Preventive Effects of a Strontium‑Containing Nano Bioactive Glass Hydrogel Against Enamel Caries: An In Vitro Study

Objective To evaluate the antibiofilm and remineralising effects of a functionalised chitosan/nano-strontium phosphosilicate hydrogel (fCS/nSPS) on enamel caries. Methods Human enamel blocks were treated with fCS/nSPS and incubated with Streptococcus mutans, Actinomyces naeslundii , and Lactobacillus acidophilus . Biofilm morphology, viability, and bacterial surface charge were assessed using scanning electron microscopy (SEM), colony-forming unit (CFU) counting, and a cytochrome C binding assay, respectively. After a cariogenic challenge, surface morphology, crystal characteristics, lesion depth, elastic modulus, and nanohardness were evaluated by SEM, X-ray diffraction (XRD), micro-computed tomography, and nanoindentation. Silver diamine fluoride (SDF) and deionised water served as positive and negative controls. Results Dense bacterial biofilms formed on water-treated enamel, whereas only sparse biofilm growth was observed on fCS/nSPS- and SDF-treated surfaces. Biofilm viability and bacterial surface charge in the fCS/nSPS and SDF groups were significantly lower than in the water group. A mineral-rich surface layer covered fCS/nSPS- and SDF-treated enamel, while enamel prisms remained exposed on water-treated enamel. XRD revealed stronger crystalline apatite on fCS/nSPS- and SDF-treated enamel, in contrast to poorly crystalline apatite on water-treated enamel. Lesion depths in fCS/nSPS- and SDF-treated enamel were significantly shallower than in water-treated enamel. The elastic modulus and nanohardness of fCS/nSPS- and SDF-treated enamel were significantly higher than those of water-treated enamel. Conclusion The fCS/nSPS hydrogel effectively inhibits cariogenic biofilms and promotes remineralisation of artificial enamel caries lesions in vitro. Clinical significance This antibacterial and mineralising nanocomposite hydrogel shows promising potential as a minimally invasive preventive treatment for early enamel caries, particularly in high-caries-risk patients.