Polymer Biomaterials

1. CHEE 340 1 Polymer Biomaterials There are a large number of uses for polymers in the biomaterials field. These arise due to the wide variety of properties possible. OBJECTIVES to introduce some fundamental polymer properties and the factors that influence them to provide an overview of the uses of polymers as biomaterials

2. CHEE 340 POLYMERS Polymers - long chain molecules of high molecular weight -(CH2)n-

3. CHEE 340 3 Definitions monomer oligomer backbone thermoplastic thermoset elastomer gel

4. CHEE 340 4 Polymers In Specific Applications

5. CHEE 340 5 Molecular weight synthetic polymers possess a molecular weight distribution

6. CHEE 340 6 The Bulk State : Solid Polymers can be either amorphous or semi-crystalline, or can exist in a glassy state. amorphous glassy state hard, brittle no melting point semi-crystalline glassy state hard, brittle crystal formation when cooled exhibit a melting point

7. CHEE 340 7 Effect of Temperature on Polymer Properties amorphous

8. CHEE 340 8 Effect of Temperature semi-crystalline

9. CHEE 340 9 Crosslinked Networks crosslinks covalent; H-bonding; entanglements crosslinking increased molecular weight swell in solvents organogel hydrogel

10. CHEE 340 10 Temperature Effects

11. CHEE 340 11 Viscoelasticity The response of polymeric materials to an imposed stress may under certain conditions resemble the behavior of a solid or a liquid.

12. CHEE 340 12 Mechanical Properties The mechanical properties of polymers depend on several factors, including the composition and structure of the macromolecular chains and their molecular weight. Table 1.6 lists some mechanical properties of selected polymeric biomaterials. Compared with metals and ceramics, polymers have much lower strengths and moduli but they can be deformed to a greater extent before failure. Consequently, polymers are generally not used in biomedical applications that bear loads (such as body weight). Ultra-high-molecular-weight polyethylene is an exception, as it is used as a bearing surface in hip and knee replacements. The mechanical properties of polymers depend on several factors, including the composition and structure of the macromolecular chains and their molecular weight. Table 1.6 lists some mechanical properties of selected polymeric biomaterials. Compared with metals and ceramics, polymers have much lower strengths and moduli but they can be deformed to a greater extent before failure. Consequently, polymers are generally not used in biomedical applications that bear loads (such as body weight). Ultra-high-molecular-weight polyethylene is an exception, as it is used as a bearing surface in hip and knee replacements.

13. CHEE 340 13 Thermal Properties

14. CHEE 340 14 Diffusion in Polymers Polymers can also act as solvents for low molecular weight compounds. The diffusion of small molecular weight components in polymers is important in a number of biomedical applications: purification of gases by membrane separation dialysis prevention of moisture loss in drugs (packaging) controlled drug delivery (transdermal patches, Ocusert) polymer degradation in vivo

15. CHEE 340 15 Diffusion in Polymers Flux is dependent on : solubility of component in polymer diffusivity of component in polymer These in turn depend on : nature of polymer temperature nature of component interaction of component with polymer

16. CHEE 340 16 Solubility Estimation From Hildebrand, the interaction parameter, c, is defined as : The solubility parameter, d, reflects the cohesive energy density of a material, or the energy of vapourization per unit volume. While a precise prediction of solubility requires an exact knowledge of the Gibbs energy of mixing, solubility parameters are frequently used as a rough estimator. In general, a polymer will dissolve a given solvent if the absolute value of the difference in d between the materials is less than 1 (cal/cm3)1/2.

17. CHEE 340 17 Diffusivity experimental observations effect of T vs Tg

18. CHEE 340 18 Diffusivity effect of permeant size

19. CHEE 340 19 Diffusivity : Effect of Crystallinity solutes do not penetrate crystals readily take path of least resistance through amorphous regions increased path length

20. CHEE 340 20 Example of Undesirable Absorption poppet-style heart valve poppet is composed of PDMS in small % of patients the poppet jammed or escaped recovered poppets were yellow, smelled, and had strut grooves recovered poppets contained up to 16% simple and complex lipids- such as cholesterol esters, troglycerides, fatty acids, cholesterol I.e., portion of the fatty components of blood. recovered poppets contained up to 16% simple and complex lipids- such as cholesterol esters, troglycerides, fatty acids, cholesterol I.e., portion of the fatty components of blood.

21. CHEE 340 21 Leaching - Undesirable polymers often contain contaminants as a result of their synthesis/manufacturing procedure/equipment may also contain plasticizers, antioxidants and so on these contaminants are a frequent cause of a polymer�s observed incompatibility

22. CHEE 340 22 Drug Delivery

23. CHEE 340 23 In Vivo Degradation of Polymers no polymer is impervious to chemical and physical actions of the body

24. CHEE 340 24 Hydrolytic Degradation hydrolysis the scission of chemical functional groups by reaction with water some polymers are very stable to hydrolysis there are a variety of hydrolyzable polymeric materials

25. CHEE 340 25 Hydrolytic Degradation degradation rate dependent on hydrophobicity crystallinity Tg impurities initial molecular weight, polydispersity degree of crosslinking manufacturing procedure geometry site of implantation

26. CHEE 340 26 Hydrolytic Degradation bulk erosion (homogeneous) uniform degradation throughout polymer process random hydrolytic cleavage (auto-catalytic) diffusion of oligomers and fragmentation of device surface erosion (heterogeneous) polymer degrades only at polymer-water interface degrading life-saver

27. CHEE 340 27 Polyesters

28. CHEE 340 28 Oxidative Degradation usually involves the abstraction of an H to yield an ion or a radical there are 2 general categories of oxidative biodegradation direct oxidation by host and/or device release of superoxide anion and hydrogen peroxide by neutrophils and macrophages release of metal ions from metal components of device oxidation induced by the external environment photo-oxidation

29. CHEE 340 29 Poly(Carbonates)

30. CHEE 340 30 Enzymatic Degradation Natural polymers degrade primarily via enzyme action collagen by collagenases, lysozyme glycosaminoglycans by hyaluronidase, lysozyme There is also evidence that degradation of synthetic polymers is due to or enhanced by enzymes. poly(e-caprolactone) elastomers