Designing advanced hydrogel inks with direct ink writing based 3D printability for engineered biostructures

The advent of 3D printing technology in the 1980s enabled the fabrication of patient-specific products with precise shape, size, and complexity. A variety of 3D printing techniques, such as direct ink writing by extrusion, particle fusion printing, light-induced printing, volumetric bioprinting, and inkjet printing, have been explored recently for the purpose of developing desired 3D printed objects. Among these techniques, direct ink writing has been used extensively due to its affordability, ease of use, potential for scaling up, and faster speed. This approach saves time and resources while allowing for the flexible use of various materials. The lack of a variety of varied printing inks, however, continues to be an impediment to its widespread commercial use. Owing to their quick gelling behaviour, extrudability, and shape fidelity, injectable hydrogels have become interesting alternatives for printing inks. Hydrogel inks can be developed by either natural or synthetic polymers, or a combination of the both, to achieve the desired rheological and mechanical properties utilizing various strategies of crosslinking and incorporation of nanoparticles like nanoclay, bioactive glass, graphene oxide and silica nanoparticles. Further, various stimuli responsive hydrogel inks responding to various external stimuli (such as temperature, pH, magnetic field) can be developed to prepare engineered biostructures that can be used in healthcare sector. This review provides a spotlight on recent advancement in developing hydrogel inks and their different physicochemical aspects for developing desired engineered biostructures. Finally, we discuss the opportunities of 4D printing, current difficulties and future prospects for hydrogel inks-based 3D printing to emphasize the significant scope for future innovations.