While thermal management is certainly not a new consideration in the design of electronics, it presents unique challenges in wearable electronics. In conventional electronics, CPUs are expected to remain under 75°C. But in wearables, users will experience pain when a device runs 42°C and discomfort at even lower temperatures.
Heat isn’t just an issue from wearables to humans, either; human-generated heat can also pose issues to embedded circuitry. Direct contact with human skin lowers the device’s thermal tolerances and can cause it to experience more heat. Additionally, wearables may experience more direct and continuous exposure to sunlight than other electronic devices, causing them to behave erroneously.
Wearables are amongst a few electronic groups that have direct exposure to human skin and sunlight. Image used courtesy of Advanced Science
What have engineering researchers done to address these thermal challenges?
Researchers Strive to Keep Wearables Cool
In the past few years, designers have tackled the issue of wearable heat in several different ways—many involving heat sinks and dissipation. For instance, researchers have investigated how two factors—heat sources/sink thermal resistances and thermoelectric materials—might affect the performance of thermoelectric coolers. These coolers are said to decrease the contact temperature between human skin and the wearable by as much as 8.2 °C.
Alternately, a Science Advances study from 2019 cites an effective method of pinpointing certain areas of a system to cool instead of targeting the whole device. These researchers used flexible thermoelectric devices to cool specific heat-generating sites in the circuitry, resulting in a decreased temperature of 10°C.
A thermoelectric cooler module in a smartwatch localizes cooling to where the device contacts the skin, obviating the need for space cooling. Image used courtesy of Nature
This now-common method may benefit from thermal modeling and simulation, which can identify those heat-prone locations and help developers assess how different device shapes, belt sizes, and materials influence thermals.
This week, however, researchers from Korea and the U.S. have turned to materials science to solve thermal challenges in wearables once and for all.
The Challenge of Metallic Heat Sinking
As mentioned, thermal considerations for wearables often involve heat sinking or dissipation based on thin metallic films. The problem is that these thin metallic layers have the potential of creating a makeshift faraday cage around the wearable, obstructing wireless communication.
Working principle of a Faraday cage. Image used courtesy of the National High Magnetic Field Lab
While this issue has been sidestepped in products on the market, it continues to roadblock the full capabilities of metallic heatsinks because of the persistent risk of blocking RF connectivity.
A New Material Cuts Heat, Keeps RF Connectivity
In a new paper published in Advanced Science, researchers from GIST in Korea in collaboration with U.S. researchers have created a new material to keep wearables cool. Nano/microvoids polymer (NMVP) is a non-metallic flexible material, which the researchers created from polymethylmetacrylate and styrene-ethylene-butylene-styrene.
Graphic of the new emissive material. Image used courtesy of Kang et al.
In combining these two materials, the team says they have created a material that has nearly 100% reflectivity of the solar spectrum, meaning that sunlight will not cause the material to heat up. The new material also features high emissivity in the atmospheric window, allowing it to radiate heat as a form of cooling.
Notably, the material is non-metallic, eliminating concerns about interfering with RF communications.
How do you anticipate thermal design in wearables will evolve in the coming years? Share your thoughts in the comments below.