## 4 Discussion  
The development of polymeric heart leaflets requires the use of polymers with superior hemocompatible properties as well as those yielding the best physical and mechanical properties. Molecular and supramolecular architectures are of key importance and drive the choice of the proper polymer. All these requirements encourage the researchers to use co-polymeric materials and composite materials, particularly in biomedicine, due to their unique properties and the ability to combine structural fragments of various functional groups [13; 39]. A particular attention should be paid to styrene and isobutylene block copolymers as promising materials since they are thermoplastic,  
Figure 8: Platelet adhesion on the surface of polymer matrices made from (**A**,**D**) polyethylene, (**B**,**E**) SIBS, and (**C**,**F**) Gore-tex(tm). elastic, possess high strength, and are resistant to hydrolytic and oxidative effects due to the carbon chain, biological inertness of the backbone and side chains (Figure 9) [40]. There are three research groups developing flexible polymeric heart valves and leaflets from original or modified SIBS (i.e., with reinforced structure, added molecules, or following phospholipid modification) [30; 33; 41]. However, the existing commercial SIBS has some specific disadvantages, including viscoelastic creeping and a tendency to thrombi formation [31; 42]. In order to meet the demands of cardiovascular surgery, it should possess unique properties [43]. By combining structural fragments of various types, we may obtain polymers with given patterns materials, i.e., hemocompatibility, biostability, increased strength, and optimal elasticity as compared with homopolymers [44]. We used the cationic polymerization method to synthesize SIBS with the preferred properties. This is the only method ensuring a simple introduction of additional units into the structure of the polymer molecule that can drastically improve the properties of the resultant material [45]. First, we analyzed the molecular weight characteristics of the polymer produced under the given conditions to assess the optimal parameters for the synthesis of a new molecule. Then, we synthesized the polymer with the number average molecular weight (M\({}_{\text{n}}\)) of 33,000 and polydispersity (M\({}_{\text{w}}\)/M\({}_{\text{n}}\)) of 1.3 (Figure 1). Gore-texTM was chosen as a control because it is currently applied in the real clinical practice when compared with other polymers undergoing experimental testing. The mechanical properties of each block copolymer may vary depending on its molecular weight, the percentage of the constituent blocks, and the sequence of the polymer chain [46]. The strength of SIBS was lower than that of Gore-texTM. Despite the superior tensile strength of SIBS that exceeds the tensile strength of the native tissues (heart and vessels), it is worth considering that synthetic materials are not capable of regeneration. As a result, the accumulated fatigue stress can lead to the destruction of the material structure. It means that an optimal material should possess a significant margin of safety [36]. Due to this, the majority of researchers used SIBS polymers with M\({}_{\text{n}}\) of 60,000 or even more [36; 46] with the ultimate tensile strength (MPa) of 10-30 and the ultimate elongation of 300-1100% [31]. However, a separate study covering this issue is required to enable the choice of the optimal molecular weight. The elasticity modulus in the physiological range of loading is considered as one of the key properties of the mechanical behavior of the material. It determines the suitability of the selected material to be used for developing artificial heart valves. More rigid polymers have a lower tendency to accumulate fatigue stress, but high rigidity of the leaflets may lead to significant hemodynamic alterations [37]. In our experiment SIBS demonstrated moderate rigidity between the biological material [38] and Gore-texTM.