Who knew? Wearables can be excessive skin-heat sources, too.

You might think that a smart watch or fitness wearable would not be a thermal concern for users. After all, they only have small rechargeable batteries and sip that battery’s energy to extend operating time as much as possible, typically at least 24 hours.

Their heat dissipation is many orders of magnitude less than that of a CPU, GPU, or other processor-core device cooking along at tens and even hundreds of watts. Nonetheless, wearables can be highly localized sources of heat and therefore cause potential skin problems.

I hadn’t thought about the extent of this localized heating on skin due to wearables until I coincidentally saw several items on the subject. The first was an IEEE conference article re- posted at InCompliance magazine, “Reduced-Order Modeling of Pennes’ Bioheat Equation for Thermal Dose Analysis.” The second was an article in Electronics Cooling, “Thermal Management and Safety Regulation of Smart Watches.”

The first paper was intensely analytic with complicated thermal models and equations, and while I didn’t want to go through it in detail, I did get the overall message: you can get surprisingly high localized skin heating from a wearable.

It pointed out that the simple term “skin” actually comprises four distinct tissue layers, and each is unique in its geometric, thermal, and physiological properties. The outermost layer is the exposed epidermis, beneath it is the dermis which is the “core” of the skin, then the subcutaneous fat (hyodermis) layer, and finally, the inner tissue muscle and bone, Figure 1.

Figure 1 The term “skin” really refers to a four-layer structure, where each layer has distinctive material, thermal, and other properties, most of which are hard to measure. Source: Cleveland Clinic

Damage to the skin is analyzed by the extent of partial or complete necrosis (death) of each layer. While that’s more than I wanted to know, I was curious about the assessment of skin damage.

It turns out that there is, as expected, a quantitative assessment of thermally induced damage and it is based on cumulative exposure at various temperatures. This thermal dose is estimated as cumulative equivalent minutes at 43°C, or CEM43°C, which provides a time and duration number:

Where T is tissue temperature, t is time, and R is a piecewise-constant function of temperature with:

R(T) = 0.25 for T ≤ 43°C and = 0.5 for T > 43°C.

So far, so good. The rest the of lengthy paper delved into models of heat flow, heat spreading through the skin, transforming surface data into three-dimensional data, and more. The analysis was complicated by the fact that heat flow through the layers is hard to measure and model, especially as the skin layers are anisotropic (the flow is different along different axes).

Cut to the chase: even a modest self-heating of the wearable can cause skin damage over time, and so must be modeled, measured, and assessed. How much heating is allowed? There are standards for that, of course, such as IEC Guide 117:2010, “Electrotechnical equipment – Temperatures of touchable hot surfaces.”

What to do?

Knowing there’s a problem is the first step to solving it. In the case of wearables, the obvious solution is to reduce dissipation even further, which would also increase run time as an added benefit. But efforts are underway to go beyond that obvious approach.

Coincident with seeing the two cited articles, I came across an article in the scholarly journal Science Advances, “Ultrathin, soft, radiative cooling interfaces for advanced thermal management in skin electronics.” A research team led by City University of Hong Kong has devised a photonic, material-based, ultrathin, soft, radiative-cooling interface (USRI) that greatly enhances heat dissipation in devices.

Their multifunctional composite polymer coating offers both radiative and non-radiative cooling capacity without using electricity and with advances in wearability and stretchability. The cooling interface coating is composed of hollow silicon dioxide (SiO2) microspheres for improving infrared radiation along with titanium dioxide (TiO2) nanoparticles and fluorescent pigments, for enhancing solar reflection. It is less than a millimeter thick, lightweight (about 1.27g/cm2), and has robust mechanical flexibility, Figure 2.

Figure 2 Overview of the USRI-enabled thermal management for wearable electronics. (A) Exploded view of the components and assembly method of the ultrathin, soft, radiative-cooling interface (USRI). (B) Photographs of a fabricated USRI layer (i) and that attached on the wrist and hand (ii). (C) Thermal exchange processes in wearable electronics seamlessly integrated with a USRI, including radiative (thermal radiation and solar reflectance) and nonradiative (convection and conduction) contributions, as well as the internal Joule heating. (D) Comparison of cooling power from the radiative and nonradiative processes in wearable devices as a function of the above-ambient temperature caused by Joule heating. (E) Conceptual graph capturing functional advantages and potential applications of USRI in wearable and stretchable electronics. Source: City University of Hong Kong

When heat is generated in a wearable fitted with this thermal interface, it flows to the cooling interface layer and dissipates to the ambient environment through both thermal radiation and air convection. The open space above the interface layer provides a cooler heat sink and an additional thermal exchange channel.

To assess its cooling capacity, they conformally coated the cooling interface layer onto a metallic resistance wire functioning as a heat source, Figure 3. With a coating thickness of 75 μm, the temperature of the wire dropped from 140.5°C to 101.3°C, compared with uncoated wire at an input current of 0.5 A with a 600-μm thickness, it dropped to 84.2°C for a temperature drop of more than 56°C. That’s fairly impressive, for sure.

Figure 3 Passive cooling for conductive interconnects in skin electronics. (A) Exploded view of a USRI-integrated flexible heating wire. (B) Photographs of the flexible heating wire before and after coating with the USRI, showing their seamless and robust integration under bending, twisting, and folding. (C) Thermal exchange processes of the USRI-coated flexible heating wire. (D and E) Measured temperature variation of the USRI-integrated flexible heating with varied interface thickness (D) and interface area (E) under different working currents. The colored shaded regions depict simulation results. (F) Image of the USRI-integrated flexible heating wire and corresponding infrared images of such devices with different thicknesses and areas. The working current was kept at 0.3 A. (G and H) Statistics of cooling temperatures of two USRI-coated flexible heating wires working at a current varying from 0.1 to 0.5 A. Both the thickness and the interface area present significant differences between the control and USRI groups (P = 0.012847 for interface thickness, P = 0.020245 for interface area, n = 3). (I) Temperature distribution of USRI-integrated flexible heating wires with varied thickness, area, and current. Source: City University of Hong Kong

Have you had to worry about excessive heat dissipation in a wearable, and the risks it might bring? Were you aware of the relevant regulatory standards for this phenomenon? How did you solve your problem?

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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The post Who knew? Wearables can be excessive skin-heat sources, too. appeared first on EDN.

Who knew? Wearables can be excessive skin-heat sources, too.

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