Multifunctional polyurethane hydrogel based on a phenol–carbamate network and an Fe3+–polyphenol coordination bond toward NIR light triggered actuators and strain sensors

Multifunctional smart hydrogels have been extensively used in wearable devices, soft robotics, tissue engineering, and information storage, especially those with good mechanical properties, desired stimuli-responsiveness, high strain sensitivity, and recyclability. Herein, a novel synergistic dual-network polyurethane hydrogel was prepared through two steps, that is, tannic acid (TA) was firstly incorporated into a polyurethane backbone comprising thermosensitive polyethylene glycol/polypropylene glycol (PEG/PPG) blocks to obtain TA-based polyurethane (TAPU) bearing a phenol–carbamate network; subsequently, TAPU film was immersed in FeCl3 aqueous solution to obtain a TAPU/Fe hydrogel bearing an Fe3+–polyphenol network. The prepared TAPU/Fe hydrogel exhibited a high tensile strength of 0.75 MPa, and large elongation at break of 550%. Benefiting from the thermosensitivity of PEG/PPG blocks and the photothermal effect of the Fe3+/TA complex, the TAPU/Fe hydrogel showed ideal near-infrared (NIR) light triggered actuating behavior and photothermal stability. Under the irradiation of NIR light, the flower-shaped hydrogel actuator can bend upward vertical to the horizontal plane within 2 min. Furthermore, Fe3+ incorporated into the hydrogel matrix can endow the TAPU/Fe hydrogel sensor with high strain sensitivity, which can be used for real-time, stable and repeated monitoring of human movement, and can also be designed as a light-driven switch to light an LED bulb. Built on the reversible cross-linked networks, the TAPU/Fe hydrogel with good biocompatibility can be recycled. Unexpectedly, the TAPU hydrogel showed blue fluorescence under UV light (365 nm) irradiation, and the fluorescence could be quenched (with Cu2+) and recovered (with ethylenediaminetetraacetic acid), resembling rewritable papers.