**1. Introduction**

Metallic biomaterials as materials implanted into a living system must fulfill stringent requirements, including good corrosion resistance, mechanical properties, and biocompatibility. Nowadays, studies on orthopedic biomaterials are focused mainly on enhancing their bioactivity and giving them new properties (e.g., antibacterial properties). It is well known that antimicrobial biomaterials prevent post-operative infections by reducing the ability of adhesion and permanent attachment of microorganisms and thus the development of biofilm, which is the main reason for the infections. Within the numerous methods used for the preparation of biomaterials with the desired properties, covering their surfaces with functionalized coatings appears to be particularly interesting with high capabilities. Titanium dioxide is one of most interesting materials, which can be applied as a coating of biomaterial.

It is a hard, corrosion resistant, and well biocompatible material with UV-induced hydrophilicity [1]. Literature studies show that several techniques for titanium dioxide coatings preparation, such as thermal and electrochemical oxidation (in case of titanium substrates) [2,3], magnetron sputtering [4,5], chemical vapor deposition [6–9], and sol-gel [2,7,10–13] have been investigated. But one of the most frequently used methods is sol-gel due to its high application potential [14]. The sol-gel TiO2 coating was proven to be a versatile platform for surface functionalization of stainless steels and other biomedical metals. The desired properties of the biomaterial can be obtained by careful control of sol-gel reaction conditions or by the use of suitable additives. One of the possibilities is to improve biocompatibility, bioactivity (related to osseointegration), and antibacterial properties of biomaterials by means of ion doping procedure. In the literature, there are reports about enhanced bioactivity of TiO2 coating by means of incorporation of Ca [15,16], Mg [17] and Sr [18,19] ions. Our previous papers [20,21] also confirmed improved bioactivity of Ca-doped TiO2 sol-gel coatings. It was also proved that, in an analogous manner, the antibacterial properties may be achieved by doping of bactericidal molecules that in most cases have antibacterial properties per se, namely Ag ions/nanoparticles [22–24], Cu ions [25,26], and Zn ions [27]. Several research studies [28–30] have shown that F doping enhances the antibacterial properties of TiO2 too. However, studies on the biocompatibility of the fluoride-modified surface have contradictory and inconclusive results. A very interesting idea is to obtain a multifunctional coating in a co-doping procedure [31–33]. In the present study, we applied the possibility of incorporating more than one modifier into the sol solution in order to achieve a multifunctional biomedical coating with bioactive and antibacterial behavior, and what is the most important exhibiting corrosion protection capability towards biomedical stainless steels. We intended to combine anticorrosion properties of titanium dioxide with the bioactive e ffect of Ca ions and the potential antibacterial e ffect of Ag ions widely reported in the literature. In the literature, there are reports on TiO2 coatings co-doped with Ca and Ag ions [34,35], but those coatings were applied to titanium rather than steel substrates, and deposition methods other than sol-gel were used. However, since the method chosen for coating preparation or its modification determines the final properties as well as the applicability of the biomaterial, we have to remember that the improvement of a certain feature or function of a biomaterial as a result of surface modification may be accompanied by deterioration of other biomaterial features. Yetim et al. reported, for example, that, using plasma nitriding, it was possible to improve the wear resistance of AISI 316L steel, but the nitriding treatment did not bring the expected improvement in the corrosion resistance of the AISI 316L steel, it was even worse [36]. Junping et al. stated that the Ce-modified 316L steel exhibits the hormesis e ffect against *Staphylococcus aureus* (the higher the Ce content, the better the antibacterial e fficacy), but it is di fficult to simultaneously obtain good corrosion resistance, antibacterial performance, and processability [37]. Based on these literature examples, it is clear how important it is to control the impact of carried out modifications on corrosion resistance, especially in the case of biomaterials, as it determines their biocompatibility.

The aim of this study was to develop a multifunctional biomedical sol-gel coating that is highly corrosion resistant, biocompatible, and reveals the bioactive properties. For that purpose, titanium dioxide coatings doubly-doped with Ca and Ag ions were deposited by dip-coating onto M30NW biomedical steel, and subsequently annealed at 450 ◦C in air. Our previous studies have shown that under these thermal oxidation conditions an e ffective crystallization of titanium dioxide occurs, and formed anatase exhibits good corrosion protection ability in case of biomedical steels [21]. In the presented study, the influence of di fferent ratios of Ca and Ag dopants on morphology, surface structure, corrosion resistance, bioactivity, wettability, and biological properties of the TiO2-based coating was investigated.

#### **2. Materials and Methods**
