A Facile Approach To Prepare Tough and Responsive Ultrathin Physical Hydrogel Films as Artificial Muscles

We report a facile approach to prepare ultrathin physical hydrogel films based on the Marangoni effect, which drives an ethanol solution of poly(stearyl acrylate-co-acrylic acid) (P(SA-co-AAc)) to rapidly spread on the water surface. The subsequent solvent exchange leads to sol-gel transition, where the long alkyl chains of SA units segregate to form physical cross-linking junctions. The resultant disk-shaped single-network (SN) gel films are uniform with tunable thickness (40-80 μm) and diameter (5-12 cm) and possess robust mechanical properties with tensile breaking stress, σ_b, and breaking strain, ε_b, being 0.3-1.1 MPa and 30-290%, respectively. The mechanical properties of SN gel films can be further improved by introducing ductile poly(N-isopropylacrylamide) (PNIPAm) into the preformed gel matrix, which forms strong hydrogen bonds with the first network. The obtained physical double-network (DN) hydrogel films are transparent and show excellent mechanical performances with σ_b of 3-5 MPa and ε_b of 100-500%. Due to the ultrathin thickness of gel films and response of PNIPAm to saline solutions, the tough DN gel films exhibit fast response (≤60 s) and large stroke force (0.5 MPa) after switching the environment from water bath to saline solution, making them an ideal material to design artificial muscles, soft actuators, and chemomechanical devices.