## 4 Discussion
The elasticity of the leaflets ensures minimal energy losses during deformation of the heart valve under the pulsatile blood flow. Therefore, elastic and deformation properties should be considered as one of the main parameters in the decision-making process. Rigidity and elasticity can vary depending on the mass fraction of styrene in the polymer. An increase in rigidity and decrease in elongation is associated with increased styrene rate, and vice versa [46]. SIBS synthesized in this study demonstrated promising results in terms of rigidity. However, its structure may be altered in case elasticity may require the further optimization. Hydrophobic surfaces are less hemocompatible, since they have a greater tendency to irreversible adsorption of proteins, including the resultant conformational changes, unlike hydrophilic surfaces, which minimally affect the conformation of proteins [40]. Hydrophilicity/hydrophobicity determines the subsequent adhesion of blood cells and cell proliferation [47]. However, there is some evidence on using of hydrophobic biomaterials in cardiovascular implants, particularly Gore-texTM in artificial  
Figure 9: ABA block copolymers of isobutylene and styrene, structural formula. heart valve leaflets, in clinical practice [48]. Our experimental data demonstrated the superiority of SIBS over Gore-texTM suggesting the former beneficial potential for the application in the clinical practice. Our results are also consistent with the recent studies [49]. Since both types of blocks are hydrophobic, hydrophilicity of the SIBS surface may be increased by adding hydrophilic groups (for example, hydroxyl) or by surface modification [49]. At the same time, the need for such modification may be confirmed by the results of hemocompatibility tests, since the process of contacting with blood is known to be a multifactorial one. The surface properties of the implanted material may also affect its hemocompatibility. Smoother surfaces with regular structures are more biocompatible due to their ability to reduce the severity of the inflammatory process and the adhesion of blood cells [50; 51]. SIBS demonstrated rather good biostability since there are no gas inclusions accelerating its biodegradation. The more pronounced porosity of the Gore-texTM sample and its previously reported biocompatibility as compared to the SIBS sample suggest the superior potential of the last according to the surface relief. We might conclude that cast molding with chloroform as a solvent for manufacturing SIBS heart valve prostheses is comparable to THF casting and melting, the well-described methods in the literature [52]. High hemocompatibility of SIBS and Gore-texTM was confirmed by cytotoxicity testing using HUVEC cell line. The preceding experiments with the L929 cell line (mouse fibroblasts) [53] also showed high cell viability on the Kaneka SIBS material. However, we used the HUVEC cell line as it suited more to the objectives of this study. The cell proliferative potential index can complement the functional activity of cells in short-term experiments, which are not able to demonstrate the full range of changes in cell viability [54]. The ability of cell adhesion and proliferation is critical for tissue-engineered heart valves, a novel trend that has recently emerged. Taking into account the obtained results, we concluded that SIBS has superior potential for the further use in the development of tissue-engineered structures unlike Gore-texTM. Moreover, SIBS has higher capacity for spontaneous endothelium. Recent literature data suggest that the presence of the methyl groups in the polymer compound structure does not contribute to the proliferation of cells adhered on the material surface [55]. Dense fibrous (connective tissue) capsule formation around the SIBS and Gore-texTM samples indicates the completion of the inflammatory process following the 60-day implantation of the biostable materials. However, the fibrous capsule around the SIBS sample was thicker than that for the Gore-texTM sample.