Using a focused beam of sound as a scalpel? That's not as far off as it sounds, according to a group of University of Michigan at Ann Arbor engineering researchers, whose research shows that a carbon-nanotube-coated lens that they developed converts light to sound and can focus high-pressure sound waves to finer points than ever before. They say it could lead to an invisible knife for noninvasive surgery.

The tools work by focusing sound waves tightly enough to generate heat, says Jay Guo, a professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering.

The team was able to concentrate high-amplitude sound waves to a speck just 75 by 400 micrometers that can blast and cut with pressure, rather than heat. Guo speculates that it might be able to operate painlessly because its beam is so finely focused it could avoid nerve fibers. The device hasn't been tested in animals or humans yet, though.

The system is unique, they say, because it performs three functions: it converts light to sound, focuses it to a tiny spot, and amplifies the sound waves. To achieve the amplification, the researchers coated their lens with a layer of carbon nanotubes and a layer of a rubbery material called polydimethylsiloxane. The carbon nanotube layer absorbs the light and generates heat from it. Then the rubbery layer, which expands when exposed to heat, drastically boosts the signal by the rapid thermal expansion.

The resulting sound waves are 10,000 times higher frequency than humans can hear. They work in tissues by creating shockwaves and microbubbles that exert pressure toward the target, which Guo envisions could be tiny cancerous tumors, artery-clogging plaques or single cells to deliver drugs. The technique might also have applications in cosmetic surgery.