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INTRODUCTIONIn the 1970's, Aoki et al. found that stoichiometric hydroxyapatite (HAp; Ca10(PO4)6(OH)2) can be sintered and that it showed excellent tissue response and good osteoconductivity, i.e., the existing bone is bonded with sintered HAp when implanted adjacent to the bone 1,2) . Although sintered HAp showed stability and relatively high mechanical strength, however, sintered HAp cannot be resorbed when implanted since sintered HAp demonstrates high crystallinity due to its crystal growth enhanced by heating at high temperature. Therefore, in recent year, fabrication of low crystalline HAp or carbonate apatite (CO 3Ap) which has similar crystallinity and chemical composition to human bone has been paid much attention. Sintering method cannot be adopted for the fabrication of low crystalline HAp and CO 3Ap since the heating induces crystal growth of HAp and decomposition of CO3Ap.An alternative method to fabricate low crystalline HAp or CO 3Ap is the phase transformation reaction using thermodynamically unstable precursor or the so-called dissolution-precipitation reaction. Our reserach group has been reported that low crystalline HAp block and HAp foam were prepared using gypsum (CaSO 4•2H2O) block and alpha tricalcium phosphate (α-TCP) foam as the precursor 3,4) . For the formation of HAp block, gypsum block was immersed in (NH4)2HPO4 and treated at 60°C, 80°C and 100°C for a prescribed period. While for the formation of HAp foam, the α-TCP foam was immersed in distilled water at 37°C for 24 h or subjected to hydrothermal treatment at 100°C and 200°C for 24 h for the dissolution-precipitation reaction to occur. Also CO 3Ap block and CO3Ap foam can be prepared by phase transformation in dissolution-precipitation reaction using CaCO 3 (calcite) block, α-TCP and calcite foam. Calcite block was immersed in Na2HPO4 solution and heated at 60°C for various periods up to 14 days while α-TCP foam was immersed in (NH 4)2CO3 solution at 200°C for 24 h 5,6) . For the treatment of calcite foam, the calcite foam was immersed in Na2HPO4 solution at 60°C for 2 weeks 7) . In the previous studies, it was found that the mechanical strength of low crystalline HAp or CO 3Ap block or foam were different up to the precursor and condition used for the dissolution-precipitation reaction. For example, diametral tensile strength (DTS) of CO 3Ap block using calcite block as a precursor was 5 MPa whereas DTS value of CO 3Ap block using gypsum as the precursor was 2 MPa. Compressive strength of CO3Ap foam obtained using calcite foam as a precursor was approximately 26 kPa while that of CO 3Ap foam using α-TCP foam as precursor was approximately 6 kPa. Of course these differences in mechanical strength are reasonable since chemical composition, densities of the precursors, treatment conditions such as temperature, type of solution and solution pH were different. Therefore, factors that affect the mechanical strength should be clarified.As an initial step to understand the factors that affect the mechanical strength, effect of precursor...

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