High-Strength, Tough, Fatigue Resistant, and Self-Healing Hydrogel Based on Dual Physically Cross-Linked Network

A bioinspired hydrogen bond crosslink strategy toward toughening ultrastrong and multifunctional nanocomposite hydrogels

A bioinspired hydrogen bond crosslink strategy enabled the physical hydrogels to possess exceptional mechanical properties, good self-recoverability, versatile adhesiveness, biocompatibility and antibacterial properties.

Dopamine enhances the mechanical and biological properties of enzyme-induced mineralized hydrogels

Enzyme-induced mineralization is an effective approach to improving the mechanical properties of acrylamide hydrogel and dopamine biofunctionalization can further significantly improve both the biological properties and the mechanical properties.

Hybrid double-network hydrogels with excellent mechanical properties

Fabrication of high performance hybrid double-network hydrogels via electrostatic interactions.

Effects of surfactant and ionic concentration on properties of dual physical crosslinking self-healing hydrogels by hydrophobic association and ionic interactions

Two kinds of dual crosslinking hydrogels have adjustable mechanical properties, self-healing and self-recovery performances.

Superstrong and Tough Hydrogel through Physical Cross-Linking and Molecular Alignment

Hydrogels are attracting increasing attention due to their potential use in various fields. However, most of the existing hydrogels have limitations in either dissipating mechanical energy or maintaining high stretchability under deformation, thus do not possess high mechanical properties. Herein, poly(vinyl alcohol) (PVA)-tannic acid (TA) hydrogels with both high mechanical strength and stretchability were obtained via a step-by-step physical cross-linking and molecular alignment method. Saline-triggered physical interactions serve as "sacrifice domains" to dissipate energy and endow PVA-based hydrogel with high mechanical strength (≈16 MPa) and stretchability (≈1000%). Due to the reversible arranging and disassociating property of physical interactions, PVA-TA hydrogels show excellent shape memory performance. We further demonstrated an effective approach to fabricate strong and aligned PVA-TA thread. The resultant well-aligned PVA-TA dry thread reveals an ultrahigh mechanical tensile strength of up to 750 MPa, nearly 45 times higher than PVA-TA thread with no alignment. Wide-angle X-ray two-dimensional diffraction images further confirmed the alignment of PVA fibers in stretching direction. In addition, we applied the PVA-TA hydrogel as suture and evaluated the cytotoxicity and biocompatibility of the PVA-TA suture.

In situ-forming, mechanically resilient hydrogels for cell delivery

Injectable, in situ-forming hydrogels can improve cell delivery in tissue engineering applications by facilitating minimally invasive delivery to irregular defect sites and improving cell retention and survival. Tissues targeted for cell delivery often undergo diverse mechanical loading including high stress, high strain, and repetitive loading conditions. This review focuses on the development of hydrogel systems that meet the requirements of mechanical resiliency, cytocompatibility, and injectability for such applications. First, we describe the most important design considerations for maintaining the viability and function of encapsulated cells, for reproducing the target tissue morphology, and for achieving degradation profiles that facilitate tissue replacement. Models describing the relationships between hydrogel structure and mechanical properties are described, focusing on design principles necessary for producing mechanically resilient hydrogels. The advantages and limitations of current strategies for preparing cytocompatible, injectable, and mechanically resilient hydrogels are reviewed, including double networks, nanocomposites, and high molecular weight amphiphilic copolymer networks. Finally, challenges and opportunities are outlined to guide future research in this developing field.

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

Assembly of Polyacrylamide-Sodium Alginate-Based Organic-Inorganic Hydrogel with Mechanical and Adsorption Properties

Hydrogels have been widely used in water purification. However, there is not much discussion and comparison about the effects of different nanofillers on the reinforcement and adsorption performances of hydrogels, which can be subjected to rapid water flow and possess strong adsorption ability. In this work, polyacrylamide (PAAM)-sodium alginate (SA) interpenetrating polymer network-structured hydrogels were prepared by in situ polymerization. PAAM formed the first flexible network and SA constructed the second rigid network. Three kinds of inorganic nanoparticles including carbon nanotubes (CNTs), nanoclays (NCs), and nanosilicas (NSs) were incorporated into a PAAM-SA matrix via hydrogen bond. The obtained hydrogels exhibited a macroporous structure with low density (≈1.4 g/cm³) and high water content (≈83%). Compared with neat PAAM-SA, the hydrogels with inorganic nanoparticles possessed excellent mechanical strengths and elasticities, and the compression strength of PAAM-SA-NS reached up to 1.3 MPa at ε = 60% by adding only 0.036 g NS in a 30 g polymer matrix. However, CNT was the best filler to improve the adsorption capacity owing to its multi-walled hollow nanostructure, and the adsorption capacity of PAAM-SA-CNT was 1.28 times higher than that of PAAM-SA. The prepared hydrogels can be potential candidates for use as absorbents to treat wastewater.

Tunable Dual Temperature-Pressure Sensing and Parameter Self-Separating Based on Ionic Hydrogel via Multisynergistic Network Design

Hydrogel-based wearable sensors have experienced an explosive development, whereas functional integration to mimic the multisignal responsiveness of skin especially for pressure and temperature remained a challenge. Herein, a functional ionic hydrogel-base flexible sensor was successfully prepared by integrating the thermal-sensitive N-isopropylacrylamide (NIPAAm) into another conductive double-network hydrogel based on polyvinyl alcohol-graphene oxide (PVA-GO) and polyacrylic acid-Fe³⁺ (PAA-Fe³⁺). Because of the multisynergistic network design, the triple-network hydrogel was endowed with excellent conductivity (∼170 Ω/mm), mechanical tolerance (1.1 MPa), and rapid recoverability (within 0.5 s), which demonstrated the potential use in pressure monitoring. Moreover, the introduction of a thermal-sensitive network allowed it to capture the changes in the human body temperature accurately simultaneously and to be further developed as a flexible temperature sensor. In particular, the unsynchronization of pressure and temperature strain (straining to stability within 0.5 s and more than 50 s, respectively) caused the two electrical signals to be automatically separated. Intuitive reading of data without involving complex parameter separation calculations allowed the hydrogel to be developed as an integrated dual temperature-pressure-sensitive flexible sensor. In addition, all above properties demonstrated that the as-prepared functional hydrogel could be extended to the practical application in human-machine interactions and personalized multisignal monitoring.

Self-healable poly(acrylic acid-co-maleic acid)/glycerol/boron nitride nanosheet composite hydrogels at low temperature with enhanced mechanical properties and water retention

Many living tissues possess excellent mechanical properties and water retention which enable them to self-heal at room temperature even below the freezing temperature of water. To mimic the unique features of living tissue, a poly(acrylic acid-co-maleic acid) composite hydrogel with enhanced mechanical properties and remarkable water retention was fabricated under accessible conditions. The hydrogel is functionalized by amino group modified boron nitride nanosheets (BNNS-NH2)/glycerol and exhibits self-healing abilities at low temperature. The self-healing process occurs through the re-establishing of hydrogen bonds and metal coordination interactions at the damaged surfaces. Its anti-freezing abilities enable the hydrogel to self-heal at -15 °C, and the self-healing efficiency based on tensile strength reaches up to ∼70%. Moreover, glycerol also endows the hydrogel with long-lasting water retention, which remains a water content of ∼99 wt% for more than 30 days. Meanwhile, the simultaneous introduction of BNNS-NH2 and glycerol significantly improved the mechanical properties of the hydrogel, which displays great stretchability (∼474%), tensile strength (∼151.3 kPa), stiffness (Young's modulus of ∼62.75 kPa) and toughness (∼355.13 kJ m-3). It is anticipated that these novel hydrogels will develop many fields and be exploited for new applications in extensive external environments.

Highly Stretchable and Transparent Double-Network Hydrogel Ionic Conductors as Flexible Thermal-Mechanical Dual Sensors and Electroluminescent Devices

The latest generation flexible devices feature materials that are conductive, highly stretchable, and transparent to meet the requirements of a reliable performance. However, the existing conductors are mostly electronic conductors, which cannot satisfy these high-performance challenges. A robust hydrogel ionic conductor was rapidly fabricated with a facile one-pot approach by employing bioinspired agar with a physically cross-linked network, polyacrylamide (PAM) with a photoinitiated cross-linked network under appropriate UV intensity, and Li⁺ as conductive ions. The resulting Li⁺/agar/PAM ionic double-network hydrogels could be fabricated into various shapes through injection. The unique ionic hydrogel showed a remarkable stretchability with over 1600% extension, high tension/compression strength (0.22 MPa/3.5 MPa), and toughness (2.2 MJ/m³). Furthermore, it was demonstrated to possess dual sensory capabilities through the combination of both temperature and mechanical deformation. This hydrogel ionic conductor exhibited high stretching sensitivity with a gauge factor of 1.8 at strain 1100% and bending sensitivity in a broad angle range of 30-150°, respectively. Such highly optical transparency and elasticity endow the hydrogel-phosphor composites with promising luminescent properties. The multifunctional ionic hydrogel can be used as soft conductors for application in flexible devices such as ionic skin for wearable sensors and luminescence display.

Synthesis of Thermo-Responsive Block-Graft Copolymer Based on PCL and PEG Analogs, and Preparation of Hydrogel via Click Chemistry

Thermo-responsive cross-linkable mPEG-b-[PCL-g-(MEO2MA-co-OEGMA)]-b-mPEG was synthesized by ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Then, the cross-linkable block-graft copolymer was used to prepare hydrogel via a copper-catalyzed 1,3-dipolar azide-alkyne cycloaddition reaction. The chemical structure and composition of copolymer were characterized by proton nuclear magnetic resonance (1H NMR), Fourier-transform infrared (FT-IR) and gel permeation chromatography (GPC). The self-assembly behaviors of the copolymer in aqueous solution were studied by UV spectrophotometer, fluorescence probes, the surface tension method, dynamic light scattering, and transmission electron microscopy. The results proved that the copolymer has excellent solubility and better temperature response. The three-dimensional network structure of the gels, observed by scanning electron microscopy at different temperatures, indicated that the gels have temperature response.

Using Synergistic Multiple Dynamic Bonds to Construct Polymers with Engineered Properties

Dynamic bonds have achieved significant attention for their ability to impart fascinating properties to polymeric materials, such as high mechanical strength, self-healing, shape memory, 3D printability, and conductivity. Incorporating multiple dynamic bonds into polymer systems affords an attractive and efficient approach to endow multiple functionalities. This mini-review focuses on the use of complementary dynamic interactions to control the properties of soft materials. Owing to the diversity in dynamic chemistries that can be explored, the scope of this article is restricted to polymers and does not include colloids, amphiphiles, liquid crystals, or biological soft matter.

Water-retaining, tough and self-healing hydrogels and their uses as fire-resistant materials

Hydrogels as fire-resistant materials have attracted great attention due to their high water content and tailored shapes that can cover various surfaces.

Radical free crosslinking of direct-write 3D printed hydrogels through a base catalyzed thiol-Michael reaction

3D printed micelle-based hydrogels were mechanically stabilized and crosslinked through the base catalyzed thiol-Michael addition in PBS buffer, without the use of potentially cytotoxic radical chemistry.

Recent advances of on-demand dissolution of hydrogel dressings

Wound management is a major global challenge and a big financial burden to the healthcare system due to the rapid growth of chronic diseases including the diabetes, obesity, and aging population. Modern solutions to wound management include hydrogels that dissolve on demand, and the development of such hydrogels is of keen research interest. The formation and subsequent on-demand dissolution of hydrogels is of keen interest to scientists and clinicians. These hydrogels have excellent properties such as tissue adhesion, swelling, and water absorption. In addition, these hydrogels have a distinctive capacity to form in situ and dissolve on-demand via physical or chemical reactions. Some of these hydrogels have been successfully used as a dressing to reduce bleeding in hepatic and aortal models, and the hydrogels remove easily afterwards. However, there is an extremely wide array of different ways to synthesize these hydrogels. Therefore, we summarize here the recent advances of hydrogels that dissolve on demand, covering both chemical cross-linking cases and physical cross-linking cases. We believe that continuous exploration of dissolution strategies will uncover new mechanisms of dissolution and extend the range of applications for hydrogel dressings.

Preparation and characterization of high-performance polyamic acid salt hydrogel in aqueous solution

Herein, we report an easy-to-prepare polyamic acid salt (PAAS) hydrogel with multiple functionalities including rapid self-healing, stimuli-responsive properties, and conductivity. This study describes a method for synthesizing PAAS hydrogel in aqueous solution. Phenylenediamine and 3,3′,4,4′-biphenyltetracarboxylic dianhydride were used to prepare a hydrogel with a fully aromatic backbone via gradual polymerization. It is possible to use the gel in the field of biomaterials in the absence of organic solvents. The prepared hydrogel was characterized using Fourier-transform infrared spectroscopy and dynamic rheometry. The gel has good toughness and the storage modulus and tensile stress are, respectively, about 0.25 and 0.45 MPa. Our study may pave the way for the manufacture of PAAS hydrogel, which may promote the development of hydrogel materials.

Dual Physically Cross-Linked κ-Carrageenan-Based Double Network Hydrogels with Superior Self-Healing Performance for Biomedical Application

Chemically linked double network (DN) hydrogels display extraordinary mechanical attributes but mostly suffer from poor self-healing property and unsatisfactory biocompatibility due to the irreversible breaks in their chemical-linked networks and the use of toxic chemical cross-linking agents. To address these limitations, we developed a novel κ-carrageenan/polyacrylamide (KC/PAM) DN hydrogel through a dual physical-cross-linking strategy, with the ductile, hydrophobically associated PAM being the first network, and the rigid potassium ion (K⁺) cross-linked KC being the second network. The dual physically cross-linked DN (DPC-DN) hydrogels with optimized KC concentration exhibit excellent fracture tensile stress (1320 ± 46 kPa) and toughness (fracture energy: 6900 ± 280 kJ/m³), comparable to those fully chemically linked DN hydrogels and physically chemically cross-linked hybrid DN hydrogels. Moreover, because of their unique dual physical-cross-linking structures, the KC/PAM hydrogels also demonstrated rapid self-recovery, remarkable notch-insensitivity, self-healing capability, as well as excellent cytocompatibility toward stem cells. Accordingly, this work presents a new strategy toward fabricating self-repairing DPC-DN hydrogels with outstanding mechanical behaviors and biocompatibility. The new type of DN hydrogels demonstrates strong potentiality in many challenging biomedical applications such as artificial diaphragm, tendon, and cartilage.

Ultratough, Self-Healing, and Tissue-Adhesive Hydrogel for Wound Dressing

A hydrogel for potential applications in wound dressing should possess several peculiar properties, such as efficient self-healing ability and mechanical toughness, so as to repair muscle and skin damage. Additionally, excellent cell affinity and tissue adhesiveness are also necessary for the hydrogel to integrate with the wound tissue in practical applications. Herein, an ultratough and self-healing hydrogel with superior cell affinity and tissue adhesiveness is prepared. The self-healing ability of the hydrogel is obtained through hydrogen bonds and dynamic Schiff cross-linking between dopamine-grafted oxidized sodium alginate (OSA-DA) and polyacrylamide (PAM) chains. The covalent cross-linking is responsible for its stable mechanical structure. The combination of physical and chemical cross-linking contributes to a novel hydrogel with efficient self-healing ability (80% mechanical recovery in 6 h), high tensile strength (0.109 MPa), and ultrastretchability (2550%), which are highly desirable properties and are superior to previously reported tough and self-healing hydrogels for wound dressing applications. More remarkably, due to plenty of catechol groups on the OSA-DA chains, the hydrogel has unique cell affinity and tissue adhesiveness. Moreover, we demonstrate the practical utility of our fabricated hydrogel via both in vivo and in vitro experiments.

Tetrapolymer Network Hydrogels via Gum Ghatti-Grafted and N-H/C-H-Activated Allocation of Monomers for Composition-Dependent Superadsorption of Metal Ions

Herein, gum ghatti (GGTI)-g-[sodium acrylate (SA)-co-3-(N-(4-(4-methyl pentanoate))acrylamido)propanoate (NMPAP)-co-4-(acrylamido)-4-methyl pentanoate (AMP)-co-N-isopropylacrylamide (NIPA)] (i.e., GGTI-g-TetraP), a novel interpenetrating tetrapolymer network-based sustainable hydrogel, possessing extraordinary physicochemical properties and excellent recyclability, has been synthesized via grafting of GGTI and in situ strategic protrusion of NMPAP and AMP during the solution polymerization of SA and NIPA, through systematic multistage optimization of ingredients and temperature, for ligand-selective superadsorption of hazardous metal ions (M(II)), such as Sr(II), Hg(II), and Cu(II). The in situ allocation of NMPAP and AMP via N-H and C-H activations, grafting of GGTI into the SA-co-NMPAP-co-AMP-co-NIPA (TetraP) matrix, the effect of comonomer compositions on ligand-selective adsorption, crystallinity, thermal stabilities, surface properties, swellability, adsorption capacities (ACs), mechanical properties, and the superadsorption mechanism have been apprehended via extensive microstructural analyses of unloaded and/or loaded GGTI-g-TetraP1 and GGTI-g-TetraP2 bearing SA/NIPA in 8:1 and 2:1 ratios, respectively, using Fourier transform infrared (FTIR), 1H/13C/DEPT-135 NMR, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, rheological analysis, and energy-dispersive X-ray spectrometry, along with measuring % gel content, pH at point of zero charge (pHPZC), and % graft ratio. The thermodynamically spontaneous chemisorption has been inferred from FTIR, XPS, fitting of kinetics data to pseudo-second-order model, and activation energies. The chemisorption data have exhibited excellent fitting to the Langmuir isotherm model. For Sr(II), Hg(II), and Cu(II), ACs were 1940.24/1748.36, 1759.50/1848.03, and 1903.64/1781.63 mg g-1, respectively, at 293 K, 0.02 g of GGTI-g-TetraP1/2, and initial concentration of M(II) = 500-1000 ppm.

Hydrogel applications for adsorption of contaminants in water and wastewater treatment

During the last decade, hydrogels have been used as potential adsorbents for removal of contaminants from aqueous solution. To improve the adsorption efficiency, there are numerous different particles that can be chosen to encapsulate into hydrogels and each particle has their respective advantages. Depending on the type of pollutants and approaching method, the particles will be used to prepare hydrogels. The hydrogels commonly applied in water/wastewater treatment was mainly classified into three classes according to their shape included hydrogel beads, hydrogel films, and hydrogel nanocomposites. In review of many recently research papers, we take a closer look at hydrogels and their applications for removal of contaminants, such as heavy metal ion, dyes, and radionuclides from water/wastewater in order to elucidate the reactions between contaminants and particles and potential for recycling and regeneration of the post-treatment hydrogels. Graphical abstract ᅟ.

Rapid Self-Recoverable Hydrogels with High Toughness and Excellent Conductivity

Hydrogels as soft and wet materials have attracted much attention in sensing and flexible electronics. However, traditional hydrogels are fragile or have unsatisfactory recovery capability, which largely limit their applications. Here, a novel hydrogen bond based sulfuric acid-poly(acrylic acid) (PAA)/poly(vinyl alcohol) physical hydrogel is developed for addressing the above drawbacks. Sulfuric acid serves two functions: one is to inhibit the ionization of carboxyl groups from PAA chains to form more hydrogen bonds and the other is to provide conductive ions to promote conductivity of hydrogel. Consequently, the hydrogel obtains comprehensive mechanical properties, including extremely rapid self-recovery (strain = 1, instantly self-recover; strain = 20, self-recover within 10 min), high fracture strength (3.1 MPa), and high toughness (18.7 MJ m^-3). In addition, we demonstrate this hydrogel as a stretchable ionic cable and pressure sensor to exhibit stable operation after repeated loadings. This work provides a new concept to synthesize physical hydrogels, which will hopefully expand applications of hydrogel in stretchable electronics.