Bioactive NiTi shape memory alloy fabricated by oxidizing in H2O2 solution and subsequent NaOH treatment

Abstract

Bioactive titanium metals can be prepared by alkaline treatment with NaOH solution [1–3] or oxidizing treatment with hydrogen peroxide (H2O2) solution [4, 5]. The former treatment produces a sodium titanate layer on titanium surface, while the latter forms a titania gel layer. Both sodium titanate layer and titania gel layer have the ability to induce deposition of bone-like apatite in vitro and in vivo and thus are considered bioactive [1–5]. Alkaline treatment is also suggested to be effective for inducing bioactivity of tantalum metal [6]. However, it is not effective for all metallic biomaterials, e.g., SUS316L stainless steel and Co-Cr-Mo alloy [2]. Chen et al. [7, 8] reported that bioactive NiTi shape memory alloy (SMA) could also be obtained by NaOH treatment. Owing to its unique shape memory effect and superelastic properties, NiTi SMA has been frequently used in medicine, such as orthopedic implants, stents, etc. [9–12]. However, it contains a large amount of Ni (about 50 at%), which can lead to the allergic and toxic responses [13, 14]. Therefore, the bioactivity induced by chemical treatment can enable NiTi implants to be used in medicine more safely and widely. In this study, a bioactive NiTi SMA with wormlike surface structure was fabricated using a composite treatment by oxidizing in H2O2 solution and subsequent NaOH treatment, whose bioactivity was investigated by the biomimetic growth of apatite on its surface after it was soaked in simulated body fluid (SBF) for a period of time. A commercially available NiTi (50.8 at% Ni) SMA plate for medical applications with a martensite start temperature (Ms) of −12.8 ◦C and an austenite finish temperature (Af) of 33.4 ◦C (Beijing Gee SMA Technology Co. Ltd, China) was cut into small rectangular blocks (10 × 10 × 1 mm3). All samples were cleaned separately in acetone, ethanol, and deionized water for 10 min, and then chemically polished to remove the native surface oxides for 10 min in Kroll’s reagent: a mixture of 2 ml