Imagine charging your Apple Watch with ... yourself.

Pacemakers, drug delivery pumps, Fitbits, and other wearable devices could soon run on a new kind of renewable energy: you.

A tiny new wearable gadget called a thermoelectric generator (TEG) directly turns your body’s heat into electrical energy. TEGs use a difference in temperature—like your body’s temperature versus the surrounding air—to turn that energy into power. To establish equilibrium, heat automatically dissipates into cooler locations, and TEGs harness the electric current produced when energized particles move from hot to cold along a chip.

That’s convenient, since body heat is a pretty stable resource. To maintain a constant temperature of about 98.6 degrees Fahrenheit, your body must regulate a tight balance between heat gain and heat loss. And because your body isn’t actually that efficient, you lose approximately 75 percent of the energy it produces through heat.

“If your body could do the same work as a watch battery...that's a win for the environment.”

While the part of the gadget that touches your skin turns your warmth into energy, the wearable shields its cold side from the sun’s rays with a wavelength-selective film to preserve the temperature differential.

But this gadget’s success as a wearable comes down to its extreme flexibility and self-healing properties, which allow it to bend with your body and bounce back from damage. A special material embedded inside the gadget heals itself from cuts by re-sealing breaks on a micro level. It’s flexible because each component is flexible in turn, like building elastic circuitry on a rubber band by using stretchable wires.

Ronggui Yang, a professor of energy and power engineering at Huazhong University of Science and Technology, who was involved in the study, says that this design vastly improves on previous, more rigid wearable thermoelectric generator designs.

That’s thanks to a few significant enhancements. For the substrate, or base, of the wearable, Yang’s team combined three commercially available compounds to synthesize a stretchy polyimine material. The resulting substrate is bendy, sort of like a rubber bracelet.

By laser-cutting slits into this polyimine substrate, the researchers created small notches for the power-generating thermoelectric chips. To create a wearable of a different size or shape, the scientists can simply cut in more notches to add more chips, or reorganize their order.

The end result: “superior stretching, self-​ healing, recycling, and Lego-like reconfiguration capabilities,” Yang says. These qualities mean that the final product is durable like never before.

The researchers say the flexible design will tile and stack, allowing for many kinds of devices with the same basic pieces. While the team of researchers—comprised of scientists from China and the University of Colorado Boulder—behind the stretchable device tested their TEG in the form of a small ring, this modularity and scalability means that the tiny generator could theoretically be larger, depending on how much power you want to generate. The greatest amount of power could be gathered by, for example, a Fitbit-like “sports bracelet” that powers a watch—or even a full sleeve of modular “generator” cells.

These new forms of TEG could create even more power, charging up devices with higher electrical requirements. Still, considering that these wearables can generate only about one volt of energy for every square centimeter of skin space, which is less voltage per area than most existing batteries, there’s still work to do. An AA or AAA battery is rated 1.5 volts, for instance, and that’s about how much power it takes to run some insulin pumps.

Batteries, for their part, are a pretty dirty technology that sometimes use rare-earth metals and corrosive materials. When batteries break down in landfills, the chemicals inside—including hydrochloric acid, the same stuff found in your digestive tract—leach into the soil, contaminating ground and surface water. Lithium-ion batteries, which are common in fitness wearables, frequently explode and catch fire in landfills, releasing noxious greenhouse gases.

If your body could do the same work as a watch battery with cleaner, more recyclable technology, that’s a win for the environment. Yang’s wearable even features completely recyclable technology. You can simply soak the TEG in a special recycling solution for six hours at room temperature, causing the polyimine substrate to break down.

Better yet? For people with implanted medical technology like pacemakers, wearable batteries like this TEG could mean a future without battery-​replacement surgery. According to Johns Hopkins Medicine, patients with pacemakers must undergo this kind of surgery every five to 10 years, as the lithium-ion battery inside their implant starts to fail.

The TEG design may seem lofty and complex. In reality, it’s just a masterfully balanced combination of technologies meant to open the door to personalization. Yang says users will be able to customize circuitry on their own using something as simple as a home soldering kit. Enthusiasts could even build out their own custom wearable tech with the exact number of battery cells they desire.

The best part? You could see these wearables in stores in the next five to 10 years, the researchers believe, with medical devices to follow. In the meantime, don’t toss that Fitbit charger.

When it comes to wearables, body heat isn’t the only human byproduct that scientists are interested in. Researchers at North Carolina State University have developed a wearable prototype that uses your sweat to provide an overall picture of your health.

Using a replaceable test strip embedded with chemical sensors, the device can measure the amounts of certain metabolites present in your sweat to analyze your body’s glucose, lactate, pH, and temperature levels. One day, that could tip off your doc to underlying health conditions. “We’re optimistic that this hardware could enable new technologies to reduce casualties during military or athletic training, by spotting health problems before they become critical,” says Michael Daniele, an assistant professor of electrical and computer engineering at NC State who was involved in the work.