SNU researchers apply the principles of mantis shrimp and fleas to create soft robots with powerful movements

Seoul National University College of Engineering announced that a research team led by Professor Kyu-Jin Cho (Director of the Soft Robotics Research Center) from the Department of Mechanical Engineering took inspiration from principles found in nature and developed the "Hyperelastic Torque Reversal Mechanism (HeTRM)," which enables robots made from rubber-like soft materials to perform rapid and powerful movements. This study was published in the prestigious international journal Science Robotics on January 29.

The mantis shrimp delivers a punch at speeds of up to 90 km/h to break its prey, while the flea can jump to heights exceeding 200 times its body length. Professor Kyu-Jin Cho explained, “The secret behind these organisms’ ability to generate powerful forces with their soft bodies lies in the ‘torque reversal mechanism,’ which enables the instantaneous switching of rotational force direction applied by muscles to their limbs.” He added, “Our research team previously developed flea-inspired robots capable of achieving high jumps both on land and water; and this latest study is particularly significant as it is an advancement that achieves powerful performance in soft, rubber-like structures.”

According to the research team, the core principle of the developed "Hyperelastic Torque Reversal Mechanism" lies in leveraging the characteristics of soft hyperelastic materials, which rapidly stiffens as they compress. The team developed upon their discovery that when compression is concentrated on one side of a flexible joint, it reaches a critical point where the stored energy is released instantaneously. They explained that even with a simple structure that connects a tendon and motor to a flexible joint, repetitive and powerful bending motions could be realized, just like the cilia found in nature.

The research team expanded this principle and demonstrated various practical applications. The soft gripper utilizing the developed principle can instantly catch falling ping-pong balls, while other applications include a robot that crawls over rough terrains like sand with strong propulsion and a robot that rapidly wraps around objects like an octopus tentacle, showcasing fast and powerful movements. Furthermore, they demonstrated a mechanical fuse that is triggered when the structure encounters unintended external forces exceeding a certain threshold.

Co-first authors of the research, Wooyoung Choi (currently at Naver Labs) and Woongbae Kim (currently at the Korea Institute of Science and Technology), explained, “The instant wrapping of slap bracelets is driven by a rapid transition between two stable states, known as snap-through. While many efforts have been made to mimic this behavior, we introduced a novel approach by leveraging material properties rather than structural designs.” Professor Kyu Jin Cho of Seoul National University, the principal investigator, expressed his optimism about the research, stating, “this technology will expand the horizons of soft robotics design and applications.”

This research has been supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIT) (RS-2023-00208052) (to K-J.C.); Korea Institute of Science and Technology (Institutional Program grant 2E33004) (to W.K.).

□ Three Key Questions About the Research, Answered

1. What inspired you to start this research? What sparked your interest in this topic?

During my graduate studies, I participated in the RoboSoft Grand Challenge, where I conducted research on soft manipulators. While experimenting to test their strength, I observed an intriguing phenomenon where the structure underwent unexpected shape transitions under applied force. Although the original design was not intended to have bistable characteristics, structural transitions occurred, causing unintended deformations.

As I analyzed this phenomenon, I discovered that such structural transitions operate similarly to how cilia function in nature. The cilium is a structural design found in nature that uses phase transitions to produce efficient and repetitive movements. Inspired by this natural design, I realized that applying the principles of cilia to soft robotics could enable the creation of new types of motion without complex mechanisms. This discovery became the foundation and motivation for initiating this research.

2. What are the next steps in your research? What do you think other researchers should focus on?

We focused on presenting the hyperelastic torque reversal mechanism principle, and expanding its applications. Going forward, we plan to focus on refining this technology and applying it to applications of various scales and environments. Future research will focus on refining the hyperelastic torque reversal mechanism and applying it to various environments and scales. To achieve this, we plan to enhance the accuracy of performance analysis through modeling, including shear stress, and verify its applicability in various environments using finite element analysis (FEM).

In addition, we will enhance the practicality by applying HeTRM to larger systems or complex multi-joint soft robots, and also the efficiency and durability of the mechanism will be improved through performance optimization of hyperelastic materials and utilizing multiple materials. Furthermore, we encourage other researchers to actively incorporate nonlinear dynamic mechanisms, such as snap-through, into soft robotic designs. Such approaches will play a crucial role in developing energy-efficient and multifunctional soft robotic systems.

3. Can you explain the developed robot in simple terms, like you would to a child?

“Our robot is made of soft, stretchy materials, kind of like rubber. Inside, it has a special part that stores energy and releases it all at once—‘BAM!’—to make the robot move super fast. It works a bit like how a bent tree branch snaps back quickly or how a flea jumps really far. This robot can grab things like a hand, crawl across the floor, or even jump high, and it all happens just by pulling on a simple muscle!”

□ Introduction to the SNU College of Engineering

Seoul National University (SNU) founded in 1946 is the first national university in South Korea. The College of Engineering at SNU has worked tirelessly to achieve its goal of ‘fostering leaders for global industry and society.’ In 12 departments, 323 internationally recognized full-time professors lead the development of cutting-edge technology in South Korea and serving as a driving force for international development.