Biodegradation mechanisms of polyurethane elastomers

After over 40 years of use in biomedical applications, polyurethanes remain one of the most popular biomaterials due to their exceptional biocompatibility, mechanical properties and versatility. However, failure of polyurethane based pacemaker leads and breast implant coatings in the late 1980s brought the long term stability of these implants under scrutiny. The biomedical device industry was faced with the need to find replacement polyurethane compositions that were biostable and maintained excellent biocompatibility and mechanical properties. The design of replacement biomedical polyurethanes is dependent on the mechanistic understanding of polyurethane biodegradation and the biological processes that govern these mechanisms. This review summarises the efforts of the authors’ lab and others to elucidate the biological mechanisms of polyurethane biodegradation and evaluate promising new elastomers based on poly(carbonate urethane) and silicone copolymer chemistries. Several of the new classes of polyurethanes reviewed here show great promise; however, attenuated total reflectance Fourier transform infrared spectroscopy analysis of explanted samples provided evidence of chain scission and cross-linking in all of the polyurethane specimens. Therefore, it was concluded that the chosen soft segment modifications were insufficient to fully inhibit biodegradation. Potential biodegradation mechanisms were explored using oxidative and hydrolytic enzyme systems. Although cholesterol esterase initiated polyurethane degradation, the effect was minimal compared to the oxidation of these polyurethane elastomers. The in vivo and accelerated degradation studies supported oxidation as the dominant mechanism of biodegradation of PEU and PCU. The H₂O₂/CoCl₂ system remains an excellent choice to accelerate oxidative biodegradation for the prediction of long term biostability in quantitative comparison with current clinical poly(ether urethanes). The biostability ranking of these four materials based on statistical comparisons of chain scission and surface pitting is as follows: PEU<PEU-S≤(PCU<PCU-S. The more biostable PCU elastomer was concluded to be a suitable choice to replace PEU in medical applications. Furthermore, the silicone modification was shown to increase the biostability of the PEU and PCU elastomers while maintaining the thermoplastic elastomeric properties.