Fabrication of silica-substituted carbonate apatite blocks through phase transformation process from silica-substituted octacalcium phosphate blocks

Although silica substitution into apatites is an attractive method for improving biocompatibility and their bone-remodeling stimulation abilities, residual organic matter from organic silica as a silica source is a disadvantage for silica doping. In this study, we fabricated silica-substituted apatite blocks from silica-substituted octacalcium phosphate (OCP-silica) blocks by immersion in (NH4)2CO3 solutions. At below 1-mol/L (NH4)2CO3, the treated blocks retained their original OCP-silica block shapes while being converted to apatites containing both silica and CO3. As the (NH4)2CO3 concentration in the treated solutions increased, the CO3 content of the apatite blocks increased, whereas their silica content decreased. The formed carbonate apatite (CO3Ap) phase was categorized as AB-type CO3Ap. The mechanical strength of the fabricated silica-substituted CO3Ap blocks was approximately 150 kPa in diametral tensile strength (DTS) values.