Design of bioactive organic-inorganic hybrids on the basis of the bonding mechanism of calcium silicate glass with living bone

Bioactive glasses and glass-ceramics have been attractive as materials for bone substitution because they spontaneously bond to living bone when implanted in bony defects. However, they are much more brittle and much less flexible than natural bone. Hybridization of essential constituents of bioactive glasses and glass-ceramics with a flexible polymer can solve this problem. The present contribution is devoted to the design of organic-inorganic hybrids that can bond to living bone. Previous studies reported that the condition for bioactive glasses and glass-ceramics to achieve direct bond to living bone is formation of a bone-like apatite layer 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) proposed by Kokubo and his colleagues. Glasses in the binary system CaO-SiO₂ showed the apatite formation in SBF. The formation of the surface apatite is induced both by dissolution of calcium ions from the bioactive glass, and by silanol (Si-OH) groups in the hydrated silica gel formed on the surface. These findings suggest that incorporation of calcium ions (Ca²⁺) and Si-OH groups into organic substances leads to a bioactive hybrid. It was confirmed that organic-inorganic hybrids containing calcium salt, which were synthesized from vinyltrimethoxysilane of 3- methacryloxypropyltrimethoxysilane (MPS), showed the apatite formation in SBF. Such type of organic-inorganic hybrids can be blended with an organic polymer. An organic-inorganic hybrid was synthesized from MPS and 2- hydroxyethylmethacrylate (HEMA) at a composition of MPS:HEMA=0.1:0.9. The MPS-HEMA hybrid also formed the apatite layer when the hybrid was incorporated with calcium chloride. These findings indicate that novel bioactive materials can be developed from organic-inorganic hybrids obtained by incorporation of calcium ions and specific kinds of functional groups such as Si-OH.