Effect of strontium substitution on the properties of mesoporous carbonate apatite for biomedical applications

Mesoporous calcium phosphate based-bioceramics with ionic substitution have attracted attention for treatment of bone diseases and bone regeneration. Carbonate apatite (CO₃Ap) is a promising candidate for bone replacement through bone remodelling, similar to that of autologous bone, owing to the chemical composition of the bone mineral. In this work, we present a dual approach to synthesize mesoporous carbonate apatite (CO₃Ap) and strontium-substituted CO₃Ap (Sr-CO₃Ap) artificial bone nanocrystals, aiming to assess the impact of strontium on CO₃Ap characteristics and biological performance. Mesoporous CO₃Ap was created by transforming hard templates composed of CaCO₃, while Sr-substituted CaCO₃ (Sr-CaCO₃) microspheres were formed using an organic complexing agent to induce pore formation. The resulting CO₃Ap and Sr-CO₃Ap granules exhibited an irregular shape, with rod-like nanocrystals composing their particles. Adsorption/desorption isotherms confirmed the mesoporous nature of the structures, featuring pore sizes ranging from 10 to 50 nm. Specifically, mesoporous CO₃Ap displayed slit-like pores, while Sr-CO₃Ap showed cylinder-like ones. High-resolution transmission electron microscopy images of Sr-CO₃Ap powder revealed mesopores within the particles, with Sr-CO₃Ap exhibiting larger pore volume and specific surface area compared to CO₃Ap. When osteoblast-like MC3T3-E1 cells were cultured on both CO₃Ap and Sr-CO₃Ap samples, they demonstrated good cell attachment, proliferation rates, and increased ALP activity, particularly evident at higher strontium concentrations. These findings suggest promising potential for further exploration of mesoporous apatite nanocrystals in bone replacement and drug delivery applications.