Page 58 - IJB-8-2

CFD Assessment of Extrusion Bioprinting Parameters
rate for the cylindrical nozzle was 0.00014 kg/s at an 3.3. Dispensing pressure
inlet pressure of 0.05 MPa. Consequently, at equivalent Cells are exposed to various mechanical forces while
flow rates, cell damage was lower in a tapered needle moving through the nozzles; this generates internal
compared to a cylindrical one [27] . pressures that can damage the cells . These forces are
[7]
3.2. Nozzle diameter directly proportional to the dispensing pressure as it
determines the force with which the extruded material
As shown in Figure 5, for both tapered conical and is being pushed. Increasing the dispensing pressure
conical nozzles, our findings agree that as the nozzle also increases the MWSS, much like decreasing the
diameter increases from 0.1 mm to 0.5 mm, the MWSS nozzle diameter. Nair et al. suggested that the effect
[31]
decreases under constant pressure, possibly leading to of increasing the inlet pressure has a more prominent
greater cell viability. As shown in Table 3, the decrease negative effect on cell viability than nozzle diameter.
of the diameter from 0.5 mm to 0.1 mm increases the However, taking into consideration, the maximum
MWSS by approximately 30% for the tapered conical percentage increase in wall shear stress on increasing the
nozzle and by 25% for the conical nozzle at a given pressure by 5 times (Table 4) and decreasing the outlet
pressure. However, for the cylindrical nozzle, the diameter by a factor of 5 at constant pressure (Table 3),
MWSS is the lowest for nozzle diameter 0.1 mm, and our results do not conclusively show the more pronounced
the maximum shear stress increases as outlet nozzle effect of inlet pressure. At a given outlet diameter, our
diameter increases. In addition, while the cylindrical findings suggest that on increasing the pressure from 0.05
nozzle with a diameter of 0.3 mm has a lower shear stress MPa to 0.25 MPa, the increase in MWSS is highest in
than the cylindrical nozzle with a diameter of 0.5 mm, the cylindrical nozzle followed by the conical and finally
for pressures higher than 0.2 MPa, the wall shear stress tapered conical nozzle. Furthermore, the variation of the
is greater in the cylindrical nozzle with outlet diameter overall percentage increase in MWSS is lower for tapered
0.3 mm. One explanation for these deviations from conical and conical nozzles as compared to the cylindrical
the expected result for the cylindrical nozzles can be nozzle of different outlet diameter, as shown in Table 4.
accounted to the choice of inlet condition as constant In addition, a very low inlet pressure will result in
pressure. There is a significant increase in the flow rate, no or little bioink being deposited, whereas a very high
as evident in Figure 6, to maintain a constant pressure pressure will result in excess bioink being deposited . It is
[7]
difference, which leads to the increased MWSS in the very important to obtain a higher volumetric flow because
nozzle with the largest outlet diameter [12] . it leads to a faster dispensing speed so as to shorten the
Cell survivability is crucial in the bioprinting time for the cells under pressure. However, the overall
process. As such, several studies have indicated the volumetric flow cannot be fully controlled by the dispensing
adverse effect on cell survivability with the decrease pressure alone as it depends on the fluid viscosity and the
in nozzle diameter due to the increased shear forces inner geometry where the fluid flows . Thus, the optimum
[8]
experienced by the cells [29-31] . Conversely, an increase dispensing pressure varies for different nozzles.
in nozzle diameter reduces the velocity gradient and From Figure 7, the best pressure ranges are in
thus reduces shear stress, in turn, increasing the cell the regions where there is a linear relationship between
survivability . Although a larger outlet diameter the pressure and the shear stress with a gentle slope. In
[22]
increases flow rate and reduces shear stress, it also results these regions, it is easier to control the cell viability and
in lower resolution; a smaller outlet diameter with higher volumetric flow. As we can see, these regions are different
inlet pressure gives higher resolution . It is crucial to for all the nozzles. When determining the optimum
[2]
balance both cell survivability and printing resolution to working pressure, it is recommended to choose the lower
obtain an optimum result. pressures from the pressure ranges to reduce the risk of
cell damage, but one must also consider the flow rate and
Table 3. Percentage increase in maximum wall shear stress (Pa) on choose the best compromise.
decreasing diameter of nozzle outlet from 0.5 mm to 0.1 mm
Pressure Tapered Conical Cylindrical Table 4. Percentage increase in maximum wall shear stress (Pa) on
(MPa) conical increasing pressure from 0.05 MPa to 0.25 MPa for different outlet
0.025 26.1 11.9 −79.4 nozzle diameter
0.050 31.7 21.1 −79.6 Diameter (mm) Tapered Conical Cylindrical
0.100 30.1 24.0 −78.2 conical
0.150 30.5 25.7 −70.9 0.1 16.5 32.6 401.9
0.200 30.9 27.1 −62.8 0.3 17.9 33.1 290.3
0.250 30.9 27.1 −55.0 0.5 17.2 26.3 127.5

50 International Journal of Bioprinting (2022)–Volume 8, Issue 2