Af Temperature And Mechanical Functionality Of Nitinol Implantable Medical Devices

Self-expanding Nitinol implantable such as stents and transcatheter heart valves are widely used for treating vascular stenosis, aneurysms and valular diseases. For stent-like devices, radial force and pinch stiffness are critical mechanical attributes affecting device-anatomy interactions. These mechanical attributes are closely related to the Nitinol superelastic mechanical property. To ensure the superelastic mechanical property is properly controlled, the industry is custom to using the A_f temperature as a surrogate for specifying Nitinol implantable devices. The A_f temperature can be measured by differential scanning calorimetry (DSC) or a bend and free recovery (BFR) test method. However, the A_f temperatures determined using these two methods differ from each other. An earlier study on wire specimens indicated that the BFR A_f temperature agrees better with the DSC R_f’ temperature than with the DSC A_f temperature. The result also suggested that Nitinol materials having similar A_f temperatures could exhibit different superelastic mechanical properties. In the present study, the correlation between DSC and BFR A_f temperatures, their effects on superelasticity as well as the radial force and pinch stiffness of laser processed stent-like specimens were investigated. The objective is to provide a better clarity on the meaning of DSC and BFR A_f temperatures and their effects on the mechanical functionality of Nitinol implantable medical devices.