Synthesis of calcium phosphate crystals in a silica hydrogel containing phosphate ions

Calcium phosphate crystals were synthesized by diffusing calcium ions into silica hydrogels containing phosphate ions. Hydroxyapatite [HAp, Ca₁₀(PO₄)₆(OH)₂] and octacalcium phosphate [OCP, Ca₈(HPO₄)₂(PO₄)₄.5H₂O] with different types of crystal morphology were formed in the gel. The HAp had an irregular or rod shape, a few micrometers in length, while the OCP had an irregular, spherulite, rod- or ribbonlike shape, ranging in size from a few micrometers to several tens of micrometers, depending on the amount of phosphoric acid added and the reaction temperature. The morphology of the OCP changed from an irregular shape to a ribbonlike or rod shape, via a spherulite shape, depending on the amount of phosphoric acid added and the reaction temperature. The degree of supersaturation of the reaction environment and the rate-determining step in the HAp and OCP crystal growth mechanism have been ascribed to the changes in crystal morphology of the HAp and OCP.