Fabrication of Si-substituted hydroxyapatite foam using calcium silicates

Silicon-substituted hydroxyapatite (Si-HAP) foam may be an ideal bone substitute since Si-HAP has stimulating effect for bone formation and the foam morphology provides interconnected macro porous structure which is ideal for bone formation. However, lower temperature was reported to be required for the fabrication of pure Si-HAP whereas higher temperature was required for the fabrication of HAP foam. In the present investigation, three calcium silicate compounds (CaSiO₃, wollastonite; Ca₂SiO₄, belite; Ca₃SiO₅, alite) were evaluated for their feasibility as a silicon source for the fabrication of Si-HAP foam. Sintering temperature at 1300°C resulted in a very weak HAP foam, whereas sintering at 1400°C resulted in HAP foam with improved compressive strength up to approximately 9 kPa. XRD analysis showed the appearance of a secondary phase, α-tricalcium phosphate, when wollastonite was used as the silicon source. In contrast, pure Si-HAP phase was obtained when belite or alite was used as the silicon source. The porosities of the Si-HAP foams were approximately 95% regardless of composition of the foam. We therefore concluded that Si-HAP foam with fully interconnecting pores could be fabricated when belite or alite was used as the silicon source. Further evaluation is expected based on this initial finding.