Nitinol Stent for Percutaneous Heart Valve Implantation: Material Shape Setting

Percutaneous aortic valve implantation has become an alternative technique to surgical valve replacement in patients with high risk for open chest surgery. NiTi vascular stents clinically used today for noninvasive aortic valve replacement are, however, too traumatic. Obtained from laser cut Nitinol material, these stents impede the dimensional changes of the compliant aortic root over the cardiac cycle. Stents braided from Nitinol wire are conversely more flexible and appear to be a less traumatic alternative for heart valve replacement. However, in order to keep the cohesion of the braid specific 3D design, and the radial flexibility of the stent, the Nitinol wire needs to undergo successive thermal treatments for shape setting purpose. This study investigates the effect of two successive and nonidentical heat treatments on constrained stent wire behavior, in terms of microstructure and mechanical properties. With a first heat treatment over 10 minutes time at 450°on a fixture plate AF could be increased to handle easily the martensitic malleable stent material at ambient temperature. A second treatment over 10 min time at 550°decreased AF to 0°C to get a very elastic austenitic structure made with interlaced wires able to undergo 20% dimensional variations.