## 3 Results and Discussion
The polar part of the energy (dipole interactions and the energy of hydrogen bonds) decreases as Ar/C\({}_{2}\)H\({}_{2}\) treatment time grows. However, argon post-treatment boosts the polar component at the interval of 30-60 s of Ar/C\({}_{2}\)H\({}_{2}\) treatment. Raman spectroscopy (Figure 4c) reveals changes in the intensity of benzene ring modes (1200, 1258, 1625, 2876, 2936 cm\({}^{-1}\)), N-C-N (1318 cm\({}^{-1}\)) [29], and CH\({}_{2}\) (2876 and 2936 cm\({}^{-1}\)) structures of the polyurethane. Peaks in the 1400-1600 and 2800 cm\({}^{-1}\) regions indicate the presence of amorphous carbon film [30; 31]. Post-treatment by argon partially destructs the formed chemical structures (the intensity of the corresponding peaks is decreased). A more detailed analysis of the chemical structure of the surface is hampered by the complex structure of the pristine polyurethane and the heterogeneity of the nanocoating. Figure 3: Thickness (**a**) and ratio of elastic modulus of hard/soft areas (**b**) of the obtained coatings. Zero treatment time corresponds to the argon plasma-activated polyurethane surface. The inset in (**a**) shows a fragment of relief with circled indentation imprints whose depth was used for the thickness estimation. The sorption of proteins onto the surface nonlinearly depends on the combination of wettability [32], free surface energy (in particular, albumin is sensitive to the polar energy component [33]), and roughness [34] (the roughness affects the available area, but may also change the conformation of protein molecules). In our case, albumin sorption was positively affected by the argon post-treatment of Ar/C\({}_{2}\)H\({}_{2}\) coatings (Figure 5). The reasons for this are the wettability as well as the polar component of the free surface energy. Local physical and chemical heterogeneities of pristine polyurethane, as well as after Ar/C\({}_{2}\)H\({}_{2}\), hamper the uniform adhesion of albumin on the surface--the nanostructures of protein films have heterogeneities, but argon post-treatment makes the protein layer smooth and uniform (see images in Figure 5). Micro- and nanocrystals are visible on uniaxially deformed (50% of elongation) surfaces after certain cases of treatment (Figure 6). The study of untreated stretched polyurethane, as well as after argon activation, did not reveal any surface defects (these images are not presented). Figure 4: Wetting contact angle (**a**), free surface energy (**b**) and Raman spectroscopy (**c**). Figure 5: Sorption activity of albumin to the surfaces. The insets show AFM images of surfaces with albumin. Figure 6: Representative optical images (70 \(\times\) 45 \(\upmu\)m) and AFM images of surfaces stretched to 50%: treatment in Ar/C\({}_{2}\)H\({}_{2}\) plasma for 30 s (**a**), 60 s (**b**), 120 s (**c**) and after subsequent argon treatment (**d–f**). The tensile axis is vertical. Only one material (60 s of Ar/C\({}_{2}\)H\({}_{2}\) treatment, Figure 6b) has no strain-induced cracks at all. This can be explained by the presence of pronounced stiff and soft areas on this surface (Figure 3b); such stiffness mismatches similarly to filler in a polymeric composite and redistributes a load. The rest of the cases show nanocracks (Figure 6a) accompanied by individual microcracks (Figure 6d,e) or a network of microcracks (Figure 6c-f). The increase in the density of cracks is associated with a rise in the elastic modulus of the coatings, as well as with local mechanical inhomogeneities (Figures 2 and 3b). Crack resistance falls slightly after argon post-treatment (the number of cracks and their sizes become higher) due to the elevated stiffness and uniformity of the coatings. The formation of cracks may lead to undesirable consequences during long-term operation of the material under real conditions. This phenomenon requires a separate study.