Wearable tech is a very popular trend. There are those which can monitor heart rate, blood pressure, and other health biosensors. There are also those which can monitor physical activity and calorie intake to help you stay fit. These gadgets are designed to be smaller; however, they require a lot of energy and thus, a larger battery.

Now, wearable tech is taking another leap with this new scientific invention—a fabric that could harness energy from body heat.

Thermoelectric Effect

To tackle this problem of “heavy and bulky” power sources, researchers at the University of Massachusetts Amherst took advantage of the so-called “thermoelectric” effect. This theory explains how body heat can produce energy by moving electrical charge from a warm region toward a cooler one.

Previously, there were also some studies which investigated the idea to create energy-harnessing fabrics. However, the results were not safe and cost-effective. They either need very expensive, toxic materials or inefficient, harvesting only small amounts of power even after an eight-hour workday.

In a paper published in the online edition of Advanced Materials Technologies, researchers created and tested knitted bands of thermoelectric fabric. When worn on the hand, this material could generate thermo-voltages more than 20 milliVolts.

Wool and Cotton

The thermoelectric fabric was made of very inexpensive and abundant materials—wool and cotton. Not only they are flexible and lightweight, but the researchers also revealed they used wool and cotton to take advantage of their naturally low heat transport properties.

They explained that this is to maintain the “thermopile,” or the temperature gradient across an electronic device. This will make sure the material can convert heat to electrical energy even after “long periods of continuous wear,” making it “mechanically and thermally stable.”

“Essentially, we capitalized on the basic insulating property of fabrics to solve a long-standing problem in the device community. 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,” the researchers wrote.

Assessing Durability and Conductivity

Digging deeper, the all-fabric thermopile was made by “vapor-printing” a polymer known as persistently p-doped poly(3,4-ethylenedioxythiophene) (PEDOT-Cl). They then integrated this weaves of fabric into a wearable band specially designed to generate thermo-voltages of more than 20 milliVolts.

To test its durability, the researchers soaked and rubbed the finished material in warm water and assessed their performance through electron micrograph scans. The result revealed that  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.”

They also tested its electrical conductivity using a custom-built probe on volunteers. They found that those with looser weave have higher conductivity. Additionally, the conductivity does not change even after rubbing and laundering

The researchers also highlighted that the wrist, palm, and upper arms are the most efficient areas to wear the fabrics as these areas produce most heat. The thermovoltage output of the material is also increased by perspiration. The researchers noted this is not surprising as cotton conducts heat better when damp.