Imagine a swarm of microrobots—tiny devices a few hair widths across—swimming through your blood vessels and repairing damage, or zipping around in computer chips as a security lock, or quickly knitting together heart tissue. Researchers at the University of California, Berkeley, Dartmouth College, and Duke University have shown how to use a single electrical signal to command a group of microrobots to self-assemble into larger structures. The researchers hope to use this method to build biological tissues. But for microrobots to do anything like that, researchers must first figure out a good way to control them.
“When things are very small, they tend to stick together,” says Jason Gorman, a robotics researcher in the Intelligent Systems Division at NIST who co-organizes an annual microrobotics competition that draws groups from around the world. “A lot of the locomotion methods that have been developed are focused on overcoming or leveraging this adhesion.”
So far, most control methods have involved pushing and pulling the tiny machines with magnetic fields. This approach has enabled them to zoom around on the face of a dime, pushing tiny objects or swim through blood vessels. However, these systems generally require complex setups of coils to generate the electromagnetic field or specialized components, and getting the robots to carry out a task can be difficult.
Bruce Donald, a professor of computer science and biochemistry at Duke, took a different approach, developing a microrobot that responds to electrostatic potential and is powered with voltage through an electric-array surface. Now he and others have demonstrated that they are able to control a group of these microrobots to create large shapes. They do this by tweaking the design of each robot a little so that each one responds to portions of the voltage with a different action, resulting in complex behaviors by the swarm.
“A good analogy is that we have multiple, remote-controlled cars but only one transmitter,” says Igor Paprotny, a post doctorate scientist at UC Berkeley and one of the lead researchers on this work, which he presented last week at a talk at Harvard University. During his talk, he passed around a container holding a wafer die the size of a thumbnail. On it were more than 100 microrobots.
“What we do is slightly change how the wheels turn,” he says. “Simple devices with a fairly simple behavior can be engineered to behave slightly different when you apply a global control signal. That allows a very complex set of behaviors.” The robots contain an actuator called a scratch drive, which bends in response to voltage supplied through the electric array. When it releases tension, it goes forward, in a movement similar to an inchworm’s. But the key to the robots’ varying behavior is the arms extending from the actuators. A steering arm on a microrobot snaps down in response to a certain amount of voltage, dragging on the surface and causing the robot to turn. By snapping the arm up and down one or two times a second, the team can control how much a given robot turns. To control a swarm, the team designed each robot with an arm that reacts differently during portions of the voltage signal. Computer algorithms vary the voltage sequence, prompting the robots to move in complex ways.