sulfides and disulfides Journal of the American Chemical Society 1951 73(8) p
3627-3632
79 Eldjarn L and A Pihl The equilibrium constants and oxidation-reduction potentials of some
thiol-disulfide systems Journal of the American Chemical Society 1957 79(17) p 4589-4593
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micelles with disulfide crosslinked cores to the liver Journal of Controlled Release 2005 109(1-
3) p 15-2381 Deng G et al Dynamic hydrogels with an environmental adaptive self-healing ability and dual
responsive solndashgel transitions ACS Macro Letters 2012 1(2) p 275-279
82 Ramachandran D F Liu and MW Urban Self-repairable copolymers that change color RSC
Advances 2012 2(1) p 135-144
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Organic Coatings 2015 85 p 189-198
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chemistry Polymer Chemistry 2013 4(7) p 2194-2205
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Journal of Polymer Science Polymer Chemistry Edition 1979 17(11) p 3415-3792
86 Laita H S Boufi and A Gandini The application of the Diels-Alder reaction to polymers
bearing furan moieties 1 Reactions with maleimides European polymer journal 1997 33(8) p1203-1211
87 Gousseacute C A Gandini and P Hodge Application of the Diels-Alder reaction to polymers
bearing furan moieties 2 Diels-Alder and retro-Diels-Alder reactions involving furan rings in
some styrene copolymers Macromolecules 1998 31(2) p 314-32188 Toncelli C et al Properties of reversible Diels-Alder furanmaleimide polymer networks as
function of crosslink density Macromolecular Chemistry and Physics 2012 213(2) p 157-165
89 Koumltteritzsch J MD Hager and US Schubert Tuning the self-healing behavior of one-
component intrinsic polymers Polymer 2015 69 p 321-329
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ACCEPTED MANUSCRIPT
90 Thakur VK and MR Kessler Self-healing polymer nanocomposite materials A review Polymer 2015 69 p 369-383
91 Schaumlfer S and G Kickelbick Self-healing polymer nanocomposites based on Diels-Alder-
reactions with silica nanoparticles The role of the polymer matrix Polymer 2015 69 p 357-
368
92 Chung C-M et al Crack healing in polymeric materials via photochemical [2+ 2]
cycloaddition Chemistry of Materials 2004 16(21) p 3982-398493 Ling J MZ Rong and MQ Zhang Photo-stimulated self-healing polyurethane containing
dihydroxyl coumarin derivatives Polymer 2012 53(13) p 2691-2698
94 Klukovich HM et al Mechanically induced scission and subsequent thermal remending of
perfluorocyclobutane polymers Journal of the American Chemical Society 2011 133(44) p17882-17888
95 Froimowicz P H Frey and K Landfester Towards the generation of self ‐ healing materials by
means of a reversible photo‐ induced approach Macromolecular Rapid Communications 2011
32(5) p 468-473
96 Ghosh B and MW Urban Self-repairing oxetane-substituted chitosan polyurethane networks
Science 2009 323(5920) p 1458-1460
97 Kawasaki T and T Iwamoto Aromatic monovinyl resin composition USPTO Editor 2005
AampM Styrene Co Ltd United States p 1598 Yuan Ce et al Self-healing of polymers via synchronous covalent bond fissionradical
recombination Chemistry of Materials 2011 23(22) p 5076-5081
99 Higaki Y H Otsuka and A Takahara Dynamic formation of graft polymers via radical
crossover reaction of alkoxyamines Macromolecules 2004 37(5) p 1696-1701
100 Higaki Y H Otsuka and A Takahara A thermodynamic polymer cross-linking system based on
radically exchangeable covalent bonds Macromolecules 2006 39(6) p 2121-2125
101 Takeda K H Unno and M Zhang Polymer reaction in polycarbonate with Na2CO3 Journal
of applied polymer science 2004 93(2) p 920-926
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Biomacromolecules 2013 14(2) p 297-301103 Nicolayuml R et al Responsive gels based on a dynamic covalent trithiocarbonate cross-linker
Macromolecules 2010 43(9) p 4355-4361104 Crivello JV Investigation of the photoactivated frontal polymerization of oxetanes using optical
pyrometry Polymer 2005 46(26) p 12109-12117
105 Ghosh B KV Chellappan and MW Urban Self-healing inside a scratch of oxetane-
substituted chitosan-polyurethane (OXE-CHI-PUR) networks Journal of Materials Chemistry
2011 21(38) p 14473-11486
106 Ghosh B KV Chellappan and M Urban UV-initiated self-healing of oxolanendashchitosanndash
polyurethane (OXOndashCHIndashPUR) networks Journal of Materials Chemistry 2012 22(31) p
16104-16113107 Penczek S P Kubisa and K Matyjaszewski Cationic activated monomer polymerization of
heterocyclic monomers Advances in Polymer Science Vol 37 1980 New York Springer-
Verlag
108 Imato K et al Self ‐ healing of chemical gels cross-linked by diarylbibenzofuranone
‐ based
trigger ‐ free dynamic covalent bonds at room temperature Angewandte Chemie
Communications 2012 51(5) p 1138-1142
109 Bejan EV E Font-Sanchis and JC Scaiano Lactone-derived carbon-centered radicals
formation and reactivity with oxygen Organic letters 2001 3(25) p 4059-4062110 Korth H-G Carbon radicals of low reactivity against oxygen radically different antioxidants
Angewandte Chemie International Edition 2007 46(28) p 5274-5276
111 de Espinosa LM et al Healable supramolecular polymer solids Progress in Polymer Science
2015 In Press p In Press
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ACCEPTED MANUSCRIPT
112 Menke W Structure and chemistry of plastids Annual Review of Plant Physiology 1962
13(1) p 27-44
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stabilities Journal of the American Chemical Society 1992 114(10) p 4010-4011
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networks Journal of Polymer Science Part A Polymer Chemistry 1999 37(19) p 3657-3670116 Sijbesma RP et al Reversible polymers formed from self-complementary monomers using
quadruple hydrogen bonding Science 1997 278(5343) p 1601-1604
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Multiple Hydrogen‐ Bonding Groups Advanced Functional hellip 2014
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335(6070) p 813-817
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10018
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p 6717-6719
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
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the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
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the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
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coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
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Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
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193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
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Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
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203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
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ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
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206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
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207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
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211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
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212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
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214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
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215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
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216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
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217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
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219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
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183-188
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6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
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glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
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Science and Manufacturing 2011 42(6) p 639-648
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239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
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241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4254
ACCEPTED MANUSCRIPT
45 Corr DT et al Biomechanical behavior of scar tissue and uninjured skin in a porcine model Wound Repair and Regeneration 2009 17(2) p 250-259
46 Corr DT and DA Hart Biomechanics of scar tissue and uninjured skin Advances in Wound
Care 2013 2(2) p 37-43
47 Brown EN NR Sottos and SR White Fracture testing of a self-healing polymer composite
Experimental Mechanics 2002 42(4) p 372-379
48 Keller MW SR White and NR Sottos A self ‐ healing poly(dimethyl siloxane) elastomer
Advanced Functional Materials 2007 17(14) p 2399-2404
49 Hamilton AR NR Sottos and SR White Self ‐ healing of internal damage in synthetic
vascular materials Advanced Materials 2010 22(45) p 5159-5163
50 Canadell J H Goossens and B Klumperman Self-healing materials based on disulfide links
Macromolecules 2011 44(8) p 2536-2541
51 Zhang H H Xia and Y Zhao Poly(vinyl alcohol) hydrogel can autonomously self-heal ACS
Macro Letters 2012 1(11) p 1233-1236
52 Chen Y et al Multiphase design of autonomic self-healing thermoplastic elastomers Nature
chemistry 2012 4 p 467-472
53 Hentschel J et al Self ‐ healing supramolecular block copolymers Angewandte Chemie 2012
124(42) p 10713-10717
54 Tuncaboylu DC et al Tough and self-healing hydrogels formed via hydrophobic interactions Macromolecules 2011 44(12) p 4997-5005
55 Cho SH et al Polydimethylsiloxane‐ based self ‐ healing materials Advanced Materials 2006
18(8) p 997-1000
56 Amamoto Y et al Repeatable photoinduced self ‐ healing of covalently cross‐ linked polymers
through reshuffling of trithiocarbonate units Angewandte Chemie International Edition in
English 2011 123(7) p 1698-1701
57 Amamoto Y et al Self ‐ healing of covalently cross‐ linked polymers by reshuffling thiuram
disulfide moieties in air under visible light Advanced Materials 2012 24(29) p 3975-3980
58 Lafont U H van Zeijl and S van der Zwaag Influence of cross-linkers on the cohesive and
adhesive self-healing ability of polysulfide-based thermosets ACS Applied Materials ampInterfaces 2012 4(11) p 6280-6288
59 Pang JWC and IP Bond A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility Composites Science and Technology 2005 65(11-12)
p 1791-1799
60 Burattini S et al A healable supramolecular polymer blend based on aromatic π minus π stacking
and hydrogen-bonding interactions Journal of the American Chemical Society 2010 132(34) p
12051-12058
61 Trask RS GJ Williams and IP Bond Bioinspired self-healing of advanced composite
structures using hollow glass fibres Journal of the Royal Society 2007 4(13) p 363-371
62 Williams G R Trask and I Bond A self-healing carbon fibre reinforced polymer for
aerospace applications Composites Part A Applied Science and Manufacturing 2007 38(6) p
1525-1532
63 Elsevier Search 2014 [cited 2014 12 December] Available from
httpwwwengineeringvillagecom64 Goacutemez DG et al In-depth numerical analysis of the TDCB specimen for characterization of
self-healing polymers International Journal of Solids and Structures 2015 64-65 p 145-154
65 Yang Y and M Urban Self-healing polymeric materials Chemical Society Reviews 2013
42(17) p 7446-7467
66 Garcia SJ Effect of polymer architecture on the intrinsic self-healing character of polymers
European Polymer Journal 2014 53 p 118-125
67 Xu H et al Competition between oxidation and coordination in cross-linking of polystyrene
copolymer containing catechol groups ACS Macro Letters 2012 1(4) p 457-760
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ACCEPTED MANUSCRIPT
68 Deng G et al Covalent cross-linked polymer gels with reversible sol-gel transition and self-
healing properties Macromolecules 2010 43(3) p 1191-1194
69 Ono T T Nobori and J-MP Lehn Dynamic polymer blendsmdashcomponent recombination
between neat dynamic covalent polymers at room temperature Chemical Communications
2005(12) p 1522-1524
70 Skene WG and J-MP Lehn Dynamers polyacylhydrazone reversible covalent polymers
component exchange and constitutional diversity proceedings of the National Academy ofSciences of the United States of America 2004 101(22) p 8270-8275
71 Rekondo A et al Catalyst-free room-temperature self-healing elastomers based on aromatic
disulfide metathesis Materials Horizons 2014 1 p 237-240
72 Yoon JA et al Self-healing polymer films based on thiolndashdisulfide exchange reactions and
self-healing kinetics measured using atomic force microscopy Macromolecules 2011 45(1) p
142-149
73 Arisawa M and M Yamaguchi Rhodium-catalyzed disulfide exchange reaction Journal of the
American Chemical Society 2003 125(22) p 6624-6625
74 Yuan YC et al Self-healing polymeric materials using epoxymercaptan as the healant
Macromolecules 2008 41(14) p 5197-5202
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
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142-149
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ACCEPTED MANUSCRIPT
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and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
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engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
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Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
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ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
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ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
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ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
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ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
90 Thakur VK and MR Kessler Self-healing polymer nanocomposite materials A review Polymer 2015 69 p 369-383
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reactions with silica nanoparticles The role of the polymer matrix Polymer 2015 69 p 357-
368
92 Chung C-M et al Crack healing in polymeric materials via photochemical [2+ 2]
cycloaddition Chemistry of Materials 2004 16(21) p 3982-398493 Ling J MZ Rong and MQ Zhang Photo-stimulated self-healing polyurethane containing
dihydroxyl coumarin derivatives Polymer 2012 53(13) p 2691-2698
94 Klukovich HM et al Mechanically induced scission and subsequent thermal remending of
perfluorocyclobutane polymers Journal of the American Chemical Society 2011 133(44) p17882-17888
95 Froimowicz P H Frey and K Landfester Towards the generation of self ‐ healing materials by
means of a reversible photo‐ induced approach Macromolecular Rapid Communications 2011
32(5) p 468-473
96 Ghosh B and MW Urban Self-repairing oxetane-substituted chitosan polyurethane networks
Science 2009 323(5920) p 1458-1460
97 Kawasaki T and T Iwamoto Aromatic monovinyl resin composition USPTO Editor 2005
AampM Styrene Co Ltd United States p 1598 Yuan Ce et al Self-healing of polymers via synchronous covalent bond fissionradical
recombination Chemistry of Materials 2011 23(22) p 5076-5081
99 Higaki Y H Otsuka and A Takahara Dynamic formation of graft polymers via radical
crossover reaction of alkoxyamines Macromolecules 2004 37(5) p 1696-1701
100 Higaki Y H Otsuka and A Takahara A thermodynamic polymer cross-linking system based on
radically exchangeable covalent bonds Macromolecules 2006 39(6) p 2121-2125
101 Takeda K H Unno and M Zhang Polymer reaction in polycarbonate with Na2CO3 Journal
of applied polymer science 2004 93(2) p 920-926
102 Krogsgaard M et al Self-healing mussel-inspired multi-pH-responsive hydrogels
Biomacromolecules 2013 14(2) p 297-301103 Nicolayuml R et al Responsive gels based on a dynamic covalent trithiocarbonate cross-linker
Macromolecules 2010 43(9) p 4355-4361104 Crivello JV Investigation of the photoactivated frontal polymerization of oxetanes using optical
pyrometry Polymer 2005 46(26) p 12109-12117
105 Ghosh B KV Chellappan and MW Urban Self-healing inside a scratch of oxetane-
substituted chitosan-polyurethane (OXE-CHI-PUR) networks Journal of Materials Chemistry
2011 21(38) p 14473-11486
106 Ghosh B KV Chellappan and M Urban UV-initiated self-healing of oxolanendashchitosanndash
polyurethane (OXOndashCHIndashPUR) networks Journal of Materials Chemistry 2012 22(31) p
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heterocyclic monomers Advances in Polymer Science Vol 37 1980 New York Springer-
Verlag
108 Imato K et al Self ‐ healing of chemical gels cross-linked by diarylbibenzofuranone
‐ based
trigger ‐ free dynamic covalent bonds at room temperature Angewandte Chemie
Communications 2012 51(5) p 1138-1142
109 Bejan EV E Font-Sanchis and JC Scaiano Lactone-derived carbon-centered radicals
formation and reactivity with oxygen Organic letters 2001 3(25) p 4059-4062110 Korth H-G Carbon radicals of low reactivity against oxygen radically different antioxidants
Angewandte Chemie International Edition 2007 46(28) p 5274-5276
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2015 In Press p In Press
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
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10018
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applied materials amp interfaces 2015 7(3) p 2064-2072
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consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
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ACCEPTED MANUSCRIPT
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6-8
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self-healing properties Soft Matter 2013 9(4) p 1091-1099140 Kalista SJ and TC Ward Thermal characteristics of the self-healing response in poly
(ethylene-co-methacrylic acid) copolymers Journal of the Royal Society 2007 4(13) p 405-
411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
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1981 52(10) p 5953-5963143 Hillewaere XKD and FED Prez Fifteen chemistries for autonomous external self-healing
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USA
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USA p 21
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2478
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Structures 1996 35(3) p 263-269
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Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
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ACCEPTED MANUSCRIPT
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nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
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nanocapsules Progress in Organic Coatings 2015 84 p 97-106
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glass bubbles Composites Science and Technology 2014 94 p 23-29
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Journal of Microencapsulation 2003 20(6) p 719-730
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formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
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2009 21(6) p 645-649
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Chemistry 2010 1(7) p 978-987
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dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
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polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
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polymer Experimental Mechanics 2006 46(6) p 725-733
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containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
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Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
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embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
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University of Illinois at Urbana Champaign Illinois USA p 290
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the Royal Society 2008 5(18) p 95-103
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ACCEPTED MANUSCRIPT
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curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
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the Royal Society 2007 4(13) p 389-393
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Macromolecules 2002 35(21) p 7878-7882
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microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
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2008 41 p 9650-9655
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coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
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and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
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Materials 2009 21(48) p 5011-5015
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and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
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Cambridge University Press
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ACCEPTED MANUSCRIPT
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shape memory alloy wires Polymer 2009 50(23) p 5533-5538
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healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
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2(2) p 152-156
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Engineers
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Engineers
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Science and Technology 2008 68(15-16) p 3337-3343
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foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
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shape memory polymer fibers Polymer 2013 54 p 5075-5086
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6(8) p 581-585
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277-286
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mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
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materials and structures 2007 16(4) p 1198-1207
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engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4554
ACCEPTED MANUSCRIPT
112 Menke W Structure and chemistry of plastids Annual Review of Plant Physiology 1962
13(1) p 27-44
113 Brunsveld L et al Supramolecular polymers Chemical Reviews 2001 101(12) p 4071-4098
114 Murray TJ and SC Zimmerman New triply hydrogen bonded complexes with highly variable
stabilities Journal of the American Chemical Society 1992 114(10) p 4010-4011
115 Lange RFM M Van Gurp and EW Meijer Hydrogen‐ bonded supramolecular polymer
networks Journal of Polymer Science Part A Polymer Chemistry 1999 37(19) p 3657-3670116 Sijbesma RP et al Reversible polymers formed from self-complementary monomers using
quadruple hydrogen bonding Science 1997 278(5343) p 1601-1604
117 Faghihnejad A et al Adhesion and Surface Interactions of a Self ‐ Healing Polymer with
Multiple Hydrogen‐ Bonding Groups Advanced Functional hellip 2014
118 Aida T EW Meijer and SI Stupp Functional supramolecular polymers Science 2012
335(6070) p 813-817
119 Beijer FH et al Strong dimerization of ureidopyrimidones via quadruple hydrogen bonding
Journal of the American Chemical Society 1998 120(27) p 6761-6769
120 Soumlntjens SHM et al Stability and lifetime of quadruply hydrogen bonded 2-ureido-4 [1 H]-
pyrimidinone dimers Journal of the American Chemical Society 2000 122(31) p 7487-7493
121 van Gemert GML et al Self ‐ Healing Supramolecular Polymers In Action Macromolecular
Chemistry and Physics 2012 213(2) p 234-242122 Bosman AW RP Sijbesma and EW Meijer Supramolecular polymers at work Materials
Today 2004 7(4) p 34-39
123 Cui J and A del Campo Multivalent H-bonds for self-healing hydrogels Chemical
Communications 2012 48 p 9302-9304
124 Biyani MV EJ Foster and C Weder Light-healable supramolecular nanocomposites based
on modified cellulose nanocrystals ACS Macro Letters 2013 2(3) p 236-240
125 Hackethal K et al Introducing polar monomers into polyisobutylene by living cationic
polymerization structural and kinetic effects Macromolecules 2010 43(4) p 1761-1770
126 Herbst F et al Aggregation and chain dynamics in supramolecular polymers by dynamic
rheology cluster formation and self-aggregation Macromolecules 2010 43(23) p 10006-
10018
127 Herbst F S Seiffert and WH Binder Dynamic supramolecular poly(isobutylene)s for self-healing materials Polymer Chemistry 2012 3(11) p 3084-3092
128 Banerjee S et al Photoinduced smart self-healing polymer sealant for photovoltaics ACS
applied materials amp interfaces 2015 7(3) p 2064-2072
129 Phadke A et al Rapid self-healing hydrogels Proceedings of the National Academy of
Sciences of the United States of America 2012 109(12) p 4383-4388
130 Cordier P et al Self-healing and thermoreversible rubber from supramolecular assembly Nature 2008 451(7181) p 977-980
131 Montarnal D et al Versatile one-pot synthesis of supramolecular plastics and self-healing
rubbers Journal of the American Chemical Society 2009 131(23) p 7966-7967
132 Maes F et al Activation and deactivation of self-healing in supramolecular rubbers Soft
Matter 2012 8(5) p 1681-1687
133 Zhang R et al Heterogeneity segmental and hydrogen bond dynamics and aging ofsupramolecular self-healing rubber Macromolecules 2013 46(5) p 1841-1850
134 Colquhoun HM and Z Zhu Recognition of polyimide sequence information by a molecular
tweezer Angewandte Chemie 2004 43(38) p 5040-5045
135 Burattini S et al A novel self-healing supramolecular polymer system Faraday Discussions2009 143 p 251-264
136 Burattini S et al A self-repairing supramolecular polymer system healability as a
consequence of donorndashacceptor π ndash π stacking interactions Chemical Communications 2009(44)
p 6717-6719
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4654
ACCEPTED MANUSCRIPT
137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
compounds Macromolecular Rapid Communications 2009 30(6) p 459-463
138 Burattini S et al A supramolecular polymer based on tweezer-type π minusπ stacking interactions
molecular design for healability and enhanced toughness Chemistry of Materials 2010 23(1) p
6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
self-healing properties Soft Matter 2013 9(4) p 1091-1099140 Kalista SJ and TC Ward Thermal characteristics of the self-healing response in poly
(ethylene-co-methacrylic acid) copolymers Journal of the Royal Society 2007 4(13) p 405-
411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
copolymers following projectile puncture Mechanics of Advanced Materials and Structures
2007 14(5) p 391-397
142 Wool RP and KM OConnor A theory crack healing in polymers Journal of Applied Physics
1981 52(10) p 5953-5963143 Hillewaere XKD and FED Prez Fifteen chemistries for autonomous external self-healing
polymers and composites Progress in Polymer Science 2015 In Press p In Press
144 Everitt DT et al Optimisation of epoxy blends for use in extrinsic self-healing fibre-reinforced
composites Polymer 2015 69 p 283-292145 Pandell NW and SC Temin Application of reactants andor catalysts to textile fabrics in
microencapsulated form 1972 Cluett Peabody amp Co Inc p 6
146 Bulbenko GF EH Sorg and JP Gallagher One-part polythiol compositions containing
encapsulated activators in Google Patents USPTO Editor 1973 Thiokol Chemical
Corporation USA p 5
147 Wolinski LE and PD Berezuk Thermoplastic polyurethane resin dissolved in an acrylic
monomer plus an additional acrylic monomer free radical catalyst in Google Patents USPTO
Editor 1978 Pratt amp Lamert Inc
148 Arnold PS Wound implant materials USPTO Editor 1995 Johnson amp Johnson Medical Inc
USA
149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
polymeric coatings Progress in Organic Coatings 2011 72 p 211-221150 Dry CM Self-repairing reinforced matrix materials USPTO Editor 1996 Carolyn M Dry
USA p 21
151 Li GL et al Silicapolymer double-walled hybrid nanotubes synthesis and application as
stimuli-responsive nanocontainers in self-healing coatings ACS nano 2013 7(3) p 24700-
2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
Structures 1996 35(3) p 263-269
153 Motuku M UK Vaidya and GM Janowski Parametric studies on self-repairing approaches
for resin infused composites subjected to low velocity impact Smart Materials and Structures
1999 8(5) p 623-638
154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
nanotubendashpolymer composites Carbon 2006 44(9) p 1624-1652
158 Wu AS et al Sensing of damage and healing in three-dimensional braided composites with
vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4754
ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
quantum mechanical mechanisms and those of empirical potentials Chemical Physics Letters
2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4854
ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4954
ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
8182019 Schein Er 2015
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4654
ACCEPTED MANUSCRIPT
137 Burattini S et al Pyrene‐ functionalised alternating copolyimide for sensing nitroaromatic
compounds Macromolecular Rapid Communications 2009 30(6) p 459-463
138 Burattini S et al A supramolecular polymer based on tweezer-type π minusπ stacking interactions
molecular design for healability and enhanced toughness Chemistry of Materials 2010 23(1) p
6-8
139 Xu Z et al Simple design but marvelous performances molecular gels of superior strength and
self-healing properties Soft Matter 2013 9(4) p 1091-1099140 Kalista SJ and TC Ward Thermal characteristics of the self-healing response in poly
(ethylene-co-methacrylic acid) copolymers Journal of the Royal Society 2007 4(13) p 405-
411
141 Kalista SJ TC Ward and Z Oyetunji Self-healing of poly (ethylene-co-methacrylic acid)
copolymers following projectile puncture Mechanics of Advanced Materials and Structures
2007 14(5) p 391-397
142 Wool RP and KM OConnor A theory crack healing in polymers Journal of Applied Physics
1981 52(10) p 5953-5963143 Hillewaere XKD and FED Prez Fifteen chemistries for autonomous external self-healing
polymers and composites Progress in Polymer Science 2015 In Press p In Press
144 Everitt DT et al Optimisation of epoxy blends for use in extrinsic self-healing fibre-reinforced
composites Polymer 2015 69 p 283-292145 Pandell NW and SC Temin Application of reactants andor catalysts to textile fabrics in
microencapsulated form 1972 Cluett Peabody amp Co Inc p 6
146 Bulbenko GF EH Sorg and JP Gallagher One-part polythiol compositions containing
encapsulated activators in Google Patents USPTO Editor 1973 Thiokol Chemical
Corporation USA p 5
147 Wolinski LE and PD Berezuk Thermoplastic polyurethane resin dissolved in an acrylic
monomer plus an additional acrylic monomer free radical catalyst in Google Patents USPTO
Editor 1978 Pratt amp Lamert Inc
148 Arnold PS Wound implant materials USPTO Editor 1995 Johnson amp Johnson Medical Inc
USA
149 Garciacutea SJ HR Fischer and Svd Zwaag A critical appraisal of the potential of self healing
polymeric coatings Progress in Organic Coatings 2011 72 p 211-221150 Dry CM Self-repairing reinforced matrix materials USPTO Editor 1996 Carolyn M Dry
USA p 21
151 Li GL et al Silicapolymer double-walled hybrid nanotubes synthesis and application as
stimuli-responsive nanocontainers in self-healing coatings ACS nano 2013 7(3) p 24700-
2478
152 Dry C Procedures developed for self-repair of polymer matrix composite materials Composite
Structures 1996 35(3) p 263-269
153 Motuku M UK Vaidya and GM Janowski Parametric studies on self-repairing approaches
for resin infused composites subjected to low velocity impact Smart Materials and Structures
1999 8(5) p 623-638
154 Bleay SM et al A smart repair system for polymer matrix composites Composites Part A
Applied Science and Manufacturing 2001 32(12) p 1767-1776155 Bond IP RS Trask and HR Williams Self-healing fiber-reinforced polymer composites
MRS bulletin 2008 33(8) p 770-774
156 Iijima S Helical microtubules of graphitic carbon Nature 1991 354(6348) p 56-58
157 Coleman JN et al Small but strong a review of the mechanical properties of carbon
nanotubendashpolymer composites Carbon 2006 44(9) p 1624-1652
158 Wu AS et al Sensing of damage and healing in three-dimensional braided composites with
vascular channels Composites Science and Technology 2012 72(13) p 1618-1626
159 Lanzara G et al Carbon nanotube reservoirs for self-healing materials Nanotechnology 2009
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4754
ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
quantum mechanical mechanisms and those of empirical potentials Chemical Physics Letters
2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4854
ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4954
ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
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ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4754
ACCEPTED MANUSCRIPT
160 Qian D et al Mechanics of carbon nanotubes Applied Mechanics Reviews 2002 55(6) p495-533
161 Troya D SL Mielke and GC Schatz Carbon nanotube fracturendash differences between
quantum mechanical mechanisms and those of empirical potentials Chemical Physics Letters
2003 382(1-2) p 133-141
162 Bass RW Synthesis and characterization of self-healing poly(carbonate urethane) carbon-
nanotube composites in Department of Chemistry 2011 University of South Florida TampaFL USA p 145
163 Kopeć M et al Self-healing epoxy coatings loaded with inhibitor-containing polyelectrolyte
nanocapsules Progress in Organic Coatings 2015 84 p 97-106
164 Zhang H P Wang and J Yang Self-healing epoxy via epoxyndashamine chemistry in dual hollow
glass bubbles Composites Science and Technology 2014 94 p 23-29
165 Brown EN et al In Situ poly(urea-formaldehyde) microencapsulation of dicyclopentadiene
Journal of Microencapsulation 2003 20(6) p 719-730
166 Wang R et al Preparation and characterization of self ‐ healing microcapsules with poly (urea‐
formaldehyde) grafted epoxy functional group shell Journal of Applied Polymer Science 2009
113(3) p 1501-1506
167 Skipor A S Scheifer and B Olson Microcapsule containing a flowable polymerizable
material and having a polymerization agent(s) in the outer surface for inducing polymerizationupon rupture of the microcapsule electronics packaging sealants coatings tire parts USPTOEditor 2006 Motorola Inc USA
168 Harris KM and M Rajagopalan Self healing polymers in sports equipment USPTO Editor
2004 Acushnet Company USA
169 Maiti S et al Continuum and molecular-level modeling of fatigue crack retardation in self-
healing polymers Journal of Engineering Materials and Technology 2006 128(4) p 595-602
170 Cho SH SR White and PV Braun Self ‐ healing polymer coatings Advanced Materials
2009 21(6) p 645-649
171 Syrett JA CR Becer and DM Haddleton Self-healing and self-mendable polymers Polymer
Chemistry 2010 1(7) p 978-987
172 Kessler MR and SR White Cure kinetics of the ring‐ opening metathesis polymerization of
dicyclopentadiene Journal of Polymer Science Part A Polymer Chemistry 2002 40(14) p2373-2383
173 Brown EN SR White and NR Sottos Microcapsule induced toughening in a self-healing
polymer composite Journal of Materials Science 2004 39(5) p 1703-1710
174 Keller MW and NR Sottos Mechanical properties of microcapsules used in a self-healing
polymer Experimental Mechanics 2006 46(6) p 725-733
175 Liu X et al Synthesis and characterization of melamine‐ urea‐ formaldehyde microcapsules
containing ENB‐ based self ‐ healing agents Macromolecular Materials and Engineering 2009
294(6-7) p 389-395
176 Jones AS et al Catalyst morphology and dissolution kinetics of self-healing polymers
Chemistry of Materials 2006 18(5) p 1312-1317177 Wilson GO et al Evaluation of ruthenium catalysts for ring-opening metathesis
polymerization-based self-healing applications Chemistry of Materials 2008 20(10) p 3288-3297
178 Liu X et al Rheokinetic evaluation of self-healing agents polymerized by Grubbs catalyst
embedded in various thermosetting systems Composites Science and Technology 2009 69(13)p 2102-2107
179 Rule JD Polymer chemistry for improved self-healing composite materials in Chemistry 2005
University of Illinois at Urbana Champaign Illinois USA p 290
180 Kamphaus JM et al A new self-healing epoxy with tungsten (VI) chloride catalyst Journal of
the Royal Society 2008 5(18) p 95-103
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4854
ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4954
ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5054
ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4854
ACCEPTED MANUSCRIPT
181 Coope TS et al Self ‐ healing of an epoxy resin using scandium (III) triflate as a catalytic
curing agent Advanced Functional Materials 2011 21(24) p 4624-4631
182 Mauldin TC et al Self-healing kinetics and the stereoisomers of dicyclopentadiene Journal of
the Royal Society 2007 4(13) p 389-393
183 Rule JD and JS Moore ROMP Reactivity of endo- and exo-Dicyclopentadiene
Macromolecules 2002 35(21) p 7878-7882
184 Liu X et al Characterization of diene monomers as healing agents for autonomic damagerepair Journal of Applied Polymer Science 2006 101(3) p 1266-1272
185 Yin T et al Self-healing epoxy compositesndashpreparation and effect of the healant consisting of
microencapsulated epoxy and latent curing agent Composites Science and Technology 2007
67(2) p 201-212
186 Yang J et al Microencapsulation of isocyanates for self-healing polymers Macromolecules
2008 41 p 9650-9655
187 Huang M and J Yang Salt spray and EIS studies on HDI microcapsule-based self-healing
anticorrosive coatings Progress in Organic Coatings 2014 77 p 168-175188 Huang M and J Yang Facile microencapsulation of HDI for self-healing anticorrosion
coatings Journal of Materials Chemistry 2011 21(30) p 11123-11130
189 Keller MW K Hampton and B McLaury Self-healing of erosion damage in a polymer
coating Wear 2013190 Zheng P and TJ McCarthy A surprise from 1954 siloxane equilibration is a simple robust
and obvious polymer self-healing mechanism Journal of the American Chemical Society 2012
134(4) p 2024-2027
191 Rule JD et al Wax‐ protected catalyst microspheres for efficient self ‐ healing materials
Advanced Materials 2005 17(2) p 205-208
192 Tagliavia G M Porfiri and N Gupta Analysis of flexural properties of hollow-particle filled
composites Composites Part B Engineering 2010 41(1) p 8693
193 Williams GJ RS Trask and IP Bond Self-healing functionality for CFRP in First
International Conference on Self Healing Materials 2007 Noordwijk ann Zee The Netherlands
Springer194 Zheludkevich ML et al Anticorrosion coatings with self-healing effect based on
nanocontainers impregnated with corrosion inhibitor Chemistry of Materials 2007 19(3) p402-411
195 Fickert J et al Design and characterization of functionalized silica nanocontainers for self-
healing materials Journal of Materials Chemistry 2012 22(5) p 2286-2291196 Corten CC and MW Urban Repairing polymers using oscillating magnetic field Advanced
Materials 2009 21(48) p 5011-5015
197 Huang L et al Multichannel and repeatable self ‐ healing of mechanical enhanced graphene‐
thermoplastic polyurethane composites Advanced Materials 2013 25(15) p 2224-2228
198 Amendola V et al Self-healing of gold nanoparticles in the presence of zinc phthalocyanines
and their very efficient nonlinear absorption performances The Journal of Physical Chemistry C
2009 113(20) p 8688-8695
199 Skorb EV et al Laser-controllable coatings for corrosion protection ACS nano 2009 3(7)
p 1753-1760200 Skorb EV et al Light responsive protective coatings Chemical Communications 2009 p
6041-6043
201 Cortie MB and AM McDonagh Synthesis and optical properties of hybrid and alloy
plasmonic nanoparticles Chemical reviews 2011 111(6) p 3713-3735
202 Rule JD NR Sottos and SR White Effect of microcapsule size on the performance of self-
healing polymers Polymer 2007 48(12) p 3520-3529
203 Otsuka K and CM Wayman Shape Memory Materials 1998 New York NY USA
Cambridge University Press
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4954
ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5054
ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 4954
ACCEPTED MANUSCRIPT
204 Kirkby EL et al Performance of self-healing epoxy with microencapsulated healing agent and
shape memory alloy wires Polymer 2009 50(23) p 5533-5538
205 Kirkby EL et al Embedded shape‐ memory alloy wires for improved performance of self ‐
healing polymers Advanced Functional Materials 2008 18(15) p 2253-2260
206 Luo X and PT Mather Shape memory assisted self-healing coating ACS Macro Letters 2013
2(2) p 152-156
207 De RG and JA Zijderveld Shape‐ memory effect and the martensitic transformation of TiNi
Journal of Applied Physics 1968 39(5) p 2195-2200
208 Sato A Y Yamaji and T Mori Physical properties controlling shape memory effect in Fe-Mn-
Si alloys Acta Metallurgica 1986 34(2) p 287-294
209 Bar-Cohen Y Electroactive polymers as artificial muscles-capabilities potentials and
challenges in Robotics 2000 Albuquerque New Mexico USA American Society of Civil
Engineers
210 Huang WM et al Water-driven programmable polyurethane shape memory polymer
demonstration and mechanism Applied Physics Letters 2005 86(11) p 114105
211 Lv H et al Shape‐ memory polymer in response to solution Advanced Engineering Materials
2008 10(6) p 592-595
212 Lendlein A et al Light-induced shape-memory polymers Nature 2005 434(7035) p 879-882
213 Zafar A and B Andrawes Manufacturing and modeling of SMA composite reinforcement forenhanced performance of concrete structures under sequential ground motion records in
Structure Congress 2013 2013 Pittsburgh Pennsylvania USA American Society of Civil
Engineers
214 Cho JW et al Electroactive shape‐ memory polyurethane composites incorporating carbon
nanotubes Macromolecular Rapid Communications 2005 26(5) p 412-416
215 Li G and M John A self-healing smart syntactic foam under multiple impacts Composites
Science and Technology 2008 68(15-16) p 3337-3343
216 Xu W and G Li Constitutive modeling of shape memory polymer based self-healing syntactic
foam International Journal of Solids and Structures 2010 47(9) p 1306-1316
217 Li G and P Zhang A self-healing particulate composite reinforced with strain hardened short
shape memory polymer fibers Polymer 2013 54 p 5075-5086
218 Nji J and G Li A biomimic shape memory polymer based self-healing particulate composite Polymer 2010 51(25) p 6021-6029
219 Rodriguez ED X Luo and PT Mather Linearnetwork poly (ε-caprolactone) blends
exhibiting shape memory assisted self-healing (SMASH) ACS Applied Materials amp Interfaces
2011 3(2) p 152-161
220 Dry C Matrix cracking repair and filling using active and passive modes for smart timed
release of chemicals from fibers into cement matrices Smart Materials and Structures 1994 3(2)p 118-123
221 Pang JWC and IP Bond Bleeding compositesmdashdamage detection and self-repair using a
biomimetic approach Composites Part A Applied Science and Manufacturing 2005 36(2) p
183-188
222 Therriault D SR White and JA Lewis Chaotic mixing in three-dimensional microvascular
networks fabricated by direct-write assembly Nature Materials 2003 2(4) p 265-271223 Toohey KS et al Self-healing materials with microvascular networks Nature Materials 2007
6(8) p 581-585
224 Toohey KS et al Delivery of two‐ part self ‐ healing chemistry via microvascular networks Advanced Functional Materials 2009 19(9) p 1399-1405
225 Hansen CJ et al Self ‐ healing materials with interpenetrating microvascular networks
Advanced Materials 2009 21(41) p 4143-4147
226 Norris CJ et al Self ‐ healing fibre reinforced composites via a bioinspired vasculature
Advanced Functional Materials 2011 21(19) p 3624-3633
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5054
ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5054
ACCEPTED MANUSCRIPT
227 Williams HR RS Trask and IP Bond Self-healing sandwich panels restoration of
compressive strength after impact Composites Science and Technology 2008 68(15-16) p
3171-3177
228 Hamilton AR NR Sottos and SR White Pressurized vascular systems for self-healing
materials Journal of the Royal Society Interface 2012 9(70) p 1020-1028
229 Kim HS and MA Khamis Fracture and impact behaviours of hollow micro-sphereepoxy
resin composites Composites Part A Applied Science and Manufacturing 2001 32(9) p 1311-1317
230 Huang C-Y RS Trask and IP Bond Characterization and analysis of carbon fibre-
reinforced polymer composite laminates with embedded circular vasculature Journal of the
Royal Society Interface 2010 7(49) p 1229-1241231 Zainuddin S et al Recovery and improvement in low-velocity impact properties of e-
glassepoxy composites through novel self-healing technique Composite Structures 2014 108 p
277-286
232 Nalla RK et al Fracture in human cortical bone local fracture criteria and toughening
mechanisms Journal of Biomechanics 2005 38(7) p 1517-1525
233 Williams HR RS Trask and IP Bond Self-healing composite sandwich structures Smart
materials and structures 2007 16(4) p 1198-1207
234 Kim S S Lorente and A Bejan Vascularized materials tree-shaped flow architecturesmatched canopy to canopy Journal of Applied Physics 2006 100(6) p 063525
235 Esser-Kahn AP et al Three-dimensional microvascular fiber reinforced composites Advanced
Materials 2011 23(32) p 3654-3658
236 Patrick JF et al Continuous self ‐ healing life cycle in vascularized structural composites
Advanced Materials 2014 26(25) p 4302-4308
237 Norris CJ IP Bond and RS Trask The role of embedded bioinspired vasculature on damage
formation in self-healing carbon fibre reinforced composites Composites Part A Applied
Science and Manufacturing 2011 42(6) p 639-648
238 Williams HR et al Biomimetic reliability strategies for self-healing vascular networks in
engineering materials Journal of the Royal Society Interface 2008 5(24) p 735-747
239 Matt AKK et al Development of Novel Self-Healing Polymer Composites for Use in Wind
Turbine Blades Journal of Energy Resources Technology 2015 137(5) p 51202240 Merzbacher CI AD Kersey and EJ Friebele Fiber optic sensors in concrete structures a
review Smart Materials and Structures 1996 5(2) p 196-208
241 Mallick PK Performance in Fiber-Reinforced Composites Materials Manufacturing and
Design 1993 CRC Press New York New York USA
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5154
ACCEPTED MANUSCRIPT
Ms Margaret Scheiner is a PhD candidate in Industrial and Manufacturing Engineering at
Florida State University with a BS in Materials Science amp Engineering from Cornell UniversityShe has contributed to research on dye-sensitized solar cells synthesis of highly
triboluminescent crystals and pulsed laser deposition of non-stoichiometric thin films Hercurrent research aims to create a self-healing composite with integrated structural health
monitoring capabilities She is a teaching assistant for the Industrial Engineering programs
Senior Design Project course is a coordinator of the summer internship programs (NSF-REUand AFRL-DREAM) and has extensive STEM outreach experience through DreamOn as well as
local chapters of the Society of Women Engineers the Society for the Advancement of Material
and Process Engineering Golden Key International Honour Society and Phi Kappa Phi
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5254
ACCEPTED MANUSCRIPT
Dr Tarik Dickensrsquo research interest include focus on cradle-to-grave production of additively
manufactured composite structurestooling and systems integration for AM performancetechnologies With development of nanostructured hybrid materials for mechanical toughening
energy conversion storage and integratedndashStructural Health Monitoring with over 20+
publications He has 2 US patent applications (awarded and pending) in the areas of advanced
composites and sensory-scaled composite manufacturing and ubiquitous real-time structuralhealth monitoring In addition he runs the Industrial Composite Engineering (ICE) lab involving
sensing techniques amp non-destructive testing of advanced materials at the High PerformanceMaterials Institute (HPMI) for failure analysis He has outreach experience in organizing and
supervising programs involved with STEM initiatives (NSF-REU and AFRL-DREAM summer
programs)
Dr Okenwa Okoli is Professor and Chair of Industrial and Manufacturing Engineering at the
Florida AampM University - Florida State University (FAMU-FSU) College of Engineering His
research group has provided extensive insight into the development of functional and affordablecomposite manufacturing technologies for which he has received several awards Dr Okolis
research efforts include the development of integrated structural health sensing within concrete
and within advanced composite structures He also focuses on the develpoment of photovoltaic
sensors innovative 3D energy conversion systems and scalable processes to allow themanufacture of customizable multifunctional composite structures He has 7 US patent
applications (awarded and pending) in the areas of advanced composites and multiscale
composites manufacturing structural ceramics and ubiquitous real-time structural healthmonitoring He is a chartered engineer and a chartered scientist
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5354
ACCEPTED MANUSCRIPT
PRODUCTS
Related to the Project
1 Dickens T J Armbrister C And Okoli O I ldquoCharacterization of triboluminescent
enhanced discontinuous glass-fiber composite beams for micro-damage detection and fracture
assessmentrdquo Journal of Luminescence doi 101016jjlumin2015020302 Roy M Joshi K Ndebele T Williams K Olawale D and Dickens T ldquoPreliminary
Investigation Additive Manufacturing Of Soluble Mold Tooling For Embedded Devices InComposite Structuresrdquo In Society for the Advancement of Material and Process Engineering
(SAMPE) (CAMX) Orlando Florida October 18 - 22
3 Okoli O Wang B Dickens T J ldquoSystems Methods and Apparatus for StructuralHealth Monitoringrdquo Florida State University Tallahassee FL 22nd November 2012 US
Patent and Trademark Office No 12691537
4 Dickens T J J Breaux D O Olawale W G Sullivan Okoli O I ldquoEffects of ZnS
Mn Concentrated Vinyl Ester Matrices under Flexural Loading on the Triboluminescent YieldrdquoJ of Lumin 132 (7) 1714-1719 doi101016jjlumin201201056
5 Dickens T J Okoli O I ldquoEnabling Damage Detection Manufacturing CompositeLaminates doped with Dispersed Triboluminescent Materialsrdquo J Rein Plastic Comp 30(2011)22 1869-1876 doi 1011770731684411413490
6 Dickens T J Okoli O I Liang Z (2008) ldquoHarnessing triboluminescence for
structural health monitoring of composite structuresrdquo In Society for the Advancement ofMaterial and Process Engineering (SAMPE) Annual Conference Long Beach California May
18 - 22 Long Beach CA SAMPE
Other Significant Products1 Yan J Uddin M J Dickens T J Daramola D E amp Okoli O I (2014) 3D Wire-
Shaped Dye-Sensitized Solar Cells in Solid State Using Carbon Nanotube Yarns with Hybrid
Photovoltaic Structure Adv Mater Interfaces 1 (6) 7 doi101002admi2014000752 Okoli O Yan J Dickens T J Uddin MJ ldquoDye-Sensitized Solar Cells Including
Carbon Nanotube Yarnsrdquo Florida State University Tallahassee FL 22nd July 2014 US
Patent and Trademark Office No 620276083 Uddin M J Daramola D E Velasquez E Dickens T J Yan J Hammel E
Cesano F amp Okoli O I (2014) A High Efficiency 3D Photovoltaic Microwire with Carbon
Nanotubes(CNT)-Quantum Dot (QD) Hybrid Interface Phys Status Solidi RRL 8 (11) 898ndash
903 doi101002pssr2014093924 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Getting Light through Cementitious Composites with in-situ Triboluminescence Damage
Sensor Structural Health Monitoring 13 (2) 177-189 doi10117714759217135139765 Olawale D O Kliewer K Okoye A Dickens T J Uddin M J amp Okoli O I
(2014) Real Time Failure Detection in Unreinforced Cementitious Composites with
Triboluminescent Sensor Journal of Luminescence 147 235-241doihttpdxdoiorg101016jjlumin2013
6 M Scheiner M McCrary-Dennis D Olawale O Okoli (2014) NSF- Retaining
Engineers through Research Entrepreneurship and Advanced-Materials Training (RETREAT)121st ASEE Annual Conference amp Exposition Proceedings Indianapolis Indiana United States
June 15-18 2014
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011
8182019 Schein Er 2015
httpslidepdfcomreaderfullschein-er-2015 5454
ACCEPTED MANUSCRIPT
7 X Xin M Scheiner M Ye Z Lin Surface-Treated TiO2 Nanoparticles for Dye-
Sensitized Solar Cells with Remarkably Enhanced Performance ACS Langmuir 27(23) 14594-14598 2011