Several kinds of ceramics called bioactive ceramics have attractive features such as direct bone-bonding in living body. However, they are much more brittle and much less flexible than natural bone. Previous studies reported that the essential condition for materials to show bone-bonding property, i.e., bioactivity, is formation of biologically active apatite on their surfaces after exposure to the body fluid. The same type of apatite formation can be observed even in a simulated body fluid (SBF, Kokubo solution) with inorganic ion concentrations similar to those of human extracellular fluid. Several functional groups such as Si-OH and Ti-OH are known to trigger nucleation of the apatite in body environment. This is significantly enhanced by the release of Ca2+ from the materials. These findings suggest that organic modification of bioactive inorganic components would provide bioactive materials with mechanical performances analogous to those of natural bone. We previously synthesized bioactive organic-inorganic hybrids from alkoxysilane that provides Si-OH groups and calcium salts that release Ca2+ by organic modification with hydroxyethylmethacrylate (HEMA). If another type of functional group can be incorporated into the hybrids, useful guideline for designing bioactive materials with different biological and mechanical properties would be obtained. In this study, we synthesized organic-inorganic hybrids from HEMA by addition of tetraisopropyl titanate (TiPT) and CaCl2. Their apatite-forming ability was examined in SBF. The obtained hybrids formed the apatite in SBF within 7 d, when they were added with appropriate amount of CaCl2. It was speculated that the apatite deposition was induced by Ti-OH groups formed on the hybrids and the release of Ca2+. The chemicovectors including the ion release govern the apatite formation on the hybrids. Mechanical properties of the hybrids were also evaluated by tensile test. They showed Young's modulus analogous to human articular cartilage.
