Hydrolytic degradation of tyrosine-derived polycarbonates, a class of new biomaterials. Part II: 3-yr study of polymeric devices

The kinetics and mechanisms of in vitro degradation of tyrosine-derived pol ycarbonates, a new class of polymeric biomaterials, were studied extensivel y at 37 degrees C. These polymers carry an alkyl eater pendent chain that a llows the fine-tuning of the polymer's material properties, its biological interactions with cells and tissue, and its degradation behavior. The polym er carrying an ethyl ester pendent chain, poly(DTE carbonate), has been est ablished as a promising orthopedic implant material, exhibiting bone apposi tion when in contact with hard tissue. Tyrosine-derived polycarbonates are relatively stable and degrade only very slowly in vitro. Therefore, acceler ated studies were conducted at 50 and 65 degrees C to observe the behavior of polymers during the later stages of degradation. Varying the pendent cha in length affected the rate of water uptake, initial degradation rate, and physical stability of the polymeric devices. During the 3-yr study, the pol ymer degraded by random chain cleavage of the carbonate bonds, accompanied by a relatively small amount of pendent chain de-esterification. No mass lo ss was observed during this period at 37 degrees C, but mass loss was readi ly evident during the accelerated studies at 50 and 65 degrees C. Thus, it is reasonable to assume that mass loss will occur also at 37 degrees C, alb eit only after extensive backbone carbonate cleavage and pendent chain este r hydrolysis. The dimension and surface area of the devices influenced the initial degradation rate, but did not significantly affect the overall rate of degradation. No evidence of "acid dumping" or the release of acidic res idues found during the degradation of poly(D,L-lactic acid) were observed f or this family of tyrosine-derived polycarbonates. (C) 2000 Elsevier Scienc e Ltd. All rights reserved.