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International Journal of Bioprinting Biomimetic scaffolds for tendon healing

its beneficial effects on lubrication, water retention, and the ink was suitable for 3D printing since, after applying a
viscoelasticity in the resulting ink. Alg, recognized for certain pressure on the ink, it became less viscous and able
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providing sturdy structural support alongside excellent to flow through the 3D printer needle. The same figure is
mechanical properties, played a pivotal role in the ink. also representative of the shear stress vs. the shear rate. A
It facilitates a crosslinking mechanism within the ink Herschel–Bulkley adjustment (R = 0.97) established that
2
through cations and guarantees efficient water retention. the σ of this ink is 107.48 Pa. This parameter indicates that
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y
Lastly, Fg, bearing similarities to natural ECM components, at stress values lower than 107.48 Pa, the ink will behave
was incorporated to enhance cellular viability within the like a solid (it will support the suspended cells while it is in
ink and to introduce a second crosslinking mechanism the cartridge) and that at higher stress values the ink will
(enzymatic, in this case). 44 start to flow (it can be extruded in a controlled way). In any
case, this value was regarded as a guide value since it was
This meticulous selection process yielded an ink
composed of polymeric natural materials, mirroring the determined by carrying out a step-like ramp. In a real 3D
printing situation, the stress is modified much faster, so σ
composition of the ECM in the target tissue, tendons value may be somewhat different. 47 y
(Figure 1). The percentage of water is high in both the
ECM (55–72%) and the scaffold (91.7%), but higher in Once extruded, the ink must recover its mechanical
the scaffold than in the ECM. The proportion of total properties to ensure that the 3D structure will form
proteins exhibited similarities in both cases, with collagen properly and will not collapse due to the force of gravity. To
dominating the ECM (65–80%) and Gel being the major determine this property, a rotational thixotropic test was
protein in the hydrogel (44.76%). Fg constituted the second performed (viscosity was measured at different shear rates).
major protein in the hydrogel at 39.30%. The percentage of This test, also known as the recovery test, showed that the
total polysaccharides showed a distinct difference between ink is very thixotropic (Figure 2B), that is, it takes a certain
tendon ECM (0.2–5%) and the hydrogel (14.85%), primarily time to reach equilibrium once a high mechanical force has
due to Alg (10.92%). While not naturally occurring in been applied to it. In this case, each of the test steps lasted
tendons, Alg was introduced for its crosslinking potential for 2 min, which were not enough for the ink to reach
and its role in conferring stiffness to the hydrogel. equilibrium (the rebuild time could not be determined
Conversely, the percentage of glycosaminoglycans from the procedure used in this test). Specifically, it
(GAGs) remained similar for both (0.2–5% in tendons recovered 36.12% of the initial viscosity the first time it
and 3.93% in the hydrogel). In summary, a high-degree was subjected to the stimulus (viscosity 455.925 Pa·s)
resemblance of the hydrogel to the tendon ECM in terms and 24.38% of the initial viscosity from the second time
of constituent compositions suggests that the hydrogel is (viscosity 307.767 Pa·s), with the initial viscosity being
an excellent candidate for tenocyte incorporation and for 1262.16 Pa·s. Regarding 3D printing, this result implies
the regeneration of damaged tendons. that once the ink has been deposited on the surface, it takes
more than 2 min to recover the initial viscosity that it had
3.2. Ink characterization in the cartridge. Even though the recovery of the solid-
3.2.1. Rheological characterization like behavior was not very fast under test conditions, the
The developed ink was rheologically characterized. These difference between the temperatures of the rheology test
rheological characterization tests helped determine some and the real 3D printing process (the cartridge temperature
of the most important mechanical properties of the ink was lower than 25°C and the printing bed temperature was
(summarized in part (B) of the Supplementary File), 4°C) suggests that the recovery after printing will become
allow comparison of the ink with other inks, and more faster, allowing good shape fidelity.
importantly predict the ink’s behavior during 3D printing.
The strain sweep allows determining the LVER, a
First, the ability of the ink to be extruded during the 3D region in which the G’, which indicates the solid-like
printing process is analyzed. By analyzing the results of the behavior, and the G’’, which indicates the liquid-like
shear rate sweep test (Figure 2A), it can be established that behavior, are independent of the applied stress (Figure 2C).
the ink has a shear-thinning behavior, that is, the viscosity The value of G’ during the LVER is 323.18 Pa, and that of
decreases as the shear rate increases. By performing a G’’ is 114.27 Pa; therefore, the values of tan ɗ and G* are
Carreau–Yasuda fit (R = 0.99), we established that the zero- 0.35 Pa and 342.79 Pa, respectively. These values are typical
2
rate viscosity is 4039.11 Pa·s while the infinite-rate viscosity of viscoelastic gels, usually between 100 Pa and 500 Pa.
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is 0.18 Pa·s. The difference between the two is possibly The linearity limit (γ ) is 67.18%. The crossover between G’
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L
attributed to the high concentration of polymers and their and G’’, also known as critical strain, was found at 345.23%
polydispersity (broad molecular weight distribution), (G’=G’’=107.21 Pa). This value indicates that from that
among other reasons. These properties confirmed that percentage of strain, the ink will start to flow. In addition,
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Volume 10 Issue 3 (2024) 449 doi: 10.36922/ijb.2632