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.