Every year, 3,500 swallowed button batteries are reported in the U.S. alone. Frequently, the batteries are digested normally, but if they come into prolonged contact with the tissue of the esophagus or stomach, they can cause an electric current that produces hydroxide, which burns the tissue.

Researchers at MIT, the University of Sheffield and the Tokyo Institute of Technology have now demonstrated a tiny origami robot that can unfold itself from a swallowed capsule and, steered by external magnetic fields, crawl across the stomach wall to remove a swallowed button battery and/or patch the resulting wound.

An example of a capsule and the unfolded origami device. Image credit: MIT.“For applications inside the body, we need a small, controllable, untethered robot system,” says Daniela Rus, MIT professor of electrical engineering and computer science. "It’s really difficult to control and place a robot inside the body if the robot is attached to a tether."

The robot consists of two layers of structural material sandwiching a material that shrinks when heated. The structural material is the type of dried pig intestine used in sausage casings; the shrinking layer is a biodegradable shrink wrap called Biolefin. A pattern of slits in the outer layers determines how the robot will fold when the middle layer contracts.

The robot can propel itself using what is called a “stick-slip” motion, in which its appendages stick to a surface through friction when it executes a move, but slip free again when its body flexes to change its weight distribution. But because the stomach is filled with fluids, the robot doesn’t rely entirely on stick-slip motion.

“In our calculation, 20% of forward motion is by propelling water—thrust—and 80% is by stick-slip motion,” says Shuhei Miyashita, who was a postdoc at MIT’s Computer Science and Artificial Intelligence Laboratory when the work was done. “In this regard, we actively introduced and applied the concept and characteristics of the fin to the body design, which you can see in the relatively flat design.”

It had to be possible to compress the robot enough that it could fit inside a capsule for swallowing; similarly, when the capsule dissolves, the forces acting on the robot need to be strong enough to cause it to fully unfold. Through trial and error, the researchers arrived at a rectangular robot design with accordion folds perpendicular to its long axis and pinched corners that act as points of traction.

In the center of one of the forward accordion folds is a permanent magnet that responds to changing magnetic fields outside the body, which control the robot’s motion. The forces applied to the robot are principally rotational. A quick rotation will make it spin in place, but a slower rotation will cause it to pivot around one of its fixed feet.

“This concept is both highly creative and highly practical, and it addresses a clinical need in an elegant way,” says Bradley Nelson, professor of robotics at the Swiss Federal Institute of Technology, in Zurich. “It is one of the most convincing applications of origami robots that I have seen.”