Dry EEG electrodes

Abstract

Electroencephalography (EEG) emerged in the second decade of the 20th century as a technique for recording the neurophysiological response. Since then, there has been little variation in the physical principles that sustain the signal acquisition probes, otherwise called electrodes. Currently, new advances in technology have brought new unexpected fields of applications apart from the clinical, for which new aspects such as usability and gel-free operation are first order priorities. Thanks to new advances in materials and integrated electronic systems technologies, a new generation of dry electrodes has been developed to fulfill the need. In this manuscript, we review current approaches to develop dry EEG electrodes for clinical and other applications, including information about measurement methods and evaluation reports. We conclude that, although a broad and non-homogeneous diversity of approaches has been evaluated without a consensus in procedures and methodology, their performances are not far from those obtained with wet electrodes, which are considered the gold standard, thus enabling the former to be a useful tool in a variety of novel applications.

Figure 1.

(a) Simple model of the electrical circuit originated by the electric dipole in a differential montage; (b) Schema of electric dipole, ionic currents and differential measure.

Figure 1.

(a) Simple model of the electrical circuit originated by the electric dipole in a differential montage; (b) Schema of electric dipole, ionic currents and differential measure.

Figure 2.

(a) Array of silicon microneedles. Adapted from [8]; (b) Multiwalled carbon nanotube arrays. Adapted from [11]; (c) Details of a microtip. Adapted from [12]; (d) 3D printed dry millielectrode. Adapted from [16].

Figure 3.

(a) Ambulatory Wireless EEG System Adapted from [17]; (b) Polymer bristles. Adapted from [18]; (c) Spring contact probes. Adapted from [19].

Figure 4.

(a) Capacitive electrode. Adapted from [22]; (b) Non-contact low Power EEE/ECG electrode. Adapted from [23]; (c) Dry active electrode made from standard Printed Circuit Board (PCB). Adapted from [24].

Figure 5.

(a) Dry foam EEG electrode. Adapted from [5]; (b) Quasi-dry electrode Adapted from [28].

Figure 6.

(a) Measure of ESEI by means of test signal. Adapted from [30]; (b) Impedance-to-voltage converter to measure the EEI. Adapted from [12]. Playback schema.

Figure 6.

(a) Measure of ESEI by means of test signal. Adapted from [30]; (b) Impedance-to-voltage converter to measure the EEI. Adapted from [12]. Playback schema.