## 3 Results and Discussion
### Rheological Characterization  
The study of bioink rheological properties is a prerequisite for a successful extrusion-based process. Indeed, it enables a preliminary evaluation of the bioink extrudability and printability [16]. Therefore, SA-G bioink rheological behavior, including its viscosity, viscoelastic properties, and shear and elastic recovery, was deeply investigated. The flow curves performed on the SA-G precursor solutions with and without cells are displayed in Figure 5a. From these curves, the presence of a pseudoplastic (or shear thinning) behavior, i.e., a decrease in the material viscosity when applying a shear stress, can be evaluated. The pseudoplastic behavior is a requirement that a bioink must fulfill to be suitable for extrusion-based 3D bioprinting [16]. Specifically, shear-thinning behavior is necessary to extrude with ease a material that has a relatively high zero-shear-rate viscosity (i.e., the viscosity of the material when no shear rate is applied) through a narrow nozzle. Indeed, during the extrusion, a high shear rate is applied to the material, thus reducing its viscosity and allowing for a more uniform filament deposition. As for the SA-G solution without cells, a zero-shear-rate viscosity plateau of 1000 Pa-s was encountered, followed by shear-thinning behavior starting from 0.1 s\({}^{-1}\). The presence of cells did not significantly impact the zero-shear-rate viscosity value nor the pseudoplastic behavior, in agreement with the findings of [PERSON] et al. [36]. Another desirable rheological feature of a bioink is represented by the presence of a yield stress, which is the stress value at which the material starts flowing and, therefore, an indication of the material's ability to maintain its own shape after the extrusion process. A high yield stress will prevent the printed structure from collapsing before cross-linking occurs. Such a characteristic was evaluated by plotting the viscoelastic properties, namely the storage modulus (G\({}^{\prime}\)) and loss modulus (G\({}^{\prime\prime}\)), of the SA-G solutions (Figure 5b). For all solutions, at low strain values, G\({}^{\prime}\) was higher than G\({}^{\prime\prime}\), thus implying that the materials have solid-like behavior, whereas starting from a strain of approximately 800%, G\({}^{\prime\prime}\) was higher than G\({}^{\prime}\), resulting in liquid-like behavior of the materials. The point at which G\({}^{\prime}\) and G\({}^{\prime\prime}\) have the same value is called the crossover point, and it indicates the presence of a yield stress. Indeed, after the crossover point, the material can flow, thus enabling extrusion during the 3D printing process. All inks exhibited a crossover, thus showing the presence of a clear yield stress and, hence, their extrudability. No significant differences in G\({}^{\prime}\) and G\({}^{\prime\prime}\) values were encountered between cell-laden bioinks and the bioink without cells. Recovery tests were performed to evaluate the material's ability to regain its original properties after printing. Since rheological properties were not affected by the presence of cells at any cell density, these tests were carried out on the SA-G bioink without cells. The results of the strain recovery test are presented in Figure 5c. The viscosity at resting conditions was about 1000 Pa-s; then, when a high value of shear rate was applied, the viscosity value dramatically dropped. These results agreed with the previous findings regarding zero-shear-rate viscosity and pseudoplastic behavior. Once the high shear was removed, the viscosity returned to its initial resting value after a small transition region. Such an outcome proved that the SA-G material has fast recovery kinetics, which is essential to ensure high shape fidelity. The good material recovery ability was confirmed by the elastic recovery test, where rapid transitions from solid-like to liquid-like behavior were found when alternating stress values were applied (Figure 5d).