No abstract

Pure soluble silica prepared by a sol-gel method induced bone-like hydroxyapatite formation onto its surface when the silica was immersed in a simulated body fluid (SBF), whereas silica glass and quartz did not. This finding directly supports the hypothesis that hydrated silica plays an important role in biologically active hydroxyapatite formation on the surfaces of bioactive glasses and glass-ceramics, which leads to bone-bonding. Gel-derived titania is also a hydroxyapatite inducer because of its abundant TiOH groups. These results provide further insight into the unique osseointegration of titanium and its alloys. It is suspected that gel-derived titania develops an apatite layer by taking calcium and phosphate from the body fluid, thus producing bone-bonding. Although sufficient AlOH groups may remain in the alumina gel, they do not serve to initiate apatite generation when immersed in SBF. This phenomenon explains the fact that an intermediate fibrous tissue is usually found to separate the alumina implant from bone. One may infer that both abundant OH groups and negatively charged surfaces of gel-derived silica and titania are important for hydroxyapatite induction. material which possesses and/or develops both a negatively charged surface and abundant OH groups in a physiologically-related fluid is most likely to be an efficient apatite inducer. Such materials are suitable candidates to serve as bone-bonding biomaterials.

It has been confirmed that the essential condition for glasses and glass-ceramics to bond to living bone is the formation of an apatite layer on their surfaces in the body. It has been proposed that a hydrated silica formed on the surfaces of these materials in the body plays an important role in forming the surface apatite layer, but this has not yet been proved. In the present study, it is shown experimentally that a pure hydrated silica gel can induce apatite formation on its surface in a simulated body fluid when its starting pH is increased from 7.2 to 7.4.

The prerequisite for glasses and glass-ceramics to bond to living bone is the formation of biologically active bonelike apatite on their surfaces in the body. Our previous study showed that a silica gel prepared by hydrolysis and polycondensation of tetraethoxysilane in aqueous solution containing poly(ethy1ene glycol) induces apatite nucleation on its surface in a simulated body fluid. In the present study, the effects of heat treatment of silica gel on its catalytic effects in apatite nucleation was investigated in a simulated body fluid. It was found that apatite forms on the surfaces of silica gels heat-treated below 8OO"C, but not on those heat-treated above 900°C. The volume of nanometer-range pores in the gel remarkably decreased by heat treatment above 900°C. The concentration of silanol groups in the silica gels gradually decreased with increasing heat treatment temperature. The rate of silica dissolution from the gel into the simulated body fluid decreased remarkably by heat treatment above 900°C. This suggested that a special type of silanol group which is formed by soaking the gel treated below 800°C into the simulated body fluid is responsible for apatite nucleation.