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International Journal of Bioprinting 3D bioprinted vascularized tissue models

are only temporarily present during the printing process, breakthrough—freeform reversible embedding of
and the removal of sacrificial networks can create hollow suspended hydrogels (FRESH)-based printing technique—
structures, which enables cells to be seeded to produce allows the freeform fabrication of more complex
lumenized vessels inside a 3D hydrogel. Because the solid structures. Lee et al. developed a coacervation approach
[33]
sacrificial network can aid in maintaining micro-channel to generate thermo-reversible gelatin micro-particles
networks, this indirect method has been explored for the used as support bath. This FRESH technique significantly
3D bioprinting of vascular channels in engineered tissues. enhanced resolution (length scales ranging from a few
For example, Homan et al. presented the sacrificial millimeters to centimeters) with the ability for the precise
[28]
bioprinting approach to fabricate 3D convoluted tubular deposition of soft hydrogels into intricate 3D biological
structures on customized perfusable chips, where the constructs. In this study, they also presented a method to
printed Pluronic F127 (PF-127)-based fugitive ink was 3D-bioprint collagen bioink using FRESH to re-build the
removed and the proximal tubule (PT) epithelial cells components of the human heart at multiple length scales,
were seeded to yield an open convoluted tubular channel from capillaries to the full organ. After printing, the gelatin
embedded within a gelatin–fibrinogen hydrogel. A similar support bath was mildly removed by placing at 37°C to
method has been adopted by Kolesky et al. to create retrieve the printed construct. Recently, this approach has
[29]
3D heterogeneous vascularized constructs containing been adapted to fabricate several 3D vascularized tissues
perfusable channels interleaved with vascular supporting such as cardiac tissues , blood vessels , and muscles .
[31]
[35]
[34]
cells (i.e., fibroblasts) within a photocurable surrounding The embedding bioprinting strategy has tremendous
matrix (i.e., gelatin methacrylate [GelMA)); this method potential for reproducing complex branched structures
presents a combinatorial approach for fabricating 3D with various diameters in 3D owing to its advantage of
tissue constructs comprising vasculature, multiple types high design flexibility and resolution. In addition, it can
of cells, and extracellular matrix (ECM). Several sacrificial broaden the range of applicable bioinks to better mimic
inks including PF-127 [28,29] , carbohydrate glass , and the structure and function of the printed tissue. However,
[30]
gelatin [31,32] have been used in this approach, all of which the complete elimination of the sacrificial support material
have been shown to successfully fabricate complex 3D in a temporally controlled fashion, the limited range
vascular structures. of available supporting materials, and the unavoidable
Sacrificial printing strategy allows the introduction of biochemical reaction between bioink and supporting bath
physical architectures, such as open and inter-connected material may be major drawbacks.
pores or perfusable micro-channels, within bulk hydrogel-
based constructs. In addition, this approach provides a high 2.4. Coaxial bioprinting
degree of freedom for designing channel geometries with Coaxial extrusion can be accomplished through a core/
a wide size range and is therefore efficient for generating shell printing configuration, which simultaneously
large-scale channel networks. However, its relatively low dispenses two or more flow streams in concentric rings. A
printing resolution in channel diameter (>100 μm) is a coaxial nozzle usually possesses an inner core into which a
key obstacle to mimicking micro-scale channels with sizes crosslinking agent or sacrificial material is dispensed; this
close to capillary vessels (10–20 μm in diameter) . enables the semi-crosslinking of the outer shell hydrogel
[9]
to create hollow micro-tubular constructs in a single-step
2.3. Embedding bioprinting procedure. With the careful selection of nozzle dimensions
In general, direct deposition of bioinks or biomaterial inks and dispensing flow rates, coaxial bioprinting can pave the
without supporting materials makes the printed tissue way to the direct printing of freestanding tubular structures
constructs prone to collapse or deformation. To address with varying wall thicknesses and lumen diameters in a
this challenge, embedding bioprinting has been proposed uniform size. Owing to its simplified manufacturing process
to meet the increasing demand for large-scale and high- and scalability, coaxial bioprinting has been increasingly
precision fabrication. In this technique, ink materials are investigated for emulating vascular constructs [36-38] . For
extruded into a liquid suspension bath (i.e., suspension example, Jia et al. reported the direct construction of
[39]
media or granular hydrogels) according to a pre-defined organized, perfusable vascular structures using a blended
pattern, thus allowing to effectively dispense low-viscosity bioink comprising GelMA, sodium alginate, and four-arm
bioinks into a support reservoir [6,10,20] . The suspension bath poly(ethylene glycol)-tetra-acrylate in combination with a
serves as a support agent to hold the printed filaments multi-layered coaxial extrusion system, which facilitated
and their designated geometry while printing, which is the accurate deposition of multi-layered 3D perfusable
beneficial for expanding the deposition ability of soft bioink hollow tubes. As a significant leap forward, Gao et al.
[40]
and increasing structural integrity. A recent technological used a triple coaxial nozzle for fabricating three-layered

Volume 9 Issue 5 (2023) 18 https://doi.org/10.18063/ijb.748