The popularity of consumer and medical wearable technologies continues to grow each year. These devices are designed for continuous use, and if not properly engineered, the heat generated can lead to malfunctions or even cause burns. To ensure user safety, it's essential to consider heat transfer during the design process. Thanks to the advanced simulation capabilities of COMSOL Multiphysics, engineers can now better predict and manage thermal behavior in wearable devices.
Wearable Technology: Use and Precautions
At last month’s International Consumer Electronics Show (CES), a wide range of innovations in wearable tech were showcased. From fitness trackers to smartwatches, wearables remain a major focus. Among the new products was a skin patch that monitors UV exposure to prevent sun damage, an accessory that alerts emergency contacts, and a baby boot that tracks vital signs to help reduce the risk of 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 must be addressed. For instance, these devices collect and store personal data, making security a top priority. Hackers have already targeted such data, and some have attempted to gain control over medical wearables used for health monitoring or drug delivery—posing serious risks. Another concern is device overheating, which could lead to burns if worn for long periods.
By analyzing heat transfer within the device and its effects on the skin, designers can create safer wearable technology. Thoratec’s research team used COMSOL Multiphysics to simulate heat transfer in electronic devices after prolonged use. Let’s take a closer look at how they approached this challenge.
Safely Designing Wearable Electronic Devices
If the part of the wearable device that touches the skin becomes hotter than other parts, it will start transferring heat to the skin until both sides reach the same temperature. Over time, excessive heat can cause skin injury. Studies from the 1930s and 1940s showed that skin damage typically occurs when temperatures exceed around 44°C. This aligns with the safety standards set by the International Electrotechnical Commission (IEC), which recommends a maximum safe temperature of 43°C for continuous use.
To meet IEC standards, Thoratec researchers used simulations to predict skin temperature and analyze the heat budget of the device. This helps determine how much heat the device can generate without causing harm.
Study Heat Transfer in Wearable Technology Design
The main goal of the simulation was to model heat transfer through the skin, subcutaneous tissues, and the device itself. The Pennes equation was used to represent heat conduction in the human body, incorporating blood flow as a factor. Blood acts as a heat source or sink 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 x 83 mm x 25 mm electronic device was included, consisting of components like the board, wires, battery, housing, and trapped air. The device was modeled using the heat conduction equation due to its conductive elements.
Figure 2. Basic model geometry, including electronic equipment, parts of the human body, and clothing layers. Image courtesy of JF Hansen, taken from his research paper presented at the COMSOL User Conference Grenoble Station 2015.
Researchers defined boundary conditions for the skin, device, and clothing. The inner surface of the device was in contact with the skin, while the outer surface was covered by fabric. The type of clothing, such as a tight spandex shirt or a loose sweatshirt, affects heat distribution. In this case, a 3 mm thick fabric was used, and heat transfer through the trapped air was also simulated.
The 3D model was analyzed using the solid heat transfer physics interface in COMSOL Multiphysics. The Pennes equation was directly applied to the heat source term. Engineers can choose between built-in material properties or input custom values for greater control—this was the approach taken by the researchers.
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