Shengwei Xiao

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.

Aqueous lubrication of poly(N-hydroxyethyl acrylamide) brushes: a strategy for their enhanced load bearing capacity and wear resistance

Surface friction is a very important property for biomaterials, especially for those with implantation purposes. Recently, a new hydrophilic polymer, namely, poly(N-hydroxyethyl acrylamide) (PHEAA) has shown significant potential as biocompatible material. Herein, the surface friction of PHEAA brushes was studied. First, PHEAA brushes with well-controlled thickness were prepared via surface-initiated atomic transfer radical polymerization (SI-ATRP). Then, the surface friction coefficients of these brushes in water were measured aiming to better understand the thickness dependence of the surface lubrication properties. The results showed that the surface lubrication of PHEAA brushes highly depends on surface morphology and hydration state. With an optimal polymer film thickness of ∼21.5 nm, PHEAA-grafted surfaces show an ultralow friction coefficient of ∼0.013. To enhance the load bearing and wear resistance of the surface lubrication, a strategy of cross-linking was proposed. Cross-linked gel brushes were prepared by adding cross-linker N,N′-methylene bis(acrylamide) during the SI-ATRP. It was shown that the presence of a cross-linker in the ATRP resulted in high surface roughness, leading to a significant increment of surface friction to ∼2, albeit the water contact angle was lower. However, the strategy of cross-linking showed great success in enhancing the load bearing and wear resistance of polymer brushes. Differing from uncross-linked polymer brushes, cross-linked gel brushes showed decreased friction coefficients when increasing the compression load. Moreover, the wear resistance of the polymer brushes was also improved, as evidenced by the fact that the friction coefficient of cross-linked gel brushes was stable at 2000 s, while the friction coefficient of uncross-linked brushes increased quickly (at ∼700 s).

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( N-isopropylacrylamide) (polyNIPAM) layer and a salt-responsive poly(3-(1-(4-vinylbenzyl)-1 H-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.