The wearable device industry is expanding rapidly. A couple of decades ago, the only common wearable device was the wristwatch. Today, we have smartwatches, fitness trackers, RFID rings and wristbands, blood sugar monitors, and more. But despite the growth of the industry, wearable devices are still held back by energy. A smartwatch may have room for a conventional lithium-ion battery — albeit a small one. But you have to charge a smartwatch every day and smaller wearable devices may not have room for a battery at all. That's why engineers at the University of California San Diego developed this novel finger wrap that can passively power wearable devices.
This finger wrap, which resembles and feels like a Band-Aid, converts sweat and physical force into electricity. The user wears the thin, flexible strip on their fingertip and then goes about their day. As they perform their daily normal activities, including sleeping, the tip of their finger will produce sweat. That is true even if they aren't doing activities that require physical exertion. The finger wrap harvests the sweat and uses it to generate electricity. It generates additional electricity when the user touches something, such as when they're typing on a keyboard or picking up an object.
The strip does this by collecting sweat in a sponge-like padding of carbon foam electrodes. Special enzymes embedded in that carbon foam trigger chemical reactions between lactate and oxygen in the sweat, which produces electricity. There is also piezoelectric material underneath the carbon foam that produces electricity in response to physical force. Energy from either source gets stored in a capacitor so it can be used later for reading a sensor or some other kind of work.
An hour of typing and clicking a mouse was enough to generate 30 millijoules of energy. Sleeping for 10 hours generated 400 millijoules of energy. For reference, the team says a wristwatch requires around 16 millijoules per hour. The benefit over triboelectric nanogenerators, which have also made headlines lately, is that this finger wrap can generate energy even when the user is completely still. That means it could power your wearable devices even when you're sitting on the couch or taking a nap. The engineering team plans to continue working on this technology to make it practical for consumer use.