3.5.2. Toxicity Evaluation

The results as displayed in Figure 11 showed that both the PUSX films and nanofibers caused less toxicity when they were in contact with cells. The cells were able to keep a spreading shape, and discrete intra-cytoplasmic granules with no cell lysis, which can be considered as a survival mechanism compared with the positive control (cells became round and layers were completely destroyed). The morphological grade of the cytotoxicity is supposed to be 0.

There was no obvious difference between the PUSX and PU materials, because of the biocompatibility of silicone. To the best of our knowledge, silicone has been extensively used in medical areas, in several products such as breast implants, contact lenses, lubricants, sealers, artificial cardiac tubes and valves, urethral and venous catheters, membranes for blood oxygenation, dialysis tubes, orthopedic applications, and facial reconstructions, because of its high biocompatibility [12,13]. The existence of the silicone group does not change the biocompatibility of the material. From these results, the PUSX materials showed a suitable biocompatibility for use as biomedical materials, such as waterproof bandages or scaffolds for tissue engineering.

Therefore, our novel nanofibrous membranes favored fibroblast cell attachment and growth by providing a stereo-structure environment that mimics the ECM, and which is considered to be biofriendly after toxicity evaluation. Indeed, there are a large amount of studies for developing biomedical materials for wound dressing and tissue engineering. For instance, Fenghua Xu et al. suggested tannic acid/chitosan/pullulan composite nanofibers, which showed synergistic antibacterial activity and the potential for deep and intricate wound healing. Compared with these materials, PUSX nanofibers show the controllable tensile strength of PU, which suggests potential applications for long-term tissue engineering, and hydrophobicity that is influenced by the silicone groups. As we know, wound infection is one of the main areas of concern in the management of the wound environment. Infection complicates treatment, and it impedes the healing process by damaging tissue, reducing wound tensile strength and inducing an undesirable inflammatory response [14–16]. PUSX nanofiber wound dressing may provide a hydrophobic effect, which helps to control bacteria by adsorbing bacteria onto the dressing surface. These results provide insight for the potential use of PUSX nanofibers for wound healing and tissue engineering into clinical practice in the future.

**Figure 11.** The toxicity evaluation of PU and PUSX samples. PU nanofibers (**a**) and films (**b**), Si01 nanofibers (**c**) and films (**d**), Si01-59 nanofibers (**e**) and films (**f**), Si04 nanofibers (**g**) and films (**h**), Si08 nanofibers (**i**) and films (**j**), compared to the negative control (**k**) and the positive control (**l**). (Magnification: 40×).

In the present study, the physical properties and biocompatible properties of PUSX nanofibers were investigated and compared with films. As a conclusion, for all of the analyses, physical properties, mechanical properties, water retention, and water contact angle (WCA) can be controlled and improved by adjusting the structure. Unfortunately, the graft-type PUSX did not show obvious changes in mechanical strength, because the side chains of silicone could not have as much influence as the main chain of PU does. Conversely, the graft-type PUSX nanofibers were the most similar alternatives to PU nanofibers in terms of mechanical properties, but with better water repellency according to the water contact angle results. Higher hydrophobicity and lower thermal conductivity were also found in the PUSX nanofibers, due to the unique advantages of nanofibers compared with films. This material can be expected to be applied in various fields. For instance, by controlling the silicone chain length and the concentration of block-type PUSX nanofibers, it can be applied in the medical field as bandages or scaffolds, the apparel field as outdoor goods and sportswear, and it can also be used for air or water filters.

In vitro biocompatible evaluation shows that cell proliferation can be performed on both the PUSX nanofibers and films. However, for cell attachment, the cells are not able to attach onto the PUSX films in a short time, nor entangle in the material, PUSX nanofibers were proven to be more appropriate for cell culture study. As we know, PU nanofibers have been developed as biomedical materials for several years. For instance, Lakshmi R. Lakshman et al. [17] and Afeesh R.Unnithan et al. [18] reported wound-dressing materials with antibacterial activity, which provided a basic understanding of the design for efficient PU nanofiber-based antibacterial wound dressing materials. Chang Hun Lee et al. [19] and Rui Chen et al. [20] demonstrated that electrospun PU nanofibers had the characteristics of a native extracellular matrix, and they may be used effectively as an alternative material for tissue engineering and functional biomaterials.
