The key component of this robot is the soft electrohydraulic actuator. "Unlike traditional rigid robots, soft robots have better environmental adaptability and safety, and electrohydraulic actuation technology is one of the breakthroughs in the field of soft robots in recent years. It has higher energy efficiency and less noise," said Fuyi Fang, a researcher at School of Mechanical Engineering, Shanghai Jiao Tong University.
The soft electrohydraulic actuator is a sealed pouch filled with silicone oil and covered with carbon electrodes on both sides. As a high-voltage electrical signal is applied, the positive and negative electrodes are attracted to each other, squeezing the silicone oil inside, and causing the actuator to expand. This design cleverly converts electrical energy into hydraulic energy, which is then converted into mechanical movement. The researchers added a skeleton to the actuator and translate the expansion motion into bend motion. The designed skeleton is in fin shape with tip-tops on the front end, which allows the robot to be adaptive to both terrestrial and aquatic environments.
"With three soft electrohydraulic flippers distributed symmetrically, our robot is able to realize multiple locomotion modes, including crawling on land, crawling underwater and swimming in water," Fang further explained the different motion modes in detail.
In land crawling mode, the robot generates forward momentum through periodic bending and recovery of the front flipper. Due to the delicately designed tip-top of the flipper, there is imbalanced friction between the robot and the ground, allowing the robot to crawl forward. Experiments show that the robot can reach a land crawling speed of 2.9 cm/s at an optimal drive frequency of 6 Hz. The underwater crawling mode utilizes a completely different propulsion principle. Due to the buoyancy of the water, the friction between the robot and the bottom of the water is greatly reduced, and propulsion is generated mainly by the interaction force between the flippers and the water. With their experiments, at a drive frequency of 1.6 Hz, the underwater crawling speed is up to 3.2 cm/s.
The most impressive mode is the swimming motion. When the three flippers work simultaneously, the robot is able to generate symmetrical upward thrusts for vertical uplift. Through fluid simulation, the team found that the oscillation of the flippers creates vortex rings underneath the robot, and the shedding and interaction of these vortex rings further enhances the propulsion efficiency. By optimizing the initial angle and drive frequency, the robot can swim at speeds of up to 5.9 cm/s.
Environmental adaptability is an important metric for evaluating exploratory robots. The research team tested this electrohydraulic flipper at extreme temperatures, and the results were surprising – it is capable of operating in temperatures ranging from -20°C to 70°C. The team further tested the robot's operation in a wide range of temperatures. In hot water at 61.3°C, the robot's locomotion performance was largely comparable to that at room temperature, thanks to the good thermal stability of the silicone oil and the high-temperature resistance of the electrode material. In 2.1°C cold water, although the increased viscosity of the silicone oil led to a decrease in the actuator response speed, the robot was still able to maintain a swimming speed of 2.7 cm/s, demonstrating excellent low-temperature adaptability.
This ability to operate over a wide temperature range allows the robot to adapt to a variety of complex natural environments, which is important for practical environmental exploration and monitoring applications.
Looking ahead, this multimodal amphibious soft robot has broad application prospects in the fields of environmental monitoring, search and rescue, and pipeline inspection. Its ability to adapt to different environments without structural reorganization makes it particularly suitable for performing tasks in complex and changing natural environments. The research team will continue to optimize the materials and manufacturing process in the future to further enhance the performance and practicality of the robot, making it a truly practical tool for environmental exploration.
The research team includes Wenming Zhang, Fuyi Fang, Junfeng Zhou, Yanran Yi, Zhen Huang and Yicheng Feng from Shanghai Jiao Tong University, Shanghai; Wenbo Li and Yuanzhen Zhang from Tongji University, Shanghai; and Kai Tao from Northwestern Polytechnical University, Xi'an.
The research is funded by National Natural Science Foundation of China (Grants no. 12472057, 12032015, 12121002), Research Project of State Key Laboratory of Mechanical System and Vibration (Grant no. MSV202407), Fundamental Research Funds for the Central Universities, China, Shanghai Gaofeng Project for University Academic Program Development.
Summary
Soft robots are exceptionally suited to exploring complex environments, including amphibious navigations, due to their flexible and adaptive characteristics. Based on a kind of soft electrohydraulic actuator, a research team present a multimodal amphibious robot with multiple locomotion modes, including crawling on land, crawling underwater and swimming in water, as well as the capability of smoothly transition between these modes. The team published their findings in Cyborg and Bionic Systems on June 9, 2025.
