Pulsed anodization of NiTi alloy to form a biofunctional Ni-free oxide layer for corrosion protection and hydrophilicity

Abstract

Abstract This study introduced an electrochemical process for the formation of a biofunctional oxide layer on an almost equiatomic NiTi alloy surface. The core technology used in the proposed process was anodization in HNO3 electrolyte under a pulsed voltage, which allowed for the formation of a poreless Ni-free TiO2 layer with a thickness of ~50 nm that exhibited hydrophilicity and corrosion protection. The acquired hydrophilicity led to enhanced endothelial cell activity, thereby beneficially promoting cell spreading and the organization of actin stress fibers. The corrosion protection significantly minimized Ni ion release from the alloy surface, further enhancing endothelial cell proliferation. The cell proliferation ratio within a cultivation period of 168 h was ~1.5 times higher than that of an untreated surface. Pulsed-anodization technology is a one-step process that does not require heating of the alloy, thereby minimizing the risk of superelasticity or shape memory deterioration in the NiTi alloy. These remarkable advantages demonstrated that pulsed anodization in HNO3 is an innovative surface modification process that overcomes the critical drawbacks typically associated with NiTi alloys.