Page 350 - IJB-9-5

International Journal of Bioprinting 3D printing of tough and self-healing hydrogels

~15 kPa. It also had self-healing properties, being able to (Anton Paar GmbH, Austria), with a parallel plate
recover from a break within 5 min. Carbon nanotubes geometry, equipped with a 25-mm plate with a distance of
(CNTs) were incorporated into the PVA/TA/PAA hydrogel ~1 mm. The shear storage modulus (G′) and loss modulus
ink as nanofillers to improve its electrical conductivity, and (G″) were measured with temperature scans ranging from
the applicability of the PVA/TA/PAA/CNT hydrogel ink 25°C to 95°C at a constant shear strain of 1% and frequency
was confirmed through in vitro biocompatibility and tissue of 10 radian/s. The shear viscosity of the hydrogel was
adhesiveness with the chemical functionalization of the measured in steady-state flow at a logarithmic sweep of
hydrogel chain. Also, light-emitting diode (LED) lighting shear rates from 0.1 s to 100 s to investigate the shear-
-1
-1
tests and resistance change via a printed circuit with PVA/ thinning behavior at 85°C.
TA/PAA/CNT hydrogel ink showed the possibility of a
strain sensor. 2.4. 3D printing of hydrogel ink
All printing was performed using an EZROBO-5GX 3D
2. Materials and methods printer with an AD3300C dispenser (Iwashita, Japan),

2.1. Preparation of PVA/TA/PAA hydrogel ink equipped with a nozzle and temperature controller. During
To prepare hydrogel ink, the 4000 mg of PVA (Sigma- printing, ink was heated to 85°C using a temperature
Aldrich, St. Louis, MO, USA, 20% w/w, Mw 89,000– control system and syringe heating pad, and various
98,000) powder was added to deionized (DI) water, heated nozzle sizes (600-, 400-, 200-, and 100-μm size of nozzles)
at 90°C, and continuously stirred to obtain a transparent were used. Printing structures were designed using Ez-
solution. After 20 min, TA (Sigma-Aldrich, St. Louis, MO, EDITOR robot communication software. The diameters
USA) at different ratios (PVA: TA = 1:0.5, 1:1, and 1:2) was of the printed hydrogels were compared with those of the
added to the PVA solution and stirred for 2 h to obtain a printing nozzle using a field-emission scanning electron
homogeneous PVA/TA solution. The PVA/TA solution was microscope (FE-SEM, JSM-IT-500HR, JEOL, Japan) at
then poured into a mold, pressed to be spread thinly and an accelerating voltage of 15.0 kV with gold sputtering to
widely, stored in a refrigerator at -20°C for 8 h, and thawed enhance image contrasts.
at 25°C for 4 h to form a PVA/TA hydrogel. The PVA/TA 2.5. Mechanical characterization of hydrogel ink
hydrogel was dried in an oven at 37°C for 1 h and annealed The mechanical properties of the hydrogels were
at 100°C for 1 h to obtain a dry PVA/TA film. To form characterized using a tensile testing machine (MultiTest
the PVA/TA/PAA network, PVA/TA film was immersed 2.5-DV, Mecmesin, UK) with a 50 N load cell at a speed
in 45 mL of aqueous acrylic acid solution (30% w/w of 50 mm/min to determine their ultimate tensile strength
acrylic acid, 0.03% w/w N, N′-bis(acryloyl)cystamine, and elongation at break. The bulk hydrogel ink samples
and 0.15% w/w 2,2′-azobis(2-methylpropionamidine) were cut to a size of 200 mm (height) × 100 mm (width).
dihydrochloride in deionized water) for 2 h. The soaked The printed hydrogel ink samples of 200 mm (height) ×
hydrogel was heated at 70°C for 30 min to form the PAA 100 mm (width) were manufactured using the above-
network. To prepare the pure PVA hydrogel and PVA/ mentioned EZROBO-5GX 3D printer with a nozzle size
PAA hydrogel, we used a PVA hydrogel without TA but of 600 μm. For the tensile tests, at least three samples of
otherwise followed the same process.
each type were tested and averaged to determine their
2.2. Preparation of PVA/TA/PAA/CNT hydrogel ink properties.
The COOH functionalized CNT powder (Nano-lab, USA,
outer diameter of 30 ± 15 nm, length of 1–5 μm) was 2.6. Swelling ratio measurement
dispersed in 10 mL of DI water to concentrations of 1, 3, The hydrogels were soaked in DI water at room temperature
6, and 9 mg/mL. Each solution was sonicated at 450 rpm to determine their swelling ratio. All hydrogel samples
for 2 h using a tip sonicator (Qsonica LLC, Newtown, CT, were prepared with similar weights and the same pattern.
USA) to obtain a homogenous CNT solution. The CNT After swelling for the desired time, the swollen hydrogels
solution was added to the PVA/TA hydrogel and stirred were removed from water and weighed. The weights of the
for an additional 2 h while heating at 90°C. The resulting samples were recorded at 0, 30, 60, 120, 240, 480, and 960
CNT/PVA/TA hydrogel underwent freezing at -20°C and min after removal, and the swelling ratio was calculated
thawing at 25°C, and the same process was used to form using the following Equation I:
the PAA network. W W
Swelling ratio % t i 100 % (I)
2.3. Rheological characterization of hydrogel ink W t
The rheological properties of the hydrogel inks were where W and W are the weights of the initial and swollen
t
i
characterized using an MCR 102 Anton Paar rheometer hydrogel, respectively.
Volume 9 Issue 5 (2023) 342 https://doi.org/10.18063/ijb.765