Macroscopic actuation of bisazo hydrogels driven by molecular photoisomerization

Authors: LI, CHUANG - University Of Science And Technology Of China; Kazem Rostami, Masoud; SEALE, JAMES - Northwestern University; ZHOU, SHUAILONG - University Of Science And Technology Of China; STUPP, SAMUEL - Northwestern University

Submitted to: Chemistry of Materials
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2023
Publication Date: 5/6/2023
Citation: Li, C., Kazem Rostami, M., Seale, J.S., Zhou, S., Stupp, S.I. 2023. Macroscopic actuation of bisazo hydrogels driven by molecular potoisomerization. Chemistry of Materials. https://doi.org/10.1021/acs.chemmater.3c00062.
DOI: https://doi.org/10.1021/acs.chemmater.3c00062

Interpretive Summary: Living organisms’ motions are generated by highly sophisticated and small mechanisms whose artificial replication has not been fully achieved by humankind. Therefore, currently, mechanical movements require the assembly of various components for generating, transmitting, and redirecting the driving forces. These components are often bulky, highly dependent on each other’s function, and fail together whenever the loosest link of the chain breaks. This research utilizes a novel approach to introduce an easy and affordable shortcut for obtaining gels that start to contract and expand when being shined by ultra-violate and blue lights, respectively. Contraction and expansion of these gels enable them to function as artificial muscles whose movements are visible to the naked eye. This publication also provides insight into the parameters influencing the performance of such artificially made muscles and their durability. The studied gels harness the energy of light and transform it to directional mechanical forces without a need for extra components and hence open a new window to the design of artificial muscles that mimic the motions of living organisms. This research contributes to the design of green, self-driven, and self-sustained machinery whose operations rely on no extra components.

Technical Abstract: 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. We have designed and synthesized a bisazobenzene-based molecular photoswitch that displays a highly directional geometrical transformation upon photoisomerization. When this photoswitch is incorporated into polymer networks as a crosslinker, its directional contraction and expansion drive the movement and rearrangement of polymer chains and amplifies these motions to the macroscale. The result is a direct macroscopic 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.