Synthesis and physical chemical characterizations of octacalcium phosphate–based biomaterials for hard-tissue regeneration

Several calcium phosphate compounds corresponding to the octacalcium phosphate (OCP) chemical composition are described. In addition to the triclinic pentahydrate structure, amorphous and apatitic OCPs have been synthesized, and their preparations and some of their physical–chemical properties are given. Other related phases, such as carbonated OCP, are discussed in relation with the characteristics of biological apatites, especially bone mineral. Biomimetic nanocrystalline apatites (BNAs) analogous to bone mineral regarding their composition, crystal shape, and specific feature, such as a surface-hydrated layer containing nonapatitic environments of the mineral ions, are presented. The correlations between this hydrated layer and the triclinic pentahydrate OCP are discussed, and the fragility of this structural feature is stressed. The determining role of carbonate ions in the evolution of BNA is shown and related to bone mineral and tooth enamel properties. The physical–chemical characterizations of these different compounds, using different techniques (chemical analyses, Fourier-transform infrared spectroscopy, X-ray diffraction, and solid-state nuclear magnetic resonance) are compared.