Addressing Rheological Issues at the Micro-Extrusion and Layer-Stacking Stages of Collagen Bioprinting

Biofabrication by 3D-printing is a promising method in tissue engineering which permits the processing of a wide range of hydrogels for restoration, replacement, and regeneration of tissues and organs. Among hydrogels, collagen is the most widely used one for 3D-printing, due to its hydrophilic structure with natural binding sites, resulting in high cell viability and proliferation rates. In this paper, we reviewed bioprinting and crosslinking of cell-laden collagen based bioinks and their shape integrities and cell viabilities in the final constructs. This paper is concerned with the role of the rheology on the bioprinting of collagens. The occurrences' of flow, gelling and crosslinking during 3D-printing are examined under two sequential stages: 1) micro-extrusion, 2) layer stacking. The main objective of this paper is to discuss the impact of rheology of collagen hydrogels on those two stages of bioprinting. In these areas, it is generally considered that characterizations by dynamic linear deformation measurements are sufficient. However, we reviewed the rheological properties of collagen solutions under dynamic linear deformations and steady-state shear flow conditions. While the dynamic measurements are more useful to characterize structures of collagen gels and their changes by crosslinking, the steady shear flow measurements are used to investigate the filament micro-extrusion and layer-stacking. For the first time to understand those stages of the collagen 3D-bio printing process, we brought the role of other non-Newtonian material functions, such as first normal stress difference and extensional viscosity in addition to shear viscosity. Extensional viscosity and the viscoelasticity manifested through normal-stress differences are significant in needle extrusion flow. We also suggested caution to use dynamic viscosity vs. oscillation frequency data in the place of steady shear viscosity vs. shear rate measurement. Finally, we discuss the role of flow conditions and crosslinking on cell viability. Those discussions are focused on collagens, nevertheless they are valid on the 3D-printing of other hydrogels.