Flexible Electronics Research Articles

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.

Sol–gel prepared ZnO: UV irradiation effect on structure and surface properties

The effect of UV irradiation on sol–gel prepared ZnO films subjected to mild thermal annealing was investigated, with special attention to their structural and surface properties. Sol–gel processes, including a high-temperature annealing stage, have been adapted to the requirements of flexible electronics for in situ synthesis of semiconductor ZnO films on polymer substrates at lower temperatures due to UV irradiation. Application of UV radiation with emission peaks at 185 and 254 nm to films annealed at 180 °C made it possible to obtain ZnO films with Zn/O ratios of ca. 1, which cannot be achieved by heat treatment alone.

Hierarchical polypyrrole@MXene (Ti3C2TX) fiber strain sensors for wearable healthcare electronics

Flexible sensors are an important component of wearable medical electronics and hold considerable promise for research. Presently, stretchable strain sensors face two primary challenges: balancing the sensing range with sensitivity, and ensuring compatibility between conductivity and mechanical properties. Here, flexible sensing fibers with high gauge factor (GF) and wide monitoring range are reported based on a hierarchical structure composed of electrostatic adsorption principle and hydrogen bonding interactions. The hierarchical structure material consists of thermoplastic polyurethane (TPU), MXene, polypyrrole (PPy) and waterborne polyurethane (WPU). Among them, MXene (2D) and PPy (3D) are uniformly distributed on the surface of the MXene/TPU fiber (1D) to form a conductive and sensing pathway. This multi-dimensional combination approach enables the fiber sensor to have a high sensing range (0 ∼ 106 %) and high sensitivity (60 ∼ 3.23 × 106). In addition, the WPU acts as a protective layer, which not only reduces the damage to the sensing layer, but also improves the stretchability (>750 %) and modulus (∼12 MPa) of the sensor. Therefore, it can function as fiber sensor for monitoring not only human movement but also swelling in the legs of patients with varicose veins. The sensor also has a joule heating effect, as well as the potential to be integrated into fabrics, providing an important case for the construction of multifunctional flexible wearable medical electronics with practical applications.

Polymer Dielectric-Based Emerging Devices: Advancements in Memory, Field-Effect Transistor, and Nanogenerator Technologies.

Polymer dielectric materials have recently attracted attention for their versatile applications in emerging electronic devices such as memory, field-effect transistors (FETs), and triboelectric nanogenerators (TENGs). This review highlights the advances in polymer dielectric materials and their integration into these devices, emphasizing their unique electrical, mechanical, and thermal properties that enable high performance and flexibility. By exploring their roles in self-sustaining technologies (e.g., artificial intelligence (AI) and Internet of Everything (IoE)), this review emphasizes the importance of polymer dielectric materials in enabling low-power, flexible, and sustainable electronic devices. The discussion covers design strategies to improve the dielectric constant, charge trapping, and overall device stability. Specific challenges, such as optimizing electrical properties, ensuring process scalability, and enhancing environmental stability, are also addressed. In addition, the review explores the synergistic integration of memory devices, FETs, and TENGs, focusing on their potential in flexible and wearable electronics, self-powered systems, and sustainable technologies. This review provides a comprehensive overview of the current state and prospects of polymer dielectric-based devices in advanced electronic applications by examining recent research breakthroughs and identifying future opportunities.

Laser‐Induced Transformation of ZIF‐8 into Highly Luminescent N‐Doped Nanocarbons for Flexible Sensors

AbstractMetal–organic frameworks (MOFs) like the zeolitic imidazolate framework (ZIF‐8) have a high surface area, tunable porosity, and robust thermal and chemical stability, making them attractive candidates for various applications. Here, a strategy is shown that spans that functionality and provides strong photoluminescence (PL) emission, unlocking ZIF‐8‐based materials for chemical and temperature sensors based on PL. The approach is based on laser processing that dramatically boosts the PL response of laser‐irradiated ZIF‐8 (LI ZIF‐8), achieving a 70‐fold increase in intensity relative to the pristine material. The PL characteristics of the irradiated material can be easily tuned by varying the laser power and irradiation time with in situ and real‐time spectroscopic analysis providing insights into the process dynamics. It is found that the observed PL enhancement is primarily due to the laser‐induced transformation of ZIF‐8 into nitrogen‐doped nanocarbons and ZnO nanostructures. The versatility of this laser processing approach is leveraged to create flexible electronics by integrating the LI ZIF‐8/nanocarbon architectures into thermoplastic polyurethane (TPU). The multifunctional composite material shows excellent performance as flexible electrodes for human‐body monitoring applications, as well as both temperature and flexure sensors with remarkable mechanical resilience.

The role of oxygen in the growth of silver nanowires using the polyol method

Silver nanowires (Ag NWs) are gaining significant interest as a promising material for applications in flexible electronics. However, the growth mechanism of Ag NWs, particularly during the polyol synthesis process, remains unclear due to the complex reduction pathways of ethylene glycol (EG). In this paper, first-principles calculation method was employed to investigate the influence of oxygen during the growth of Ag NWs. Specifically, oxygen affects the decomposition products of ethylene glycol and thus the reducibility of EG. This research aims to reveal the mechanism of oxygen promoting the growth of Ag NWs.

Conformal printed electronics on flexible substrates and inflatable catheters using lathe-based aerosol jet printing

With the growth of additive manufacturing (AM), there has been increasing demand for fabricating conformal electronics that directly integrate with larger components to enable unique functionality. However, fabrication of conformal electronics is challenging because devices must merge with host substrates regardless of curvilinearity, topography, or substrate material. In this work, we employ aerosol jet (AJ) printing, an AM method for jet printing electronics using ink-based materials, and a custom-made lathe mechanism for mounting flexible substrates and 3D objects on a rotating axis. Using this method of lathe-based AJ printing, conformal electronics are printed around the circumference of rotational bodies with 3D curvilinear surfaces through cylindrical-coordinate motion. We characterize the diverse capabilities of lathe AJ (LAJ) printing and demonstrate flexible conformal electronics including multilayer carbon nanotube transistors. Lastly, a graphene sensor is conformally printed on an inflated catheter balloon for temperature and inflation monitoring, thus highlighting the versatilities of LAJ printing.

Oxygen-defective electrostrictors for soft electromechanics.

Electromechanical metal oxides, such as piezoceramics, are often incompatible with soft polymers due to their crystallinity requirements, leading to high processing temperatures. This study explores the potential of ceria-based thin films as electromechanical actuators for flexible electronics. Oxygen-deficient fluorites, like cerium oxide, are centrosymmetric nonpiezoelectric crystalline metal oxides that demonstrate giant electrostriction. These films, deposited at low temperatures, integrate seamlessly with various soft substrates like polyimide and PET. Ceria thin films exhibit remarkable electrostriction (M33 > 10-16 m2 V-2) and inverse pseudo-piezo coefficients (e33 > 500 pmV-1), enabling large displacements in soft electromechanical systems. Our study explores resonant and off-resonant configurations in the low-frequency regime (<1 kHz), demonstrating versatility for three-dimensional and transparent electronics. This work advances the understanding of oxygen-defective metal oxide electromechanical properties and paves the way for developing versatile and efficient electromechanical systems for applications in biomedical devices, optical devices, and beyond.

OFETs Electric Characteristics with different Organic Material for Low Power Flexible Electronics Design

OFETs Electric Characteristics with different Organic Material for Low Power Flexible Electronics Design

A Comprehensive Review of Functional Gel Polymer Electrolytes and Applications in Lithium-Ion Battery.

The rapid expansion of flexible and wearable electronics has necessitated a focus on ensuring their safety and operational reliability. Gel polymer electrolytes (GPEs) have become preferred alternatives to traditional liquid electrolytes, offering enhanced safety features and adaptability to the design requirements of flexible lithium-ion batteries. This review provides a comprehensive and critical overview of recent advancements in GPE technology, highlighting significant improvements in its physicochemical properties, which contribute to superior long-term cycling stability and high-rate capacity compared with traditional organic liquid electrolytes. Special attention is given to the development of smart GPEs endowed with advanced functionalities such as self-protection, thermotolerance, and self-healing properties, which further enhance battery safety and reliability. This review also critically examines the application of GPEs in high-energy cathode materials, including lithium nickel cobalt manganese (NCM), lithium nickel cobalt aluminum (NCA), and thermally stable lithium iron phosphate (LiFePO4). Despite the advancements, several challenges in GPE development remain unresolved, such as improving ionic conductivity at low temperatures and ensuring mechanical integrity and interfacial compatibility. This review concludes by outlining future research directions and the remaining technical hurdles, providing valuable insights to guide ongoing and future efforts in the field of GPEs for lithium-ion batteries, with a particular emphasis on applications in high-energy and thermally stable cathodes.

Ultra-stable ionogels based on hydrogen networks with broad temperature, environment, and long-term stability

Ionogels with broad temperature, environment, and long-term stability are desirable in intelligent flexible electronics. Herein, we have developed a novel ultra-stable ionogel through hydrogen bonding between the amphiphilic ionic liquid--1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EMIM][NTf2]) and butyl acrylate (BA)-ethyl ethoxyethyl acrylate (EO) copolymer (P(BA-co-EO)). The ionogel demonstrates high decomposition voltage, strong adhesion stability, non-corrosive stability, and cyclic strain sensing stability (500 cycles at 120 % strain). Additionally, it exhibits broad temperature and environment adaptability, capable of stretching up to 7 times at −20 °C, maintaining consistent weight after 30 days of storage in extreme environments such as low/high temperatures (−20∼100 °C), high vacuum (6 × 10−4 Pa), and underwater (without additional sealed packaging), while also maintaining electrophysiological monitoring at −20 °C or 100 °C. Most importantly, the ionogel without sealing can self-adhere to the human skin for continuous and high-quality electrophysiological monitoring for 1 month under daily life conditions. We have utilized the ionogel to fabricate an ionic skin with multifunctional sensing capabilities for strain, pressure, and temperature, which has been successfully employed in human motion and pressure detection. It is believed that ionogels with long-term stability will pave the way for developing next-generation intelligent flexible electronics in future research endeavors.