This Acoustic Tractor Beam Can Levitate Small Objects With Sound

The device allows researchers to float and manipulate targets with just a single array of ultrasound emitters

An artist's rendering shows an acoustic hologram trapping a particle over a levitation device. Courtesy Asier Marzo, Bruce Drinkwater and Sriram Subramanian

It's no Mattel hoverboard. But a device built by a team in Spain and the U.K. can levitate and manipulate small objects in air, and possibly in water and human tissue, using high-frequency sound waves. The technology holds promise in a variety of fields ranging from medicine to space exploration.

Scientists already knew that sound waves create oscillating pockets of pressurized air, which can produce a force on an object capable of counteracting the pull of gravity. But while ultrasound levitation devices do exist, they all rely on standing waves, which are created when two sound waves of the same frequency are emitted from opposite directions and superimposed on one another. That means all previous devices require two sets of transducers.

“All previous levitators had to surround the particle with acoustic elements, which was cumbersome for some kind of manipulations,” says study leader Asier Marzo at Public University of Navarre in Spain. “Our technique, however, only requires sound waves from one side. It’s like a laser—you can levitate particles, but with a single beam.”

To develop their technology, Marzo and his colleagues drew inspiration from visual holograms, in which a light field is projected from a flat surface to produce a series of “interference patterns” that form a 3D image. Sound waves are also capable of making interference patterns, so the same principle can be applied.

“Basically we copied the principle of light holograms to create these acoustic holograms,” says Marzo, whose team describes their work this week in Nature Communications.

Marzo and his team arranged 64 small 16-volt transducers in a grid-like pattern. Each transducer was calibrated to emit sound waves at 40,000 Hertz, a frequency that far exceeds the maximum sensitivity of the human ear (20,000 Hz) but is audible to other animals such as dogs, cats and bats.

Though the frequency and power of each transducer was identical, the scientists crafted an algorithm that varied the relative peaks and troughs of each wave to generate interference patterns and create acoustic objects.

The challenge was that these acoustic objects were inaudible and invisible to humans, so the team had to develop various simulations to “see” the sound. In an approach that would make any synesthete proud, Marzo used a microphone to sample ultrasound waves emitted by the transducers and then fed the data through a 3D printer, which they used to create digital visualizations of the auditory objects.

After testing a variety of acoustic shapes, the research team discovered three that were most effective: the twin trap, which resembles a pair of tweezers; the vortex trap, analogous to a tornado that suspends a spinning object in its center; and the bottle trap, which levitates the object in the empty space inside the bottle. 

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Though the current experiment only lifted small Styrofoam beads, Marzo believes the technology can be scaled for different objects by manipulating the frequency of the sound waves, which determines the size of the acoustic objects, as well as the overall power of the system, which allows the levitation of lighter or heavier objects over longer distances.

“The levitation of particles by one-sided transducers is an amazing result that opens new possibilities for acoustic levitation technology,” says Marco Aurélio Brizzotti Andrade, an assistant professor of physics at the University of São Paulo who has previously worked on sound-based levitation.

“One application of scaling down is in vivo manipulation—meaning levitating and manipulating particles inside the body,” says Marzo. “And these particles could be kidney stones, clots, tumors and even capsules for targeted drug delivery.” Ultrasonic levitation does not interfere with magnetic resonance imaging, so doctors could instantaneously image the action during in vivo manipulation.

And when it comes to these micromanipulations in the human body, the one-sided beam technology has tremendous advantage over the two-sided standing wave technology. For starters, levitation devices based on standing waves can accidentally trap more particles than the intended targets. “However, with one-sided levitators, and there is only a single trapping point,” he says.

Marzo points out, though, that ultrasound is limited in its ability levitate larger objects: “To pick up a beach ball-sized object would require 1,000 Hz. But that enters the audible range, which could be annoying or even dangerous to the human ear.”

The technology also has some promising applications in outer space, where it can suspend larger objects in lower gravity and prevent them from drifting around uncontrolled. But Marzo dismisses any notions of a Star Trek-like tractor beam capable of manipulating humans on Earth.

Under normal gravity, “the power required to lift a human would probably be lethal,” says Marzo. “If you apply too much ultrasound power to a liquid, you will create microbubbles.” In other words, too much sound power can make your blood boil.

In future studies, Marzo hopes to collaborate with ultrasound specialists to refine the technology for medical applications and further expand the approach to different sized objects.

“That's the nice thing about sound," he says. "You have a wide range of frequencies that you can utilize for a variety of applications.”

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