Figure 3.

(A) Reversible hydrogel–solution transition of silk nanofiber-based system and their nanostructure changes through ultrasonic-culturing treatment cycle. The samples were as follows: (a) untreated 2 wt % hydrogels or the recycled hydrogels after ultrasonic-culturing process after 7 days (similar nanofiber structures were achieved again after the cycle, so the typical image is shown in a); (b) hydrogels after ultrasonic treament for 10 min with the intensity of 100 W; (c) transformed solutions after ultrasonic treatment for 10 min with the intensity of 200 W; (d) transformed solutions after ultrasonic treatment for 10 min with the intensity of 500 W; (e) solutions after the sample in panel d was cultured at 60 °C for 1 day; (f) reversed hydrogels after the sample in panel d was cultured at 60 °C for 5 days. The cycle process could be repeated many times. (B) The secondary structures of silk in the reversible hydrogel–solution transition process (CD spectra, FTIR spectra, and XRD patterns). Both CD spectra derived from silk gel or solution, and FTIR and XRD spectra derived from freeze-dried samples showed that silk kept stable β-sheet structures in the reversible process. (C) Schematic models for the reversible hydrogel–solution transition of silk with high β-sheet content. The repulsive force derived from negative charge of silk could restrain further assembly of silk nanofibers with high β-sheet content, resulting in the flowing behaviors of hydrogels in higher concentrations and further the transformation from hydrogel to solution states when the solutions were diluted or the nanofibers were broken into shorter fiber/particles.