Reversible Hydrogel–Solution System of Silk with High Beta-Sheet Content

Abstract

Silkworm silk has been widely used as a textile fiber, as biomaterials and in optically functional materials due to its extraordinary properties. The β-sheet-rich natural nanofiber units of about 10–50 nm in diameter are often considered the origin of these properties, yet it remains unclear how silk self-assembles into these hierarchical structures. A new system composed of β-sheet-rich silk nanofibers about 10–20 nm in diameter is reported here, where these nanofibers formed into “flowing hydrogels” at 0.5–2% solutions and could be transformed back into the solution state at lower concentrations, even with a high β-sheet content. This is in contrast with other silk processed materials, where significant β-sheet content negates reversibility between solution and solid states. These fibers are formed by regulating the self-assembly process of silk in aqueous solution, which changes the distribution of negative charges while still supporting β-sheet formation in the structures. Mechanistically, there appears to be a shift toward negative charges along the outside of the silk nanofibers in our present study, resulting in a higher zeta potential (above −50 mV) than previous silk materials which tend to be below −30 mV. The higher negative charge on silk nanofibers resulted in electrostatic repulsion strong enough to negate further assembly of the nanofibers. Changing silk concentration changed the balance between hydrophobic interactions and electrostatic repulsion of β-sheet-rich silk nanofibers, resulting in reversible hydrogel–solution transitions. Furthermore, the silk nanofibers could be disassembled into shorter fibers and even nanoparticles upon ultrasonic treatment following the transition from hydrogel to solution due to the increased dispersion of hydrophobic smaller particles, without the loss of β-sheet content, and with retention of the ability to transition between hydrogel and solution states through reversion to longer nanofibers during self-assembly. These reversible solution-hydrogel transitions were tunable with ultrasonic intensity, time, or temperature.

Silk Reconstitution Disrupts Fibroin Self-Assembly

“…All natural spider and silkworm silks appear to display a nanofibrillar substructure, and there is ample evidence that these fibrils contribute significantly to the prodigious mechanical properties of silks. 64,65 Assembly of RSF into fibrils with native-like structure has been demonstrated; 34,[38][39][40][41]43,44,54,57 however, while such fibrillar artificial silks are relatively robust compared to other biological materials and promise a novel range of properties in composite applications, 38,44 their mechanical properties still fall short of their natural silk counterparts. 9,38,40,43,44,54,57 The diminished capability of RSF to self-assemble is in line with past studies that have revealed deviations in the structure and bonding character of the RSF molecule.…”

Bioactive natural protein–hydroxyapatite nanocarriers for optimizing osteogenic differentiation of mesenchymal stem cells

“…28–30 Silks were boiled in 0.02 M aqueous Na 2 CO 3 solution for 20 min, and then rinsed thoroughly with distilled water to extract the residual sericin. The extracted silk was placed in 9.3 M LiBr solution at 60°C for 4 h to achieve a 20% (w/v) solution.…”

Injectable and pH-Responsive Silk Nanofiber Hydrogels for Sustained Anticancer Drug Delivery

“…36 The hydrogels were composed of silk nanofibers with beta-sheet content as the dominant conformation and their gel state was depended on silk concentration (Fig. 1).…”

“…1C), as in our previous study. 36 Large amplitude oscillatory rheology revealed the thixotropic properties of the silk nanofiber hydrogels. In a wide strain sweep, the G' of the hydrogels decreased rapidly following increased strain, suggesting a collapse of the gels to a quasi-liquid ( Fig.…”