Wear Of Nitinol Wires As A Function Of Nitinol Micro-Cleanliness And Surface Treatment

Nitinol is a well-known material for cardiovascular implants like stents and heart valve frames. Beside lasercut devices from nitinol tubing or sheets, there is now an increasing number of medical devices made of nitinol wires by various textile processes such as weaving and braiding. For these wires a wide variety of nitinol bulk material and surface finishing is used depending on the application. In contrast to lasercut implants, nitinol wires touch each other at crossover points of the braided stent and experience micro movements in vivo. Here, fretting damage can occur.

Already published data (D. A. Siddiqui et al, 2017) concludes that fretting of electropolished nitinol wires leads to damage that changes corrosion behavior, but the corrosion of the wires is still in an acceptable range. However, the effect of fretting on corrosion resistance of different nitinol material and surface finishes has yet to be investigated.

For long-term in vivo applications, fretting corrosion has to be controlled. Therefore, we compare in this study the degree of fretting damage and corrosion behavior after ASTM F2129 for nitinol material with various degree of micro-cleanliness as well as different surface treatments. To simulate fretting damage on wires, we present a newly designed set-up with one fixed and one rotating wire for the investigation of fretting under controlled environment and variable fretting frequency (up to 25 Hz), normal force between the wires (up to 0.3 N) and fretting depth.