Researchers from North Carolina State University have created teardrop-shaped soft robots that leap upward or forward when exposed to infrared light – and will keep jumping as long as the light is present. The work demonstrates a new mechanism for self-resetting jumping behavior in soft robotics.

“This ‘ring leaper’ design is very simple,” says Jie Yin, corresponding author of a paper on the work and a professor of mechanical and aerospace engineering at NC State. “We use torsion to store elastic energy and then release that energy all at once. And the nature of the design means that it does not need to be reset between jumps. It resets itself.”

The robots are made of a liquid crystal elastomer ribbon shaped like a teardrop, with a thin aluminum tube shaped like a V at one end. When exposed to light from an infrared lamp, the surface of the ribbon contracts, causing the ribbon to rotate. The stiff V at one end of the robot prevents the ribbon from simply rolling in place, causing the ribbon to twist tighter and tighter. This stores energy until the twist reaches a critical point when the ribbon releases that energy, causing the V at one end of the teardrop to snap downward and strike the surface. This launches the teardrop into the air.

Having released the stored energy, the teardrop returns to its original shape. This process will keep repeating itself as long as the robot is exposed to the infrared light. Video of the robots can be found here: https://youtu.be/7dl-f9SbNEk.

“We found that we can control the nature of the robot’s movement by making minor design changes,” says Fangjie Qi, first author of the paper and a postdoctoral researcher at NC State. “If the V at one end of the teardrop is very wide – an angle of 120 degrees – the robot simply crawls forward. If you reduce the angle to 90 degrees, it jumps forward. And if you reduce the angle further to 50 degrees, the robot leaps vertically. In other words, a single geometric parameter determines whether it crawls, jumps forward, or leaps upward.

“We could also improve the robot’s jumping distance by adding a small amount of weight to the rounded end of the teardrop,” says Qi. “This changes the robot’s center of mass and leads to a more powerful and stable forward motion – much like a swimmer leaning forward as they dive off the starting block.”

In addition, the researchers found that the intensity of the infrared light plays an important role.

“It has to be strong enough to induce the jumping behavior, but not too strong – because that can cause the robot to jump erratically and in unpredictable directions,” says Qi.

In proof-of-concept testing, the researchers found that the teardrop-shaped robots could leap across slopes, hurdles, and a wide variety of surfaces – including grass, sand, rocks and mulch.

“There are no immediate applications for this work, but it’s a fundamental advance that is worth exploring for potential use in environmental navigation, swarm robotics, and unstructured terrain navigation,” says Yin.

The paper, “A Self-Resetting Soft Ring for Autonomous, Continuous Leaping in Unstructured Environments,” is published in PNAS. The paper was co-authored by Caizhi Zhou and Haoze Sun, both Ph.D. students at NC State; and Haitao Qing and Yaoye Hong, both Ph.D. graduates from NC State.

This work was done with support from the National Science Foundation under grants 2329674, 2445551 and 2527304.