Covalent-supramolecular hybrid polymers as muscle-inspired anisotropic actuators

Skeletal muscle provides inspiration on how to achieve reversible, macroscopic, anisotropicmotion in soft materials. Here we report on the bottom-up design of macroscopic tubes thatexhibit anisotropic actuation driven by a thermal stimulus. The tube is built from a hydrogel inwhich extremely long supramolecular nanofibers are aligned using weak shear forces, followedby radial growth of thermoresponsive polymers from their surfaces. The hierarchicallyordered tube exhibits reversible anisotropic actuation with changes in temperature, withmuch greater contraction perpendicular to the direction of nanofiber alignment. We identifytwo critical factors for the anisotropic actuation, macroscopic alignment of the supramolecularscaffold and its covalent bonding to polymer chains. Using finite element analysis andmolecular calculations, we conclude polymer chain confinement and mechanical reinforcementby rigid supramolecular nanofibers are responsible for the anisotropic actuation. Thework reported suggests strategies to create soft active matter with molecularly encodedcapacity to perform complex tasks.