Integrated Functional High-Strength Hydrogels with Metal-Coordination Complexes and H-Bonding Dual Physically Cross-linked Networks

Functionalization of chitosan and its application in flame retardants: A review

In recent years, bio-based flame retardants have gained significant attention as sustainable alternatives, achieving important breakthroughs in flame retardancy and becoming a key focus for future development. Derived from biomass, chitosan (CS) has been widely used in the field of advanced functional materials. However, in the field of flame retardancy, chitosan alone shows limited effectiveness, leading researchers to explore its reactive functional groups for creating multifunctional flame retardant chitosan composites (FRCC). This review examines FRCC modifications, focusing on how physical and chemical techniques enhance flame retardancy based on combustion mechanisms. Particular emphasis is placed on the impact of embedding flame-retardant elements. Additionally, this paper outlines FRCC performance characteristics, addressing operational requirements in varied environments, and discusses key challenges. This study offers researchers a comprehensive overview of FRCC, serving as a valuable resource for ongoing research and development.

Fabrication of Zinc(II) Mediated Poly(Acrylamide Co Acrylic Acid) Hydrogel with Thixotropic and Tribological Properties

Hydrogels have emerged as promising candidates for biomedical applications, such as replacing natural articular cartilage, owing to their unique viscoelastic properties. However, sufficient mechanical properties, self-healing ability, and adhesive nature are some issues limiting its application window. Here, a facile one-pot synthesis of dual cross-linked zinc-coordinated copolymer hydrogels is presented. The network structure of the copolymer hydrogels is strategically developed via dynamic and reversible physical cross-linking by Zn2+ ions and simultaneous covalent cross-linking through a covalent cross-linker viz methylene bisacrylamide. Fourier-transform infrared (FTIR), X-ray diffraction (XRD) scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analysis have thoroughly characterized the structure of the synthesized hydrogels. The introduction of Zn2+ offers dynamic and reversible complexation, leading to excellent mechanical properties and self-healing features. Moreover, the percentage of the equilibrium water content of zinc-coordinated copolymer hydrogel samples is comparable with that of natural articular cartilage. The Shear sliding study shows the dominant adhesive behavior of HGel-Zn(NO3)2 sample compared to the parent HGel sample. This facile dual cross-linked hydrogel, HGel-Zn(NO3)2, with a combination of good mechanical properties, efficient self-recovery, adequate water content, and favorable adhesive nature, seems very promising to mimic the articular cartilage.

Leaftronics: Natural lignocellulose scaffolds for sustainable electronics

The global rise in electronic waste is alarming, driven by the persistent use of glass, epoxy, and plastic substrates owing to their cost, stability, flexibility, and transparency. This underscores the need for biodegradable alternatives with similar properties. This study shows that leaf-derived lignocellulose scaffolds can stabilize bio-sourced, solution-processed polymers by acting as natural sequestering media. Such reinforced films, even when based on gelatin (Tg ~ 60°C), can endure processes over 200°C. We demonstrate dip-coated ethyl cellulose films for commercially viable reflow soldered circuitry. The films offer high flexibility, more than 80% transparency, and surface roughness below 5.5 nm. Advanced OPDs and OECTs fabricated on these films perform comparably to those on glass and the low material cost and simple fabrication process yields a minimal carbon footprint of 1.6 kgCO2/m2. This work thus opens a vista of possibilities for biodegradable polymers heretofore considered unsuitable for making temperature-stable substrates for state-of-the-art electronics applications.

Induction of polymer-grafted cellulose nanocrystals in hydrogel nanocomposites to increase anti-swelling, mechanical properties and conductive self-recovery for underwater strain sensing

Anti-swelling conductive hydrogels with simultaneous high tensile strength (>1 MPa) and fast self-recovery are promising candidates for underwater strain sensing, but their preparation remains challenging. Herein, novel anti-swelling conductive nanocomposite hydrogels were fabricated based on poly(acrylamide-co-acrylic acid) (P(AM-co-AA)), polymer-grafted cellulose nanocrystals (CNCs) and Fe3+ ions through a strategy combining nano-reinforcing and multiple physical crosslinking. Due to the presence of interfacial H-bonds, polymer-grafted cellulose nanocrystals played important role in endowing hydrogels with anti-swelling capacity and enhanced mechanical performance. The obtained nanocomposite hydrogels exhibited relatively low swelling ratio (2.9-3.3 g/g), high tensile strength (>1.5 MPa), fast self-recovery (86 % recovery of hysteresis within 5 min) and conductivities of 0.0534-0.0593 S/m. The combination of excellent tensile properties and conductivity endowed the hydrogel-based strain sensors with good sensitivity (GF ≈ 0.8) and reliable cycling repeatability in 0-100 % strain range. Notably, the nanocomposite hydrogels can maintain their mechanical and sensing performance after soaking in water for 14 days, making them applicable for human motion detection both in air and underwater. Hence, this work provided a facile method to construct highly robust and anti-swelling CNC-reinforced conductive hydrogels, which have potential applications in underwater strain sensing and beyond.

Hydrogels Based on Proteins Cross-Linked with Carbonyl Derivatives of Polysaccharides, with Biomedical Applications

Adding carbonyl groups into the hydrogel matrix improves the stability and biocompatibility of the hydrogels, making them suitable for different biomedical applications. In this review article, we will discuss the use of hydrogels based on polysaccharides modified by oxidation, with particular attention paid to the introduction of carbonyl groups. These hydrogels have been developed for several applications in tissue engineering, drug delivery, and wound healing. The review article discusses the mechanism by which oxidized polysaccharides can introduce carbonyl groups, leading to the development of hydrogels through cross-linking with proteins. These hydrogels have tunable mechanical properties and improved biocompatibility. Hydrogels have dynamic properties that make them promising biomaterials for various biomedical applications. This paper comprehensively analyzes hydrogels based on cross-linked proteins with carbonyl groups derived from oxidized polysaccharides, including microparticles, nanoparticles, and films. The applications of these hydrogels in tissue engineering, drug delivery, and wound healing are also discussed.

Flexible and Stretchable Organic Electrochemical Transistors for Physiological Sensing Devices

Flexible and stretchable bioelectronics provides a biocompatible interface between electronics and biological systems and has received tremendous attention for in situ monitoring of various biological systems. Considerable progress in organic electronics has made organic semiconductors, as well as other organic electronic materials, ideal candidates for developing wearable, implantable, and biocompatible electronic circuits due to their potential mechanical compliance and biocompatibility. Organic electrochemical transistors (OECTs), as an emerging class of organic electronic building blocks, exhibit significant advantages in biological sensing due to the ionic nature at the basis of the switching behavior, low driving voltage (<1 V), and high transconductance (in millisiemens range). During the past few years, significant progress in constructing flexible/stretchable OECTs (FSOECTs) for both biochemical and bioelectrical sensors has been reported. In this regard, to summarize major research accomplishments in this emerging field, this review first discusses structure and critical features of FSOECTs, including working principles, materials, and architectural engineering. Next, a wide spectrum of relevant physiological sensing applications, where FSOECTs are the key components, are summarized. Last, major challenges and opportunities for further advancing FSOECT physiological sensors are discussed.

A Comprehensive Review of Polysaccharide-Based Hydrogels as Promising Biomaterials

Polysaccharides have emerged as a promising material for hydrogel preparation due to their biocompatibility, biodegradability, and low cost. This review focuses on polysaccharide-based hydrogels' synthesis, characterization, and applications. The various synthetic methods used to prepare polysaccharide-based hydrogels are discussed. The characterization techniques are also highlighted to evaluate the physical and chemical properties of polysaccharide-based hydrogels. Finally, the applications of SAPs in various fields are discussed, along with their potential benefits and limitations. Due to environmental concerns, this review shows a growing interest in developing bio-sourced hydrogels made from natural materials such as polysaccharides. SAPs have many beneficial properties, including good mechanical and morphological properties, thermal stability, biocompatibility, biodegradability, non-toxicity, abundance, economic viability, and good swelling ability. However, some challenges remain to be overcome, such as limiting the formulation complexity of some SAPs and establishing a general protocol for calculating their water absorption and retention capacity. Furthermore, the development of SAPs requires a multidisciplinary approach and research should focus on improving their synthesis, modification, and characterization as well as exploring their potential applications. Biocompatibility, biodegradation, and the regulatory approval pathway of SAPs should be carefully evaluated to ensure their safety and efficacy.

Hydrogels with ultra-highly additive adjustable toughness under quasi-isochoric conditions

Bioinspired smart hydrogels with additive-switchable mechanical properties have been attracting increasing attention in recent years. However, most existing hydrogel systems suffer from limited stiffening amplitude and dramatic volume change upon response to environmental triggers. Herein, we propose a novel strategy to prepare additive-responsive hydrogels with ultra-highly adjustable toughness under quasi-isochoric conditions. The key point lies in tuning the softening transition temperature of the hydrogels with non-covalent interactions between the polymer networks and additives, shifting the hydrogels from glassy to rubbery states. As a proof of concept, a variety of glassy hydrogels are prepared and exposed to additives to trigger responsive performances. Young's modulus of the same hydrogel demonstrates up to 36 000 times ultra-broad-range tunability, ranging from 0.0042 to 150 MPa in response to different additives. Meanwhile, negligible volume changes occur, keeping the hydrogels in quasi-isochoric conditions. Interestingly, the mechanical behaviors of the hydrogels manifest remarkable dependence on the additive type and concentration since both the Hofmeister effect and hydrophobicity of the additives play pivotal roles according to mechanism investigations. Furthermore, the regulation with additives reveals satisfactory reversibility and universality. Taken together, this simple and effective approach provides a novel strategy to fabricate hydrogels with highly tunable toughness for versatile applications, including spatially patterned conductive gels and anti-icing coatings.

Advancements in the Use of Hydrogels for Regenerative Medicine: Properties and Biomedical Applications

Due to their particular water absorption capacity, hydrogels are the most widely used scaffolds in biomedical studies to regenerate damaged tissue. Hydrogels can be used in tissue engineering to design scaffolds for three-dimensional cell culture, providing a novel alternative to the traditional two-dimensional cell culture as hydrogels have a three-dimensional biomimetic structure. This material property is crucial in regenerative medicine, especially for the nervous system, since it is a highly complex and delicate structure. Hydrogels can move quickly within the human body without physically disturbing the environment and possess essential biocompatible properties, as well as the ability to form a mimetic scaffold in situ. Therefore, hydrogels are perfect candidates for biomedical applications. Hydrogels represent a potential alternative to regenerating tissue lost after removing a brain tumor and/or brain injuries. This reason presents them as an exciting alternative to highly complex human physiological problems, such as injuries to the central nervous system and neurodegenerative disease.

Nanocomposite conductive tough hydrogel based on metal coordination reinforced covalent Pluronic F-127 micelle network for human motion sensing

The design of conductive hydrogels integrating anti-fatigue, high sensitivity, strong mechanical property and good sterilization performance remains a challenge. We innovatively introduced metal coordination in covalently crosslinked Pluronic F-127 micelle network and synthesized nanocomposite conductive tough hydrogel through the combination of covalent crosslinking, metal coordination and silver nanowire reinforcement. Compared with pure diacylated PF127 hydrogel (PF127), the tensile strength of PF-AA-AM-Al³⁺/Ag0.25 hydrogel reaching 1.4 MPa was about 10 times than that of PF127. The toughness of PF-AA-AM-Al³⁺/Ag0.25 reaches 1.88 MJ/m³. Compared with PF-AA-AM-Al³⁺, the introduction of silver nanowires increased the fatigue life of PF-AA-AM-Al³⁺/Ag0.25 by 200% (31837 cycles), 170% (12804 cycles) and 1022% (511 cycles) under 100%, 120% and 150% ultimate tensile strains, respectively. Besides, the PF-AA-AM-Al³⁺/Ag0.25 showed strain sensitivity to small deformation (Gauge factor = 2.42) in wearable tests on hands and knees. In addition, the PF-AA-AM-Al³⁺/Ag0.25 had good cytocompatibility and antibacterial performance that bacteria killing ratio of 98% to S. aureus and 99% to E. coli. Finally, a viscoelastic numerical constitutive model was established based on finite element method to study the damage failure history of the material. Comparative analysis showed that local stress concentration was the main factor leading to the failure of hydrogel.

Mechanically Robust Elastomers Enabled by a Facile Interfacial Interactions-Driven Sacrificial Network

Strength and toughness are usually mutually exclusive for materials. The sacrificial bond strategy is used to address the trade-off between strength and toughness. However, the complex construction process of sacrificial network limits the application of sacrificial network. This work develops a facile strategy to construct an interfacial interactions-driven sacrificial network. The authors' group finds that there are the interfacial interactions between arginines (A) aggregates and molecular chains. Such interfacial interactions result in the mechanical properties of samples having a strong dependence on extension rates, which shows that A aggregates construct a network structure by interfacial interactions. The interfacial interactions between A aggregates and chains improve the strength of samples; while the A aggregate network driven by interfacial interactions preferentially ruptures to dissipate large energy for the improvement of fracture toughness, which can be considered as a sacrificial network. Therefore, their designed elastomers have both high strength and high toughness. This work provides an easier strategy for the construction of sacrificial networks, which can promote the industrial application of sacrificial networks in elastomer materials.

High-strength, thermosensitive double network hydrogels with antibacterial functionality

Herein, we report a method of fabricating strong and thermosensitive double network (T-DN) poly(N-isopropyl acrylamide) (PNIPAM)-based hydrogels, i.e. rigid and brittle poly(2-acrylamido-2-methylpropanesulfonic acid sodium salt) (PNaAMPS) as the first and soft and ductile poly(N-isopropyl acrylamide-co-acrylamide) (P(NIPAM-co-AAm)) as the second interpenetrating each other. In particular, NIPAM was deliberately integrated into the double network as an adjustor of elastic modulus and hydrophilicity, besides thermosensitivity. Such double network construction strategy resulted in PNaAMPS/P(NIPAM-co-AAm) T-DN hydrogels of excellent mechanical properties (0.83-1.37 MPa) and desirable temperature-dependent swellabilities. Besides, T-DN hydrogels with various NIPAM contents exhibited good biocompatibility with high cell survival rates around normal body temperatures. Furthermore, crystal violet (CV) could be readily loaded to impart antibacterial functionality to the T-DN hydrogels against E. coli. The double network construction strategy could be adapted to fabricating high-strength antibacterial hydrogels for a broad range of biomedical applications.

Selected Phase Separation Renders High Strength and Toughness to Polyacrylamide/Alginate Hydrogels with Large-Scale Cross-Linking Zones

High water content usually contradicts the mechanics for hydrogels, and achieving both characteristics is extremely challenging. Herein, a novel confined-chain-aggregation (CCA) strategy is developed to fabricate ultrastrong and tough hydrogels without sacrificing their inherent water capacity. Based on the popular polyacrylamide/alginate (PAAm/Alg) system with a double network (DN), a poor solvent exchange is induced once PAAm is fully cross-linked but prior to ionic cross-linking of alginate. In this case, the alginate chains are restricted by the chemical PAAm network and undergo a confined-chain aggregation, which guarantees an interpenetrating network of both polymers and simultaneously generates micron-scale aggregates. In addition, after the subsequent water uptake, the accompanying formation of hydrogen bonds and metal-ligand coordination stabilizes the newly formed alginate aggregates, serving as large-scale cross-linking zones. However, the PAAm chains are anchored by the preformed cross-linking points and convert back to the uniformly distributed, high-water-content state, achieving a selected phase separation in a DN system. The combined CCA and hybrid cation cross-linking method gives mechanical strength and toughness to the PAAm/Alg hydrogels to reach approximately 30 and 5 times the traditional methods, respectively. This investigation provides a general strategy for the development of a new generation of double-network hydrogels, which will expand their application as structural materials for cartilage and soft robotics.

Dually cross-linked single networks: structures and applications

Cross-linked polymers have attracted an immense attention over the years, however, there are many flaws of these systems, e.g. softness and brittleness; such materials possess non-adjustable properties and cannot recover from damage and thus are limited in their practical applications. Supramolecular chemistry offers a variety of dynamic interactions that when integrated into polymeric gels endow the systems with reversibility and responsiveness to external stimuli. A combination of different cross-links in a single gel could be the key to tackle these drawbacks, since covalent or chemical cross-linking serve to maintain the permanent shape of the material and to improve overall mechanical performance, whereas non-covalent cross-links impart dynamicity, reversibility, stimuli-responsiveness and often toughness to the material. In the present review we sought to give a comprehensive overview of the progress in design strategies of different types of dually cross-linked single gels made by researchers over the past decade as well as the successful implementations of these advances in many demanding fields where versatile multifunctional materials are required, such as tissue engineering, drug delivery, self-healing and adhesive systems, sensors as well as shape memory materials and actuators.

Recent Advances in Design Strategies for Tough and Stretchable Hydrogels

The development of multifunctional hydrogels with excellent stretchability and toughness is one of the most fascinating subjects in soft matter research. Numerous research efforts have focused on the design of new hydrogel systems with superior mechanical properties because of their potential applications in diverse fields. In this Minireview, we consider the most up-to-date mechanically strong hydrogels and summarize their design strategies based on the formation of double networks and dual physical crosslinking. Based on the synthetic approaches and different toughening mechanisms, double-network hydrogels can be further classified into three different categories, namely chemically crosslinked, hybrid physically-chemically crosslinked, and fully physically crosslinked. In addition to the above-mentioned methods, we also discuss few uniquely designed hydrogels with the intention of guiding the future development of these fascinating materials for superior mechanical performance.

Material innovation and mechanics design for substrates and encapsulation of flexible electronics: a review

Advances in materials and mechanics designs have led to the development of flexible electronics, which have important applications to human healthcare due to their good biocompatibility and conformal integration with biological tissue. Material innovation and mechanics design have played a key role in designing the substrates and encapsulations of flexible electronics for various bio-integrated systems. This review first introduces the inorganic materials and novel organic materials used for the substrates and encapsulation of flexible electronics, and summarizes their mechanics properties, permeability and optical transmission properties. The structural designs of the substrates are then introduced to ensure the reliability of flexible electronics, including the patterned and pre-strained designs to improve the stretchability, and the strain-isolation and -limiting substrates to reduce the deformation. Some emerging encapsulations are presented to protect the flexible electronics from degradation, environmental erosion or contamination, though they may slightly reduce the stretchability of flexible electronics.

High-Performance Photochromic Hydrogels for Rewritable Information Record

Rewritable information record materials usually demand not only reversibly stimuli-responsive ability, but also strong mechanical properties. To achieve one photochromic hydrogel with super-strong mechanical strength, hydrophobic molecule spiropyran (SP) has been introduced into a copolymer based on ion-hybrid crosslink. The hydrogels exhibit both photoinduced reversible color changes and excellent mechanical properties, i.e., the tensile stress of 3.22 MPa, work of tension of 12.8 MJ m^-3 , and modulus of elasticity of 8.6 MPa. Moreover, the SP-based Ca²⁺ crosslinked hydrogels can be enhanced further when exposed to UV-light via ionic interaction coordination between Ca²⁺ , merocyanine (MC) with polar copolymer chain. In particular, hydrogels have excellent reversible conversion behavior, which can be used to realize repeatable writing of optical information. Thus, the novel design is demonstrated to support future applications in writing repeatable optical information, optical displays, information storage, artificial intelligence systems, and flexible wearable devices.

Integration of Macro-Cross-Linker and Metal Coordination: A Super Stretchable Hydrogel with High Toughness

The development of multifunctional hydrogels with high strength and stretchability is one of the most important topics in soft-matter research owing to their potential applications in various fields. In this work, a dual physically cross-linked network was designed for the fabrication of ultrastretchable tough hydrogels. The hydrogels were prepared through in situ polymerization of acrylic acid and acrylamide in the presence of positively charged quaternary poly(ethylene imine) (Q-PEI) and micelle-forming Pluronic F127 diacrylate, thus introducing electrostatic interactions between the positively charged Q-PEI and negatively charged poly(acrylic acid-co-acrylamide). For further mechanical reinforcement, Ca²⁺ and Cu²⁺ ions were introduced into the hydrogel network to construct coordination bonds, significantly enhancing tensile strength as well as stretchability. The hydrogel prepared with Ca²⁺ ion coordination bonds was found to be stretchable to 108 times its original length and exhibited a maximum toughness of 177 MJ·m^-3, representing one of the most robust systems with both extraordinary toughness and superstretchability prepared to date. The hydrogels also exhibited excellent recovery of dimensions and reproducibility in terms of mechanical properties, providing a promising ultrastretchable soft-matter system with outstanding mechanical strength.

A facile method to synthesize strong salt-enhanced hydrogels based on reversible physical interaction

To overcome the adverse effects of salt on the mechanical properties of hydrogels, a facile double cross-linking method has been proposed to synthesize salt-enhanced tough hydrogels. Herein, a poly(hexafluorobutyl methacrylate-acrylamide) hydrogel [P(AAm-co-HFBMA) hydrogel] is prepared by the copolymerization of acrylamide (AAm) and hexafluorobutyl methacrylate (HFBMA) with N,N'-methylene bisacrylamide (NMBA) as a cross-linking agent in a dimethylformamide (DMF)/aqueous solution; DMF is then replaced by water. The results indicate that the tensile fracture stress of the P(AAm-co-HFBMA) hydrogel (20 mol% HFBMA) is as high as 0.43 MPa, which is far better than that of the PAAm hydrogel (ca. 30 kPa). Additionally, with a further increase in the hydrophobic structural units (25 mol% HFBMA), the tensile fracture stress of the P(AAm-co-HFBMA) hydrogel can be increased up to 2.34 MPa. The mechanical strength of the P(AAm-co-HFBMA) hydrogel is significantly enhanced to 3.50 MPa (2 M) from 2.34 MPa (0 M) after it is soaked in aqueous NaCl solutions with various salt concentrations. The mechanical properties and the results of the DSC analysis indicate that the main reason for its mechanical strength to exhibit a unique salt-enhancement trend can be explained as follows. After the P(AAm-co-HFBMA) hydrogel is soaked in the salt solution, the network gradually collapses with the penetration of the small molecules of salt. Thus, the hydrophobic C-F units easily form dynamic cross-linking junctions due to the switchable hydrophobic interaction between C-F groups, which can endow the P(AAm-co-HFBMA) hydrogel with a more effective dynamic energy dissipation mechanism in salt solution.

Mechanically strong and tough hydrogels with pH-triggered self-healing and shape memory properties based on a dual physically crosslinked network

A kind of dual physically crosslinked hydrogel with pH-triggered self-healing and shape memory properties is reported.

Engineering hydrogels by soaking: from mechanical strengthening to environmental adaptation

The soaking strategy could not only strengthen hydrogels with superior mechanical properties but also provide the hydrogels with environmentally adapting properties.

Synthesis of biscarboxylic acid functionalised EDTA mimicking polymers and their ability to form Zr(iv) chelation mediated nanostructures

We present a new biscarboxylic acid acrylate, which is used for the synthesis of double hydrophilic EDTA-mimicking block copolymers capable of self-assembly upon zirconium complexation.

High strength and antibacterial polyelectrolyte complex CS/HS hydrogel films for wound healing

We report a simple and facile self-assembly approach to fabricate polyelectrolyte complex (PEC) hydrogel films with positively charged chitosan (CS) and negatively charged heparin sodium (HS) by combining hydrogen bonding and electrostatic interactions. The CS/HS hydrogel films exhibited excellent tensile strength and toughness, good self-recovery ability, superior water absorbency, and pH-dependent surface charge characteristics. The gelation mechanism was investigated by zeta potential measurements. The CS/HS hydrogel films exhibited high antibacterial efficacy against E. coli at selected pHs or when coordinated with various metal ions and a significant effect on accelerating wound healing. The self-assembly approach presented in this work may serve as a generic strategy for the fabrication of novel multi-functional PEC hydrogels for broad biomedical applications.

Cooking-Inspired Versatile Design of an Ultrastrong and Tough Polysaccharide Hydrogel through Programmed Supramolecular Interactions

Simultaneously achieving strength and toughness in soft materials remains a challenge, especially for physically crosslinked hydrogels with many inactive interaction sites. In this work, inspired by the cooking of thick soup in China, a facile method that includes free water evaporation of the diluted pregel solution followed by crosslinking (WEC) is proposed to fabricate polysaccharide hydrogels. Herein, without the constraints of viscosity and crosslinking, polymer chains can homogenously approach as much as possible, thereby enabling the transformation of inactive supramolecular interaction (H-bonding and ionic coordination) sites into active sites until reaching the maximum level. Through facilely tuning the concentrating degree, programmed supramolecular interactions, serving as energy-dissipating sacrificial bonds, impart the hydrogels with strength and toughness over a very wide range, where a "ductile-to-tough" transition is discovered to occur first. Using WEC in alginate, the concentration can be as high as 25 wt% without sacrificing processing ability, a result that is significantly beyond common value (3-7 wt%), and the extremely stiff and tough hydrogels are obtained, superior to isotropic alginate hydrogels ever reported. This research offers a facile and versatile strategy to fabricate isotropic polysaccharide hydrogels, which become ideal matrix materials for further fabrication of hybrid or anisotropic hydrogels.

Conductive Tough Hydrogels with a Staggered Ion-Coordinating Structure for High Self-Recovery Rate

Conductive hydrogels are attracting increasing attention owing to their great potential for applications in flexible devices. For practical use, these high-water-content materials should not only show good conductivity but also be strong, stretchable, tough, and elastic. Herein, we describe a class of novel conductive tough hydrogels based on strong staggered Fe³⁺-carboxyl coordinating interactions. They are made from copolymers of acrylamide and N-acryloyl glutamic acid, a bidentate-based comonomer. The design of the staggered structure of Fe³⁺ and bidentate units is expected to enable energy dissipation and also results in a synergetic effect of two binding sites for fast self-recovery. We demonstrate that the equilibrated hydrogels with a water content of 53 wt % exhibit superior mechanical properties (e.g., highest tensile strength, 12.1 MPa; Young's modulus, 36.1 MPa; work of extension, 42.1 MJ m^-3; fracture energy, 10,691 J m^-2; compressive strength, 65.1 MPa at 98% strain without a macroscopic fracture) compared to the ion-coordinated hydrogels reported to date, including elasticity at small strain, fast self-recoverability at room temperature (∼25 °C), a high dielectric constant (k = 341-1395 at 100 kHz), and good electrical conductivity (0.0018-0.024 S cm^-1). Given their extraordinary overall characteristics, we envision their potential applications in flexible electronic devices.

Ultrastiff, Tough, and Healable Ionic-Hydrogen Bond Cross-Linked Hydrogels and Their Uses as Building Blocks To Construct Complex Hydrogel Structures

We report the ultrastiff and tough poly(acrylamide- co-acrylic acid)/Na-alginate/Fe3+ (P(AM- co-AA)/Na-alginate/Fe3+) hydrogel via the formation of hybrid ionic-hydrogen bond cross-linking networks. The optimal P(AM- co-AA)/Na-alginate/Fe3+ hydrogel possessed super high elastic modulus (∼24.6 MPa), tensile strength (∼10.4 MPa), compression strength (∼44 MPa), and toughness (∼4800 J/m2). The P(AM- co-AA)/Na-alginate/Fe3+ hydrogel was highly stable and maintained its superior mechanical properties in 0.5-2 M NaCl solution, aqueous solution with pH ranging from 4 to 10. The ionic cross-linking networks of the P(AM- co-AA)/Na-alginate/Fe3+ hydrogels can be locally and selectively dissociated by treating with aqueous NaOH solution with pH of 13 for 1 min and reformed by locally adding the additional Fe3+ solutions, making the hydrogels healable and cohesive. The healed hydrogels from the cutting surfaces can bear a tensile strength of up to 7.1 MPa. Various complex hydrogel structures can be constructed by using the P(AM- co-AA)/Na-alginate/Fe3+ hydrogels as building blocks via the adhesion of as-prepared hydrogels.