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Concentric bioprinting of alginate-based tubular constructs using multi-nozzle extrusion-based technique

complete gelation after printing. Although some re-
search has been done on cross-linking alginate gels
simultaneously during printing, the process was
shown to be relatively slow as they were done in the
cross-linking solution or on the surface of the alginate
solution. Thus, the resolution of the fabricated struc-
ture is influenced by parameters such as surface ten-
sion and the speed of the extrusion, rendering a
non-direct way to control resolution.
In this study, XG was selected to formulate an op- Figure 2. Viscosity as a function of shear rate for alginate-
timal viscosity for alginate to retain its shape fidelity xanthan gum gel. Xanthan gum concentrations varied from 1.0%
after printing. XG is an anionic polysaccharide pro- (blue diamond), 1.5% (orange square), 2.0% (grey triangle), 2.5%
(yellow cross) and 3.0% (blue asterisk).
duced by the bacterium Xanthomonas capestris. XG
has been used in multiple applications ranging from computer-aided design profile was replicated suc-
food [35] , agriculture [36] , petroleum [37] and in the phar- cessfully in the bioprinted hydrogel. The printing
maceutical industry [38] . It is used mainly as a viscosity quality was represented using the following measure-
enhancer and stabilizer in blends and has been re- ments, namely tubular length (t), wall thickness (w),
ported to contain bio-adhesive properties [39] . Since XG and roundness (R). For calculation of roundness in the
has only weak interactions with CaCl 2, it serves as a equation 2, we require the perimeter (P) and the area
pure filler and will have minimum effect in the (A) of the inner cavity of the tube. Printing effects
cross-linking process. To understand the viscosity of in Figure 3 such as Spreading (e1 and e2) and opaque
the hydrogel and its shear thinning properties, the rhe- layer thickness (OW) were also proposed and dis-
ology behaviour of the hydrogel was characterized cussed.
(Figure 2). With increasing amounts of XG, the solu- Tubular length (t) helps to quantify if the layer
tion tends toward a more viscous behavior as expected. thickness was calibrated correctly such that sufficient
This viscosity will affect the volume of hydrogel ex- hydrogel was deposited to form layers additively to
truded and the overall printing fidelity of the con- eventually achieve the desired tubular height in ver-
struct. tical configuration. To optimize the printing, a group
of printing parameters such as extrusion speed, nozzle
3.3 Quality of Printing
diameter and pressure of the extrusion were synchro-
Tubular constructs were bioprinted successfully in the nized to enable the calibrated system to deposit material
vertical configuration using the concentric printing me- accurately on top of the previously printed layers [40] .
thod developed in this study. In general, the designed With proper calibration, defects and delamination

Figure 3. Parameters for measurements recorded to quantify the printing quality of a bioprinted tubular construct.

52 International Journal of Bioprinting (2015)–Volume 1, Issue 1