**4. Conclusions**

The roles of SF on characteristics of HS-IPN hydrogels were examined. The schematic procedures for preparing HS-IPN hydrogels were presented in Scheme 1. The pore size of the model hydrogels was 38.96 ± 5.05 μm between those of H and S hydrogels (Figure 2). The swelling ratios for HS-IPN hydrogels (e.g., 185.9 ± 24.4%, *n* = 3) were similar to those for H/PEI hydrogels (Figure 3) which were much less than those for HA hydrogels. The hydrogels composed of SF of strain A showed higher G and G" values than those composed of SF of strain B and C (Tables 1 and 2, respectively). Moreover, the model hydrogels consisted of the highest weight ratios of SF to HA showed significantly higher G, G" and |G\*|values than other hydrogels consisted of those of low weight ratios. (Figure 4A). The model hydrogel was biocompatible for TE applications (Figure 5). Moreover, the model hydrogels significantly promoted the differentiations of hBMSC to NP cells with increasing the expressions of GAG and collagen type II for 7 d and 14 d (Figure 6). Moreover, the induced hBMSC in the hydrogels increased the gene expressions of AGN and COL II while decreased those of COL I for 7 d of cultivations (Figure 7). Hence, the model hydrogels developed herein were suitable to tissue engineering for NP regeneration.

**Author Contributions:** Conceptualization, T.-W.C., W.-P.C.; methodology, T.-W.C., P.-W.T., T.-Y.W.; validation, P.-W.T., T.-Y.W.; investigation, T.-Y.W., H.-Y.L.; data curation, P.-W.T., H.-Y.L.; writing—original draft preparation, T.-W.C., W.-P.C.; writing—review and editing, T.-W.C., W.-P.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ministry of Science and Technology of the ROC, Taiwan, for financially supporting this research under Contracts Nos. MOST 104-2221-E010-004-MY3 and 106-2221-E-027-029-MY3.

**Acknowledgments:** The authors would like to thank the Miaoli District Agricultural Research and Extension Station, Council of Agriculture, Executive Yuan, Taiwan for providing silk fibroin.

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