Modeling heat transfer in humans for body heat harvesting and personal thermal management

The human thermoregulatory system dictates the transfer of heat, produced within organs and skeletal muscles, through body layers and its dissipation to the environment. With the remarkable progress in the field of heat transfer within human beings (bioheat transfer), numerical modeling of the thermal behavior of humans became possible. Such models allow a rough diagnosis of the subject's health in various clinical conditions (humans undergoing hyperthermia or hypothermia, humans under sleep, newborn infants, etc.) and more accurate estimation of power generated from body heat harvesting devices. In this review, elements that accompany the bioheat transfer models (e.g., heat conduction through body layers, heat generation due to metabolism and blood perfusion, and heat loss mechanisms) are discussed, along with how they have been integrated to develop various numerical methods for estimating the human thermal behavior under different environmental conditions. More importantly, we introduce two representative applications of bioheat transfer models: body heat harvesting and core body temperature measuring devices. The human body is a reliable and continuous heat source that can be used for operating wearable sensors and devices. Accurately determining the amount of heat flowing into the device is crucial for estimating the power output. Furthermore, noninvasive core body temperature sensors based on bioheat transfer mechanisms are explained. Reportedly, the measurements taken with these sensors agree well with the actual core temperatures. This review will be beneficial for heat transfer engineers interested in bioheat transfer as well as for those working in the field of wearable sensors and devices pursuing a reliable energy harvester to operate their sensors.