Human Skin-Inspired Electrospun Patterned Robust Strain-Insensitive Pressure Sensors and Wearable Flexible Light-Emitting Diodes

Advanced Morphological and Material Engineering for High-Performance Interfacial Iontronic Pressure Sensors

High-performance flexible pressure sensors are crucial for applications such as wearable electronics, interactive systems, and healthcare technologies. Among these, iontronic pressure sensors have garnered particular attention due to their superior sensitivity, enabled by the giant capacitance variation of the electric double layer (EDL) at the ionic-electronic interface under deformation. Key advancements, such as incorporating microstructures into ionic layers and employing diverse materials, have significantly improved sensor properties like sensitivity, accuracy, stability, and response time. This review highlights advancements in flexible EDL pressure sensors, focusing on structural designs and material engineering. These strategies are tailored to optimize key metrics such as sensitivity, detection limit, linearity, stability, response speed, hysteresis, transparency, wearability, selectivity, and multifunctionality. Key fabrication techniques, including micropatterning and externally assisted methods, are reviewed, along with strategies for sensor comparison and guidelines for selecting appropriate sensors. Emerging applications in healthcare, environmental and aerodynamic sensing, human-machine interaction, robotics, and machine learning-assisted intelligent sensing are explored. Finally, this review discusses the challenges and future directions for advancing EDL-based pressure sensors.

Optical Adhesives and Screen Sealants for Foldable Displays: Analysis, Progress and Trends

The realm of flexible display devices, particularly centered around folding screen smartphones, is undergoing rapid advancements. As integral components, optical adhesives and screen sealants for these devices play pivotal roles in determining their overall performance. This paper provides a comprehensive overview of the evolution of display technology and display screens, delving into the critical function of optical adhesives within this framework. Notably, light-curing adhesives stand out for their paramount importance in display screen manufacturing, attributed to their swift curing capabilities. We synthesize the key research achievements and concomitant limitations pertaining to the characteristics of diverse flexible optical adhesives compositions over recent years. Furthermore, we delve into the influence of chemical modification techniques applied to various adhesive systems and the integration of physical doping fillers on enhancing the performance of screen sealants.

Research Progress of Flexible Electronic Devices Based on Electrospun Nanofibers

Electrospun nanofibers have become an important component in fabricating flexible electronic devices because of their permeability, flexibility, stretchability, and conformability to three-dimensional curved surfaces. This review delves into the advancements in adaptable and flexible electronic devices using electrospun nanofibers as the substrates and explores their diverse and innovative applications. The primary development of key substrates for flexible devices is summarized. After briefly discussing the principle of electrospinning, process parameters that affect electrospinning, and two major electrospinning techniques (i.e., single-fluid electrospinning and multifluid electrospinning), the review shines a spotlight on the recent breakthroughs in multifunctional and stretchable electronic devices that are based on electrospun substrates. These advancements include flexible sensors, flexible energy harvesting and storage devices, flexible accessories for electronic devices, and flexible environmental monitoring devices. In particular, the review outlines the challenges and potential solutions of developing electrospun nanofibers for flexible electronic devices, including overcoming the incompatibility of multiple interfaces, developing 3D microstructure sensor arrays with gradient geometry for various imperceptible on-skin devices, etc. This review may provide a comprehensive understanding of the rational design of application-oriented flexible electronic devices based on electrospun nanofibers.

Capturing forceful interaction with deformable objects using a deep learning-powered stretchable tactile array

Capturing forceful interaction with deformable objects during manipulation benefits applications like virtual reality, telemedicine, and robotics. Replicating full hand-object states with complete geometry is challenging because of the occluded object deformations. Here, we report a visual-tactile recording and tracking system for manipulation featuring a stretchable tactile glove with 1152 force-sensing channels and a visual-tactile joint learning framework to estimate dynamic hand-object states during manipulation. To overcome the strain interference caused by contact with deformable objects, an active suppression method based on symmetric response detection and adaptive calibration is proposed and achieves 97.6% accuracy in force measurement, contributing to an improvement of 45.3%. The learning framework processes the visual-tactile sequence and reconstructs hand-object states. We experiment on 24 objects from 6 categories including both deformable and rigid ones with an average reconstruction error of 1.8 cm for all sequences, demonstrating a universal ability to replicate human knowledge in manipulating objects with varying degrees of deformability.

Encapsulated stretchable amphibious strain sensors

Soft and stretchable strain sensors have found wide applications in health monitoring, motion tracking, and robotic sensing. There is a growing demand for strain sensors in amphibious environments, such as implantable sensors, wearable sensors for swimmers/divers, and underwater robotic sensors. However, developing a sensitive, stretchable, and robust amphibious strain sensor remains challenging. This work presents an encapsulated stretchable amphibious strain sensor. The conductive layer, made of silver nanowires embedded below the surface of polydimethylsiloxane, was sandwiched by two layers of thermoplastic polyurethane. Periodic sharp cuts were introduced to change the direction of flow from across the sensor to along the conductive path defined by the opening cracks. The crack advancing and opening is controlled by a unique combination of weak/strong interfaces within the sandwich structure. The cut design and the interfacial interactions between the layers were investigated. The strain sensor exhibited a high gauge factor up to 289, a linear sensing response, a fast response time (53 ms), excellent robustness against over-strain, and stability after 16 000 loading cycles and 20 days in an aqueous saline solution. The functionality of this amphibious strain sensor was demonstrated by tracking the motion of a robotic fish, undertaking language recognition underwater, and monitoring the blood pressure of a porcine aorta. This illustrates the promising potential for this strain sensor for both underwater use and surgically implantable applications.

Highly Sensitive and Flexible Capacitive Pressure Sensors Combined with Porous Structure and Hole Array Using Sacrificial Templates and Laser Ablation

Flexible, wearable pressure sensors offer numerous benefits, including superior sensing capabilities, a lightweight and compact design, and exceptional conformal properties, making them highly sought after in various applications including medical monitoring, human-computer interactions, and electronic skins. Because of their excellent characteristics, such as simple fabrication, low power consumption, and short response time, capacitive pressure sensors have received widespread attention. As a flexible polymer material, polydimethylsiloxane (PDMS) is widely used in the preparation of dielectric layers for capacitive pressure sensors. The Young's modulus of the flexible polymer can be effectively decreased through the synergistic application of sacrificial template and laser ablation techniques, thereby improving the functionality of capacitive pressure sensors. In this study, a novel sensor was introduced. Its dielectric layer was developed through a series of processes, including the use of a sacrificial template method using NaCl microparticles and subsequent CO2 laser ablation. This porous PDMS dielectric layer, featuring an array of holes, was then sandwiched between two flexible electrodes to create a capacitive pressure sensor. The sensor demonstrates a sensitivity of 0.694 kPa-1 within the pressure range of 0-1 kPa and can effectively detect pressures ranging from 3 Pa to 200 kPa. The sensor demonstrates stability for up to 500 cycles, with a rapid response time of 96 ms and a recovery time of 118 ms, coupled with a low hysteresis of 6.8%. Furthermore, our testing indicates that the sensor possesses limitless potential for use in detecting human physiological activities and delivering signals.

Materials, Structure, and Interface of Stretchable Interconnects for Wearable Bioelectronics

Since wearable technologies for telemedicine have emerged to tackle global health concerns, the demand for well-attested wearable healthcare devices with high user comfort also arises. Skin-wearables for health monitoring require mechanical flexibility and stretchability for not only high compatibility with the skin's dynamic nature but also a robust collection of fine health signals from within. Stretchable electrical interconnects, which determine the device's overall integrity, are one of the fundamental units being understated in wearable bioelectronics. In this review, a broad class of materials and engineering methodologies recently researched and developed are presented, and their respective attributes, limitations, and opportunities in designing stretchable interconnects for wearable bioelectronics are offered. Specifically, the electrical and mechanical characteristics of various materials (metals, polymers, carbons, and their composites) are highlighted, along with their compatibility with diverse geometric configurations. Detailed insights into fabrication techniques that are compatible with soft substrates are also provided. Importantly, successful examples of establishing reliable interfacial connections between soft and rigid elements using novel interconnects are reviewed. Lastly, some perspectives and prospects of remaining research challenges and potential pathways for practical utilization of interconnects in wearables are laid out.

Advancements in stretchable organic optoelectronic devices and flexible transparent conducting electrodes: Current progress and future prospects

The rapid evolution of flexible optoelectronic devices in consumer markets, such as solar cells, photonic skins, displays, lighting, supercapacitors, and smart windows, has spurred global innovation in the design and development of Stretchable Transparent Conducting Electrode (STCE) materials. These materials, which combine the flexibility of organic materials with the functionality of optoelectronic components, have drawn a lot of attention because of their potential uses in a variety of disciplines, such as medical equipment, wearable electronics, and soft robotics. Recent advancements in material science and device design have significantly improving performance, durability, and functionality of these stretchable organic optoelectronic devices. Furthermore, flexible conducting transparent electrodes play an essential role in a wide range of flexible and transparent electronics, including touch screens, displays, and solar cells. Traditional materials like indium tin oxide (ITO) electrodes, while effective, and constrained by their fragility and high cost. Recent innovations in alternative materials, such as metal mesh, nanowires, conducting polymers and graphene have ushered in a new era of affordable, flexible, and transparent conductive electrodes. Materials like graphene, metal nanowires, metallic grids, metal meshes, and dielectric-metal-dielectric electrodes are explored as potential substitutes for fragile ITO electrodes, thanks to their excellent combination of mechanical flexibility and electrical conductivity. This abstract delves into the opportunities and challenges in the development of flexible and transparent organic optoelectronic devices and flexible conducting transparent electrodes. In this review, we explain the technological advancements of transparent and stretchable electrodes, as well as their applications in organic optoelectronic devices such as organic and perovskite solar cells, OLED, heaters, and supercapacitors. We will specifically examine the basic characteristics, optoelectronic properties, and manufacturing procedures of transparent conducting electrodes. We also discuss the key criteria for evaluating proposals for new research lines in this burgeoning sector.

Research on high sensitivity piezoresistive sensor based on structural design

With the popularity of smart terminals, wearable electronic devices have shown great market prospects, especially high-sensitivity pressure sensors, which can monitor micro-stimuli and high-precision dynamic external stimuli, and will have an important impact on future functional development. Compressible flexible sensors have attracted wide attention due to their simple sensing mechanism and the advantages of light weight and convenience. Sensors with high sensitivity are very sensitive to pressure and can detect resistance/current changes under pressure, which has been widely studied. On this basis, this review focuses on analyzing the performance impact of device structure design strategies on high sensitivity pressure sensors. The design of structures can be divided into interface microstructures and three-dimensional framework structures. The preparation methods of various structures are introduced in detail, and the current research status and future development challenges are summarized.

Degradable silk fibroin based piezoresistive sensor for wearable biomonitoring

Degradable wearable electronics are attracting increasing attention to weaken or eliminate the negative effect of waste e-wastes and promote the development of medical implants without secondary post-treatment. Although various degradable materials have been explored for wearable electronics, the development of degradable wearable electronics with integrated characteristics of highly sensing performances and low-cost manufacture remains challenging. Herein, we developed a facile, low-cost, and environmentally friendly approach to fabricate a biocompatible and degradable silk fibroin based wearable electronics (SFWE) for on-body monitoring. A combination of rose petal templating and hollow carbon nanospheres endows as-fabricated SFWE with good sensitivity (5.63 kPa-1), a fast response time (147 ms), and stable durability (15,000 cycles). The degradable phenomenon has been observed in the solution of 1 M NaOH, confirming that silk fibroin based wearable electronics possess degradable property. Furthermore, the as-fabricated SFWE have been demonstrated that have abilities to monitor knuckle bending, muscle movement, and facial expression. This work offers an ecologically-benign and cost-effective approach to fabricate high-performance wearable electronics.

Programmable and Weldable Superelastic EGaIn/TPU Composite Fiber by Wet Spinning for Flexible Electronics

As an essential component of flexible electronics, superelastic conductive fibers with good mechanical and electrical properties have drawn significant attention, especially in their preparation. In this study, we prepared a superelastic conductive fiber composed of eutectic gallium-indium (EGaIn) and thermoplastic polyurethane (TPU) by simple wet spinning. The composite conductive fiber with a liquid metal (LM) content of 85 wt % achieved a maximum strain at a break of 659.2%, and after the conductive pathway in the porous structure of the composite fibers was fully activated, high conductivity (1.2 × 105 S/m) was achieved with 95 wt % LM by mechanical sintering and training processes. The prepared conductive fibers exhibited a stable resistive response as the fibers were strained and could be sewn into fabrics and used as wearable strain sensors to monitor various human motions. These conductive fibers can be molded into helical by heating, and they have excellent electrical properties at a maximum mechanical strain of 3400% (resistance change <0.27%) with a helical index of 11. Moreover, the conductive fibers can be welded to various two or three-dimensional conductors. In summary, with a scalable manufacturing process, weldability, superelasticity, and high electrical conductivity, EGaIn/TPU composite fibers fabricated by wet spinning have considerable potential for flexible electronics.

High-sensitivity and ultralow-hysteresis fluorine-rich ionogel strain sensors for multi-environment contact and contactless sensing

Information transduction via soft strain sensors under harsh conditions such as marine, oily liquid, vacuum, and extreme temperatures without excess encapsulation facilitates modern scientific and military exploration. However, most reported soft strain sensors struggle to meet these requirements, especially in complex environments. Herein, a class of fluorine-rich ionogels with tunable ultimate strain, high conductivity, and multi-environment tolerance are designed. Abundant ion-dipole and dipole-dipole interactions lead to excellent miscibility between the hydrophobic ionic liquid and the fluorinated polyacrylate matrix, as well as adhesion to diverse substrates in amphibious environments. The ionogel-based sensors, even in encapsulation-free form, exhibit stable operation with a negligible hysteresis (as low as 0.119%) and high sensitivity (gauge factor of up to 6.54) under amphibious conditions. Multi-environment sensing instances in contact and even contactless forms are also demonstrated. This study opens the door for the artificial syntheses of multi-environment tolerance ionic skins with robust sensing applications in soft electronics and robotics.

Highly stable and strain-insensitive metal film conductors via manipulating strain distribution

Metal film-based stretchable conductors are essential elements of flexible electronics for wearable, biomedical, and robotic applications, which require strain-insensitive high conductivity over a wide strain range and excellent cyclic stability. However, they suffer from serious electrical failure under monotonic and cyclic tensile loading at a small strain due to the uncontrolled film cracking behavior. Here, we propose a novel in-plane crack control strategy of engineering hierarchical microstructures to achieve outstanding electromechanical performance via harnessing the strain distribution in metal films. The wrinkles delay the crack initiation at undercuts which should be the most vulnerable sites during the stretching process. The surface protrusions/grooves/undercuts inhibit the crack propagation because of the effective strain redistribution. In addition, hierarchical microstructures significantly improve cyclic stability due to the strong interfacial adhesion and stable crack patterns. The metal film-based conductors exhibit ultrahigh strain-insensitive conductivity (1.7 × 107 S m-1), negligible resistance change (ΔR/R0 = 0.007) over an ultra-wide strain range (>200%), and excellent cyclic strain durability (>15 000 cycles at 100% strain). A range of metal films was explored to establish the universality of this strategy, including ductile copper and silver, as well as brittle molybdenum and high entropy alloy. We demonstrate the strain-insensitive electrical functionality of a metal film-based conductor in a flexible light-emitting diode circuit.

Highly adhesive chitosan/poly(vinyl alcohol) hydrogels via the synergy of phytic acid and boric acid and their application as highly sensitive and widely linear strain sensors

In recent years, flexible strain sensors have attracted increasing interest, and accurate sensing and comfortable wearables are highly demanded. However, current flexible strain sensors fail to have wide linearity and high sensitivity simultaneously, and their adhesion is insufficient for convenient wear and precise motion monitoring. Herein, chitosan/poly(vinyl alcohol) hydrogels with phytic acid (PA) and boric acid (BA) as crosslinkers (CS/PVA-PA-BA hydrogels) were fabricated. The synergy of phytic acid and boric acid not only improved the mechanical properties of the obtained hydrogels (1070% of fracture strain and 0.83 MPa of fracture stress), but also provided them with outstandingly strong adhesion. Their adhesive strength was up to 527 kPa for a variety of materials, including glass, silica rubber, steel, polytetrafluoroethylene (PTFE), and skin. In addition, the hydrogel-based strain sensor demonstrated high sensitivity (gauge factor = 4.61), a wide linear strain range (up to 1000%, R2 = 0.996), fast response time (90 ms), and good stability. A flexible strain sensor with such high sensitivity and wide linear range simultaneously, to the best of our knowledge, has never been reported before. The development of CS/PVA-PA-BA hydrogels is expected to inspire a novel method for high-adhesive and high-sensing-performance wearable electronics.

Stretchable Sensors: Novel Human Motion Monitoring Wearables

A human body monitoring system remains a significant focus, and to address the challenges in wearable sensors, a nanotechnology-enhanced strategy is proposed for designing stretchable metal-organic polymer nanocomposites. The nanocomposite comprises reduced graphene oxide (rGO) and in-situ generated silver nanoparticles (AgNPs) within elastic electrospun polystyrene-butadiene-polystyrene (SBS) fibers. The resulting Sandwich Structure Piezoresistive Woven Nanofabric (SSPWN) is a tactile-sensitive wearable sensor with remarkable performance. It exhibits a rapid response time (less than three milliseconds) and high reproducible stability over 5500 cycles. The nanocomposite also demonstrates exceptional thermal stability due to effective connections between rGO and AgNPs, making it suitable for wearable electronic applications. Furthermore, the SSPWN is successfully applied to human motion monitoring, including various areas of the hand and RGB sensing shoes for foot motion monitoring. This nanotechnology-enhanced strategy shows promising potential for intelligent healthcare, health monitoring, gait detection, and analysis, offering exciting prospects for future wearable electronic products.

Dynamic competitive strains enabled self-supporting Janus nanostructured films for high-performance airflow perception

Recently, piezoresistive airflow sensing systems have shown extensive potential applications in aerospace, weather forecasting, mineral enterprises, and wearable electronics. However, the achievement of both an ultralow detection limit and broad monitoring range still remains challenging. Here, we propose a self-supported Janus film based on a graphene/carbon sphere-elastomer hybrid, which allows us to sensitively and efficiently perceive tiny and strong airflows via responding with opposite current variations enabled by the dynamic competition of transverse and longitudinal strains. The achieved film enables an ultralow detection limit of ∼0.0087 m s^-1, a wide detection range of 0.0087-23 m s^-1, favorable response speed as fast as ∼0.1 s, and signal stability for 1150 cycles. Furthermore, an artificial smart spiderweb array system is delicately designed to efficiently distinguish the position and intensity of the applied airflow for efficient non-contact manipulation, enabling significant potential in the development of advanced soft electronics and smart biomimetic systems.

Recent Progress in Self-Healable Hydrogel-Based Electroluminescent Devices: A Comprehensive Review

Flexible electronics have gained significant research attention in recent years due to their potential applications as smart and functional materials. Typically, electroluminescence devices produced by hydrogel-based materials are among the most notable flexible electronics. With their excellent flexibility and their remarkable electrical, adaptable mechanical and self-healing properties, functional hydrogels offer a wealth of insights and opportunities for the fabrication of electroluminescent devices that can be easily integrated into wearable electronics for various applications. Various strategies have been developed and adapted to obtain functional hydrogels, and at the same time, high-performance electroluminescent devices have been fabricated based on these functional hydrogels. This review provides a comprehensive overview of various functional hydrogels that have been used for the development of electroluminescent devices. It also highlights some challenges and future research prospects for hydrogel-based electroluminescent devices.

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate's hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics.

Review of Bacterial Nanocellulose as Suitable Substrate for Conformable and Flexible Organic Light-Emitting Diodes

As the development of nanotechnology progresses, organic electronics have gained momentum in recent years, and the production and rapid development of electronic devices based on organic semiconductors, such as organic light-emitting diodes (OLEDs), organic photovoltaic cells (OPVs), and organic field effect transistors (OFETs), among others, have excelled. Their uses extend to the fabrication of intelligent screens for televisions and portable devices, due to their flexibility and versatility. Lately, great efforts have been reported in the literature to use them in the biomedical field, such as in photodynamic therapy. In tandem, there has been considerable interest in the design of advanced materials originating from natural sources. Bacterial nanocellulose (BNC) is a natural polymer synthesized by many microorganisms, notably by non-pathogenic strains of Komagataeibacter (K. xylinus, K. hansenii, and K. rhaeticus). BNC shows distinct physical and mechanical properties, including its insolubility, rapid biodegradability, tensile strength, elasticity, durability, and nontoxic and nonallergenic features, which make BNC ideal for many areas, including active and intelligent food packaging, sensors, water remediation, drug delivery, wound healing, and as conformable/flexible substrates for application in organic electronics. Here, we review BNC production methods, properties, and applications, focusing on electronic devices, especially OLEDs and flexible OLEDs (FOLEDs). Furthermore, we discuss the future progress of BNC-based flexible substrate nanocomposites.

Progression of Quantum Dots Confined Polymeric Systems for Sensorics

The substantial fluorescence (FL) capabilities, exceptional photophysical qualities, and long-term colloidal stability of quantum dots (QDs) have aroused a lot of interest in recent years. QDs have strong and wide optical absorption, good chemical stability, quick transfer characteristics, and facile customization. Adding polymeric materials to QDs improves their effectiveness. QDs/polymer hybrids have implications in sensors, photonics, transistors, pharmaceutical transport, and other domains. There are a great number of review articles available online discussing the creation of CDs and their many uses. There are certain review papers that can be found online that describe the creation of composites as well as their many different uses. For QDs/polymer hybrids, the emission spectra were nearly equal to those of QDs, indicating that the optical characteristics of QDs were substantially preserved. They performed well as biochemical and biophysical detectors/sensors for a variety of targets because of their FL quenching efficacy. This article concludes by discussing the difficulties that still need to be overcome as well as the outlook for the future of QDs/polymer hybrids.

Effects of Surface Roughness on Direct Plasma Bonding between Silicone Rubbers Fabricated with 3D-Printed Molds

This study presents the effects of surface roughness on the adhesion strength of plasma-treated rubbers that are widely used in soft robotics. The rubbers are designed with 11 molds of different patterns and fabricated from liquid silicones for mutual comparison. Several specimens with nonperiodic and periodic surface waveforms are quantitatively analyzed based on the correlation between surface roughness and adhesion strength. The surface roughness of three-dimensional (3D) printed molds under different printing conditions is compared to that of the standard specimens molded by a smooth acrylic plate and four sandpapers. The surface profiles are measured by a stylus profiler, analyzed using fast Fourier transform, and subsequently quantified using the experimental roughness parameters, R _a and R _ku ^*. The kurtosis ratio R _ku ^* is proposed to simultaneously evaluate the sharpness, total height, and peak density to identify contact surfaces. A 90° peel test is also conducted to evaluate the adhesion strength, considering the designed pattern and printing orientation relative to the peeling direction. Microstructural analysis of the specimens is performed to investigate the peeling mechanism and molding quality using scanning electron and digital microscopes. Correlations between adhesion strength and surface roughness are obtained through the evaluation of the plasma-treated silicone specimens. R _ku ^* is significant in determining the surface properties of the effective contact area, particularly for rough surfaces, and further contributes to an effective evaluation when the parameter R _a is used simultaneously. The results suggest that the plasma bonding of silicone rubbers fabricated with 3D-printed molds is effective in enhancing the adhesion strength of soft robots or stretchable devices.