Swallowable sensors that sustain power from stomach acid

A tiny sensor capable of transmitting information from inside the body and powered by stomach acid has been unveiled by US scientists.

The device was tested in a pig over the course of a week. It wirelessly transmitted the body temperature of the animal every twelve seconds to an external receiver. The MIT and Harvard-based team behind the work, which was published this week in Nature Biomedical Engineering, say this represents a step towards safer, cheaper ingestible sensors that could even be used to dispense drugs inside the body.

Swallowable sensors or “ingestible electronics” can tell us about what's going on inside us, including pressure, acidity and respiration. They can even shoot videos of the internal anatomy. Current designs fit into capsules about 1cm x 3cm in size, but researchers hope to shrink that down to the size of an ordinary pill. However, long term sensing using electronics poses a problem, since devices that rely on conventional batteries could pose a safety risk were they to break open inside the body.

Now a new frontier is opening up in so-called “bio-compatible” cells that harvest energy from a current generated in stomach acid between two electrical contacts on the device, similar to the “lemon-" or “potato-battery” experiment typically conducted in school science labs.

One of the metal contacts on the sensor loses positively charged zinc atoms to the acid leaving behind electrons in the zinc. The other contact, made of copper, absorbs positively charged hydrogen atoms in the acid. This charge difference between the two metals drives the electrons through the device generating a few nanowatts of power.

While this corresponds to a current billions of times tinier than that needed to run a lightbulb, it can be used to supply lower-power sensor devices like those being pioneered by study lead author Phillip Nadeau, whose group at MIT are seeking alternatives to using ordinary batteries.

One approach is to collect the small amounts of energy generated by the intestinal battery. When sufficient energy has been stored the device can turn on temporarily, make and transmit some information and then return to a stand-by mode while it recharges.

However, as the sensor moves into the small intestine, the environment becomes a lot less acidic and less useful for energy harvesting, only producing 1/100 of the power it can in the stomach.

"But there's still power there, which you could harvest over a longer period of time and use to transmit less frequent packets of information," Giovanni Traverso, a senior author on the study, explains.

Nevertheless, there are still hurdles to clear before you can expect your own stomach to become a mini power plant for one of these tiny machines, including safety considerations. Although the metal used by the battery, zinc, is routinely absorbed by the body from our food, further studies would need to be done to make sure that the levels of exposure aren’t too high. And doctors will need be to be reassured that the device sitting in the stomach for long periods of time will not cause irritation or injury.

Moreover, there are also potential security risks associated with having these devices emitting information about your body to eavesdropping hackers, a problem encountered with insulin pumps and pacemakers that share similar vulnerabilities.

"We didn't examine this problem in our work, but it is a very critical issue to consider if we eventually create a medical device.  The good news is there is a vast field of security research that is being adopted in this area,” says Nadeau.