Forget Lasers. The Hot New Tool for Physicists Is Sound - 6 minutes read
Yushun Zeng squishes cancer cells in a petri dish at work. No, not with his ungainly, macroscopic human fingers. Zeng, an engineering graduate student at the University of Southern California, has [built a that traps and compresses the cells using acoustic waves—otherwise known as sound.
The purpose of the experiment is to test a hypothesis that cancer cells are softer than healthy ones, says Zeng. suggest cancer cells deform more easily, which allows them to migrate and metastasize throughout the body. If that’s the case, these experiments could help researchers design therapies that stiffen cancer cells to make them “harder to spread in the human body,” he says.
The use of sound to squish objects makes perfect sense, when you remember what a sound is: a vibration that travels through matter, whether it be through air, water, or a tin can pressed to your ear. (Technically, Zeng uses ultrasound—acoustic frequencies too high to be audible to humans.) Zeng’s device is known as an “acoustic tweezer.” The tweezer deforms the cancer cells by making use of sound as a pressure wave, and it’s one example of how scientists are expanding the uses of sound as a tool.
Acoustics, or the science of sound, “is an old and very established field,” says physicist Andrea Alù of the City University of New York. Early technologies, dating back centuries, largely revolved around music, from building better acoustics for theaters to designing tuning forks. In the 20th century, people reconceived sound as an imaging tool. Military researchers developed sonar to find enemy submarines, which medical engineers later adapted to image fetuses during pregnancy. People began to use sound to map spaces, whether they were in the ocean or in a human body.
These days, engineers have taken a fresh perspective on sound—in analogy with light. Sound, just like light, is a wave. Consequently, both exhibit many parallel phenomena: Your voice echoing in a canyon, for example, is mathematically analogous to light bouncing off a mirror. Over the last half-century, engineers have achieved unprecedented control over light, with inventions ranging from lasers to fiber optics to one-way mirrors to holograms. Now, engineers are adapting the tools for manipulating sound waves instead. “Many groups have been translating ideas from optics to acoustics,” says Alù.
The acoustic tweezer, for example, was inspired by a tool known as an “optical tweezer,” invented in the 1980s, which is basically a laser focused to a tight point. An object placed in a laser beam feels a push from the photons pelting it. Engineers shape the beam so that the object feels a balance of forces at the laser’s focus. This apparatus is handy for gripping the super-small: Scientists have trapped and manipulated [single and in optical tweezers, and even used them to [measure the springiness of double helix.
Instead of a laser producing a train of photons, acoustic tweezers vibrate an object like a bell, producing a train of sound waves in a medium. This creates pockets of high and low pressure. Similar to focusing a laser, Zeng engineers the shape of the sound waves to control the location of those pressure pockets. By positioning a low pressure zone over a cluster of cancer cells, for example, Zeng can squish them by causing the surrounding fluid from a high-pressure zone to rush in.
Sound waves can also steer objects inside organisms. Daniel Ahmed, an engineer at ETH Zurich in Switzerland, [recently used to move hollow plastic beads inside a live zebrafish embryo. By doing these experiments, Ahmed aims to demonstrate the potential of using sound to guide drugs to a target site within an animal, such as a tumor. Similar to the acoustic tweezer, the ultrasound creates a repeating pattern of low and high pressure areas within the embryo, allowing Ahmed to use the pressure pockets to push the beads around. Other researchers are investigating the steering capability of sound to treat kidney stones. [A 2020 for example, used ultrasound to move the stones around in the bladders of living pigs.
Other researchers are developing a technology known as acoustic holography to shape sound waves, in order to more precisely design the location and shape of the pressure zones in a medium. Scientists project sound waves through a patterned plate known as an acoustic hologram, which is often 3D-printed and computer-designed. It shapes the sound waves in an intricate, predefined way, just like an optical hologram does for light. In particular, researchers are investigating how they can [use acoustic for brain research, focusing ultrasound waves to target a precise place in the head, which could be useful for imaging and therapeutic purposes.
Andrea Alù also explores new ways of shaping sound waves, but not necessarily tailored to specific applications. In one recent demonstration, his team [controlled sound with order to control sound propagation in new ways, his team stacked the plastic blocks on a platter in a grid pattern, making them stick up like trees in a forest. By shaking the platter, they produced sound waves on its surface. But sound traveled bizarrely over the platter. Normally, a sound wave should disperse symmetrically in concentric circles, like the ripple from a pebble falling into a pond. Alù could make the sound only travel in particular patterns.
Alù’s project draws inspiration not from light, but from the electron—which, according to quantum mechanics, is both a wave and a particle. In particular, the Legos were designed to mimic the crystal pattern of a type of material known as twisted bilayer graphene, which restricts the motion of its electrons in a distinctive way. Under certain conditions, electrons only flow on the edges of this material. Under others, the material becomes superconducting, and the electrons form pairs and move through it without electrical resistance.
Because electrons move so strangely in this material, Alù’s team predicted that the crystal geometry, scaled up to Lego size, would also restrict the movement of sound. In an experiment, the team found that they could make the sound emanate in an elongated egg shape, or in ripples that curve outward like the tips of a slingshot.
These unusual acoustic trajectories illustrated surprising parallels between sound and electrons, and hint at more versatile ways of controlling sound propagation, which could prove useful for ultrasound imaging or the acoustic technology that cell phones rely on for communicating with cell towers, says Alù. For example, Alù has [created a with similar principles that allows sound to only propagate in one direction. Thus, the device can distinguish a transmission signal from a return signal, which means it can enable technology to transmit and receive signals of the same frequency simultaneously. That’s unlike sonar, which sends out an acoustic wave and has to wait for the echo to return before pinging the environment again.
But applications aside, these experiments have changed how scientists think about sound. It’s not just something you can blast from the rooftops, whisper in someone’s ear, or even use to map an undersea environment. It’s becoming a precision tool that scientists can mold, direct, and manipulate for their needs.
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