A team of researchers at the University of California, San Diego, has designed and demonstrated a wearable patch with long-term, energy-efficient cooling and heating effects on human skin. Their work was published in the journal Science Advances.
Prototype of the cooling and heating patch embedded in a mesh armband. Image credit: David Baillot / Jacobs School of Engineering, University of California, San Diego.
There are a variety of personal cooling and heating devices on the market, but they are not the most convenient to wear or carry around. Some use a fan, and some need to be soaked or filled with fluid such as water.
Dr. Renkun Chen and colleagues designed their device to be comfortable and convenient to wear. It’s flexible, lightweight and can be easily integrated into clothing.
“This type of device can improve your personal thermal comfort whether you are commuting on a hot day or feeling too cold in your office,” Dr. Chen said.
“If wearing this device can make you feel comfortable within a wider temperature range, you won’t need to turn down the thermostat as much in the summer or crank up the heat as much in the winter.”
The scientists built the patch by taking small pillars of thermoelectric materials (made of bismuth telluride alloys), soldering them to thin copper electrode strips, and sandwiching them between two elastomer sheets. The sheets are specially engineered to conduct heat while being soft and stretchy.
They created the sheets by mixing a rubber material called Ecoflex with aluminum nitride powder, a material with high thermal conductivity.
The patch uses an electric current to move heat from one elastomer sheet to the other. As the current flows across the bismuth telluride pillars, it drives heat along with it, causing one side of the patch to heat up and the other to cool down.
“To do cooling, we have the current pump heat from the skin side to the layer facing outside,” Dr. Chen said.
“To do heating, we just reverse the current so heat pumps in the other direction.”
One patch measures 2 x 2 inches (5 x 5 cm) in size and uses up to 0.2 watts worth of power.
The patch is powered by a flexible battery pack. It is made of an array of coin cells all connected by spring-shaped copper wires and embedded in a stretchable material.
The team embedded a prototype of the patch into a mesh armband and tested it on a male subject. Tests were performed in a temperature-controlled environment.
In two minutes, the patch cooled the tester’s skin to a set temperature of 89.6 degrees Fahrenheit (32 degrees Celsius). It kept the tester’s skin at that temperature as the ambient temperature was varied between 71.6 and 96.8 degrees Fahrenheit (22-365 degrees Cerlsius).
“You could place this on spots that tend to warm up or cool down faster than the rest of the body, such as the back, neck, feet or arms, in order to stay comfortable when it gets too hot or cold,” said Dr. Sahngki Hong, first author of the study.
The researchers estimate that it would take 144 patches to create a cooling vest. This would use about 26 watts total to keep an individual cool on an average hot day.
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Sahngki Hong et al. 2019. Wearable thermoelectrics for personalized thermoregulation. Science Advances 5 (5): eaaw0536; doi: 10.1126/sciadv.aaw0536
