Effects of the amount of Ni by powder metallurgy TiNi alloy aimed for thinned stents in superficial femoral artery and biological evaluation by animal test

Abstract

Stents are one of devices for the invasive cardiac catheterization (Percutaneous Transluminal Coronary Angioplasty) and especially for pediatric catheterization, the stent is manufactured by superelastic alloys including TiNi alloy. Transradial coronary intervention (TRI) is an invasive treatment in comparison with transfemoral coronary intervention (TFI), as TRI can reduce the risk of bleeding complications. To perform TRI, it is necessary to reduce the thickness of stent and diameters of delivery system. Therefore, one of the solution is to improve the strength of the materials for the stent. In this study, the medical application to stents was investigated by manufacturing high-strengthened Ni-rich powder metallurgy TiNi alloy using pure Ti powder and pure Ti pre-mixed powder. Ni-rich TiNi shape memory alloys showed much high plateau stress and enough shape recovery rate compared to the conventional ingot metallurgy TiNi alloy used in the commercial stent devices. In the simulation of the stent radial force by a finite element analysis (FEA), it was clarified that the radial forces of Ti-52.0 at.%Ni (stent wall thickness; 80 μm) and Ti-50.5 at.%Ni (stent wall thickness; 200 μm) were equivalent. The improvement of plateau stress of TiNi alloys successfully reduced the thickness of the stent by 120 μm (60 %). In fact, the stent using Ni-rich TiNi alloy resulted in the thinned stent with a high radial expansion force. When implanting the stents made of Ni-rich TiNi alloy into the external iliac artery of pigs, no stent fracture was observed, and in-stent occlusion never occurred after 1-month and 3-month placement of the stents. They are similar biocompatibility to commercial stents because of the similar results of inflammation.