Researchers at Penn Engineering have developed tiny, soft robots using opposable materials that are capable of leaping meters into the air with flexibility, responsiveness, and programmability.
2 min. read
When a knot lets go, it doesn’t just fall apart. It snaps.
That simple observation led Penn Engineers to rethink what a knot can do. Instead of treating it as something that holds tension, they asked a different question: what happens when you design a knot to release it?
Jiarui Wang, a member of the Yang Lab, holds up one of these programmable knot robots with forceps showing how something not much bigger than a grain of rice can pack a serious punch in the world of soft, automated robotics.
(Image: Courtesy of Penn Engineering)
The answer is a tiny, soft robot capable of leaping meters into the air, flipping mid-flight, spinning like a propeller or even gliding back to where it started.
At the center of this work, published in Science and led by Shu Yang, Joseph Bordogna Professor and Chair of Materials Science and Engineering, and postdoctoral associate Yaoye Hong, is a fiber no thicker than a millimeter, made from two seemingly opposing materials: a Kevlar core provides strength and stiffness, and a surrounding shell of liquid crystal elastomer (LCE) adds flexibility, responsiveness, and programmability. Together, they store energy when twisted and knotted, then release it all at once when heated.
The system behaves like a spring held in place by a latch. In this case, the latch is the knot. When the temperature rises to about 60-90° 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.
By adjusting the knot’s topology, the material that is used and even the way the fiber is pre-twisted before tying, the researchers can tune how the robot behaves after takeoff. This means the motion is not only powerful but programmable.
“Knotting the fiber allowed us to store much more energy,” Hong says. “And by changing the topology of the knot itself, we could control how that energy is released.”
“People think of a knotted fiber as something passive,” says Yang. “But if you design the elasticity and materials carefully, the knot itself becomes an active system.” A knot just a few millimeters long can launch itself nearly two meters into the air, reaching heights hundreds of times its own size. Adding a wing extends that control into the air. Inspired by the autorotation descent of maple seeds, the team attached a thin, leaf-like appendage to the fiber. In their robot, the wing stabilizes the structure and ensures continuous self-rotation during descent. More importantly, the forceful kinetic energy carried during jumping drives the slender rod’s dive into the soil, nearly vertically, achieving high local pressure that is essential for seeding.
That last behavior points to one of the most promising applications of this system: planting seeds.
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