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.