Crystallization of Calcium Phosphate in Agar Hydrogels in the Presence of Polyacrylic Acid under Double Diffusion Conditions

We studied the precipitation of calcium phosphate (CaP) in the presence or absence of polyacrylic acid (PAA) under double diffusion conditions in agar hydrogels. Waiting times and first precipitation positions were measured to analyze the CaP crystallization behavior, with the equivalency rule governing the precipitation in gel for a double diffusion process. We speculated that amorphous dicalcium phosphate (ADCP, CaHPO4·nH2O) nucleates at the first precipitation position via the reaction that the Ca2+ ion interacts with the HPO42– ion, subsequently transforming into octacalcium phosphate (OCP) according to the Ostwald rule of stages and finally being converted to the thermodynamically more stable hydroxyapatite (HAp) via dissolution. The irregularly shaped aggregates formed at an early stage of crystallization were a mixture of OCP and HAp. In the later stage, spherulites with a hierarchical structure precipitated, with their inner part composed of OCP crystals and the outer part composed of HAp crystals aligned with the c-axis. The dissolution of the inner part advanced the growth of the needle-like crystals comprising the outer layer. PAA addition favored the formation of HAp as the final precipitate. We proposed a mechanism for the crystallization of ADCP, OCP, and HAp in the presence or absence of PAA under double diffusion conditions.