**5. Conclusions**

A toothpaste treatment based on substituted n-HAp showed an in vitro antibacterial effect both on natural and RBC surfaces, which was more pronounced after 12 h than after 24 h. On RBC surfaces, toothpaste remnants seemed to show selective affinity with some particles, such as alumina and barium glass fillers, rather than silica particles. This observation is worthy of future investigations aimed at clarifying the interactions between substituted n-HAp and RBC fillers from the perspective of secondary caries prevention. This phenomenon provides a basis to design clinical studies aimed to confirm in vivo the preventive efficacy of these active principles.

**Author Contributions:** Conceptualization, A.C.I. and E.B.; Data curation, A.C.I., G.C. and M.O.; Formal analysis, M.O., F.G.-G., and E.B.; Funding acquisition, E.B.; Investigation, A.C.I. and G.C.; Methodology, A.C.I., G.C. and M.O.; Project administration, E.B.; Resources, E.B.; Software, G.C. and M.O.; Supervision, A.C.I. and E.B.; Validation, A.C.I.; Visualization, E.B., F.G.-G.; Writing—original draft, A.C.I., G.C. and M.O.; Writing—review & editing, F.G.-G., and E.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors are grateful to Massimo Tagliaferro and to Nanovision S.R.L. for providing the SEM-EDS equipment.

**Conflicts of Interest:** The authors declare no conflict of interest.
