Targeting medical treatment to an ailing body part is a practice as old as medicine itself. A Band-Aid is placed on a skinned knee. Drops go into itchy eyes. A broken arm goes into a cast.

But often what ails us is inside the body and is not so easy to reach. In such cases, a treatment like surgery or chemotherapy might be called for. A pair of researchers in Caltech's Division of Engineering and Applied Science are working on an entirely new form of treatment—microrobots that can deliver drugs to specific spots inside the body while being monitored and controlled from outside the body.

"The microrobot concept is really cool because you can get micromachinery right to where you need it," says Lihong Wang, Caltech's Bren Professor of Medical Engineering and Electrical Engineering. "It could be drug delivery, or a predesigned microsurgery."

The microrobots are a joint research project of Wang and Wei Gao, assistant professor of medical engineering, and are intended for treating tumors in the digestive tract.

The microrobots consist of microscopic spheres of magnesium metal coated with thin layers of gold and parylene, a polymer that resists digestion. The layers leave a circular portion of the sphere uncovered, kind of like a porthole. The uncovered portion of the magnesium reacts with the fluids in the digestive tract, generating small bubbles. The stream of bubbles acts like a jet and propels the sphere forward until it collides with nearby tissue.

On their own, magnesium spherical microrobots that can zoom around might be interesting, but they are not especially useful. To turn them from a novelty into a vehicle for delivering medication, Wang and Gao made some modifications to them.