Biomedical engineers at the University of Michigan are testing "acoustic tweezers" that use ultrasound waves to gently shuttle cells around. The cells are grown on a polymer that turns pulsed laser light into high-frequency vibrations; the vibrations heat the polymer slightly, and it responds by expanding. Projecting a pulsing ring of laser light around a cell deforms the underlying polymer into a tiny hill, and the cell slides down its slope. Moving the laser nudges the cell in any direction. One advantage of the technique over other cell manipulation schemes is that the process can be reversed and an isolated cell returned to its place in the cell culture. Project leaders Matt O'Donnell and Tak Buma hope to initially license the technology to drug companies. Drug researchers could quickly isolate liver cells, say, treat them with potential drugs, and return them safely to their cultures to see if they suffer any toxic effects.