Thermoelectric Fabric can Power Wearables and Sensors

January 25, 2019 by Scott McMahan

Numerous wearable data transmitters, and biosensors and similar tech advances for personal health monitoring have now been miniaturized according to materials chemist Trisha Andrew at the University of Massachusetts Amherst. However, they require energy, and power sources can be bulky and heavy. Without power, they become essentially useless, like in the image above of a group of wearable devices without any power.

Now Trisha and her Ph.D. student Linden Allison report that they have created a unique fabric that can harvest body heat to produce enough electicity to power small, wearable microelectronics like activity trackers.

In an early online version of Advanced Materials Technologies, Andrew and Allison describe how in theory, body heat can produce power by using the "thermoelectric" effect that takes advantage of the difference between body temperature and ambient cooler air. In this way, materials with high electrical conductivity and low thermal conductivity can move electrical charge from a warm region toward a cooler area.

While some research has demonstrated that small amounts of power can be harvested from a human body over an eight-hour workday, the unique materials needed are either very costly, toxic, or inefficient, they note. Andrew says, "What we have developed is a way to inexpensively vapor-print biocompatible, flexible and lightweight polymer films made of everyday, abundant materials onto cotton fabrics that have high enough thermoelectric properties to yield fairly high thermal voltage, enough to power a small device."

For this development, the researchers exploited the naturally low heat transport properties of cotton and wool to produce thermoelectric garments that can maintain a temperature gradient across an electronic device known as a thermopile. The thermopile they created converts heat to electrical energy even over long periods of continuous wear.

This long-term reliability is a practical concern to ensure that the conductive material will be mechanically, electrically, and thermally stable over time, Andrew indicated.

"Essentially, we capitalized on the basic insulating property of fabrics to solve a long-standing problem in the device community," she and Allison summarize. "We believe this work will be interesting to device engineers who seek to explore new energy sources for wearable electronics and designers interested in creating smart garments."

Specifically, they produced their all-fabric thermopile by vapor-printing a conducting polymer known as persistently p-doped poly(3,4-ethylenedioxythiophene) (PEDOT-Cl) onto one tight-weave and one medium-weave kind of commercial cotton fabric. Then, they integrated this thermopile into a custom-designed, wearable band that generates thermo-voltages greater than 20mV when worn on the hand.

They tested the durability of the PEDOT-CI coating by rubbing or laundering coated fabrics in warm water and evaluating performance using a scanning electron micrograph. This testing revealed that the coating "did not crack, delaminate or mechanically wash away upon being laundered or abraded, confirming the mechanical ruggedness of the vapor-printed PEDOT-CI."

They used a custom-built probe to measure the surface electrical conductivity of the coatings and observed that the looser weave cotton showed higher conductivity than the tighter weave fabric. The conductivities of both materials "remained largely unchanged after rubbing and laundering," they added.

With a thermal camera, they proved that the wrist, palm and upper arms of volunteers radiated the most heat. So, Andrew and Allison made stretchy knitted bands of the thermoelectric fabric that can be worn in these areas. The yarn's thickness insulates the air-exposed outer side of the band from body heat. Only the uncoated side of the thermopile touches the skin to reduce the risk of allergic reaction to PEDOT-CI, they noted.

The researchers pointed out that perspiration significantly raised the thermovoltage output of the stretchy armband, which was expected, as damp cotton is known to better conduct heat than dry fabrics. They were also able to deliberately turn off heat transfer by inserting a heat-reflective plastic layer between the wearer's skin and the band.

Overall, they say, "We show that the reactive vapor coating process creates mechanically-rugged fabric thermopiles" with "notably-high thermoelectric power factors" at low temperature differentials compared to conventionally produced devices. "Further, we describe best practices for naturally integrating thermopiles into garments, which allow for significant temperature gradients to be maintained across the thermopile despite continuous wear."

Funding for this work came from the National Science Foundation and the David and Lucille Packard Foundation.