Polymer Hydrolysis Stresses Out Medical Implants

Polymer scientists tracking how well siloxane-polyether-urethane block co­polymers used in medical implants stand the test of time have found that troubling degradation occurs by simple hydrolysis of the polymer backbones (Macromolecules, DOI: 10.1021/ma301965y). Kimberly A. Chaffin of medical device manufacturer Medtronic, in collaboration with Marc A. Hillmyer and Frank S. Bates of the University of Minnesota, Minneapolis, led a team that studied the commercial thermoelastic polymers Elast-Eon E2A and PurSil 35. The researchers exposed the polymers for up to a year in buffered aqueous solutions ranging from 37 °C (body temperature) to 85 °C. They monitored the polymers via X-ray scattering and various spectroscopic techniques, size-exclusion chromatography, and mechanical tests. The polymers are stable to oxidation, but the researchers found that long-term exposure to water leads to both polymer mass reduction and degradation of tensile strength. They suppose that hydrolysis is occurring at the Si–O bonds of polysiloxane units, C–O bonds of polyether units, and N(H)–C(O)–O bonds of polyurethane units. The silver lining in the observation, Hillmyer says, is that further work determining the specific bonds being broken under hydrolytic aging will allow researchers to design more resistant and mechanically stable biomedical materials.