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Page 10 of 25 Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12

that they are bulkier, more costly, and more prone to movement aberrations because there is no liquid
[77]
contact .
Due to the lack of humid and electrolyte media in dry electrodes, an array of structural designs is
[78]
implemented for improving the contact area, including the spongy , nanowire [79,80] , microtip, and variable
structural designs, or achieving highly flexible and stretchable structures and deformable conductors.

A wide range of brain-computer interactions can be performed using flexible dry EEG electrodes, as
described by Grozea et al. , including recording alpha rhythms in the occipital region, testing event-related
[81]
potentials using an oddball auditory paradigm, and implementing sensory-motor rhythms-based event-
related desynchronization paradigms. The electrode was made by coating thin polymer bristles with silver-
based conductive ink. Its bristles almost flex as easily as those in toothbrushes, and it is hard enough to pass
through hair and reach the surface.

Compared to gel-based electrodes, the bristle sensors produced almost the same signal in the recorded
frequency range between 7 to 44 Hz. Furthermore, it maintained mechanical and electrical contact, which is
recommended for long-term usability. Compared to both gel-based electrodes and arrays of pin electrodes,
this electrode demonstrated excellent comfortability.

To overcome the drawbacks of wet electrodes, a novel flexible dry electrode was proposed by Wang et al.,
which did not need any conductive gel and skin preparation . This PDMS-based flexible electrode with
[82]
pins structure indicated superior contact impedance compared to a standard wet electrode without skin
preparation, while it was higher than that with a skin preparation. This flexible dry electrode is safer and
more comfortable and can meet the requirement of high-quality EEG measurement.

Furthermore, Li et al. fabricated a unique flexible Ag/AgCl dry electrode array with a sweat-absorbable
[83]
sponge for frontal EEG monitoring . This coating exhibited exceptional non-polarizability and adhesion
performance. Sweat absorption can be substantially aided by the sponge. Furthermore, it uses sweat as the
electrolyte, effectively eliminating the risk of cross-interference or short circuits while lowering the contact
impedance. All of the findings supported the viability of forehead EEG recording. Forehead EEG recording
techniques make wearability more accessible. Golparvar et al. developed a graphene-based e-textile interface
[84]
that can record brain waves . This graphene-based textile EEG interface is made with a Dip-Dry-Reduce
method. The main materials used are hydrophilic nylon textile and graphene oxide solution. The reliability
of this wearable device was demonstrated by comparing it with commercial dry electrodes in EEG recording
experiments. Another wearable e-textile EEG recording device developed by Carneiro et al. contains 11
electrodes for interfacing with the forehead site and an electronic circuit for signal processing . Their
[85]
design and fabrication process allows this soft latex-based interface to have two open sites, one to contact
the epidermis and the other to allow connection with the electronics. The inclusion of the electronics system
along with Lithium-Polymer batteries allows 24 hours of operation time, which is favorable for wearable
device applications. For a variety of EEG recording and monitoring applications, flexible dry electrodes
provide quick setup, user-friendliness, self-application, and wearer comfort. In general, the flexible dry
electrodes presented rapid setup, user-friendliness, self-application, and wearer comfort for various
applications of EEG recording and monitoring.

Semi-dry soft EEG interface
Several non-invasive electrode types, such as spring-like, porous, or sponge-like structures, and materials
consisting of elastomers and hydrogels are known as semi-dry or quasi-dry electrodes [86-88] .