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Kim et al. Soft Sci 2023;3:18 https://dx.doi.org/10.20517/ss.2023.08 Page 15 of 19

Although PEDOT:PSS is not biodegradable and is reported to circulate through the renal system to be
[92]
cleared from the body , it has been used in many neural engineering applications due to its low
immunogenicity [93,94] . Conversely, the main route of biodegradation of ICH is postulated to be achieved via
its enzymatic degradation from hyaluronidase and by increased hydrolysis at the hydrophilic domains of
HA [95,96] . Thus, to evaluate its biodegradation, ICH was imaged initially after the 1-mL injection of ICH over
the left hemisphere of the cerebral cortex. After calibrating the brightness and contrast relative to 0 week as
the baseline, the minimum and maximum brightness values were set to 0 and 30,000, respectively
[Figure 5A]. After 4 weeks, the same rodent was imaged and analyzed using calibrated brightness and
contrast, and approximately 0.4 mL of the injected volume was found to remain [Figure 5B]. These results
demonstrate that ICH is safely biodegraded, further confirming the feasibility of ICH as a controllable
degrading electrode array.

Next, rodents implanted with an electrode array device were imaged three days after surgery. In the coronal
view, ICH and PVDF-HFP were clearly visible individually without the disturbance of resonance, which is
unavoidable when using metal-based electrodes , and showed conformal contact. In the axial and sagittal
[33]
views, the ICH channels were also evident and did not inundate outside of the channel [Figure 5C].

CONCLUSIONS
In this study, an unconventional injectable bioelectronic material composed of purely organic polymers was
developed to implement as a MRI-compatible brain-interfacing device. The ICH has numerous advantages
that are essential for implantable bioelectronics. First, ICH is injectable in situ into a high-resolution (less
than 200 μm diameter) linear array pattern due to the ionically crosslinked (hydrogen bonding between
tyramine and sulfone) hydrogel. Second, it is highly biocompatible both in vitro (95.9% cell viability) and in
vivo (no signs of inflammation and fibrosis), along with a rate of degradation of up to 40% of its original
volume within four weeks. Third, brain conformal, soft neural electrodes for ECoG recording are
demonstrated using a rodent model with a clear VEP signal from the visual cortex. Finally, no artifacts were
generated by the device on MRI, as ICH and PVDF-HFP were clearly imaged without resonance distortion.
As a result, based on its brain-like soft moduli and excellent ionic conductivity, MRI-compatible ICH neural
electrodes realized stable ECoG monitoring with highly sensitive documentation of neural responses to
visual stimulation, which could be further expanded to recording other neural responses from various
senses, such as olfactory, tactile, and auditory senses, using our ICH-based brain-interfacing platform.

DECLARATIONS
Acknowledgments
We thank Chanhee Lee and the Center for Neuroscience Imaging Research for the imaging of 9.4T animal
MRI. We would also like to thank Jin-Hwan Jeon of the Center for Neuroscience Imaging Research for the
3D printing of the substrate mold.

Authors’ contributions
Substantial contributions to the conception and design of the study and performed data analysis and
interpretation: Kim SD, Park K, Lee S
Data acquisition and administrative, technical, and material support: Kim SD, Park K, Lee S, Kum J, Ahn S,
Kim Y, Kim J
Review, editing, and supervision of this project: Shin M, Son D
Kim SD, Park K, and Lee S contributed equally to the article.