A thermal conductivity sensor for the measurement of skin blood flow

The 'Hematron' is a non-invasive sensor (disc 25 mm o.d., 4 mm thick) for the measurement of skin blood flow (SBF) in the capillary layers from the surface of the skin. A constant temperature difference is maintained between the center and the periphery of the sensor by a PID control device. The heat power injected into the skin by a micro-fold heater is equal to the heat power washed out by the blood flow in the capillary network. SBF may be recorded from continuous measurement up to 1 Hz. The results are expressed in units of thermal conductivity (mW/cm °C): minimum value 2.5 mW/cm °C (null blood flow); maximum value 10 mW/cm °C (vasodilation). The calibration is carried out by physical models: media, the thermal conductivity of which is constant and well defined (vaseline, polyacrylamide gel), rotating water tank with low linear speed, perfused catheter bunch. The high repeatability of this quantitative method allows long-term measurement and inter- or intrasubject comparison. This sensor is used for the study of physiological tests on SBF (valsalva maneuver, effect of heat, posture, etc…), for the study of the vascularization of burnt skin and graft, the effects of pharmacological or cosmetic agents and for per and post operative monitoring. As SBF is related to brain activity, the sensor is used for the study of the vigilance status and the emotional response (sport, driving of vehicle, mental imagery).