Unraveling the role of calcium in the osteogenic behavior of mesoporous bioactive glass nanoparticles

The use of bioactive materials has emerged as a promising strategy to circumvent bone-related diseases. Because of their chemical composition, calcium-containing bioactive glasses, including mesoporous bioactive glass nanoparticles (nMBG), have long demonstrated their bone regeneration features. In this work, SiO₂-CaO nMBG were synthesized varying Si/Ca ratio from 10 % to 40 % to explore the role of Ca in the osteogenic properties of such materials. We have performed an in-depth physicochemical and biological evaluation of samples by TEM, FTIR, adsorption nitrogen and solid state NMR, revealing that increasing calcium weakens the silica network and consequently, the osteogenic properties. In addition, we have evaluated the protein corona in human serum, obtaining varying protein patterns depending on the Si/Ca ratio and the incubation time. The cellular studies have shown that only certain amounts of calcium up-regulate the osteogenic differentiation, although exceeding such concentrations does not provide improved effects. Finally, All Ca-containing samples promoted calcium phosphate mineralization in biological fluids, while those with higher Si/Ca ratios enhanced significantly hMSC and hOB mineralization. Calcium also modulated hMSC gene expression, with samples containing up to 20 % calcium up-regulating OC and RUNX2. Furthermore, nMBG exhibited immunomodulatory properties, inducing a shift toward the M2 reparative phenotype. Overall, this comprehensive study highlights the crucial role of calcium in osteogenic responses, demonstrating that calcium quantity alone does not surpass the importance of structural and compositional quality in nanosized MBG. STATEMENT OF SIGNIFICANCE: Bone-related diseases are becoming a major socioeconomic issue owing to the increased aging of our society. Therefore, bioactive materials based on silicon, calcium and phosphorus have been used for years due to the osteogenic properties of these elements. In the last few years, the preparation of these materials as nanoparticles has increased their range of applications. In this sense, the novelty of our work relies on the in-depth physicochemical and biological evaluation of those mesoporous bioactive glass nanoparticles based on silicon and calcium, which remained unexplored so far. Couple with the establishment of the range of atomic percentage of calcium with respect to silicon that up-regulate the osteogenic differentiation, although exceeding such concentrations does not provide improved effects.