Experimental evaluation of materials for electrodes for transcutaneous detection of bioelectric signals

Abstract

Reliability of results of electrophysiological examination significantly depends on the quality and physical and chemical properties of the electrodes used. Electrodes make contact with human body and thus complete the electric circuit between a generator of biopotentials and a measuring device. The electrodes used for transcutaneous detection of bioelectric signals are presently made of various materials (stainless steel, Nichrome, silver, gold, graphite, etc). Although there is a broad assortment of electrodes commercially available [6], it is general practice to use stainless steel or chrome-plated electrodes for recording cutaneogalvanic reflex [3]. Microelectrodes made of silver plates coated with AgCI are described in [1]. The possibility of use of electrodes made of graphite materials is presently studied [10]. Comparative analysis of stability and reproducibility of the AgCI electrode potential difference (EPD) is made in [6, 12]. These electrodes are used for recording stable or super slow biosignals. It should be noted that mathematical methods used for processing bioelectric signals include estimation of amplitude-phase characteristics of electrographic activity of transient states (phase shift analysis of electrograms). Therefore, the frequencydependent errors of measurement occurring by electrographic methods of examination of biopotentials should be taken into account. These errors are inevitable when measurements are taken using electrodes made of stainless steel, graphite or polymeric metals.The area of contact between the electrode and skin is usually regarded as a series of layers with different potentials:subcutaneous tissues--skin--(intermediate contact substance)--electrode--lead. Intermediate contact substance is not universally observed.An electrochemical half-cell is produced by the contact area between the electrode and skin. This cell is characterized by an interface potential difference (electrode potential difference, EPD), which is usually as much as 0.3-1 V. Compensation of EPD is used to reduce the error caused by EPD. However, EPD can not be compensated completely. Residual EPD varies with time between 0.1 and 0.4 V [9]. Therefore, when wide-band amplifiers of biopotentials are used, EPD can exceed useful signal several hundred or thousand times and occupy most of the dynamic range.There are at least 3 sources of changes in EPD during the process of measurement: changes in solid conductive material of electrodes, changes in the liquid phase at the interface between solid and liquid phases, changes in the area of contact between the electrode and skin. Changes in EPD caused by a polarizing current source connected to the measuring circuit also contribute to the total error [8].The character and limits of changes in the solid conductive material of electrodes depend mainly on the process of its preparation. Changes in ion activity are the main cause of changes in the liquid phase, especially in the presence of extrinsic conductivity or additional sources of polarizing current (cutane...

Quasi-periodic patterns (QPP): Large-scale dynamics in resting state fMRI that correlate with local infraslow electrical activity

“…Ten rats were anesthetized with isoflurane anesthesia with fine-tip glass microelectrodes (1 - 5 MΩ impedance) implanted into left and right (interhemispheric) primary somatosensory cortex of the forelimb region (S1FL) for simultaneous LFP-fMRI recording (Pan et al, 2010). Low-frequency recording was enabled by use of direct-current capable amplifiers and silver/silver chloride leads in glass electrodes (Agranovskii et al, 1998; Geddes, 2001, 2003). The procedures used here are described in detail elsewhere (Pan et al, 2013).…”

“…These hypothetically neural frequencies are directly comparable to the frequency range of the spontaneous BOLD fluctuations used to map functional networks; however these frequencies are rarely examined as most in vivo neural electrophysiology is limited by electrode materials (Agranovskii et al, 1998; Geddes, 2001, 2003) or amplifier hardware to frequencies above 0.1Hz.…”