Monetite-Assisted Growth of Micrometric Ca-Hydroxyapatite Crystals from Mild Hydrothermal Conditions

Calcium hydroxyapatite, HAp hereinafter, Ca₅(OH)­(PO₄)₃, the main component of vertebrate bones and teeth, plays a strategic role mainly in biomedical applications because of its bioactivity, biocompatibility, and slow-degradation rate. It is a critical bioceramic material due to its properties of osteo-conduction, -integration, and -induction. Moreover, HAp has a role in catalysis, agricultural and pharmaceutical products, protein chromatography, and water and soil treatment as well. The bulk of investigations about HAp concerns nanosized crystals, owing to the difficulties encountered when growing large laboratory crystals. Then, deep information about surface and even bulk properties are unavoidably lacking. In this paper, we investigate the relationship between the HAp polymorphism and its growth morphology both from the experimental and theoretical point of view. The micron-sized and well grown crystals we obtained are exploitable for morphological investigations, in order to better understand detailed surface properties determining the crystal reactivity. Further, a clean and effective HAp method of chemical synthesis is proposed, and the involved crystal-growth mechanisms are extensively investigated as well. Finally, the unexpected synergic effect between the low supersaturation of the HAp solution and the templating effect of the monetite (CaHPO₄) crystal, used as precursors, is recognized.