Ionoskins: Nonvolatile,Highly Transparent,Ultrastretchable Ionic Sensory Platforms for Wearable Electronics

The primary technology of next‐generation wearable electronics pursues the development of highly deformable and stable systems. Here, nonvolatile, highly transparent, and ultrastretchable ionic conductors based on polymeric gelators [poly(methyl methacrylate‐ran‐butyl acrylate), PMMA‐r‐PBA] and ionic liquids (IL) are proposed. A crucial strategy in the molecular design of polymer gelators is copolymerization of PMMA and IL‐insoluble low glass transition temperature (T_g) polymers that can be deformed and effectively dissipate applied strains. Highly stretchable (elongation limit ≈850%), mechanically robust (elastic modulus ≈3.1 × 10⁵ Pa), and deformation durable (recovery ratio ≈96.1% after 500 stretching/releasing cycles) gels are obtained by judiciously adjusting the molecular characteristics of polymer gelators and gel composition. An extremely simple “ionic” strain sensory platform is fabricated by directly connecting the stretchable gel and a digital multimeter, exhibiting high sensitivity (gauge factor ≈2.73), stable operation (>13 000 cycles), and nonvolatility (>10 d in air). Moreover, the skin‐type strain sensor, referred to as ionoskin, is demonstrated. The gels are attached to a part of the body (e.g., finger, elbow, knee, or ankle) and various human movements are successfully monitored. The ionoskin renders the opportunity to achieve wearable ubiquitous electronics such as healthcare devices and smart textile systems.     

Thermal Properties of the Binary‐Filler Hybrid Composites with Graphene and Copper Nanoparticles

The thermal properties of epoxy‐based binary composites comprised of graphene and copper nanoparticles are reported. It is found that the “synergistic” filler effect, revealed as a strong enhancement of the thermal conductivity of composites with the size‐dissimilar fillers, has a well‐defined filler loading threshold. The thermal conductivity of composites with a moderate graphene concentration of f_g = 15 wt% exhibits an abrupt increase as the loading of copper nanoparticles approaches f_Cu ≈ 40 wt%, followed by saturation. The effect is attributed to intercalation of spherical copper nanoparticles between the large graphene flakes, resulting in formation of the highly thermally conductive percolation network. In contrast, in composites with a high graphene concentration, f_g = 40 wt%, the thermal conductivity increases linearly with addition of copper nanoparticles. A thermal conductivity of 13.5 ± 1.6 Wm^?1K^?1 is achieved in composites with binary fillers of f_g = 40 wt% and f_Cu = 35 wt%. It has also been demonstrated that the thermal percolation can occur prior to electrical percolation even in composites with electrically conductive fillers. The obtained results shed light on the interaction between graphene fillers and copper nanoparticles in the composites and demonstrate potential of such hybrid epoxy composites for practical applications in thermal interface materials and adhesives.     

Knittable and Washable Multifunctional MXene‐Coated Cellulose Yarns

Textile‐based electronics enable the next generation of wearable devices, which have the potential to transform the architecture of consumer electronics. Highly conductive yarns that can be manufactured using industrial‐scale processing and be washed like everyday yarns are needed to fulfill the promise and rapid growth of the smart textile industry. By coating cellulose yarns with Ti₃C₂T_x MXene, highly conductive and electroactive yarns are produced, which can be knitted into textiles using an industrial knitting machine. It is shown that yarns with MXene loading of ≈77 wt% (≈2.2 mg cm^?1) have conductivity of up to 440 S cm^?1. After washing for 45 cycles at temperatures ranging from 30 to 80 °C, MXene‐coated cotton yarns exhibit a minimal increase in resistance while maintaining constant MXene loading. The MXene‐coated cotton yarn electrode offers a specific capacitance of 759.5 mF cm^?1 at 2 mV s^?1. A fully knitted textile‐based capacitive pressure sensor is also prepared, which offers high sensitivity (gauge factor of ≈6.02), wide sensing range of up to ≈20% compression, and excellent cycling stability (2000 cycles at ≈14% compression strain). This work provides new and practical insights toward the development of platform technology that can integrate MXene in cellulose‐based yarns for textile‐based devices.     

Enhancing the Sensitivity of Percolative Graphene Films for Flexible and Transparent Pressure Sensor Arrays

Flexible and transparent pressure sensor arrays can find applications in many places such as touch panels, artificial skin, or human motion detection. However, conventional strain gauges are rigid and opaque and are not suitable for such applications. Graphene‐based percolative strain gauges can overcome these challenges but currently are still in the infancy of their development. In this work, the performance of graphene‐based percolative strain gauges is investigated and guidelines to improve the durability and sensitivity of graphene films as sensing elements are developed. It is found that the gauge factor depends on the initial resistance of the graphene film. For the same film resistance, it is found that graphene flake size and film morphology also play a role in determining the gauge factor. Increasing the flake–flake resistance through assembly of surfactant molecules between graphene flakes provides an additional route to enhance the gauge factor. Furthermore, encapsulating the percolative film in micrometer‐thin Poly(methyl methacrylate) does not disrupt the sensing process but significantly improves the sensor's durability. Finally, thus enhanced graphene strain gauges are integrated into flexible and transparent pressure sensor arrays that exhibit high reproducibility and sensitivity.     

3D Graphene Films Enable Simultaneously High Sensitivity and Large Stretchability for Strain Sensors

Integration of 2D membranes into 3D macroscopic structures is essential to overcome the intrinsically low stretchability of graphene for the applications in flexible and wearable electronics. Herein, the synthesis of 3D graphene films (3D‐GFs) using chemical vapor deposition (CVD) is reported, in which a porous copper foil (PCF) is chosen as a template in the atmospheric‐pressure CVD preparation. When the 3D‐GF prepared at 1000 °C (noted as 3D‐GF‐1000) is transferred onto a polydimethylsiloxane (PDMS) membrane, the obtained 3D‐GF‐1000/PDMS hybrid film shows an electrical conductivity of 11.6 S cm^?1 with good flexibility, indicated by small relative resistance changes (ΔR/R₀) of 2.67 and 0.36 under a tensile strain of 50% and a bending radius of 1.6 mm, respectively. When the CVD temperature is reduced to 900 °C (generating a sample noted as 3D‐GF‐900), the 3D‐GF‐900/PDMS hybrid film exhibits an excellent strain‐sensing performance with a workable strain range of up to 187% and simultaneously a gauge factor of up to ≈1500. The 3D‐GF‐900/PDMS also shows a remarkable durability in resistance in repeated 5000 stretching‐releasing cycles. Kinetics studies show that the response of ΔR/R₀ upon strain is related to the graphitization and conductivity of 3D‐GF which are sensitive to the CVD preparation temperature.     

MXene Composite and Coaxial Fibers with High Stretchability and Conductivity for Wearable Strain Sensing Textiles

The integration of nanomaterials with high conductivity into stretchable polymer fibers can achieve novel functionalities such as sensing physical deformations. With a metallic conductivity that exceeds other solution‐processed nanomaterials, 2D titanium carbide MXene is an attractive material to produce conducting and stretchable fibers. Here, a scalable wet‐spinning technique is used to produce Ti₃C₂T_x MXene/polyurethane (PU) composite fibers that show both conductivity and high stretchability. The conductivity at a very low percolation threshold of ≈1 wt% is demonstrated, which is lower than the previously reported values for MXene‐based polymer composites. When used as a strain sensor, the MXene/PU composite fibers show a high gauge factor of ≈12900 (≈238 at 50% strain) and a large sensing strain of ≈152%. The cyclic strain sensing performance is further improved by producing fibers with MXene/PU sheath and pure PU core using a coaxial wet‐spinning process. Using a commercial‐scale knitting machine, MXene/PU fibers are knitted into a one‐piece elbow sleeve, which can track various movements of the wearer's elbow. This study establishes fundamental insights into the behavior of MXene in elastomeric composites and presents strategies to achieve MXene‐based fibers and textiles with strain sensing properties suitable for applications in health, sports, and entertainment.     

Highly Elastic and Conductive N‐Doped Monolithic Graphene Aerogels for Multifunctional Applications

The simple synthesis of ultralow‐density (≈2.32 mg cm^?3) 3D reduced graphene oxide (rGO) aerogels that exhibit high electrical conductivity and excellent compressibility are described herein. Aerogels are synthesized using a combined hydrothermal and thermal annealing method in which hexamethylenetetramine is employed as a reducer, nitrogen source, and graphene dispersion stabilizer. The N‐binding configurations of rGO aerogels increase dramatically, as evidenced by the change in pyridinic‐N/quaternary‐N ratio. The conductivity of this graphene aerogel is ≈11.74 S m^?1 at zero strain, whereas the conductivity at a compressive strain of ≈80% is ≈704.23 S m^?1, which is the largest electrical conductivity reported so far in any 3D sponge‐like low‐density carbon material. In addition, the aerogel has excellent hydrophobicity (with a water contact angle of 137.4°) as well as selective absorption for organic solvents and oils. The compressive modulus (94.5 kPa; ρ ≈ 2.32 mg cm^?3) of the rGO aerogel is higher than that of other carbon‐based aerogels. The physical and chemical properties (such as high conductivity, elasticity, high surface area, open pore structure, and chemical stability) of the aerogel suggest that it is a viable candidate for the use in energy storage, electrodes for fuel cells, photocatalysis, environmental protection, energy absorption, and sensing applications.     

Morphology‐Dependent Magnetism in Nanographene: Beyond Nanoribbons

Imprinting self‐sustainable magnetic features into graphene has recently generated much interest owing to its potential application in spintronics. Several strategies for imprinting magnetic features into graphene are proposed theoretically. However, only a few of them are realized experimentally. Here, the first scalable synthesis of magnetic graphene nanoplatelets with diverse morphologies, including nanoribbons and triangular, pentagonal, hexagonal, and other polyhedral shapes, is reported. This material enters the ferromagnetic regime at a temperature of ≈37 K with magnetization approaching ≈0.45 emu g^?1 under high external magnetic fields. Theoretical calculations are used to explain this sort of morphology‐driven magnetism of graphene nanoplatelets, which emerges from the synergistic effects of the size, geometry of nanographenes, edge terminations, and angle between adjacent edges. In addition, they suggest a new way for preparing magnetically ordered graphene nanoplatelets with a higher transition temperature. In this respect, triangular motifs with zigzag edges represent the most promising morphology of graphene nanoplatelets, which can remain magnetically ordered up to ≈107 K. Based on these challenging results, further tuning of the size and morphology in spatially confined nanographenes combined with doping and sp³ functionalization will enable the preparation of magnetically ordered half‐metallic carbon sustainable up to room temperature, thus opening new opportunities in spintronics.     

Biocompatible Soft Fluidic Strain and Force Sensors for Wearable Devices

Fluidic soft sensors have been widely used in wearable devices for human motion capturing. However, thus far, the biocompatibility of the conductive liquid, the linearity of the sensing signal, and the hysteresis between the loading and release processes have limited the sensing quality as well as the applications of these sensors. In this paper, silicone based strain and force sensors composed of a novel biocompatible conductive liquid (potassium iodide and glycerol solution) are introduced. The strain sensors exhibit negligible hysteresis up to 5 Hz, with a gauge factor of 2.2 at 1 Hz. The force sensors feature a novel multifunctional layered structure, with microcylinder‐filled channels to achieve high linearity, low hysteresis (5.3% hysteresis at 1 Hz), and good sensitivity (100% resistance increase at a 5 N load). The sensors' gauge factors are stable at various temperatures and humidity levels. These biocompatible, low hysteresis, and high linearity sensors are promising for safe and reliable diagnostic devices, wearable motion capture, and compliant human–computer interfaces.     

Lowering Internal Friction of 0D–1D–2D Ternary Nanocomposite‐Based Strain Sensor by Fullerene to Boost the Sensing Performance

The development of strain sensors with both large strain range (>50%) and high gauge factor (>100) is a grand challenge. High sensitivity requires material to perform considerable structural deformation under tiny strain, whereas high stretchability demands structural connection or morphological integrity for materials upon large deformation, yet both features are hard to be achieved in one thin film. A new 0D–1D–2D ternary nanocomposite–based strain sensor is developed that possesses high sensitivity in broad working strain range (gauge factor 2392.9 at 62%), low hysteresis, good linearity, and long‐term durability. The skin‐mountable strain sensor, fabricated through one‐step screen‐printing process, is made of 1D silver nanowire offering high electrical conductivity, 2D graphene oxide offering brittle layered structure, and 0D fullerene offering lubricity. The fullerene constitutes a critical component that lowers the friction between graphene oxide–based layers and facilitates the sliding between adjacent layers without hurting the brittle nature of the nanocomposite film. When stretching, layer slippage induced by fullerene can accommodate partial applied stress and boost the strain, while cracks originating and propagating in the brittle nanocomposite film ensure large resistance change over the whole working strain range. Such high comprehensive performance renders the strain sensor applicable to full‐spectrum human motion detection.     

Sandwich,Vertical‐Channeled Thick Electrodes with High Rate and Cycle Performance

3D thick electrode design is a promising strategy to increase the energy density of lithium‐ion batteries but faces challenges such as poor rate and limited cycle life. Herein, a coassembly method is employed to construct low‐tortuosity, mechanically robust 3D thick electrodes. LiFe_0.7Mn_0.3PO₄ nanoplates (LFMP NPs) and graphene are aligned along the growth direction of ice crystals during freezing and assembled into sandwich frameworks with vertical channels, which prompts fast ion transfer within the entire electrode and reveals a 2.5‐fold increase in ion transfer performance as opposed to that of random structured electrodes. In the sandwich framework, LFMP NPs are entrapped in the graphene wall in a “plate‐on‐sheet” contact mode, which avoids the detachment of NPs during cycling and also constitutes electron transfer highways for the thick electrode. Such vertical‐channel sandwich electrodes with mass loading of 21.2 mg cm^?2 exhibit a superior rate capability (0.2C–20C) and ultralong cycle life (1000 cycles). Even under an ultrahigh mass loading of 72 mg cm^?2, the electrode still delivers an areal capacity up to 9.4 mAh cm^?2, ≈2.4 times higher than that of conventional electrodes. This study provides a novel strategy for designing thick electrodes toward high performance batteries.     

Enhanced Reversible Sodium‐Ion Intercalation by Synergistic Coupling of Few‐Layered MoS2 and S‐Doped Graphene

Sodium‐ion batteries (SIBs) are regarded as the best alternative to lithium‐ion batteries due to their low cost and similar Na⁺ insertion chemistry. It is still challenging but greatly desired to design and develop novel electrode materials with high reversible capacity, long cycling life, and good rate capability toward high‐performance SIBs. This work demonstrates an innovative design strategy and a development of few‐layered molybdenum disulfide/sulfur‐doped graphene nanosheets (MoS₂/SG) composites as the SIB anode material providing a high specific capacity of 587 mA h g^?1 calculated based on the total composite mass and an extremely long cycling stability over 1000 cycles at a current density of 1.0 A g^?1 with a high capacity retention of ≈85%. Systematic characterizations reveal that the outstanding performance is mainly attributed to the unique and robust composite architecture where few‐layered MoS₂ and S‐doped graphene are intimately bridged at the hetero‐interface through a synergistic coupling effect via the covalently doped S atoms. The design strategy and mechanism understanding at the molecular level outlined here can be readily applied to other layered transition metal oxides for SIBs anode and play a key role in contributing to the development of high‐performance SIBs.     

New Silicone Composites for Dielectric Elastomer Actuator Applications In Competition with Acrylic Foil

Several polydimethylsiloxane elastomers were developed and investigated regarding their potential use as materials in dielectric elastomer actuators (DEA). A hydroxyl end‐functionalized polydimethylsiloxane was reacted with different crosslinkers and the electromechanical properties of the resulting elastomers were investigated. The silicone showing the best actuation at the lowest electric field was further used as matrix and compounded with encapsulated conductive polyaniline particles. These composites have enhanced properties including increased strain at break, higher dielectric constant as well as, gratifyingly, breakdown fields higher than that of the matrix. One of the newly synthesized composites is compared to the commercially available acrylic foil VHB 4905 (3M) which is currently the most commonly used elastomer for DEA applications. It was found that this material has little hysteresis and can be activated at lower voltages compared to VHB 4905. For example, when the newly synthesized composite was 30% prestrained, a lateral actuation strain of about 12% at 40 V μm^?1 was measured while half of this actuation strain at the same voltage was measured for VHB 4905 film that was 300% prestrained. It also survived more than 100 000 cycles at voltages which are close to the breakdown field. Such materials might find applications wherever small forces but large strains at low voltages are required, in, for example, tactile displays.     

Liquid‐Gated Transistors Based on Reduced Graphene Oxide for Flexible and Wearable Electronics

Graphene is regarded as the ultimate material for future flexible, high‐performance, and wearable electronics. Herein, a novel, robust, all‐green, highly reliable (yield ≥ 99%), and upscalable technology is reported for wearable applications comprising reduced graphene oxide (rGO) as the electroactive component in liquid‐gated transistors (LGTs). rGO is a formidable material for future flexible and wearable applications due to its easy processability, excellent surface reactivity, and large‐area coverage. A novel protocol is established toward the high‐yield fabrication of flexible rGO LGTs combining high robustness (>1.5 h of continuous operation) with state‐of‐the‐art performances, being similar to those of their rigid counterparts operated under liquid gating, including field‐effect mobility of ≈10^?1 cm² V^?1 s^?1 and transconductance of ≈25 µS. Permeable membranes have been proven crucial to operate flexible LGTs under mechanical stress with reduced amounts of solution (<20 µL). Our rGO LGTs are operated in artificial sweat exploiting two different layouts based on lateral‐flow paper fluidics. These approaches pave the road toward future real‐time tracking of perspiration via a simple and cost‐effective approach. The reported findings contribute to the robust and scalable production of novel graphene‐based flexible devices, whose features fulfill the requirements of wearable electronics.     

Smart and Multifunctional Fiber-Reinforced Composites of 2D Heterostructure-Based Textiles

Smart and multifunctional fiber reinforced polymer (FRP) composites with energy storage, sensing, and heating capabilities have gained significant interest for automotive, civil, and aerospace applications. However, achieving smart and multifunctional capabilities in an FRP composite while maintaining desired mechanical properties remains challenging. Here, a novel approach for layer-by-layer (LBL) deposition of 2D material (graphene and molybdenum disulfide, MoS₂)-based heterostructure onto glass fiber fabric using a highly scalable manufacturing technique at a remarkable speed of ≈150 m min⁻¹ is reported. This process enables the creation of smart textiles with integrated energy storage, sensing, and heating functionalities. This methodology combines gel-based electrolyte with a vacuum resin infusion technique, resulting in an efficient and stable smart FRP composite with an areal capacitance of up to ≈182 µF cm⁻² at 10 mV s⁻¹. The composite exhibits exceptional cyclic stability, maintaining ≈90% capacitance after 1000 cycles. Moreover, the smart composite demonstrates joule heating, reaching from ≈24 to ≈27 °C within 120 s at 25 V. Additionally, the smart composite displays strain sensitivity by altering electrical resistance with longitudinal strain, enabling structural health monitoring. These findings highlight the potential of smart composites for multifunctional applications and provide an important step toward realizing their actual real-world applications.     

Hierarchical Composite Electrodes of Nickel Oxide Nanoflake 3D Graphene for High‐Performance Pseudocapacitors

NiO nanoflakes are created with a simple hydrothermal method on 3D (three‐dimensional) graphene scaffolds grown on Ni foams by microwave plasma enhanced chemical vapor deposition (MPCVD). Such as‐grown NiO‐3D graphene hierarchical composites are then applied as monolithic electrodes for a pseudo‐supercapacitor application without needing binders or metal‐based current collectors. Electrochemical measurements impart that the hierarchical NiO‐3D graphene composite delivers a high specific capacitance of ≈1829 F g^?1 at a current density of 3 A g^?1 (the theoretical capacitance of NiO is 2584 F g^?1). Furthermore, a full‐cell is realized with an energy density of 138 Wh kg^?1 at a power density of 5.25 kW kg^?1, which is much superior to commercial ones as well as reported devices in asymmetric capacitors of NiO. More attractively, this asymmetric supercapacitor exhibits capacitance retention of 85% after 5000 cycles relative to the initial value of the 1^st cycle.     

BN–Graphene Composites Generated by Covalent Cross‐Linking with Organic Linkers

Composites of boron nitride (BN) and carboxylated graphene are prepared for the first time using covalent cross‐linking employing the carbodiimide reaction. The BN_1–xG_x (x ≈ 0.25, 0.5, and 0.75) obtained are characterized using a variety of spectroscopic techniques and thermogravimetric analysis. The composites show composition‐dependent electrical resistivity, the resistivity decreasing with increase in graphene content. The composites exhibit microporosity and the x ≈ 0.75 composite especially exhibits satisfactory performance with high stability as an electrode in supercapacitors. The x ≈ 0.75 composite is also found to be a good electrocatalyst for the oxygen reduction reaction in fuel cells.     

Wireless Monitoring of Small Strains in Intelligent Robots via a Joule Heating Effect in Stretchable Graphene–Polymer Nanocomposites

Flexible strain sensors are an important component for future intelligent robotics. However, the majority of current strain sensors must be electrically connected to a corresponding monitoring system via conducting wires, which increases system complexity and restricts the working environment for monitoring strains. Here, stretchable graphene–polymer nanocomposites that act as strain sensors using a Joule heating effect are reported. When the resistance of the sensor changes in response to a strain, the resulting change in temperature is wirelessly detected in an intelligent robot. By engineering and optimizing the surface structure of graphene–polymer nanocomposites, the fabricated strain sensors exhibit excellent stability when subjected to periodic temperature signals over 400 cycles while being periodically strained and deliver a high strain sensitivity of 7.03 × 10^?4 °C^?1 %^?1 for strain levels of 0% to 30%. As a wearable electronic device, the approach provides the capability to wirelessly monitor small strains for intelligent robots at a high strain resolution of ≈0.1%. Moreover, when the strain sensing system operates as a multichannel structure, it allows precise strain detection simultaneously, or in sequence, for each finger of an intelligent robot.     

Tuneable Piezoresistance of Graphene-Based 2D:2D Nanocomposite Networks

Piezoresistive nanocomposites are an important class of materials that allow the production of very sensitive strain sensors. Herein, a new class of piezoresistive nanocomposites prepared by mixing different types of 2D nanosheets is explored. In this way, three distinct types of nanocomposite are produced by mixing conducting and insulating nanosheets (graphene, Gr and boron nitride, BN), conducting and semiconducting nanosheets (graphene and tungsten diselenide, WSe₂ or tungsten disulfide, WS₂) as well as mixing two different types of conducting nanosheets (graphene and silver, Ag). For each nanocomposite type, a different dependence of composite conductivity on filler volume fraction is observed although all behaviors can be fully described by percolation theory. In addition, each composite type shows different piezoresistive properties. Interestingly, while the conductor insulator composites show the standard monotonic relationship between gauge factor and conductivity, both conductor:semi-conductor and conductor:conductor composites show very unusual behavior, in each case displaying a peak engage factor at the percolation threshold. In each case, percolation theory is used to develop simple equations for gauge factor as a function of both volume fraction and conductivity that fully describes all experimental data. This work expands the understanding of piezoresistive nanocomposites and provides a platform for the engineering of high-performance strain sensors.     

Enhanced Dielectric and Electromechanical Responses in High Dielectric Constant All‐Polymer Percolative Composites

A type of all‐polymer percolative composite is introduced which exhibits a very high dielectric constant (> 7000). The experimental results also show that the dielectric behavior of this new class of percolative composites follows the predictions of the percolation theory and the analysis of conductive percolation phenomena. The very high dielectric constant of the all‐polymer composites, which are also very flexible and possesses an elastic modulus close to that of the insulation polymer matrix, makes it possible to induce a high electromechanical response under a very reduced electric field (a strain of 2.65 % with an elastic energy density of 0.18 J cm^–3 can be achieved under a field of 16 MV m^–1). Data analysis also suggests that within the composites, the non‐uniform local field distribution as well as interface effects can significantly enhance the strain responses. Furthermore, the experimental data as well as the data analysis indicate that conduction loss in the composites will not affect the strain hysteresis.