Researchers at Penn Engineering have turned a common nuisance—a knotted string—into a high-performance, heat-activated robot.

These tiny, soft robots can leap up to two meters (over six foot) into the air, flip mid-flight, and even “boomerang” back to their starting point. It offers a new method for autonomous reforestation and agriculture.

The study reveals how a simple change in knot “topology” can transform a millimeter-thick fiber into a programmable machine capable of performing complex gymnastic routines without a single electronic component.

The research team, led by Shu Yang and Yaoye Hong, utilized a dual-material fiber consisting of a stiff Kevlar core and a surrounding shell of liquid crystal elastomer (LCE).

This pairing allows the fiber to store elastic energy when twisted and knotted, acting as a spring held in place by a friction-based latch.

Using mathematical topology

The system is activated by heat rather than electronics.

“When the temperature rises to about 60 to 90 degrees Celsius, the LCE shell contracts and untwists, loosening the knot just enough to trigger an abrupt untying. In a fraction of a second, stored elastic energy converts into kinetic energy for rapid motion,” said the researchers in a press release.

A knot only a few millimeters long can reach heights hundreds of times its own size.

The specific movement of the robot is controlled by its mathematical topology. For instance, an overhand knot produces a flipping motion, while a figure-eight knot causes the robot to spin.

More intricate knots can be designed to untie in stages, resulting in sequential movements similar to a gymnastic routine.

“People think of a knotted fiber as something passive,” said Yang. “But if you design the elasticity and materials carefully, the knot itself becomes an active system.”

Useful for reforestation, agriculture

To control the flight and descent, the team added a thin wing inspired by the autorotation of maple seeds.

Depending on the positioning of this wing, the robot can glide forward or curve back toward its starting point. This kinetic energy is particularly useful for reforestation, as it drives the robot into the soil with high local pressure.

This method generates penetration pressures roughly 30 times greater than previous seed-carrying systems that relied on rainfall to expand wood veneers. 

Because the new robots are triggered by heat, they can be activated by predictable sunlight in arid environments where rain is infrequent. The project originated from an interest in how rigid and flexible materials interact.

The addition of the Kevlar core allowed the fiber to resist deformation and store enough energy to double its jumping height, which matches the capabilities of soil-dwelling insects like springtails.

Developing adaptive, power-free machines

“Future versions may use more environmentally friendly components, especially if deployed outdoors. Researchers are also working to lower the activation temperature and refine the way the fibers interact with soil,” highlighted the researchers.

The long-term goal is to develop a series of adaptive, power-free machines that can navigate complex environments to solve ecological problems.

“The broader goal is to build a suite of small, adaptive machines that can operate in complex environments without electronics or external power,” concluded the press release.