Page 165 - IJB-10-5

International Journal of Bioprinting Nozzle optimization for multi-ink bioprinting

ranging from 0.5 to 2.0 wt% concentration flowed from switching phenomenon. The Se for the acute-angle nozzles
Inlet 1 into the conjunction area, which was already filled (15°, 45°) was higher than that for the obtuse-angle nozzle
with SA solution within the same concentration range. The (75°) (Figure 2C, depicted in white bars). Furthermore, the
Se was evaluated using the method previously described. Se of the T-junction nozzle (90°) was higher than that of
the obtuse-angle nozzle. The highest switching efficiency
2.4.5. Evaluation of the Se of the nozzle developed via recorded was 0.28 at a 15° conjunction angle, while the
numerical simulation lowest was 0.02 at 75°. In obtuse-angle nozzles, the fluid
Based on the results from Section 2.2.6., the same nozzle was proposed to flow along the outlet wall, taking a longer
designs were replicated, and their Se was measured using time to fill the conjunction area, thereby resulting in a
the method previously described to verify the consistency lower Se.
of the numerical simulations. SA solutions with different
viscosities (0.5 and 1.0 wt%) flowed into the conjunction The same analysis was conducted experimentally
area of these nozzles, and the Se for each was calculated. with nozzles of different conjunction angles (Figure 2B).
These results were compared with the Se obtained from the The experimental results corresponded well with the
T-junction nozzle to assess performance differences. simulation results (Figure 2C, shown by black bars). The
Se of the obtuse-angle nozzle (75°) was 0.03 ± 0.00, while
2.4.6. Multi-ink printing of hydrogel structure with the Se of acute-angle nozzles (15°, 45°) and the T-junction
different viscous inks nozzle (90°) were 0.25 ± 0.02, 0.10 ± 0.02, and 0.21 ± 0.04,
Line structures utilizing two types of SA-Ph inks were respectively, all higher than that of the obtuse-angle nozzle.
printed using the nozzle designed through numerical The discrepancies between the simulation and experimental
simulation. The two inlets of the nozzle were connected results were likely arise from the fluid model used. In the
to syringe cartridges containing SA-Ph inks dyed pink simulation, the power law model was employed to describe
and green. SA-Ph ink with pink dye was first extruded fluid behavior due to its simplicity. This model has been
from Inlet 1 at a rate of 2.71 × 10 cm /s, forming a line widely used for modeling SA solutions. 42,43 Although the
3
−2
structure through the nozzle. After stopping the flow from model is sufficient for SA solutions, the Herschel–Bulkley
Inlet 1, SA-Ph ink with green dye from Inlet 2 was similarly model can provide higher accuracy in reproducing
extruded at 2.71 × 10 cm /s. The extruded inks underwent liquid behaviors when applied to various types of liquids,
3
−2
gelation upon exposure to visible light (λ = 450 nm). including Bingham and Newtonian fluids. 44,45 This study
Subsequently, line structures featuring alternating demonstrated that numerical simulation can effectively
colors were printed, and images of the transition zone reveal the effect of the shape of the conjunction area in a
between the green and pink inks were captured. These single nozzle on switching behavior using Se.
images were analyzed using the image processing software 3.2. Effect of viscosity on switching efficiency
ImageJ (version 1.53a). The red color intensity along the We also hypothesized that viscosity would affect the
printed line was plotted against its length. The transition switching behavior. The effect of the viscosity of the SA
22
length is the segment where the red intensity rapidly solution on switching efficiency within the nozzle was
changes, as illustrated in Figure 1E.
investigated. The combinations of inks are depicted in
2.5. Statistical analysis Figure 3A. The results showed that viscosity is an important
Data is presented as the mean ± standard deviation. factor in switching. In numerical simulations (Figure
Statistical analysis was performed using analysis of variance 3B, shown by white bars), the Se was lower when a low-
(ANOVA) followed by Tukey’s post-hoc comparison to viscosity ink (0.5 wt%) flowed against a high-viscosity ink
assess differences between groups. Statistical significance (1.0 wt%) compared with when a high-viscosity ink flowed
was set at p < 0.05. All analyses were conducted using Excel against a low-viscosity ink (Figure 3C, shown in I and II).
(ver16.79, Microsoft, WA, USA). Notably, the ink did not switch even after 20 s when a 0.5
wt% SA solution flowed against a 2.0 wt% SA solution in
3. Results and discussion the conjunction area (Figure 3B, labeled as N).
3.1. Effect of conjunction angle on switching The same analysis was conducted experimentally
efficiency (Figure 3B, shown by black bars). The Se was higher
First, we hypothesized that the conjunction angle of the when a high-viscosity ink (1.0 wt%) flowed against a
single nozzle would be a crucial factor in the switching low-viscosity ink (0.5 wt%) than when a low-viscosity
behavior of bioink. The effect of the conjunction angle on ink flowed against a high-viscosity ink. Furthermore, the
Se was investigated through numerical simulations (Figure 0.5 wt% SA solution did not switch against the 2.0 wt%
2A). The conjunction angle significantly impacted the ink within 20 s. The experimental results closely matched

Volume 10 Issue 5 (2024) 157 doi: 10.36922/ijb.4091