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Sweat-sipping wearable aims to charge electronics without couch potatoes lifting a finger

What's wrong with plugging it in?

Brainiacs at UC San Diego say they have created a wearable designed to turn your horrid sweaty hands into a charge for your electronic devices – while you barely have to lift a finger.

"We envision that this can be used in any daily activity involving touch, things that a person would normally do anyway while at work, at home, while watching TV or eating," said Joseph Wang, professor of nanoengineering at UC San Diego and the study's senior author. "The goal is that this wearable will naturally work for you and you don't even have to think about it."

Previous efforts to generate energy from sweat, including earlier work at UC San Diego on a sweat-sucking shirt with embedded microgeneration, focused on the really sweaty types who exercise. This time around, it's for everyone – including couch potatoes.

"Normally, you want maximum return on investment in energy. You don't want to expend a lot of energy through exercise to get only a little energy back," Wang explained. "But here we wanted to create a device adapted to daily activity that requires almost no energy investment – you can completely forget about the device and go to sleep or do desk work like typing, yet still continue to generate energy. You can call it 'power from doing nothing.'"

To prove the concept, the team wrapped the wearable – a flexible strip of carbon foam electrodes treated with enzymes which trigger energy-generating reactions in sweat, with a piezoelectric layer added for additional energy harvesting from pressure – around a subject's fingertip. During a 10-hour sleep period, the gadget gathered about 400 mJ/cm2 of energy from the perspiration – enough, the researchers said, to power an electronic wristwatch for a full day.

The same device gathered at least 30 mJ per hour "from a single press of a finger" on a keyboard or mouse, an action "that consumes merely 0.5 mJ mechanical energy," the team reported. Times that by 10 fingers, the team pointed out, and that could be useful.

"Compare this to a device that harvests energy as you exercise," said Lu Yin, nanoengineering PhD student and co-first author of the paper. "When you are running, you are investing hundreds of joules of energy only for the device to generate millijoules of energy. In that case, your energy return on investment is very low. But with this device, your return is very high. When you are sleeping, you are putting in no work. Even with a single finger press, you are only investing about half a millijoule."

The team picked the fingertips as the wearable's target thanks to their surprising position as one of the body's most sweaty spots – yes, even more than you-know-where in the summer. Each fingertip has more than a thousand sweat glands, which spew out two to three orders of magnitude more sweat than most other areas of the body.

"The reason we feel sweatier on other parts of the body is because those spots are not well ventilated," Yin explained. "By contrast, the fingertips are always exposed to air, so the sweat evaporates as it comes out. So rather than letting it evaporate, we use our device to collect this sweat, and it can generate a significant amount of energy."

The team's work has been published under open-access terms in the journal Joule. ®

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