Soft robots, driven by actuators like hydrogels, offer advantages in complex environments over rigid robots. Hydrogels are attractive due to their biocompatibility, softness, and designability for various stimuli responses (temperature, pH, light, magnetic fields, electricity). However, their actuation speed is typically limited by water diffusion kinetics, often requiring minutes to hours for a single actuation cycle. Researchers have attempted to increase speed by reducing hydrogel thickness or introducing porosity, but these methods often compromise design flexibility and actuation output. Other approaches, such as incorporating cofacially oriented electrolyte nanosheets, offer faster actuation but rely on complex magnetic field control. Furthermore, converting isotropic volume changes in hydrogels to complex motions requires spatial material heterogeneity, often fixed during synthesis and lacking reprogrammability. Existing reprogrammable hydrogels, while offering some flexibility, still suffer from slow actuation times (tens of minutes). This research aims to overcome these limitations by developing a high-performance hydrogel robot that combines fast actuation with programmable complex motions.

The literature extensively covers stimuli-responsive hydrogels for soft robotics. Many studies focus on improving actuation speed through strategies like reducing hydrogel thickness, introducing porosity, or utilizing unique mechanisms independent of water diffusion, such as those involving electrostatic repulsion between nanosheets. However, these methods often present challenges in achieving complex motions or require sophisticated control systems (e.g., magnetic fields). Reprogrammability, the ability to repeatedly alter the robot's motion, is another significant challenge. Existing approaches often involve slow cooling-induced actuation steps, limiting the overall speed. This paper addresses these limitations by introducing a new approach based on photo-mechanical programming and dynamic bond exchange.

The researchers developed a thermo-responsive hydrogel crosslinked by disulfide bonds. The dynamic nature of these bonds, allowing UV-triggered break-revolve processes, enables photo-mechanical programming to introduce spatio-selective network anisotropy. This results in an actuation mechanism driven by thermally controlled chain conformation changes, rather than water diffusion. The hydrogel precursors include poly(N-isopropylacrylamide) (PNIPAM), a disulfide crosslinker (BISS), and poly(vinyl alcohol) (PVA). The process involves dissolving these components in an aqueous solution, polymerization, and subsequent crosslinking with aluminum ions. The programming step involves deforming the hydrogel and irradiating it with UV light to trigger disulfide bond exchange and shape reconfiguration. The addition of photothermal fillers (e.g., carbon black) enables rapid heating via near-infrared (NIR) light. The researchers investigated various factors affecting shape reconfiguration and actuation, including BISS content, UV irradiation time, type of metal ions for crosslinking, and PVA concentration. They conducted dimensional investigations, tensile tests, and DSC characterization to optimize the hydrogel properties. The actuation mechanism was compared to a standard bilayer hydrogel actuator to highlight the difference in speed and kinetics. The versatility of the photo-mechanical programming was demonstrated by creating different actuation modes and regionally selective actuations. Finally, the hydrogel was used to create high-speed underwater robots with programmable multimodal actuations.

The key findings of this study demonstrate a significant advancement in hydrogel-based soft robotics. The researchers successfully achieved high-speed and programmable actuation in a thermo-responsive hydrogel using a novel photo-mechanical programming approach. The actuation mechanism, based on thermally driven chain conformation changes instead of water diffusion, resulted in actuation speeds three orders of magnitude faster than conventional hydrogels. The optimized hydrogel exhibited actuation frequencies as high as 1.7 Hz. The photo-mechanical programming allowed for the creation of robots with diverse high-speed motions, including continuous swimming, step-wise walking, and rotating. The shape retention and actuation amplitude were optimized by controlling the BISS content, UV irradiation time, and type of metal ions used for crosslinking. The addition of PVA improved the hydrogel's mechanical properties and prevented blister formation. A comparison with a standard bilayer hydrogel actuator clearly demonstrated the superior actuation speed of the proposed hydrogel, reaching near 100% actuation degree in seconds, compared to only 13% for the bilayer hydrogel under the same conditions. Furthermore, experiments conducted in oil confirmed that the actuation mechanism was independent of water diffusion. The study's figures and supplementary materials provide detailed experimental results supporting these key findings.

The findings address the long-standing challenge of combining fast actuation with programmable complex motions in hydrogel robots. The unique actuation mechanism, based on thermally driven chain conformation changes, offers a significant improvement over diffusion-limited actuation, achieving speeds that were previously unattainable. The versatility of photo-mechanical programming enables the creation of robots with a wide range of functionalities, exceeding the capabilities of existing hydrogel actuators. The high actuation frequency achieved (up to 1.7 Hz) opens up new possibilities for applications requiring fast and precise movements. The demonstrated underwater robots showcase the potential of this technology for various tasks in aquatic environments. The results have implications for the broader field of soft robotics and offer a pathway towards developing more sophisticated and efficient hydrogel-based devices.

This research successfully demonstrated high-speed, programmable underwater hydrogel robots powered by light. The novel actuation mechanism, based on photo-mechanical programming and thermally driven conformational changes, overcomes limitations of traditional hydrogel actuators. Future work could focus on improving light powering strategies (e.g., integrated photothermal capabilities) and enhancing actuation amplitude without sacrificing speed. Exploring biocompatible materials and integrating advanced sensing capabilities would further expand the applicability of this technology in biomedical and environmental applications.

The current hydrogel system presents two main limitations. First, the light-powering mechanism requires focused NIR light, which can be challenging to control, especially for complex motions. Second, high-speed actuation is currently achieved at the expense of actuation amplitude, limiting applications demanding both high speed and large amplitude. While these limitations do not significantly affect the demonstrated underwater robot functionalities, they should be addressed for broader applications.