Balloon-expandable coronary stents may be made of stainless steel conforming to ASTM or ISO specifications, referred to by ASTM as UNS S31673 alloys. A need exists for an alloy with enhanced radiopacity to make stents more visible radiographically and more effective clinically. A research program was initiated with the objectives of enhancing fluoroscopic radiopacity while maintaining adherence to the ASTM and ISO specifications. These objectives were ultimately achieved by adding a noble metal, platinum, to UNS S31673 by vacuum induction melting a commercially-available alloy. Freedom of the resulting microstructure from formation of harmful topologically close packed phases was ensured by use of phase computation methodology (New PHACOMP), and confirmed by X-ray diffraction and transmission electron microscopy. Platinum was chosen since it is over twice as dense as nickel and, with approximately half its effect as an austenitizer, allows nickel content to be reduced to a minimum level.
A platinum-enhanced variant of UNS S31673 stainless steel has been eveloped for use in fabricating balloon-expandable coronary stents. PERSS® is sufficiently radiopaque to be detected by current radiographic methods and maintains compliance with ASTM requirements. Boston Scientific Corporation/Interventional Technologies (BSC/IVT) has implemented a pilot program for the development of PERSS with the intent of achieving vertical integration of the manufacturing process. PERSS ingots containing 5 wt% platinum were cast by vacuum induction melting, refined by vacuum arc remelting, upset in a forge, then hot and cold-rolled into sheet and foil. The sheet was cold-rolled on a 20-high cluster mill to a foil thickness of less than 0.25 mm (0.01 inches). While the final product otherwise meets the ASTM specification for UNS S31673 alloys, intentional addition of unspecified additional elements precludes its compliance with same.
Dilation-balloon expandable coronary stents are made of implant grade stainless steels, UNS S31673, e.g., BioDur ® 316LS. Boston Scientific / Interventional Technologies (BS/IVT) determined that addition of platinum to UNS S31673 could produce a stainless steel with enhanced radiopacity, which made such stents more visible radiographically. A goal of the program was to ensure the platinum additions would not adversely affect the corrosion resistance of the resulting 5-6 wt % PERSS ® alloys. Corrosion resistance of PERSS and BioDur 316LS was determined using electrochemical tests for general, pitting, crevice and intergranular corrosion. Experimental methods included A262E, F746, F2129, and potentiodynamic polarization. The ~ 6 wt % PERSS alloy (IVT 78) had a resistance to pitting, crevice and intergranular corrosion similar to base materials. IVT 78 was a single-phase austenitic PERSS alloy with no evidence of inclusions or precipitates; it was more resistant to pitting corrosion than the ~ 5 wt % PERSS alloys. PERSS performance was not a function of oxygen content in the range 0.01 to 0.03 wt %.
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