Homogenization in supercritical carbon dioxide enhances the diffusion of vitamin E in ultrahigh-molecular-weight polyethylene

Vitamin E stabilization of radiation-crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) joint implants was successfully introduced to improve long-term oxidation resistance. Current clinically available vitamin E stabilized UHMWPE implants were prepared by the postirradiation diffusion of vitamin E into 100-kGy-irradiated UHMWPE by a two-step process, which included doping in pure vitamin E at an elevated temperature below the melting point followed by an annealing step at an elevated temperature in inert gas to homogenize the antioxidant throughout components of desired thickness. We hypothesized that the diffusion of vitamin E could be enhanced with supercritical carbon dioxide (SC-CO2) during homogenization without an increase in the surface vitamin E concentration, which would thus result in faster homogenization. Our hypothesis tested positive; crosslinked UHMWPE doped with vitamin E at 120 degrees C and homogenized in SC-CO2 at 10-12 MPa had a greater penetration of vitamin E than those homogenized in inert gas. We attributed the faster diffusion of vitamin E in irradiated UHMWPE in SC-CO2 to the dissolution of vitamin E in the supercritical fluid and a rate of diffusion that was closer to that of the supercritical fluid in the polymer. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012