Coming soon: 'Invisible knife' that lets doctors cut with sound

New technology uses a carbon nanotube-coated lens to convert light into sound waves that are then focused enough to cut through skin.

Scientists have made an 'invisible knife' that uses sound, instead of a sharp edge, to cut, paving the way for non-invasive detailed medical procedures.

The technology concentrates high-amplitude sound waves that are strong enough to cut flesh onto an area of 75 by 400 micrometers (0.075 by 0.4 millimeters), scientists at the University of Michigan announced on Wednesday.

"We believe this could be used as an invisible knife for noninvasive surgery," said Jay Guo, co-author of the paper on the research. "Nothing pokes into your body, just the ultrasound beam. And it is so tightly focused you can disrupt individual cells."

The beam is generated by converting light from a pulsed laser into sound by beaming it through a lens coated with carbon nanotubes and a rubbery material called polydimethylsiloxane.

Light is absorbed by the carbon-nanotube layer and turned into heat; the rubbery material then boosts the resultant signal via rapid thermal expansion; finally, the lens focuses the sound to a very high level.

"A major drawback of current strongly focused ultrasound technology is a bulky focal spot, which is on the order of several millimeters," noted Hyoung Won Baac, a Harvard Medical School research fellow who worked on the technology as a doctoral student in Guo's lab. "A few centimeters is typical. Therefore, it can be difficult to treat tissue objects in a high-precision manner, for targeting delicate vasculature, thin tissue layer and cellular texture. We can enhance the focal accuracy 100-fold."

The generated sound waves are 10,000 times higher than the frequency limit for human hearing.

In early tests the technology was able to detach a single ovarian cancer cell and blast a sub-150 micrometer hole in an artificial kidney stone.

"This work opens a way to probe cells or tissues in much smaller scale," Guo said.

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