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Nagwade et al. Soft Sci 2023;3:24 https://dx.doi.org/10.20517/ss.2023.12 Page 11 of 25

The self-storage and gradual release of electrolytes make this type of electrode stand out from wet and dry
[89]
electrodes . Indeed, Semi-dry electrodes release electrolytes at a slower pace than wet electrode types,
depending on the particular structural design or substance. Therefore, this value will have an impact on
preserving low contact impedance and preventing short-circuit interference between electrodes. Despite
these advantages, large-scale manufacture is limited due to specific structural designs or particular materials.

Furthermore, regarding the usage of soft materials, the mechanical mismatch between electrodes and the
human scalp is low . As a result, semi-dry electrodes are advised for prolonged therapeutic usage,
[90]
[91]
including sleep monitoring and rehabilitative treatment .
In 2019, Lin et al. reported a type of Gel-Free EEG Electrode with a high conductivity of 917 S/m due to the
surface metallization by the silver nanowires (AgNWs). Compared to the conventional electrodes that
contained conductive gel, the new electrode showed almost the same steady-state visual evoked potential
(SSVEP) as that of the conventional one. In conclusion, the cost-effectiveness, simplicity of manufacture,
flexibility, robustness, relatively low electrode-skin impedance, and mechanical stability of this electrode
were its main strengths.

In addition, Li et al. proposed innovative passive semi-dry electrodes for recording EEG data from the hairy
scalp. This ceramic-based electrode was able to attain a low and stable impedance and an SSVEP paradigm.
Due to the capillary force through porous ceramic pillars, a regulated amount of saline solution could be
released, which eliminates skin preparation and gel application. EEG signals were consistent, and electrode
polarization voltage demonstrated a steady state that was similar to commercial gel-based Ag/AgCl
electrodes. Semi-dry electrodes were applied to real-world practical EEG applications, such as brain-
computer interfaces and wearable technology.

A quasi-dry electrode for EEG is fabricated by Mota et al. . This polymer-based electrode was able to
[77]
discharge 30 μl of a hydrating agent, which decreased the volume of gel needed to simply cover the contact
sites of electrodes. The obtained legitimate EEG signals were comparable to those of commercial Ag/AgCl
reference electrodes, demonstrating the suitability of electrodes for BCI applications.

Soft ECG interfaces
ECG is a non-invasive technique used for obtaining data on the electrical activity of a heart through small
[92]
electrical changes caused by the continuous depolarization-repolarization cycle of the cardiac muscle . In
layman’s terms, an ECG signal is the recording of the electrical activity of the heart. This technique provides
information on the autonomic nervous system responsible for the electrical activity of the heart [93,94] . An
ECG signal has multiple waves that represent the specific state of the heart during a heartbeat cycle. This
signal or wave is called the PQRST wave, where P indicates atrial contractions, QRS indicates ventricle
contraction, and T indicates ventricle expansion . Abnormalities in PQRST intervals can indicate heart-
[95]
related problems. ECG recordings have been extensively used in the medical field to record or monitor the
electrical activity of the heart in patients . Abnormalities in ECG readings have proven to be essential for
[96]
medical experts to determine the health condition of the patient .
[97]
Since ECG is mostly used for health monitoring, factors such as long-term usage, high stability, and stable
attachment to the epidermal layer are of great importance and should be considered when developing
wearable devices. Ag/AgCl electrodes are the industry standard for recording ECG signals, but they lack the
wearability aspects mentioned in the previous section.