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

transmission. To accurately achieve the human–machine process revealed that resistance immediately recovered
interface, not only flexible, stretchable, and self-healing to the initial value after self-healing, which was possible
features but also adhesion capabilities play a very significant because the reversible network within the hydrogel allows
role. Recently, Iversen et al. presented a flexible, wearable rapid rearrangement of the conductive pathway of the
[44]
smart patch with Polydimethylsiloxane was defined as PDMS hydrogel. Moreover, conductive hydrogels are suitable
(PDMS) substrate and CNT electrode for pH and hydration for application in resistivity sensors owing to their high
sensing. Although they confirmed the pH and hydration sensitivity to mechanical deformation (Figure 5C). As
sensing ability, they did not feature adhesion functionality shown in Figure 5D, the relative resistance change ratio
which is crucial to wearable sensors. Thus, to enhance the upon strain change, ∆R(R-R )/R ×100 (%), was measured,
0
0
wearability of PVA/TA/PAA/CNT hydrogel, additional where the gauge factor (GF) is defined as (∆R/R ) ×100/ε.
0
surface functionalization via chemical treatment was treated. It presented a high GF of 1.356 for 0–300% strain and a
NHS was introduced on the PAA chain in hydrogel since GF of 4.457 for 300–400% strain, indicating its comparable
it has abundant carboxyl groups, which enable the surface performance to the previously reported hydrogel-based
having amine groups including tissue to covalently bond . strain sensor [46,47] .
[45]
After the contact between the NHS-treated hydrogel and the Furthermore, the change in resistance of the printed
tissue, the two surfaces are strongly bonded after pressing PVA/TA/PAA/CNT hydrogel ink was evaluated in response
for 5 min (Figure 4D). As shown in Figure 4E, the PVA/TA/ to different mechanical inputs, including compression,
PAA/CNT hydrogel was successfully attached to porcine tension, and stretching. The relative resistance change
skin, and also PVA/TA/PAA hydrogel showed strong ratio upon deformation was measured, and the results
adhesion (Figure S10 in Supplementary File). for compression, tension, and stretching are shown in
An in vitro biocompatibility test of the hydrogel was Figure 5E. The resistance signal was read and recorded by
also conducted to verify its toxicity for reliable applications an electrometer, while the printed circuit was in the bending
to humans. PVA/TA/PAA/CNT hydrogels with NIH 3T3 and stretching conditions. As in the earlier experiment,
fibroblasts were incubated in a cell culture media for the PVA/TA/PAA/CNT hydrogel sample was printed
5 days, and a live/dead assay was carried out after 1, 3, with dimensions of 100 mm (height) × 200 mm (width)
and 5 days in culture. As shown in Figure 4F, most cells using a 600-μm-diameter nozzle. The compressive, tensile
remained alive (green fluorescence) that no obvious cell bending, and stretching resistance change ratios reached
death was observed on the hydrogel, suggesting excellent approximately 30%, 10%, and 100%, respectively. From the
biocompatibility. The cell viability tests also showed that electrical performance investigation, we conclude that the
no significant differences existed between fibroblasts with hydrogel could be utilized as a strain sensor to accurately
the hydrogel and the control group for 5 days (Figure 4G), detect body movements.
and as expected, PVA/TA/PAA hydrogel also showed non-
toxicity (Figure S11 in Supplementary File). We confirmed 4. Conclusion
that the proposed hydrogel is biocompatible with tissue In this study, we developed a 3D-printable, tough, self-
and is a safe material for the application. healing, and electrically conductive hydrogel made from
PVA, TA, and PAA. The PVA/TA/PAA hydrogel ink
3.8. Applications of 3D-printed hydrogel: LED has improved mechanical properties and self-healing
lighting test and strain sensing capabilities compared to traditional hydrogels due to its
Figure 5A presents photographs of the circuits printed double network structure based on reversible hydrogen
with PVA/TA/PAA/CNT hydrogel. The printed circuits bonds. TA acts as a crosslinker, forming weak hydrogen bonds
enable visualization of the proposed hydrogel ink’s on PVA chains, while PAA forms strong hydrogen
electrically conductive property through the illumination bonds to create a double network. The PVA/TA /PAA
1:1
of an LED. With a printed circuit and power source, the ink has impressive toughness, stretchability, and self-
LED bulb was successfully switched on, and when the healing properties. In addition, the developed hydrogel
circuit was physically cut, the LED turned off. However, ink showed excellent printability with a 3D-printing
when the split circuit was put together, the LED turned on structure and high resolution (~100 μm), compared to
instantly through self-healing of the printed hydrogel, and existing tough and self-healing hydrogels that are not
in stretching, the brightness of the light was maintained. 3D-printed and have low resolution. Rheological tests
Then, as shown in Figure 5B, electrochemical tests were were performed to assess the suitability of the hydrogel
conducted to evaluate the relation between electrical ink for 3D printing and showed that the PVA/TA /
1:1
resistance and mechanical deformations. The real-time PAA ink has ideal properties for precise 2D/3D printing.
resistance measurement of the cutting and contacting Additionally, we successfully added CNT fillers to the

Volume 9 Issue 5 (2023) 350 https://doi.org/10.18063/ijb.765