麻省理工做出了一块可以吞下去的电池：它在猪的身体里工作了三天，然后自己化掉了

Let's begin with a piece of common sense almost everyone has been taught.

Do not swallow batteries.

A small child swallowing a button battery is one of the situations an emergency department least wants to see.

The reason is that the chemicals inside will burn the oesophagus and stomach wall within a very short time.

So when a research team announces they have made a battery you can swallow, the first reaction is that the sentence itself is faulty.

On the twenty-first of September this year, the work was formally published.

The test subjects were pigs; the result was that it worked inside the body for three days and then dissolved.

It sounds like a joke, but it is real.

To understand why anyone would set out to make an edible battery, you first have to see clearly what problem it solves.

For several decades, progress in medical imaging has followed essentially one path: looking in from outside the body.

But that path has its limits.

So a second line of thinking emerged: stop looking through the body and send the instrument in directly.

The capsule endoscope is what that line produced once it worked.

It is a camera the size of a capsule; the patient swallows it, it photographs the whole way, and it is passed naturally at the end.

This entire category is called ingestible electronic devices, and they are all stuck on the same problem: power.

Any electronic device needs electricity, and a standard battery contains chemicals that must not contact human tissue.

The current approach is really avoidance: seal the battery tightly so it never touches the body at any point, and pass it out with the device.

In the overwhelming majority of cases, this is safe.

The trouble is that the approach depends on that outer layer not breaking.

The seal can fail, and the device can lodge in the digestive tract and fail to pass.

If either of those happens, the risk that was being avoided immediately becomes a real accident.

Which is to say, part of whether this approach is safe comes down to luck.

So a genuinely complete solution has only one direction: make the battery out of materials that are not toxic to begin with.

Better still, have it degrade safely inside the body once its work is finished.

The answer from the MIT team is a battery made of paper.

More precisely, a degradable paper-based battery using magnesium and molybdenum trioxide as electrodes.

Choosing magnesium was not incidental.

Magnesium is an element the body needs anyway, which fundamentally changes the starting point of any safety discussion.

The battery is wrapped in a thin cellulose film, whose job is to hold off stomach acid for a set period.

The inspiration for this design is remarkably plain.

The researchers mention that what they had in mind was the edible rice paper wrapped around a sweet.

That paper dissolves over time, and that is exactly what they wanted.

In other words, what they wanted was for it to fail at the end.

Two numbers are worth remembering.

The first is volume.

The whole battery occupies only a quarter of a standard capsule.

Which means the other three quarters remain available for sensors, transmitters, or the drug itself.

The second is voltage.

Inside the body it can output around one point eight four volts, slightly above a household AA.

Validation proceeded in two stages, and they deliberately chose two tasks with very different power demands.

The first was the low-power kind.

They sealed the battery into a capsule alongside an electronic tag that transmits a signal, and had a pig swallow it.

A receiver outside the body successfully read the tag's signal, proving the power chain was intact.

The second was the power-hungry kind.

They used the battery to drive a device delivering electrical stimulation inside the stomach.

Clinically, that kind of stimulation is used to promote gastric emptying.

In the experiment, the pig's hunger hormone levels rose, showing the device really was having an effect.

Together the two stages establish one thing: this battery genuinely works.

Last comes the time dimension.

The battery supplied power inside the pig for about three days.

After three days, gastric fluid penetrated the coating and the electrode materials began to corrode and dissolve.

It should be stressed that this is exactly the result the design was aiming for, not a failure.

But one academic who was not involved in the research raised the hardest question this work currently faces.

He pointed out that degrading is not the same as disappearing.

We still need to know what it actually breaks down into, how much the body absorbs, and whether those products irritate or damage the gut.

Repeated use matters especially here, because a patient may need to swallow such devices again and again.

There is no answer to that at present.

So it cannot yet be said that this is ready for use in people.

Worth adding: making the battery edible is only one of the technical routes.

The other route runs on the opposite logic: use food to make the battery directly, rather than making the battery resemble food.

A team in Milan has already built a degradable battery from riboflavin, caper extract and nori seaweed.

Its output is still nowhere near enough to drive anything practically useful, but it has already been certified as edible.

The end point of both routes is in fact the same.

Turning an electronic device that enters the body from a foreign object requiring careful isolation into something the body was always capable of handling.

Once you have thought all that through, only one question is left.

Now put yourself in the scene.

A doctor hands you a capsule and tells you there is a battery inside, that it will work for three days, and that it will then dissolve in your body.

Do you swallow it, or not?