Barely visible to the naked eye, each robot measures about 200 by 300 by 50 micrometers, smaller than a grain of salt. Operating at the scale of many biological microorganisms, the robots could advance medicine by monitoring the health of individual cells and manufacturing by helping construct microscale devices.
Powered by light, the robots carry microscopic computers and can be programmed to move in complex patterns, sense local temperatures and adjust their paths accordingly.
Described in Science Robotics and Proceedings of the National Academy of Sciences (PNAS), the robots operate without tethers, magnetic fields or joystick-like control from the outside, making them the first truly autonomous, programmable robots at this scale.
"We've made autonomous robots 10,000 times smaller," says Marc Miskin, Assistant Professor in Electrical and Systems Engineering at Penn Engineering and the papers' senior author. "That opens up an entirely new scale for programmable robots."
The forces that dominate the human world, like gravity and inertia, depend on volume. Shrink down to the size of a cell, however, and forces tied to surface area, like drag and viscosity, take over. "If you're small enough, pushing on water is like pushing through tar," says Miskin.
In other words, at the microscale, strategies that move larger robots, like limbs, rarely succeed. "Very tiny legs and arms are easy to break," says Miskin. "They're also very hard to build."
So the team had to design an entirely new propulsion system, one that worked with - rather than against - the unique physics of locomotion in the microscopic realm.
The new robots, by contrast, don't flex their bodies at all. Rather, they generate an electrical field that nudges ions in the surrounding solution. Those ions, in turn, push on nearby water molecules, animating the water around the robot's body. "It's as if the robot is in a moving river," says Miskin, "but the robot is also causing the river to move."
The robots can adjust the electrical field that causes the effect, allowing them to move in complex patterns and even travel in coordinated groups, much like a school of fish, at speeds of up to one body length per second.
And because the electrodes that generate the field have no moving parts, the robots are extremely durable. "You can repeatedly transfer these robots from one sample to another using a micropipette without damaging them," says Miskin. Charged by the glow of an LED, the robots can keep swimming for months on end.
Blaauw's lab holds the record for the world's smallest computer. When Miskin and Blaauw first met at a presentation hosted by the Defense Advanced Research Projects Agency (DARPA) five years ago, the pair immediately realized that their technologies were a perfect match. "We saw that Penn Engineering's propulsion system and our tiny electronic computers were just made for each other," says Blaauw. Still, it took five years of hard work on both sides to deliver their first working robot.
"The key challenge for the electronics," says Blaauw, "is that the solar panels are tiny and produce only 75 nanowatts of power. That is over 100,000 times less power than what a smart watch consumes." To run the robot's computer on such little power, the Michigan team developed special circuits that operate at extremely low voltages and bring down the computer's power consumption by more than 1000 times.
Still, the solar panels occupy the majority of the space on the robot. Therefore, the second challenge was to cram the processor and memory to store a program in the little space that remained. "We had to totally rethink the computer program instructions," says Blaauw, "condensing what conventionally would require many instructions for propulsion control into a single, special instruction to shrink the program's length to fit in the robot's tiny memory space."
The robots have electronic sensors that can detect the temperature to within a third of a degree Celsius. This lets robots move towards areas of increasing temperature, or report the temperature - a proxy for cellular activity - allowing them to monitor the health of individual cells.
"To report out their temperature measurements, we designed a special computer instruction that encodes a value, such as the measured temperature, in the wiggles of a little dance the robot performs," says Blaauw. "We then look at this dance through a microscope with a camera and decode from the wiggles what the robots are saying to us. It's very similar to how honey bees communicate with each other."
The robots are programmed by pulses of light that also power them. Each robot has a unique address that allows the researchers to load different programs on each robot. "This opens up a host of possibilities," adds Blaauw, "with each robot potentially performing a different role in a larger, joint task."
"This is really just the first chapter," says Miskin. "We've shown that you can put a brain, a sensor and a motor into something almost too small to see, and have it survive and work for months. Once you have that foundation, you can layer on all kinds of intelligence and functionality. It opens the door to a whole new future for robotics at the microscale."
Research Report:Microscopic Robots That Sense, Think, Act, and Compute
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University of Pennsylvania School of Engineering and Applied Science
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