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International Journal of Bioprinting CFD analysis for multimaterial bioprinting conditions

The most commonly used additive manufacturing (3D rapidly expanding field to overcome the restrictions of
printing) techniques [5,6] to create cell-laden constructs mixing highly viscous biomaterials .
[25]
are material jetting [7,8] , extrusion-based [2,9] , and light- Computational fluids dynamics (CFD), simulations
based technologies [10,11] . Multimodal systems, integrating of the biopolymer flow in extrusion-based bioprinting
different printing principles into a single machine, have processes, are widely used to understand the relationship
been also proposed . Among these printing strategies, between printing parameters, nozzle size and geometry,
[6]
extrusion-based bioprinting is one of the most commonly viscous forces, and material properties during the
used approach as it allows to print materials with a bioprinting process. More specifically, it enables to
wide range of viscosities and to fabricate multimaterial/ determine inner parameters that are experimentally
cellular volumetric biological constructs [12-14] . In extrusion difficult to evaluate, such as pressure, velocity, flow rate,
bioprinting, biocompatible cell-laden hydrogels are and also shear stresses on cells in the case of bioinks. It
loaded into the cartridges (reservoirs) and deposited on is known that the dispensing pressure, and particularly
a bioprinting platform using one or several nozzles via shear stresses, have a significant influence on cell
pneumatic, mechanical, or solenoid actuation . survival [26-29] . However, only few simulation studies have
[5]
A key object of tissue engineering applications is the investigated shear stresses and cell viability on extrusion-
fabrication of multimaterial and multiscale heterogeneous based bioprinting processes, considering different nozzle
constructs, mimicking the organized cellular architecture geometries and dispensing pressures [30-34] . Moreover, all
and functionality of natural tissues . However, this is not these studies have only considered the simulation of a
[15]
achievable with single-material bioprinting approaches, single-material bioprinting process.
as they often fail to replicate the complexity and variety In this work, we extensively investigated the extrusion
of real tissues consisting of multiple layers of different cell process of non-Newtonian alginate and gelatin solutions,
types . To overcome this shortcoming, multimaterial through an entire and novel dispensing system consisting
[16]
bioprinting emerged as a promising approach [6,15-17] . of two cartridges, KSM integrated mixing chamber and
Multimaterial 3D bioprinting technique refers to the a single nozzle. To the best of our knowledge, to date, no
simultaneous or sequential deposition of two or more numerical study has yet investigated the inner parameters
biomaterials in a predetermined manner to create region- such as shear stress, pressure, and velocity field as well
specific characteristics and performances . These type as mixing index within the KSM-embedded 3D model
[16]
of heterogeneous bioconstructs have been fabricated of printing head flow domain. The spatial distribution
using multireservoir and multinozzle printing systems . of the two different polymer solutions was quantitatively
[18]
However, the fundamental limitation of these multinozzle characterized by determining the mixing index. To
bioprinting systems is the considerably long printing time understand the effect of the printing needle geometry on
while switching between different bioinks, in addition to the inner parameters, cylindrical and conical nozzles with
the need of accurate calibration of all printheads before varying outlet diameters were investigated considering
the deposition process [3,19,20] . Moreover, multinozzle different printing pressures. The simulation results were
bioprinting systems result in a discontinuity in the printed validated by comparing predicted pressure drop results
filament morphology that adversely affect the mechanical at different Reynolds numbers with existing empirical
integrity of the 3D-printed structures . To overcome these correlations. This model can be easily adapted to different
[19]
limitations, several researchers focused on bioprinting of biomaterials.
multiple hydrogels through a single nozzle [20-23] .
Implementing multireservoir single-nozzle systems is 2. Materials and methods
simple, though this strategy is ineffective for systematically
printing tissue engineering structures with continuous 2.1. Computational fluid dynamics analysis
gradient features. A variety of mixers have been used to A computational fluid dynamics (CFD) analysis was
tackle this limitation by blending two or more biomaterials performed by using the simulation software package
in various concentrations to facilitate multimaterial ANSYS® Academic Research Fluent 19.2 (ANSYS,
bioprinting tasks . In this case, active and passive or Canonsburg, PA, USA) to numerically evaluate the mixing
[23]
static mixers have been extensively studied. Static mixers mechanism of two different biomaterials through a static
are usually easier to assemble and more biocompatible mixer integrated printing head, and both the velocity and
compared to active mixers, since they cause less shear shear stress distribution profiles at the needle outlet during
stresses to the encapsulated cells . Recently, Kenics static the extrusion process. Figure 1 shows the computer-aided
[24]
mixers (KSM) have been employed to enable chaotic design (CAD) model of the printing head equipped with
bioprinting of multimaterial constructs and this is a cartridges, static mixer, and a cylindrical or a conical
[15]

Volume 9 Issue 6 (2023) 12 https://doi.org/10.36922/ijb.0219