Thin film sensor for transepidermal water loss measurement - calibration method and application to human skin

In the past we demonstrated the measuring principle of a new thin film sensor which is capable of measuring the evaporation rate of the human skin. This rate is called transepidermal water loss (TEWL). The sensor consists of electrodes with interdigital structure. Water evaporated from human skin precipitates on the sensor´s surface and alters the electrical impedance of the sensor. The change of the impedance is used to evaluate the TEWL. In order to correlate the electrical impedance and the TEWL it was necessary to develop suitable calibration measurements. We established an experimental setup to measure the evaporation rate of distinct cellulose tissues which were selected such that they serve as references for different types of skin. The evaporation rate was determined using a precision balance. By recording the loss of weight together with the change of impedance we received the respective calibration curve of our sensor system. Due to the strong dependence on the air humidity we calibrated the system at different levels of relative humidity and received a set of calibration curves each of which is valid for the respective air humidity. These calibration curves were approximated by polynomials to allow a fast and easy evaluation of the measured data. Finally, we demonstrate and discuss measurement results obtained at persons with different types of skin (normal and atopic skin)