Reactivity of Ca and P precursors to form hydroxyapatite and its influence on the properties of the obtained powders

Synthesis of hydroxyapatite has been profusely studied in the literature due to the similarities between this calcium phosphate and the inorganic component of natural bone. Despite the numerous synthesis methods explored, most of the previous studies are focused on the effect of experimental conditions reducing the importance of the chemical reagents selected as precursors. In this work, four combinations of chemical reagents were used for synthesizing hydroxyapatite powders by a sol-gel method. Conditions for removal of residual functional groups from precursors and its influence on the evolution from amorphous to crystalline powders were studied. Nanocrystalline hydroxyapatite with high thermal stability was obtained by using calcium nitrate or calcium acetate with phosphoric acid or ethyl phosphite as precursors of Ca and P respectively. The use of calcium acetate combined with ethyl phosphite delays the formation of the inorganic network due to retention of acetate groups requiring higher calcining temperatures.