An Environment‑Tolerant Ion‑Conducting Double‑Network Composite Hydrogel for High‑Performance Flexible Electronic Devices

High-performance ion-conducting hydrogels(ICHs)are vital for developing flexible electronic devices.However,the robustness and ion-conducting behavior of ICHs deteriorate at extreme tempera-tures,hampering their use in soft electronics.To resolve these issues,a method involving freeze–thawing and ionizing radiation technology is reported herein for synthesizing a novel double-network(DN)ICH based on a poly(ionic liquid)/MXene/poly(vinyl alcohol)(PMP DN ICH)system.The well-designed ICH exhibits outstanding ionic conductivity(63.89 mS cm^(-1) at 25℃),excellent temperature resistance(-60–80℃),prolonged stability(30 d at ambient temperature),high oxidation resist-ance,remarkable antibacterial activity,decent mechanical performance,and adhesion.Additionally,the ICH performs effectively in a flexible wireless strain sensor,thermal sensor,all-solid-state supercapacitor,and single-electrode triboelectric nanogenerator,thereby highlighting its viability in constructing soft electronic devices.The highly integrated gel structure endows these flexible electronic devices with stable,reliable signal output performance.In particular,the all-solid-state supercapacitor containing the PMP DN ICH electrolyte exhibits a high areal specific capacitance of 253.38 mF cm^(-2)(current density,1 mA cm^(-2))and excellent environmental adaptability.This study paves the way for the design and fabrication of high-performance mul-tifunctional/flexible ICHs for wearable sensing,energy-storage,and energy-harvesting applications.

Low-temperature metal–oxide thin-film transistor technologies for implementing flexible electronic circuits and systems

Here we review two 300℃metal–oxide(MO)thin-film transistor(TFT)technologies for the implementation of flexible electronic circuits and systems.Fluorination-enhanced TFTs for suppressing the variation and shift of turn-on voltage(VON),and dual-gate TFTs for acquiring sensor signals and modulating VON have been deployed to improve the robustness and performance of the systems in which they are deployed.Digital circuit building blocks based on fluorinated TFTs have been designed,fabricated,and characterized,which demonstrate the utility of the proposed low-temperature TFT technologies for implementing flexible electronic systems.The construction and characterization of an analog front-end system for the acquisition of bio-potential signals and an active-matrix sensor array for the acquisition of tactile images have been reported recently.

Mass-producible low-cost flexible electronic fabrics for azo dye wastewater treatment by electrocoagulation

Electrocoagulation is progressively becoming an ecologically friendly water treatment method owing to its lack of secondary pollution,high active ingredient concentration,high treatment effectiveness,simple equipment,and simplicity of automated control implementation.Herein,electrocoagulation is offered as a method for treating wastewater containing azo dyes using a revolutionary flexible electronic fabric that can be mass-producible at a reasonable price.A computer-controlled machine embroiders 316L stainless steel fiber(316L SSF)onto an insulating fabric to manufacture a flexible electronic device of cathode and anode with a monopolar arrangement on the fabric surface.Using methyl orange(MO)solution to simulate azo dye wastewater,the decolorization rate of 500 ml MO reached 99.25% under the conditions of 50 mg·L^(-1)initial mass concentration,120 min electrolysis time,15 mA·g^(-1)current density,1 cm electrode spacing,0.1 mol·L^(-1)NaCl,pH 7.6,200 r·min^(-1)rotational speed of the stirrer,and 22-25℃ room temperature.In addition,it is feasible to embroider flexible electronic fabrics with varied sizes and numbers of electrodes based on the amount of treated sewage to increase the degradation rate,which has significant practical application value.

Flexible electronics and optoelectronics of 2D van der Waals materials

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries,including healthcare and wellness,electronic skins,the automotive industry,and foldable or rollable displays.Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface.Two-dimensional van der Waals materials(2D VdWMs)have exceptional multifunctional properties,including large specific area,dangling-bond-free interface,plane-to-plane van der Waals interactions,and excellent mechanical,electrical,and optical properties.Thus,2D VdWMs have considerable application potential in functional intelligent flexible devices.To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures,new designs and configurations of electronics and optoelectronics have emerged.However,these new designs and configurations do not consider lattice mismatch and process incompatibility issues.In this review,we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly.Moreover,we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics.

Structured Microgrids (SμGs) and Flexible Electronic Large Power Transformers (FeLPTs)

Structured microgrids(SμGs)and Flexible electronic large power transformers(FeLPTs)are emerging as two essential technologies for renewable energy integration,flexible power transmission,and active control.SμGs provide the integration of renewable energy and storage to balance the energy demand and supply as needed for a given system design.FeLPT’s flexibility for processing,control,and re-configurability offers the capability for flexible transmission for effective flow control and enable SμGs connectivity while still keeping multiscale system level control.Early adaptors for combined heat and power have demonstrated significant economic benefits while reducing environmental foot prints.They bring tremendous benefits to utility companies also.With storage and active control capabilities,a 300-percent increase in bulk transmission and distribution lines are possible without having to increase capacity.SμGs and FeLPTs will also enable the utility industry to be better prepared for the emerging large increase in base load demand from electric transportation and data centers.This is a win-win-win situation for the consumer,the utilities(grid operators),and the environment.SμGs and FeLPTs provide value in power substation,energy surety,reliability,resiliency,and security.It is also shown that the initial cost associated with SμG and FeLPTs deployment can be easily offset with reduced operating cost,which in turn reduces the total life-cycle cost by 33%to 67%.

Polymer-Sandwich Ultra-Thin Silicon(100) Platform for Flexible Electronics

As a potential flexible substrate for flexible electronics, a polymer-sandwiched ultra-thin silicon platform is stud- ied. SU-8 photoresist coated on the silicon membrane improves its flexibility as shown by an ANSYS simulation. Using the plasma enhanced chemical vapor deposited Si02/Si3N4 composite film as an etching mask, a 4＂ silicon- （100） wafer is thinned to 26[tm without rupture in a 30wt.% KOH solution. The thinned wafer is coated on both sides with 20 pm of SU-8 photoresist and is cut into strips. Then the strips are bent by a caliper to measure its bending radius. A sector model of bending deformation is adopted to estimate the radius of curvature. The determined minimal bending radius of the polymer-sandwiched ultra-thin silicon layer is no more than 3.3mm. The fabrication process of this sandwich structure can be used as a post-fabrication process for high performance flexible electronics.

Third-order polynomial model for analyzing stickup state laminated structure in flexible electronics

Laminated hard-soft integrated structures play a significant role in the fabrication and development of flexible electronics devices. Flexible electronics have advantageous characteristics such as soft and light-weight, can be folded,twisted, flipped inside-out, or be pasted onto other surfaces of arbitrary shapes. In this paper, an analytical model is presented to study the mechanics of laminated hard-soft structures in flexible electronics under a stickup state. Thirdorder polynomials are used to describe the displacement field,and the principle of virtual work is adopted to derive the governing equations and boundary conditions. The normal strain and the shear stress along the thickness direction in the bimaterial region are obtained analytically, which agree well with the results from finite element analysis. The analytical model can be used to analyze stickup state laminated structures, and can serve as a valuable reference for the failure prediction and optimal design of flexible electronics in the future.

In situ reduction strategy towards high conductivity,anti-freezing and super-stretchable rGO based hydrogel for diverse flexible electronics

Hydrogels electrolytes with flexibility and high conductivity have been widely used in kinds of flexible electronics.However,hydrogels always suffer from the inevitable freezing of water at subzero temperatures,which results in the sacrificing of their electrical properties.Herein,an anti-freezing,flexible hydrogel based on in situ reduction of graphene oxide(GO)and laponite has been developed as electrolyte for high performance supercapacitor and sensitive sensors.The crosslinked GO and laponite in polyacrylamide(PAM)resulted in an enhanced mechanical property,while the in-situ reduction of GO in the hydrogel enhanced the conductivity and diminishes the aggregated of GO.These features guarantee a reliable electro signal as sensor and a high performance of the supercapacitor.Besides,in the process of preparation of reduced graphene oxide(rGO)hydrogel,the addition of ethylene glycol(EG)and KOH,endows the hydrogel antifreeze properties.This anti-freezing electrolyte can be stretched to a strain of 1600%and maintained a specific capacitance of 37.38 F·g^(-1) at-20℃.In addition,the photothermal conversion character of rGO in the hydrogel,endows it’s the potential application in wound healing.The overall merits of the hydrogel will open up a new avenue for sensitive sensor and energy storage device in practical applications.

Electrohydrodynamic printing for high resolution patterning of flexible electronics toward industrial applications

Electrohydrodynamic(EHD)printing technique,which deposits micro/nanostructures through high electric force,has recently attracted significant research interest owing to their fascinating characteristics in high resolution(<1μm),wide material applicability(ink viscosity 1–10000 cps),tunable printing modes(electrospray,electrospinning,and EHD jet printing),and compatibility with flexible/wearable applications.Since the laboratory level of the EHD printed electronics'resolution and efficiency is gradually approaching the commercial application level,an urgent need for developing EHD technique from laboratory into industrialization have been put forward.Herein,we first discuss the EHD printing technique,including the ink design,droplet formation,and key technologies for promoting printing efficiency/accuracy.Then we summarize the recent progress of EHD printing in fabrication of displays,organic field-effect transistors(OFETs),transparent electrodes,and sensors and actuators.Finally,a brief summary and the outlook for future research effort are presented.

Biological Tissue-Inspired Ultrasoft,Ultrathin,and Mechanically Enhanced Microfiber Composite Hydrogel for Flexible Bioelectronics

Hydrogels offer tissue-like softness,stretchability,fracture toughness,ionic conductivity,and compatibility with biological tissues,which make them promising candidates for fabricating flexible bioelectronics.A soft hydrogel film offers an ideal interface to directly bridge thin-film electronics with the soft tissues.However,it remains difficult to fabricate a soft hydrogel film with an ultrathin configuration and excellent mechanical strength.Here we report a biological tissue-inspired ultrasoft microfiber composite ultrathin(<5μm)hydrogel film,which is currently the thinnest hydrogel film as far as we know.The embedded microfibers endow the composite hydrogel with prominent mechanical strength(tensile stress~6 MPa)and anti-tearing property.Moreover,our microfiber composite hydrogel offers the capability of tunable mechanical properties in a broad range,allowing for matching the modulus of most biological tissues and organs.The incorporation of glycerol and salt ions imparts the microfiber composite hydrogel with high ionic conductivity and prominent anti-dehydration behavior.Such microfiber composite hydrogels are promising for constructing attaching-type flexible bioelectronics to monitor biosignals.

Flexible electronics based on one-dimensional inorganic semiconductor nanowires and two-dimensional transition metal dichalcogenides

Flexible electronics technology is considered as a revolutionary technology to unlock the bottleneck of traditional rigid electronics that prevalent for decades,thereby fueling the next-generation electronics.In the past few decades,the research on flexible electronic devices based on organic materials has witnessed rapid development and substantial achievements,and inorganic semiconductors are also now beginning to shine in the field of flexible electronics.As validated by the latest research,some of the inorganic semiconductors,particularly those at low dimension,unexpectedly exhibited excellent mechanical flexibility on top of superior electrical properties.Herein,we bring together a comprehensive analysis on the recently burgeoning low-dimension inorganic semiconductor materials in flexible electronics,including one-dimensional(1D)inorganic semiconductor nanowires(NWs)and two-dimensional(2D)transition metal dichalcogenides(TMDs).The fundamental electrical properties,optical properties,mechanical properties and strain engineering of materials,and their performance in flexible device applications are discussed in detail.We also propose current challenges and predict future development directions including material synthesis and device fabrication and integration.

Failure mechanisms in flexible electronics

The rapid evolution of flexible electronic devices promises to revolutionize numerous fields by expanding the applications of smart devices.Nevertheless,despite this vast potential,the reliability of these innovative devices currently falls short,especially in light of demanding operation environment and the intrinsic challenges associated with their fabrication techniques.The heterogeneity in these processes and environments gives rise to unique failure modes throughout the devices'lifespan.To significantly enhance the reliability of these devices and assure long-term performance,it is paramount to comprehend the underpinning failure mechanisms thoroughly,thereby,enabling,optimal design solutions.A myriad of investigative efforts have been dedicated to unravel these failure mechanisms,utilizing a spectrum of tools from analytical models,numerical methods,to advanced characterization methods.This review delves into the root causes of device failure,scrutinizing both the fabrication process and the operation environment.Next,We subsequently address the failure mechanisms across four commonly observed modes:strength failure,fatigue failure,interfacial failure,and electrical failure,followed by an overview of targeted characterization methods associated with each mechanism.Concluding with an outlook,we spotlight ongoing challenges and promising directions for future research in our pursuit of highly resilient flexible electronic devices.

Viologen-based flexible electrochromic devices

Electrochromic technology has gained significant attention in various fields such as displays,smart windows,biomedical monitoring,military camouflage,human-machine interaction,and electronic skin due to its ability to provide reversible and fast color changes under applied voltage.With the rapid development and increasing demand for flexible electronics,flexible electrochromic devices(FECDs)that offer smarter and more controllable light modulation hold great promise for practical applications.The electrochromic material(ECM)undergoing color changes during the electrochemical reactions is one of the key components in electrochromic devices.Among the ECMs,viologens,a family of organic small molecules with 1,1'-disubstituted-4,4'-dipyridinium salts,have garnered extensive research interest,due to their well-reversible redox reactions,excellent electron acceptance ability,and the ability to produce multiple colors.Notably,viologen-based FECDs demonstrate color changes in the liquid or semisolid electrolyte layer,eliminating the need for two solid electrodes and thus simplifying the device structure.Consequently,viologens offer significant potential for the development of FECDs with high optical contrast,fast response speed,and excellent stability.This review aims to provide a comprehensive overview of the progress and perspectives of viologen-based FECDs.It begins by summarizing the typical structure and recent exciting developments in viologen-based FECDs,along with their advantages and disadvantages.Furthermore,the review discusses recent advancements in FECDs with additional functionalities such as sensing,photochromism,and energy storage.Finally,the remaining challenges and potential research directions for the future of viologen-based FECDs are addressed.

Artificial Intelligence Meets Flexible Sensors:Emerging Smart Flexible Sensing Systems Driven by Machine Learning and Artificial Synapses

The recent wave of the artificial intelligence(AI)revolution has aroused unprecedented interest in the intelligentialize of human society.As an essential component that bridges the physical world and digital signals,flexible sensors are evolving from a single sensing element to a smarter system,which is capable of highly efficient acquisition,analysis,and even perception of vast,multifaceted data.While challenging from a manual perspective,the development of intelligent flexible sensing has been remarkably facilitated owing to the rapid advances of brain-inspired AI innovations from both the algorithm(machine learning)and the framework(artificial synapses)level.This review presents the recent progress of the emerging AI-driven,intelligent flexible sensing systems.The basic concept of machine learning and artificial synapses are introduced.The new enabling features induced by the fusion of AI and flexible sensing are comprehensively reviewed,which significantly advances the applications such as flexible sensory systems,soft/humanoid robotics,and human activity monitoring.As two of the most profound innovations in the twenty-first century,the deep incorporation of flexible sensing and AI technology holds tremendous potential for creating a smarter world for human beings.

Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

In the past decade,the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare,Internet of Things,human–machine interfaces,artificial intelligence and soft robotics.Among them,flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change.This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring.Four categories of humidity sensors are highlighted based on resistive,capacitive,impedance-type and voltage-type working mechanisms.Furthermore,typical strategies including chemical doping,structural design and Joule heating are introduced to enhance the performance of humidity sensors.Drawing on the noncontact perception capability,human/plant healthcare management,human-machine interactions as well as integrated humidity sensor-based feedback systems are presented.The burgeoning innovations in this research field will benefit human society,especially during the COVID-19 epidemic,where cross-infection should be averted and contactless sensation is highly desired.

Recent advances in designing conductive hydrogels for flexible electronics

Flexible electronics have emerged as an exciting research area in recent years,serving as ideal interfaces bridging biological systems and conventional electronic devices.Flexible electronics can not only collect physiological signals for human health monitoring but also enrich our daily life with multifunctional smart materials and devices.Conductive hydrogels(CHs)have become promising candidates for the fabrication of flexible electronics owing to their biocompatibility,adjustable mechanical flexibility,good conductivity,and multiple stimuli-responsive properties.To achieve on-demand mechanical properties such as stretchability,compressibility,and elasticity,the rational design of polymer networks via modulating chemical and physical intermolecular interactions is required.Moreover,the type of conductive components(eg,electron-conductive materials,ions)and the incorporation method also play an important role in the conductivity of CHs.Electron-CHs usually possess excellent conductivity,while ion-CHs are generally transparent and can generate ion gradients within the hydrogel matrices.This mini review focuses on the recent advances in the design of CHs,introducing various design strategies for electron-CHs and ion-CHs employed in flexible electronics and highlighting their versatile applications such as biosensors,batteries,supercapacitors,nanogenerators,actuators,touch panels,and displays.

A transparent, conducting tape for flexible electronics

Transparent electrodes are essential components for optoelectronic devices such as displays and thin-film solar cells. Traditionally, the deposition of transparent conducting layers and the sealing of the device are separate steps. Here we report on a highly transparent, conductive, and flexible ＂tape＂, which can be obtained by transferring silver nanowire networks to conventional transparent tape. We utilized the viscidity of the tape to reduce the junction resistance between silver nanowires and further protect the nanowires from corrosion, oxidation and mechanical damage. By this simple method, we obtained a flexible tape with high transparency （-90% at 550 nm wavelength） and low sheet resistance （approaching 22 Ω.sq^-1）. The transparent tape can be attached and stuck firmly on complex surfaces, making the surface highly conductive. We demonstrated the use of the tape as both a conducting layer and a sealing layer for flexible electronics applications including in-situ temperature monitoring and electrochromic devices.

Development of flexible Li-ion batteries for flexible electronics

We provide a critical review on the recent development of flexible lithium-ion batteries(FLIBs)for flexible electronic devices.The innovative designs of cell configuration for bendable and stretchable FLIBs,selection of active materials,and evaluation methods for FLIBs are discussed.The grand challenges for FLIBs are energy density and scale-up fabrication as demonstrated in the review.Furthermore,the lack of quantitative evaluation methods for FLIBs'performance and nondestructive tools to probe the mechanical degradation may significantly hinder the development of FLIB technologies.Perspectives for future research directions,based on the current state of progress,are discussed.

Two-dimensional MXenes:New frontier of wearable and flexible electronics

Wearable electronics offer incredible benefits in mobile healthcare monitoring,sensing,portable energy harvesting and storage,human-machine interactions,etc.,due to the evolution of rigid electronics structure to flexible and stretchable devices.Lately,transition metal carbides and nitrides(MXenes)are highly regarded as a group of thriving two-dimensional nanomaterials and extraordinary building blocks for emerging flexible electronics platforms because of their excellent electrical conductivity,enriched surface functionalities,and large surface area.This article reviews the most recent developments in MXene-enabled flexible electronics for wearable electronics.Several MXeneenabled electronic devices designed on a nanometric scale are highlighted by drawing attention to widely developed nonstructural attributes,including 3D configured devices,textile and planer substrates,bioinspired structures,and printed materials.Furthermore,the unique progress of these nanodevices is highlighted by representative applications in healthcare,energy,electromagnetic interference(EMI)shielding,and humanoid control of machines.The emerging prospects of MXene nanomaterials as a key frontier in nextgeneration wearable electronics are envisioned and the design challenges of these electronic systems are also discussed,followed by proposed solutions.

Flexible electronics based on one-dimensional and two-dimensional hybrid nanomaterials

Research on flexible or wearable electronics has been grown remarkably due to the advent of nanomaterials,such as metal nanowires,graphene,or transition metal dichalcogenides.Although each nanomaterial has mechanical and electrical characteristics that can be applied into flexible electronics,the limitations of each nanomaterial are also clear.In order to overcome the limitations of these nanomaterials,research on the hybrid structures of nanomaterials has been extensively conducted.In this study,we introduce the properties of one-dimensional nanomaterials,twodimensional nanomaterials,and their hybrid nanomaterials.And then,we provide information concerning various flexible electronics based on these nanomaterials.