Inspired by embryos, these disk-shaped robots use magnets, motors, and light to shift between rigid and fluid states. The result? A self-healing, shape-shifting system that could change how we build and interact with materials.

Robots That Behave Like Materials
"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 UCSB mechanical engineering professor Elliot Hawkes, and the lead author of a study published in the journal Science on February 20. 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 of the biggest challenges the team tackled was creating a robotic material that could be both stiff and strong while also able to flow into new configurations. "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." In the past, robots tightly connected in a collective couldn't easily rearrange themselves. Now, that has changed.

Taking Inspiration from Embryos

For inspiration, the researchers looked to how embryos form in nature, drawing on the work of Otger Campàs, former UCSB professor and now director of PoL at the Dresden University of Technology. "Living embryonic tissues are the ultimate smart materials," Campàs said. "They have the ability to self-shape, self-heal, and even control their material strength in space and time." His lab previously discovered that embryos can temporarily soften — almost like melting glass — to sculpt their final forms. "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 allow 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.

Magnets and Motors: The Key to Shape-Shifting

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.