A stretchable, wearable device that lights an LED using only skin heat

One of the downsides of fitness trackers and other wearables is that their batteries eventually run out. But what if, in the future, wearable tech could use body heat to power itself?

Researchers at the University of Washington have developed a flexible and durable electronic prototype that can capture energy from body heat and convert it into electricity that can be used to power small electronic devices, such as batteries, sensors or LEDs. The device is also durable: it continues to work even after being punctured multiple times and then stretched 2,000 times.

The team detailed these prototypes in a paper published August 30 in Advanced materials.

“I had this vision a long time ago,” said lead author Mohammad Malakooti, ​​an assistant professor of mechanical engineering at the University of Washington. “When you put this device on your skin, it uses your body heat to directly power an LED. As soon as you turn the device on, the LED lights up. That wasn’t possible before.”

Traditionally, devices that use heat to generate electricity are rigid and brittle, but Malakooti and his team previously created one that is highly flexible and pliable so it can conform to the shape of a person’s arm.

The device was designed from scratch. The researchers started with simulations to determine the best combination of materials and structures for the device, then created almost all of the components in the lab.

It is composed of three main layers. At the center are rigid thermoelectric semiconductors that convert heat into electricity. These semiconductors are surrounded by 3D printed composites with low thermal conductivity, which improves energy conversion and reduces the weight of the device.

To ensure stretchability, conductivity, and electrical self-healing, the semiconductors are connected to printed traces of liquid metal. Additionally, liquid metal droplets are embedded in the outer layers to improve heat transfer to the semiconductors and maintain flexibility because the metal remains liquid at room temperature. Everything except the semiconductors was designed and developed in Malakooti’s lab.

In addition to connected objects, these devices could be useful in other applications, Malakooti said. One idea is to use these devices with electronics that heat up.

“You could imagine sticking these things on hot electronics and using that excess heat to power small sensors,” Malakooti said. “This could be particularly useful in data centers, where servers and computing equipment use a lot of electricity and generate heat, requiring even more electricity to keep them cool.”

“Our devices can capture this heat and reuse it to power temperature and humidity sensors. This approach is more sustainable because it creates a self-contained system that monitors conditions while reducing overall energy consumption. Plus, there’s no need to worry about maintenance, changing batteries, or adding new cables.”

These devices also work in reverse, in that adding electricity allows them to heat or cool surfaces, opening up another avenue of applications.

“We hope to eventually integrate this technology into virtual reality systems and other wearables to create hot and cold sensations on the skin or improve overall comfort,” Malakooti said. “But we’re not there yet. For now, we’re starting with wearables that are efficient, durable and provide temperature feedback.”