Sound waves can also direct objects in organisms. Daniel Ahmed, an engineer at ETH Zurich in Switzerland, recently used ultrasound to move hollow plastic beads into a living zebrafish embryo. By doing these experiments, Ahmed aims to demonstrate the potential of using sound to direct drugs to a target site in an animal, such as a tumor. Similar to the acoustic tweezers, the ultrasound creates a repeating pattern of low and high pressure areas in the embryo, allowing Ahmed to use the pressure pockets to push the beads around. Other researchers are investigating the directing ability of sound to treat kidney stones. A study from 2020for example, used ultrasound to move the stones in the bladders of live pigs.
Other researchers are developing a technology known as acoustic holography to shape sound waves, to more accurately 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, much like an optical hologram does for light. In particular, researchers are investigating how they can use acoustic holograms for brain research, where ultrasound waves are focused at a precise location in the head, which may 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 a recent demonstration, his team controlled sound with Lego†
To control the propagation of sound in new ways, his team stacked the plastic blocks on a saucer in a grid pattern, making them rise up like trees in a forest. By shaking the shell, they produced sound waves on the surface. But sound traveled bizarrely across the saucer. Normally, a sound wave should spread symmetrically in concentric circles, like the ripple of a pebble falling into a pond. Alù could only make sound travel in certain patterns.
Alù’s project is not inspired by light, but from the electron, which according to quantum mechanics is both a wave and a particle. Specifically, the Legos are designed to mimic the crystal pattern of a type of material known as twisted bilayer graphene, which restricts the movement of its electrons in a distinctive way. Under certain conditions, electrons only flow along the edges of this material. Among other things, the material becomes superconducting and the electrons pair 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 limit the movement of sound. In an experiment, the team found that they could make the sound go out 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, pointing to more versatile ways to control sound propagation, which could be useful for ultrasound imaging or the acoustic technology mobile phones rely on to communicate with cell towers, Alù says. For example, Alù has made a device with similar principles that allow sound to propagate in only one direction. Thus, the device can distinguish a transmit signal from a return signal, which means it can enable technology to simultaneously transmit and receive signals of the same frequency. That’s unlike sonar, which emits an acoustic wave and has to wait for the echo to return before pinging the environment again.
But applications aside, these experiments have changed the way scientists think about sound. It’s not just something you can blast off the rooftops, whisper in someone’s ear, or even use to map an undersea environment. It will be a precision instrument that scientists can shape, steer and manipulate for their needs.