On bio activity related signals from contactless electrode measurements

Abstract

In a recent paper on a displacement current sensor for contactless detection of bio-activity related signals [1], it was stated that “A potential means for human presence detection…is via sensing…human biopotentials” [because] “physiological events associated with the biological functions of many human organs produce electric fields,” and that “the ECG is relatively easier to measure compared to other biopotentials and, hence, can potentially provide an important [underscore added] means of detection of human presence.” Contactless biopotential measurement using off-body electrodes has attracted many authors that have proposed various solutions, some of which are far more expensive than that devised in [1]. Usually, one or two aluminium discs at a few centimetres from the chest are connected to a high-impedance voltage amplifier and the large impedance between these electrodes and ground makes the circuit very susceptible to capacitive (electric-field) interference hence it requires electric shields or driven guards. Measuring displacement current instead of electrode voltage in contactless biopotential measurements, as proposed in [1], avoids high impedance nodes hence measurements should be far less susceptible to electric field interference. Consequently, it is stated in [1] that “[Contactless] capacitive sensors therefore can be used to sense the displacement current induced by the time-varying electric fields associated with various human biopotentials.” This is in principle an interesting approach worth being considered and the authors deserve praise for that.