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The small beads of sweat your fingertips produce while you sleep could power wearable sensors that measure glucose, vitamin C, or other health indicators. That’s the promise of a new advance—a thin, flexible device that wraps around fingertips like a Band-Aid—that its creators say is the most efficient sweat-powered energy harvester yet.  

“The ability to harvest tiny amounts of sweat from the fingertips is really unique,” says Roozbeh Ghaffari, a biomedical engineer at Northwestern University who was not involved with the work.

Researchers around the world are currently developing wearable sensors to measure anything from a runner’s acceleration to a diabetic’s glucose levels.

A big hurdle to these sensors’ widespread use, however, is the ability to power them sustainably. Batteries are bulky and short-lived, and solar power doesn’t work at night. More recently, researchers have looked to the human body itself to generate power.

Sweat is a particularly promising energy source because it contains a natural byproduct of anaerobic respiration—the method used by your body to get energy quickly when you exert yourself—called lactate that can be broken down by an enzyme to produce energy. However, existing devices need a lot of sweat to work, and most people don’t want to hit the gym whenever their electronics run out of power.

“The [energy] return on investment is really low,” says Lu Yin, a nanoengineer with the University of California, San Diego.

So Yin and his team turned to fingertip sweat. Our fingertips are home to the highest concentration of sweat glands on our body—even higher than in our armpits—and they make sweat constantly, regardless of whether you’re exerting yourself. We typically don’t notice this sweat because it evaporates almost instantly. 

The new device, reported today in Joule, is just 1 square centimeter wide and flexible enough to wrap around a fingertip. It captures sweat with a flexible hydrogel that sits against the skin. Three foam blocks on top of the gel serve as electrodes. Two contain an enzyme that takes electrons from lactate and the other contains platinum that uses those electrons to convert oxygen into water. This process creates a flow of electrons through the device that generates electricity.

The device can generate 300 millijoules of energy per square centimeter during a night’s sleep—enough to power a wristwatch for a day. What’s more, if a person wearing the device applies pressure by pinching two fingers together, it can produce 30 millijoules per square centimeter thanks to generators that turn the mechanical energy into electricity. The team demonstrated that these little bursts of energy are enough to power a wearable vitamin C sensor and its display.

“It allows the user to continuously harvest energy from the human body,” says Wei Gao, a medical engineer at the California Institute of Technology. “From a power point of view, this makes wearable sensors more practical.”

Wearable sensors are still in the early stages of development, but research like this is helping push them toward becoming an everyday reality, Ghaffari says.