The classic ‘90s cartoon Magic School Bus predicted the future of medicine when it shrunk its eponymous bus into a nanobot and injected it into a human body. But it didn’t address the question of fuel: What powered this thing? Perhaps Miss Frizzle had also foreseen that stomach acid can power a tiny robot’s generator, as a team of researchers from MIT discovered this week.
In a new paper in the journal Nature Biomedical Engineering, the scientists describe the first ingestible battery that can power itself using the body’s stomach acid for prolonged periods of time. Currently, tiny devices used to deliver drugs to specific parts of the body or to measure internal physiological conditions, like temperature and breathing rate, are powered by chemical batteries. But these take up space, are dangerous when they leak, and eventually run out of juice. Using the body’s natural resources as fuel, in contrast, is much like harvesting energy from the sun or the wind: It’s relatively safe and endlessly renewable.
The new device was inspired by another childhood classic: The lemon battery. Elementary school kids, learning that electricity is just the flow of electrons from one metal to another, are taught that lemon juice can act as an electrolyte that separates the electrons from a copper penny and zinc nail so that they can move freely. Stomach juice is acidic, just like lemon juice, so when the MIT researchers stuck zinc and copper electrodes to the surface of its sensor, electricity was generated as soon as it reached the gut of the pigs that swallowed them. Each battery can generate enough electricity to power a temperature sensor and the 900-megahertz transmitter needed to send that data to a computer two meters away every 12 seconds. On average, each battery lasted about six days.
Stomach juice is profoundly acidic (it has to be, in order to break down the food we ingest) and can help voltaic cells generate a lot of electricity, but that acidity is not particularly helpful when the device leaves the gut. Once the devices passed out of the stomach into the small intestine, the amount of electricity they produced decreased to 1 percent of what they were generating in the gut. To get around this hurdle, the researchers have to optimize the device’s ability to store energy without taking up too much space: Their current device has dimensions of about 40 by 12 millimeters — about the size of a small screw — but they’re hoping to shrink it down to one-third of the size while also adding more sensors to it.
In 2015, a team from the University of California also built a stomach acid-powered device, but it wasn’t able to survive very long. MIT’s breakthrough has the potential to keep tiny robots collecting data in our bodies indefinitely — or, at least, without having to resort to bacterial wind turbines as a power source. Miss Frizzle would be proud.