## 3 Results and Discussion
### Cell Viability on Nanocomposites  
Fibroblasts are one of the important types of heart cells. They are responsible for the production of proteins that make up the extracellular matrix, which supports all other cells in the heart and is responsible for the elasticity of the heart tissue, as well as the ability to withstand stress. Fibroblasts express a wide range of growth factors and cytokines [57]. A number of studies indicate the possibility of electromechanical interaction of fibroblasts with neighboring myocytes, which can potentially be used for therapeutic purposes [58]. Thus, the compatibility of the cardiac tissue regeneration material with fibroblasts is an important requirement. In this regard, a comparative analysis of the viability of heart fibroblasts in nanocomposites based on SWCNT1 (0.001/0.01 wt.%) and SWCNT2 (0.001/0.01 wt.%) with control was carried out. The nanocomposites were formed in layers of a flat shape on a cover slip. Next, MTT assay and fluorescence microscopy of fibroblasts were performed (Figure 7).  
Figure 6: Specific electrical conductivity of nanocomposites.  
Figure 7: Diagram of MTT assay and fluorescence microscopy images of heart fibroblasts on nanocomposites based on SWCNT1 (0.001 wt.%) (**a**) SWCNT1 (0.01 wt.%) (**b**) SWCNT2 (0.001 wt.%) (**c**) and SWCNT2 (0.01 wt.%) (**d**) relative to control (cover slip) (**e**) after 72 h of incubation.  
After 72 h of incubation, the following results were obtained. The best fibroblast survival was observed on SWCNT1 nanocomposites (0.001 wt.%). The increase in concentration led to a decrease in the viability of fibroblasts. For both samples, the viability of fibroblasts was 14 and 6% higher than on the control. The situation with nanocomposites based on SWCNT2 (0.001/0.01 wt.%) was different. Cell viability decreased compared to samples from SWCNT1, but for SWCNT2 (0.001 wt.%), it exceeded by 4% than in control. The viability of fibroblasts on SWCNT2 nanocomposites with an increased concentration of 0.01 wt.% was at the control level, and in some cases lower. The high viability on the SWCNT1 samples can be explained by highest purity of this type tubes and their homogeneous distribution over the volume of nanocomposites. The images of fluorescence microscopy show that the cells on the nanocomposites were inhomogeneously distributed in height and filled the entire porous volume of the nanocomposites after 72 h. Therefore, some fibroblasts were not in focus of the objective of the fluorescence microscope. The cells on the control were evenly distributed over the coverslip area and, accordingly, in height. Therefore, their fluorescence image was clearer with visualization of nuclei. Fibroblasts on nanocomposites formed groups (Figure 7a-d). The cells were close to each other in these groups. The control fibroblasts were at some distance from each other. The close arrangement of cells in groups promotes the formation of tissue monolayer, as seen in Figure 7a. The cells were structured in rows and were ready to form a tissue monolayer on the SWCNT1 sample (0.001 wt.%). Fluorescence microscopy images confirmed the results of MTT assay. Thus, using MTT assay and fluorescence microscopy, the compatibility of cardiac fibroblasts with nanocomposites based on SWCNT1 (0.001/0.01 wt.%) and SWCNT2 (0.001/0.01 wt.%) was demonstrated.