Page 519 - IJB-10-5

International Journal of Bioprinting 3D model of neurogenesis in Alzheimer’s disease

required for extrusion on its structure, and its ability to liquid-like state. 65,66 At low oscillatory strain, the ALG-GEL
recover post-extrusion, maintaining the original shape of hydrogel reported tan δ > 3, which reduced with increasing
the 3D-bioprinted material. Mechanical and rheological strain stress but remained greater than 1. As the tan δ value
56
characterizations are also fundamental for understanding approached 1 (upon 60% strain), the hydrogel complex
the influence of hydrogel network structures on the viscosity increased, indicating a potential transition from a
biological properties in the 3D-bioprinted model. 56,57 liquid-like to a solid-like state (where Gʹ dominates G˝ and
Nevertheless, according to a systematic review, only 12.1% tan δ < 1) (Figure 2F).
58
out of the 118 analyzed papers performed rheological To assess the mechanical strength of the hydrogel
characterization or detailed the viscosity of the hydrogels composition before and after crosslinking with 2% CaCl
developed for 3D bioprinting. solution (construct), the storage modulus and phase angle
2
A continuous shear rate sweep was performed to simulate of the hydrogel and the construct were determined at 1
the shear stress that the bioink undergoes in a 3D extrusion Hz frequency and 1% strain, respectively. Figure 2G and
bioprinting process as it passes through the nozzle. The H indicates that constructs had G’ values nearly tenfold
flow behavior and viscosity of the hydrogel obtained in this higher than the hydrogel (10 kPa), suggesting that the
analysis are plotted as rheograms (Figure 2A and B). The presence of an ionic crosslinker promoted the formation
ALG-GEL hydrogel behaved as a non-Newtonian fluid with of a harder/stiffer structure with elastic character (tan δ <
pseudoplastic behavior and thixotropy. A pseudoplastic 1), most likely due to interactions between Ca ions and
2+
fluid (also called a shear-thinning material) exhibits a ALG chains. Although the presence of GEL may enhance
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decrease in viscosity when the shear rate increases 59,60 which the biological and physicochemical properties of ALG
is crucial for 3D printing since the viscosity of the hydrogel due to the tripeptide Arg-Gly-Asp (RGD) sequence that
decreases during extrusion. This viscosity reduction avoids facilitates cell attachment, 68,69 the uncrosslinked hydrogel
excessive extrusion pressures that can negatively affect cell did not present viscoelastic solid-gel behavior (G’ < 10 Pa;
viability during the printing process. The hydrogel also tan δ > 1), possibly due to weak internal chemical bonds
61
displayed minimal thixotropy with a narrow hysteresis between the two biopolymers.
loop, indicating that after extrusion, the hydrogel will The hydrogel developed in the present study exhibited
recover its original structure at rest rather than continuing viscoelastic characteristics (Gʹ < G˝ tan δ > 1) that differ
to flow as a fluid. 62
from other ALG-GEL hydrogels reported in the literature.
The presence of yield stress, as depicted in Figure 2B, Several studies characterize these hydrogels as having gel-
is another important component. Hydrogels composed like behavior and elastic mechanical characteristics. 70–73
of materials with solid-like properties typically have However, when comparing these results, various aspects
yield stress, making them promising candidates for 3D must be considered, the most important of which is the GEL:
bioprinting. The hydrogel behaves like a solid when it is ALG ratio. Blending GEL with ALG boosts the hydrogel’s
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at rest, e.g., when the bioink containing the cell suspension viscosity and elastic behavior, as the ideal mechanical
is in the syringe. It will not flow unless it is subjected to characteristics of ALG hydrogels are heavily influenced
a specific stress, such as pressing the syringe plunger, by the polymer’s molecular weight and concentration,
that surpasses the yield stress. This behavior enables the as well as the ionic crosslinker used. 72,74,75 Furthermore,
hydrogel to be extruded in a controlled manner and rapidly some researchers noted that cooling the hydrogels before
recover its solid-like characteristics after the applied force printing could result in overall gel properties that are
is removed, right before crosslinking. 56,64 more suitable for higher print resolution due to GEL’s
Although the hydrogel demonstrated flow behavior thermoresponsive qualities. 76
and a viscosity profile that are critical for 3D bioprinting, Chung et al. described that GEL hydrogels with low
76
the oscillatory studies yielded unexpected results. ALG concentration (2%) behave like fluids (G˝ > Gʹ) at
Figure 2D and E illustrates that the hydrogel behaves room temperature (25°C) or higher, consistent with our
as a viscous material (G˝ > Gʹ) across a wide range of findings. Maihemuti et al. demonstrated that when fish
77
frequencies and strains. In both oscillatory sweeps, the GEL is blended with ALG, the printability is determined
loss modulus (G˝) was greater than the storage modulus by the concentration of ALG rather than the GEL itself,
(G’), indicating that the hydrogel has a fluid structure and the finding of which supports our results. In their study,
may be classified as a viscoelastic liquid. Figure 2E also hydrogels were only printable and stable when they
65
depicts the phase angle or loss tangent (tan δ = G˝/Gʹ), included 6% ALG, which kept the viscosity within a
which confirms these results, where tan δ < 1 indicates a printable range that was neither too high to extrude nor
solid-like state of the hydrogel, and tan δ > 1 indicates a too low to maintain the shape.

Volume 10 Issue 5 (2024) 511 doi: 10.36922/ijb.3751