Bioinspired Tunable Sacrificial Bonds Endowing Tetra‐PEG Based PU Hydrogel with Tunable Mechanical Properties,Shape‐Memory,and Self‐Healing Functions

Development of hydrogels with excellent and tunable mechanical properties combining with multifunctions is an intriguing issue in material science and engineering. Herein, bioinspired tunable sacrificial bonds are introduced into the tetra‐poly(ethylene glycol) (PEG) based polyurethane (PU) (TP) network to afford a hydrogel with tunable mechanical properties, shape‐memory, and self‐healing functions. The mussel‐inspired compound of Lysine‐dopamine (LDA) is introduced into the network of TP hydrogel through polyurethane/polyurea chemistry to form LDA‐tetra‐PEG‐PU (LTP) hydrogel. As catechol groups in LDAs can intermolecularly interact with each other and can also coordinate with ferric ions with different coordination ratios, these physical interactions with different strengths in the afforded LTP hydrogel construct kinds of sequentially tuned sacrificial bonds. As a result, these sacrificial bonds preferentially rupture prior to the covalent network upon external loading, which dissipate the energy and endow the hydrogel with advanced and postadjustable mechanical properties. This mechanism is investigated in detail. Furthermore, the LTP hydrogel shows multifunctions such as shape‐memory and self‐healing abilities. In addition, the tetra‐PEG based hydrogel shows remarkable thermoresponsiveness that the hydrogel distinctly contracts with the increase of the temperature. The improved mechanical strength and multifunctions should enlarge the application areas of the tetra‐PEG based hydrogel in various fields.     

A Facile Strategy for Preparing Tough,Self‐Healing Double‐Network Hyaluronic Acid Hydrogels Inspired by Mussel Cuticles

Compared with hydrogel‐like biological tissues such as cartilage, muscles, and blood vessels, current hyaluronic acid hydrogels often suffer from poor toughness and limited self‐healing properties. Herein, a facile and generalizable strategy inspired by mussel cuticles is presented to fabricate tough and self‐healing double‐network hyaluronic acid hydrogels. These hydrogels are composed of ductile, reversible Fe³⁺‐catechol interaction primary networks, and secondarily formed brittle, irreversible covalent networks. Based on this design strategy, the hyaluronic acid hydrogels are demonstrated to exhibit reinforced mechanical strength while maintaining a rapid self‐healing property. In addition, by simply regulating pH or UV irradiation time, the mechanical properties of the hydrogels can be regulated conveniently through variations between the primary and secondary networks.     

Physically Linked PVA/P(AAm‐co‐SMA) Hydrogels: Tough,Fast Recovery,Antifatigue, and Self‐Healing Soft Materials

The application of traditional chemically crosslinked hydrogels is often limited by poor mechanical properties because of their own inhomogeneous network and irreversible crosslinking bonds. Herein, physical interactions are applied to crosslink the interpenetrating network hydrogel, i.e., hydrogen bonding and crystalline domain for polyvinyl alcohol network, and hydrophobic interaction inside micelle for poly (acrylamide‐co‐stearyl methyl acrylate) [P(AAm‐co‐SMA)] network. In this gel network system, reversible energy dissipation mechanism is realized by dissociation and reassociation of weak interactions including hydrogen bonding and hydrophobic interaction inside the micelle. Strong crystalline domains serve as permanent crosslinking interactions to maintain network integrity under large extension. As a result, the synergy of weak and strong interactions leads to tough, antifatigue, fast recovery, and self‐healing properties of the hydrogel. This proposed strategy of achieving versatile hydrogels can broaden the use of hydrogels into load‐bearing applications.     

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     

Super‐Tough,Self‐Healing Polyurethane Based on Diels‐Alder Bonds and Dynamic Zinc–Ligand Interactions

Self‐healing polymer materials have attracted extensive attention and have been explored due to their ability of crack repairing in materials. This paper aims to develop a novel polyurethane‐based material with high self‐healing efficiency and excellent mechanical properties under 80 °C on the basis of reversible Diels–Alder bonds as well as zinc–ligand structure (DA‐ZN‐PU). By integrating DA bonds and zinc–ligand structure, as‐prepared DA‐ZN‐PU samples reach the maximum tensile strength as much as 28.45 MPa. After self‐healing, the tensile strength is 25.85 MPa, leading to the high self‐healing efficiency of 90.8%. In addition, by introducing carbonyl iron powder (CIP), a new polyurethane containing carbonyl iron powder (DA‐ZN‐CIP‐PU) can be achieved, exhibiting microwave‐assisted self‐healing property. And the self‐healing efficiency can be reached to 92.6% in 3 min. Due to high self‐healing efficiency and excellent mechanical properties of the prepared novel polyurethane, it has application attributes in crack repair of functional composite materials.     

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     

A Self‐Healing PAM/CMHPG System for Unconventional Reservoir Stimulation

A hybrid chemically and physically linked polyacrylamide (PAM)/carboxymethyl hydroxypropyl guar gum (CMHPG) system is prepared via a fast and controllable one‐pot strategy. Due to the synergetic effect of the non‐covalent interactions between chains, these systems show improved, balanced mechanical properties. The apparent morphology, storage modulus G′, and loss modulus G″ show that these systems have rapid and almost full recovery ability (the self‐healing efficiency can reach 95%) with several hydrogen‐bonding interactions between two networks. This self‐healing property can cover the shortage of G′, G″, and viscosity loss at high shear force, which will help the system keep enough viscosity to create fractures or carry proppants during the whole fracturing process. Meanwhile, the self‐healing fracturing fluid can be broken easily and flow back to surface with little damage to the fracture conductivity, indicating great potential in unconventional reservoir which is sensitive to the fracturing fluid damage.     

Hydrogen‐bonded supramolecular polymers as self‐healing hydrogels: Effect of a bulky adamantyl substituent in the ureido‐pyrimidinone monomer

In an attempt to generate supramolecular assemblies able to function as self‐healing hydrogels, a novel ureido‐pyrimidinone (UPy) monomer, 2‐(N ′‐methacryloyloxyethylureido)‐6‐(1‐adamantyl)‐4[1H]‐pyrimidinone, was synthesized and then copolymerized with N,N‐dimethylacrylamide at four different feed compositions, using a solution of lithium chloride in N,N‐dimethylacetamide as the polymerization medium. The assembling process in the resulting copolymers is based on crosslinking through the reversible quadruple hydrogen bonding between side‐chain UPy modules. The adamantyl substituent was introduced in order to create a “hydrophobic pocket” that may protect the hydrogen bonds against the disruptive effect of water molecules. Upon hydration to equilibrium, all copolymers generated typical hydrogels when their concentration in the hydrated system was at least 15%. The small‐deformation rheometry showed that all hydrated copolymers were hydrogels that maintained a solid‐like behavior, and that their extrusion through a syringe needle did not affect significantly this behavior, suggesting a self‐healing capacity in these materials. An application as injectable substitutes for the eye's vitreous humor was proposed. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39932.     

Synthesis and self‐healing behavior of thermoreversible epoxy resins modified with nitrile butadiene rubber

Cracks may generate in epoxy resins, which can affect the comprehensive property and shorten service life. The problem is expected to be resolved by endowing epoxy resin with self‐healing performance. Herein, a new kind of self‐healing epoxy resin containing both Diels–Alder (DA) bonds and nitrile butadiene rubber (NBR) has been developed. The self‐healing performance and mechanical properties of as‐prepared epoxy resins are investigated by qualitative observation and quantitative measuring. Results reveal that the as‐prepared epoxy resins exhibit excellent self‐healing performance and multiple repair ability, and the self‐healing behavior is based on dual actions of thermal reversibility of DA reaction and thermal movement of molecular chains. Furthermore, the thermoreversible DA bonds contribute much to the recovery of mechanical property, while the incorporated thermoplastic NBR accelerates the whole healing process. The self‐healing efficiency of epoxy resins can be enhanced markedly by introducing thermoplastic NBR. In addition, the self‐healing epoxy resins also exhibit outstanding reprocessing performance, which makes it possible of recycling waste epoxy resin. POLYM. ENG. SCI., 59:1603–1610 2019. © 2019 Society of Plastics Engineers     

Double dynamic self‐healing polymers: supramolecular and covalent dynamic polymers based on the bis‐iminocarbohydrazide motif

Self‐healing polymers are a class of functional polymers that, by the virtue of the presence of certain dynamic chemical linkages, may undergo self‐repair at a mechanically cut surface. Herein we report the synthesis of a self‐healing polymer giving access to double dynamicity within the polymer network by making use simultaneously of reversible covalent bonds and dynamic non‐covalent hydrogen bonding interactions. These features are provided, respectively, by doubly dynamic cassettes comprising chemically reversible imine linkages and multiply hydrogen‐bonded urea groups, connected by a siloxane‐based backbone that imparts softness to the material. Such a system can be envisaged to give access to a broad spectrum of functional materials, which can be tuned by convenient modulation of the structural motifs of the polymer. © 2013 Society of Chemical Industry     

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.     

A seawater‐assisted self‐healing metal–catechol polyurethane with tunable mechanical properties

Recently, self‐healing polymers have been one of the most intriguing academic fields due to the fact that they can increase their service lives and reduce the amount of waste. Here we designed and synthesized a novel telechelic polyurethane with dopamine (DA) end groups that are coordinated with Ca²⁺ to form dynamic non‐covalent bonds. The tensile stress of the designed polyurethanes increases with increase in the amount of metal cation added, while the strain at break slightly decreases. Rheological tests show that the ionic coordination between Ca²⁺ and catechol can dynamically break and recombine under the stimulation of seawater, endowing the polymer with superior self‐healing properties (up to 84% based on toughness). Therefore, the seawater‐triggered self‐healable, super tough polyurethane presented here is very intriguing as it has many potential applications especially in the marine environment. © 2019 Society of Chemical Industry     

Self‐healing properties of layer‐by‐layer assembled multilayers

The review is focused on the formation and the self‐healing properties of polymer and hybrid multilayers formed via the layer‐by‐layer approach. In the first part of the review the recent developments in the construction of polymer multilayers are highlighted. In the second part the design and the self‐healing properties of inorganic ? polymer hybrid multilayers are described. It is shown that self‐healing multilayers have a broad spectrum of applications including corrosion protection, as elements of antifouling and antimicrobial coatings and bio‐inspired superhydrophobic interfaces. It is demonstrated that dynamic functional interfaces have a complex hierarchical organization of non‐covalently bonded polymers and colloidal particles. Mechanisms of self‐healing behavior of the multilayers and the role of water and external stimuli (pH, ionic strength and temperature, light) in swelling of multilayers and rearrangement of polymer segments are discussed. Future trends, perspectives and research strategies for the design of ‘smart’ self‐assemblies with self‐healing properties are proposed. © 2015 Society of Chemical Industry     

An autonomous self‐healing hydrogel based on surfactant‐free hydrophobic association

Self‐healing hydrogels are attractive for a variety of applications including wound dressings and coatings. This paper describes the facile preparation and characterization of an autonomous self‐healing hydrogel system comprising surfactant‐free hydrophobic associations. The hydrogel comprised a copolymer of benzyl methacrylate (B), octadecyl methacrylate (O), and methacrylic acid (MA). The hydrogels were prepared via a controlled dehydration procedure to achieve the formation of strong intermolecular hydrophobic associations of the octadecyl groups above a critical polymer concentration. Fractured hydrogels healed within 30 min without any external intervention. Increasing hydrogel polymer content from 31 wt % to 39 wt % resulted in a threefold increase in the shear modulus and 50% reduction of the relaxation time. Addition of 4 mM NaCl to a hydrogel of 31 wt % polymer content resulted in 2.5 times longer relaxation time and 24% decrease in shear modulus. The hydrogels swelled up water by up to four times its weight, which corroborates the robustness of the hydrophobic association crosslinks. The bulk properties of the hydrogels are discussed in terms of the hydrophobic associations of the O‐groups and the electrostatic interaction of the MA‐groups in the polymer chains. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44800.     

Highly Stretchable,Transparent, and Bio‐Friendly Strain Sensor Based on Self‐Recovery Ionic‐Covalent Hydrogels for Human Motion Monitoring

Stretchable, flexible, and strain‐sensitive hydrogels have gained tremendous attention due to their potential application in health monitoring devices and artificial intelligence. Nevertheless, it is still a huge challenge to develop an integrated strain sensor with excellent mechanical properties, broad sensing range, high transparency, biocompatibility, and self‐recovery. Herein, a simple paradigm of stretchable strain sensor based on multifunctional hydrogels is prepared by constructing synergistic effects among polyacrylamide (PAM), biocompatible macromolecule sodium alginate (SA), and Ca ion in covalently and ionically crosslinked networks. Under large deformation, the dynamic SA‐Ca²⁺ bonds effectively dissipate energy, serving as sacrificial bonds, while the PAM chains bridge the crack and stabilize the network, endowing hydrogels with outstanding mechanical performances, for instance, high stretchability and compressibility, as well as excellent self‐recovery performance. The hydrogel is assembled to be a transparent and wearable strain sensor, which has good sensitivity and very wide sensing range (0–1700%), and can precisely detect dynamic strains, including both low and high strains (20–800% strain). It also exhibits fast response time (800 ms) and long‐time stability (200 cycles). The sensor can monitor and distinguish complicated human motions, opening up a new route for broad potential applications of eco‐friendly flexible strain‐sensing devices.     

Increased stability in self‐healing polymer networks based on reversible Michael addition reactions

A reversible thiol‐ene click reaction is utilized to design novel self‐healing polymers. These materials are based on a new methacrylate monomer featuring a benzylcyanoacetamide derivative, which is copolymerized with butyl methacrylate. Afterwards, the crosslinking is performed by the addition of a dithiol and a tetrathiol, respectively. Self‐healing behavior is obtained by heating the crosslinked polymers to 100 °C (150 °C) for several hours and is monitored by scratch healing experiments utilizing an optical microscope. The thermal properties are studied in detail by differential scanning calorimetry as well as thermogravimetric analysis. Moreover, depth‐sensing indentation measurements are performed to determine the mechanical properties. The healing process is based on the reversible cleavage/closing of the bonds (i.e., thiol‐ene reaction), which could be demonstrated by Raman spectroscopy. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44805.     

β‐cyclodextrin‐conjugated hyaluronan hydrogel as a potential drug sustained delivery carrier for wound healing

In order to develop a potential drug sustained delivery carrier suitable for wound healing, a series of β‐cyclodextrin conjugated hyaluronan hydrogels (β‐CD‐HA) with adjustable crosslink densities were synthesized and characterized, meanwhile the delivery kinetics and mechanism of diclofenac as a model anti‐inflammatory drug from these hydrogels were investigated. By controlling the feeding molar ratio of β‐CD/HA, a β‐CD substitution degree of 4.65% was obtained by ¹H‐NMR analysis. The incorporation of β‐CD modification had little effect on the internal porous structure, water swelling ratio, and rheological property of HA hydrogel, which however were influenced by the crosslink density. Although the crosslink density had an influence on the drug loading and release profile by altering the water swelling property, the interaction between β‐CD and drug was the primary factor for the high loading capacity and long‐term sustained delivery of diclofenac. The semiempirical equation fit showed that the release of diclofenac from HA‐based hydrogels followed a pseudo‐Fickian diffusion mechanism. By the aid of β‐CD and controlled crosslink density, a β‐CD‐HA hydrogel with a diclofenac sustained delivery period of over 28 days and desirable physicochemical properties was achieved, which will be a promising drug sustained delivery carrier for wound healing. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43072.     

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.     

Evaluation of biodegradability of epoxy‐guar gum composites

Epoxy composites incorporating natural components have been mainly limited to the use of natural fibers. However, there have been a few instances where polysaccharides have been used as particulate fillers in thermoset compositions. The present study investigated the effect of guar gum/hydroxypropyl guar gum as a filler on the degradative properties of epoxy composites at various filler concentrations, with reference to fungal degradation and soil burial tests. It was found that at higher filler concentrations, the degradation increased. Composites based on hydroxypropyl guar gum showed increased degradation initially but on prolonged exposure to the fungal environment, the difference between guar gum and the hydroxypropyl guar gum‐based composites was found to be marginal. Microscopic evaluation of the composites showed that the degradation occurred at both the composite surface and in the bulk. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Heat‐triggered poly(siloxane‐urethane)s based on disulfide bonds for self‐healing application

Polydimethylsiloxane (PDMS) is one of the most widely employed silicon‐based polymers for its high flexibility, low usage temperature, excellent water resistance, outstanding electrical insulting property, and physiological inert, etc. However, the covalent‐bonded Si? O bonds are unable to heal automatically when damaged, which would result in the failure of the materials and devices. Disulfide bond based polymers show high healing efficiency at moderate temperature and have been investigated intensively. Herein, we report a PDMS‐based polyurethane self‐healing polymer (PDMS‐PU) modified with disulfide bonds, which exhibited a reinforced thermal stability, excellent stretchability, and satisfactory self‐healing ability. The effect of different ratio of PDMS and disulfide bond contents on the elastomer properties was investigated. With the increase of PDMS content, the decomposition temperature of the PDMS‐PU‐3 (332 °C) elastomer with highest content of PDMS was increased by 34 °C compared to PDMS‐PU‐1 (298 °C) with lowest content of PDMS and exhibited a largest elongation at break of 1204%. PDMS‐PU‐1 with highest content of disulfide bond possessed a highest healing efficiency of 97%. The results indicated the PDMS‐PU elastomers can be used as self‐healing flexible substrate for flexible electronics. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46532.