Engineers have developed a wearable patch that could provide personalized cooling and heating at home, work, or on the go. The soft, stretchy patch cools or warms a user’s skin to a comfortable temperature 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.
The device, which is at the proof-of-concept stage, could also save energy. 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.
The patch is made of thermoelectric alloys — materials that use electricity to create a temperature difference and vice versa — sandwiched between stretchy elastomer sheets. The device physically cools or heats the skin to a temperature that the wearer chooses.
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 degrees Fahrenheit. It kept the tester’s skin at that temperature as the ambient temperature was varied between 71.6 and 96.8 °F.
The ultimate goal is to combine multiple patches together to create smart clothing that can be worn for personalized cooling and heating. So, engineers designed “soft” electronic devices that can stretch, bend and twist without compromising their electronic functions.
The work is a collaboration between several research groups at the UC San Diego Jacobs School of Engineering. The researchers 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. Researchers 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.
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 system also includes a stretchable circuit board. One patch measures 5 × 5 centimeters in size and uses up to 0.2 W worth of power. Chen’s team estimates that it would take 144 patches to create a cooling vest. This would use about 26 W total to keep an individual cool on an average hot day (during extreme heat, estimated power use would climb up to 80 W, which is about how much a laptop uses). By comparison, a conventional air conditioning system uses tens of kilowatts to cool down an entire office.