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Page 8 of 19 Kim et al. Soft Sci 2023;3:18 https://dx.doi.org/10.20517/ss.2023.08

Feasibility of ICH in electrocardiogram and electromyogram
Both electrocardiogram (ECG) and electromyogram (EMG) measurements were performed as previously
reported [67,70,71] . For the ECG measurement, three sensing and reference electrodes were attached to the left
arm, right arm, and left foot, respectively, of a volunteer. Subsequently, the ECG signals were recorded using
a bio-signal amplifier (Bio Amp FE231; AD Instruments, Oxford, UK) and a data acquisition device (DAQ)
(PowerLab 8/35; AD Instruments, Oxford, UK). The recorded ECG signals were processed using a low-pass
filter of 1,500 Hz in accordance with the International Society of Electrophysiology and Kinesiology (ISEK)
standards.

For EMG measurements, the three electrodes were attached to the upper arm, elbow, and lower arm,
respectively. The EMG signals were recorded from flexion and extension of the forearm at intervals of 3 s
using the measurement setup comprising of an amplifier, DAQ, and low-pass filter.

Statistical analysis
All statistical analyses were performed by analysis of variance using the software Origin, and all of the
results are presented as mean ± standard deviation (SD). Differences are defined as ns for “not significant”
and significant at *P < 0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001. Unless stated otherwise, at least
three independent tests were performed for each result.

RESULTS AND DISCUSSION
Characterization of ICH
Conjugation of tyramine to HA was first conducted to synthesize HATYR. The results of UV-vis
1
[Supplementary Figure 1A] and H-NMR spectroscopy [Supplementary Figure 1B] indicated that the degree
of substitution for tyramine to HA was approximately 6.29% ± 0.31% and 7.24% ± 0.14 %, respectively.

For the gelation of HATYR and PEDOT:PSS into a soft hydrogel, the two polymers were mixed together in
water. Hydrogel is a highly porous and biocompatible structure with ionic conductivity and tissue-like
properties . Particularly, hydrogel-based implantable electrode arrays have their advantages in implantable
[72]
devices due to their ability to conformally interface with target biological tissues with matching mechanical
modulus. Moreover, hydrogels are intrinsically soft and water-rich, therefore allowing for injectability and
patternability, along with biocompatibility, when implanted in the body [73,74] .

Hydrogen bonds formed between hydroxyl groups in HATYR and the sulfonate group of PSS, with the
sulfonate group of PSS mainly acting as a hydrogen bond acceptor to crosslink with the phenol group of
tyramine [Figure 2A]. This was first observed in the FT-IR spectrum, where the O-H , C-C, and C=C
[75]
peaks at 3439, 2936, and 2866 cm , respectively, increased as the concentration of HATYR increased
-1
[Figure 2B, left]. In addition, the C=O carboxyl peak at 1,736 cm indicates that the HA backbone also
-1
participates in ionic crosslinking with PEDOT:PSS. More importantly, the C=O amide shifted from 1,632
cm to 1,654 cm as the concentration of HATYR increased, indicating that the amide bond between the
-1
-1
tyramine and the HA backbone stretched due to hydrogen bonding at the phenol end of tyramine
[Figure 2B, right] .
[76]
The chemical difference between HATYR/PEDOT:PSS and ICH, that is, the presence of glycerol, was
evaluated in the FT-IR spectrum, and the disappearance of C=C and C-O-C of PEDOT at 1,550 and
[77]
[78]
1,490 cm , respectively, was attributed to the broken bonds between PEDOT and PSS due to glycerol.
-1
However, -SO 3 -[77,79] appeared at 1,296 and 1,180 cm , which indicated stretching of the sulfonate due to the
-1
covalent crosslinking of PSS with glycerol as the sulfonic acid group of PSS and the hydroxyl group of