pH-Sensitive Janus Hydrogels with Bidirectional Bending Behaviors

This study introduces a novel design for pH-sensitive bilayer Janus hydrogels with bidirectional bending capabilities, achieved through the layer-by-layer polymerization of two distinct hydrogel layers. The upper layer, composed of P(HEMA-co-4VP), swells in acidic environments due to the protonation of pyridine groups, inducing bending toward one side. Conversely, the lower layer, made of P(HEMA-co-AA), expands in alkaline conditions by deprotonation, causing bending toward the opposite direction. The bilayer hydrogel demonstrates significant shape deformations, influenced by variations in layer thickness and pH conditions. Detailed analyses of swelling behaviors, compressive toughness, and rheological properties reveal that the unique composition ensures distinct and tunable mechanical responses. Furthermore, hydrogels fabricated into complex geometries, such as flower- and starfish-shaped structures, showcase practical applications in soft robotics and biomimetic devices. The versatility of the design is further confirmed by substituting P(HEMA-co-4VP) with P(HEMA-co-DMAEMA), which maintained the bidirectional bending responses under both acidic and alkaline conditions. This research expands the theoretical understanding of stimulus-responsive hydrogels and offers an innovative platform for designing intelligent actuators adaptable to diverse and dynamic environments.