“I lost my charger” has become the most convenient excuse for FitBit slackers, but it won’t hold up for long: Scientists at the Massachusetts Institute of Technology have figured out how to harvest energy from human movements to make sure our devices are always as charged up as we are.

While wearable tech has given us sweat-reading headbands and pulse-tracking bras, they’re not exactly life-changing when they’re low on power — and the MIT team hopes to change that.

The team’s paper, published recently in Nature Communications, describes a stamp-sized device that uses our bending movements to generate electricity. Using our movements to generate electricity isn’t anything revolutionary — in one Black Mirror dystopia, human hamster wheels comprised a self-sustaining power plant — but doing it has not been easy to do efficiently.

To tackle this problem, the MIT scientists focused on optimizing the size and chemical makeup of their movement-harvesting tools. Embedded in their adhesive strip are two tiny electrodes made of lithium-alloyed silicon, separated by an electrolyte-soaked polymer sheet. Electricity is generated after you stick the device onto, say, the crook of your inner elbow, and flex repeatedly, causing lithium ions to move back and forth from the compressed to the tensed electrode. Electrical current, after all, is simply the torrent of electrons that circle around these ions.

(a) Schematic view of the device design. Compressed region is illustrated in red while the tensile region is illustrated in blue. Lithium ions migrating from the compressed plate to the tensile plate are shown with arrows. The electrolyte soaked separator is drawn in yellow. (b) An image of the actual device with a bending unit. Both scale bars indicate 1 cm. (c) Cross sectional image of the device showing polypropylene electrolyte layer (A in the figure), LixSi electrode on Ag current collector (B in the figure) and polyimide adhesion layer (C in the figure). The scale bars on the left and right indicate 40 and 2 μm, respectively.
(a) Schematic view of the device design. Compressed region is illustrated in red while the tensile region is illustrated in blue. Lithium ions migrating from the compressed plate to the tensile plate are shown with arrows. The electrolyte soaked separator is drawn in yellow. (b) An image of the actual device with a bending unit. Both scale bars indicate 1 cm. (c) Cross sectional image of the device showing polypropylene electrolyte layer (A in the figure), LixSi electrode on Ag current collector (B in the figure) and polyimide adhesion layer (C in the figure). The scale bars on the left and right indicate 40 and 2 μm, respectively.

The maximum generating capacity — that is, the amount of bending energy that ends up being converted into electricity — of the MIT device is about 15 percent.

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In practice, however, using it seems to generate electricity at an average of about 0.6 percent, as lead author Sangtae Kim, Ph.D. told The Guardian.

Ramping it up to 6 percent efficiency would render it powerful enough to charge a wristband or even a smartphone.

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Efficiency aside, the major obstacle to producing energy efficiently from human movement is, well, the fact that we have to move — a lot. Generating electricity, after all, is really an exercise in energy conversion — you need consistent input to produce consistent output, and we’re not exactly good at that. Still, when you consider that the other alt-energy solution engineers have come up with is an electricity-generating sock that runs on pee (seriously), ramping up daily activity doesn’t seem like such a bad option.

Photos via Nature/Sangtae Kim, Andrew Burton/Getty