Multimaterial Hydrogel 3D Printing

In recent years, hydrogels have emerged as quintessential 3D printing materials. Coupled with their inherent printability, the unique mechanical properties, network structure, and biocompatibility of hydrogels make them ideal for a wide range of 3D printing applications. Also in recent years, the rise of multimaterial 3D printing, an additive manufacturing technology which involves at least two different materials in the fabrication process, has been witnessed. Advanced multimaterial 3D printing protocols have brought the field closer than ever before to a new industrial revolution. In this review, the use of hydrogels in multimaterial 3D printing is investigated. The review is sectioned by discussing three major multimaterial 3D printing methods: direct‐ink‐writing, vat‐switching, and coaxial extrusion. Subsections detail three common domains of multimaterial hydrogel 3D printing: bioprinting, 4D printing, and particle‐polymer composite printing. In the second part of the review, recent advancements in both multimaterial 3D printing hardware and hydrogel inks which are expected to steer the field in exciting new directions are explored. Finally, a case is made for coaxial extrusion and light‐responsive printers being the best choice for multimaterial hydrogel 3D printing in the long run, due to their gradient and greyscale printing capabilities.