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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As representative soft materials with widespread applications, gels with various functions have been developed. However, traditional gels are vulnerable to stress-induced formation of cracks. The propagation of these cracks may affect the integrity of network structures of gels, resulting in the loss of functionality and limiting the service life of the gels. To address this challenge, self-healing gels that can restore their functionalities and structures after damage have been developed as "smart" soft materials. In this paper, we present an overview of the current strategies for synthesizing self-healing gels based on the concept of constitutional dynamic chemistry, which involves molecular structures capable of establishing dynamic networks based upon physical interactions or chemical reactions. The characterization methods of self-healing gels and the key factors that affect self-healing properties are analyzed. We also illustrate the emerging applications of self-healing gels, with emphasis on their usage in industry (coatings, sealants) and biomedicine (tissue adhesives, agents for drug or cell delivery). We conclude with a perspective on challenges facing the field, along with prospects for future development.

Double‐network (DN) hydrogels with high mechanical strength have been synthesized using the natural polymers bacterial cellulose (BC) and gelatin. As‐prepared BC contains 90 % water that can easily be squeezed out, with no more recovery in its swelling property. Gelatin gel is brittle and is easily broken into fragments under a modest compression. In contrast, the fracture strength and elastic modulus of a BC–gelatin DN gel under compressive stress are on the order of megapascals, which are several orders of magnitude higher than those of gelatin gel, and almost equivalent to those of articular cartilage. A similar enhancement in the mechanical strength was also observed for the combination of BC with polysaccharides, such as sodium alginate, gellan gum, and ι‐carrageenan.