A health sensor powered by sweat

The flexible electronic device wrapped around the finger in this photo can track four common biomarkers: glucose, lactic acid, vitamin C, and levodopa—compounds that are medically relevant to monitoring a person’s metabolism and exercise. The measurements are not new, but other sensor designs have drawbacks: They may require bulky batteries that limit wearability, or the small form factor may demand that the batteries be frequently charged. This prototype device, made by Joseph Wang and Shichao Ding, of the University of California, San Diego, and their colleagues, avoids those problems. Even without physical activity, human fingers secrete sweat filled with lactate and oxygen, which can serve as biofuels. The device collects energy and stores it in two batteries that conform to the wearer’s skin. Made of silver chloride and zinc, the nonflammable and nontoxic batteries operate in safe pH-neutral conditions.

First, a hydrogel layer withdraws sweat from the skin near the fingertip. Then, the sweat collects in four fan-shaped biofuel cells that convert the chemical energy into electricity, which is stored by the batteries. The sweat, colored blue in the photo, is pumped into the squiggly white microfluidic channel before it reaches the biomarker sensor. Measurements show that the energy harvested by the device provides enough power to continuously monitor the wearer for longer than 16 hours. Now that the researchers have an operational prototype, they’re working on improving performance, stability, and comfort. (S. Ding et al., Nat. Electron. 7, 788, 2024, doi:10.1038/s41928-024-01236-7 ; image courtesy of Shichao Ding.)