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International Journal of Bioprinting Fluid mechanics of extrusion bioprinting

level of mixing. These microfluidic systems draw different complexity. Idaszek et al. introduced an innovative
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biomaterials from separate reservoirs and extrude coaxial 3D extrusion bioprinting system that incorporated
them as a single multi-material fiber, ensuring precise a microfluidic mixing unit to fabricate 3D constructs with
control over the concentration of precursor biomaterials gradients of cells and biomaterials, specifically designed for
at the outlet. 143,170 Colosi utilized a simple Y-shaped chondral defect repair. This platform enabled the separate
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microfluidic chip with coaxial outlet to print a cell-laden delivery of various bioinks or their quick mixing within
heterogeneous construct composed of core–shell fibers. the microfluidic mixer prior to extrusion through a coaxial
Microfluidic systems are among the fastest methods for nozzle. The system facilitated the generation of gradients
bioprinting. However, due to the single-nozzle design, these in composition, mechanical properties, and biological
systems can only print one bioink or a mixture of bioinks cues, resulting in constructs with high structural integrity
at a time. While designs with co-flow of biomaterials and enhanced bioactivity.
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inside a Y-microchannel (Figure 10A) promotes mixing,
designs equipped with micromixer elements (Figure 10C 5. Numerical simulation of bioprinting
and D) effectively facilitate the mixing of biomaterials
during the extrusion process. Although both active and Due to the high cost of most cell-containing bioinks and the
passive micromixers can work in microfluidic printing time-consuming nature of bioprinting tasks, researchers
systems, passive micromixers are more popular due to have increasingly turned to computational fluid dynamics
their simplicity and relatively mild effect on cells. Passive (CFD) simulations as a valuable complementary approach.
micromixers operate based on interdigitation mechanism, These simulations analyze the extrusion, mixing, and
which can be achieved through split-and-recombine (SAR) deposition processes involved in bioprinting by leveraging
flow guidance methods or chaotic mixing. 160 the detailed information from the numerical solutions of
the governing equations. 88
Strook et al. demonstrated that a grooved-wall
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micromixer can benefit from chaotic advection to Numerical simulations of bioprinting processes allow
decrease the mixing distance. They reported that for for a deeper understanding of the relationships between
grooved-wall micromixers, nozzle geometry, bioink properties, printing parameters,
and extrusion forces. These simulations play a crucial
role in predicting the impact of the printing process on

lnPe < (XXXV) cell viability and the shape of the extruded fiber. CFD
d techniques have the potential to optimize dispenser
which shortens the length of mixing channel required geometry, printing conditions, and even facilitate the
for complete mixing of precursors. development of new bioink compositions, reducing the
need for extensive experimental testing. 63,174
Angelozzi et al. demonstrated that adding a
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serpentine micromixer to the Y-shaped microfluidic chip In order to model the flow of bioinks during the extrusion
(Figure 10D) enhances the mixing of osteoblast cells bioprinting process, the governing equations of fluid flow,
within the alginate solution and facilitates the deposition along with appropriate boundary conditions, should be
of fibers with homogeneous distribution of cells. The solved numerically using commonly employed methods,
serpentine micromixer can facilitate the chaotic mixing such as finite-volume or finite-element approaches.
process by generating Dean vortices in the flow cross- The numerical solution method enables researchers to
section. The straight channels (Figure 10A) were found to simulate and analyze the intricate fluid dynamics within
cause cell segregation along one side of the printed fibers the bioprinting system, providing valuable insights into the
due to the laminar dispersion of cells in the flow inside the behavior of bioinks and their interaction with the printing
microfluidic device. 148,152 environment. By leveraging these simulations, researchers
Based on the intended purpose and the desired can make informed decisions regarding printing
structure of the printed scaffold, a microfluidic chip parameters, biomaterial compositions, and optimization
with a specialized function can be custom-designed and strategies, ultimately advancing the capabilities of
integrated into the printing head. For instance, combining bioprinting technology. 63
microfluidic chips with coaxial printing heads can harness
the advantages of both approaches for bioprinting. The Assuming laminar, incompressible flow, the governing
integration of the microfluidic chip upstream of the equations for the flow of biomaterials can be written
as follows:
coaxial dispensing nozzle enables the printing of low-
viscosity bioinks while attaining high architectural (i) Continuity :
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Volume 10 Issue 6 (2024) 137 doi: 10.36922/ijb.3973