Polyurethanes are used in biomedical devices because they constitute one of the few classes of polymers that are highly elastomeric and show good biocompatibility. However, there is continuing debate in the literature regarding their long term stability in biomedical environments, and the validity of using polyurethanes for extended implant applications is still very much in question. While evidence of polymer degradation in explanted devices has been reported by a number of workers, other studies conclude that degradation is minimal. This review discusses possible pathways of biomedical degradation, such as enzymatic and oxidative attack, as well as observed effects such as cracking and calcification. Metal catalyzed degradation may also occur in devices such as pacemaker leads. While a number of model studies have assessed various modes of degradation, reliable accelerated tests for the prediction of the rates of biomedical ageing effects have apparently not been established. The role of additives and surface contaminants in biocompatibility and degradation is also discussed. Finally, as important interactions between a polyurethane polymer and the host body occur at the interface, the surface analysis of polyurethanes is discussed, and it is concluded that proper characterization of the implant surface before and after degradation studies is important for the understanding of degradation, in particular to exclude effects caused not by the polyurethane itself, but by surface contaminants which appear to have been present on the majority of materials subjected to detailed characterization so far.
