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Hydrogels usually are fabricated by using monomers or preexisting polymers in precursor solutions. Here, a polyelectrolyte complex biohydrogel (Bio-PEC hydrogel) made from a precursor dough, by kneading, annealing, and crosslinking the dough of two oppositely charged polysaccharides, cationic chitosan quaternary ammonium salt (HACC) and anionic sodium hyaluronate (HA), photoinitiator (α-ketoglutaric acid), crosslinker glycidyl methacrylate (GMA), and water of very small quantity is reported. Controlled kneading and annealing homogenized the dough with respect to transforming randomly distributed, individual polymer chains into tightly wound double-stranded structures, which, upon UV irradiation, covalently sparsely crosslinked into a highly entangled network and subsequently, upon fully swollen in water, results in Bio-PEC hydrogel, HACC/HA, exhibiting near-perfect elasticity, high tensile strength, and high swelling resistance. Via the same kneading and annealing, tetracarboxyphenylporphyrin iron (Fe-TCPP) metal nanoclusters are incorporated into HACC/HA to obtain photocatalytic, antibacterial, and biocompatible Bio-PEC hydrogel composite, Fe-TCPP@HACC/HA. Using SD rat models, the efficacy of Fe-TCPP@HACC/HA in inhibiting Escherichia coli (E. coli) growth in vitro and the ability to promote wound healing and scar-free skin regeneration in vivo, or its high potential as a wound dressing material for biomedical applications are demonstrated., (© 2023 Wiley‐VCH GmbH.)

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.

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 Ca 2+ crosslinked hydrogels can be enhanced further when exposed to UV-light via ionic interaction coordination between Ca 2+ , 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., (© 2021 Wiley-VCH GmbH.)

The research on multiple hydrogen bonds (H-bonds) hydrogels have gradually aroused wide interest. In this paper, a multiple H-bonds-reinforced poly(acrylamide-co-2-vinyl-4,6-diamino-2-vinyl-1,3,5-triazine)/tannic acid (P(Am-co-VDT)/TA) hydrogels are prepared. The results suggest that the prepared hydrogel has two types of H-bonds crosslinking regions: A "soft" region of H-bonds between the diaminotriazine (DAT) moieties on the polymer chains and the TA pyrogallol/catechol groups, and a "hard" region of H-bonds forming by DAT moieties with itself. The hard crosslinking region exhibits significantly higher activation energy than the soft region. Such soft and hard dual physically crosslinked networks dramatically enhance the mechanical properties of P(Am-co-VDT)/TA hydrogels in a synergistic manner (tensile strength is 2.34 MPa, elongation at break is 410%). Due to the multiple hydrogen bonds, the hydrogel has good pH sensitivity and rapid response to shape memory within a few minutes. In addition, the hydrogels have the capacity of physical adsorption of the anti-inflammatory drug diclofenac sodium and other molecules with a specific spatially arranged chemical composition. These hydrogels with high mechanical strength, excellent shape memory behavior, and capacity of adsorption of anti-inflammatory drug could be attractive candidates for applications in the fields of biomedicine, tissue engineering, and medical materials., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A versatile ionic crosslink lithography (ICL) approach is reported to achieve geometry transitions of strong polyvinyl alcohol/sodium alginate (PVA/SA) hydrogels in a controllable and programmable manner. Specifically, localized PVA/SA and PVA/SA/Fe 3+ hydrogel domains of significantly different swellabilities (i.e., in-plane gradient) are created by patterning and selective ionic crosslinking of one single type of PVA/SA hydrogel. A simple two-step sequential pre- and free-swelling, or each alone, directs the patterned, inhomogeneous hydrogel to transform in various programmable and quasi-quantitative ways through local bulging and/or global buckling. All types of shape changing are reversible and repeatable due to the reversible nature of ionic coordination in the hydrogel networks. The flexibility and versatility of 3D printing is also demonstrated in creating through-thickness gradient in PVA and PVA/SA hydrogel assemblies with similar morphing capability. The ICL approach developed in this work may help shed some light on developing strong and shape morphing hydrogels as soft sensors and actuators and for potentially biomimetic transformations. The ICL approach may also be transferable to fabrication of many other types of hydrogel materials for similar applications., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Shape-shifting polymers are widely used in various fields such as intelligent switches, soft robots and sensors, which require both multiple stimulus-response functions and qualified mechanical strength. In this study, a novel near-infrared-light (NIR)-responsible shape-shifting hydrogel system was designed and fabricated through embedding vinylsilane-modified carbon nanotubes (CNTs) into particle double-network (P-DN) hydrogels by micellar copolymerisation. The dispersed brittle Poly(sodium 2-acrylamido-2-methylpropane-1-sulfonate) (PNaAMPS) network of the microgels can serve as sacrificial bonds to toughen the hydrogels, and the CNTs endow it with NIR photothermal conversion ability. The results show that the CNTs embedded in the P-DN hydrogels present excellent mechanical strength, i.e., a fracture strength of 312 kPa and a fracture strain of 357%. Moreover, an asymmetric bilayer hydrogel, where the active layer contains CNTs, can achieve 0°–110° bending deformation within 10 min under NIR irradiation and can realise complex deformation movement. This study provides a theoretical and experimental basis for the design and manufacture of photoresponsive soft actuators. [ABSTRACT FROM AUTHOR]

Shape-shifting polymers usually require not only reversible stimuli-responsive ability, but also strong mechanical properties. A novel shape-shifting photochromic hydrogel system was designed and fabricated by embedding hydrophobic spiropyran (SP) into double polymeric network (DN) through micellar copolymerisation. Here, sodium alginate (Alg) and poly acrylate-co-methyl acrylate-co-spiropyran (P(SA-co-MA-co-SPMA)) were employed as the first network and the second network, respectively, to realise high mechanical strength. After being soaked in the CaCl2 solution, the carboxyl groups in the system underwent metal complexation with Ca2+ to enhance the hydrogel. Moreover, after the hydrogel was exposed to UV-light, the closed isomer of spiropyran in the hydrogel network could be converted into an open zwitterionic isomer merocyanine (MC), which was considered to interact with Ca2+ ions. Interestingly, Ca2+ and UV-light responsive programmable shape of the copolymer hydrogel could recover to its original form via immersion in pure water. Given its excellent metal ion and UV light stimuli-responsive and mechanical properties, the hydrogel has potential applications in the field of soft actuators. [ABSTRACT FROM AUTHOR]

This review critically evaluates advancements in multifunctional hydrogels, particularly focusing on their applications in osteoarthritis (OA) therapy. As research evolves from traditional natural materials, there is a significant shift towards synthetic and composite hydrogels, known for their superior mechanical properties and enhanced biodegradability. This review spotlights novel applications such as injectable hydrogels, microneedle technology, and responsive hydrogels, which have revolutionized OA treatment through targeted and efficient therapeutic delivery. Moreover, it discusses innovative hydrogel materials, including protein-based and superlubricating hydrogels, for their potential to reduce joint friction and inflammation. The integration of bioactive compounds within hydrogels to augment therapeutic efficacy is also examined. Furthermore, the review anticipates continued technological advancements and a deeper understanding of hydrogel-based OA therapies. It emphasizes the potential of hydrogels to provide tailored, minimally invasive treatments, thus highlighting their critical role in advancing the dynamic field of biomaterial science for OA management. [ABSTRACT FROM AUTHOR]

Polyampholyte (PA) hydrogels are randomly copolymerized from anionic and cationic monomers, showing good mechanical properties owing to the existence of numerous ionic bonds in the networks. However, relatively tough PA gels can be synthesized successfully only at high monomer concentrations (CM), where relatively strong chain entanglements exist to stabilize the primary supramolecular networks. This study aims to toughen weak PA gels with relatively weak primary topological entanglements (at relatively low CM) via a secondary equilibrium approach. According to this approach, an as-prepared PA gel is first dialyzed in a FeCl3 solution to reach a swelling equilibrium and then dialyzed in sufficient deionized water to remove excess free ions to achieve a new equilibrium, resulting in the modified PA gels. It is proved that the modified PA gels are eventually constructed by both ionic and metal coordination bonds, which could synergistically enhance the chain interactions and enable the network toughening. Systematic studies indicate that both CM and FeCl3 concentration ( C FeCl 3 ) influence the enhancement effectiveness of the modified PA gels, although all the gels could be dramatically enhanced. The mechanical properties of the modified PA gel could be optimized at CM = 2.0 M and C FeCl 3 = 0.3 M, where the Young's modulus, tensile fracture strength, and work of tension are improved by 1800%, 600%, and 820%, respectively, comparing to these of the original PA gel. By selecting a different PA gel system and diverse metal ions (i.e., Al3+, Mg2+, Ca2+), we further prove that the proposed approach is generally appliable. A theoretical model is used to understand the toughening mechanism. This work well extends the simple yet general approach for the toughening of weak PA gels with relatively weak chain entanglements. [ABSTRACT FROM AUTHOR]

Many living tissues, such as muscle, become mechanically stronger with growth. Yet, synthetic hydrogels usually exhibit an opposite size‐mechanical property relation, that is, swelling‐weakening behavior. Herein, a series of swollen yet strengthened polyampholyte (PA) hydrogels are developed via a simple metal‐ion solution soaking strategy. In this strategy, a dynamic PA hydrogel (with ionic bonds) is dialyzed in ZrOCl2 solutions (Step‐I) and deionized water (Step‐II) successively to obtain equilibrated hydrogels. Due to the specific Zr4+ ions and PA network structure, Step‐I takes several months with sample size and mechanical performance increasing continuously, while Step‐II only needs several days. Through this strategy, the resultant hydrogel networks are reorganized and eventually constructed by ionic and metalligand bonds, enabling the swelling yet strengthening behavior. A systematic study confirms that dialysis time in Step‐I and corresponding ZrOCl2 concentration can significantly affect the multiphase microstructures of the hydrogels, resulting in different mechanical enhancements. The optimized hydrogel possesses 39.2 MPa of Young's modulus and 3.7 MPa of tensile strength, which are 302 and 5.5 times these of the original PA gel, respectively. Despite distinct swelling, these hydrogels still mechanically surpass many existing high‐performance hydrogels. This study opens a novel pathway for fabricating swollen yet strengthened hydrogels. [ABSTRACT FROM AUTHOR]

Stimuli-responsive hydrogel actuators have attracted tremendous interest in switches and microrobots. Based on N-isopropylacrylamide (NIPAM) monomers with LCST phase separation and photochromic molecule spiropyran which can respond to ultraviolet light and H+, we develop a novel multistimuli-responsive co-polymer anisotropic bilayer hydrogel, which can undergo complex deformation behavior under environmental stimuli. Diverse bending angles were achieved based on inhomogeneous swelling. By controlling the environmental temperature, the bilayer hydrogels achieved bending angles of 83.4° and −162.4° below and above the critical temperature of PNIPAM. Stimulated by ultraviolet light and H+, the bilayer hydrogels showed bending angles of −19.4° and −17.3°, respectively. In addition, we designed a strategy to enhance the mechanical properties of the hydrogel via double network (DN). The mechanical properties and microscopic Fourier transform infrared (micro-FTIR) spectrum showed that the bilayer hydrogel can be well bonded at the interfaces of such bilayers. This work will inspire the design and fabrication of novel soft actuators with synergistic functions. [ABSTRACT FROM AUTHOR]

Hydrogels with high mechanical strength, good crack resistance, and good adhesion are highly desirable in various areas, such as soft electronics and wound dressing. Yet, these properties are usually mutually exclusive, so achieving such hydrogels is difficult. Herein, we fabricate a series of strong, tough, and adhesive composite hydrogels from polyampholyte (PA) gel reinforced by nonwoven cellulose-based fiber fabric (CF) via a simple composite strategy. In this strategy, CF could form a good interface with the relatively tough PA gel matrix, providing high load-bearing capability and good crack resistance for the composite gels. The relatively soft, sticky PA gel matrix could also provide a large effective contact area to achieve good adhesion. The effect of CF content on the mechanical and adhesion properties of composite gels is systematically studied. The optimized composite gel possesses 35.2 MPa of Young's modulus, 4.3 MPa of tensile strength, 8.1 kJ m−2 of tearing energy, 943 kPa of self-adhesive strength, and 1.4 kJ m−2 of self-adhesive energy, which is 22.1, 2.3, 1.8, 6.0, and 4.2 times those of the gel matrix, respectively. The samples could also form good adhesion to diverse substrates. This work opens a simple route for fabricating strong, tough, and adhesive hydrogels. [ABSTRACT FROM AUTHOR]

Chitosan (CS) has become a favorable choice for synthesizing hydrogels due to its hydrophilic and biocompatible nature. However, achieving robust CS-based hydrogels remains challenging. Herein, we propose a multiple mechanisms-based network design for mechanical enhancement in CS-based double-network (DN) composite hydrogels. In the DN composite gels, graphene oxide-filled CS-based network serves as the first network and polyacrylamide network is the second network. The multi-structure of the first network enhances its energy dissipation capacity; when combining with the second network to form DN structure, the coupling effect between the multiple mechanisms-based networks allows for much more energy dissipation, enabling synergistic enhancement in mechanical properties of the DN composite gels. By regulating the mechanics, we further explore the role of the individual networks. The results indicate that both the individual networks are essential: The first network primarily contributes to the energy dissipation, while the second network plays a role to the synergistic enhancement in the DN composite gels. This work provides an effective reinforcing strategy for CS-based composite hydrogels. [ABSTRACT FROM AUTHOR]

Tough hydrogels have shown strong potential as structural biomaterials. These hydrogels alone, however, possess limited mechanical properties (such as low modulus) when compared to some load-bearing tissues, e.g., ligaments and tendons. Developing both strong and tough soft materials is still a challenge. To overcome this obstacle, a new material design strategy has been recently introduced by combining tough hydrogels with woven fiber fabric to create fiber reinforced soft composites (FRSCs). The new FRSCs exhibit extremely high toughness and tensile properties, far superior to those of the neat components, indicating a synergistic effect. Here, focus is on understanding the role of energy dissipation of the soft matrix in the synergistic toughening of FRSCs. By selecting a range of soft matrix materials, from tough hydrogels to weak hydrogels and even a commercially available elastomer, the toughness of the matrix is determined to play a critical role in achieving extremely tough FRSCs. This work provides a good guide toward the universal design of soft composites with extraordinary fracture resistance capacity. [ABSTRACT FROM AUTHOR]

The novel double-network (DN) hydrogels were prepared using the chitosan- g-poly(acrylic acid) as the first network, and polyacrylamide as the second. The effects of the concentrations of the second network on chemical structure, intermolecular interactions and mechanical properties for the DN gels were investigated. The DN hydrogels had decreased swelling capacities and significantly improved glassy modulus and strength with the increase of the acrylamide concentration, owing to the enhanced intermolecular interaction and physical entanglement, and reduced molecular motion. It is worth noting that DN hydrogels with 5.50 mol/L acrylamide content had the greatest mechanical strength and still relatively high water content (~82 wt%), resulting from the effectiveness of the intermolecular penetration and intermolecular interactions between two independent polymer networks. Therefore, the reported novel chitosan-based DN hydrogels exhibit good potentials in some applications, for example biomedical engineering applications. [ABSTRACT FROM AUTHOR]

(Size: 5 cm height × 13 cm width). [Display omitted] • Developing a modified polyampholyte hydrogel with a swelling-strengthening behavior. • Hydrogel exhibiting high elastic modulus (18.5 MPa) and tensile strength (1.85 MPa). • Providing a simple pathway for achieving strong and stiff swollen hydrogels. Hydrogel networks are usually diluted upon swelling, weakening their mechanical performance. The mechanical weakness limits their practical applications. Herein, we report a modified polyampholyte (PA) hydrogel with a swelling-strengthening behavior via a secondary equilibrium strategy, based on the synergy of ionic and metal–ligand bonds in the network by introducing Zr4+ ions. Swelling and mechanical properties of the hydrogels are investigated, and the structure–property relationship is discussed. The modified PA gel possesses 18.5 MPa Young's modulus and 1.85 MPa tensile strength, highly surpassing the original PA gel. This work provides a simple route to achieve strong and stiff swollen hydrogels. [ABSTRACT FROM AUTHOR]

A simple, multiple‐hydrogen‐bond approach to fabricating physically crosslinked, Amylopectin reinforced polyacrylamide/poly(vinyl alcohol) (Amy/PAAm/PVA) double‐network (DN) hydrogels with super toughness in bulk and at solid interfaces is reported. The Amy/PAAm/PVA DN hydrogels exhibit high tensile strength (854.1 kPa), high extensibility (≈eight times), high bulk toughness (4094.8 kJ m−3), good self‐recovery property (≈92% of self‐recovery at room temperature), and strong adhesion to nonporous glass surfaces (≈158 kPa). Such tough and adhesive DN hydrogels have great potential for various applications in engineering artificial soft tissues, flexible electronics, and wearable devices. [ABSTRACT FROM AUTHOR]

Chitosan (CS), a linear natural polysaccharide, is often chosen to prepare hydrogels due to its hydrophilic nature and biocompatible merits. However, mechanically weak CS-based hydrogels show limited applications. Herein, we show a new multi-structural network design for mechanically enhancing graphene oxide (GO) filled CS-based hydrogel nanocomposites. The designed multi-structural hydrogel network shows a relatively dense structure with surrounding polymer chains at the local zone of self-assembled GO-CS units and a relatively loose structure at the far zone, resulting in the mechanical enhancement. By tuning GO loading and molecular weight of CS in a specific range, we intend to understand the effect of the specific multiple network structures on the mechanical properties of the composites. The experimental results demonstrate that the multi-structural network design provides an effective reinforcement mechanism for the hydrogel composites. The composite (containing only 0.30 wt% GO and ≈90 wt% water) has achieved 163% and 91% increases in compressive strength and work of compression, respectively, comparing to those of the neat gel. The findings may be generally applicable for the structural design and understanding of nanofiller-reinforced hydrogel and elastomer composites. [ABSTRACT FROM AUTHOR]

Polytetrafluoroethylene (PTFE) hollow fiber membranes are a promising candidate in highly contaminated wastewater treatment and management because PTFE is intrinsically chemically inert and hydrophobic. However, low water flux and fouling (contamination due to protein attachment and accumulation), both associated with low surface energy, pose great limits in filtration efficiency and the long-term use of PTFE hollow fiber membranes. Here we report a surface engineering strategy of coating PTFE hollow fiber membranes with polyvinyl alcohol/oxidized sodium alginate (PVA/OSA) double network (DN) hydrogels via N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTS) hydrolysis and grafting. The as-prepared PVA/OSA DN hydrogel coated PTFE hollow fiber membranes exhibited high efficiency in oil-water separation and excellent antifouling performance due primarily to the favorable interfacial electrostatic interaction and the hydration coating layer on the membrane surfaces. Moreover, the PVA/OSA DN hydrogel coatings remained high hydrophilicity and structural intergrity and operation stability after extensive time of soaking in strong acidic and alkaline solutions, all in great favor of the performance they were initially designed and intended for. [Display omitted] • The PVA/OSA DN coating enhances durability and superhydrophilicity simultaneously. • High oil-water separation efficiency (oil rejection rate up to 99.17%). • Excellent anti-protein-fouling performance. • Operation stability in harsh acidic and alkali environments. [ABSTRACT FROM AUTHOR]

Thermoresponsive hydrogel-based actuators have shown great potential in tissue engineering, biosensors and soft robots, while the preparation of temperature-driven bilayer hydrogel actuators with rapid responseness and recovery properties remains a challenge. Inspired by the layered heterostructure of human skin, we developed a Janus bilayer structure actuator consisting of thermoresponsive double network (TDN) hydrogel and polydimethylsiloxane (PDMS) elastomer with tough interface adhesion. Specifically, PNaAMPS/P(NIPAM- co -AAm) TDN hydrogel via photocopolymerization of N-isopropyl acrylamide (NIPAM) and acrylamide (AAm) and reinforced by microgel poly(2-acrylamide-2-methylpropane sulfonate sodium salt) (PNaAMPS) was in situ grafted onto benzophenone-activated PDMS surface, exhibiting strong interface adhesion (628 Jm-2 adhesion energy). The thermoresponsive shape deformations of the double-layer structure actuator were controllable by adjusting hydrogel/elastomer thickness ratio and introduction of metal ions, taking advantage of the different swelling and shrinkage characteristics of the hydrogel layer and the elastomer layer. The TDN/PDMS bilayer structure actuator, which integrates the "Janus" structure, thermal responsiveness and photothermal conversion, provides new ideas for the application and remote control of soft robots and soft actuators. [Display omitted] • Thermoresponsive bilayer actuator composed of hydrophilic TDN hydrogel and hydrophobic PDMS elastomer. • The bilayer hybrid laminates exhibites reversible and controllable shape deformation behavior. • Addition of copper ions improved photothermal conversion capability of TDN/PDMS bilayer hybrid laminates. • Janus, thermal responsiveness, and photothermal transformation realized shape transformation. • High toughness and reversible shape deformation controlled by temperature, thickness ratio and metal ion coordination. [ABSTRACT FROM AUTHOR]

This work demonstrates a facile approach to achieve ultralong anti-dehydration lifespan and ion-conductivity in a swollen yet strengthened polyampholyte (PA) hydrogel via the synergy of ionic and two kinds of metal–ligand bonds. The gel is made into a flexible electronic skin as an environmental-adaptable strain sensor for monitoring human activity in the −60 to 80 ℃ range. [Display omitted] • Proposing a strengthened PA hydrogel via synergy of ionic and metal–ligand bonds. • Presenting a viscoelastic model of the mechanical properties of the hydrogels. • Achieving an ultralong anti-dehydration lifespan of > 300 days. • Achieving a high ion conductivity of 50 S/m and GF of 4.9 at 0%-300% strain. • Achieving 100% accuracy of body motions classification in physical exercises. In order to adapt environmental changes, a hydrogel that possesses high mechanical strength, long-lasting moisture, good temperature tolerance, and exceptional electrical conductivity simultaneously is extremely desirable but remains challenging. Inspired by muscles becoming more ion-rich and stronger after swelling with blood, herein we propose a simple secondary soaking method to achieve ultralong anti-dehydration lifespan and high conductivity in a swollen yet strengthened polyampholytes (PA) hydrogel via the synergy of ionic and two types of metal–ligand bonds. An as-prepared PA gel is first dialyzed in a ZnCl 2 solution to reach a swelling equilibrium, and then moved to a FeCl 3 solution to achieve a new equilibrium. Through this approach, the as-prepared PA gel network could be optimized and eventually constructed by ionic and two kinds of metal–ligand bonds, enabling a synergistic reinforcement. To understand the reinforcing mechanisms, we also discuss the viscoelastic and elastic contributions to the mechanical properties of the hydrogels by a viscoelastic model. Additionally, the hydrogels possess ultralong anti-dehydration lifespan (>300 days), high ion conductivity (≈50 S/m) and temperature- and humidity-responsiveness, exhibiting high potential as e-skin to monitor human activity. The proposed strategy is simple but effective for achieving anti-dehydration and highly ion-conductive tough hydrogels. [ABSTRACT FROM AUTHOR]

Polyampholyte (PA) hydrogels in general possess excellent mechanical properties, but their underwater applications are largely limited due to swelling induced dramatically reduced strength and poor wet adhesion, especially in high salinity environments. Here, we report our approach to fabricating an anti-swelling poly(acryloyloxyethyltrimethyl ammonium chloride (DAC) -co- sodium p-styrenesulfonate (NaSS) -co- 2-Vinyl-4,6-diamino-1,3,5-triazine (VDT), or P(DAC -co- NaSS -co- VDT), PA hydrogel by introducing multi-hydrogen bonding capable functional VDT moieties into the DAC/NaSS coexisting and cation/anion ratio easily tunable PA network. The mechanical properties of the P(DAC -co- NaSS -co- VDT) hydrogel in bulk and at interface were controllable with dimethyl sulfoxide (DMSO)/H 2 O solvent displacement and Hofmeister effect in a wide salinity range, exhibiting maximum tensile strength, Young's modulus, and wet adhesion toughness of 7.6 MPa, 200 MPa, and 350 J m−2, respectively. The anti-swelling and strong P(DAC -co- NaSS -co- VDT) PA hydrogel developed in this work may find great potential in underwater sensing and sewage treatment as well as for many biomedical applications. • Polyelectrolyte P(DAC -co- NaSS -co- VDT) hydrogels can maintain volume stability in different salt solutions. • Hofmeister effect improves the mechanical properties of hydrogels. • The underwater adhesion of hydrogel can be regulated by solvent exchange. [ABSTRACT FROM AUTHOR]

Tough hydrogels with adjustable stiffness are expected for adapting application as various biomaterials. Oppositely charged polyelectrolytes form tough and self-healing physical polyion-complex (PIC) hydrogels via formation of inter-chain ionic bonds with a wide distribution in bond strength. The strong bonds serve as permanent crosslinking to impart elasticity and the weak bonds as reversible sacrificial bonds to dissipate energy and to self-heal. In this work, we fabricate four PIC hydrogels using four positively charged trimethyl-ammonium monomers with slightly different chemical moieties and a same negatively charged polymer. The obtained PIC hydrogels all show high toughness but large difference in stiffness, extensibility, and self-recovery kinetics. With slight difference in the monomer structure of the polycations, the modulus of the hydrogels varies over two orders in magnitude, from 0.36 to 56 MPa, and the difference in elongation at break is up to five times. The presence of acryloyl moiety and methyl moiety increase the stiffness of the hydrogels. In the temperature range studied, all the four PIC hydrogels exhibit the rheological simple behaviours, following the time-temperature superposition principle. The four samples show quite different dynamic relaxation spectra over wide frequency range, revealing large difference in the strength distribution of dynamic ionic bonds. SEM observation reveals quite different phase separation structure for the four samples, in which the polymer chain stiffness should play an important role. This understanding of structure-properties of the PIC hydrogels will merit the designing of various supramolecular tough hydrogels and therefore broaden the scope of hydrogels for the applications as biomaterials. [ABSTRACT FROM AUTHOR]

Most of the currently developed flexible energy storage devices lack sufficient flexibility to withstand various deformations, such as stretching, compression, and bending under the action of external forces. It also lacks sufficient and stable energy output. In this work, we use the flexible material hydrogel as electrode and electrolyte to design an all-hydrogel integrated flexible supercapacitor. Both the electrode and the electrolyte contain the same polyampholyte hydrogel P(NaSS- co -DMAEA-Q) matrix, making them possess superb self-adhesion due to the electrostatic interaction between the anion and cation group, and highly softness/toughness thanks to the energy-dissipative mechanism. The electrode of supercapacitor is a composite of activated carbon and hydrogel, which has e xcellent mechanical and electrical properties. Supercapacitors prepared from hydrogel electrodes and hydrogel electrolytes are inherently scalable/compressible, and simultaneously deliver high areal capacitance (128.9 mF cm−2 at 1 mV s−1 and 340.18 mF cm−2 at 0.1 mA cm−2) and maintain stable energy output. The combination of simple device structure, stable mechanical properties and excellent electrical properties makes flexible supercapacitors promising for applications in wearable electronics/devices and energy storage. • The same PA-based hydrogel as base material reduce the Interfacial contact resistance to a great extent. • The device has excellent overall stress and strain tolerance and durability. • Satisfied electrochemical performance of the device is evidenced by high areal capacitanceand moderately high power density/energy density. [ABSTRACT FROM AUTHOR]

Naturally occurring polysaccharide-based polyelectrolyte complex (PEC) physical hydrogels have broad applications in biomedicine. Yet, obtaining high-strength, strong-adhesion, and antibacterial PEC hydrogels via physically crosslinking of natural polysaccharides still remains a challenge. In this work, a chitosan (CS)/sodium alginate (SA) PEC physical hydrogel film was developed by using two oppositely charged natural polysaccharides, CS and SA, of equivalently high molecular weight (M w , approx. 105 Da), the tensile stress of which was achieved as high as 12.3 MPa due to CS-CS intermolecular hydrogen bonding and CS-SA electrostatic interaction. In addition, the hydrogel films, upon protonation at acidic conditions (e.g. pH 2.6), exhibited excellent antibacterial activity against Escherichia coli (E. coli) and the CS/SA hydrogel films also demonstrated strong adhesion to biological tissue. The high M w -matching, two-component natural polysaccharide approach presented in this work to preparing high-strength, strong-adhesion, and antibacterial PEC hydrogels may shed some lights on converting renewable biomass to value-added biomaterials. The hydrogel films (CS/SA) prepared by self-assembly of chitosan (CS) and sodium alginate (SA) with high molecular weight have a large number of hydrogen bonds and electrostatic interactions, the synergistic effect of those two makes the hydrogel have excellent mechanical properties. This hydrogel films also have strong tissue adhesion and bacteriostatic properties, with which the hydrogel films could be used for converting renewable biomass to value-added biomaterials. [Display omitted] • The hydrogen bonds and electrostatic interactions of CS/SA gel made it have excellent mechanical properties. • Ag+ made the CS/SA hydrogel films exhibited excellent mechanical properties and antibacterial activity. • The CS/SA hydrogels demonstrated excellent adhesion to biological tissues and promptly closed a wound. [ABSTRACT FROM AUTHOR]

Hydrogels that possess adequate toughness in bulk and at soft interface are highly desirable. We report a fully physically crosslinked dual Amylopectin (Amy, the first network)/poly(N-hydroxyethyl acrylamide) (PHEAA, the second network) hydrogel, where both networks were hydrogen bonding (H-bonding) crosslinked, with super toughness in bulk and tunable adhesion at hydrogel-soft tissue interface. Through adjustment of hydrogen bond density, strong and tough hydrogels were prepared that exhibited different levels of adhesion strength and three adhesive failure mechanisms. We replaced Amy with oxidized starch (OSA) to synthesize dual and hybrid chemically/physically crosslinked OSA/PHEAA double network (DN) hydrogels that were capable of forming Schiff's bases at hydrogel-hydrogel or hydrogel-tissue interfaces, aside from interfacial H-bonding, incurring sustained, high interfacial toughness (1850 J m−2 hydrogel-hydrogel and 610 J m−2 hydrogel-tissue) in wet environments. Such tough and adhesive DN hydrogels have great potential in various applications such as engineering of artificial soft tissues or implantable devices that are intended to adhere to internal organs or tissues. • Fully physically crosslinked hydrogels via H-bonding. • Tunable adhesion and three adhesive failure mechanisms of hydrogels. • The sacrificial bonds including H-bonds and reversible Schiff's bases at the interface. • High interfacial toughness at hydrogel-hydrogel or hydrogel-tissue interfaces in wet environments. [ABSTRACT FROM AUTHOR]