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Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34 Page 7 of 28

To enhance the conductivity and mechanical properties of hydrogels, Shin et al. proposed a strategy to
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optimize the materials by incorporating polymers and conductive fillers . Figure 3F-H illustrates the
conductive network structure of a conductive hydrogel based on poly(3,4-ethylenedioxythiophene)-
poly(styrenesulfonate) (PEDOT:PSS) and polyaniline (PANI), wherein PEDOT:PSS serves as the conductive
matrix and PANI further enhances the electron transport efficiency. The combination not only improves
the electrical conductivity of the hydrogel, but also maintains its mechanical flexibility. Figure 3G illustrates
the variation in homogeneity and conductivity of the hydrogels under different PEDOT:PSS doping
concentrations. The experimental results demonstrate that the synergistic effect of PEDOT:PSS and PANI
effectively optimizes the conductive pathways, while improving the homogeneity of the material and
ensuring its stability under different application conditions.

Electronic conduction mechanism is also mainstream for the design and fabrication of conductive
bioelectronics. Zhang et al. developed a liquid metal (LM)-doped polyvinyl alcohol (PVA)-based hydrogel,
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which achieved high electrical conductivity and strong toughness through a self-sintering mechanism .
Figure 3I illustrates the settling of LM microdroplets under the influence of gravity, which subsequently
undergo self-sintering to form a conductive network. Figure 3J illustrates the morphology of the hydrogel
with cyclic stretching, which demonstrates the adaptive nature of the material. Figure 3K depicts the
resistance change of the hydrogel under 1,000 cycles of stretching to show stability under prolonged
mechanical strain. This LM-doped hydrogel exhibits excellent electrical conductivity and mechanical
strength, which indicates its potential for use in flexible brain electrodes.

The conduction mechanism of hydrogels is essential for their effectiveness in BCIs, which directly
influences the performance in terms of conductivity, impedance, and user comfort during long-term
monitoring. Adhesive ionic conductive hydrogel has the advantages in the enhancement of adhesion and
conductivity upon skin application and low impedance can be obtained. The electronic and ionic
conductive hydrogel can create a stable network and this synergy results in sustained low impedance and
high-quality EEG signal acquisition. The innovative use of LM-doped hydrogels demonstrates a self-
sintering mechanism that achieves high conductivity and mechanical strength. Overall, the advancements in
hydrogels with different conduction mechanisms significantly enhance their functionality, which can
expand their potential applications in biomedical sensing and long-term neural monitoring.

Biocompatibility of hydrogels in bioelectronics
The biocompatibility of hydrogel materials is of great significance for long-term monitoring, especially for
implantable electronics. As shown in Figure 4A and B, our team conducted systematic in vitro cytotoxicity
experiments on the hydrogel to validate its biocompatibility . Mouse skeletal muscle (MSkM) cells were
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cultured in vitro on the surface of the STEHy-6 hydrogel to assess their cellular compatibility with tissue
culture plates (TCP) as a control. Figure 4A shows that the growth rate of MSkM cells on both TCP and the
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hydrogel is consistent . In detail, in the initial three days, there is no significant difference in cell
proliferation between TCP and STEHy-6, indicating that STEHy-6 has no significant impact on cell growth.
MSkM cells exhibited the ability to adhere to and proliferate on both the STEHy-6 and TCP surfaces, which
indicates the excellent biocompatibility of the materials. Subsequently, cell proliferation was further
confirmed using the CCK-8 assay. Figure 4B demonstrates that with extended culture time, both the control
and experimental groups showed a significant increase in skeletal muscle cells . Additionally, live/dead
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assays were conducted after one, two, and three days of cultivation to identify live cells (green) and dead
cells (red). The results showed that the number of cells continued to increase during the cultivation, and few
dead cells were observed even after three days of culture. Importantly, the number of MSkM cells was
equivalent to that of the TCP control group, indicating that the STEHy-6 hydrogel is suitable for the
survival and proliferation of isolated skeletal muscle cells and the material has excellent biocompatibility.