Flexible Electronics Research Articles

Boosting Zinc‐Ion Storage Capability in Longitudinally Aligned MXene Arrays with Microchannel Architecture

AbstractFlexible Zn2+ ion hybrid capacitors (ZHCs) will play a crucial role in developing next‐generation wearable products, which demand portability, durability, and environmental adaptability. To further meet these requirements, Ti3C2Tx MXene with exceptional conductivity and robust mechanical properties can be utilized as cathodes, except for challenges such as the dense stacking of Ti3C2Tx nanosheets. In this study, a novel MXene cathode architecture has been developed with the facilitation of an ice template, which creates longitudinally aligned Ti3C2Tx arrays with microchannels. The introduction of hydrochloric acid to the Ti3C2Tx slurry induces a crumpled morphology, increasing active sites and enhancing ion transport with expanded interlayer spacing. Interestingly, experimental results and COMSOL simulations verify that the cathode structure also has effective impacts on the Zn anode with a weakened ion concentration gradient and suppressed dendrite formation. Consequently, the ZHCs exhibit enhanced electrochemical performance with excellent rate performance and long‐term cycling stability (enduring over 50 000 cycles at 5 A g−1) and further deliver a reliable low‐temperature operation by applying an anti‐freezing electrolyte. Moreover, flexible ZHCs assembled with gel electrolytes demonstrate excellent flexibility, improved rate performance, and mechanical stability, making them well‐suited for flexible electronics that power flexible LED panels.

Liquid Metal Aerogel With Janus Architecture for Selective Direction Recognition and High‐Efficiency Moisture Energy Harvesting

AbstractLiquid metal (LM) micro‐nano droplets show the superiority in reducing the high surface tension applicable in conductive inks for flexible electronics. However, the dynamic surface makes LM droplets susceptible to be oxidized, facing challenges in storage and applications. Herein, a bifunctional groups cross‐linking strategy is adopted to encapsulate LM droplets into a robust shell. During sonicating LM in carboxymethyl chitosan (CMCS) solution with bifunctional groups (i.e., carboxyl and amino groups), a robust interface shell is constructed by anchoring effect, thereby providing high chemical stability (>7d). When directionally freezing‐drying LM dispersions, aerogel with Janus architecture is produced driven by the gravity of LM droplets. Due to the unique heterogeneous structure, the resultant aerogel exhibits a selective multifunctional response toward directional bending. In addition, owing to the gradient distribution of CMCS within aerogel, moisture electricity generator can be built with a high output voltage of 460 mV. Thus, this bifunctional groups cross‐linking strategy not only improves the chemical stability of LM micro‐nano droplets, but also renders a new method for producing multifunctional aerogel with energy harvesting and selective directional recognition, and applicable in smart sensors and power supplying devices.

A review on preparation, properties and applications of graphene composites

The flexible and bending materials technology is focusing on graphene-based materials. A popular technique for its production is chemical vapour deposition. Raman spectroscopy indicated its high quality; oxidation experiments demonstrate its substantial impermeability, and scanning electron microscopy reveals that the films are continuous over broad areas. Due to its unique properties, such as mechanical, electrical, and optical characteristics and highly readily functionalizable derivatives, graphene has emerged as a perfect candidate for the realisation of flexible electronics. Biomedical assistance, membranes, flexible wearable sensors, actuators, electronics, etc, show great promise for using graphene-based materials and their composites. This study delves into the synthesis methods of graphene composites to explore their various applications and overcome technological problems. It explores the discoveries of better mechanical characteristics and conductivity, providing insight into the future of advanced materials. The findings show that graphene and its derivatives can create nanocomposites with various polymer matrices. Graphene varieties, including pristine graphene, graphene oxide, and reduced graphene oxide, were introduced in our study. This study also investigates nanocomposites containing multiple graphene, inorganic, and polymeric components, including polymer/GR, activated carbon/GR, metal oxide/GR, metal/graphene, and carbon fibre/GR.

Dual‐Mode Optical Thermometers Based on the Thermal‐Stable Perovskite Nanocrystals Embedded in Robust Metal Halide Salts

AbstractPhotothermal sensing is crucial for the creation of smart integrated devices. All‐inorganic metal halide perovskites with the formula CsPbX3 (X═Cl, Br, I) have excellent photo‐physical performance, offering exciting opportunities for flexible electronics. Hence, a supersaturated precipitation strategy is proposed for the preparation of salt‐shelled metal halide solids. The well‐designed CsPbX3@MX (M═K, Cs) products exhibit bright narrow visible emissions and favorable thermal stability up to 195 °C. Co‐doping with Ni2+ and Mn2+ ions, the CsPbCl3@KCl products have realized dual‐mode thermometry. Utilizing the blue emission from the CsPbCl3 host and the red emission from the Mn2+ ion, a fluorescence intensity ratio technique is obtained to monitor the temperature of good stability and repeatability. Based on the regular fluorescence decay of Mn2+ ions with rising temperature, the lifetime of Mn2+ ions can also be used for temperature sensing. It is believed that such stable metal halides with dual‐mode thermometry will provide a new sight for optical thermometers and, more importantly, will unleash the possibility of a broad variety of applications in lightweight and integrated functional devices.

Recoverability degradation of adhesion between soft matters under uniaxial cyclic bonding–debonding: Modified cohesive interface model and numerical implementation

The main objective of this work is to propose a partial recoverable cohesive interface model, coupled with a bi-potential contact algorithm, to simulate the phenomenon of adhesion recoverability degradation under cyclic bonding–debonding between hyperelastic bodies. For this end, the proposed adhesion recoverability degradation model is constructed by defining the recovery of interface damage during the rebonding when two bodies come into contact, and a degradation factor related to the number of bonding–debonding cycles is introduced into the fully recoverable adhesion model to reduce energy dissipation after multiple cycles. Recoverability degradation includes adhesive stiffness and strength degradation, which is physically described as parallel, series and mixed arrays of adhesive bonds. Then, a finite element framework coupling the adhesion recoverability degradation model and the bi-potential contact algorithm is proposed, with Mooney–Rivlin hyperelastic material used to describe the soft matters. This framework is implemented in an in-house finite element code, with numerical examples demonstrating the model’s reliability. The proposed approach could be applied to investigate interfacial adhesion effects in fields such as flexible electronics and intelligent robotics.

Transformable 3D curved high-density liquid metal coils – an integrated unit for general soft actuation, sensing and communication

Rigid solenoid coils have long been indispensable in modern intelligent devices. However, their sparse structure and challenging preparation of flexible coils for soft robots impose limitations. Here, a transformable 3D curved high-density liquid metal coil (HD-LMC) is introduced that surpasses the structural density level of enameled wire. The fabrication technique employed for high-density channels in elastomers is universally applicable. Such HD-LMCs demonstrated excellent performance in pressure, temperature, non-contact distance sensors, and near-field communication. Soft electromagnetic actuators thus achieved significantly improved the electromagnetic force and power density. Moreover, precise control of swinging tail motion enables a bionic pufferfish to swim. Finally, HD-LMC is further utilized to successfully implement a soft rotary robot with integrated sensing and actuation capabilities. This groundbreaking research provides a theoretical and experimental basis for expanding the applications of liquid metal-based multi-dimensional complex flexible electronics and is expected to be widely used in liquid metal-integrated robotic systems.

Strong and anti-swelling nanofibrous hydrogel composites inspired by biological tissue for amphibious motion sensors.

Conductive hydrogel-based sensors are increasingly favored for flexible electronics due to their skin-like characteristics. However, conventional hydrogels suffer from significant swelling in humid environments and poor mechanical properties which largely restrict their applications in wearable electronic devices, especially for underwater sensing. Herein, drawing inspiration from the extracellular matrix (ECM) structure, a TPU-PVAc@AgNPs/MXene nanofibrous hydrogel composite (TPAMH) with excellent mechanical robustness and anti-swelling properties is developed. The TPAMH nanofibrous hydrogel composite is created by integrating the silver nanoparticles (AgNPs) and MXene nanosheets into an interwoven network comprising of stiff TPU nanofibers as the fibril scaffold and formic acid-crosslinked PVA hydrogel fibers as the elastic matrix (PVAc). Benefiting from the unique ECM structure, the obtained nanofibrous hydrogel composites exhibit exceptional tensile strength (4.47 MPa), remarkable elongation at break (621%), excellent anti-swelling properties, and high detection sensitivity (maximum gauge factor = 105.02), which are sufficient to monitor body motions in both air and water environments effectively. They can detect large strain movements of fingers, elbows, wrists, and knees, as well as small strain physiological signals such as frown, smile, and pulse beats, with high accuracy. Particularly noteworthy is their ability to accurately identify underwater multidirectional motions and facilitate underwater smart alarms using Morse code.

Flexible memristors with low-operation voltage and high bending stability based on Cu2AgBiI6 perovskite

Cu2AgBiI6 films were prepared by a one-step spin coating method, and flexible memristors with an Ag/PMMA/Cu2AgBiI6/ITO structure were constructed. The devices showed a bipolar resistive switching behavior with low switching voltage, which is beneficial for reducing energy consumption. Furthermore, this study found that the device exhibits an endurance of about 900 cycles, a higher ON/OFF ratio of over 103, a long retention time (∼104 s), and high stabilities against mechanical stress. Remarkably, the present flexible memristor displayed extraordinary flexibility and stability, with no significant change for the resistive switching behavior even at various bending angles or after undergoing 900 bending cycles. This study establishes that the lead-free halide perovskite Cu2AgBiI6 can be used for the resistive random-access memory of flexible electronics.

Biaxial Strain Transfer in Monolayer MoS2 and WSe2 Transistor Structures.

Monolayer transition metal dichalcogenides are intensely explored as active materials in 2D material-based devices due to their potential to overcome device size limitations, sub-nanometric thickness, and robust mechanical properties. Considering their large band gap sensitivity to mechanical strain, single-layered TMDs are well-suited for strain-engineered devices. While the impact of various types of mechanical strain on the properties of a variety of TMDs has been studied in the past, TMD-based devices have rarely been studied under mechanical deformations, with uniaxial strain being the most common one. Biaxial strain on the other hand, which is an important mode of deformation, remains scarcely studied as far as 2D material devices are concerned. Here, we study the strain transfer efficiency in MoS2- and WSe2-based flexible transistor structures under biaxial deformation. Utilizing Raman spectroscopy, we identify that strains as high as 0.55% can be efficiently and homogeneously transferred from the substrate to the material in the transistor channel. In particular, for the WSe2 transistors, we capture the strain dependence of the higher-order Raman modes and show that they are up to five times more sensitive compared to the first-order ones. Our work demonstrates Raman spectroscopy as a nondestructive probe for strain detection in 2D material-based flexible electronics and deepens our understanding of the strain transfer effects on 2D TMD devices.

Role of Wearable Health Devices in Public Health: Developing Flexible Electronics for Seamless and Continuous Health Monitoring

The strategy for personalized medicine is the incorporation of regular health check-ups and wearable health devices into public health campaigns. These innovations have the potential to reduce the burden on healthcare systems by enabling early detection, improved management of chronic conditions, and provision of real-time personal health data to individuals. To achieve this, problems related to the accuracy of devices used, security of data stored in these devices, compliance with usage requirements by consumers, and relations with the current healthcare system must be addressed. A solution to these barriers has been proposed as a Non-Invasive Flexible Sensors for Health Monitoring (N-IFS-HM) approach which involves making sensors that are lightweight, attractive looking, and provide accurate continuous health information without disturbing the users during their daily activities. Through detailed simulation studies conducted in different healthcare settings, this paper examine the dependability and effectiveness of N-IFS-HM implementation. Consequently, based on simulation results done on delicate sensors such as these, vital signs, activities and minimal discomforting healthcare products can be traced accurately. According to this finding, wearable digital technology with sophisticated flexible electronics can revolutionize how public health is assessed.

Correction to “Electrohydrodynamic Printing of Ultrafine and Highly Conductive Ag Electrodes for Various Flexible Electronics”

Correction to “Electrohydrodynamic Printing of Ultrafine and Highly Conductive Ag Electrodes for Various Flexible Electronics”

Differential growth-induced center waving of film-substrate system

Various intriguing morphological patterns emerge on the surfaces of growing, developing, or aging tissues, organs, and colonies of microorganisms. Our research draws inspiration from biological growth processes and applies differential growth to film-substrate systems. The strain mismatch induced by this approach leads to distinctive buckling patterns. Through a combination of finite element analysis (FEA) and theoretical analysis, we derive a comprehensive expression for the strain energy in a buckled film-substrate system subjected to differential growth. By solving the energy equations, our study investigates the intricate interplay of material properties, growth functions, and geometric characteristics. This exploration provides valuable insights into the deformations induced by growth in membrane structures and soft materials. Additionally, we establish a phase diagram that characterizes the buckling patterns. It opens avenues for innovative geometric design and 3D self-assembly techniques. Furthermore, our findings have the potential to inspire applications in fields such as soft robotics and flexible electronics.

Microwave reflective properties of amorphous nanogranular composites (CoTaNb)x(MgO)1-x

The most important task of flexible and stretchable electronics is to modify the structure in nanoheterogeneous metal-dielectric media to control the properties of materials and create new small-sized devices based on them. In this paper, we present the results of experimental studies of the structure and microwave reflective properties in the frequency range of 8–12 GHz of amorphous nanogranulated composites (CoTaNb)x(MgO)1−x, 21.73 ≤ x ≤ 73.27 at.% with a thickness of 311–1040 nm. Niobium and tantalum, which are part of the metal alloy, are the most promising metals used in aircraft and rocket engineering, as well as in radio electronics and microprocessor technology, for example, as capacitors. The AFM method revealed a granular structure with a granule size from 70 to 200 nm at a metal alloy content of up to 51.70 at.%. The spectra of the microwave reflectivity as a function of frequency, the dependence of the reflectivity on the metallic phase content and the effective thickness of the composite layer are obtained. It is shown that a significant increase in the microwave reflectivity and its saturation are determined to a greater extent by the metallic phase content than by the effective layer thickness. The results of the experimental effect of magnetic fields with induction up to 0.3 T on the microwave reflectivity of composite films are presented. It is shown that magnetic fields can most effectively change the reflective properties of composites (from 8 % to 29 %) at the extremes of the microwave reflectivity of films with the lowest metallic phase content of 22.61–21.73 at.%. With an increase in the metallic phase content, the maximum change in the microwave reflectivity decreases exponentially.

Ultraviolet curing technology plus chemical copper plating: A novel method for producing highly durable fabric‐based flexible circuit

AbstractThe construction of flexible circuits is a crucial and challenging aspect in the design and fabrication of fabric‐based flexible electronics, which hold significant potential for various applications. In this study, we successfully developed high‐precision and durable fabric‐based flexible circuits by ingeniously combining ultraviolet light (UV)‐curing technology with chemical plating. Specifically, a UV coating containing Ag/Fe3O4 catalysts was applied onto polyester fabric surface, followed by printing the designed circuit structure diagram onto the fabric using UV light‐directed curing of the coating, and fabric‐based flexible circuits were then fabricated through chemical plating process. The fabric‐based flexible circuits exhibit only minimal increases in resistance following durability testing, including bending (8000 times), abrasion (2000 times), high and low temperature stability (−30 to 60°C), and high temperature/humidity stability (65°C, RH = 95%, 48 h), which remains consistently stable. This developed technology holds immense potential across various applications for smart wearable devices.

Bipolar Textile Composite Electrodes Enabling Flexible Tandem Solid-State Lithium Metal Batteries.

A majority of flexible and wearable electronics require high operational voltage that is conventionally achieved by serial connection of battery unit cells using external wires. However, this inevitably decreases the energy density of the battery module and may cause additional safety hazards. Herein, a bipolar textile composite electrode (BTCE) that enables internal tandem-stacking configuration to yield high-voltage (6 to 12V class) solid-state lithium metal batteries (SSLMBs) is reported. BTCE is comprised of a nickel-coated poly(ethylene terephthalate) fabric (NiPET) core layer, a cathode coated on one side of the NiPET, and a Li metal anode coated on the other side of the NiPET. Stacking BTCEs with solid-state electrolytes alternatively leads to the extension of output voltage and decreased usage of inert package materials, which in turn significantly boosts the energy density of the battery. More importantly, the BTCE-based SSLMB possesses remarkable capacity retention per cycle of over 99.98% over cycling. The composite structure of BTCE also enables outstanding flexibility; the battery keeps stable charge/discharge characteristics over thousands of bending and folding. BTCE shows great promise for future safe, high-energy-density, and flexible SSLMBs for a wide range of flexible and wearable electronics.

Multifunctional double network hydrogel with multi-directional actuation and high-precision sensing

Conductive hydrogels have gained great attention for applications in flexible electronics, electronic skins, and as human-machine interfaces. However, poor environmental tolerance of traditional hydrogels and defects in hydrogel actuator make them unfit for use in certain environments. Herein, a small natural molecule, proline (ZP), was introduced into the PAA/CS dual network system to prepare multifunctional hydrogel materials with high concentration of physically crosslinked metal coordination points. The elongation at break and mechanical strength of the hydrogel with optimized structure were 1277 % and 202 kPa, respectively. The effect of ZP on the destruction of hydrogen bonds of free water in the hydrogel provided the hydrogel with excellent temperature resistance. Multiple reversible interactions endowed the PAA/CS/Al0.5/ZP8 hydrogel with excellent self-healing properties at room temperature and even at −40 ℃. The PAA/CS/Al0.5/ZP8 hydrogel sensor with excellent comprehensive performance could be applied for the monitoring of a variety of human motions, small shape changes of flexible objects, and detection of vibrations in rigid pipes. The interactions between Fe3+, ascorbic acid, and -COOH groups on the hydrogel surface were used to record information and erase it repeatedly at low temperature and showed excellent shape memory. Based on the self-healing property, the hydrogel with gradients of swelling rates and water loss rates was assembled into a double-layer actuator, which could complete the actuation process in opposite directions in air and also underwater. This work provides a new idea for the design of soft robots and intelligent switches.

On the flexibility of 1,4-bis(phenylethynyl)benzene butyl ester organic crystal featuring mobile components

The quest for organic materials with compliant mechanical properties has significantly increased in the past few years in materials science and engineering. Crystalline organic compounds are excellent candidates for exploring and developing those long-lasting properties in the solid state since their fine structural functionalization may provide them with unique flexible behaviors. This study describes the 1,4-bis(phenylethynyl)benzene butyl ester synthesized in four steps and whose crystal can display elastic and elasto-plastic responses upon action with an external stimulus. A detailed characterization by single crystal X-ray diffraction revealed a layered crystal array that results from π-π stacking interactions, C–H⋯O and C-H⋯π bonds. Complementarily, variable temperature solid-state 13C and 2H NMR studies were carried out to probe the motion of the different components of the molecule and explore their possible association with the mechanical features. The compiled evidence indicates that the central phenylenes and the butyl groups experience molecular motion at different timescales, which, combined with the crystal array, facilitate the observed elastic and elasto-plastic flexibility. The results presented here not only contribute to advancing our understanding of flexible behavior but also could inspire potential applications such as flexible electronics.

In Silico Toxicity Screening as a Tool for the Development of Sustainable Electronics, Exemplified with Organic Light‐Emitting Electrochemical Cells

AbstractElectrical and electronic equipment (EEE) have revolutionized our lives, however, their associated waste (WEEE) presents a global challenge because at this time EEE relies heavily on metals that are not commonly found in the living environment (Biosphere), which find their way into the environment during both production/disposal of EEE/WEEE. The use of organic components in EEE is increasingly common, particularly with the growing interest in flexible electronics. Here we describe an approach to device design employing in silico toxicity screening to assess the toxicity of the components chosen for use in EEE that is exemplified using inorganic and organic components known in the literature for the production of prototype organic light‐emitting electrochemical cells. This approach could easily be employed to screen a variety of components for which datasets to produce safety data sheets (SDSs) don't yet exist because they have not been produced in large scale or in a regulatory environment which necessitates this. The approach has significant potential to improve high throughput screening of components for EEE that are “safe‐by‐design”, potentially in combination with AI and ML approaches.

Biometric-Tuned E-Skin Sensor with Real Fingerprints Provides Insights on Tactile Perception: Rosa Parks Had Better Surface Vibrational Sensation than Richard Nixon.

The dense mechanoreceptors in human fingertips enable texture discrimination. Recent advances in flexible electronics have created tactile sensors that effectively replicate slowly adapting (SA) and rapidly adapting (RA) mechanoreceptors. However, the influence of dermatoglyphic structures on tactile signal transmission, such as the effect of fingerprint ridge filtering on friction-induced vibration frequencies, remains unexplored. A novel multi-layer flexible sensor with an artificially synthesized skin surface capable of replicating arbitrary fingerprints is developed. This sensor simultaneously detects pressure (SA response) and vibration (RA response), enabling texture recognition. Fingerprint ridge patterns from notable historical figures - Rosa Parks, Richard Nixon, Martin Luther King Jr., and Ronald Reagan - are fabricated on the sensor surface. Vibration frequency responses to assorted fabric textures are measured and compared between fingerprint replicas. Results demonstrate that fingerprint topography substantially impacts skin-surface vibrational transmission. Specifically, Parks' fingerprint structure conveyed higher frequencies more clearly than those of Nixon, King, or Reagan. This work suggests individual fingerprint ridge morphological variation influences tactile perception and can confer adaptive advantages for fine texture discrimination. The flexible bioinspired sensor provides new insights into human vibrotactile processing by modeling fingerprint-filtered mechanical signals at the finger-object interface.

Durable polylactic acid bionanocomposites with biomass-derived nanocellulose additives: Recent advances in production

BackgroundPlastic is vital in our modern society as it is considered a versatile polymer that has been an indispensable part of daily life. Nevertheless, the nonstop production of plastics derived from petroleum has resulted in an unprecedented crisis in waste management, environmental pollution, and individual health. Global plastic production exceeds 400 million tons annually, and plastic waste is expected to reach 850 million tons annually by 2050. To address this, shifting to durable and sustainable bioplastics from renewable sources is crucial for replacing petrochemical plastics and advancing the bioeconomy. MethodsThis review evaluates the development of durable, green PLA-based bionanocomposites using nanocellulose. It covers DES modification and electrospinning techniques and assesses their impact on PLA's mechanical properties, thermal stability, and biodegradability. The review highlights progress and identifies areas for further research. FindingsIncorporating nanocellulose into PLA significantly improved its strength and heat resistance while keeping it biodegradable. Surface treatment with DES enhanced the even distribution of nanocellulose in the PLA, reducing clumping and improving compatibility. Electrospinning created well-aligned nanocellulose fibers, further strengthening the composite. These improvements make PLA-based bionanocomposites suitable for flexible electronics, medical applications, and packaging.