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, σb, and breaking strain, εb, 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 σb of 3–5 MPa and εb 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.