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.

We successfully synthesized a family of alginate/polyacrylamide hydrogels using various multivalent cations. These hydrogels exhibit exceptional mechanical properties. In particular, we discovered that the hydrogels cross-linked by trivalent cations are much stronger than those cross-linked by divalent cations. We demonstrate stretchability and toughness of the hydrogels by inflating a hydrogel sheet into a large balloon, and the elasticity by using a hydrogel block as a vibration isolator in a forced vibration test. The excellent mechanical properties of these hydrogels may open up applications for hydrogels.

1352 wileyonlinelibrary.com maintain the integrity of network structures and mechanical properties of bulk gels, leading to their long-term use with stable functionality. [9][10][11][12][13] The scientifi c community nowadays focus on two major approaches, based on dynamic covalent bond [14][15][16][17] and noncovalent bond, [18][19][20][21][22][23][24][25][26][27][28] to design self-healing hydrogels. Dynamic covalent bond integrates both the stability of covalent bond and the reversibility of noncovalent bond in one system. [ 29 ] These dynamic covalent bonds can build an intrinsic dynamic equilibrium of bond generation and dissociation in hydrogel networks, endowing self-healing performance to the hydrogels. Despite a few examples of self-healing hydrogels based on the dynamic covalent bonds (e.g., phenylboronate esters, [30][31][32] acylhydrazone bonds, [ 29,33 ] disulfi de bonds, [34][35][36] and Diels-Alder reactions, [ 37,38 ] the diffi culty of manipulating in vivo due to their nonautonomous self-healing characteristics, impedes their applications. For instance, selfhealing hydrogel based on dynamically restructuring of phenylboronic esters needs an acid environment (pH 4.2), [ 30 ] while hydrogel based on dynamic disulfi de bonds usually needs an alkali environment (pH 9), [ 34 ] to trigger the corresponding healing process. Moreover, complicated synthetic procedures and unconfi rmed biocompatibility of these self-healing hydrogels may limit their applications. For instance, the self-healing A novel biocompatible polysaccharide-based self-healing hydrogel, CEC-l-OSA-l-ADH hydrogel ("l" means "linked-by"), is developed by exploiting the dynamic reaction of N -carboxyethyl chitosan (CEC) and adipic acid dihydrazide (ADH) with oxidized sodium alginate (OSA). The self-healing ability, as demonstrated by rheological recovery, macroscopic observation, and beam-shaped strain compression measurement, is attributed to the coexistence of dynamic imine and acylhydrazone bonds in the hydrogel networks. The CEC-l-OSA-l-ADH hydrogel shows excellent self-healing ability under physiological conditions with a high healing effi ciency (up to 95%) without need for any external stimuli. In addition, the CEC-l-OSA-l-ADH hydrogel exhibits good cytocompatibility and cell release as demonstrated by threedimensional cell encapsulation. With these superior properties, the developed hydrogel holds great potential for applications in various biomedical fi elds, e.g., as cell or drug delivery carriers.

Tough Al-alginate/poly(N-isopropylacrylamide) (PNIPAM) hydrogel has been synthesized by introducing an interpenetrating network with hybrid physically cross-linked alginate and chemically cross-linked PNIPAM. Varying the concentration of AlCl3 regulates the mechanical properties of the tough hydrogel and tunes its lower critical solution temperature (LCST) as well. The tough Al-alginate/PNIPAM exhibits 6.3 ± 0.3 MPa of compressive stress and 9.95 of uniaxial stretch. Tunability of LCST is also achieved in a wide range within 22.5-32 °C. A bending beam actuator and a four-arm gripper made of bilayer (Na-alginate/PNIPAM)/(Al-alginate/PNIPAM) hydrogel as prototype of all-hydrogel soft robotics are demonstrated. A finite element (FE) simulation model is developed to simulate the deformation of the soft robotics. The FE simulation not only reproduces the deformation process of performed experiments but also predicts more complicated devices that can be explored in the future. This work broadens the application of temperature-responsive PNIPAM-based hydrogels.

A new type of electroluminescent device achieves giant stretchability by integrating electronic and ionic components. The device uses phosphor powders as electroluminescent materials, and hydrogels as stretchable and transparent ionic conductors. Subject to cyclic voltage, the phosphor powders luminesce, but the ionic conductors do not electrolyze. The device produces constant luminance when stretched up to an area strain of 1500%.