Wearables are very hot right now, with consumers amassing more than 100 million units of smartwatches, fitness trackers, augmented reality glasses, and similar technology in the first quarter of 2021 alone. Sales in that category rose 34.4 percent in the second quarter of Q2 2020, making it one of the fastest growing personal electronics categories.
The increase comes with increased demand for practical and efficient energy harvesters that are able to continuously energize the wearer’s tools. Now, a group of engineers at the University of California San Diego have designed a new type of biofuel cell that harnesses energy from the sweat of your fingertips, according to a recent paper published in the journal Joule. These cells can also be combined with a piezoelectric generator to get energy from pressing the fingertips. This breakthrough will one day make it possible to increase the wearability of your clothes while you type or sleep.
The vast majority of wearable electronics today are powered by small electrochemical storage devices such as batteries and super capacitors, but this is limited to how long they can turn on the electronics due to prolonged use. In addition to finding smart ways to reduce the energy consumption of such devices, the researchers see energy harvesters as a potentially promising solution. Some of the available self -powered sensors manage to reduce energy consumption, according to the authors, but the device is unable to turn on the signals needed for efficient data transmission.
Therefore, there is much interest in developing self -harvesting systems that draw energy from sunlight, body movements, biofuels, or temperature gradients (i.e., good ancient thermodynamics). Each type has its limitations. The sun does not always shine, for example, and thermoelectric devices require a temperature difference between the wearer and its surroundings. And harvesters who rely on body movement usually require vigorous exercise to extract the energy needed.
The main challenge facing all these new energy sources is a metric called energy return on investment (EROI), which is basically the ratio between how much energy is actively invested in a system that is not needed and how much energy is ultimately harvested. Movement -dependent energy harvesters, for example, utilize less than 1 percent of all the energy a person puts into a tool through vigorous exercise. Ideally, you want an energy harvester that relies on passive continuous input from the human body.
“Usually, you want the maximum return on investment in energy. You don’t want to spend a lot of energy through training to get some energy back,” said co-author Joseph Wang of UCSD, whose lab develops biofuel cells that draw energy from high lactate concentrations in human sweat eight years ago. “But here, we want to make a device that is adapted to daily activities that requires almost no energy investment – you can completely forget about this device and sleep or do desk work like typing, yet still continue to generate energy. nothing. ‘ “
Actually, according to Wang et al., their device has the best EROI for bioenergy harvesters. They believe it represents a paradigm shift from what they call “working to be powerful” to “living to be powerful.”
Their secret: passive sweat known as fingertip sweat. Our hands and fingers produce more sweat than we realize because our fingers are constantly exposed to air, so the sweat can evaporate quickly. In fact, fingertips have the highest concentration of sweat glands anywhere in the human body, including under the arms. The fingertips produce sweat at a rate as high as a few microliters per centimeter.
“Even with a small amount of sweat compared to the sweat you get from a very intense workout, this strength is still very large,” co -author Lu Yin told New Scientist. “No matter how clean your hands are, it’s very easy to leave your fingerprints anywhere. That’s basically the waste of your sweat, with a lot of metabolites. What we’re doing is leveraging this.”
The new UCSD biofuel cell is a thin, flexible strip that easily wraps around fingertips like a Band-Aid. There is an electrode base made of carbon foam and a sweat -absorbent hydrogel. The enzyme in the electrode then triggers a chemical reaction between the lactate and oxygen molecules in the sweat to produce electricity. The researchers next added a piezoelectric chip so that more energy could be generated simply by pressing one’s fingertips onto an object. The energy is then stored in small capacitors until needed.
The UCSD team found that their biofuel cells were able to produce nearly 400 milijoules of energy per square centimeter (enough to turn on an electric watch for 24 hours) because test subjects slept for ten hours. An hour of light typing or mouse clicking produces nearly 30 milijoules from one fingertip. Adding strips to other parts of the finger has the potential to produce ten times more energy, resulting in a large return on energy investment. “While you’re asleep, you’re not going to work,” Yin said. “Even with one finger, you only invest about half a milijoule.”
To demonstrate the practicality of their device, the UCSD researchers connected their biofuel cells to a chemical sensor with a low -power electrochromic display that provided readings of the data collected by the sensor. They connected the system to the subjects to monitor their Vitamin C levels after taking the supplement. Another experiment involved activating a sodium sensor to monitor how many sodium ions were in a saltwater sample. The researchers found that both the sensor and the display could be turned on by pressing the device ten times every ten seconds or simply by wearing a strip on a fingertip for two minutes.
The next step is to increase the efficiency of these novel biofuel cells and integrate them with other types of harvesters for specifically targeted situations. Hydrogel components can also be upgraded for better durability and moisture from repeated and repetitive operations. “Highly efficient, user -friendly, biocompatible energy harvesting technology, combined with appropriate system consolidation and energy budgeting, offers considerable promise for creating an independent, reliable and independent next -generation epidermal electronic system for tracking health and wellness,” the authors conclude.
DOI: Joule, 2021. 10.1016 / j.joule.2021.06.004 (Regarding DOI).
Listing photo by UCSD Jacobs School of Engineering