Ultrastretchable,Super Tough,and Rapidly Recoverable Nanocomposite Double‐Network Hydrogels by Dual Physically Hydrogen Bond and Vinyl‐Functionalized Silica Nanoparticles Macro‐Crosslinking

It remains a challenge to develop tough hydrogels with recoverable or healable properties after damage. Herein, a new nanocomposite double‐network hydrogel (NC‐DN) consisting of first agar network and a homogeneous vinyl‐functionalized silica nanoparticles (VSNPs) macro‐crosslinked polyacrylamide (PAM) second network is reported. VSNPs are prepared via sol‐gel process using vinyltriethoxysilane as a silicon source. Then, Agar/PAM‐SiO₂ NC‐DN hydrogels are fabricated by dual physically hydrogen bonds and VSNPs macro‐crosslinking. Under deformation, the reversible hydrogen bonds in agar network and PAM nanocomposite network successively break to dissipate energy and then recombine to recover the network, while VSNPs in the second network could effectively transfer stress to the network chains grafted on their surfaces and maintain the gel network. As a result, the optimal NC‐DN hydrogels exhibit ultrastretchable (fracture strain 7822%), super tough (fracture toughness 18.22 MJ m^‐3, tensile strength 431 kPa), rapidly recoverable (≈92% toughness recovery after 5 min resting at room temperature), and self‐healable (can be stretched to 1331% after healing) properties. The newly designed Agar/PAM‐SiO₂ NC‐DN hydrogels with tunable network structure and mechanical properties by multi‐bond crosslinking provide a new avenue to better understand the fundamental structure‐property relationship of DN hydrogels and broaden the current hydrogel research and applications.     

Highly Mechanical and Fatigue‐Resistant Double Network Hydrogels by Dual Physically Hydrophobic Association and Ionic Crosslinking

Double network (DN) hydrogels with high strength and toughness are considered as promising soft materials. Herein, a dual physically cross‐linked hydrophobic association polyacrylamide (HPAAm)/alginate‐Ca²⁺ DN hydrogel is reported, consisting of a HPAAm network and a Ca²⁺ cross‐linked alginate network. The HPAAm/alginate‐Ca²⁺ DN hydrogel exhibits excellent mechanical properties with the fracture stress of 1.16 MPa (3.0 and 1.7 times higher than that of HPAAm hydrogel and HPAAm/alginate hydrogel, respectively), fracture strain of 2604%, elastic modulus of 71.79 kPa, and toughness of 14.20 MJ m^?3. HPAAm/alginate‐Ca²⁺ DN hydrogels also demonstrate self‐recovery, notch‐insensitivity, and fatigue resistance properties without any external stimuli at room temperature through reversible physical bonds consisting of hydrophobic association and ionic crosslinking. As a result, the dual physical crosslinking would offer an avenue to design DN hydrogels with desirable properties for broadening current applications of soft materials.     

Composite double network hydrogels with thermoresponsive colloidal nanoemulsions

We report the formulation and mechanical characterization of double network (DN) composite hydrogels. The first network consists of covalently crosslinked poly(ethylene glycol diacrylate) (PEGDA), which forms a strong, brittle network that provides elasticity to the gel. The second network, sodium alginate, is ionically crosslinked with Ca²⁺ to allow increased dissipation of mechanical energy. The novelty of this system over existing DN hydrogels is the additional incorporation of a third mesoscale network, composed of thermoresponsive poly(dimethyl siloxane) (PDMS) nanoemulsions, which undergo colloidal gelation through the bridging of the PEGDA hydrophobic end groups into the PDMS droplets. The colloidally gelled microstructures are photopolymerized into a solid hydrogel by crosslinking the precursors with ultraviolet (UV) light. Tensile mechanical experiments performed on the crosslinked DN nanoemulsion hydrogels show that their rupture stress (0.17–0.34 MPa), fracture energy (144–421 J/m²), and Young's modulus (1–2.1 MPa) are comparable to similar systems in the literature. These mechanical measurements suggest that the gels may be suitable for manufacturing processes in which large shear rates and deformations are encountered.     

Tough and recoverable triple‐network hydrogels based on multiple pairs of toughing mechanisms with excellent ionic conductivity as stable strain sensors

The emerging applications of hydrogels in flexible devices require it possess multifunctional properties including stable mechanical and functions under various deformations or external environments. Herein, a multifunctional polyvinyl alcohol/M‐alginate/PAM hydrogel with very excellent mechanical properties and sensing functions was fabricated by introducing multiple pairs of toughing mechanisms into triple network (TN). The multiple supramolecular physical networks work as sacrificial networks to toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties and functions with the assistance of the elastic covalent network. The optimal TN hydrogels are extremely tough (a fracture strength of 512 kPa, a fracture toughness of 3 MJ/m³) and recoverable from fatigue damage (~77% toughness recovery after 5 min resting at room temperature). The presence of abundant ionic species endows the tough and recoverable TN hydrogels high ionic conductivity and high sensitivity as strain sensors. Moreover, such TN hydrogels with multi‐bond crosslinking in three networks can potentially guarantee stable mechanical and sensor functions under various deformations or external environments compared to the DN candidates. This work provides a simple strategy for fabricating multifunctional hydrogels with high stability to fulfill its flexible devices applications. POLYM. ENG. SCI., 59:1657–1666 2019. © 2019 Society of Plastics Engineers     

Highly stretchable,ionic conductive and self-recoverable zwitterionic polyelectrolyte-based hydrogels by introducing multiple supramolecular sacrificial bonds in double network

Zwitterionic hydrogels have been explored for applications in electrochemical devices very recently due to their high water retention ability and interesting electrochemical properties. The use of zwitterionic hydrogels in devices requires them tough and recoverable or healable from fatigue damage. Herein, a double network zwitterionic hydrogel contains a reversible noncovalent interaction crosslinked polyvinyl alcohol (PVA) first network, together with a covalent/noncovalent hybrid crosslinked acrylamide and sulfobetaine methacrylate copolymer (P(AM-co-SBMA)) second network, was fabricated by a simple two-steps methods of copolymerization and freezing/thawing. The reversible hydrogen bonds, crystalline domain, and electrostatic interactions in the double networks work as sacrificial bonds to dissipate energy and toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties with the assistance of the elastic covalent network. The optimal hydrogels are highly stretchable (fracture strain 970%), tough (fracture toughness 693 kJ m⁻³), rapidly recoverable (65% toughness recovery and 75% stiffness recovery after resting 5 min at room temperature) and with widely tunable mechanical properties by multibond crosslinking. Meanwhile, the zwitterionic counterions of SBMA moieties endow the tough and recoverable hydrogels extremely high intrinsic ionic conductivities (7.49 S m⁻¹) at room temperature. This work not only provides a simple strategy for fabricating tough and recoverable zwitterionic hydrogels but also demonstrates multifunctional properties of the zwitterionic hydrogels, which possess a great potential to fulfill flexible devices applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47783.     

Nanoclay Reinforced Self‐Cross‐Linking Poly(N‐Hydroxyethyl Acrylamide) Hydrogels with Integrated High Performances

Despite recent significant progress in fabricating tough hydrogels, it is still a challenge to realize high strength, large stretchability, high toughness, rapid recoverability, and good self‐healing simultaneously in a single hydrogel. Herein, Laponite reinforced self‐cross‐linking poly(N‐hydroxyethyl acrylamide) (PHEAA) hydrogels (i.e., PHEAA/Laponite nanocomposite [NC] gels) with dual physically cross‐linked network structures, where PHEAA chains can be self‐cross‐linked by themselves and also cross‐linked by Laponite nanoplatelets, demonstrate integrated high performances. At optimal conditions, PHEAA/Laponite NC gels exhibit high tensile strength of 1.31 MPa, ultrahigh tensile strain of 52.23 mm mm^?1, high toughness of 2238 J m^?2, rapid self‐recoverability (toughness recovery of 79% and stiffness recovery of 74% at room temperature for 2 min recovery without any external stimuli), and good self‐healing properties (strain healing efficiency of 42%). The work provides a promising and simple strategy for the fabrication of dual physically cross‐linked NC gels with integrated high performances, and helps to expand the fundamentals and applications of NC gels.     

Super tough double network hydrogels and their application as biomaterials

The double network (DN) technique, developed by authors’ group, provides an innovative and universal pass way to fabricate hydrogels with super high toughness comparable to rubbers. The excellent mechanical performances of DN hydrogels originate from the specific combination of two networks with contrasting structures. The first brittle network serves as sacrificial bonds, which breaks into small clusters to efficiently disperse the stress around the crack tip into the surrounding damage zone, while the second ductile polymer chains act as hidden length, which extends extensively to sustain large deformation. Based on the principle of DN hydrogel, the author’s group recently has developed several novel systems and techniques, which has greatly expanded the practical accessibility of DN technique for practical use. The DN principle and the DN gel have already attracted much attention in the soft matter community. Inspired by the DN principle, many research groups have also designed and developed some innovative hydrogels with large enhancement in their mechanical strength and toughness. Some tough hydrogels fabricated by the DN technique also exhibit good biocompatibility and low friction resistance with promising prospective in industrial and medicine fields, especially for load-bearing artificial soft tissues such as artificial cartilage. In this feature article, we address the major concept and toughening mechanism of DN gel, then we describe some recent novel hydrogel systems based on the DN concept, and finally the applicability of DN gel as soft biomaterials is discussed.     

A Dual‐Crosslinked Strategy to Construct Physical Hydrogels with High Strength,Toughness, Good Mechanical Recoverability,and Shape‐Memory Ability

A novel type of physical hydrogel based on dual‐crosslinked strategy is successfully synthesized by micellar copolymerization of stearyl methacrylate, acrylamide, and acrylic acid, and subsequent introduction of Fe³⁺. Strong hydrophobic associations among poly(stearyl methacrylate) blocks form the first crosslinking point and ionic coordination bonds between carboxyl groups and Fe³⁺ serve as the second crosslinking point. The mechanical properties of the hydrogel can be tuned in a wide range by controlling the densities of two crosslinks. The optimal hydrogel shows excellent mechanical properties (tensile strength of ≈6.8 MPa, elastic modulus of ≈8.0 MPa, elongation of ≈1000%, toughness of 53 MJ m^?3) and good self‐recovery property. Furthermore, owing to stimuli responsiveness of physical interaction, this hydrogel also shows a triple shape memory effect. The combination of two different physical interactions in a single network provides a general strategy for designing of high‐strength hydrogels with functionalities.     

One-pot preparation of ultrastrong double network hydrogels

A double-network (DN) polyethylene glycol/polyacrylic acid (PEG/PAA) hydrogel with high compressive strength was synthesized by one-pot solution polymerization. The PEG network crosslinked by glutaraldehyde (GA) was fabricated via condensation reaction while the PAA network crosslinked by N,N’-dimethylenebiacrylamide (MBAM) via free-radical polymerization. The components of hydrogel were analyzed with Fourier transform infrared spectroscopy (FTIR). Mechanical strength of PEG/PAA hydrogels was examined, and the results showed that the addition of GA and PEG endowed the DN hydrogel with a high compressive strength of 10.9 MPa in a water content of 90 wt% due to lightly crosslinking and special entangled bundles morphology. Morphological studies showed that the hydrogels exhibited various pore structures when they were synthesized using different molar ratio of GA to PEG. This work provided a simple way to prepare ultrastrong DN hydrogels.     

High‐Strength,Rapidly Self‐Recoverable,and Antifatigue Nano‐SiO2/Poly(Acrylamide–Lauryl Methacrylate) Composite Hydrogels

Creating load‐bearing hydrogels with superior mechanical strength and toughness is of vital importance for promoting the development of polymer hydrogels toward practical applications. Herein, a type of composite hydrogel is facilely fabricated employing simple and effective UV irradiation one‐pot method by introducing cheap and available nanosilica sol into hydrophobic association poly(acrylamide–lauryl methacrylate) (HAPAM gels). Composite hydrogels exhibit enhanced mechanical strength (compression stress reaching 4.4 MPa) and toughness (compression hysteresis energy achieved is 151.15 kJ m^?3) compared to HAPAM gels. Composite hydrogels also demonstrate rapid self‐recovery behavior (95.91% stress recovery and 92.19% hysteresis energy recovery after restoration for 15 min, respectively) and favorable fatigue‐resistant ability without the help of external stimuli at room temperature based on the cyclic loading–unloading compression measurements. The simple and effective design strategy may help the development of hydrogel materials toward practical applications for soft sensors, tissue engineering, and actuators.     

Preparation and characterization of PAM/SA tough hydrogels reinforced by IPN technique based on covalent/ionic crosslinking

In order to fabricate tough hydrogels with superior formability, polyacrylamide/sodium alginate (PAM/SA) interpenetrating polymer network (IPN) hydrogels were produced with ionically crosslinked SA interpenetrated in covalently crosslinked PAM. TGA results show that the heat resistance of PAM/SA IPN hydrogel is improved as compared to that of the individual component. Swelling studies indicate that increasing either chemical crosslinker content or ionic crosslinking via adding more N,N′‐methylenebisacrylamide (MBA) or SA results in lower ESR. It is concluded by tensile test that loosely crosslinked PAM coupled with tightly crosslinked SA improve mechanical strength for hydrogels based on covalent/ionic crosslinking. PAM/SA hydrogels via “one‐pot” method can form different complex shapes with mechanical properties comparable to conventional double network (DN) gels. The fracture strength of PAM_0.05/SA₂₀ reaches level of MPa, approaching 2.0 MPa. The work strives to provide method to tune mechanical and physical properties for hydrogels, which is hopefully to guide the design of hydrogel material with desirable properties. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41342.     

CO2‐responsive swelling behavior and metal‐ion adsorption properties in novel histamine‐conjugated polyaspartamide hydrogel

Novel polyaspartamide copolymers containing histamine pendants (PHEA‐HIS) were prepared from polysuccinimide, which is the thermal polycondensation product of aspartic acid, via a successive ring‐opening reaction using histamine (HIS) and ethanolamine (EA). The prepared water‐soluble copolymer was then crosslinked by reacting it with hexamethylene diisocyanate in order to provide a hydrogel with both good gel strength and reversible CO₂ absorption characteristics. PHEA‐HIS gel is also pH‐sensitive and eligible to coordinate to metal ions such as Pb²⁺, Cu²⁺, and Ni²⁺ due to the imidazole units in its structure. The CO₂‐responsive swelling behavior, metal‐ion adsorption, and morphology of the crosslinked gels were investigated. The approach described here results is a promising hydrogel with potential for a variety of industrial and biomedical applications including CO₂ capture, CO₂‐responsive and switchable sensors, and smart drug delivery systems. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43305.     

Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels with Bio‐Friendly Toughening Agents

Inspired by the avoidance of toxic chemical crosslinkers and harsh reaction conditions, this work describes a poly(vinyl alcohol)‐based (PVA) double‐network (DN) hydrogel aimed at maintaining biocompatibility through the combined use of bio‐friendly additives and freezing–thawing cyclic processing for the application of synthetic soft‐polymer implants. This DN hydrogel is studied using techniques that characterize both its chemical and mechanical behavior. A variety of bio‐friendly additives are screened for their effectiveness at improving the toughness of the PVA hydrogel system in monotonic tension. Starch is selected as the best additive for further tensile testing as it brings about a near 30% increase in ultimate tensile strength and maintains ease of processing. This PVA–starch DN sample is then studied for its tensile fatigue properties through cyclic, strain‐controlled testing to develop a fatigue life curve. Though an increase in monotonic tensile strength is observed, the PVA–starch DN hydrogel does not bring about an improvement in the fatigue behavior as compared to the control. Although synthetic hydrogel reinforcement is widely researched, this work presents the first fatigue analysis of its kind and it is intended to serve as a guide for future fatigue studies of reinforced hydrogels.     

Flexible Polysaccharide Hydrogel with pH‐Regulated Recovery of Self‐Healing and Mechanical Properties

A self‐healable hydrogel with recoverable self‐healing and mechanical properties is reported. The hydrogel (coded as ACSH) crosslinked by Schiff base linkage contains two polysaccharides of acrylamide‐modified chitosan (AMCS) and oxidized alginate (ADA). Self‐healing and mechanical properties are heavily influenced by the crosslinking time. The hydrogel crosslinked for 2 h possesses better mechanical and self‐healing properties than hydrogel crosslinked for 24 h. Macroscopic test shows that hydrogel without self‐healing ability can recover the self‐repair and mechanical properties by adjusting the pHs. The recovery of self‐healing and mechanical properties relies on the pH sensitivity of the Schiff base linkage. Adjusting the pH to acid, the Schiff base linkage becomes unstable and breaks. Regulating the pH to neutral, reconstruction of Schiff base linkage leads to recovery of the self‐repair and mechanical properties. The recoverable self‐healing property can be cycled once breakage and reconstruction of the Schiff base linkage can be conducted. In addition, this study demonstrates that the hydrogel can be remodeled into different shapes based on self‐healing property of the hydrogel. It is anticipated that this self‐healable hydrogel with recoverable self‐healing and mechanical properties may open a new way to investigate self‐healing hydrogel and find potential applications in different biomedical fields.     

Mechanical and Rheological Behavior of Hybrid Cross‐Linked Polyacrylamide/Cationic Micelle Hydrogels

In this work, a hybrid cross‐linked polyacrylamide (PAM)/cationic micelle hydrogel is fabricated by introducing the cationic micelles into the chemically cross‐linked PAM network. The cationic micelles act as the physical cross‐linking points through the strong electrostatic interaction with anionic initiator potassium persulfate. Thereafter, in situ free radical polymerization is initiated thermally from the cationic micelle surface to form the hybrid cross‐linked network. The synergistic effect between chemical and physical cross‐link endows the hydrogel with excellent mechanical and recoverable properties. The resulting hydrogel exhibits tensile stress of 481 kPa and fracture toughness of 1.65 MJ m⁻³. It is found that the chemical cross‐linking can inhibit the hysteresis of the hybrid hydrogel, exhibiting good elasticity in the tensile loading–unloading test. Moreover, dynamic rheological measurements show that the hybrid hydrogels possess fewer defects of network and exhibit excellent self‐recovery behavior. Thus, this investigation provides a different view for the design of new high elastic and tough hydrogels containing hybrid physical and chemical cross‐linking networks.     

Robust hydrogel of regenerated cellulose by chemical crosslinking coupled with polyacrylamide network

Regenerated cellulose/polyacrylamide (RC/PAAm) double network (DN) hydrogels are composed of cellulose crosslinked by epichlorohydrin (ECH) and chemical-crosslinked PAAm. The prepared RC/PAAm DN hydrogels present enhanced strength, good shape recovery property, excellent energy dissipation properties, decreased equilibrium water content, and low equilibrium swelling ratio (SR). The compressive strength and modulus of RC/PAAm hydrogel are about 4.3 and 11.5 times compared to that of RC hydrogel, respectively. Intriguingly, the chemical crosslinking between ECH and cellulose chains could increase the distance between cellulose chains. Consequently, the increasing molar ratio of ECH to glucose leads to larger SRs and decreased mechanical strength of the hydrogels. Additionally, higher PAAm contents lead to more densely crosslinked networks, and thus decreasing the SRs and improving the mechanical strength of the hydrogels. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47811.     

Agar/PAAc-Fe<Superscript>3+</Superscript> hydrogels with pH-sensitivity and high toughness using dual physical cross-linking

As promising structural materials, various tough hydrogels have been developed recently by incorporating various kinds of bonds. An important challenge is to use dual physical cross-linking to develop both toughness and self-recovery in a single material. Here we report smart, strain-responsive hydrogels composed of a fully physically linked agarose/poly(acrylic acid)-ferric ion (agar/PAAc-Fe³⁺) double network (DN) with high toughness and pH-sensitivity. These hydrogels were fabricated in a one-pot reaction to generate dual physical cross-linking through, first, a hydrogen-bonded cross-linked agarose network, and, second, a physically linked PAAc-Fe³⁺ network via Fe³⁺ coordination interactions. The DN hydrogels possessed high toughness, with breaking strain of 1130%, fast self-recovery properties in ambient conditions (100% recovery in 30 min) and self-healing properties (the healed hydrogels can be manually stretched up to 700% of their original length after self-healing for 60 h from the cut-off state). In addition, the hydrogels exhibited pH-sensitivity due to the dissociation of ionic coordinate bonds between –COO^? ions of the PAAc chains and Fe³⁺ ions. Double-layer hydrogel strips with two different concentrations of PAAc formed a “C”-shaped material when initially immersed in pH 7 solution and then soaked in a pH 3 solution. These characteristics make the hydrogels attractive candidates for tissue engineering, soft actuators and flexible electronics.     

Synthesis and properties of self-healing hydrogel plugging agent

The P(AM-co-AMPS)/SA DN hydrogel was synthesized through aqueous polymerization in this study. It formed a crosslinking network with hydrophobic associations between acrylamide (AM) and lauryl methacrylate (LMA), as well as an ionic bond network involving sodium alginate and Ca²⁺. To enhance its high-temperature resistance, 2-acrylamide-2-methylpropane sulfonic acid (AMPS) was incorporated into the hydrogel formulation. The structure of the hydrogel was characterized using Fourier transform infrared spectrometer (FTIR), thermogravimetric analyzer (TGA), and scanning electron microscopy (SEM) techniques. Results demonstrated that the hydrogel exhibited excellent temperature resistance and possessed a porous structure. Mechanical testing revealed a high tensile strength of 110 kPa, elongation at break of 995.31%, along with good fatigue resistance and self-recovery performance during multiple cyclic stretching. Healing experiments indicated that the healing strength of the hydrogel was influenced by temperature variations. Furthermore, pressure plugging tests were conducted on steel models with crack widths of 0.5 and 1 mm, respectively; it was found that the 0.8%P(AM-co-AMPS)/SA DN hydrogel could withstand pressures up to 4.5 MPa at a temperature of 70°C. This novel hydrogel material exhibits remarkable mechanical properties along with certain self-healing capabilities, making it suitable for leak control applications.     

Self-assembly and metal ions-assisted one step fabrication of recoverable gelatin hydrogel with high mechanical strength

ABSTRACT

Gelatin hydrogel has been widely applied in bio-applications due to their good biocompatibility and high water content. However, poor mechanical properties of gelatin hydrogel greatly limit their application. Here we present a facile one-step soaking method to fabricate a recoverable gelatin hydrogel with high mechanical property, which is based on hydrogen bonds and metal ionic interaction. The mechanical properties of gelatin hydrogels can be tuned with different metal ions, temperatures and soaking times. Especially, gelatin-Fe³⁺ hydrogel can reach to 65 MPa compression stress with the compressive strain over 99% and possess good fatigue resistance under cyclic loadings. Besides, hydrogels crosslinked with metal ions show better antibacterial ability against Escherichia coli and Staphylococcus aureus. This work suggested an alternative for the design of tough gelatin-based hydrogels with desirable properties, which may hold promising for potential bio-applications under physiological conditions.