You'll Get a Charge Out of This

We are all so used to charging our phones, earbuds, smartwatches, tablets, laptops, Bluetooth speakers, and fitness trackers that we hardly think about it anymore. Not that anyone likes to keep all their devices topped off, of course, but it is just a part of life now. Even still, everyone has their limits; we have to draw a line in the sand somewhere. And clothing is most definitely on the wrong side of that line. No one, and I mean no one, wants to charge their shirt before they get dressed in the morning.

But with the rise of wearable electronic devices, that may be exactly where we are headed. Most of the components needed to build on-body health trackers, personal assistants, and much more have been made small enough, cheap enough, and comfortable enough for use in a wearable. Batteries, however, are one glaring exception. Every electronic device needs them, but they are too stiff and bulky for many wearables. The best we can do is use as small of a battery as we can get away with, but then they need to be frequently recharged.

This situation may soon change, however. A team led by researchers at Southwest University has put forward a new idea that may change how we think about powering wearables. Rather than incorporating traditional batteries into textiles, they have made the fabric itself a battery. The good news is that these stretchable yarn batteries are self-powered, so they never need to be charged. The bad news is that they are powered by sweat, so they may not be too popular for use in areas outside of athletic wear.

Called the Stretchable Sweat-Activated Yarn Battery (S-SAYB), the new device offers an unusual combination of traits: it is flexible, stretchable, washable, and, most importantly, maintains an ultra-stable power output even when the wearer is moving. This stands in contrast to textile-based batteries of the past, in which the power output significantly fluctuates when stretched or twisted during use.

The S-SAYB has an elastic yarn core as the foundation, which is then wrapped in a sequence of materials to create a functional battery that activates when exposed to sweat. These materials include a zinc wire anode and a carbon yarn cathode, which are coiled tightly around the elastic core. Between them lies a cotton fiber layer, which acts as both a separator and a sweat-absorbing medium.

To enhance the stability of the power output, the elastic core is coated with a hydrophilic polyester fiber layer, which can retain sweat even during stretching. This prevents gaps in the conductive pathway that would otherwise cause voltage drops. Furthermore, the electrodes are wrapped around the core at a high density, keeping them close enough together that they remain electrically connected even under significant strain. It was shown that, due to these design choices, the yarn can be stretched by up to 80% with a voltage drop of less than half a percent.

When integrated into fabric, the yarn can be woven, knitted, or stitched using standard textile manufacturing techniques. In a series of trials, the researchers produced S-SAYB headbands and sports shirts that generated enough power from sweat to run small devices like LEDs and pedometers throughout a workout session. The output was steady enough that the devices worked without a hiccup.

While this technology might not appeal to everyone, it could be important for sportswear, outdoor gear, or even medical monitoring clothing. And as the team explores integration with more kinds of devices, we may see it being incorporated into more intelligent wearable systems that do their job without ever needing to be plugged in.