Nitinol stents: does the level of coldwork on the nitinol tube have an impact on the thermo-mechanical properties of the final device?

The purpose of the present study is to investigate the impact of different raw material properties on radial forces and thermo-mechanical properties achieved by the final stent. In particular, tubes with three different levels of cold-work (low, mid and high) have been considered and the tube size (4.00 mm OD / 0.25 mm WT) has been chosen in order to be in the middle between the range of sizes typically used for peripheral stents (smaller) and for heart valve frames (larger), allowing to extend the findings to both the devices. Thermal analysis, mechanical test (tensile and three-point bending test) and radial forces evaluation have been performed on specimens and stents obtained from these tubes. At each step of thermal treatment required for stent production, from untreated to fully treated conditions (30 minutes of cumulated time), samples have been tested. Even if some differences are visible on the thermal analysis, the overall behavior among the different cold-work levels is similar. Furthermore, the mechanical tests and radial forces evaluation show a full equivalency of characteristics among the three cold-work levels considered. The analysis performed on the results reveals that the thermo-mechanical characteristics of the specimens and the radial forces on the stents do not seem to be signicantly influenced by the initial level of cold-work on the Nitinol tube.