Band-Aid-like strip worn on fingertips generates energy during sleep
There is an exciting field of research emerging around how the wearables of the future could be powered by human sweat, and at the vanguard of these advances is a team of engineers at the University of California (UC), San Diego. The group's latest creation leverages the surprisingly large sweat output of the fingertips to generate electricity when the user is sedentary or even sleeping, providing a potentially round-the-clock source of power.
Among the many wearable sensors and devices that are powered by sweat we've looked at over the years, the UC San Diego team has put forward quite a few promising examples. Way back in 2014 the scientists showed off a temporary tattoo that worked as a sweat-powered bio-battery, and last year they demonstrated a wearable vitamin C sensor powered by similar means. More recently, the team developed a smart shirt that generates electricity via sweat and movement.
The newly revealed wearable device is described as the first of its kind, in that it can generate power even when the user sleeps or is sitting still. This could open up some very interesting possibilities in the wearable space, as the device could potentially act as a power source in any place, at any time.
“Unlike other sweat-powered wearables, this one requires no exercise, no physical input from the wearer in order to be useful," says co-first author Lu Yin. "This work is a step forward to making wearables more practical, convenient and accessible for the everyday person.”
In developing this new type of wearable, the team had to get creative in combining a mix of components that can both absorb sweat and convert it into energy. The thin Band-Aid-like strip consists of carbon foam electrodes that absorb sweat and use embedded enzymes to trigger chemical reactions between lactate and oxygen molecules within it, which in turn generates electricity that is stored in a small capacitor.
Fitted to the fingertips, the device takes advantage of the more than a thousand sweat glands on each fingertip that produce 100 to 1,000 times more sweat than most other parts of the body. The authors therefore describe the fingertips as 24-hour factories of perspiration.
“The reason we feel sweatier on other parts of the body is because those spots are not well ventilated,” says Yin. “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.”
While the device generates most of its electricity in this way, it is no one-trick pony. Underneath its electrodes the device also features a piezoelectric material that generates extra electricity in response to pressure. This means that activities like typing, texting or playing piano can also result in energy gains.
In one experiment, subjects wore the device on one fingertip during 10 hours of sleep, which generated almost 400 millijoules of energy, which the scientists say is enough to power an electronic watch for 24 hours. One hour of "casual" typing and clicking with a mouse allowed the device to generate almost 30 millijoules.
“Compare this to a device that harvests energy as you exercise,” explains Yin. “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 separate experiments, the team used their novel energy harvester to power chemical sensors and displays, including the aforementioned vitamin C sensor developed previously. They are now working to improve the device to make it both more efficient and durable, and hope to combine it with other energy harvesters to form new types of self-powered wearable devices.
“Our goal is to make this a practical device,” says Yin. “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.”
The study was published in the journal Joule, while the video below offers an overview of the research.