New Biomaterials For Tissue Engineering
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In the United States, poly(glycolic acid) (PGA), poly(lactic acid) (PLA), polydioxanone, and copolymers thereof are the only synthetic, degradable polymers with an extensive FDA approval history. It is therefore not surprising that tissue engineers have employed almost exclusively PGA, PLA, and other poly(ahydroxy ester)s for the construction of degradable polymeric scaffolds. Although the utility of these materials as sutures and in a number of drug-delivery applications is well-established, a wide range of material needs cannot be addressed by the use of these few polyesters. For example all polyesters release acidic-degradation products that can adversely effect biocompatibility.4'. These polyesters tend to be relatively stiff materials.7 This can be an advantage
in load-bearing applications but is a disadvantage when mechanical compliance with soft tissue or blood vessels is required. Finally none of these polyesters provides a chemically reactive pendent chain for the easy attachment of drugs, crosslinkers, or biologically active moieties. Thus simple poly(o:-hydroxy ester)s have performed well in establishing the foundation and feasibility of tissue engineering 8 1 , but may not be optimally suited for the construction of polymeric cell scaffolds serving a variety of applications. Tyrosine-Derived Polycarbonates and Polyarylates: New Biomaterials for Tissue Engineering Tyrosine-derived polycarbonates and polyarylates are part of a new group of polymers referred to as pseudopoly(amino acid)s.14 Like the polymers based on lactic and glycolic acid, pseudopoly(amino acid)s are based on natural metabolites — amino acids. However rather than linking amino acids solely through amide bonds, pseudo-poly(amino acid)s incorporate non-amide bonds such as urethane, ester, iminocarbonate, and carbonate linkages. This has been shown to significantly improve physicomechanical properties when compared to conventional poly(amino acid)s. The synthesis of pseudo-poly(amino acid)s is typically based on the polymerization of trifunctional amino acids or dipeptides using the functional groups located on the amino-acid side chains.14-15
Table I: Some Commonly Investigated Degradable Polymers. Single Polymers poly(e-caprolactone) polydioxanone poly(glycolic acid) poly(lactic acid)
Polymer Families poly(amino acid)s lactide-glycolide copolymers polyanhydrides polyhydroxybuty rates poly(ortho ester)s poly(phospho ester)s
Table II: Currently Available " "Pseudo"-Poly(Amino Acid)s. Polymers poly(N-acyl trans-4-hydroxy-L-proline ester) poly(N-acyl L-serine ester) tyrosine-derived polyiminocarbonates tyrosine-derived polycarbonates tyrosine-derived polyarylates
Literature Citations Yu,33Yu-Kwon,34Kohn14 Gelbin,35Zhou,36Fietier37 Kohnetal. 38 ' 39 Kohnetal. 16 ' 24 - 39 ' 40 Kohnetal. ,7 ' 4,
MRS BULLETIN/NOVEMBER 1996
New Biomaterials For Tissue Engineering
The pseudo-poly(amino acid)s that have been synthesized and characterized to date are listed in Table II. Of these polymers, the tyrosine-derived polycarbonates and poiyaryiates
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