Can conical nozzles improve print quality in embedded 3D bioprinting?

Embedded 3D printing is a fabrication technique wherein a nozzle is embedded into a support bath and extrudes either continuous filaments (Embedded Ink Writing) or droplets (Embedded Droplet Printing). The support bath, which is usually a viscoelastic hydrogel, holds the form of the printed part until the part is cured and removed from the bath. Embedded 3D printing enables more flexibility in the rheology of the ink than freestanding direct ink writing, allowing for low-viscosity inks and inks without yield stresses. As a result, this technique is particularly useful for bioprinting. In this work, we use computational fluid dynamics simulations in OpenFOAM to compare the effects of conical and cylindrical nozzles on print quality. Commonly, conical nozzles are used to improve cell survival during printing of bio-inks. Confirming this practice, these simulations indicate that with increasing nozzle angle, the shear stresses inside of the nozzle decrease. However, larger nozzle angles can lead to surface roughness and anisotropy in printed parts. As such, during nozzle selection for bioprinting applications, the trade-offs between shear stress and filament shape defects should be considered.