Facile solution-based synthesis of impurity-free hydroxyapatite nanocrystals at ambient conditions

Synthesis of nanocrystalline hydroxyapatite with high degree of crystallinity results in improved bioactivity responses due to its high specific surface area and low ion release. Therefore, the innovative methods resulting in these qualities are of significant importance especially for 3D printing of biomaterials and biocomponents made by hydroxyapatite. Addressing the qualities essential for bioperformance of hydroxyapatite, a new method was developed in this work, which is able to prepare impurity-free hydroxyapatite nanocrystals at ambient conditions. Characterization of role of processing conditions and their successive optimization resulted in a facile cost-effective process in contrast to the literature. Stoichiometric nanoscale hydroxyapatite free from by-products, unwanted materials (unwanted phase formation) and impurities was obtained by the approach. It was found that the longer drying time increases the crystallinity of the as-washed precipitate; however, to reach a high degree of crystallinity, application of a heat treatment process is essential. The designed short time heat treatment was able to enhance the crystallinity of the dried precipitate while its structure remained nanoscale after heat treatment. Keeping the nanoscale structure of the samples even after heat treatment allows using its benefits for improvement of biocompatibility, osteoconductivity and tissue in-growth of the biomaterials made by these nanoscale hydroxyapatite powder products. The results showed that the amino acid (glycine) has a significant effect on the crystallinity of the hydroxyapatite due to its effect on kinetic of hydroxyapatite crystallization (nucleation of new crystals) by enhancing the surface diffusion mechanism in the precipitate. In contrast, the glycine also controlled the growth process of the nucleated crystals by its adsorption on the growing surfaces and/or by introducing the strain to the growing crystals. These stresses controlled the growth of the nanocrystals and induced directional growth and morphological changes in the hydroxyapatite powder products.