Microfabricated thermal sensors for skin perfusion measurements

Perfusion, which is the flow of blood through a volume of tissue, is an important clinical measurement because the blood supplies the oxygen and nutrients necessary to sustain life in tissues. The goal of this work was to create a noninvasive sensor and the associated instrumentation to measure perfusion in the skin and subcutaneous tissue by determining convective heat flux. One unique aspect of this device is that the temperature sensors are made from relatively thin films, so they have relatively large surface area but low thermal mass; therefore the volume of tissue in which the perfusion is measured is larger than just a shell of tissue surrounding a small sensor. The second unique aspect of this device is that it measures the heat flux going into the tissue rather than heat losses as a whole, thus eliminating any errors caused by heat loss to the environment.;To measure perfusion, the sensor introduces a small heat flux into the skin. Of the two mechanisms of heat loss, the conduction of heat through the tissue remains relatively constant; therefore changes in the measured heat flux will be directly related to changes in convective losses due to tissue perfusion. A two dimensional steady state analytical model was created to approximate the heat transfer in the tissue, to validate the relationship between the surface heat flux and perfusion, and to determine the sensor design parameters. The analytical model was validated by a physical model of water flowing through a porous media and an agar gel static conduction model. The maximum difference between the physical and analytical models under the conditions of conduction was less than 1°C. The analytical model was also in close agreement with the physical data under combined conditions of conduction and convection with a mean error of 2.75%.;Thick- and thin-film temperature sensors were developed and configured in a sandwich structure with a thermal resistance layer between them. A heater placed on one side of the sandwich introduces the heat flux into the skin and the temperature sensors in the sandwich measure the flux. The heat flux measured by this sensor was found to correlate well with a commercial heat flux sensor and had a sensitivity of 18 W/m2 per mL/min. Preliminary results demonstrated qualitatively that the heat flux sensor was capable of distinguishing different levels of perfusion in human skin.