The increasing popularity of consumer and medical wearable technologies is a clear trend that continues to grow year after year. These devices are designed for long-term, continuous use, which makes their thermal management a critical design consideration. If the heat generated by the device is not properly managed, it can lead to malfunction or even cause burns to the user. Therefore, understanding and simulating heat transfer during the design phase is essential to ensure both safety and performance.
Wearable Technology: Use and Precautions
At last month's International Consumer Electronics Show (CES), numerous innovations in wearable technology were on display, highlighting the exciting future of this field. From fitness trackers and smartwatches to more advanced medical devices, wearables are becoming more integrated into daily life. One notable example was a skin patch that monitors UV exposure and helps prevent sun damage. Another innovation was a clothing accessory that alerts emergency contacts when needed, and a baby boot that tracks vital signs to help prevent sudden infant death syndrome (SIDS).
**Figure 1.** The fitness tracker is a more common wearable technology application designed to monitor your physical activity throughout the day.
As wearable technology advances, several challenges remain. One major concern is data security, as these devices collect and store sensitive personal information. This makes them attractive targets for hackers who may attempt to access or manipulate the data. In the case of medical wearables, such as those used for health monitoring or drug delivery, unauthorized access could have serious consequences. Another issue is thermal safety—devices worn for extended periods must be carefully designed to avoid overheating, which can cause discomfort or even burns to the skin.
To address these concerns, researchers at Thoratec used COMSOL Multiphysics to simulate heat transfer in wearable electronic devices. Their goal was to better understand how heat is distributed between the device and the skin, ensuring safer and more comfortable use.
**Safely Wearable Electronic Devices**
When a wearable device comes into contact with the skin, heat will naturally transfer from the device to the skin until both reach equilibrium. If the device becomes too hot, it can cause thermal injury. Studies dating back to the 1930s and 1940s have shown that skin damage can occur when temperatures exceed about 44°C. The International Electrotechnical Commission (IEC) sets safety standards for such devices, specifying a maximum safe temperature of 43°C for continuous use. This aligns closely with the known threshold for skin damage.
According to IEC guidelines, the device should maintain steady-state contact with the skin for up to 10 minutes, which is typically enough time for most wearable devices to operate normally. To meet these standards, Thoratec’s team conducted simulations to predict skin temperature and analyze the heat budget of the device, ensuring it would not exceed safe limits.
**Study Heat Transfer in Wearable Technology Design**
The primary objective of the simulation was to model heat transfer within the human body and around the wearable device. The researchers used the Pennes bioheat equation, which accounts for heat conduction and blood flow effects. Blood itself is not a heat source, but it acts as a heat sink or source depending on its temperature relative to the body.
In the model, the human body was divided into four layers: skin, fat, muscle, and internal organs. A 141 mm × 83 mm × 25 mm electronic device, consisting of a circuit board, wires, battery, housing, and trapped air, was placed in contact with the skin. The device was modeled using the heat conduction equation due to its conductive components.
**Figure 2.** Basic model geometry, including electronics, parts of the human body, and clothing layers. Image courtesy of JF Hansen, from his research paper presented at the COMSOL User Conference Grenoble Station 2015.
Boundary conditions were set for the skin, device, and clothing. The inner surface of the device was in direct contact with the skin, while the outer surface was covered by a 3 mm thick fabric. The tightness of the clothing affected the thermal behavior, with tighter fits potentially leading to higher temperatures. The researchers simulated a snug-fitting fabric and included heat transfer through the trapped air.
Using the solid heat transfer physics interface in COMSOL Multiphysics, the team modeled the heat distribution. They incorporated the Pennes equation directly into the source term to account for blood flow effects. For materials and heat transfer coefficients, they had the option to use built-in values or customize them, which they did to achieve greater control over the simulation.
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