A new wearable power-harvesting device turns the touch of a finger or even micro-droplets of sweat into a power-harvesting source for small devices and sensors, according to a new study published today in Joule journal.
Engineers at the University of California San Diego created this thin, flexible strip that can be worn on a fingertip. They generate small amounts of electricity when a person’s finger sweats or the wearer presses on it.
What makes this device special is that it generates electricity even when the wearer is sleeping or sitting still. This is potentially a big thing for the wearables space, as researchers have now figured out how to harness the energy that can be extracted from human sweat even when a person is not moving.
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Separate from, but complementary to, sweat-based energy generation, the device also harvests additional energy from everyday movements such as z Type or click.
The most efficient wearable energy harvesting device yet
To date, the device is the most efficient wearable energy harvesting device ever invented. For example, it can generate 300 millijoules (mJ) of energy per square centimeter during a 10-hour sleep session—without any mechanical energy input. And a single finger press generates an additional 30 mJ of energy.
The device is the first of its kind and represents a significant advance in self-sustaining wearable electronics, said co-author Lu Yin.
“Unlike other sweat-powered wearables, this one requires no practice, no physical effort on the part of the wearer to be useful,” he said. “This work is a step forward in making wearables more convenient, convenient, and accessible for everyday people.”
Previous sweat-based energy devices required intense physical activity, such as running or cycling, before the user sweated enough to activate power generation . But the large amount of energy expended during exercise can easily offset the energy generated, often resulting in an energy return of less than 1%.
In contrast, this device falls into what the authors as the “Holy Grail” category of Energy Harvester. Rather than relying on external, erratic sources like sunlight or motion, it only takes finger contact to collect more than 300 mJ of energy during sleep, which the authors say is enough to power some small portable electronics.
Energy harvesting through mouse clicks or playing the piano
The device also generates additional energy through light finger pressure, meaning that activities such as typing, texting, playing the piano or even tapping in Morse code can also become sources of energy
“We envision this being used in any daily activity involving touch, things that a person would normally do anyway at work, at home, or while watching TV or eating,” said Co-author Joseph Wang. “The goal is for this wearable to work naturally for you, without even having to think about it.”
This wearable energy-harvesting device, which can generate electricity with the click of a mouse or playing the piano, gets even more Power from the sweat of your fingertips, which are sweat factories around the clock. It’s a little-known fact that the fingertips are some of the sweatiest parts of the body; each one is packed with over a thousand sweat glands.
But not every sweat-powered device can work at your fingertip. Collecting and harnessing sweat in such a small area required innovative material engineering, Yin explained. The researchers had to build various parts of the device to be super absorbent and efficient at converting the chemicals in human sweat into electrical energy.
This latest energy-harvesting technology is particularly unique in that it could serve as a power source anytime, anywhere. It doesn’t have the same limitations as, say, solar cells, which only work under sunlight, or thermoelectric generators, which only work when there’s a large temperature difference between the device and the environment.
So how much power? A night’s sleep can power a smartwatch for 24 hours
Researchers had subjects wear the device on a fingertip while they sedated or slept. From 10 hours of sleep, the device collected almost 400 millijoules of energy, enough to power an electronic wristwatch for 24 hours. For comparison: one millijoule corresponds to the amount of energy that a 1 cent euro coin emits when it falls from a height of 5 centimetres.
The device collected almost an hour of casual mouse tapping and clicking 30 millijoules.
And all from just one fingertip.Strapping devices to the rest of your fingertips would generate 10 times more energy, the researchers said.
A much higher return than previous energy-harvesting devices
“By using the With sweat at the fingertips naturally expelling no matter where you are or what you are doing, this technology offers a net gain in energy without the user having to exert themselves. That’s what we call maximum energy return,” Wang said.
“Contrast this with a device that harvests energy during exercise,” explained Yin. “When you run, you invest hundreds of joules of energy just for the device to generate millijoules of energy. In that case, your return on energy is very low.”
“But with this device, your return on energy is very high,” he said. “When you sleep, you don’t work. Even with a single touch of your finger, you only invest about half a millijoule.”
How these energy harvesting strips work
The device that the researchers developed in this study is a type of energy collector called a biofuel cell (BFC) that runs on lactate, a compound dissolved in sweat.
From the outside, it looks like a simple piece of foam connected to a circuit with electrodes, each attached to a fingertip. The foam is made from carbon nanotube material and the device also contains a hydrogel that helps maximize sweat absorption.
The device is a thin, flexible strip that can be wrapped around the fingertip like a band-aid. Padding made of carbon foam electrodes absorbs sweat and converts it into electrical energy.
“The size of the device is approximately 1 square centimeter. The material is flexible too, so you don’t have to worry about it being too rigid or feeling weird. You can wear it comfortably for long periods of time,” says Yin.
The electrodes are equipped with enzymes that trigger chemical reactions between lactate and oxygen molecules in sweat to generate electricity. Below the electrodes is a chip made of a so-called piezoelectric material that generates additional electrical energy when pressed.
When the wearer sweats or presses on the strip, the electrical energy is stored in a small capacitor and is at Discharge to other devices as needed.
“Not just another cool thing that can generate a small amount of energy.”
“Our goal is to make this a practical device ‘ Yin said. “We want to show that this isn’t 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 like sensors and displays.”
The researchers were able to use the device to power effective vitamin C and sodium sensing systems, and they are optimistic about improving the device to have even greater capabilities in the future, which could benefit health – and wellness treatments such as glucose meters could make suitable for people with diabetes.
“We would like to integrate this device more closely into wearable forms such as gloves. We are also investigating the possibility of enabling wireless connection to mobile devices for enhanced continuous detection,” says Yin.
To that end, the team is making further improvements to the device to make it more efficient and durable . Future studies will include combining it with other types of energy harvesting devices to create a new generation of portable, self-powered systems.
You can watch a video of this new energy harvesting device in action below.
Study: “A Passive Perspiration Biofuel Cell: High Energy Return on Investment” Authors: Muyang Lin, Mengzhu Cao, Alexander Trifonov, Fangyu Zhang, Zhiyuan Lou, Jae- Min Jeong, Sang-Jin Lee and Sheng XuPublished in: JoulePublished date: 13. July 2021 DOI: https://doi.org/10.1016/j.joule.2021.06.004
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