A team created microscopic robots that incorporate semiconductor components, allowing them to be controlled — and made to walk — with standard electronic signals. The robots are about 5 microns thick, 40 microns wide, and range from 40 to 70 microns in length — roughly the same size as microorganisms like paramecium. These robots provide a template for building even more complex versions that utilize silicon-based intelligence, can be mass-produced, and may someday travel through human tissue and blood.
The new robots each consist of a simple circuit made from silicon photovoltaics — which essentially functions as the torso and brain — and four electrochemical actuators that function as legs. Since there were no small, electrically activatable actuators that could be used, the team had to invent them and then combine them with the electronics.
Using atomic layer deposition and lithography, they 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. The robots are compatible with standard microchip fabrication and operate with low voltage (200 millivolts) and low power (10 nanowatts). Because they are made with standard lithographic processes, they can be fabricated in parallel: about 1 million bots fit on a 4” silicon wafer.
The researchers are exploring ways to equip 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, suturing blood vessels, or being dispatched en masse to probe large swaths of the human brain.