Biomaterials for Medical Applications Reporter: AGNES Purwidyantri Student ID no: D Biomedical Engineering Dept.

A MATERIAL INTENTED TO INTERFACE WITH BIOLOGICAL SYSTEMS TO EVALUATE, TREAT, AUGMENT OR REPLACE ANY TISSUE, ORGAN OR FUNCTION OF THE BODY. What is a Biomaterial?

Biomaterials cover all classes of materials – metals, ceramics, polymers

Medical Devices Values (Biomaterials Science: An Introduction to Materials in Medicine, 2nd ed., B.D. Ratner et al., eds., Elsevier, NY 2004)

Structural Hierarchy

Polymer Crystallinity 6 Ex: polyethylene unit cell Crystals must contain the polymer chains in some way  Chain folded structure 10 nm Adapted from Fig , Callister 7e. Adapted from Fig , Callister 7e.

Polymer Crystallinity 7 Polymers rarely 100% crystalline Too difficult to get all those chains aligned % Crystallinity: % of material that is crystalline. -- TS and E often increase with % crystallinity. -- Annealing causes crystalline regions to grow. % crystallinity increases. Adapted from Fig , Callister 6e. (Fig is from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.) crystalline region amorphous region

Polymer Crystal Forms 8 Single crystals – only if slow careful growth Adapted from Fig , Callister 7e.

Spherulites – crossed polarizers 9 Adapted from Fig , Callister 7e. Maltese cross

Mechanical Properties 10 i.e. stress-strain behavior of polymers brittle polymer plastic elastomer  FS of polymer ca. 10% that of metals Strains – deformations > 1000% possible (for metals, maximum strain ca. 10% or less) elastic modulus – less than metal Adapted from Fig. 15.1, Callister 7e.

Tensile Response: Brittle & Plastic 11 brittle failure plastic failure  (MPa)  x x crystalline regions slide fibrillar structure near failure crystalline regions align onset of necking Initial Near Failure semi- crystalline case aligned, cross- linked case networked case amorphous regions elongate unload/reload Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from Figs & 15.13, Callister 7e. (Figs & are from J.M. Schultz, Polymer Materials Science, Prentice- Hall, Inc., 1974, pp )

Predeformation by Drawing 12 Drawing…(ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) Annealing after drawing decreases alignment -- reverses effects of drawing. Compare to cold working in metals! Adapted from Fig , Callister 7e. (Fig is from J.M. Schultz, Polymer Materials Science, Prentice-Hall, Inc., 1974, pp )

13 Tensile Response: Elastomer Case Compare to responses of other polymers: -- brittle response (aligned, crosslinked & networked polymer) -- plastic response (semi-crystalline polymers) Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig , Callister 7e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)  (MPa)  initial: amorphous chains are kinked, cross-linked. x final: chains are straight, still cross-linked elastomer Deformation is reversible! brittle failure plastic failure x x

14 Thermoplastics vs. Thermosets Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: -- large crosslinking (10 to 50% of mers) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin Adapted from Fig , Callister 7e. (Fig is from F.W. Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc., 1984.) Callister, Fig T Molecular weight TgTg TmTm mobile liquid viscous liquid rubber tough plastic partially crystalline solid crystalline solid

15 T and Strain Rate: Thermoplastics Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. Adapted from Fig. 15.3, Callister 7e. (Fig is from T.S. Carswell and J.K. Nason, 'Effect of Environmental Conditions on the Mechanical Properties of Organic Plastics", Symposium on Plastics, American Society for Testing and Materials, Philadelphia, PA, 1944.) °C 20°C 40°C 60°C to 1.3  (MPa)  Data for the semicrystalline polymer: PMMA (Plexiglas)

Melting vs. Glass Transition Temp. 16 What factors affect T m and T g ? Both T m and T g increase with increasing chain stiffness Chain stiffness increased by 1. Bulky sidegroups 2. Polar groups or sidegroups 3. Double bonds or aromatic chain groups Regularity – effects T m only Adapted from Fig , Callister 7e.

17 Time Dependent Deformation Stress relaxation test: -- strain to   and hold. -- observe decrease in stress with time. Relaxation modulus: Sample T g (  C) values: PE (low density) PE (high density) PVC PS PC Selected values from Table 15.2, Callister 7e. time strain tensile test oo (t)(t) Data: Large drop in E r for T > T g. (amorphous polystyrene) Adapted from Fig. 15.7, Callister 7e. (Fig is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.) rigid solid (small relax) transition region T(°C) TgTg E r (10s) in MPa viscous liquid (large relax)

Polymer Fracture 18 fibrillar bridges microvoids crack alligned chains Adapted from Fig. 15.9, Callister 7e. Crazing  Griffith cracks in metals – spherulites plastically deform to fibrillar structure – microvoids and fibrillar bridges form

Addition (Chain) Polymerization 19 – Initiation – Propagation – Termination

Condensation (Step) Polymerization 20

Polymer Types 21 Coatings – thin film on surface – i.e. paint, varnish  To protect item  Improve appearance  Electrical insulation Adhesives – produce bond between two adherands – Usually bonded by: 1. Secondary bonds 2. Mechanical bonding Films – blown film extrusion Foams – gas bubbles in plastic

Advanced Polymers 22 Ultrahigh molecular weight polyethylene (UHMWPE)  Molecular weight ca. 4 x 10 6 g/mol  Excellent properties for variety of applications  bullet-proof vest, golf ball covers, hip joints, etc. UHMWPE Adapted from chapter- opening photograph, Chapter 22, Callister 7e.

Polymeric Biomaterials: Easy to make complicated items Tailorable physical & mechanical properties Surface modification Immobilize cell etc. Biodegradable Leachable compounds Absorb water & proteins etc. Surface contamination Wear & breakdown Biodegradation Difficult to sterilize AdvantagesDisadvantages

General Criteria for materials selection Mechanical and chemicals properties No undersirable biological effects carcinogenic, toxic, allergenic or immunogenic Possible to process, fabricate and sterilize with a godd reproducibility Acceptable cost/benefit ratio

Material Properties Compresssive strength Tensile strength Bending strength E-Modulus Coefficient of thermal expansion Coefficient of thermal coductivity Surface tension Hardness and density Hydrophobic/philic Water sorption/solubility Surface friction Creep Bonding properties

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