Sr²⁺-dependent microstructure regulation of biodegradable Sr-doped hydroxyapatite microspheres with interconnected porosity for sustained drug delivery

The microstructure and compositional tunability of hydroxyapatite (HA) microspheres, as well as their limited biodegradability, have been challenges for their use as carriers in drug delivery systems. In this study, strontium-doped HA (Sr-HA) microspheres with interconnected porosity and pore channels were successfully prepared under the regulation of Sr²⁺ via a hydrothermal method. The morphologies, scales, constituents, and pore geometries of Sr-HA microspheres were modulated by the concentration of Sr²⁺. SEM and TEM observations revealed that diverging, dense flower-like, hierarchical porous, and trumpet-shaped macroporous microspheres were obtained as Sr/(Sr + Ca) ratio increased. Correspondingly, the size of the microspheres decreased from approximately 120 μm–15 μm. The introduction of Sr²⁺ significantly enhanced the biodegradability of the microspheres. Furthermore, the porous Sr-HA microspheres showed superior vancomycin loading capacity (167.5 mg/g) and sustained release properties. This possible microstructure regulation mechanism was discussed based on the experimental results. This study provides a new perspective on material design for various potential applications.