Microgel-Reinforced Hydrogel Films with High Mechanical Strength and Their Visible Mesoscale Fracture Structure

The poor mechanical properties remain the largest barrier to traditional synthetic hydrogels for extensive practical applications, such as tissue scaffolds. In this work, we have synthesized the hydrogel films in the presence of microgel precursors of various chemical species with different charges. The hydrogels fabricated have a novel two-phase composite structure, where the continuous phase is a loosely cross-linked polyacrylamide (PAAm) matrix and the disperse phase is virtually double-network (DN) microgels. Named as microgel-reinforced (MR) hydrogels, they exhibited dramatic enhancement in mechanical strength and toughness, in comparison to the hydrogels with no microgels. MR hydrogels showed the comparable mechanical properties with the conventional bicontinuous DN hydrogels. By visualizing the embedded microgels before, during, and after the elongation, mesoscale fractures of the microgels phase were confirmed, which should effectively blunt the crack and enhance the fracture propagation resistance. Therefore, we conclude that the essential reinforcement principle of MR gels roots in the sacrificial bonds effect contributed by the microgels. This work provides a novel universal pathway to synthesize hydrogel thin films with high strength and toughness from various microgels and may open a new avenue for the application of hydrogels in various fields, such as fast responsive actuators, fuel cell films, wound dressings, etc.