Fabrication of bioactive titanium oxide coating on nickel-titanium using plasma electrolytic oxidation

Abstract

When nickel titanium (NiTi) was introduced as a biomaterial because of its unique shape memory and super-elastic properties, researchers began to focus their attention on the safe use and apatite-forming ability of this material. Due to its high content of nickel, nickel inevitably exists on the surface of untreated NiTi. Therefore, surface treatment is necessary to reduce the nickel content on the surface of NiTi to ensure the safety of implantation. Apart from the risk of nickel ions release that may cause allergic effects, NiTi is a bio-inert and poor osteoinductive material. The growth of body tissue on NiTi implants is not easy unless a bioactive coating exists. In order to enhance the stability of implantation, a lot of research work has been focused on the surface modification of NiTi. This project investigated the feasibility of forming a thick and porous coating on NiTi by plasma electrolytic oxidation (PEO) which is an effective technique that can be conducted at room temperature. At such low temperatures, the bulk properties of the thermally sensitive NiTi would not be affected. The Taguchi experimental design approach was implemented to optimize the process parameters for PEO. Two L16 and one L27 Taguchi experiments were conducted and optimized parameters in terms of concentration of electrolytes, voltage and processing time were found. The results from the Taguchi experiments indicated that an alkaline environment was more suitable for conducting PEO treatment on NiTi than an acidic one. Titanium oxide coatings of around 10μm thick and with a porous structure were successfully fabricated in a Na₂SO₄/NaOH electrolyte by an AC power source. XPS analyses showed that the surface of the treated NiTi contained only a small amount of Ni when compared to the substrate. In addition, the influences of different parameters on the surface morphology, phase composition and corrosion resistance were investigated. Crystalline titanium oxide coatings with enhanced corrosion resistance were obtained. The immersion test results in a simulated body fluid demonstrated significant improvement in terms of apatite-forming ability of the PEO-treated samples. Hydroxyapatite (HA) particles were observed on the treated NiTi after 28 days of immersion whereas no particle was found on the bare NiTi samples. This indicated that the PEO treatment developed in this study can improve the bioactivity of NiTi. It is concluded that such a bioactive coating can reduce nickel content at implant surface and enhance the apatite-forming ability, thus improving the performance of orthopedic and dental implants.