Calibe Thompson CC BY-NC 2.0

Okay, we’ve all been there: you’re in the middle of your workout, completely in the zone, and all of a sudden the battery on your mp3 player cuts out. While that’s among the most frustrating things in the Universe now, that might not always be the case. A team from the University of California, San Diego has developed a battery that is worn like a tattoo, and it is actually able to generate energy from sweat. Additionally, the device can also indicate changes in electrolyte levels during the workout. The research was published in the journal Angewandte Chemie and was presented at the 248th National Meeting & Exposition of the American Chemical Society.

When the researchers began developing this device, it was only meant to monitor the changing levels of lactate produced while exercising. Typically, monitoring lactate involves a series of blood tests at variable intervals during a workout, which is not ideal. However, Jia’s team was able to develop a device that could be applied and worn just like a temporary tattoo. The device reads the lactate levels that are present in sweat, giving scientists, physicians, and coaches realtime results.

"Lactate is a very important indicator of how you are doing during exercise," Jia explained in a statement. 

Not content to create a device that only tracks lactate levels, they also managed to turn it into a biobattery that runs on sweat. The battery’s anode is an enzyme that pulls electrons out of lactate transfers the current to an electron-accepting molecule that serves as the cathode.

Image credit: Joseph Wang

This might sound like the perfect device for fitness buffs, but testing revealed that those who were less physically fit were able to produce the most energy. Lactate is produced after the body breaks down glucose for energy, and too much lactate buildup is responsible for the sore, burning muscles experienced during exercise. Those who are more in shape do not produce as much lactate as their more sedentary counterparts, so they aren’t able to maximize the amount of energy generated. 

Even when used on someone who is out of shape, this device still has a ways to go before it can be used to charge heart rate monitors or mp3 players. However, the team is looking forward to a time when it will be capable of that function.

"The current produced is not that high, but we are working on enhancing it so that eventually we could power some small electronic devices," Jia explained. "Right now, we can get a maximum of 70 microWatts per cm2, but our electrodes are only 2 by 3 millimeters in size and generate about 4 microWatts — a bit small to generate enough power to run a watch, for example, which requires at least 10 microWatts. So besides working to get higher power, we also need to leverage electronics to store the generated current and make it sufficient for these requirements."

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