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International Journal of Bioprinting New challenges in liver tissue engineering

authors propose this hydrogel mixture to build a viscoelastic Hydrogels have achieved progress in the in vitro culture
approach with native adhesion domains and fibrillary of hepatic cells since hepatic functionality and the culture
architecture to provide a mimetic microenvironment for time of 3D hepatocytes have improved in comparison to
the cell culture of HepG2 cells, as an in vitro artificial liver the traditional monolayer culture. However, hydrogels have
model to study alcohol-induced liver disease. HA has also also posed new challenges related to the construction of
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been combined with Gel for the culture of HepaRG cells. more physiologically relevant in vitro models. For instance,
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there is a need to recreate the native liver microarchitecture,
5.2. Synthetic hydrogels for liver tissue engineering with a heterogeneous ECM that can be recreated by building
PEG hydrogels are the most frequently used micropatterned hydrogels. 111,112 Although the hydrogels
synthetic hydrogels in tissue engineering due to their described have a good affinity for hepatic cells or have
biocompatibility, neutral charge, and precise control been decorated with biomolecules that provide cell–matrix
of the crosslinking density, and they form networks interactions, the inclusion of growth factors presented by
with reproducible mechanical properties. Compared the hydrogel in synergy with the interaction domains can
to natural origin hydrogels, PEG hydrogels are limited represent a step forward in hepatic functionality. New
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by their disadvantages such as non-biodegradability hydrogel formulations that more accurately reproduce the
and lack of recognition sites to interact with cells. These viscoelastic behavior of the native tissue are expected in
drawbacks can be solved, on the one hand, by providing the future 114,115 to adequately mimic the in vivo situation in
PEG with MMP-sensitive peptides (as crosslinking agents order to improve the current outcomes.
or in the polymer backbone), allowing PEG degradation
in cell microenvironment. On the other hand, the 6. Bioprinting applied to designing
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chemical modification of PEG by including bioactive
factors, such as the adhesion sequence RGD, permits hepatotoxicity assessment platforms and
the interaction with cells, in general, and modifications 3D in vitro disease models
by incorporating hepatic cells, in particular, improve the 6.1. Bioprinting methods and main
hepatic functionality of hepatocytes (in co-culture with control parameters
fibroblasts). Orthogonally crosslinked thiol-ene hydrogel Bioprinting is the automatic 3D spatial deposition of
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PEG systems have been synthesized as a cytocompatible cells (and other biological molecules such as growth
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material for the culture of hepatic cells (Huh7 and HepG2) factors) through a layer-by-layer process of a hydrogel that
in three dimensions. When these hydrogels contained crosslinks by non-cytotoxic reactions. As the bioprinter
bioactive MMP-sensitive peptide linkers, they could better automatically manufactures the samples, multiple samples
modulate the expression of different hepatocyte-specific can be produced in a very short time for scaling up, while
genes and enhance hepatocyte functions than bioinert complex tissue-like structures can also be formed. Bioink
hydrogels of the same nature. Including RGD motifs was is a mixture of a hydrogel precursor solution, cells, and
also conducive for augmenting hepatic functionality. bioactive molecules. When it is injected onto the bioprinter

There are not many options of single-component hydrogels plate, a crosslinking reaction takes place that keeps the
that can mimic mechanical and biological properties of the shape of the printed structure, homogeneously entrapping
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natural ECM. Thus, there is an increasing tendency to use the cells and biomolecules. There are three main
multicomponent hydrogels, especially in the development bioprinting strategies: inkjet-based, laser-assisted, and
of bioinks to improve printability at the same time as the extrusion-based. Inkjet bioprinting works by depositing
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hydrogels keep the biological performance. Of importance microdroplets formed in the print head by a piezoelectric
is the case of the use of dECM in combination with other actuator onto the plate. In laser bioprinting, a bioink
components to enable its printability, as it has a slow gelation droplet is formed in a template material that is vaporized by
time and the preparation of complex or large structures with the laser and transferred to the receiving substrate. During
enough fidelity is quite difficult. For example, Kim et al. extrusion bioprinting, a continuous filament, produced in a
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combined a liver dECM bioink with gelatin and observed a syringe that can move in three directions, is deposited onto
significant increase in its viscosity, allowing the improvement the collecting plate. The filament is formed by pneumatic
of the printing resolution and enabling the preparation of or mechanical screw pressure in the syringe. Highly viscous
higher structures in the case of using single-component materials with different cell densities can be extruded by
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dECM bioink. Another study incorporated gelatin into the this method, the most widely used bioprinting technique.
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dECM bioink with the aid of silk fibronectin with promising The printing resolution depends on the method used for
results in hepatocyte functionality. Incorporation of obtaining the structure; the literature describes 50 µm
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HA into dECM bioinks was also possible thanks to the for inkjet bioprinting, 10 µm for laser bioprinting, and
modification of HA with tyramine. 110 100 µm for extrusion bioprinting. The ideal structure
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Volume 10 Issue 3 (2024) 125 doi: 10.36922/ijb.2706