XPS study of the process of apatite formation on bioactive Ti-6Al-4V alloy in simulated body fluid

Bioactive Ti-6Al-4V alloy, which spontaneously forms a bonelike apatite layer on its surface in the body and bonds to living bone through this apatite layer, can be prepared by producing an amorphous sodium titanate on its surface by NaOH and heat treatments. In this study, the process of apatite formation on the bioactive Ti-6Al-4V alloy was investigated in vitro, by analyzing its surface with X-ray photoelectron spectroscopy as a function of soaking time in a simulated body fluid (SBF). Thin-film X-ray diffractometry of the alloy surface and atomic emission spectroscopy of the fluid were also performed complementarily. It was found that immediately after immersion in the SBF, the alloy exchanged Na+ ions from the surface sodium titanate with H3O+ ions in the fluid to form Ti-OH groups on its surface. The Ti-OH groups, immediately after their formation, incorporated the calcium ions in the fluid to form calcium titanate. The calcium titanate thereafter incorporated the phosphate ions in the fluid to form an amorphous calcium phosphate, which was later crystallized into bonelike apatite. This process of apatite formation on the alloy was the same as on the pure titanium metal, because the alloy formed the sodium titanate free of Al and V by the NaOH and heat treatments. The initial formation of the calcium titanate is proposed to be a consequence of the electrostatic interaction of negatively charged units of titania dissociated from the Ti-OH groups with the positively charged calcium ions in the fluid. The calcium titanate is postulated to gain a positive charge and interact with the negatively charged phosphate ions in the fluid to form amorphous calcium phosphate, which eventually stabilizes into crystalline apatite. (C) 2001 Elsevier Science Ltd. All rights reserved.