Clothes generate electricity to continuously power medical sensors on the body

The new circuit patterns created by the researchers can be printed on a variety of soft conductive polymers, such as clothing, to produce materials that harvest energy from body heat and can be used to power simple biosensors that measure vital signs such as heart rate and breathing.

Wearable textile structures imprinted with p-type and n-type semiconductors could allow clothing fabrics to convert body heat into electricity to power biosensors or conversely to cool the wearer, according to researchers at the Georgia Institute of Technology.

By using the geometric spaces made famous by German mathematician David Hilbert - specifically his space-filling curves - as patterns for imprinting circuits, the researchers demonstrated that the output voltage and power of a printable thermoelectric energy-harvesting wearable device can be fine-tuned to meet the precise needs of a particular application.

The first demonstration at Georgia Tech was on paper, but the researchers say the circuit patterns could also be printed onto clothing or a variety of soft conductive polymers, creating materials that could harvest energy from body heat and be used to power simple biosensors that measure vital signs such as heart rate and respiration.

"The Hilbert pattern we used for the interconnects is fractally symmetric, a well-known mathematical structure. This basically means that any subset is exactly the same as the whole -- self-similarity at different scales," said Akanksha Menon, a doctoral candidate who conducted the research in the lab of Georgia Tech professor Shannon Yee. "When applied to thermoelectric components, the design creates lines of symmetry along the module that can be patterned to provide a specific voltage output. This allows us to print components with many elements at a large scale, and then cut along these lines of symmetry to get the desired voltage." 

Georgia Tech researchers measure the electrical conductivity of a thermoelectric polymer film module (Credit: Candler Hobbs/Georgia Tech)

Menon and Yee call their Hilbert curve a fractal wiring pattern because it can be printed at a variety of densities and then cut to size according to the voltage and power requirements of the target application.

While the researchers have yet to prove the concept of reversing the process to create personalized air conditioning, they believe that Hilbert curve-imprinted fabrics could focus temperature gradients only on the wearer's skin, resulting in huge energy savings on currently inefficient whole-room air conditioning.

“In theory, the device could work in reverse,” Menon said. “You would have to send an electric current through the device, which would result in one end cooling [inside the garment] and the other end heating [outside the garment]. The challenge is that to get a lot of cooling you need very specific materials. So we have another research program going on in the lab specifically for this application.” 

Akanksha Menon, a doctoral student in Georgia Tech's Woodruff Department of Mechanical Engineering, measures a thermoelectric polymer film module. (Credit: Candler Hobbs/Georgia Tech)

Thermoelectric materials have been used in reverse for a long time. For example, coolers that plug into car cigarette lighters use flexible strips that heat up when the cooler is plugged in at the other end while the inside gets cold. But these products use toxic inorganic bulk materials. Menon and Yee are exploring using the same approach but in a non-toxic organic thin-film polymer form instead, making it possible to use inkjet printers to print prototypes or roll-to-roll printing for mass production.

The Hilbert plot allows the material to be customized for applications after it is mass-produced or printed onto textiles. This would make it possible to increase the efficiency of heat-to-electricity conversion by eliminating the need for voltage or power converters, thereby generating hundreds of microwatts (uW) or even milliwatts (mW) of power, depending on the size of the garment. The material's p-type and n-type polarities are more closely spaced than in bulk, making it efficient enough to power medical monitoring sensors woven into clothing, the researchers say. 20171030-Garment-Pattern-Biosensor-Nets-2The image shows 3,600 inkjet-printed thermoelectric pins in an area the size of a U.S. quarter (36.0- x 31.2-mm). The red and blue dots represent n-type and p-type polymers, respectively.

As for the future, the research team hopes to find optimized materials for specific wearable applications and prove that well-fitting and comfortable clothing can harvest enough heat from the wearer's body to operate the sensor networks woven into the clothing.

The research was funded by the Air Force Office of Scientific Research and PepsiCo Inc. Details of the research are published in the Journal of Applied Physics, “Interconnect Patterns for Printed Organic Thermoelectric Devices with Large Fill Factors.”