Bioactive materials bond to bone through a bonelike apatite layer which is formed on their surfaces in living boy. Amorphous phases play an important role in forming the apatite layer for most of the materials. It has been proposed that Si-OH and Ti-OH groups at the surfaces of the amorphous phases induce the apatite nucleation. In the present study, processes of apatite formations induced by the Si-OH and Ti-OH groups were investigated in a model system in a simulated body fluid (SBF) by using transmission electron microscopy and X-ray photoelectron spectroscopy. Apatite formation was found to progress in the sequence: 1) binding of calcium ions in SBF with the Si-OH or Ti-OH groups to form a calcium silicate or a calcium titanate, 2) late binding of phosphate ions in SBF with the calcium silicate or the calcium titanate to form apatite nuclei, and 3) spontaneous growth of the apatite nuclei by consuming calcium and phosphate ions in SBF. This indicates that Si-OH or Ti-OH groups on amorphous phases do not directly induce the apatite nucleation, but via formation of a calcium silicate or calcium titanate.
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