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International Journal of Bioprinting Nozzle optimization for multi-ink bioprinting

directed through each of the three types of asymmetrical SA ink (Figure 5C; T-junction nozzle, category 1, white
nozzles (Figure 4C; labeled I–III). The Se values were bar). Conversely, all proposed asymmetrical nozzles (I–III)
compared with those of a symmetrical or T-junction nozzle. demonstrated shorter transition lengths compared with the
The results showed that the asymmetrical nozzles exhibited T-junction nozzle under the same conditions (p < 0.05).
better Se than the T-junction nozzle. Asymmetrical nozzles Specifically, the transition lengths for nozzles I, II, and III
I, II, and III achieved Se values of 0.46, 0.37, and 0.34, were 8.5 ± 2.5, 9.2 ± 2.3, and 6.9 ± 2.6 mm, respectively,
respectively, (Figure 4D; labeled I–III, category 1, white when 0.5 wt% SA ink flowed against 1.0 wt% SA ink (Figure
bars) when low-viscosity ink flowed against high-viscosity 5C; nozzles "I–III, category 1, black bars). When 1.0 wt%
ink, outperforming the T-junction nozzle. Conversely, the SA ink flowed against 0.5 wt% SA ink, the transition lengths
Se values for the three asymmetrical nozzles were 0.34, were 6.9 ± 2.6, 10.9 ± 4.3, and 7.5 ± 1.3 mm, respectively
0.20, and 0.83, respectively, when high-viscosity ink flowed (no significant difference, Figure 5C; nozzles I–III, category
against low-viscosity ink (Figure 4D; labeled I–III, category 2, white bars), which were comparable to those observed
2, white bars), which were comparable to or higher than with the T-junction nozzle. These results indicate that the
those of the T-junction nozzle. These simulation results asymmetrical nozzles facilitated more efficient switching
demonstrated that asymmetrical nozzles could switch inks of lower-viscosity ink against higher-viscosity ink with
with higher efficiency than T-junction nozzles. shorter transition lengths. Skylar–Scott et al. created a
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Experimentally, the Se values of asymmetrical nozzles single-nozzle printing system with high fidelity by utilizing
I, II, and III were 0.39 ± 0.08, 0.33 ± 0.06, and 0.30 ± 0.04, a pressure-controlling system that required experimental
respectively (Figure 4D; labeled I–III, category 1, black adjustments for each ink, which features a complex control
bars) when low-viscosity ink flowed against high-viscosity system. Considering that most bioinks have low viscosity
ink, again surpassing the performance of the T-junction and their behavior is significantly influenced by pressure,
nozzle. However, when high-viscosity ink flowed against a complex system is necessary to manage such inks in
low-viscosity ink, the Se values were 0.35 ± 0.081 (nozzle microfluidic channels, such as bioprinting nozzles. Thus,
I, statistically comparable to the T-junction nozzle), our proposed nozzle, which effectively leverages the offset
0.28 ± 0.10 (nozzle II, no significant difference from the of effects on viscosity and flow direction, offers a simpler
T-junction nozzle), and 0.65 ± 0.14 (nozzle III), showing and more efficient solution for controlling the switching of
that performance was comparable to or better than that of different viscous bioinks.
the T-junction nozzle (Figure 4D; labeled I–III, category 3.5. Investigation and proposal of nozzle design for
2, black bars). These experimental results aligned well further improvement in single-nozzle printing
with the simulation findings, confirming that our strategy In this study, we primarily focused on the effects of the
to balance the effects of viscosity and flow direction is nozzle shape and ink viscosities on Se. It is worthy to
effective in achieving high-efficiency ink switching. note that switching behavior could also be influenced
3.4. Multi-ink printing by a repertoire of factors, which should be considered in
Multi-ink printing was tested using the proposed further numerical simulation-based investigations with
asymmetrical nozzles to switch between inks of different the aim to enhance the resolution of single-nozzle multi-
viscosities. High-viscosity ink (1.0 wt% SA) flowed ink bioprinting. For example, when switching 0.5 wt%
horizontally toward the conjunction area, while low- SA ink against 2.0 wt% SA ink in various nozzles (Figure
viscosity ink (0.5 wt% SA) flowed vertically (Figure 5A). 6A), the 0.5 wt% ink did not effectively switch against the
The asymmetrical nozzle designed through simulation was 2.0 wt% ink in the T-junction, asymmetrical, and cross
fabricated (Figure 5B), and line structures were printed nozzles, resulting in low Se (Figure 6B). The simulations
using two different viscous inks. Transition lengths were revealed that the liquid remained on one side of the nozzle
measured to evaluate the effectiveness of ink switching. (Figure 6A, indicated by a red circle), contributing to a
For comparison, a standard T-junction channel was used low Se in both T-junction and asymmetrical nozzles. To
for multi-ink bioprinting. The widths of the transition address this, a cross nozzle design was proposed, allowing
areas of printed line structures were 1.8–2.1 mm, showing lower-viscosity ink to be extruded from the top of the
no significant difference (Figure S4A, Supporting conjunction area, with a higher-viscosity ink arriving from
Information). Figure S4B, Supporting Information, both sides of the nozzle.
presents the transition areas of printed line structures. In Compared to the T-junction and asymmetrical nozzles,
the T-junction setup, the transition length was 23.2 ± 4.2 the cross nozzle demonstrated improved switching
mm when 0.5 wt% SA ink flowed against 1.0 wt% SA ink between 2.0 and 0.5 wt% SA inks (Figure 6B). Moreover,
(Figure 5C; T-junction, category 1, black bar), and it was transition lengths were measured in the cross nozzle
7.5 ± 1.3 mm when 1.0 wt% SA ink flowed against 0.5 wt% (Figure 6C) setup, where the transition length was 31.7 ±

Volume 10 Issue 5 (2024) 161 doi: 10.36922/ijb.4091