The U.S. Food and Drug Administration (FDA) has finalized their guidance related to medical devices that contain nitinol, the draft of which was published on April 19, 2019. The guidance recommends that companies clarify the material composition (including conformance to ASTM F2063), manufacturing steps (heat treatments and surface treatments), and characterize transformation temperatures per ASTM F2004 or F2082 (for weight bearing devices). The guidance also details mechanical test considerations that are unique to nitinol including precondition loading, testing at body temperature in saline, and cyclic shape memory testing for devices utilizing the shape memory property of nitinol.
Similarly unique considerations for computation modeling are also detailed, including need to obtain model parameters per ASTM F2516, model validation, and use of a constant life curve to estimate fatigue life. Consistent with FDA regulation to date, six of the worst-case, final, finished, preconditioned devices should undergo pitting corrosion testing per ASTM F2129 to assess the robustness of the oxide layer. So long as adequate breakdown potentials are obtained (the guidance includes several literature articles that can be used to establish acceptance criteria) and a standard surface finishing method is used (electropolishing, chemical etch, or mechanical polishing is used), nickel leach testing per ASTM F3306 should not be required. There are no specific considerations for biocompatibility unless nickel leaching testing was performed, in which case a toxicological assessment of nickel exposure will be needed. The only labeling consideration detailed in the guidance is a warning of possible allergy/hypersensitivity to nickel.
Use of nitinol significantly increases the amount of bench testing that will be required to obtain marketing clearance. This guidance details the corrosion testing that has been common FDA practice, but also significantly expands the type and amount of mechanical and characterization tests that will be required. As these tests typically need to be performed on final, finished devices, most of the testing burden will fall on the medical device company, not the material supplier. Accordingly, use of nitinol in medical devices should be avoided unless its unique material properties (superelasticity and/or shape memory) are critical to device function.