This Smart Pill Uses A Stomach Acid Battery

[Curt White] is working on a smart pill whose copper-zinc battery will use his own stomach acid as the electrolyte. It’s not that unusual of an idea, MIT tested a similar approach in a pig. It’s also better than using lithium ion batteries, something we covered in this PSA.

Smartpill circuit diagram
Smartpill circuit diagram

His starting point is a small, hacked activity tracker with its Nordic nRF51822 ARM Cortex-M0 and Bluetooth LE SoC. Most everything else is removed. The battery electrodes are sewn onto a plastic mesh cut to the activity tracker’s dimensions. Three coin type super capacitors and a boost converter sit between the battery and the SoC.

He uses the Bluetooth LE for communication, sort of. BLE devices constantly transmit information about themselves and it’s this which you see when scanning for available devices. Included in that transmission is a UUID (Universally Unique Identifier) and a name (e.g. “smartpillxyz”). He has the pill transmit data by putting it in that name. This saves power by minimizing the time which the pill’s Bluetooth radio is turned on. The smartphone app extracts the data from these transmissions without ever connecting.

His goal is to monitor the voltage and the maximum current. This will tell him if his stomach acid battery works and what can be powered by it. First tests will use regurgitated gastric fluid and then later he’ll swallow the pill himself. As he puts it, why not, “people swallow and pass all kinds of weird stuff without a problem.” Thay may sound cavalier but judging by his hackaday.io page, he’s doing his homework.

33 thoughts on “This Smart Pill Uses A Stomach Acid Battery

    1. Can you elaborate? This has to be open source and accessible to anyone, so assume use of BLE over a hybrid app framework (React Native/Cordova) or Web Bluetooth. How would I get data in, let’s say, 1/10 of a second?

      1. With the device as a BLE slave, crank latency up. The master listens every connection interval, the slave need only turn up every N, (and N x latency can be more than the max advertising interval.. ) when it has something to say. Terminology may be a bit off but you get the idea. It’s been a few years since I played with BLE now…

  1. If that pill goes through your digestive system it’s in and out in less than a day, something lithium cell can handle without any problems. If you use copper and zinc electrodes for producing electricity you’re dissolving those elements which is probably a bad idea to do inside your stomach.

      1. And copper and hydrochloric although a much slower reaction (no where near as vigorous as Zn+HCl) will produce CuCl and CuCl2 which is considered toxic to humans. Although concentrations less than 5 ppm is legally allowed in drinking water in the US. Depending on the reactive surface area, it may just make you feel very ill.

        And Zinc Chloride, if the concentration is low enough, not to kill you, will be self evacuated by the body (ref: https://en.wikipedia.org/wiki/Zinc_chloride#Safety ).

        I’m starting to think this is either a bad joke, or a method of execution.

    1. Yup. You could maybe put in a 1-way valve, but then you’d be limited to just as much acid as would fill the chamber once, not enough to react with all the metals there. And the valve would need to be medically reliable (ie very very very), and be able to work on such a tiny scale without being blocked by hydrogen or air bubbles.

      If you think about the poisonous aspects, and the fact that existing tiny batteries can easily power modern smart-pill endo-sensors, then for one thing there’s no need for this, for a second it’s fairly dangerous and you’d be insane to swallow it.

      I realise the idea “stomach = free acid!” occurred to Curt, but this is a dead end. It has no practical use and is actively dangerous! It’s also huge! There’s not much use for a medical sensor when your patient’s choked to death.

      Maybe Curt could experiment with other power-scavenging things, there’s plenty of alternative power methods still not well explored.

  2. I’d be very, very careful about swallowing small metal objects, particularly anything with any other kind of battery in it. They can cause major issues, including bowel perforation. (Oh, and don’t google “swallowing money”, at least if you do, make sure your content filtering is cranked up to 11).

  3. At the Bluetooth frequency (2.4 GHz), attenuation of the signal in human tissue is 20 dB per centimeter.

    And the attenuation is made even worse by the horrible mismatch and reflection at the skin-air boundary: another 12-15 dB loss.

    I hope he’s really skinny.

  4. He is showing some foresight; if it goes terribly wrong this is just a modern day version of Charon’s obol, where the ancient Greeks and Romans would put the copper coin in the mouth after the person was dead, to pay the boatman.

  5. So this project is still at an early stage…. I haven’t had time to mess with it much recently, but what with this article, I’ll benchmark some of my cells and post results tomorrow.

    1. The description says the cells are wired in series but I think the shared electrolyte may cause a significant loss of voltage. I suppose a worst case scenario would be using a boost converter that can use the voltage from a single cell directly. On the plus side then you could just use 2 larger electrodes instead of 6 smaller. Also if the cells are mostly covered anyway, maybe you could just use stomach acid strength HCI and seal the unit completely, so if it leaks it is no more toxic than it would have been if open but now it can work in a neutral environment. I wonder how much voltage you will get as it crosses from stomach acid to the alkaline environment of the intenstine. Perhaps if you can find a biologically compatible and o2 permeable membrane a zinc-air type cell could be viable. Hmm, further reading indicates but reaching the large intestine there isn’t much o2 left. Well I’ll just concede it is a tricky problem.

  6. Also, let it be known: if I should die a horrible death testing this out, it was only in an effort to prove a Hackaday article comment section wrong. My blood will be on your hands. Seriously though, I’m not going to do anything harmful to myself. But if I did, it would your fault. You know who you are.

    1. We will avenge your death.

      What kind of data are you trying to collect from your insides? If it is not too data rate intensive you may be better off with a much lower frequency transmission. You could even do some kind of inductive power coupling if you keep the frequency low enough. Think of an inductive belt you wear, or wrap around your torso, that would send power and receive data.

      There is a lot of movement and “gurgling” in the digestive tract, so you could also look into piezoelectric power. A linear piezo element with a proof mass on its end power a voltage converter (Linear Technology LTC3588 for example), or perhaps even a polymer piezo element could form part of the housing of the pill.

    2. “Testing” doesn’t mean you’re going to actually swallow it, does it? I really really really recommend you don’t.

      If you must, though, stream it live on Youtube (so you don’t have to be capable of pressing “upload” afterward). Check you live in a land with socialised medicine, and have someone keep an eye on you all the time. Perform the experiment somewhere quite close to a hospital. Perhaps sat outside one, on a bench. Hospitals often have really nice gardens. You can share your results with the frantic doctors, as they drag you inside and fill you full of charcoal.

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