title: 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.
authors: National Engineering Laboratory for Modern Silk & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University , Suzhou 215123, People's Republic of China.
authors: Shumeng
authors: S
authors: Bai
authors: Xiuli
authors: X
authors: Zhang
authors: Qiang
authors: Q
authors: Lu
authors: Weiqin
authors: W
authors: Sheng
authors: Lijie
authors: L
authors: Liu
authors: Boju
authors: B
authors: Dong
authors: David L
authors: DL
authors: Kaplan
authors: Hesun
authors: H
authors: Zhu
mesh_terms: D000818:Animals; D001672:Biocompatible Materials / Q000737:chemistry; D012831:Bombyx; D002942:Circular Dichroism; D005349:Fibroins / Q000737:chemistry; D020100:Hydrogels / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D057139:Nanofibers / Q000737:chemistry; D047011:Silk / Q000737:chemistry*; D012996:Solutions; D017550:Spectroscopy, Fourier Transform Infrared; D014961:X-Ray Diffraction
chemical_list: D001672:Biocompatible Materials; D020100:Hydrogels; D047011:Silk; D012996:Solutions; D005349:Fibroins