1 wileyonlinelibrary.com recoverability and self-healing property, due to their intrinsic structural heterogeneity and/or lack of effi cient energydissipation mechanisms, [ 13 ] which greatly limit their uses for other applications requiring highly mechanical properties such as cartilage, tendon, muscle, and blood vessel.Many efforts have been made to develop tough hydrogels with new microstructures and toughening mechanisms, such as double network hydrogels, [ 14 ] nanocomposite hydrogels, [ 15 ] sliding-ring hydrogels, [ 16 ] macromolecular microsphere composite hydrogels, [ 17 ] tetrapolyethylene glycol hydrogels, [ 18 ] hydrophobically associated hydrogels, [ 19,20 ] and dipole-dipole or hydrogen bonding enhanced hydrogels. [ 21,22 ] Among them, double network (DN) hydrogels have demonstrated their excellent mechanical properties. The existing knowledge of DN gels from synthesis methods, network structures, to toughening mechanisms mainly comes from chemically cross-linked DN gels. [ 23 ] Both networks with contrasting structures in DN gels are separately crosslinked by covalent bonds, [ 24 ] and the interpenetration of two contrasting networks makes the chemically linked DN gels both tough and soft, as evidenced by stiffness (elastic modulus of 0.1-1.0 MPa), strength (failure tensile stress of 1-10 MPa, strain 1000%-2000%, failure compressive stress 20-60 MPa, strain 90%-95%), and toughness (tearing fracture energy of 10 2 -10 3 J m −2 ). [ 23 ] Chemically linked DN gels have comparable toughness to cartilage and rubber. The toughening mechanisms are largely based on "sacrifi cial bonds" that break from the fi rst network to effectively dissipate energy, protect the second network, sustain stress, and store elastic energy, thus to reinforce the gels. However, the fracture of the fi rst network also causes irreversible and permanent bond breaks, making the gels very diffi cult to be repaired and recovered from damages and fatigues. [ 25 ] Thus, the internal fracture process of the fi rst network is considered to be critical for toughness enhancement, because relatively large damage zones formed in the fi rst network allow for more accumulated damage before macroscopic crack propagation occurs throughout whole networks. [ 26,27 ] Double network (DN) hydrogels with two strong asymmetric networks being chemically linked have demonstrated their excellent mechanical properties as the toughest hydrogels, but chemically linked DN gels often exhibit negligible fatigue resistance and poor self-healing property due to the irreversible chain breaks in covalent-linked networks. Here, a new design strategy is proposed and demonstrated to improve both fatigue resistance and self-healing property of DN gels by introducing a ductile, nonsoft gel with strong hydrophobic interactions as the second network. Based on this design strategy, a new type of fully physically cross-linked Agar/hydrophobically associated polyacrylamide (HPAAm) DN gels are synthesized by a simple one-pot method. Agar/ HPAAm DN gels exhibit excellent mech...

Double network hydrogels (DN gels) are considered as one of the toughest soft materials. However, conventional chemically linked DN gels often lack high self-recovery and fatigue resistance properties due to permanent damage of covalent bonds upon deformation. Current strategies to improve selfrecovery and fatigue resistance properties of tough DN gels mainly focus on the manipulation of the first network structure. In this work, we proposed a new design strategy to synthesize a new type of Agar/PAMAAc-Fe 3+ DN gels, consisting of an agar gel as the first physical network and a PAMAAc-Fe 3+ gel as the second chemical−physical network. By introducing Fe 3+ ions into the second network to form strong coordination interactions, at optimal conditions, Agar/PAMAAc-Fe 3+ DN gels can achieve extremely high mechanical properties (σ f of ∼8 MPa, E of ∼8.8 MPa, and W of ∼16.7 MJ/m 3 ), fast self-recovery (∼50% toughness recovery after 1 min of resting), and good fatigue resistance compared to properties of cyclic loadings by simply controlling acrylic acid (AAc) content in the second network. The high toughness and fast recovery of Agar/PAMAAc-Fe 3+ DN gel is mainly attributed to energy dissipation through reversible noncovalent bonds in both networks (i.e., hydrogen bonds in the agar network and Fe 3+ coordination interactions in the PAMAAc network). The time-dependent recovery of Agar/PAMAAc-Fe 3+ gels at room temperature and the absence of recovery in Agar/PAMAAc gels also confirm the important role of Fe 3+ coordination interactions in mechanical strength, self-recovery, and fatigue resistance of DN gels. Different mechanistic models were proposed to elucidate the mechanical behaviors of different agar-based DN gels. Our results offer a new design strategy to improve strength, selfrecovery, and fatigue resistance of DN gels by controlling the structures and interactions in the second network. We hope that this work will provide an alterative view for the design of tough hydrogels with desirable properties.

Gelatin/polycrylamide double-network (DN) hydrogels composed of two different polymer networks with strong asymmetry are excellent structural platforms to integrate different mechanical properties into a single material.

Combining both chemical and physical cross-links in a double-network hydrogel (DN gel) has emerged as a promising design strategy to obtain highly mechanically strong hydrogels. Unlike chemically cross-linked DN gels, little is known about the fracture process and toughening mechanisms of hybrid chemically physically linked DN gels. In this work, we engineered tough hybrid DN gels of agar/polyacrylamide (Agar/PAAm) by combining two types of cross-linked polymer networks: a physically linked, first agar network and a chemical-linked, second PAAm network. The resulting Agar/PAAm exhibited high stiffness of 313 kPa and high toughness of 1089 J/m2. We then specifically examined the effect of the first agar network on the mechanical properties of hybrid Agar/PAAm gels. We found that by controlling agar concentrations above a critical value, the physically linked agar network can simultaneously enhance both stiffness and toughness of Agar/PAAm DN gels, as evidenced by a linear relationship of elastic modulus and tearing energies of the gels as the increase of agar concentration. This toughening behavior is different from that of chemically linked DN gels. Complement to chemically linked DN gels, this work provides a different view for the design of new stiff and tough hydrogels using hybrid physical and chemical networks.

It is a challenge to realize high strength, toughness, and energy storage, as well as excellent capacitive self-recovery, fatigue-resistant, and self-healing performances simultaneously in a single all-in-one supercapacitor aiming for wearable electronics. Herein, based on the self-crosslinking and molecular template, a supramolecular poly­(vinyl alcohol)/poly (N-hydroxyethyl acrylamide) (PVA/PHEA) hydrogel electrolyte (HGE) decorated by polyaniline (PANI) was prepared by in situ rapid polymerization of high-concentration aniline on the PVA/PHEA gel containing H2SO4. The multiple hydrogen bonds, rapid polymerization, and decoration endowed PANI-decorated PVA/PHEA HGE-based all-in-one flexible supercapacitor with the integrated high performances, which include high specific capacitance, good cycling stability, high strength, excellent toughness, rapid self-recovery, excellent fatigue-resistant, and self-healing capabilities, as well as high capacitance retention during or after the large deformations or after the self-healing. Thus, the current work presents a novel and promising strategy to design the integrated high-performance supercapacitors aiming for wearable electronics.

Developing functional hybrids of globular proteins and synthetic polymers into multipurpose tough hydrogels remains challenging. Here, we propose a new strategy combining double-network and protein misfolding concepts to create diverse protein–polymer double-network (DN) hydrogels with both high bulk and interfacial toughness. The method integrates an intrinsic heat-induced protein denaturation/aggregation feature and a double-network concept, which produces different bovine serum albumin (BSA)-based DN hydrogels with hybrid physical–chemical cross-linking or fully physical cross-linking to achieve a high modulus of 252–1199 kPa, high strength of 0.24–0.48 MPa, high fracture energy of 3.56–16.88 MJ/m3, high extensibility of 7.7–79.9 mm/mm, fast self-recovery (stiffness/toughness recovery of 94/80% after heat treatment at 80 °C for 30 min), and strong surface adhesion to various nonporous solid surfaces (interfacial toughness of 1176–2827 J/m2). Such tough and adhesive protein–polymer hydrogels have great potential for different applications, such as artificial soft tissues, flexible electronics, and wearable devices.

Low-molecular-weight gelator (LMWG)-based supramolecular hydrogels, self-assembled by small molecules via noncovalent interactions, have recently attracted great attention due to their unique structure–property relationship and potential applications spanning from functional materials to biomedical devices. Unfortunately, many LMWG-based supramolecular hydrogels are mechanically weak and can not even be handled by conventional tensile and tearing tests. Here, we propose several design principles to fabricate new LMWG-based hydrogels with a true double-network structure (G4·K+/PDMAAm DN gels), consisting of the supramolecular self-assembly of guanosine, B­(OH)3 and KOH as the first, physical G4·K+ network and the covalently cross-linked poly­(N,N′-dimethyacrylamide) (PDMAAm) as the second, chemical network. Different from those LMWG-based supramolecular hydrogels, G4·K+/PDMAAm DN gels exhibit high tensile properties (elastic modulus = 0.307 MPa, tensile stress = 0.273 MPa, tensile strain = 17.62 mm/mm, and work of extension = 3.23 MJ/m3) and high toughness (tearing energies = 1640 J/m2). Meanwhile, the dynamic, noncovalent bonds in the G4·K+ network can reorganize and reform after being broken, resulting in rapid self-recovery property and excellent fatigue resistance. The stiffness/toughness of G4·K+/PDMAAm DN gels can be recovered by 65%/58% with 1 min resting at room temperature, and the recovery rates are further improved with the increase of temperatures and resting times. Interestingly, G4·K+/PDMAAm DN gels also exhibit UV-triggered luminescence due to the unique G4-quartet structure in the G4·K+ supramolecular first network. A new toughening mechanism is proposed to interpret the high strength and toughness of G4·K+/PDMAAm DN gels. We believe that our design principles, along with new G4·K+/PDMAAm DN gel system, will provide a new viewpoint for realizing the tough and strong LMWG-based gels.

Protein-based hydrogels have attracted great attention due to their excellent biocompatible properties, but often suffer from weak mechanical strength. Conventional strengthening strategies for protein-based hydrogels are to introduce nanoparticles or synthetic polymers for improving their mechanical strength, but often compromise their biocompatibility. Here, a new, general, protein unfolding-chemical coupling (PNC) strategy is developed to fabricate pure protein hydrogels without any additives to achieve both high mechanical strength and excellent cell biocompatibility. This PNC strategy combines thermal-induced protein unfolding/gelation to form a physically-crosslinked network and a -NH2/-COOH coupling reaction to generate a chemicallycrosslinked network. Using bovine serum albumin (BSA) as a globular protein, PNC-BSA hydrogels show macroscopic transparency, high stability, high mechanical properties (compressive/tensile strength of 115/0.43 MPa), fast stiffness/toughness recovery of 85%/91% at room temperature, good fatigue resistance, and low cell cytotoxicity and red blood cell hemolysis. More importantly, the PNC strategy can be not only generally applied to silk fibroin, ovalbumin, and milk albumin protein to form different, high strength protein hydrogels, but also modified with PEDOT/PSS nanoparticles as strain sensors and fluorescent fillers as color sensors. This work demonstrates a new, universal, PNC method to prepare high strength, multi-functional, pure protein hydrogels beyond a few available today.