**4. Conclusions**

In this research, the physical properties and the biocompatibility of PUSX nanofibers were discussed and compared with PUSX films. Tensile strength increased with the increase of silicone chain length, and decreased with the increase of silicone concentration. The elongation at break decreased with the increase of both the silicone chain length and the silicone concentration in block-type PUSX nanofibers, while the Young's modulus showed the entirely opposite trend. Higher hydrophobicity and lower thermal conductivity were found in PUSX nanofibers, compared with PU nanofibers and films. An in vitro biocompatible evaluation shows that PUSX nanofibers are able to be applied in the cell culture field, and that it shows very little toxicity. Compared with films, PUSX nanofibers shows better cell attachment over a short period of time. Therefore, electrospun PUSX nanofibers with highly controllable physical properties can be expected as ideal alternatives to PU membranes applications of the biomedical field, such as for wound dressings and tissue engineering.

**Author Contributions:** Conceptualization, H.H., M.T., H.S., S.I., C.Y. and T.T.; Resources, H.H., M.T., H.S., and S.I.; Investigation, C.Y., S.R., R.O. and M.K.; Formal analysis, C.Y., S.R., R.O. and M.K.; Data curation, C.Y., H.S., H.H. and T.T.; Supervision, Y.T. and T.T.; Writing—original draft, C.Y.; Writing—review & editing, C.Y. and T.T.; Funding acquisition, C.Y. and T.T.

**Funding:** This work, including APC, was supported by a Grant-in-Aid for the Shinshu University Advanced Leading Graduate Program by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

**Acknowledgments:** The authors gratefully acknowledge financial support from Grant-in-Aid for the Shinshu University Advanced Leading Graduate Program by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. In addition, the authors would like to expressly thank Shin-Etsu Chemical Co., Ltd (Tokyo, Japan) and Dainichiseika Color & Chemicals Mfg. Co., Ltd (Tokyo, Japan) for kindly providing the materials.

**Conflicts of Interest:** The authors declare no conflict of interest.
