Synthesis of poly(acrylic acid)–Fe3+/gelatin/poly(vinyl alcohol) triple‐network supramolecular hydrogels with high toughness,high strength and self‐healing properties

Hydrogels with good mechanical and self‐healing properties are of great importance for various applications. Poly(acrylic acid)–Fe³⁺/gelatin/poly(vinyl alcohol) (PAA‐Fe³⁺/Gelatin/PVA) triple‐network supramolecular hydrogels were synthesized by a simple one‐pot method of copolymerization, cooling and freezing/thawing. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels exhibit superior toughness, strength and recovery capacity compared to single‐ and double‐network hydrogels. The mechanical properties of the synthesized hydrogels could be tailored by adjusting the compositions. The PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogel with 0.20 mmol Fe³⁺, 3% gelatin and 15% PVA could achieve good mechanical properties, the tensile strength and elongation at break being 239.6 kPa and 12.8 mm mm^?1, respectively, and the compression strength reaching 16.7 MPa under a deformation of about 91.5%. The synthesized PAA‐Fe³⁺/Gelatin/PVA triple‐network hydrogels have good self‐healing properties owing to metal coordination between Fe³⁺ and carboxylic groups, hydrogen bonding between the gelatin chains and hydrogen bonding between the PVA chains. Healed PAA‐Fe³⁺(0.20)/Gelatin3%/PVA15% triple‐network hydrogels sustain a tensile strength of up to 231.4 kPa, which is around 96.6% of the tensile strength of the original samples. Therefore, the synthesized triple‐network supramolecular hydrogels would provide a new strategy for gel research and expand the potential for their application. © 2019 Society of Chemical Industry     

Calcium ion‐induced self‐healing pattern of chemically crosslinked poly(acrylic acid) hydrogels

Poly(acrylic acid) hydrogels crosslinked with N,N′‐methylenebisacrylamide were synthesized by free radical polymerization. Polymerization conditions had a significant influence over the gel content and swelling behaviour of the hydrogels. The incorporation of calcium ions led to the origin of a self‐healing feature. The self‐healing behaviour and mechanical performance of the hydrogels were systematically investigated. The hydrogels showed good tensile strength of 1 MPa and excellent stretchable behaviour where hydrogels regained instantaneously. Hydrogel pieces joined together to become an integrated matrix as soon as two cut pieces were brought in contact. The hydrogels possessed a marked healing efficiency of 97% within 6 h at room temperature without any external intervention. The results are explained in terms of the dynamic mobility of calcium ions within the dual‐crosslinked networks of the poly(acrylic acid) hydrogels. © 2017 Society of Chemical Industry     

Highly stretchable,tough, and self‐recoverable and self‐healable dual physically crosslinked hydrogels with synergistic “soft and hard” networks

Strength, toughness and self‐recoverability are among the most important properties of hydrogels for tissue‐engineering applications. Yet, it remains a challenge to achieve these desired properties from the synthesis of a single‐polymer hydrogel. Here, we report our one‐pot, a monomer‐polymerization approach to addressing the challenge by creating dual physically crosslinked hybrid networks, in particular, synergistic “soft and hard” polyacrylic acid‐Fe³⁺ hydrogels (SHPAAc‐Fe³⁺). Favorable mechanical properties achieved from such SHPAAc‐Fe³⁺ hydrogels included high tensile strength (about 1.08 MPa), large elongation at break (about 38 times), excellent work of extension (about 19 MJ m^?3), and full self‐recoverability (100% recovery of initial properties within 15 min at 50°C and within 60 min in ambient conditions, respectively). In addition, the hydrogels exhibited good self‐healing capabilities at ambient conditions (about 40% tensile strength recovery without any external stimuli). This work demonstrates that dual physical crosslinking combining hydrophobic interaction and ionic association can be achieved in single‐polymer hydrogels with significantly improved mechanical performance but without sacrificing favorable properties. POLYM. ENG. SCI., 59:145–154, 2019. © 2018 Society of Plastics Engineers     

Novel collagen‐based hydrogels with injectable,self‐healing,wound‐healing properties via a dynamic crosslinking interaction

Collagen‐based hydrogels have gained significant popularity in biomedical applications; however, traditional collagen hydrogels are easily disabled for lack of self‐healing properties due to their non‐reversible bonds. Here, a self‐healing collagen‐based hydrogel has been developed based on dynamic network chemistry, consisting of dynamic imine linkages between collagen and dialdehyde guar gum, as well as diol‐borate ester bonds between guar gum and borax. In addition, macromolecular interactions amongst macromolecules are involved. The above‐mentioned interactions were validated by Fourier transform infrared spectroscopy, sodium dodecyl sulfate polyacrylamide gel electrophoresis and DSC. The as‐prepared collagen‐based hydrogels showed good injectability and rapid self‐healing capacity (within 3 min) as reflected from injection tests, optical microscope observations, rheological measurements, as well as self‐healing studies. In addition, the collagen‐based hydrogels showed accelerated wound‐healing properties. This study offers a facile strategy to endow self‐healing ability on collagen‐based hydrogels without any external stimulus, which show great application potential as wound dressings. © 2020 Society of Chemical Industry     

High‐Performance Flexible Sensors of Self‐Healing,Reversibly Adhesive,and Stretchable Hydrogels for Monitoring Large and Subtle Strains

Flexible sensors are becoming required in heath monitoring and human–machine interfaces, but it is still a challenge to develop flexible sensors with integrated high performances. Herein, high‐performance flexible sensors are fabricated that are self‐healing, reversibly adhesive, and utilizing stretchable hydrogels, which are composed of a pluronic F127 diacrylate (F127DA) cross‐linked poly(acrylic acid) (PAA) network and polydopamine (PDA), and further cross‐linked by Fe³⁺. The unique structure endows the resulting hydrogels (PAA‐PDA‐Fe³⁺ hydrogels) excellent self‐healing property, reversible adhesion property, mechanical stretchability, and electrical conductivity. On the basis of the excellent properties of PAA‐PDA‐Fe³⁺ hydrogels, flexible sensors with large sensing range (0–575%), high sensitivity (GF = 6.31), low response time (0.25 s), and excellent robustness (>500 cycles) are assembled and further applied in detecting both large and subtle strains induced by human motions and water ripple. Overall, this work not only provides an alternative clue to construct multi‐functional hydrogels, but also offers a new kind of high‐performance materials for flexible electronic devices, especially those for health monitoring and human–machine interface.     

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.     

Dual Cross‐Linked Hydrogels with High Strength,Toughness, and Rapid Self‐Recovery Using Dynamic Metal–Ligand Interactions

A dual cross‐linking design principle enables access to hydrogels with high strength, toughness, fast self‐recovery, and robust fatigue resistant properties. Imidazole (IMZ) containing random poly(acrylamide‐co‐vinylimidazole) based hydrogels are synthesized in the presence of Ni²⁺ ions with low density of chemical cross‐linking. The IMZ‐Ni²⁺ metal–ligand cross‐links act as sacrificial motifs to effectively dissipate energy during mechanical loading of the hydrogel. The hydrogel mechanical properties can be tuned by varying the mol% of vinylimidazole (VIMZ) in the copolymer and by changing the VIMZ/Ni²⁺ ratio. The resultant metallogels under optimal conditions (15 mol% VIMZ and VIMZ/Ni²⁺ = 2:1) show the best mechanical properties such as high tensile strength (750 kPa) and elastic modulus (190 kPa), combined with high fracture energy (1580 J m^?2) and stretchability (800–900% strain). The hydrogels are pH responsive and the extent of energy dissipation can be drastically reduced by exposure to acidic pH. These hydrogels also exhibit excellent anti‐fatigue properties (complete recovery of dissipated energy within 10 min after ten successive loading–unloading cycles at 400% strain), high compressive strength without fracture (17 MPa at 96% strain), and self‐healing capability due to the reversible dissociation and re‐association of the metal ion mediated cross‐links.     

Dual‐Crosslinking of Hyaluronic Acid–Calcium Phosphate Nanocomposite Hydrogels for Enhanced Mechanical Properties and Biological Performance

Herein, a facile approach for synthesizing mechanically enhanced nanocomposite hydrogels via a dual‐crosslinking process is described. Additional ionic crosslinking using various cations is introduced after an in situ precipitation process for hydroxyapatite immobilization in hyaluronic acid hydrogels (HAc–CaP). Ca²⁺, Ba²⁺, and Sr²⁺ ions exhibit the highest efficiencies in reinforcing the mechanical properties of HAc–CaP hydrogels. In addition, the dual‐crosslinked HAc–CaP hydrogels promote the biological responses of preosteoblast cells, which exhibit highly stretched shapes and greatly enhanced proliferation. Furthermore, the nanocomposite hydrogels achieve enhanced bioactivity by supporting osteogenic differentiation. Thus, enhancement on both the mechanical and biological properties of hyaluronic‐acid‐based nanocomposite hydrogels is achieved through this dual‐crosslinking process, extending the potential application of these materials to hard tissue engineering.     

Highly Stretchable,Fast Self‐Healing,Responsive Conductive Hydrogels for Supercapacitor Electrode and Motion Sensor

Conductive hydrogels as potential soft materials have attracted tremendous attention in wearable electronic devices. Nonetheless, manufacturing intelligent materials that integrate mouldability, stretchability, responsive ability, fast self‐healing ability, as well as mechanical and electrochemical properties is still a challenge. Here, multifunctional conductive hydrogels composed of poly(vinyl alcohol) (PVA) and polypyrrole (PPy) nanotube are prepared using borax as cross‐linker. The existence of multicomplexation, entangled PVA chains, and interconnected PPy nanotubes, as well as extensive hydrogen bonding results in the fabrication of hierarchical network of PVA‐PPy hydrogels. PVA‐PPy hydrogels exhibit high stretchability (more than 1000%), multiresponsiveness, low density (0.95 g cm^?3), high water content (96%), and 15 s self‐healing features. Furthermore, the self‐healing supercapacitor electrode and motion sensor based on PVA‐PPy hydrogels demonstrate ideal performances. This facile strategy in this work would be promising to construct an excellent multifunctional soft material for various flexible electrode and biosensor.     

Photo‐Induced Hydrogel Formation Based on g‐C3N4 Nanosheets with Self‐Cross‐Linked 3D Framework for UV Protection Application

Composite hydrogels of graphitic carbon nitride nanosheets (CNNS) and polyacrylamide (PAM) with superior UV absorption and visible transparence capabilities are reported. CNNS is employed not only as a photocatalytic initiator to trigger the polymerization of acrylamide, but also as a cross‐linker to 3D connected PAM chains via hydrogen bonds. The obtained CNNS/PAM hydrogels are highly moldable for preparing various forms, and have good mechanical properties, self‐healing ability, and photo‐stability. Furthermore, the composite hydrogels have a wide spectral range for UV absorption compared to conventional UV protective materials. Besides the complete screening of UVB (280–315 nm) in sun radiation, the CNNS/PAM hydrogel film can also filter >95% UVA radiation (315–400 nm) by regulating the coating thickness, meanwhile retaining a high visible transmittance. Therefore, the CNNS/PAM hydrogels have potential applications for shielding UV radiation. Additionally, this strategy provides a common and facile route to fabricate functional composite hydrogels via photo‐induced polymerization.     

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.     

Site‐Specific Oxidation‐Induced Stiffening and Shape Morphing of Soft Tough Hydrogels

Living biological tissues are made of structures with properly defined mechanical properties (toughness and stiffness) toward specific biological functions. Herein, a chemical manipulation strategy is developed to locally vary the oxidation state of Fe ions from divalent to trivalent in the tough hydrogels. The resultant trivalent ionically cross‐linked networks become less flexible and lead to a significant enhancement of the stiffness of the tough hydrogels. The mechanical strengthening of Fe²⁺/Ca²⁺‐alginate/polyacrylamide tough hydrogels is demonstrated by the oxidation with ammonium persulfate (APS). Moreover, by applying surface patterning, the mechanical properties of the tough hydrogels are spatially stiffened and thus can serve as anisotropic elements to guide the shape morphing of tough hydrogels into complex 3D structures. This method opens up a simple strategy not only to dynamically vary the mechanics of tough hydrogels, both in bulk and locally from prefabricated soft tough hydrogels, but also toward their shape morphing behaviors on demand.     

Tannin‐Tethered Gelatin Hydrogels with Considerable Self‐Healing and Adhesive Performances

Hydrogels, especially the ones with self‐recovery and adhesive performances, have attracted more and more attention owing to their wide practical potential in the biomedical field involving cell delivery, wound filling, and tissue engineering. Tannic acid (TA), a nature‐derived gallol‐rich polyphenol, exhibits not only unique chelating properties with transition metal cations but also desirable anti‐oxidation properties and strong bonding capability to proteins and gelatin. Thus, taking advantage of the versatility of TA, a one‐pot method is proposed herein to produce TA‐modified gelatin hydrogels with the aid of NaIO₄ under basic conditions. By changing the amount of NaIO₄ used, the obtained hydrogels are covalently cross‐linked to different degrees and consequently exhibit diversity in their self‐healing and adhesive properties. The gelling time, viscoelasticity, and morphology of hydrogels are investigated, and when the feed molar ratio of NaIO₄ to TA is adjusted to 15:1, the fabricated hydrogel shows optimum self‐healing efficiency of 73% and adhesive strength of 36 kPa. Additionally, considering the completely natural origin of TA and gelatin, this study offers an original way for the fabrication of biocompatible self‐healing and adhesive materials.     

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.     

An injectable and self‐healing novel chitosan hydrogel with low adamantane substitution degree

Chitosan (CS) is a semi‐natural polymer with supreme biological function, while the strong interchain hydrogen bonds cause poor water solubility and limit its broader use. To break the semi‐rigid structure of CS, a kind of CS modified by adamantane (AD‐CS) was successfully synthesized by amidation reaction with 1‐(3‐dimethylaminopropyl)‐3‐ethylcarbodiimide hydrochloride as catalyst. The chemical structure of AD‐CS was characterized by Fourier transform infrared spectroscopy and ¹H NMR. The AD substitution degree of CS is around 2%, calculated by ¹H NMR. A soft and transparent hydrogel composed of hydrogen bonds was obtained directly by simply adding a certain amount of water under mild conditions. Rheological measurements were carried out to research the mechanism of hydrogel formation by measuring the influence of different additives and conditions on the AD‐CS hydrogel. Reinforced hydrogels were prepared by freezing and thawing. The mechanical strength and self‐healing property of reinforced and pristine hydrogels were assessed with an oscillatory rheometer. The modulus of the reinforced hydrogel was obviously enhanced without much loss of self‐healing property. Other properties such as adhesion, injectability and temperature response were also studied. These injectable and self‐healing hydrogels show potential value in medical care. Additionally, this is a new method to design CS hydrogels with their original interchain hydrogen bonds. © 2019 Society of Chemical Industry     

Ultra‐Fast Microwave Assisted Self‐Healing of Covalent Adaptive Polyurethane Networks with Carbon Nanotubes

Vitrimers are a class of covalent adaptive networks which, unlike other covalent networks, can be thermally reprocessed, recycled, and are self‐healing. In this research, a polyurethane vitrimer network is prepared using 1,4‐phenylene diisocyanate and excess amount of polycaprolactone polyol. The dynamic nature of this network is provided by a dual effect of dynamic transesterification reactions as well as dynamic transcarbamoylation reactions. This vitrimer can be reshaped, be recycled, and heal potential defects at high enough temperatures. A fast healing strategy is developed by the addition of small amounts (0.05 wt%) of carbon nanotubes (CNTs) which enables the use of microwave radiation for an efficient fast healing process. Using this strategy the healing time decreases more than 30 times compared to using a conventional oven. CNTs also enhance the vitrimer mechanical properties and compensate for the mechanical property loss of the dynamic PU network in comparison to the permanent PU network.     

Graphene Oxide Hybrid Supramolecular Hydrogels with Self‐Healable,Bioadhesive and Stimuli‐Responsive Properties and Drug Delivery Application

Self‐healable hydrogels are promising soft materials with great potential in biomedical applications due to their autonomous self‐repairing capability. Although many attempts are made to develop new hydrogels with good self‐healing performance, to integrate this characteristic along with other responsive multifunctions into one hydrogel still remains difficult. Here, a self‐healable hybrid supramolecular hydrogel (HSH) with tunable bioadhesive and stimuli‐responsive properties is reported. The strategy is imparting graphene oxide (GO) nanosheets and quadruple hydrogen bonding ureido‐pyrimidinone (UPy) moieties into a thermoresponsive poly(N‐isopropylacrylamide) (PNIPAM) polymer matrix. The obtained GO–HSH hydrogel shows rapid self‐healing behavior and good adhesion to various surfaces from synthetic materials to biological tissue. In addition, doxorubicin hydrochloride (DOX) release profiles reveal the dual thermo‐ and pH‐responsiveness of the GO–HSH hydrogel. The DOX‐loaded hydrogel can further directly adhere to titanium substrate, and the released DOX from this thin hydrogel coating remains biologically active and has high capability to kill tumor cells.     

Highly Flexible,Tough, and Self‐Healable Hydrogels Enabled by Dual Cross‐Linking of Triblock Copolymer Micelles and Ionic Interactions

Development of artificial soft materials that have good mechanical performances and autonomous healing ability is a longstanding pursuit but remains challenging. This work reports a kind of highly flexible, tough, and self‐healable poly(acrylic acid)/Fe(III) (PAA/Fe(III)) hydrogels. The hydrogels are dually cross‐linked by triblock copolymer micelles and ionic interaction between Fe(III) and carboxyl groups. Due to the coexistence of these two cross‐linking points, the resulting PAA/Fe(III) hydrogels are tough and can be flexibly stretched, bent, knotted, and twisted. The hydrogels can withstand a deformation of 600% and an ultimate stress as high as 250 kPa. Moreover, the dynamic ionic interaction also endows the hydrogels self‐healing properties. By varying the ratio of Fe(III)/AA, a compromised healing efficiency of 73% and an ultimate stress of 200 kPa are obtained.

     

Microwave‐Assisted Reversible Coordination‐Mediated Polymerization for Self‐Healing Hybrid Materials: RGO@PDA Simultaneous as Catalyst and Nanocomposites in One‐Pot

In this article, polydopamine (PDA) is efficiently adhered on the surface of graphene oxide (GO) by mussel‐inspired chemistry. The obtained reduced GO/PDA (RGO@PDA) nanocomposites are used for catalyzing reversible coordination‐mediated polymerization under microwave radiation. Well‐defined and iodine‐terminated polyacrylonitrile‐co‐poly(n‐butyl acrylate) (PAN‐co‐PnBA) is successfully fabricated by using RGO@PDA nanocomposites as catalysts. Importantly, green and novel strategy of PAN‐co‐PnBA‐type self‐healing nanocomposite materials is further fabricated with RGO@PDA as additive after polymerization as catalyst in one‐pot. As a reinforcement agent, RGO@PDA can also improve the mechanical and self‐healing properties of hybrid materials, which opens up a novel and green methodology for the preparation of self‐healing hybrid materials.     

Superfast Self‐Healing and Photothermal Active Hydrogel with Nondefective Graphene as Effective Additive

2D graphene with high quality holds great promise in improving the performance of the hydrogels owing to its exceptional electronic, thermal, and mechanical properties. However, the structure defects existed in graphene restrict its further applications. Herein, a simple and green method of fabricating defect‐free graphene nanosheets with the assistance of supercritical carbon dioxide (SC CO₂) is designed. The graphene nanosheets directly assemble with acrylic acid monomer and clay, and a flexible semitransparent hydrogel is fabricated. Benefiting from the excellent properties of the defect‐free graphene, the hydrogel exhibits the high mechanical performance, superfast self‐healing capability, excellent conductivity, and super photothermal conversion efficiency. According to the advantages above, the graphene/poly(acrylic acid)/clay hydrogels can be used for intelligent sensors for disease diagnosis, artificial electronic skin, and military stealth materials in the near future.