A device which turns sweaty fingers into a power source could recharge batteries while people sleep, scientists reveal.
A study found wearing it for 10 hours at night produces enough juice to run a watch for 24 hours.
Most power producing wearable devices require wearers to perform intense exercise or depend on external sources such as sunlight or large changes in temperature.
But now, researchers at the University of California San Diego, have invented the energy harvester which generates power even when the wearer is sat still.
The flexible strip wraps around the finger like a sticky plaster and produces small amounts of electricity when the wearer presses down or starts to sweat.
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It also produces power from light finger tapping, converting activities like typing, texting, playing the piano or tapping in morse code, into extra charge.
Co-first author doctoral student Lu Yin said: “Unlike other sweat-powered wearables, this one requires no exercise, no physical input from the wearer in order to be useful.
“This work is a step forward to making wearables more practical, convenient and accessible for the everyday person.”
Fingertips have one of the highest concentrations of sweat glands in the body, with each digit producing between 100 and 1000 times more perspiration than most other areas.
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Mr Yin said: “The reason we feel sweatier on other parts of the body is because those spots are not well ventilated.
“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.”
But collecting sweat from such a small area and converting it into electricity was a challenge, requiring special materials and engineering.
The device is equipped with electrical conductors - or electrodes, made from a carbon foam, which absorbs any finger sweat.
Enzymes on the electrodes then trigger a chemical reaction between sweat molecules - lactate and oxygen, to generate electricity.
A small chip under the electrodes, made of ‘piezoelectric material’, also generates power when pressed.
The juice is then stored in a small capacitor and can be discharged to other devices when needed.
Mr Yin said: “The size of the device is about one 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.”
To test their device, a volunteer was asked to perform various tasks while wearing it on their finger.
From 10 hours of sleep, the device collected almost 400 millijoules of energy, enough to power an electronic wristwatch for 24 hours.
From one hour of casual typing and clicking on a mouse, the device collected almost 30 millijoules.
Senior author Professor Joseph Wang said : “By using the sweat on the fingertip, which flows out naturally regardless of where you are or what you’re doing, this technology provides a net gain in energy with no effort from the user.
“This is what we call a maximum energy return on investment.”
Most devices which turn sweat into power require the wearer to perform intense exercise like running or biking, but not this one.
Mr Yin added: “Compare this to a device that harvests energy as you exercise.
“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.”
In one experiment, the finger-charger was connected to a chemical sensor and a small low-powered screen.
Pressing it 10 times every 10 seconds or simply wearing it for two minutes was enough to power both, the researchers found.
In another experiment, a volunteer was asked to swallow a vitamin C tablet.
The device was then used to power a sensor the researchers had developed in their laboratory which read their vitamin C levels.
Another sensor designed to measure sodium ion levels in saltwater was also successfully powered by the device.
Mr Yin said: “Our goal is to make this a practical device.
“We want to show that this is not just another cool thing that can generate a small amount of energy and then that’s it.
“We can actually use the energy to power useful electronics such as sensors and displays.”
Further improvements are being made to make the device more efficient and durable.
It is also being combined with other types of energy harvesters to create a new generation of self-powered wearable systems.
