Limitations and Challenges In Corrosion Testing and Surface Characterization Protocols For Implantable Nitinol Medical Devices

After over two decades of using nitinol in an ever increasing variety of implantable medical device applications, there are still significant challenges in designing relevant test methods to assess a device’s expected in vivo corrosion resistance and biocompatibility.  Because the corrosion behavior of nitinol is heavily dependent on surface condition, testing is important in order to assure that the finished device will be safe and effective in use.  However, determining what tests to do, how to do them and, especially, how to interpret the results is not necessarily straightforward.  Some specific bench testing protocols (such as ASTM F 2129 for assessing susceptibility to pitting corrosion) have been developed for devices intended for implantation and these provide (at least in theory) standardization of testing methodology.  However even for ASTM F 2129, which has been widely used for more than a decade, many questions remain regarding interpretation of results.  A galvanic testing protocol is currently being balloted at ASTM, yet even as this standard is being developed, FDA guidelines are being updated that may affect how this test should be used.  Other assessments, such as for nickel ion release, general surface characterization, and the effects of fretting or fatigue on the corrosion resistance of a device, are generally deemed important, but no established protocols exist that would standardize methodology.  In this paper, current perspectives on these various testing strategies will be presented to highlight the challenges and limitations being faced by those designing, testing, evaluating and, ultimately, approving nitinol medical device implants.