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

Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures

Development of smart soft actuators is highly important for fundamental research and industrial applications but has proved to be extremely challenging. In this work, we present a facile, one-pot, one-step method to prepare dual-responsive bilayer hydrogels, consisting of a thermoresponsive poly­(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly­(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)­propane-1-sulfonate) (polyVBIPS) layer. Both polyNIPAM and polyVBIPS layers exhibit a completely opposite swelling/shrinking behavior, where polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional bending in response to temperatures, salt concentrations, and salt types. Such bidirectional bending, bending orientation, and degree can be reversibly, repeatedly, and precisely controlled by salt- or temperature-induced cooperative swelling–shrinking properties from both layers. Based on their fast, reversible, and bidirectional bending behavior, we further design two conceptual hybrid hydrogel actuators, serving as a six-arm gripper to capture, transport, and release an object and an electrical circuit switch to turn on-and-off a lamp. Different from the conventional two- or multistep methods for preparation of bilayer hydrogels, our simple, one-pot, one-step method and a new bilayer hydrogel system provide an innovative concept to explore new hydrogel-based actuators through combining different responsive materials that allow us to program different stimuli for soft and intelligent materials applications

Structural Dependence of Salt-Responsive Polyzwitterionic Brushes with an Anti-Polyelectrolyte Effect

Some polyzwitterionic brushes exhibit a strong “anti-polyelectrolyte effect” and ionic specificity that make them versatile platforms to build smart surfaces for many applications. However, the structure–property relationship of zwitterionic polymer brushes still remains to be elucidated. Herein, we aim to study the structure-dependent relationship between different zwitterionic polymers and the anti-polyelectrolyte effect. To this end, a series of polyzwitterionic brushes with different cationic moieties (e.g., imidazolium, ammonium, and pyridinium) in their monomeric units and with different carbon spacer lengths (e.g., CSL = 1, 3, and 4) between the cation and anion were designed and synthesized to form polymer brushes via the surface-initiated atom transfer radical polymerization. All zwitterionic brushes were carefully characterized for their surface morphologies, compositions, wettability, and film thicknesses by atomic force microscopy, contact angle measurement, and ellipsometry, respectively. The salt-responsiveness of all zwitterionic brushes to surface hydration and friction was further examined and compared both in water and in salt solutions with different salt concentrations and counterion types. The collective data showed that zwitterionic brushes with different cationic moieties and shorter CSLs in salt solution induced higher surface friction and lower surface hydration than those in water, exhibiting strong anti-polyelectrolyte effect salt-responsive behaviors. By tuning the CSLs, cationic moieties, and salt concentrations and types, the surface wettability can be changed from a highly hydrophobic surface (∼60°) to a highly hydrophilic surface (∼9°), while interfacial friction can be changed from ultrahigh friction (μ ≈ 4.5) to superior lubrication (μ ≈ 10^–3). This work provides important structural insights into how subtle structural changes in zwitterionic polymers can yield great changes in the salt-responsive properties at the interface, which could be used for the development of smart surfaces for different applications

Dual Salt- and Thermoresponsive Programmable Bilayer Hydrogel Actuators with Pseudo-Interpenetrating Double-Network Structures

Development of smart soft actuators is highly important for fundamental research and industrial applications but has proved to be extremely challenging. In this work, we present a facile, one-pot, one-step method to prepare dual-responsive bilayer hydrogels, consisting of a thermoresponsive poly­(<i>N</i>-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly­(3-(1-(4-vinylbenzyl)-1<i>H</i>-imidazol-3-ium-3-yl)­propane-1-sulfonate) (polyVBIPS) layer. Both polyNIPAM and polyVBIPS layers exhibit a completely opposite swelling/shrinking behavior, where polyNIPAM shrinks (swells) but polyVBIPS swells (shrinks) in salt solution (water) or at high (low) temperatures. By tuning NIPAM:VBIPS ratios, the resulting polyNIPAM/polyVBIPS bilayer hydrogels enable us to achieve fast and large-amplitude bidirectional bending in response to temperatures, salt concentrations, and salt types. Such bidirectional bending, bending orientation, and degree can be reversibly, repeatedly, and precisely controlled by salt- or temperature-induced cooperative swelling–shrinking properties from both layers. Based on their fast, reversible, and bidirectional bending behavior, we further design two conceptual hybrid hydrogel actuators, serving as a six-arm gripper to capture, transport, and release an object and an electrical circuit switch to turn on-and-off a lamp. Different from the conventional two- or multistep methods for preparation of bilayer hydrogels, our simple, one-pot, one-step method and a new bilayer hydrogel system provide an innovative concept to explore new hydrogel-based actuators through combining different responsive materials that allow us to program different stimuli for soft and intelligent materials applications

General Strategy To Fabricate Strong and Tough Low-Molecular-Weight Gelator-Based Supramolecular Hydrogels with Double Network Structure

Low-molecular-weight gelator (LMWG)-based supramolecular hydrogels, self-assembled by small molecules via noncovalent interactions, have recently attracted great attention due to their unique structure–property relationship and potential applications spanning from functional materials to biomedical devices. Unfortunately, many LMWG-based supramolecular hydrogels are mechanically weak and can not even be handled by conventional tensile and tearing tests. Here, we propose several design principles to fabricate new LMWG-based hydrogels with a true double-network structure (G4·K⁺/PDMAAm DN gels), consisting of the supramolecular self-assembly of guanosine, B­(OH)₃ and KOH as the first, physical G4·K⁺ network and the covalently cross-linked poly­(<i>N</i>,<i>N</i>′-dimethyacrylamide) (PDMAAm) as the second, chemical network. Different from those LMWG-based supramolecular hydrogels, G4·K⁺/PDMAAm DN gels exhibit high tensile properties (elastic modulus = 0.307 MPa, tensile stress = 0.273 MPa, tensile strain = 17.62 mm/mm, and work of extension = 3.23 MJ/m³) and high toughness (tearing energies = 1640 J/m²). Meanwhile, the dynamic, noncovalent bonds in the G4·K⁺ network can reorganize and reform after being broken, resulting in rapid self-recovery property and excellent fatigue resistance. The stiffness/toughness of G4·K⁺/PDMAAm DN gels can be recovered by 65%/58% with 1 min resting at room temperature, and the recovery rates are further improved with the increase of temperatures and resting times. Interestingly, G4·K⁺/PDMAAm DN gels also exhibit UV-triggered luminescence due to the unique G4-quartet structure in the G4·K⁺ supramolecular first network. A new toughening mechanism is proposed to interpret the high strength and toughness of G4·K⁺/PDMAAm DN gels. We believe that our design principles, along with new G4·K⁺/PDMAAm DN gel system, will provide a new viewpoint for realizing the tough and strong LMWG-based gels