Improving exposure of anodically ordered Ni–Ti–O and corrosion resistance and biological properties of NiTi alloys by substrate electropolishing

Although Ni–Ti–O nanopores (NPs) can be fabricated by anodization of mechanically polished NiTi alloys, the top disordered layer is difficult to remove thus hindering the functionality of the Ni–Ti–O NPs. In this work, an electropolishing (EP) pretreatment was performed on the NiTi substrate prior to anodization to thoroughly expose the NPs. Our results show that the EP pretreatment for 5 min perfectly removes the top disordered layer on the Ni–Ti–O NPs to expose the underlying NPs and consequently, the corrosion resistance and antibacterial ability are enhanced. The exposed NPs can elongate bone marrow mesenchymal stem cells, which may be responsible for the upregulated alkaline phosphatase activity, secretion of Type I collagen, and extracellular matrix mineralization. These results suggest that EP is a desirable pretreatment before anodization of the NiTi alloys because the irregular surface layer on the Ni–Ti–O NPs can be removed to enhance the corrosion resistance and biological functions.