Researchers from Boston Scientific report that they joined with other researchers to evaluate micro-cleanliness and rotary bend fatigue in a sampling of current and future generation Nitinol wire, including VAR and VIM products from multiple suppliers. All materials were manufactured to the requirements of ASTM F2063 with comparable raw wire mechanical properties and diameter.
In contrast to ASTM F2063, which describes metallurgical micro-cleanliness on semi-finished mill product for material suppliers, micro-cleanliness was assessed on the actual final wire, the form of interest to a device manufacturer.
Rotary Bend Fatigue (RBT), R=-1 [1], was performed at 37°C on electro-polished samples of each wire type, and then heat-treated to the same Af, to allow direct comparison between materials at physiological temperatures.
Nitinol is commonly used in medical devices due to its unique shape memory and superelastic 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 such as self-expanding stents, heart valves, and other devices. The evolution of more complex devices with higher performance is driving the development of next generation nitinol with consistent and improved high-cycle fatigue performance.
There are a variety of nitinol materials available on the market today with a broad range of fatigue properties and distinctly different micro-cleanliness characteristics. Next generation nitinol continues to evolve with higher levels of inclusion control and fatigue resistance, close to twice that of standard type ASTM F2063 materials. This improved fatigue performance results from smaller average inclusion size, smaller maximum inclusion size and a lower incidence of larger inclusions.
The variation between available materials signals to device manufacturers that certification to the ASTM standard alone is inadequate. For more sophisticated and demanding devices, additional controls are required to ensure materials consistency. Supplying a next generation superior fatigue performing nitinol is just the first step and shows feasibility. The greater challenge is consistently supplying that material for commercial medical devices.
This abstract is from a presentation at SMST 2018, the International Conference on Shape Memory and Superelastic Technologies, which took place in Ireland in May.
Characterization of Current and Future Generation Nitinol Wire.
Authors include
Adrian McMahon, Minh Phan, Ali Salahieh, Boston Scientific, Galway, Ireland.
Boston Scientific, Los Gatos, California, USA
Siobhan Carroll, G.Rau Inc. Santa Clara, California, USA
Jay Yang, Independent Nitinol Consultant, Los Gatos, California, USA
