Polyether polyurethanes are subject to oxidation catalyzed by and through direct (redox) reaction with transition metal ions (cobalt), released by corrosion of metallic parts in an implanted device. Replacing part of the polyether with polysiloxane appears to reduce susceptibility to metal ion oxidation (MIO). In vitro studies indicated that polyurethanes containing 20-35% polysiloxane (PS-20 and PS-35) are about optimum. We implanted tubing samples containing cobalt mandrels in the subcutis of rabbits for periods up to 2 years. After 2 years, only traces of microscopic cracks were seen on half the PS-35 samples, PS-20 significantly delayed MIO, while the polysiloxane-free control was very severely degraded. Infrared spectroscopy established that polyether soft segment oxidation was occurring in PS-20. We could not directly evaluate oxidation in PS-35 because siloxane bands mask the aliphatic ether. Indirect FTIR evidence suggests that there is very slight polyether oxidation that develops early, and then seems to stabilize. The molecular weight of degraded PS-20 decreased. That of microcracked PS-35 decreased negligibly while that of undamaged PS-35 increased slightly after 2-year in vivo. The polysiloxane-free control was profoundly degraded. PS-20 has much improved MIO resistance, while that for PS-35 is highly MIO resistant compared with its polysiloxane-free control.

Polyethylene oxide (PEO) on polymer surfaces has been reported to reduce cellular adhesion, a very desirable property for cardiac pacing leads. A Shore 80A polyether polyurethane with up to 6% PEO surface-modifying end groups (SME) was evaluated for its chronic in vivo biostability. In a short-term (12 week) screening test, strained samples appeared to develop the same surface oxidation as unmodified polymer, but did not produce visible cracking > or =500x, prompting a longer-term study. By the time the longer-term study was initiated, most of the PEO SME had disappeared from the starting material's surface. After 1 year in vivo, surface oxidation, shallow surface cracking, and environmental stress cracking (ESC) developed on highly strained samples to the point of failure, so that there was no significant difference between the SME polymer and its control (the same polymer without SME). No further change was seen for up to 2 years of implantation. Unstrained PEO SME polymer developed shallow surface cracking, but no ESC up to 2 years of implantation. Thus, PEO SME slightly delayed, but did not stop biodegradation, and under unstrained conditions, has no adverse effect on biostability.