Page 71 - IJB-10-1

International Journal of Bioprinting Microfluidic-assisted 3D bioprinting

highlighted in the theoretical model, the introduction cell concentration. In the first case, fast switching (500
197
of a sheath flow protects encapsulated cells from harsh ms) and seamless transition both between alginate-based
spinning conditions, minimizing the level of shear materials and photocrosslinkable resins were achieved.
188
stress imparted to the central stream, thus enabling the By alternating inks with different Young’s moduli, the
successful printing of high cell density bioinks. Adopting authors demonstrated the possibility to create thin slabs
this strategy, the microfluidic extruder and the upstream with patterned mechanical properties. The flow-focusing
microfluidic modules can be connected through flexible geometry, instead, was employed to extrude alginate fibers
tubes or even integrated into a single platform, minimizing crosslinked using a calcium chloride solution as sheath
flow perturbations and dead volumes. fluid, resulting in fiber diameters between 200 and 800 µm.
In 2013, Beyer et al. presented the first connector-free However, only single-layer structures could be printed,
microfluidic device employed as a printhead to fabricate suggesting that the deposition of additional layers was
and deposit alginate fibers with diameters ranging from hindered by the excessive crosslinking solution gathered
75 to 300 µm with a calcium chloride sheath flow. around the fibers. Finally, a unique approach to adjust
185
Moreover, the prototype was incorporated with pneumatic printed cell concentration on-the-fly was proposed using
198
valves to perform accurate switching between different a microfluidic cell concentrator. The latter consisted of
bioinks. Extruded fibers were deposited into simple a main channel fitted with micropillars on the sides at a
189
3D structures (ring-shaped or cuboid) onto a porous fixed distance of 1.5 µm. Excess liquid was drained from
substrate equipped with a vacuum pump to remove the the main channel after imposing a negative flow from a
significant build-up of the sheath solution overflow, secondary inlet, thereby creating a colander-like filtering
drastically limiting the ultimate 3D printing resolution. system.
Harnessing the aforementioned microfluidic printhead In this way, NOR-10 fibroblasts initially injected at a
and a fibrin-based biomaterial, the fabrication of a density of 2×10 cells/ml reached a final concentration
6
functional neural tissue model using induced pluripotent of 20×10 cells/ml and were subsequently embedded
6
stem cells (iPSCs) or mesenchymal stem cells (MSCs) in collagen using a passive micromixer. Despite the
190
186
along with a 3D-printed glioblastoma multiforme (GBM) adopted solution enabling high cell density printing while
model 191,192 was reported (Figure 7e). To increase the guaranteeing 97% of cell viability, further improvements
model complexity, the same bioink was loaded with are required to enhance the resolution of the ultimate
guggulsterone-releasing microspheres to encourage stem construct and the deposition of multiple layers.
cell differentiation, boosting the mechanical strength of
193
the final 3D construct as well as the bioink printability. Even though a3DMB is ushering in a new paradigm
194
Using the same commercial microfluidic printhead, for extrusion-based biofabrication, this technology has
another study reported the successful printing of smooth not gained traction among biofabrication communities
muscle cells with high viability. 195 since most monolithic microfluidic printheads still do not
achieve the deposition accuracy of conventional extrusion
The serial combination of flow-focusing junctions bioprinters. Moreover, owing to the extreme sensitivity of
makes microfluidic printheads suitable for core- such systems, manufacturing flaws or air bubbles present
shell fibers biofabrication, overcoming the technical in microchannels induce flow disturbances that may alter
limitations of multiple coaxial needles. A 3D in vitro the internal fluid patterns, compromising the stability
kidney model was created by Addario et al. to investigate of the spinning process and the accuracy of the entire
renal physiopathological conditions (Figure 7f). Core- printing. Moreover, since coaxial flow is not imposed by
196
shell fibers are produced through two serial T-junctions, physical constraints but rather by hydrodynamic forces,
one to surround renal tubule-derived primary cells with the resulting cross-section of extruded fibers cannot depart
HUVECs and one to crosslink the filament with a CaCl - from cylindrical-based geometries. 85
2
based solution. After 14 days, a tubular structure following
the natural cell rearrangement observed in the renal tissue
is formed. 5. Conclusions and future perspectives

The microfluidic approach creates countless solutions The technological advancements achieved in manufacturing
to increase the complexity of printed constructs by microfluidic chips have fostered the use of MST as a
controlling both biomaterial and cell deposition. Serex versatile approach for fabricating biological tissue models.
et al. recently suggested a set of smart microfluidic Additionally, the incorporation of microfluidic printheads
printheads to perform (i) multi-material printing, (ii) into 3D bioprinters has enabled the manufacture of 3D
flow focusing, (iii) mixing of biomaterial inks, and (iv) structures with a great degree of control, resolution, and

Volume 10 Issue 1 (2024) 63 https://doi.org/10.36922/ijb.1404