*3.1. Fibers Characterization*

Prior to the cell culture study, the quality and morphology of the produced electrospun fibers were verified with SEM; see Figure 2. The average fiber diameter for PS was 4.62 ± 0.3 μm (Figure 2C) and 0.101 ± 0.018 μm for PA6 (Figure 2D). The size of PS fibers was similar in the fibrous composite. However, the average fiber diameter of PA6 fibers was increased to 0.145 ± 0.030 μm, due to the slight adjustment of the electrospinning parameters; see Table 1 and Figure 2G,H, as previously described [44]. The water contact angle measurement confirmed PA6 hydrophilicity (45.9 ± 4.9◦) and PS (139.7 ± 4.7◦) hydrophobicity and also showed the hydrophobic character of produced composite (132.8 ± 3.5◦); see Figure 2. According to the previous study, the larger the average fiber diameter, the higher the surface roughness (Ra), which reached 15.535 ± 2.197 μm for PS, 0.205 ± 0.222 μm for PA6, and 8.848 ± 0.960 μm for PS-PA6 [38]. Additionally, Ra strongly influences the wetting behavior of electrospun membranes [31]. Here, the hydrophobic character was obtained for PS-PA6 composite mesh mainly due to the roughness effect [44], and the water droplets still kept the contact points with PS fibers. The presence of PS and PA6 fibers was already confirmed by the X-ray photoelectron microscopy analysis (XPS) reported in our studies previously [38], where also the roughness and water contact angle were investigated according

to PA6 content. Based on the reported data in [38], the increased PA6 fraction of nanofibers that usually forms a compact layer of membrane lowers the surface roughness once combined with PS microfibers. The increase in PA6 nanofibers fraction, controlled with a longer electrospinning time, decreased the roughness and water contact angle only slightly. Importantly, the PA6 meshes are characterized by relatively small pore sizes of 1.7 μm, and a very high porosity of 96% in meshes [45]. In addition to the morphology, the mechanical properties of manufactured PS-PA6 hierarchical composite meshes were also investigated in various configurations, showing higher tensile stress for PA6 (1.24 MPa) than for PS (0.3 MPa) fibers. The incorporation of PA6 fibers into PS meshes significantly improved the mechanical properties of composite meshes reaching 0.6 MPa [38].

**Figure 2.** SEM micrographs of electrospun fibers of (**A**) PS, (**B**) PA6, (**E**) PS-PA6 composites, and (**F**) PS-PA6 composite with the higher magnification showing a few PA6 nanofibers on the individual PS microfiber. The fiber diameter distribution showed in histograms for (**C**) -PS; (**D**) -PA6, (**G**) -PS in the composite, and (**H**) -PA6 in the composite meshes.
