Calcium phosphate crystals were synthesized in polyacrylamide (PAAm) hydrogel, and the effects of the concentrations of calcium and phosphate ions on the crystalline phases and morphology were investigated. PAAm hydrogels containing diammonium hydrogen phosphate ((NH4)2HPO4) were transformed into calcified materials by diffusion of calcium ions from calcium nitrate (Ca(NO3)2) aqueous solution into the gels. Several kinds of calcium phosphate crystals were precipitated at various Ca(NO3)2 concentrations (0.5-4.0 mol·dm- 3), or (NH4)2HPO4 contents (3.6-21.6 mmol) in the gels. The crystalline phases were mainly determined by the (NH4)2HPO4 content in the gels. When the (NH4)2HPO4 content was ≥ 10.8 mmol, hydroxyapatite (HAp) formed near the interfaces between Ca(NO3)2 solution and the gels, whereas octacalcium phosphate (OCP) formed in gels with ≤ 10.8 mmol (NH4)2HPO4. HAp crystals were granular in form and about 200 nm in diameter, and OCP crystals were spherulitic with diameter 10-70 μm.
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