Why scientists want to harvest your sweat while you sleep

Less than a month into summer and sweat is already our constant companion.

Whether we’re drenched in the salty stuff at our desks, damp during errands, or putting up with sticky sleep, there’s seemingly little solace to be found.

In the future, this sweaty season may actually be an energy gold mine. On Tuesday, researchers at the University of California, San Diego (UCSD) announced the invention of a sweat-slurping electronic device — a wearable that can transform sweat into usable electricity.

Small enough to wrap around your finger like a Band-Aid, the device collects sweat from perspiring fingertips and turns it into energy. In a paper published in the journal Joule, the study team reports their device can be used to power small electronics and wellness sensors, including vitamin C or sodium detectors.

Joseph Wang is a senior author on the paper and a nanoengineering professor at UCSD. What makes the device especially unique, he explains in an accompanying statement, is that this whole process can take place while you sleep.

“You can call it 'power from doing nothing’,” Wang said.

What’s new — Sweat-powered wearables are hot right now — we’ve reported on several previous designs. What’s new about this latest approach is that it doesn’t require users to be covered in sweat for it to work — in turn, positioning this new design as more practical and accessible.

Forgoing sweat-drenched laboratory conditions also makes their wearables the most energy-efficient sweat-based wearable to date, explains co-first author Lu Yin, a nanoengineering Ph.D. student at UCSD working in Wang’s lab, in a statement.

“Compare this to a device that harvests energy as you exercise," Yin said.

"When you are running, you are investing hundreds of joules of energy only for the device to generate millijoules of energy. In that case, your energy return on investment is very low. But with this device, your return is very high. When you are sleeping, you are putting in no work. Even with a single finger press, you are only investing about half a millijoule."

This energy savings boosted the wearable’s return on investment (i.e. how much energy it requires versus produces) to 6,000 percent. And that’s just from one finger.

Why it matters — Eventually, it’s possible tech like this will be incorporated into gloves for ultra off-grid charging of our devices. But the more salient effect of wearables like these will be the creation of autonomously powered health monitors.

In their trials, the team looked at sensors to monitor vitamin C and sodium levels in the body. In the future, these wearables could detect a number of other invisible wellness markers.

“... we use our device to collect this sweat, and it can generate a significant amount of energy.”

Yin tells Inverse the team recently developed touch-based sensors for easy glucose monitoring for people with diabetes, cortisol monitoring for stress, and monitoring of levodopa for Parkinson’s.

How it works — Fingertips might not jump out as the best sweaty location for these sensors, but fingertips are actually a lot sweatier than we realize. The main difference is that — in comparison to our underarms which are usually constricted unless our arms are raised — the sweat on our fingertips evaporates before we notice it pooling up.

“The reason we feel sweatier on other parts of the body is because those spots are not well ventilated,” Yin explains. "By contrast, the fingertips are always exposed to air, so the sweat evaporates as it comes out. So rather than letting it evaporate, we use our device to collect this sweat, and it can generate a significant amount of energy."

This captured sweat can equal a few microliters per square centimeter. This is roughly 1/600th of an eighth teaspoon. These microliters of sweat are absorbed by padding made from carbon foam electrodes. Once soaked up, the sweat undergoes a chemical reaction to create usable electricity:

• Enzymes on the device’s anodes (the negative side of a battery) remove electrons from lactate — a dissolved component of sweat
• The device’s positively charged cathodes then use electrons to reduce oxygen in the system into water
• From there, electrons removed from the lactate can flow through the circuit from anode to cathode to create an electric current

The team tested these wearables in a number of different scenarios, including both real human trials and trials using “artificial sweat.”

They found that pressing as well as resting fingers on the device was effective and could produce up to 300 millijoules of energy per square centimeter.

What’s next — Despite their promising results, the team says that not all users would necessarily generate the same amount of energy. Due in part to the fact that not all sweaty fingers are created equal. And Yin tells Inverse not to hold your breath hoping your sweat may charge your phone.

“Currently our fingertip biofuel cell generates power on the level of 0.1-1 mW, whereas cell phones consume 10^2-10^3 mW,” Yin explains. “Which is 2-3 orders of magnitude higher. Therefore we do not think it is realistic to use fingertip sweat to power cell phones or larger electronic devices.”

Nevertheless, Wang said that this is only just the beginning of what such a device could do. Right now, the team is developing new sensors and low-powered technology that can be powered by the fingertip device.

“There's a lot of exciting potentials," said Wang. "We have ten fingers to play with."

Abstract: Self-powered wearable systems that rely on bioenergy harvesters commonly require excessive energy inputs from the human body and are highly inefficient when accounting for the overall energy expenses. A harvester independent from the external environment for sedentary states has yet to be developed. Herein, we present a touch-based lactate biofuel cell that leverages the high passive perspiration rate of fingertips for bioenergy harvesting. Powered by finger contact, such a bioenergy-harvesting process can continuously collect hundreds of mJ of energy during sleep without movements, representing the most efficient approach compared to any reported on-body harvesters. To maximize the energy harvesting, complementary piezoelectric generators were integrated under the biofuel cell to further scavenge mechanical energy from the finger presses. The harvesters can rapidly and efficiently power sensors and electrochromic displays to enable independent self-powered sensing. The passive perspiration-based harvester establishes a practical example of remarkably high energy return on investment for future self-sustainable electronic systems.