Page 424 - IJB-9-4

International Journal of Bioprinting 3D bioprinting of artificial blood vessel

longer than 1 h at room temperature, which decreases and other methods. Meanwhile, photoinitiators such as
the structural fidelity and mechanical strength. Thus, lithium phenyl (2,4,6-trimethylbenzoyl) phosphinate
enhancing the storage modulus of the ink is the principal (LAP) and Irgacure 2959 are more suitable for bioprinting
direction of improving the collagen bioinks. One of the the arterial blood vessel [113] . The advantages of Irgacure
methods is to increase the collagen concentration in 2959, which is the most common photoinitiator, include
solution. Some studies showed that single-component being slightly soluble in water and having low cytotoxicity
collagen with a concentration of higher than 20 mg/mL without UV irradiation [114] . After irradiation, Irgacure 2959
could be used to create accurate 3D structures, and it is can efficiently generate free radicals and cross-link the
important to note that the cell encapsulation and activity required content, culminating with an acceptable level of
could always be influenced by higher concentrations. cell survival [115] . Nevertheless, the UV (337 nm) may induce
Therefore, using the methacrylate group to modify the endogenous oxidative damage to deoxyribonucleic acid
collagen allows the photopolymerization of the hydrogel (DNA) through the action of reactive oxygen species [116] .
without causing protein denaturation, and this kind of The cross-linking method of HAMA and GelMA is
hydrogel is known as methacrylated collagen (ColMA) similar, and using the mixture of the two hydrogels could
or carboxymethyl agarose. ColMA contains small create adjustable 3D microenvironment to regulate cell
randomly oriented fibers, which allow spontaneous behavior [117] . Due to the very low viscosity, the pure HAMA
fibrous self-assembly and keep cell activity as normal is difficult to maintain the shape of the printed object.
collagen with enzymatic biodegradability. ColMA still Adding 6% – 12% GelMA into 4% HAMA could increase
contains small randomly oriented fibers, which could the print resolution and regulate the tissue stiffness. The
spontaneous fibrous self-assembly and keep cell activity mixture could also support the encapsulated cells and
as normal collagen with enzymatic biodegradability [104] . regulate the cellular response [118] . In theory, the ideal ink
Another frequently used method is adding cross- should exhibit gelatinous properties (the storage modulus
linking agent: For example, the collagen cross-linked by G’ is dominant and higher than 200 Pa) and shear-thinning
1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and behavior for high-fidelity printing. To further reinforce the
N-hydroxysuccinimide had the strength of 853 kPa and bioink made by HAMA-GelMA, the cellulose nanocrystals
1000 kPa, respectively [105,106] . Muthusamy et al. added the (CNCs) were added into the system. The structure printed
xanthan gum into the collagen to form the collagen-XG as by HAMA-GelMA-CNCs could keep the printed structure
bioink, and the hydrogel could support the EC to form a stable after 20% strain circulation [119] . Besides, the fidelity
network of interconnected blood vessels [107] .
of printed filaments could be improved by mixing HAMA
(C) Gelatin with GelMA. Mouser et al. found that the yield stress of
GelMA/gellan/HAMA was relatively high and the addition
Gelatin is a denatured form of collagen, with high viscosity, of HAMA increased the stability of the filament [120] . The
good biocompatibility, and degradation ability in vivo. filaments made of 10% GelMA began to collapse after
However, gelatin is a thermal gel that turns into a solution bridging an 8 mm gap, but following the addition of
above 37°C, and the printing accuracy is so low that it HAMA, a single filament can close a gap of 16 mm [120] .
cannot be directly used as bioink [108] . Therefore, combining Nguyen et al. studied the cytotoxicity under 405 nm light
gelatin with other biological materials or chemical and found that the concentration below 17 mmol/L (0.5
modification is the main method of applying gelatin [109] . wt%) did not induce obvious cytotoxicity [121] . Yin et al.
GelMA is a photo-cross-linked hydrogel formed by used LAP as the photoinitiator to prepare the GelMA/
modified gelatin with methacryloyl group (MA), and the gelatin bioink [122] , as shown in Figure 6. The shear-thinning
chemical modification of gelatin only involves less than behavior and the biocompatibility of the 5% GelMA and
5% of the amino acid residues in molar ratio, which will 30% gelatin were higher than those of the pure 5% GelMA,
not significantly decrease the arginyl-glycyl-aspartic and the swelling behavior of the GelMA/gelatin was lower
acid (RGD) content of the gelatin [110] . The RGD, which than that of the 5% GelMA [122] .
is a fragment of GelMA, gives EC better adhesion and

differentiation abilities [111] . The matrix metalloproteinases The proportion of methacryloyl substituents in GelMA
on the GelMA enable the marking and degradation of is another factor that influences cross-linking density and
the complex by enzyme [112] . Due to the exiting lots of modulus of compressibility [123] . After light curing the low
primary amine (-NH ) and hydroxy (−OH), which are concentration (5 wt%) GelMA with 0.5 wt% Irgacure 2959
2
unsaturated photo-cross-linkable groups, the GelMA is at 5 mW/cm for 300 s, the cells had better embedded
2
easy to polymerize under the light-induced condition. cell properties. On the other hand, it is very difficult to
Besides, GelMA could be cured by reduction-oxidation culture cells with high concentration GelMA. Therefore,
reaction, heating, gamma irradiation or electron beam, the mechanical properties of GelMA-based 3D-bioprinted

Volume 9 Issue 4 (2023) 416 https://doi.org/10.18063/ijb.740