## 3 Results and Discussion
### Printability Assessment  
Once it was verified that the SA-G bioink met the rheological requirements needed for the extrusion-based process, filament formation and printing accuracy were assessed. The first test consisted in printing four lines at different speeds and pressures with a 22G nozzle to evaluate filament continuity and the average line width using an optical microscope (Figure 7a). At all applied pressures, the average line width decreased as the speed increased (Figure 7b), achieving an average line width of 0.45 \(\pm\) 0.124 mm at 25 kPa and of 0.5 \(\pm\) 0.2 mm at 30 kPa for a printing speed of 20 mm/s, thus approximating, in both cases, the value of the nozzle inner diameter. The printing of narrower lines is due to a smaller amount of deposited material when higher speeds are applied. However, these speeds led to filament discontinuity and inhomogeneities. In contrast, filaments printed at 5 and 10 mm/s were continuous and homogeneous. Applying a higher pressure resulted in the deposition of a greater amount of material, as it induced higher flow rates. Therefore, at increasing pressures, broader lines were obtained. For instance, at a printing speed of 10 mm/s, the average line width was 0.592 \(\pm\) 0.079 and 0.7 \(\pm\) 0.1 for a pressure of 25 and 30 kPa, respectively. Thus, when using the 22G nozzle, the best compromise between printing resolution and filament continuity was achieved at a printing speed of 10 mm/s and an applied pressure of 25 kPa. Finally, the effect of nozzle diameter on printing resolution was evaluated by repeating the test using a 25G conical nozzle. In this case, to obtain a continuous filament, a minimum pressure of 120 kPa, nearly five times the pressure needed with the 22G nozzle, was required. Consistent with the results obtained using a 22G nozzle, the average line width decreased with increasing speed (Figure S1). The 25G nozzle provided higher printing resolution compared with the 22G nozzle, achieving an average line width of 0.30 \(\pm\) 0.091 at a printing speed of 20 mm/s. These findings indicate that nozzle diameter significantly impacts the printing conditions and resolution. The choice of nozzle diameter should be tailored to the desired application. For bioinks containing cells, and especially primary cells, it is crucial to minimize cell death by optimizing the printing conditions. According to the literature, increased dispensing pressure is a major cause of cell damage due to elevated shear stresses on the cells [38]. As future studies will focus on assessing the formation of a functional photoreceptor layer that includes primary stem cells within the SA-G bioink, the 22G nozzle, a printing speed of 10 mm/s, and a pressure of 25 kPa were selected for further printability tests and for 3D bioprinting. Printing accuracy was investigated through the filament fusion test, calculation of the printability index, and the comparison of real dimensions with the digital ones. Based on the results of the filament fusion test, filaments merged only when printed at a filament spacing of 0.5 mm (Figure 7c). The same outcome was obtained when printing Nivea Creme, considered in the literature as a demonstration ink, proving SA-G bioink's  
Figure 6: Viability (%) of cells determined by the alamarBlue assay after 1, 5, and 7 days of contact with SA-G degradation products. Error bars represent the standard deviation. SA-G sodium alginate-gelatin. ability to print fine details like small pores with sharp angles [39]. To better investigate such capabilities, small pore geometries were obtained by printing porous circular discs (Figure 7d). Consequently, the printability index (Pr), which depends on the pore perimeter and area, was derived. The average Pr (1.080 \(\pm\) 0.0025) was close to the ideal Pr, showing a high geometric accuracy. Ultimately, no relevant difference was encountered between the average diameter of the printed constructs (30.25 \(\pm\) 0.056 mm) and that of the digital model (30 mm).