Thermal Acceptability Limits for Wearable Electronic Devices

A sophisticated human thermal simulation tool was used to identify the upper limit of the acceptable temperature range for a wearable electronic device. A model of a generic device was developed to evaluate the heat transfer between a wearable device and a human subject. Heat transfer was characterized by the surface temperature and the contact resistance between the device and the right forearm. The human thermal model was used to simulate a subject exerting a low level of activity while wearing typical summer attire within a neutral “office” environment. A series of simulated exposures were conducted in which the device temperature was varied from a near neutral temperature to a maximum value corresponding to the pain threshold ($44^{\circ}\mathrm{C}$). The device was evaluated by the impact on local (forearm) skin temperature, mean skin temperature, core temperature, thermal sensation, and thermal comfort. The results of the investigation show that the device had a significant impact on local skin temperature. At lower device temperatures, the local skin temperature matched almost exactly the device temperature. At higher device temperatures, in particular, at the upper-temperature limit considered in the investigation, it was predicted that the skin temperature was more than $1.5^{\circ}\mathrm{C}$ below the pain threshold. Although the device had a minimal influence on whole-body mean skin temperature and negligible influence on core temperature, which are two key indications of the thermal state of a human, the simulation predicted that device temperatures of $34^{\circ}\mathrm{C}$ and greater can negatively impact overall thermal comfort due to the local discomfort experienced by the body segment (in this study, the forearm) in contact with the heated device.