Reversible Hydrogel–Solution System of Silkwith High Beta-Sheet Content.

Silkwormsilk has been widely used as a textile fiber, as biomaterialsand in optically functional materials due to its extraordinary properties.The β-sheet-rich natural nanofiber units of about 10–50nm in diameter are often considered the origin of these properties,yet it remains unclear how silk self-assembles into these hierarchicalstructures. A new system composed of β-sheet-rich silk nanofibersabout 10–20 nm in diameter is reported here, where these nanofibersformed into “flowing hydrogels” at 0.5–2% solutionsand could be transformed back into the solution state at lower concentrations,even with a high β-sheet content. This is in contrast with othersilk processed materials, where significant β-sheet contentnegates reversibility between solution and solid states. These fibersare formed by regulating the self-assembly process of silk in aqueoussolution, which changes the distribution of negative charges whilestill supporting β-sheet formation in the structures. Mechanistically,there appears to be a shift toward negative charges along the outsideof the silk nanofibers in our present study, resulting in a higherzeta potential (above −50 mV) than previous silk materialswhich tend to be below −30 mV. The higher negative charge onsilk nanofibers resulted in electrostatic repulsion strong enoughto negate further assembly of the nanofibers. Changing silk concentrationchanged the balance between hydrophobic interactions and electrostaticrepulsion of β-sheet-rich silk nanofibers, resulting in reversiblehydrogel–solution transitions. Furthermore, the silk nanofiberscould be disassembled into shorter fibers and even nanoparticles uponultrasonic treatment following the transition from hydrogel to solutiondue to the increased dispersion of hydrophobic smaller particles,without the loss of β-sheet content, and with retention of theability to transition between hydrogel and solution states throughreversion to longer nanofibers during self-assembly. These reversiblesolution-hydrogel transitions were tunable with ultrasonic intensity,time, or temperature. [ABSTRACT FROM AUTHOR]