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CFD Assessment of Extrusion Bioprinting Parameters
Nozzles are very important in determining the shear stress as a measure of cell viability and concluded
printability of bioinks and survivability of cells; therefore, that convergence angle of the nozzle and exit diameter
the nozzle design is critical and takes into consideration had the greatest effect on printability and cell viability.
various aspects, including viscosity of bioink, shear- Emmermacher et al. used computational simulations
[9]
thinning property, and shear stress induced during the and analytical calculations to predict mechanical stress,
printing process. Cells are exposed to various mechanical pressure gradient, and flow rate for optimizing the
forces, and among these, shear stress is regarded as bioprinting process and developing new bioinks. Göhl
especially significant since it is the main cause of cell et al. simulated the flow of bioink from a nozzle onto a
[10]
damage and death. These forces are directly proportional printing plate using a proprietary CFD simulation tool IPS
to the inlet pressure of the nozzle and an increased IBOFlow to evaluate the effect of bioprinting parameters,
pressure corresponds to an increased shear stress endured such as printing speed and nozzle height, on the printed
by the cells. The cells near the wall experience greater strand resolution. Gómez-Blanco et al. investigated
[11]
shear stress compared to the cells in the center of the the effect of inlet velocity, which is proportional to flow
nozzle, and the cell viability decreases in an exponential rate and can affect the extent of pressure endured by cells
manner as shear stress increases . On the other hand, an during the bioprinting process. Reina-Romo et al.
[12]
[6]
overly low inlet pressure will result in no or little bioink studied the effect of conical and blunted nozzle geometry
being deposited, whereas a pressure too high will result in on cell viability through computational simulations and
excess bioink being deposited . conducted additional comparisons with experimental
[7]
The behavior of the bioink flowing inside a nozzle results. Most computational studies are limited by the
is an important aspect to determine, but is difficult to fact that they are specific to certain bioinks and/or
achieve with experimental tests, mainly due to the small nozzle geometries and some by the use of proprietary
size of the nozzles, which make it harder to directly software that hinders reproducibility. Our study aims to
probe without interfering with the measurements. not only characterize holistically the effect of bioprinting
Experimental tests of bioink behavior are usually parameters but also investigate whether the trends in the
focused on bioprinting results, such as printability, results observed for a particular bioink are transferable to
shape fidelity, or cell viability, but studies on influential and reproducible in other bioinks.
bioprinting parameters, such as shear stress, pressure, In this paper, we will use CFD to provide an
and velocity, are not as common experimentally ; overview of the effects of dispensing pressure, nozzle
[8]
therefore, computational simulations are increasingly diameter, and nozzle geometry, which are bioprinting
being used to address this gap. parameters known to greatly affect the shear stress
Computational fluid dynamics (CFD) can provide experienced by cells in bioink . Using the wall shear
[6]
key insights into the effect of specific bioprinting stress as a measure of cell viability, we will analyze the
parameters that cannot be measured while running an effect of these parameters taking into consideration the
experiment. For instance, using CFD, we can calculate rheological properties of the bioinks. We also investigate
microfluidics inner parameters, such as velocity, pressure, the impact of printing speed and dynamic behavior of the
or shear stress, which are experimentally difficult to extruded bioink through transient simulations.
measure . Experimental tests focus on bioprinting
[7]
results (printability, shape fidelity, or cell viability) but 2. Materials and methodology
require a large number of iterations, thereby increasing 2.1. Modeling
the cost, especially if the bioink is prepared using
expensive materials. CFD can reduce such iterations, We adapted the procedure outlined by Magalhães et al.
[2]
thereby making the process cost and time efficient. As using three distinct nozzle geometries, namely, tapered
previously mentioned, doing a measurement itself can conical, conical, and cylindrical, as shown in Figure 1.
affect the parameters since the measurement devices have Three-dimensional models were created for each of
a non-negligible size compared to the conditions of the the nozzle designs using 3D Computer-Aided Design
experiment . CFD is widely used to obtain flow behavior software, Solidworks. The inlet diameter (D ) was kept
[7]
in
in simple or more complicated designs and can pinpoint constant at 10.0 mm across all three nozzle geometries,
the exact spatial coordinates where forces are exerted on whereas the outlet diameter (D ) was varied as 0.1 mm,
out
the cells, facilitating bioprinter optimization for complex 0.3 mm, and 0.5 mm, which correspond to the nominal
geometries. inner diameter of 32G, 24G, and 21G commercial nozzles
In recent years, several papers have discussed commonly used in bioprinting, respectively. The angle of
optimizing extrusion bioprinting using computational convergence was noted as a driven variable. The complete
simulations. Magalhães et al. looked at optimizing specification of the nozzles is provided in Table 1. The
[2]
nozzle geometry through CFD simulation using wall 3D models were imported into Ansys Fluent 2021 R1
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46 International Journal of Bioprinting (2022)–Volume 8, Issue 2