When your body is cold, its involuntary feedback systems kick in to warm you up. Tiny muscles in your skin contract and cause your hairs to stand on end, creating an insulating layer to trap heat – because humans have very little hair, you can see the contracted muscles as goose bumps. And little blood vessels close to the skin’s surface, called arterioles, shrink down to reroute your blood to your body’s warmer core, in a process called vasoconstriction.

The trouble is, it’s impossible to completely prevent heat loss from your body. Even with four or five layers on, some heat will still escape to the air around you. But could this heat be captured and used to your advantage? An intriguing study published in Nature suggests that one day we could harness our lost body heat and use it to charge our phones.

Researchers from Northwestern University in the United States and Donghua University in China discovered how to embed technology into the fabric of your clothing. These devices (thermoelectric generators) could generate electricity from your body heat. So in just a few years’ time, you may be able to buy jumpers or shirts that use your body heat to charge everyday devices, such as your phone.

As we use phones and laptops often, portable battery packs have become a staple in our bags and backpacks. These devices provide a convenient way for us to charge our phones, watches and laptops while we’re on the go. However, like any handheld device, it can easily be lost or stolen. And if you forget to charge the battery, you’re left with a bag full of dead devices.

That is why a wearable thermoelectric generator would be desirable. All over the world, scientists have struggled to find a quick and effective way to make a ‘human generator’.  Creating both a working device and a wearable product has been a fickle process. Some thermoelectric devices are too stiff and bulky to be incorporated into a comfortable jumper or shirt, while others are too fragile to be embedded into mediums that are constantly in motion.

In the past, scientists attempted to use a two-dimensional wave-like fabric pattern to maximize the amount of body heat captured for charging the device. They also considered inserting the thermoelectric generator into a random section of the clothing. Although these trials were somewhat successful, the final product had many disadvantages. For example, the clothing was vulnerable to an uneven distribution of heating, making the device unchargeable. In addition, everyday fabrics such as wool or cotton could not utilize heat in an effective way. The synthetic materials used to create the device made the clothing inflexible. Achieving a balance between generator efficiency and the wearer’s comfort was proving difficult.

Researchers at Northwestern and Donghua solved these problems by deciding that the generator must be woven into the threads of the shirt or sweater. They use a blend of materials, including carbon nanotubes. The key is to strategically arrange these materials to maximise efficiency, and they found that a looped, almost knitted pattern works the best. Using the three-dimensional threads, heat is retained.

The clothing is attractive and comfortable enough to wear in public. In this design, the thermoelectric modules can successfully match the direction of heat flow. It also displayed the highest power output reported out of all the thermoelectric prototypes, having the potential to fully recharge a dead iPhone. All the wearers need to do is to warm themselves up and then place their device on their clothes.

There is still work ahead to make thermoelectric clothing more practical to use. Just because it was successful in a lab doesn’t mean it can translate easily into retail clothing stores. How will the thermoelectric clothing react in a washing and drying machine? How much will the clothes cost? Which devices charge the best? There is also the question of whether or not these clothes can be mass-produced. Scientists have yet to form a conclusion

However, the possibility that Apple Watches, Fitbits and phones could one day be charged with something as simple as body heat is groundbreaking – no more searching for power outlets. This concept could also motivate people to exercise more, improving their health while charging their phone, and perhaps at a larger scale, thermoelectric clothing could help reduce the world’s high energy consumption.

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