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Kim et al. Soft Sci 2023;3:18 https://dx.doi.org/10.20517/ss.2023.08 Page 11 of 19

printing electrodes. Secondly, ICH was not only able to withstand complete dissolution for a longer
duration in aqueous conditions but also showed high confidence in conductivity [Supplementary Figure 6A
and Figure 2H]. In addition, the normalized resistivity values (Δρ/ρ ) and conductivity values (Δσ/σ ) of ICH,
0
0
while soaked in a PBS buffer from 0 to 16 h, showed that the conductivity of ICH was increased until 2 h
(Δσ/σ = 2.248 ± 0.062), decreased via dissociation from 2 to 8 h, and dissociated completely after 16 h
0
[Supplementary Figure 6B]. Third, cyclic strain-resistance tests were conducted to evaluate improvements
in conductivity after the addition of glycerol, which improved from 6,092 Ω (1.45 × 10 S/cm) to 4,932 Ω
-2
-2
(1.79 × 10 S/cm), and the relative normalized resistance values (ΔR/R ) after glycerol addition exhibited
0
negligible changes of about 0.92 after 100 times cyclic strains [Figure 2I, Supplementary Figure 6C and
Supplementary Figure 7]. Lastly, we assessed the feasibility of ICH in terms of electrophysiology (EMG and
ECG). We first proceeded with the measurement of EMG signals using ICH to confirm the feasibility of the
electrode for the recording of electrophysiological signals. In correspondence with the soft-hand gestures of
the volunteer, the SNR of the enveloped EMG data was 3.87[Figure 2J]. Furthermore, the ECG signal
measurement showed clear P-Q-R-S-T wave peaks, which are the typical shapes of ECG signal, confirming
the feasibility of ICH as a recording electrode [Figure 2K].

Fabrication and physicochemical evaluation of ICH-based soft and stretchable ECoG arrays
After establishing the feasibility of ICH as an electrode for implantable array devices, we proceeded with the
device fabrication. Through the stable injection property of ICH, soft and stretchable 4-channel ECoG array
fabrication was performed by simply extruding ICH along the 100 μm-thick molded PVDF-HFP substrate,
and the channels were purposely left open for the in situ injection of ICH for interface with the brain tissue
[Figure 3A]. Finally, the patterned soft and stretchable ECoG arrays were connected to a printed circuit
board (PCB) for an implant-ready 4-channel ECoG device [Figure 3B].

The completed 4-channel array device was first optimized in UTM to evaluate the PVDF-HFP substrate for
a soft and stretchable ECoG interface [Figure 3C]. The PVDF-HFP film with ICH (e.g., soft and stretchable
brain interface) endured an appropriate tensile strain of more than 400% with similar toughness (576.83
3
kJ/m ) compared to the pristine PVDF-HFP film (471.05 kJ/m ). Specifically, Young’s modulus of PVDF-
3
HFP shows a minimum value of around 0.2 MPa, which is ideal for tissue mechanical modulus
matching [87,88] . Therefore, these results indicate that PVDF-HFP is the most efficient substrate for brain
[89]
interfaces compared to the other widely applied current elastomeric substrates (e.g., PDMS and SEBS) .
Satisfying the aforementioned standards, PVDF-HFP was chosen as an appropriate substrate for a soft and
stretchable ECoG interface [Figure 3D].

The conformability of the soft and stretchable ECoG arrays due to the substrate deformation markedly
improved the contact area between the ICH electrode and the brain phantom model [Figure 3E]. Crucially,
impedance measurements at a varying range of frequencies in the PBS solution predicted the efficiency of
[90]
monitoring electrophysiological signals similar to those generated by living tissues [Figure 3F] . Unlike
purely electrical conductivity from metallic components, conductive hydrogels contain both ionic and
0
[39]
electrical conductivity , which results in a low impedance value of approximately 10 kΩ even in low
frequencies under 1 Hz . Our ICH showed similarly low impedance values (~6 × 10 kΩ at 10 Hz) and a
0
[91]
0
phase angle close to 0° (maximum -10° from long connection wire and -6° from short connection wire)
0
across all frequency ranges between 10 and 10 Hz . These results highlight the highly stable
[91]
5
electrophysiological performance of the hydrogel arrays for ECoG monitoring [Figure 3F and
Supplementary Figure 8].