Scientists create ‘odd’ objects that adapt and move over obstacles

Locomotion, the ability to move from one place to another, is an essential survival strategy for virtually every organism. Adapting to the unpredictable terrain they run into, cells, fungi and microorganisms autonomously move and change shape to explore their environments, while animals run, crawl, slither, roll and jump.

But despite advances in computing power and AI, human-made robots still struggle to imitate this movement, especially in new and unpredictable terrain.

In a paper published March 12 in Nature, physicists from the University of Amsterdam and the University of Chicago showcase a series of ‘odd’ objects that are remarkably good at moving across any terrain they encounter—including uphill and over obstacles placed in their way. But unlike traditionally designed robots, these have no centralized control or brain; they are simply responding to small forces on each other and the terrain.

This offers a new way to accomplish locomotion, the scientists said, and could solve problems in robot movement.

"The striking thing about this is its minimalism," said Colin Scheibner (PhD’23), co-first author on the paper. "There’s not a complicated algorithm powering its decisions. I feel there’s something powerful about its simplicity, which approaches the question of movement in a different way."

Brainless motion from odd elasticity

Instead of a traditional robot design, the ‘odd’ robots are instead a set of simple motorized devices, connected by elastic springs. When turned on, the motions of the building blocks interacting with each other propels the whole set forward together.

The key difference is that these objects aren’t being steered or controlled by a centralized "brain," which sets them apart from most previous attempts at robotic movement. Instead, the motion comes solely from the interactions between the objects’ motorised building blocks.

"The building blocks exert forces that are nonsymmetric and nonreciprocal. This means that a building block A reacts to its neighbouring building block B differently than how B reacts to A," explained Jonas Veenstra, co-first author of the publication and a PhD student at the University of Amsterdam.

As the object moves, a self-reinforcing cycle is created. The terrain deforms the object, the object’s building blocks sense and respond by deforming the object further—and the object moves forward and encounters new terrain.

(Video by Jonas Veenstra)

The object was able to move easily over very challenging terrain, such as piles of sand or a field of ball bearings.

Using the same principles, the researchers could build different shapes: A wormlike ‘odd chain’ of linked building blocks will wiggle through a bendy tunnel and over uneven ground. Similarly, an ‘odd ball’ made of the building blocks connected in a hexagonal grid will roll on flat terrain, but changes to a crawling gait to move uphill.

Unlike ordinary elastic materials, which compress along the same direction as an applied force and bulge out in the perpendicular direction, the objects made of these building blocks always stretch diagonally at a fixed angle, a phenomenon known as "odd elasticity."

‘A sense of shock’

Don’t confuse the objects’ wiggly motions for randomness, the scientists said: Their locomotion is reliable and robust thanks to their decentralised nature, and to the fact that the solids actively sense and respond to their environment.

"There was a sense of shock when we discovered how robust they were, given their simplicity," said Scheibner, who conducted the research as a graduate student at UChicago and is now a research fellow at Princeton University.

Even when the scientists turned off more than half of the individual units, the robot as a whole could keep going.

(Video by Jonas Veenstra)

The research is part of a burgeoning field known as "active metamaterials"—referring to artificial structures made up of individual units powered by motors, which can demonstrate interesting abilities.

"You can imagine many situations where you need a machine to venture into rough territory autonomously," said Vincenzo Vitelli, professor of physics at the University of Chicago and study co-author. "The idea of one that is extremely robust to damage, and gains more energy the more obstacles it encounters, is very appealing."

The basic principle is intriguing, too, the scientists said; it might be a way to think about how locomotion first evolved, in animals that don’t have centralized brains, like starfish or slime molds. It could also suggest applications in other fields such as material science and chemistry.

"It’s curious to think about how small you could make this building block," said Scheibner. "For example, since it doesn’t require complex algorithms, could you go down to just a few molecules? Just a few chemical bonds? Harnessing broken symmetries for minimal feedback loops could inspire techniques at small scales."

The research was partly conducted with the University of Chicago Materials Research Science and Engineering Center, the Chan Zuckerberg Initiative, the Physics Frontier Center for Living Systems, and the National Institute for Theory and Mathematics in Biology.

Citation: "Adaptive locomotion of active solids." Veenstra, Scheibner, Brandenbourger, Binysh, Souslov, Vitelli, and Coulais, Nature, March 12, 2025.

Funding: European Research Council, Netherlands Organisation for Scientific Research, Army Research Office, National Science Foundation, Chan Zuckerberg Initiative, European Union and Simons Foundation.

—Adapted in part from an article published by the University of Amsterdam.