6. 3D Bioprinting of Non-viscous Bioink

Photo-crosslinkable hydrogels have great potential as bioinks. These materials have been developed over the past few decades to encompass a wide range of properties, and they have been of significant interest for their applicability in cell encapsulation and tissue formation. Despite the plethora of photo-crosslinkable hydrogels under development in the biomaterials field, their application to bioprinting is hindered through their generally low initial viscosity and challenges in polymerizing fast enough to maintain printed structures. To overcome this limitation, photo-crosslinkable hydrogels have been combined with polymers that gel through other mechanisms, such as with supramolecular assembly, temperature, or exposure to ions. This is not ideal, as it alters the material environment for cells. To address these challenges in printing photo-crosslinkable materials, here we present a generalizable bioprinting method to enable 3D printing of hydrogel structures from photosensitive precursors. In this approach, we introduce the light through a photo-permeable capillary (e.g., silicon tubing, glass) to crosslink the hydrogel immediately prior to leaving the needle and before deposition, which we termed in situ crosslinking. Advantages to this approach are (i) that it does not include any viscosity modulation or copolymerization with other polymers, (ii) that it can be generalized to different photo-crosslinkable hydrogel formulations, (iii) that it permits the encapsulation of viable cells, and (iv) that it can be used to print heterogeneous and complex structures.