Abstract
The effect of colloidal silica (SiO2; CS) addition on the formation of porous spherical α-calcium orthophosphate (α-Ca3(PO4)2; α-TCP) agglomerates has been examined. The starting powder was prepared by the spray pyrolysis of calcium phosphate (Ca/P ratio = 1.50) solution containing 0.9 mol·dm-3 Ca(NO3)2, 0.6 mol·dm-3 (NH4)2HPO4, CS (mean particle size; 14, 24 and 39 nm) and concentrated HNO3 at 600°C, using an air-liquid nozzle; the amounts of CS in the spray-pyrolyzed powder were adjusted to be 0.1-0.4 mass%. The heat-treatment of spray-pyrolyzed power with 0.4 mass% of CS addition at 1200°C for 10 min showed that the transformation from β- to α-TCP was restricted as the diameter of CS decreased from 24 to 14 nm. The heat-treatment of spray-pyrolyzed powder containing 0.1 mass% of CS (mean diameter: 24 nm) at 1200°C for 10 min resulted in not only the complete transformation of β- to α-TCP but also preservation of original spherical framework. The immersion of porous spherical α-TCP agglomerates in simulated body fluid (SBF) brought about the rapid formation of hydroxyapatite, due to higher specific surface area (17.0 m2·g-1) compared to that of commercial powder (10.9 m2·g-1). The hardened body with the porosity of 66.3% could be fabricated by mixing the calcium-phosphate paste with 50 mass% of the present porous α-TCP agglomerates, using the malaxation liquid. Immersion of this hardened body in SBF at 37.0°C resulted in the formation of hydroxyapatite within three days.
