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

the best printing quality with the material as seen The photos of the printed construct are shown in
in Figure 6. best printing quality was achieved for Figure 8. When the concentration of XG used is too
formulation of 1.5% to 2.5%. Spreading of the materi- low (1%), we can see that there is an overall spreading
al increased at low and high viscosity indicating an of the hydrogel as compared to the rest of the con-
optimal threshold of spreading could be related to the struct. Additionally, in Figure 8(E), the hydrogel
viscosity of the material. The decrease in the spread- printed showed substantial shrinkage after cross-link-
ing angle at 3% could indicate that the initial layer did ing. This further strengthens the need of an optimal
not have sufficient surface area to provide sufficient material viscosity as cross-linking diffusion was too
compressive strength to withstand the weight of the slow at high viscosity, thus affecting the overall shape.
hydrogel that was layered on it. This can be seen from the defects and cave-in shown
in Figure 8(J).
3.7 Opaque Layer Thickness The constructs developed an opaque interior due to
The clear variation between the opaque area and the the cross-linking effect of CaCl 2 on the alginate-XG
transparent area indicates that the hydrogel is rela- gel, creating a gradient of mechanical properties ra-
tively viscous with the diffusion coefficient of the gel dially towards the outer wall of the gel wall. From the
reaching close to 1 [41] .The constructs develop an opa- results, it could be concluded that using 2% XG pro-
que interior due to the cross-linking effect of calcium duced the best printing quality with optimal roundness,
ion on the alginate-XG gel. The opaque wall is related minimum spreading and optimal diffusion rate.
to the diffusion rate of the cross-linking agent in the 3.8 Extending Process Capability
hydrogel. Based on estimation from equation (1), the
thickness of the opaque area should be inversely pro- Demonstrating the process capability of this novel
portional to viscosity. The increase in concentration of approach, a tubular construct of 15 mm length was
XG increases the number of molecular chains block- fabricated in the vertical configuration using the 2%
ing the path of the ions as it diffuses into the gel. Thus, XG-alginate gel as shown in Figure 9.
the viscosity increases, reducing the diffusion rate as
shown in Figure 7.

Figure 9. A tubular construct of 15 mm in the vertical configu-
ration.

Compared to printing of the 4.8 mm tubular struc-
Figure 7. Opaque layer thickness of tubular construct at dif- ture, there seems to be a shrinkage effect caused by
ferent concentrations of xanthan gum.
the CaCl 2 gelation interaction at higher tube height.
This shrinkage could potentially be resolved by using
a lower concentration of CaCl 2. In summary, printing
at 2% XG is effective despite an increase in printed
length of tubular structure with minimal deviation
from its designated shape and minimal spreading.
4. Conclusion

This paper has shown that tubular constructs can be
successfully bioprinted in vertical configuration by
Figure 8. Printed alginate-xanthan gum tubular structure at
different concentrations of xanthan gum. controlling the viscosity of the hydrogel and through
54 International Journal of Bioprinting (2015)–Volume 1, Issue 1