Programmable Contractile Actuations of Twisted Spider Dragline Silk Yarns.

The contraction behavior of spider dragline silk upon water exposure has drawn particular interest in developing humidity-responsive smart materials. We report herein that the spider dragline silk yarns with moderate twists can generate much improved lengthwise contraction of 60% or an isometric stress of 11 MPa when wetted by water. Upon the removal of the absorbed water, the dried and contracted spider silk yarns showed programmable contractile actuations. These yarns can be plastically stretched to any specified lengths between the fully contracted state and the state before supercontraction and return to the fully contracted state when wetted. Moreover, the generated isometric stress of these yarns is also programmable, depending on the stretching ratio. The mechanism of the programmable reversible contraction is based on the plastic mechanical property of the dried and contracted spider silk yarns, which can be explained by the variation of the hydrogen bonds and the secondary structures of the proteins in spider dragline silk. Humidity alarm switches, smart doors, and wound healing devices based on the programmable contractile actuations of the spider silk yarns were demonstrated, which provide application scenarios for the supercontraction of spider dragline silk.