High-Resolution 3D Printing of Stretchable Hydrogel Structures Using Optical Projection Lithography

Double-network (DN) hydrogels, with their unique combination of mechanical strength and toughness, have emerged as promising materials for soft robotics and tissue engineering. In the past decade, significant effort has been devoted to synthesizing DN hydrogels with high stretchability and toughness; however, shaping the DN hydrogels into complex and often necessary user-defined two-dimensional (2D) and three-dimensional (3D) geometries remains a fabrication challenge. Here, we report a new fabrication method based on optical projection lithography to print DN hydrogels into customizable 2D and 3D structures within minutes. DN hydrogels were printed by first photo-crosslinking a single network structure via spatially modulated light patterns followed by immersing the printed structure in a calcium bath to induce ionic cross-linking. Results show that DN structures made by this method can stretch four times their original lengths. We show that strain and the elastic modulus of printed structures can be tuned based on the hydrogel composition, cross-linker and photoinitiator concentrations, and laser light intensity. To our knowledge, this is the first report demonstrating quick lithography and high-resolution printing of DN (covalent and ionic) hydrogels within minutes. The ability to shape tough and stretchable DN hydrogels in complex structures will be potentially useful in a broad range of applications.