Surface characteristics, properties and in vitro biological assessment of a NiTi shape memory alloy after high temperature heat treatment or surface H2O2-oxidation: A comparative study

To improve the biocompatibility and bioactivity of a NiTi shape memory alloy (SMA), an oxide layer was fabricated on its surface using either a heat treatment at 600 °C for 1 h or surface H2O2-oxidation with subsequent hot water aging. The oxide layer synthesized by the heat treatment was not uniform and had a thickness from 1.5 μm to 4.0 μm, whereas the titania gel layer fabricated through the H2O2-oxidation method was homogeneous and about 4.0 μm thick. The oxide layer formed on heat treated NiTi SMA was composed of both anatase and rutile titania with a small amount of Ni2O3. On the contrary, the gel layer made by H2O2-oxidation mainly consisted of poorly crystalline anatase and was relatively Ni-free. Abundant Ti–OH functional groups existed on the titania gel layer, contributing to the improvement of bioactivity. Mechanical properties of the two oxide layers were drastically different. The phase transformation temperatures of NiTi SMA were altered by the heat treatment. The titania gel layer on H2O2-oxidized NiTi SMA was more chemically stable than the oxide layer on heat treated NiTi SMA. Although both the heat treatment and the H2O2-oxidation method greatly improved the bioactivity of NiTi SMA, H2O2-oxidized samples were found to be more bioactive. Moreover, in vitro cytotoxicity evaluation based on MTT assay revealed that both heat treated and H2O2-oxidized samples did not have acute cytotoxicity. Results from live and dead cells staining showed that more MCF-7 cells attached and spread extensively on H2O2-oxidized samples in comparison to either heat treated or untreated samples.