Robust Near-Infrared-Responsive Composite Hydrogel Actuator Using Fe³⁺/Tannic Acid as the Photothermal Transducer

Light-driven hydrogel actuators show potential applications because their spatiotemporal precision and contact-free manner, especially for near-infrared light (NIR), can be focused on a specific area, which possesses tunable intensity and strong penetrability. Herein, we propose a novel NIR-responsive hydrogel actuator incorporating Fe³⁺/tannic acid (Fe³⁺/TA) as a photothermal transducer into the poly(N-isopropylacrylamide) (PNIPAAm) hydrogel via photo-cross-linking and subsequent immersion in FeCl₃ solution. TA contains abundant pyrogallol and catechol groups, which can be linked to PNIPAAm through hydrogen bonds during in situ polymerization; moreover, as a mediator, TA can form metal-phenolic networks with Fe³⁺ via the coordination between catechol and metal ions, endowing the PNIPAAm gel with enhanced mechanical properties as well as NIR-responsive photothermal effect. We demonstrated that introduction of Fe³⁺/TA maintained the volume phase transition temperature of the hydrogel around 32 °C and guaranteed its deformation behaviors upon NIR irradiation. Furthermore, a higher concentration level of BIS and Fe³⁺ were verified to facilitate a stronger photothermal capacity of the hydrogels. Therefore, under NIR irradiation, Fe³⁺/TA within the hydrogel converted NIR light into heat, and the local high temperature in the irradiated region would cause the petals of the "snowflake"-shaped hydrogel to bend upward perpendicular to the horizontal plane within 1 min, possessing excellent repeatability. This study puts forward a new idea of preparing NIR-responsive hydrogel actuators based on Fe³⁺/TA, which show promising application in the fields of biomimetic devices, flowing control, and soft robotics.