Rapid Self-healing Ability Research Articles

Bioprocess-inspired synthesis of printable, self-healing mineral hydrogels for rapidly responsive, wearable ionic skin

Oyster reefs, which can maintain coastal ecosystems by absorbing storm surge energy, are composed of aggregated oyster shells bonded by bioadhesive secretions containing inorganic minerals. Bioadhesive secretions are formed by cross-linking extracellular liquid, which is rich in organic and inorganic ions, with sand, bacteria, or diatoms. Inspired by such bioprocesses, a mineral hydrogel with excellent 3D printability, rapid self-healing ability (85% recovery within 1 min), high stretchability (>1500% tensile elongation) and ionic conductivity was synthesized by the chelation of polyacrylic acid (PAA) with calcium ions (Ca2+) and subsequent physical cross-linking of PAA with modified amorphous calcium phosphate (ACP) nanoparticles. With such mineral hydrogels, ionic skin was successfully prepared, which could recognize the bending degrees of the index finger and sensitively sense the tensile strain (approximately 33 ms). Moreover, the ionic skin can distinguish external temperature stimuli and work stably at a temperature of 75 °C. These performances exhibit the great mechano-sensing and thermosensing abilities of ionic skin, showing promising applications in various fields, such as wearable sensors, for withstanding high ambient temperatures and the next generation of soft intelligent robots.

Epidermis microstructure inspired mica-based coatings for smart corrosion protection

Two-dimensional (2D) graphene-like nanomaterials have gained interest in anticorrosion coatings due to their enable to act as physical barriers to aggressive species, yet 2D coatings are not always effective or sustainable. Inspired by the epidermis microstructure, herein we designed a high-performance anticorrosion coating with rapid self-healing ability via integrating the excellent adhesion property of polydopamine (PDA) and superior barrier properties of natural mica nanosheets (MNSs) for the first time. Such the coating comprises a protective hard top layer with MNSs constituents (“sealing” agents) and a bottom hybrid soft polymer multilayer, forming a stratified epidermis microstructure, can realize a complete restoration of coating defects under water owing to their mutual benefit. These MNSs not only can enhance the barrier performance of the coating matrix to retard the diffusion of aggressive species and hamper polymer release into water, but also induce an anisotropic diffusion to promote the self-healing of the polymer chains. The microstructures of the bioinspired coatings were systemically investigated by scanning electron microscopy (SEM) and optical microscopy (OM) measurements. Electrochemical results confirmed the anticorrosion performance of the coatings were significantly improved. This work provides a new insight for the design and fabrication of 2D nanomaterials reinforced high-performance and smart anticorrosion coatings.

High-Strength and High-Toughness Silk Fibroin Hydrogels: A Strategy Using Dynamic Host-Guest Interactions.

Natural polymer-based hydrogels attract great attention because of their inherent biocompatibility and controllable biodegradability. However, the broad applications of these hydrogels require a combination of high mechanical strength, high toughness, fatigue resistance, as well as self-healing. The integration of this combination into one natural polymer-based hydrogel remains challenging. Here, a molecular design strategy was proposed to fabricate mechanically robust silk fibroin-based hydrogels using host-guest interactions. Silk fibroin molecules was chemically modified with cholesterol (Chol, guest) or β-cyclodextrin (β-CD, host), and host-guest interaction between Chol and β-CD moieties drove the supramolecular assemblies of hydrogels. The dissociation/reassociation behavior of host-guest complexation, serving as sacrificial bonds, endowed hydrogels with effective energy dissipation and rapid self-healing ability. The prepared silk fibroin-based hydrogels exhibited high mechanical strength, high toughness, and remarkable fatigue resistance, superior to conventional silk fibroin hydrogels. Moreover, due to reversible host-guest interactions, hydrogels achieved facile functional recovery after damage without any external stimuli. This design strategy provides an avenue to develop natural polymer-based materials with robust mechanical properties, thus broadening current hydrogel applications.

Study on mussel-inspired tough TA/PANI@CNCs nanocomposite hydrogels with superior self-healing and self-adhesive properties for strain sensors

Tough, self-adhesive and conductive hydrogels have recently attracted considerable interest due to their promising applications in electronic skins, wearable devices and flexible sensors. In this work, we designed a mussel-inspired nanocomposite hydrogel derived from water-soluble tannic acid/polyaniline coated cellulose nanocrystals (TA/PANI@CNCs) and various functional acrylic monomers. The incorporated TA/PANI@CNCs not only endowed the nanocomposite hydrogels with enhanced electrical conductive network and mechanical performance, but also played an important role in repeatable and durable adhesiveness to human skin without additional fixation. By taking the advantage of double networks based on various supramolecular interactions and dynamic borate ester bonds, the hydrogels exhibited a dramatic breaking strain of 974%, fracture stress of 759 kPa and rapid self-healing ability at room temperature without external stimuli. In addition, the as-prepared nanocomposite hydrogels had great strain sensitivity and excellent electrical conductivity, which can be employed as flexible strain sensors for tracking human body motion with a broad range of strain or directly assembled on other materials surfaces as electrical circuits. Compared with previous conductive hydrogels, this work will provide a novel pathway to fabricate nanocomposite hydrogels with self-healing, self-adhesive, strain sensitive and robust mechanical properties for potential applications in wearable strain sensors, flexible electrical interconnection and human motion monitoring.

Rapid gelling, self-healing, and fluorescence-responsive chitosan hydrogels formed by dynamic covalent crosslinking

Chitosan hydrogels fabricated by covalent crosslinking exhibit tough mechanical properties and chemical stability. In this paper, debranched starch (DBS) is oxidized to dialdehyde debranched starch (DADBS), which is used as a new type of a crosslinking agent to prepare hydrogels. Chitosan hydrogels with excellent properties are prepared by dynamic Schiff-base crosslinking between the aldehyde groups in DADBS and the amino groups in chitosan. Hence, chitosan hydrogels exhibit a rapid gelation ability, with a gelation time of less than 30 s, and their storage modulus increases with the gelation time. By adjusting the molar ratio of the amino group of chitosan to the aldehyde group of DADBS and the reaction temperature, the hydrogels exhibit tunable elasticity and mechanical properties. Notably, scanning electron microscopy revealed the presence of 100–200 nm microgels in the hydrogel network, which could exert a strengthening effect on the mechanical properties of the hydrogels. In addition, chitosan hydrogels exhibit a rapid self-healing ability and remarkable fluorescence properties; also, they can be 3D printed in different shapes. Overall, the DADBS cross-linked chitosan hydrogels demonstrate potential applications in food, medicine, agriculture, and materials.

Biomass polyamide elastomers based on hydrogen bonds with rapid self-healing properties

By the polycondensation reaction of high-purity dimer acid (DA) and diethylene triamine (DETA), a polyamide elastomer (DPE) with rapid self-healing ability was successfully prepared. In the DPE networks, long polymer chains are physically entangled with each other. Chemical crosslink also occurred between amide groups and secondary amine structures. Besides, numerous hydrogen bonds formed among the amide bonds of different segments endowed the DPE with self-healing ability at room temperature. As a result, triple effects of cross-linking, physical entanglement and hydrogen bonding improve the mechanical properties of DPE. The tensile strength and elongation at break reached 4.5 MPa and 1700%, respectively. Self-healing efficiency of DPE can reach 80% within 2 h. This material can be utilized for many practical applications, such as smart wearable devices, e-skins and so on.

Self-healing supramolecular hydrogels through host-guest interaction between cyclodextrin and carborane.

New self-healing hydrogels based on the strong host-guest interaction of carborane (CB) and β-cylcodextrin (CD) were constructed through CB-grafted dextran and β-CD-grafted poly(acrylic acid). The storage modulus of the hydrogels could reach as high as 10 kPa, and the hydrogels exhibited an outstanding self-healing rate in minutes.

A super-stretchable, self-healing and injectable supramolecular hydrogel constructed by a host-guest crosslinker.

Supramolecular hydrogels based on host-guest interactions have drawn considerable attention due to their unique properties and promising applications. However, it is still a great challenge to construct supramolecular hydrogels that simultaneously achieve mechanical strength, processability, and biocompatibility. Herein, we present a rational design of a "supramolecular crosslinker" approach to fabricate a new host-guest hydrogel with super-stretchability, self-healing, and injectable properties and excellent biocompatibility. The star-shaped supramolecular crosslinker is formed by the host-guest interactions between octa-cyclodextrin polyhedral oligomeric silsesquioxane (OCDPOSS) and acrylamide-modified adamantane (Ad-AAm). Supramolecular hydrogels can be briefly prepared by UV-initiated copolymerization of acrylamide and supramolecular crosslinkers. Supramolecular hydrogels present impressive mechanical properties due to rigid POSS as the core of the supramolecular crosslinker. Moreover, multivalent host-guest interactions improve the ductility, rapid self-healing and injectable ability of these hydrogels. Simultaneously, these supramolecular hydrogels possess good biocompatibility and can be utilized as carriers for the sustained release of hydrophobic drugs. Thus, such supramolecular hydrogels will have potential applications for tissue engineering and drug delivery systems.

Supramolecular medical antibacterial tissue adhesive prepared based on natural small molecules.

In the field of biomedicine, tissue bio-adhesives require the use of polymer materials with integrated functions to meet changing practical applications. However, the currently available tissue glues cannot balance mechanical properties and biocompatibility. Inspired by the conversion of lipoic acid from small molecular biological sources into high-performance supramolecular polymeric materials, thioctic acid (TA) was modified and polyethylene glycol diacrylate (PEGDA) was introduced. Successfully constructed a dry gel with antibacterial effect and promote infection for wound regeneration. The prepared modified lipoic acid is mixed with PEGDA, melted under mild heating and self-assembled, and then directly extruded on both sides of the wound. It quickly cures at 37 °C and firmly adheres to both sides of the wound. The material exhibits good processability and rapid self-healing ability due to the cross-linked structure of the internal disulfide bonds and thioether bonds. In addition, the characteristics of TA make the prepared xerogels have good tissue adhesion and good antibacterial properties. This work proposes an innovative material with mechanical strength and biocompatible tissue glue, which provides broad prospects for application in wound treatment.

Self-Healing and Highly Stretchable Gelatin Hydrogel for Self-Powered Strain Sensor.

Hydrogels that electronically respond to mechanical changes can be used as strain sensors. However, these systems usually require external power to convert changes in strain into electrical signals. Here, a self-powered strain sensor is developed based on a gelatin-based hydrogel and a galvanic cell. In the hydrogel matrix, hydrophobic interactions and hydrogen bonding between tannic acid and gelatin give the prepared hydrogel great potential for elongation (1600%). The hydrogel also has a rapid self-healing ability (within 0.65 s) and high self-healing efficiency (95%). The hydrogel operates as an efficient electrolyte material and forms a hydrogel battery when assembled with a thin layer of zinc and an air electrode. This device had excellent tolerance to large compressional strain without sacrificing open-circuit voltage. On the basis of this hydrogel battery, we fabricated a self-powered strain sensor by connecting the hydrogel battery to a fixed resistor to form a closed loop. By converting its chemical energy into electrical energy, the self-powered sensor efficiently converted resistance changes, caused by stretching or compression of the hydrogel, into changes in the voltage output signals without external power. Owing to the stretchability of the hydrogel, the self-powered sensor exhibited good response and flexibility. Self-healing and continuous cycling tests confirmed the long-term stability of the device. These properties suggest that our self-powered sensor has a potential for applications to portable and wearable electronic devices.

Conductive double-crosslinked network hydrogel with superior stretchability and self-healing ability

Conductive self-healing hydrogels are a new type of functional material that combine self-healing with conductive properties. They are widely used in flexible and stretchable devices, which require not only self-healing properties and electrical conductivity but also good stretchability. In this study, we used a one-pot process to crosslink and polymerize trivalent iron (Fe3+) with acrylic acid (AA) and acrylamide (AM) into a dual network hydrogel P(AA–co–AM)/Fe3+, with both chemically and physically crosslinked networks. The obtained hydrogels exhibited excellent tensile properties (∼2000% stretchability) and rapid self-healing ability (complete healing at room temperature within 8 h). Furthermore, the Fe3+ gives the hydrogel good electrical conductivity. In addition, a capacitive pressure sensor using the hydrogel as a conductive active layer was fabricated, which proved its application in the field of wearable devices.

Effect of ionic liquid structure on viscoelastic behavior of hydrogen-bonded micellar ion gels

Ion gels based on block copolymer self-assembly in ionic liquids (ILs) are attractive, processable, non-volatile, ion-conducting soft materials. Recently, a new class of ion gels based on hydrogen bonding between jammed block copolymer micelles in ILs that shows good mechanical strength and rapid self-healing ability has been developed. Herein, the effect of IL cation and anion structures on the microstructures and viscoelastic properties of a hydrogen-bonded micellar ion gel is reported. It was clarified that competition among cations, anions, and polymer chains for hydrogen bonding significantly affects the viscoelastic behavior of the ion gel. In the case of weak hydrogen bonding for ILs, macroscopic phase separation occurs due to strong hydrogen bonding between diblock copolymer micelles, resulting in an opaque and fragile ion gel, whereas storage and loss moduli are significantly reduced for ILs showing strong hydrogen bonding. Control of competitive hydrogen bonding interaction between polymers and ILs is thus critical to obtaining the desirable physical properties for ion gels.

Rapid self-healing, stretchable, moldable, antioxidant and antibacterial tannic acid-cellulose nanofibril composite hydrogels

Here, we designed a self-healing composite hydrogel with antioxidant and antibacterial activities by using cellulose nanofibrils (CNF) and tannic acid (TA) as functional additives. Excellent mechanical stability, moldability, stretchability and rapid self-healing ability without any external intervention were realized in one system due to the combined dynamic borate ester bonding between polyvinyl alcohol-borax (PB) and multi-hydrogen bonding between different components. The rheological measurements indicated the incorporation of CNF and TA to PB system substantially affected the viscoelasticity of hydrogels. The unique antioxidant and antibacterial properties were achieved due to the complexation of TA. These high performance multifunctional hydrogels opens a window for a broad application in the field of smart devices and surface engineering.

Highly Stretchable and Self-Healing Strain Sensors Based on Nanocellulose-Supported Graphene Dispersed in Electro-Conductive Hydrogels.

Intrinsic self-healing and highly stretchable electro-conductive hydrogels demonstrate wide-ranging utilization in intelligent electronic skin. Herein, we propose a new class of strain sensors prepared by cellulose nanofibers (CNFs) and graphene (GN) co-incorporated poly (vinyl alcohol)-borax (GN-CNF@PVA) hydrogel. The borax can reversibly and dynamically associate with poly (vinyl alcohol) (PVA) and GN-CNF nanocomplexes as a cross-linking agent, providing a tough and flexible network with the hydrogels. CNFs act as a bio-template and dispersant to support GN to create homogeneous GN-CNF aqueous dispersion, endowing the GN-CNF@PVA gels with promoted mechanical flexibility, strength and good conductivity. The resulting composite gels have high stretchability (break-up elongation up to 1000%), excellent viscoelasticity (storage modulus up to 3.7 kPa), rapid self-healing ability (20 s) and high healing efficiency (97.7 ± 1.2%). Due to effective electric pathways provided by GN-CNF nanocomplexes, the strain sensors integrated by GN-CNF@PVA hydrogel with good responsiveness, stability and repeatability can efficiently identify and monitor the various human motions with the gauge factor (GF) of about 3.8, showing promising applications in the field of wearable sensing devices.

Ultrastretchable Wearable Strain and Pressure Sensors Based on Adhesive, Tough, and Self-healing Hydrogels for Human Motion Monitoring.

Currently, flexible wearable hydrogel-based sensors have attracted considerable attention due to their promising applications in a variety of fields. However, concurrently integrating toughness, adhesiveness, self-healing ability, and conductivity into the hydrogel is still a great challenge. Here, casein sodium salt from bovine milk (sodium casein, SC) and polydopamine (PDA, inspired by mussels) were successfully introduced into the polyacrylamide (PAAm) hydrogel system to fabricate a tough and adhesive SC-PDA hydrogel. The hydrogel exhibits splendidly reversible adhesive behavioral bonding toward various materials and even human skin. Moreover, based on the dynamic cross-linking of SC and PDA in the system, the hydrogel has superstretching ability, excellent fatigue resistance, and rapid self-healing ability. In addition, the existence of sodium ions also endowed the SC-PDA hydrogel with sensitive deformation-dependent conductivity to act as a flexible strain and pressure sensor for directly monitoring large-scale human motions (e.g., joint bending) and tiny physiological signals (e.g., speaking and breathing). Therefore, the strategy would broaden the path of a new generation of hydrogel-based sensors for wide applications.

Tough, Adhesive, Self-Healable, and Transparent Ionically Conductive Zwitterionic Nanocomposite Hydrogels as Skin Strain Sensors.

It is desired to create skin strain sensors composed of multifunctional conductive hydrogels with excellent toughness and adhesion properties to sustain cyclic loadings during use and facilitate the electrical signal transmission. Herein, we prepared transparent, compliant, and adhesive zwitterionic nanocomposite hydrogels with excellent mechanical properties. The incorporated zwitterionic polymers can form interchain dipole-dipole associations to offer additional physical cross-linking of the network. The hydrogels show a high fracture elongation up to 2000%, a fracture strength up to 0.27 MPa, and a fracture toughness up to 2.45 MJ/m3. Moreover, the reversible physical interaction imparts the hydrogels with rapid self-healing ability without any stimuli. The hydrogels are adhesive to many surfaces including polyelectrolyte hydrogels, skin, glasses, silicone rubbers, and nitrile rubbers. The presence of abundant zwitterionic groups facilitates ionic conductivity in the hydrogels. The combination of these properties enables the hydrogels to act as strain sensors with high sensitivity (gauge factor = 1.8). The strategy to design the tough, adhesive, self-healable, and conductive hydrogels as skin strain sensors by the zwitterionic nanocomposite hydrogels is promising for practical applications.

Facile Access to Guar Gum Based Supramolecular Hydrogels with Rapid Self-Healing Ability and Multistimuli Responsive Gel-Sol Transitions.

In this work, we prepare guar gum-based supramolecular hydrogel through the formation of borate/didiol bonds. This dynamic and reversible noncovalent borate/didiol interaction is critical for the multifunctional properties of supramolecular hydrogel. The FT-IR and XRD analysis verified the existence of boronate ester interactions between borax and guar gum. Moreover, the viscoelastic and mechanical behaviors of the hydrogels with different guar gum concentrations showed that the storage modulus and compressive stress were highest at guar gum concentration of 2 wt %. Besides, due to the dynamic and reservable properties of boronate ester, these guar gum-based hydrogels had excellent self-healing property, outstanding reformable and injectable capability. In addition, hydrogels also exhibited reversible gel-sol transformations by the application of physicochemical stimuli such as thermal and pH value. The coupling of these multifunctional properties made from low-cost, environment friendly, and readily available materials could have potential applications in many biomedical areas in the future. We expect that this simple strategy of fabricating the self-healing guar gum hydrogels with multistimuli responsive properties may enrich the avenue in the exploration of multifunctional guar gum based hydrogels to expand their potential applications in various fields.

Degradable conductive self-healing hydrogels based on dextran-graft-tetraaniline and N-carboxyethyl chitosan as injectable carriers for myoblast cell therapy and muscle regeneration

Injectable conductive hydrogels have great potential as tissue engineering scaffolds and delivery vehicles for electrical signal sensitive cell therapy. In this work, we present the synthesis of a series of injectable electroactive degradable hydrogels with rapid self-healing ability and their potential application as cell delivery vehicles for skeletal muscle regeneration. Self-healable conductive injectable hydrogels based on dextran-graft-aniline tetramer-graft-4-formylbenzoic acid and N-carboxyethyl chitosan were synthesized at physiological conditions. The dynamic Schiff base bonds between the formylbenzoic acid and amine group from N-carboxyethyl chitosan endowed the hydrogels with rapid self-healing ability, which was verified by rheological test. Equilibrated swelling ratio, morphology, mechanical strength, electrochemistry and conductivity of the injectable hydrogels were fully investigated. The self-healable conductive hydrogels showed an in vivo injectability and a linear-like degradation behavior. Two different kinds of cells (C2C12 myoblasts and human umbilical vein endothelial cells (HUVEC)) were encapsulated in the hydrogels by self-healing effect. The L929 fibroblast cell culture results indicated the biocompatibility of the hydrogels. Moreover, the C2C12 myoblast cells were released from the conductive hydrogels with a linear-like profile. The in vivo skeletal muscle regeneration was also studied in a volumetric muscle loss injury model. All these data indicated that these biodegradable self-healing conductive hydrogels are potential candidates as cell delivery vehicles and scaffolds for skeletal muscle repair. Statement of SignificanceInjectable hydrogels with self-healing and electrical conductivity properties are excellent candidates as tissue-engineered scaffolds for myoblast cell therapy and skeletal muscle regeneration. The self-healing property of these hydrogels can prolong their lifespan. However, most of the reported conductive hydrogels are not degradable or do not have the self-healing ability. Herein, we synthesized antibacterial conductive self-healing hydrogels as a cell delivery carrier for cardiac cell therapy based on chitosan-grafted-tetraaniline hydrogels synthesized in our previous work. However, an acid solution was used to dissolve the polymers in that study, which may induce toxicity to cells. In this work, we synthesized a series of injectable electroactive biodegradable hydrogels with rapid self-healing ability composed of N-carboxyethyl chitosan (CECS) and dextran-graft-aniline oligomers, and these hydrogel precusor can dissolve in PBS solution of pH 7.4; we further demonstrated their potential application as cell delivery vehicles for skeletal muscle regeneration.

Nanocellulose-Mediated Electroconductive Self-Healing Hydrogels with High Strength, Plasticity, Viscoelasticity, Stretchability, and Biocompatibility toward Multifunctional Applications.

Conducting polymer hydrogels (CPHs) have emerged as a fascinating class of smart soft matters important for various advanced applications. However, achieving the synergistic characteristics of conductivity, self-healing ability, biocompatibility, viscoelasticity, and high mechanical performance still remains a critical challenge. Here, we develop for the first time a type of multifunctional hybrid CPHs based on a viscoelastic polyvinyl alcohol (PVA)-borax (PB) gel matrix and nanostructured CNFs-PPy (cellulose nanofibers-polypyrrole) complexes that synergizes the biotemplate role of CNFs and the conductive nature of PPy. The CNF-PPy complexes are synthesized through in situ oxidative polymerization of pyrrole on the surface of CNF templates, which are further well-dispersed into the PB matrix to synthesize homogeneous CNF-PPy/PB hybrid hydrogels. The CNF-PPy complexes not only tangle with PVA chains though hydrogen bonds, but also form reversibly cross-linked complexes with borate ions. The multi-complexation between each component leads to the formation of a hierarchical three-dimensional network. The CNF-PPy/PB-3 hydrogel prepared by 2.0 wt % of PVA, 0.4 wt % of borax, and CNF-PPy complexes with a mass ratio of 3.75/1 exhibits the highest viscoelasticity and mechanical strength. Because of a combined reinforcing and conductive network inside the hydrogel, its maximum storage modulus (∼0.1 MPa) and nominal compression stress (∼22 MPa) are 60 and 2240 times higher than those of pure CNF/PB hydrogel, respectively. The CNF-PPy/PB-3 electrode with a conductivity of 3.65 ± 0.08 S m-1 has a maximum specific capacitance of 236.9 F g-1, and its specific capacitance degradation is less than 14% after 1500 cycles. The CNF-PPy/PB hybrid hydrogels also demonstrate attractive characteristics, including high water content (∼94%), low density (∼1.2 g cm-3), excellent biocompatibility, plasticity, pH sensitivity, and rapid self-healing ability without additional external stimuli. Taken together, the combination of such unique properties endows the newly developed CPHs with potential applications in flexible bioelectronics and provides a practical platform to design multifunctional smart soft materials.

Self-Healing and Multistimuli-Responsive Hydrogels Formed via a Cooperation Strategy and Their Application in Detecting Biogenic Amines.

We reported here a new platform of supramolecular hydrogels cross-linked by the cooperation of metal-ligand coordination and hydrophobic interaction. A salicylaldehyde benzoyl hydrazone-terminal poly(ethylene glycol) (2SBH-PEG) was synthesized and formed small micelles in an aqueous environment. Addition of Ni2+ connected the low-molecular-weight 2SBH-PEG into a metallopolymer via metal-ligand coordination and led to micelle aggregation, resulting in gelation due to the enhancement of hydrophobic interaction. The forming hydrogel, Ni-PEGel, exhibited rapid self-healing ability and reversible pH-responsive property. Because of the containing metal coordination bond, it was also sensitive to the strong competing ligands, such as ethylenediaminetetraacetic acid (EDTA)and pyridine. In addition, Ni-PEGel showed colorimetric changes when exposed to biogenic amine (BA) vapor. The color development of Ni-PEGel toward BAs makes it a good candidate in monitoring food spoilage.