In experiments involving a simulation of the human esophagus and stomach, researchers at MIT, the University of Sheffield, and the Tokyo Institute of Technology have 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 or patch a wound.

The work builds on a long sequence of papers on origami robots from the research group of Daniela Rus, the Andrew and Erna Viterbi Professor in MIT’s Department of Electrical Engineering and Computer Science.

Rus, who also directs MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL). feels that for applications inside the body, there is need for a small, controllable, untethered robot system. It’s really difficult to control and place a robot inside the body if the robot is attached to a tether.

The robot can propel itself using what’s 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.

Also like its predecessor — and like several other origami robots from the Rus group — the new robot consists of two layers of structural material sandwiching a material that shrinks when heated. A pattern of slits in the outer layers determines how the robot will fold when the middle layer contracts.

Material difference

The robot’s envisioned use also dictated a host of structural modifications. Stick-slip only works when, one, the robot is small enough and, two, the robot is stiff enough. With the original Mylar design, it was much stiffer than the new design, which is based on a biocompatible material.

To compensate for the biocompatible material’s relative malleability, the researchers had to come up with a design that required fewer slits. At the same time, the robot’s folds increase its stiffness along certain axes.

But because the stomach is filled with fluids, the robot doesn’t rely entirely on stick-slip motion. 20 percent of forward motion is by propelling water — thrust — and 80 percent is by stick-slip motion. It also had to be possible to compress the robot enough that it could fit inside a capsule for swallowing; similarly, when the capsule dissolved, the forces acting on the robot had to be strong enough to cause it to fully unfold.

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. In the researchers’ experiments, the robot uses the same magnet to pick up the button battery.

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

Source: Ingestible Origami Robot