This review has highlighted the WR properties of different materials and their related actuators, which may exhibit remarkable performance in various fields, such as artificial muscles, smart sensors, robots, and new energy generators.

Fish can adaptively adjust their body kinematics and swimming modes by sensing to realize optimal propulsion. However, most soft robotic fish have an unchangeable swimming mode through simple structure design, making them difficult to adapt to dynamic and complex fluid environments. Here, inspired by the multiple muscle synergy and lateral line sensing function of fish, we developed a soft robotic fish with multiple actuating units and embedded sensing elements. By collaboratively controlling the amplitude and phase of excitation from the multiple flexible actuating units, the soft robotic fish can successfully realize various swimming modes very similar to those of natural fish. Additionally, the embedded flexible sensing elements enable the robotic fish to sense the swimming state and the surrounding fluid environment in real time. The multiple actuation and embedded sensing allow the soft robotic fish to adaptively switch to an optimal swimming mode in a certain fluid environment. The multi-mode swimming and perception capabilities proposed in this work not only make soft robotic fish more intelligent and adaptable to complex fluid environments, but also contribute to the future implementation of autonomous control capabilities for robotic fish.

The pectoral fins of manta rays are driven by bilateral muscles to generate up-down flapping movements that allow for efficient and flexible swimming. However, current soft underwater biomimetic flapping robots are mostly driven by unilateral muscle actuators, and the pectoral fin can only realize simple downward bending motion, the biomimicry, motion flexibility, and swimming speed of these robots need to be further enhanced. Here, a soft manta ray robot driven by bilateral dielectric elastomer bionic muscle actuators is proposed. The bilateral muscle-driven pectoral fin can not only achieve a larger bending angle (37°), but also present a 3D morphology similar to that of a natural manta ray during the swimming process. Compared to the simple downward flapping used by most robots, the simulation shows that the pectoral fins driven by bilateral muscles can obtain larger vortex intensity, flow field velocity, and thrust. The fastest swimming speed of the designed manta ray robot driven by a bilateral bionic actuator is 42 mm/s (0.76 body length per second BL/s), which is about 0.7 times higher than the swimming speed of the robot driven by a unilateral bionic muscle actuator (25 mm/s). What's more, by strategically controlling multiple actuating units of the actuators, the manta ray robot can achieve more flexible swimming motions, including continuous turns, C-shaped, and S-shaped swimming trajectories. This study not only reveals the effectiveness of the bidirectional flapping mode in improving the performance of underwater swimming robots but also the proposed bilateral muscle-driven mode can be applied to other soft robots and flexible electronic devices.

Soft actuators based on smart materials can utilize external stimuli, especially clean energy sources such as light and humidity in nature, to achieve controllable actuation and perception, which has attracted much attention in the development of smart robots. However, most existing soft actuators based on light or humidity stimulation may suffer from several shortcomings, such as low response sensitivity, a single response mode, and an inability to achieve a multi-stimulus cooperative response. Hence, inspired by the biological structure of plants, we have fabricated soft double-layer actuators with ultrahigh sensitivity by casting a graphene oxide (GO) solution on a low-density polyethylene (LDPE) film. With just 40 mW cm⁻² of NIR light, the actuator can quickly change from the curled state to the flat state, achieving an angle change of 500°, which is less than the natural light intensity (80 mW cm⁻²). Moreover, the actuator also showed a rapid response (∼2.6 s) and large deformation (500°) under humidity stimulation. As an application concept demonstration, the actuator can be assembled into a crawling robot that mimics inchworms crawling on a leaf, an adaptive soft gripper that grabs different shapes of cargo, and a rotating robot driving a boat forward. The soft actuator will have great potential in smart bionics, energy conversion, and other emerging fields in the future.