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Hydrogels have been extensively used for tissue engineering applications due to their versatility in structure and physical properties, which can mimic native tissues. Although significant progress has been made toward designing hydrogels for soft tissue repair, engineering hydrogels that resemble load‐bearing tissues is still considered a great challenge due to their specific mechanophysical demands. Herein, microporous, tough, yet highly compressible poly(vinyl alcohol) (PVA)‐based hydrogels are reported for potential applications in repairing or replacing different load‐bearing tissues. The synergy of freeze‐thawing and the Hofmeister effect, which controlled the spatial arrangement and aggregation of polymer chains, facilitated the formation of microstructured frameworks with tunable porosity. While the maximum mechanical strength, toughness, and stretchability of the engineered hydrogel were ≈390 kPa, ≈388 kJ m−3, and ≈170%, respectively, Young's modulus based on compression testing wasfound to be in the range of ≈0.02–0.30 MPa, highlighting the all‐in‐one mechanically enriched nature of the hydrogel. Furthermore, the minimal swelling and degradation rate of the engineered hydrogel met the specific requirements for load‐bearing tissues. Finally, excellent antibacterial resistance as well as in vitro biocompatibility of the hydrogel demonstrates its potential for the replacement of load‐bearing tissues. [ABSTRACT FROM AUTHOR]

The design of lignin nanostructures where interfacial interactions enable enhanced entanglement of colloidal networks can broaden their applications in hydrogel‐based materials and light‐based 3D printing. Herein, an approach for fabricating surface‐active dendritic colloidal microparticles (DCMs) characterized by fibrous structures using nanostructured allylated lignin is proposed for the development of lignin‐based photocurable resins. With allyl‐terminated surface functionality of 0.61 mmol g−1, the entanglement between lignin‐DCM fibrils with a size of 1.4 µm successfully produces only lignin‐based hydrogels with structural integrity through photo‐crosslinking. The colloidal network of lignin dendricolloids reinforces the poly(ethylene glycol) (PEG) hydrogels during a digital light processing (DLP) 3D printing process by generating bicontinuous morphologies, resulting in six‐fold increases in toughness values with respect to the neat PEG hydrogel. The dual effectiveness of photoabsorption and free‐radical reactivity of lignin‐DCMs allow the light‐patterning of rather dilute PEG hydrogels (5–10%) with high geometric fidelity and structural complexity via DLP 3D printing. This study demonstrates a green and effective strategy for the design of 1D lignin‐DCMs that increases the versatility of the nanostructured biopolymer, opening up numerous opportunities for formulating functional hydrogels with robust structure‐property correlations. [ABSTRACT FROM AUTHOR]

Tough hydrogels have emerged as a promising class of materials to target load‐bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high‐performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state‐of‐the‐art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high‐performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high‐performance hydrogels are compared with existing materials, and promising future opportunities are discussed. [ABSTRACT FROM AUTHOR]

Emerging soft ionotronics better match the human body mechanically and electrically compared to conventional rigid electronics. They hold great potential for human‐machine interfaces, wearable and implantable devices, and soft machines. Among various ionotronic devices, ionic junctions play critical roles in rectifying currents as electrical p–n junctions. Existing ionic junctions, however, are limited in electrical and mechanical performance, and are difficult to fabricate and degrade. Herein, the design, fabrication, and characterization of tough transient ionic junctions fabricated via 3D ionic microgel printing is reported. The 3D printing method demonstrates excellent printability and allows one to fabricate ionic junctions of various configurations with high fidelity. By combining ionic microgels, degradable networks, and highly charged biopolymers, the ionic junctions feature high stretchability (stretch limit 27), high fracture energy (>1000 Jm−2), excellent electrical performance (current rectification ratio >100), and transient stability (degrade in 1 week). A variety of ionotronic devices, including ionic diodes, ionic bipolar junction transistors, ionic full‐wave rectifiers, and ionic touchpads are further demonstrated. This study merges ionotronics, 3D printing, and degradable hydrogels, and will motivate the future development of high‐performance transient ionotronics. [ABSTRACT FROM AUTHOR]

Conductive hydrogels have attracted much attention because of their adjustable mechanical properties and excellent electrical conductivity. However, the variability and conductivity of traditional conductive hydrogels will be greatly reduced in low temperature environment, which seriously affects their application range. In this study, we fabricated a dual network conductive hydrogel with frost resistance. Here, water and glycerol binary solvents are introduced into the hydrogel to give the hydrogel antifreezing properties. The double-network structure of PAA/CMCs-Ca2+(W-G) hydrogel can provide good tensile and energy dissipation properties. The hydrogel strain sensor has high sensitivity (strain coefficient is 3.294 in the range of 100 ~ 250%, strain coefficient is 6.986 in the range of 250 ~ 400%) and good stability. PAA/CMCs-Ca2+(W-G) hydrogel has a great potential application in human signal detection. [ABSTRACT FROM AUTHOR]

Growing polymer materials emerge as a class of new intelligent systems, but they are all covalently cross‐linked. Herein, an alternating growth strategy for enabling tough supramolecular hydrogels is described to integrate externally provided compounds to change their sizes, geometric shapes, mechanical properties, etc. The hydrogels have dynamic acid–base complex (ABC) structures made from acidic and basic monomers, in which the acid–base pairs equilibrate between ionic and nonionic states to show either strong electrostatic or weak acid–base interactions. The growth process includes swelling of acidic/basic monomers, polymerization of the absorbed monomers, and homogenization of the original and newborn polymers, in which the balance between electrostatic and acid–base interactions is reversibly shifted to enable the process. As a result of such repeatable growth cycles, the hydrogels can continuously expand without compromising material mechanical properties. The growing sub‐processes and the monomer compositions can be used to post‐modulate the structure and properties of the grown products. It is demonstrated that the hydrogels may be designed as artificial tissues capable of growth under force‐bearing states. [ABSTRACT FROM AUTHOR]

Hydrogels are promising material candidates in engineering soft robotics, mechanical sensors, biomimetic regenerative medicine, etc. However, developing multinetwork hydrogels with high mechanical properties and excellent printability are still challenging. Here, a bifunctional phenol‐enabled sequential polymerization (BPSP) strategy is reported to fabricate high‐performance multinetwork hydrogels under the orthogonal catalysis of efficient ruthenium photochemistry. Benefiting from this bifunctional design, phenols can sequentially polymerize with typical monomers and themselves to fabricate various phenol‐containing polymers (Ph‐Ps) and Ph‐Ps‐based multinetwork tough hydrogels, respectively. The as‐prepared hydrogels have maximum stress of 0.75 MPa and toughness of 2.2 MJ m–3 under the critical strain of 800%. These property parameters are a maximum of 16 times higher than those of the phenol‐postmodified and phenol‐free hydrogels. Moreover, the rapid coupling polymerization of phenols can shorten the gelation times of hydrogels to as low as ≈4 s, which enables its printable property for customizable applications. As a proof of concept, a 3D scaffold‐like structure is optimized as highly sensitive mechanical sensors for detecting various human motions. [ABSTRACT FROM AUTHOR]

Hydrogels are a fascinating class of materials popular in numerous fields, including tissue engineering, drug delivery, soft robotics, and sensors, thanks to their 3D network porous structure containing a significant amount of water. However, traditional hydrogels exhibit poor mechanical strength, limiting their practical applications. Thus, many researchers have focused on the development of mechanically enhanced hydrogels. This review describes the design considerations for constructing tough hydrogels and some of the latest strategies in recent years. These tough hydrogels have an up‐and‐coming prospect and bring great hope to the fields of biomedicine and others. Nonetheless, it is still no small challenge to realize hydrogel materials that are tough, multifunctional, intelligent, and with zero defects. [ABSTRACT FROM AUTHOR]

Natural gels are constrained by a limited number of building blocks, yet based on time and space organization, they perform diverse functions. In contrast, the properties of synthetic hydrogels are frequently tuned through substantial changes in their chemical make‐up, causing complex interplay between composition, structure, and properties. This work fabricates a series of hydrogels with identical compositions but disparate properties by selectively quenching the depth and path of a water vapor‐induced phase separation process. These hydrogels are solely comprised of short alkyl side‐modified polyvinyl alcohol at the same volume fraction, but they exhibit hierarchical differences across multiple length scales, including porous morphology (≈µm), hydrophobic clusters (≈10 nm), and molecular packing (subnm). The hierarchical discrepancy is explicitly related to the striking contrast in terms of turbidity, permeability, stretchability, and viscoelasticity, thus advancing the understanding of the relationship between multiscale structures and properties without interference from chemical formulations. In addition, the hydrogels exhibit excellent biocompatibility, acid‐aided degradability, in situ healability, and underwater malleability. This work exploits a design principle to imitate the hierarchically specific tenet in nature, that is, the spatiotemporal organization of a single type of polymer via kinetic arrest of network‐forming phase separation, rather than modulation of the chemical make‐ups. [ABSTRACT FROM AUTHOR]

Adhesives can stably hold adherent components together. However, a simple design of a high‐performance printable tough adhesive (PTA) with instant fatigue‐resistant adhesion on various substrates is still a significant challenge. Here, the authors report a rapid design to meet this requirement that enables one‐step adhesion of PTA on wet and dry, porous, and nonporous surfaces low as 5 s. This design gives an interfacial toughness of ≈3000 J m−2 and comparable mechanical and fatigue‐resistant properties. The adhesive precursor can be used as an adhesive paint for bonding materials even underwater and maintains strong adhesion under several thousand cycles of large deformation or in a wide temperature range. Moreover, the PTA precursor is chemically stable and controllable, thus benefiting the printing of heterogeneous, 4D shape‐memory composites and a wearable electroluminescent sensor. It is anticipated that this rapid one‐step design of PTA opens new horizons to tough adhesion of diverse surfaces. [ABSTRACT FROM AUTHOR]

High‐resolution 3D‐printable hydrogels with high mechanical strength and biocompatibility are in great demand because of their potential applications in numerous fields. In this study, a material system comprising Pluronic F‐127 dimethacrylate (FDMA) is developed to function as a direct ink writing (DIW) hydrogel for 3D printing. FDMA is a triblock copolymer that transforms into micelles at elevated temperatures. The transformation increases the viscosity of FDMA and preserves its structure during DIW 3D printing, whereupon the printed structure is solidified through photopolymerization. Because of this viscosity shift, various functionalities can be incorporated through the addition of other materials in the solution state. Acrylic acid is incorporated into the pregel solution to enhance the mechanical strength, because the carboxylate group of poly(acrylic acid) ionically crosslinks with Fe3+, increasing the toughness of the DIW hydrogel 37 times to 2.46 MJ m−3. Tough conductive hydrogels are also 3D printed by homogenizing poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate into the pregel solution. Furthermore, the FDMA platform developed herein uses DIW, which facilitates multicartridges 3D printing, and because all the materials included are biocompatible, the platform may be used to fabricate complex structures for biological applications. [ABSTRACT FROM AUTHOR]

Biological tissues hinge on blood perfusion and mechanical toughness to function. Injectable hydrogels that possess both high permeability and toughness have profound impacts on regenerative medicine but remain a long‐standing challenge. To address this issue, injectable, pore‐forming double‐network hydrogels are fabricated by orchestrating stepwise gelation and phase separation processes. The interconnected pores of the resulting hydrogels enable direct medium perfusion through organ‐sized matrices. The hydrogels are amenable to cell encapsulation and delivery while promoting cell proliferation and spreading. They are also pore insensitive, tough, and fatigue resistant. When tested in biomimetic perfusion bioreactors, the hydrogels maintain physical integrity under prolonged, high‐frequency biomechanical stimulations (>6000 000 cycles at 120 Hz). The excellent biomechanical performance suggests the great potential of the new injectable hydrogel technology for repairing mechanically dynamic tissues, such as vocal folds, and other applications, such as tissue engineering, biofabrication, organs‐on‐chips, drug delivery, and disease modeling. [ABSTRACT FROM AUTHOR]

Hydrogel‐based soft electronics (HSE) is promising as implantable devices due to the similarity of hydrogel substrates to biologic tissues. Most existing HSE devices are based on conducting hydrogels that usually have weak mechanical properties, low conductivity, and poor patternability. Reported here is an HSE with good mechanical performance, high sensitivity, and versatile functions by stencil printing of liquid metal on a tough hydrogel, facilitating integration of multiple sensing units. Self‐shaping ability is imparted to the HSE by creating gradient structure in the hydrogel substrate. The resultant HSE actively deforms into 3D configurations with zero or nonzero Gaussian curvature to fix on objects or organs with sophisticated geometries and maintains the sensing functions. The versatilities and potential applications of this HSE are demonstrated by monitoring motions of a rice field eel and beatings of a rabbit heart. Such HSE based on morphing substrate should pave the way for implantable electronics with better fixation and interfacial contact with the organs. The concept of morphing hydrogel devices can be extended to other soft electronics with responsive polymer films or elastomers as the substrates. [ABSTRACT FROM AUTHOR]

Natural polysaccharides (NPS) are regarded as biomolecular and structural components for preparing high‐performance tough hydrogels. But the one‐step fabrication of NPS‐containing hydrogels in seconds and the template‐free design of complicated high‐resolution structures are still significant challenges in this field. To meet these requirements, various NPS‐containing tough hydrogels are fabricated and processed into 2D/3D structures via the combination of Ru(bpy)32+‐mediated photochemistry and extrusion 3D printing technique. The whole fabrication process is one‐step, completed in tens of seconds under visible light irradiation. It is found that the used NPS plays a key role in achieving the fabrication of high‐performance structured tough hydrogels. The high reactivity of functional groups in the used NPS can shorten their gelation times. Long rigid chains of the used NPS, their hierarchical assemblies, and contrasting multinetworks benefit from the efficient dissipation of mechanical energy and enhancement of its operational stability. Strong supramolecular interactions enable hydrogel precursors to have high viscosities, therefore providing good controllability to design high‐resolution and complicated tough hydrogel structures via extrusion 3D printing. It is anticipated that this straightforward fabrication strategy and findings will open new horizons for NPS‐containing materials. [ABSTRACT FROM AUTHOR]

Synthetic hydrogels are unique tissue mimics but rarely reproduce the strain‐stiffening properties of native tissues. This mechanical mismatch impairs the performance of hydrogels in practical applications. Inspired by the crimped structure of collagenous tissues, a series of strain‐stiffening hydrogels composed of curved parallel fibers are developed. These fibers are constructed from a bundle of intertwisted nanofibrils composed of short alkyl side chain‐modified polymer chains. This hierarchical organization enables exquisitely cascaded deformation that facilitates soft‐to‐firm and resilience‐to‐viscoelasticity transitions, thus synergically mimicking the strain‐adaptive stiffening and damping behaviors of natural tissues. Together with structural evolution and a constitutive model, rationally tuning the tortuosity and flexibility of the curved fibers produces a diverse combination of strain‐stiffening properties and unprecedented penetration into the regions of several tissues. The crimped structure and the resultant stiffening properties constitute major improvements to nanofiber‐based scaffolds for use in collagenous tissue repair. [ABSTRACT FROM AUTHOR]

In this work, doubly physically crosslinked hydrogels were synthesized, based on poly(N-isopropylacrylamide) containing 10 mol% of methacrylate co-monomer, in which nano-platelets of hectorite clay acted as the primary strong physical crosslinker, and trivalent cations La3+, Fe3+, and Al3+ as an additional dynamic one. Due to their excellent mechanical and tensile properties, such gels might be of interest as advanced structural materials for soft robotics. Of especial scientific interest was the comparison of the seemingly similar trivalent cations embedded in analogous gels, where their different bonding to carboxylate made dramatic differences concerning the material properties: La3+—purely electrostatic and highly dynamic bonding, Fe3+—with tendency to coordination-covalent-, and Al3+—with a rather covalent bonding. The cations incorporated into the hydrogels caused a marked, or even very strong improvement of tensile toughness, which in case of La3+ occurred without reducing the extensibility. All the cations expectedly raised the modulus: the effect was most pronounced with Al3+, which tends to covalent bonding. Most importantly, all the cations improved the kinetics of internal self-healing (self-recovery) of the gels, which was fastest with the dynamically (purely electrostatically) bonding La3+ (complete recovery of properties in 1 h). All the cations were incorporated via impregnation of precursor gels. An original synthesis aspect was, that the impregnation was conducted for a brief period of time, which was found necessary to achieve maximum modulus (the modulus dropped again at longer times). In this way, a non-stoichiometric (usually sub-stoichiometric) amount was incorporated, which made a subsequent extraction of cation excess unnecessary. Additionally, the non-stoichiometry helped the self-healing (network reorganization), which was most striking in case of the rather covalently bonding Al-impregnated gel. [ABSTRACT FROM AUTHOR]

Many biological tissues including cartilage and tendons are composed of polymeric hydrogels, which exhibit high toughness and rapid self‐recovery. However, developing hydrogels with both high toughness and rapid recovery remains a challenge. Inspired by the nacre of abalone shell, two non‐covalent cross‐linkers (Al3+ and diethylenetriamine [DETA]) with different relaxation times are introduced into a polyacrylic acid network. Compared with mono cross‐linked hydrogels, the dual physical cross‐linked hydrogel exhibits both high toughness (work of extension at fracture up to 8.0 MJ m−3) and rapid self‐recovery ability without loss of extensibility. Strong carboxylate‐DETA ionic cross‐links with longer relaxation time endow the hydrogels with high strength and help to localize the reformation of carboxylate‐Al3+ coordinate bonds; weak carboxylate‐Al3+ coordinate bonds with shorter relaxation time dissociate and reform to dissipate energy. Therefore, the hydrogels can dissipate massive energy effectively without any residual strain. More notably, the toughness and hysteresis of hydrogels can be completely recovered in 20 min. This finding unravels a new path to reinforce the hydrogels by the cooperation of different non‐covalent interactions, which can be applied in more material systems. [ABSTRACT FROM AUTHOR]

This chapter reviews the preparation, structures, properties, and applications of tough and responsive hydrogels crosslinked by triblock copolymer micelles. An amphiphilic triblock copolymer, Pluronic F127, is functionalized with reactive groups. The functional F127 chains form polyfunctional micelles in aqueous solution, which are used for reaction with hydrophilic monomers or polymer chains to form hydrophobically crosslinked network. Free radical polymerization, host-guest interaction, and dynamic bonds have been utilized to connect the polymer chains with the reactive micelle coronae. The obtained polymer hydrogels show outstanding strength and toughness, with the micelles serving as energy dissipation centers. The structural evolution and energy dissipation mechanisms have been investigated in detail. By incorporating functional monomers into the network, a series of functional hydrogels responsive to pH, salt, electric field, and temperature have been developed. The hydrogels have been utilized as building blocks to fabricate soft actuators, shape-morphing devices, and biomaterials for tissue repair. [ABSTRACT FROM AUTHOR]

Due to their outstanding physical and mechanical properties, double-network (DN) hydrogels have gained a most attention among various synthetic tough hydrogels. The Mullins effect and buckling instability are two frequent phenomena that may occur simultaneously in slender DN hydrogel structures as high load-bearing candidates. The swelling/deswelling degree of the DN hydrogel affects the coupling phenomena. It is essential to understanding the interplay between these behaviors. In this work, a physically based damage constitutive model for a DN polymer under water-diffusion equilibrium and cyclic loadings is developed. This theory of coupled swelling and load-induced deformations show good capability in capturing the Mullins effect of swollen DN hydrogels and the corresponding swelling ratio. The model parameters are determined by fitting experimental data of a freely swollen DN hydrogel under cyclic compression. Based on the constitutive model and determined parameters, the buckling instability of DN hydrogel beams is studied via the analytical formula of incremental modulus. The stability diagrams of the DN hydrogel beams under virgin loading and reloading are presented. The influences of stress softening, strain stiffening and chemical potential on buckling conditions for compressive stress and slenderness ratio are thoroughly analyzed. It demonstrates that the stress softening is dramatically against the stability of the beam, but the strain-stiffening effect would conversely help widen the stable range. Besides, it is found that a DN gel beam immersed in a sufficient low chemical potential environment has better buckling stability. These theoretical results are valuable in the preparation and structural design of DN hydrogels for repeated use purpose. • A constitutive model for double-network (DN) polymers under swelling and cyclic loads. • Global instability diagrams of DN hydrogel beams with various damage degrees. • Analysis of impacts of the Mullins effect and water content on buckling conditions. [ABSTRACT FROM AUTHOR]

Polymer network structures in the design of bilayer hydrogels for oral drug delivery systems contribute to the mechanical stability as well as the stimulus responsive drug release. We report a strategy to improve the stability of polyelectrolyte layer interfaces in bilayer hydrogels by employing concepts from tough interpenetrating hydrogel network design. Polyvinyl alcohol as the neutral substrate and Polyacrylic acid as the pH-sensitive layer were used to prepare bilayers capable of pH-sensitive bending behavior, and inverse double network structures were compared to conventional thin film coated bilayers in this study. The morphology of the bilayers observed after exposure to stimulus revealed that the polyacrylic acid layer was fragmented in conventional thin films. The delamination of the coating also had an adverse impact on bending behavior. Additionally, bilayers with inverse double network structure had a better pH-sensitive swelling behavior which was lacking in regular thin films. Controllable, pH-specific drug release behavior was a direct benefit of using the inverse double network structure, and was demonstrated by controlled vancomycin release under different pHs chosen to simulate the physiological milieu of exposure corresponding to hydrogel passage along the gastrointestinal tract. Additionally, the inverse double networks showed improved mucoadhesive properties and mechanical stability compared to the thin film bilayers. The use of inverse double network structured hydrogels has many potential applications in improving bilayer hydrogel stability; specifically, for drug delivery systems, designing hydrogel origamis, and to prepare soft material actuators. [ABSTRACT FROM AUTHOR]

Recently, strong polymer-based hydrogels have been intensively investigated. However, the development of tough protein hydrogels with controlled degradation for bone regeneration has rarely been reported. Here, regenerated silk fibroin/gelatin (RSF/G) hydrogels with both strength and controlled degradation are prepared via physically and chemically double-crosslinked networks. As a representative example, the 9%RSF/3%G hydrogel shows approximately 80% elongation and a compressive and tensile modulus of up to 0.25 and 0.21 MPa, respectively. It also shows a degradation rate that can be adjusted to approximately three months in vivo, a value between that of the rapidly degrading gelatin hydrogel and the slowly degrading RSF hydrogel. The 9%RSF/3%G hydrogel has good biocompatibility and promotes the proliferation and differentiation of bone marrow--derived stem cells compared with the control and pure RSF hydrogels. At 12 weeks after implantation of the gel in a calvarial defect, micro-computed tomography shows greater bone volume and bone mineral density in the 9%RSF/3%G group. More importantly, histology reveals more mineralization and enhancements in the quality and rate of bone regeneration with less of a tissue response in the 9%RSF/3%G group. These results indicate the promising potential of this tough protein hydrogel with controlled degradation for bone regeneration applications. [ABSTRACT FROM AUTHOR]

Based on the strategy of dynamic metal-coordination for improving mechanical properties, a further ideal was taken to prepare the tough hydrogels with the combination of self-crosslinking monomer and metal-coordination complexes. Herein, a series of hydrogels of poly(acrylamide-co-acrylic acid-co- N -hydroxymethyl acrylamide) (P(AM-co-AAc-co-NMAM)) consisting the chemical crosslinking induced by NMAM units and the physical crosslinking derived from the coordination complexes of carboxyl-Fe3+ were prepared. The molecular structure was investigated with the ATR-FTIR, Raman and UV-vis spectra. These hydrogels with different water content of 57–93% possess good mechanical performances. The optimal hydrogels possess high tensile strength (8.56 MPa), prominent modulus (15.5 MPa), remarkable toughness (37.85 MJ/m3) and superb tearing energy (7062 J/m2). The tough hydrogels also display excellent self-recovery (95% toughness recovery within 50 min), pH-triggered healing, shape memory and plasticity abilities. These hydrogels having high strength and toughness may broaden range of potential applications in load-bearing soft actuators, flexible electronics, etc. Image 1 • A series of novel hydrogels were fabricated by introducing metal-coordination interaction and self-crosslinking. • The hydrogels display outstanding mechanical properties. • The hydrogels possess excellent self-recovery, pH-triggered healing and shape memory abilities. [ABSTRACT FROM AUTHOR]

Abstract Human skin exhibits high stiffness of up to 100 MPa and high toughness of up to 3600 J m−2 despite its high water content of 40–70 wt%. Engineering hydrogels have rarely possessed both high stiffness and toughness, because compliant hydrogels usually become brittle when excess crosslinker is added to make the gel stiff. Furthermore, conventional hydrogels usually swell under physiological conditions, weakening their mechanical properties. Here, we designed a non-swellable hydrogel with high stiffness and toughness by interpenetrating covalently and ionically crosslinked networks. The stiffness is enhanced by utilizing ionic crosslinking sites fully, and the toughness is enhanced by adopting synergistic effects between energy-dissipation by ionic networks and crack-bridging by covalent networks. Non-swelling behaviors of the gel are achieved by densifying covalent and ionic crosslinks. The hybrid gel shows high elastic moduli (up to 108 MPa) and high fracture energies (up to 8850 J m−2). In vitro and in vivo swelling tests prove non-swelling behaviors of the gel. Live/dead assays show 99% cell viability over a period of 60 days. Graphical abstract Unlabelled Image Highlights • Enhanced stiffness (~108 MPa) and toughness (~8,850 J m-2) of pHEMA-alginate hydrogels were comparable with those of human skin. • Non-swelling behaviors of the gels were stable under in vivo conditions. • 99% cell viability over a period of 60 days was shown via live/dead assays. [ABSTRACT FROM AUTHOR]

Novel tough hydrogel materials are required for 3D-printing applications. Here, a series of thermoplastic polyurethanes (TPUs) based on poly(e-caprolactone)-b-poly(ethylene glycol)-b-poly(e-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymers and hexamethylene diisocyanate (HDI) were developed with PEG contents varying between 30 and 70 mol%. These showed excellent mechanical properties not only when dry, but also when hydrated: TPUs prepared from PCL-b-PEG-b-PCL with PEG of Mn 6 kg/mol (PCL7-PEG6-PCL7) took up 122 wt.% upon hydration and had an E-modulus of 52 ± 10 MPa, a tensile strength of 17 ± 2 MPa, and a strain at break of 1553 ± 155% in the hydrated state. They had a fracture energy of 17976 ± 3011 N/mm² and a high tearing energy of 72 kJ/m². TPUs prepared using PEG with Mn of 10 kg/mol (PCL5-PEG10-PCL5) took up 534% water and were more flexible. When wet, they had an E-modulus of 7 ± 2 MPa, a tensile strength of 4 ± 1 MPa, and a strain at break of 147 ± 41%. These hydrogels had a fracture energy of 513 ± 267 N/mm² and a tearing energy of 16 kJ/m². The latter TPU was first extruded into filaments and then processed into designed porous hydrogel structures by 3D-printing. These hydrogels can be used in 3D printing of tissue engineering scaffolds with high fracture toughness. [ABSTRACT FROM AUTHOR]

Controlling traumatic bleeding from damaged internal organs while effectively sealing the wound is critical for saving the lives of patients. Existing bioadhesives suffer from blood incompatibility, insufficient adhesion to wet surfaces, weak mechanical properties, and complex application procedures. Here, we engineered a ready-to-use hemostatic bioadhesive with ultra-strengthened mechanical properties and fatigue resistance, robust adhesion to wet tissues within a few seconds of gentle pressing, deformability to accommodate physiological function and action, and the ability to stop bleeding efficiently. The engineered hydrogel, which demonstrated high elasticity (>900%) and toughness (>4600 kJ/m3), was formed by fine-tuning a series of molecular interactions and crosslinking mechanisms involving N -hydroxysuccinimide (NHS) conjugated alginate (Alg-NHS), poly (ethylene glycol) diacrylate (PEGDA), tannic acid (TA), and Fe3+ ions. Dual adhesive moieties including mussel-inspired pyrogallol/catechol and NHS synergistically enhanced wet tissue adhesion (>400 kPa in a wound closure test). In conjunction with physical sealing, the high affinity of TA/Fe3+ for blood could further augment hemostasis. The engineered bioadhesive demonstrated excellent in vitro and in vivo biocompatibility as well as improved hemostatic efficacy as compared to commercial Surgicel®. Overall, the hydrogel design strategy described herein holds great promise for overcoming existing obstacles impeding clinical translation of engineered hemostatic bioadhesives. [ABSTRACT FROM AUTHOR]

In this article, we report a synergistic strategy to develop dual physically cross‐linked tough hydrogels via one‐pot bulk copolymerization of N‐vinyl‐2‐pyrrolidone, acrylic acid, and stearyl methylacrylate (SMA) without any adscititious surfactant. Due to synergic effects of hydrogen bonding and hydrophobic association, the resulted dual physically cross‐linked hydrogels (DP Gel) with ultra‐wide range adjustable Young's modulus (0.08–45.6 MPa), tensile stress (0.7–6.9 MPa), and toughness (3.3–23.1 MJ m−3). Stretching to 300%, DP Gel exhibited fast recoverability that remained ~95% of initial dissipated energy after resting in 60 °C for 3 min. Finally, scanning electron microscopy revealed that the microstructure of hydrogel changed from phase separation structure to micro phase separation as SMA added, which accounted for excellent performance of DP Gel. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1469–1474 A facile method to fabricate dual physically cross‐linked hydrogels with ultra‐wide range adjustable Young's modulus (0.08–45.6 MPa), tensile stress (0.7–6.9 MPa), toughness (3.3–23.1 MJ·m−3), and fast recoverability is presented. The hydrogel was developed via a one‐pot bulk copolymerization of N‐vinyl‐2‐pyrrolidone (NVP), acrylic acid (AAc), and stearyl methylacrylate (SMA) without any added surfactant. [ABSTRACT FROM AUTHOR]

Hydrogen bonds are known to play an important role in prescribing the mechanical performance of certain hydrogels such as polyether‐based polyurethanes. The quantitative contribution of hydrogen bonds to the toughness of polymer networks, however, has not been elucidated to date. Here, a new physical model is developed to predict the threshold fracture energies of hydrogels physically crosslinked via hydrogen bonds. The model is based on consecutive and sequential dissociation of hydrogen‐bonded crosslinks during crack propagation. It is proposed that the scission of hydrogen bonds during crack propagation allows polymer strands in the deformation zone to partially relax and release stored elastic energy. The summation of these partial chain relaxations leads to amplified threshold fracture energies which are 10–45 times larger than those predicted by the classical Lake–Thomas theory. Experiments were performed on a hydrophilic polyurethane hydrogel where urea additions were used to control the density of hydrogen bonds. The measured fracture energies were in good agreement with the calculated values. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1287–1293 Hydrogen‐bonded hydrogels are easily fabricated highly swollen polymer networks that are mechanically robust. The origin of their high toughness is explored by using urea to control the density of hydrogen bonds. The gel toughness is shown to be much greater than predicted by the classical Lake‐Thomas theory. A new model supported by experiments is introduced based on the sequential debonding of successive hydrogen bonds along polymer chains that span the crack plane. [ABSTRACT FROM AUTHOR]

Tough polymer hydrogels have great potential applications in soft actuators, artificial muscles, tissue engineering, and so forth. To improve the strength and toughness of hydrogels, numerous strategies have been developed to integrate efficient energy dissipation mechanisms into the hydrophilic networks. Among them, the use of macro‐crosslinkers to replace conventional chemical ones has become promising to develop tough hydrogels. Polymer colloids—including nano‐/microparticles, nano‐/microgels, hydrophobic associates, and block copolymer assemblies—have been employed in literature as multi‐functional macro‐crosslinkers that link polymer chains through covalent bonds or noncovalent interactions. The dislocation, deformation, desociation, and rupture of polymer colloids upon loadings are the major mechanisms to dissipate energy. This article provides a comprehensive account of most recent progresses on tough hydrogels crosslinked by polymer colloids, and explores the toughening mechanisms. It aims to inspire novel designs of tough hydrogels with multi‐functionalities. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1336–1350 Multi‐functional polymer colloids including nano‐/micoparticles, microgels, micelles, and vesicles are widely used as macro‐crosslinkers to synthesize tough hydrogels by crosslinking hydrophilic polymer chains through covalent bonds, dynamic bonds, hydrophobic association, or supramolecular recognition. Upon loadings, the colloids dislocate, deform, or dissociate to dissipate energy and toughen the hydrogels. Some energy dissipation mechanisms are discussed in detail. This article aims to inspire novel design and synthesis of tough and functional hydrogels for applications in smart devices. [ABSTRACT FROM AUTHOR]

Development of tough hydrogels has greatly expanded their applications as load‐bearing materials. However, the elastic modulus of tough hydrogels is usually lower than 1 MPa. It remains a challenge to design tough hydrogels with high modulus. We report here a series of tough double‐network (DN) hydrogels with ultrahigh elastic modulus (up to 200 MPa) by forming robust hydrogen bonds between the first poly(acrylic acid) network and the second poly(N‐isopropyl acrylamide) network. The dense cooperative hydrogen bonds greatly reduce the segmental mobility and thus improve the rigidity of gel matrix. Owing to the dynamic nature of hydrogen bonds, the modulus of hydrogels is strongly influenced by temperature and pH, affording the gels shape memory property. The strategy by forming robust noncovalent bonds between interpenetrating networks should be applicable to other systems for designing tough and versatile hydrogels with diverse promising applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1281–1286 Ultrastiff and tough hydrogels are developed by forming dense hydrogen bonds between the interpenetrating networks, which show shape memory behavior due to the temperature‐mediated huge variation of elastic modulus. [ABSTRACT FROM AUTHOR]

Abstract: Shape‐morphing hydrogels have emerging applications in biomedical devices, soft robotics, and so on. However, successful applications require a combination of excellent mechanical properties and fast responding speed, which are usually a trade‐off in hydrogel‐based devices. Here, a facile approach to fabricate 3D gel constructs by extrusion‐based printing of tough physical hydrogels, which show programmable deformations with high response speed and large output force, is described. Highly viscoelastic poly(acrylic acid‐co‐acrylamide) (P(AAc‐co‐AAm)) and poly(acrylic acid‐co‐N‐isopropyl acrylamide) (P(AAc‐co‐NIPAm)) solutions or their mixtures are printed into 3D constructs by using multiple nozzles, which are then transferred into FeCl3 solution to gel the structures by forming robust carboxyl–Fe3+ coordination complexes. The printed gel fibers containing poly(N‐isopropyl acrylamide) segment exhibit considerable volume contraction in concentrated saline solution, whereas the P(AAc‐co‐AAm) ones do not contract. The mismatch in responsiveness of the gel fibers affords the integrated 3D gel constructs the shape‐morphing ability. Because of the small diameter of gel fibers, the printed gel structures deform and recover with a fast speed. A four‐armed gripper is designed to clamp plastic balls with considerable holding force, as large as 115 times the weight of the gripper. This strategy should be applicable to other tough hydrogels and broaden their applications. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A tough hybrid hydrogel was based on covalent crosslinking and ionic coordination. • Latex microspheres with carboxylic groups acted as crosslinking centers. • The hydrogel exhibited excellent mechanical behaviors. Abstract In this work, carboxylic groups were introduced to latex microspheres (LMs) to obtain poly(styrene-butyl acrylate) (P(St-BA)-COOH) LMs with negative charges by a soap-free emulsion polymerization method. Then P(St-BA)-COOH LMs were added into an aqueous solution composed of acrylamide (AAm) and acrylic acid (AAc). The P(AAm-co-AAc)-MBA/LMs hydrogel was fabricated after adding redox initiators and N,N’-methylene diacrylamide (MBA) as covalent crosslinker by a free radical copolymerization. Subsequently, P(AAm-co-AAc)-MBA/LMs hydrogels were soaked into a ferric solution to achieve a tough hybrid hydrogel with covalent crosslinking and ionic coordination interaction between ferric ions and negative carboxylic groups arising from LMs and P(AAm-co-AAc) chains. The resultant P(AAm-co-AAc)-MBA/LMs-Fe3+ hydrogels possessed high fracture stress of 1440 kPa, large strain of 1140% and fracture energy of 6983 kJ/m3. This would help us to deeply understand the relationship between the structure and performance of LMs-reinforced hydrogels. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical properties (e.g., stiffness, stretchability) of prefabricated hydrogels are of pivotal importance for diverse applications in tissue engineering, soft robotics, and medicine. This study reports a feasible method to fabricate ultrasoft and highly stretchable structures from stiff and tough hydrogels of low stretchability and the application of these switchable hydrogels in programmable shape‐morphing systems. Stiff and tough hydrogel structures are first fabricated by the mechanical strengthening of Ca2+‐alginate/polyacrylamide tough hydrogels by addition of Fe3+ ions, which introduces Fe3+ ionically cross‐linked centers into the Ca2+ divalent cross‐linked hydrogel, forming an additional and much less flexible trivalent ionically cross‐linked network. The resulting stiff and tough hydrogels are exposed to an L‐ascorbic acid (vitamin C, VC) solution to rapidly reduce Fe3+ to Fe2+. As a result, flexible divalent ionically cross‐linked networks are formed, leading to swift softening of the stiff and tough hydrogels. Moreover, localized stiffness variation of the tough hydrogels can be realized by precise patterning of the VC solution. To validate this concept, sequential steps of VC patterning are carried out for local tuning of the stiffness of the hydrogels. With this strategy, localized softening, unfolding, and sequential folding of the tough hydrogels into complex 3D structures is demonstrated. [ABSTRACT FROM AUTHOR]

Tough hydrogels, polymeric network structures with excellent mechanical properties (such as high stretchability and toughness), are emerging soft materials. Despite their remarkably mechanical features, tough hydrogels exhibit two flaws (freezing around the icing temperatures of water and drying under arid conditions). Inspired by cryoprotectants (CPAs) used in the inhibition of the icing of water in biological samples, a versatile and straightforward method is reported to fabricate extreme anti-freezing, non-drying CPA-based organohydrogels with long-term stability by partially displacing water molecules within the pre-fabricated hydrogels. CPAbased Ca-alginate/polyacrylamide (PAAm) tough hydrogels were successfully fabricated with glycerol, glycol, and sorbitol. The CPA-based organohydrogels remain unfrozen and mechanically flexible even up to @70 8C and are stable under ambient conditions or even vacuum. [ABSTRACT FROM AUTHOR]

Spherical polymer brushes, poly(acrylic acid) (PAA)-grafted polystyrene nanoparticle (PAA@PS), are employed as the macro-crosslinker to prepare PAA hydrogels. Benefitting from the innumerable hydrogen bonds between highly entangled PAA chains both in bulk and on the polymer brush, the PAA/PAA@PS hydrogels combine desirable stretchability, toughness, and notch-insensitivity. The uniaxial tensile tests show a very high fracture elongation up to 9.1 × 103% while the fracture toughness reaches 3.0 MJ m−3 and the maximum swelling ratio of the hydrogel can be 2.0 × 103 as well. After being loaded with silver nanoparticles, the PAA/PAA@PS hydrogels are employed as a recyclable catalyst successfully. [ABSTRACT FROM AUTHOR]

Despite their potential in various fields of bioapplications, such as drug/cell delivery, tissue engineering, and regenerative medicine, hydrogels have often suffered from their weak mechanical properties, which are attributed to their single network of polymers. Here, supertough composite hydrogels are proposed consisting of alginate/polyacrylamide double-network hydrogels embedded with mesoporous silica particles (SBA-15). The supertoughness is derived from efficient energy dissipation through the multiple bondings, such as ionic crosslinking of alginate, covalent crosslinking of polyacrylamide, and van der Waals interactions and hydrogen bondings between SBA-15 and the polymers. The superior mechanical properties of these hybrid hydrogels make it possible to maintain the hydrogel structure for a long period of time in a physiological solution. Based on their high mechanical stability, these hybrid hydrogels are demonstrated to exhibit on-demand drug release, which is controlled by an external mechanical stimulation (both in vitro and in vivo). Moreover, different types of drugs can be separately loaded into the hydrogel network and mesopores of SBA-15 and can be released with different speeds, suggesting that these hydrogels can also be used for multiple drug release. [ABSTRACT FROM AUTHOR]

The chitosan based on purified regeneration could be dissolved in 6 wt% aqueous NaOH without freeze-thawing cycles and acetylation processing, and such a solution system was effective and different from other dissolving methods Upon heating, a tough hydrogel was constructed from the chitosan (purified regeneration) alkaline solution. The results of XRD, TEM, SEM and rheology analysis proved that chitosan easily aggregated in the solution and formed a nanofibers network to gelate at elevated temperature and concentration. The merely chitosan hydrogel had a uniform network structure and its (5 wt%) compressive fracture stress could reach 0.2 MPa. Furthermore, the hydrogels exhibited excellent biodegradability, blood compatibility and cellular compatibility. Therefore, the tough chitosan hydrogels may have a wide range of applications in biomedicine. [ABSTRACT FROM AUTHOR]

The developments of tough hydrogels in recent years have greatly expanded the applications of hydrogels as structural materials. However, most of the tough hydrogels are made of synthetic polymers. To develop biopolymer-based tough hydrogels has both fundamental and practical significances. Here we report a series of polysaccharides-based tough hydrogel films prepared by polyion complexation and solvent evaporation of chondroitin sulfate (CS) and protonated chitosan (CHT) solutions with different weight ratios. The obtained CS/CHT gel films with thickness of 40−80 μm and water content of 66 wt%−81 wt% possess excellent mechanical properties, with tensile breaking stress and breaking strain being 0.4−3 MPa and 160%−320%, respectively. We found that in the mixture solutions there are large amounts of excess CHT in terms of charges; after swelling the films in water, the acetic acid, which is used to protonate the amino groups of CHT, diffuses out of the gel matrix, enhancing the intermolecular interactions between CHT molecules and thus improving the mechanical properties of gel films, besides the ionic bonds between CS and CHT. Antimicrobial tests also showed that the gel films with low weight ratio of CS to CHT, corresponding to the case with excess CHT, have evident antimicrobial effect. These CS/CHT gel films with good mechanical properties and antimicrobial effect should extend the applications of hydrogels in biomedical fields. [ABSTRACT FROM AUTHOR]

Polyacrylamide (PAAm) and polyacrylic acid (PAAc) hydrophobic association hydrogels were synthetized via free radical polymerization, respectively. Also, the hydrophobic alkyl groups were introduced into PAAm and PAAc to enhance the mechanical properties of hydrogels. It was found that PAAm and PAAc hydrophobic association hydrogels didn't exhibit any self-healing behaviors. When PAAm and PAAc samples were touched each other for a few seconds, however, PAAm hydrophobic association hydrogels exhibited a strong adhesive force with PAAc hydrophobic association hydrogels. Subsequently, the adhesive behavior between PAAm and PAAc hydrophobic association hydrogels was investigated. It indicated that the adhesion between PAAm and PAAc hydrophobic association hydrogels was based on hydrogen bonding between amide groups of PAAm and carboxyl groups of PAAc. Moreover, the hydrophobic groups also had the cooperative function to enhance the adhesive strength of PAAm/PAAc adhesive hydrogels via hydrophobic association. It was envisioned that more adhesive hydrogels would be developed in the future and more applications would be possibly achieved in repairing tissue engineering or interface. [ABSTRACT FROM AUTHOR]

A series of chemically cross-linked microgel composite hydrogels (MCH gels) with excellent toughness and stretchability were prepared using core–shell polymer microspheres as cross-linking junctions. In our strategy, MCH gels are obtained by connecting microspheres with polyacrylamide (PAAm) chains chemically grafted onto their surfaces, where an organic cross-linking agent is completely unnecessary. The mechanical behavior of the MCH gels was analyzed, and superresolution fluorescence microscopy and scanning electron microscopy were used to investigate their toughening mechanism. The results indicated that the homogeneous network structure resulting from the good compatibility between the core–shell microspheres and matrix was an important reason for the excellent toughness of the MCH gels. In addition to interactions among H bonds in the grafted PAAm chains, reversible deformation of the core–shell microspheres acting as cross-linking junctions, which arises from the flexibility of the microspheres, and the effect of cavitation between the microspheres and matrix could also effectively dissipate energy during deformation of the MCH gels. [ABSTRACT FROM AUTHOR]

In our previous work, poly(butyl acrylate) (PBA) latex microspheres (LMs) were embedded within hydrogels to enhance their mechanical strength. Herein, acrylonitrile (AN) components were added to the latex system to prepare novel poly(butyl acrylate-acrylonitrile) (P(BA-AN)) LMs. LMs were able to adsorb hydrophobic hexadecyl methacrylate (HMA) due to hydrophobic interactions, stabilized by using sodium dodecyl sulfate as a surfactants. The HMA could occur radical copolymerization with acrylamide (AAm) under the redox initiators to form P(HMA-AAm)-P(BA-AN) hydrogels. The intra and interchain non-permanent binding could be promoted in P(BA-AN) LMs due to the dipole-dipole interactions of -CN groups from acrylonitrile. As a result, P(BA-AN) LMs were utilized to fabricate tough hydrogels as crosslinking centers for hydrophobic association. The mechanical properties of the hydrogels exhibited a fracture stress of 775 KPa, an ultrastretchable strain of 3600% and a fracture energy of approximately 7600 KJ/m 3 . This innovative design strategy for LMs capable of strong physicochemical interactions, including electrostatic and hydrogen-bond interactions and coordination effects, may open a novel direction for the production of tough hydrogels. [ABSTRACT FROM AUTHOR]

Keywords: 3D printing; granular hydrogels; microgels; ionotronics; tough hydrogels EN 3D printing granular hydrogels microgels ionotronics tough hydrogels 1 1 1 05/16/23 20230512 NES 230512 B Ionotronics b An hourglass measures time by a regulated flow of sand through a narrow neck. Tough Transient Ionic Junctions Printed with Ionic Microgels (Adv. Funct. In article number 2213677, Jianyu Li and co-workers show that with similar shapes, ionic junctions regulate ion flow, allowing unidirectional current conduction. [Extracted from the article]

Recyclable and degradable supercapacitors have promising applications for a sustainable energy storage industry. Herein, we prepare a dual-physical crosslinking (DP) carboxymethyl cellulose (CMC) hydrogel with high-toughness, healability, and electric conductivity by integrating abundant ions into the matrix. The prepared hydrogel displays a maximum compressive fracture stress of 4.42 MPa, fast healing in five seconds, and full degradation within eight days. Moreover, the fabricated supercapacitor shows high specific capacitance (309 F g−1) and volumetric capacitance (2.60 F cm−3). The supercapacitor achieves a healing efficiency of 93.9 % after five cuttings, and exhibits a cycling stability of 84.6 % capacitance retention after 1000 cycles. These merits ensure that the all-cellulose-based supercapacitor can operate in case of sudden collision and deformation, which contribute to reducing the environmental hazards from supercapacitor's preparation to its abandonment. A dual-physical crosslinking strategy for the fabrication of super-tough, self-healable, and fast degradable CMC hydrogel as quasi-solid-state electrolyte with outstanding electrochemical properties. [Display omitted] • Fully degradable all-cellulose electrolytes are fabricated as supercapacitors. • The supercapacitors combine superior mechanical and electrochemical performance. • Dual-physical networks provide its strength, self healing, and ions conduction. • A route avoids petrochemical polymers, chemical crosslinking agents, and UV light. [ABSTRACT FROM AUTHOR]

The rapid developments of tough hydrogels have promoted the applications of this kind of soft materials in loading bearing systems. However, tough gels usually have poor processibility, which is crucial for the construction of complex structures with gels. In this paper, we demonstrate the processing of tough polyion complex (PIC) hydrogels by simple compression moulding or extruding, which relies on the distinct strength of ionic bonds during and after the processing. The PIC precipitates formed by mixing the solutions of polycation and polyanion, poly(3-(methacryloylamino)propyl-trimethylammonium chloride) and poly(sodium p-styrenesulfonate), are plasticized by saline solution with selected concentration. These raw materials, initially weak and viscoelastic, are easily processed into different shapes. After swelling the as-prepared gels in water to dialyze out the salt and counterions of PIC, tough equilibrated gels are obtained with the imposed shapes during processing. These gels with 50–70 wt% water show good mechanical properties, with tensile fracture stress, fracture strain and tearing fracture energy being 3.7 MPa, 700% and 8 × 10 3 J/m 2 , respectively. These mechanical properties can be effectively tailored by tuning the charge ratio of PIC. However, the concentration of saline water has negligible influence on the mechanical properties of final equilibrated gels. The resultant PIC gels also exhibit stimuli-triggered healing properties owing to the dynamic and reversible nature of the ionic bonds. This study should promote the applications of tough PIC gels in structural elements of soft actuators and synthetic tissues. [ABSTRACT FROM AUTHOR]

Many active proteins and growth factors exist in extracellular matrix (ECM), regulating important physiological processes in the body. The conversion of ECM into hydrogels will significantly expand their application scope. Given the complexity of the environment in vivo , fabrication of ECM hydrogels with regulable coagulation/anticoagulation is very important yet challenging. Herein, we utilize phosphate buffer (PBS) as a concentration switch to realize the coagulation/anticoagulation transformation of ECM hydrogels. Benefiting from their regulable characteristic, these ECM hydrogels are respectively converted into hemostatic sponges/powders and tough anticoagulant films. The tensile strength of the film reached 39.23 ± 3.21 MPa. Especially, blood flow signals were still detected after two weeks transplantation of ECM films onto rat carotid arteries (diameter < 1 mm) as a vascular patch, suggesting the potential application of ECM anticoagulant films in small-caliber vascular grafts. This work provided a facile and sustainable strategy for developing ECM hydrogels with regulable coagulant/anticoagulant properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Over the past two decades, the development of hydrogels with high mechanical strength and toughness shows both fundamental and practical significances. It is challenging yet highly desired to process these tough hydrogels into structural elements with desired architectures, which are important for real applications. Three-dimensional (3D) printing, as an emerging additive manufacturing strategy, demonstrates great potentials for engineering tough hydrogels with versatile applications. This paper aims to review the recent progress on 3D printing of tough hydrogels and their relevant applications. We give an overview on the advantages and requirements of tough hydrogel systems for specific 3D printing technologies, followed by detailed summary of the network structures, mechanical performances, toughening mechanisms, and representative applications of the printed tough hydrogels. Finally, recent advances of 4D printing of tough hydrogels are discussed as a prospective direction to devise soft actuators and robots. [ABSTRACT FROM AUTHOR]

Appropriate mechanical properties and highly interconnected porosity are important properties for tissue engineering scaffolds. However, most existing hydrogel scaffolds suffer from poor mechanical properties limiting their application. Furthermore, it is relatively infrequent that precision control is achieved over pore size and structure of the scaffold because there are relatively few current technologies that allow such control and there is not a general appreciation that such control is important. To address these shortcomings, by combining double network polymerization and sphere-templating fabrication techniques, we developed a tough, intelligent scaffold based on poly(acrylic acid) and poly(N-isopropyl acrylamide) with a controllable, uniform, and interconnected porous structure. A mechanical assessment showed the toughness of the hydrogel and scaffold to be up to ~1.4 × 107 Jm-3 and ~1.5 × 106 Jm-3 respectively, as compared with 104-105 Jm-3 for most synthetic hydrogels. The thermosensitivity and pH-sensitivity were explored in a swelling study. In vitro testing demonstrated the scaffold matrices supported NIH-3T3 cell adhesion, proliferation and infiltration. An in vivorabbit study showed the scaffolds promote strong cellular integration by allowing cells to migrate into the porous structure from the surrounding tissues. These data suggest that the poly(acrylic acid)/poly(N-isopropyl acrylamide)-based scaffold could be an attractive candidate for tissue engineering. [ABSTRACT FROM AUTHOR]

A combination of multiple non-covalent interactions has been used to fabricate strong and tough hydrogels. However, mechanisms behind toughening remain unclear. In this study, a series of single-network (SN), double-network (DN), and triple-network (TN) hydrogels sequentially crosslinked by chemical bonds, freeze-thawed polyvinyl alcohol (PVA) crystallites, and ion coordination of alginate chains are systematically prepared. The contributions of hydrogen bonding, entanglements, crystallite crosslinking, and ion coordination to the dynamics of the hydrogels are comparatively studied. Dynamic mechanical analysis (DMA) reveals the apparent activation energy of hydrogen bonding from 41.1 kJ/mol to 50.6, 56.8, and 65.5 kJ/mol for SN gels with increasing PVA concentration (from 0 to 2, 6, 10 wt/vol%, respectively), whereas the corresponding apparent activation energy of chain entanglements increases from 148.3 to 160.2, 163.5, and 167.0 kJ/mol. PVA crystallites formed upon freeze-thawing act as physical crosslinks to further improve the strength and toughness of gels, as well as the activation energy (251.7 kJ/mol). Subsequent ion coordination with alginate further enhances the activation energy to 304.7 kJ/mol (Cu2+), 309.3 kJ/mol (Fe3+). Structural evolution studies on pre-cracked hydrogels reveal that PVA crystallites render crack blunting upon stretching. Polarized optical microscopy and scanning electron microscopy reveal the rearrangement and reconstruction of internal structures during crack propagation, including orientation and alignment of PVA crystallites. This study provides first insights to synergetic energy dissipation by non-covalent interactions of multi-network hydrogels. [Display omitted] • Multi-network hydrogels were prepared with multiple non-covalent interactions. • Energy dissipation through hydrogen bonding, entanglement, crystallization, and ionic complexation were studied. • Structural evolution at the crack tip of the hydrogels was identified to dissipate energy and enhance toughness. [ABSTRACT FROM AUTHOR]

In modern society, information leakage is a very important issue during our dairy life. Multi-dimensional information encryption and storage are effective ways to increase the security level. Herein, a serials of rapidly stimuli-responsive, tough fluorescent hydrogels have been fabricated by doping Ln3+ ions (Ln = Eu, Tb, Eu/Tb) into poly(N -acryloyl glycinamide- co -potassium 6-acryl-amidopicolinate) (P(NAGA- co -K6APA)) hydrogel networks which are prepared by one-pot copolymerization. The dynamic lanthanide complexes could not only serve as photoluminescent emitters but also cross-linking points to provide additional strength to the hydrogel network. The tensile fracture stress and Young's modulus can reach up to 3.9 MPa and 0.66 MPa, respectively, comparable to previously reported tough hydrogels. Moreover, because of these dynamic lanthanide coordination bonds, the luminescent hydrogels showed very fast and reversible responsiveness via direct alternating exposure to acid and base without luminance fatigue for at least five cycles. Remarkably, time-dependent luminescence fading process of this dynamic lanthanide complexes provides these hydrogels to be applied in multi-dimensional information encryption and decryption. A tough and rapidly pH-responsive color-tunable luminescent hydrogels reinforced by the robust hydrogen bonds and dynamic lanthanide complexes were prepared. These luminescent hydrogels hold great potential in programmable information protection and multi-dimensional information encryption and decryption. [Display omitted] •Tough, color-tunable luminescent hydrogels are prepared by a facile strategy. •The lanthanide containing hydrogels showed reversible and rapid stimuli-response. •The luminescence switch can repeat at least five times without luminance fatigue. •The hydrogels show the time-dependent luminescence fading process. •The hydrogels hold potential application in multi-dimensional information encryption. [ABSTRACT FROM AUTHOR]

Herein we present a CMC based hydrogel that can be engineered for texture by applying a soaking strategy after freeze-thaw. These two processes lead to a hierarchal structure within the CMC polymer hydrogels. Such hydrogels can be strain hardening; that is the gel modulus increases with higher strain levels. Both the failure stress and energy are high (1.54 MPa and 9.94 kJm−2 respectively). At sub-failure strains, the hydrogels show a high degree of recoverability as evidenced by their minimal hysteresis in a stress-strain test. Furthermore, these hydrogels are electrically conductive making them good candidates for tough sensors and wearable electronic devices. A key soaking strategy for the fabrication of strong hierarchical hydrogels with outstanding mechanical and electrical properties, which can function as wearable strain sensors. [Display omitted] • Strong carboxymethyl cellulose (CMC) electronic sensors are built by applying a soaking strategy. • The soaking step introduces strong physical interactions between the polymer chains. • The CMC hydrogels show excellent conductivity, high electronic sensitivity, and good durability. • Ionic crosslinking CMC hydrogels with eminent mechanical performance are obtained. [ABSTRACT FROM AUTHOR]

Mimicking the natural actuating systems artificially provides ample opportunities in the field of soft robotics. In this domain, recent advancements in tough hydrogels along with advanced manufacturing techniques have offered us new tools for creating hybrid and soft artificial muscles that resemble the attributes of natural muscles and biological organisms. This review article provides a summary of the recent developments in the field of hydrogel-based actuators and soft artificial muscles and presents a current account of their emerging applications. Further, the fundamentals behind the actuation of hydrogels are discussed with a focus on hydrogels swelling, diffusion, and mechanical behaviour. The design details of different tough hydrogel systems are highlighted, and their physical and mechanical properties are quantitatively compared with biological tissues. Finally, a brief conclusion is provided with remarks on the remaining challenges in this field. [Display omitted] • In-depth overview of hydrogel-based actuators, focusing on their fundamental attributes necessary for robotic systems. • Providing a comprehensive comparison between tough hydrogels and soft tissues as a selection guide for soft robotics. • Details of the new ground-breaking innovations taken place in the field of soft robotic systems based on tough hydrogels. [ABSTRACT FROM AUTHOR]