Surface characteristics, Ni ion release, and antibacterial efficacy of anodized NiTi alloy using HNO3 electrolyte of various concentrations

A titanium oxide layer containing a small amount of Ni can be formed on a NiTi surface through anodization when using HNO3 as an electrolyte. In this study, the effect of different HNO3 concentrations on the surface characteristics of the alloy, including hydrophilicity, Ni-ion release, and antibacterial efficacy, was investigated. An oxide layer was fabricated, in which tiny pores were generated by pitting corrosion. The layer thickness and pore size varied with the concentration of the HNO3 electrolyte. The hydrophilicity of the NiTi surface was dramatically improved through anodization owing to the formation of the oxide layer. The layer formed was expected to serve as a corrosion protection barrier; however, the Ni release rate from the anodized surface was higher than that of an untreated surface due to the generation of pores. The Ni release rate from the anodized surfaces varied depending on the electrolyte concentrations used, owing to the balance between the layer thickness and pore size. Ni ion release was advantageous from the view of antibacterial efficacies; the efficacies of NiTi alloy against E. coli and S. aureus were actually enhanced by anodization. Ni ion release was not beneficial with regards to allergenic reaction and cytotoxicity; the Ni ion release rate decreased rapidly when the alloy was soaked in a simulated body fluid. Thus, we concluded that anodization in HNO3 is one possible approach to fabricate ideal NiTi medical devices with excellent antibacterial performance and biosafety.