Sweat-Powered Batteries Can Be Tattooed on Your Arm

A diagram shows how electrical current is generated when electrons are stripped from lactate in the sweat of a person wearing the tattoo biosensor/biobattery. Joseph Wang/University of California San Diego

A temporary tattoo that generates and stores electrical energy from sweat will be a hot topic of conversation.

"Here we show the first…noninvasive approach to harvest energy [from the body]," says Joseph Wang, whose team from the University of California San Diego will present their yet-to-be-published work at the 248th National Meeting & Exposition of the American Chemical Society in San Francisco. Their particular design, applied to the arm via the same peel-dampen-and-stick method used for temporary tattoos, is a fashionable U.C. logo no bigger than a quarter. The science behind it, however, lends itself to a wide range of products that you won't have to get tatted up to use.

At the heart of the device is a sensor that measures lactate, a chemical naturally present in sweat. Measuring lactate levels is nothing new. Professional athletes monitor their lactate during performance testing as a way to evaluate their fitness and training program (in general, the higher the lactate level, the higher the workout intensity). In addition, doctors measure lactate during exercise testing of patients for heart or lung disease, which are marked by abnormally high lactate levels. However, there has never been a skin-level sensor for this. Currently, lactate testing requires blood to be drawn from the subject at different times throughout the exercise regime and the samples to be sent to a lab for analysis. The new sensor that Wang and his team have designed provides an alternative method that is not only less painful and more convenient but also allows lactate levels to be monitored continuously (as opposed to measured incrementally).

Then there's the power production part of the sensor. The sensor contains an enzyme that strips electrons from the lactate to generate electrical current across an anode and cathode, which, in the case of this design, are printed on tattoo transfer paper. The electrical current generated is stored in a battery built into the sensor.

"Anything in contact with the body" can harness energy using this technology, so long as it gets sweaty, says Wang. "We are working on other options beyond tattoos," he says, mentioning wristbands and headbands in addition to the paper that his team has submitted "on using biofuel cells in textiles, [specifically] underwear." Explaining that with "a larger area or multiple electrodes, you can generate more total power," Wang says, "On the underwear we are placing multiple…cells."

While in the future your smelly, drenched running shorts may help charge your phone post-workout, it will be some time before the biobattery has that much power density. Wang says that for "the moment, it's more to power on-body devices, like a sensor or pacemaker…artificial pancreas, etc." But even to power these smaller electronics, Wang's team will need to figure out how to get their device to harness and store more energy.

During lab testing, Wang's team applied the temporary tattoo to the upper arms of 15 healthy volunteers who agreed to ride a stationary bike at increasing resistance levels for 30 minutes. Subjects were divided into three groups in accordance with their fitness levels (i.e., how often they exercise); less fit people actually excrete more lactate when they exercise. A person in the low-fitness group, then, generated the most electrical energy. However, the amount of energy that person produced in the half-hour session still was not enough to even power a wristwatch. The biobattery design works; its power capacity just needs to be enhanced.

Biobatteries offer several advantages over conventional batteries; they recharge more quickly, use renewable energy sources and are safer because they do not explode or leak toxic chemicals. Until now, the only batteries fueled by man got their energy from glucose in the blood, which meant they had to be implanted in the body. This new sweat-fueled battery is tiny, temporary, removable, external and inexpensive. In fact, the tattoo version is so cheap that "it's actually single-use," Wang says, calling it "disposable."

The future development of this technology may depend on the specific needs of the companies that choose to invest in it. But it's easy to imagine all kinds of sport, military, biomedical and wearable electronics consumer applications of this technology. After all, in today's world, almost everything comes with batteries.