Page 356 - IJB-9-5

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

together. The samples were brought into contact, and after our proposed hydrogel in combination with additional
3 min, they had self-healed to form an integrated structure CNT fillers. Compared to previously reported hydrogel
and could withstand stretching, and also the printed with multi-functionality (Table S1 in Supplementary File),
PVA/TA /PAA hydrogels displayed self-healing ability the proposed hydrogel, which implemented the features of
1:1
within 3 min (Figure S5 in Supplementary File). Then, conductivity, toughness, stretchability, self-healing ability,
by comparing the original maximum (N) tensile strength and 3D printability in one platform showed excellent
and the tensile strength after self-healing (N’), the self- performance especially in self-healing with rapid recovery
healing efficiency of the bulk hydrogel, (N’/N) ×100 (%), and printability with high resolution.
was calculated at various time points (Figure 3I). It
revealed a 30% recovery in 30 s, and the healing efficiency CNT functionalized with a carboxyl group was used
increased dramatically as the time increased, showing self- since it is water soluble and can be distributed within our
healed 80% after 5 min. Figure 3J is the results of strain– PVA, TA, and PAA-based hydrogel network via H-bond
[42]
stress curve tested with the hydrogels after self-healing through their carboxyl and hydroxyl groups . As shown
times at 30, 60, 180, and 300 s. As previously explained, in Figure 4A, the carboxyl functionalized CNT with DI
this rapid autonomous self-healing ability is a result of water was sonicated and poured into the PVA/TA hydrogel
the weak H-bond formation of TA in the PVA/TA/PAA at a mass ratio of 1:1. PAA was then introduced into the
network and the disulfide bond within PAA, which enables PVA/TA/CNT hydrogel to obtain the PVA/TA/PAA/
the breakage and reformation of reversible bonds . CNT ink. To prove whether the CNT was mixed into the
[29]
Afterward, the maximum tensile tests after self-healing PVA/TA/PAA hydrogel without significant impairment of
in 180 s with different value of pH and temperature were existing features, the rheological properties of an ink with
also investigated (Figure S6 in Supplementary File). The CNT concentration of 6 mg/mL were investigated in terms
pH did not significantly affect the self-healing property, of its printability and mechanical properties. It exhibited
which is due to the non-swellable characteristics of PVA/ a shear-thinning effect in which viscosity decreased as
TA /PAA hydrogel, as confirmed in the previous swelling the shear rate increased and both G’ and G’’ decreased
1:1
ratio test. Meanwhile, when the strain–stress curve tests of monotonously as temperature increased, with similar
the hydrogel according to temperature were conducted, it values of G’ and G’’ (Figure S7 in Supplementary File).
showed that the higher the temperature applied, the lower Based on these rheological measurements, the printability
the tensile strength of the hydrogel, but the better the of PVA/TA /PAA/CNT hydrogel inks was verified by
1:1
stretchability. It elongated to 890% when heated to 85°C. examining printing fidelity using nozzles of various
As shown in Figure 2D of the rheological property, when diameters and successfully printed a 3D structure through
the temperature is applied, the storage and loss modulus layer-by-layer stacking (Figure S8 in Supplementary
of the hydrogel decreases. Hydrogels with a lower storage File). The mechanical properties of bulk PVA/TA /PAA/
1:1
and loss modulus tend to be more stretchable, as they can CNT hydrogels with CNT concentrations of 0 (without
dissipate less energy and withstand larger deformation. CNT), 1, 3, 6, and 9 mg/mL were also verified as shown in
Figure 4B. The measured stress–strain curves represented
3.6. Synthesis process and characterization of PVA/ that as CNT incorporation increased, the maximum tensile
TA/PAA/CNT hydrogel ink strength improved while the elongation at break was
Electrically conductive hydrogels using various types reduced. Although the stretchability of the hydrogel was
of conductive materials such as carbon nanotube-based weakened compared to without CNT hydrogel, the results
and graphene-based materials have been developed for showed a consistent tendency that the tensile strength was
bioelectronics . However, one of the main challenges enhanced according to the CNT concentration. It indicates
[39]
of the incorporating such nanoparticles or nanotubule- that the CNTs were homogeneously dispersed inside the
doped hydrogels is achieving a homogeneous dispersion PVA/TA/PAA hydrogel network without breaking the
of the particles within the hydrogel matrix . Aggregation crosslinking structure. In addition, it presents that the
[40]
of the particles hinders the dense crosslinking within the mechanical properties can be tuned by the concentration
hydrogel matrix, which leads to poor mechanical properties of CNTs. Then, the self-healing behavior was further tested
and rheological properties, thus preventing the tough, and displayed in Figure S9 (Supplementary File). The
self-healing, and printable hydrogel implementation. In samples were attached for 3 min (180 s), and they showed
addition, some nanoparticles, such as certain types of metal successfully self-healed to form an integrated structure and
nanoparticles, have been shown to have toxic effects on could withstand stretching. Compared to without CNT
tissues , therefore not suitable for biomedical applications. hydrogel, as expected, it showed higher tensile strength
[41]
To address these problems, we attempted to realize a tough, while the value of elongation at break decreased. Then,
self-healing, and printable conductive hydrogel ink using to investigate the potential applicability of PVA/TA/PAA/

Volume 9 Issue 5 (2023) 348 https://doi.org/10.18063/ijb.765