100M-Cycle Bending Durability of VAR/EBR-Nitinol for Medical Device Implants

During the past decade, manufacturers have accelerated the pace to provide "ultra-clean" Nitinol products to the medical device industry. High-purity Nitinol is required to provide implant durability in challenging in-vivo applications such as in structural heart implants, and further enables the development of novel implant designs. Extensive research has shown that the size and volume fraction of Ti₄Ni₂O_x and TiC inclusions are the limiting factor for long-term durability for these medical devices. Specifically, it was demonstrated that fatigue survival probability inversely scales with inclusion length. The present study provides a statistical comparison of the bending fatigue performance of VAR/EBR-processed Nitinol with custom-designed “diamond-shaped” coupons manufactured from 7mm x 0.5mm Nitinol tubing. Test conditions of 8.6% crimp strain, with mean strains between -0.5 and 7% and alternating strains from 0.5 to 2.75% are used to characterize the effects of inclusion size and distribution on the fatigue behavior to 100,000,000 cycles. These current results demonstrate a fatigue strain limit of up to 2.25% strain amplitude that confirms that the effects of inclusion size and distribution have a significant impact on the high-cycle fatigue life for Nitinol medical devices. Furthermore, optimized thermomechanical processing of the tubing and devices (with resultant grain size less than 20µm) is also shown to enhance the fatigue behavior. Tensile properties, DSC measurements, as well as microstructure (LOM, SEM, TEM) and grain orientation (EBSD) will be discussed as contributing factors for the unparalleled fatigue behavior.