Electrical properties of human skin: From linear recordings of exogenous electrodermal activity to non-linear memristor measurements

Abstract

This thesis is all about applying an electrical voltage to human skin and measuring the corresponding electrical current. A recording (on any tissue) is non-linear when the electrical properties are affected by the applied signal itself; The measurement is linear otherwise. Non-linear measurements on human skin initiate a very new field of research. Dependent on the polarity of the applied voltage, the sweat inside the ducts is moved towards the skin surface or towards deeper skin layers. Here we confirmed that the sweat ducts are memristors (memory resistor) and found that information can actually be stored inside. If you are a forgetful person, this might be a way to store your bank pin number in the future. A second memristor can be found in the stratum corneum (the tissue in the outer layer of the skin that surrounds the sweat ducts). However, the voltage-current characteristics recorded from human skin vary largely among subjects with potential applications in Diagnostics. Further, we found that human skin can act as a frequency doubler and halve-wave rectifier. An example of linear measurements will be given as an introduction. Changes in sweating due to sympathetic nervous activity are subject of (linear) electrodermal activity (EDA) recordings. Here, the test subjects were asked to do continuous subtractions and deep breaths, to stimulate their sweat activity. By doing so, it was possible to find out that EDA recordings using the alternating current (AC) method are comparable with recordings using the direct current (DC) method.