Researchers at the University of California, Berkeley, have just unveiled the world’s smallest flying robot. With a wingspan of just 9.4 millimeters and weighing 21 milligrams — smaller than a grain of rice and lighter than a single raindrop — this robot is the smallest and lightest untethered flying machine ever built.

The insect-scale machine can hover, turn, and even recover after a midair collision without any wires grounding it. The robot lifts off under the influence of a magnetic field and stays upright through the steady spinning of tiny propeller blades.

Shrinking Flight

For years, roboticists have dreamed of insect-sized machines. They could fly freely through forests, disaster zones, or even deep inside the crevices of a collapsed building. With this dream in mind, one way to make them smaller is to ditch the battery altogether. But this often means tethering the robot to a power source with a wire — a solution that limits movement and usefulness.

“The dream was to make flying robots to fly anywhere and anytime without using an electrical wire for the power source,” Liwei Lin, a professor of mechanical engineering at UC Berkeley, told IEEE Spectrum.

Now, Lin and his team have demonstrated a new way forward. Their prototype flits through the air using an external magnetic field as its invisible engine.

The robot is designed with a 3D-printed rotor: four tiny blades encircled by a stabilizing ring. On top sit two minuscule permanent magnets. When placed within a changing magnetic field, the magnets cause the rotor to spin — and with enough spin, the robot takes off.

Small in Size, Big on Potential

Creating a robot this small that can fly untethered is a major feat of engineering. For comparison, the previous record holder for the smallest flying robot had a wingspan of 28 millimeters — which is three times wider than the new design. Nature, of course, still holds the record for smallest flier. The parasitic wasp Nasonia vitripennis has a 3-millimeter wingspan and weighs less than a milligram.

Of all flying robots under 1 gram, only three have ever achieved untethered flight: the Robobee X-Wing (256 mg), a laser-powered drone (190 mg), and now, this 21-mg magnetic marvel.

Despite its size, the robot can do more than just hover. By tuning the frequency of the magnetic field — 310 hertz for hovering, 340 hertz for climbing — the team can control its motion. By manipulating the magnetic field’s orientation, they can steer it side to side.

Remarkably, the robot can recover from gentle collisions without any sensors or onboard controllers. That means it can keep flying even after bumping into a wall, as long as the impact isn’t too strong.

At optimal settings, the robot achieves a lift-to-drag ratio of 0.7 — respectable for such a tiny craft — and a lift-to-power ratio of 0.072 newtons per watt. That’s enough to keep it aloft, and even carry tiny payloads, like sensors.

In one test, a 20.5-millimeter version of the robot weighing 162 milligrams carried an infrared sensor weighing 110 milligrams. That’s almost 70% of its own weight.

And it’s efficient — more so than other reported flying robots, and even more than fruit flies or hummingbirds, according to the team’s measurements of lift generation.

What’s Next for the Micro-Fliers?

Right now, there’s a catch. These robots can only fly about 10 centimeters (4 inches) from their magnetic power source. That’s far from the range needed for most real-world applications.

But the researchers have ideas. “It could be possible to drive micro flying robots using electromagnetic waves such as those in radio or cell phone transmission signals,” says Lin.

The team is also exploring ways to extend flight range. One approach is to increase the strength and shape of the magnetic field by using more coils or using “beamforming” techniques, which direct magnetic energy precisely. With those improvements, the robots could potentially fly up to a meter from the coils.

There’s also the possibility of shrinking them even more. Making the robots lighter would reduce the magnetic energy needed to lift them, opening up new ways to power flight wirelessly.

Looking even further ahead, the team envisions robots with onboard devices that can convert magnetic energy into electricity. That would open the door for adding tiny cameras, sensors, or processors — creating fully autonomous micro-fliers.

From inspecting fragile industrial machines to pollinating flowers in greenhouses, their future roles could be as varied as they are small. With further advances in power delivery and miniaturization, these robots might one day slip through spaces too small for drones, too dangerous for people, and too delicate for anything else.

The findings appeared in Science Advances.