Surface analyses and biocompatibility study of 500 °C oxidized Ni50Ti50 and Ni40Ti50Cu10 shape memory alloys

Ni50Ti50 and Ni40Ti50Cu10 shape memory alloys (SMAs) are oxidized at 500 °C. Considering the surface roughness, the thicknesses of oxide layer and Ni-free layer, the surface Ni concentration, the proper oxidation times for oxidized specimens are found to be 60 min for Ni50Ti50 and 30 min for Ni40Ti50Cu10. Experimental results reveal that the oxidation is diffusion-controlled with its oxide layer containing titanium oxide and that the surface Ni concentration is much lower than the nominal composition. When Ni50Ti50 and Ni40Ti50Cu10 SMAs are oxidized at these times, the latter has better corrosion resistance than the former in Hanks' solution at 27 °C. However, the results of cytotoxicity and cell proliferation assays indicate that the biocompatibility of unoxidized Ni40Ti50Cu10 is worse than that of unoxidized Ni50Ti50, but that of oxidized Ni40Ti50Cu10 ranks as good as that of oxidized Ni50Ti50.