**1. Introduction**

Electrospun nanofibers have been used in various fields such as filtration, catalysis, clothing, and biomedical applications, because of their submicron size and high surface area, along with their porous architecture [1,2]. Especially for biomedical applications, electrospun mats provide the lightness in weight, porosity, flexibility in technique, as well as a good support for cells to attach and grow. Their capacity to exchange nutrients and gases makes them suitable for tissue engineering, wound dressing, drug delivery, health care, etc. [3]. A non-woven matrix composed of nanofibers is easily produced via electrospinning, and it is architecturally similar to the nanofibrous structure of extracellular matrix [4]. If necessary, the nanofibers can be further functionalized via the incorporation of bioactive species (e.g. enzymes, DNAs, and growth factors) to better control the proliferation and differentiation of cells

seeded on the scaffolds [5]. These attributes make electrospun nanofibers well-suited as scaffolds for tissue engineering.

Among various kinds of nanofibers, polyurethane nanofibers were selected as one of the most suitable choices for biomedical applications, thanks to the unique properties of polyurethane. Electrospun polyurethane nanofibers have been successfully used in wound dressing, due to an excellent oxygen permeability and barrier properties [6]. Water permeability is also important, as it keeps the wound moist and prevents the accumulation of fluid around the wound and on its cover. These covers perform a preventive function against infection with microorganisms, to absorb blood and wound fluids to contribute to the healing process, and in some cases, to apply medical treatment to the wound [7–9].

However, there are still several limitations and disadvantages of polyurethane nanofibers to be applied in the biomedical field, such as poor thermal capability, poor weatherability, and flammability. In order to improve the properties of polyurethane nanofibers, silicone groups were introduced into polyurethane polymer chains to fabricate silicone-modified polyurethane (PUSX) and to optimize the electrospinning parameters. In this study, PUSX nanofibers were evaluated by physical properties and cell culture studies, and they were compared with the films. The advantages of polyurethane, silicone, and nanofibers are very attractive for this work. This new material is expected to be applied in many fields as an improved alternative for polyurethane nanofibers, such as wound dressing and tissue engineering, due to the biocompatibility of silicone. Before going for the in vitro cell attachment and proliferation applications, all of the prepared nanofibers were analyzed in detail by various methods, and compared with films. To investigate the effects of different structure (block and graft type), chain lengths, and silicone concentration, evaluation of the physical properties was performed. Tensile tests were performed to investigate mechanical properties such as tensile strength, elongation at break, and Young's modulus. The water contact angle (WCA) measurement and water retention tests were carried out to determine the hydrophobicity of the PUSX material. The thermal conductivity was analyzed in order to discuss the heat retention ability of the PUSX nanofibers and films. In order to reveal the potential for cell adhesion and proliferation, NIH3T3 mouse embryonic fibroblasts cells were cultured on all the samples, followed by lactate dehydrogenase (LDH) activity. The toxicity of the PUSX nanofibers and films were evaluated by using direct contact based on ISO 10993-5. Therefore, as the purpose of our research, the influence of PUSX structures on the physical properties and biocompatibility is investigated. PUSX nanofibers might be expected as an ideal alternative for PU nanofibrous membranes or films in the biomedical fields.
