Exercise sweat can charge your electronics

When you workout, people say you burn a lot of energy. What if some of that could be harvested to power, say, your iPhone? A team of engineers from UC San Diego say they’ve developed stretchable fuel cells that can pull energy from sweat and can charge electronics. These aren’t the first so-called biofuel cells, but the engineers say they generate 10x more power per surface area than any other wearable around, thanks to some tricky chemistry, material science and advanced electronic interfaces.

First the researchers used lithography to build a stretchable electronic base featuring carbon nanotube-based cathode and anode arrays. To this they added an enzyme that oxidizes lactic acid in human sweat to generate electricity, making the sweat a replenishable source of power. To be practical for wearable devices, the fuel cells needed to be stretchable, so the team employed a “bridge and island” structure made up of rows of dots that are each connected by spring-like structures. Half of the dots are anode, the other half cathode, while the springs can bend and stretch between them. Engineers then deposited layers of biofuel material on top of the electronic dots.

A big challenge was maxing out the fuel cells’ energy density — how much juice it can produce per square centimeter. Said researcher Amay Bandodkar: “We needed to figure out the best combination of materials to use and in what ratio to use them.” The electronic/enzyme layering let researchers load each anode dot with more of the lactic-acid reacting enzyme. It also made for better electron transfer, upping the cell’s performance.

The final step was connecting the stretchy, layered fuel cells to a custom circuit board. The board evened out the fluctuating power generated by the cells as they encountered greater and lesser amounts of sweat. Wearing the devices, test subjects were able to successfully light blue LEDs while exercising on stationary bicycles.

Results of the study were reported in Engineers report their results in the June issue of Energy & Environmental Science.