Abstract
Amorphous calcium phosphate (ACP) prepared in unstable supersaturated solutions was found to transform to brushite at physiological pH when introduced to phosphate solutions the concentration of which were comparable with those of the inorganic phosphate reported in matrix vesicles. Comparison of non-treated dry ACP and pressed ACP after being reground showed that brushite forms as a result of a slow dissolution of ACP. In this regard, ACP acts as calcium reservior, resulting in a slow release of calcium in phosphate solutions.
Thermodynamic analysis of the solution of calcium and phosphate at the concentrations used confirmed that brushite was stable phase even at pH higher than 7.20. Scanning electron micrograms of the solid samples taken from the phosphate solutions with the initial pH adjusted at 7.40 showed plate-like crystals characteristic of brushite. With time, however, the thermodynamically most stable phase of HAp precipitated on to the brushite crystals.
From the data presented it is suggested that macromolecules and ions which inhibit ACP transformation, possibly to hydroxyapatite, may play an important role on ACP dissolution, which favors brushite formation in phosphate solutions.
