Devices that harness electricity using sweat, attached to fingertips.

Electricity from sweaty fingertips?

The device on a user's fingers while they sleep, harvesting energy.
The device on a user’s fingers while they sleep, harvesting energy. Credit: Jong-Min Moon

A team of US researchers have made a tool that can generate electricity from sweat. Worn as a small cap on a fingertip, it could be used to power small medical devices while sleeping.

Engineers around the world have previously explored the potential of human sweat to generate energy. But previously developed devices are “hindered by the inherent inaccessibility of natural sweat,” according to a paper published in Joule. This device, while it doesn’t generate much electricity, also doesn’t need much sweat to work: it can be powered from passive activities.

A fingertip Vitamin-C sensing device with an electrochromic display, powered by the sweat-powered device. Credit: Lu Yin

“We wanted to create a device adapted to daily activity that requires almost no energy investment,” says Joseph Wang, a nanoengineering professor at the University of California, US, and senior author on the paper.

“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’.”

“The size of the device is about 1 centimetre squared. Its material is flexible as well, so you don’t need to worry about it being too rigid or feeling weird. You can comfortably wear it for an extended period of time,” says Lu Yin, a nanoengineering PhD student in Wang’s lab and co-author on the paper.

A hand with four sweat power-harvesting devices wrapped onto it. Credit: Lu Yin

The device uses a compound in sweat called lactate. When oxidised, the lactate releases a small amount of energy.

The researchers developed a battery-style setup, including an anode made with an enzyme called lactate oxidase (which, unsurprisingly, oxidises lactate), and a cathode made of platinum which completes the oxidation cycle. As long as there is a consistent sweat supply, electrons flow through the circuit spontaneously without any extra energy input.

A small hydrogel (right) collecting sweat from the fingertip for the vitamin-C sensor (left), then displaying the result on the electrochromic display. Credit: Lu Yin

The device also includes piezoelectric generators, which can make electricity from the mechanical movements of fingers. This adds up to 20% more power to the device.

In total, the device can collect 300 millijoules from a centimetre of skin over a 10-hour sleep period, or 30 millijoules from the press of a fingertip. While it won’t be charging your smartphone anytime soon, it’s enough energy to power small wearable medical devices– in their paper, the researchers used it to run vitamin C and sodium sensing devices.

The researchers focussed on the fingertips because they have the highest proportion of sweat glands.

“Sweat rates on the finger can reach as high as a few microlitres [0.001 millilitres] per square centimetre per minute,” says Yin. “This is significant compared to other locations on the body, where sweat rates are maybe two or three orders of magnitude smaller.”

Yin adds that they’re hoping to integrate the energy harvester into gloves, among other things.

“There’s a lot of exciting potential,” says Wang. “We have ten fingers to play with.”

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