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Applying the double network principle to develop tough hydrogels with different polymer chemistries is important for the potential application of hydrogel materials. Synthesis of the two interpenetrated networks with contrasting structure and properties required for double networks usually involves a two-step polymerization process. In this work, we present a new method to synthesize tough double network hydrogels by post-physical crosslinking of linear semi-rigid polyelectrolytes entrapped in a chemically crosslinked neutral network. Owing to their semi-rigid structure, the linear polyelectrolytes form a brittle physical network above their overlap concentration in multi-valent ZrCl2O ion solutions without macroscopic phase separation within the flexible neutral network. The double network hydrogels thus prepared exhibit high modulus (~1.7 MPa), strength (~1.3 MPa), fracture strain (~7.3), and strain energy density (~5.9 MJ/m3), while containing over 80% water. These materials also exhibit modest self-healing ability (~51% after 30 minutes), demonstrating an additional benefit of a physical sacrificial network. This method is simpler than the conventional two-step polymerization and could be applied to develop tough hydrogels from rigid polyelectrolytes, including biopolymers such as DNA, HA, and chondroitin sulfate.

We report a method to create anisotropic double-network (DN) hydrogels, through the controlled orientation of a physical sacrificial network. A cross-linked polyacrylamide hydrogel is synthesized from a solution containing a semirigid anionic polyelectrolyte. Subsequently, the gel is stretched to orient the semirigid polyelectrolyte, which does not relax in the stretched state because of the high contour length in comparison to the mesh size of the polyacrylamide network. The polyelectrolyte is then physically cross-linked with a multivalent cation, ZrCl2O, to fix the anisotropy. Anisotropy was visualized by observing birefringence and quantified by small-angle X-ray scattering. By comparing the scattering in the oriented direction versus perpendicular to the oriented direction, a structural anisotropy factor was calculated. Uniaxial tensile testing was performed on samples of varying prestretch, both parallel and perpendicular to the stretching direction. Young's modulus, fracture stress, fracture strain, and work of extension were characterized, and the resulting mechanical anisotropy was compared to the structural anisotropy factor. We find that the anisotropy of Young's modulus and fracture stress is directly controlled by the anisotropy of the sacrificial network, while fracture strain and work of extension show little influence from structural anisotropy. The results of this work demonstrate that prestretching of a physical sacrificial network is a controllable and simple method to create anisotropic DN hydrogels.

Synthetic hydrogels possessing both macroscopic anisotropic structure and toughness, which are analogous to the load-bearing bio-tissues such as muscles and tendons, are rarely available. Studying the molecular mechanism of tough and anisotropic hydrogel under deformation is beneficial to understand the load-deformation functions of soft bio-tissues. In this work, the deformation-induced structure transformation of a macroscopically anisotropic and tough hydrogel has been investigated to understand the role of structure evolution for enhanced toughness. At rest, the hydrogel possesses a well-defined hierarchical structure in which self-assembled nanometer thick lamellar bilayers are alternatively stacked in hundred nanometer thick hydrogel matrixes. Stretching along the lamellar direction induces structure transformation from lamellar bilayers to hierarchical fibrous structures aligned along the deformation axis. The generated hierarchical structures consist of micrometer thick fiber bundles made from nanometer thick fibrils analogous to tropo-collagen bundles or microfibrils of the tendon. The fibrous structure formed at large elongation is associated with damage and rupture of the bilayers, which underpins the molecular mechanism of the unique mechanical behaviors of the tough lamellar hydrogel.

The double network concept, based on the fracture of sacrificial bonds, has been revolutionary toward the creation of robust soft materials. Based on the essence of double network hydrogels, macroscale, three-dimensional printed rigid sacrificial networks are embedded within silicone rubber stretchable matrices. Preferential fracture of the sacrificial network results in a , similar to 60 time increase in stiffness and a similar to 50% increase in the work of extension compared with the neat matrix. Maximizing yield strength while maintaining multistep internal fracture occurs when the strength of the sacrificial network approaches the strength of the matrix. Upon determining the optimal sacrificial network strength, the sacrificial bond section density can be increased to maximize energy dissipation and toughening efficiencies up to similar to 70% of the maximum theoretical toughness are achieved. High toughness and dissipation are achieved because topological interlocking enables significant force transmission to the sacrificial network at smaller length scales than interfacial adhesion, allowing much higher sacrificial bond density. This method is general and can be used with a variety of materials systems, without requiring strong interfacial adhesion, contrasting traditional composite systems. Demonstrating that the double network concept can be used at length scales far beyond the molecular scale will have important implications toward the development of future structural materials.

The human body is composed of composite structures made of water-rich hydrophilic domains and hydrophobic barriers. Combining hydrogels with elastomers results in similar synthetic biomaterials applicable to fields ranging from stretchable electronics to actuators. Here, we report a method to combine hydrogels with elastomers via a glass fiber fabric interphase. The interphase plays two roles: it enables chemically different materials to be robustly bound without chemical treatment while also dramatically improving the mechanical properties of the composite. Maximum interfacial adhesion energies of similar to 1,000 N m(-1) between the hydrogel and fabric and similar to 360 N m(-1) between the elastomer and fabric approach adhesion values of chemically bound soft materials. The composite tearing toughness (143 kJ m(-2)) vastly exceeds that of all neat components. In these materials, Young's modulus is high (similar to 1.2 GPa), while bending modulus is low (similar to 7 MPa), resulting in structures that can serve as actuators through controlled solvent exposure.

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.

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 and self‐recoverable hydrogel membranes with micrometer‐scale thickness are promising for biomedical applications, which, however, rarely be realized due to the intrinsic brittleness of hydrogels. In this work, for the first time, by combing noncovalent DN strategy and spin‐coating method, we successfully fabricated thin (thickness: 5–100 µm), yet tough (work of extension at fracture: 105–107 J m−3) and 100% self‐recoverable hydrogel membranes with high water content (62–97 wt%) in large size (≈100 cm2). Amphiphilic triblock copolymers, which form physical gels by self‐assembly, were used for the first network. Linear polymers that physically associate with the hydrophilic midblocks of the first network, were chosen for the second network. The inter‐network associations serve as reversible sacrificial bonds that impart toughness and self‐recovery properties on the hydrogel membranes. The excellent mechanical properties of these obtained tough and thin gel membranes are comparable, or even superior to many biological membranes. The in vitro and in vivo tests show that these hydrogel membranes are biocompatible, and postoperative nonadhesive to neighboring organs. The excellent mechanical and biocompatible properties make these thin hydrogel membranes potentially suitable for use as biological or postoperative antiadhesive membranes.