**4. Conclusions**

Application of the sol-gel method made it possible to obtain homogeneous titanium dioxide coatings co-doped with calcium and silver ions in a di fferent molar ratio. The thermal treatment at 450 ◦C allowed crystalline coatings of anatase structure to be obtained at a thickness of approx. 70–80 nm, regardless of the amount of each dopant. It was found that the amount of crystalline anatase phase increases with increasing silver content. All sol-gel TiO2-based coatings investigated in this study have hydrophilic properties regardless of the type of dopant. Thus, each of the produced coatings is beneficial to the adsorption of osteoblast cells and thus for bone-bonding properties of implants. The doping with calcium and silver ions a ffects the topography of the TiO2-based coatings. Undoped coatings and coatings doped only with calcium ions are characterized by small surface development. Along with the increase of silver content in the coating, its surface development increases as well, due to pores existing in the coating. The corrosion tests confirmed anticorrosive properties of TiO2-based coatings. The best protective (anticorrosive) properties were registered for the coating doped with calcium ions. Analysis of corrosion results showed that the increase in silver content resulted in increasing of the electrochemical activity of the investigated samples in PBS solution. The immersion test in the SBF solution confirmed the bioactivity of the tested coatings—the apatite layer was found on the samples' surface. It was found that, with the increase in calcium ion content in the coating, the Ca/P ratio increases and approaches the value of 1.67, characteristic of stoichiometric hydroxyapatite.

Silver is recognized as an antibacterial element, but at the same time can be also cytotoxic for cells. Calcium is very well known as an element that can improve the osseointegration processes. Our results showed that coatings containing Ca and Ag particles, independently of their molar ratio in TiO2 coating, are biocompatible and do not significantly reduce the proliferation ability of the osteoblast cells, compared to the pure material, as the M30NW steel is. The lack of toxicity and viability of cells above 80% for all coatings may indicate that these doubly-doped coatings meet at least one of the requirements to define them as biocompatible materials. As the other results indicated, they may also show the ability for bone-like apatite formation and significantly improved corrosion resistance in comparison to the steel biomaterial.

**Author Contributions:** B.B.: Conceptualization, methodology, investigations (surface characterization and corrosion investigations), results analysis, visualization, writing, editing, and supervising the manuscript; P.O.: Resources, corrosion investigations, and data analysis; D.B. (Damian Batory): Surface characterization, data and results analysis, discussion, and editing the manuscript; M.C.: Wetting investigations, data and results analysis, discussion; M.K.: biological investigations, data analysis, and discussion; D.B. (Dorota Bociaga): Biological investigations, data and results analysis, discussion, writing, and editing the manuscript. All authors have read and agreed to the published version of the manuscript.

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

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