Aspartic Acid-Assisted Synthesis of Multifunctional Strontium-Substituted Hydroxyapatite Microspheres

Strontium-substituted hydroxyapatite (SrHAP) microspheres with three-dimensional (3D) structures were successfully prepared via hydrothermal method using self-assembled poly(aspartic acid) (PASP) as a template. By controlling various parameters, including hydrothermal reaction time, amount of L-aspartic acid (L-Asp), and ratio of Sr ions, we were able to investigate the influences of the additive L-Asp on morphology and properties of final products as well as the role of self-assembled PASP template on the formation of HAP microspheres. The change in the amount of Sr substitution significantly affected the particle size, morphology, and concurrent surface area. This difference caused variation in the drug-release properties. In addition, substitution of Sr ions into Ca ion sites affected luminescence of HAP powders. Particularly, multifunctional SrHAP with molar ratios (Sr/[Ca+Sr]) of 0.25 possessed the strongest luminescence as well as superior drug-loading and sustained-releasing properties. These properties were associated with large surface area and large pore size of the SrHAP. This study suggests that the optical and structural properties of the HAP particles can be carefully tuned by controlling the amount of Sr ions doped into HAP particles during synthesis. This work provides new opportunities to synthesize HAP particles suitable for diverse applications including bone regeneration and drug delivery.