Surface preparation of bioactive Ni–Ti alloy using alkali, thermal treatments and spark oxidation

The primary aim of this study was to compare different surface treatments used for bioactivation of pure titanium surfaces––thermal, alkali treatment and spark oxidation, and to assess their suitability as treatments for Ni–Ti alloys. This was considered by examining the surface properties, calcium phosphate precipitation from a physiological solution, and nickel ion release. Additionally, changes in the transformation temperature were measured for thermally treated samples. These studies indicate that the native surface of Ni–Ti alloy is highly bioactive when assessing the precipitation of calcium phosphates from Hank’s solution. Low temperature heat treatments also produced promising surfaces while high temperature treatment resulted in a very low rate of Ca and P precipitation. Alkali treatment and spark oxidation resulted in some bioactivity. Nickel ion release was greatest for alkali treated and sparks oxidized samples, and the rate of its release from these two samples was on the verge of daily safe dose for adolescent human. The other analyzed samples revealed very low rates of nickel ion release. Heat treatment at 400°C resulted in significant increase in the transformation temperatures, and a further increase of the treatment temperature up to 600°C caused a drop of the transformation temperature.