Human Intestinal Tract
© SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY, VIA GETTY IMAGES
Some things don’t break down when left to their own devices.
Medical devices can be inserted into the human gastrointestinal tract to treat, diagnose or monitor a range of disorders.

The problem is you usually have to get them out again, which involves endoscopic surgery.

US engineers may have a neat alternative, however.

A team from Massachusetts Institute of Technology says it has come up with a way to make devices break down inside the body when exposed to light from an ingestible LED - and has tested it successfully in pigs.

The secret, they report in a paper in the journal Science Advances, is a light-sensitive hydrogel developed in their labs. It includes a chemical bond that is broken when exposed to a wavelength of light between 405 and 365 nanometres (blue to ultraviolet).

In recent years, MIT researchers have developed a number of ingestible devices, while also trialling different ways to control their breakdown, including methods based on changes in pH or temperature, or exposure to certain chemicals.

A light-based trigger has particular appeal because it doesn't need to come into direct contact with the material being broken down. Also, light normally does not penetrate the gastrointestinal tract, removing the risk of accidental triggering.

In the latest work, a team led by Ritu Raman decided not to make a material composed exclusively of the light-sensitive polymer, but to use it to link together stronger components such as polyacrylamide.

This, Raman says, makes the overall material more durable, while still allowing it to break apart or weaken when exposed to the right wavelength of light. She also constructed the material as a "double network", in which one polymer network surrounds another.

"You're forming one polymer network and then forming another polymer network around it, so it's really entangled," she says. "That makes it very tough and stretchy."

The material's properties can be tuned by varying the composition of the gel. When the light-sensitive linker makes up a higher percentage of the material, it breaks down faster in response to light but is also mechanically weaker.

The researchers say they also can control how long it takes to break down the material by using different wavelengths of light. Blue light works more slowly but poses less risk to cells that are sensitive to damage from ultraviolet light.

The gel and its breakdown products are biocompatible, and the gel can be moulded into a variety of shapes.

In their study, Raman and colleagues used it to demonstrate two possible applications: a seal for a bariatric balloon (sometimes used to help treat obesity) and an oesophageal stent.

They believe, however, that their approach could be used to create other kinds of degradable devices, such as vehicles for delivering drugs to the gastrointestinal tract.