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Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34 Page 17 of 28

Figure 9. Non-invasive hydrogel interfaces with in-suit gelling method. (A and B) The hydrogel-based electrodes exhibit exceptional
[34]
performance in impedance and resistance to noise and other interference ; (C and D) The phase transition property of the hydrogen
can be used for EEG monitoring on hairy scalps [35] ; (E-G) PAAS-MXene significantly reduces the impedance between electrode and skin
and improves the quality of bioelectrical signals [97] . EEG: Electroencephalogram; PAAS: sodium polyacrylate.

ensuring a conformal fit with the scalp and mitigating issues such as motion artifacts and sweat interference.
The ability to achieve a fluid-to-gel transition enhances the interface’s adaptability, making it suitable for
diverse anatomical variations and promoting stable signal acquisition over extended periods. Additionally,
rapid solidification methods significantly reduce electrode-skin impedance, further improving the quality of
recorded bioelectrical signals. Overall, these developments pave the way for more reliable and user-friendly
EEG monitoring systems, facilitating advancements in both clinical diagnostics and neuroscience research.

Implantable hydrogel interfaces for ECoG
Compared with non-invasive applications, hydrogel shows unique advantages in invasive electrodes to
monitor the ECoG and LFP signals benefiting from biocompatibility. Therefore, numerous studies have
been carried out for the design and fabrication of implantable devices with hydrogels.

Wang reported a hydrophilic conductive hydrogel composed of poly(3,4-ethylenedioxythiophene)
nanoparticles (dPEDOT NPs) synthesized through a dopamine-limited area polymerization process as
shown in Figure 10A and B . With the introduction of nanoparticles into a hydrogel network comprised of
[98]
κ-carrageenan (CA), PDA and PAM, a highly flexible, tissue-adhesive, and biocompatible conductive
hydrogel (dPEDOT-CA-PDA-PAM) was obtained. The hydrogel was used as a flexible conductive interface
to seamlessly connect the rigid microcircuits with soft brain tissue. Also, due to the in-situ self-cure
property, microcircuits can be transferred to arbitrary surfaces without sacrificing integrity. Benefiting from
its biocompatibility, the hydrogel suppresses neural inflammation during implantation and enhances the
stability of the BCI in long-term recording of neural signals.