The Effect of Boron Oxide on the Biocompatibility, Cellular Response, and Antimicrobial Properties of Phosphosilicate Bioactive Glasses for Metallic Implants’ Coatings

Bioactive glasses remain promising candidates for enhancing osseointegration on metallic implants. However, achieving a composition that combines controlled dissolution, cytocompatibility, and antimicrobial functionality remains an ongoing challenge. Building upon the prior structural and thermal characterization of boron-substituted 6P55 phosphosilicate glasses, this study investigates the biological consequences of incorporating 0, 5, 10, and 15 mol% B₂O₃ to determine their suitability as coatings for Ti6Al4V. Glass extracts were evaluated using L-929 fibroblast cultures (MTT assay and ImageJ-based cell counting), antimicrobial assays against Escherichia coli and Staphylococcus aureus using a semi-quantitative dilution-plating method, and SBF immersion studies to assess pH evolution, surface mineralization, and Ca/P ratio development. FTIR and SEM analyses revealed composition-dependent formation of phosphate-, carbonate-, and silicate-rich surface layers, with 5B exhibiting the most consistent early-stage hydroxyapatite-like signatures, supported by Ca/P ratios approaching the stoichiometric value. The pH measurements showed rapid alkalization for 5B and moderate buffering behavior at higher boron contents, consistent with boron-dependent modifications to network connectivity. Cytocompatibility studies demonstrated a dose- and time-dependent reduction in cell number at elevated B₂O₃ levels, whereas the 0B and 5B extracts maintained higher viability and preserved cell morphology. Antibacterial assays revealed strain-dependent and sub-lethal inhibitory effects, with E. coli exhibiting stronger sensitivity than S. aureus, likely due to differences in cell wall architecture and susceptibility to ionic osmotic microenvironment changes. When considered alongside previously published computational and physicochemical results, the biological data indicate that moderate boron incorporation (5 mol%) provides the most favorable balance between dissolution kinetics, apatite formation, cytocompatibility, and antimicrobial modulation. These findings identify the 5B composition as a strong candidate for further optimization toward bioactive glass coatings on Ti6Al4V implants.