THE starship disengages its warp drive and draws up alongside an enemy destroyer. This civilisation doesn’t look friendly and the destroyer appears to be powering up its weapons systems. The captain gives the order to make a quick getaway – but what’s this? Our heroes appear to be caught in a tractor beam! Slowly but surely, they are being sucked towards their doom.
This is prime material for the plot of a science fiction movie. But there is one part of the scene that is a little closer to science fact than you might realise: tractor beams are becoming a reality. We have long been able to levitate objects; all you need is a big enough fan. But now we are building devices that can go much further, floating objects into the air and manipulating them with speed and precision.
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It is more than just a nifty trick. Tractor beams function like invisible robot arms, letting us move, separate and purify sensitive material, such as living cells or electronic components. This could be the basis of a form of contactless manufacturing that near eliminates the risk of contamination or breakage. They can also be used to make a new kind of holographic display.
“Magnets were used to suspend a number of objects in air – including a frog”
The most exciting application of all might involve moving things not in mid air but within living tissue. We could move tiny cameras around inside people, expel foreign objects, maybe even build minute machines inside our bodies – all without a scalpel in sight. Tractor beams may not be capable of snaring a starship, but they look to be extremely useful nonetheless.
We have been fascinated with the idea of objects hovering in space for decades. In 1933, two scientists in Poland used , where the peaks and troughs remained in the same position. This produced spots of low pressure in empty space, and the pair found that droplets of alcohol could be held there, in defiance of gravity. Impressive, but not hugely useful, because the object couldn’t be moved around and had to be almost completely surrounded by the crystal speakers.
Then there was the infamous levitating frog incident. In 1997, Andre Geim and his colleague Michael Berry, both then at the University of Bristol, UK, discovered it was possible to repel certain kinds of molecules, including water, with a hefty magnetic field. They used magnets to suspend a number of objects in the air, including a frog – which, like other animals, is mostly made of water. The magnets had to be extremely strong, though, and this method won’t work on any old material.
The origins of the precision tractor beam devices that are making waves today can be traced back to around 2010, when and Drinkwater were working in the same corridor as each other at the University of Bristol. They were using arrays of ultrasonic speakers like those found in a car’s parking sensor to create what is called haptic technology. The idea is to produce patterns of pressure in the air that people can feel. It might seem like an invisible finger drawing a circle on your palm, say, or a wave rippling over your fingers.
Haptics is handy in itself. It can work in conjunction with gesture recognition to provide a way of controlling machines, for instance. Subramanian, who is now based at University College London, recently founded a company called Ultraleap to develop it. Touchless control of in-car gadgets could reduce the amount of time people spend glancing away from the road and make driving safer. Haptic interfaces for public ticket machines or self-service checkouts could also be a win. “Nowadays, everyone thinks about hygiene,” says Subramanian.
He and Drinkwater began to wonder if they could use sound waves to levitate and move objects. It began as a bit of fun. Drinkwater thought it might make a cute demo to show off on university open days. But the researchers soon found that they could go way beyond simple standing waves. Their panels of speakers could produce complex 3D shapes made of sound, and the devices could be programmed so that the shapes changed. Objects such as small polystyrene balls or droplets of liquid could be held at low-pressure points and would float around as the shapes changed. They produced a pair of acoustic “tweezers”, made of two finger-like projections of sound, that could pinch a ball and move it. Then they formed a vortex of sound that could make balls spin. They even managed to create multiple low-pressure points, allowing them to .
Acoustic wizardry
Drinkwater enjoyed demonstrating the toy to the public, but he began to wonder if this could be more than a party trick. One limitation of the speaker array was that there couldn’t be anything between it and the object being levitated. In 2018, Subramanian dreamed up a solution. The key was to use acoustic metamaterials – that is, materials with sonic properties not normally found in nature. He 3D-printed 16 specially designed plastic bricks that each had an internal structure that manipulates sound in a specific way. These could then be used to bend sound around an object. Subramanian showed off the tech by having it bat so that a little ball bobbed up and down and side-to-side in the air over its head.
How could this acoustic wizardry be put to good use? One of the first ideas was to make what are known as volumetric displays. These are floating 3D images of the sort you might see glowing on the dashboard of a spacecraft in science fiction movies. Drinkwater and his team made a polystyrene ball zip through the air so quickly that, to the eye, it looked like an image – a bit like tracing patterns in the dark night air with a sparkler. They could also illuminate the ball using lights to have it change colour. In separate work, Subramanian used two parallel arrays of speakers to whizz a ball around in patterns at speeds of up to 9 metres per second. As he showed in 2019, this is fast enough to , such as smiley faces and number countdowns.
These days, others are beginning to consider more ambitious applications for tractor beams. thinks they could also be used for contactless manufacturing in fields like electronics, pharmaceuticals and biomedical sciences, where delicate components can easily be broken or contaminated.
Marzo, who previously worked with Drinkwater, has published acoustic tractor beam that costs less than £150. He has been experimenting with levitating and joining together sticky balls and rods to make simple structures. He hopes researchers from many fields will start exploring how they can use it too. “I want to put out the system so other people can build it,” he says. “I’m sure that they will come up with great ideas.”
We are always going to struggle to use acoustic tractor beams to levitate big, heavy objects. But that may not be too much of a problem. The kinds of things that we would find it most useful to levitate are small and light anyway – things like cells and bits of living tissue.
We have ways of manipulating these materials already, but they aren’t perfect. Take centrifuges, spinning devices used to separate fluids like blood into different fractions. This kind of purification makes it easier to analyse living tissues and run tests. But centrifuges struggle when it comes to the tiniest components. University in North Carolina wondered if an invisible centrifuge made of sound could do better.
He was especially interested in isolating exosomes, packets of proteins and DNA that cells release. It is thought these could be useful diagnostic markers of cancer and Alzheimer’s disease, but they are so minuscule that centrifuges struggle to isolate them. Last year, Huang of fluid. By varying the frequency of the sound, Huang and his team could control the size of the nanoparticles held in the vortex. They then showed that the . This process would take about 8 hours using a conventional centrifuge, says Huang. “But now, using acoustics, we can do it within 1 minute”.
Acoustic tractor beams could even be used as a way to manipulate things inside living tissue. In 2020, engineer and his colleagues demonstrated a proof of principle experiment. They used acoustic tractor beams like those developed by Drinkwater and Subramanian to of sedated pigs. Using vortex-shaped beams of sound generated by a speaker array outside the pigs’ bodies, they were able to steer the spheres along complex 3D paths, such as figures-of-eight and circles.
Blast and nudge
Bailey is also working with medics at a couple of US hospitals on a . It is already routine to blast kidney stones with ultrasound to break them up, but bits of them can be left behind. Bailey’s trial aims to blast these fragments with sound again in order to nudge them into positions where they will be naturally cleared from the kidneys. They observe the process using a normal ultrasound scan to keep track of what is happening. “These stones hop a centimetre, or something like that,” says Bailey. “Most people, when they see it, are kind of startled.” The longer-term plan is to use acoustic tractor beams to move the fragments in a more controlled way. Much of Bailey’s work is sponsored by NASA; the space agency considers kidney stones to be a serious health risk to astronauts, particularly on future deep-space and interplanetary missions.
It might be possible to move all manner of other things inside the body. Take microbubbles, spheres made of fatty molecules. These could be loaded with a drug, injected, moved to particular places in the body with a tractor beam and then popped with a blast of ultrasound. The method could prove a great way of administering chemotherapy, for example, which can harm healthy tissues. Mechanical engineer Diego Baresch at the University of Bordeaux in France is interested in this idea. Last year, he tested it out using a mock-up of real human tissue and bubbles loaded with nanoparticles as a practice exercise. Working with , Baresch showed it was possible to , move them around in complex patterns and burst them on demand.
Yi-Ju Ho at Chung Yuan Christian University in Taoyuan City, Taiwan, researches microbubbles in medicine. She says they can be used to release oxygen near tumours, which can improve the effectiveness of cancer treatments. She thinks Baresch’s idea of controlling the microbubble’s location with a tractor beam is a good one that “could avoid systemic toxicity and reduce the side effects of gas therapy”.
We might pull off a similar trick with cameras. It is becoming increasingly common to use pill-sized cameras that patients swallow to examine the digestive tract. The NHS in England is beginning a trial of this technique as a means of screening for bowel cancer in 11,000 patients. But you can’t control these pill cams once they are inside the body. “You might want to stop it, spin it for a bit or go back up if you have missed something,” says Bailey. He thinks an acoustic tractor beam would be the perfect aide. “There is a lot of room to make that much more sophisticated than hoping it just washes through and captures everything.”
Subramanian, who helped start it all, is still thinking about other ways to put his acoustic tractor beams to use. He is working on a quirky idea in collaboration with the Shanghai Academy of Fine Arts in China. Together, they are designing a series of acoustic-powered displays that will show a selection of talking and singing heads as an art installation. Subramanian is excited about how we can use sonic tractor beams – but for him, they have always been partly about having fun.
