Page 354 - IJB-9-5

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

dried hydrogels by the wet hydrogels; dried hydrogels were and fine printing structure. The literature showed a high
all prepared being frozen and dried in a vacuum freeze resolution (~60 μm) of the resultant printed structure with
dryer. The water content of the printed PVA/TA/PAA high stretchability (stretched up to over 800%). However, it
hydrogels with the mass ratio of 1:0.5, 1:1, and 1:2 was required about 10–20 min after printing for the curing and
about 70.8%, 92.4%, and 83.5%, respectively (Figure S2 cleaning time, which is the typical disadvantage of digital
in Supplementary File). The results showed less increment light processing printing that leads the process complicated
of swelling ratio in higher water content, and these can and hampers the practicality. Compared to this printing
be attributed to the crosslinking density and the internal process, the proposed PVA/TA/PAA hydrogel ink can be
hydrogel network. In particular, PVA/TA /PAA hydrogel efficiently printed in 3D and retain its structure without
1:1
showed high water content and non-swellable property post-process while maintaining mechanical properties
after immersing DI water. It indicates that hydrogel can including toughness, stretchability, and self-healing
operate stably even in a wet environment in the application ability. Furthermore, regarding the resolution of hydrogel,
of bioelectronics. PVA/TA/PAA hydrogel has a high printing performance
compared to other studied printable multi-functional
3.4. Printability and 2D-, 3D-printing performance hydrogels .
[37]
We utilized the rheological properties of the hydrogel ink
to print a certain architecture for evaluated PVA/TA mass 3.5. Mechanical and self-healing properties of
ratios. Figure 3A presents the results of printing fidelity PVA/TA/PAA hydrogel ink
for the various hydrogel ink compositions. All inks were To minimize the mismatch between human tissue and
printed using the same design with a 600-μm diameter electronic devices, it is important for the mechanical
nozzle, and the PVA/TA /PAA hydrogel ink produced properties of the hydrogels used to be similar to those of
1:1
uniform and precise results. The filament extruded from human tissue while still being strong enough for long-term
the nozzle was uniform, achieving a width similar to that bioelectronics functionality . The mechanical properties
[38]
of the nozzle, and the designed structure was precisely of the hydrogel inks with different PVA to TA ratios
printed with a sharp edge. Its high viscosity and moduli (1:0.5, 1:1, and 1:2) were evaluated through tensile tests.
allowed it to maintain its structure after being extruded Figure 3E shows the typical stress–strain curve for each
from the nozzle. In contrast, the PVA/TA 1:0.5 /PAA and ratio of the printed hydrogel. It is apparent that there is a
PVA/TA /PAA hydrogel inks had low printing accuracy, trade-off between toughness and stretchability; as the TA
1:2
with the filaments not retaining their shape and the printed ratio increases, elongation increases but maximum tensile
structures losing their details and sharpness. As explained strength decreases (Figure S2 in Supplementary File). For
above, TA did not form dense crosslinks owing to the low low TA ratios (PVA/TA 1:0.5 /PAA), crosslinking sites cannot
concentration of TA and not homogeneously mixed in be sufficiently formed in the hydrogel network, resulting in
ink due to too high a concentration of TA. Consequently, poor stretchability. In addition, for high TA ratios (PVA/
when the ink was heated for extrusion from the nozzle and TA /PAA), the hydrogel network does not homogeneously
1:2
the weak H-bonds were broken, the bonds were unable to mix with TA, causing it to aggregate and reduce toughness
rapidly reform the H-bond to maintain a robust printed (Figure S3 in Supplementary File). The PVA/TA /
1:1
structure. The high-resolution printing capability of the PAA hydrogel ink was found to have the most balanced
PVA/TA /PAA hydrogel was demonstrated by printing 2D mechanical properties, offering both high toughness and
1:1
shapes through 400-, 200-, and 100-μm diameter nozzles stretchability. Its maximum tensile strength and elongation
(Figure 3B) and mesh structures using a 100-μm diameter at break were 45.6 kPa and 656%, respectively. Then, the
nozzle (Figure 3C). The SEM images confirmed the high mechanical properties of the printed PVA/TA /PAA
1:1
pattern fidelity of the printed scaffolds, with a continuous hydrogel ink were compared to those of the bulk hydrogel
filament and precise grid. Additionally, the favorable (Figure 3F). As shown in Figure 3G, the maximum
rheological properties of the PVA/TA /PAA hydrogel ink tensile strength of the bulk and printed hydrogel is not
1:1
allowed us to fabricate multi-layered 3D structures through significantly differed and showed both high toughness
layer-by-layer stacking with a 600-μm nozzle, which was with stretchability. Despite the heat applied to the hydrogel
strong enough to support their own weight (Figure 3D). In for printing, the printed hydrogel showed high toughness
particular, all printing procedures were conducted without and elongation at break up to 600%. However, Young’s
any post-processing but showed high resolution and 3D modulus of printed hydrogel was decreased compared to
printability. Recently, Zhou et al. presented 3D printing the bulk hydrogel. This is attributed to the dehydration of
[36]
of a UV-curable elastomer with digital light processing. 3D hydrogel after printing (Figure S4 in Supplementary File).
printing with digital light processing is a rapidly developing When a hydrogel is dehydrated, as its volume decreases,
area since it enables the hydrogel with high resolution the hydrogel shrinks and the network of the hydrogel

Volume 9 Issue 5 (2023) 346 https://doi.org/10.18063/ijb.765