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Big Idea

Your workouts could charge your smartwatch

New biofuel cell technology can use sweat — and maybe more — to power small electronics.

By Eoin O’Carroll

A dead fitness tracker can put a damper on your morning run — and, if you use your tracker as an alarm clock, your whole morning. But what if you could charge it with your own dampness?

The Problem

The past decade has witnessed a surge in wearable electronics, including fitness trackers, Bluetooth earbuds, and biosensors that monitor your blood pressure, EKG readings, and other vital signs.

But wearables need batteries. And USB cables. And AC adapters. And wall sockets. Forget to plug these together, and your wearable is just jewelry.  

Ever since Motorola introduced its two-pound DynaTAC 8000X  phone in 1983,  mobile device performance has improved exponentially.  But batteries have not kept up. 

“Batteries just aren’t improving at the same level that circuits are,” says Patrick Mercier, a professor of electrical and computer engineering at the University of California, San Diego, and the co-director of the UCSD’s Center for Wearable Sensors.

What’s more, most modern mobile batteries contain lithium, the mining of which comes at a steep environmental and social cost. From the grasslands in Tibet to the plateau, our insatiable demand for batteries has befouled rivers and depleted water supplies for some of the planet’s poorest inhabitants. 

The Solution

Your body is jam-packed with chemical energy, and some of it leaks from your pores. So far, all that fuel has gone to waste. Mercier and his colleagues at UCSD hope to change that, with a thin, flexible biofuel cell powered by sweat. 

The biofuel cells use an enzyme to oxidize lactate, a compound that muscles produce as they exercise. Your sweat’s lactate concentrations indicate how hard you’ve been working out, so the biofuel cell doubles as a self-powering exertion sensor.

“We get two things for the price of one,” says Mercier.

Don’t expect a sweat-powered Tesla any time soon. This technology is for sweating the small stuff.

In 2014, Mercier and his colleagues screen-printed the biofuel cells onto a headband, a wristband, and a temporary tattoo, then wired the cells to the battery terminals of a digital watch.

“And nothing happened,” he says. “But then we started exercising. As soon as we started sweating, the watch turned on.”

Those early biofuel cells harvested about a tenth of a milliwatt per square centimeter. Today, they can generate 10 times that amount. A few square centimeters would be more than enough to power a Bluetooth radio, the most power-hungry component of most wearables. 

“Certainly for someone doing exercise, there should be enough lactate there,” says Ahmed Busnaina, a professor of mechanical and industrial engineering at Northeastern University and the director of Northeastern’s Center for High-rate Nanomanufacturing. After all, he says, “you don’t need it anymore. You’re not taking anything from the body.”

The Challenge

Don’t expect a sweat-powered Tesla any time soon. This technology is for sweating the small stuff, like fitness trackers.

“Our biofuel cell can, at most, under maximum sweat conditions barely power a whole smartwatch,” says Mercier.

The biofuel cells generate a current only as long as you keep sweating. What’s more, the enzymes in the biofuel cells degrade after a few days, so they would need to be continually replaced.

“It’s still not even close to being in a place where we’re going to be seeing a lot of commercial projects,” says Mercier. “Just because you see something working in an academic lab doesn’t mean it’s ready for prime time.”

Unlike batteries, however, the biofuel cells can be screen-printed, which means they can be cheaply manufactured. “Once it scales up, I expect the manufacturing costs would be only a few cents,” Mercier says.

The Big Picture

For all its limitations, sweat power could be a useful backup, Mercier says. 

“You could imagine putting on your Apple Watch in the morning and putting a sticker on that,” he says. “Maybe as an auxiliary for energy for when you need to charge your battery.”

The biofuel cells could also be paired with other types of energy harvesters. Mercier’s colleagues have recently developed a Tron-like washable suit with lactate biofuel cells and generators that convert motion into electricity. Tiny supercapacitors on the chest store the electric charge for when the wearer isn’t moving or sweating.

The biofuel cells can also be repurposed for a wide variety of human excretions. In the future, you might get a notification on your tear-powered augmented-reality contact lens sent by your baby’s urine-powered smart diaper.”It’s all flexible, which is great,” says Busnaina. “The question is, is it needed? I don’t know.”

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Eoin O’Carroll is a writer and podcaster based in Amherst, Massachusetts.


Illustration by Francesco Zorzi