Gel scientists are facing challenges in providing synthetic connective tissues that serve a predominantly biomechanical role in the body, such as articular cartilage, semilunar cartilage, tendons, and ligaments. However, in order to replace the natural tissues with hydrogels, a number of significant engineering questions should be addressed, such as the provision of low surface friction and wear, a suitable elastic modulus, and high mechanical strength, both in vivo and in vitro. For instance, an articular cartilage that is a gel containing 70 % water exhibits little wear under a loading as high as several to a hundred megapascals and millions of cycles with a wide range of sliding velocity.[1] Our recent study shows that if a gel has free dangling polymer chains on its surface, its frictional coefficient becomes as low as 10 ±4 .[2]From this viewpoint, gels have a high potential as an artificial articular cartilage. Although poly(vinyl alcohol), PVA, hydrogel has been found to be mechanically strong and serves as a candidate for artificial articular cartilage, [3,4] most hydrogels derived from either natural or synthetic sources suffer from lack of mechanical strength. We report a general method of obtaining very strong hydrogels by inducing a double-network (DN) structure for various combinations of hydrophilic polymers. These DN hydrogels, containing 60±90 % water, exhibit a fracture strength as high as a few to several tens of megapascals and show high wear resistance due to their extremely low coefficient of friction.These gels might open new era of soft and wet materials as substitutes for articular cartilage and other tissues. Hereafter, the DN gels are referred to as P 1 -x 1 -y 1 /P 2 -x 2 -y 2 , where P i , x i , and y i (i = 1,2) are the abbreviated polymer name, molar monomer concentration, and the crosslinker concentration in mol-% with respect to the monomer for the ith network, respectively. The DN hydrogels with an optimized network structure can sustain a compressive pressure as high as several tens of megapascals. This is in stark contrast to most common hydrogels with a single network, which are easily broken either by pressing with a finger or pulling with the hands. As shown in Figure 1, for example, the PAMPS-1-4/ PAAm-2-0.1 DN gel is so tough that it is resistant to slicing with a cutter, despite containing 90 wt.-% water. Here, PAMPS stands for poly(2-acrylamido-2-methylpropanesulfon-

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Recent progresses in developing tough hydrogels promise the great potential of this class of soft and wet materials as biomaterials, soft robotics, smart actuators and sensors. [1][2][3][4][5][6][7][8][9][10] Any practical application of these hydrogels as smart materials, however, requires a combination of mechanical properties including stiffness, strength, toughness, and self-healing. For instance, along with the high strength and toughness, a cartilage substitute material requires a high stiffness to bear the load, while a blood vessel substitute material requires flexibility.Self-healing not only merits the long term durability of the load-bearing materials, but also gives possibility to reconstruct the material with desired shape from its microgels. While many hydrogels possess some of these requirements, it is a challenge to develop hydrogels satisfying all of these criteria. [2,[11][12][13] In this work, we report a new class of physical hydrogels that possess these multiple functions. These hydrogels are obtained from concentrated solution of oppositely charged polyelectrolytes. After dialysis of their small counter ions, the oppositely charged polyelectrolytes form polyion complexes of a wide strength distribution, which give dynamic crosslinking of an extremely wide life time scale. The strong, long life time bonds serve as permanent cross-linking, imparting elasticity, whereas the weak, short life

The double-network (DN) technique, developed by authors' group, provides an innovative and universal pass way to fabricate hydrogels with super high toughness comparable to rubbers. The excellent mechanical performances of DN hydrogels originate from the specific combination of two networks with contrasting structures. The first brittle network serves as sacrificial bonds, which breaks into small clusters to efficiently disperse the stress around the crack tip into the surrounding damage zone, while the second ductile polymer chains act as hidden length, which extends extensively to sustain large deformation. Based on the principle of DN hydrogel, the author's group recently has developed several novel systems and techniques, which has greatly expanded the practical accessibility of DN technique for practical use. The DN principle and the DN gel have already attracted much attention in the soft matter community. Inspired by the DN principle, many research groups have also designed and developed some innovative hydrogels with large enhancement in their mechanical strength and toughness. Some tough hydrogels fabricated by the DN technique also exhibit good biocompatibility and low friction resistance with promising prospective in industrial and medicine fields, especially for load-bearing artificial soft tissues such as artificial cartilage. In this feature article, we address the major concept and toughening mechanism of DN gel, then we describe some recent novel hydrogel systems based on the DN concept, and finally the applicability of DN gel as soft biomaterials is discussed

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

A novel anisotropic hydrogel, consisting of lamellar bilayers and a polymer network, with unidirectional alignment of the bilayer structure has been synthesized. The unidirectional orientation of bilayer in the gels leads to one‐dimensional swelling, strong anisotropy in elastic modulus, and exhibits excellent visible color. The gel shows reversibly tunable structural color under mechanical stimulation and could be the basis for a deformation‐based color display.

We report the extraordinary toughness, hysteresis, self-recovery, and persistent fatigue resistance of an anisotropic hydrogel with single-domain lamellar structure, consisting of periodical stacking of several thousands of rigid, hydrophobic bilayers in the ductile, hydrophilic polymer matrix. The stratified lamellar bilayers not only diffract light to exhibit magnificent structural color but also serve as reversible sacrificial bonds that dissociate upon deformation, exhibiting large hysteresis as an energy dissipation mechanism. Both the molecular dissociation and lipid-like mobile nature of bilayers dramatically enhance the resistance to crack propagation by suppressing the stress concentration at the crack tip with the formation of extraordinary crack blunting. This unique toughening phenomenon could allow deep insight into the toughening mechanism of the hydrogel-like soft materials such as biological soft tissues.

5Previously we revealed that the high toughness of double network hydrogels (DN gels) derives from the internal fracture of the brittle network during deformation, which dissipates energy as sacrificial bonds. In this study, we intend to elucidate the detailed internal fracture process of DN gels. We quantitatively analysed the tensile hysteresis and re-swelling behaviour of a DN gel that shows a well-defined necking and strain hardening, and obtained the following new findings: 1) Fracture of the 1 st network PAMPS 10 starts far below the yielding strain, and 90% of the initially load-bearing PAMPS chains already breaks at the necking point.2) The dominant internal fracture process occurs in the necking and hardening region although the softening mainly occurs before necking.3) The internal fracture efficiency is very high, 85% of the work is used for the internal fracture and 9% of all PAMPS chains break at sample failure. 4) The internal fracture is anisotropic, fracture occurs preferentially perpendicular to the tensile direction than 15 other two directions, but the fracture anisotropy decreases in the hardening region. Result 1) and 2) is in agreement with a hierarchical structural model of PAMPS network. Based on these findings, we present a revised description of the fracture process of DN gels.