Development of bioactive organic–inorganic hybrid for bone substitutes

Abstract

Bioactive glasses and glass–ceramics have been attractive because they spontaneously bond to living bone when implanted in bony defect. However, they are much more brittle and much less flexible than natural bone. Hybridization of essential constituents of bioactive glasses and glass–ceramics with flexible polymer can solve this problem. The present topic 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 (Kokubo solution) proposed by Kokubo and his colleagues. Glasses in the binary system CaO–SiO2 showed the apatite formation in Kokubo solution. The formation of the surface apatite is induced both by dissolution of calcium ions from the bioactive glass, and by silanol (Si–OH) group in the hydrated silica gel formed on the surface. These findings bring an idea that incorporation of calcium ion (Ca2+) and Si–OH group into organic substances leads to a bioactive hybrid. It was confirmed that organic–inorganic hybrids containing calcium salt, which were synthesized from vinyltrimethoxysilane (VS) or 3-methacryloxypropyltrimethoxysilane (MPS), showed the apatite formation in Kokubo solution. Such type of organic–inorganic hybrids can be blended with 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. Incorporation of MPS and calcium salt also provides conventional PMMA-based bone cement with the ability of apatite formation. These findings indicate that novel bioactive materials can be developed from organic–inorganic hybrids obtained by incorporation of calcium ion and specific kinds of functional groups such as Si–OH.