A MICRO-THERMAL DIFFUSION SENSOR FOR NONINVASIVE SKIN CHARACTERIZATION

The probe consists of a disc (8 mm diameter, 2.5 mm thickness) with a thermistor (0.45 mm diameter) located at its center, and operating in self-heating mode. A cyclic principle is used with two phases: a passi ve phase for baseline skin temperature measurement and an active phase for thermal conductivity measurement. During the active phase, the el ectrical heating power is dissipated in the thermistor at a rate suffi cient to maintain its temperature at a fixed increment above the basel ine skin temperature, as measured by the same thermistor just prior to heating. The thermal conductivity of each skin layer is determined by analysis of the thermal power time course which depends on the therma l field propagation into the skin layers. Calibration experiments were performed on five media of known stable thermal conductivity (polysty rene foam, vaseline and three polyacrylamide gels). Preliminary experi ments on a bilayer phantom (polyacrylamide gel covered with polyvinyl chloride) which simulates the skin have shown the possibility of this non-invasive instrument-probe system to characterize a medium with lay ers of different thermal conductivities.