Composite as Biomaterials

Transcription

1 MME 297: Lecture 19 Composite as Biomaterials Dr. A. K. M. Bazlur Rashid Professor, Department of MME BUET, Dhaka Topics to discuss today Introduction 2. Structure of Composites 3. Mechanics of Composites 4. Application of Composite Biomaterials 5. Biocompatibility of Composite Biomaterials References: Joon Park and R S Lakes. Biomaterials An Introduction, 3rd Editions, Springer, 2007,

2 1. Introduction Composite materials are solids, which contain two or more distinct constituent materials or phases on a scale larger than the atomic. The term composite is usually reserved for those materials in which the phases are microscopically or macroscopically distinct, and properties such as the elastic modulus are significantly altered in comparison with those of a homogeneous material Accordingly, reinforced plastics such as fiberglass as well as natural materials such as bone are viewed as composite materials, but metallic alloys such as brass are not. All natural biological materials are composites (e.g., bone, wood, dentin, skin, etc.) Foams or cellular solids are also composites (mixture of solid and air) 1.1 Classification of Composites 1. Based on matrix material: (a) (b) (c) Metal matrix composite (MMC) e.g., SiC reinforced Aluminium Ceramic matrix composite (CMC) e.g., Ag toughened Alumina, Zirconia toughened Alumina Polymer matrix composite (PMC) e.g., Glass-fibre reinforced plastics (GFRP), Carbon particle reinforced rubber 2. Based on reinforcing material: (a) (b) (c) (d) Particulate reinforced composite e.g., SiC reinforced Aluminium, Zirconia toughened Alumina Fibre reinforced composite e.g., Glass-fibre reinforced plastics (GFRP) Structural composite (Laminar / Sandwich / Honeycome) e.g., Plywood Foams or Cellular composite (a mixture of solid and air) 2

3 FIGURE 4.1: Morphology of basic composite inclusions. (a) platelet or lamina (with two long dimensions), (b) fiber (with one long dimension), and (c) particle (with no long dimension). Figure Dental composite. The particles are silica (SiO 2 ) and the matrix is polymeric. Figure Structure of a cross-ply laminate. Figure Glass-fiber-epoxy composite. 3

4 Plywood a laminar composite honeycomb sandwich structure Cellular solids structures. Left: Synthetic cellular solids: (a) open-cell polyurethane, (b) closed-cell polyethylene, (c) foamed nickel, (d) foamed copper, (e) foamed zirconia, (f) foamed mullite, (g) foamed glass, (h) polyester foam with both open and closed cells. Right: Natural cellular solids: (a) cork, (b) balsa wood, (c) sponge, (d) cancellous bone, (e) coral, (f) cuttlefish bone, (g) iris leaf, (h) plant stalk. 4

5 2. Structure of Composite The properties of composite materials depend upon their structure (as they do in homogeneous materials). Composites differ from homogeneous materials in that considerable control can be exerted over the larger scale structure, and hence over the desired properties. The properties of a composite material depend upon 1. the properties of the homogeneous matrix and reinforcing materials, 2. the shape, volume fraction, and orientation of the reinforcing materials, 3. the interface among the constituents, and 4. the adhesion of the reinforcing materials with the matrix. 3. Mechanics of Composites Mechanical properties in many composite materials depend on structure in a complex way. However, for some structures the prediction of properties is relatively simple. 5

6 3.1 Fiber-Reinforced Composites stress parallel to fibers (iso-strain condition) stress perpendicular to fibers (iso-stress condition) FIGURE 4.2 Voigt (a, laminar; b, fibrous) and Reuss (c) composite models, subjected to tension force indicated by arrows. Iso-strain Condition The total force acting on the composite is the sum of the forces carried by each constituent: F c = F m + F f σ c A c = σ m A m + σ f A f ( F = σ A) σ c = σ m A m A c + σ f A f A c If the fibers have a uniform cross-section, the area fraction equals the volume fraction: V c = V m + V f σ c = σ m V m + σ f V f 6

7 σ c = σ m V m + σ f V f Using Hooke s law, σ = Eε E c ε c = E m ε m V m + E f ε f V f If the fibers are rigidly bonded to the matrix, both the fibers and the matrix must stretch equal amounts (iso-strain conditions): ε c = ε m = ε f E c = E m V m + E f V f ρ c = ρ m V m + ρ f V f Upper Bound Value of E c (Voigt model) Rule of Mixture property of composite is weighted sum of its constituent materials Iso-stress Condition stress is applied perpendicular to the axis of the fiber The stresses in each component are equal But, the strains are no longer equal Instead, the weighted sum of the strains in each component equals the total strain in the composite ε c = ε m V m + ε f V f σ c E c = σ m E m V m + σ f E f 1 E c = V m E m + V f E f V f Lower Bound Value of E c (Reuss model) ( σ c = σ m = σ f ) 7

8 Problem 1 Boron coated with SiC (or, Borsic) reinforced aluminum containing 40 vol% fibres is an important high-temperature, lightweight composite material. The fibres are aligned parallel to the loading direction. Estimate the density, modulus of elasticity, and tensile strength parallel to the fibre axis. Also estimate the modulus of elasticity perpendicular to the fibres. Material Density Young s Modulus Tensile Strength (g/cc) (psi) (psi) Fibre ,000, ,000 Aluminium ,000,000 5,000 From the rule of mixtures: ρ c = (2.36 g/cc) (0.40) + (2.70 g/cc) (0.60) = 2.56 g/cc E c = (55*10 6 psi) (0.40) + (10*10 6 psi) (0.60) = 28*10 6 psi TS c = (400*10 3 psi) (0.40) + (5*10 3 psi) (0.60) = 163*10 3 psi Perpendicular to the fibers: 1 E c = psi psi = psi 1 E c = psi 8

9 Problem 2 In a 30 vol% glass fibre reinforced nylon matrix composite, the fibres are aligned parallel to the loading direction. What proportion of the load and stress are carried by the fibres? The modulus of elasticity for each component of the composite is E glass = 72.4 GPa and E nylon = 2.8 GPa. Both the nylon and the glass fibers have equal strain if bonding is good. ε c = ε m = ε f ε m = σ m E m = ε f = σ f E f σ f σ m = E f E m = 72.4 GPa 2.8 GPa = σ f σ m = E f E m = Fraction of stress = σ f σ f + σ m = σ = m σf = 0.96 Fraction of load = F f F f + F m = σ f A f σ f A f + σ m A m = σ f 0.3 σ f σ m 0.7 = σ = m σf = 0.92 Almost all of the stress and load are carried by the glass fibers. 9

10 Controlling the Characteristics of Fibre-Reinforced Composites 1. Aspect Ratio (the length of a fiber divided by its diameter) Fibres can be short, long, or continuous Properties of composite improved when the aspect ratio is large 2. Volume Fraction of Fibre A greater volume fraction of fibre increases strength and stiffness of composite For over 80 vol % fibre, the matrix can no longer completely surrounds the fibres 4. Properties of Fibre and Matrix Stiff, strong, light fibre Tough and ductile matrix 5. Bonding and Failure Good bonding between fibre and matrix is essential to transmit load from matrix to the fibre If bonding is poor, fibre pull out occurs Special coating on fibre may be used to improve bonding 3. Orientation of Fibre Unidirectionally aligned fibre optimum properties along fibre direction; poor properties across fibre direction (anisotropic properties) Randomly oriented fibre isotropic properties; properties not optimum Increasing the length of chopped E-glass fibers in an epoxy matrix increases the strength of the composite. In this example, the volume fraction of glass fibers is about 0.5. Effect of fiber orientation on the tensile strength of E-glass fiber-reinforced epoxy composites. 10

11 Scanning electron micrograph of the fracture surface of a silver-copper alloy reinforced with carbon fibers. Poor bonding causes much of the fracture surface to follow the interface between the metal matrix and the carbon tows ( 3000). 11

12 3.2 Particulate Composites It is often convenient to stiffen or harden a material, commonly a polymer, by the incorporation of particulate inclusions. Properties follows the rule of mixture: ρ c = V i ρ i = V 1 ρ 1 + V 2 ρ V n ρ n The effect of clay on the properties of polyethylene The shape of the particles is important. In isotropic systems, stiff platelet (or flake) inclusions are the most effective in creating a stiff composite, followed by fibers; the least effective geometry for stiff inclusions is the spherical particle, Microstructure of a dental composite, 50% by volume; filler: barium glass and colloidal silica Particle inclusions do not increase the composite stiffness as much as inclusions of other shapes Often used for reasons of simplicity of preparation or availability of inclusions of that shape. 12

13 Dilute Composites A dilute concentration of spherical particulate inclusions of stiffness E i and volume fraction V i, in a matrix (with Poisson s ratio assumed to be 0.5) denoted by the subscript m, gives rise to a composite with a stiffness E: E = 5 E i E m V i E i Em + E m 13

14 4. Application of Composite Biomaterials it is important that each constituent of the composite be biocompatible, and that the interface between constituents not be degraded by the body environment. Common composite biomaterials: dental filling composites bone particle or carbon fiber reinforced MMA bone cement and UHMPE porous surface orthopedic implants very fine silica particles reinforced rubber (used in catheters, rubber gloves, etc.) 14

15 4.1 Dental Filling Composites and Cements Previously used dental filling material silver amalgam and gold for posterior teeth Acrylic resins and silicate cements for anterior teeth Poor material properties led to short service life and clinical failures Dental composite resins have virtually replaced these materials consist of a polymer matrix and stiff inorganic inclusions Inorganic inclusions / fillers confer a relatively high stiffness and high wear resistance to the material by virtue of their translucence and index of refraction similar to that of dental enamel, they are cosmetically acceptable Common inorganic inclusions: Barium glass, silica [quartz, SiO 2 ] Particle size range: 0.04 to 13 μm Concentration range: 33 to 78% by weight 15

16 The matrix consists of BIS-GMA, an addition reaction product of bis(4-hydroxyphenol), dimethylmethane, and glycidyl methacrylate. The viscosity must be sufficiently low and the polymerization reaction controllable. Low-viscosity liquids such as triethylene glycol dimethacrylate (TEGDMA) are used to lower the viscosity Inhibitors such as BHT (butylated trioxytoluene) are used to prevent premature polymerization. Polymerization can be initiated by a thermochemical initiator (such as benzoyl peroxide), or by a photochemical initiator (benzoin alkyl ether), which generates free radicals when subjected to ultraviolet light from a lamp used by the dentist. 16

17 Dental composites are considerably less stiff than natural enamel, which contains about 99% mineral. difficult to produce synthetic composites with such high concentrations of mineral particles particles do not pack densely the viscosity of the unpolymerized paste increased (which prevents the dentist from adequately packing the paste into the prepared cavity) Higher thermal expansion than that of the tooth structure. There is also a contraction up to 1.6% during polymerization. this contribute to leakage of saliva, bacteria, etc., at the interface margins, which can cause further decay of the tooth. All dental composites exhibit creep stiffness changes by a factor of from 2.5 to 4 over a time period from 10 sec to 3 hr under steady load this creep may result in indentation of the restoration and wear Dental composites tend to be brittle and relatively weak in tension They are also subject to mechanical fatigue, so they can break or become loose at stress levels below the static fracture strength 17

18 Dental cements used to attach dental crowns to the remaining tooth structure resin-based cements, with 65 to 74% by weight filler high elastic modulus required 4.2 Porous Implants Porous implants allow tissue ingrowth considered desirable in many contexts, since it allows a relatively permanent anchorage of the implant to the surrounding tissues Two composites to be considered in porous implants: 1. the implant prior to ingrowth the pores are filled with tissue fluid which is ordinarily of no mechanical consequence, and the stiffness and strength of the composite are much less than in the case of the solid from which it is derived 2. the implant filled with tissue 18

19 Porous materials have a high ratio of surface area to volume demands upon inertness and biocompatibility are likely to be greater for a porous material than a homogeneous one The pore size of a cellular solid has no influence on its stiffness or strength, although it does influence toughness. Pore size can be of considerable biological importance. in orthopedic implants with pores larger than about 150 μm, bony ingrowth into the pores occurs and this is useful to anchor the implant pores less than 75 μm in size did not permit the ingrowth of bone tissue Some other applications of porous materials where porosity encourages tissue ingrowth: soft tissue applications include polyurethane, polyimide, and polyester velours used in percutaneous devices. porous reconstituted collagen in artificial skin braided polypropylene has been used in artificial ligaments porous blood vessel replacements 19

20 Manufacturing Processes of Porous Implants Sintering of beads or wires in the case of bone compatible surfaces. Vascular and soft tissue implants are produced by weaving or braiding fibers as well as by nonwoven felting methods. Protective foams for external use are usually produced by blow moulding process where a blowing agent, is used to produce gas during polymerization of the foam. Microporous materials are produced using replamineform process (meaning replicated life forms e.g., skeletal structure of coral) where replication of structures found in biological materials are made by casting of metals, ceramic or polymers. 4.3 Fibrous and Particulate Composites in Orthopedic Implants Incorporating stiff inclusions in a polymer matrix results in increased stiffness, strength, fatigue life, and other properties. Carbon fibers reinforced high-density polyethylene (HDPE) used in total knee replacements and replaces the standard ultrahigh-molecular-weight polyethylene (UHMWPE). UHMWPE provides adequate wear resistance over ten years' use While this is sufficient for implantation in older patients, longer wear-free lifetime is desirable in implants to be used in younger patients. Improvement in the resistance to creep of the polymeric component is also considered desirable, since excessive creep results in an indentation of the polymeric component after long-term use. 20

21 Enhancements of various properties by a factor of two The mechanical properties of a fiber-reinforced composite depend on the properties of the fiber the degree to which an applied load is transmitted to the fibers by the matrix phase Length of fibre (some critical fiber length is necessary for effective strengthening and stiffening of the composite material) Critical fibre length: l c = σ f d σ f = fibre tensile strength d = fibre diameter 2 τ c τ c = fibre-matrix surface shear/bond strength For fibers longer than critical length, load is efficiently transferred to them, to achieve the maximum strength of the composite 21

22 Composites have been considered for bone plates and in the femoral component of total hip replacements. Currently used metal implants are much stiffer than bone. They therefore shield the nearby bone from mechanical stress. Such stress-shielding results in a kind of disuse atrophy: the bone resorbs. Composite materials can be made more compliant than metal, and deform elastically to a higher strain (to about 0.01 compared with for a mild steel). Flexible composite bone plates are effective in promoting healing. Some composites hip replacement prostheses are made with carbon fibers in a matrix of polysulfone and polyetherether ketone. Although in polymer matrix composites, creep behavior due to the polymer component is a matter of concern, prototype composite femoral components exhibited creep of small magnitude limited by the fibers, which do not creep much. Creep is not expected to limit the life of the implant. 22

23 5. Biocompatibility of Composite Biomaterials Each constituent of the composite must be biocompatible, and the interface between constituents must not be degraded by the body environment. Carbon as inclusion material has good compatibility and is used successfully. Carbon fibers are inert in aqueous and even seawater environments; however, they do not have a long track record as biomaterials. Substantial electrochemical activity occurs in carbon fiber composites in aqueous media. If placed near a metallic implant, may cause galvanic corrosion. Inclusions in dental composites are minerals and ceramics with a good record of compatibility. But polymer matrix tends to absorb water when placed in a hydrated environment. Water acts as a plasticizer of the matrix and lowers the glass transition temperature. This causes a reduction in stiffness and an increase in mechanical damping. Water absorption also causes swelling in polymers; this can be beneficial in dental composites since it neutralizes some of the shrinkage due to polymerization. 23