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3D Bioprinted Organoids
fibrillated filaments, rather than just individual droplets, supplant extrusion bioprinting as the foremost standard
resulting in greater mechanical pressure, and shear biological 3D printing in the future.
stress on biomaterials and cells. Therefore, using this
technique can reduce the survival rate of printed cells, 3. Bioinks for bioprinting organoids
which is more obvious when printing bioinks with Bioinks are crucial for bioprinting. The ideal bioink
high cell density. Extrusion bioprinting is currently a should meet certain printability requirements,
common method used to construct organoids, and new demonstrate suitable mechanical properties; and possess
bioprinting methods have emerged based on traditional sufficient levels of biodegradability, biocompatibility, and
extrusion bioprinting methods. Researchers have built cytocompatibility. For bioprinting organoids, the bioink is
an extrusion bioprinter equipped with two nozzles and a selected based on the printability of the ink and its effect on
motorized X-Z robot. Using hepatocyte- and fibroblast- cell behavior. Printability implies that during bioprinting,
loaded GelMA hydrogels, the feasibility of the technique bioinks are required to exhibit proper flowability and
was demonstrated for bioprinting organoids or cellular the capability to quickly mold into a shape after printing
aggregates that maintained a certain level of cellular and cross-linking (photo cross-linking, chemical cross-
activity over time . In addition, a prominent study linking, and physical cross-linking). Increasing the
[19]
has recently proposed a printing method referred to hydrogel concentration can accelerate the curing time and
as bioprinting-assisted tissue emergence (BATE) that improve the hydrogel strength, which is conducive for
combines an extrusion printing system and a microscope better forming. However, it reduces the gel water content
system to build a printing system with its own microscope and narrows the micro-pore size inside the gel, which
image for real-time observation and precise control of is not conducive for cell survival and deposition of the
tissue development temporally and spatially . extracellular matrix (ECM). In addition, different types of
[20]
2.4. Photo‑curing bioprinting bioinks produce cells with different microenvironments,
affecting cell proliferation, differentiation, migration,
Photo-curing bioprinting is a biological 3D printing and self-organization. Therefore, bioinks have a suitable
method based on surface projection, which is now often printing window for printing complex geometric organ
subdivided into stereolithography (SLA) and digital shapes.
light processing (DLP). Both methods use light-induced Both natural and synthetic polymeric materials can
photopolymer molding. SLA applies this molding to be used as raw materials for bioprinting. The commonly
light-cure using laser light from point to line and line used bioinks are agarose-based, alginate-based, collagen-
to surface, while DLP uses a projector to irradiate the based, hyaluronic acid-based, fibrin-based, cellulose-
photopolymer and light-cure it layer-by-layer . The based, silk protein-based, and ECM bioinks. Each
[21]
photo-curing printing device uses a digital light projector bioink possess its own advantages and disadvantages
to solidify the entire surface of the bioink with high (Table 2). Agarose, a marine polysaccharide extracted
efficiency. Regardless of the complexity of the single- from seaweed, exhibits suitable mechanical properties.
layer structure, the printing time is the same, and the However, its ability to support cell growth is limited,
printing accuracy is high . The printer requires only which is not conducive to organoid construction.
[22]
a vertically moving platform. Compared with other Therefore, its use often requires mixing with other
methods, the device is relatively simple and easy to biomaterials to improve its biocompatibility. The earliest
control, and the printing mode without nozzles does reported strategy for bioprinting blood vessels by Norotte
not cause problems, such as nozzle blockage and shear et al. was the use of 300–500 µm diameter blood vessels
force affecting cell activity . Photo-curing bioprinting and supporting cell spheres. These spheres were then
[23]
is a promising printing method for cell assemblies and allowed to deposit on each other on molds printed with
organoid construction because of its ability to guide cell agarose to form a single blood vessel . Alginate is a
[25]
self-organization and relatively controlled differentiation. negatively charged polysaccharide that can be transiently
Creff et al. used SLA technology and a photosensitive cross-linked with divalent cations to form hydrogels.
polymer hydrogel (polyethylene glycol diacrylate/acrylic However, it lacks cell adhesion sites , and different
[26]
acid polymer) that supports the growth of intestinal polymers, such as PCL and gelatin, are often mixed with
cell lines to construct intestinal epithelial structures, alginate to form different structures for 3D printed tissues.
demonstrating that these structures support small Zhang et al. used alginate and nano-hydroxyapatite to
intestinal epithelial cell proliferation and differentiation promote osteochondral repair . Collagen is the primary
[27]
for 3 weeks . However, the disadvantage is that component of the ECM, which exhibits appropriate
[24]
ultraviolet light and its initiator can damage the cells. biocompatibility. It can be cross-linked by changing
Photo-curing bioprinting is a progressively imperative the temperature or pH. Using a mixture of collagen and
part of cell printing strategies, and it is anticipated to alginate bioink produces a stronger effect than using

22 International Journal of Bioprinting (2021)–Volume 7, Issue 3