Mechanical properties and microstructure of platinum enhanced radiopaque stainless steel (PERSS) alloys

Abstract

Balloon-expandable coronary stents may be made from austenitic stainless steel conforming to ISO, BSI and ASTM specifications (UNS S31673). These stents are not sufficiently visible, radiographically, under certain operational conditions, therefore a range of platinum-enhanced radiopaque stainless steel (PERSS) alloys were developed, using UNS S31673 alloys as a base, which are more visible when used as stents. This paper gives the methodology used to develop the PERSS alloys in order to ensure that the properties are those required for coronary stents. These properties include an ability to be cold worked, mechanical strength and ductility similar to those of implant grades of austenitic stainless steel, an absence of magnetic phases, such as ferrite and martensite, as well as an absence of brittle intermetallic phases, particularly σ. Biocompatibility must also be acceptable, comparable to UNS S31673 alloys. Microscopy and X-ray diffraction analysis confirmed predictions that the microstructure of the alloys would be fully austenitic, free from potentially damaging topologically close packed intermetallic phases, and that the addition of platinum would reduce the amounts of magnetic and embrittling phases present in the alloys. Mechanical tests revealed platinum does not significantly affect the mechanical properties, except that there is about a 14% increase in ultimate tensile strength with the addition of 10 wt% platinum.