Characterization of Current and Future Generation Nitinol Wire

Nitinol is commonly used in medical devices due to its shape memory, mechanical and fatigue properties. Medical devices continue to evolve to treat new and often more challenging conditions and anatomies and also younger patient populations. Current generation materials and nitinol forms such as wire, tube, sheet and rod have served the industry well with established products ranging from self-expanding stents, heart valves and other devices. The evolution of more complex devices with higher performance demands is driving the development of next generation nitinol with consistent and improved high cycle fatigue performance. In this paper, current and future generation nitinol wire products have been evaluated in terms of microcleanliness and fatigue performance by rotary bend testing. Nitinol from multiple suppliers, manufactured by a variety of methods was assessed. In contrast to ASTM F2063 which describes metallurgical microcleanliness on semi-finished mill product for material suppliers, microcleanliness was assessed on the actual final wire, the form of interest to a device manufacturer. Rotary Bend Fatigue followed by SEM fractography was assessed similarly on heat-treated and electro-polished samples of each wire type to allow direct comparison.