Researchers fed mice a small drop of a 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.