Engineers at the University of California, San Diego have shown that a micromotor fueled by stomach acid can take a bubble-powered ride inside a mouse. These tiny motors may someday offer a safer and more efficient way to deliver drugs or diagnose tumors. The experiment is the first to show that these micromotors can operate safely in a living animal. Stomach acid reacts with the zinc body of the motors to generate a stream of hydrogen microbubbles that propel the motors forward. The researchers say that the motors lodged themselves firmly in the stomach lining of mice. As the zinc motors are dissolved by the acid, they disappear within a few days leaving no toxic chemical traces.