## 3 Results
### Effect of Printing Speed and Tubular Structure Diameter on Structural Irregularity and Stability  
In the previous sections, an optimal setting of printing parameters is identified to ensure the good printing quality of the tubular structure. However, the printing time efficiency is also critical to the whole printing process, which is related to the printing travel speed. Firstly, in this section, the tubular structure irregularity as a function of printing speed is investigated. The optimal setting of printing parameters determined from previous sections is as follows: bath temperature at 22 \({}^{\circ}\)C, extrusion flowrate is 4.8 mL/h, travel speed is 200 mm/min, layer height is set as 0.5 mm (to ensure a strong connection between layers), and the hydrogel temperature is always maintained at 37 \({}^{\circ}\)C. When altering the travel speed, the ratio between flowrate and travel speed should be maintained at a fixed value to ensure a constant filament cross sectional area. In Figure 7a-c, the 3 pairs of flowrates and travel speeds are (a) 0.96 mL/h, 40 mm/min; (b) 4.8 mL/h, 200 mm/min, (a) 9.6 mL/h, 400 mm/min. The (a)-(c) shows the cross section view and side view of printed tubular structures at different flowrate and travel speeds. The images indicate that the irregularity is different between 3 prints. The tubular structure printed at high travel speed exhibits more uniform surface characteristics than at lower speeds. The underlying mechanism can be described as follows. Since the ratio between travel speed and the extrusion flowrate is fixed, higher travel speeds correspond to higher extrusion flowrates. Based on the fact that the gelatin hydrogel is a shear-thinning biomaterial [27], the high extrusion  
Figure 6: Effect of support bath temperature on the printed tubular structure quality. (**a**) The plot of the printed tubular structures quality as a function of support bath temperature and layer height; (**b**) Color image of the 3 types (“oversize”, “well-printed”, “weak connection”) tubular structures; (**c**) Color image of “break” tubular structure. flowrate imparts a high shear rate of the gelatin-alginate hydrogel, which corresponds to low viscosity. During extrusion, material at a lower viscosity experiences less intermolecular internal friction, yielding an irregular printed filament surface. Similarly, the support bath is also a shear-thinning material. When the travel speed is high, the shear-thinning region experiences lower viscosities accompanied by a larger surface tension [28]. In summary, the filaments extruded at high flowrates are less viscous and exhibit a smooth surface (larger surface tension) when exiting the nozzle, and are deposited within a less viscous support bath environment with large surface tension. This mechanism results in a more uniform surface for the layered structure at high travel speeds. To explore differences in the filaments between the 3 prints, the (d)-(f) shows cross-section views with boundary boxes of filaments printed at the 3 different pairs of parameters. In Figure 7g shows a combined plot of the edge irregularity and edge ratio (defined as length/width of the boundary boxes in (d)-(f)). The larger irregularity corresponds to a lower edge ratio. When printing at a lower speed, the filament is more similar to a circle, which causes the fused filaments to be more irregular. The other structural property investigated here is the overall structural stability. It is demonstrated that printing within a support bath medium enables complex structure fabrication. However, the designed tissue construct itself should be structurally stable. Otherwise, the structure may collapse upon extraction of the support material.