Investigation of the Influence of the Moisture Content of Clothes on Capacitive Electrocardiography

For vital sign monitoring in private spaces, the usage of unobtrusive measurement techniques is desirable to avoid restrictions with respect to free movement of the users.One of these techniques is capacitive electrocardiography (cECG) which instead of adhesive electrodes uses metal electrodes so that a capacitive coupling with the body is achieved through the clothes which reduces the signal-tonoise ratio (SNR).It is known that the relative humidity and thus the moisture content of the worn clothes has a strong influence on the cECG.However, to the best knowledge of the authors, the effects have not been modelled in detail, yet.In this paper, models for the change of permittivity and resistance of textiles with moisture content are used to obtain a range of values for them.Furthermore, a model for the commonly used active electrode is used for simulations with Simscape, such that the influence of the changing properties on the obtained cECG signal can be quantified by means of the SNR.It can be shown that the resistance of the texiles changes from several giga-ohms to several kilo-ohms.Furthermore, it can be seen that while the permittivity of cotton changes over roughly one magnitude (approx.10 pF to 140 pF), the permittivity of polyester does not significantly change.From the simulations, it can be seen that the SNR for cotton is generally higher than for polyester and that the SNR rises for higher moisture contents.