**4. Conclusions**

On the basis of the performed experiments, it can be concluded that the modification of the LDPE in plasma conditions obtaining multi-layer coatings based on DLC structures resulted in profitable changes of physicochemical and biological properties for their medical applications. In particular, it allowed for a required hardness gradient on the modified surface to be obtained, and as a result made the polymer surface harder and less elastic. It was shown that all modifications in which DLC structures (doped and undoped with Si or N atoms) were obtained improved the wettability of the polymer surface, which is an important property in osseointegration processes. Importantly, in the case of N-DLC/DLC (PE\_3 series) and DLC/Si-DLC (PE\_4 series) multi-layers deposition on the LDPE surface, a significant improvement in hardness was observed—up to ca. 2.3 and 2.5 GPa, respectively. In addition, a high content of Si (ca. 27 at.%) in the top layer of PE\_4 series enabled the incorporation of oxygen in the DLC structure after the modification process at ambient air conditions. All obtained surface modifications resulted in considerable increase in the mechanical properties of the tested surface by improving the PE substrate strengthening at a distance of ca. 1.7 μm from the surface. It is noteworthy that for all tested series no significant changes in cell viability in vitro were observed.

**Acknowledgments:** This work has been supported by the statutory research of Department of Physical Chemistry and Modeling of Faculty of Materials Science and Ceramics, AGH-UST (subject number 11.11.160.768).

**Author Contributions:** Karol Kyzioł conceived the article concept, designed and performed the experiments, and wrote the paper; Julia Oczkowska analyzed the data and prepared part of the manuscript; Daniel Kottfer, Marek Klich, Łukasz Kaczmarek and Zbigniew Grzesik analyzed the data, participated in discussion; Agnieszka Kyzioł designed and performed biological experiments and analyzed the obtained results.

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