PolyHydroxyAlkanoatesNatural Biodegredable polymers

ByMohamed Mahmoud Abdul-Monem

Assistant lecturerDental biomaterials Department

Faculty of DentistryAlexandria University

Mohamed_mahmoud.biomaterials@yahoo.com2017

History Definition Structure Classification Biosynthesis Properties Biodegradation Applications References

Contents

The first example of polyhydroxyalkanoates to be discovered was polyhydroxybutyrate (PHB) in the year 1926 by Maurice Lemoigne

History of PHA

Polyhydroxyalkanoates (PHAs) have recently been the focus of attention as a biodegradable and biocompatible substitute for conventional non-degradable plastics.

Polyhydroxyalkanoates (PHAs) are a family of biopolyesters, of hydroxyalkanoic acids synthesized as intracellular products by prokaryotic genera encompassing :

Gram-positive and Gram-negative bacteria

PHA production is reported in plants and yeasts .

Definition

 Any salt or ester formed from the reaction of an alkanoic acid and alkanol

Alkanoic acid : Aliphatic carboxylic acid Alkanol : Simple aliphatic alcohol

Alkanoates

Structure The pendant group (R)

varies from methyl (C1) to tridecyl (C13).

classification Short chain length (scl)

polymers consisting of 3–5 carbon atoms

Medium chain length (mcl) polymers, consisting of 6–14 carbon atoms

The mcl-PHA have a much lower level of crystallinity than scl-PHA and are more elastic.

Due to the stereospecifity of the biosynthetic enzymes (PHA synthases), they are D configured.

Reports of any L-configured PHAs do not exist.

Most PHAs are optically active (levorotatry).

These monomers can display saturated, unsaturated straight, cyclic or branched side-chains.

In general, these side chains are aliphatic but, when cultivated on unusual substrates, certain microbes (mainly found among the bacterial group of fluorescent pseudomonades) can synthesize PHAs with pendant functionalities like : aromatic, halogenic, or alkoxy groups .

Many of these ‘functional’ side chains provide the possibility for post-synthetic modification by chemical or enzymatic methods.

This way, functional biopolymers are accessible with properties tailor-made for special applications.

Prime examples are provided by the conversion of unsaturated side chains of monomeric PHA building blocks to carboxylate, epoxide, or hydroxyl groups that provide the polyester with enhanced hydrophilicity.

PHA biosynthesis starts from renewable feedstocks like hexoses, pentoses, starch, sucrose, lactose, maltose, lipids, alcohols, organic acids, or gases like methane or CO2, classically under unfavourable growth conditions due to imbalanced nutrient supply .

The biosynthesis of PHA is usually caused by certain deficiency conditions (e.g. lack of macro elements such as phosphorus, nitrogen, trace elements, or lack of oxygen) and the excess supply of carbon sources.

Accumulating PHAs is a natural way for bacteria to store carbon and energy, when nutrient supplies are imbalanced.

Biosynthesis

Properties of PHA

PHB is a homopolyester consisting merely of 3HB building blocks; this material features rather high crystallinity (60-70 % ) and restricted processability .

Crystallinity

PHB has had only limited use mainly because of its intrinsic brittleness (presenting low strain at break).

Reasons for brittleness:1. The secondary crystallization (re-crystallization) of

the amorphous phase takes place during storage at room temperature;

2. The glass transition temperature (Tg) of PHB is close to room temperature;

Brittleness

For these reasons, efforts in compounding PHB are mainly focused on the search of plasticizers and nucleating agents capable of reducing the crystallization process and improving flexibility.

These obstacles can be overcome by interrupting the crystalline PHB matrix by additional building blocks like 3HV or the achiral building block 4HB, resulting in co-polyesters with enhanced material properties and a broader range of applications.

The glass transition temperature is usually well below room temperature, ranging from 4°C to -48°C

Glass transition temperature

Melting temperature

PHB thermally decomposes at (270°C) just above its melting point(180°C).

A short exposure of PHB to temperature near 180°C could induce a severe degradation accompanied by production of the degraded products of olefinic and carboxylic acid compounds, e.g., crotonic acid and various oligomers: through the random chain scission reaction.

Thermal Degradation

The very low resistance to thermal degradation seems to be the most serious problem related to the processing of PHB.

The main reaction involves chain scission, which results in a rapid decrease in molecular weight.

During processing, the degradation of the chains may be reduced by the addition of a lubricant that prevents the degradation of the chains in processing,so that the material can be processed at 170– 180°C, because PHB is sensitive to high processing temperatures.

TGA of PHB

They are hydrophobic

The conversion of unsaturated side chains of monomeric PHA building blocks to carboxylate, epoxide, or hydroxyl groups that provide the polyester with enhanced hydrophilicity .

Hydrophcobicity

They are insoluble in water but soluble in halogenated solvents such  as :

chloroform, dichloromethane or dichloroethane .

Solubility

They are UV stable i.e resist UV degradation.

Stable ?? They reflect the ultraviolet rays before penetrating into the polymer .

UV degradation : Theoretical explanation is based upon absorption of ultraviolet energy, raising some bonds to an energy level which exceeds their stability, and thus initiating their breakdown, usually involving atmospheric oxidation and sometimes hydrolysis as well.

UV Stability

Properties of PHB

Microorganisms in nature are able to degrade PHAs using PHA depolymerazes, however, the activities of these enzymes vary depending on the composition, crystallinity, additives and the surface area of the polymer.

The extracellular PHA depolymerases hydrolyse the PHAs into water-soluble oligomers and monomers and subsequently utilize the resulting products as nutrients for biomass accumulation.

Biodegradation

The end product of PHA degradation in an aerobic environment is carbon dioxide and water while in anaerobic conditions it is methane .

In the context of medical applications, PHAs are established as biocompatible and biodegradable polymers.

The polymers are degraded by enzymatic hydrolysis, and P(3HB), on degradation, forms the 3-hydroxybutyric acid, a known constituent of blood plasma.

In mammals, the hydrolysis of the polymer happens gradually (6 months or more).

This was shown as the result of 6 month’s implantation of P(3HB) in mice, which indicated a mass loss of less than 1.6% w/w.

The population of aliphatic polymer-degrading microorganisms in different ecosystems was found to be in the following order:

PHB = PCL > PBS > PLA

PBS :Polybutylene succinate

biodegradable aliphatic polyester with properties that are comparable to polypropylene

For use in medical applications, materials must be biocompatible, which means they cannot cause severe immune reactions when introduced to soft tissues or blood of a host organism.

PHA materials must also not elicit immune responses during degradation in the body to be considered biocompatible.

Furthermore, sterilization of PHA-based materials does not appear to affect the average molecular weight (Mw), tensile strength, or other properties.

Applications1.Medical Applications

a. Drug delivery : Degradation of PHA matrices in the tissues of the host organism offers the possibility of coupling this phenomenon with release of bioactive compounds, such as antibiotic or anti-tumor drug. (Pellets and microshperes).

b. Sutures (fibers) A common PHA type used for fabrication of surgical

materials is poly(4-hydroxybutyrate) (P4HB).

As suture material, oriented P4HB fibers (545 MPa) are stronger than polypropylene sutures (410-460 MPa).

c. Scaffolding material for tissue enginering : Surface properties of PHA films have been shown to be favorable

for proliferation and attachment of tissue culture cells .

Fibroblast cells have been shown to adhere and proliferate on PHA membranes .

Also, mesenchymal stem cells were shown to adhere and proliferate on several PHA substrates, with a terpolymer poly(hydroxybutyrate-cohydroxyvalerate- co-hydroxyhexanoate) yielding the best results .

This polymer exhibited the greatest surface roughness, as well as the highest water contact angle, suggesting that these characteristics are important for adherence and proliferation of cells on PHA surfaces.

d. Bone implant materials :

Osseoinductive due to piezoelectric properties. Piezoelectricity :  the electric charge that

accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress.

Electricity resulting from pressure .

e. Pulmonary valve leaflets and pulmonary artery scaffolds.

f. Post-surgery recovery (Films) : Healing of oral wounds .

a. Food packaging The high barrier of PHAs towards oxygen permeation

is of enormous interest for producing packaging materials, which prevent the oxidative spoiling of products.

b. Diapersc. Potential agricultural applications include : encapsulation of seeds, encapsulation of fertilizers for slow release, biodegradable plastic films for crop protection

2.Other applications

A Journal Paper on PHAs

Development ternary bioactive composite materials composed of poly(3-hydroxybutyrate-co-3-hydroxyvalerate), calcium silicate and poly(lactide-co-glycolide) (PHBV/CS/PLGA), which merged the good bioactivity of CS/PHBV composite and the improved degradation velocity of PHBV/PLGA blend .

Aim of the study

Composite films were prepared.

For degradation studies and measurements of mechanical properties dumbbell-shaped samples were prepared.

Materials and methods

The degradation study was carried out in simulated body fluid

The SBF was replaced after 7, 14, 21, and 28 days. At each time point on average 5 samples were removed from SBF, rinsed with deionized water, and dried in vacuum oven at 40 °C for 24 h.

Every sample was characterized with respect to change of its mass, mechanical properties,molecular weight, and surface morphology.

Degradation study

Addition of CS had an adverse effect on the mechanical properties of PHBV and decreased the ultimate strength by 31%, Young's modulus by 10% and elongation at break by 67%, in comparison to neat PHBV.

Addition of PLGA, however, improved strength and stiffness of the composites

Results

The reduction of the mechanical properties of the composite can be explained by agglomeration of the

CS particles within the polymer solution and poor adhesion between filler particles and polymer matrix.

Agglomerates can act as stress concentrators, which effectively decreases the mechanical properties of the composite.

The ultimate tensile strength (UTS) of neat PHBV remained relativelyconstant over the degradation period, whereas it was found to continuously decrease for all composites

The change of mass of all tested materials is shown in the figure. All composites, except PHBV65, gained mass over the time of degradation.

Molecular weight :the highest decrease was observed for PHBV75 (by 48% of initial value), followed by PHBV95 (by 24% of initial value) and PHBV (decrease of Mw by 15% of initial value).

In order to accelerate the degradation velocity of PHBV–CS composite, a third phase, i.e. PLGA was added.

The PLGA not only increased degradation rate of the ternary composites but also additionally improved their initial mechanical properties .

Organic PLGA phase should adhere better to the organic PHBV matrix than the inorganic CS particles. Better

adhesion at the interface leads to an effective transfer of the load through the whole composite.

Incorporation of CS and PLGA to PHBV matrix allowed us to fabricate bioactive composites with good in vitro

bioacceptance, tailored degradation rates and improved mechanical properties.

The tailored degradation was proven both at the molecular (decrease of molecular weight) and macroscopic levels (decrease of mechanical properties )

Thus the ternary composite PHBV75 is a promising material for bone regeneration.

Conclusion

1. Bugnicourt E, Cinelli P, Lazzeri A, Alvarez V. Polyhydroxyalkanoate (PHA): Review of synthesis, characteristics, processing and potential applications in Packaging. eXPRESS Polymer Letters .2014;8(11):791-808 

2. Christopher J and Anthony J. Sinskey. Applications of Polyhydroxyalkanoates in the Medical Industry.International Journal of Biotechnology for Wellness Industries. 2012;1:53-60

3. Reddy C, Ghai. R, Kalia.V .Polyhydroxyalkanoates: an overview. Bioresource Technology.2003; 87 :137–46

4. Idaszek J ,Zinn M, Obarzanek-Fojt M et al . Tailored degradation of biocompatible poly(3-hydroxybutyrateco- 3-hydroxyvalerate)/calcium silicate/poly(lactide-co-glycolide) ternary composites: An in vitro study. Materials Science and Engineering .2013;33: 4352–60

References

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