A. M. Kueck, J. Muskivitch, L. Vien, J. Fino, C. Trepanier, A. Pelton, Nitinol Devices & Components, Fremont, CA
Laser cutting and wire forming are two of the most commonly used processes in the manufacture of Nitinol medical devices. Because these two fabrication methods subject the part to very different external conditions, it is prudent to understand how post-process surface treatment methods may need to be adjusted to achieve optimal in vivo fatigue behavior. This study explores how varying the amount of material removed during the final surface treatment steps affects the fatigue life of Z-type stents that have either been laser-cut from tube or shape set from wire. The parts were subjected to a typical heat treatment process necessary to achieve an Austenite finish temperature of 28°C, and were subsequently post-processed with one of two different surface treatments, an electrochemical passivation or a chemical passivation. The total weight loss during post-processing was recorded and the process adjusted to create groups with low, medium, and high amounts of weight loss. Z-stent samples, produced from both tube and wire, were processed from each weight loss group to analyze the extent of the heat-affected zone (HAZ) and to perform fatigue testing. The HAZ was evaluated by preparing metallographic cross-sections of both as-cut and as-processed devices and then observing with a scanning electron microscope (SEM) under high magnification. The thickness of the HAZ, if present, was measured and correlated with the amount of material removed during the post-process. Fatigue testing was conducted on the laser-cut stents and compared to those stents formed by wire. Fatigue data were collected for a maximum of ten million cycles using mean and alternating strains that are consistent with the physiological strains encountered in vivo.
Summary: Two methods of manufacturing Nitinol medical devices, laser cutting and wire forming, are compared to correlate the removal of surface layers including the heat-affected zone to the fatigue life.
