**4. Conclusions**

This study is a continuation and extension of a previously published work [13] where anti-fungal properties of ZnO-PMMA nanocomposites, were demonstrated and the fact that they increase with increasing content of these zinc oxide nanoparticles was highlighted. The aim of the work was to investigate some properties of new biomaterials that could explain the origin of these properties. The obtained results of laboratory tests are encouraging. However, they must be clinically validated. It would be now interesting to carry out research to find out whether or not the addition of ZnO-NPs contributes at the same time to deterioration of the material mechanical properties. Therefore, prior to clinical application of the new biomaterial further mechanical and cytotoxic studies are required. Finally, it should be noted that the study showed no significant deterioration in the properties of acrylic resin, which could disqualify nanocomposite for clinical use. The increased hydrophilicity and hardness with absorbability within the normal range can explain the reduced microorganisms' growth on the denture base what has been proven in a previous work. The presented study brings much closer the possibility of clinical use of new nanobiomaterials.

**Author Contributions:** M.C., I.O., A.K., J.W. and D.S. conceived and designed the experiments; M.C., I.O., A.K., J.W. and D.S. performed the experiments; M.C. analyzed the data; M.C. and J.W. contributed reagents and materials; M.C., I.O., A.K., J.W. and D.S. wrote the manuscript; W.Ł., K.K., K.A. and E.M.-N. provided support and critically reviewed the manuscript.

**Acknowledgments:** The work was supported by a research project carried out in the years 2017–2018, funded by a statutory gran<sup>t</sup> obtained by the Faculty of Medicine and Dentistry, Medical University of Warsaw. The paper was partially prepared as a result of execution of "PRELUDIUM 10" research project, ref. no. UMO-2015/19/N/ST5/03668, financed by the National Science Centre, Poland. Part of the research project was carried out with the use of equipment funded by the project CePT, reference: POIG.02.02.00-14-024/08, financed by the European Regional Development Fund within the Operational Programme "Innovative Economy" for 2007–2013.

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