Researchers at MIT, the University of Sheffield, and the Tokyo Institute of Technology conducted experiments involving a simulation of the human esophagus and stomach through which they demonstrated a miniature origami robot that can unfold itself from a ingested capsule and, maneuvered by external magnetic fields, crawl across the stomach wall to patch a wound or remove an ingested button battery.
The new research, which the team will present this week at the International Conference on Robotics and Automation, builds on lengthy 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.
It’s really exciting to see our small origami robots doing something with potential important applications to health care. For applications inside the body, we need 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.
Daniela Rus, Professor, MIT
Rus, also directs MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL).
Ingestible origami robot
Researchers at MIT and elsewhere developed 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. (Video: Melanie Gonick/MIT)
Collaborating with Rus on the paper are first author Shuhei Miyashita, who was a postdoc at CSAIL when the study was done and is currently a lecturer in electronics at the University of York, in England; Steven Guitron, a graduate student in mechanical engineering; Shuguang Li, a CSAIL postdoc; Kazuhiro Yoshida of Tokyo Institute of Technology, who was visiting MIT on sabbatical when the study was done; and Dana Damian of the University of Sheffield, in England.
The new robot is a successor to the one exhibited at the same conference the year before; however, the present robot’s body design is drastically different. Similar to its predecessor, this robot can propel itself using a “stick-slip” motion, in which its appendages adhere to a surface through friction when it performs a move, but slip free again when its body flexes to alter its weight distribution.
The new robot, like its predecessor and like many other origami robots from the Rus group, comprises of two layers of structural material sandwiching a material that gets smaller when heated. A pattern of slits in the outer layers establishes how the robot will fold when the middle layer contracts.
The robot’s contemplated use also dictated a number 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.
Steven Guitron, Graduate Student, MIT
To make up for the relative malleability of the biocompatible material, the team had to prepare a design that required lesser slits. Simultaneously, the folds of the robot increase its stiffness along certain axes.
But since the stomach is filled with fluids, the robot does not rely fully on stick-slip motion.
“In our calculation, 20 percent of forward motion is by propelling water — thrust — and 80 percent is by stick-slip motion,” says Miyashita. “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 was also necessary to compress the robot enough that it could fit within a capsule for swallowing; similarly, when the capsule dissolved, the forces acting on the robot had to be strong enough to cause it to completely unfold. Through a design process that Guitron explains as “mostly trial and error,” the researchers finally designed a rectangular robot with accordion folds perpendicular to its long axis and pinched corners that serve as points of traction.
In the middle of one of the forward accordion folds is a permanent magnet that reacts to changing magnetic fields outside the body, which steer the robot’s motion. The forces applied to the robot are mainly rotational. The robot will spin in place if the rotation is quick. A slower rotation will make it to pivot around one of its fixed feet.
In the experiments conducted by the researchers, the robot only used the same magnet to collect the button battery.
The researchers analyzed nearly 12 different possibilities for the structural material before choosing a type of dried pig intestine used in sausage casings. “We spent a lot of time at Asian markets and the Chinatown market looking for materials,” Li says. The shrinking layer is a biodegradable shrink wrap termed Biolefin.
To design their synthetic stomach, the team purchased a pig stomach and examined its mechanical properties. Their open cross-section of the stomach and esophagus model was molded from a silicone rubber and possessed the same mechanical profile. A mixture of lemon juice and water simulates the acidic fluids found in the stomach.
Annually, 3,500 cases of swallowed button batteries are reported in the U.S. alone. In most cases, the batteries are digested normally, but if they have prolonged contact with the tissue of the stomach or esophagus, they can cause an electric current that generates hydroxide, which would burn the tissue.
Miyashita applied a smart strategy to persuade Rus that the removal of swallowed button batteries and the treatment of ensuing wounds was a compelling application of their origami robot.
Shuhei bought a piece of ham, and he put the battery on the ham. Within half an hour, the battery was fully submerged in the ham. So that made me realize that, yes, this is important. If you have a battery in your body, you really want it out as soon as possible.
Daniela Rus, Professor, MIT
“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.”