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Page 16 of 28 Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34

Traditional dry electrodes are challenging for signal acquisition because of their high contact impedance
and unstable interface, especially in the condition of dynamic movement. In contrast, wet electrodes, which
utilize conductive gels to reduce impedance, can cause skin irritation with extended periods and require
frequent rehydration to maintain conductivity. Semi-dry electrodes can achieve low impedance between the
skin and electrodes benefiting from the water retention capabilities of hydrogel, while also offering excellent
flexibility and adhesion. This design ensures stable signal quality and a comfortable wearing experience and
semi-dry electrodes are considered as an ideal alternative to traditional dry and wet electrodes.

Non-invasive hydrogel interfaces with in-suit gelling method
Long-term EEG signal monitoring with high fidelity is of great significance for clinical applications and
neuroscience research. However, conventional electrodes are based on conductive liquids or hydrogels and
incorporation with commercial EEG caps to realize multi-channel monitoring of EEG signals, which lacks
adaptability in deformation and hairy scalp conditions. As a result, long-term EEG recordings remain a
challenge. Therefore, there has been extensive research in the development of self-adhesive for multi-
channel EEG interfaces based on the in-suit gelling hydrogel to overcome the challenge.

As illustrated in Figure 9A and B, Han et al. reported a multifunctional hydrogel with high conductivity,
adhesiveness, and biocompatibility . The proposed hydrogel can realize in-suit gelling and bridges the
[34]
mechanical mismatch between human tissues and electrodes and offers an efficient and stable method for
brain signal acquisition. In brief, they proposed a strategy based on free radical oxidation to prepare mussel-
inspired polydopamine (PDA) nanoparticles. The PDA nanoparticles are uniformly distributed throughout
the hydrogel network, with carboxyl groups ionized into carboxylate anions, which serve as fixed ions.
Simultaneously, positive ions, such as hydrogen and sodium ions, are introduced during the preparation of
the hydrogel and uniformly dispersed in the hydrogel network. As a result, it provides excellent and stable
conductivity. The EEG signal recording indicates that the hydrogel-based electrodes exhibit exceptional
performance in impedance and resistance to noise and other interference, such as sweat and motion
artifacts.

Wang et al. have developed a conductive bio-gel, which realizes the reversible fluid-to-gel transition
characteristics at a desired temperature by adjusting the collagen content and cross-linking density as
[35]
depicted in Figure 9C and D . The phase transition provides unique skin adaptability and in-situ gelling
capability for the bio-gel, which enables conformal contact between the electrodes and the scalp. The phase
transition property is particularly suitable for EEG monitoring on hairy scalps for long-term stable EEG
recordings with high quality.

Also, Luo et al. reported a highly flowable pre-polymerized sodium polyacrylate (PAAS)-based ion-
conductive hydrogel that rapidly conforms to the structure of the scalp as shown in Figure 9E and F . They
[97]
used MXene as a cross-linking agent, which allows the PAAS-MXene to solidify on the skin within five
seconds. This method significantly reduces the electrode-skin impedance and improves the quality of
bioelectrical signals. In addition, PAAS-MXene demonstrates excellent electrical properties, including
outstanding polarization potential stability, extremely low impedance and stable electrical performance after
stretching for 1,000 cycles.

Non-invasive hydrogel interfaces utilizing in-suit gelling methods represent a transformative approach to
long-term EEG signal monitoring. Traditional electrodes often struggle with adaptability and stability, but
advancements in self-adhesive hydrogels have led to more effective solutions. These innovative materials
not only provide high conductivity and biocompatibility but also enable real-time gelling upon application,