Flexible Electronics Technology Research Articles

Advances in Wearable Multifunctional Devices Based on Human‐Body Energy Harvesting

AbstractWearable electronics with multi‐functionalities are widely utilized in various domains, including everyday living, healthcare, military training, and sports. Advances in flexible electronic technology, new materials, artificial intelligence technology, and sensor technology have accelerated the rapid development of smart wearable devices toward multifunctional and highly integrated trends. The energy supply technology based on the human‐body energy harvesting method endows wearable, multifunctional electronic devices with sustainable, renewable, and self‐powered characteristics, which proposes a solution strategy for the function expansion and energy supply of wearable devices. Herein, this paper discusses recent research on various methods of harvesting human body energy and wearing parts respectively, focusing on the new materials, structures, and processes involved in the representative studies, as well as the impact on energy harvesting and output, and functional applications. Furthermore, the challenges and obstacles faced in the creation of wearable multifunctional devices based on human self‐sufficiency and propose solution strategies to propel them in order to advance the creation of the next wave of intelligent wearable technology are also discussed.

Synthesis of PVA@S-BNNSs flexible fiber backbone by electrostatic spinning assisting the construction of excellent ductile epoxy thermal interface materials

With the emergence of flexible electronics technologies based on stretchable, foldable, and bendable properties, there is a growing need for new thermal interface materials (TIMs) that possess better flexibility and stretchability. Epoxy resins, as thermal management materials commonly used to improve heat transfer and heat transport, need to have high thermal conductivity as well as excellent flexibility and a degree of stretchability. In order to devise TIMs that can be employed with flexible electronic devices, here we propose a strategy to form a flexible thermal conductive network structure with the shape of a “rice plant” inside the flexible epoxy resin matrix. The polyvinyl alcohol (PVA) flexible network skeleton constructed by the electrostatic spinning technique is regarded as the branches of the rice plant, and the ultra-sonicated boron nitride nanosheets (BNNS) are assumed to be the granular rice seeds. As a result, a perfect interconnected thermal conductivity path can be constructed, and the thermal conductivity is greatly enhanced with a relatively low boron nitride loading. Additionally, the prepared EP/PVA@S-BNNSs epoxy composites can exhibit a low elastic modulus and 153 % elongation at the break, demonstrating superior electrical insulation and excellent ductility, making them suitable for a wide range of applications in flexible electronic and wearable devices.

Application of flexible electronic technology in real-time monitoring for wheel<bold>-</bold>rail contact forces of high-speed trains

Application of flexible electronic technology in real-time monitoring for wheel<bold>-</bold>rail contact forces of high-speed trains

Enhanced sensitivity and linear-response in iontronic pressure sensors for non-contact, high-frequency vibration recognition

Monitoring non-contact high-frequency vibrations requires improving the sensitivity and linear response of iontronic pressure sensors (IPSs). In this study, we incorporate composite electrodes comprising silver nanowires (Ag NWs) and MXene into IPSs to enhance electronic conduction and pseudocapacitance. Moreover, we utilize a novel surface-pillar microstructure, along with an internally randomized multi-bubble structure within the dielectric layer, to significantly expand the linear response range of the sensor. The resulting IPS device demonstrates exceptional linear sensitivity, measuring approximately 153.83 kPa−1, across a broad pressure range of up to 260 kPa. Additionally, it exhibits long-term stability, rapid response and recovery characteristics, and remains functional underwater. Notably, these devices exhibit remarkable capabilities in monitoring ultrasonic vibrations and accurately identifying sound wave vibrations. The integration of composite electrodes, microstructure designs, and their compatibility with underwater applications positions these IPSs as highly promising tools for precise measurements and advancements in flexible electronics technology.

Designs and Applications for the Multimodal Flexible Hybrid Epidermal Electronic Systems.

Research on the flexible hybrid epidermal electronic system (FHEES) has attracted considerable attention due to its potential applications in human-machine interaction and healthcare. Through material and structural innovations, FHEES combines the advantages of traditional stiff electronic devices and flexible electronic technology, enabling it to be worn conformally on the skin while retaining complex system functionality. FHEESs use multimodal sensing to enhance the identification accuracy of the wearer's motion modes, intentions, or health status, thus realizing more comprehensive physiological signal acquisition. However, the heterogeneous integration of soft and stiff components makes balancing comfort and performance in designing and implementing multimodal FHEESs challenging. Herein, multimodal FHEESs are first introduced in 2 types based on their different system structure: all-in-one and assembled, reflecting totally different heterogeneous integration strategies. Characteristics and the key design issues (such as interconnect design, interface strategy, substrate selection, etc.) of the 2 multimodal FHEESs are emphasized. Besides, the applications and advantages of the 2 multimodal FHEESs in recent research have been presented, with a focus on the control and medical fields. Finally, the prospects and challenges of the multimodal FHEES are discussed.

Highly elastic relaxor ferroelectrics for wearable energy storage.

Polymer-based relaxor ferroelectrics with high dielectric constant are pivotal in cutting-edge electronic devices, power systems, and miniaturized pulsed electronics. The surge in flexible electronics technology has intensified the demand for elastic ferroelectric materials that exhibit excellent electrical properties and mechanical resilience, particularly for wearable devices and flexible displays. However, as an indispensable component, intrinsic elastomers featuring high dielectric constant and outstanding resilience specifically tailored for elastic energy storage remain undeveloped. Elastification of relaxor ferroelectric materials presents a promising strategy to obtain high-dielectric elastomers. In this study, we present a strain-insensitive, high elastic relaxor ferroelectric material prepared via peroxide crosslinking of a poly(vinylidene fluoride) (PVDF)-based copolymer at low temperature, which exhibits an intrinsic high dielectric constant (∼20 at 100 Hz) alongside remarkable thermal, chemical, and mechanical stability. This relaxor ferroelectric elastomer maintains a stable energy density (>8 J cm-3) and energy storage efficiency (>75%) under strains ranging from 0 to 80%. This strain-insensitive, high elastic relaxor ferroelectric elastomer holds significant potential for flexible electronic applications, offering superior performance in soft robotics, smart clothing, smart textiles, and electronic skin.

ТЕХНОЛОГИИ НА ОСНОВЕ ГИБКОЙ ЭЛЕКТРОНИКИ: МАТЕРИАЛЫ, ПРОИЗВОДСТВО И ПРИМЕНЕНИЯ

The article reviews modern flexible electronics technologies, including materials, manufacturing methods, and their main applications. It provides an analysis of key materials, such as flexible substrates and conductive materials, which play an important role in the creation of flexible electronics. Printing and deposition technologies used to produce flexible electronic devices are described. Application areas include consumer electronics, medical devices, industrial and military solutions, as well as the role of flexible electronics in the Internet of Things (IoT). Particular attention is paid to current challenges and prospects for technology development.

Imine-vitrimer elastomer enabled triboelectric nanogenerators with remarkable electrical output and circular life cycles

Triboelectric nanogenerator (TENG) has garnered significant attention as a promising technology for energy harvesters and flexible electronics. However, the current state-of-the-art TENGs suffer from the low electrical output and limited lifespan, which pose significant obstacles to their broader implementation. Therefore, a TENG with ultra-high electrical output and circular life cycles is developed by combining dynamic covalent chemistry and TENG technologies. Specifically, imine-vitrimer elastomers (IVEs) with excellent compression resilience are synthesized and utilized to fabricate TENG as an electronegative tribolayer. The assembled TENG can generate ∼18-fold higher electrical charge density than traditional PTFE-based TENG. This substantial improvement enables the TENG to power over 2400 commercial LEDs, showcasing the great potential for high-sensitivity traffic sensors and energy intensive devices. Moreover, to solve inevitable wear or tear of the tribolayers during mechanical friction, efficient self-healing and recycling of the tribolayer are achieved, ensuring consistently high electrical output and durability. This study provides a new sight to fabricate the next generation TENG with high performance and circular life cycles.

Triboelectric Nanogenerator Enabled Sweat Extraction and Power Activation for Sweat Monitoring

AbstractWearable sweat sensors can detect and monitor various substances in sweat, providing valuable information for healthcare monitoring and clinical diagnostics. Recent advances in flexible electronic technologies have enabled the development of wearable sweat sensors that can measure sweat rate and biochemical substances in real time, although several challenges remain, such as power management and sweat extraction issues. Here, a passive sweat extraction strategy as well as a self‐powered monitoring system (SEMS) is reported to be designed for sedentary individuals, i.e., elders. The SEMS system comprises a wearable triboelectric nanogenerator (TENG) for sweat extraction, a sweat‐activated battery (SAB) as the integrated power source, carbachol‐loaded iontophoresis electrodes for sweat extraction, microfluidics with biosensors for detecting physiological information in sweat, and near field communication (NFC)‐based wireless microelectronics for data communication, processing, and collection. By tapping the TENG, sedentary people can passively extract sweat based on the iontophoresis process, allowing the sensors to detect biological information in sweat. The good flexibility of the SEMS device enables real‐time and non‐invasive detection of sweat analytes in a wearable format. This system offers a new strategy of sweat collection and analysis for the elderly group, and therefore can help to understand human physiology and personalize health monitoring deeply.

A flexible electrode with tetra-chiral structure connected in serpentine shape

Flexible electrodes are usually well-matched to the substrate as the core component of flexible electronics technology. However, when large deformation occurs, the conductive layer of flexible electrodes can mechanically fracture due to insufficient mechanical properties, leading to the failure of flexible electronic devices. Improving the mechanical properties of flexible electrodes is essential for the development of flexible electronics. Herein, a flexible electrode with a serpentine shape tetra-chiral structure is designed and fabricated via screen-printed process based on existing tetra-chiral structure. The research demonstrates that the serpentine tetra-chiral structure can improve the mechanical properties of the flexible electrode and the maximum tensile stress is reduced by 87.19 % relative to existing tetra-chiral structure. The flexible electrode is still conductive when the strain of that is 70 %, and the resistance is only 5 Ω and 4.4 Ω, respectively, at a bending angle of 180° and following 1000 times fold cycles. In addition, the flexible electrodes fabricated in this paper are utilized to realize the collection of ECG signals and pulse signals, showing greater potential in biological signal monitoring. The strategy proposed here is expected to maintain electrodes’ original functions under repeated use and complex deformation.

Graphene as advanced applied material in flexible display applications

Flexible display technology has grown in popularity as a study area in the electronics industry as a result of the quick development of flexible electronics technology. Transparent electrodes, the main element of flexible display technology, are crucial for high resolution, high brightness, low power consumption, and interference resistance. A new kind of transparent electrode material is urgently required because conventional applied transparent electrode materials have issues like expensive cost, difficult preparation, and low dependability. Due to its exceptional performance in terms of transparency, electrical conductivity, and flexible bendability, graphene has gained a much interest as an emerging material. In order to provide guidance on the future development of cutting-edge materials for flexible display applications, this paper investigates the use of graphene in flexible displays. It examines graphene's use as a transparent electrode material and a thin film transistor material in flexible displays, as well as its application prospects and development trends. It is found that graphene has a high transparency of 97.7% and high conductivity, which outperforms the generally used indium tin oxide and copper materials. However, further advancements in the mass production of graphene is necessary for the industrialization of graphene in flexible display applications.

The flexible and distributed pressure sensor with 64 units for on-line gait recognition analysis

With the rapid development of flexible electronic technology, human gait recognition device is also developing in the direction of flexibility, refinement, and intelligence. In this paper, a gait recognition system with the advantages of being non-invasive and flexible is proposed and prepared to collect complex pressure information of human plantar and identify different gaits accurately. A flexible and distributed pressure sensor with a structure of eight rows and eight columns is designed and fabricated by the screen printing process and micro-nano manufacturing technology. The insole shape can be placed directly into a sneaker for easy wearing, while 64 pressure sensing units allow for more accurate detection of complex plantar pressure information. In addition, based on the “voltage mirror” principle and the feature threshold analysis method, a distributed pressure acquisition circuit and an online gait recognition software are developed respectively to realize time-sharing decoupling of complex output signals from multiple pressure sensing units and online real-time display of different gaits. Finally, Different human gait recognition experiments have also been carried out. The results show that the test error of the pressure sensing unit is better than 8.28%. The gait recognition system can realize the reliable and accurate online real-time identification of slow walking, fast walking, and different leaning directions, which shows great potential and value in the fields of medical diagnosis, health monitoring, gait correction and artificial intelligence.

Indirect method for preparing dual crosslinked eutectogels with high strength, stretchability, conductivity and rapid self-recovery capability as flexible and freeze-resistant strain sensors

The growing demand for stretchable conductor materials in flexible electronic technologies has led to increased focus on developing conductive gels. However, existing hydrogels have poor temperature resistance, and ionogels may have potential toxicity, limiting their use in bio-related applications. This work focuses on the development of the emerging eutectogels with both mechanical and electrical properties, which have traditionally been challenging to achieve. We present an indirect method to prepare dual crosslinked eutectogels that possess high strength, stretchability, conductivity and rapid self-recovery capability. The approach involves using water molecules as a co-solvent to form a deep eutectic solvent through a three-step process, followed by ionically cross-linking poly(acrylamide-co-acrylic acid) with Fe3+. The resulting eutectogels have a balanced mechanical strength with a tensile strength up to 6.17 MPa, large stretchability up to 1067%, a modulus of 1.25 MPa, and excellent toughness of 20.95 MJ/m3. In addition, the eutectogels exhibit high ionic conductivity of 10.29 mS/cm, rapid self-recovery and low temperature tolerance capability (up to −50 °C). To our knowledge, the eutectogels with balanced tensile strength, toughness, conductivity, and low-temperature tolerance can hardly be achieved. Moreover, the conductive eutectogel also exhibit high sensitivity with a gauge factor of 9.47 at strain of 700% with fast response time, making them promising candidates for soft human-motion sensors that can accurately detect human motions with high sensitivity and good durability over a wide temperature range. The findings provide new insights into developing eutectogels with both mechanical and electrical properties, opening opportunities in flexible electronics, soft robotics, and energy storage devices.

Call for Papers: Special Issue on Selected Papers from the 5th IEEE International Flexible Electronics Technology Conference (IFETC) 2023

Call for Papers: Special Issue on Selected Papers from the 5th IEEE International Flexible Electronics Technology Conference (IFETC) 2023

Fabrication of flexible planar micro-supercapacitors using laser scribed graphene electrodes incorporated with MXene

The surge in demand for personalized electronic products has fueled the emergence and development of flexible electronic technology, giving rise to flexible energy storage devices. An integral component in these devices is planar micro-supercapacitors (MSCs), which hold immense promise for compatibility with flexible electronic products, especially in terms of their miniaturization, flexibility, integration, and customization. In this study, a fast, efficient, and environmentally friendly laser scribing technique has been employed to pattern MXene and graphene oxide (MXene/GO) composite films to prepare shape-controllable MXene and laser scribed graphene (MXene/LSG) patterned electrodes on a flexible substrate, which were then constructed into flexible MSCs. Based on the synergistic effect of these two-dimensional materials, MXene and LSG, the resulting adaptable devices exhibit exceptional electrochemical performance, including high areal capacitance, long cycle stability, superior flexibility, and mechanical stability. Furthermore, the use of a laser scribing process has pivotal implications for the scalable production of patterned electrodes and modular integration.

Wearable Chemosensors in Physiological Monitoring

The development of flexible electronic technology has led to significant advancements in wearable sensors. In the past decades, wearable chemosensors have received much attention from researchers worldwide due to their high portability, flexibility, lightweight, and adaptability. It allows real-time access to the user’s physiological status at the molecular level to analyze their health status. Therefore, it can be widely used in the field of precision medicine. This review introduces the sensing mechanisms of wearable chemosensors and recent progress in wearable sweat and interstitial fluid-based chemosensors. The complexities of wearable chemosensors are not to be underestimated, as there are considerable challenges in this field. This review aims to shed light on the difficulties associated with designing wearable sweat and interstitial fluid-based chemosensors and their potential development directions.

Ice-Template-Induced Highly Interconnected Porous Polymer Gel Electrolytes for Dendrite-Free Flexible Zinc-Air Batteries.

Improving the performance of quasi-solid-state gel polymer electrolytes is critical for addressing issues at the Zn anode-electrolyte interface of high-performance flexible Zn-air batteries (FZABs). In this study, a highly interconnected porous poly(vinyl alcohol)/poly(ethylene glycol) (PVA/PEG) hydrogel electrolyte was fabricated via an ice-crystal template for FZABs. The mechanical toughness and stability of the gel electrolytes can be reinforced by the formation of a PEG-PVA cross-linking network. The three-dimensional PVA/PEG porous skeleton greatly increased electrolyte uptake and accelerated ion transport, leading to high ionic conductivity (42.5 mS cm-1). In-situ synchrotron radiation X-ray imaging revealed that the PVA/PEG network can effectively inhibit dendrite growth and the hydrogen evolution reaction. The assembled FZABs exhibited superior cycle stability, high power density (109 mW cm-3), and excellent flexibility and structural stability under bending conditions, thus showing great potential for future applications in flexible and wearable electronic device technologies.

Liquid metal assisted fabrication of MXene-based films: Toward superior electromagnetic interference shielding and thermal management

The development of thin and flexible films that possess both electromagnetic interference (EMI) shielding and thermal management capabilities has always been an intriguing pursuit, but itisnevertheless a crucialproblemtoaddress. Inspired by the deformability of liquid metal (LM) and film forming capacity of MXene, here we present a series of ternary compositing films prepared via cellulose nanofiber (CNF) assisted vacuum filtration technology. Originating from the highly conductive LM/MXene network, the MLMC film presents a maximum EMI shielding effectiness (EMI SE) of 78 dB at a tiny thickness of 45 μm, together with a high specific EMI SE of 3046 dB mm−1. Meanwhile, these compositing films also deliver excellent flexibility and mechanical reliability, showing no obvious decline in EMI shielding performance even after 1000 bending and 500 folding cycles, respectively. Moreover, notable anisotropic thermal conductive property was successfully achieved, allowing for a highly desirable in-plane thermal conductivity of 7.8 W m−1 K−1. This accomplishment also yielded an exceptional electro-thermal conversion capacity, enabling efficient low-voltage (3 V) heating capabilities. These captivating features are expected to greatly drive the widespread adoption of LM-based films in future flexible electronic and wearable technologies.

Graphene and its application in flexible electronics

With the rapid development of technology and the dramatic improvement of living quality in recent years, traditional electronic technology is increasingly difficult to meet people's needs, and more portable flexible electronic (FE) technology products also stand out. However, with the development of research and application in the field of FEs, people put forward higher requirements for FE products in terms of electronic performance, light transmission performance, and mechanical tensile performance. As a new carbon nanomaterial, graphene has great mechanical, thermal, electrical and optical properties and is an excellent raw material for preparing FE devices, which has broad application prospects. This paper introduces the development of graphene preparation and application and its application results in the field of FEs summarizes the current research progress of graphene in the field of FEs and focuses on discussing and comparing the technical characteristics and development potential of different methods in each application. This review is expected to provide theoretical and data support for the application and preparation of graphene, especially in the field of FEs and provide new ideas for the development of graphene applications in the field of FEs.

Recent Progress in Mechanically Robust and Conductive‐Hydrogel‐Based Sensors

Flexible electronic technology has developed rapidly in recent years, showing broad application prospects in motion monitoring, wearable devices, and personalized medicine. Consequently, the demand for high sensitivity and wide sensing range has gradually increased. Conductive hydrogels have high flexibility, excellent conductivity, and good biocompatibility, making them ideal candidates for fabricating flexible sensors. However, conductive hydrogels exhibit weak mechanical stability, which limits their applications. Therefore, sufficient mechanical properties and fatigue resistance are usually needed to fulfill their application requirements. Herein, the research frontiers of sensors based on mechanically robust conductive hydrogels are reviewed. While published papers always focus on the configuration design and application of sensors and the improvement of sensing performance, research on the network design of hydrogels and their effects on mechanical properties and sensing performance are limited. It is attempted in this review to fill this gap by focusing on the design principles of mechanically enhanced conductive hydrogels and their applications in flexible electronic devices. Herein, hydrogels’ structural designs, toughening mechanisms, mechanical properties, and sensing applications are discussed. The different working mechanisms of flexible sensors composed of tough conductive hydrogels and their applications are also reviewed. Finally, the future development directions and challenges in this field are highlighted.