Turning robotic ensembles into smart materials that mimic life

Researchers have engineered groups of robots that behave as smart materials with tunable shape and strength, mimicking living systems. “We’ve figured out a way for robots to behave more like a material,” said Matthew Devlin, a former doctoral researcher in the lab of University of California, Santa Barbara (USCB) mechanical engineering professor Elliot Hawkes, and the lead author of the article published in the journal Science.

Composed of individual, disk-shaped autonomous robots that look like small hockey pucks, the members of the collective are programmed to assemble themselves together into various forms with different material strengths.

One challenge of particular interest to the research team was creating a robotic material that could both be stiff and strong, yet be able to flow when a new form is needed. “Robotic materials should be able to take a shape and hold it” Hawkes explained, “but also able to selectively flow themselves into a new shape.” However, when robots are strongly held to each other in a group, it was not possible to reconfigure the group in a way that can flow and change shape at will. Until now.

For inspiration, the researchers tapped into previous work on how embryos are physically shaped by Otger Campàs, a former UCSB professor and currently the director of the Cluster of Excellence Physics of Life (PoL) at the Dresden University of Technology.

“Living embryonic tissues are the ultimate smart materials,” he said. “They have the ability to self-shape, self-heal and even control their material strength in space and time.”

While at UCSB, his laboratory discovered that embryos can melt like glass to shape themselves. “To sculpt an embryo, cells in tissues can switch between fluid and solid states; a phenomenon known as rigidity transitions in physics,” he added.

During the development of an embryo, cells have the remarkable ability of arranging themselves around each other, turning the organism from a blob of undifferentiated cells into a collection of discrete forms—like hands and feet—and of various consistencies, like bones and brain.

The researchers concentrated on enabling three biological processes behind these rigidity transitions: the active forces developing cells apply to one another that allow them to move around; the biochemical signaling that allows these cells to coordinate their movements in space and time; and their ability to adhere to each other, which ultimately lends the stiffness of the organism’s final form.

In the world of robots, the equivalent of cell-cell adhesion is achieved with magnets, which are incorporated into the perimeter of the robotic units. These allow the robots to hold onto to each other, and the entire group to behave as a rigid material. Additional forces between cells are encoded into tangential forces between robotic units, enabled by eight motorized gears along each robot’s circular exterior.

By modulating these forces between robots, the research team was able to enable reconfigurations in otherwise completely locked and rigid collectives, allowing them to reshape. The introduction of dynamic inter-unit forces overcame the challenge of turning rigid robotic collectives into malleable robotic materials, mirroring living embryonic tissues.

The biochemical signaling, meanwhile, is akin to a global coordinate system. “Each cell ‘knows’ its head and tail, so then it knows which way to squeeze and apply forces,” Hawkes explained. In this way, the collective of cells manages to change the shape of the tissue, such as when they line up next to each other and elongate the body. In the robots, this feat is accomplished by light sensors on the top of each robot, with polarized filters.

When light is shone on these sensors, the polarization of the light tells them which direction to spin its gears and thus how to change shape. “You can just tell them all at once under a constant light field which direction you want them to go, and they can all line up and do whatever they need to do,” Devlin added.

With all this in mind, the researchers were able to tune and control the group of robots to act like a smart material: sections of the group would turn on dynamic forces between robots and fluidize the collective, while in other sections the robots would simply hold onto each other to create a rigid material. Modulating these behaviors across the group of robots over time allowed the researchers to create robotic materials that support heavy loads but can also reshape, manipulate objects, and even self-heal.

Currently, the proof-of-concept robotic group comprises a small set of relatively large units (20). However, simulations conducted by former postdoctoral fellow Sangwoo Kim in the Campàs laboratory, and now assistant professor at EPFL, indicate the system can be scaled to larger numbers of miniaturized units. This could enable the development of robotic materials comprising thousands of units, that can take on myriad shapes and tune their physical characteristics at will, changing the concept of objects that we have today.

In addition to applications beyond robotics, such as the study of active matter in physics or collective behavior in biology, the combination of these robotic ensembles with machine learning strategies to control them could yield remarkable capabilities in robotic materials, bringing a science fiction dream to reality.