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Polymer Biomaterials

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

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Polymer Biomaterials

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  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. POLYMERS • Polymers - long chain molecules of high molecular weight • -(CH2)n-

  3. Common Polymer Biomaterials

  4. Polymers In Specific Applications

  5. Ni Mi Properties: Molecular weight • synthetic polymers possess a molecular weight distribution polydispersity index = Mw/Mn

  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. Glass transition temperature, Tg • related to chain mobility • increased flexibility, lower Tg • factors : • flexible links in backbone • size of pendent groups • interaction between chains • plasticizers interfere with bonding, increase chain movement, decrease Tg

  8. Tg • effect of Molecular weight • Fox-Flory eqn. • K = constant for given polymer • Tg∞ = Tg for infinite M • for copolymers (Fox-Flory) • w = weight fraction of monomer in copolymer

  9. Effect of Temperature on Polymer Properties • amorphous viscous liquid rubbery Tg T glassy Mw

  10. Effect of Temperature • semi-crystalline Rubber Liquid Viscous Liquid Tm tough plastic T Tg semi-crystalline plastic crystalline solid 10 1000 100000 1000000 molecular weight (g/mol)

  11. Crosslinked Networks • crosslinks • covalent; H-bonding; entanglements • crosslinking • increased molecular weight • swell in solvents • organogel • hydrogel

  12. Thermal Properties

  13. Temperature Effects Tg Tm semicrystalline log(Modulus) crosslinked T linear amorphous Temperature

  14. Viscoelasticity • The response of polymeric materials to an imposed stress may under certain conditions resemble the behavior of a solid or a liquid. Stress Strain

  15. Mechanical Properties

  16. 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 fields : • purification of gases by membrane separation • dialysis • prevention of moisture loss in food and drugs (packaging) • controlled drug delivery (transdermal patches, Ocusert) • polymer degradation

  17. 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

  18. 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 solute if the absolute value of the difference in d between the materials is less than 1 (cal/cm3)1/2.

  19. Diffusivity • experimental observations • effect of T vs Tg

  20. Diffusivity • effect of permeant size

  21. Diffusivity : Effect of Crystallinity • solutes • do not penetrate crystals readily • take path of least resistance • through amorphous regions • increased path length D1,c = diffusivity in semi-crystalline polymer D1,a = diffusivity in amorphous polymer fc = volume fraction of crystals x = shape factor (=2 for spheres) (Mathematics of Diffusion)

  22. 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

  23. 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

  24. Drug Delivery Ocusert TD - Scopolamine

  25. In Vivo Degradation of Polymers • no polymer is impervious to chemical and physical actions of the body Mechanisms causing degradation

  26. Hydrolytic Degradation • hydrolysis • the scission of chemical functional groups by reaction with water • there are a variety of hydrolyzable polymeric materials: • esters • amides • anhydrides • carbonates • urethanes

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

  28. 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

  29. Polyesters

  30. Polyesters fractional change in molecular weight

  31. Oxidative Degradation • usually involves the abstraction of an H to yield an ion or a radical • direct oxidation by host and/or device • release of superoxide anion and hydrogen peroxide by neutrophils and macrophages • catalyzed by presence of metal ions from corrosion

  32. Poly(Carbonates) PEC in vivo M. Acemoglu, In. J. Pharm. 277 (2004) 133-139

  33. 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. Z Gan et al., Polymer 40 (1999) 2859 C.G. Pitt et al., J. Control. Rel. 1(1984) 3-14

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