The Effect of Hard Segment Chemistry on the In-Vivo Biostability of Polyurethanes

Abstract

Four polyurethanes that were synthesized with different hard segments as well as four commercial polyurethanes were investigated for in-vivo biostability. The four polyurethanes with the varying hard segments were based on a 3/2/1/ mole ratio of methylene diphenylene diisocyanate (MDI) or methylene dicyclohexylene diisocyanate (H₁₂MDI); butanediol (BD) or ethylene diamine (ED); and polytetramethylene oxide (PTMO) (MW=1000). Two commercial polyurethanes were also used: Biomer (Ethicon, Inc.) and Pellethane (Dow).

Films of the polymers were implanted subcutaneously in rats for up to three months to assess their biostability. Polymer films were implanted with either a 100% strain applied or in the unstressed state. Measurement of tensile properties and surface properties before and after implantation was used to assess the stability of each of the polymers. Surface cracking was observed with scanning electron microscopy and the extent and depth of cracking was determined.

It was found that BD chain extended polymers maintained their tensile properties better than the ED chain extended polymers after implantation. H₁₂MDI-based polyurethanes were more susceptible to surface cracking than MDI-based polyurethanes, possibly due to the lack of a crystallizable hard segment.