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Ch11 Ch11Biomaterials

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Definition of Biomaterial Classes of Materials Crystal Structure Mechanical Behavior Wear Resistance Calcium-phosphate Bioactive Glasses 2

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A biomaterial is a nonviable material used in medical device, so its intended to interact with a biological systems. Biomaterials are manufactured substitutes for natural tissues. They are used in implants or catheters.artificial organs. drug delivery. wound dressing. 4

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Biomaterials can be conveniently grouped into three classes: metals, polymers, and ceramics. Composites, representing another group of materials, consist of combinations of two or more metals, polymers, or ceramics. 5

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metallic bond As the electrons can move easily in metals, making metals easily deformable. The independent electrons in the metallic bonds can quickly transfer electric charge and thermal energy. 6

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it is often desirable to coat metallic implants with a bioactive ceramic film in order to improve implant fixation the difference in the thermal expansion coefficient between metals and ceramics results in interfacial shear stresses that can create microcracks at the interface during cooling subsequent to plasma-spraying a calcium phosphate coating onto a metallic implant. http://www.gordonengland.co.uk/img/ps1.gif 7 http://www.azom.com/work/amvxiMoDGM6y1N7EBNXZ_files/image003.jpg

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As the mechanical properties (and also the chemical and physical properties) of metals can be improved by alloying, most metals used in orthopedic surgery are alloyed. Obvious examples are the alloys based either on titanium or on cobalt. 8

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Ceramics are refractory polycrystalline compounds Usually inorganic Highly inert Hard and brittle High compressive strength Generally good electric and thermal insulators Good aesthetic appearance Applications: orthopaedic implants dental applications 9 http://www.azonano.com/images/Article_Images/ImageForArticle_2702(2).jpg

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10 Synthetic HABone HA

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covalent bond These large molecules contain many repeating units, from which comes the word polymers. The polymeric molecular arrangements can be linear, branched, or cross-linked

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catheters artificial trachea

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Composite materials are solids that contain two or more distinct constituent materials or phases Most composite biomaterials have been developed to enhance mechanical and biocompatibility behavior. The shape of the phases in a composite material is classified into three categories. 14 platelet fiber particle

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Some applications of composites in biomaterial applications are: (1) dental filling composites (2) reinforced methyl methacrylate bone cement (3) orthopedic implants with porous surfaces. 15 http://seis.irsm.cas.cz/images/IMG/Bull/Fig_6.jpg

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(unit cells) 17

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Body-centered cubic (BCC) Face-centered cubic (FCC) Hexagonal close-packed (HCP) 19

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( )=8 2 =0.68 20

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(coordination number) (atomic packing factor, APF) APF APF 21

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a R 8 22

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( )=12 4 =0.74 23

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a R (0.74) (0.68) 12 24

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(hexagonal close-packed HCP) 6 12 2 3 25

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( )=12 6 =0.74 26

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x, y, z x y z a b c r a b c (lattice parameters) 27

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All metals, most ceramics, and some polymers crystallize when they solidify. A crystalline material is characterized by long- range order and an infinitely repeating unit cell of atoms/ions. 31

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TABLE 11-2. Some Materials and Their Representative Equilibrium Crystal Structure at Body Temperature MaterialStructure Cobalt-chromium alloyFCC Stainless steel AISI 316LFCC Titanium (Ti)HCP Tantalum (Ta)BCC Niobium (Nb)BCC Gold (Au)FCC Alumina (Al 2 O 3 )HCP Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 ]HCP 32

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Hydroxyapatite (HA), a bone bioactive ceramic. HA structure is considerably complex, as a result of which, displacement of atoms within the lattice is difficult. Thus the structure is resistant to deformation and, when overloaded, fractures rather than deforming permanently. 33

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As the extent of polymerization increases and the molecular chains become longer, the relative mobility of the chains in the structure decreases. As a result, alignment of the chains and formation of long-range order is difficult. 35

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Factors affecting the strength of polymers further include chemical composition, side groups, cross-linking, copolymerization, and blending. 38

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Mechanical properties including elasticity and strength are important properties to consider in the selection of a material for a specific implant design. 39

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TABLE 11-3. Typical Values of Tensile Strength for Various Materials at Room Temperature MaterialTensile Strength (MPa) Diamond1.05 10 6 Kevlar4,000 High-strength carbon fiber4,500 High-tensile steel2,000 Superalloy1,300 Spider webs (drag line)1,000 Ti-6Al-4V860 CoCr alloy (F75)655 Aluminum570 Titanium (grade 4)550 316L SS (F745, annealed)485 (Cold forged)1,351 Bone200 Nylon100 Rubber100 41

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TABLE 11-5. Elastic Properties of Some Typical Materials MaterialsDirectionality Properties Modulus of Elasticity (MPa) Cortical boneAnisotropicLongitudinal axis, 17,000 Trabecular boneAnisotropic Longitudinal axis of femur intertrochanteric 316 293 High-density polyethyleneIsotropic410-1,240 PMMAIsotropic3,000-10,000 Stainless steel AISI 316LIsotropic200,000 Cobalt-chromium alloyIsotropic220,000 TitaniumIsotropic107,000 Ti-6A1-4VIsotropic110,000 Carbon fiber-reinforced graphite fibersAnisotropicParallel to unidirectional 140,000 AluminaIsotropic Single crystal, 362,700 Polycrystal, 408,900 42

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( ) ( ) (ductile) (brittle) 43

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( ) 44

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(a) (b) 46

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(a) (b) (cleavage) (transgranular transcrystalline) 47

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(a) (b) 50 (intergranular) 48

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Stress Concentration (stress raiser) 49

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(static fatigue) (delayed fracture) ( 10 ) 50

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Fatigue cyclic load fatigue strength fatigue limit (Static fatigue) (delayed fracture) 51

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TABLE 11-4. Fatigue Properties of Implant Metals MaterialASTM DesignationCondition Fatigue Endurance Limit (at 10 7 Cycles, R = -1) (MPa) Stainless steel F745Annealed221-280 F55, F56, F138, F139Annealed241-276 30% Cold worked310-448 Cold forged820 Co-Cr alloysF75As-cast/annealed207-310 P/M HIP a 725-950 F799Hot forged600-896 F90AnnealedNot available 44% Cold worked586 F562Hot forged500 Cold worked, aged 689-793 (axial tension R = 0.05, 30 Hz) Ti alloysF67 30% Cold-worked grade 300 F136Forged annealed620 Forged, heat treated620-689 a P/M HIP, Powder metallurgy produced, hot-isostatically pressed. 52

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Ultrahigh-molecular-weight Polyethylene (UH-MWPE) (MW above 2 10 6 g/mol) The success of UHM-WPE is due to its favorable properties, including abrasion resistance, impact strength, low coefficient of friction, chemical inertness, and resistance to stress cracking. It has long been used for total hip prostheses and knee prostheses.

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In the majority of contemporary total joint replacements, a metallic component articulates against UHMWPE. As a patient may be expected to take an average of 1,200 steps per day, the joint replacement is expected to withstand millions of loading cycles during its service lifetime.

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In total hip replacements, where the articulating geometries consist of conforming spherical surfaces, the wear occurs at a microscopic length scale (m or less). Wear resistance has been related to its resistance to multidirectional stresses.

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In total knee replacements, where the articulating surfaces consist of nonconforming cylindrical, toroidal, or flat surfaces, the wear process includes various surface damage mechanisms ranging from pitting and delamination to burnishing and adhesive wear.

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Adhesive wear is a process in which surface asperities of the polymer adhere to the metal surface and subsequently are torn off. As a result, either a polymer film is formed on the metal surface or polymer particles are released and entrapped in the joint. Such particles as well as remaining cement fragments can generate abrasive wear, a second mode of wear.

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Abrasive wear can also be generated by asperities on the gliding metal partner. A third mechanism is fatigue wear; as a result of creep or plastic flow, folds or cracks are formed that cause small polymer particles to break off.

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It is generally accepted that particulate debris generated by mechanical wear of prosthetic components stimulates the generation of a pseudosynovial membrane at the interface between implant and bone and the infiltration of fibrocytes and macrophages. In the presence of debris, these cells release various cytokines and mediators (such as IL-1 , TNF , collagenase, and prostaglandin E 2 )

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These cytokines have been shown to be involved in bone resorption by activating osteoclasts. A recent study showed a positive correlation between cytokine concentration in the loosening membrane and the degree of underlying osteolysis. Many reports have investigated the failed total joint arthroplasty and demonstrated that phagocytosis and cell mortality increase with particle size and concentration.

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The mechanism that results in this cell death remains unknown, although a potential role for apoptosis in the pathogenesis of wear debris-associated osteolysis has recently been suggested. Aseptic loosening, which is the single most common cause for long-term failure of TJA, is associated with periprosthetic osteolysis with the incidence of up to 25% of implant recipients.

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Multiple factors can affect polyethylene wear and the production of wear debris in vivo after joint arthroplasty. Such factors include the roughness and material of the femoral head, the method of polyethylene sterilization, and the mechanical properties of the polyethylene itself.

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Recent laboratory studies have confirmed that the wear resistance of UHMWPE can be significantly increased when applying additives or high-dose irradiation. Many studies reporting extremely low quantities of wear, typically as low as 2.0 mm 3 per million cycles for highly cross-linked polyethylenes.

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High-density, high-purity alumina is used in load-bearing hip prostheses because of its outstanding wear resistance and excellent corrosion resistance. It has a high Young's modulus and a hardness second only to that of diamond. These properties have made the alumina-on- alumina couple for femoral heads and acetabular cups a materials combination of considerable importance.

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Most alumina devices are very fine-grained polycrystalline -Al 2 O 3 produced by pressing and sintering at high temperature (1600C to 1700C). A very small amount of MgO (