D. L. Steffey, L. E. Eiselstein, A. Nissan, Exponent, Menlo Park, CA; N. Corlett, Exponent International, Harrogate, United Kingdom
ASTM F 2129 test method, nor the FDA provides any guidance as to what constitutes an acceptance criterion. Neither provides any guidance on how many samples to test or how to handle censored data, i.e. data sets where there are only a few tests that breakdown. The development of both a statistically valid acceptance criterion for corrosion resistance and a method of evaluation would be of significant benefit to the medical device community.
This study, which builds on previous research that was presented at SMST 2007, investigates the effect of long-term exposure to simulated in-vivo environments with differing degrees of aeration. This was accomplished by pre-exposing electropolished nitinol to phosphate buffered saline (PBS) at 37°C that had been sparged with either UHP nitrogen, lab air, or simulated blood gas. Immersion times ranged from 1 hour up to 30 days. A large number of samples were tested at each exposure time in order to obtain a reasonable number of samples with breakdown. This data allows us to test our statistical model that was presented at SMST 2007. This model takes into account the probability of breakdown per unit of exposed surface area and, if breakdown occurs, predicts the probability that Eb-Er is greater than some threshold value.
Summary: The development of both a statistically valid acceptance criterion for corrosion resistance and a method of evaluation would be of significant benefit to the medical device community. This study is a continuation and validation of a methodology that we first presented at SMST 2007. We have collected additional experimental data on the breakdown potential (Eb) of electropolished nitinol, the variation in rest potential (Er) over time, and the effect of different pre-exposure environments on these parameters. Using these data, we have constructed a statistical model that models the implications of in-vivo exposure on device performance—specifically, the probability that the rest potential would exceed the breakdown potential during the life of the device. The mixed statistical model we described at SMST 2007 was further evaluated through testing even larger sample sets and expanding the model to take in to account the effect of exposure to aerated environments as is expected in-vivo.
