Salt-Responsive Bilayer Hydrogels with Pseudo-Double-Network Structure Actuated by Polyelectrolyte and Antipolyelectrolyte Effects

Development of stimuli-responsive, shape-transformable materials is fundamentally and practically important for smart actuators. Herein, we design and synthesize a bilayer hydrogel by assembling a polycationic (polyMETAC/HEAA) layer with polyelectrolyte effect and a polyzwitterionic (polyVBIPS) layer with antipolyelectrolyte effect together. The bilayer hydrogels adopt a pseudo-double-network structure, and both polyelectrolyte and polyzwitterionic layers have salt-responsive swelling and shrinkage properties, but in a completely opposite way. The resulting polyMETAC/HEAA-polyVBIPS bilayer hydrogels exhibit bidirectional bending in response to salt solutions, salt concentrations, and counterion types. Such bidirectional bending of this bilayer hydrogel is fully reversible and triggered between salt solution and pure water multiple times. The bending orientation and degree of the bilayer hydrogel is driven by the opposite volume changes between the volume shrinking (swelling) of polyMETAC/HEAA layer and the volume swelling (shrinking) of polyVBIPS layer. Such cooperative, not competitive, salt-responsive swelling-shrinking properties of the two layers are contributed to by the polyelectrolyte and antipolyelectrolyte effects from the respective layers. Moreover, an eight-arm gripper made of this bilayer hydrogel is fabricated and demonstrates its ability to grasp an object in salt solution and release the object in water. This work provides a new shape-regulated, stimuli-responsive asymmetric hydrogel for actuator-based applications.