Macroscopic Actuation of Bisazo Hydrogels Driven by Molecular Photoisomerization

Living organisms use the amplification of molecular motions over orders of magnitude to produce deformation, motion, and function on the macroscale. The design of artificial molecular machines that can mimic their biological counterparts in the production of such macroscopic actuation is of great interest. Here, we designed a polymerizable bisazobenzene-based molecular photoswitch that displays a highly directional geometrical transformation upon photoisomerization. When this photoswitch is incorporated into polymer networks as a cross-linker, its directional contraction and expansion drive the movement and rearrangement of polymer chains and amplifies these motions to the macroscale, resulting into a direct volume change and deformation of the bulk polymeric hydrogels. These photoswitchable hydrogels therefore mimic macroscopic biological motions such as muscular contraction and light-driven bending similar to phototropism. Furthermore, the use of this photoswitch to tune the macroscopic properties of hydrogels is in principle transferable to a variety of polymeric systems. This work develops a clear connection between molecular motion and macroscopic actuation, providing a platform to investigate this relationship further.