Thin, tough, pH-sensitive hydrogel films with rapid load recovery.

Stimuli-responsive hydrogels are used as the building blocks of actuators and sensors. Their application has been limited, however, by their lack of mechanical strength and recovery from loading. Here, we report the preparation of pH-sensitive hydrogels as thin as 20 μm. The hydrogels are made of a polyether-based polyurethane and poly(acrylic acid). A simple method was employed to create hydrogels with thicknesses in the range of 20-570 μm. The hydrogel films volume changed by a factor of ∼2 when the pH was switched around the transition point (pH 4). Tensile extensibilities of up to ∼350% were maintained at each pH, and the average Young's modulus and tensile strength were in the range of 580-910 and 715-1320 kPa, respectively, depending on the pH. Repeated tensile loading and unloading to 100% extension showed little permanent damage, unlike analogous double-network hydrogels, and with immediate recovery (up to 75-85% of the first loading cycle), unlike hybrid ionic-covalent interpenetrating network hydrogels.