Structural and thermal effects of controlled sodium substitution in nanocrystalline carbonate hydroxyapatite synthesized by precipitation with spray-based reactant delivery

Hydroxyapatite (HAp) is often used in biomedical applications due to its biocompatibility, bioactivity, and similarity to the mineral component of bone. Naturally occurring apatites contain trace elements such as sodium, which play an important role in bone remodeling and ionic balance. However, the effects of controlled sodium incorporation on the structural and thermal properties of synthetic HAp have not yet been sufficiently investigated. In this study, nanometric hydroxyapatite (HAp) and sodium-substituted hydroxyapatite (HAp-Na) were synthesized by wet chemical precipitation using spray, to produce biomimetic materials with physicochemical properties comparable to biogenic HAp and to investigate the physicochemical effects of controlled Na + incorporation on the structural and thermal stability of HAp. Sodium was incorporated at concentrations ranging from 9500 to 34,000 ppm to mimic physiological levels as well as those found in human bone. The structural, morphological, thermal, and vibrational properties of the materials were characterized by XRD, HR-SEM, FTIR, TGA/DSC, and ICP-OES. The results showed that sodium substitution at the Ca 2+ sites led to a decrease in Ca and P content and caused lattice expansion in conjunction with the incorporation of carbonate as a charge-compensation mechanism. The crystallite size increased from 20 to 45 nm during calcination at 400 °C, while maintaining the hexagonal P6 3 /m structure. FTIR and TGA/DSC confirmed the successful elimination of synthesis by-products and showed shifts in the vibrational modes associated with ion substitution. These results demonstrate that Na + doping modulates the lattice geometry and thermal behavior of HAp and provides a tunable route for tailoring apatite-based materials for functional applications beyond bone regeneration. Age-dependent structural optimization of biogenic hydroxyapatite: implications for the design of nanocrystalline bone grafts and implant coatings • Crystallinity and Ca/P ratio peak at 25 weeks in rat bone-derived HAp. • Donor age impacts porosity, crystallite size, and bioactive potential. • Calcination must be adjusted to age to preserve nanostructures. • Hydroxylation and Mg content vary with bone maturation. • Optimal HAp for implants found in young adult rat femurs.