Hydrogel-based inks for extrusion 3D printing: A rheological viewpoint

Extrusion 3D printing has achieved significant progress, emerging as one of the most important 3D printing methods for designing biologically relevant organs or tissue substitutes by bioprinting cell-laden inks. Swollen polymeric networks, or hydrogels, have emerged as the preferred biomaterial for fabricating cell-encapsulated inks appropriate for layer-by-layer extrusion through nozzles. The design aspects of the hydrogels play a crucial role in determining the flow behavior of these inks. The review first overviews the fundamentals of rheological measurements in extrusion-based 3D printing, followed by hydrogel ink design approaches, and their implications on the rheological properties. We also discuss the effect of cell density on rheology and 3D bioprinting outcomes. We identify the existing challenges in the field of extrusion bioprinting and discuss future directions to address them.