The speed of the fist launched by the mantis shrimp when it crushes hard prey reaches approximately 90 km/h. Fleas can jump more than 200 times their body length. The secret of how a small, soft-bodied creature can exert a strong force in an instant lies in the 'torque reversal mechanism,' which instantaneously shifts the direction of the rotational force exerted by muscles in the arms or legs.
Recently, a research team led by Professor Kyu-jin Cho from the Department of Mechanical Engineering at Seoul National University showcased a technology that utilizes this torque reversal mechanism to enable a robot body made of soft materials, like rubber, to perform fast and powerful movements. Professor Cho's research team named this technology the 'Hyperelastic Torque Reversal Mechanism (HeTRM).'
The research team discovered the core principle of 'HeTRM' in the unique property of elastic polymers, which become significantly harder as they are condensed. By intensely condensing one part of a soft joint, they developed soft robot technology utilizing the characteristics of superelastic materials that instantaneously release stored energy upon reaching a critical point.
According to the research team, a simple structure that connects a pair of motors and tendons to a flexible joint allows it to move quickly and powerfully, much like natural cilia, which perform repetitive and strong flexing motions. The research team confirmed the practical applicability of the technology, moving beyond mere theoretical outcomes.
Examples utilizing the fast and strong movements enabled by the 'Hyperelastic Torque Reversal Mechanism' include a soft gripper that can catch a falling ping pong ball in an instant, a robot that can crawl with strong propulsion on rugged terrain like a sandpit, and a robot that quickly wraps around an object like an octopus arm. They also implemented a mechanical fuse that automatically cuts off contact when receiving unintended strong forces.
Research Institute’s joint authors Choi Woo-young and Kim Woong-bae noted, "A toy with a magical bracelet that wraps around quickly when the wrist is tapped is an example of the snap-through phenomenon, which rapidly moves between two structurally stable points, and many attempts have been made to apply such bistable structures to robots to realize fast movements. This study presents new possibilities for soft robotics by implementing the movement through the properties of materials instead of structural design."
Choi Woo-young earned a master’s degree in mechanical engineering from Seoul National University and is currently working as a robotic software engineer at Naver Labs. Kim Woong-bae earned a Ph.D. in mechanical engineering from Seoul National University and is currently conducting research in the field of soft robotics at the Korea Institute of Science and Technology (KIST).
Reference material
Science Robotics (2025), DOI: https://www.science.org/doi/10.1126/scirobotics.ado7696