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International Journal of Bioprinting Biomimetic scaffolds for tendon healing

elastic behavior dominates). Both parameters have in gel stiffness. The intermolecular interactions can be
51
a very high frequency dependence (with increasing increased in the case of the HA and Gel, facilitating the
frequency, both moduli increase), which is typical associations between these molecules. 52
in complex materials such as polymers and gels
(Figure 2F). The η* decreased at higher frequencies. 3.2.2. Printability
50
The combination of these two results (increasing G’ The printability assay was performed to determine if the ink
and G’’ and decreasing η* with frequency) indicates was printable under each test condition (assay explained in
that the ink has a complex viscoelastic shear-thinning part (C) of the Supplementary File). Taking into account
behavior (as previously indicated). the results of the temperature sweep (T = 28.14°C and T m
g
= 23.56°C), the studies were carried out at six different
It is important to analyze the behavior of the ink temperatures: 18°C, 20°C, 22°C, 24°C, 26°C, and 28°C.
with the change of temperatures since all the constituent The results in Figure 3A demonstrate that the ink can be
materials of the ink are thermally sensitive. To analyze extruded at any of the tested temperatures. Despite this,
the ink’s behavior and to define the best protocol for notable differences can be observed in the printed filaments
obtaining and printing the ink, a temperature sweep due to the properties of the ink. At 18°C and 20°C, the ink
assay was performed (Figure 2G). The G’ and G’’, was over-gelated, causing incomplete filling and structural
when starting the test at 37°C, were almost similar, at disruption of the filaments. On the contrary, at 26°C and
18.7 Pa and 16.9 Pa, respectively. When the temperature 28°C, the ink was under-gelated; therefore, continuous
dropped to 4°C, both moduli increased to 2200 Pa and filaments could hardly be obtained. Optimal gelation was
441 Pa, respectively. During this cooling period, with observed in the filaments extruded at 22°C and 24°C, with
a 5% change in the storage modulus value, the T was the situation at 24°C being slightly better than that at 22°C,
g
determined, which was established at 28.14°C. In the
subsequent heating process, the moduli rose again to where there was still some minimal degree of gelation.
2.94 × 10 Pa and 4.13 × 10 Pa, respectively, and with a The strand printability and shape fidelity analysis
4
5
5% change in the value of the G’, the T was determined, enabled the determination of the optimal 3D printing
m
which was established at 23.56°C. This increase in pressure and temperature at the selected speed (7 mm s )
-1
the value of the moduli indicates that changes in and needle diameter (22 G) (Figure 3B). In the case of strand
temperature are irreversible (or at least requires more printability, the diameter of the obtained filaments was
time than the one used in this assay). In the last step, the analyzed and represented as a percentage when compared
temperature was decreased again, producing a change to the theoretical diameter size (Figure 3B-i). For the shape
in the values of the moduli, with a slight drop in this fidelity, the area of the squares that were formed on the
case. The information extracted from this test allows diagonal of the printed design (as described in Figure S4
for the determination of a suitable temperature window in the Supplementary File) was analyzed and represented
for achieving good printing while maintaining shape as a percentage when compared to the theoretical area size
fidelity and cell viability, i.e., 23.56–28.14°C. (Figure 3B-ii). For both parameters, the optimal pressure
Once the temperatures that are critical for ink increased while the temperature decreased. Taking into
development and 3D printing process had been estimated, account the results obtained during the extrudability and
the G* for each of those temperatures at different filament formation assay, the values obtained for strand
frequencies was determined (Figure 2H). The G* increased printability and shape fidelity under optimal gelation
with temperature, with the results being very similar conditions were analyzed in detail, that is, at 22°C and
for 18°C and 20°C. A higher G* was observed when the 24°C. In the case of 22°C, the best percentage of strand
temperature was raised to 22°C. The values kept increasing printability and shape fidelity was observed at 140 kPa,
with temperature until reaching the maximum value at measuring 108.68% and 90.98%, respectively. In the case of
28°C. This behavior is not uncommon in hydrogel-based 24°C, the best percentage of strand printability and shape
inks. Specifically, this can be accounted for by different fidelity was observed at 120 kPa, measuring 108.51% and
possible explanations surrounding the diverse materials 89.92%, respectively. These results indicate that the strand
used to compose the hydrogel. For instance, the increase printability and the shape fidelity obtained at these two
in temperature can augment the mobility of the Alg chains temperatures are very similar. In order to select the best
and the diffusion rate of ions. This mobility improves printing parameters, two aspects were taken into account:
the interactions and crosslinking between those chains, the temperature per se and the used pressure. As previously
leading to a stiffer network. Likewise, the gelation process described, high pressures during the printing process
50
can also be enhanced. The fibrin clot formation can, in negatively affect cell viability. 53-55 In the same way, a decrease
addition, be thermally activated, resulting in an increase in temperature can also negatively affect cell viability.
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Volume 10 Issue 3 (2024) 452 doi: 10.36922/ijb.2632