Page 124 - Read Online

Page 6 of 19 Kim et al. Soft Sci 2023;3:18 https://dx.doi.org/10.20517/ss.2023.08

perform repeated cyclic strain tests (100 times) from 0 to 100%. The sample was injected in lengths of 15
(ø = 3 mm) onto the stretching stage, followed by wiring an end-to-end line with cables to connect to an
inductance/capacitance/resistance meter to determine the electrical performance of the stretched hydrogels.
The resistance-strain values were obtained and evaluated using Origin software.

Preparation and characterization of brain surface interface
Next, 1 g of PVDF-HFP was dissolved in 20 mL of acetone and stirred overnight. The dissolved PVDF-HFP
was then poured onto a 100 mm × 100 mm square glass coated with low-density polyethylene. The solution
was left to dry for 6 h to fabricate a substrate with a thickness of 100 μm. Then, mechanical tests of the
prepared films were analyzed using a universal tensile machine (UTM) (Instron 34SC-1; Norwood, MA,
[67]
USA), as previously reported .
A previously designed four-electrode array mold was 3D-printed using Ultimaker S7 (Ultimaker, Utrecht,
Netherlands) with acrylonitrile butadiene styrene (ABS) as the printing material. Then, 100-μm thick
PVDF-HFP was cut into a cuboid with a length and width of 13 mm × 9 mm, respectively. The printed four-
electrode array mold was then gently stamped onto the cut PVDF-HFP film to leave patterns on the
substrate. Then, holes at the end of each array were made using a 1-mm biopsy punch. Finally, ICH was
injected along the patterned substrate, leaving approximately 200 μm (diameter) of ICH along 7-mm long
outer lines and 2-mm long inner lines.

The electrochemical impedance of the electrode array device
Electrochemical impedance spectroscopy (EIS) was performed by utilizing a potentiostat (SP1, ZIVE Lab
Co., Ltd., Seoul, Korea) according to a previously reported method . A saturated calomel electrode was
[68]
used as the reference, and a Pt plate (RDE0021, AT Frontier Co., Ltd., Anyang, Korea) was used as the
counter electrodes. With a reference and counter electrode soaked in PBS, a soft electrode was gently placed
on the surface of the solution. The impedance of the devices according to frequency was profiled for a range
of 100 kHz to 10 Hz using a 10 mV input signal.
rms
Conformal contact evaluation
To evaluate the characteristic conformal contact ability of our electrode array device, fully dissolved 1 w/v %
agarose gel was poured over a brain mold to fabricate brain-like grooves and fissures. The device was then
mounted on the agarose brain to observe the conformal contact. The device was then left overnight to
observe the degree of conformal contact according to the curves of the agarose brain phantom model.

In vitro cell viability assay
The cell viability assay was performed in accordance with the ISO 10993-5:2009 in vitro cytotoxicity
evaluation and previously tested procedure. First, a HT22 cell suspension (50,000 cells per well) was seeded
in a 24-well plate with 1 mL of Dulbecco’s modified Eagle’s medium supplemented with 1% sodium
pyruvate, 10% fetal bovine serum, and 1% penicillin/streptomycin and preincubated at 37 °C and 5% CO
2
for 24 h until 80% confluency was achieved. Then, 100 μL of UV-irradiated ICH and a 20 mm × 20 mm
PVDF-HFP film were placed in different respective wells and immersed in 1 mL of the supplemented
medium before culturing at 37 °C and 5% CO for 24 h. Finally, the cells were washed with Dulbecco’s
2
phosphate-buffered saline (DPBS), and a cytotoxicity assay was performed following the staining of live cells
with 0.5 mL of calcein AM solution (2 μM) and dead cells with 0.5 mL of ethidium homodimer-1 solution
(4 μM). Fluorescent images were obtained using a Leica DMi 8 fluorescent microscope (Leica, Wetzlar,
Germany). Using ImageJ, cell viability (%) was calculated as the percentage of the live cells to the total
number of cells, including dead cells.