Fig. 10. Assessment of printability using different nozzle diameters. A: Post-printing accuracy (%) calculated for alginate constructs printed using various nozzle diameters (15G, 18G and 23G), 25G not included in graph. B: Post-printing photographs of alginate constructs printed using (ii) 25G, (iii) 23G, (iv) 18G and (v) 15G needle gauge. (i) CAD file for printed construct. Scale bars represent 5mm. Data points are means; error bars represent SEM for n=3. No statistical significance was found between groups (p = 0.0522; p > 0.05).
Figure 10 shows the images of the printed constructs (post-printing and cross-linking), printed using various needle gauges; 15G, 18G and 23G. The inner diameters for each needle gauge are as follows; 15G = 1.372 mm, 18G = 0.838 mm, 23G = 0.337 mm. A final needle gauge was included in the experiment; 25G (inner diameter = 0.260 mm), however, the printed constructs produced were not uniform and therefore printing accuracies based on filament width were unable to be calculated; Fig. 10b(ii). The printing accuracies (%) produced for each needle gauge experiment include; 136.53 %, 86.01 % and 129.93 % for 15G, 18G and 23G, respectively. Although no statistical significance was found between groups, the 18G needle produced a printing accuracy closest to 100 % (Fig. 10a).
Flow rates
The spreading ratio was determined at different flow percentages (100%, 125%, 150%, 200%) using the following equation:
\begin{equation} Spreading\ ratio=\frac{\text{Filament\ diameter}}{\text{Nozzle\ diameter}}\nonumber \\ \end{equation}
The flow percentages listed are extrusion multipliers. These values instruct the printers to extrude more material during the prints. Post-printing images were taken immediately after printing. Calcium chloride was not added to the printed constructs for cross-linking. A nozzle diameter of 0.838 mm (18G needle) was used for the syringe extruder based on the results obtained from the needle gauge printability experiment. Three constructs were printed for each flow percentage tested. An average of the width, measured at various positions along the filament, of the printed filaments were divided by the needle diameter which remained constant throughout the experiment; 0.838 mm. The constructs were printed using 6% (w/v) alginate hydrogel with no post-printing cross-linking methods employed.