A robot that uses its spiked feet to cling to the mucus-covered lining of the gut could one day climb through the human body to install monitoring devices or deliver therapies
27 May 2022
By Christa Lesté-Lasserre
Tiny robots with soft, flexible bodies and spiky feet can climb along the moist, slippery inner walls of the lungs and the gut where they could one day deliver drugs and medical sensors in hard-to-reach places.
The new “millirobot” – which is a few millimetres long – has feet that stick to tissue surfaces without losing their grip. The robot can resist being dislodged by jarring movements and can even cling to a surface as liquids flush over it, resembling the movement of fluids associated with breathing and digestion.
Capable of climbing straight up – and even upside down – inside the human body, the wireless device represents “a significant milestone in soft robotics”, says Metin Sitti at the Max Planck Institute for Intelligent Systems in Germany.
Sitti’s previous millirobot could walk, roll, swim, jump and crawl along biological tissues, he says. But it couldn’t climb up the complex 3D surfaces in the body, which is critical for reaching hard-to-access areas within the heart, lungs and digestive system.
Co-author Xiaoguang Dong at Vanderbilt University in Tennessee says their team first tried to add footpads that clamp down on tissue, inspired by the way some intestinal parasites attach themselves. But they had trouble reproducing the high forces required – and getting the robot to release its grip was also tricky, he says.
Instead, the team equipped the robot with two spiked footpads – reminiscent of the prickly plant burrs that stick to clothing during a country walk, says Sitti. Once one foot is down, the robot pulls its other foot off the surface and flips its body over to take a “step”— a mechanism they call peeling and loading.
When covered with a thin layer of chitosan (a substance found in shrimp shells), the foot’s “microspikes” created just enough increased friction and stickiness for the feet to latch on to the mucus layer inside pigs’ lungs and digestive tracts – including the bronchial tubes, oesophagus, stomach and intestines – and then pull away to take a new step.
In a series of laboratory tests, the researchers found that the robot will keep climbing along and clinging to “highly slippery” and often wrinkled biological tissue, even when the tissue is shaken or flushed with water. “I was very excited and surprised,” Sitti says of the results.
The researchers controlled the robot’s movement inside the organs by using a nearby machine that manipulates magnetic fields. Because the robot’s body is made of an elasticised magnetic metal, it bends and turns in response to the machine’s commands, says Dong.
The millirobot is ultrathin, with a body 3.7 mm long and 1.5 mm wide. It can carry “cargo” three times its own volume and up to 20 times its own weight, says co-author Yingdan Wu, also at the Max Planck Institute. That means it could transport medications, wireless electronic sensors or even possibly microneedles and biopsy materials. It can also release microscopic drug particles through its microspikes.
Because the microspikes stick only to the mucus layer, they cause no damage to the tissue itself, says Wu.