A tiny origami robot that unfolds from a swallowed capsule and can steer itself around a human body can be used to remove obstacles in the digestive tract and patch up internal wounds.
The robot moves via external magnetic fields and has been used to remove a swallowed button battery and patch a wound in a simulated human oesophagus.
The robot is the successor to an earlier model that uses the same propulsion technique, 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.
“It’s really exciting to see our small origami robots doing something with potential important applications to health care,” said professor Daniela Rus with the Massachusetts Institute of Technology, who worked on the project.
“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.”
The robot is a significant upgrade from the earlier model and has been given a series of structural modifications.
“Stick-slip only works when, one, the robot is small enough and, two, the robot is stiff enough,” said student Steven Guitron, who helped develop the device. “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, its folds increase stiffness along certain axes.
But the robot doesn’t rely entirely on stick-slip motion because the stomach is filled with fluids. Twenty per cent of its forward motion is carried out by thrusting itself through the body’s liquid using a fin which is attached to its body.
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.
Through a design process that Guitron describes as ‘mostly trial and error’, the researchers arrived at a rectangular robot with accordion folds perpendicular to its long axis and pinched corners that act as points of traction.
In the centre 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
The team created a synthetic stomach to test the device using a pig intestine and a mixture of water and lemon juice to simulate the acidic fluids.
“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.”