New device turns the body into a battery

While thermoelectric generators are proving useful in a growing number of modern applications, ranging from electric vehicles to spacecraft, a new potential use for the technology has emerged that could give a whole new meaning to the phrase, "personal electronic device."

Researchers at the University of Colorado Boulder have developed a new, low-cost wearable device that uses tiny thermoelectric generators with liquid metal wiring to transform the human body into a biological battery.

The device, described in a research paper published Feb. 10 in the journal "Science Advances," can be worn like a ring, a bracelet or any other accessory that touches your skin. It taps into a person's natural heat, using thermoelectric generators to convert the body's internal temperature into electricity.

Jianliang Xiao, senior author of the paper, says thermoelectric generators are excellent candidates for powering wearable electronics and internet of things devices because they are capable of directly converting heat to electrical energy.

"In the future, we want to be able to power your wearable electronics without having to include a battery," said Xiao, who is an associate professor in the Paul M. Rady Department of Mechanical Engineering at CU Boulder.

The device can generate about 1 volt of energy for every square centimeter of skin space – less voltage per area than what most existing batteries provide but still enough to power small electronics such as watches or fitness trackers. Xiao's colleagues calculated, for example, that a person taking a brisk walk could use a device the size of a typical sports wristband to generate about 5 volts of electricity – which is more power than what many watch batteries can generate.

Materials marvel

The project is not Xiao's first attempt to meld human and artificial capabilities, according to a university statement, as Xiao and his colleagues previously experimented with designing "electronic skin," wearable devices that look, and behave, much like real human skin. That android epidermis, however, has to be connected to an external power source to work.

The researchers' latest innovation begins with a base made of a stretchy material called polyimine. The scientists then plug a series of thin thermoelectric chips into that base, connecting them all with liquid metal wires. The final product looks like a cross between a plastic bracelet and a miniature computer motherboard or maybe a techy diamond ring.

"Our design makes the whole system stretchable without introducing much strain to the thermoelectric material, which can be really brittle," Xiao said.

Like Xiao's electronic skin, the new device is as resilient as biological tissue. If it tears, for example, you can pinch together the broken ends, and they will seal together in just a few minutes.

Device design and fabrication

The thermoelectric generator, or TEG, is composed of modular thermoelectric chips, liquid metal as electrical wiring, and dynamic covalent thermoset polyimine as both the underlying surface and the coating for the liquid-metal wiring.

The polyimine material can be synthesized by cross-linking three commercially available compounds terephthalaldehyde, 3,3′-diamino-N-methyldipropylamine, and tris(2-aminoethyl)amine. To fabricate the thermoelectric chips, the researchers deposited thin film bismuth and antimony chalcogenides onto polyimide films using a thermal evaporator. Chalcogenides consist of at least one negatively charged chalcogen – oxygen, sulfur, selenium, tellurium, and polonium – and one electropositive element.

To improve crystallinity and performance, the team then treated the thermoelectric films at 320 degrees Celsius for 26 minutes in an argon atmosphere. Then, gold-germanium electrodes were deposited using a thermal evaporator to form connections, which completed the fabrication of the thermoelectric chips.

The process of assembling the modular thermoelectric chips into a TEG starts with laser-cutting the polyimine base to create slots, followed by screen-printing patterned liquid-metal electrical wirings. Then, the modular thermoelectric chips were inserted into the slots of the polyimine base, and a small amount of the polyimine with methanol in solution was applied to bond the thermoelectric chips with the base and to coat the liquid-metal wiring.

Among other minerals and compounds used by the researchers in creating the wearable TEGs are tellurium, selenium, gallium, indium, tin, and silicon dioxide in a multi-stage process that includes smelting and thermal evaporation at very high temperatures.

Easy to reconfigure, recycle

A TEG can also be easily reconfigured or modified to boost power by adding in more blocks of generators. In that sense, Xiao said his design is like Legos, the popular children's toy.

"What I can do is combine these smaller units to get a bigger unit," he explained. "It's like putting together a bunch of small Lego pieces to make a large structure. It gives you a lot of options for customization.

When you're done with the device, you can dunk it into a special solution that will separate out the electronic components and dissolve the polyimine base. All the materials in the device can be reused.

We're trying to make our devices as cheap and reliable as possible, while also having as close to zero impact on the environment as possible," explained Xiao.

These TEG devices are integrated with a wavelength-selective metamaterial film on the cold side, leading to greatly improved device performance under solar irradiation, which is critically important for wearable "energy-harvesting" during outdoor activities.

Xiao also observed that the optimal properties and design concepts of the TEG device can pave the way for delivering the next-generation of high-performance, adaptable, customizable, durable, economical, and eco-friendly energy-harvesting devices with wide applications.

With some kinks yet to be resolved in the design, he said the research group's innovation could appear on the market as early as 2026.