Crystallization of calcium phosphate in polyacrylamide hydrogels containing phosphate ions

Calcium phosphate crystals were formed in polyacrylamide (PAAm) hydrogels containing phosphate ions by diffusion of calcium ions from calcium nitrate (Ca(NO3)(2)) solutions covering the gels. Changes in crystalline phases and crystal morphology of calcium phosphate, and in ion concentrations of the Ca(NO3)(2) solutions were investigated as a function of reaction time. Single or two coexisting crystalline phases of calcium phosphate, hydroxyapatite (HAp), HAp/dicalcium phosphate dihydrate (DCPD) or octacalcium phosphate (OCP)/DCPD were formed in the gels. HAp crystals are formed near the surface of the gels. The dense HAp layer and HAp/DCPD layer prevented diffusion of calcium ions from the Ca(NO3)(2) solution, thus formation of calcium phosphate in the gel phase was inhibited. Formation of DCPD was observed to follow the formation of OCP or HAp. The size of the OCP crystals gradually increased with reaction time, while changes in size of HAp crystals were not observed. The reaction time required for DCPD formation depended on the degree of supersaturation with respect to DCPD in the systems. DCPD formed within 1 day under high supersaturation conditions, whereas it formed at 10 days in low supersaturation conditions. (C) 2010 Elsevier B.V. All rights reserved.