The effects of annealing temperature on corrosion behavior, Ni2+ release, cytocompatibility, and antibacterial ability of Ni-Ti-O nanopores on NiTi alloy

The present work investigated the effects of annealing on corrosion behavior, Ni2+ release, cytocompatibility, and antibacterial ability of nickel-titanium-oxygen (Ni-Ti-O) nanopores (NPs) anodically grown on nearly equiatomic NiTi alloy, aiming at optimizing annealing process to yield favorable comprehensive performance. The morphology and crystal structure of the NPs were characterized by scanning electron microscopy and X-ray diffraction respectively. It was found that when the annealing temperature was <600 °C, the NP layer could be well preserved on the substrate surface, and annealed at 400 °C led to the transformation of amorphous phase to anatase. Annealing at 200 °C significantly enhanced the corrosion resistance and at 400 °C drastically reduced Ni2+ release of the NiTi alloy, but their cytocompatibility had no appreciable difference, indicating the release level of Ni2+ is well tolerated by osteoblasts. Although the release amount of Ni2+ is reduced after annealing especially the sample annealed at 400 °C, their antibacterial ability is even better when compared with that of the unannealed sample. These results suggest the NPs annealed as at 200-400 °C are promising as coatings of biomedical NiTi alloy.