In the near future, when you work up a sweat, you could be charging your smartwatch, even as it measures your calories burned. A team of engineers at the University of California San Diego has developed stretchable fuel cells that extract energy from body sweat and use it to power electronics, such as LEDs and Bluetooth radios.

The breakthrough comes from a combination of chemistry, advanced materials and electronic interfaces. The researchers built a stretchable electronic foundation using lithography and screen printing to make 3-D carbon nanotube-based cathode and anode arrays. The biofuel cells are equipped with an enzyme that oxidizes the lactic acid present in human sweat to generate current, turning sweat into a power source.

Researchers made the fuel cells flexible by creating what they called a “bridge and island” structure, where the cell is composed of rows of dots, each connected by spring-shaped structures. Half the dots make up the anode, and the other half are the cathode. Thanks to the springs in between, the cell is bendable, without deforming the anode or cathode. The basis for the islands and bridges structure was made of gold, using lithography. As a second step, the researchers used screen printing to deposit layers of biofuel materials on top of the anode and cathode dots.

The biggest challenge was increasing the energy-generating power of the biofuel cell. “We needed to figure out the best combination of materials to use and in what ratio to use them,” says Amay Bandodkar, one of the researchers who has since moved on to purse postdoctoral work at Northwestern University.

To increase power density, engineers screen printed a 3-D carbon nanotube structure onto the anodes and cathodes, which allows them to load each anodic dot with more of the enzyme that reacts to lactic acid. The tubes also allow for easier electron transfer, further improving biofuel cell performance.

The researchers reported their results in the June issue of Energy & Environmental Science, describing how they connected the biofuel cells to a custom-made circuit board to show how a person on a stationary bike could power an LED for about four minutes. In the future, researchers hope to find a way to store the energy produced via sweat and release it gradually, so the fuel cell can power electronics for a longer duration.