Embedded Tutorial: Realistic Thermal Model for Human Skin in Contact with a Wearable Electronic Device

Makers of electronic devices try to provide as much performance and functionality in them as possible, consistent with certain limits for internal chip temperatures. For wearables, the external temperatures of these devices are also critical for user comfort and safety. For accuracy in a thermal model for the wearable device, it is necessary to accurately account for the transfer of heat into human skin. The commonly used ad-hoc assumption of an isothermal boundary condition representing the region of contact between a wearable device and human skin is no longer adequate. In the medical and biological fields, modeling the transfer of heat into or out of living tissue is a mature area of study. The dominant methodology in this regard is referred to as the Pennes biothermal model, named after its creator. It is a conduction model supplemented by a mechanism for cooling the tissue by blood flow, which Pennes called "perfusion'.'The application of the Pennes model requires that certain specified material properties be measured for each of the different tissue types involved in the heat flow, namely: thickness, thermal conductivity, specific heat, and perfusion rate. In the case of human skin all three layers (epidermis, dermis, and hypodermis) are separately represented. Despite its wide use in the life sciences, the Pennes biothermal model is virtually unknown in the electronics cooling sector. It's the intent of this presentation to provide sufficient background information and details in its implementation that the attendees will be able to apply it immediately to their work.