Stimulus-responsive gradient hydrogel microactuators fabricated by two-photon polymerization-based 4D printing

The growing field of 4D printing has spurred extensive exploration into applications of stimulus-responsive materials, such as hydrogels for micro-actuators. However, the hydrogel-based micro-actuators fabricated by one-step, single-material printing are typically bilayer, and their actuation capabilities are limited. This study proposes a novel gradient printing strategy via two-photon polymerization (2PP) based 4D printing to enhance the actuation performance of stimulus-responsive hydrogel micro-actuators. The feasibility of this approach was demonstrated by investigating the shrinkage rates and elastic moduli of the poly(N-isopropylacrylamide) (PNIPAm) hydrogel micro-cuboids printed at different laser doses using the confocal laser scanning microscope and atomic force microscopy based nano-indentation respectively. The 2PP-based gradient printing was used to fabricate bilayer and trilayer PNIPAm hydrogel micro-actuators, with the laser dose programmed to modulate the crosslinking degree of each layer. These micro-actuators were actuated by near-infrared (NIR) light in the gold nanorods (AuNRs) solutions. The effects of the NIR light powers, micro-actuator sizes, and layer thicknesses on the actuation behaviors were systematically investigated. Compared with 12-µm-thickness bilayer micro-actuation, the introduction of the transitional layer into the gradient trilayer one significantly enhanced the actuation amplitude and speed (the bending angle and curvature increased by about 150 and 70%, respectively, and the cycle time of actuation and recovery shortened by 35%). These advancements have significant implications for printing microscale gradient materials and enhancing their applications.