Influence of manufacturing methods on the homogeneity and properties of nitinol tubular stents

Abstract
Nitinol has found growing applications in vascular stents, with the recent introduction on the market of a number of new nitinol tubular devices for coronary and peripheral usage. These stents can be produced either through the expansion of a pattern cut on a small tube (pre-cut) or by cutting the deployed design on a tube of a larger size (pre-expanded) [l].The aim of the study was to analyze the differences of properties induced by the two manufacturing methods. For this purpose, Differential Scanning Calorimetry (DSC) measurements were performed on various specimens from the two types of stents. A first set of measurements shows that pre-expanded stents exhibit usual A ↔ R ↔ M transformations whereas multi-stage transformations occur for pre-cut stents. Two types of DSC specimens were then prepared for each stent, corresponding to the straight sections and the curved parts of the stent. It is shown that the pre-cut stents exhibit less homogeneous thermomechanical properties of the material within each stent compared to the pre-expanded stents. This is attributed to the manufacturing method which involves non-homogeneous bending stress-state for the pre-cut stents.