Engineers say the new lens can focus high-pressure sound waves to finer points than ever before.

Doctors routinely use focused sound waves to blast apart kidney stones and prostate tumors. The tools work primarily by focusing sound waves tightly enough to generate heat, but the beams can be unwieldy.

“A major drawback of current strongly focused ultrasound technology is a bulky focal spot, which is on the order of several millimeters,” says Hyoung Won Baac, a research fellow at Harvard Medical School who worked on the project as a doctoral student in the lab of Jay Guo, a professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering at the University of Michigan.