Gripping and Buoyancy Control of Underwater Soft Robot Using Residual Stress

A soft underwater robot can easily adapt to the environment and interact with various underwater organisms and structures. The soft robot proposed in this study uses the stress difference between two soft polymers. The residual stress formed during fabrication plays the role of gripping and buoyancy control. When the polymer membrane with high elasticity is fixed in a stretched state and then a polymer with relatively low elasticity is poured and cured, the structure forms a curvature and hardened inward due to the stress difference between the two materials. This curling phenomenon became the gripping mechanism of the soft robot. If an air layer is formed using a lubricant between the two polymer layers, the grip force and buoyancy can be adjusted with the amount of air injected into the device. The residual stress generated during the curing process causes the soft robot to maintain a structural curling state, thereby providing a tension that can be stably gripped even by small objects such as screws. When the amount of fluid is small, the soft robot can grip the object and put the object down with increased internal volume. By changing the volume of the internal fluid, buoyancy can be adjusted, and accordingly, the robot's position in the water can be moved up and down. Through the buoyancy model applying the Archimedes principle, the effect of the volume change of the internal fluid on the robot's density was analyzed. The soft robot rose from the bottom of the tank with increased buoyancy. In particular, the ability to adjust the position of an object to float or sink was experimentally verified by the amount of air injected. This soft robot can manage buoyancy in the water and move its position up and down by changing the volume of the internal fluid without relying on external devices. The buoyancy control system of this soft robot enables diversity and adaptability in water and can be used for underwater exploration and research.<br/><br/>This work was supported by the National Science Foundation through the Harvard University’s Materials Research Science and Engineering Center DMR-2011754, the MOTIE (Ministry of Trade, Industry, and Energy) in Korea, under the Fostering Global Talents for Innovative Growth Program (P0017307) supervised by the Korea Institute for Advancement of Technology (KIAT) and National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2018R1A5A7023490).