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Tough hydrogels have emerged as a promising class of materials to target load‐bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high‐performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state‐of‐the‐art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high‐performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high‐performance hydrogels are compared with existing materials, and promising future opportunities are discussed. [ABSTRACT FROM AUTHOR]

Conductive hydrogels have attracted much attention because of their adjustable mechanical properties and excellent electrical conductivity. However, the variability and conductivity of traditional conductive hydrogels will be greatly reduced in low temperature environment, which seriously affects their application range. In this study, we fabricated a dual network conductive hydrogel with frost resistance. Here, water and glycerol binary solvents are introduced into the hydrogel to give the hydrogel antifreezing properties. The double-network structure of PAA/CMCs-Ca2+(W-G) hydrogel can provide good tensile and energy dissipation properties. The hydrogel strain sensor has high sensitivity (strain coefficient is 3.294 in the range of 100 ~ 250%, strain coefficient is 6.986 in the range of 250 ~ 400%) and good stability. PAA/CMCs-Ca2+(W-G) hydrogel has a great potential application in human signal detection. [ABSTRACT FROM AUTHOR]

Natural polysaccharides (NPS) are regarded as biomolecular and structural components for preparing high‐performance tough hydrogels. But the one‐step fabrication of NPS‐containing hydrogels in seconds and the template‐free design of complicated high‐resolution structures are still significant challenges in this field. To meet these requirements, various NPS‐containing tough hydrogels are fabricated and processed into 2D/3D structures via the combination of Ru(bpy)32+‐mediated photochemistry and extrusion 3D printing technique. The whole fabrication process is one‐step, completed in tens of seconds under visible light irradiation. It is found that the used NPS plays a key role in achieving the fabrication of high‐performance structured tough hydrogels. The high reactivity of functional groups in the used NPS can shorten their gelation times. Long rigid chains of the used NPS, their hierarchical assemblies, and contrasting multinetworks benefit from the efficient dissipation of mechanical energy and enhancement of its operational stability. Strong supramolecular interactions enable hydrogel precursors to have high viscosities, therefore providing good controllability to design high‐resolution and complicated tough hydrogel structures via extrusion 3D printing. It is anticipated that this straightforward fabrication strategy and findings will open new horizons for NPS‐containing materials. [ABSTRACT FROM AUTHOR]

This chapter reviews the preparation, structures, properties, and applications of tough and responsive hydrogels crosslinked by triblock copolymer micelles. An amphiphilic triblock copolymer, Pluronic F127, is functionalized with reactive groups. The functional F127 chains form polyfunctional micelles in aqueous solution, which are used for reaction with hydrophilic monomers or polymer chains to form hydrophobically crosslinked network. Free radical polymerization, host-guest interaction, and dynamic bonds have been utilized to connect the polymer chains with the reactive micelle coronae. The obtained polymer hydrogels show outstanding strength and toughness, with the micelles serving as energy dissipation centers. The structural evolution and energy dissipation mechanisms have been investigated in detail. By incorporating functional monomers into the network, a series of functional hydrogels responsive to pH, salt, electric field, and temperature have been developed. The hydrogels have been utilized as building blocks to fabricate soft actuators, shape-morphing devices, and biomaterials for tissue repair. [ABSTRACT FROM AUTHOR]

Abstract Human skin exhibits high stiffness of up to 100 MPa and high toughness of up to 3600 J m−2 despite its high water content of 40–70 wt%. Engineering hydrogels have rarely possessed both high stiffness and toughness, because compliant hydrogels usually become brittle when excess crosslinker is added to make the gel stiff. Furthermore, conventional hydrogels usually swell under physiological conditions, weakening their mechanical properties. Here, we designed a non-swellable hydrogel with high stiffness and toughness by interpenetrating covalently and ionically crosslinked networks. The stiffness is enhanced by utilizing ionic crosslinking sites fully, and the toughness is enhanced by adopting synergistic effects between energy-dissipation by ionic networks and crack-bridging by covalent networks. Non-swelling behaviors of the gel are achieved by densifying covalent and ionic crosslinks. The hybrid gel shows high elastic moduli (up to 108 MPa) and high fracture energies (up to 8850 J m−2). In vitro and in vivo swelling tests prove non-swelling behaviors of the gel. Live/dead assays show 99% cell viability over a period of 60 days. Graphical abstract Unlabelled Image Highlights • Enhanced stiffness (~108 MPa) and toughness (~8,850 J m-2) of pHEMA-alginate hydrogels were comparable with those of human skin. • Non-swelling behaviors of the gels were stable under in vivo conditions. • 99% cell viability over a period of 60 days was shown via live/dead assays. [ABSTRACT FROM AUTHOR]

A combination of multiple non-covalent interactions has been used to fabricate strong and tough hydrogels. However, mechanisms behind toughening remain unclear. In this study, a series of single-network (SN), double-network (DN), and triple-network (TN) hydrogels sequentially crosslinked by chemical bonds, freeze-thawed polyvinyl alcohol (PVA) crystallites, and ion coordination of alginate chains are systematically prepared. The contributions of hydrogen bonding, entanglements, crystallite crosslinking, and ion coordination to the dynamics of the hydrogels are comparatively studied. Dynamic mechanical analysis (DMA) reveals the apparent activation energy of hydrogen bonding from 41.1 kJ/mol to 50.6, 56.8, and 65.5 kJ/mol for SN gels with increasing PVA concentration (from 0 to 2, 6, 10 wt/vol%, respectively), whereas the corresponding apparent activation energy of chain entanglements increases from 148.3 to 160.2, 163.5, and 167.0 kJ/mol. PVA crystallites formed upon freeze-thawing act as physical crosslinks to further improve the strength and toughness of gels, as well as the activation energy (251.7 kJ/mol). Subsequent ion coordination with alginate further enhances the activation energy to 304.7 kJ/mol (Cu2+), 309.3 kJ/mol (Fe3+). Structural evolution studies on pre-cracked hydrogels reveal that PVA crystallites render crack blunting upon stretching. Polarized optical microscopy and scanning electron microscopy reveal the rearrangement and reconstruction of internal structures during crack propagation, including orientation and alignment of PVA crystallites. This study provides first insights to synergetic energy dissipation by non-covalent interactions of multi-network hydrogels. [Display omitted] • Multi-network hydrogels were prepared with multiple non-covalent interactions. • Energy dissipation through hydrogen bonding, entanglement, crystallization, and ionic complexation were studied. • Structural evolution at the crack tip of the hydrogels was identified to dissipate energy and enhance toughness. [ABSTRACT FROM AUTHOR]

Incorporating sacrificial bonds into hydrogels networks has been demonstrated an efficient way to obtain ultrahigh strength, but is usually at the expense of flexibility and recoverability. This study demonstrates novel triple physically crosslinked hydrogels with very high stretchability, toughness and rapid self-recovery at ambient conditions. Physically crosslinked κ-carrageenan acts as the first network to host free radical copolymerization of polymers with Pluronic F127 diacrylate micelles. Hydrophobic association of micelles in the second network contributes to outstanding stretchability (over 1100%) and toughness (10.83 MJ/m3). Further crosslinking of the second network through tridentate Fe3+–COO− coordination significantly enhances the fracture strength up to 2.7 MPa, and elastic modulus 1171 kPa. Systematic investigations demonstrate the key roles played by non-covalent crosslinking on energy dissipation during loadings and the damaged non-covalent networks could recover 40% toughness in 1 min at ambient conditions. These hydrogels are promising for applications as biocompatible load-bearing materials. Image 1 • Novel triple physically crosslinked hydrogels were prepared with double network structures. • Supramolecular helices, hydrophobic associations and metal coordination were used. • The hydrogels showed excellent mechanical properties and fast recovery at room temperature. [ABSTRACT FROM AUTHOR]