Fabrication of interconnected porous calcium-deficient hydroxyapatite using the setting reaction of α tricalcium phosphate spherical granules

Interconnected porous calcium-deficient hydroxyapatite (cdHAp) blocks may be an ideal biomaterial to repair bone defects because of their greater similarity to human bone than that of sintered hydroxyapatite (HAp) with respect to calcium content and crystallinity. In particular, the interconnected pores in cdHAp may provide pathways for cell migration and tissue ingrowth. In this study, the feasibility of fabricating interconnected porous cdHAp blocks through the setting reaction of alpha-tricalcium phosphate (αTCP) spherical granules was investigated. It was found that regulation of cdHAp formation was important to fabricate interconnected porous cdHAp blocks. That is, cdHAp needed to precipitate preferentially at the contacting areas between αTCP spherical granules. Exposure of αTCP spherical granules to steam under appropriate pressure was effective for this purpose. When αTCP spherical granules were immersed in water at 100 °C, the setting reaction resulted in dense cdHAp blocks because of the free crystal growth of cdHAp in water. Therefore, steam was used to localize the water at the contacting areas between αTCP spherical granules, which was driven by the surface tension of the water. Without an applied load, no setting reaction was observed when αTCP spherical granules were exposed to steam at 100 °C for 12 h. In contrast, under a load of 20 MPa, cdHAp precipitated to bridge spherical granules, providing an interconnected porous cdHAp block. The porosity and diametral tensile strength of this block were approximately 63% and 1.5 MPa, respectively.