Does the Size of Microgels Influence the Toughness of Microgel-Reinforced Hydrogels?

Rapid advances in the biomedical field increasingly often demand soft materials that can be processed into complex 3D shapes while being able to reliably bear significant loads. Granular hydrogels have the potential to serve as artificial tissues because they can be 3D printed into complex shapes and their composition can be tuned over short length scales. Unfortunately, granular hydrogels are typically soft such that they cannot be used for load-bearing applications. To address this shortcoming, individual microgels can be connected through a percolating network, such that they introduce the double network toughening mechanism into granular hydrogels. However, the influence of the microgel size and concentration on the processing and toughness of microgel-reinforced hydrogels (MRHs) remains to be elucidated. Here, it is demonstrated that processing and toughness depend on the inter-microgel connectivity, while the stress at break is solely dependent on the microgel size. These findings offer an in-depth understanding of how liquid- and paste-like precursors containing soft, deformable microgels can be processed into bulk microstructured soft materials and how the size and concentration of these microgels influence the mechanical properties of microgel-reinforced hydrogels.