High Strain Accelerated Fatigue Failure Testing of NiTi Implantable Devices

Accelerated testing, using frequencies higher than clinically observed, has been used to reduce time and cost of fatigue testing. Unfortunately, changes in a device design or manufacturing process may require repeating fatigue testing to show that the change does not adversely affect the specified device lifetime. OBJECTIVE: There were three main objectives of this study: To test implantable Nitinol (NiTi) devices in an accelerated fatigue failure test at frequencies and strains higher than seen clinically; To investigate the use of this testing to decrease fatigue testing time; To investigate the use of higher strains as a screening tool for design or processes that may affect fatigue lifetime. METHODS: Different groups of NiTi devices were built with different processing parameters. Quality of electropolish, radius of curvature and pre-test strain were varied and tested against a control group to obtain fatigue lifetime or StrainNumber of cycles to failure (S-N) curves. The resulting S-N curves were generated for each group of data. The S-N curves were overlaid to determine the relative difference in fatigue life between a test group and its control group. RESULTS: The divergence of the two fatigue lifetime (S-N) curves reveals which processes or designs produce devices with longer fatigue life. For certain testing groups, optimal fatigue testing strain values can highlight the differences between groups better than other fatigue testing strain values. Accelerated high strain testing produced results in 1 day of testing while clinically equivalent strain testing took 28 days. CONCLUSIONS: High strain fatigue testing can produce relevant fatigue data much faster than testing at clinically equivalent strains. Optimal strains for determining the difference in fatigue life between processes or designs can be determined by overlaying the S-N curves for two or more groups of samples. Future design and process changes or improvements can be tested using an optimal alternating strain based on the divergence of the S-N curves. The results of the testing could be used to determine if the new design or process has compromised the fatigue lifetime of the device.