A Nanotube Lens Focuses Sound Waves Into An Invisible Sonic Scalpel

Therapeutic ultrasound can now blast and cut with targeted precision.

Today’s ultrasound machines are a long way beyond just checking out babies in the womb–they’re used to break up kidney stones, ease sore muscles and more. Now, with a new nanotube lens, they can also serve as an invisible sonic knife.

Though targeted ultrasound is useful, it can be unwieldy, with a relatively large focal area. Aim a beam of sound at a kidney stone and you’ll likely hit the centimeter-sized object, but better precision–like hitting a cholesterol deposit in a blood vessel, or a specific clump of cancer cells–is hard to achieve. To improve matters, University of Michigan researchers turned to nanotubes and started with light instead of sound.

First Jay Guo and colleagues coated a specially designed optoacoustic lens, used to convert laser light into high-amplitude sound waves, with a layer of carbon nanotubes. The nanotubes absorb the laser light and grow warm as a result. The second layer was a rubbery synthetic material that expands when it gets warm. This serves as an amplifier. To produce the ultrasonic waves, Guo and his colleagues pulsed laser light through the 6-millimeter lens, which converted the optical energy into sonic energy. The graphene absorbed the laser’s heat and the amplifier boosted the signal. The result were sound waves with a frequency 10,000 times the hearing capability of humans.

What’s more, the waves were super-focused–the researchers controlled their target range from around 6 to 15 microns up to 300 to 400 μm.

Where other ultrasonic therapy uses heat to provide a stimulus, this one creates shockwaves that force pressure toward a target. Its superfine focus can blast away anything from blood clots to tumor growth–all non-invasively. In tests, Guo and colleagues detached a single cancer cell from an ovary and blasted a 150-micron hole in an artificial kidney stone. They say this type of ultra-precise ultrasound can be a new way to deliver drugs, fight cancer or even perform cosmetic surgery.

A paper describing their methods is published in Nature Scientific Reports. They plan to present their work at the upcoming SPIE Photonics West meeting in San Francisco.

University of Michigan