Enhanced radiopacity austenitic stainless steel foil

Abstract

Austenitic stainless steel designed for implant applications is used to fabricate balloon expandable coronary stents. The alloy was not designed for this purpose but has been found to work well except for relatively low radiopacity in the energy range used for stent deployment, typically 80kV to 100kV. Stents made of more dense elements such as tantalum exhibit high radiopacity in this energy range. Low radiopacity is due to a combination of tubular stents having a thin wall (strut) thickness (less than 0.13mm) and the alloy being comprised of low-density elements, approximately 2/3 iron and 1/3 chromium and nickel. To retain the desired thickness and increase radiopacity, alloy density may be increased by partial substitution with dense element(s). The new alloy must maintain the biocompatibility, corrosion resistance, non-ferromagnetic structure, strength, ductility, and fatigue- and fracture-resistant characteristics that made the original alloy attractive to stent designers. Coronary stents are subject to intensive review by regulatory authorities prior to being approved for human use, thus stent designers are hesitant to depart from accepted standards in selecting new alloys. Revising an existing alloy is the preferred approach to achieve subtle feature changes. A set of criteria was set that maintained chromium, nickel, and molybdenum within prescribed compositional ranges and diminished iron to its minimum level, allowing platinum to be substituted for approximately 1/3 the total elemental weight (wt%). Above 20wt% platinum, undesirable precipitates were found. An alloy containing 20wt% platinum, in the form of foil and at a thickness of 0.127mm, was found to be free of precipitates not found in the base or original alloy and to provide approximately 20% radiopacity increase at 80kV and 15% radiopacity increase at 100kV, exceeding minimum programme goals at 80kV and equaling those at 100kV.