National Geographic Daily News
A machine containing a grid of steel spheres.

A prototype acoustic lens for generating "sound bullets."

Photograph courtesy PNAS

Ker Than

for National Geographic News

Published April 5, 2010

A new machine inspired by a common office toy could one day allow doctors to zap cancerous tumors using "sound bullets," scientists say.

Dubbed an acoustic lens, the device could also be used to create near photo-quality images of internal organs that surpass the resolution of modern ultrasounds.

(Related: "Exploding Bubbles Can Trim Enlarged Prostate.")

The design is based on the Newton's cradle, which features several identically sized metal balls suspended so that the balls barely touch each other.

Due to Newton's laws of motion, when an end ball is pulled back and released, the ball at the other end swings outward with the same speed, even though none of the middle balls move.

The toy inspired study co-author Chiara Daraio to invent the acoustic lens, which uses 0.95-centimeter stainless steel spheres aligned in parallel chains. But instead of channeling motion, the new machine manipulates sound.

When a sound wave strikes the spheres at one end of the acoustic lens, the sound gets converted into a type of shock wave known as a solitary wave.

The solitary wave propagates through the chains in the same way motion moves through the balls in a Newton's cradle. But because of the length of the chains, the solitary wave exits the last sphere as a sound wave instead of bouncing back through the chain.

Converting the sound wave into a solitary wave is crucial, because solitary waves are easier to control, said Daraio, a physicist at the California Institute of Technology. (Related: "Laser 'Light Bullets' Made to Curve.")

For example, by squeezing the balls in a chain closer together—a process called static precompression—scientists can adjust the solitary wave so that the emitted sound wave travels in a particular direction and at a given speed.

"The goal was to create an acoustic lens that could achieve very high focal intensities and at the same time be able to control the focal position without having to change the structure of the lens itself," Daraio said.

Rapid-Fire Sound Bullets

By tweaking each chain of metal balls in the acoustic lens separately, a barrage of sound bullets can be made to converge onto a single spot.

"We squeeze the outer chains of the lens more than the chains in the middle, and this causes the solitary waves to travel faster in the outer chains than in the inner ones," ultimately releasing successive sound bullets, Daraio explained.

In addition, changing certain parameters in the machine allows scientists to alter the intensity of the sound bullets: The emitted waves can be gentle enough to probe internal organs or powerful enough to serve as "sonic scalpels" for cleaving off tumors.

As an imaging tool, the acoustic lens beats ultrasound imaging because the sound pulses can be focused much more tightly and can be easily repositioned.

"You could have 3-D images that are clearer and much more resolved," Daraio said. (Related: "Humans Can Learn to 'See' With Sound, Study Says.")

But such applications are still a ways off, she said, as the acoustic lens has not yet been tested on cells or living animals.

The research is detailed in this week's issue of the journal Proceedings of the National Academy of Sciences.

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