'Tractor beam' grabs beads with sound waves

Engineers in the UK have developed a system that can grab, hold and move small objects without touching them, using "holograms" made of sound waves.

It offers the kind of remote control that naturally draws comparison with the "tractor beams" of science fiction.

So far the team has tested the design on small pea-sized objects, which they can manipulate from 30-40cm away.

Writing in Nature Communications, they suggest the work could help develop remote surgical instruments.

In essence, an object sitting in a "quiet" region of space can be held there if it is surrounded by very high-intensity sound waves. As the pattern of that boundary shifts, the object can be moved around.

The researchers programmed a grid of small speakers to emit ultrasound in intricate, shifting patterns, crafting shapes from the interacting waves that resembled tweezers, bottles, and tiny tornado-like twisters.

These "holograms" were able to control small beads up to 5mm across. Crucially, the design works from just one side - including above or below the beads - instead of requiring the object to be surrounded by loudspeakers.

Prof Bruce Drinkwater from the University of Bristol, one of the study's authors, said the holy grail in this field is to use this sort of manipulation in, for example, targeted drug delivery.

"Our method, we hope, will now be applied, both at a smaller scale - maybe for medical purposes - and at a larger scale, potentially for handling dangerous materials in some sort of non-contact production line."

Prof Drinkwater developed the system with collaborators at Bristol company Ultrahaptics, as well as the University of Sussex.

He said the effort was first inspired by similar research from the University of Dundee, published last year, which demonstrated that sound waves could tug an object towards a sound source.

"That was the starting point for us thinking, actually, this could be possible," he told the BBC. "They showed that the force existed and they measured it experimentally - but they didn't hold anything stably.

"Our mission, as it were, was to try and take that and make it into a stable, working tractor beam, that could actually hold objects and move and manipulate them."

One of the authors of the 2014 Dundee study, Dr Mike MacDonald, said the work from Bristol was "a very nice demonstration" of the power of this technology.

The main difference between their work, he explained, was that the Bristol team had built their own speaker system from scratch - which made it very adaptable.

"We used a clinically approved device, which was designed for doing focused ultrasound surgery. We programmed that to create the same sort of beams that they did.

"[But] because they had their own system, it was more programmable. That allowed them to change the beams that they were creating in real time.

"It is quite different from what we did, in that it's moving individual particles around and it's reconfigurable. But in other ways, the beams it's producing are not millions of miles away from what's been done already by others - including us."

The invisible manipulation of objects certainly sets imaginations racing. When looking for ways to demonstrate what their contraption could do, Prof Drinkwater and his colleagues understandably settled on a sci-fi analogy.

"We wanted to demonstrate that we could do it upside down. We had a discussion and we thought that everyone thinks of a tractor beam as people being sucked up into space.

"So we mounted the array upside-down in a cardboard UFO, and the particle gets sucked up into it."

The Dundee researchers, before developing their own tractor beam in 2014, trod a similar line when they described their ultrasound device for pushing and turning as a "sonic screwdriver".

Nobody is more aware than engineers, it seems, that their research is increasingly blurring our experience of science fiction and fact.

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