Wearables are so hot right now, with consumers collecting more than 100 million units of smartwatches, exercise trackers, augmented reality glasses and similar technology in the first quarter of 2021 alone. Sales in the category increased by 34.4 percent in the second quarter from the second quarter 2020, making it one of the fastest growing categories of personal electronics.
This increase comes with an increased need for practical and efficient energy harvesters that are able to continuously operate these portable products. Now a team of engineers at the University of California San Diego has designed a new type of biofuel cell that utilizes energy from sweat at your fingertips, according to a recent article published in the journal Joule. The cell can also be integrated with piezoelectric generators to harvest energy from pressure at the fingertips. The breakthrough could one day make it possible to turn on your laptop while typing or sleeping.
Most portable electronics these days are powered by small electrochemical storage devices such as batteries and supercapacitors, but these are limited in how long they can operate the electronics over a longer period of time. In addition to finding smart ways to further reduce the energy consumption of such devices, researchers see energy harvesters as a promising potential solution. Some existing self-powered sensors successfully reduce energy consumption, per authors, but these devices fail to drive the necessary signals for efficient data transfer.
So there is considerable interest in developing self-supporting harvesting systems that draw energy from sunlight, body movements, biofuels or temperature gradients (ie good old-fashioned thermodynamics). Each type has its limitations. The sun does not always shine, for example, and thermoelectric devices need a temperature difference between users and the environment. And the harvests that depend on body movements usually require vigorous training to extract the necessary energy.
The main challenge for all these new power sources is a calculation called energy return (EROI), which is basically the ratio between how much energy is actively invested in a system that would not otherwise be necessary and how much energy is eventually harvested. An energy harvester that depends on movement, for example, uses less than 1
“Normally you want maximum energy return. You do not want to use a lot of energy through exercise to regain some energy,” said co-author Joseph Wang of UCSD, whose laboratory developed biofuel cells that draw energy from the high concentration of lactate in human sweat for eight years ago. “But here we wanted to create a device adapted to daily activity that requires almost no energy investment – you can completely forget the device and go to sleep or do desk work like writing, but still continue to generate energy. You can call it” power from doing nothing. “”
According to Wang et al., Their unit has the most favorable EROI yet for bioenergy harvesters. They believe it represents a paradigm shift from what they call “work for power” to “living for power.”
Their secret: passive sweating known as fingertip sweating. Our hands and fingers produce much more sweat than we realize because our fingers are constantly exposed to air, so that sweat evaporates quickly. In fact, the fingertips have the highest concentration of sweat glands anywhere on the human body, including under the arms. The fingertips produce sweat at speeds as high as a few microliters per centimeter.
“Even with the sweaty amount of sweat compared to the sweat you got from a very intense workout, this power is still very significant,” co-author Lu Yin told New Scientist. “No matter how clean your hand is, it is very easy to leave your fingerprint everywhere. It’s basically the rest of your sweat, with a lot of metabolites. What we did is take advantage of this. ”
The new UCSD biofuel cell is a thin, flexible strip that is easily wrapped around the fingertip like a patch. It is a pad with electrodes made of carbon foam and a hydrogel that absorbs sweat. Enzymes in the electrodes then trigger chemical reactions between the lactate and oxygen molecules in the sweat to produce electricity. The researchers then added a piezoelectric chip so that even more energy could be produced just by pressing your fingertips against an object. That energy is then stored in a small capacitor until needed.
The UCSD team found that their biofuel cell could produce almost 400 millijoules of energy per square centimeter (sufficient to power an electric wristwatch for 24 hours) as the subject slept for ten hours. An hour of easy typing or clicking of a mouse produced nearly 30 million dollars from a single fingertip. Adding stripes to the rest of your fingers can potentially produce ten times more energy, giving a good return on energy investment. “When you sleep, you do not put in any work,” Yin said. “Even with a single finger press, you only invest about half a million.”
To demonstrate the practicality of their device, UCSD researchers connected their biofuel cell to a chemical sensor with a small, low-power, electrically efficient screen that provides a reading of the data collected by the sensor. They connected the system to a subject to monitor vitamin C levels after taking a supplement. Another experiment involved operating a sodium sensor to monitor the amount of sodium ions in a sample of saline. The researchers found that both the sensor and the screen could be operated by pressing the device ten times every ten seconds or by simply using the strip on the fingertip for two minutes.
The next step is to improve the efficiency of this new biofuel cell and integrate it with other types of harvesters for particularly targeted situations. The hydrogel component can also be improved for better durability and moisture retention during prolonged, repeated use. “Such high-efficiency, user-friendly, biocompatible energy harvesting technologies, combined with system integration and similarly sensible energy budgeting, offer significant promise in establishing self-sustaining, reliable and independent next-generation epidermal electronics systems for health and wellness tracking,” the authors concluded.
DOI: Joule, 2021. 10.1016 / j.joule.2021.06.004 (Om DOI).
Listing image of UCSD Jacobs School of Engineering