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Scientists from the University of Massachusetts have developed a fabric that can harvest body heat to power small wearable devices such as fitness trackers.
Materials chemist Trisha Andrew and her Ph.D. student Linden Allison have developed the fabric that uses the “thermoelectric” effect that occurs when there is a difference between body temperature and ambient cooler air.
The fabric has a high electrical conductivity and a low thermal conductivity which enables it to move electrical charges from a warm region toward a cooler one using this effect while many scientists have identified the heat from a human body as a possible source of power for wearables.
Body heat is an infinite energy source
The existing fabrics that can harvest this energy source are either costly, toxic or inefficient.
Andrew says, “What we have developed is a way to inexpensively vapor-print biocompatible, flexible and lightweight polymer films made of every day, abundant materials onto cotton fabrics that have high enough thermoelectric properties to yield fairly high thermal voltage, enough to power a small device.”
The two scientists leveraged the naturally low heat transport properties of wool and cotton to create thermoelectric garments that can maintain a temperature gradient across an electronic device known as a thermopile.
This 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,” 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.”
Fabric takes advantage of wool's natural properties
The scientists developed a prototype of their idea 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 piece of commercially available cotton fabric.
This was then integrated into a wearable band that generates thermo-voltages greater than 20 milliVolts when worn on the hand.
The prototype was manipulated and laundered to test its strength and durability the material. The performance of the material “did not crack, delaminate or mechanically wash away upon being laundered or abraded, confirming the mechanical ruggedness of the vapor-printed PEDOT-CI.”
Sweat can be useful to increase thermovoltage
Sweat increased the thermovoltage output of the fabric armband, as damp materials are known to better heat conductor than dry fabrics. They could mitigate excessive heat transfer by adding a heat-reflective plastic layer between the wearer’s skin and the band.
The research has been published in Advanced Materials Technologies.
In summary, the researchers 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 traditionally 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.”
The research could pave the way for more affordable and non-toxic fabrics that could be used to power wearables.