(Inside Science) -- Four-legged robots smaller than some microbes can walk when zapped with laser light, a new study reports.
Scientists have long sought to create microscopic robots invisible to the naked eye. However, powering the motions of such machines has proven difficult. Microscopic robots driven by heat do not work well because heat diffuses rapidly. Materials similar to organic muscle are difficult to integrate into robots because they are damaged by the standard chemicals used in microfabrication. And so on.
The new robots possess leg muscles composed of platinum strips just 7 nanometers thick. When the robots are submerged in water, electrically charged ions in the liquid attach to the strips, making them swell and flex. A tiny amount of electricity can drive the ions off the platinum, straightening the strips.
The robots each carry photovoltaic devices that convert light to electricity. By illuminating these robots with a series of near-infrared laser pulses, the researchers can make the robots' legs curl and straighten, driving the machines to walk at an average speed of about 60 microns per minute. (The average human hair is about 100 microns wide.)
"Zapping small invisible robots with lasers? There's something really cool about that," said study lead author Marc Miskin, an electrical engineer at the University of Pennsylvania in Philadelphia.
One of the microscopic robots walks in a circle. Video credit: Marc Miskin
The bodies of the robots are as small as 40 microns long, with legs 30 microns in length when unfolded. In comparison, the single-celled microbes known as paramecia can range between 50 and 320 microns long. The researchers said that to the best of their knowledge, these are the first robots of such tiny size to use onboard electronics to drive their movements.
Experiments revealed the robots could withstand strong acids, as well as temperatures up to 275 C. They are also small enough to fit in hypodermic needles, offering the chance for the robots to be injected into people's bodies.
However, "all the robots can do right now is walk," Miskin said. "They are far from any applications any time soon. Everything we build right now is still very primitive compared to biology."
The researchers suggested microscopic robots could one day be powered only by ambient sunlight and manufactured at much less than a penny per robot using commercial silicon foundries. "That's the dream -- they get power from the sun, and you can program it to find a chemical or other target and it would start moving," Miskin said.
The scientists detailed their findings in the Aug. 27 issue of the journal Nature.