Fig. 9. Assessing printability.; (i) CAD file of construct, (ii) Post-printing photographs of the printed construct; printed using a syringe-based extruder, (iii) Post-printing accuracy (%) calculated for the printed alginate constructs (using height and width). Scale bars represent 5 mm. Data points are means; error bars represent SEM for n = 6.
Using the measurements of height and width (mm), of both Fig.9(i) and Fig.9(ii), the printing accuracies (%) were calculated for the assembled hydrogel extruder (Fig.9(iii)). Printing accuracies (%) for height and width were calculated at 64.13 % and 64.58 %, respectively. The settings chosen turned out to be suitable for experimental trials based on cell-free 6 % (w/v) concentrated hydrogel inks, hence these parameters were kept for the remaining experiments except for flow rate and needle gauge optimization studies.
Optimization studies: Does needle gauge and flow rate affect printability?
Needle gauge
The printing accuracy (%) was determined using the following equation;
\begin{equation} \text{Printing\ Accuracy\ }\left(\%\right)\frac{\text{Experimental\ Value}}{\text{Theoretical\ value}}x\ 100\nonumber \\ \end{equation}
A 6% (w/v) alginate solution was used as extrusion material to print the designed constructs with subsequent cross-linking in CaCl2 solution for 2 minutes. Images were taken of the cross-linked constructs and measurements determined with ImageJ software (v1.4.6.r). Prints were repeated three times for each needle gauge tested (15G, 18G, 23G and 25G). The flow percentage used in the experiment was 100%. This experiment aimed to determine the effect of nozzle diameter (using different needle gauges) on the printed construct shape quality, under certain constant parameters (flow rate = 100%, print speed = 10 mm/s).
Kahl et al. (2019) found that the needle diameters are essential in optimizing printing resolutions such that; the larger the needle diameter, the smaller the resolution and therefore the lower the printing accuracy. The choice in needle diameter, length and shape (i.e. conically shaped or cylindrical) has additional consequences in terms of cell viabilities. Findings suggest shorter, conically shaped needles allow for higher cell viabilities, post-printing, compared to cylindrically shaped needles [11,22,23]. These findings have major implications in cell-laden bioprinting approaches but little implications in cell-free approaches, as the cells are seeded onto the bioprinted constructs post-printing.