North Carolina State University team develops self-resetting soft ring jumping robot that jumps continuously over 1,000 times under constant illumination

Move, take off, flip, land, and take off again... Under constant infrared illumination, a millimeter-scale soft robot is autonomously performing continuous jumps. Recently, a research team at North Carolina State University in the United States published results in the Proceedings of the National Academy of Sciences, proposing a new type of self-resetting soft ring jumping robot. This teardrop-shaped "small-statured" robot athlete requires no manual resetting and no complex control system; relying solely on the "physical intelligence" of the material and structure itself, it can jump continuously more than 1,000 times under constant illumination, with a maximum vertical jump height of up to 80 times its own height and a horizontal jump distance of more than 3 times its own length. "The key to this work is not relying on complex mechanical mechanisms or electronic control to achieve continuous jumping, but rather combining liquid crystal elastomer smart materials with a ring structure to 'write' the entire motion cycle of energy storage, energy release, takeoff, and resetting into the robot itself," said Yin Jie, corresponding author of the study and a professor in the Department of Mechanical and Aerospace Engineering at North Carolina State University, in an interview with our reporter.

Continuous jumping is an effective mode of locomotion for traversing complex, unstructured environments. But for soft robots, achieving autonomous continuous jumping is not easy: it requires cyclable energy storage and release, automatic resetting after landing, and maintaining a stable posture in the air while adapting to different terrains. In the past, many soft jumping robots could only complete a single jump; to jump continuously, they often required manual resetting or continuous switching of external stimuli. Due to their complex structures, they are also susceptible to disturbances during takeoff and landing, making it difficult to stably recover a posture suitable for the next takeoff.

In response, the research team started with structural design and provided a concise solution. The main body of the soft ring is made of liquid crystal elastomer material; after being bent into a ring shape, its two ends are connected by an aluminum tube, forming a closed-loop structure with a V-shaped rigid tail. Liquid crystal elastomer is a smart material that can convert light or heat stimuli into shape changes. Under constant infrared illumination, the material contracts when heated; because there is a temperature difference between the illuminated side and the shaded side, the soft ring spontaneously twists, storing elastic energy like winding a spring.

As the twisting continues to accumulate, the V-shaped rigid tail gradually bends and lifts. When a critical state is reached, the structure suddenly becomes unstable, the stored elastic energy is rapidly released, and the tail strikes the ground at high speed, catapulting the soft ring into the air. After leaving the ground, the twisted soft ring automatically untwists in the air and returns to its original shape, and after landing it can store energy again and take off again. The entire process requires no onboard sensors, electronic controllers, or repeated manual intervention.

"The ring structure solves the problem of 'whether it can jump continuously,' while center-of-mass adjustment helps solve the problem of 'whether it can jump stably toward the target direction,'" Yin Jie said. So-called center-of-mass adjustment refers to adding a small amount of counterweight to the head of the ring body so that the robot's mass distribution shifts forward, thereby suppressing large flips and improving directional stability during continuous jumping. By adjusting the tail angle, counterweight ratio, and light intensity, the robot can switch among crawling, directional jumping, and vertical jumping.

Chen Tao, a researcher at the Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, and the National Key Laboratory of Marine Critical Materials, told our reporter that in the past, soft robots required manual resetting or external modulation for each jump, because under constant external excitation it is extremely difficult to achieve a cyclable "energy storage-energy release-resetting" closed loop, which requires simultaneously achieving energy recovery, aerial posture stability, and energy replenishment after landing. This work replaces bending and stretching energy storage with torsional energy storage, and "encodes" resetting into the structure itself through geometric constraints, providing a minimalist path for continuous soft jumping.

This structure is not limited to land jumping. The research team further modified the material and tail design so that the soft ring can float, paddle, and return to shore, achieving "jump-swim-jump again" amphibious locomotion. On the water surface, the tail changes from "striking the ground to take off" to "striking the water for propulsion"; when reaching a sloped shore, the thrust generated by the tail's bouncing can also help it overcome surface tension and return to land.

Regarding application prospects, Yin Jie believes that this type of millimeter-scale soft robot is small, lightweight, and structurally compliant, making it suitable for entering complex environments such as grasslands, gravel, mud, shallow water, and land-water interfaces to carry out environmental sensing, target search, and localized monitoring. If multi-robot coordination is achieved in the future, it may also expand to swarm environmental exploration and large-scale distributed monitoring.

There is still a distance to go from the laboratory to practical application. Next, the research team hopes to explore the use of natural light to free the robot from dependence on fixed artificial light sources. Yin Jie said that the significance of this work lies not in forming a mature outdoor robot product, but in proving that a very simple millimeter-scale soft robot can also rely on the physical intelligence of materials and structures themselves to obtain sustained, adaptive autonomous locomotion capabilities, providing a new design approach for future micro-robots entering real unstructured environments.