**1. Introduction**

The vastly growing field of regenerative medicine is continuously looking for new materials and novel ways to improve currently used materials for cell culture studies. Many studies showed the importance of cells–materials interactions and how to modify the surface to enhance cell adhesion and proliferation, which are responsible for tissue growth [1–3]. In regenerative medicine, one of the most frequently used polymers is polystyrene (PS), which is hydrophobic and thus, often, its surface needs modification by entering hydroxyl groups to achieve hydrophilic behavior [4]. This modified PS is a so-called tissue culture polystyrene (TCPS) that enables easy cell attachment and proliferation. Therefore, it is widely used in cell culture experiments in the form of a flat surface [5,6]. Films, which are 2D structures, are mostly applied for in vitro tests, whereas for tissue engineering, 3D porous constructs are more preferable for cell development. A variety of methods can be applied for 3D meshes manufacturing [7]. Those produced via electrospinning have a structure with high porosity and a high surface area to volume ratio [8,9]. The porosity can be controlled via fiber size governed by electrospinning parameters, as well as the application of various collectors [10]. This technique allows the fabrication of both random and aligned fibers for different applications with a wide range of sizes from nano to micrometers, which influence different cell behavior on manufactured material [7,11–13]. Aligned fibers fabricated via electrospinning were applied to build the ligament tissue based on the hierarchical structure [14]. A combination of nano with micro electrospun fibers in the scaffold was

found to be a promising material for bone tissue regeneration [15]. A two-nozzle electrospinning set-up [16] can be used to obtain composite structures [17] made of nano and microfibers [18].

Polyamides are commercially used as surgical struts [19], in many cardiovascular applications [20], and also for the production of artificial tendons, ligaments, joints [21] and inguinal meshes [22]. PS with nylon 6 (PA6) is known for its high mechanical strength, biocompatibility, flexibility, and similarity to the peptides concerning amide bonds. Electrospun PA6 fibers were blended with other polymer fibers [23,24], which resulted in increased cell proliferation [25,26] applied in the wound and burn treatment [27]. Additionally, the wetting behavior of PA6 fibers can be controlled via electrospinning itself [28,29], and the wetting properties of a material are crucial factors in biomedical applications. Both the chemistry in the meaning of hydrophobicity or hydrophilicity and roughness influence surface wettability [30,31].

PS is mostly used for in vitro studies [4,5,32], but without any surface modification, for example, sliver negative ion implementation [33], protein absorption [34], or plasma treatment [35,36], does not enhance cell development. PS fibers have been already combined with PA6 fibers [37] in the fog collector's meshes, comparing fiber diameter, roughness, contact angle, and the showing mechanical properties of PS, PA6, and PS-PA6 mats of maximum stress 0.03, 1.24 and 0.07 MPa, respectively [38]. Similar designs of nondegradable polymers have the potential to be used in vascular tissue engineering [39] or hernia meshes [40,41]. Moreover, electrospinning was often used to produce highly porous materials with controlled morphology and mechanical properties for vascular grafts [42].

Therefore, the goal of this study was to electrospin hierarchical constructs containing PS in the form of microfibers with the addition of hydrophilic PA6 nanofibers to produce 3D structures. To fabricate our meshes, we applied a two-nozzle system to electrospun both polymers at the same time, aiming towards the biomimetic extracellular matrix (ECM) in terms of wetting and roughness [43]. Importantly, we showed that the chemical or oxidation modifications of hydrophobic PS modification can be replaced by adding hydrophilic PA6 nanofibers into meshes. The engineered hierarchical and fibers-based composite meshes can be applied in regenerative medicine to control cell behavior and to firmly integrate with living tissue.

#### **2. Materials and Methods**
