Potential Applications In Wearable Electronics Research Articles

Novel conductive polymer composites based on CNTs/CNFs bridged liquid metal

Conductive polymer composites (CPCs) are vital and indispensable for the emerging field of soft electronics. In this work, a new strategy for using carbon nanotubes (CNTs)/carbon nanofibers (CNFs) and liquid metal droplets as multiphase hybrid fillers for CPCs is presented. We found that CPCs with multiphase hybrid conductive fillers had advantages in electromechanical properties over those with single solid-phase conductive fillers. CNTs/CNFs can easily form conductive paths in the polymer due to their excellent electrical conductivity with large aspect ratio. Self-repair of conductive networks was realized since the exposed Galinstan under strains could bridge the gap between CNTs/CNFs to form new conductive pathways. In addition, the influences of the size and content of conductive fillers on the electromechanical properties of CPCs were investigated. We found CPCs with liquid metals can be served as a flexible strain sensor under mild strain, while as a conductor under large strain due to the robust stability of resistance, which has not been reported yet in the literature. The CPCs presented in this work could provide potential applications in wearable electronics and flexible electronics.

Patterning of graphene for highly sensitive strain sensing on various curved surfaces

AbstractIn recent years, graphene based flexible strain sensors have attracted great attention. Although various designed micro/nanostructure morphologies have been introduced to optimize the performance parameters such as sensitivity and working range, there are relatively limited studies on the effective patterning of graphene on various substrates on curved surfaces to manufacture functional devices. In this work, we report a general route to realize the patterning of graphene into predesigned structure on a variety of complex curved substrates. The methodology involves a combination of polydopamine surface modification of the substrate and the spraying process of graphene. The polydopamine modification method preserves the intrinsic electrical properties and stability of graphene, and the modification process by self‐polymerization of dopamine is available on many types of materials of complex shape. This general approach of interface modification to achieve free patterning has capability of large‐scale fabrication. By this interface‐enhancement patterning method, sensing structure of cut‐through channel crack has been obtained, resulting in flexible strain sensors with adjusted high sensitivity (gauge factor of 100–2000 at 1% strain), little hysteresis and conformality of complex surfaces. In addition, the strain sensor array has also been fabricated to detect the deformation spatial distribution of complex curved structural components, thereby expanding their potential applications in wearable electronics.

Multi-functional and highly conductive textiles with ultra-high durability through ‘green’ fabrication process

Conductive textiles with mechanical flexibility, long-term durability and stability under harsh conditions are highly desired for potential applications in wearable electronics and devices. One challenge associated with the development of such materials is their fabrication method, which requires to be low-cost, scalable, and environmental-friendly. Herein, we developed a full “green” route to fabricate machine-washable conductive textiles by coating textiles with a novel crosslinked and conductive polymer composite coating, using single-walled carbon nanotubes (SWNTs) and bio-mass derived glucaric acid/chitosan (GA-chitosan) organic salt aqueous solution with dip-coating or spray-coating. The crosslinked SWNTs/GA-chitosan polyamide coatings exhibit a high electrical conductivity of up to 7.4 × 102 S/m and high water/organic solvents resistance. The conductive textiles can achieve an exceptional Joule heating performance driven by moderate voltage and exhibit a high electromagnetic interference shielding efficiency of approximately 30 dB at X-band under optimized formulation. The high adhesive energy between the polymer composite coatings and textile substrates enables the ultra-high durability and stability of textiles, confirmed by mechanical deformation, rubbing, and washing tests. This simple and organic solvent-free processing method provides an environmentally friendly, cost-effective fabrication approach, holding great promise for large-scale production of multifunctional conductive wearable textiles for EMI shielding and/or personal heating applications.

Processing Natural Wood into a High-Performance Flexible Pressure Sensor

Flexible pressure sensors have received wide attention because of their potential applications in wearable electronics and electronic skins (e-skins). However, the high performance of the pressure sensors relies principally on the introduction of complex surface microstructures, which often involves either complicated procedures or costly microfabrication methods. Moreover, these devices predominantly use synthetic polymers as flexible substrates, which are generally nonbiodegradable or not ecofriendly. Here, we report a facile and scalable processing strategy to convert naturally rigid wood into reduced graphene oxide (rGO)-modified flexible wood (FW/rGO) via saw cutting, chemical treatment, and rGO coating, resulting in high-performance wood-based flexible piezoresistive pressure sensors. Benefiting from the largely deformable ribbon-like surface microstructures, the obtained wood-based pressure sensor displayed a high sensitivity of 1.85 kPa-1 over a broad linear range up to 60 kPa and showed high stability over 10 000 cyclic pressings. The favorable sensing performance of the pressure sensor allows for accurate recognition of finger movements, acoustic vibrations, and real-time pulse waves. Moreover, a large-area pressure sensor array has been successfully assembled on one piece of flexible wood for spatial pressure mapping. The proposed strategy of directly using natural wood for high-performance flexible pressure sensors is simple, low-cost, sustainable, and scalable, opening up a new avenue for the development of next-generation wearable electronics and e-skins.

High-Crystallinity Epitaxial Sb2Se3 Thin Films on Mica for Flexible Near-Infrared Photodetectors.

The V-VI binary chalcogenide, Sb2Se3, has attracted considerable attention for its applications in thin film optoelectronic devices because of its unique 1D structure and remarkable optoelectronic properties. Herein, we report an Sb2Se3 thin film epitaxially grown on a flexible mica substrate through a relatively weak (van der Waals) interaction by vapor transport deposition. The epitaxial Sb2Se3 thin films exhibit a single (120) out-of-plane orientation and a 0.25° full width at half-maximum of (120) rocking curve in X-ray diffraction, confirming the high crystallinity of the epitaxial films. The Sb2Se3(120) plane is epitaxially aligned on mica(001) surface with the in-plane relationship of Sb2Se3[2̅10]//mica[010] and Sb2Se3[001]//mica[100]. Compared to the photodetector made of a nonepitaxial Sb2Se3 film, the photocurrent of the epitaxial Sb2Se3 film photodetector is almost doubled. Furthermore, because of the flexibility and high sensitivity of the epitaxial Sb2Se3 film photodetector on mica, it has been successfully employed to detect the heart rate of a person. These encouraging results will facilitate the development of epitaxial Sb2Se3 film-based devices and potential applications in wearable electronics.

NIR-responsive multi-healing HMPAM/dextran/AgNWs hydrogel sensor with recoverable mechanics and conductivity for human-machine interaction

Conductive and self-healing hydrogel sensor is perspective in human-machine interaction applications. However, the design of ideal self-healing hydrogels are always challenging. Herein, by introducing disulfide modified Ag nanowires (AgNWs), we show a novel self-healing hydrogel strain sensor with superior mechanics, conductivity, antibacterial property, and firstly realizing of self-healing with both recovery of mechanics and sensing properties. We demonstrate that the covalent and reversible non-covalent hydrophobic blocks in hydrophobic modified polyacrylamide (HMPAM) achieves the basic self-healing network; dextran with plentiful hydroxyl groups synergistic helps the self-healing by hydrogen bonds; disulfide on the AgNWs surface forms a NIR-responsive and dynamic Ag-S coordination bridge between HMPAM and AgNWs. The resulted hydrogel sensor exhibits comprehensive electromechanical properties, and precisely monitors human motion and subtle electromyography (EMG) signals. Importantly, we firstly achieved the recovery of sensing properties on human motion detection and EMG signal detection after self-healing. This work provides a promising exploration to manufacture bionic strain sensors for potential applications in wearable electronics.

A facile and cost-effective approach to fabrication of high performance pressure sensor based on graphene-textile network structure

High performance flexible pressure sensors have received tremendous attention due to the potential applications in wearable electronics and humanoid robotics. Herein, a low-cost, time-saving fabrication strategy is reported to efficiently construct highly sensitive and scalable pressure sensors based on graphene-textile. The flexible pressure sensor was prepared through a dip coating approach where the graphene ink was used as the active material and textile with ordered network structure, serves as the flexible support matrix. By increasing the number of layers of the textile, the pressure sensor could achieve a high linear sensitivity value of 0.23 kPa−1, a broad pressure range from 0 to 140 kPa, and a durability over 800 cycles. It was found that the network structure of the graphene-textile contributed to the enhancement of the sensing performance due to the piezoresistive effect resulting from the deformation of conductive graphene-textile. Moreover, by virtue of its exceptional properties, it is demonstrate that the high performance flexible pressure sensor could be further used to detect human physiological signals, such as wrist pulse, finger press detection and walking state monitoring, indicating its promising potential to flexible and wearable electronics.

Two-terminal photonic neuromorphic devices with short/long-term plasticity based on interface charge effects

Synaptic devices have attracted intense research interests recently due to their potentials in overcoming the Von Neumann bottleneck. The light-stimulated synaptic devices based on three-terminal transistors have advantages of sustainability and gate modulation, however, the simultaneous control of multiterminal is complicated for operation. In this work, a new type of two-terminal photoresponsive and synaptic device is developed with easy fabrication process by utilizing the interfacial charge trapping effect introduced by the dielectric layer. This field-effect two-terminal device (FETD) is derived from the three-terminal transistor structure, and hence maintains the field effect advantages of the traditional transistor synaptic device, but has simplified operation. Moreover, the light employed as the additional input signal enables the reading and writing process to be implemented simultaneously. The synaptic functionalities including short-term plasticity and transition from short-term memory to long-term memory are successfully imitated. The flexible device is further developed for the potential application in wearable electronics. Therefore, this work has enlightened a novel structure, which not only retains the advantages of three-terminal synaptic devices with simple operation but also realizes the synchronization of information transfer and learning process by introducing light as input signals. • A two-terminal photoresponsive and synaptic device is developed, with the simple operation and advantages of transistors. • Synaptic functions including short-term plasticity and transition from short-term to long-term memory are imitated. • By using PET-ITO as substrate, the device with low operation voltage below 1.5 V shows good flexibility and stability.

Fingerprint-inspired graphene pressure sensor with wrinkled structure

Nowadays, flexible pressure sensors have broad applications in healthcare monitoring, wearable electronics and so on. However, conventional fabrications are mostly time-consuming, complicated and even use toxic chemical reagents. It is in urgent need to fabricate pressure sensors with high sensitivity in a facile way. In nature, the fingertip skin of human has many epidermal ridges, which can sense small external stimuli. Here, inspired from the fingerprint, we propose a new type of pressure sensor made with reduced graphene oxide (rGO). The rGO film is designed to form a wrinkled structure by a facile stretching-releasing process, which mimics the morphology of fingertip skin. Owing to the fingertip-skin-like wrinkled structure, the pressure sensor exhibits high sensitivity of 5.77 kPa−1 (0 - 490 Pa). It also has a lot of attractive properties, such as fast response (< 100 ms), small detecting pressure (3 Pa) and good repeatability. We show the ability of the pressure sensor in small pressure detecting, human motion monitoring and motion pattern recognition. Furthermore, a translation system and a multi-touch device with spatially resolved detection based on the pressure sensor are developed. This pressure sensor is hopeful to have potential applications in wearable electronics and human–machine interaction systems.

Highly sensitive printed crack-enhanced strain sensor as dual-directional bending detector

Strain sensors with good flexibility and high sensitivity attracted a lot of interest because of their potential applications in wearable electronics. In this paper, we demonstrate a high-performance crack-based strain sensor based on printable polyvinyl chloride/carbon black (PVC/CB) composite material. Flexible conductive circuit and PVC/CB composite were screen-printed to a desirable geometry on a polyimide (PI) substrate. After 5000 bends, a stable cracked morphology formed on the PVC/CB surface, which greatly enhanced the sensitivity of our strain sensor: corresponding gauge factors were equal to 741 and 1563 gauge under the tensile and compressive strains, respectively. Our crack-based strain sensor also demonstrated high stability. Additionally, this work shows the application of this crack-based sensor for human activity detection to demonstrate their application prospects for flexible and wearable electronic devices.

Cost-Effective Yarn-Shaped Lithium-Ion Battery with High Wearability.

Because of flexibility, compactness, weavability, and ergonomic design, yarn-shaped lithium-ion batteries (LIBs) have enormous potential applications in wearable electronics. Still, the yarn-shaped LIB with the ability to meet commercialization requirements has never been reported, owing to the current challenge in complex material synthesis technologies, expensive raw material costs, poor safety performance, and nonstandard manufacturing equipment. Herein, we propose a yarn-shaped LIB that meets the aforementioned requirements. With a highly conductive and flexible stainless-steel yarn acting as the current collector, the electrode active materials and the gel electrolyte, which are commercially available at low cost, are uniformly coated onto the stainless-steel yarn by a simple and facile dipping-drying method. Even at different deformation conditions (i.e., bending or knotting), the specific capacity of the yarn-shaped LIB (7 cm long, <2 mm in diameter) assembled from graphite and lithium iron phosphate electrodes is maintained >85%. After charged treatment, it can successfully power up an electronic watch and an electronic thermo-hygrometer. Thanks to the simple preparation process, low cost of raw materials, and good safety performance, this work can promote the commercialization of wearable energy storage devices.

Superhydrophobic, flame-retardant and conductive cotton fabrics via layer-by-layer assembly of carbon nanotubes for flexible sensing electronics

Functional textiles are ideal substrates for wearable electronics. Herein, superhydrophobic, flame-retardant and conductive cotton fabrics were fabricated by sequential assembly of poly(ethylenimine), ammonium polyphosphate and carbon nanotubes, followed by post-treatment with poly(dimethylsiloxane). The resulting fabrics possessed excellent superhydrophobic stability toward acid, alkali, organic solvent, UV irradiation, abrasion and long-time laundering. Meanwhile, when suffering to fire, the coated fabric could generate an efficient char layer and extinguish the fire to protect the cotton fiber from forming flame. Furthermore, this conductive cotton fabric exhibited stable sensing ability in contact with water droplets, showing wide potential application in wearable electronics as multifunctional smart textiles.

Stretchable supercapacitors: Electrodes, electrolytes, and devices

Stretchable supercapacitors have received more and more attention due to their potential applications in wearable electronics and health monitoring. The stretchable supercapacitors have not only the advantages of high power density, long cycle life, safety and low cost of ordinary supercapacitor, but also good flexibility and stretchability to integrate well with wearable system. In this review, according to the structures of supercapacitors, the methods of preparing stretchable electrodes/devices reported in the literature are categorized and analyzed. We particularly highlight the key findings of creating stretchable electrodes/devices, which include elastic polymer substrates, tensile structure design and elastic polymer + tensile structure. In addition, the research progress of multi-functional stretchable supercapacitors and high elastic gel electrolytes are discussed. Finally, the challenges to the future development of the stretchable supercapacitors are analyzed and summarized. We expect to stimulate more research in creating stretchable supercapacitors for wide practical applications.

Amorphous Oxide Semiconductor Transistors with Air Dielectrics for Transparent and Wearable Pressure Sensor Arrays

AbstractFlexible, transparent pressure sensors have numerous potential applications in wearable electronics, soft robotics, health monitoring. In particular, highly sensitive and reliable pressure sensors that cover wide ranges of pressures are promising because they can undergo various external stimulations. Here an unconventional approach is presented for fabricating the active‐matrix array of air‐dielectric, amorphous oxide semiconductor transistors for transparent, wearable pressure sensors. In the structure of these pressure‐sensitive field‐effect transistors (FETs), the clean interface between the air and the oxide semiconductor channel enables the FETs to have outstanding mobility and negligible electrical hysteresis with rapid and reliable responses as pressure sensors for an extensive range of pressures from 200 Pa to 5 MPa. Also, low processing temperature and high transparency of oxide semiconductors make it possible to fabricate them on plastics with flexibility and transparency. The fabrication of active‐matrix pressure sensor arrays demonstrated the real‐time monitoring of freely moving, ultralight, liquid droplets on the sensor. In addition, this can be integrated into the fingertips of gloves to monitor the pressure changes that occur during grasping objects. These results illustrate the potential of pressure sensors to provide robust solutions in the next‐generation electronics including remote surgery, health monitoring, and robotics.

Ecofriendly high-performance ionic soft actuators based on graphene-mediated cellulose acetate

Ionic-type artificial muscles with eco-friendly, biodegradable, and biocompatible functionalities have attracted attention for a wide range of potential applications in wearable electronics, soft haptic-feedback systems, and active biomedical devices. Here, we report on the development of an ecofriendly high-performance ionic soft actuator based on biofriendly cellulose acetate (CA), graphene nanopowders (GN), ionic liquid (IL) as a plasticizer, and biofriendly-flexible-nonmetallic conducting polymer poly(3,4-ethylene-dioxythiopene)-polystyrenesulfonate (PEDOT: PSS) as an electrode, thereby realizing a novel ecofriendly CA-IL-GN actuator with a large bending mechanical deformation and a fast response time. The proposed CA-IL-GN (0.2 wt%) nanocomposite membrane exhibited dramatic increments in specific capacitance (2.92 times) and Young’s modulus (2.38 times), thus leading to a 2.9 times larger bending deformation and a 4.8 times faster response than those of pure CA-IL actuator. Therefore, the developed ecofriendly high-performance CA-IL-GN actuator can be considered to be a promising candidate for human-friendly electronics, including artificial muscles, flexible haptic devices, soft wearable devices, and bio-medical devices, due to its cost-effectiveness, large bending mechanical actuation, fast response, and bio-friendly functionalities.

Carbon Black from Diesel Soot for High‐Performance Wearable Pressure Sensors

AbstractThe rational design of high‐performance flexible pressure sensors with both high sensitivity and large workable range attracts great attentions due to the potential applications in wearable electronics and human–machine interfaces. Here, carbon black (CB) from diesel soot, an air pollutant produced during incomplete combustion of hydrocarbon fuels, is used as the active material to construct high‐performance flexible all‐textile pressure sensors. Due to the unique hierarchical structures and large surface roughness of the CB‐coated fabric, the pressure sensor exhibits outstanding performances, such as high sensitivity (81.61 kPa−1 within the range of 0–10 kPa), extrawide workable pressure regime (0–100 kPa), and rapid response and relaxation time (6 and 30 ms, respectively). Based on these superior sensing properties, its practical applications are demonstrated in detecting the signals of musical sound and of human physiology, showing its potential for health monitoring as wearable electronics.

Compromised Power Capability of Yarn-Shaped Supercapacitors and Its Mitigation Strategies

Supercapacitors are known to have superior power densities over batteries due to their surface-centered charge-discharge processes. Recently, increasing attention has accumulated in the fiber or yarn shaped supercapacitor (FSC) systems, mainly due to their high flexibility and pliable nature, leading to potential applications in wearable electronics, microrobots, medical implants, and smart textiles. However, one of the major limitations that occurs when supercapacitors are made into fiber/yarn shapes, is the increased equivalent series resistance (ESR) due to the fibrous geometry adopted here. For instance, even the best FSC reported will exhibit several hundred ohms of resistance at one-meter length, which however is usually neglected by researchers in their publication. To directly tackle this issue, here we report our strategies of using a metallic core filament in the yarn supercapacitors development in our lab. With detailed materials and process engineering, we are able to boost the power density by one order of magnitude. Sealing strategies and other alternative solutions for this issue will also be discussed.

Polyaniline Nanofiber Wrapped Fabric for High Performance Flexible Pressure Sensors.

The rational design of high-performance flexible pressure sensors with both high sensitivity and wide linear range attracts great attention because of their potential applications in wearable electronics and human-machine interfaces. Here, polyaniline nanofiber wrapped nonwoven fabric was used as the active material to construct high performance, flexible, all fabric pressure sensors with a bottom interdigitated textile electrode. Due to the unique hierarchical structures, large surface roughness of the polyaniline coated fabric and high conductivity of the interdigitated textile electrodes, the obtained pressure sensor shows superior performance, including ultrahigh sensitivity of 46.48 kPa−1 in a wide linear range (<4.5 kPa), rapid response/relaxation time (7/16 ms) and low detection limit (0.46 Pa). Based on these merits, the practical applications in monitoring human physiological signals and detecting spatial distribution of subtle pressure are demonstrated, showing its potential for health monitoring as wearable electronics.

Highly conductive 1D-2D composite film for skin-mountable strain sensor and stretchable triboelectric nanogenerator

The growing demand of intelligent wearable devices has triggered rapid development of highly stretchable strain sensors and power sources. Here, we fabricated a sandwich-structured stretchable device, where a composite film is encapsulated in two layers of polydimethylsiloxane (PDMS). The highly conductive composite film is composed of one-dimensional (1D) silver nanowires (AgNWs) network wrapped with two-dimensional (2D) metallic MoS2 nanosheets. The combination of 1D AgNWs and 2D metallic MoS2 nanosheets endows the composite film with excellent flexibility and conductivity, which enables their use as high-performance skin-mountable strain sensor with the maximum gauge factor of 215.4 and high stretchability up to 70%. Furthermore, the sandwich-structured device can serve as a stretchable triboelectric nanogenerator (STENG). Results show that the STENG with an area of 1 × 2.5 cm2 could produce an average power density of 0.16 W/m2. In addition, the STENG could retain stable output performance even under a stretching ratio of 50%. With the outstanding stretchability, it can be fully conformal on nonplanar irregular object such as plant leaves. Moreover, we demonstrated that the fabricated STENG could harvest wind energy and serve as a self-powered wind speed sensor when attached to plant leaves. This work provides new prospects for stretchable power sources and shows potential applications in wearable electronics.

Transfer-Medium-Free Nanofiber-Reinforced Graphene Film and Applications in Wearable Transparent Pressure Sensors.

Graphene exhibits properties of atomic thickness, high transparency, and high carrier mobility, which is highly desirable for a flexible transparent conductive material. However, the electronic properties of large-area chemical vapor deposition grown graphene film suffer from insulated polymer contaminations introduced by the transfer process and the easily cracked nature. Here, we report a preparation method of a transfer-medium-free large-area nanofiber-reinforced graphene (a-PAN/G) film simply by annealing the electrostatically spun polyacrylonitrile (PAN) nanofibers on the graphene film. The film could be free-standing on water and suspended in air with high transparency and enhanced electrical and mechanical properties compared to that of a monolayer graphene film. The flexible transparent a-PAN/G films were demonstrated as active materials for sensitive pressure sensors. The obtained pressure sensors demonstrate high sensitivity (44.5 kPa-1 within 1.2 kPa), low operating voltage (0.01-0.5 V), and excellent stability for 5500 loading-unloading cycles, revealing promising potential applications in wearable electronics.