Spider dragline silk as torsional actuator driven by humidity

Self-powered actuation driven by ambient humidity is of practical interest for applications such as hygroscopic artificial muscles. We demonstrate that spider dragline silk exhibits a humidity-induced torsional deformation of more than 300°/mm. When the relative humidity reaches a threshold of about 70%, the dragline silk starts to generate a large twist deformation independent of spider species. The torsional actuation can be precisely controlled by regulating the relative humidity. The behavior of humidity-induced twist is related to the supercontraction behavior of spider dragline silk. Specifically, molecular simulations of MaSp1 and MaSp2 proteins in dragline silk reveal that the unique torsional property originates from the presence of proline in MaSp2. The large proline rings also contribute to steric exclusion and disruption of hydrogen bonding in the molecule. This property of dragline silk and its structural origin can inspire novel design of torsional actuators or artificial muscles and enable the development of designer biomaterials.

Torsional failure of water-filled carbon nanotubes

The failure behavior of water-filled carbon nanotubes under the torsional moment is investigated by molecular dynamics simulations. The computational results indicate that the shear modulus can be slightly increased but the critical buckling angle can be greatly enhanced by the water filling as compared to the empty carbon nanotubes. The enhancement in the critical buckling angle is almost comparable to that induced by the copper filling. However, the critical failure angle of carbon nanotubes decreases with the increase in the packing density of water, and it is reduced by about 50% when the water density reaches 1.2 g/cm3. Moreover, the researches on the defective water-filled carbon nanotubes reveal that the torsional property of the carbon nanotubes can be influenced by the density and distribution of the Stone–Wales defects. The present findings can provide a theoretical guidance for the stability assessment of nanochannels and the potential application for the drug delivery and release.