## 3 Results
### Surface Characterization
#### 3.1.3 Coating Stability  
The stability of the coating was assessed by immersing the samples in water for 48 h. In particular, water contact angle measurements were conducted on the as-deposited coatings and on coatings after being immersed in water (Figure 5). It was found that all the analyzed coatings exhibited lower water contact angles when compared with the pristine Ti. Furthermore, an increase of the water contact angle was observed for all films after immersion in water. This was most likely due to the immersion in water, during which fragments of the non-polymerized HEMA oligomers were removed or, depending of the stability of the coating, thickness losses occurred. Hence, XPS analysis was conducted to study the chemical composition of the HEMA-based polymer films, depending on storage in air or in water for 24 h and 48 h, respectively, as well as after 5 min of ultrasonic treatment in water (Figure 6). The elemental composition was determined on three different spots in the center of the coating. While the coatings stored in water showed no differences compared to the as-deposited coating, the storage in water caused a decrease of the carbon signal and an increase of the oxygen signal. Figure 4: Line scan of the atomic percent concentrations of all elements identified on the sample surfaces S1 (**A**), S2 (**B**), S3 (**C**) and S4 (**D**). The dashed lines indicate the stoichiometric composition of Poly(2-hydroxyethyl methacrylate (PHEMA). The inset in Figure 4a presents a photograph of the generated coating. The yellow area highlights the center of the coating. High-resolution C 1s spectra of coatings as deposited and after storage in air or deionized water for 24 h and 48 h respectively, were fitted with several peaks associated with different chemical species (Figure 7). For the as-deposited film and those stored in air, four componets were assumed, in particular hydrocarbon (i.e., sp\({}^{3}\), C-H,C (B.E. = 285.0 eV)), the ether group C-O (B.E. = 286.6 eV), the carbonyl group C=O (B.E. = 287.8 eV) and the ester group O-C=O (B.E. = 289.2 eV). The additional peak at B.E. = 285.7 eV was assigned to the secondary (\(\beta\)) chemical shift produced by the ester carbon (4.2 eV primary chemical shift) [58]. In contrast, for the HEMA-based polymer films stored in water, only three functional components were identified which were attributed to hydrocarbon, ether and ester carbon species. Furthermore, the C 1s spectra showed an increased intensity of the oxygen-containing components when immersed in water. Quantification data derived from the high resolution C 1s spectra are shown in Figure 8. Compared with the values reported in the literature [59], the films immersed in water proved that the poly(HEMA) structure can be retained even after plasma polymerization. Figure 5: Water contact angle of the coatings as-deposited and after 48 h of immersion in deionized water. Figure 6: Elemental composition of the plasma-polymerized HEMA coatings as deposited after 5 min of ultrasonic treatment (USB) and of the coatings being stored in air or water for 24 h and 48 h, respectively. To investigate the elemental and chemical distributions at greater depths, destructive XPS depth profiling was performed in the center of the coating by sputtering with a large Ar cluster ion beam. In this approach, Ar cluster ions (10 keV beam energy) were used to etch a few nanometers of the sample surface. The elemental depth profiles were determined by sequential measurements of the elemental intensities after each sputter cycle. In Figure 9A, the XPS depth profile of the HEMA-based film, deposited on titanium, is depicted. It was found that the elemental composition near the surface and in the bulk of the coating was comparable. Furthermore, high-resolution C 1s spectra were recorded at regular intervals during depth profiling, shown in Figure 9B.