Mesoscale bicontinuous networks in self-healing hydrogels delay fatigue fracture.

Load-bearing biological tissues, such as muscles, are highly fatigue-resistant, but how the exquisite hierarchical structures of biological tissues contribute to their excellent fatigue resistance is not well understood. In this work, we study antifatigue properties of soft materials with hierarchical structures using polyampholyte hydrogels (PA gels) as a simple model system. PA gels are tough and self-healing, consisting of reversible ionic bonds at the 1-nm scale, a cross-linked polymer network at the 10-nm scale, and bicontinuous hard/soft phase networks at the 100-nm scale. We find that the polymer network at the 10-nm scale determines the threshold of energy release rate G0 above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transition Gtran far above G0 In situ small-angle X-ray scattering analysis reveals that the hard phase network suppresses the crack advance to show decelerated fatigue fracture, and Gtran corresponds to the rupture of the hard phase network.

Dynamic bonds enable high toughness and multifunctionality in gelatin/tannic acid-based hydrogels with tunable mechanical properties.

Biopolymer-based functional hydrogels with excellent mechanical properties are desired, but their fabrication remains a challenge. Learning from the tofu-making process, we developed a freely formable hydrogel with high toughness and stiffness from the hydrogen bond-rich coacervation of tannic acid and gelatin through a simple hot-pressing process that transforms the coacervate particles into a bulk hydrogel. The mechanical properties of the obtained gelatin/tannic acid hydrogel (G/T gel) can be controlled by tuning the weight ratio of tannic acid to gelatin in the gel. The G/T gel with optimum mechanical properties possesses high Young's modulus, fracture strain, and fracture energy of ∼60 MPa, ∼10, and ∼24 kJ m-2, respectively. These properties arise from the phase-separated structure and high concentration of dynamic hydrogen bonds with widely distributed bond strengths. These dynamic hydrogen bonds also enable multifunctional properties of the gel, such as self-recovery, self-healing, rebuildability and shape memory. The combination of excellent mechanical properties, good biocompatibility, and useful functionalities into one hydrogel that comes from renewable sources demonstrates the great potential of G/T gels.

Polymer Topology Reinforced Synergistic Interactions among Nanoscale Molecular Clusters for Impact Resistance with Facile Processability and Recoverability.

The intrinsic conflicts between mechanical performances and processability are main challenges to develop cost-effective impact-resistant materials from polymers and their composites. Herein, polyhedral oligomeric silsesquioxanes (POSSs) are integrated as side chains to the polymer backbones. The one-dimension (1D) rigid topology imposes strong space confinements to realize synergistic interactions among POSS units, reinforcing the correlations among polymer chains. The afforded composites demonstrate unprecedented mechanical properties with ultra-stretchability, high rate-dependent strength, superior impact-resistant capacity as well as feasible processability/recoverability. The hierarchical structures of the hybrid polymers enable the co-existence of multiple dynamic relaxations that are responsible for fast energy dissipation and high mechanical strengths. The effective synergistic correlation strategy paves a new pathway for the design of advanced cluster-based materials.

Unexpected Elasticity in Assemblies of Glassy Supra-Nanoparticle Clusters.

Granular materials, composed of densely packed particles, are known to possess unique mechanical properties that are highly dependent on the surface structure of the particles. A microscopic understanding of the structure-property relationship in these systems remains unclear. Here, supra-nanoparticle clusters (SNPCs) with precise structures are developed as model systems to elucidate the unexpected elastic behaviors. SNPCs are prepared by coordination-driven assembly of polyhedral oligomeric silsesquioxane (POSS) with metal-organic polyhedron (MOP). Due to the disparity in sizes, the POSS-MOP assemblies, like their classic nanoparticles counterparts, ordering is suppressed, and the POSS-MOP mixtures will vitrify or jam as a function of decreasing temperature. An unexpected elasticity is observed for the SNPC assemblies with a high modulus that is maintained at temperatures far beyond the glass transition temperature. From studies on the dynamics of the hierarchical structures of SNPCs and molecular dynamic simulation, the elasticity has its origins in the interpenetration of POSS-ended arms. The physical molecular interpenetration and inter-locking phenomenon favors the convenient solution or pressing processing of the novel cluster-based elastomers.

Multiscale Energy Dissipation Mechanism in Tough and Self-Healing Hydrogels.

Understanding the energy dissipation mechanism during deformation is essential for the design and application of tough soft materials. We show that, in a class of tough and self-healing polyampholyte hydrogels, a bicontinuous network structure, consisting of a hard network and a soft network, is formed, independently of the chemical details of the hydrogels. Multiscale internal rupture processes, in which the double-network effect plays an important role, are found to be responsible for the large energy dissipation of these hydrogels.

Stretching-induced ion complexation in physical polyampholyte hydrogels.

Recently, we have developed a series of charge balanced polyampholyte (PA) physical hydrogels by random copolymerization in water, which show extraordinarily high toughness, self-healing ability and viscoelasticity. The excellent performance of PA hydrogels is ascribed to dynamic ionic bond formation through inter- and intra-chain interactions. The randomness results in ionic bonds of wide strength distribution, the strong bonds, which serve as permanent crosslinking, imparting the elasticity, while the weak bonds reversibly break and re-form, dissipating energy. In this work, we developed a simple physical method, called a pre-stretching method, to promote the performance of PA hydrogels. By imposing a pre-stretching on the sample in the as-prepared state, ion complexation during dialysis is prominently accelerated and the final performance is largely promoted. Further analysis suggests that the strong bond formation induced by pre-stretching is responsible for the change in final performance. Pre-stretching decreases the entropy of the system and increases the chain alignment, resulting in an increased possibility for strong bond formation.

Molecular structure of self-healing polyampholyte hydrogels analyzed from tensile behaviors.

Recently, charge balanced polyampholytes (PA) have been found to form tough and self-healing hydrogels. This class of physical hydrogels have a very high equilibrated polymer concentration in water (ca. 40-50 wt%), and are strongly viscoelastic. They are synthesized by random copolymerization of equal amounts of oppositely charged monomers at a high concentration, followed by a dialysis process of the small counter-ions and co-ions in water. The randomly distributed, opposite charges of the polymer form multiple ionic bonds of intra- and inter-chains with strength distribution. The strong inter-chain bonds, stabilized by topological entanglement, serve as quasi-permanent crosslinks, imparting the elasticity, while the weak bonds, both inter- and intra-chains, reversibly break and re-form to dissipate energy to toughen the materials. In this work, we intend to clarify the structure of the physical PA hydrogels from the tensile behaviors of the PA hydrogels. To clarify the structure and its formation mechanism, we analysed the tensile behaviors of the samples before and after the dialysis. We separated the quasi-permanent crosslinking of strong inter-chain bonds and the dynamic crosslinking of weak inter-chain bonds by using a combined model that consists of the Upper Convected Maxwell model and the Gent strain hardening model. The model fitting of the tensile behaviors extracts quantitative structural parameters, including the densities of weak and strong inter-chain bonds and the theoretical finite extensibility of polymer chains. Based on the fitting results of the combined model, the structural parameters of partial chains at a fixed observation time, including the Kuhn number, Kuhn length, and chain conformation, are determined using the scaling theory. The effects of monomer concentration at preparation, the effect of dialysis and the initial strain rate on the dynamic structure of PA gels, are discussed based on these analyses.

Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity.

Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biological tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mechanical properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mechanical properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These physical hydrogels of supramolecular structure can be tuned to change multiple mechanical properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.

Dimerization of sub-nanoscale molecular clusters affords broadly tuneable viscoelasticity above the glass transition temperature.

Materials with promising mechanical performance generally demonstrate requirements for the critical sizes of their key building units, e.g. entanglements and crystal grains. Herein, only with van der Waals interaction, viscoelasticity with broad tunability has been facilely achieved below the critical size limits: the dimers of ∼1 nm polyhedral oligomeric silsesquioxane (POSS) with M w < 4 kD and size < 5 nm, which demonstrate distinct material physics compared to that of polymer nanocomposites of POSS. The dimeric POSSs are confirmed by scattering and calorimetrical measurements to be intrinsic glassy materials with glass transition temperatures (T gs) lower than room temperature. From rheological studies, their viscoelasticity can be broadly tuned through the simple tailoring of the dimer linker structures above their T g. In dimer bulks, each POSS cluster is spatially confined by the POSSs from other dimers and therefore, the correlation of the dynamics of the two linked POSS clusters, which, as indicated by dynamics analysis, is regulated by the length and flexibilities of linkers, contributes to the caging dynamics of POSS confined by their neighbours and the resulting unique viscoelasticity. Our discoveries update the understanding of the structural origin of viscoelasticity and open avenues to fabricate structural materials from the design of sub-nanoscale building blocks.

A Transparent, Highly Stretchable, Solvent-Resistant, Recyclable Multifunctional Ionogel with Underwater Self-Healing and Adhesion for Reliable Strain Sensors.

Ionogels have gained increasing attentions as a flexible conductive material. However, it remains a big challenge to integrate multiple functions into one gel that can be widely applied in various complex scenes. Herein, a kind of multifunctional ionogels with a combination of desirable properties, including transparency, high stretchability, solvent and temperature resistance, recyclability, high conductivity, underwater self-healing ability, and underwater adhesiveness is reported. The ionogels are prepared via one-step photoinitiated polymerization of 2,2,2-trifluoroethyl acrylate and acrylamide in a hydrophobic ionic liquid. The abundant noncovalent interactions including hydrogen bonding and ion-dipole interactions endow the ionogels with excellent mechanical strength, resilience, and rapid self-healing capability at room temperature, while the fluorine-rich polymeric matrix brings in high tolerance against water and various organic solvents, as well as tough underwater adhesion on different substrates. Wearable strain sensors based on the ionogels can sensitively detect and differentiate large body motions, such as bending of limbs, walking and jumping, as well as subtle muscle movements, such as pronunciation and pulse. It is believed that the designed ionogels will show great promises in wearable devices and ionotronics.

Effect of mesoscale phase contrast on fatigue-delaying behavior of self-healing hydrogels.

We investigate the fatigue resistance of chemically cross-linked polyampholyte hydrogels with a hierarchical structure due to phase separation and find that the details of the structure, as characterized by SAXS, control the mechanisms of crack propagation. When gels exhibit a strong phase contrast and a low cross-linking level, the stress singularity around the crack tip is gradually eliminated with increasing fatigue cycles and this suppresses crack growth, beneficial for high fatigue resistance. On the contrary, the stress concentration persists in weakly phase-separated gels, resulting in low fatigue resistance. A material parameter, λtran, is identified, correlated to the onset of non-affine deformation of the mesophase structure in a hydrogel without crack, which governs the slow-to-fast transition in fatigue crack growth. The detailed role played by the mesoscale structure on fatigue resistance provides design principles for developing self-healing, tough, and fatigue-resistant soft materials.

Polyzwitterions as a Versatile Building Block of Tough Hydrogels: From Polyelectrolyte Complex Gels to Double-Network Gels.

The high water content of hydrogels makes them important as synthetic biomaterials, and tuning the mechanical properties of hydrogels to match those of natural tissues without changing chemistry is usually difficult. In this study, we have developed a series of hydrogels with varied stiffness, strength, and toughness based on a combination of poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), a strong acidic polyelectrolyte, and poly-N-(carboxymethyl)-N,N-dimethyl-2-(methacryloyloxy) ethanaminium) (PCDME), a polyzwitterion with a weak acidic moiety. We demonstrate that modifying the true molar ratio, R, of PCDME to PAMPS results in four unique categories of hydrogels with different swelling ratios and Young's moduli. When R < 1, a negatively charged polyelectrolyte gel (PE) is formed; when 1 < R < 3, a tough and viscoelastic polyelectrolyte complex gel (PEC) is formed; when 3 < R < 6.5, a conventional, elastic interpenetrating network gel (IPN) is formed; and when R > 6.5, a tough and stiff double-network gel (DN) is formed. Both the PEC and DN gels exhibit high toughness and fracture stress, up to 1.8 and 1.5 MPa, respectively. Importantly, the PEC gels exhibit strong recovery properties along with high toughness, distinguishing them from DN gels. Without requiring a change in chemistry, we can tune the mechanical response of hydrogels over a wide spectrum, making this a useful system of soft and hydrated biomaterials.

Fiber-Reinforced Viscoelastomers Show Extraordinary Crack Resistance That Exceeds Metals.

Soft fiber-reinforced polymers (FRPs), consisting of rubbery matrices and rigid fabrics, are widely utilized in industry because they possess high specific strength in tension while allowing flexural deformation under bending or twisting. Nevertheless, existing soft FRPs are relatively weak against crack propagation due to interfacial delamination, which substantially increases their risk of failure during use. In this work, a class of soft FRPs that possess high specific strength while simultaneously showing extraordinary crack resistance are developed. The strategy is to synthesize tough viscoelastic matrices from acrylate monomers in the presence of woven fabrics, which generates soft composites with a strong interface and interlocking structure. Such composites exhibit fracture energy, Γ, of up to 2500 kJ m-2 , exceeding the toughest existing materials. Experimental elucidation shows that the fracture energy obeys a simple relation, Γ = W · lT , where W is the volume-weighted average of work of extension at fracture of the two components and lT is the force transfer length that scales with the square root of fiber/matrix modulus ratio. Superior Γ is achieved through a combination of extraordinarily large lT (10-100 mm), resulting from the extremely high fiber/matrix modulus ratios (104 -105 ), and the maximized energy dissipation density, W. The elucidated quantitative relationship provides guidance toward the design of extremely tough soft composites.

Tough Bonding, On-Demand Debonding, and Facile Rebonding between Hydrogels and Diverse Metal Surfaces.

Hybrid systems of hydrogels and metals with tough bonding may find widespread applications. Here, a simple and universal method to obtain strong adhesion between hydrogels and diverse metal surfaces, such as titanium, steel, nickel, tantalum, argentum, and aluminum, with adhesion energy up to >1000 J m-2 is reported. To achieve such, the metal surfaces are instantly modified with a linker molecule via soaking, dip-coating, or drop-casting. The designed linker molecule has a carboxylic acid group to bind with a metal surface, and a methacrylic group to crosslink with a hydrogel, thus bridging the interface between them. In addition, by introducing a stimulus-responsive disulfide bond into the linker molecule, the on-demand debonding between toughly bonded hydrogel and metal surface, which is enabled by reductive cleavage of the disulfide chemical linkage, is also demonstrated. More interestingly, after the reductive debonding, the resulting metal surface with free thiol groups can be easily rebonded with a second hydrogel without any further surface modification. The strategy may provide unique opportunities in designing hybrid devices that are suitable for complex and dynamic environments.

Elastic-Plastic Transformation of Polyelectrolyte Complex Hydrogels from Chitosan and Sodium Hyaluronate.

Hydrogels formed by polyelectrolyte complexation (PEC) of oppositely charged biopolymers, free of any chemical additives, are promising biomaterials. In this work, the mechanical behavior of hydrogels consisting of positively charged chitosan and negatively charged sodium hyaluronate (HA) at balanced charge composition is investigated. These hydrogels exhibit strong tensile strain and strain rate dependence. They are elastic-like, independent of the strain rate at small strain, but exhibit plastic-like behavior above the yield point by showing a monotonous decrease of the stress. The cyclic tensile test demonstrates that these hydrogels exhibit small and quickly recoverable hysteresis in the elastic-like region, but large and partially recoverable hysteresis above the yield point. The stress relaxation experiment shows a plateau in the reduced stress followed by an abrupt fracture, and the time-to-failure decreases exponentially with increasing applied step-strain. Such elastic-to-plastic-like transformation of the biopolymer PEC gels is quite different from the behaviors of PEC hydrogels formed by oppositely charged vinyl-type synthetic polyelectrolytes due to the difference in flexibility, charge density, and ionic bond strength of these polymers.

Creating Stiff, Tough, and Functional Hydrogel Composites with Low-Melting-Point Alloys.

Reinforcing hydrogels with a rigid scaffold is a promising method to greatly expand the mechanical and physical properties of hydrogels. One of the challenges of creating hydrogel composites is the significant stress that occurs due to swelling mismatch between the water-swollen hydrogel matrix and the rigid skeleton in aqueous media. This stress can cause physical deformation (wrinkling, buckling, or fracture), preventing the fabrication of robust composites. Here, a simple yet versatile method is introduced to create "macroscale" hydrogel composites, by utilizing a rigid reinforcing phase that can relieve stress-induced deformation. A low-melting-point alloy that can transform from a load-bearing solid state to a free-deformable liquid state at relatively low temperature is used as a reinforcing skeleton, which enables the release of any swelling mismatch, regardless of the matrix swelling degree in liquid media. This design can generally provide hydrogels with hybridized functions, including excellent mechanical properties, shape memory, and thermal healing, which are often difficult or impossible to achieve with single-component hydrogel systems. Furthermore, this technique enables controlled electrochemical reactions and channel-structure templating in hydrogel matrices. This work may play an important role in the future design of soft robots, wearable electronics, and biocompatible functional materials.

Tough Hydrogels with Fast, Strong, and Reversible Underwater Adhesion Based on a Multiscale Design.

Hydrogels have promising applications in diverse areas, especially wet environments including tissue engineering, wound dressing, biomedical devices, and underwater soft robotics. Despite strong demands in such applications and great progress in irreversible bonding of robust hydrogels to diverse synthetic and biological surfaces, tough hydrogels with fast, strong, and reversible underwater adhesion are still not available. Herein, a strategy to develop hydrogels demonstrating such characteristics by combining macroscale surface engineering and nanoscale dynamic bonds is proposed. Based on this strategy, excellent underwater adhesion performance of tough hydrogels with dynamic ionic and hydrogen bonds, on diverse substrates, including hard glasses, soft hydrogels, and biological tissues is obtained. The proposed strategy can be generalized to develop other soft materials with underwater adhesion.

Tough Physical Double-Network Hydrogels Based on Amphiphilic Triblock Copolymers.

A series of physical double-network hydrogels is synthesized based on an amphiphilic triblock copolymer. The gel, which contains strong hydrophobic domains and sacrificial dynamic bonds of hydrogen bonds, is stiff and tough, and even stiffens in concentrated saline solution. Furthermore, due to its supramolecular structure, the gel features improved self-healing and self-recovery abilities.

Self-Adjustable Adhesion of Polyampholyte Hydrogels.

Developing nonspecific, fast, and strong adhesives that can glue hydrogels and biotissues substantially promotes the application of hydrogels as biomaterials. Inspired by the ubiquitous adhesiveness of bacteria, it is reported that neutral polyampholyte hydrogels, through their self-adjustable surface, can show rapid, strong, and reversible adhesion to charged hydrogels and biological tissues through the Coulombic interaction.

Free Reprocessability of Tough and Self-Healing Hydrogels Based on Polyion Complex.

Tough hydrogels with facile processability to reform into various shapes are required in many practical applications. In this work, we reported that a novel, tough, and self-healing physical hydrogel based on polyion complex (PIC) can be dissolved in 4 M NaCl solution to form a PIC solution. The PIC solution can be easily reprocessed into various shapes, such as thin films, sheets, fibers, and capsules, by using simple methods, such as casting and injection, while maintaining excellent mechanical properties comparable to, or even better than, the original hydrogel. The reprocessability and robust mechanical properties of PIC hydrogels are promising for practical applications in soft materials, especially in 3D/4D printing technology.