Reducing Ni Ion Release From Anodized NiTi Alloy in a Biological Environment by Altering the Surface Characteristics

ABSTRACT Anodization of the superelastic NiTi alloy forms an oxide layer mainly consisting of TiO 2 , thereby reducing Ni ion release in a biological environment. To optimize the anodization process, this study investigated the dominant layer characteristics contributing to the reduction of Ni ion release as well as the influence of anodization parameters. The surface and electrochemical analyses revealed that the concentration of Ni (OH) 2 in the topmost surface has the most influence on Ni ion release, whereas other layer characteristics, such as surface roughness, are less influential. The electrolyte contributes to the removal of Ni (OH) 2 by chemical reaction, and H 2 SO 4 was a better electrolyte than HNO 3 and H 3 PO 4 . Additionally, adjustment of the applied voltage mode enhances the chemical reaction with the electrolyte. Here, applying a pulsed voltage, rather than a continuous voltage, enhanced the layer performance for all electrolytes. An anodized layer that minimizes Ni ion release from the NiTi alloy can be obtained by optimizing the electrolyte and the applied voltage mode to facilitate Ni (OH) 2 removal.