Scientists have created a tiny origami robot that can unfold itself from a swallowed capsule and crawl across the stomach wall to patch a wound.
In the experiments, the robot was to pick up a button battery lodged inside a synthetic stomach and oesophagus model.
"It's really exciting to see our small origami robots doing something with potential important applications to health care," said Daniela Rus, from the Massachusetts Institute of Technology (MIT) in US.
"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," said Rus.
The new 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.
It 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.
The robot's envisioned use also dictated a host of structural modifications over an earlier origami robot built by the researchers.
"Stick-slip only works when, one, the robot is small enough and, two, the robot is stiff enough," said Steven Guitron, a graduate student at MIT.
However, because the stomach is filled with fluids, the robot does not rely entirely on stick-slip motion.
"In our calculation, 20 per cent of forward motion is by propelling water - thrust - and 80 per cent is by stick-slip motion," said Shuhei Miyashita, formerly at MIT.
"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," Miyashita, now a lecturer at the University of York in UK.
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.
The researchers tested about a dozen different possibilities for the structural material before settling on the type of dried pig intestine used in sausage casings.
The robot was attached with a permanent magnet that responds to changing magnetic fields outside the body, which control the robot's motion.
To design the synthetic stomach, the researchers bought a pig stomach and tested its mechanical properties. Their model is an open cross-section of the stomach and oesophagus, molded from a silicone rubber with the same mechanical profile.
A mixture of water and lemon juice simulates the acidic fluids in the stomach.
