Bioinspired 4D printing of multi-responsive and tough hydrogels with micelle-enhanced double networks

Digital light processing (DLP)-based 4D printing of hydrogels has been widely applied in various fields. However, the poor mechanical properties of conventional hydrogels limit their recovery force during swollen-induced shape memory and recovery processes, restricting their propulsive performance and further applications. In this work, we develop a 4D printable PF127-diacrylate (PF127-DA) micelle-enhanced hydrogel system. The UV-polymerized PF127-DA micelle-enhanced single-network hydrogels exhibit exceptional mechanical properties, including an elongation at break of 2350 % and a toughness of 3.63 MJ/m³. Also, the UV-triggered polymerization enables compatibility with DLP-based 3D printing, facilitating the fabrication of high-resolution, complex 3D hydrogel structures. Inspired by the elastic actuation mechanism of jumping animals, the hydrogel is pre-deformed and immersed in an Fe³⁺ solution, enabling coordination between Fe³⁺ and COO⁻ groups in the hydrogel to build the second crosslinking network. This network fixes the hydrogel in the pre-deformed state, achieving a shape memory effect. The results demonstrate that the hydrogel achieves effective shape fixation under large deformations (500 % pre-deformation with a shape fixation rate of 80 %). Then, the coordination network can then be dissociated through acid or UV light stimulation, triggering shape recovery with a recovery rate exceeding 90 % within 15 min under 1 mol/L acid conditions. Additionally, the designed shape memory measurement yields a maximum recovery stress of approximately 30 kPa. By using this strategy, we achieve 4D-printed hydrogels with large deformability, high recovery force, and programmable shape transformations, showcasing their potential for advanced actuation devices.