Last year, in a lab in sunny San Diego, researchers fed a dozen mice a small drop each of a very special liquid. Inside the drops, invisible to the naked eye, were thousands of tube-shaped, microscopic motors. The motors made their way to the mice's stomachs, embedded in their stomach linings, and released their tiny payloads: nano-size flakes of gold. The research represented a major step toward putting microbots to work in human medicine, where they could one day ferry drugs efficiently into specific organs or even specific cells.

Strange as such research may sound, it's actually been going on for about a decade. A number of research teams around the world have worked on making engines, many of them a fraction of the width of a human hair, that are able to move around in blood, bodily fluids, and other interesting liquids. The objects are all prototypes, steps toward a more ambitious idea: That in the future, engineers could make fleets of tiny, programmable motors to perform tasks such as killing individual cancer cells or cleaning up molecules in an oil spill.

The research is still in its early stages. Other research teams have only tested their motors in solutions they've mixed up in lab, or in cells they've grown in Petri dishes. The University of California, San Diego, study is the first to try such motors in animals.