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Zhang et al. Soft Sci 2024;4:39 https://dx.doi.org/10.20517/ss.2024.34 Page 21 of 28

Figure 12. Multimodal implantable hydrogel interfaces. (A and B) Highly stretchable hydrogels as implantable electrodes for the
recording of neural signals [102] ; (C-F) In situ multimodal transparent electrophysiological hydrogel for in vivo miniature two-photon
neuroimaging and neural signal analysis [85] ; (G-I) MRI compatible ICH electrodes [76] . MRI: Magnetic resonance imaging; ICH: injectable
conductive hydrogel.

biocompatibility. Also, the hydrogel, containing abundant water and ions, exhibits mechanical and chemical
properties similar to those of biological tissues. As a result, it can significantly reduce neural inflammation
reactions and damage to brain tissue. When the hydrogel is used as the electrodes, it can adhere tightly to
brain tissue and exhibits low interface impedance with brain tissue (150 Ω at 1 kHz). In addition, there is a
high transparency of the hydrogel with a transmission of 93% in the range of 300 to 1,100 nm, and no
degradation is observed in the structure and electrochemical performance under exposure to light and
electrical stimulation. As a proof of concept, this MTEHy-3 was used with MRI scanning technology,
achieving a rapid and precise localization of brain regions using MRI scans. In brief, in-vivo MRI was
conducted on the brains of mice after the hydrogel electrode was implanted on the surface of the cerebral
cortex for seven and thirty days as shown in Figure 11D. In addition, the real-time two-photon images for
the neuronal activity in resting and active states of the mice were also monitored benefiting from the high
transparency of the MTEHy-3 electrode as shown in Figure 12E in conjunction with the two-photon
imaging . The ECoG signal with high SNR during rest and movement is measured and shown in
[85]
Figure 12F.

Son at Sungkyunkwan University developed an injectable conductive hydrogel (ICH) based on the ionic
cross-linking properties of hydrogels . The ICH can be injected in-situ to form high-resolution array with
[76]
a diameter less than 200 micrometers. In addition, it demonstrates high biocompatibility for both in-vitro