A New Facile Route to Flexible and Semi‐Transparent Electrodes Based on Water Exfoliated Graphene and their Single‐Electrode Triboelectric Nanogenerator

Wearable technologies are driving current research efforts to self‐powered electronics, for which novel high‐performance materials such as graphene and low‐cost fabrication processes are highly sought.The integration of high‐quality graphene films obtained from scalable water processing approaches in emerging applications for flexible and wearable electronics is demonstrated. A novel method for the assembly of shear exfoliated graphene in water, comprising a direct transfer process assisted by evaporation of isopropyl alcohol is developed. It is shown that graphene films can be easily transferred to any target substrate such as paper, flexible polymeric sheets and fibers, glass, and Si substrates. By combining graphene as the electrode and poly(dimethylsiloxane) as the active layer, a flexible and semi‐transparent triboelectric nanogenerator (TENG) is demonstrated for harvesting energy. The results constitute a new step toward the realization of energy harvesting devices that could be integrated with a wide range of wearable and flexible technologies, and opens new possibilities for the use of TENGs in many applications such as electronic skin and wearable electronics.     

Spatially Controlled Amyloid Reactions Using Organic Electronics

Abnormal protein aggregates, so called amyloid fibrils, are mainly known as pathological hallmarks of a wide range of diseases, but in addition these robust well‐ordered self‐assembled natural nanostructures can also be utilized for creating distinct nanomaterials for bioelectronic devices. However, current methods for producing amyloid fibrils in vitro offer no spatial control. Herein, we demonstrate a new way to produce and spatially control the assembly of amyloid‐like structures using an organic electronic ion pump (OEIP) to pump distinct cations to a reservoir containing a negatively charged polypeptide. The morphology and kinetics of the created proteinaceous nanomaterials depends on the ion and current used, which we leveraged to create layers incorporating different conjugated thiophene derivatives, one fluorescent (p‐FTAA) and one conducting (PEDOT‐S). We anticipate that this new application for the OEIP will be useful for both biological studies of amyloid assembly and fibrillogenesis as well as for creating new bioelectronic nanomaterials and devices.     

Stable and Biocompatible Carbon Nanotube Ink Mediated by Silk Protein for Printed Electronics

Ink-based processes, which enable scalable fabrication of flexible devices based on nanomaterials, are one of the practical approaches for the production of wearable electronics. However, carbon nanotubes (CNTs), which possess great potential for flexible electronics, are facing challenges for use in inks due to their low dispersity in most solvents and suspicious cytotoxicity. Here, a stable and biocompatible CNT ink, which is stabilized by sustainable silk sericin and free from any artificial chemicals, is reported. The ink shows stability up to months, which can be attributed to the formation of sericin–CNT (SSCNT) hybrid through non-covalent interactions. It is demonstrated that the SSCNT ink can be used for fabricating versatile circuits on textile, paper, and plastic films through various techniques. As proofs of concept, electrocardiogram electrodes, breath sensors, and electrochemical sensors for monitoring human health and activity are fabricated, demonstrating the great potential of the SSCNT ink for smart wearables.     

Self‐Assembly of Enzyme‐Like Nanofibrous G‐Molecular Hydrogel for Printed Flexible Electrochemical Sensors

Conducting hydrogels provide great potential for creating designer shape‐morphing architectures for biomedical applications owing to their unique solid–liquid interface and ease of processability. Here, a novel nanofibrous hydrogel with significant enzyme‐like activity that can be used as “ink” to print flexible electrochemical devices is developed. The nanofibrous hydrogel is self‐assembled from guanosine (G) and KB(OH)₄ with simultaneous incorporation of hemin into the G‐quartet scaffold, giving rise to significant enzyme‐like activity. The rapid switching between the sol and gel states responsive to shear stress enables free‐form fabrication of different patterns. Furthermore, the replication of the G‐quartet wires into a conductive matrix by in situ catalytic deposition of polyaniline on nanofibers is demonstrated, which can be directly printed into a flexible electrochemical electrode. By loading glucose oxidase into this novel hydrogel, a flexible glucose biosensor is developed. This study sheds new light on developing artificial enzymes with new functionalities and on fabrication of flexible bioelectronics.     

Wetting‐Induced Climbing for Transferring Interfacially Assembled Large‐Area Ultrathin Pristine Graphene Film

Owing to inherent 2D structure, marvelous mechanical, electrical, and thermal properties, graphene has great potential as a macroscopic thin film for surface coating, composite, flexible electrode, and sensor. Nevertheless, the production of large‐area graphene‐based thin film from pristine graphene dispersion is severely impeded by its poor solution processability. In this study, a robust wetting‐induced climbing strategy is reported for transferring the interfacially assembled large‐area ultrathin pristine graphene film. This strategy can quickly convert solvent‐exfoliated pristine graphene dispersion into ultrathin graphene film on various substrates with different materials (glass, metal, plastics, and cloth), shapes (film, fiber, and bulk), and hydrophobic/hydrophilic patterns. It is also applicable to nanoparticles, nanofibers, and other exfoliated 2D nanomaterials for fabricating large‐area ultrathin films. Alternate climbing of different ultrathin nanomaterial films allows a layer‐by‐layer transfer, forming a well‐ordered layered composite film with the integration of multiple pristine nanomaterials at nanometer scale. This powerful strategy would greatly promote the development of solvent‐exfoliated pristine nanomaterials from dispersions to macroscopic thin film materials.     

Chemically Integrated Inorganic‐Graphene Two‐Dimensional Hybrid Materials for Flexible Energy Storage Devices

State‐of‐the‐art energy storage devices are capable of delivering reasonably high energy density (lithium ion batteries) or high power density (supercapacitors). There is an increasing need for these power sources with not only superior electrochemical performance, but also exceptional flexibility. Graphene has come on to the scene and advancements are being made in integration of various electrochemically active compounds onto graphene or its derivatives so as to utilize their flexibility. Many innovative synthesis techniques have led to novel graphene‐based hybrid two‐dimensional nanostructures. Here, the chemically integrated inorganic‐graphene hybrid two‐dimensional materials and their applications for energy storage devices are examined. First, the synthesis and characterization of different kinds of inorganic‐graphene hybrid nanostructures are summarized, and then the most relevant applications of inorganic‐graphene hybrid materials in flexible energy storage devices are reviewed. The general design rules of using graphene‐based hybrid 2D materials for energy storage devices and their current limitations and future potential to advance energy storage technologies are also discussed.     

Scalable 2D Hierarchical Porous Carbon Nanosheets for Flexible Supercapacitors with Ultrahigh Energy Density

2D carbon nanomaterials such as graphene and its derivatives, have gained tremendous research interests in energy storage because of their high capacitance and chemical stability. However, scalable synthesis of ultrathin carbon nanosheets with well‐defined pore architectures remains a great challenge. Herein, the first synthesis of 2D hierarchical porous carbon nanosheets (2D‐HPCs) with rich nitrogen dopants is reported, which is prepared with high scalability through a rapid polymerization of a nitrogen‐containing thermoset and a subsequent one‐step pyrolysis and activation into 2D porous nanosheets. 2D‐HPCs, which are typically 1.5 nm thick and 1–3 µm wide, show a high surface area (2406 m² g^?1) and with hierarchical micro‐, meso‐, and macropores. This 2D and hierarchical porous structure leads to robust flexibility and good energy‐storage capability, being 139 Wh kg^?1 for a symmetric supercapacitor. Flexible supercapacitor devices fabricated by these 2D‐HPCs also present an ultrahigh volumetric energy density of 8.4 mWh cm^?3 at a power density of 24.9 mW cm^?3, which is retained at 80% even when the power density is increased by 20‐fold. The devices show very high electrochemical life (96% retention after 10000 charge/discharge cycles) and excellent mechanical flexibility.     

Laser–Material Interactions for Flexible Applications

The use of lasers for industrial, scientific, and medical applications has received an enormous amount of attention due to the advantageous ability of precise parameter control for heat transfer. Laser‐beam‐induced photothermal heating and reactions can modify nanomaterials such as nanoparticles, nanowires, and two‐dimensional materials including graphene, in a controlled manner. There have been numerous efforts to incorporate lasers into advanced electronic processing, especially for inorganic‐based flexible electronics. In order to resolve temperature issues with plastic substrates, laser–material processing has been adopted for various applications in flexible electronics including energy devices, processors, displays, and other peripheral electronic components. Here, recent advances in laser–material interactions for inorganic‐based flexible applications with regard to both materials and processes are presented.     

Highly Concentrated,Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices

Solution‐based techniques are considered as a promising strategy for scalable fabrication of flexible electronics owing to their low‐cost and high processing speed. The key to the success of these techniques is dominated by the ink formulation of active nanomaterials. This work successfully prepares a highly concentrated two dimensional (2D) crystal ink comprised of ultrathin nickel hydroxide (Ni(OH)₂) nanosheets with an average lateral size of 34 nm. The maximum concentration of Ni(OH)₂ nanosheets in water without adding any additives reaches as high as 50 mg mL^?1, which can be printed on arbitrary substrates to form Ni(OH)₂ thin films. As a proof‐of‐concept application, Ni(OH)₂ nanosheet ink is coated on commercialized carbon fiber yarns to fabricate wearable energy storage devices. The thus‐fabricated hybrid supercapacitors exhibit excellent flexibility with a capacitance retention of 96% after 5000 bending–unbending cycles, and good weavability with a high volumetric capacitance of 36.3 F cm^?3 at a current density of 0.4 A cm^?3, and an energy density of 11.3 mWh cm^?3 at a power density of 0.3 W cm^?3. As a demonstration of practical application, a red light emitting diode can be lighted up by three hybrid devices connected in series.     

Superamphiphobic Bionic Proboscis for Contamination‐Free Manipulation of Nano and Core–Shell Droplets

Manipulation of nanoliter droplets is a key step for many emerging technologies including ultracompact microfluidics devices, 3D and flexible electronic printing. Despite progress, contamination‐free generation and release of nanoliter droplets by compact low‐cost devices remains elusive. In the present study, inspired by butterflies' minute manipulation of fluids, the authors have engineered a superamphiphobic bionic proboscis (SAP) layout that surpasses synthetic and natural designs. The authors demonstrate the scalable fabrication of SAPs with tunable inner diameters down to 50 µ m by the rapid gas‐phase nanotexturing of the outer and inner surfaces of readily available hypodermic needles. Optimized SAPs achieve contamination‐free manipulation of water and oil droplets down to a liquid surface tension of 26.56 mN m^?1 and a volume of 10 nL. The unique potential of this layout is showcased by the rapid and carefully controlled in‐air synthesis of core‐shell droplets with well‐controlled compositions. These findings provide a new low‐cost tool for high‐precision manipulation of nanoliter droplets, offering a powerful alternative to established thermal‐ and electrodynamic‐based devices.     

Soft Langmuir–Blodgett Technique for Hard Nanomaterials

Materials and their assemblies of dimensions down to a few nanometers have attracted considerable scientific interest in physical, chemical, and biological sciences because of unique properties not available in their bulk counterparts. The Langmuir–Blodgett (LB) technique allows rigid nanomaterials to be aligned in particular structures through a flexible assembly process at liquid interfaces. In this review, we summarize the development of assembly of hard nanomaterials using soft LB techniques. An initial summary of the basic features of nanomaterials will include dimension‐related effects, synthesis, characterization, and analysis, and will be followed by examples of LB assemblies of nanomaterials described according to their morphology: nanoparticles, nanorods, nanowires, nanotubes, and nanosheets. Some of the nanomaterials have been fabricated in orientation‐controlled morphologies, and have been incorporated into prototype devices for gas sensing and photocurrent transport. In the final part of this review, the challenges remaining for LB techniques of hard nanomaterials will be overviewed, and will include a comparison with the widely‐used LB technique involving soft materials.     

Crab Chitin‐Based 2D Soft Nanomaterials for Fully Biobased Electric Devices

2D nanomaterials have various size/morphology‐dependent properties applicable in electronics, optics, sensing, and actuating. However, intensively studied inorganic 2D nanomaterials are frequently hindered to apply in some particular and industrial fields, owing to harsh synthesis, high‐cost, cytotoxicity, and nondegradability. Endeavor has been made to search for biobased 2D nanomaterials with biocompatibility, sustainability, and biodegradability. A method of hydrophobization‐induced interfacial‐assembly is reported to produce an unprecedented type of nanosheets from marine chitin. During this process, two layers of chitin aggregations assemble into nanosheets with high aspect ratio. With super stability and amphiphilicity, these nanosheets have super ability in creating highly stable Pickering emulsions with internal phase up to 83.4% and droplet size up to 140 μm, in analogue to graphene oxide. Combining emulsifying and carbonization can further convert these 2D precursors to carbon nanosheets with thickness as low as ≈3.8 nm. Having biologic origin, conductivity, and dispersibility in various solvents, resultant carbon nanosheets start a new scenario of exploiting marine resources for fully biobased electric devices with sustainability and biodegradability, e.g., supercapacitor, flexible circuits, and electronic sensors. Hybrid films of chitin and carbon nanosheets also offer low‐cost and environment‐friendly alternative of conductive components desirable in green electronics, wearable electronics, biodegradable circuits, and biologic devices.     

Direct Laser‐Patterned Micro‐Supercapacitors from Paintable MoS2 Films

Micrometer‐sized electrochemical capacitors have recently attracted attention due to their possible applications in micro‐electronic devices. Here, a new approach to large‐scale fabrication of high‐capacitance, two‐dimensional MoS₂ film‐based micro‐supercapacitors is demonstrated via simple and low‐cost spray painting of MoS₂ nanosheets on Si/SiO₂ chip and subsequent laser patterning. The obtained micro‐supercapacitors are well defined by ten interdigitated electrodes (five electrodes per polarity) with 4.5 mm length, 820 μm wide for each electrode, 200 μm spacing between two electrodes and the thickness of electrode is ～0.45 μm. The optimum MoS₂‐based micro‐supercapacitor exhibits excellent electrochemical performance for energy storage with aqueous electrolytes, with a high area capacitance of 8 mF cm^?2 (volumetric capacitance of 178 F cm^?3) and excellent cyclic performance, superior to reported graphene‐based micro‐supercapacitors. This strategy could provide a good opportunity to develop various micro‐/nanosized energy storage devices to satisfy the requirements of portable, flexible, and transparent micro‐electronic devices.     

Porous Graphene Materials for Advanced Electrochemical Energy Storage and Conversion Devices

Combining the advantages from both porous materials and graphene, porous graphene materials have attracted vast interests due to their large surface areas, unique porous structures, diversified compositions and excellent electronic conductivity. These unordinary features enable porous graphene materials to serve as key components in high‐performance electrochemical energy storage and conversion devices such as lithium ion batteries, supercapacitors, and fuel cells. This progress report summarizes the typical fabrication methods for porous graphene materials with micro‐, meso‐, and macro‐porous structures. The structure–property relationships of these materials and their application in advanced electrochemical devices are also discussed.     

Graphene‐Bonded and ‐Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes

Silicon (Si) has been considered a very promising anode material for lithium ion batteries due to its high theoretical capacity. However, high‐capacity Si nanoparticles usually suffer from low electronic conductivity, large volume change, and severe aggregation problems during lithiation and delithiation. In this paper, a unique nanostructured anode with Si nanoparticles bonded and wrapped by graphene is synthesized by a one‐step aerosol spraying of surface‐modified Si nanoparticles and graphene oxide suspension. The functional groups on the surface of Si nanoparticles (50–100 nm) not only react with graphene oxide and bind Si nanoparticles to the graphene oxide shell, but also prevent Si nanoparticles from aggregation, thus contributing to a uniform Si suspension. A homogeneous graphene‐encapsulated Si nanoparticle morphology forms during the aerosol spraying process. The open‐ended graphene shell with defects allows fast electrochemical lithiation/delithiation, and the void space inside the graphene shell accompanied by its strong mechanical strength can effectively accommodate the volume expansion of Si upon lithiation. The graphene shell provides good electronic conductivity for Si nanoparticles and prevents them from aggregating during charge/discharge cycles. The functionalized Si encapsulated by graphene sample exhibits a capacity of 2250 mAh g^?1 (based on the total mass of graphene and Si) at 0.1C and 1000 mAh g^?1 at 10C, and retains 85% of its initial capacity even after 120 charge/discharge cycles. The exceptional performance of graphene‐encapsulated Si anodes combined with the scalable and one‐step aerosol synthesis technique makes this material very promising for lithium ion batteries.     

Direct CVD Graphene Growth on Semiconductors and Dielectrics for Transfer‐Free Device Fabrication

Graphene is the most broadly discussed and studied two‐dimensional material because of its preeminent physical, mechanical, optical, and thermal properties. Until now, metal‐catalyzed chemical vapor deposition (CVD) has been widely employed for the scalable production of high‐quality graphene. However, in order to incorporate the graphene into electronic devices, a transfer process from metal substrates to targeted substrates is inevitable. This process usually results in contamination, wrinkling, and breakage of graphene samples ‐ undesirable in graphene‐based technology and not compatible with industrial production. Therefore, direct graphene growth on desired semiconductor and dielectric substrates is considered as an effective alternative. Over the past years, there have been intensive investigations to realize direct graphene growth using CVD methods without the catalytic role of metals. Owing to the low catalytic activity of non‐metal substrates for carbon precursor decomposition and graphene growth, several strategies have been designed to facilitate and engineer graphene fabrication on semiconductors and insulators. Here, those developed strategies for direct CVD graphene growth on semiconductors and dielectrics for transfer‐free fabrication of electronic devices are reviewed. By employing these methods, various graphene‐related structures can be directly prepared on desired substrates and exhibit excellent performance, providing versatile routes for varied graphene‐based materials fabrication.     

Implantable Organic Electronic Ion Pump Enables ABA Hormone Delivery for Control of Stomata in an Intact Tobacco Plant

Electronic control of biological processes with bioelectronic devices holds promise for sophisticated regulation of physiology, for gaining fundamental understanding of biological systems, providing new therapeutic solutions, and digitally mediating adaptations of organisms to external factors. The organic electronic ion pump (OEIP) provides a unique means for electronically‐controlled, flow‐free delivery of ions, and biomolecules at cellular scale. Here, a miniaturized OEIP device based on glass capillary fibers (c‐OEIP) is implanted in a biological organism. The capillary form factor at the sub‐100 µm scale of the device enables it to be implanted in soft tissue, while its hyperbranched polyelectrolyte channel and addressing protocol allows efficient delivery of a large aromatic molecule. In the first example of an implantable bioelectronic device in plants, the c‐OEIP readily penetrates the leaf of an intact tobacco plant with no significant wound response (evaluated up to 24 h) and effectively delivers the hormone abscisic acid (ABA) into the leaf apoplast. OEIP‐mediated delivery of ABA, the phytohormone that regulates plant's tolerance to stress, induces closure of stomata, the microscopic pores in leaf's epidermis that play a vital role in photosynthesis and transpiration. Efficient and localized ABA delivery reveals previously unreported kinetics of ABA‐induced signal propagation.     

Photosynthetic Bioelectronic Sensors for Touch Perception,UV‐Detection,and Nanopower Generation: Toward Self‐Powered E‐Skins

Energy self‐sufficiency is an inspirational design feature of biological systems that fulfills sensory functions. Plants such as the “touch‐me‐not” and “Venus flytrap” not only sustain life by photosynthesis, but also execute specialized sensory responses to touch. Photosynthesis enables these organisms to sustainably harvest and expend energy, powering their sensory abilities. Photosynthesis therefore provides a promising model for self‐powered sensory devices like electronic skins (e‐skins). While the natural sensory abilities of human skin have been emulated in man‐made materials for advanced prosthetics and soft‐robotics, no previous e‐skin has incorporated phototransduction and photosensory functions that could extend the sensory abilities of human skin. A proof‐of‐concept bioelectronic device integrated with natural photosynthetic pigment‐proteins is presented that shows the ability to sense not only touch stimuli (down to 3000 Pa), but also low‐intensity ultraviolet radiation (down to 0.01 mW cm^‐2) and generate an electrical power of ≈260 nW cm^‐2. The scalability of this device is demonstrated through the fabrication of flexible, multipixel, bioelectronic sensors capable of touch registration and tracking. The polysensory abilities, energy self‐sufficiency, and additional nanopower generation exhibited by this bioelectronic system make it particularly promising for applications like smart e‐skins and wearable sensors, where the photogenerated power can enable remote data transmission.     

Carbon‐Nanomaterial‐Based Flexible Batteries for Wearable Electronics

Wearable electronics have received considerable attention in recent years. These devices have penetrated every aspect of our daily lives and stimulated interest in futuristic electronics. Thus, flexible batteries that can be bent or folded are desperately needed, and their electrochemical functions should be maintained stably under the deformation states, given the increasing demands for wearable electronics. Carbon nanomaterials, such as carbon nanotubes, graphene, and/or their composites, as flexible materials exhibit excellent properties that make them suitable for use in flexible batteries. Herein, the most recent progress on flexible batteries using carbon nanomaterials is discussed from the viewpoint of materials fabrication, structure design, and property optimization. Based on the current progress, the existing advantages, challenges, and prospects are outlined and highlighted.     

Photocatalytic Reduction of Graphene Oxide–TiO2 Nanocomposites for Improving Resistive‐Switching Memory Behaviors

Graphene oxide (GO)‐based resistive‐switching (RS) memories offer the promise of low‐temperature solution‐processability and high mechanical flexibility, making them ideally suited for future flexible electronic devices. The RS of GO can be recognized as electric‐field‐induced connection/disconnection of nanoscale reduced graphene oxide (RGO) conducting filaments (CFs). Instead of operating an electrical FORMING process, which generally results in high randomness of RGO CFs due to current overshoot, a TiO₂‐assisted photocatalytic reduction method is used to generate RGO‐domains locally through controlling the UV irradiation time and TiO₂ concentration. The elimination of the FORMING process successfully suppresses the RGO overgrowth and improved RS memory characteristics are achieved in graphene oxide–TiO₂ (Go‐TiO₂) nanocomposites, including reduced SET voltage, improved switching variability, and increased switching speed. Furthermore, the room‐temperature process of this method is compatible with flexible plastic substrates and the memory cells exhibit excellent flexibility. Experimental results evidence that the combined advantages of reducing the oxygen‐migration barrier and enhancing the local‐electric‐field with RGO‐manipulation are responsible for the improved RS behaviors. These results offer valuable insight into the role of RGO‐domains in GO memory devices, and also, this mild photoreduction method can be extended to the development of carbon‐based flexible electronics.