Flexible Electronics Technology Research Articles

Multifunctional composite flexible films of hydroxypropyl cellulose and silver nanowires

Flexible wearable devices have aroused attentions since the booming flexible electronic technologies, as well as the requirements for improved comfortableness and real-time monitoring for complex motion state of human body. Developing a flexible integrated wearable device with multi functions provides a promising route for next-generation wearable devices. Meanwhile, transparency and biocompatibility are also considered a lot. Herein, hydroxypropyl cellulose/silver nanowires (HPC/AgNWs) flexible composite films were fabricated with a facile two-step method. The fabricated composite films exhibited sufficient transparency and mechanical strength. More importantly, the HPC/AgNWs composite films afforded good motion sensing performances with current sensitivity as high as 350 %. It can accurately identify different bending angles and frequencies, and remained stable for more than 1000 bending cycles. Apart from being employed as motion sensors, the fabricated HPC/AgNWs could be also used for electrical heating and electromagnetic shielding. The films could be heated to more than 60°C at 3.0 V in 57 seconds, and the electromagnetic shielding efficiency reached more than 99.9 %. This study contributed a feasible strategy for the preparation of multifunctional flexible films which are expected to be further applicated in the field of wearable devices.

Synthesis of novel Stachytarpheta jamaicensis flower liked hexagonal boron nitride nanoribbons (SJF-BNNBs) to efficiently improve the thermal/mechanical/electrical properties of flexible polyimide films

Heat dissipation has become a bottleneck restricting large-scale integration and flexible electronic technologies. This study reports a novel fabrication strategy to prepare flexible stachytarpheta jamaicensis flower-like hexagonal boron nitride nanoribbons (SJF-BNNBs) and their composite films with polyimide (PI). The effect of synthesis conditions on the morphology of SJF-BNNBs was investigated, and the growth mechanism was proposed. The in-plane and through plane thermal conductivities of the 4 wt% SJF-BNNBs/PI composite film reach 5.98 W m−1 K−1 and 0.79 W m−1 K−1, which are 3047 % and 316 % higher than those of the pure PI film, respectively. The in-plane and through plane thermal conduction enhancement efficiencies reach 761.8 % and 78.9 %, respectively. The tensile strength of the composite film is 61.7 % higher than that of pure PI, and maintains excellent electrical insulating and thermal stability properties. Infrared imaging tests verified the excellent heat dissipation performance of the composite film as a thermal interface material and a flexible copper clad laminate substrate, where the operating temperature of device could be reduced by 17.3 °C. To the best of our knowledge, SJF-BNNBs outperform the most efficient thermally conductive electrically insulating filler and are to break the overcome of polymer composites in heat dissipation applications.

Triboelectrically active hydrogel drives self-charging zinc-ion battery and human motion sensing

The fast advancement of flexible electronic technology hastens an ever-growing demand for power sources that are adaptable and flexible, and the arrival of innovative energy harvesting technologies such as triboelectric nanogenerator (TENG) further endow the potable electronics with versatility and sustainability. Herein, polyvinyl alcohol/ polyacrylamide/ zinc sulfate (PPZ) hydrogel was synthesized via an one-step UV-initiated polymerization. The hydrogel demonstrates exceptional tensile properties (with a large strain of 915 % and a corresponding tensile strength of 231 KPa), excellent electrical conductivity and light transmittance. A highly flexible and easily adaptable single-electrode TENG was thus fabricated on the base of the as-synthetized PPZ (PPZ-TENG), where the PPZ hydrogel functions as both the charging layer and current collector. While promoting a high electrical output performance of the PPZ-TENG (an open circuit voltage of 176 V and a power density of 328 mW/m2), the PPZ hydrogel also serves as an efficient ion-conductive electrolyte to construct highly flexible self-charging zinc ion batteries (ZIBs). Besides, highly sensitive strain response toward environmental stimuli was also demonstrated, which makes the integration strategy a promising solution for future development self-powered flexible wearable electronics toward the sustainable monitoring of human vital signs such as pulse and limb movement.

Compliant Iontronic Triboelectric Gels with Phase-Locked Structure Enabled by Competitive Hydrogen Bonding.

Rapid advancements in flexible electronics technology propel soft tactile sensing devices toward high-level biointegration, even attaining tactile perception capabilities surpassing human skin. However, the inherent mechanical mismatch resulting from deficient biomimetic mechanical properties of sensing materials poses a challenge to the application of wearable tactile sensing devices in human-machine interaction. Inspired by the innate biphasic structure of human subcutaneous tissue, this study discloses a skin-compliant wearable iontronic triboelectric gel via phase separation induced by competitive hydrogen bonding. Solvent-nonsolvent interactions are used to construct competitive hydrogen bonding systems to trigger phase separation, and the resulting soft-hard alternating phase-locked structure confers the iontronic triboelectric gel with Young's modulus (6.8-281.9kPa) and high tensile properties (880%) compatible with human skin. The abundance of reactive hydroxyl groups gives the gel excellent tribopositive and self-adhesive properties (peel strength > 70 N m-1). The self-powered tactile sensing skin based on this gel maintains favorable interface and mechanical stability with the working object, which greatly ensures the high fidelity and reliability of soft tactile sensing signals. This strategy, enabling skin-compliant design and broad dynamic tunability of the mechanical properties of sensing materials, presents a universal platform for broad applications from soft robots to wearable electronics.

Wireless charging flexible in-situ optical sensing for food monitoring

Food safety is an important public health issue, real-time monitoring and identification of food freshness is essential to ensure food quality and consumer health. This study presents a wireless charging flexible in-situ optical sensing system (FIOS). FIOS integrates visible (VIS)/near-infrared (NIR) spectral sensors, electronic components, and microcontrollers into flexible circuits by flexible electronic technology, and obtains energy through wireless charging to solve the problem of battery replacement and frequent charging of electronic systems. FIOS exhibits excellent flexibility and bendability which can be attached well to the surface of the food for efficient, accurate, non-destructive, and in-situ monitoring. A prediction model between food quality and spectral data was established by chemometrics to achieve accurate prediction of food quality. It can obtain the spectral information of the food in real-time and transmit the data wirelessly to a cloud platform, which enables the visualization of food quality and facilitates quality traceability. FIOS is characterized by simple craft, low cost, and real-time monitoring, and can be widely deployed as an Internet of Things (IoT) node in food transportation, storage, and sales to improve the transparency of food at all stages and ensure the quality and safety of food. The FIOS has the advantages of portable and non-destructive monitoring, which improves the accuracy of food quality monitoring and provides a new technological program in the field of food quality monitoring. It is expected to promote the development of food quality and safety monitoring in the future and provide consumers with a more reliable guarantee of food quality.

Flexible electronic materials and devices toward portable immunoassays

AbstractBiomarker identification is a tried‐and‐true method that can provide precise biological information for disease diagnosis. Prompt diagnosis, disease progression monitoring, therapy efficacy evaluation, and prognosis assessment of cancers all benefit from sensitive, rapid, and precise measurement of significant biomarkers employing chemical and immunological approaches. The study of biomolecules and immunoassay evaluations can profit greatly from recent advancements in flexible electronic materials and technologies, which provide amazing flexibility, affordability, mobility, and integration. However, an overview of the implementation of portable immunoassays in conjunction with flexible electronic devices is rare to come by. This review focuses on recent breakthroughs in flexible electronic materials and devices for portable biomarker testing, which provides an extensive summary of flexible electrical components and sensing‐capable devices, emphasizing their adaptability in the construction of biosensing platforms. These platforms employ various signal transduction systems to record biological affinity recognition events, including pressure, temperature, electrical parameters, colorimetric signals, and other physical features. The challenges for portable, integrated, intelligent, and multifunctional immunoassays based on flexible sensing devices are also discussed. The portable immunoassays with flexible electronics would unlock the potential to transform clinical diagnostics into non‐clinical personalized treatments and achieve home‐based point‐of‐care testing for daily monitoring.

A one step method to prepare polymer‐based flexible wires by continuous spatial‐confining network assembly technique

AbstractIn recent years, flexible electronics technology has been developing rapidly, and flexible electronic devices have been widely used in wearable gadgets and robotics. Flexible wires are the basic components of these devices, which usually consist of metallic materials with flexibility and ductility. However, the introduction of flexible conductive materials has broadened the application scope of flexible wires. In this paper, PDMS/SCF conductive composites exhibiting different conductivity properties on both sides are fabricated by adding meshes based on the continuous spatial‐confining network assembly method. The conductivity of PDMS/SCF (6 wt%) composites can be exhibited as 173.45 S/m in the conductive layer and 6.25 × 10−11 S/m in the insulating layer, which can be used as core and surface layers of conductors, respectively. A mathematical model is also developed to predict the electrical conductivity of the PDMS/SCF conductive composites, and the experimental data are in good agreement with the predicted results. This work provides a method for the continuous preparation of multifunctional composites that can be molded in one step with conductive layer and insulating layer, which can be used as the core and insulating layers of wires, thus simplifying the preparation process of composite‐based wires and expanding the range of conductive composites applications.

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.

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.