Scientists built a swallowable paper battery that can power medical devices inside the body and then gradually break down after its job is done.

So far, the battery has been tested only in pigs, in which it powered devices for up to three days. If proven safe and effective in people, the battery could someday power temporary devices inside the gut while avoiding surgery to retrieve a conventional battery from the body when the device is no longer needed.

"I'm very excited about this work," said Reza Ghodssi, a professor of electrical and computer engineering at the University of Maryland who was not involved in the study. "The battery is one component that takes up most of the space in an ingestible device, so anything that can provide the required power while reducing the size of the capsule is very promising."

Examples of ingestible medical devices include those that detect bleeding, dispense medicines, or stimulate specific tissues or organs.

How does the battery work?

Conventional batteries used in ingestible devices are not only large; they also need to stay sealed to prevent their internal materials from leaking into surrounding tissue and causing damage. The new battery, described Monday (Sept. 21) in the journal Nature Chemical Engineering, is made from materials that gradually dissolve in the acidic gastrointestinal tract and can then be safely absorbed without leaving behind harmful fragments or toxic byproducts.

The battery is built in layers. A magnesium alloy forms the anode (the battery's negative terminal), while the cathode (positive terminal) contains molybdenum trioxide and activated carbon. Between the two sides is a biodegradable electrolyte that allows the battery to generate electric current. These materials have been used in earlier biodegradable battery prototypes for temporary medical and wearable electronics.

The new design adapts them into a thin, porous, paper-like battery using cellulose nanofibrils as a binder, whereas earlier versions used larger binders. "The paper structure improves the battery's strength and control over degradation while still allowing it to produce electricity," study co-author Giovanni Traverso, director of the Laboratory for Translational Engineering at MIT, told Live Science in an email.

To keep the battery from breaking down too quickly in stomach acid, the researchers coated it with beeswax. Some versions also got a layer of candelilla wax — derived from the desert shrub Euphorbia antisyphilitica — for longer protection.

"The wax coating is not simply packaging," Traverso said. "It is a key design element that controls the functional lifetime."

The team made two versions of the battery, including one that was small enough to fit inside a standard gelatin capsule. In lab tests, it produced about 1.77 volts and could store and deliver 2 milliampere-hours per square centimeter, which is enough capacity to power low-power electronics.

A larger version of the battery produced about 1.84 volts and had a maximum capacity of 3.5 milliampere-hours. That could power devices that need a bit more power. However, the battery's capacity is still low, Ghodssi noted. "They need to have an order of magnitude higher capacity for this technology to be even more promising," he said.

Testing the battery inside the body

The researchers placed the battery prototypes inside 3D-printed capsules and administered them to pigs orally using an endoscope — essentially a long tube. Both versions of the battery worked for up to three days, although their voltage and capacity dropped as they gradually degraded. The larger battery's voltage dropped from about 1.8 volts to 1.6 volts after one day and then down to about 1.45 volts by the third day, while the smaller battery fell from 1.7 volts to 1.35 volts over the same period.

The team used the batteries to power two types of medical devices inside the pigs. The smaller battery powered a wireless RFID (radio-frequency identification) tag. Similar RFID-based sensors have been tested in people before, but the one used in this study was experimental. Researchers positioned it in the esophagus, where the tag communicated with a receiver up to 5 feet (1.5 meters) away and allowed researchers to detect when the pigs swallowed a medication.

The larger battery powered a swallowable capsule that electrically stimulated the stomach; this increased levels of the hunger-stimulating hormone ghrelin in the blood without causing visible tissue damage at the stimulation site. Gastric electrical stimulation is already used in people with severe gastroparesis, a condition that slows the rate at which the stomach empties; the battery-powered capsule tested in this study is experimental.

"It's quite impressive that the battery can operate in a stable and reliable fashion as the overall device passes through the GI system in a large animal model," said John Rogers, a pioneer in bioelectronics and a materials scientist at Northwestern University who was not involved in the study.

The battery and its biodegradable components broke down, but the electronic circuit board used for the stomach-stimulation experiment did not; instead, the pigs passed it naturally. "Making every part of the device bioresorbable could eliminate the risk of a leftover component becoming lodged in the gastrointestinal tract," Rogers said.

The researchers' biggest remaining challenge is controlling the battery's working lifetime and making its breakdown more predictable, Traverso said.

The researchers also found some variability between the batteries, which they attributed to factors in their construction, including the amount of contact between layers, their electrolyte distribution, and differences in the thickness of the wax coating. The team is now working to standardize manufacturing and adjust the coating so the batteries can be designed to function for specific periods of time, ranging from hours to days.

The team also plans to conduct longer tests under conditions that more closely mimic the human gastrointestinal tract. They are working toward an initial clinical trial of the RFID system that could begin in about two years, Traverso said.