Investigation of Variability in Skin Tissue Intrinsic Thermal Conductivity Measurements

The Thermal Diffusion Probe 200 (TDP200) is a thermal properties characterization device that is capable of measuring the thermal conductivity of phantom materials with less than 0.2% variability. However, this device gives 1.5% to 2% variability in thermal conductivity values when the medium being measured is human skin tissue. Six different sources of variability have been identified that might contribute this order of magnitude increase in measurement variability as the medium being measured is changed from phantom materials to skin tissue. The sources of variability include: the low sampling rate of the TDP200 system; baseline temperature fluctuations of the skin tissue; random perfusion effects of the blood vessels within the skin tissue; variable thermal contact between the measuring thermistor probe and the skin tissue; the dependence of thermal conductivity on the applied incremental heat temperature step; and inherent hydration induced skin tissue intrinsic thermal conductivity changes. Each of these possible sources of variability are carefully examined through simulations with Matlab, experiments both in-vitro and in-vivo, and theoretical calculations. The contribution from each of these variability sources are then quantified. This investigation is able to show that these six sources of variability account for the majority of the observed skin tissue intrinsic thermal conductivity measurement variability. On average, these variability sources combine to cause about 1.2% variability in the measurements. Thesis Advisor: Dr. H. Frederick Bowman Title: Director, MIT Hyperthermia Program Senior Academic Administrator for the Harvard-MIT Division of Health Science and Technology