Mechanism of apatite formation induced by silanol groups - TEM observation

The essential requirement for an artificial material to bond to bone is formation of bonelike apatite on its surface in body environment. The formation of the apatite has been proposed to be induced by silanol groups on the surface of the material. In the present study, the mechanism of apatite formation induced by the silanol groups was investigated in a biomimetic process. A collodion film supported by a titanium grid which faced to a CaO-SiO2-based glass was soaked for various periods in an acellular simulated body fluid with ion concentrations nearly equal to those of the human blood plasma. The him was observed by a transmission electron microscope equipped with energy dispersive X-ray spectrometer. Silicate ions rich in silanol groups, which were released from the glass, were attached onto the collodion film. They first combined only with calcium ions in the fluid to form a calcium silicate. Later, the calcium ions in the calcium silicate combined with phosphate ions in the fluid to form an amorphous calcium phosphate with a low Ca/P atomic ratio. The amorphous calcium phosphate, increasing its Ca/P atomic ratio, eventually transformed into a crystalline apatite, This indicates that the silanol groups induce the apatite formation not directly but through the formation of calcium silicate and amorphous calcium phosphate with a low Ca/P atomic ratio. The formation of calcium silicate at the initial stage might be attributed to interaction between negatively charged silanol groups and positively charged calcium ions.