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'Sound bullets' could blast cancer
An old toy has inspired a powerful new weapon that could destroy submarines or annihilate tumours.

Using a modified version of Newton's cradle, a series of stainless steel balls suspended by fishing wire, scientists from the California Institute of Technology have created a powerful weapon for soldiers and doctors known as a "sound bullet."

"The beauty of this system is that it's just a bunch of ball bearings that we control with weights," says Professor Chiara Daraio, co-author of the study, which appears this week in Proceedings of the National Academy of Sciences.

According to Daraio, by varying the pressure on the ball bearings, the scientists can dramatically amplify and focus sound waves to make them "extremely destructive".

Newton's cradle is an old toy, but the Cal Tech scientists, including Alessandro Spadonia, have adapted it for a new field of science known as metamaterials, which includes the Harry Potter-style invisibility cloak. Metamaterials are difficult and time-consuming to create.

For their study, the researchers lined up 21 rows of stainless steel ball bearings in an area the size and shape of a laptop computer, with weights attached at the ends to vary the pressure on each row. Then the scientists dropped a small 20-centimetre ball onto the ball bearings.

Using a high-speed camera and a pressure sensitive material, the scientists watched the pressure waves focus on a single spot several inches away from the metamaterial.
Concentrated waves

The simple set up belies the power of the new metamaterial. Not only did the scientists focus all of the sound waves onto one specific area; they also amplified those waves more than 100 times than what any other metamaterial had previously produced. Those numbers could easily go higher, says Daraio.

The sound waves Daraio and Spadonia manipulated were too high pitched for human ears to detect. Properly adapted to audible sound, the new metamaterial could turn a normal sentence into a split-second ear drum rupturing explosion.

If these sound bullets were actual bullets, the metamaterial would be like transforming hot lead projectiles into rocket propelled grenades, all converging on one place at one time. The damage such concentrated waves of pressure could create would be devastating.

Like normal bullets, sound bullets can travel through air. Unlike normal bullets, sound bullets can also easily travel through liquids and solids. Sound bullets could be used by the military to create submarine melting waves of pressure, or shock waves powerful enough to destroy caves otherwise untouchable by conventional weapons.

Sound bullets won't just destroy large objects. Daraio says that powerful, focused sound waves could obliterate hard kidney stones or destroy cancerous tumours without any damage to surrounding tissue.

Other, less destructive applications are also possible, says Daraio. Focused sound waves could let engineers see weak bridge supports or future potholes in roads.
'Significant development'

Military or medical use of the new metamaterial is still years away, says Daraio. Yet the simplicity and effectiveness of the sound bullet makes it a "significant development," according to Professor L B Freund of Brown University, who was not involved in the research.

"People have focused sound waves for a long time," says Freund. "But if you want to do anything with a sound wave, what you can do is determined by the intensity of the sound wave."

Now that Daraio and Spadonia have found a new way to create these powerful sound waves, the potential range of applications for sound bullets is huge.

"The research has opened a new way to developed applied devices, and was done by young researchers in a very creative way," says Freund.

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