IT'S harder than finding a needle in a haystack. Unusual or rare cells, such as those that cause the spread of cancer, are difficult to isolate from thousands of other cells in a sample.

Now a new device has been developed which can direct and focus streams of cells in a liquid, and even separate them out according to size. "We can take a stream of cells and focus, defocus and reflect it as if it's a light beam," says Robert Austin of Princeton University, who developed the device with colleagues from Princeton and Boston University, Massachusetts.