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Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12 Page 9 of 25

Figure 6. Flexible dry and semi-dry electrodes: (A) Silver-coated polymer conductive bristles dry electrode instead. Reproduced with
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permission . Copyright 2011, the Authors. Published by IOP Publishing. (B) Schematic diagram of assembled screen-printed flexible
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Ag/AgCl electrode array. Reproduced with permission . Copyright 2020, the Authors. Published by IOP Publishing. (C) Photo and
schematic diagram of a single semi-dry electrode including porous ceramic pillars (a), a built-in reservoir (b), 3.5% saline solution (c),
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and sintered Ag/AgCl electrode (d). Reproduced with permission . Copyright 2016, Elsevier. (D) Hollow cylinder electrode consisting
of a PVC shell, infiltrated normal saline, and an AgPMS contact and assembled EEG electrode. Reproduced under the terms of an ACS
AuthorChoice License [78] . Copyright 2019, Copyright American Chemical Society. (E) Diagram illustrating a semi-dry electrode that
expels the hydrating agent. Manufactured polyurethane electrodes. Reproduced with permission [77] . Copyright 2013, Elsevier. EEG:
electroencephalography;

Wet flexible EEG interface
In literature, this kind of electrode requires electrolytic substances to improve the conductivity of the scalp-
electrode. In the commercial stage, these electrodes are composed of Ag/ AgCl disk and an infiltration
substance, i.e., conductive gel, which contains electrochemical potential stability and infiltration ability. Due
to the advantages mentioned above, wet electrodes are currently prevalently used.

However, a list of drawbacks exists that should be considered and solved. One of the common problems is
the time-consuming procedure of cleaning up and controlling the amount of conductive gel for each
electrode point. Another critical issue is reducing the moisturizing level of gel during the experiment, which
results in poor EEG signal quality as an increasing impedance .
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To mitigate these problems, dry and semi-dry electrodes were suggested.

Dry flexible EEG interface
Dry flexible electrodes are leveraged to overcome the inconvenience and unstable recording conditions of
wet electrodes, as mentioned before. Indeed, dry electrodes make dry connections with skin that do not
require conductive gel or any skin preparation and are desirable for portable and wearable electronic
devices [74-76] . However, in addition to these benefits, dry electrodes have several drawbacks, including the fact