A micromotor just one-fifth as wide as a human hair has taken a ride through a mouse's stomach, suggesting a possible future method of delivering drugs or diagnosing tumors in humans, researchers say.

Engineers at the University of California, San Diego, say their successful experiment shows that such micromotors are capable of operating safely inside a living creature.

In their laboratory, Liangfang Zhang and Joseph Wang of the university's NanoEngineering Department have experimented on different designs and different fuels for their tiny motors, letting them travel through lab samples of blood, water and other bodily fluids.

"But this is the first example of loading and releasing a cargo in vivo," says Wang of the motors in the mouse. "We thought it was the logical extension of the work we have done, to see if these motors might be able to swim in stomach acid."

Stomach acid serves as the fuel for the micromotors, reacting with the motor's zinc bodies to generate continuous microbubbles of hydrogen that push the tiny devices forward, the researchers report in the journal ACS Nano.

After moving, the micromotors lodged themselves into the lining of the mice stomachs. The stomach acid eventually dissolves the motors in a few days, leaving behind no traces of any toxic chemicals.

This suggests such micromotors could serve as ideal drug delivery systems, the researchers say.

To test this out they filled up some of their micromotors with "payloads" of nanoparticles of gold, then released them into the mice.

More of the particles reach the stomach lining when they were carried as micromotor payloads than when the mice swallowed the gold particles alone, the researchers said -- 168 nanograms as opposed to 53.6 nanograms.

"This initial work verifies that this motor can function in a real animal and is safe to use," says Zhang.

The cone shape of the motors helped them pierce and stick in the mucous covering of the stomach wall, he says.

"It's the motor that can punch into this viscous layer and stay there, which is an advantage over more passive delivery systems," he said.

Tiny drops of a liquid solution containing hundreds of the micromotors were fed to the mice, and the tiny devices began moving the moment they encountered acid in the stomach, moving toward to stomach lining at around 60 micrometers per second, the researchers said.

This self-propelled movement can go on for as long as 10 minutes, they said.

To increase their targeting potential, it might be possible to eventually add navigation abilities to the micromotors, Wang says, making them useful for diagnostics, drug delivery, nanosurgery and obtaining biopsies of hard-to-reach tumors.