A rapid and efficient self-healing thermo-reversible elastomer crosslinked with graphene oxide

2D Nanomaterials Reinforced Organic Coatings for Marine Corrosion Protection: State of the Art, Challenges, and Future Prospectives

2D nanomaterials, with extraordinary physical and chemical characteristics, have long been regarded as promising nanofillers in organic coatings for marine corrosion protection. The past decade has witnessed the high-speed progress of 2D nanomaterial-reinforced organic composite coatings, and plenty of breakthroughs have been achieved as yet. This review covers an in-depth and all-around outline of the up-to-date advances in 2D nanomaterial-modified organic coatings employed for the marine corrosion protection realm. Starting from a brief introduction to 2D nanomaterials, the preparation strategies and properties are illustrated. Subsequently, diverse protection models based on composite coatings for marine corrosion protection are also introduced, including physical barrier, self-healing, as well as cathodic protection, respectively. Furthermore, computational simulations and critical factors on the corrosion protection properties of composite coatings are clarified in detail. Finally, the remaining challenges and prospects for marine corrosion protection based on 2D nanomaterials reinforced organic coatings are highlighted.

Grafting photochromic spiropyran polymer brushes on graphene oxide surfaces via surface-initiated ring-opening metathesis polymerization

A practical "grafting-from" strategy is described to grow photochromic polymer brushes bearing spiropyran (SP) functional groups on graphene oxide (GO) surfaces via surface-initiated ring-opening metathesis polymerization (SI-ROMP). The Grubbs II catalyst was fixed on the GO surface, and the norbornene derivatives functionalized using spiropyran were synthesized from this active site via the ROMP method. The results indicated that the spiropyran-modified polymer brushes were obtained on the GO surface in the form of thin films. The solubility of GO modified by spiropyran polymers (GO-SPs) in organic solvents was significantly improved. The GO-SPs exhibited excellent photochromic properties, including fast coloration/decoloration. The modified GO with an isomeric structure was colored in 90 s under ultraviolet irradiation and decolored in 360 s under white light. The fading kinetic rate in the dark was slow and the kinetic attenuation curve followed bi-exponential decay. The GO-SP composite materials took more than 2 h to return to thermodynamically stable forms. The reversible change in the water contact angle reached 8° after continuous cycling with ultraviolet and visible light. GO-SP maintained its photochromic performance and possessed excellent fatigue resistance after more than six successive UV/light cycles. This work describes a practical strategy for the preparation of photochromic polymer brush modified GO composite materials and extends the applications of GO in photochromic materials.

Liquid Metal Actuators: A Comparative Analysis of Surface Tension Controlled Actuation

Liquid metals, with their unique combination of electrical and mechanical properties, offer great opportunities for actuation based on surface tension modulation. Thanks to the scaling laws of surface tension, which can be electrochemically controlled at low voltages, liquid metal actuators stand out from other soft actuators for their remarkable characteristics such as high contractile strain rates and higher work densities at smaller length scales. This review summarizes the principles of liquid metal actuators and discusses their performance as well as theoretical pathways toward higher performances. The objective is to provide a comparative analysis of the ongoing development of liquid metal actuators. The design principles of the liquid metal actuators are analyzed, including low-level elemental principles (kinematics and electrochemistry), mid-level structural principles (reversibility, integrity, and scalability), and high-level functionalities. A wide range of practical use cases of liquid metal actuators from robotic locomotion and object manipulation to logic and computation is reviewed. From an energy perspective, strategies are compared for coupling the liquid metal actuators with an energy source toward fully untethered robots. The review concludes by offering a roadmap of future research directions of liquid metal actuators.

Self-Healing MXene- and Graphene-Based Composites: Properties and Applications

Today, self-healing graphene- and MXene-based composites have attracted researchers due to the increase in durability as well as the cost reduction in long-time applications. Different studies have focused on designing novel self-healing graphene- and MXene-based composites with enhanced sensitivity, stretchability, and flexibility as well as improved electrical conductivity, healing efficacy, mechanical properties, and energy conversion efficacy. These composites with self-healing properties can be employed in the field of wearable sensors, supercapacitors, anticorrosive coatings, electromagnetic interference shielding, electronic-skin, soft robotics, etc. However, it appears that more explorations are still needed to achieve composites with excellent arbitrary shape adaptability, suitable adhesiveness, ideal durability, high stretchability, immediate self-healing responsibility, and outstanding electromagnetic features. Besides, optimizing reaction/synthesis conditions and finding suitable strategies for functionalization/modification are crucial aspects that should be comprehensively investigated. MXenes and graphene exhibited superior electrochemical properties with abundant surface terminations and great surface area, which are important to evolve biomedical and sensing applications. However, flexibility and stretchability are important criteria that need to be improved for their future applications. Herein, the most recent advancements pertaining to the applications and properties of self-healing graphene- and MXene-based composites are deliberated, focusing on crucial challenges and future perspectives.

Advances and Challenges of Self-Healing Elastomers: A Mini Review

In the last few decades, self-healing polymeric materials have been widely investigated because they can heal the damages spontaneously and thereby prolong their service lifetime. Many ingenious synthetic procedures have been developed for fabricating self-healing polymers with high performance. This mini review provides an impressive summary of the self-healing polymers with fast self-healing speed, which exhibits an irreplaceable role in many intriguing applications, such as flexible electronics. After a brief introduction to the development of self-healing polymers, we divide the development of self-healing polymers into five stages through the perspective of their research priorities at different periods. Subsequently, we elaborated the underlying healing mechanism of polymers, including the self-healing origins, the influencing factors, and direct evidence of healing at nanoscopic level. Following this, recent advance in realizing the fast self-healing speed of polymers through physical and chemical approaches is extensively overviewed. In particular, the methodology for balancing the mechanical strength and healing ability in fast self-healing elastomers is summarized. We hope that it could afford useful information for research people in promoting the further technical development of new strategies and technologies to prepare the high performance self-healing elastomers for advanced applications.

Repulsive segregation of fluoroalkyl side chains turns a cohesive polymer into a mechanically tough, ultrafast self-healable, nonsticky elastomer

Dynamic crosslinking of flexible polymer chains via attractive and reversible interactions is widely employed to obtain autonomously self-healable elastomers. However, this design leads to a trade-off relationship between the strength and self-healing speed of the material, i.e., strong crosslinks provide a mechanically strong elastomer with slow self-healing property. To address this issue, we report an "inversion" concept, in which attractive poly(ethyl acrylate-random-methyl acrylate) chains are dynamically crosslinked via repulsively segregated fluoroalkyl side chains attached along the main chain. The resulting elastomer self-heals rapidly (> 90% within 15 min) via weak but abundant van der Waals interactions among matrix polymers, while the dynamic crosslinking provides high fracture stress (≈2 MPa) and good toughness (≈17 MJ m^-3). The elastomer has a nonsticky surface and selectively self-heals only at the damaged faces due to the surface segregation of the fluoroalkyl chains. Moreover, our elastomer strongly adheres to polytetrafluoroethylene plates (≈60 N cm^-2) via hot pressing.

Advances in nanoenabled 3D matrices for cartilage repair

Cartilage repair strategies are evolving at a fast pace with technology development. Matrices that offer multifaceted functions and a full adaption to the cartilage defect are of pivotal interest. Current cartilage repair strategies face numerous challenges, mostly related to the development of highly biomimetic materials, non-invasive injectable solutions, and adequate degradation rates. These strategies often fail due to feeble mechanical properties, the inability to sustain cell adhesion, growth, and differentiation or by underestimating other players of cartilage degeneration, such as the installed pro-inflammatory microenvironment. The integration of nanomaterials (NMs) into 3D scaffolds, hydrogels and bioinks hold great potential in the improvement of key features of materials that are currently applied in cartilage tissue engineering strategies. NMs offer a high surface to volume ratio and their multiple applications can be explored to enhance cartilage mechanical properties, biocompatibility, cell differentiation, inflammation modulation, infection prevention and even to function as diagnostic tools or as stimuli-responsive cues in these 3D structures. In this review, we have critically reviewed the latest advances in the development of nanoenabled 3D matrices - enhanced by means of NMs - in the context of cartilage regeneration. We have provided a wide perspective of the synergistic effect of combining 3D strategies with NMs, with emphasis on the benefits brought by NMs in achieving functional and enhanced therapeutic outcomes. STATEMENT OF SIGNIFICANCE: Cartilage is one of the most challenging tissues to treat owing to its limited self-regeneration potential. Novel strategies using nanoenabled 3D matrices have emerged from the need to design more efficient solutions for cartilage repair, that take into consideration its unique mechanical properties and can direct specific cell behaviours. Here we aim to provide a comprehensive review on the synergistic effects of 3D matrices nanoenrichment in the context of cartilage regeneration, with emphasis on the heightening brought by nanomaterials in achieving functional and enhanced therapeutic outcomes. We anticipate this review to provide a wide perspective on the past years' research on the field, demonstrating the great potential of these approaches in the treatment and diagnosis of cartilage-related disorders.

Shape memory polymer/graphene nanocomposites: State-of-the-art

Graphene is one of most exceptional type of nanocarbon. It is a two-dimensional, one atom thick, nanosheet of sp2 hybridized carbon atoms. Graphene has been employed as nanofiller for shape memory polymeric nanocomposites due to outstanding electrical conductivity, mechanical strength, flexibility, and thermal stability characteristics. Consequently, graphene nanostructures have been reinforced in the polymer matrices to attain superior structural, physical, and shape recovery properties. This review basically addresses the important class of shape memory polymer (SMP)/graphene nanocomposites. This assessment is revolutionary to portray the scientific development and advancement in the field of polymer and graphene-based shape memory nanocomposites. In SMP/graphene nanocomposites, polymer shape has been fixed at above transition temperature and then converted to memorized shape through desired external stimuli. Presence of graphene has caused fast switching of temporary shape to original shape in polymer/graphene nanocomposites. In this regard, better graphene dispersion, interactions between matrix-nanofiller, and well-matched interface formation leading to high performance stimuli-responsive graphene derived nanocomposites, have been described. Incidentally, the fabrication, properties, actuation ways, and relevance of the SMP/graphene nanocomposite have been discussed here. The potential applications of these materials have been perceived for the aerospace/automotive components, self-healing nanocomposites, textiles, civil engineering, and biomaterials.

Progress and Roadmap for Intelligent Self-Healing Materials in Autonomous Robotics

Robots are increasingly assisting humans in performing various tasks. Like special agents with elite skills, they can venture to distant locations and adverse environments, such as the deep sea and outer space. Micro/nanobots can also act as intrabody agents for healthcare applications. Self-healing materials that can autonomously perform repair functions are useful to address the unpredictability of the environment and the increasing drive toward the autonomous operation. Having self-healable robotic materials can potentially reduce costs, electronic wastes, and improve a robot endowed with such materials longevity. This review aims to serve as a roadmap driven by past advances and inspire future cross-disciplinary research in robotic materials and electronics. By first charting the history of self-healing materials, new avenues are provided to classify the various self-healing materials proposed over several decades. The materials and strategies for self-healing in robotics and stretchable electronics are also reviewed and discussed. It is believed that this article encourages further innovation in this exciting and emerging branch in robotics interfacing with material science and electronics.

Bioinspired modified graphene oxide/polyurethane composites with rapid self-healing performance and excellent mechanical properties

Self-healing efficiency and mechanical strength are always a pair of mechanical contradictions of a polymer. Herein, a series of novel mussel-inspired modified graphene oxide/polyurethane composites were successfully fabricated via rational molecular design and introducing hyperbranched polymer-modified graphene oxide. The composites exhibit outstanding self-healing performances with a self-healing efficiency of 87.9%. Especially, their self-healing properties possess exceptional water-insensitivity, which presents a high self-healing efficiency of 92.5% under 60 °C water for 2 h and 74.6% under 25 °C water for 6 h. Furthermore, the tensile strength of the composites increased by 107.7% with a high strain of 2170%. In addition, the composites show a remarkable recovery capability of 76.3% and 83.7% under tensile and compression loading, respectively, after 20 cycles. This strategy shows prominent application potential in high-performance solid propellants, protective coating, electronic skin, soft sensors and other water-insensitive devices.

We successfully modified graphene oxide with amino-terminated hyperbranched polyamide(MGO), and obtained novel mussel-inspired MGO/polyurethane composites with outstanding self-healing and mechanical performances via rational molecular design.

Mussel-inspired and aromatic disulfide-mediated polyurea-urethane with rapid self-healing performance and water-resistance

Although lots of methods have been developed for self-healing materials, it remains a formidable challenge to achieve a thermosetting material with water-insensitive and self-healing properties at room temperature. Nature always provides intelligent strategies for developing advanced materials with superior properties. Herein, a novel self-healable polyurea-urethane was rationally designed by combining mussel adhesive protein-mimetic structure and dynamic aromatic disulfide bonds. It achieves high self-healing efficiency of 98.4% at room temperature for only 6 h and 90% at 60℃ for only 30 min without any external stimuli. Impressively, this self-healing capability possesses exceptional water-resistance, which presents high self-healing efficiency of 98.1% for 2 h and 82.1% for 6 h in 60℃ and 25℃ water, respectively. Besides, the designed polyurea-urethane exhibits excellent mechanical properties such as high elongation at break of 2400%, notch-insensitive stretching elongation of 1500% and notable recovery capability. This strategy shows promising application potential in solid propellants, protective coating, electronic skin, soft sensors and other water-resistant devices.

The nonequilibrium behaviors of covalent adaptable network polymers during the topology transition

Vitrimers with bond exchange reactions (BERs) are a class of covalent adaptable network (CAN) polymers at the forefront of recent polymer research. They exhibit malleable and self-healable behaviors and combine the advantages of easy processability of thermoplastics and excellent mechanical properties of thermosets. For thermally sensitive vitrimers, a molecular topology melting/frozen transition is triggered when the BERs are activated to rearrange the network architecture. Notable volume expansion and stress relaxation are accompanied, which can be used to identify the BER activation temperature and rate as well as to determine the malleability and interfacial welding kinetics of vitrimers. Existing works on vitrimers reveal the rate-dependent behaviors of the nonequilibrium network during the topology transition. However, it remains unclear what the quantitative relationship with heating rate is, and how it will affect the macroscopic stress relaxation. In this paper, we study the responses of an epoxy-based vitrimer subjected to a change in temperature and mechanical loading during the topology transition. Using thermal expansion tests, the thermal strain evolution is shown to depend on the temperature-changing rate, which reveals the nonequilibrium states with rate-dependent structural relaxation. The influences of structural relaxation on the stress relaxation behaviors are examined in both uniaxial tension and compression modes. Assisted by a theoretical model, the study reveals how to tune the material and thermal-temporal conditions to promote the contribution of BERs during the reprocessing of vitrimers.

An Extremely Stretchable and Self-Healable Supramolecular Polymer Network

The construction of a single polymer network with extreme stretchability, relatively high mechanical strength, and fast and facile autonomous room-temperature self-healing capability still remains a challenge. Herein, supramolecular polymer networks are fabricated by synergistically incorporating metal-ligand and hydrogen bonds in poly(propylene glycol) (PPG). The representative specimen, PPG-Im-MDA-1.5-0.25-Cu, shows a combination of notable mechanical properties involving an extreme stretching ratio of 346 ± 14× and a Young's modulus of 2.10 ± 0.14 MPa, which are superior to the previously reported extremely stretchable polymeric materials. Notably, the destroyed specimen can fully recover mechanical performances within 1 h. The tunability of mechanical properties and self-healing capability has been actualized by merely tailoring the content of a chain extender. The application of the as-prepared supramolecular PPG network in constructing a flexible and self-healable conductor has been demonstrated. This strategy provides some insights for preparing extremely stretchable and self-healable polymeric materials.

Self-healing solid polymer electrolyte based on imine bonds for high safety and stable lithium metal batteries

Due to their low flammability, good dimensional stability and chemical stability, solid polymer electrolytes are currently attracting extensive interest for building lithium metal batteries. But severe safety issues such as cracks or breakage, resulting in short circuits will prevent their widespread application. Here, we report a new design of self-healing solid polymer electrolyte (ShSPE) based on imine bonds, fabricated from varying amounts of polyoxyethylenebis(amine) and terephthalaldehyde through a simple Schiff base reaction. Moreover, adding diglycidyl ether of bisphenol A improves the flexibility and high stretchability of the polymer electrolyte. The polymer networks exhibit good thermal stability and excellent self-healing characteristics. The ShSPE with the highest NH2-PEG-NH2 content (ShSPE-3) has an improved lithium ion transference number of 0.39, and exhibits an electrochemical stability up to 4.5 V vs. Li/Li+. ShSPE-3 shows the highest ionic conductivity of 1.67 × 10-4 S cm-1 at 60 °C. Besides, the interfacial stability of ShSPE-3 is promoted and the electrolyte membrane exhibits good cycling performance with LiFePO4, and the LiFePO4/Li cell exhibits an initial discharge capacity of 141.3 mA h g -1. These results suggest that self-healing solid polymer electrolytes are promising candidates for high safety and stable lithium metal batteries.

Mechanically tough yet self-healing transparent conductive elastomers obtained using a synergic dual cross-linking strategy

A tough yet self-healing transparent conductive elastomer was synthesized by introducing Al(iii)-carboxyl complexes into photo-polymerizable deep eutectic solvent (PDES).

Recent Advances in High-Performance Microbatteries: Construction, Application, and Perspective

High-performance miniaturized energy storage devices have developed rapidly in recent years. Different from conventional energy storage devices, microbatteries assume the main responsibility for micropower supply, functionalization, and characterization platforms. Evolving from the essential goals for battery design of high power density, high energy density, and long lifetime, further practical demands for microbatteries (MBs) have been raised for the microfabrication technique and device design. Numerous studies have generally focused on specific aspects of the microelectrode structures or certain microfabrication techniques, while the connection from techniques to functional applications is rarely involved. This Review generally fills such blanks from an application-oriented perspective. First, some basic micromachining techniques with different compatible features are summarized. Afterward, device designs including diversified battery reaction types, configuration, and assembly are highlighted, as well as microbatteries serving powering resources or further complicated functional systems. Finally, through providing the overall design concept based on requirements in application, this Review offers innovative insights for further development of microbatteries.

Multifunctional Poly(vinyl alcohol) Nanocomposite Organohydrogel for Flexible Strain and Temperature Sensor

Hydrogels are important for stretchable and wearable multifunctional sensors, but their application is limited by their low mechanical strength and poor long-term stability. Herein, a conductive organohydrogel with a 3D honeycomb structure was prepared by integrating carbon nanotubes (CNTs) and carbon black (CB) into a poly(vinyl alcohol)/glycerol (PVA/Gly) organohydrogel. Such a nanocomposite organohydrogel is built on a physical cross-linking network formed by the hydrogen bonds among PVA, glycerol, and water. CNTs and CB had an add-in synergistic impact on the mechanical and electrical performances of the PVA/Gly organohydrogel because of the distinct aspect ratios and geometric shapes. The prepared organohydrogel integrated with a tensile strength of 4.8 MPa, a toughness of 15.93 MJ m^-3, and flexibility with an elongation at break up to 640%. The organohydrogels also showed good antifreezing feature, long-term moisture retention, self-healing, and thermoplasticity. Sensors designed from these organohydrogels displayed high stretching sensitivity to tensile strain and temperature, with a gauge factor of 2.1 within a relatively broad strain range (up to ∼600% strain), a temperature coefficient of resistance of -0.935%·°C^-1, and long-term durability. The sensors could detect full-range human physiological signals and respond to the change in temperature, which are highly desired for multifunctional wearable electronic devices.

High-Strength, Fast Self-Healing, Aging-Insensitive Elastomers with Shape Memory Effect

Self-healing materials, which can autonomously repair damages like living organisms, have undergone extensive development in the last decades. The bottleneck problems for their practical applications are long healing time, inferior mechanical strength, and aging sensitivity. Here, a new class of polymeric materials rationally grafted with oligo poly(ethylene glycol) is reported. The network based on the formation of multiple weak hydrogen bonds endows the elastomers with good comprehensive performances including high stretchability (725%), high strength (4.20 MPa), and fast self-healing process (40 s). Benefiting from characteristic multiple weak hydrogen bond interactions, the fractured elastomers maintain high healing efficiency even after aging for 192 h, showing an unusual aging insensitivity. A Sudoku structural device based on the developed elastomer shows shape memory effect, providing a new avenue for fabricating novel smart devices with complicated shapes.

Direct Ink Writing Based 4D Printing of Materials and Their Applications

4D printing has attracted academic interest in the recent years because it endows static printed structures with dynamic properties with the change of time. The shapes, functionalities, or properties of the 4D printed objects could alter under various stimuli such as heat, light, electric, and magnetic field. Briefly, 4D printing is the development of 3D printing with the fourth dimension of time. Among the fabrication techniques that have been employed for 4D printing, the direct ink writing technique shows superiority due to its open source for various types of materials. Herein, the state-of-the-art achievements about the topic of 4D printing through direct ink writing are summarized. The types of materials, printing strategies, actuated methods, and their potential applications are discussed in detail. To date, most efforts have been devoted to shape-shifting materials, including shape memory polymers, hydrogels, and liquid crystal elastomers, showing great prospects in areas ranging from the biomedical field to robotics. Finally, the current challenges and outlook toward 4D printing based on direct ink writing are also pointed out to leave open a significant space for future innovation.

Preparation, characterization and properties of intrinsic self-healing elastomers

Significant advances have been made in the development of self-healing synthetic polymer materials in recent years. This review article discusses the recent progress in preparation, characterization and properties of different kinds of intrinsic self-healing elastomers based on reversible covalent bonds and dynamic supramolecular chemistry. Healing conditions, mechanical property recovery and healing efficiency are the main discussion topics. Potential applications, challenges and future prospects in self-healing elastomer fields are also discussed in the last part of this review.

Universally autonomous self-healing elastomer with high stretchability

Developing autonomous self-healing materials for applications in harsh conditions is challenging because the reconstruction of interaction in material for self-healing will experience significant resistance and fail. Herein, a universally self-healing and highly stretchable supramolecular elastomer is designed by synergistically incorporating multi-strength H-bonds and disulfide metathesis in polydimethylsiloxane polymers. The resultant elastomer exhibits high stretchability for both unnotched (14000%) and notched (1300%) samples. It achieves fast autonomous self-healing under universal conditions, including at room temperature (10 min for healing), ultralow temperature (-40 °C), underwater (93% healing efficiency), supercooled high-concentrated saltwater (30% NaCl solution at -10 °C, 89% efficiency), and strong acid/alkali environment (pH = 0 or 14, 88% or 84% efficiency). These properties are attributable to synergistic interaction of the dynamic strong and weak H-bonds and stronger disulfide bonds. A self-healing and stretchable conducting device built with the developed elastomer is demonstrated, thereby providing a direction for future e-skin applications.

Advances in intrinsic self-healing polyurethanes and related composites

Fascinating and challenging, the development of repairable materials with long-lasting, sustainable and high-performance properties is a key-parameter to provide new advanced materials. To date, the concept of self-healing includes capsule-based healing systems, vascular healing systems, and intrinsic healing systems. Polyurethanes have emerged as a promising class of polymeric materials in this context due to their ease of synthesis and their outstanding properties. This review thereby focuses on the current research and developments in intrinsic self-healing polyurethanes and related composites. The chronological development of such advanced materials as well as the different strategies employed to confer living-like healing properties are discussed. Particular attention will be paid on chemical reactions utilized for self-healing purposes. Potential applications, challenges and future prospects in self-healing polyurethane fields are also provided.

Constructing Electrically and Mechanically Self-Healing Elastomers by Hydrogen Bonded Intermolecular Network

One key limitation of artificial skin-like materials is the shortened service life caused by mechanical damages during practical applications. The ability to self-heal can effectively extend the material service life, reduce the maintenance cost, and ensure safety. Therefore, it is important and necessary to fabricate materials with simultaneously mechanical and electrical self-healing behavior in a facile and convenient way. Herein, we report a stretchable and conductive self-healing elastomer based on intermolecular networks between poly(acrylic acid) (PAA) and reduced graphene oxide (rGO) through a facile and convenient postreduction and one-pot method. The introduction of rGO provides the PAA-GO elastomers with good mechanical stability and electrical properties. Moreover, this material exhibited both electrical and mechanical self-healing properties. After cutting, the elastomers self-healed quickly (∼30 s) and efficiently (∼95%) at room temperature. The elastomers were accurate and reliable in detecting external strain even after healing. The elastomers were further applied for strain sensors, which were attached directly to human skin to monitor external movements, including finger bending and wrist twisting.

Free-Form and Deformable Energy Storage as a Forerunner to Next-Generation Smart Electronics

Planar and rigid conventional electronics are intrinsically incompatible with curvilinear and deformable devices. The recent development of organic and inorganic flexible and stretchable electronics enables the production of various applications, such as soft robots, flexible displays, wearable electronics, electronic skins, bendable phones, and implantable medical devices. To power these devices, persistent efforts have thus been expended to develop a flexible energy storage system that can be ideally deformed while maintaining its electrochemical performance. In this review, the enabling technologies of the electrochemical and mechanical performances of flexible devices are summarized. The investigations demonstrate the improvement of electrochemical performance via the adoption of new materials and alternative reactions. Moreover, the strategies used to develop novel materials and distinct design configurations are introduced in the following sections.

Improvement of Mechanical and Self-Healing Properties for Polymethacrylate Derivatives Containing Maleimide Modified Graphene Oxide

In this paper, a self-healable nanocomposite based on the Diels-Alder reaction is developed. A graphene-based nanofiller is introduced to improve the self-healing efficiency, as well as the mechanical properties of the nanocomposite. Graphene oxide (GO) is modified with maleimide functional groups, and the maleimide-modified GO (mGO) enhanced the compatibility of the polymer matrix and nanofiller. The tensile strength of the nanocomposite containing 0.030 wt% mGO is improved by 172%, compared to that of a polymer film incorporating both furan-functionalized polymer and bismaleimide without any nanofiller. Moreover, maleimide groups of the surface on mGO participate in the Diels-Alder reaction, which improves the self-healing efficiency. The mechanical and self-healing properties are significantly improved by using a small amount of mGO.

Design of Rubber Composites with Autonomous Self-Healing Capability

The development of self-healing rubbers is currently under investigation as a strategy to promote their reuse and, hence, reduce their waste. However, autonomous, multicycle self-healing rubbers with good mechanical properties have so far proven difficult to achieve. Here, mechanically robust composites based on epoxidized natural rubber (ENR) and thermally reduced graphene oxide (TRGO) were successfully designed and prepared with a high healing efficiency of up to 85% at room temperature without applying external stimuli. The incorporation of TRGO not only improves the mechanical performance in more than 100% in relation to pristine ENR but also promotes the hydrogen bonding interactions with the rubber. This leads to a homogenous dispersion of TRGO within the ENR matrix, which further increases its self-healing capability.

Hydrogen bonding derived self-healing polymer composites reinforced with amidation carbon fibers

Self-healing polymer materials (SHPM) have aroused great interests in recent years. Ideal SHPM should have not only simple operations, but also high elongations at break, tensile strain and self-healing properties at room temperature. Herein, the amidated carbon fibers (CFs) reinforced self-healing polymer composites were designed by hydrogen bonding interaction between functionalized CFs and hyperbranched polymers. The amidated CFs were prepared by transformation of hydroxyl to acylamino through a one-step amidation. By introducing amidated CFs, amidated CFs self-healing polymer composites (called AD-CF) exhibited many desirable characteristics compared to pure polymer composites, such as a better elasticity, lower healing temperatures, and higher self-healing efficiencies. The stress-strain test was selected to carefully study the self-healing property of the AD-CF. The observed same recovery condition, i.e. without any mechanical breakdown after the 10 sequential cycles of cutting and healing indicates no aging of the AD-CF. The ability of AD-CF to exhibit a soft state and rapid self-healing at room temperature makes it possible for much wider applications.

Imine and metal-ligand dynamic bonds in soft polymers for autonomous self-healing capacitive-based pressure sensors

In this work, a facile and simple yet effective method to generate intrinsic autonomous self-healing polymers was developed, leading to new materials that can be easily fine-tuned both mechanically and chemically. The new materials were designed to incorporate two dynamic and reversible types of chemical bonds, namely dynamic imine and metal-coordinating bonds, to enable autonomous self-healing, controlled degradability and ultra-high tunable stretchability (up to 800% strain) based on the ratio of metal to ligand incorporated. Through an easy condensation reaction, imine bonds are generated at the end-termini of a short siloxane chain. The new dynamic system was characterized by a variety of techniques, including tensile-pull strain testing, atomic force microscopy and UV-Vis spectroscopy, which showed that the highly dynamic imine bonds, combined with coordination with Fe²⁺ ions, allow for the material to regenerate 88% of its mechanical strength after physical damage. The materials were also controlled to be degraded in mild acidic conditions. Lastly, application in self-healable electronics was demonstrated through the fabrication of a capacitive-based pressure sensor, which shows good sensitivity and dynamic response (∼0.33 kPa^-1) before and after healing.

Room temperature readily self-healing polymer via rationally designing molecular chain and crosslinking bond for flexible electrical sensor

Mechanically tough polymers with excellent room temperature self-healing capacity have aroused strong interest in soft electronics, electronic skins and flexible energy storage devices. However, achieving such polymers remains a challenge due to tardy diffusion dynamics. Herein, a robust and readily self-healing polymer, which is synthesized by one-pot polymerization among 2,4'-tolylene diisocyanate, isophorone diisocyanate, and poly(oxy-1,4-butanediyl), is achieved through reasonably tuning the hardness of the molecular segment and the strength of the dynamic crosslinking bond. The poly(oxy-1,4-butanediyl) that act as a soft segment can effectively avoid the microphase separation, enabling rapid chain mobility of the polymer at the room temperature. Furthermore, the dual H-bonding from 2,4'-tolylene diisocyanate segment acting as a relatively strong crosslinking bond contributes to high mechanical strength, while the weaker single H-bonding from isophorone diisocyanate segment can efficiently dissipate strain energy by bond rupture, endowing the polymer with rapid room temperature self-healing ability. Featuring state-of-the-art of robust stress strength (≈1.3 MPa), high self-healing efficiency (97% within 6 h), and large tensile strain (≈2100%), the resulting polymers are used for the fabrication of stretchable and self-healable electrical sensor, which can be employed to monitor a variety of physiological activities in real time. The described strategy is promising and universal for healable materials, displaying great potential for developing soft electronics.

Self-Healing and Recyclable Hydrogels Reinforced with in Situ-Formed Organic Nanofibrils Exhibit Simultaneously Enhanced Mechanical Strength and Stretchability

In this work, self-healing and recyclable polymer hydrogels with simultaneously enhanced mechanical strength and stretchability are fabricated through the complexation of poly(acrylic acid) (PAA) with complexes of branched poly(ethylenimine) and 1-pyrenybutyric acid (PEI-PYA) to generate PAA/PEI-PYA complexes, which are further molded, dried, and rehydrated. The in situ-formed PYA nanofibrils with aggregated structures during the complexation process enable the simultaneous enhancement of the tensile strength and stretchability of the PAA/PEI-PYA hydrogels. The PAA/PEI-PYA hydrogels have a tensile strength of 1.13 ± 0.04 MPa and stretchability of 2970 ± 154%, which are 2.2 and 2.1 times higher than those of the PAA/PEI hydrogels. Meanwhile, the damaged PAA/PEI-PYA hydrogels can be efficiently healed or recycled at room temperature to regain their original mechanical strength and integrity because the dynamic nature of hydrogen-bonding and electrostatic interactions among PAA, PEI, and PYA endows the hydrogels with excellent healing and recycling capacity. This strategy of using aggregated nanofibrils to simultaneously enhance the tensile strength and stretchability of hydrogels can be extended to PAA/PEI hydrogels reinforced with aggregated nanofibrils of 9-anthracenecarboxylic acid and N,N'-di(propanoic acid)-perylene-3,4,9,10-tetracarboxylic diimide, demonstrating its generality for fabricating hydrogels with enhanced mechanical properties.

Transparent and Scratch-Resistant Antifogging Coatings with Rapid Self-Healing Capability

Typical antifogging coatings based on hydrophilic polymers are soft and susceptible to mechanical damage. In this paper, an antifogging coating that is both scratch-resistant and self-healing is fabricated by copolymerizing sulfobetaine methacrylate and 2-hydroxyethyl methacrylate in the presence of sulfobetaine-modified silica nanoparticles in one pot. The coating is highly efficient in preventing fog formation at the surface and reducing ice adhesion, and is resistant to fouling by oil and protein, due to the strong hydration ability of the zwitterionic moieties. The composite coating is resistant to scratching and abrasion under normal use conditions to maintain its transparency due to increased hardness by the filled silica nanoparticles and is able to heal completely within several minutes severe scratches and cuts inflicted in harsh conditions, owing to the water-assisted reversibility of the electrostatic and hydrogen-bonding interactions holding together the polymer components and the silica nanoparticles. The multiple desirable properties demonstrated and the simple fabrication process of the coating offers great potential in many practical applications.

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

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

Advances in self-healing supramolecular soft materials and nanocomposites

The ability to rationally tune and add new end-groups in polymers can lead to transformative advances in emerging self-healing materials. Self-healing networks manipulated by supramolecular strategies such as hydrogen bonding and metal coordination have received significant attention in recent years because of their ability to extend materials lifetime, improve safety and ensure sustainability. This review describes the recent advancements in supramolecular polymers self-healing networks based on hydrogen bonding, metal-containing polymers and their nanocomposites. Collectively, the aim of this review is to provide a panoramic overview of the conceptual framework for the interesting nexus between hydrogen bonding and metal-ligand interactions for enabling supramolecular self-healing soft materials networks and nanocomposites. In addition, insights on the current challenges and future perspectives of this field to propel the development of self-healing materials will be provided.

Plasmonic Ti3C2Tx MXene Enables Highly Efficient Photothermal Conversion for Healable and Transparent Wearable Device

Skin-mountable and transparent devices are highly desired for next-generation electronic applications but are susceptible to unexpected ruptures or undesired scratches, which can drastically reduce the device lifetime. Developing wearable and transparent materials with healable function that can recover their original functionality after mechanical damage under mild and noninvasive repairing operation is thus imperative. Herein, we demonstrate that the incorporation of ultrasmall quantities of plasmonic silver nanoparticle (AgNP)@MXene nanosheet hybrids to serve as photothermal fillers in waterborne elastic polyurethane enables high transparency as well as effective light-triggered healing capabilities for wearable composite coatings. The AgNP@MXene hybrid functions as a highly effective photon captor, energy transformer, and molecular heater due to the amalgamation of (1) ultrahigh photothermal conversion efficiency, high thermal conductivity, and structural properties of MXene, (2) the outstanding plasmonic effect of AgNPs, and (3) the synergistic effects from their hybrids. The resulting wearable composite coating with ultralow loading of plasmonic AgNP@MXene hybrids (0.08 wt % or 0.024 vol %) can produce a significant temperature increase of ∼111 ± 2.6 °C after the application of 600 mW cm^-2 light irradiation for 5 min, while maintaining a high optical transmittance of ∼83% at a thickness of ∼60 μm. This local temperature increase can rapidly heal the mechanical damage to the composite coating, with a healing efficiency above 97%.

Overviews of Biomimetic Medical Materials

This chapter describes the overviews of biomimetic medical materials which covers innovation and significance of terminology, diverse fabrication methods, and technologies ranges from nanotechnology to 3D printing to develop biomimetic materials for medical applications. It also depicts specific fundamental characteristics required for a material to be a model biomimetic material for particular medical application. It basically outlines current statuses of biomimetic medical materials used for tissue engineering and regenerative medicine, drug/protein delivery, bioimaging, biosensing, and 3D bioprinting technology. It also illustrates the effect of functionalization of a material through chemical and biological approaches towards different applications. Not only, the key properties and potential applications of the biomimetic materials, but it also explains the protection and utilization of intellectual property associated with biomedical materials.

An interfacially polymerized self-healing organo/hydro copolymer with shape memory

Organo/hydro copolymer materials have recently received significant attention in the fields of energy, environment and healthcare. Herein, we report the fabrication of a robust organo/hydro copolymer with rapid self-healing and shape memory by emulsion interfacial polymerization. The emulsion interfacial polymerization allowed the formation of a crosslinked organo/hydro copolymer with hydrogen-bonded networks, significantly enhancing the mechanical properties; the proposed organo/hydro copolymer substantially outperformed most of the synthetic self-healing polymers based on hydrogen bonding interactions. We showed that the interfacially polymerized organo/hydro copolymer exhibited good self-healing capacity, i.e. achieved self-healing in less than 2 h, with a healing efficiency of 95.6%. Moreover, it presented shape memory, with a complete shape memory time less than 5 min.

Tough Self-Healing Elastomers Based on the Host-Guest Interaction of Polycyclodextrin

Inspired by animal muscles, we developed a kind of tough elastomers combining high strength and high stretchability with autonomous self-healing capability. A key structural feature is the construction of a double network (DN) connected by the hydrogen bond and host-guest interactions. The first network is the classic elastomer polyacrylate matrix cross-linked by strong hydrogen bonding. The second network is formed through the host-guest interactions between polycyclodextrin and the adamantane (Ad) groups on the side of the polyacrylate chain. Supramolecular interactions between two networks make them miscible and interpenetrate in the molecular level and then can share the load as the sample was stretched. The host-guest interactions act not only as sacrificial bonds for energy dissipation but also as self-healing driving forces. The tensile strength of the DN elastomer reaches about 6.7 MPa and the strain is as high as about 950%. The DN elastomer can be easy to repair by touching the damaged surface together at ambient conditions when broken or cut. The recovered tensile strength can reach over 4.5 MPa, which is better than the most pristine strength of existing spontaneous self-healing elastomers.

Highly Stretchable Room-Temperature Self-Healing Conductors Based on Wrinkled Graphene Films for Flexible Electronics

Flexible conductors are emerging soft materials for diverse electrical applications. However, it still remains a great challenge to fabricate high-performance soft conductors that are highly conductive, largely stretchable, and rapid room-temperature self-healable. Here, we design and fabricate flexible conductive bilayer composite films composed of healable elastomeric substrates and wrinkled graphenes. The elastomeric substrates, obtained by a facile bulk copolymerization of N-isopropylacrylamide and 2-methoxyethyl acrylate, show fast room-temperature self-healing efficiency of up to 96%, imparted by the reversible hydrogen bonds. Importantly, the substrates also display strong interfacial adhesion crucial to the formation of stable bilayer composite films based on a prestrain route. The synergy between self-healing of the substrates and wrinkled structures of graphene is endowed to the composite films for mechanical and electrical healing. By adjusting the prestrain ratio of the substrates, the composite films could display the tunable stretchability, conductivity, and self-healing. The optimal bilayer composite film exhibits a high conductivity of 126 S cm^-1, a large stretchability of 300%, and rapid room-temperature self-healing. Moreover, it is demonstrated that the composite films are strain-sensitive and can be used as strain sensors to monitor stretching deformation and human motion. These prominent demonstrations suggest a great potential of the bilayer composite films in next-generation wearable electronics.