Fabrication of Asymmetric Tubular Hydrogels through Polymerization-Assisted Welding for Thermal Flow Actuated Artificial Muscles

By nature, diversified motions in most muscle systems are accompanied by fluid transportation. Inspired by the fluid-induced actuation behavior of Urechis unicinctus, we introduce a two-step fixed-site selective surface catalytically initiated radical polymerization strategy to prepare asymmetric thermoresponsive hydrogel tubes (as-TRTs) containing the responsive PNIPAAm_x-PAA_y part and nonresponsive PAM_x-PAA_y part with perfect interface fusion, including uniform structure (I), Janus structure (II), and block structure (III). In a typical case, we can easily obtain three kinds of multiple-channel as-TRTs with complex geometries by using multiple wires as a growth template and three kinds of TRTs with patterned, thread, and spiral structures. Meanwhile, the as-TRTs can achieve dynamic bending based on the generated interface stress difference from up to 7 times modulus value change for its thermoresponsive part upon immersing the tubes into a hot water bath or injecting the hot fluid into the channels. The prepared as-TRTs can bend or twist depending on their flexible geometric design. Devices made with these asymmetric hydrogel tubes can capture/release or lift objects both underwater and in air, convert thermal stimulation to the transportation capability of the fluid, gas or mix different kinds of liquids, and act as intelligent multichannel fluidic switchers. The current work is highly anticipated to open new frontiers for developing stimuli-responsive "smart" soft robots.