Thermal-Responsive Hydrogel Actuators with Photo-Programmable Shapes and Actuating Trajectories

Thermal-responsive hydrogel actuators have aroused a wide scope of research interest and have been extensively studied. However, their actuating behaviors are usually monotonous due to their unchangeable shapes and structures. Here, we report thermal-responsive poly(isopropylacrylamide-co-2-(dimethylamino)ethyl methacrylate)/alginate hydrogels with programmable external shapes and internal actuating trajectories. The volume phase transition temperatures of the resulting hydrogels can be tuned in a wide temperature range from 32 to above 50 °C by adjusting the monomer composition. While the formation and photo-dissociation of Fe³⁺-carboxylate tri-coordinates within the entire hydrogel network enable photo-responsive shape memory property, the insufficient dissociation of the tri-coordinates along the irradiation path gives rise to gradient crosslinking for realizing thermal-responsive actuation. Controlling the evolution of the gradient structure facilitates the regulation of the actuating amplitude. Furthermore, we show that the combination of these two types of shape-changing functionalities leads to more flexible and intricate shape-changing behaviors. One interesting application, a programmable hook with changeable actuating behaviors for lifting different objects with specific shapes, is also demonstrated. The proposed strategy can be extended to other types of actuating hydrogels with more advanced actuating behaviors.