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Kim et al. Soft Sci 2023;3:18 https://dx.doi.org/10.20517/ss.2023.08 Page 5 of 19

Design of HATYR/PEDOT:PSS hydrogel and ICH
HATYR/PEDOT:PSS hydrogel was prepared by simply mixing different weights (0.5, 1, 2, and 3 w/v %) of
HATYR with 1.1% of PEDOT:PSS solution using a vortex mixer. Then, the ICH was prepared by adding
glycerol to the HATYR/PEDOT:PSS hydrogel at a 2:1 molar ratio to the free PSS moles unlinked to the TYR
moieties. Immediately after the addition of glycerol, the hydrogel was thoroughly mixed to form the final
ICH formulation.

Physicochemical evaluation of ICH
To investigate the chemical interactions in the prepared hydrogels, freeze-dried HATYR, PEDOT:PSS,
HATYR/PEDOT:PSS, and ICH were observed by Fourier transform infrared (FT-IR) spectral analysis
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(TENSOR27; Bruker, Bremen, Germany) using the KBr method at a wavenumber resolution of 4 cm of 32
scans for each spectrum.

Additionally, to examine the morphological changes of the hydrogel, freeze-dried HATYR, PEDOT:PSS,
and HATYR/PEDOT:PSS, a field-emission scanning electron microscopy (FE-SEM) (Jeol JSM-IT800,
Tokyo, Japan) was used. In addition, the specific intensities of sulfur elements were further evaluated using
energy-dispersive X-ray spectroscopy (EDS).

Rheological measurements
For the rheological examination of all hydrogel samples, a rheometer (Discovery Hybrid Rheometer, TA
Instruments, New Castle, DE, USA) was used. At the settings of 1% strain from 0.01 to 10 Hz, a frequency
sweep test was performed to determine the storage modulus, loss modulus, and damping factor of the
hydrogels, denoted as G', G", and tan δ, respectively. For the measurement of shear viscosity of the
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hydrogels, an oscillation test in the shear rate range of 0.01 to 100 s was performed. The self-healing
behavior of ICH was observed in the rheometer at oscillating strains of 0.5% and 1,000% for every 180 s at a
1 Hz oscillation frequency.

Injectability and printability characterization of ICH
The injectability of the hydrogels prepared with varying concentrations (0.5, 1, 2, and 3 w/v %) of HATYR
was observed using a 30 G needle in a 1-mL syringe. Each hydrogel was tested for extrudability and
evaluated using the frequency sweep and oscillation test methods of rheological measurements.

The feasibility of 3D printing was observed using ICH. First, four paths of the electrode array with a total
area of 11 mm × 7 mm were designed using the Blender software and exported as a .stl file. The file was then
sliced in the NewCreatorK software as a .gcode file. Then, a 15-mL syringe filled with the hydrogel
formulation was mounted onto a 3D printer (Dr. INVIVO 4D2; Rokit Healthcare, Seoul, Korea) and
printed at a printing speed of 1 mm/s.

Dissolution and conductive properties ICH
The dissolution resistance of ICH due to addition of glycerol was demonstrated by submerging HATYR/
PEDOT:PSS and ICH in 1% phosphate buffer solution (PBS) for 30 min. Photographs were taken to
qualitatively observe changes in the morphology of the injected hydrogels. Furthermore, normalized
resistivity and conductivity calculations of ICH were performed while soaking in a PBS buffer from 0 to 16
h to observe the electrical durability of ICH from dissolution.

Strain-dependent conductivity tests were performed to measure the electrical performance of glycerol using
TM
the following previously reported protocol : a piece of Ecoflex 00-30 film was attached to the Motorizer
[66]
X-translation auto-stretching stage (Jaeil Optical System, Incheon, Korea). The stage was preprogrammed to