Page 119 - IJB-9-2

International Journal of Bioprinting Photocurable pullulan-based bioink for 3D printing

The role of crosslinkers on the formation of the gel of lyophilized Pul-NB hydrogels was characterized by
network structure was also assessed through rheological scanning electron microscopy (SEM; Figure 6A). The
studies, and the results are presented in Figure 5B. statistical data of pore diameter were obtained using Nano
Although the G′ of Pul-NB hydrogel cross-linked by Measurement software (Figure 6B). All of the samples have
HDT was slightly higher than that cross-linked by DTT, 3D inner connected macroporous structure with ordered
the DTT was considered the preferable crosslinker in this pore wall. Large pore size distribution (33–170 μm) and
experiment. Due to the poor solubility of HDT in water, thin wall part were the structural features of 5% Pul-NB,
the Pul-NB hydrogels cross-linked by HDT underwent which indicate the poor stability of gel morphology of
an uneven gelation reaction. As shown in Figure S1, the 5% Pul-NB. In contrast, the 10% Pul-NB hydrogel had
Pul-NB hydrogel cross-linked by HDT turned a non- a narrow pore size distribution of 13–65 μm according
transparent milky-white by UV irradiation. to the result of the statistical data of pore diameter. A

To understand the rheology of Pul-NB precursor higher solid content of the hydrogel had a denser network
solution, the viscosity of the Pul-NB precursor solution at structure, which was confirmed by SEM observation. The
different frequency (Figure 5C) and the time dependence of Pul-NB hydrogels presented structure with microchannels,
G′ and G″ of Pul-NB precursor solution with different solid which were conducive to the exchange of the substances,
content (Figure 5D) were performed. It could be found from for example, loading and release of drugs, swelling and
Figure 5C that the viscosity increased with the increase of deswelling of solvents, as well as nutrient diffusion of cell
[46,47]
the Pul-NB precursor concentration. For the determined culture medium . In general, the pore size of 8% and
concentration of Pul-NB precursor, the viscosity had a slight 10% (w/v) Pul-NB hydrogel was larger than the size of
change at a lower shear rate range (1–10 s ) and then remained cells, allowing for encapsulating cells in hydrogel. Since the
-1
constant, indicating the independence of shear frequency. recommended concentration of the GelMA, a commonly
It was observed that G′ of Pul-NB precursor solution was used photocurable hydrogel for 3D bioprinting, is 5% to
much lower than its G″ and remained stable throughout the 10% (w/v), 8% and 10% solid content were also appropriate
[44,45]
measurement time of 10 min (Figure 5D), indicating that the working solutions for cell encapsulation . The
Pul-NB precursor solution remained in the liquid state. characteristics of Pul-NB hydrogels can further expand
their applications in biomedical fields.
The Pul-NB precursor solutions with different solid
content were further irradiated by 405 nm UV light. 3.5. Water absorption and swelling properties of
Frequency-dependence of G′ and G″ of Pul-NB hydrogels Pul-NB hydrogels
with different solid content are shown in Figure 5E and The water absorption and swelling behavior of Pul-NB
F. It was also observed that the difference between Pul- hydrogels were investigated because the water absorption
NB precursor solutions and Pul-NB hydrogels were very properties of a hydrogel determine its potential applications.
significant. G′ of Pul-NB precursor solution after UV curing Figure 7A shows the changes of water-absorption rate of
was larger than G″ on the frequency in the range from 1 to Pul-NB hydrogels with different DS with increasing soaking
100 rad/s, indicating that the Pul-NB precursor solution time at 25°C. Due to the low degree of crosslinking, Pul-
turned into gel state after UV curing. In addition, the G′ NB 2 hydrogel was fractured after being dipped in water
of the hydrogel was increased with the increase of Pul-NB for 8 h. Pul-NB 3 and Pul-NB 4 hydrogels absorbed water
concentration, confirming that the solid content of precursor rapidly at the initial stage (0–10 h) and reached equilibrium
solution played a key role in the strength of the Pul-NB after 20 h. The equilibrium swelling values of Pul-NB 3
hydrogels. The hydrogels with desired strength can be and Pul-NB 4 were 12884% and 13160%, respectively.
obtained by adjusting the Pul-NB concentration. It is worth Figure 7B shows the changes in water-absorption rate
noting that the viscosity of Pul-NB precursor solution was of Pul-NB hydrogels with different solid content with
lower than the other photocurable bioprinting hydrogels. increasing soaking time. With increasing solid content,
Therefore, Pul-NB hydrogel could be sterilized by the easy the water-absorption rate of the hydrogels increased,
and simple membrane filtration of its precursor solution. where Pul-NB hydrogels with 8% and 10% solid content
However, for GelMA hydrogel, when the concentration of are comparable in water absorption. Owing to the dense
its precursor solution exceeded 5%, it would become too network structure that was beneficial for water storage,
viscous for filter membrane sterilization [44,45] . the Pul-NB hydrogels with 8% and 10% solid content
exhibited equilibrium water absorption values of 13160%
3.4. Morphology characterization of and 12962%, respectively. Figure 7C displays the photos
Pul-NB hydrogels of water absorption experiment of a freeze-dried hydrogel
In order to understand the effect of solid content on at different time point. The results of water absorption
the structure of hydrogel network, the morphology test indicated that the Pul-NB hydrogels were capable of

Volume 9 Issue 2 (2023) 111 https://doi.org/10.18063/ijb.v9i2.657