Stomach acid helps power tiny, ingestible sensors

There’s no doubt stomach acid is powerful – it’s capable of digesting metal. But it’s also powerful in the electric sense, helping sustain energy for swallowable electronic sensors that monitor vital signs such as temperature.

Researchers from Massachusetts Institute of Technology in the US and KTH Royal Institute of Technology in Sweden successfully took the temperature of a pig using an ingestible device that harvested power from a mixture of metal and stomach acid. It even broadcast the information to a receiver two metres away.

The work, published in Nature Biomedical Engineering, offers a low-cost and safer alternative to traditional battery-powered devices.

“You could have a self-powered pill that would monitor your vital signs […] and transmit them to your phone,” says Phillip Nadeau, a biomedical engineer at Massachusetts Institute of Technology and lead author of the paper.

Ingestible devices capable of monitoring heart rate, temperature and breathing rate are gaining popularity as small electronic components become available.

The problem is these electronic pills normally rely on power from small batteries that contain toxic elements, posing a safety risk should the battery leak.

Taking inspiration from a common high-school experiment – the lemon battery – Nadeau and his crew created their own miniaturised device.

To turn a lemon into a battery you need three things: a lemon (obviously), and a copper coin and a nail to act as electrodes. In this configuration, citric acid in the lemon carries the small electric current between the electrodes.

To make a power source suitable for a pill only 40 millimetres long and 12 millimetres wide, the researchers attached small strips of zinc and copper to the outside of the pill.

Once in the pig’s stomach, the zinc dissolved, releasing zinc ions – about the same amount released by a common vitamin tablet – to create a circuit.

The researchers found that this method generated enough energy to run a temperature sensor and transmitter in the pig’s stomach and intestines for up to seven days. It also let the device transmit information to a receiver every 12 seconds.

Kourosh Kalantar-Zadeh, a biomedical engineer from RMIT in Australia and who was not involved with the study, says “an energy source that that prolongs the lifetime of a device, by even by a few hours, will result in much more information [from the patient]”.

Although the battery performed well in the stomach, power dropped to almost nothing once it entered the small intestine. This is because the small intestine contains less acid so the device wasn’t able to send data anywhere near as frequently.

Still, the researchers anticipate that one pill could hold more than one sensor at a time and even be used for controlled drug delivery for diseases such as malaria.