Amorphous oxide film on stainless steel for cardiovascular applications

Surface oxide film could be the most critical factor that determines the biocompatibility for the metallic cardiovascular devices. Properties of oxide films found on the marketed stents were studied by a combination of electrochemical approaches and spectroscopy: electrochemical properties using cyclic polarization, open-circuit potential and current density at open-circuit potential, and electrochemical impedance spectroscopy (EIS); oxide particle sizes and structures by transmission electron microscopy (TEM); surface morphologies by scanning electron microscopy (SEM), chemical bonding by electron spectroscopy for chemical analysis (ESCA), and lastly, the elemental depth profile by auger electron spectroscopy (AES). Degree of thrombosis was determined from the weight difference before and after 4-days in-vivo experiment. Degree of restenosis was judged from the intima/media ratio of stented iliac artery after a 28-day and a 56-day implantations in New Zealand white rabbits. Efficiency of drug elution from the amorphous oxide was detected by cyclic voltammetry, HPLC, and ESCA. Experimental evidences show that stainless steel passivated with amorphous oxide film has the oxide particle in the range of nanoscale. This amorphous oxide film displays the following characteristics: a higher time constant, a lower open-circuit potential, negative current density at open-circuit potential, a lower degree of thrombosis, minimum inflammation after stent deployment, and a ∼60% reduction of restenosis compared to currently marketed bare stents. Furthermore, it can serve effectively as the platform for drug elution without the complication of polymer coating. Taken together, these distinguishing characteristics make amorphous oxide film a stellar feature for metallic cardiovascular devices.