## 4 Discussion
The obtained results were consistent with the results of other experiments [52]. Therefore, the molecular weight of the polymer and the structural quantitative composition of the block copolymer did not affect the biocompatibility of the polymer in the short-term experiments. Connective tissue is formed around the artificial heart valve leaflets creating a protective barrier, separating foreign material from the body environment. Moreover, it ensures a tight connection with the aortic root to prevent paravalvular regurgitation and secure the device fixation. Foreign body giant cells visualized in the Gore-texTM and GA-treated xenopericardium samples may secrete reactive oxygen species and other chemical agents, contributing to oxidative damage and destruction of the implanted devices [56]. There were no such cells found in the SIBS sample 60 days after the implantation. The biocompatibility of SIBS most likely is related to a thin (10 nm) layer of pure polyisobutylene covering the polymer surface contacting with biological tissues, while the polystyrene phases are buried within the material [57]. High biocompatibility of SIBS and Gore-texTM has recently been confirmed in the experimental and clinical studies [48; 58]. Calcification is the principal cause leading to stenosis of native and tissue aortic valves [6]. Polymers, unlike biological tissue, do not contain phosphorus-rich cellular garbage and destroyed collagen provoking the mineralization process. Therefore, they are more resistant to calcification. Our results demonstrated the absence of calcium crystals provoking calcification in the studied polymer samples. The results of the quantitative analysis of calcium content in the in vivo model reported higher calcium content in GA-treated xenopericardium (the positive control) as compared to the polymer materials. Our finding was consistent with the literature data [59]. Gore-texTM demonstrated the presence of calcite deposits at the interface between the fibrous capsule and the polymer, while SIBS had isolated calcium crystals within the connective tissue capsule [60]. The obtained resultssuggest superiority of SIBS to Gore-tex(tm) in terms of minimal calcification rate, stating the benefits of the former for manufacturing synthetic heart valve prosthesis. The evaluation of the level of RBC hemolysis induced by SIBS demonstrated the absence of toxic effects on RBCs. Gore-tex(tm), being the control sample, did not cause the destruction of RBC membrane. The level of RBC hemolysis after direct contact with polyethylene was 1.82%, suggesting its thrombogenicity. The obtained data are consistent with the results of other studies on the impact of SIBS and Gore-tex(tm) on RBC hemolysis. The level of RBC hemolysis after contact with Gore-tex(tm) was also significantly lower than those for the positive control with 100% hemolysis [41; 61]. The cationic polymerization method used to synthesize the block copolymer ensures the safety of the produced medical materials as it provides a high degree of monomer conversion. Moreover, none of the solvents and catalytic residues were identified in the resultant material after isolation procedure. Platelet adhesion is the initial stage of blood clot formation. Importantly, adhesion alone is not enough to trigger the thrombogenic cascade. Activated platelets are able to release substances into the blood that can lead to irreversible aggregation. The adhesion of platelets with a regular round shape that have not yet been deformed, does not carry a thrombogenic hazard, since the adsorption of platelets type I is reversible and they can be easily returned to the bloodstream [62]. Our results on the assessment of adhesion properties showed that platelets adhere to all polymers, regardless of their type.