Competing Roles of Substrate Composition, Microstructure, and Sustained Strontium Release in Directing Osteogenic Differentiation of hMSCs

Strontium releasing bioactive ceramics constitute an important class of biomaterials for osteoporosis treatment. In the present study, we evaluated the synthesis, phase assemblage, and magnetic properties of strontium hexaferrite, SrFe₁₂O₁₉, (SrFe) nanoparticles. On the biocompatibility front, the size- and dose-dependent cytotoxicity of SrFe against human mesenchymal stem cells (hMSCs) were investigated. After establishing their non-toxic nature, we used the strontium hexaferrite nanoparticles (SrFeNPs) in varying amount (x = 0, 10, and 20 wt %) to consolidate bioactive composites with hydroxyapatite (HA) by multi-stage spark plasma sintering (SPS). Rietveld refinement of these spark plasma sintered composites revealed a near complete decomposition of SrFe₁₂O₁₉ to magnetite (Fe₃O₄) along with a marked increase in the unit cell volume of HA, commensurate with strontium-doped HA. The cytocompatibility of SrHA-Fe composites with hMSCs was assessed using qualitative and quantitative morphological analysis along with phenotypic and genotypic expression for stem cell differentiation. A marked decrease in the stemness of hMSCs, indicated by reduced vimentin expression and acquisition of osteogenic phenotype, evinced by alkaline phosphatase (ALP) and collagen deposition was recorded on SrHA-Fe composites in osteoinductive culture. A significant upregulation of osteogenic marker genes (Runx2, ALP and OPN) was detected in case of the SrHA-Fe composites, whereas OCN and Col IA expression were similarly high for baseline HA. However, matrix mineralization was elevated on SrHA-Fe composites in commensurate with the release of Sr²⁺ and Fe²⁺. Summarizing, the current work is the first report of strontium hexaferrite as a non-toxic nanobiomaterial. Also, SrHA-based iron oxide composites can potentially better facilitate bone formation, when compared to pristine HA.