Endothelium Cell Responses on Pulsed-Anodized NiTi Alloy with HNO<sub>3</sub>, NH<sub>4</sub>NO<sub>3</sub>, H<sub>2</sub>SO<sub>4</sub>, and (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> as Electrolytes

Abstract

Pulsed anodization allows to form a Ni-free TiO2 layer on an almost equiatomic NiTi alloy. The surface is then hydrophilized and impedes the release of Ni ions from the alloy. This study aimed to assess the suitability of electrolytes used in the pulsed anodization process with the view of application as a biomaterial. Toward this end, the NiTi alloy was pulsed-anodized in four different aqueous electrolytes, HNO3, NH4NO3, H2SO4, and (NH4)2SO4, after which the endothelium cell behavior was compared. The use of H2SO4 as the electrolyte resulted in the formation of a TiO2 layer with a groove-like structure of several tens of nanometers wide, and this surface inhibited the activity of endothelium cells. The ability to prevent the release of Ni ions was diminished when using the HNO3 electrolyte, resulted in inferior cell proliferation. The other two electrolytes, NH4NO3 and (NH4)2SO4, formed a Ni-free TiO2 layer with a comparatively smooth surface, which more effectively suppressed the release of Ni ions; thus, the cell attachment as well as proliferation were superior to that of the other two electrolytes. The surface smoothness and Ni suppression were thus concluded to be the factors governing the selection of an electrolyte.