EPFL's Tiny Robots Move Using Only Sound, No Batteries or Motors

As electronics get smaller, they run into a basic problem: some devices become too small to fit a conventional battery, motor or other moving parts. Researchers at the MicroBioRobotic Systems (MICROBS) Lab in EPFL's School of Engineering have found an unusual answer: sound.

The technique relies on Helmholtz resonance, the same effect that makes a humming sound when you blow air across the neck of a bottle. Air trapped inside a hollow structure, called an acoustic cavity, vibrates strongly at a certain frequency. When sound waves make that air vibrate, the cavity pushes out a concentrated stream of air, while the air flowing back in is more spread out. That imbalance creates thrust.

Scientists have long known how to levitate objects using sound waves, but in those experiments the objects are passively pushed around by external sound. “Instead of pushing devices around with sound waves, we have created acoustic resonators that are tuned to harness sound at specific frequencies to generate directional thrust and controlled motion,” said Selman Sakar, the lab's head.

The team first built miniature boats, each fitted with three cavities tuned to different frequencies, so changing the sound frequency could steer the boat in a chosen direction. They also built 'microfliers,' ultralight flying drones weighing as little as 150 micrograms, each with three microscopic cavities, powered by ultrasonic frequencies too high for humans to hear.

In another design, the researchers combined the cavities with tiny propeller blades. The cavities generated enough force to spin the blades at 13,000 revolutions per minute, faster than the several thousand RPM typical of regular drone propellers. The team says the approach could eventually build devices and robots that function without conventional batteries and motors, including flexible devices with several sound-responsive parts that move independently when triggered by different frequencies.