New methods for the assessment of in vitro and in vivo stress cracking in biomedical polyurethanes

This article describes a new test method for the assessment of the severity of environmental stress cracking of biomedical polyurethanes in a manner t hat minimizes the degree of subjectivity involved. The effect of applied st rain and acetone pre-treatment on degradation of Pellethane 2363 80A and Pe llethane 2363 55D polyurethanes under in vitro and in vivo conditions is st udied. The results are presented using a magnification-weighted image ratin g system that allows the semi-quantitative rating of degradation based on d istribution and severity of surface damage. Devices for applying controlled strain to both flat sheet and tubing samples are described. The new rating system consistently discriminated between. the effects of acetone pre-trea tments, strain and exposure times in both in vitro and in vivo experiments. As expected, P80A underwent considerable stress cracking compared with P55 D. P80A produced similar stress crack ratings in both in vivo and in vitro experiments, however P55D performed worse under in vitro conditions compare d with in vivo. This result indicated that care must be taken when interpre ting in vitro results in the absence of in vivo data. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.