To create their unorthodox battery, the team members Tom Schroeder and Anirvan Guha began by reading up on how the eel’s electrocytes work. These cells are stacked in long rows with fluid-filled spaces between them. Picture a very tall tower of syrup-smothered pancakes, turned on its side, and you’ll get the idea.

When the eel’s at rest, each electrocyte pumps positively charged ions out of both its front-facing and back-facing sides. This creates two opposing voltages that cancel each other out. But at the eel’s command, the back side of each electrocyte flips, and starts pumping positive ions in the opposite direction, creating a small voltage across the entire cell. And crucially, every electrocyte performs this flip at the same time, so their tiny voltages add up to something far more powerful. It’s as if the eel has thousands of small batteries in its tail; half are pointing in the wrong direction but it can flip them at a whim, so that all of them align. “It’s insanely specialized,” says Schroeder.

He and his colleagues first thought about re-creating the entire electric organ in a lab, but soon realized that it’s far too complicated. Next, they considered setting up a massive series of membranes to mimic the stacks of electrocytes—but these are delicate materials that are hard to engineer in the thousands. If one broke, the whole series would shut down. “You’d run into the string-of-Christmas-lights problem,” says Schroeder.

In the end, he and Guha opted for a much simpler setup, involving lumps of gel that are arranged on two separate sheets. Look at the image below, and focus on the bottom sheet. The red gels contain saltwater, while blue ones contain freshwater. Ions would flow from the former to the latter, but they can’t because the gels are separated. That changes when the green and yellow gels on the other sheet bridge the gaps between the blue and red ones, providing channels through which ions can travel.

Here’s the clever bit: The green gel lumps only allow positive ions to flow through them, while the yellow ones only let negative ions pass. This means (as the inset in the image shows) that positive ions flow into the blue gels from only one side, while negative ions flow in from the other. This creates a voltage across the blue gel, exactly as if it was an electrocyte. And just as in the electrocytes, each gel only produces a tiny voltage, but thousands of them, arranged in a row, can produce up to 110 volts.

Schroeder et al. / Nature

The eel’s electrocytes fire when they receive a signal from the animal’s neurons. But in Schroeder’s gels, the trigger is far simpler—all he needs to do is to press the gels together.

It would be cumbersome to have incredibly large sheets of these gels. But Max Shtein, an engineer at the University of Michigan, suggested a clever solution—origami. Using a special folding pattern that’s also used to pack solar panels into satellites, he devised a way of folding a flat sheet of gels so the right colors come into contact in the right order. That allowed the team to generate the same amount of power in a much smaller space—in something like a contact lens, which might one day be realistically worn.