Electrostatically Reinforced Double Network Granular Hydrogels

Rapid advances in biomedical applications and soft robotics demand load-bearing soft materials that can be processed into complex 3D shapes. Direct ink writing (DIW) enables the fabrication of customizable shapes with locally varying compositions. Hydrogels that are formulated as microgels meet the rheological requirements that DIW imparts on the inks if they are jammed. However, most granular hydrogels are soft because inter-particle interactions are weak. These hydrogels can be reinforced with a second hydrogel, yielding double network granular hydrogels (DNGHs). Yet, DNGHs suffer from low fracture energy. This limitation is addressed by electrostatically reinforcing them. The resulting materials exhibit Young's moduli and fracture energies similar to values of cartilage and muscles. An empirical model is proposed to predict the fracture energy of these reinforced DNGHs, based on the dissipation zone size, contact area, and adhesion energy. These DNGHs can be 3D-N, N-methylene bisacrylamideprinted into free-standing structures exhibiting tuneable mechanical properties at the centimeter scale without the need for supporting structures.