Tough Supramolecular Hydrogels Research Articles

Tough Supramolecular Hydrogels Crafted via Lignin-Induced Self-Assembly.

Supramolecular hydrogels are typically assembled through weak non-covalent interactions, posing a significant challenge in achieving ultra strength. Developing a higher strength based on molecular/nanoscale engineering concepts is a potential improvement strategy. Herein, a super-tough supramolecular hydrogel is assembled by gradually diffusing lignosulfonate sodium (LS) into a polyvinyl alcohol (PVA) solution. Both simulations and analytical results indicate that the assembly and subsequent enhancement of the crosslinked network are primarily attributed to LS-induced formation and gradual densification of strong crystalline domains within the hydrogel. The optimized hydrogel exhibits impressive mechanical properties with tensile strength of ≈20MPa, Young's modulus of ≈14MPa, and toughness of ≈50MJm⁻3, making it the strongest lignin-PVA/polymer hydrogel known so far. Moreover, LS provides the supramolecular hydrogel with excellent low-temperature stability (<-60°C), antibacterial, and UV-blocking capability (≈100%). Interestingly, the diffusion ability of LS is demonstrated for self-restructuring damaged supramolecular hydrogel, achieving 3D patterning on hydrogel surfaces, and enhancing the local strength of the freeze-thaw PVA hydrogel. The goal is to foster a versatile hydrogel platform by combining eco-friendly LS with biocompatible PVA, paving the way for innovation and interdisciplinarity in biomedicine, engineering materials, and forestry science.

Engineering Tough Supramolecular Hydrogels with Structured Micropillars for Tunable Wetting and Adhesion Properties.

Soft-lithography is widely used to fabricate microstructured surfaces on plastics and elastomers for designable physical properties such as wetting and adhesions. However, it remains a big challenge to construct high-aspect-ratio microstructures on the surface of hydrogels due to the difficulty in demolding from the gel with low strength and stiffness. Demonstrated here is the engineering of tough hydrogels by soft-lithography to form well-defined micropillars. The mechanical properties of poly(acrylamide-co-methacrylic acid) hydrogels with dense hydrogen-bond associations severely depend on temperature, with Young's modulus increasing from 8.1MPa at 15°C to 821.8MPa at -30°C, enabling easy demolding at low temperatures. Arrays of micropillars are maintained on the surface of the gel, and can be used at room temperature when the gel restores soft and stretchable. The hydrogel also exhibits good shape-memory property, favoring tailoring the morphology with a switchable tilt angle of micropillars. Consequently, the hydrogel shows tunable wetting and adhesion properties, as manifested by varying contact angles and adhesion strengths. These surface properties can also be tuned by geometry and arrangement of micropillars. This facile strategy by harnessing tunable viscoelasticity of supramolecular hydrogels should be applicable to other soft materials, and broaden their applications in biomedical and engineering fields.

Tough supramolecular hydrogels of poly(N,N-dimethylacrylamide)-grafted poly(methacrylic acid) with cooperative hydrogen bonds as physical crosslinks.

Incorporating associative interactions as the energy dissipation units has been recognized as an effective strategy to develop tough hydrogels. For hydrogen-bond associations, however, it is highly challenging to stabilize them under aqueous conditions. Although affording cooperativity can enhance and stabilize the hydrogen bonds, it usually requires stepwise polymerization to form these cooperative associations between different polymers and networks. Here, we report a series of tough supramolecular hydrogels with robust hydrogen-bond associations between grafted polymers that are synthesized by polymerization of a macromonomer of poly(N,N-dimethylacrylamide) (PDMAA) and a small monomer of methacrylic acid. The grafted chains of PDMAA form cooperative hydrogen bonds with the main chain of poly(methacrylic acid) (PMAAc), forming supramolecular hydrogels with high toughness and good stability. The tough and stiff hydrogels are in a glassy state, exhibit forced elastic deformation at room temperature, and remain stable over a wide pH range. In contrast, hydrogels prepared by the copolymerization of DMAA and MAAc are swollen and weak in water due to the lack of successive hydrogen donor/acceptor units and the absence of cooperative hydrogen bonds. In addition, these tough hydrogels exhibit good recyclability and shape memory properties, owing to the supramolecular nature of the network and the temperature-dependent mechanical properties. The influence of polymer structure on the associative interactions and macroscopic properties of the hydrogels should be informative for the design of tough soft materials with versatile applications.

Alternating Growth for InSitu Post‐Programing Hydrogels’ Sizes and Performance

AbstractGrowing polymer materials emerge as a class of new intelligent systems, but they are all covalently cross‐linked. Herein, an alternating growth strategy for enabling tough supramolecular hydrogels is described to integrate externally provided compounds to change their sizes, geometric shapes, mechanical properties, etc. The hydrogels have dynamic acid–base complex (ABC) structures made from acidic and basic monomers, in which the acid–base pairs equilibrate between ionic and nonionic states to show either strong electrostatic or weak acid–base interactions. The growth process includes swelling of acidic/basic monomers, polymerization of the absorbed monomers, and homogenization of the original and newborn polymers, in which the balance between electrostatic and acid–base interactions is reversibly shifted to enable the process. As a result of such repeatable growth cycles, the hydrogels can continuously expand without compromising material mechanical properties. The growing sub‐processes and the monomer compositions can be used to post‐modulate the structure and properties of the grown products. It is demonstrated that the hydrogels may be designed as artificial tissues capable of growth under force‐bearing states.

Digital Light Processing 3D Printing of Tough Supramolecular Hydrogels with Sophisticated Architectures as Impact-Absorption Elements.

Processing tough hydrogels into sophisticated architectures is crucial for their applications as structural elements. However, Digital Light Processing (DLP) printing of tough hydrogels is challenging because of the low-speed gelation and toughening process. Described here is a simple yet versatile system suitable for DLP printing to form tough hydrogel architectures. The aqueous precursor consists of commercial photoinitiator, acrylic acid, and zirconium ion (Zr4+ ), readily forming tough metallo-supramolecular hydrogel under digital light because of in situ formation of carboxyl-Zr4+ coordination complexes. The high-stiffness and antiswelling properties of as-printed gel enable high-efficiency printing to form high-fidelity constructs. Furthermore, swelling-induced morphing of the gel is also achieved by encoding structure gradients during the printing with grayscale digital light. Mechanical properties of the printed hydrogels are further improved after incubation in water due to the variation of local pH and rearrangement of coordination complex. The swelling-enhanced stiffness affords the printed hydrogel with shape fixation ability after manual deformations, and thereby provides an additional avenue to form more complex configurations. These printed hydrogels are used to devise an impact-absorption element or a high-sensitivity pressure sensor as proof-of-concept examples. This work should merit engineering of other tough gels and extend their scope of applications in diverse fields.

Tough, Anti-Swelling Supramolecular Hydrogels Mediated by Surfactant-Polymer Interactions for Underwater Sensors.

It is a great challenge for traditional hydrogel-based sensors to be effective underwater due to unsatisfactory water resistance and insufficient wet adhesion. Herein, a tough supramolecular hydrogel aiming at underwater sensing is prepared by the modification of hydrophilic poly(acrylic acid) (PAA) with a small amount of hydrophobic lauryl methacrylate (LMA) in the presence of high concentrations of the cationic surfactant cetyltrimethylammonium bromide (CTAB). Owing to the synergistic effects of the electrostatic interactions and hydrophobic associations of CTAB with the P(AA-co-LMA) copolymer, the hydrogel with a water content of approximately 58.5 wt % demonstrates outstanding anti-swelling feature, superior tensile strength (≈1.6 MPa), large stretchability (>900%), rapid room-temperature self-recovery (≈3 min at 100% strain), and robust wet adhesion to diverse substrates. Moreover, the strain sensor based on the hydrogel displays keen sensitivity in a sensing range of 0-900% (gauge factor is 0.42, 3.44, 5.44, and 7.39 in the strain range of 0-100, 100-300, 300-500, and 500-900%, respectively) and pronounced stability both in air and underwater. Additionally, the hydrogel can be easily recycled by dissolving in anhydrous ethanol. This work provides a facile strategy to fabricate eco-friendly, tough supramolecular hydrogels for underwater sensing.

Integrated multifunctional flexible electronics based on tough supramolecular hydrogels with patterned silver nanowires

A hybrid conductive hydrogel system was facilely integrated with complex circuits. The obtained hydrogel electronics show excellent mechanical and electrical performances, enable monitoring tensile strain, pressure, and temperature.

Tough supramolecular hydrogels with excellent self-recovery behavior mediated by metal-coordination interaction

Based on the strategy of dynamic metal-coordination for improving mechanical properties, a further ideal was taken to prepare the tough hydrogels with the combination of self-crosslinking monomer and metal-coordination complexes. Herein, a series of hydrogels of poly(acrylamide-co-acrylic acid-co- N-hydroxymethyl acrylamide) (P(AM-co-AAc-co-NMAM)) consisting the chemical crosslinking induced by NMAM units and the physical crosslinking derived from the coordination complexes of carboxyl-Fe3+ were prepared. The molecular structure was investigated with the ATR-FTIR, Raman and UV-vis spectra. These hydrogels with different water content of 57–93% possess good mechanical performances. The optimal hydrogels possess high tensile strength (8.56 MPa), prominent modulus (15.5 MPa), remarkable toughness (37.85 MJ/m3) and superb tearing energy (7062 J/m2). The tough hydrogels also display excellent self-recovery (95% toughness recovery within 50 min), pH-triggered healing, shape memory and plasticity abilities. These hydrogels having high strength and toughness may broaden range of potential applications in load-bearing soft actuators, flexible electronics, etc.

Ultrastiff and Tough Supramolecular Hydrogels with a Dense and Robust Hydrogen Bond Network

Design of tough hydrogels has made great progress in the past two decades. However, the synthetic tough gels are usually much softer than some biotissues (e.g., skins with modulus up to 100 MPa). Here we report a new class of ultrastiff and tough supramolecular hydrogels facilely prepared by copolymerization of methacrylic acid and methacrylamide. The gels with water content of approximately 50–70 wt % possessed remarkable mechanical properties, with Young’s modulus of 2.3–217.3 MPa, tensile breaking stress of 1.2–8.3 MPa, breaking strain of 200–620%, and tearing fracture energy of 2.9–23.5 kJ/m2, superior to most existing hydrogels, especially in terms of modulus. Typical yielding and crazing were observed in the gel under tensile loading, indicating the forced elastic deformation of these hydrogels in a glassy state, as confirmed by dynamic mechanical analysis. The ultrahigh stiffness was attributed to the dense cross-linking and reduced segmental mobility caused by the robust intra- and interchain hydr...

Stretchy and strong polyurethane-urea supramolecular (PUUS) hydrogels with various stimulus-responsive behaviours: the effect of chain-extenders.

The development of an easy synthetic strategy combined with straightforward tailoring of physical properties and functionalities, such that optimal performance can be targeted for various applications, still remains challenging. Previously, we reported the construction of thermo- and water-responsive strong and tough supramolecular hydrogels based on the cooperatively enhancing effect between H-bonding and hydrophobic forces. Recently, the strategy was greatly simplified to a one-pot two-step approach. In this work, by simply changing the chain extenders used in the second step, different kinds of functional units are easily introduced into the hard segments in the main chain of the copolymers to endow the resultant strong and tough supramolecular hydrogels with various stimulus-responsive properties. A dynamic covalent bond (disulphide or imine bond, e.g.-S-S- or -C[double bond, length as m-dash]N-) in the main chain provides the resulting hydrogels with reduction or pH responsive degradation (gel-to-sol transition) on demand behaviors, respectively; the azobenzene unit endows the yielding hydrogels with UV-Vis controlled stiffness; the pyridine group containing supramolecular hydrogel shows metal ion responsive mechanical and fluorescent behavior. These results provide progress toward addressing the challenges of achieving structural and synthetic simplicity married with sophisticated functionalities.

Self-healable tough supramolecular hydrogels crosslinked by poly-cyclodextrin through host-guest interaction

A new supramolecular hydrogel with superior self-healing and shape memory properties was synthesized via in-situ copolymerization of the adamantane-containing monomer N-adamantylacrylamide (Ad-AAm) and monomer acrylic acid (AAc) in the polycyclodextrin (PCD) aqueous solution, where PCD served as the polyfunctional physical cross-linker. The PCD-Ad hydrogels showed strength of 40 kPa with breaking strain of 600%, but small tensile loading-unloading hysteresis and residual deformation benefited from the reversible crosslinking structure. Excellent self-healing property was realized at 70 °C and humid condition. The healing percentage reached over 70% after 120 min, which showed enhanced self-healing efficiency compared with the reported supramolecular hydrogels crosslinked by CD and Ad. This may benefit from the polyfunctional PCD crosslinking reagent, which provided more host sites and binding opportunities for the complexation during the self-healing process. Shape memory behavior was also realized through the reversible second crosslinked network by the carboxyl groups of PAAc with Fe3+ ions. The present study provides a new method to improve self-healing ability of the supramolecular hydrogels, which will find the applications in self-healing coating, biomedical materials and soft actuators.

Ultrastrong and Tough Supramolecular Hydrogels from Multiurea Linkage Segmented Copolymers with Tractable Processablity and Recyclability.

Strong and tough synthetic hydrogels have received ever-increasing interests due to their potential applications as load-bearing structural materials. However, strong, tough, and recyclable hydrogels in different forms that can be generated by different methods according to various practical applications still remain an intrinsic bottleneck. A simple one-pot synthesis of multiurea linkage segmented linear copolymers with easy recyclability, hybridization, and processability, including compression molding, solution casting, and spinning methods, to yield ultrastrong and tough hydrogel films or stretchable hydrogel fibers with diameters ranged from macro-, micro-, to nanoscale, is reported here.