## 4 Discussion  
An atmospheric pressure plasma jet was used for the polymerization of liquid-phase HEMA to generate acrylate-based coatings on titanium substrates. The chemical compositions, wetting properties and stability of the coatings deposited at different process conditions were investigated. The coatings obtained showed hydrophilic surface properties with a similar chemical composition when compared to the original precursor. The coatings'  
Figure 11: Adhesion of hFFs on the plasma-polymerized, HEMA-coated samples. For each time point, a indicates statistically significant differences compared with Ti, b compared with S1, S2 and S3 and c compared with S2 and S3 (_p_ < 0.05). Figure 12: _S. aureus_ (**a**) and _E. coli_ (**b**) adhesion on plasma-polymerized HEMA samples. \(a\) indicates statistically significant differences compared with Ti in the _S. aureus_ assay, and \(b\) indicates statistically significant differences compared with Ti in the _E. coli_ assay. (_p_ < 0.05). surface free energy was more than twofold higher than that of pristine titanium, resulting in an improved wettability. Similar results were found in previous studies for PHEMA coatings [60], with a higher polar component and a lower dispersive component. This fact indicates that the coatings presented here had a higher amount of polar groups when compared with other methods employed to synthesize HEMA-based polymer films. The higher wettability enhances the suitability of the coating for biomedical applications [61; 62]. Regarding the chemical analysis by ATR-FTIR, the main peaks of a PHEMA coating could be detected [63]. The elemental compositions of the coatings, determined by XPS, are comparable to the one reported by other authors for PHEMA coatings, obtained by plasma polymerization [64; 65], and they are similar to the theoretical one of the precursor (67% carbon, 33% oxygen). However, the stoichiometry of the different carbon-oxygen bonds is slightly different to the theoretical one, indicating that some changes on these functionalities might have occurred during the plasma treatment. The stability studies showed that the coating was stable after being stored in air and immersed in water. This can be associated with the formation of a chemical bond between the polymer and the substrate during the plasma treatment. No toxicity was observed in the in vitro cytotoxicity assay, with a cell viability above 80% for all samples. It has been reported that HEMA exerts cell toxicity via apoptosis, among other effects [66; 67; 68]. In contrast, its polymerized counterpart, PHEMA, is biocompatible and has no toxic effects [69; 70]. From the results obtained in this study, it can be concluded that the HEMA precursor was polymerized during the plasma treatment, at least to the extent that the toxicity effect was not observed. Moreover, an indicator of the stability of the coating can also be extracted from these results, since no changes in the cell viability were observed after its immersion in a cell culture medium for 72 h.  
The hFF adhesion on the HEMA-based coatings was higher than the control, indicating the possibility of better tissue integration around the material. Even though PHEMA coatings have been reported to be antifouling and reduce the protein adsorption and cell adhesion [65], there are also reports regarding the ability of such coatings to allow cell adhesion [60]. Moreover, some modifications of the coatings, such as UV cross-linking, can impart chemical functionalities which enhance cell adhesion [71]. In the case of fibroblasts, a good adhesion brings forward the possibility of optimum bioesaling around the implant, which further avoids bacterial infiltration and infection [72; 73]. The sealing has been also observed to be promoted by the lower bacterial adhesion, both of _E. coli_ and _S. aureus_, in similar coatings [35; 74; 75]. This is due to the water retention of the coating, which in turn produces an antifouling effect, reducing the number of bacteria adhered.