A Game Changer: Functional Nano/Micromaterials for Smart Rechargeable Batteries

The imperative to electrify the transport sector in the past few decades has put millions of electric vehicles on the road worldwide with an extended mile range from critical technological breakthroughs in developing the rechargeable energy storage systems, which also covers electronic devices and smart grid applications. However, the available energy density of prevailing systems in the market (i.e., batteries) is reaching its boundaries due to the limited choice of electrochemical reactions that necessarily depend on the thermodynamics and kinetics of the components (e.g., cathode, anode, electrolyte, separator, and current collectors). Reaching the high energy density of batteries exploits new redox chemistry such as sensitive metal anodes, insulating and highly dissolving sulfur cathodes, etc., thus requiring novel designs of various multiscale functional materials to address the corresponding issues. Here, the recent achievements on the designs of smart functional materials for emerging problems in the whole range of systems are discussed: i) interfacial control/kinetic regulation of Li–S battery; ii) self‐healing‐driven structural stability in the electrode and electrolyte; iii) ion‐sieving functional membranes for selective scavenging capability; and iv) functional materials to ensure battery safety.     

Flexible and Stretchable Smart Display: Materials,Fabrication, Device Design,and System Integration

Recent technological advances in nanomaterials have driven the development of high‐performance light‐emitting devices with flexible and stretchable form factors. Deformability in such devices is mainly achieved by replacing the rigid materials in the device components with flexible nanomaterials and their assemblies (e.g., carbon nanotubes, silver nanowires, graphene, and quantum dots) or with intrinsically soft materials and their composites (e.g., polymers and elastomers). Downscaling the dimensions of the functional materials to the nanometer range dramatically decreases their flexural rigidity, and production of polymer/elastomer composites with functional nanomaterials provides light‐emitting devices with flexibility and stretchability. Furthermore, monolithic integration of these light‐emitting devices with deformable sensors furnishes the resulting display with various smart functions such as force/capacitive touch‐based data input, personalized health monitoring, and interactive human–machine interfacing. These ultrathin, lightweight, and deformable smart optoelectronic devices have attracted widespread interest from materials scientists and device engineers. Here, a comprehensive review of recent progress concerning these flexible and stretchable smart displays is presented with a focus on materials development, fabrication techniques, and device designs. Brief overviews of an integrated system of advanced smart displays and cutting‐edge wearable sensors are also presented, and, to conclude, a discussion of the future research outlook is given.     

Self‐Healing Actuating Adhesive Based on Polyelectrolyte Multilayers

Creating actuators capable of mechanical motion in response to external stimuli is a key for design and preparation of smart materials. The lifetime of such materials is limited by their eventual wear. Here, self‐healable and adhesive actuating materials are demonstrated by taking advantage of the solvent responsive of weak polyelectrolyte multilayers consisting of branched poly(ethylenimine)/poly(acrylic acid) (BPEI/PAA). BPEI/PAA multilayers are dehydrated and contract upon contact with organic solvent and become sticky when wetted with water. By constructing an asymmetric heterostructure consisting of a responsive BPEI/PAA multilayer block and a nonresponsive component through either layer‐by‐layer assembly or the paste‐to‐curl process, smart films that actuate upon exposure to alcohol are realized. The curl degree, defined as degrees from horizontal that the actuated material reaches, can be as high as ≈228.9°. With evaporation of the ethanol, the curled film returns to its initial state, and water triggers fast self‐healing extends the actuator's lifetime. Meanwhile, the adhesive nature of the wet material allows it to be attached to various substrates for possible combination with hydrophobic functional surfaces and/or applications in biological environments. This self‐healable adhesive for controlled fast actuation represents a considerable advance in polyelectrolyte multilayers for design and fabrication of robust smart advanced materials.     

Smart Passivation Materials with a Liquid Metal Microcapsule as Self‐Healing Conductors for Sustainable and Flexible Perovskite Solar Cells

The electrical repair of device circuits has been considered a main issue in the area of electronic packaging. Demand for self‐healing conductors as cost‐effective and promising materials for prolonging the durability of devices has increased. Recently, diverse designs of self‐healing and deformable circuits have been introduced in virtue of their high stretchability and conductivity. However, encapsulating a liquid metal with a polymer in a micro‐size container is essential for real applications. In this work, core–shell‐structured liquid metal microcapsules (LMCs, diameter = 2–10 µm) are synthesized via in situ polymerization of urea‐formaldehyde onto liquid metal colloids. Passivation films comprising LMC/polymer composites are simply prepared using phase separation between the capsules and the liquid prepolymer. Capsules ruptured by cutting or pressing release and transport liquid metal to the damaged sites, leading to effective recovery of electrical pathways. Such self‐healing of the metal contacts shows the high potential of LMCs for smart passivation of electronic devices. As an example, flexible perovskite solar cells incorporated with the passivation film demonstrate perfect recovery of the photovoltaic parameters immediately after cutting the metal contact, exhibiting a power conversion efficiency (PCE) retention of 99% relative to the initial value (PCE = 15.07%).     

Highly Efficient and Water‐Insensitive Self‐Healing Elastomer for Wet and Underwater Electronics

Integrating self‐healing capabilities into soft electronic devices increases their durability and long‐term reliability. Although some advances have been made, the use of self‐healing electronics in wet and/or (under)water environments has proven to be quite challenging, and has not yet been fully realized. Herein, a new highly water insensitive self‐healing elastomer with high stretchability and mechanical strength that can reach 1100% and ≈6.5 MPa, respectively, is reported. The elastomer exhibits a high (>80%) self‐healing efficiency (after ≈ 24 h) in high humidity and/or different (under)water conditions without the assistance of an external physical and/or chemical triggers. Soft electronic devices made from this elastomer are shown to be highly robust and able to recover their electrical properties after damages in both ambient and aqueous conditions. Moreover, once operated in extreme wet or underwater conditions (e.g., salty sea water), the self‐healing capability leads to the elimination of significant electrical leakage that would be caused by structural damages. This highly efficient self‐healing elastomer can help extend the use of soft electronics outside of the laboratory and allow a wide variety of wet and submarine applications.     

Electrical and Mechanical Self‐Healing in High‐Performance Dielectric Elastomer Actuator Materials

Dielectric elastomers are of interest for actuator applications due to their large actuation strain, high bandwidth, high energy density, and their flexible nature. If future dielectric elastomers are to be used reliably in applications that include soft robotics, medical devices, artificial muscles, and electronic skins, there is a need to design devices that are tolerant to electrical and mechanical damage. In this paper, the first report of self‐healing of both electrical breakdown and mechanical damage in dielectric actuators using a thermoplastic methyl thioglycolate–modified styrene–butadiene–styrene (MGSBS) elastomer is provided. The self‐healing functions are examined from the material to device level by detailed examination of the healing process, and characterization of electrical properties and actuator response before and after healing. It is demonstrated that after dielectric breakdown, the initial dielectric strength can be recovered by up to 67%, and after mechanical damage, a 39% recovery can be achieved with no degradation of the strain–voltage response of the actuators. The elastomer can also heal a combination of mechanical and electrical failures. This work provides a route to create robust and damage tolerant dielectric elastomers for soft robotic and other applications related to actuator and energy‐harvesting systems.     

2D Janus Hybrid Materials of Polymer‐Grafted Carbon Nanotube/Graphene Oxide Thin Film as Flexible,Miniature Electric Carpet

Ultrathin, freestanding polymer hybrid film with macroscopic sizes and molecular thicknesses have received significant interest due to their applications as functional devices, microsensors or nanoactuators. Herein, a 2D Janus hybrid of polymer‐grafted carbon nanotubes/graphene oxide (CNTs/GO) thin film is fabricated using microcontact printed CNTs/GO as photo active surface to grow polymer brushes by self‐initiated photografting and photopolymerization selectively from one side of CNTs/GO film. This achieved 2D Janus hybrid materials with grafted polymer layer as insulative carpet and supported CNTs/GO thin film as conductive element have the potential application as flexible and miniature electric carpet for heating micro‐/nano devices locally.     

Bioinspired Graphene Actuators Prepared by Unilateral UV Irradiation of Graphene Oxide Papers

Inspired by natural autonomous systems that demonstrate controllable shape, appearance, and actuation under external stimuli, a facile preparation of moisture responsive graphene‐based smart actuators by unilateral UV irradiation of graphene oxide (GO) papers is reported. UV irradiation of GO is found to be an effective protocol to trigger the reduction of GO; however, due to the limited light transmittance and thermal relaxation, thick GO paper cannot be fully reduced. Consequently, by tuning the photoreduction gradient, anisotropic GO/reduced GO (RGO) bilayer structure can be easily prepared toward actuation application. To get better control over the responsive properties, GO/RGO bilayer paper with a certain curvature and RGO patterns are successfully prepared for actuator design. As representative examples, smart humidity‐driven graphene actuators that mimic the cilia of respiratory tract and tendril climber plant are successfully developed for controllable objects transport.     

Biomimetic Chitin–Silk Hybrids: An Optically Transparent Structural Platform for Wearable Devices and Advanced Electronics

The cuticles of insects and marine crustaceans are fascinating models for man‐made advanced functional composites. The excellent mechanical properties of these biological structures rest on the exquisite self‐assembly of natural ingredients, such as biominerals, polysaccharides, and proteins. Among them, the two commonly found building blocks in the model biocomposites are chitin nanofibers and silk‐like proteins with β‐sheet structure. Despite being wholly organic, the chitinous protein complex plays a key role for the biocomposites by contributing to the overall mechanical robustness and structural integrity. Moreover, the chitinous protein complex alone without biominerals is optically transparent (e.g., dragonfly wings), thereby making it a brilliant model material system for engineering applications where optical transparency is essentially required. Here, inspired by the chitinous protein complex of arthropods cuticles, an optically transparent biomimetic composite that hybridizes chitin nanofibers and silk fibroin (β‐sheet) is introduced, and its potential as a biocompatible structural platform for emerging wearable devices (e.g., smart contact lenses) and advanced displays (e.g., transparent plastic cover window) is demonstrated.     

Recent Progress in Functional Dye-Doped Liquid Crystal Devices

As a typical representative of dopants, organic functional dyes have demonstrated their significant roles in novel smart liquid crystal (LC) devices, and dye-doped LCs have also been a source of inspiration for scientists to design and fabricate stimuli-gated materials or devices for envisioned applications in a wide range of areas. In this review, the focus on dichroic dyes, fluorescent dyes, and photothermal dyes, and the recent progress of the LC devices employing these dyes as dopants are overviewed. The review highlights the developments of the novel LC devices doped with these dyes. The structures, designs, and applications of these devices are outlined. The underlying principles of dichroic dyes, fluorescent dyes, and photothermal dyes which are utilized as functional dopants in LC devices are first introduced. Subsequently, the novel developments of functional dye-doped LC devices in the application fields of smart windows, attenuators for augmented reality (AR) systems, color-changeable textiles, dichroic color filters, dual-mode circular polarizers, chirality detectors, optical limiters, switchable luminescent solar concentrators, multiple information encryption, anti-counterfeiting, photo-addressed transparent displays, circularly polarized luminescence, tunable lasers, and light-driven soft actuators are discussed. Finally, the challenge and the strategies for the future improvement of dye-doped LC devices are also discussed.     

Microstructured Actuation of Liquid Crystal Polymer Networks

Smart microstructured materials enable functions such as actuation, detection, transportation, and sensing with potential applications ranging from robotics and photonics to biomedical devices. Of the many materials systems, liquid crystal polymer networks (LCN) are fascinating owing to their ability to exhibit reversible macroscopic deformation driven by a molecular order–disorder phase transition. LCN have been increasingly explored for their utility in the design and fabrication of smart actuating devices capable of complex shape changes or motions upon external stimulation of humidity, heat, light, and other stimuli, and recent studies in this field show that their actuation complexity can be enriched and actuation performance enhanced by having some sort of microstructures. Herein, the recent progress in microstructured actuation of LCN materials with substructures in scale ranging from micrometer to millimeter is reported, placing the emphasis on the main approaches to generating a microstructure in LCN, which include patterned LC director fields, patterned chain crosslinking in LCN with uniaxial orientation of mesogens, 3D/4D printing, and replica molding. The potential applications in microstructured 3D actuators and devices as well as functional LCN surfaces are also highlighted, with an outlook on important issues and future trends in smart microstructured LCN materials and actuators.     

Ultrafast Self‐Assembly of Graphene Oxide‐Induced Monolithic NiCo–Carbonate Hydroxide Nanowire Architectures with a Superior Volumetric Capacitance for Supercapacitors

The monolithic electrodes with high volumetric capacitance demonstrate a great potential in practical industrial applications for supercapacitors. Herein, a novel strategy for ultrafast self‐assembly of graphene oxides (GO)‐induced monolithic NiCo–carbonate hydroxide (NiCo–CH) nanowire composite films (G–CH) is reported. The oxygen‐containing functional groups on the GO surface help effectively to induce formation of the monodisperse NiCo–CH nanowires. Such a nanowire‐shaped structure further functions as a scaffold and/or support, leading to 25 s of ultrafast self‐assembly for G–CH composite films and a relatively loose and open channel that contributes to fast electrolyte transport. The as‐obtained monolithic G–CH architectures show an excellent supercapacitor performance as binder‐ and conductive agent‐free electrode, evidenced by a superior volumetric capacitance of 2936 F cm^?3 and good electrochemical stability. Combining highly conductive carbon nanotubes (CNTs) into the monolithic composite films can further create well‐interconnected conductive networks within the electrode matrix, thus to improve the reaction kinetics and rate capability. The present strategy that can modulate the growth of the high‐electroactive pseudocapacitive hydroxides and achieve an ultrafast self‐assembly of monolithic composites may pave a promising new way for development of high‐performance supercapacitors and shed a new light on the configuration of carbon‐based electrode materials in energy storage and conversion devices.     

A Versatile Approach towards Multifunctional Robust Microcapsules with Tunable,Restorable, and Solvent‐Proof Superhydrophobicity for Self‐Healing and Self‐Cleaning Coatings

Numerous microencapsulation techniques have been developed to encase various chemicals, for which specific processing parameters are required to address the widely differing features of the encapsulated materials. Microencapsulation of reactive agents is a powerful technique that has been extensively applied to self‐healing materials. However, the poor solvent compatibility and insufficient thermal stability of microcapsules continue to pose challenges for long‐term storage, processing, and service in practical applications. Here, an easily modifiable and highly versatile method is reported for preparing various chemicals filled poly(urea‐formaldehyde) microcapsules that exhibit superior tightness against solvents and heat and that possess widely tunable, repetitiously self‐restorable, and solvent‐proof superhydrophobicity. In addition, the low‐cost fabrication of biomimetic multifunctional smart coatings is demonstrated for self‐healing anticorrosion and self‐cleaning antifouling applications by directly dispersing the superhydrophobic microcapsules into and onto a polymer matrix. The methodology presented in this study should inspire the development of multifunctional intelligent materials for applications in related fields.     

Dry‐Type Artificial Muscles Based on Pendent Sulfonated Chitosan and Functionalized Graphene Oxide for Greatly Enhanced Ionic Interactions and Mechanical Stiffness

Biopolymer‐based artificial muscles are promising candidates for biomedical applications and smart electronic textiles due to their multifaceted advantages like natural abundance, eco‐friendliness, cost‐effectiveness, easy chemical modification and high electical reactivity. However, the biopolymer‐based actuators are showing relatively low actuation performance compared with synthetic electroactive polymers because of inadequate mechanical stiffness, low ionic conductivity and ionic exchange capacity (IEC), and poor durability over long‐term activation. This paper reports a high‐performance electro‐active nano‐biopolymer based on pendent sulfonated chitosan (PSC) and functionalized graphene oxide (GO), exhibiting strong electro‐chemo‐mechanical interations with ionic liquid (IL) in open air environment. The proposed GO‐PSC‐IL nano‐biopolymer membrane shows an icnreased tensile strength and ionic exchange capacity of up to 44.8% and 83.1%, respectively, and increased ionic conductivity of over 18 times, resulting in two times larger bending actuation than the pure chitosan actuator under electrical input signals. Eventually, the GO‐PSC‐IL actuators could show robust and high‐performance actuation even at the very low applied voltages that are required in realistic applications.     

Self‐Healing Materials for Energy‐Storage Devices

The booming development of electronics, electric vehicles, and grid storage stations has led to a high demand for advanced energy‐storage devices (ESDs) and accompanied attention to their reliability under various circumstances. Self‐healing is the ability of an organism to repair damage and restore function through its own internal vitality. Inspired by this, brilliant designs have emerged in recent years using self‐healing materials to significantly improve the lifespan, durability, and safety of ESDs. Extrinsic and intrinsic self‐healing materials and their working principles are first introduced. Then, the application of self‐healing materials in ESDs according to their self‐healing chemistry, including hydrogen bonds, electrostatic interactions, and borate ester bonds, are described in detail. Based on these, critical challenges and important future directions of self‐healing ESDs are discussed.     

Ionic Liquid–Polymer Composites: A New Platform for Multifunctional Applications

Ionic liquids (ILs) have emerged as a novel class of chemical compounds for the development of advanced (multi)functional materials with outstanding potential in applications of several areas due to their unique properties and functionalities. The combination of ILs with polymers, in a composite, allows for developing smart materials, which synergistically combine the features of specific polymers and ILs. Moreover, ILs can be extensively modified by the incorporation of functional groups with specific properties into the cation, anion, or both. Thus, it is possible to tune the IL, the polymer, or both to obtain a broad spectrum of multifunctional composites and address the specific requirements of many applications. This work focusses on advanced materials and strategies concerning ILs and polymers for the development of smart IL/polymer‐based materials for applications including responsive and sensitive sensors, actuators, environment, batteries, fuel cells, and biomedical applications.     

Wood‐Derived Materials for Advanced Electrochemical Energy Storage Devices

Over the past decade, wood‐derived materials have attracted enormous interest for both fundamental research and practical applications in various functional devices. In addition to being renewable, environmentally benign, naturally abundant, and biodegradable, wood‐derived materials have several unique advantages, including hierarchically porous structures, excellent mechanical flexibility and integrity, and tunable multifunctionality, making them ideally suited for efficient energy storage and conversion. In this article, the latest advances in the development of wood‐derived materials are discussed for electrochemical energy storage systems and devices (e.g., supercapacitors and rechargeable batteries), highlighting their micro/nanostructures, strategies for tailoring the structures and morphologies, as well as their impact on electrochemical performance (energy and power density and long‐term durability). Furthermore, the scientific and technical challenges, together with new directions of future research in this exciting field, are also outlined for electrochemical energy storage applications.     

Cucurbit[n]uril Supramolecular Hydrogel Networks as Tough and Healable Adhesives

Supramolecular noncovalent interactions are widely found in natural adhesion phenomena to control macroscopic adhesion and accomplish a variety of complex functions. Such supramolecular adhesives could impart the interfaces with intriguing properties, e.g., energy dissipation and self‐healing, on account of their dynamic nature. Here, we demonstrate that cucurbit[8]uril (CB[8])‐based supramolecular hydrogel networks can function as dynamic adhesives for diverse nonporous (e.g., glass, stainless steel, aluminum, copper, and titanium) and porous substrates (wood and bone). Without any surface prefunctionalization or introduction of curing agents, these CB[8] hydrogel networks can be readily applied by curing around the softening temperature, forming a tough and healable adhesive interlayer. The ability to fabricate a robust sandwich model consisting of substrate–CB[8] hydrogel network–substrate enables a number of applications including stretchable and wearable electronics, hybrid systems for biomedical devices or tissue/bone regeneration.     

Plasmonic‐Assisted Graphene Oxide Films with Enhanced Photothermal Actuation for Soft Robots

Carbon‐based materials are widely used as light‐driven soft actuators relying on their thermal desorption or expansion. However, applying a passive layer in such film construction greatly limits the actuating efficiency, e.g., bending amplitude and speed. In this work, a dual active layer strengthened bilayer composite film made of graphene oxide (GO)–polydopamine (PDA)–gold nanoparticles (Au NPs)/polydimethylsiloxane (PDMS) is developed. In this film, the conventional passive layer is replaced by another AuNPs‐enhanced thermal responsive layer. When applying NIR light exposure, the whole film deforms controllably resulting from the water loss in the GO–PDA–Au NPs layer and thermal expansion in the PDMS layer. Benefiting from the dual active bilayer mechanism, the thin film's actuating efficiency is dramatically improved compared with that of conventional methods. Specifically, the bending amplitude is enhanced up to 173%, and the actuating speed is improved to 3.5‐fold. The soft actuator can act as an artificial arm with high actuating strength and can be used as a wireless gripper. Moreover, the film can be designed as soft robots with various locomotion modes including linear, rolling, and steering motions. The developed composite film offers new opportunities for biomimetic soft robotics as well as future applications.     

Smart Nanostructured Materials based on Self‐Assembly of Block Copolymers

Nanoscale fabrication of smart materials relying on the molecular self‐assembly of block copolymers (BCPs) has been recognized as a valuable platform for various next‐generation functional structures. In this Progress Report, the recent advances in the BCP self‐assembly process, which has paved the way for viable applications of emerging nanotechnologies, are highlighted. Effective light‐induced self‐assembly based on photothermal annealing of high‐χ BCPs and conformal 3D surface nanopatterning exploiting chemically modified graphene flexible substrates are reviewed as the typical instances of advanced BCP‐based nanofabrication methodologies. Additionally, relevant potential application fields are suggested, namely, graphene nanoribbon field effect transistors, highly tunable refractive index metasurfaces for visible light, high‐sensitivity surface‐enhanced Raman spectroscopy, 2D transition metal dichalcogenide nanopatterning, sequential infiltration synthesis, and organic photovoltaics. Finally, the future research direction as well as innovative applications of these smart nanostructured materials is proposed.