A robot does not always need a motor to move. Scientists have now demonstrated tiny machines that can travel through water and even fly using sound waves instead of conventional motors.
The technology, developed by researchers at EPFL's MicroBioRobotic Systems Lab, uses carefully shaped cavities built into the robots themselves. When exposed to particular sound frequencies, the cavities generate forces that push the machines forward.
That could be useful at a scale where conventional motors become difficult to fit. The smaller a robot gets, the harder it becomes to accommodate the moving parts, power sources, and other hardware normally needed to make it move.
The researchers' approach takes a different route: rather than shrinking a motor, they built the mechanism for producing movement directly into the robot's structure.
How Sound Waves Produce Movement
The idea is based on a phenomenon known as Helmholtz resonance.
The same basic effect can be heard when someone blows across the opening of an empty bottle. Air inside the cavity vibrates at a particular frequency, creating a distinct sound.
The researchers adapted that principle for propulsion. Their tiny robots contain hollow chambers designed to resonate when exposed to specific sound frequencies. The resulting air movement produces thrust, allowing the machines to move without conventional mechanical propulsion.
Because the cavities are part of the robot itself, there is no need for a separate motor with gears or other moving components.
That becomes particularly important as the machines shrink.
Why Eliminating Motors Matters
A conventional miniature robot still needs a way to generate movement. At larger scales, motors, batteries, and mechanical components can be relatively easy to accommodate. At very small scales, however, those same components can consume much of the available space.
The EPFL researchers used advanced 3D printing to create their tiny structures with precisely shaped internal cavities. Some of their airborne devices, called microfliers, measured roughly 1 millimeter across.
The team also built small boats that could move across water.
Instead of carrying a miniature engine, these machines use their shape to interact with sound.
That design gives researchers another way to approach a problem that has become increasingly important as robotics moves into smaller and smaller spaces.
Different Frequencies Can Change How They Move
The sound does more than simply make the robots move.
The researchers designed different resonating cavities to respond to different frequencies. Changing the frequency of the sound can therefore activate different parts of the robot and alter the direction of its movement.
In experiments with the miniature boats, the system allowed the researchers to steer the machines around obstacles.
The airborne designs used different arrangements to generate lift. One produced upward thrust through acoustic cavities, while another incorporated sound-driven rotating blades to create a helicopter-like motion.
The demonstrations show that acoustic energy can be turned into controlled movement without putting a conventional motor inside the robot.
That raises a more interesting question: where could machines this small eventually be used?
Could Tiny Robots Eventually Be Used in Medicine?
The researchers have not demonstrated these robots performing medical procedures or travelling through the human body. Any medical application remains a future possibility rather than a current capability.
Even so, the technology could be relevant to biomedical engineering because size matters when a device has to operate in a confined environment.
A miniature medical robot would have limited room for propulsion, sensors, electronics and other equipment. Removing a conventional motor could potentially leave more space for components that perform the actual task.
In the longer term, researchers could investigate whether similar designs might be adapted for miniature biomedical devices, including systems intended to move through difficult-to-reach environments.
Such applications could involve delivering materials, sensing conditions or manipulating objects, although none of these uses has been demonstrated in humans with this technology.
Before that could happen, researchers would have to solve several major problems. A medical robot would need to be precisely controllable, safe around living tissue and made from materials suitable for use inside the body. Scientists would also need to determine how the machines could be powered and controlled in environments very different from a laboratory.
A New Way to Build Smaller Machines
For now, the work is less about creating a medical robot than demonstrating a new way to make very small machines move.
The researchers showed that carefully engineered cavities can turn sound into mechanical force, allowing propulsion to become part of the robot's physical structure rather than a separate motorized component.
That could give engineers another option as they push robotics toward smaller scales.
The next step is not simply making these machines smaller. It is finding out what they can actually do once they are small enough to reach places conventional robots cannot.
