There’s more to ultrasound machines than simply watching babies develop inside the womb. They’re used to blast apart kidney stones and prostate tumors, for example. And now, with a nanotube-coated lens, the technology could be honed to create an invisible sonic blade for noninvasive – perhaps even painless – microsurgery. Ultrasound beams can be unwieldy. Aim a beam of sound at a tissue, and you’re more than likely to hit it, but aiming at something much smaller, like a patch of cancer cells, is a much more difficult feat. To improve the precision, University of Michigan researchers turned to nanotubes for converting light to sound for finer ultrasound waves than have ever before been produced. A new technique uses tightly focused sound waves for microsurgery. University of Michigan engineering researchers have used it to drill a 150-µm hole in a confetti-sized artificial kidney stone. Courtesy of L. Jay Guo, University of Michigan. “Light scatters more severely in tissue, but sound waves much less and therefore can penetrate deeper,” said engineering professor Jay Guo. To achieve a superfine beam, Guo and colleagues took an optoacoustic approach that converts pulsed laser light to high-amplitude sound waves through a specially designed lens. These sound waves are then concentrated to a speck measuring just 75 by 400 µm. “We used a concave optical lens and grew CNTs [carbon nanotubes] over its surface to function as a light-absorbing layer, which becomes heated upon absorbing the nanosecond laser light,” Guo told BioPhotonics. “We further coated a PDMS [polydimethylsiloxane] elastomer to help with the thermal expansion to generate a stronger acoustic wave. The concave shape of the lens brings the ultrasound to a tight focus.” The resulting sound waves have a frequency 10,000 times higher than humans can hear. The beam does its blasting and cutting with pressure, rather than heat; the sound waves create shockwaves and microbubbles in tissues that exert pressure toward a target, which could be tiny cancerous tumors, artery-clogging plaque or single cells to deliver drugs, Guo said. The technique also could have applications in cosmetic surgery. Pain-free surgery could be possible with the technique, he said, because the beam is so finely focused that it could avoid nerve fibers. It hasn’t yet been tested in animals or humans, but experiments to test the device on living tissue are in the works. In the lab, the researchers have demonstrated micro-ultrasonic surgery, accurately detaching a single ovarian cancer cell and blasting a hole less than 150 µm in an artificial kidney stone in less than a minute. “The current depth is a few millimeters,” Guo said. “One can focus deeper using a larger lens, but it may compromise the resolution. For example, the current lens can focus down to about 100 µm, but going deeper may limit it to, say, 1 mm, but that should still be quite useful.” Next, the team will evaluate materials other than carbon nanotubes, “and try to understand the mechanism of cell detachment and cell disruption – e.g., investigating the cavitation effect – in more detail,” Guo said. The work, funded by the National Science Foundation and the National Institutes of Health, appeared in Scientific Reports (doi:10.1038/srep00989).