Incorporating Mn²⁺ ions in bioactive sol-gel coatings: Impact on cell adhesion, inflammation and bone regeneration

Manganese (Mn), an essential trace element involved in bone metabolism, plays a crucial role in key biological functions, including the regulation of cell adhesion, modulation of immune responses, and promotion of osteogenesis. In this study, sol-gel coatings with increasing concentrations of MnCl₂ (0.5, 1 and 1.5 % wt) were synthesised and applied onto titanium (Ti) surfaces. The materials were characterised physicochemically, and in vitro responses were assessed using human osteoblasts (HOb) and THP-1-derived macrophages. Protein adsorption from human serum was analysed by nLC-MS/MS. The incorporation of MnCl₂ did not disrupt the sol-gel silica network and enabled a sustained release of Mn²⁺ ions, and all coatings showed good biocompatibility with no cytotoxicity. Immune response analysis revealed that 0.5Mn promoted anti-inflammatory markers (IL-10, TGF-β), while 1Mn and 1.5Mn induced strong proinflammatory profiles, reflected in increased TNF-α and IL-1β. Proteomics identified preferential adsorption of immune-related proteins such as complement components, ficolins, acute-phase proteins and apolipoproteins to 1Mn and 1.5Mn. Conversely, 0.5Mn enhanced the adsorption of proteins linked to anti-inflammatory effects and oxidative stress regulation. Mn-doped surfaces also enhanced gene expression related to cell adhesion (CTNNB1, ITG1B, PTK2) and osteogenic markers (RUNX2, BMP2, BGLAP), particularly on 1.5Mn, correlating with increased calcium deposition and adsorption of mineralisation-related proteins (FETUA, ECM1, IGF2). All Mn sol-gel coatings promoted the coagulation cascade through increased adsorption of FA9, FA12 and ZPI. These results demonstrate the capacity of Mn-doped sol-gel coatings to modulate immune and osteogenic responses, underscoring the relevance of optimising Mn concentration to improve bone-implant integration.