What if your walking stick could change itself into a foldable stool while you’re out on a wilderness hike? Or if a piece of wrapping paper could fold itself around a package? The idea of shape-shifting active matter has been a staple of sci-fi programming for decades (see: the liquid-metal T-1000 in Terminator 2, or Spidey foe “the Sandman”). But now experiments by teams in Georgia and the Netherlands are finding that the idea of active matter might not be such a stretch. Using agglomerations of worms as a biological model, researchers are designing mechanical devices that one day may be able to form and re-form into different shapes, just like the wrigglers.
In one recent experiment, researchers started by observing tens of thousands of California blackworms, humble creatures that form living masses to escape dehydration and heat, to study the principle of how active matter works. They watched the worms assembling into spherical balls to protect themselves, then sending out a few worms together to form a limb-like feeler to search for moisture or a cooler spot.
“If you take a blob of these worms and you hold them in your hand, they behave like Silly Putty and they flow through your fingers,” says Saad Bhamla, assistant professor of chemical and biomolecular engineering at Georgia Tech. “But if you make a ball of them and throw it at the ground, it won’t splatter. They bounce, but they won’t die. They will maintain their integrity and shape. They have solid and liquid-like behavior. They can move across flat surfaces. They can crawl over things together.”
Bhamla and postdoctoral fellow Yasemin Ozkan-Aydin actually constructed a tiny “worm gymnasium” in their lab to see how individual worms pulled and twisted the others together to form these structures. (Don’t watch this video of the California blackworms at work if squigglies creep you out.)
Ozkan-Aydin’s worm gym included tiny levers and poles that allowed her to measure some of the forces used by two or three “puller” worms that were capable of moving a 15-worm blob across a surface. Some worms were stronger than others, but they became lazy and stopped working as she increased the temperature in the container. She also observed the worms moving as a group away from bright lights, toward colder and darker areas. Some worms acted as pullers, others helped reduce the friction of the entire blob by acting as lifters.
While bees, birds, and ants swarm together to mate or protect themselves against predators, these worms are able to braid themselves together to accomplish tasks that unconnected individuals can’t handle. They live at the bottom of freshwater ponds, feeding on bacteria and other microorganisms. During periods of sustained drought, when the water in the ponds runs low, the blob-forming is a kind of collective decisionmaking that enables worms to survive longer without drying out. The worm sphere is able to conserve water, since it exposes less surface area to the air than the worms would if they remained solo. Some of these balls can grow as large as 100,000 worms.
In fact, Bhamla says he first encountered the worms while walking by a dried-out pond on the Stanford University campus in 2017 as a graduate student. He was curious about what kind of life might return to a drought-stricken lake. “It had just rained, and I was excited because California had a lot of drought,” Bhamla remembers. “I was curious about this pond—when it is dry for so long, what happens when water arrives? What kind of life might emerge?”
Bhamla returned to the pond with a bottle of water and a pipette to collect rejuvenated worms that were starting to form small tangles of life. After graduating with a doctoral degree in molecular engineering from Stanford, Bhamla moved to a position at Georgia Tech and has been conducting experiments on the worm blobs ever since.
By studying these worms in the lab, the Georgia Tech team was also able to construct simple mechanical analogs of the worm blobs. Using the worm behavior as a blueprint, Ozkan-Aydin devised six 3D-printed robots, each about 3 to 4 inches long. (Unlike actual worms, each device had two arms and two light sensors.) Then, they could be programmed to perform various movements and observed as they tangled with each other.
Hoping to gain some insight into how to develop future robotic swarms with better energy efficiency, the experimenters measured the energy used by each individual robot. The team determined that the robots used less power while wiggling than crawling. The Georgia Tech researchers published the results of their experiments with the worm blobs and their robotic counterparts this month in the journal Proceedings of the National Academy of Sciences.
This kind of work might one day lead to programmable active matter, says Daniel Goldman, a professor of physics at Georgia Tech. Active matter is a hypothetical material that would shape-shift just like the worm blobs—in which tiny particles of material would organize themselves in response to a stimulus or a program. Imagine that self-wrapping paper, for example, or a liquid-metal tool that could reshape itself depending on what kind of job you need to do. “These robot models can act like theoretical and computational models to test biological hypotheses,” Goldman says. “Once you get the robot physical system going, it can inspire engineers to create better engineered devices.”
The idea of active matter is also being studied by Antoine Deblais, a postdoctoral researcher at the University of Amsterdam who performed experiments on another species of worm that are used as food for aquarium fish. He, too, observed the collective behavior of the worms as a way of understanding movement—but in this case, the behavior of particles in polymers, the large molecules that make up many plastics. In an experiment published last year in Physical Review Letters. Deblais and colleagues put a blob of these sludge worms into a special device to measure the viscosity of the liquid around them. As the temperature dropped, the worms could alter the viscosity or thickness of the liquid, turning the liquid into an oozy blob of sorts, just like what happens with plastics and other chemical polymers when they are cooled. Deblais says the next step is developing a kind of “active polymer,” or plastic that can reshape itself quickly depending on its use.
Deblais says the Georgia Tech team’s experiments with both the worms and their robotic analogs might lead to new kinds of devices or substances that can be programmed to squeeze into tight spaces, just like a sci-fi blob monster sneaking from one room to another. “If you place these worms or the bots in an area, you can do a lot of things,” Deblais says. “In a hostile environment or war zone, they could flow through constrictions in buildings or over a rough landscape. I can imagine many such things.”
Of course, designing and building active matter or a programmable shape-shifting substance is still a long way away. Still, Bhamal and Goldman see a day when the microscopic strands and particles that make up materials can be cued to change into different shapes and strengths, or behave like a fluid and a solid, just like the living worms in the lab.
But until then, Bhamla says he’s happy watching the tangle of crawlers act in very strange ways. “If you were to think about a shape-shifting living system that existed, this worm blob is as close as it comes,” says Bhamla. “They are doing all kinds of crazy things to get from point A to point B. If I ever want to make a shape-shifting robot, what better system than this to study?”
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