Nature creates hydrogels--soft, water-rich materials--with stiffness and toughness spanning over six orders of magnitude. These materials fulfill diverse biological functions tailored to mechanical demands. Inspired by this versatility, synthetic hydrogels have been developed for a wide range of uses. However, conventional single-network hydrogels often suffer from a stiffness-toughness trade-off. This can be partially addressed by designing double network (DN) hydrogels, which combine a rigid first network with a ductile second network. Their interplay results in improved strength and fracture resistance. Still, DN hydrogels are typically limited to casting, preventing use in applications requiring complex 3D structures. To enable such structures, hydrogels can be formulated as double network granular hydrogels (DNGHs), which are processable via extrusion-based 3D printing or casting. DNGHs are composed of microgels--hydrogel microparticles--loaded with precursors of the second network. Once shaped, the precursor is polymerized to form a continuous network interpenetrating the microgels. This architecture imparts DNGHs with greater strength and toughness than conventional hydrogels. However, the underlying reinforcement mechanisms and network architecture are not yet fully understood. As a result, DNGHs often exhibit brittle failure below 200% strain and limited control over mechanical behavior, hindering broader application where tunable properties are essential.

This thesis addresses these challenges by investigating the molecular and microstructural factors that govern the fracture and tensile behavior of DNGHs and by developing strategies to control their toughness and stiffness. The first part explores the role of microgel connectivity and packing density in toughening. The results show that in the jammed state, microgels are strongly linked through the second network, forming a structure analogous to conventional DN gels. The second part focuses on the effects of second network composition and interfacial interactions on fracture energy. A more ductile second network expands the dissipation zone, greatly increasing fracture energy, while reinforced microgel interfaces double the fracture energy. The final part demonstrates how DNGHs can be made piezoresistive by functionalizing interstitial spaces with conductive fillers. Tuning the second network allows precise control of toughness and stiffness over a wide range. The softest formulations detect light touch, while the stiffest detect and withstand body weight. Multi-material 3D printing enables fabrication of sensors with spatially varying properties, allowing detection of both small and large forces within a single device.

This work establishes a framework for engineering the mechanical behavior of microstructured hydrogels and demonstrates their potential in flexible electronics. The developed DNGHs achieve stiffness from tens to hundreds of kilopascals and over a 30-fold increase in fracture energy compared to single-network gels, offering promising opportunities for applications requiring both toughness and rigidity.