Many biological organisms navigate complex environments using highly coordinated muscle movements, often with anisotropic structures. Artificial robots typically require multiple independent actuators to achieve similar locomotion. This research seeks to create a biomimetic soft robot capable of multimodal locomotion driven and steered by light. The central hypothesis is that a carefully designed anisotropic hydrogel, incorporating photothermal agents, can exhibit self-coordinated shape changes and friction modulation in response to light irradiation, enabling versatile and controllable movement. The successful creation of such a robot would be a significant advancement in soft robotics, offering a more biologically inspired and potentially more efficient approach to locomotion than current methods.

Existing soft robots often rely on multiple independently activated actuators to achieve complex movement. While some progress has been made in creating soft robots with simpler actuation mechanisms, such as those using temperature-responsive hydrogels or light-activated materials, these have limitations in terms of speed, controllability, and the range of motion. The literature reveals a need for soft robots that mimic the efficient and adaptable locomotion of biological organisms. Studies exploring the use of anisotropic materials and the coupling of shape change with friction modulation to achieve locomotion are relatively few, suggesting a significant opportunity for innovation in this area.

The researchers prepared a muscle-like poly(N-isopropylacrylamide) (PNIPAAm) nanocomposite hydrogel. Nanosheets were electrically oriented and then gelled to create an anisotropic structure. Multi-step electrical orientation and photolithographic polymerization were used to create patterned anisotropic hydrogels with programmed deformation capabilities. Gold nanoparticles (AuNPs) were incorporated into the hydrogel to provide photothermal responsiveness. The temperature- and light-responsiveness of the hydrogels were characterized using polarizing optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometry. The mechanical properties of the hydrogels were also assessed using a tensile tester. The locomotion of the hydrogel robots was tested on different substrates (hydrophobic PVC and hydrophilic glass) under different light irradiation patterns. Numerical simulations were conducted to model the locomotion and help understand the underlying mechanisms.

The researchers successfully fabricated a muscle-like hydrogel with anisotropic properties. The incorporation of AuNPs made the hydrogel responsive to light irradiation, resulting in fast isochoric deformation (volume-constant shape change). Upon light irradiation, the hydrogel exhibited a rapid increase in friction against a hydrophobic substrate. By controlling the spatial and temporal patterns of light irradiation, the researchers demonstrated three distinct locomotion gaits: crawling, walking, and turning. Crawling was achieved by unidirectional scanning of a light beam across a slender hydrogel sheet. The direction of crawling depended on the orientation of the nanosheets within the hydrogel. Walking was demonstrated using a stripe-patterned hydrogel, with each end acting as a "foot." The direction of walking could be reversed by changing the direction of light scanning. Turning was achieved by scanning the light beam diagonally across the stripe-patterned hydrogel. The friction coefficient measurements confirmed that the light-induced temperature change significantly altered the friction between the hydrogel and the substrate, playing a critical role in the locomotion. Numerical simulations based on a hyperelastic model with a temperature-dependent friction coefficient successfully reproduced the observed locomotion behaviors, further validating the proposed mechanism. The locomotion speeds observed were comparable to or faster than other hydrogel-based soft robots reported in literature.

The findings demonstrate a novel approach to creating biomimetic soft robots that utilize light-induced shape change and friction modulation for locomotion. The ability to control the locomotion gait (crawling, walking, turning) using spatiotemporal light patterns highlights the versatility of this approach. The fast response time and high controllability of the light-actuated hydrogel robot represent a significant improvement over existing soft robots. The successful modeling of the locomotion through numerical simulations reinforces the understanding of the underlying mechanisms. The reported locomotion speeds are competitive with or surpass those reported for existing hydrogel-based robots, suggesting the potential for further optimization and advancement.

This study successfully demonstrated light-steered locomotion in a biomimetic soft robot constructed from a muscle-like hydrogel. The robot exhibits versatile movement capabilities, including crawling, walking, and turning, which are controlled by the spatiotemporal patterns of light irradiation. This work advances the field of soft robotics by presenting a novel locomotion mechanism based on the interplay of light-induced shape change and dynamic friction modulation. Future research could focus on miniaturizing the robot for biomedical applications, exploring more complex locomotion gaits, and integrating sensory feedback for autonomous navigation.

The current study primarily focuses on demonstrating the feasibility of light-steered locomotion. While the locomotion speeds are competitive, they might be limited by the power of the light source and the properties of the hydrogel. The experimental setup involves external light control, which may not be suitable for all applications. Future work should address these limitations through optimizing hydrogel materials and developing integrated light sources or wireless control mechanisms.