In Vivo Thermal Conductivity Measurement of Blood-Perfused Tissue Accounting for The Effect of Blood Flow

Understanding the thermal properties of human tissue is crucial for evaluating their ability to regulate temperature, assessing the effectiveness of thermotherapy techniques, designing skin-contact medical devices, and optimizing thermal comfort in different environments. Traditional measurement methods primarily conducted on nonliving materials involve modifying the material’s geometry and placing the material in a vacuum environment, which is not achievable for living human beings. Moreover, the presence of blood flow and perfusion of tissue introduces advection effects described by Pennes’ bioheat equation, and its influence on thermal response at the skin must be considered when measuring thermal conductivity. Previous studies have often been unable to isolate the effect of blood perfusion, resulting in the quantification of effective thermal conductivity, which exhibits a strong temperature dependence due to vasomotion's role in dissipating excess heat or conserving heat in cold conditions. In this study, we employ sinusoidal heating and transient plane source heating to subsequently measure the blood perfusion rate and thermal conductivity exclusively at the skin by controlling the thermal penetration depth. Analytical solution to the transient bioheat equation allows us to separate the influence of blood perfusion when obtaining the thermal conductivity attributed solely to conduction. Tissue temperatures ranging from 30.5°C to 35.5°C yielded tissue thermal conductivity averaged at 0.334 ± 0.035 W/m-K. This value of thermal conductivity agrees well with that of excised human epidermis (0.21 – 0.41 W/m-K) [1]. As expected, the blood perfusion rate increased with increasing tissue temperature from 0.143×10^-3 to 3.421×10^-3 m³/s/m³, which is typical for convective heat transfer coefficients ranging from 5 to 15 W/m²-K. By measuring the thermal conductivity of blood-perfused skin, we hope to enhance the design and performance of wearable devices, improve the efficacy of thermoregulation techniques, and advance the understanding of various skin-related disorders and conditions. The insights gained from these studies have the potential to revolutionize fields such as biomedical engineering, personalized medicine, and thermal comfort optimization, ultimately benefiting human health and well-being.<br/>[1] Xu, F., Lu, T. J., Seffen, K. A., & Ng, E. Y. K. (2009). Mathematical modeling of skin bioheat transfer. <i>Applied mechanics reviews</i>, <b>62</b> (5).