Ingestible, expanding pill monitors the stomach for up to a month

Engineers have designed an ingestible, Jell-O-like pill that, upon reaching the stomach, quickly swells to the size of a soft, squishy ping-pong ball big enough to stay in the stomach for an extended period of time.

The inflatable pill is embedded with a sensor that continuously tracks the stomach's temperature for up to 30 days. If the pill needs to be removed from the stomach, a patient can drink a solution of calcium that triggers the pill to quickly shrink to its original size and pass safely out of the body.

The new pill is made from two types of hydrogels -- mixtures of polymers and water that resemble the consistency of Jell-O. The combination enables the pill to quickly swell in the stomach while remaining impervious to the stomach's churning acidic environment.

The hydrogel-based design is softer, more biocompatible, and longer-lasting than current ingestible sensors, which either can only remain in the stomach for a few days, or are made from hard plastics or metals that are orders of magnitude stiffer than the gastrointestinal tract.The inner material contains sodium polyacrylate -- superabsorbent particles that are used in commercial products such as diapers for their ability to rapidly soak up liquid and inflate.

The researchers realized, however, that if the pill were made only from these particles, it would immediately break apart and pass out of the stomach as individual beads. So they designed a second, protective hydrogel layer to encapsulate the fast-swelling particles. This outer membrane is made from a multitude of nanoscopic, crystalline chains, each folded over another, in a nearly impenetrable, gridlock pattern.

"You would have to crack through many crystalline domains to break this membrane," the author says. "That's what makes this hydrogel extremely robust, and at the same time, soft."

In the lab, the researchers dunked the pill in various solutions of water and fluid resembling gastric juices, and found the pill inflated to 100 times its original size in about 15 minutes -- much faster than existing swellable hydrogels. Once inflated, the pill is about the softness of tofu or Jell-O, yet surprisingly strong.

To test the pill's toughness, the researchers mechanically squeezed it thousands of times, at forces even greater than what the pill would experience from regular contractions in the stomach.

"The stomach applies thousands to millions of cycles of load to grind food down," the author explains. "And we found that even when we make a small cut in the membrane, and then stretch and squeeze it thousands of times, the cut does not grow larger. Our design is very robust."

The researchers further determined that a solution of calcium ions, at a concentration higher than what's in milk, can shrink the swollen particles. This triggers the pill to deflate and pass out of the stomach.
Finally, the authors embedded small, commercial temperature sensors into several pills, and fed the pills to pigs, which have stomachs and gastrointestinal tracts very similar to humans. The team later retrieved the temperature sensors from the pigs' stool and plotted the sensors' temperature measurements over time. They found that the sensor was able to accurately track the animals' daily activity patterns up to 30 days.

Down the road, the researchers envision the pill may safely deliver a number of different sensors to the stomach to monitor, for instance, pH levels, or signs of certain bacteria or viruses. Tiny cameras may also be embedded into the pills to image the progress of tumors or ulcers, over the course of several weeks. The author says the pill might also be used as a safer, more comfortable alternative to the gastric balloon diet, a form of diet control in which a balloon is threaded through a patient's esophagus and into the stomach, using an endoscope.

"With our design, you wouldn't need to go through a painful process to implant a rigid balloon," the author says. "Maybe you can take a few of these pills instead, to help fill out your stomach, and lose weight. We see many possibilities for this hydrogel device."