A reactive vapor coating process can produce mechanically rugged fabric thermopiles with high thermoelectric power factors that could use body heat to power wearable electronic devices. [Linden, K., et al., Adv. Mater. Technol. (2019); DOI: 10.1002/admt.201800615]

Wearable biosensors, data transmitters and other personalized devices for health monitoring, for instance, have been creatively miniaturized over the last few years, but still require a lot of energy, according to materials chemist Trisha Andrew at the University of Massachusetts Amherst, USA. Harnessing body heat could be enough to turn the heat up on this emerging area of technology and preclude the need for bulky battery packs.

Writing in an early online edition of Advanced Materials Technologies, the team explains how they have exploited the difference between body temperature and ambient air to generate power. In this "thermoelectric" effect a material with a high electrical conductivity but a low thermal conductivity moves electrical charges from a warm region to a cooler generating a current to power a device. The new work uses biocompatible materials rather than the expensive, toxic and inefficient thermoelectric materials investigated to date. The biocompatible materials are essentially the wool and cotton, the flexible and lightweight polymer fibers, we already use in clothing. Wool and cotton have naturally low heat transport properties and can maintain a temperature gradient across an electronic device known as a thermopile, which converts heat to electrical energy even over long periods of continuous wear.

"Essentially, we capitalized on the basic insulating property of fabrics to solve a long-standing problem in the device community," Andrew explains. "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, the team created an all-fabric thermopile by vapor-printing a conducing polymer known as persistently p-doped poly(3,4-ethylenedioxythiophene) (PEDOT-Cl) on to one tight-weave and one medium-weave form of commercial cotton fabric. They then integrated this thermopile into a wearable band that can generate 20 millivolts when worn on the hand. The coating is resistant to everyday wear and tear as well as laundering. The air-exposed outer side of the band is insulated from body heat by yarn thickness, while only the uncoated side of the thermopile touches the wearer's skin. They could switch off the "device" by sliding a heat-reflective plastic layer under the band.