Oppositely charged polyelectrolytes form tough, self-healing, and rebuildable hydrogels

Tough Materials Through Ionic Interactions

This article introduces butyl acrylate-based materials that are toughened with dynamic crosslinkers. These dynamic crosslinkers are salts where both the anion and cation polymerize. The ion pairs between the polymerized anions and cations form dynamic crosslinks that break and reform under deformation. Chemical crosslinker was used to bring shape stability. The extent of dynamic and chemical crosslinking was related to the mechanical and thermal properties of the materials. Furthermore, the dependence of the material properties on different dynamic crosslinkers—tributyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C4ASA) and trihexyl-(4-vinylbenzyl)ammonium sulfopropyl acrylate (C6ASA)—was studied. The materials’ mechanical and thermal properties were characterized by means of tensile tests, dynamic mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. The dynamic crosslinks strengthened the materials considerably. Chemical crosslinks decreased the elasticity of the materials but did not significantly affect their strength. Comparison of the two ionic crosslinkers revealed that changing the crosslinker from C4ASA to C6ASA results in more elastic, but slightly weaker materials. In conclusion, dynamic crosslinks provide substantial enhancement of mechanical properties of the materials. This is a unique approach that is utilizable for a wide variety of polymer materials.

Enhanced Mechanical Properties by Ionomeric Complexation in Interpenetrating Network Hydrogels of Hydrolyzed Poly (N-vinyl Formamide) and Polyacrylamide

Tough hydrogels were made by hydrolysis of a neutral interpenetrating network (IPN) of poly (N-vinyl formamide) PNVF and polyacrylamide (PAAm) networks to form an IPN of polyvinylamine (PVAm) and poly (acrylic acid) (PAAc) capable of intermolecular ionic complexation. Single network (SN) PAAm and SN PNVF have similar chemical structures, parameters and physical properties. The hypothesis was that starting with neutral IPN networks of isomeric monomers that hydrolyze to comparable extents under similar conditions would lead to formation of networks with minimal phase separation and maximize potential for charge-charge interactions of the networks. Sequential IPNs of both PNVF/PAAm and PAAm/PNVF were synthesized and were optically transparent, an indication of homogeneity at submicron length scales. Both IPNs were hydrolyzed in base to form PVAm/PAAc and PAAc/PVAm IPNs. These underwent ~5-fold or greater decrease in swelling at intermediate pH values (3-6), consistent with the hypothesis of intermolecular charge complexation, and as hypothesized, the globally neutral, charge-complexed gel states showed substantial increases in failure properties upon compression, including an order of magnitude increases in toughness when compared to their unhydrolyzed states or the swollen states at high or low pH values. There was no loss of mechanical performance upon repeated compression over 95% strain.

3D architected temperature-tolerant organohydrogels with ultra-tunable energy absorption

The properties of mechanical metamaterials such as strength and energy absorption are often “locked” upon being manufactured. While there have been attempts to achieve tunable mechanical properties, state-of-the-art approaches still cannot achieve high strength/energy absorption with versatile tunability simultaneously. Herein, we fabricate for the first time, 3D architected organohydrogels with specific energy absorption that is readily tunable in an unprecedented range up to 5 × 10³ (from 0.0035 to 18.5 J g⁻¹) by leveraging on the energy dissipation induced by the synergistic combination of hydrogen bonding and metal coordination. The 3D architected organohydrogels also possess anti-freezing and non-drying properties facilitated by the hydrogen bonding between ethylene glycol and water. In a broader perspective, this work demonstrates a new type of architected metamaterials with the ability to produce a large range of mechanical properties using only a single material system, pushing forward the applications of mechanical metamaterials to broader possibilities.

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The first fabrication of 3D architected organohydrogels by Digital Light Processing

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Two-step toughening effect of organohydrogels by metal coordination and hydrogen bonding

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3D structures achieved ultra-tunable range of specific energy absorption up to 5000 x

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3D architected organohydrogels were demonstrated as tunable impact attenuators

Preparation of Chitosan/Calcium Alginate/Bentonite Composite Hydrogel and Its Heavy Metal Ions Adsorption Properties

In order to avoid the secondary pollution of the toxic residue of chemical crosslinking agent accompanied by chemical hydrogel adsorbent and enhance the adsorption performance of physical hydrogel, chitosan/calcium alginate/bentonite (CTS/CA/BT) composite physical hydrogel was constructed. The formation mechanism and structure of the composite hydrogel were determined by FTIR, XRD and SEM. Adsorption performances of the hydrogel toward Pb2+, Cu2+ and Cd2+ in water under different condition as well as multi-ion competitive sorption were investigated. The adsorption processes were described with the canonical adsorption kinetics and isotherms models. With the utilization of XPS analysis and adsorption thermodynamics analysis, it was found that the adsorptions were spontaneous physico-chemical adsorptions. The results showed that the maximum adsorption capacity of the hydrogel for Pb2+, Cu2+ and Cd2+ reached up to 434.89, 115.30 and 102.38 mg·g-1, respectively, better than those of other physical hydrogels or chitosan/bentonite composite. Moreover, the composite hydrogel improved the collectability of bentonite and showed a good reusability. The modification of bentonite and the formation of hydrogel were completed simultaneously, which greatly simplifies the operation process compared with the prior similar works. These suggest that the CTS/CA/BT composite hydrogel has promising application prospects for removal of heavy metal ions from water.

Ultrafast Fabrication of Self-Healing and Injectable Carboxymethyl Chitosan Hydrogel Dressing for Wound Healing

Herein, a new type of injectable carboxymethyl chitosan (CMCh) hydrogel wound dressing with self-healing properties is constructed. First, CMCh samples are homogeneously synthesized in alkali/urea aqueous solutions. Subsequently, trivalent metal ions of Fe³⁺ and Al³⁺ are introduced to form coordination bonds with CMCh, leading to an ultrafast gelation process. A series of hydrogels can be obtained by altering the concentration of CMCh and the relative content of metal ions. Owing to the dynamic and reversible characteristics of the coordination bonds, the hydrogel exhibits self-healing, self-adaption, and thermoresponsive ability. Moreover, due to the interaction between the amino groups on CMCh and SO₄^2-, the hydrogel undergoes phase separation and can be painlessly detached from the skin with little residue. Taking advantage of all these characteristics, the hydrogel is used as a wound dressing and can significantly accelerate skin tissue regeneration and wound closure. This hydrogel has great potential in the application of tissue engineering.

Hydrolyzed Hydrogels with Super Stretchability, High Strength, and Fast Self-Recovery for Flexible Sensors

Polyacrylamide is widely employed in constructing functional hydrogels. However, the volume expansion of this hydrogel in water weakens its mechanical properties and restricts its application. Herein, we report a strategy to convert the swollen and weak polyacrylamide/carboxymethyl chitosan hydrogel into a strong and tough one by hydrolysis in acid solution with an elevated temperature. The obtained hydrolyzed hydrogels possess a high strength, toughness, and tearing fracture energy of 5.9 MPa, 22 MJ/m³ and 7517 J/m², which are 254, 535 and 186 times higher than those of the original swollen one, respectively. In addition, the gels demonstrate low residual strain and rapid self-recovery abilities. Moreover, the gels have good shape memory behavior controlled by temperature. Furthermore, the gels can be worked as strain sensors with a broad strain window, high sensitivity, excellent linear response, and great durability in monitoring human motions after immersing treatment in a normal saline solution. This work provides a new method for preparing the stretchable and tough polyacrylamide-based hydrogels used in the areas of soft actuators and flexible electronics.

Hydrophobically-Modified PEG Hydrogels with Controllable Hydrophilic/Hydrophobic Balance

This work reports on a novel method to synthesize hydrophobically-modified hydrogels by curing epoxy monomers with amines. The resulting networks contain hydrophilic poly(ethylene glycol) (PEG) segments, poly(propylene glycol) (PPG) segments, and C₁₈ alkyl segments. By varying the content of C₁₈ segments, networks with different hydrophilic-lipophilic balance (HLB) are obtained. All networks show an amphiphilic behavior, swelling considerably both in organic solvents and in aqueous media. In the latter they display a thermosensitive behavior, which is highly affected by the network HLB and the pH of the solution. A decrease in HLB results in an increment of the polymer weight content (w_p) due to hydrophobic association. Furthermore, a reduction in HLB induces a remarkable increase in initial modulus, elongation at break and tensile strength, especially when w_p becomes greater than about 10%. Low field nuclear magnetic resonance (LF-NMR) experiments evidence that, when HLB decreases, a sudden and considerable increase in hydrogel heterogeneity takes place due to occurrence of extensive physical crosslinking. Available data suggest that in systems with w_p ≳ 10% a continuous physical network superimposes to the pre-existing chemical network and leads to a sort of double network capable of considerably improving hydrogel toughness.

Constitutive modeling of strain-dependent bond breaking and healing kinetics of chemical polyampholyte (PA) gel

A finite strain nonlinear viscoelastic constitutive model is used to study the uniaxial tension behaviour of chemical polyampholyte (PA) gel. This PA gel is cross-linked by chemical and physical bonds. Our constitutive model attempts to capture the time and strain dependent breaking and healing kinetics of physical bonds. We compare model prediction by uniaxial tension, cyclic and relaxation tests. Material parameters in our model are obtained by least squares optimization. These parameters gave fits that are in good agreement with the experiments.

Modulation of Properties through Covalent Bond Induced Formation of Strong Ion Pairing between Polyelectrolytes in Injectable Conetwork Hydrogels

In situ simultaneous formation of both covalent linkages and ion pair is challenging yet necessary to control the biological properties of a hydrogel. We report that the generation of covalent linkages (⁺N-C) facilitates the simultaneous formation of ion pairs between polyelectrolytes (PEs) in a hydrogel network. Co-injection of tertiary amine functional macromolecules and reactive poly(ethylene glycol) (PEG) containing negatively charged PE leads to the formation of hydrogel conetworks consisting of covalent junctions and ion pairs. Our design is based on the gradual appearance of ⁺N-C junctions followed by formation of ion pairs. This strategy provides an easy access to hydrogel networks bearing a predetermined proportion of ion pair and covalent cross-linking junction. The proportion of ion pair could be varied by introducing a precalculated proportion of mono- and difunctional reactive PEG in the hydrogel system. The topology of the prepolymer and the hydrogel could be modulated (graft) during hydrogel formation. This approach is applicable to obtain covalent/ionic, covalent bond induced purely ionic, and purely covalent hydrogels of several macromolecular entities. The effect of ion pairing in the hydrogels is strongly reflected in the modulus, strain bearing, degradation, free volume, swelling, and drug release properties. The hydrogels exhibit microscopic recovery of modulus after application of high amplitude strain depending on the prepolymer concentration (chain entanglement) and nature of hydrogel network. The hydrogels are hemocompatible, and the covalent/ionic hydrogels show a slower release of methotrexate than that of the purely covalent hydrogel. This work provides an understanding for the in situ construction and manipulation of biological properties of hydrogels through the covalent bond induced formation of a strong ion pair.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

Highly tough and ultrafast self-healable dual physically crosslinked sulfated alginate-based polyurethane elastomers for vascular tissue engineering

Since vascular diseases are regarded as a major cause of death worldwide, developing engineered biomimetic elastomers with physicochemical and biological properties resembling those of the natural vascular tissues, is vital for vascular tissue engineering (VTE). This study reports synthesis of highly tough supramolecular biologically active alginate-based supramolecular polyurethane (BASPU) elastomers that benefit from the presence of two physical networks with different strength of soft tertiary ammonium-soft sulfate pairs, as strong ionic bonds, and soft tertiary ammonium-hard carboxylate groups, as the weak bonds. The presence of sulfate groups resulted in low Young's modulus, high toughness and stretchability, proper energy dissipation, ultrafast self-healing and complete healing efficiency of BASPU. In vitro studies showed higher endothelial cells attachment, higher anticoagulation ability and significantly less platelet adhesion for BASPUs compared to the commercial vascular prosthesis. The histological studies of subcutaneously implanted scaffolds confirmed their low fibrosis and gradual biodegradation during 2 months of following.

Self-healing and easy-to-shape mineralized hydrogels for iontronics

Hydrogel-based multifunctional materials have attracted much attention. In this work, novel mineralized hydrogels were fabricated through physically cross-linked polyvinylpyrrolidone (PVP) and CaCO3. The mineralized hydrogels were prepared by simply mixing CaCl2, Na2CO3, and PVP in aqueous solutions. The CO32- induced the aggregation of the PVP chains and the CaCO3 particles in situ generated in the aqueous solution worked as fillers to strengthen the hydrogels. Based on this method, other kinds of mineralized hydrogels were prepared by replacing the Ca2+ with different metal ions. The mineralized hydrogels displayed shapeable, self-healing and thixotropic properties. Moreover, the mineralized hydrogel-based sensor showed good and stable sensitivity to compressive pressure, and could be used to monitor human actions. This work presents a facile method for preparing mineralized hydrogels, which are promising for various applications due to their outstanding properties.

Photo-induced programmable degradation of carboxymethyl chitosan-based hydrogels

Hydrogels are widely used in the biomedical field, due to their high similarity to native extracellular matrix (ECM). Most responsive hydrogels could only passively receive stimuli and independently change their properties. In this study, a photosensitive o-nitrobenzyl (NB) ester linker of polyethylene glycol (PEG) with maleimido (Mal) as terminal groups (PEG-NB-Mal) and a 5-methylfurfuryl (mF) grafted carboxymethyl chitosan (CMCS) derivative (CMCS-mF) were synthesized and used to prepare functional hydrogels via Diels-Alder (DA) reactions. The hydrogel exhibited programmable degradation properties after sequential exposure to UV light and acid treatments. It can maintain high integrity upon the single stimuli, the cascade acid and UV light treatments or the cascade UV light and alkaline treatments. Moreover, the hydrogel exhibited well controlled release profile of rhodamine B (RB). In summary, such CMCS-based hydrogels show great potential in biomedical applications. In addition, the usage of photo-induced cascade reaction in sequential degradation hydrogels can be extended to design other types of programmable smart materials.

A robust, freeze-resistant and highly ion conductive ionogel electrolyte towards lithium metal batteries workable at -30 °C

The wide applications of lithium metal batteries have encountered a severe conductivity issue when operating in cold weather. Here we report a freeze-resistant lithium metal battery, which displays outstanding rate performance, negligible polarization deterioration, and a good capacity retention of 94.25% after 700-cycles of use at -30 °C, the lowest temperature ever reported for gel electrolyte-based lithium metal batteries. Remarkably, the lithium metal batteries are even workable at temperatures down to -60 °C. The key point of the innovative design is the utilization of a newly created anti-freezing ionogel as an electrolyte, which is produced by gelation of an electrochemically inert ionic liquid, 1-butyl-3-methylimidazolium tetrafluoro-borate ([BMIM]BF₄), via dynamic condensation of a specially designed benzaldehyde-terminated polyethylene glycol (PEG-CHOs) with the tetra-hydrazide derivative of p-tert-butyl-calix[4]arene (CTH). The as-prepared ionogel electrolyte demonstrates a high ionic conductivity (0.43 mS cm^-1), a broad stability window (2.4-4.3 V vs. Li⁺/Li), and high flexibility at -30 °C. The outstanding property of the ionogel electrolyte is ascribed to its unique gel network structure as it enables enrichment of Li⁺ and enhances its efficient transportation. Further tests demonstrate that the ionogel electrolyte could be also used for the assembly of flexible lithium metal batteries.

A Multifunctional, Self-Healing, Self-Adhesive, and Conductive Sodium Alginate/Poly(vinyl alcohol) Composite Hydrogel as a Flexible Strain Sensor

Hydrogel-based wearable devices have attracted tremendous interest due to their potential applications in electronic skins, soft robotics, and sensors. However, it is still a challenge for hydrogel-based wearable devices to be integrated with high conductivity, a self-healing ability, remoldability, self-adhesiveness, good mechanical strength and high stretchability, good biocompatibility, and stimulus-responsiveness. Herein, multifunctional conductive composite hydrogels were fabricated by a simple one-pot method based on poly(vinyl alcohol) (PVA), sodium alginate (SA), and tannic acid (TA) using borax as a cross-linker. The composite hydrogel network was built by borate ester bonds and hydrogen bonds. The obtained hydrogel exhibited pH- and sugar-responsiveness, high stretchability (780% strain), and fast self-healing performance with healing efficiency (HE) as high as 93.56% without any external stimulus. Additionally, the hydrogel displayed considerable conductive behavior and stable changes of resistance with high sensitivity (gauge factor (GF) = 15.98 at a strain of 780%). The hydrogel was further applied as a strain sensor for monitoring large and tiny human motions with durable stability. Significantly, the healed hydrogel also showed good sensing behavior. This work broadens the avenue for the design and preparation of biocompatible polymer-based hydrogels to promote the application of hydrogel sensors with comfortable wearing feel and high sensitivity.

pH Responsive Strong Polyion Complex Shape Memory Hydrogel with Spontaneous Shape Changing and Information Encryption

Polyion complex (PIC) hydrogels attract lots of studies because of the relatively definite network and excellent mechanical strength. However, the stability of the PIC hydrogel is poor in salt solutions due to the counter-ion screening effect, which restricts their applications. Besides, novel functions of the PIC hydrogels also need to be explored. In this work, a multifunctional PIC hydrogel is prepared by polymerizing a hydrophobic monomer 2-(diethylamino)ethyl methacrylate in poly(styrene sulfonic acid) aqueous solution with the presence of counter-ion NaCl. Fourier transform infrared (FTIR) spectra, water content, and mechanical properties of the hydrogel are investigated. The introduction of hydrophobic weak electrolyte into the hydrogel brings stable excellent mechanical strength even in NaCl solutions with high concentration and pH modulated softening and strengthening. Surprisingly, the hydrogel swells but is strengthened in HCl, while it shrinks but is softened in NaOH. pH induced shape memory and unique spontaneous shape changing is thus presented benefiting from this synergistic effect. Moreover, information encryption is realized on the PIC hydrogel owing to the transmittance change and the different water absorption capability of the hydrogel at different states. This new kind of PIC hydrogel proposes a new smart material in continuously actuating soft machines and secretive information transformation.

Development of a Tough, Self-Healing Polyampholyte Terpolymer Hydrogel Patch with Enhanced Skin Adhesion via Tuning the Density and Strength of Ion-Pair Associations

Polyampholyte (PA) hydrogels have great potential for biomedical applications, owing to their high toughness and good self-recovery and self-healing (SELF) behavior in addition to their physical properties similar to human tissue. However, their implementation as practical biomedical skin patches or wearable devices has so far been limited by their insufficient transdermal adhesion strength. In this work, a new polyampholytic terpolymer (PAT) hydrogel with enhanced skin adhesion was developed using a novel and simple strategy that tunes the structure of ion-pair associations (IPAs), acting as cross-links, in the hydrogel via adding an extra neutral monomer component into the network without changing the total charge balance. The PAT hydrogels were synthesized by the terpolymerization of the neutral monomer N,N-dimethylacrylamide (DMAAm) (or 2-hydroxyethyl methacrylate (HEMA)) as well as the cationic monomer 3-(methacryloylamino) propyl-trimethylammonium chloride (MPTC) and the anionic monomer sodium p-styrenesulfonate (NaSS). Their IPA, which determines their network structure, was modulated by varying the feed concentration of the neutral monomer, C_nm. An increase of C_nm within an optimized C_nm window (0.3-0.4 M) decreased the cross-linking density (strength and density of the IPAs) of the PAT hydrogels, reducing the softening temperature and Young's modulus, which increased compliance but maintained sufficient mechanical strength and thereby maximized the contact surface and enhanced skin adhesion. The DMAAm monomers, compared to the HEMA monomers, produced the higher skin adhesion of the PAT hydrogel, which was explained by the difference in their reactivity to the MPTC and NaSS. This study demonstrated this new method to develop the PAT hydrogels with excellent skin adhesion and biocompatibility while maintaining good toughness, compliance, and SELF behavior and the potential of the PAT hydrogels for biomedical skin patches and wearable devices.

Can oppositely charged polyelectrolyte stars form a gel? A simulational study

We present a Langevin molecular dynamics study of an equimolar mixture of monodispersed oppositely charged di-block four-armed polyelectrolyte stars. We used an implicit solvent coarse-grained representation of the polyelectrolyte stars, and varied the length of the terminal charged blocks that reside on each arm. By varying the polymer concentration, we computed PV diagrams and determined the free-swelling equilibrium concentration with respect to a pure water reservoir as a function of the charged block length. We investigated various structural properties of the resulting equilibrium structures, like the number of ionic bonds, dangling arms, isolated stars, and cluster sizes. The ionic bonds featured a broad distribution of the number of arms involved and also displayed a distribution of net charges peaked around the neutral ionic bond. Our main result is that for charged block length equal to 4 and 5 ionized beads the resulting macro-aggregate spans the box and forms a network phase. Furthermore, we investigated the restructuring dynamics of ionic bonds; the results suggested both short bond lifetimes and a high frequency of ballistic association/dissociation events. Bonds result strong enough to yield a stable gel phase, but they are still weak enough to allow network restructuring under thermal fluctuations.

Tissue Adhesives: From Research to Clinical Translation

Sutures, staples, clips and skin closure strips are used as the gold standard to close wounds after an injury. In spite of being the present standard of care, the utilization of these conventional methods is precarious amid complicated and sensitive surgeries such as vascular anastomosis, ocular surgeries, nerve repair, or due to the high-risk components included. Tissue adhesives function as an interface to connect the surfaces of wound edges and prevent them from separation. They are fluid or semi-fluid mixtures that can be easily used to seal any wound of any morphology - uniform or irregular. As such, they provide alternatives to new and novel platforms for wound closure methods. In this review, we offer a background on the improvement of distinctive tissue adhesives focusing on the chemistry of some of these products that have been a commercial success from the clinical application perspective. This review is aimed to provide a guide toward innovation of tissue bioadhesive materials and their associated biomedical applications.

A novel dual crosslinked polysaccharide hydrogel with self-healing and stretchable properties

We synthesized oxidatively modified acetoacetyl cellulose OCAA, and then a double-network polysaccharide complex hydrogel was prepared. The hydrogel exhibited very good mechanical strength, self-healing behavior, and good biocompatibility.

Reversibly Transforming a Highly Swollen Polyelectrolyte Hydrogel to an Extremely Tough One and its Application as a Tubular Grasper

Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and its copolymer hydrogels are typical polyelectrolyte gels with extremely high swelling capacity that are widely used in industry. It's common to consider these hydrogels as weak materials that are difficult to toughen. Reported here is a facile strategy to transform swollen and weak poly(acrylamide-co-2-acrylamido-2-methyl-1-propanesulfonic acid) [P(AAm-co-AMPS)] hydrogels to tough ones by forming strong sulfonate-Zr⁴⁺ metal-coordination complexes. The resultant hydrogels with moderate water content possess high stiffness, strength, and fracture energy, which can be tuned over 3-4 orders of magnitude by controlling the composition and metal-to-ligand ratio. Owing to the dynamic nature of the coordination bonds, these hydrogels show rate- and temperature-dependent mechanical performances, as well as good self-recovery properties. This strategy is universal, as manifested by the drastically improved mechanical properties of hydrogels of various natural and synthetic sulfonate-containing polymers. The toughened hydrogels can be converted to the original swollen ones by breaking up the metal-coordination complexes in alkaline solutions. The reversible brittle-tough transition and concomitant dramatic volume change of polyelectrolyte hydrogels afford diverse applications, as demonstrated by the design of a tubular grasper with holding force a thousand times its own weight for objects with different geometries. It is envisioned that these hydrogels enable versatile applications in the biomedical and engineering fields.

A Solvent-Exchange Strategy to Regulate Noncovalent Interactions for Strong and Antiswelling Hydrogels

Physical hydrogels from existing polymers consisting of noncovalent interacting networks are highly desired due to their well-controlled compositions and environmental friendliness; and therefore, applied as adhesives, artificial tissues, and soft machines. Nevertheless, these gels have suffered from weak mechanical strength and low water resistance. Current methodologies used to fabricate these hydrogels mainly involve the freezing-thawing process (cryogels), which are complicated in preparation and short in adjustment of polymer conformation. Here, taking the merits of noncovalent bonds in adjustability and reversibility, a solvent-exchange strategy is developed to construct a class of exogels. Based on the exchange from a good solvent subsequently to a poor one, the intra- and interpolymer interactions are initially suppressed and then recovered, resulting in dissolving and cross-linking to polymers, respectively. Key to this approach is the good solvent, which favors of a stretched polymer conformation to homogenize the network, forming cross-linked hydrogel networks with remarkable stiffness, toughness, antiswelling properties, and thus underwater adhesive performance. The exogels highlight a facile but highly effective strategy of turning the solvent and consequently the noncovalent interactions to achieve the rational design of enhanced hydrogels and hydrogel-based soft materials.

Stress Relaxation and Underlying Structure Evolution in Tough and Self-Healing Hydrogels

The tough and self-healing hydrogels composed of polyampholytes (PA gels) are drawing great attention due to their multiscale structures and the resultant multiple mechanical properties. This work studies the stress relaxation behavior of PA gels and reveals the underlying multiscale structure evolutions by combining birefringence and small-angle X-ray scattering measurements. The PA gels show a fast and strong stress relaxation that obeys the stress-optical rule, which could be associated with relaxation of chain segment orientation by the breaking of ionic bonds. A slow and weak relaxation of phase structure (∼100 nm) is also observed, which tells that the stress redistributes and local strain amplification gradually builds in the phase network at long relaxation times as a result of synergetic breaking of multiple ionic bonds. This work gives insight into exploring the formation of the crack precursor that is important in the fracture and fatigue of self-healing hydrogels.

Fundamental Concepts of Hydrogels: Synthesis, Properties, and Their Applications

In the present review, we focused on the fundamental concepts of hydrogels-classification, the polymers involved, synthesis methods, types of hydrogels, properties, and applications of the hydrogel. Hydrogels can be synthesized from natural polymers, synthetic polymers, polymerizable synthetic monomers, and a combination of natural and synthetic polymers. Synthesis of hydrogels involves physical, chemical, and hybrid bonding. The bonding is formed via different routes, such as solution casting, solution mixing, bulk polymerization, free radical mechanism, radiation method, and interpenetrating network formation. The synthesized hydrogels have significant properties, such as mechanical strength, biocompatibility, biodegradability, swellability, and stimuli sensitivity. These properties are substantial for electrochemical and biomedical applications. Furthermore, this review emphasizes flexible and self-healable hydrogels as electrolytes for energy storage and energy conversion applications. Insufficient adhesiveness (less interfacial interaction) between electrodes and electrolytes and mechanical strength pose serious challenges, such as delamination of the supercapacitors, batteries, and solar cells. Owing to smart and aqueous hydrogels, robust mechanical strength, adhesiveness, stretchability, strain sensitivity, and self-healability are the critical factors that can identify the reliability and robustness of the energy storage and conversion devices. These devices are highly efficient and convenient for smart, light-weight, foldable electronics and modern pollution-free transportation in the current decade.

Polyzwitterions as a Versatile Building Block of Tough Hydrogels: From Polyelectrolyte Complex Gels to Double-Network Gels

The high water content of hydrogels makes them important as synthetic biomaterials, and tuning the mechanical properties of hydrogels to match those of natural tissues without changing chemistry is usually difficult. In this study, we have developed a series of hydrogels with varied stiffness, strength, and toughness based on a combination of poly(2-acrylamido-2-methylpropane sulfonic acid) (PAMPS), a strong acidic polyelectrolyte, and poly-N-(carboxymethyl)-N,N-dimethyl-2-(methacryloyloxy) ethanaminium) (PCDME), a polyzwitterion with a weak acidic moiety. We demonstrate that modifying the true molar ratio, R, of PCDME to PAMPS results in four unique categories of hydrogels with different swelling ratios and Young's moduli. When R < 1, a negatively charged polyelectrolyte gel (PE) is formed; when 1 < R < 3, a tough and viscoelastic polyelectrolyte complex gel (PEC) is formed; when 3 < R < 6.5, a conventional, elastic interpenetrating network gel (IPN) is formed; and when R > 6.5, a tough and stiff double-network gel (DN) is formed. Both the PEC and DN gels exhibit high toughness and fracture stress, up to 1.8 and 1.5 MPa, respectively. Importantly, the PEC gels exhibit strong recovery properties along with high toughness, distinguishing them from DN gels. Without requiring a change in chemistry, we can tune the mechanical response of hydrogels over a wide spectrum, making this a useful system of soft and hydrated biomaterials.

A Regenerable Hydrogel Electrolyte for Flexible Supercapacitors

Easy regenerability of core components such as electrode and electrolyte is highly required in advanced electrochemical devices. This work reports a reliable, regenerable, and stretchable hydrogel electrolyte based on ionic bonds between polyacrylic acid (PAA) and polyallylamine (PAH). PAA-PAH electrolyte (1M LiCl addition) exhibits high ionic conductivity (0.050 S·cm-1) and excellent mechanical property (fracture strain of 1,688%). Notably, the electrolyte can be regenerated to any desired shape under mild conditions and remains 96% and 90% of the initial ionic conductivity after the first and second regeneration, respectively. PAA-PAH/LiCl-based supercapacitor exhibits nearly 100% capacitance retention upon rolling, stretching, and 5,000 charge-discharge cycles, whereas the regenerated device holds 97.6% capacitance of the initial device and 90.9% after 5,000 cycles. This low-cost, high-efficiency, and regenerable hydrogel electrolyte reveals very promising use in solid-state/flexible supercapacitors and possibly becomes a standard commercial hydrogel electrolyte for sustainable electrochemical energy devices.

Fatigue-Resistant, Notch-Insensitive Zwitterionic Polymer Hydrogels with High Self-Healing Ability

Introducing self-healing properties into hydrogels can prolong their application lifetime. However, achieving mechanical strength without sacrificing self-healing properties is still a major challenge. We prepared a series of zwitterionic polymer hydrogels by random copolymerization of zwitterionic ionic monomer (SBMA), cationic monomer (DAC) and hydrophilic monomer (HEMA). The ionic bonds and hydrogen bonds formed in the hydrogels can efficiently dissipate energy and rebuild the network. The resulting hydrogels show high mechanical strength (289-396 KPa of fracture stress, 433-864 % of fracture stress) and have great fatigue resistance. The hydrogel with a 1 : 1 molar ratio of SBMA:DAC possesses the best self-healing properties (self-healing efficiency up to 96.5 % at room temperature for 10 h). The self-healing process is completely spontaneous and does not require external factors to assist. In addition, the hydrogel also possesses notch insensitivity with a fracture energy of 12000 J m^-2 . After combining the conductivity of RGO aerogel, the hydrogel/RGO composites show good strain sensitivity with high reliability and self-healing ability, which has certain significance in broadening the application of these zwitterionic hydrogels.

Quantitative Insights into the Effects of Post-Cross-Linking on Physical Performance Improvement and Surface-Cracking Healing of a Hydrogel

We report a post-cross-linking protocol that can improve the mechanical properties, freezing resistance, and fracture energies of a covalent cross-linking hydrogel and can also enable its surface-cracking healing. We design a covalent cross-linking reaction based on 3-(methacryloylamino) propyl-trimethylammonium chloride (MPTC) and sodium acrylate (SA) to give rise to a PMPTC@PSA model hydrogel. After post-cross-linking treatment, the mechanical stress is improved by 9.0-fold, accompanied by a 3.5-fold improvement in elongation; the freezing resistance is increased by 2.5-fold, which is reflected by the stretchability improvement at -35 °C. In addition, the fracture energy increased from 266 to 4686 J/m², an ∼17-fold improvement. Importantly, a surface-cracking hydrogel can be healed through the post-cross-linking treatment that enables the healing efficiency to approach 100% in terms of mechanical modulus and >81% in terms of maximum mechanical stress. This protocol is expected to provide a new option for physical performance improvement and crack healing of hydrogels in soft actuator, sensing device, and robotic applications.

Extrusion 3D Printing of Polymeric Materials with Advanced Properties

3D printing is a rapidly growing technology that has an enormous potential to impact a wide range of industries such as engineering, art, education, medicine, and aerospace. The flexibility in design provided by this technique offers many opportunities for manufacturing sophisticated 3D devices. The most widely utilized method is an extrusion-based solid-freeform fabrication approach, which is an extremely attractive additive manufacturing technology in both academic and industrial research communities. This method is versatile, with the ability to print a range of dimensions, multimaterial, and multifunctional 3D structures. It is also a very affordable technique in prototyping. However, the lack of variety in printable polymers with advanced material properties becomes the main bottleneck in further development of this technology. Herein, a comprehensive review is provided, focusing on material design strategies to achieve or enhance the 3D printability of a range of polymers including thermoplastics, thermosets, hydrogels, and other polymers by extrusion techniques. Moreover, diverse advanced properties exhibited by such printed polymers, such as mechanical strength, conductance, self-healing, as well as other integrated properties are highlighted. Lastly, the stimuli responsiveness of the 3D printed polymeric materials including shape morphing, degradability, and color changing is also discussed.

Conductive Hydrogels-A Novel Material: Recent Advances and Future Perspectives

A conductive hydrogel is a kind of polymer material having substantial potential applications with various properties, including high toughness, self-recoverability, electrical conductivity, transparency, freezing resistance, stimuli responsiveness, stretchability, self-healing, and strain sensitivity. Herein, according to the current research status of conductive hydrogels, properties of conductive hydrogels, preparation methods of different conductive hydrogels, and their application in different fields, such as sensor and actuator fabrication, biomedicine, and soft electronics, are introduced. Furthermore, the development direction and application prospects of conductive hydrogels are proposed.

Chemically Coupled Interfacial Adhesion in Multimaterial Printing of Hydrogels and Elastomers

Functional devices that use hydrogels as ionic conductors and elastomers as dielectrics have the advantage of being soft, stretchable, transparent, and biocompatible, making them ideal for biomedical applications. These devices are typically fabricated by manual assembly. Techniques for the manufacturing of soft materials have generally not looked at integrating multiple dissimilar materials. Silane coupling agents have recently shown promise for creating strong bonds between hydrogels and elastomers but have yet to be used in the extrusion printing of complex devices that integrate both hydrogels and elastomers. Here, we demonstrate the viability of silane coupling agents in a system with the rheology and functional composition necessary for three-dimensional (3D) extrusion printing of hydrogel-elastomer materials, specifically polyacrylamide (PAAm) hydrogel and poly(dimethylsiloxane) (PDMS) hydrophobic elastomer. By introducing a charge-neutral surfactant in the PDMS and adjusting silane concentrations in the PAAm, cast material samples demonstrate strong adhesion. We were also able to achieve an interfacial toughness of up to Γ = 193 ± 6.3 J/m² for a fully extrusion printed PAAm hydrogel-on-PDMS bilayer. This result demonstrates that an integration strategy based on silane coupling agents makes it possible for extrusion printing of a wide variety of hydrogel and silicone elastomers.

Chondroitin sulfate hydrogels based on electrostatic interactions with enhanced adhesive properties: exploring the bulk and interfacial contributions

Adhesive polysaccharide gels have highlighted their potential in biomedicine, tissue engineering, and wearable/implantable devices due to their tissue adhesive nature and excellent biocompatibility. However, the weak adhesive strength caused by the unclear relationship between the structure and the adhesive properties seriously hinders their further practical application. Here, a facile one-step synthesis method for adhesive and self-healing hydrogels with chondroitin sulfate (CS) and poly (methyl chloride quarternized N,N-dimethylamino ethylacrylate) (PDMAEA-Q) by ultraviolet light irradiation has been presented. We investigate the mechanism of the adhesion enhancement including improving the mechanical strength of gels (cohesion) and gel/substrate interfacial interactions (interfacial adhesion) by tailoring the compliance and cohesive energy density of the gel. The resultant soft and viscoelastic hydrogels displayed favorable adhesion ability on various substrates, and the adhesive strength to the iron substrate and porcine skin can reach 49.4 kPa and 15.4 kPa, respectively. Additionally, the gels also exhibited rapid self-healing properties and good cytocompatibility. We believe that the adhesive PDMAEA-Q/CS gel would be an ideal candidate for hydrogel glues for human-machine interfaces and biological tissues, and this design idea can open a new path for the preparation of adhesive polysaccharide hydrogels.

Ionically Conductive Hydrogel with Fast Self-Recovery and Low Residual Strain as Strain and Pressure Sensors

Hydrogel-based sensors have attracted enormous interest due to their broad applications in wearable devices. However, existing hydrogel-based sensors cannot integrate satisfying mechanical performances with excellent conductivity to meet the requirements for practical application. Herein, an ionically conductive hydrogel with high strength, fast self-recovery, and low residual strain is constructed through a facile soaking strategy. The proposed ionically conductive double network hydrogel is achieved by combining chemically crosslinked polyacrylamide and physically crosslinked gelatin network followed by sodium citrate solution immersing. The obtained hydrogel has a tensile strength of 1.66 MPa and an elongation of 849%. The ionically conductive hydrogels can be utilized as both strain and pressure sensors with high sensitivity. Moreover, they can be used as ionic skin to monitor various human movements precisely, demonstrating their promising potential in wearable devices and flexible electronics.

Developing super tough gelatin-based hydrogels by incorporating linear poly(methacrylic acid) to facilitate sacrificial hydrogen bonding

Mechanically robust protein-based hydrogels are strongly desired but their construction remains a significant challenge. In this work, gelatin, together with methacrylic acid, is used to construct a novel hydrogen-bonded hydrogel through a facile low-temperature polymerization and a subsequent dry-swell process. The obtained gel is extremely stiff and tough with a high Young's modulus and a fracture energy of 11 MPa and 8.5 kJ m^-2, respectively, which are comparable to the performance of tough synthetic hydrogels, rubber, cartilage, and skin. These gels also show recovery and healing properties as well as biocompatibility and stability in physiological saline solutions. The gel is easy to prepare and exhibits a wide range of functional properties, making it a promising load-bearing material for medical applications.