“But the legs did not exist before,” McEuen said. “There were no small, electrically activatable actuators that you could use. So we had to invent those and then combine them with the electronics.”

Using atomic layer deposition and lithography, the team constructed the legs from strips of platinum only a few dozen atoms thick, capped on one side by a thin layer of inert titanium. Upon applying a positive electric charge to the platinum, negatively charged ions adsorb onto the exposed surface from the surrounding solution to neutralize the charge. These ions force the exposed platinum to expand, making the strip bend. The ultra-thinness of the strips enables the material to bend sharply without breaking. To help control the 3D limb motion, the researchers patterned rigid polymer panels on top of the strips. The gaps between the panels function like a knee or ankle, allowing the legs to bend in a controlled manner and thus generate motion.

The researchers control the robots by flashing laser pulses at different photovoltaics, each of which charges up a separate set of legs. By toggling the laser back and forth between the front and back photovoltaics, the robot walks.

“While these robots are primitive in their function – they’re not very fast, they don’t have a lot of computational capability – the innovations that we made to make them compatible with standard microchip fabrication open the door to making these microscopic robots smart, fast and mass producible,” Cohen said. “This is really just the first shot across the bow that, hey, we can do electronic integration on a tiny robot.”

The robots are certainly high-tech, but they operate with low voltage (200 millivolts) and low power (10 nanowatts), and remain strong and robust for their size. Because they are made with standard lithographic processes, they can be fabricated in parallel: About 1 million bots fit on a 4-inch silicon wafer.

Radical Collaboration
The researchers are exploring ways to soup up the robots with more complicated electronics and onboard computation – improvements that could one day result in swarms of microscopic robots crawling through and restructuring materials, or suturing blood vessels, or being dispatched en masse to probe large swaths of the human brain.

“Controlling a tiny robot is maybe as close as you can come to shrinking yourself down. I think machines like these are going to take us into all kinds of amazing worlds that are too small to see,” said Miskin, the study’s lead author.

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