*2.2. Electrospinning Method of PUSX Nanofibers*

All the electrospinning solutions were prepared by diluting the PUSX solutions (30 wt%) in DMF/MEK mixed solvent (v/v = 64:36), and stirring at room temperature for 48 h in order to obtain homogeneous solutions. All electrospinning experiments were performed at room temperature (22 ◦C) under the optimized parameters of our previous study [10], and the deposited nanofibers were collected on a moving metallic collector. A 10–20 kV voltage was applied while the needle tip-to-collector distance was 10 cm with an irradiation angle of 30◦, and the air flow rate in the spinning environment was 0.1 mL/min.

The surface morphology of the nanofibers was investigated by scanning electron microscope (SEM, JSM-6010LA JEOL, Tokyo, Japan) at an accelerating voltage of 10 kV. Before SEM analysis, the prepared samples were coated by using a platinum sputter coater (Ion sputter JFC-1600 JEOL, Ltd, Tokyo, Japan) under 30 mA for 60 s. The diameters of the nanofibers were measured by ImageJ (National Institutes of Health, Bethesda, MD, USA). The average fiber diameters were calculated from data of at least 50 measurements per sample.

#### *2.3. Physical Properties*

Tensile tests were performed by a compact tabletop universal tensile tester (EZTest/EZ-S, Shimadzu Corporation, Kyoto, Japan) for samples 10 mm long and 5 mm wide, at a crosshead

speed of 10 mm/min. At least 10 specimens were tested for each sample. To compare the mechanical properties of the PUSX nanofibers with PUSX films, the same tests were performed on the PUSX films.

Thermal conductivities were determined by a KES-F7 Thermal LaboIIB precision rapid thermal property measurement unit (KES-F7 Thermal Labo, Kato Tech Co., Ltd, Kyoto, Japan). The temperature of the water box was set to room temperature. Samples (5 × 5 cm2) were then placed on the water box, and the heat plate of the bottom temperature box (B. T. box) was placed on the upper surface of the samples. After reaching a constant value, the heat flow loss W (watts) of the B.T. was recorded using a panel meter. Steady heat flow lost was calculated as the following equation:

$$\mathbf{W} = \mathbf{K} \cdot \mathbf{A} \cdot \triangle \mathbf{T} / \mathbf{D}$$

where D is the thickness of the samples (cm), A is the area of the B.T. heat plate (cm2), T is the temperature difference of sample (◦C), and K is the thermal conductivity. The thermal conductivity K was calculated by the following equation:

$$\mathbf{K} = \mathbf{W} \cdot \mathbf{D} / \mathbf{A} \triangle \mathbf{T} \text{ (W/cm } ^\circ \text{C)} = 100 \text{ W} \cdot \text{D} / \mathbf{A} \triangle \text{T(W/mK)}.$$

While taking measurements with the B. T. Box, the applied pressure could be adjusted. The standard value was set as 6 g/cm2. The temperature of the B. T. Box heat plate was controlled with an error of less than 0.1 ◦C.

Water retention tests were performed based on JIS L 1913 6.9.2. The electrospun PUSX nanofibers, and films of dimensions 100 × 100 mm2 were incubated in distilled water for a period of 15 min, and then weighed. Water retention capacity was determined as the increase in the weight of the fibers. The percentage of water absorption was calculated as in the following equation:

$$\mathbf{m} = (\mathbf{m}2 - \mathbf{m}1)/\mathbf{m}1 \times 100\%$$

where m2 and m1 are the weights of the samples in wet and dry environments, respectively.

WCA is an easy measurement for determining the wettability of the materials by a liquid. The static contact angle of pure water for the surfaces of the PUSX samples was measured by an automated contact angle meter (DM-501Hi, Kyowa Interface Science Co., Ltd, Saitama, Japan) after randomly dripping 2 μL of purified water on the surfaces of the samples. The droplets on the samples were captured after 1000 ms through an image analyzer, and the WCA, θ, was calculated by the software through analyzing the shape of the drop. When depositing a droplet onto the material, the water will form a droplet shape. The point where the surface, the water, and the vapor meet, is called the three-face point, and it determines the contact angle. The relationship between the contact angle, the surface free energy, the liquid surface tension, and the interfacial tension between solid and liquid is defined by the Young equation:

$$
\gamma \mathbf{S} = \gamma \mathbf{L} \cos \theta + \gamma \mathbf{S} \mathbf{L},
$$

where θ is the contact angle, γL is the surface free energy of the solid, and γSL is the interfacial tension between the solid and liquid.

Usually, when the WCA is less than 90◦, the material can be considered to be hydrophilic while the material is hydrophobic. It is worth mentioning that if the WCA is between 150◦ and 180◦, it shows the high water-repellency of the material.

#### *2.4. In Vitro Biocompatible Evaluation*
