A simple idea has led to a surprising invention: tiny robots made from knotted fibers that can jump, spin, glide and even plant seeds.
Instead of using motors or electronics, these soft robots rely on stored energy inside a knot.
When the knot suddenly releases, the energy is unleashed in a powerful burst of motion.
The work, led by Shu Yang at University of Pennsylvania and published in Science, reimagines what a knot can do.
Normally, knots are used to hold things together. But the researchers asked a different question: what if a knot is designed to come undone in a controlled way?
The robot itself is extremely small, made from a fiber thinner than a millimeter. It combines two very different materials.
At the center is Kevlar, which gives strength and stiffness. Around it is a soft outer layer made of liquid crystal elastomer, a material that changes shape when heated. This combination allows the fiber to store energy when it is twisted and tied into a knot.
When the temperature rises to about 60 to 90 degrees Celsius, the outer layer contracts and begins to untwist.
This loosens the knot just enough to trigger a sudden release. In a split second, the stored energy turns into motion.
Even a knot only a few millimeters long can launch itself nearly two meters into the air, reaching heights hundreds of times its size.
The motion is not random. By changing the type of knot or how tightly the fiber is twisted beforehand, the researchers can control how the robot moves.
A simple knot might cause it to flip through the air. A more complex knot can make it spin like a propeller or perform a sequence of movements, almost like a tiny gymnast.
The team also added a small wing inspired by falling maple seeds. This wing helps the robot glide and control its landing. In some cases, the robot can even curve back toward where it started, similar to a boomerang. The wing also stabilizes the robot as it falls.
One of the most exciting uses for this technology is planting seeds. Earlier designs for self-burying seed carriers relied on rain to activate their movement, which made them unreliable.
This new system uses heat instead, which can come from sunlight. In hot environments, the fibers can easily reach the temperature needed to trigger the jump.
When the robot lands, the force of the jump drives it into the soil. The pressure it generates is much stronger than earlier designs, helping it anchor firmly in the ground. In early tests, seeds attached to the robots were successfully planted and later germinated, showing promise for future use in farming or reforestation.
The idea started as a curiosity about how twisted fibers behave. But by combining different materials and studying how knots release energy, the researchers created something entirely new. These tiny machines show that even simple structures, like a knot, can become powerful tools when carefully designed.
In the future, the team hopes to improve the materials and make the robots more environmentally friendly.
Their long-term goal is to develop small, smart systems that can work in nature without batteries or electronics. Sometimes, the most advanced technology begins with the simplest idea—and in this case, it was just a knot.
