Ultrastrong Bioinspired Graphene-Based Fibers via Synergistic Toughening

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

High Performance Nacre Fibers by Engineering Interfacial Entanglement

Biological materials exhibit fascinating mechanical properties for intricate interactions at multiple interfaces to combine superb toughness with wondrous strength and stiffness. Recently, strong interlayer entanglement has emerged to replicate the powerful dissipation of natural proteins and alleviate the conflict between strength and toughness. However, designing intricate interactions in a strong entanglement network needs to be further explored. Here, we modulate interlayer entanglement by introducing multiple interactions, including hydrogen and ionic bonding, and achieve ultrahigh mechanical performance of graphene-based nacre fibers. Two essential modulating trends are directed. One is modulating dynamic hydrogen bonding to improve the strength and toughness up to 1.58 GPa and 52 MJ/m3, simultaneously. The other is tailoring ionic coordinating bonding to raise the strength and stiffness, reaching 2.3 and 253 GPa. Modulating various interactions within robust entanglement provides an effective approach to extend performance limits of bioinspired nacre and optimize multiscale interfaces in diverse composites.

Bioinspired Macroporous Materials of MXene Nanosheets: Ice-Templated Assembly and Multifunctional Applications

Biological macroporous materials, such as stems of the plants and bone of the animals, possess outstanding properties for powerful guarantee of creatures' survival through the well-aligned architecture constructed from limited components. Transition metal carbides or nitrides (MXenes), as novel 2D assemblies, have attracted numerous attentions in various applications due to their unique properties. Therefore, mimicking the bioinspired architecture with MXenes will boost the development of human-made materials with unparalleled properties. Freeze casting has been widely applied to fabricate bioinspired MXene-based materials and achieve the assembly of MXene nanosheets into 3D forms. This process solves the inherent restacking problems of MXenes, simultaneously preserving the unique properties of MXenes with a physical process. Here, the ice-templated assembly of MXene in terms of the freezing processes and their potential mechanisms is summarized. In addition, applications of MXene-based materials in electromagnetic interference shielding and absorption, energy storage and conversion, as well as piezoresistive pressure sensors are also reviewed. Finally, the current challenges and bottlenecks of ice-templated assembly of MXene are further discussed to guide the development of bioinspired MXene-based materials.

High Toughness Combined with High Strength in Oxide Ceramic Nanofibers

Traditional oxide ceramics are inherently brittle and highly sensitive to defects, making them vulnerable to failure under external stress. As such, endowing these materials with high strength and high toughness simultaneously is crucial to improve their performance in most safety-critical applications. Fibrillation of the ceramic materials and further refinement of the fiber diameter, as realized by electrospinning, are expected to achieve the transformation from brittleness to flexibility owing to the structural uniqueness. Currently, the synthesis of electrospun oxide ceramic nanofibers must rely on an organic polymer template to regulate the spinnability of the inorganic sol, whose thermal decomposition during ceramization will inevitably lead to pore defects, and seriously weaken the mechanical properties of the final nanofibers. Here, a self-templated electrospinning strategy is proposed for the formation of oxide ceramic nanofibers without adding any organic polymer template. An example is given to show that individual silica nanofibers have an ideally homogeneous, dense, and defect-free structure, with tensile strength as high as 1.41 GPa and toughness up to 34.29 MJ m-3 , both of which are far superior to the counterparts prepared by polymer-templated electrospinning. This work provides a new strategy to develop oxide ceramic materials that are strong and tough.

Superior Strong and Tough Nacre-Inspired Materials by Interlayer Entanglement

Natural materials teach that mechanical dissipative interactions relieve the conflict between strength and toughness and enable fabrication of strong yet tough artificial materials. Replicating natural nacre structure has yielded rich biomimetic materials; however, stronger interlayer dissipation still waits to be exploited to extend the performance limits of artificial nacre materials. Here, we introduce strong entanglement as a new artificial interlayer dissipative mechanism and fabricate entangled nacre materials with superior strength and toughness, across molecular to nanoscale nacre structures. The entangled graphene nacre fibers achieved high strength of 1.2 GPa and toughness of 47 MJ/m³, and films reached 1.5 GPa and 25 MJ/m³. Experiments and simulations reveal that strong entanglement can effectively dissipate interlayer energy to relieve the conflict between strength and toughness, acting as natural folded proteins. The strong interlayer entanglement opens up a new path for designing stronger and tougher artificial materials to mimic but surpass natural materials.

Extremely Robust and Multifunctional Nanocomposite Fibers for Strain-Unperturbed Textile Electronics

Textile electronics are needed that can achieve strain-unaltered performance when they undergo irregular and repeated strain deformation. Such strain-unaltered textile electronics require advanced fibers that simultaneously have high functionalities and extreme robustness as fabric materials. Current synthetic nanocomposite fibers based on inorganic matrix have remarkable functionalities but often suffer from low robustness and poor tolerance against crack formation. Here, we present a design for a high-performance multifunctional nanocomposite fiber that is mechanically and electrically robust, which was realized by crosslinking titanium carbide (MXene) nanosheets with a slide-ring polyrotaxane to form an internal mechanically-interlocked network. This inorganic matrix nanocomposite fiber featured distinct strain-hardening mechanical behavior and exceptional load-bearing capability (toughness approaching 60 MJ m^-3 and ductility over 27%). It retained 100% of its ductility after cyclic strain loading. Moreover, the high electrical conductivity (>1.1 × 10⁵ S m^-1 ) and electrochemical performance (>360 F cm^-3 ) of the nanocomposite fiber can be well retained after subjecting the fiber to extensive (>25% strain) and long-term repeated (10 000 cycles) dimensional changes. Such superior robustness allowed for the fabrication of the nanocomposite fibers into various robust wearable devices, such as textile-based electromechanical sensors with strain-unalterable sensing performance and fiber-shaped supercapacitors with invariant electrochemical performance for 10 000 strain loading cycles.

Carbonene Fibers: Toward Next-Generation Fiber Materials

The development of human society has set unprecedented demands for advanced fiber materials, such as lightweight and high-performance fibers for reinforcement of composite materials in frontier fields and functional and intelligent fibers in wearable electronics. Carbonene materials composed of sp²-hybridized carbon atoms have been demonstrated to be ideal building blocks for advanced fiber materials, which are referred to as carbonene fibers. Carbonene fibers that generally include pristine carbonene fibers, composite carbonene fibers, and carbonene-modified fibers hold great promise in transferring the extraordinary properties of nanoscale carbonene materials to macroscopic applications. Herein, we give a comprehensive discussion on the conception, classification, and design strategies of carbonene fibers and then summarize recent progress regarding the preparations and applications of carbonene fibers. Finally, we provide insights into developing lightweight, high-performance, functional, and intelligent carbonene fibers for next-generation fiber materials in the near future.

Photocuring Graphene Oxide Liquid Crystals for High-Strength Structural Materials

Graphene is the strongest known material. However, the challenge of translating that strength from the microscale to the more useful macroscale remains unmet. Preparing solid structures from self-assembled graphene oxide liquid crystals has allowed the creation of paper and fibers with excellent mechanical properties. Conventionally, vacuum filtration, wet spinning, and freeze-drying are used to prepare such structures from graphene oxide liquid crystals. Here, we introduce photocuring as an additional option to create solid structures of self-assembled graphene oxide liquid crystals that allows for thicker samples and other shapes to be realized. The photocured graphene oxide paper prepared here exhibited mechanical properties comparable to those of benchmark samples prepared by vacuum filtration.

Functionalized Graphene Fiber Modified With MOF-Derived Rime-Like Hierarchical Nanozyme for Electrochemical Biosensing of H2O2 in Cancer Cells

The rational design and construction of high-performance flexible electrochemical sensors based on hierarchical nanostructure functionalized microelectrode systems are of vital importance for sensitive in situ and real-time detection of biomolecules released from living cells. Herein, we report a novel and facile strategy to synthesize a new kind of high-performance microelectrode functionalized by dual nanozyme composed of rime-like Cu2(OH)3NO3 wrapped ZnO nanorods assembly [Cu2(OH)3NO3@ZnO], and explore its practical application in electrochemical detection of hydrogen peroxide (H2O2) released from living cells. Benefiting from the merits of the unique hierarchical nanohybrid structure and high catalytic activities, the resultant Cu2(OH)3NO3@ZnO-modified AGF microelectrode shows remarkable electrochemical sensing performance towards H2O2 with a low detection limit of 1 μM and a high sensitivity of 272 μA cm-2 mM-1, as well as good anti-interference capability, long-term stability, and reproducibility. These properties enabled the proposed microelectrode-based electrochemical platform to be applied for in situ amperometric tracking of H2O2 released from different types of human colon cells, thus demonstrating its great prospect as a sensitive cancer cell detection probe for the early diagnosis and management of various cancer diseases.

Recent Progress in Flexible Graphene-Based Composite Fiber Electrodes for Supercapacitors

Graphene has shown the world its fascinating properties, including high specific surface area, high conductivity, and extraordinary mechanical properties, which enable graphene to be a competent candidate for electrode materials. However, some challenges remain in the real applications of graphene-based electrodes, such as continuous preparation of graphene fibers with highly ordered graphene sheets as well as strong interlayer interactions. The combination of graphene with other materials or functional guests hence appears as a more promising pathway via post-treatment and in situ hybridism to produce composite fibers. This article firstly provides a full account of the classification of graphene-based composite fiber electrodes, including carbon allotropy, conductive polymer, metal oxide and other two-dimensional (2D) materials. The preparation methods of graphene-based composite fibers are then discussed in detail. The context further demonstrates the performance optimization of graphene-based composite fiber electrodes, involving microstructure design and surface modification, followed by the elaboration of the application of graphene-based composite fiber electrodes in supercapacitors. Finally, we present the remaining challenges that exist to date in order to provide meaningful guidelines in the development process and prospects of graphene-based composite fiber electrodes.

Multi-Scale Structure-Mechanical Property Relations of Graphene-Based Layer Materials

Pristine graphene is one of the strongest materials known in the world, and may play important roles in structural and functional materials. In order to utilize the extraordinary mechanical properties in practical engineering structures, graphene should be assembled into macroscopic structures such as graphene-based papers, fibers, foams, etc. However, the mechanical properties of graphene-based materials such as Young’s modulus and strength are 1–2 orders lower than those of pristine monolayer graphene. Many efforts have been made to unveil the multi-scale structure–property relations of graphene-based materials with hierarchical structures spanning the nanoscale to macroscale, and significant achievements have been obtained to improve the mechanical performance of graphene-based materials through composition and structure optimization across multi-scale. This review aims at summarizing the currently theoretical, simulation, and experimental efforts devoted to the multi-scale structure–property relation of graphene-based layer materials including defective monolayer graphene, nacre-like and laminar nanostructures of multilayer graphene, graphene-based papers, fibers, aerogels, and graphene/polymer composites. The mechanisms of mechanical property degradation across the multi-scale are discussed, based on which some multi-scale optimization strategies are presented to further improve the mechanical properties of graphene-based layer materials. We expect that this review can provide useful insights into the continuous improvement of mechanical properties of graphene-based layer materials.

Smart fibers for energy conversion and storage

Fibers have played a critical role in the long history of human development. They are the basic building blocks of textiles. Synthetic fibers not only make clothes stronger and more durable, but are also customizable and cheaper. The growth of miniature and wearable electronics has promoted the development of smart and multifunctional fibers. Particularly, the incorporation of functional semiconductors and electroactive materials in fibers has opened up the field of fiber electronics. The energy supply system is the key branch for fiber electronics. Herein, after a brief introduction on the history of smart and functional fibers, we review the current state of advanced functional fibers for their application in energy conversion and storage, focusing on nanogenerators, solar cells, supercapacitors and batteries. Subsequently, the importance of the integration of fiber-shaped energy conversion and storage devices via smart structure design is discussed. Finally, the challenges and future direction in this field are highlighted. Through this review, we hope to inspire scientists with different research backgrounds to enter this multi-disciplinary field to promote its prosperity and development and usher in a truly new era of smart fibers.

Chemical Strategies for Making Strong Graphene Materials

Graphene materials have been widely applied in various fields because of their remarkable mechanical and electrical properties. However, two obstacles arise during the assembly of graphene platelets into macroscale graphene materials and composites that impair the performance of the resultant graphene materials: 1) the voids between the graphene platelets, and 2) the wrinkling of the graphene platelets. In the past decade, several strategies have been developed to eliminate these obstacles. These strategies result in strong macroscale graphene materials, such as graphene fibers with tensile strengths of over 3.4 GPa and sheets with tensile strengths of over 1.5 GPa, which have many practical applications. This Minireview summarizes the effective strategies for assembling graphene materials and compares their advantages and drawbacks. The preparation processes as well as the resulting fundamental mechanical properties and wide spectrum of electrical and magnetic properties are also discussed. Finally, our outlook for the future of this field is presented.

Bioproduced Polymers Self-Assemble with Graphene Oxide into Nanocomposite Films with Enhanced Mechanical Performance

Graphene oxide (GO) has recently been highlighted as a promising multipurpose two-dimensional material. However, free-standing graphene oxide films suffer from poor strength and flexibility, which limits scaling-up of production and lifetime structural robustness in applications. Inspired by the relationship between the organic and inorganic components of the hierarchical structure of nacre found in mollusk shells, we have fabricated self-assembled, layered graphene-based composite films. The organic phase of our composite is produced via environmentally friendly and economical methods based on bacterial production of γ-poly(glutamic acid) (PGA). Composite films made of GO, PGA, and divalent cations (Ca²⁺) were prepared through a slow solvent evaporation method at ambient temperature, resulting in a nacre-like layered structure. These biobased nanocomposite films showed impressive mechanical properties, which resulted from a synergistic combination of hydrogen bonding with the bacterially produced PGA and ionic bonding with calcium ions (Ca²⁺). The GO/PGA/Ca²⁺ composite films possessed a high strength of 150 ± 51.9 MPa and a high Young's modulus of 21.4 ± 8.7 GPa, which represents an increase of 120% and over 70% with respect to pure GO films. We provide rational design strategies for the production of graphene-based films with improved mechanical performance, which can be applied in filtration purification of wastewater in the paper, food, beverage, pigment, and pharmaceuticals industries, as well as for manufacturing of functional membranes and surface coatings.

Boosting storage properties of reduced graphene oxide fiber modified with MOFs-derived porous carbon through a wet-spinning fiber strategy

Supercapacitors that are light weight and flexible, while occupying a low volume and demonstrating good mechanical properties are in demand for portable energy storage devices. Graphene composite fibers are supposed to be ideal electrodes for flexible fiber-shaped supercapacitors. Integration of MOFs-derived porous carbon into graphene fibers provides desirable electrochemical and mechanical properties. Herein, a general strategy is shown for the preparation of MOFs-derived porous carbon/reduced graphene oxide fibers. Close-packed and aligned graphene sheets along with porous MOFs-derived porous carbon can achieve outstanding mechanical properties through synergistic effects. Consequently, a large specific capacitance of 56.05 F cm^-3, a good tensile property of 86.5 MPa and a high retention of 96.6% after 10 000 cycles can be achieved with the composite fibers. Moreover, a further deposition of polyaniline (PANI) and manganese dioxide (MnO₂) by in situ growth on the fabricated composite fibers provide an improvement in specific capacitance with value of 74.21 F cm^-3 and 65.08 F cm^-3, respectively. The above results demonstrate the promising application of composite fibers as a flexible and stable electrode and substrate for energy storage devices.

Hierarchical Assembly of Nanocellulose into Filaments by Flow-Assisted Alignment and Interfacial Complexation: Conquering the Conflicts between Strength and Toughness

Filaments comprising solely cellulose nanofibrils (CNFs) have been fabricated by flow-assisted assembling, where the strength can be improved greatly with the sacrifice of toughness. Inspired by the architecture of natural nacre and plant cell wall, the combined technique of convergent microfluidic spinning and in situ interfacial complexation between CNF and chitosan molecules was used to construct the filaments with hierarchical assembly of highly oriented CNFs locked by chitosan complexes, showing simultaneous enhancements of strength and toughness. In specific, the best performing filament exhibited a toughness of 88.9 kJ/m³ and a tensile strength of 1289 MPa because of the strong interfacial complexation interactions between CNFs and chitosan molecules. The tensile strength was further raised to 1627 MPa when the filaments were cross-linked synergistically by using Ca²⁺, surpassing the reported values in the literature. Molecular dynamics simulations revealed the possible fracture mechanism of the filaments under tension. With excellent mechanical performance and biocompatibility, the resulting CNF/chitosan filament system showed a promising application potential as nonabsorbable surgical sutures. The demonstrated spinning technology also offered a new avenue for the fabrication of high-performance filaments.

Hierarchical cross-linking for synergetic toughening in crustacean-mimetic nanocomposites

The twisted plywood structure as found in crustacean shells possesses excellent mechanical properties with high stiffness and toughness. Synthetic mimics can be produced by evaporation-induced self-assembly of cellulose nanocrystals (CNCs) with polymer components into bulk films with a cholesteric liquid crystal structure. However, these are often excessively brittle and it has remained challenging to make materials combining high stiffness and toughness. Here, we describe self-assembling cholesteric CNC/polymer nanocomposites with a crustacean-mimetic structure and tunable photonic band gap, in which we engineer combinations of thermo-activated covalent and supramolecular hydrogen-bonded crosslinks to tailor the energy dissipation properties by precise molecular design. Toughening occurs upon increasing the polymer fractions in the nanocomposites, and, critically, combinations of both molecular bonding mechanisms lead to a considerable synergetic increase of stiffness and toughness - beyond the common rule of mixtures. Our concept following careful molecular design allows one to enter previously unreached areas of mechanical property charts for cholesteric CNC-based nanocomposites. The study shows that the subtle engineering of molecular energy dissipation units using sophisticated chemical approaches enables efficient enhancing of the properties of bioinspired CNC/polymer nanocomposites, and opens the design space for future molecular enhancement using tailor-made interactions.

Electrical and Thermal and Self-Healing Properties of Graphene-Thermopolyurethane Flexible Conductive Films

We fabricated graphene-thermopolyurethane (G-TPU) flexible conductive film by a blending method and systematically investigated the electrical, thermal and self-healing properties of the G-TPU flexible conductive film by infrared light and electricity. The experimental results demonstrate that the G-TPU composite films have good conductivity and thermal conductivity in the appropriate mass content of graphene in the composite film. The composite films have the good electro-thermal and infrared light thermal response performances and electro-thermal response performance is closely related to the mass content of graphene in the composite film, but the infrared light thermal response performance is not. The scratch on the composite film can be completely healed, using electricity or infrared light. The healing efficiency of the composite film healed using infrared light is higher than that of using the electricity, while the healing time of the composite film is shorter. Regardless of the self-healing method, the temperature of the self-healing is a very important factor. The self-healing conductive composite film still exhibits a good conductivity.

Metal Mimics: Lightweight, Strong, and Tough Nanocomposites and Nanomaterial Assemblies

The ideal structural material would have high strength and stiffness with a tough ductile failure, all with a low density. Historically, no such material exists, and materials engineers have had to sacrifice a desired property during materials selection, with metals (high density), fiber composites (brittle failure), and polymers (low stiffness) having fundamental limitations on at least one front. The ongoing revolution of nanomaterials provides a potential route to build on the potential of fiber-reinforced composites, matching their strength while integrating toughening behaviors akin to metal deformations, all while using low-weight constituents. Here, the challenges, approaches, and recent developments of nanomaterials for structural applications are discussed, with an emphasis on improving toughening mechanisms, which is often the neglected factor in a field that chases strength and stiffness.

Coordination-Driven Hierarchical Assembly of Hybrid Nanostructures Based on 2D Materials

2D materials have received tremendous scientific and engineering interests due to their remarkable properties and broad-ranging applications such as energy storage and conversion, catalysis, biomedicine, electronics, and so forth. To further enhance their performance and endow them with new functions, 2D materials are proposed to hybridize with other nanostructured building blocks, resulting in hybrid nanostructures with various morphologies and structures. The properties and functions of these hybrid nanostructures depend strongly on the interfacial interactions between 2D materials and other building blocks. Covalent and coordination bonds are two strong interactions that hold high potential in constructing these robust hybrid nanostructures based on 2D materials. However, most 2D materials are chemically inert, posing problems for the covalent assembly with other building blocks. There are usually coordination atoms in most of 2D materials and their derivatives, thus coordination interaction as a strong interfacial interaction has attracted much attention. In this review, recent progress on the coordination-driven hierarchical assembly based on 2D materials is summarized, focusing on the synthesis approaches, various architectures, and structure-property relationship. Furthermore, insights into the present challenges and future research directions are also presented.

Graphene-Based Fibers: Recent Advances in Preparation and Application

Graphene-based fibers (GBFs) are macroscopic 1D assemblies formed by using microscopic 2D graphene sheets as building blocks. Their unique structure exhibits the same merits as graphene such as low weight, high specific surface area, excellent mechanical/electrical properties, and ease of functionalization. Furthermore, the fibrous nature of GBFs is intrinsically compatible with existing textile technologies, making them suitable for applications in flexible and wearable electronics. Recently, novel synthetic methods have endowed GBFs with new structures and functions, further improving their mechanical and electrical properties. These improvements have rapidly bridged the gaps between laboratory demonstrations and real-life applications in fiber-shaped batteries, supercapacitors, and electrochemical sensors. Recent advances in the fabrication, optimization, and application of GBFs are systematically reviewed and a perspective on their future development is given.

A Review on Graphene Fibers: Expectations, Advances, and Prospects

Graphene fiber (GF) is a macroscopically assembled fibrous material made of individual units of graphene and its derivatives. Beyond traditional carbon fibers, graphene building blocks consisting of regulable sizes and regular orientations of GF are expected to generate extreme mechanical and transport properties, as well as multiple functions in smart electronic fibrous devices and textiles. Here, the features of GF are presented along four lines: preparation, morphology, structure-performance correlations, and state-of-the-art applications as flexible and wearable electronics. The principles, experiments, and keys of fabricating GF from graphite with different methods, focusing on the industrially viable mainstream strategy, wet spinning, are introduced. Then, the fundamental relationship between the mechanical and transport properties and the structure, including both highly condensed structures for high-performance and hierarchical structures for multiple functions, is presented. The advances of GF based on structure-performance formulas boost its functional applications, especially in electronic devices. Finally, the possible promotion methods and structural-functional integrated applications of GF are discussed.

Bio-Inspired Stretchable and Contractible Tough Fiber by the Hybridization of GO/MWNT/Polyurethane

Spider silks represent stretchable and contractible fibers with high toughness. Those tough fibers with stretchability and contractibility are attractive as energy absorption materials, and they are needed for wearable applications, artificial muscles, and soft robotics. Although carbon-based materials and poly(vinyl alcohol) (PVA) composite fibers exhibit high toughness, they are still limited in low extensibility and an inability to operate in the wet-state condition. Herein, we report stretchable and contractible fiber with toughness that is inspired by the structure of spider silk. The bioinspired tough fiber provides 495 J/g of gravimetric toughness, which exceeds 165 J/g of spider silk. Besides, the tough fiber was reversibly stretched to ∼80% strain without damage. This toughness and stretchability are realized by hybridization of aligned graphene oxide/multiwalled carbon nanotubes in a polyurethane matrix as elastic amorphous regions and β-sheet segments of spider silk. Interestingly, the bioinspired tough fiber contracted up to 60% in response to water and humidity similar to supercontraction of the spider silk. It exhibited 610 kJ/m³ of contractile energy density, which is higher than previously reported moisture driven actuators. Therefore, this stretchable and contractible tough fiber could be utilized as an artificial muscle in soft robotics and wearable devices.

Infrared induced repeatable self-healing and removability of mechanically enhanced graphene-epoxy flexible materials

A repeatable self-healing epoxy composite mechanically enhanced by graphene nanosheets (GNS) was prepared from an epoxy monomer with Diels-Alder (DA) bonds, octanediol glycidyl ether (OGE) and polyether amine (D230). The GNS/epoxy composites, with a maximum tensile modulus of 14.52 ± 0.45 MPa and elongation at break more than 100%, could be healed several times under Infrared (IR) light with the healing efficiency as high as 90% through the molecule chain mobility and the rebonding of reversible DA bonds between furan and maleimide. Also, they displayed excellent recyclable ability by transforming into a soluble polymer, which offers a wide range of possibilities to produce epoxy flexible materials with healing and removable abilities.

Dual-Phase Super-Strong and Elastic Ceramic

Ceramic materials exhibit very high stiffness and extraordinary strength, but they typically suffer from brittleness. Amorphization and size confinement are commonly used to reinforce materials. However, the inverse Hall-Petch effect and the shear-band softening effect usually limit further improvement of their performance under a critical size. With an optimum structure design, we demonstrate that dual-phase zirconia nanowires (DP-ZrO₂ NWs) with nanocrystals embedded in an amorphous matrix as a strengthening phase can overcome these problems simultaneously. As a result of this structure, in situ tensile tests demonstrate that the mechanical properties have been enormously improved in a way that does not follow both the inverse Hall-Petch effect and the shear band softening effect. The elastic strain approaches ∼7%, and the ultimate strength is 3.52 GPa, accompanied by a high toughness of ∼151 MJ m^-3, making the DP-ZrO₂ NW composite the strongest and toughest ZrO₂ ever achieved. The findings provide a way to improve the mechanical properties of ceramics in a controllable manner, which may serve as a pervasive approach to be broadly applied to a variety of materials.

"Cut-and-paste" method for the rapid prototyping of soft electronics

Unlike wafer-based rigid electronics, soft electronics have many unique advantages including thinness, flexibility, stretchability, conformability, lightweight, large area, as well as low cost. As a result, they have demonstrated many emerging capabilities in healthcare devices, soft robotics, and human-machine interface. Instead of conventional microfabrication, there is an evergrowing interest in the freeform or digital manufacture of soft electronics. This review provides a survey for a cost- and time-effective subtractive manufacturing process called the "cut-and-paste" method. It employs a mechanical cutter plotter to form patterns on various electronically functional membranes such as sheets of metals, functional polymers, and even two-dimensional (2D) materials, supported by a temporary tape. The patterned membranes can then be pasted on soft substrates such as medical tapes or even human skin. This process is completely dry and desktop. It does not involve any rigid wafers and is hence capable of making large-area electronics. The process can be repeated to integrate multiple materials on a single substrate. Integrated circuits (ICs) and rigid components can be added through a "cut-solder-paste" process. Multilayer devices can also be fabricated through lamination. We therefore advocate that the "cut-and-paste" method is a very versatile approach for the rapid prototyping of soft electronics for various applications.

Ultratough Bioinspired Graphene Fiber via Sequential Toughening of Hydrogen and Ionic Bonding

Graphene-based fibers synthesized under ambient temperature have not achieved excellent mechanical properties of high toughness or tensile strength compared with those synthesized by hydrothermal strategy or graphitization and annealing treatment. Inspired by the relationship between organic/inorganic hierarchical structure, interfacial interactions, and moderate growth temperature of natural nacre, we fabricate an ultratough graphene fiber via sequential toughening of hydrogen and ionic bonding through a wet-spinning method under ambient temperature. A slight amount of chitosan is introduced to form hydrogen bonding with graphene oxide nanosheets, and the ionic bonding is formed between graphene oxide nanosheets and divalent calcium ions. The optimized sequential toughening of hydrogen and ionic bonding results in an ultratough graphene fiber with toughness of 26.3 MJ/m³ and ultimate tensile strength of 743.6 MPa. Meanwhile, the electrical conductivity of the resultant graphene fiber is as high as 179.0 S/cm. This kind of multifunctional graphene fiber shows promising applications in photovoltaic wires, flexible supercapacitor electrodes, wearable electronic textiles, fiber motors, etc. Furthermore, the strategy of sequential toughening of hydrogen and ionic bonding interactions also offers an avenue for constructing high-performance graphene-based fibers in the near future.

Bioinspired Supertough Graphene Fiber through Sequential Interfacial Interactions

Natural nacre exhibits extraordinary functional and structural diversity, combining high strength and toughness. The mechanical properties of nacre are attributed to (i) a highly arranged hierarchical layered structure of inorganic minerals (95 vol %) containing a small amount only of organic materials (5 vol %), (ii) abundant synergistic interfacial interactions, and (iii) formation under ambient temperature. Herein, inspired by these three design principles originating from natural nacre, the supertough bioinspired graphene-based nanocomposite fibers (BGNFs) are prepared under room temperature via sequential interfacial interactions of ionic bonding and π-π interactions. The resultant synergistic effect leads to a super toughness of 18.7 MJ m^-3 as well as a high tensile strength of 740.1 MPa. In addition, the electrical conductivity of these supertough BGNFs is as high as 384.3 S cm^-1. They can retain almost 80% of this conductivity even after 1000 cycles of loading-unloading testing, which makes these BGNFs promising candidates for application in flexible and stable electrical devices, such as strain sensors and actuators.

Programmable wettability on photocontrolled graphene film

Surface materials with specific wettability play important roles in a wide variety of areas from science to industry. We present a novel paraffin-infused porous graphene film (PIPGF) with programmable wettability. Because of graphene's photothermal property, the paraffin in the PIPGF was in transition between liquid and solid in response to near-infrared (NIR) light irradiation. Thus, we imparted the film with a dynamic and reversible transition between a slippery and a rough surface as the remotely tunable wettability. In addition, with the integration of NIR masks, the paraffin could melt at corresponding patterns on the PIPGF, which formed special flow pathways for the slipping droplets. Therefore, the PIPGF could provide programmable wettability pathways for the spatiotemporal droplet manipulation by flexibly changing the NIR masks. We demonstrated these programmable wettability pathways to not only simplify liquid handling in the microplates and droplet microarrays technology but also to provide distinctly microfluidic microreactors for different purposes, such as practical blood grouping diagnosis. These features indicated that the photocontrollable PIPGF would be amenable to a variety of applications, such as microfluidic systems, laboratory-on-a-chip settings, and droplet manipulations.

Trends towards Biomimicry in Theranostics

Over the years, imaging and therapeutic modalities have seen considerable progress as a result of advances in nanotechnology. Theranostics, or the marrying of diagnostics and therapy, has increasingly been employing nano-based approaches to treat cancer. While first-generation nanoparticles offered considerable promise in the imaging and treatment of cancer, toxicity and non-specific distribution hindered their true potential. More recently, multistage nanovectors have been strategically designed to shield and carry a payload to its intended site. However, detection by the immune system and sequestration by filtration organs (i.e., liver and spleen) remains a major obstacle. In an effort to circumvent these biological barriers, recent trends have taken inspiration from biology. These bioinspired approaches often involve the use of biologically-derived cellular components in the design and fabrication of biomimetic nanoparticles. In this review, we provide insight into early nanoparticles and how they have steadily evolved to include bioinspired approaches to increase their theranostic potential.

A Hydrogel-Film Casting to Fabricate Platelet-Reinforced Polymer Composite Films Exhibiting Superior Mechanical Properties

The fabrication of mechanically superior polymer composite films with controllable shapes on various scales is difficult. Despite recent research on polymer composites consisting of organic matrices and inorganic materials with layered structures, these films suffer from complex preparations and limited mechanical properties that do not have even integration of high strength, stiffness, and toughness. Herein, a hydrogel-film casting approach to achieve fabrication of simultaneously strong, stiff, and tough polymer composite films with well-defined microstructure, inspired from a layer-by-layer structure of nacre is reported. Ca²⁺ -crosslinked alginate hydrogels incorporated with platelet-like alumina particles are dried to form composite films composed of horizontally aligned alumina platelets and alginate matrix with uniformly layered microstructure. Alumina platelets are evenly distributed parallel without precipitations and contribute to synergistic enhancements of strength, stiffness and toughness in the resultant film. Consequentially, Ca²⁺ -crosslinked alginate/alumina (Ca²⁺ -Alg/Alu) films show exceptional tensile strength (267 MPa), modulus (17.9 GPa), and toughness (3.60 MJ m^-3 ). Furthermore, the hydrogel-film casting allows facile preparation of polymer composite films with controllable shapes and various scales. The results suggest an alternative approach to design and prepare polymer composites with the layer-by-layer structure for superior mechanical properties.

Bio-inspired graphene-derived membranes with strain-controlled interlayer spacing

The precisely controlled size of nanoscale fluidic channels plays a critical role in resolving the permeation-selectivity trade-off in separation and filtration applications, where highly efficient gas separation and water desalination are targeted. Inspired by natural nacre where the spacing between mineral platelets changes upon applying tension as fractured mineral bridges climb over each other, bio-inspired graphene-derived membranes with sheets cross-linked by aligned covalent bonds are proposed in design, to ensure a controlled interlayer spacing ranging from 4 Å to 14 Å while preserving structural and mechanical stabilities by prohibiting swelling. The underlying mechanism is that the tension applied to the membrane is transferred between finite-sized graphene sheets through interlayer shear of the cross-links, which expands the interlayer gallery. First-principles calculations and continuum mechanics based model analysis are combined to explore the feasibility of this protocol, by considering the microstructures of graphene-derived membranes that have recently been demonstrated to offer exceptional performance in selective mass transport. The results show that the critical size range in molecular sieving is covered by this synergetic interface- and strain-engineering approach.

Fire Alarm Wallpaper Based on Fire-Resistant Hydroxyapatite Nanowire Inorganic Paper and Graphene Oxide Thermosensitive Sensor

Wallpaper with multiple functions, such as fire resistance and an automatic alarm in fire disasters, will be attractive for the interior decoration of houses. Herein, we report a smart fire alarm wallpaper prepared using fire-resistant inorganic paper based on ultralong hydroxyapatite nanowires (HNs) and graphene oxide (GO) thermosensitive sensors. At room temperature, the GO thermosensitive sensor is in a state of electrical insulation; however, it becomes electrically conductive at high temperatures. In a fire disaster, high temperature will rapidly remove the oxygen-containing groups of GO, leading to the transformation process of GO from an electrically insulated state into an electrically conductive one. In this way, the alarm lamp and alarm buzzer connected with the GO thermosensitive sensor will send out the alerts to people immediately for taking emergency actions. After the surface modification with polydopamine of GO (PGO), the sensitivity and flame retardancy of the GO thermosensitive sensor are further improved, resulting in a low responsive temperature (126.9 °C), fast response (2 s), and sustained working time in the flame (at least 5 min). Compared with combustible commercial wallpaper, the smart fire alarm wallpaper based on HNs and GO (or PGO) is superior owing to excellent nonflammability and high-temperature resistance of HNs, which can protect the GO (or PGO) thermosensitive sensor from the flames. The smart fire alarm wallpaper can be processed into various shapes, dyed with different colors, and printed with the commercial printer and thus has promising applications in high-safety interior decoration of houses.

A Bioinspired Interface Design for Improving the Strength and Electrical Conductivity of Graphene-Based Fibers

Graphene-based fibers (GBFs) are attractive for next-generation wearable electronics due to their potentially high mechanical strength, superior flexibility, and excellent electrical and thermal conductivity. Many efforts have been devoted to improving these properties of GBFs in the past few years. However, fabricating GBFs with high strength and electrical conductivity simultaneously remains as a great challenge. Herein, inspired by nacre-like multilevel structural design, an interface-reinforced method is developed to improve both the mechanical property and electrical conductivity of the GBFs by introducing polydopamine-derived N-doped carbon species as resistance enhancers, binding agents, and conductive connection "bridges." Remarkably, both the tensile strength and electrical conductivity of the obtained GBFs are significantly improved to ≈724 MPa and ≈6.6 × 10⁴ S m^-1 , respectively, demonstrating great superiority compared to previously reported similar GBFs. These outstanding integrated performances of the GBFs provide it with great application potential in the fields of flexible and wearable microdevices such as sensors, actuators, supercapacitors, and batteries.

Analysis of optimal crosslink density and platelet size insensitivity in graphene-based artificial nacres

Exploration of graphene-based artificial nacres with excellent mechanical properties demonstrates the potential to surpass natural nacre. Recent experimental studies report that optimal crosslink density defined as concentration of the surface functional groups is usually observed in these artificial nacres towards superb mechanical performance. A hybrid model integrating a nonlinear shear-lag model and atomistic simulations reveals the emergence of an optimal crosslink density at which the maximum strength and toughness are achieved. The origin is due to the balance among the reduction of in-plane tensile properties of the graphene sheets, the enhancement of the shear strength of the interlayer and the reduction of interface plasticity. In addition, our results also reveal that the size insensitivity of the graphene sheet appears when the shear stress of the interlayer is highly localized, the increase of the crosslink density intensifies the nonuniformity of the shear stress and the optimal mechanical properties of the artificial nacre cannot be further enhanced by tuning the size of the graphene sheets. Three kinds of interface molecular interactions with their optimal crosslink densities are also proposed to simultaneously maximize the strength and toughness of graphene-based artificial nacres.

High-Performance Nanocomposites Inspired by Nature

Natural materials, including nacre, bone, and the lobster cuticle, exhibit excellent mechanical properties, combining high strength and toughness. Such materials have the added benefit of being light in weight. These advantageous features are due to such natural materials' orderly hierarchical architectures and abundant interface interactions. How to utilize these design principles created by nature to fabricate high-performance bioinspired nanocomposites remains a great research challenge. A logical roadmap for developing these nanocomposites can be described as "discovery, invention, and creation." Here, the discovery of the relationship between natural materials' design principles and such materials' extraordinary mechanical properties is discussed. Then, the invention of bioinspired strategies for mimicking natural materials is considered and representative strategies addressed. Next, the creation of multifunctional nanocomposites is discussed and bioinspired nanocomposites, including fiber nanocomposites, 2D film nanocomposites, and 3D bulk nanocomposites reviewed. Finally, a perspective and outlook for future directions in making bioinspired nanocomposites is provided to offer inspiration to the community and a clear vision for future research.

Dry spinning approach to continuous graphene fibers with high toughness

Graphene fiber (GF) has emerged as a new carbonaceous fiber species since graphene-based liquid crystals were discovered. The growing performances of GFs in terms of their mechanical performance and their functionalities have assured their extensive applications in structural materials and functional textiles. To date, many spinning strategies utilizing coagulation baths have been applied in GF, which necessitates a complicated washing process. Dry spinning is a more convenient and green method for use with fibers in the chemical fiber industry, and should be a good option for GFs; however, this technique has never been used in a system of GF. In this research, first the dry spinning technique was used to fabricate continuous GFs and the dry spun GFs showed good toughness and flexibility. The dry spinnability of graphene oxide liquid crystals was achieved by choosing dispersive solvents with low surface tension and high volatility. The dry spun neat GFs possessed high toughness up to 19.12 MJ m^-3, outperforming the wet spun neat GFs. This dry spinning methodology facilitates the green fabrication of fibers of graphene and graphene-beyond two-dimensional nanomaterials, and it may also be extended to other printing technologies for complex graphene architectures.

Graphene and Other 2D Colloids: Liquid Crystals and Macroscopic Fibers

Two-dimensional colloidal nanomaterials are running into renaissance after the enlightening researches of graphene. Macroscopic one-dimensional fiber is an optimal ordered structural form to express the in-plane merits of 2D nanomaterials, and the formation of liquid crystals (LCs) allows the creation of continuous fibers. In the correlated system from LCs to fibers, understanding their macroscopic organizing behavior and transforming them into new solid fibers is greatly significant for applications. Herein, we retrospect the history of 2D colloids and discuss about the concept of 2D nanomaterial fibers in the context of LCs, elaborating the motivation, principle and possible strategies of fabrication. Then we highlight the creation, development and typical applications of graphene fibers. Additionally, the latest advances of other 2D nanomaterial fibers are also summarized. Finally, conclusions, challenges and perspectives are provided to show great expectations of better and more fibrous materials of 2D nanomaterials. This review gives a comprehensive retrospect of the past century-long effort about the whole development of 2D colloids, and plots a clear roadmap - "lamellar solid - LCs - macroscopic fibers - flexible devices", which will certainly open a new era of structural-multifunctional application for the conventional 2D colloids.

A biomimetic ion-crosslinked layered double hydroxide/alginate hybrid film

Inspired by the structure of nacre, a biomimetic Ca²⁺-crosslinked layered double hydroxide/alginate (LDH/ALG-Ca²⁺) hybrid film is prepared, which exhibits excellent mechanical property and high transparency.

A simple and large-scale method to prepare flexible hollow graphene fibers for a high-performance all-solid fiber supercapacitor

The fabrication of hollow graphene fibers (HGFs) via simple spray deposition of GO on silk fiber templates for a flexible supercapacitor.

Space-Confined Growth of Defect-Rich Molybdenum Disulfide Nanosheets Within Graphene: Application in The Removal of Smoke Particles and Toxic Volatiles

In this work, molybdenum disulfide/reduced graphene oxide (MoS₂/RGO) hybrids are synthesized by a spatially confined reaction to insert the growth of defect-rich MoS₂ nanosheets within graphene to enable incorporation into the polymer matrix for the application in the removal of smoke particles and toxic volatiles. The steady-state tube furnace result demonstrates that MoS₂/RGO hybrid could considerably reduce the yield of CO and smoke particles. The TG-IR coupling technique was utilized to identify species of toxic volatiles including aromatic compounds, CO, and hydrocarbons and to investigate the removal effect of MoS₂/RGO hybrids on reducing toxic volatiles. The removal of smoke particles and toxic volatiles was attributed to the adsorption capacity derived from edges sites of MoS₂ and the honeycomb lattice of graphene, as well as the inhibition of nanobarrier resulting from two-dimensional structure. The work will offer a strategy for fabricating graphene-based hybrids by the space-confined synthesis and exploiting the application of space-confined graphene-based hybrid.

High-strength carbon nanotube/carbon composite fibers via chemical vapor infiltration

In this study, we have developed an efficient and scalable method for improving the mechanical properties of carbon nanotube (CNT) fibers. The mechanical properties of as-synthesized CNT fibers are primarily limited by their porous structures and the weak bonding between adjacent CNTs. These result in inefficient load transfer, leading to low tensile strength and modulus. In order to overcome these limitations, we have adopted chemical vapor infiltration (CVI) to efficiently fill the internal voids of the CNT fibers with carbon species which are thermally decomposed from gas phase hydrocarbon. Through the optimization of the processing time, temperature, and gas flow velocity, we have confirmed that carbon species formed by the thermal decomposition of acetylene (C₂H₂) gas successfully infiltrated into porous CNT fibers and densified them at relatively low temperatures (650-750 °C). As a result, after CVI processing of the as-synthesized CNT fibers under optimum conditions, the tensile strength and modulus increased from 0.6 GPa to 1.7 GPa and from 25 GPa to 127 GPa, respectively. The CVI technique, combined with the direct spinning of CNT fibers, can open up a route to the fast and scalable fabrication of high performance CNT/C composite fibers. In addition, the CVI technique is a platform technology that can be easily adapted into other nano-carbon based yarn-like fibers such as graphene fibers.

Ca2+ Enhanced Nacre-Inspired Montmorillonite-Alginate Film with Superior Mechanical, Transparent, Fire Retardancy, and Shape Memory Properties

Inspired by nacre, this is the first time that using the cross-linking of alginate with Ca ions to fabricate organic-inorganic nacre-inspired films we have successfully prepared a new class of Ca²⁺ ion enhanced montmorillonite (MMT)-alginate (ALG) composites, realizing an optimum combination of high strength (∼280 MPa) and high toughness (∼7.2 MJ m^-3) compared with other MMT based artificial nacre. Furthermore, high temperature performance of the composites (with a maximum strength of ∼170 MPa at 100 °C) along with excellent transmittance, fire retardancy, and unique shape memory response to alcohols could greatly expand the application of the mutilfunctional composites, which are believed to show competitive advantages in transportion, construction, and insulations, protection of a flammable biological material, etc.