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International Journal of Bioprinting Liver printing: from structure to application

printing accuracy of <50 μm, effectively overcoming Adding vascular endothelial growth factor (VEGF), basic
challenges related to printing collagen. 163,164 Fang et al. fibroblast growth factor (bFGF), platelet-derived growth
developed a dual-phase bioink based on densely packed factor (PDGF), and inducing hypoxia in the culture
cell aggregates for embedded printing in a suspension medium promotes angiogenesis. However, constructing
bath, aiming to construct dense liver tissue constructs. large-scale, perfusable blood vessels requires the use of
This bioink features high cell density (~1.7×10 cells/cm) sacrificial materials. The general process of sacrificial
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without compromising cell viability (~83%). 165 bioprinting involves first printing the geometric structure
using sacrificial materials. Subsequently, after the entire
4.8. Coaxial printing structure is printed, the sacrificial material is removed
Coaxial printing is a specialized form of extrusion-based by dissolution or temperature induction to induce phase
printing, where an outer layer of bioink and an inner layer of transition, leaving behind a sacrificial template. Finally,
sacrificial material are simultaneously extruded through a endothelial cells are introduced to endothelialize the
coaxial nozzle. After removal of the inner sacrificial material, channels. Hepatic sinusoidal endothelial cells are the ideal
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a hollow tubular structure is formed. This technology source for endothelial cells, as they are highly specialized
enables the printing of concentric multi-material and endothelial cells with fenestrations on their membranes
multi-cellular structures. By adjusting the nozzle diameter, and an absence of an organized basement membrane.
the size of the printed structure can be altered, presenting These features enhance membrane permeability,
significant potential for constructing biological tissues facilitating direct substance exchange between hepatocytes
with perfusable vascular networks. Taymour et al. utilized and the bloodstream. However, due to the difficulty in
seaweed alginate and methylcellulose loaded with HepG2 isolating hepatic sinusoidal endothelial cells and their low
spheroids dissolved in human fresh frozen plasma as shell purity, HUVECs are often used as substitutes in printing
material, and coaxially printed collagen-, fibronectin-, and processes. Commonly used sacrificial template materials
gelatin-encapsulated fibroblast and endothelial cells as include gelatin, Pluronic F127, carbohydrate glass, sodium
core material to construct 3D vascularized liver tissue. This alginate, and agarose. Additionally, the team led by Miller
triple-cell co-culture model enhances the secretion of ALB utilized laser-sintered carbohydrate-based compounds as
from hepatocytes and serves as a valuable tool for studying sacrificial templates, which were then dissolved in water
cell–cell interactions. However, coaxial printing still faces or phosphate-buffered saline (PBS) to construct dendritic
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significant challenges in constructing branched vascular vascular networks. These dendritic vascular networks were
networks. Table 4 summarizes the recent representative capable of sustaining cellular metabolic activity within
studies in liver bioprinting. tissue models exceeding 1 cm in thickness.
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5. Typical applications of 3D bioprinting in There has been significant progress in constructing
the liver vascularized liver tissues using various extrusion-based
printing strategies, including the development of complex
5.1. Vascularization vascular structures resembling hepatic lobules. 27,99 As
Blood vessels are a critical component in maintaining the aforementioned, Xiong et al. constructed functional
vitality and function of various tissues and organs within liver tissue with high cell density and perfusable vascular
the body. In addition to transporting oxygen and nutrients networks by simultaneously printing granular cell
and clearing metabolic wastes, the vascular system also aggregate-based biphasic (GCAB) bioink and gelatin
regulates blood clotting, the transport of inflammatory bioink loaded with endothelial cells. The GCAB bioink
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cells, and crucial cell signaling interactions in an organ- ensures high cell density, while the gelatin bioink serves
specific manner. The liver harbors a rich network of as a sacrificial material to create vascular channels
blood vessels, including major vessels (e.g., hepatic (Figure 8A). Taymour et al. selected collagen-, fibrin-,
artery, portal vein, and central vein) and various hepatic and gelatin-encapsulating fibroblasts and endothelial
sinusoids surrounding the hepatic cords within the liver cells as the core materials for coaxial printing. Collagen
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lobules, thereby facilitating material exchange between and fibrin support angiogenesis, while gelatin increases
hepatocytes and blood. Therefore, the introduction of the viscosity of the bioink (Figure 8B). Similarly, Liu et
vascular structures is necessary when constructing liver al. used gelatin as a sacrificial material and employed a
tissue in vitro. Furthermore, in tissue engineering, the combination of cell-laden bioink (3% GelMA + 0.25%
diffusion limit for oxygen and nutrients is 200 μm. To fibrin), fugitive bioink (10% gelatin), and elastic bioink
construct large-scale engineered tissues, a hierarchical (10% GelMA) for multi-material bioprinting to create
vascular network is indispensable. Small-scale capillary perfusable, surgically anastomotic 3D-vascularized liver
networks can self-organize through endothelial cells. tissue (Figure 8C). Miller’s team developed a technique
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Volume 10 Issue 5 (2024) 136 doi: 10.36922/ijb.3819