Packaging of a thin-film sensor for transepidermal water loss measurements

The water content in the superficial part of the human skin is one index to evaluate the skin health. In the past we have developed a packaging concept which brings a sensor chip carrying a comb-shaped electrode system into intimate contact with the skin. The electrical impedance between these electrodes can be used as a measure for the water content of the skin. However, we have recently found that the same sensor chip used with a slightly altered packaging concept can measure the transepidermal water loss (TEWL), which is another indicator for the skin health. In our paper we report on the interconnection and packaging technique of this advanced measuring system for the evaluation of human skin. Furthermore, we present some first measurement results. The basic structure of the sensor consists of a thin film interdigital electrode system deposited on a ceramic substrate by rf-sputtering. Thin wires are guided through funnel-shaped holes arranged in the center of the contact pads. A CO₂ laser with a wavelength in the medium infrared range is used to produce these holes. The wires are bonded to the contact metallizations by using conductive adhesives. In the case of TEWL measurements the sensor chip does not touch the skin. The chip is placed in a distance of a few millimeters away from the skin. The electrical impedance of the electrode system depends on the stream of water molecules emitted from the skin. This kind of measurement does not suffer from pollution of the sensor chip by the oily substances of the skin surface. Therefore, the results are more reproducible compared to classical skin impedance measurements.