Bioactive Microarc Oxidized TiO2-based Coatings for Biomedical Implication

Abstract

Hydroxyapatite (Ca10(PO4)6(OH)2, HA) and bioactive glass-ceramic (SiO2-CaO-P2O5-MO (M=Na, Mg, etc.)) both exhibit good bioactivity. Unfortunately, these bioactive ceramic materials are not suitable for load-bearing conditions on account of their poor mechanical properties (Liu et al., 2004; Morais et al., 2007). As compared to these brittle ceramics, metal materials such as titanium and its alloys exhibit excellent mechanical toughness and strength (Liu et al., 2004). However, they have poor osteoinductive properties because of their bioinert feature. Thus, to prepare bioactive coatings on titanium and its alloys is an approach to resolving the disadvantages of ceramic and metal biomaterials. Many surface modifying techniques (e.g., plasma spraying (Zheng et al., 2000), sol-gel method (Wen et al., 2007; Balamurugan et al., 2007), biomimetic and electrochemical deposition (Zhang et al., 2005)) have been developed to deposit bioactive coating on titanium and its alloys. Microarc oxidation (MAO) is a relatively convenient and effective technique to deposit bioceramic coatings on the surfaces of Ti and its alloys (Yerokhin et al., 1999). This technique can introduce various desired elements into titania-based coatings and produce various functional coatings with a porous structure (Yerokhin et al., 1999). Additionally, MAO coatings usually exhibit good interfacial bonding to substrates (Wang et al., 2009). Moreover, it is very suitable to modify various substrates with complex geometries. Most of MAO coatings mainly contained anatase, rutile and amorphous or crystalline calcium phosphate phase (at high applied voltage). It is difficult to form HA phase during MAO process because of a high temperature and a rapid cooling rate at anodic surfaces. And the apatite-forming ability of the MAO coatings is not very good. Thus, the subsequent modifications have been developed such as sol-gel, ultraviolet (UV) irradiation and hydrothermal treatment (Li et al., 2005; Han et al., 2008; Ishizawa et al., 1995). Authors have developed a simple method of chemical-treatment to modify the surfaces of the MAO coatings for improving the inducation ability for the formation of biomimetic apatite (Weia et al., 2007; Wei et al., 2008). In addition, the formation process of biomimetic apatite on the bioactive surface has been noted for long time. However, some important problems were also not solved until now. Thus, systematic thermodynamic and kinetic calculations of Gibbs free energy (∆G) and nucleation rate (logJ) for the formation of various calcium phosphates (CaPs) in different simulated body fluids (SBFs) was conducted by authors. The structure and formation process of biomimetic apatite were reported.