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International Journal of Bioprinting Droplet-based bioprinting of tumor spheroids

and precisely controlled the compositions with microfluidic 20–100 μm from the blood vessel (Figure 4B, left). They
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single-cell printing (Figure 4A). They found that collagen observed that GSC spheroids moved toward the blood
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generated by HSCs improved the compactness of spheroids, vessel, and higher concentration laminin facilitated
and the morphology was closely related to the ratios of HepG2 the movement and proliferation of GSCs. Trondle et
and HSC cells in the spheroid. They utilized the spheroids al. utilized bioink-containing EC spheroids to fabricate
for drug testing with TGF-β1 and demonstrated a higher customized blood vessel tubes (Figure 4B, right). The
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precision of fluorescence readouts compared with randomly endothelial tubes had an elliptical cross-section, with an
loaded spheroids. Ota and Miki observed the activation area of hundreds to thousands of square micrometers, and
of detoxifying enzymes (CYP1A1) in the liver spheroids. a wavy longitudinal cross-section, and manually induced
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They concluded that the activation level of CYP1A1 in the convective flow could be generated in these tubes.
spheroid with a diameter of less than ~180 μm reached the Besides, Hong et al. fabricated hepatic lobular spheroid,
maximum level within 2 days and then remained for more in which lobule-like structures were manufactured, thus
than 16 days. Dornhof et al. developed an oxygen detection benefitting the formation of vascular structures. They
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system based on microfluidic chips to detect oxygen implanted these spheroids into BALB/c nude mice for
consumption by the single MCF-7 breast tumor spheroid a 2-week culture, and then observed richer functional
during cultivation and verified the inhibition effect of microvascular networks inside the structured spheroids
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antimycin A on tumor cellular respiration. compared with non-structured spheroids. Although
Although spheroids can mimic certain aspects of some advances have been achieved, however, the formed
heterogeneity, it is still a challenge to completely recapitulate vascular networks in tumor tissue are mostly chaotic and
the real conditions in vivo. For instance, as suggested deformed, which results in two drawbacks: (i) increased
by Dornhof et al., the detection of tumor physiological permeability makes it easier for tumor cells to pass
activity was mainly observed through fluorescent staining, through the vascular wall and enter the bloodstream,
which was scattered and static. However, fluorescence leading to metastasis; and (ii) poor vascular perfusion
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images may not clearly reveal the entire process of tumor hinders the delivery of targeted drugs, which reduces
physiological activity, and thus, it is of great importance to the accuracy of anti-cancer drug screening. Besides, it is
develop high-throughput, sensitive, and stable monitoring still a challenge to address the issue of hypoxia in large
systems. Additionally, these in vitro tumor spheroid models spheroids, due to the lack in the formation of microvessels
lack vascular structures, which are essential for the study of inside the tumor spheroid.
long-term proliferation of tumors. 4.3. Modeling tumor invasion and metastasis

4.2. Modeling tumor angiogenesis The invasion and metastasis of tumors are the fundamental
Blood vessels are responsible for the transport of oxygen, reasons why cancer is difficult to cure. Tumor cells
nutrients, and cellular metabolites in tissues. They usually invade surrounding tissues as individuals (single-
are different from endothelial cells (ECs) and have a cell invasion) or cell clusters, and sometimes enter
dense network structure. Tumor angiogenesis is mainly lymphatic tissues through lymphatic vessels, in which
regulated by pro-angiogenic and anti-angiogenic factors. mesenchymal migration is a typical single-cell invasion
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The penetration depth of oxygen and nutrients in tissue strategy. Another important step for tumor cells to
usually does not exceed 250 μm ; thus, relatively large 3D migrate to new sites and land successfully is to establish
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tissues often suffer from hypoxia in the inner part owing to a premetastatic niche. Hypoxia can induce tumor cell
the lack of vascular structures. Homogeneous spheroids metastasis, and it can promote metastasis by regulating
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undergo sufficient material exchange within the ECM, c-Met protein expression. Efforts have been made to
and thus, they possess long-term viability and meet the model tumor invasion and metastasis using spheroids.
requirement of angiogenesis. Chen et al. bioprinted (CAL27)–CAFs co-culture models,
and they observed some sprouts on the periphery of the
Some scaffold-based bioprinting methods have tumor spheroid on day 3, indicating that CAF cells were
been utilized to construct the vascular system required initially recruited by the tumor cells. On day 5, they
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for tumor growth. Glioblastoma (GBM) is a highly found two invasion traces that CAL27 cells invaded the
vascularized tumor. Glioma stem cells (GSCs) are gap of CAF cells and CAL27 cells followed CAF cells,
commonly used to simulate tumor angiogenesis in GBM respectively (Figure 4C, left). These results indicated that
tumors by secreting vascular endothelial growth factor CAFs have a significant effect on tumor invasion. Chen
(VEGF). Lee et al. prepared a fluidic vascular channel et al. demonstrated that 5-fluorouracil (5-FU) showed
using gelatin as sacrificial materials in the protein matrix inhibition on the tumor microtissues-CAFs invasion
and cultured GSC tumor spheroids with a distance of modeling (Figure 4C, right). Quantitative analysis of
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Volume 10 Issue 1 (2024) 116 https://doi.org/10.36922/ijb.1214