What if you could recharge your smartphone by going for a hike or job? Turning exercise into electricity to power small devices may be possible in the not-too-distant future, with nanoengineers developing a wearable ‘microgrid’ that harvests and stores energy from sweat and movement.
The wearable microgrid system is made of sweat-powered biofuel cells, motion-powered devices called triboelectric generators, and energy-storing supercapacitors. All of the parts are flexible and washable, making them ideal for clothing.
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“We’re applying the concept of the microgrid to create wearable systems that are powered sustainably, reliably and independently. Just like a city microgrid integrates a variety of local, renewable power sources like wind and solar, a wearable microgrid integrates devices that locally harvest energy from different parts of the body, like sweat and movement, while containing energy storage,” says Lu Yin, a nanoengineering doctoral student at the University of California San Diego and one of the creators of the new technology.
The team first discovered sweat-harvesting wearables in 2013, but have since developed them to become stretchable and powerful enough to run small electronics. In a test, the microgrid was able to continue powering an LCD wristwatch throughout a 30 minute exercise session made up of 10 minutes of running and 20 minutes of rest. While the triboelectric generators provide power as soon as the user starts moving, before breaking a sweat, the biofuel cells kick in when the wearer begins to sweat.
Biofuel cells that harvest energy from sweat are located inside the shirt at the chest. Triboelectric generators that convert energy from movement into electricity are positioned outside the shirt on the forearms and sides of the torso near the waist. Both harvest energy from the swinging movement of the arms against the torso while walking or running, after which supercapacitors outside the shirt on the chest temporarily store energy and discharge it to power small electronics.
“When you add these two together, they make up for each other’s shortcomings. They are complementary and synergistic to enable fast startup and continuous power,” Yin said.
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The biofuel cells are equipped with enzymes that trigger a swapping of electrons between lactate and oxygen molecules in human sweat to generate electricity. Meanwhile, the triboelectric generators are made of a negatively charged material, placed on the forearms, and a positively charged material, placed on the sides of the torso. As the arms swing against the torso while walking or running, the oppositely charged materials rub against each other and generate electricity.
While the biofuel cells provide continuous low voltage, the triboelectric generators provide pulses of high voltage. Then, both are combined and regulated into a stable voltage by the supercapacitors which act as a reservoir that temporarily stores the energy and discharges it as needed.
