Surfactant induced bilayer-micelle transition for emergence of functions in anisotropic hydrogel

Multiresponsive Bilayer Hydrogel Actuator with Switchable Shape Morphing Capability and Visible Color/Fluorescence Change

Bilayer hydrogels, endowed with multiresponsive and switchable color-changing properties, have garnered significant attention for bioinspired artificial intelligent materials. However, the design and fabrication of such hydrogels that can fully mimic the adaptation of the live organism, i.e., simultaneous changes in shape, fluorescent, and/or visible color, still remain significant challenges. Herein, a multiresponsive (e.g., temperature, salt, and pH) and multiadaptive (shape, fluorescent color, and visible color changes) hydrogel was fabricated by employing monomers featuring pH-responsive fluorescence 4-(2-(4-(dimethylamino) phenyl)-1-isocyanovinyl) phenol (DP) and switchable color-changing 4-(2-sulfethyl) -1-(4-vinylbenzyl) pyridinium betaine (VPES). The bilayer hydrogel comprises a temperature- and pH-responsive gel layer, poly(N-isopropylacrylamide-co-2-(dimethylamino) ethyl methacrylate), along with a pH-, temperature-, and salt-responsive gel layer, poly(acrylamide-co-2-(dimethylamino)ethyl methacrylate-co-VPES)@DP. Due to the opposite swelling/shrinking behavior between the two layers, the prepared hydrogel exhibits shape changes in response to thermal, salt, and pH stimuli, along with switchable fluorescent color and visible color change that originate from DP and polyVPES, respectively. Apart from multiresponsive behavior, this hydrogel also shows an excellent antifatigue property and high sensitivity, which makes it hold significant potential in many applications. We anticipate that this strategy to realize multiresponsive capability in this work can also inspire the design of the biomimetic smart materials.

Thermoresponsive Lamellar Hydrogels with Tunable Turbidity, Structural Color, and Anisotropic Swelling

We report a thermoresponsive anisotropic photonic hydrogel: poly(dodecyl glyceryl itaconate)/polyacrylamide-poly(N-isopropylacrylamide) hydrogel (PDGI/PAAm-PNIPAM hydrogel). Hydrogels with uniaxially aligned lamellar bilayers possess bright structural color and swelling anisotropy, while PNIPAM-based hydrogels exhibit distinct thermoresponsive properties around a lower critical solution temperature (LCST). Hybridization of thermoresponsive PNIPAM with the lamellar hydrogel can give the anisotropic photonic hydrogel various fascinating thermoresponsive properties, such as structural color/turbid transition, thermoresponsive structural color, and anisotropic deswelling/reswelling behavior by temperature stimuli. The temperature-induced changes in turbidity, structural color, and anisotropic swelling of the gel around the LCST can be tuned by controlling the incorporated PNIPAM density. PNIPAM can be regioselectively incorporated into the specific region of the lamellar hydrogels by photomasking during UV polymerization. The PDGI/PAAm-PNIPAM hydrogel can find diverse promising applications such as smart windows and smart displays.

Mechanical Reinforcement of Lamellar Bilayer Hydrogels by Small Amounts of Co-surfactants

Anisotropic photonic hydrogels with alternatively stacked poly(dodecyl glyceryl itaconate) (PDGI) bilayers and polyacrylamide (PAAm) gel layers are unique soft materials with various functions. It is known that to form the lamellar phase of bilayers, a small amount of co-surfactant sodium dodecyl sulfate (SDS) should be present in the precursor monomer solutions of the gels. However, little is known about the influence of the co-surfactant on the structure of bilayers and on the mechanical properties of such photonic hydrogels. Herein, we chose several co-surfactants and studied the effect of the co-surfactants on the self-assembly behavior of the bilayers and on the mechanical properties of the resulting photonic hydrogels. A macroscopically aligned lamellar phase could be induced for all the co-surfactants. Interestingly, the mechanical response of the photonic hydrogels sensitively depends on the chemical structure of the co-surfactant, especially at large deformation. We hypothesize that doping by small amounts of co-surfactants dramatically changes the anchoring strength and density of PAAm strands onto the bilayer surface, thereby influencing the load transfer efficiency from the bilayer to the PAAm gel layer at large deformation and the rupture of the bilayer. This work provides new understanding in the molecular mechanisms of deformation and strengthening in this soft and anisotropic nanocomposite, helping to design more robust photonic hydrogels.