Wearable Energy-Efficient Cooling and Heating Patch

University of California San Diego engineers have created a wearable patch capable of providing personalized heating and cooling at home, work, or on the go. The soft, stretchy patch warms or cools a user’s skin to a comfortable temperature and maintains it there as the ambient temperature alters. It is driven by a flexible, stretchable battery pack and can be implanted in clothing. Scientists say wearing it could help conserve energy on heating and air conditioning.

The research paper has been published in the May 17^th issue of the journal Science Advances.

“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 step, could also conserve 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,” Chen said. Maintaining a building’s set temperature 12° higher in the summer, for instance, could bring down cooling costs by nearly 70%, he noted.

There are a range of commercial personal heating and cooling devices, but they are not the most portable or convenient to wear. A few use a fan, and others need to be filled or soaked with fluid such as water.

Chen and a team of scientists at the UC San Diego Jacobs School of Engineering engineered their device to be comfortable and easy to wear. It is flexible, lightweight, and can be easily incorporated into clothing.

The patch is composed of thermoelectric alloys—materials that utilize electricity to form a temperature difference and vice versa—sandwiched between elastic elastomer sheets. The device physically heats or cools the skin to a temperature that the wearer prefers.

“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 first author Sahngki Hong, a UC San Diego mechanical engineering alumnus who worked on the project as a PhD student in Chen’s lab.

The scientists implanted a prototype of the patch into a mesh armband and tried it on a male subject. Tests were carried out in a temperature-regulated environment. Within two minutes, the patch cooled the tester’s skin to a pre-programmed temperature of 89.6 °F. It maintained the tester’s skin at that temperature as the ambient temperature fluctuated between 71.6 and 96.8 °F.

A Building Block for Smart Clothing

The eventual goal is to integrate multiple patches together to form smart clothing that can be worn for customized heating and cooling. Therefore, engineers built "soft" electronic devices that can stretch, bend, and twist without sacrificing their electronic functions.

The research is a partnership between a number of research groups at the UC San Diego Jacobs School of Engineering. Chen’s lab, which focuses on heat transfer technology, led the research. They partnered with nanoengineering professors Sheng Xu, an expert in stretchable electronics; Shirley Meng, an expert in battery technology; Ping Liu, who is also a battery expert; and Joseph Wang with expertise in wearable sensors.

The scientists engineered the patch by taking small pillars of thermoelectric materials (composed of bismuth telluride alloys), soldering them to thin copper electrode strips, and placing them between two elastomer sheets.

The sheets are particularly engineered to convey heat while being soft and stretchy. Scientists developed the sheets by blending a rubber material known as Ecoflex with aluminum nitride powder, a material possessing high thermal conductivity.

The patch makes use of an electric current to transfer heat from one elastomer sheet to the other. As the current flows across the bismuth telluride pillars, it drives heat along with it, resulting in 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. To do heating, we just reverse the current so heat pumps in the other direction.

Renkun Chen, Professor of Mechanical and Aerospace Engineering, UC San Diego

The patch is driven by a versatile battery pack. It is composed of an arrangement of coin cells all linked by spring-shaped copper wires and implanted in a stretchable material. The system also comprises of a stretchable circuit board.

Saving Energy

One patch measures 5×5 cm in size and consumes up to 0.2 Watts worth of power. Chen’s team estimates that it would require 144 patches to form a cooling vest. This would use around 26 Watts total to keep a person cool on an average hot day (in extreme heat, projected power use would go up to 80 Watts, which is about the same amount a laptop uses). By comparison, a conventional air conditioning system uses tens of kilowatts to cool down a full office.

It is more energy-efficient to cool down one person than a large room, scientists emphasized. “If there are just a handful of occupants in that room, you are essentially consuming thousands of watts per person for cooling. A device like the patch could drastically cut down on cooling bills,” Chen said.

The team is presently working on patches that could be embedded into a prototype heating and cooling vest. They wish to market the technology in a few years.

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.

Renkun Chen, Professor of Mechanical and Aerospace Engineering, UC San Diego

This research is supported by the Advanced Research Project Agency–Energy (ARPA-E, grant DE-AR0000535) and UC San Diego startup funds.