Converting light to sound with a nanotube-coated lens can create finer ultrasound waves than ever before, and the new optoacoustic technique could someday be honed to create an invisible blade for noninvasive — and maybe even painless — microsurgery. There's more to ultrasound than glimpses into the womb: In therapeutic ultrasound, focused sound waves regularly blast apart kidney stones and prostate tumors, for example. The tools work primarily by focusing sound waves tightly enough to generate heat, said Jay Guo, an engineering professor at the University of Michigan. The problem is that ultrasound beams today can be unwieldy, said Hyoung Won Baac, a research fellow at Harvard Medical School who worked on this project as a doctoral student in Guo's lab. "A major drawback of current strongly focused ultrasound technology is a bulky focal spot, which is on the order of several millimeters," Baac said. "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." To achieve a superfine beam, Guo's team took an optoacoustic approach that converts light from a pulsed laser to high-amplitude sound waves through a specially designed lens, concentrating high-amplitude sound waves to a speck just 75 by 400 µm. The general technique has been around since Thomas Edison's time and has advanced over the centuries, but the process doesn't normally generate a sound signal strong enough to be useful for medical applications. The new system is unique because it performs three functions: It converts the light to sound, focuses it to a tiny spot and amplifies the sound waves. To achieve the amplification, the researchers coated a lens with a layer of carbon nanotubes and a layer of polydimethylsiloxane. The carbon nanotube layer absorbs the light and generates heat from it. The rubbery polydimethylsiloxane layer expands when exposed to heat, thereby drastically boosting the signal through rapid thermal expansion. 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 plaques 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, Guo said, because the beam is so finely focused that it could avoid nerve fibers; it hasn't yet been tested in animals or humans, however. "We believe this could be used as an invisible knife for noninvasive surgery," Guo said. "Nothing pokes into your body, just the ultrasound beam. And it is so tightly focused, you can disrupt individual cells." 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 Hyoung Won Baac. In experiments, the researchers 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. "This is just the beginning," Guo said. "This work opens a way to probe cells or tissues in much smaller scale." The work was published in the current issue of Nature's Scientific Reports, and the researchers will present the work in February at SPIE Photonics West in San Francisco. The research was funded by the National Science Foundation and the National Institutes of Health. For more information, visit: http://web.eecs.umich.edu/~guo