3D Printing of Strong and Tough Double Network Granular Hydrogels

Many soft natural tissues display a fascinating set of mechanical properties that remains unmatched by manmade counterparts. These unprecedented mechanical properties are achieved through an intricate interplay between the structure and locally varying the composition of these natural tissues. This level of control cannot be achieved in soft synthetic materials. To address this shortcoming, a novel 3D printing approach to fabricate strong and tough soft materials is introduced, namely double network granular hydrogels (DNGHs) made from compartmentalized reagents. This is achieved with an ink composed of microgels that are swollen in a monomer-containing solution; after the ink is additive manufactured, these monomers are converted into a percolating network, resulting in a DNGH. These DNGHs are sufficiently stiff to repetitively support tensile loads up to 1.3 MPa. Moreover, they are more than an order of magnitude tougher than each of the pure polymeric networks they are made from. It is demonstrated that this ink enables printing macroscopic, strong, and tough objects, which can optionally be rendered responsive, with high shape fidelity. The modular and robust fabrication of DNGHs opens up new possibilities to design adaptive, strong, and tough hydrogels that have the potential to advance, for example, soft robotic applications.