The soft, stretchy patch cools or warms a user's skin to a comfortable temperature (chosen by the user) and keeps it there as the ambient temperature changes. It is powered by a flexible, stretchable battery pack and can be embedded in clothing. Researchers say wearing it could help save energy on air conditioning and heating.
"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," said Renkun Chen, a professor of mechanical and aerospace engineering at UC San Diego who led the study.
The device, which is at the proof-of-concept stage, could also save energy. "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," Prof Chen added. Keeping a building's set temperature 12 degrees higher during the summer, for example, could cut cooling costs by about 70 per cent, according to the team.
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, noted the researchers.
The patch is made of thermoelectric alloys – materials that use electricity to create a temperature difference and vice versa. These are soldering to thin copper electrode strips and sandwiched between two elastomer sheets.
The sheets are specially engineered to conduct heat while being soft and stretchy. Researchers created the sheets by mixing a rubber material called Ecoflex with aluminum nitride powder, a material with high thermal conductivity.
In a demon, the researchers 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°F. It kept the tester's skin at that temperature as the ambient temperature was varied between 71.6 and 96.8°F.
The soft electronic patch can stretch, bend and twist without compromising its electronic function, with the ultimate goal to combine multiple patches to create smart clothing that can be worn for personalised cooling and heating.
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," Prof Chen explained. "To do heating, we just reverse the current so heat pumps in the other direction."
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.
One patch measures 5 × 5cm in size and uses up to 0.2 Watts worth of power. Prof Chen's team estimates that it would take 144 patches to create a cooling vest. This would use about 26W total to keep an individual cool on an average hot day. By comparison, a conventional air conditioning system uses tens of kilowatts to cool down an entire office.
"We've solved the fundamental problems, now we're tackling the big engineering issues, the electronics, hardware, and developing a mobile app to control the temperature," Prof Chen said.
