Antibacterial efficiency of alkali-free bio-glasses incorporating ZnO and/or SrO as therapeutic agents

A series of seven alkali-free silica-based bioactive glasses (SBG) with ZnO and/or SrO additives (in concentrations of 0–12 mol%) were synthesized by melt-quenching, aiming to delineate a candidate formulation possessing (i) a coefficient of thermal expansion (CTE) similar to the one of titanium (Ti) and its medical grade super-alloys (crucial for the future development of mechanically adherent implant-type SBG coatings) and (ii) antibacterial efficiency, while (iii) conserving a good cytocompatibility. The SBGs powders were multi-parametrically evaluated by X-ray diffraction, Fourier transform infrared and micro-Raman spectroscopy, dilatometry, inductively coupled plasma mass spectrometry, antibacterial (against Staphylococcus aureus and Escherichia coli strains) suspension inhibition and agar diffusion tests, and human mesenchymal stem cells cytocompatibility assays. The results showed that the coupled incorporation of zinc and strontium ions into the parent glass composition has a combinatorial and additive benefit. In particular, the “Z6S4” formulation (mol%: SiO2—38.49, CaO—32.07, P2O5—5.61, MgO—13.24, CaF2—0.59, ZnO—6.0, SrO—4.0) conferred strong antimicrobial activity against both types of strains, minimal cytotoxicity combined with good stem cells viability and proliferation, and a CTE (~ 8.7 × 10−6 × °C−1) matching well those of the Ti-based implant materials.