Engineers at EPFL, the École Polytechnique Fédérale de Lausanne, have sent two microscopic vehicles into the air without a motor, a battery, a wire or a single electronic component on board. The craft weigh 150 and 184 micrograms and consist of nothing more than hollow polymer shells, printed in one piece, that lift off when an ultrasonic field is switched on. The research appeared in August in Science Advances.
The fine print, however, sits on the other side of the bench. Beneath the flyers, an array of 256 transducers generates the acoustic field that does the pushing. The motor has not disappeared — it has been relocated outside the aircraft.
- No levitation involved: The authors explicitly reject the acoustic-levitation label; spheres of the same polymer do not rise in the identical field.
- Helmholtz resonance provides the thrust: Each part holds a cavity with a narrow neck whose air column oscillates asymmetrically, generating a directional jet.
- It scales cleanly: The same effect works in centimetre-sized parts resonating at 200 Hz and in micron-scale parts responding at 40 kHz, beyond human hearing.
Why an ultrasound field can push a solid object
Inside every flyer sits a hollow cavity capped by a narrow neck — the geometry of a bottle that whistles when you blow across its opening. The trapped air column oscillates in step with the incoming sound and, because it enters and leaves the cavity asymmetrically, produces a steady directional jet. That jet is the thrust.
The elegant part is the scaling behaviour: pushing force grows almost perfectly linearly with cavity volume, so shrinking the device does not kill the effect, it shifts the operating frequency upward. Micron-scale structures answer to 40 kilohertz, inaudible to humans.
Two flyers and a fleet of frequency-steered boats
Both airborne demonstrations were nanoprinted as single pieces. One, at 150 micrograms, carries three microresonators that generate vertical thrust with a thrust-to-weight ratio of 4.9; it flies unstably and occasionally flips over. The second, at 184 micrograms, is shaped like a rotor, spins at up to 13,000 revolutions per minute and self-stabilises like a spinning top. Both operate inside the focus of the array, roughly three centimetres from its surface: the first tops out near 12 millimetres and settles around seven, while raising the array voltage lifts the rotor’s equilibrium height without changing its spin much — it holds near 6,500 rpm. Magnetic flyers of similar dimensions, the authors note, weigh three orders of magnitude more.
The second half of the paper is arguably more striking. The team built boats about five centimetres long fitted with up to three cavities tuned to different notes: at 1.2 kHz the craft moves forward, at 1.8 kHz it turns one way and at 2.4 kHz the other. Steering happens purely by changing pitch, with no moving parts. The boats traced a figure-eight course with a mean tracking error of 5.7 millimetres and a maximum of 17.4. Knocked off course by a blast of air, they could be reoriented remotely and still complete the path. One variant does carry a battery, a board and an Arduino microcontroller, with two contact transducers bonded to the resonators — proof that in that configuration sound replaces the motor, not the power supply.
Impressive physics, constrained by the bench
None of this works outside the laboratory yet, and the authors are candid about why. The system “depends on the power and precision of the external hardware,” and the working volume is bounded by the actuation and imaging equipment; without the array underneath, there is no flight. The array also runs at a single frequency, ruling out in-flight steering, and further miniaturisation hits a physical wall: below a certain size, air viscosity dominates and thrust collapses. Autonomy would mean packing in transducers, power and sensors without exceeding the available lift — what the paper calls “substantial practical challenges.”
“Our work demonstrates the feasibility of transforming a simple mechanical component, cleverly designed, into robotic matter,” said Mahmut Selman Sakar, who heads EPFL’s MICROBS laboratory.
First author Junsun Hwang, a doctoral researcher, added that the concept “is compatible with even greater miniaturisation.” The applications the team sketches are prospective: micromachines free of motors and magnetic materials, and surfaces with embedded resonators that bend, vibrate or shift depending on the frequency they receive.
The catchy framing — drones that fly on sound — both overpromises and undersells. It hints at autonomy where there is total dependence on lab hardware. The real breakthrough is not the aircraft but the actuator: a wireless way to move matter that needs no magnets, no electronics and no exotic materials, and behaves alike at centimetre and micron scales. The likeliest destination is not free flight but the manipulation of tiny objects in spaces too small for a motor to fit.