Ultrasensitive and Highly Stretchable Multifunctional Strain Sensors with Timbre‐Recognition Ability Based on Vertical Graphene

Stretchable/wearable strain sensors are attracting growing interest due to their broad applications in physical and physiological measurements. However, the development of a multifunctional highly stretchable sensor satisfying the requirements of ultrahigh sensitivity (able to distinguish sound frequency) remains a challenge. An ultrasensitive and highly stretchable multifunctional strain sensor with timbre‐recognition ability based on high‐crack‐density vertical graphene (VGr) is fabricated using an ultrasonic peeling (UP) method. It can distinguish frequencies of sounds higher than 2500 Hz. Detailed microscopic examinations reveal that their ultrahigh sensitivity stems from the formation of high‐density nanocracks in the graphitic base layer, which is bridged by the top branched VGr nanowalls. These nanocracks cut the VGr film into a large number of nanopieces, which increase the natural frequency of the sensors, enabling the sensors to distinguish the sound frequency. Demonstrations are presented to highlight the sensors' potential as wearable devices for human physiological signal and timbre detections. This is the first multifunctional highly stretchable strain sensor with timbre‐recognition ability.     

Highly Elastic Graphene‐Based Electronics Toward Electronic Skin

Epidermal electronics are extensively explored as an important platform for future biomedical engineering. Epidermal devices are typically fabricated using high‐cost methods employing complex vacuum microfabrication processes, limiting their widespread potential in wearable electronics. Here, a low‐cost, solution‐based approach using electroconductive reduced graphene oxide (RGO) sheets on elastic and porous poly(dimethylsiloxane) (PDMS) thin films for multifunctional, high‐performance, graphene‐based epidermal bioelectrodes and strain sensors is presented. These devices are fabricated employing simple coatings and direct patterning without using any complicated microfabrication processes. The graphene bioelectrodes show a superior stretchability (up to 150% strain), with mechanical durability up to 5000 cycles of stretching and releasing, and low sheet resistance (1.5 kΩ per square), and the graphene strain sensors exhibit a high sensitivity (a gauge factor of 7 to 173) with a wide sensing range (up to 40% strain). Fully functional applications of dry bioelectrodes in monitoring human electrophysiological signals (i.e., electrocardiogram, electroencephalography, and electromyogram) and highly sensitive strain sensors for precise detection of large‐scale human motions are demonstrated. It is believed that our unique processing capability and multifunctional device platform based on RGO/porous PDMS will pave the way for low‐cost processing and integration of 2D materials for future wearable electronic skin.     

Carbonized Cotton Fabric for High‐Performance Wearable Strain Sensors

Recent years have witnessed the booming development of flexible strain sensors. To date, it is still a great challenge to fabricate strain sensors with both large workable strain range and high sensitivity. Cotton is an abundant supplied natural material composed of cellulose fibers and has been widely used for textiles and clothing. In this work, the fabrication of highly sensitive wearable strain sensors based on commercial plain weave cotton fabric, which is the most popular fabric for clothes, is demonstrated through a low‐cost and scalable process. The strain sensors based on carbonized cotton fabric exhibit fascinating performance, including large workable strain range (>140%), superior sensitivity (gauge factor of 25 in strain of 0%–80% and that of 64 in strain of 80%–140%), inconspicuous drift, and long‐term stability, simultaneously offering advantages of low cost and simplicity in device fabrication and versatility in applications. Notably, the strain sensor can detect a subtle strain of as low as 0.02%. Based on its superior performance, its applications in monitoring both vigorous and subtle human motions are demonstrated, showing its tremendous potential for applications in wearable electronics and intelligent robots.     

Microporous Polypyrrole‐Coated Graphene Foam for High‐Performance Multifunctional Sensors and Flexible Supercapacitors

This study reports on the fabrication of pressure/temperature/strain sensors and all‐solid‐state flexible supercapacitors using only polydimethylsiloxane coated microporous polypyrrole/graphene foam composite (PDMS/PPy/GF) as a common material. A dual‐mode sensor is designed with PDMS/PPy/GF, which measures pressure and temperature with the changes of current and voltage, respectively, without interference to each other. The fabricated dual‐mode sensor shows high sensitivity, fast response/recovery, and high durability during 10 000 cycles of pressure loading. The pressure is estimated using the thermoelectric voltage induced by simultaneous increase in temperature caused by a finger touch on the sensor. Additionally, a resistor‐type strain sensor fabricated using the same PDMS/PPy/GF could detect the strain up to 50%. Flexible, high performance supercapacitor used as a power supply is fabricated with electrodes of PPy/GF for its high surface area and pseudocapacitance. Furthermore, an integrated system of such fabricated multifunctional sensors and a supercapacitor on a skin‐attachable flexible substrate using liquid–metal interconnections operates well, whereas sensors are driven by the power of the supercapacitor. This study clearly demonstrates that the appropriate choice of a single functional material enables fabrication of active multifunctional sensors for pressure, temperature, and strain, as well as the supercapacitor, that could be used in wirelessly powered wearable devices.     

Stretchable,Skin‐Mountable,and Wearable Strain Sensors and Their Potential Applications: A Review

There is a growing demand for flexible and soft electronic devices. In particular, stretchable, skin‐mountable, and wearable strain sensors are needed for several potential applications including personalized health‐monitoring, human motion detection, human‐machine interfaces, soft robotics, and so forth. This Feature Article presents recent advancements in the development of flexible and stretchable strain sensors. The article shows that highly stretchable strain sensors are successfully being developed by new mechanisms such as disconnection between overlapped nanomaterials, crack propagation in thin films, and tunneling effect, different from traditional strain sensing mechanisms. Strain sensing performances of recently reported strain sensors are comprehensively studied and discussed, showing that appropriate choice of composite structures as well as suitable interaction between functional nanomaterials and polymers are essential for the high performance strain sensing. Next, simulation results of piezoresistivity of stretchable strain sensors by computational models are reported. Finally, potential applications of flexible strain sensors are described. This survey reveals that flexible, skin‐mountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome.     

Flexible Transparent Reduced Graphene Oxide Sensor Coupled with Organic Dye Molecules for Rapid Dual‐Mode Ammonia Gas Detection

Flexible chemical sensors utilizing chemically sensitive nanomaterials are of great interest for wearable sensing applications. However, obtaining high performance flexible chemical sensors with high sensitivity, fast response, transparency, stability, and workability at ambient conditions is still challenging. Herein, a newly designed flexible and transparent chemical sensor of reduced graphene oxide (R‐GO) coupled with organic dye molecules (bromophenol blue) is introduced. This device has promising properties such as high mechanical flexibility (>5000 bending cycles with a bending radius of 0.95 cm) and optical transparency (>60% in the visible region). Furthermore, stacking the water‐trapping dye layer on R‐GO enables a higher response as well as workability in a large relative humidity range (up to 80%), and dual‐mode detection capabilities of colorimetric and electrical sensing for NH₃ gas (5–40 ppm). These advantageous attributes of the flexible and transparent R‐GO sensor coupled with organic dye molecules provide great potential for real‐time monitoring of toxic gas/vapor in future practical chemical sensing at room conditions in wearable electronics.     

Flexible,Graphene‐Coated Biocomposite for Highly Sensitive,Real‐Time Molecular Detection

Wearable biosensors hold significant potential for healthcare and environmental applications, and the development of flexible and biocompatible sensing platforms for high accuracy detection of physiological biomarkers remains an elusive goal. Herein, an ultrasensitive, flexible sensor is described that is based on a 3D hierarchical biocomposite comprised of hollow, natural pollen microcapsules that are coated with a conductive graphene layer. Modular assembly of the graphene‐coated microcapsules onto an ultrathin polyethylene terephthalate layer enables a highly flexible sensor configuration with tunable selectivity afforded by subsequent covalent immobilization of antibodies against target antigens. In a proof‐of‐concept example, the biosensor demonstrates ultrahigh sensitivity detection of prostate specific antigen (PSA) down to 1.7 × 10^?15m with real‐time feedback and superior performance over conventional 2D graphene‐coated sensors. Importantly, the device performance is consistently high across various bending conditions. Taken together, the results demonstrated in this work highlight the merits of employing lightweight biocomposites as modular building blocks for the design of flexible biosensors with highly responsive and sensitive molecular detection capabilities.     

Wearable and Highly Sensitive Graphene Strain Sensors for Human Motion Monitoring

Sensing strain of soft materials in small scale has attracted increasing attention. In this work, graphene woven fabrics (GWFs) are explored for highly sensitive sensing. A flexible and wearable strain sensor is assembled by adhering the GWFs on polymer and medical tape composite film. The sensor exhibits the following features: ultra‐light, relatively good sensitivity, high reversibility, superior physical robustness, easy fabrication, ease to follow human skin deformation, and so on. Some weak human motions are chosen to test the notable resistance change, including hand clenching, phonation, expression change, blink, breath, and pulse. Because of the distinctive features of high sensitivity and reversible extensibility, the GWFs based piezoresistive sensors have wide potential applications in fields of the displays, robotics, fatigue detection, body monitoring, and so forth.     

Flexible and Highly Sensitive Pressure Sensors Based on Bionic Hierarchical Structures

The rational design of high‐performance flexible pressure sensors attracts attention because of the potential applications in wearable electronics and human–machine interfacing. For practical applications, pressure sensors with high sensitivity and low detection limit are desired. Here, ta simple process to fabricate high‐performance pressure sensors based on biomimetic hierarchical structures and highly conductive active membranes is presented. Aligned carbon nanotubes/graphene (ACNT/G) is used as the active material and microstructured polydimethylsiloxane (m‐PDMS) molded from natural leaves is used as the flexible matrix. The highly conductive ACNT/G films with unique coalescent structures, which are directly grown using chemical vapor deposition, can be conformably coated on the m‐PDMS films with hierarchical protuberances. Flexible ACNT/G pressure sensors are then constructed by putting two ACNT/G/PDMS films face to face with the orientation of the ACNTs in the two films perpendicular to each other. Due to the unique hierarchical structures of both the ACNT/G and m‐PDMS films, the obtained pressure sensors demonstrate high sensitivity (19.8 kPa^?1, <0.3 kPa), low detection limit (0.6 Pa), fast response time (<16.7 ms), low operating voltage (0.03 V), and excellent stability for more than 35 000 loading–unloading cycles, thus promising potential applications in wearable electronics.     

Wide‐Area Strain Sensors based upon Graphene‐Polymer Composite Coatings Probed by Raman Spectroscopy

Functional graphene optical sensors are now viable due to the recent developments in hand‐held Raman spectroscopy and the chemical vapor deposition (CVD) of graphene films. Herein, the strain in graphene/poly (methyl methacrylate) sensor coatings is followed using Raman band shifts. The performance of an “ideal” mechanically‐exfoliated single crystal graphene flake is compared to a scalable CVD graphene film. The dry‐transferred mechanically exfoliated sample has no residual stresses, whereas the CVD sample is in compression following the solvent evaporation during its transfer. The behavior of the sensors under cyclic deformation shows an initial breakdown of the graphene‐polymer interface with the interface then stabilizing after several cycles. The Raman 2D band shift rates per unit strain of the exfoliated graphene are ≈35% higher than CVD graphene making the former more strain sensitive. However, for practical wide‐area applications, CVD graphene coatings are still viable candidates as a Raman system can be used to read the strain in any 5 μm diameter spot in the coating to an absolute accuracy of ≈0.01% strain and resolution of ≈27 microstrains (μs), which compares favorably to commercial photoelastic systems.     

Highly Conductive,Scalable, and Machine Washable Graphene‐Based E‐Textiles for Multifunctional Wearable Electronic Applications

Graphene‐based textiles show promise for next‐generation wearable electronic applications due to their advantages over metal‐based technologies. However, current reduced graphene oxide (rGO)‐based electronic textiles (e‐textiles) suffer from poor electrical conductivity and higher power consumption. Here, highly conductive, ultraflexible, and machine washable graphene‐based wearable e‐textiles are reported. A simple and scalable pad?dry?cure method with subsequent roller compression and a fine encapsulation of graphene flakes is used. The graphene‐based wearable e‐textiles thus produced provide lowest sheet resistance (≈11.9 Ω sq^?1) ever reported on graphene e‐textiles, and highly conductive even after 10 home laundry washing cycles. Moreover, it exhibits extremely high flexibility, bendability, and compressibility as it shows repeatable response in both forward and backward directions before and after home laundry washing cycles. The scalability and multifunctional applications of such highly conductive graphene‐based wearable e‐textiles are demonstrated as ultraflexible supercapacitor and skin‐mounted strain sensors.     

Bubble‐Decorated Honeycomb‐Like Graphene Film as Ultrahigh Sensitivity Pressure Sensors

Recently, macroporous graphene monoliths (MGMs), with ultralow density and good electrical conductivity, have been considered as excellent pressure sensors due to their excellent elasticity with a rapid rate of recovery. However, MGMs can only exhibit good sensitivity when the strain is higher than 20%, which is undesirable for touch‐type pressure sensors, such as artificial skin. Here, an innovative method for the fabrication of freestanding flexible graphene film with bubbles decorated on honeycomb‐like network is demonstrated. Due to the switching effect depended on “point‐to‐point” and “point‐to‐face” contact modes, the graphene pressure sensor has an ultrahigh sensitivity of 161.6 kPa^?1 at a strain less than 4%, several hundred times higher than most previously reported pressure sensors. Moreover, the graphene pressure sensor can monitor human motions such as finger bending and pulse with a very low operating voltage of 10 mV, which is sufficiently low to allow for powering by energy‐harvesting devices, such as triboelectric generators. Therefore, the high sensitivity, low operating voltage, long cycling life, and large‐scale fabrication of the pressure sensors make it a promising candidate for manufacturing low‐cost artificial skin.     

Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

Multiaxial and Transparent Strain Sensors Based on Synergetically Reinforced and Orthogonally Cracked Hetero‐Nanocrystal Solids

Wearable strain sensors are widely researched as core components in electronic skin. However, their limited capability of detecting only a single axial strain, and their low sensitivity, stability, opacity, and high production costs hinder their use in advanced applications. Herein, multiaxially highly sensitive, optically transparent, chemically stable, and solution‐processed strain sensors are demonstrated. Transparent indium tin oxide and zinc oxide nanocrystals serve as metallic and insulating components in a metal–insulator matrix and as active materials for strain gauges. Synergetic sensitivity‐ and stability‐reinforcing agents are developed using a transparent SU‐8 polymer to enhance the sensitivity and encapsulate the devices, elevating the gauge factor up to over 3000 by blocking the reconnection of cracks caused by the Poisson effect. Cross‐shaped patterns with an orthogonal crack strategy are developed to detect a complex multiaxial strain, efficiently distinguishing strains applied in various directions with high sensitivity and selectivity. Finally, all‐transparent wearable strain sensors with Ag nanowire electrodes are fabricated using an all‐solution process, which effectively measure not only the human motion or emotion, but also the multiaxial strains occurring during human motion in real time. The strategies can provide a pathway to realize cost‐effective and high‐performance wearable sensors for advanced applications such as bio‐integrated devices.     

Highly Sensitive,Wearable, Durable Strain Sensors and Stretchable Conductors Using Graphene/Silicon Rubber Composites

Highly sensitive, wearable and durable strain sensors are vital to the development of health monitoring systems, smart robots and human machine interfaces. The recent sensor fabrication progress is respectable, but it is limited by complexity, low sensitivity and unideal service life. Herein a facile, cost‐effective and scalable method is presented for the development of high‐performance strain sensors and stretchable conductors based on a composite film consisting of graphene platelets (GnPs) and silicon rubber. Through calculation by the tunneling theory using experimental data, the composite film has demonstrated ideal linear and reproducible sensitivity to tensile strains, which is contributed by the superior piezoresistivity of GnPs having tunable gauge factors 27.7–164.5. The composite sensors fabricated in different days demonstrate pretty similar performance, enabling applications as a health‐monitoring device to detect various human motions from finger bending to pulse. They can be used as electronic skin, a vibration sensor and a human‐machine interface controller. Stretchable conductors are made by coating and encapsulating GnPs with polydimethyl siloxane to create another composite; this structure allows the conductor to be readily bent and stretched with sufficient mechanical robustness and cyclability.     

Superelastic and Arbitrary‐Shaped Graphene Aerogels with Sacrificial Skeleton of Melamine Foam for Varied Applications

Elastic graphene aerogels are lightweight and offer excellent and electrical performance, expanding their significance in many applications. Recently, elastic graphene aerogels have been fabricated via various methods. However, for most reported elastic graphene aerogels, the fabrication processes are complicated and the applications are usually limited by the brittle mechanical properties. Thus, it still remains a challenge to explore facile processes for the fabrication of graphene aerogels with low density and high compressibility. Herein, arbitrary‐shaped, superelastic, and durable graphene aerogels are fabricated using melamine foam as sacrificial skeleton. The resulting graphene aerogels possess high elasticity under compressive stress of 0.556 MPa and compressive strain of 95%. Thanks to the superelasticity, high strength, excellent flexibility, outstanding thermal stability, and good electrical conductivity of graphene aerogels, they can be applied in sorbents and pressure/strain sensors. The as‐assembled graphene aerogels can adsorb various organic solvents at 176–513 g g^?1 depending on the solvent type and density. Moreover, both the squeezing and combustion methods can be adopted for reusing the graphene aerogels. Finally, the graphene aerogels exhibit stable and sensitive current responses, making them the ideal candidates for applications as multifunctional pressure/strain sensors such as wearable devices.     

Polyelectrolyte Dielectrics for Flexible Low‐Voltage Organic Thin‐Film Transistors in Highly Sensitive Pressure Sensing

Organic thin‐film transistors (OTFTs) can provide an effective platform to develop flexible pressure sensors in wearable electronics due to their good signal amplification function. However, it is particularly difficult to realize OTFT‐based pressure sensors with both low‐voltage operation and high sensitivity. Here, controllable polyelectrolyte composites based on poly(ethylene glycol) (PEG) and polyacrylic acid (PAA) are developed as a type of high‐capacitance dielectrics for flexible OTFTs and ultrasensitive pressure sensors with sub‐1 V operation. Flexible OTFTs using the PAA:PEG dielectrics show good universality and greatly enhanced electrical performance under a much smaller operating voltage of ?0.7 V than those with a pristine PAA dielectric. The low‐voltage OTFTs also exhibit excellent flexibility and bending stability under various bending radii and long cycles. Flexible OTFT‐based pressure sensors with low‐voltage operation and superhigh sensitivity are demonstrated by using a suspended semiconductor/dielectric/gate structure in combination with the PAA:PEG dielectric. The sensors deliver a record high sensitivity of 452.7 kPa^?1 under a low‐voltage of ?0.7 V, and excellent operating stability over 5000 cycles. The OTFT sensors can be built into a wearable sensor array for spatial pressure mapping, which shows a bright potential in flexible electronics such as wearable devices and smart skins.     

High-sensitivity crack-based flexible strain sensor with dual hydrogen bond-assisted structure for monitoring tiny human motions and writing behavior

Owing to their ultrahigh sensitivity, crack-based flexible strain sensors have garnered considerable attention in recent years. In this study, a practical, and reliable chemical bonding-based dip-coating method is proposed to fabricate high sensitivity and high stability crack-based flexible strain sensor with dual hydrogen bond-assisted structure. The strain sensor has a sandwich structure, which is composed of graphene nanoplatelets (GNPs)/poly (sodium-p-styrenesulfonate) (PSS) conductive layer, ultra-violet (UV) adhesive substrate layer, and UV adhesive covering layer. The fabrication process, principle of dual hydrogen bond-assisted structure, strain sensing mechanism, and various properties of the proposed sensor are examined. It is demonstrated that the cracks and the dual hydrogen bond-assisted structure facilitate a practical strain sensor with high sensitivity (gauge factor of 19.65 in the strain range of 0–30%), long-term stability (over 10,000 cycles), good linearity, negligible drift, fast response time (~50 ms), and low detection limit (0.10%). Meanwhile, the proposed crack-based flexible strain sensor can be used as a wearable device, which can be directly mounted on human skin to monitor tiny human motions and writing behavior. Consequently, it exhibits immense potential for wearable applications including artificial skin, human-machine interfaces, and medical healthcare.     

Self‐Healable Multifunctional Electronic Tattoos Based on Silk and Graphene

Electronic tattoos (E‐tattoos), which can be intimately mounted on human skin for noninvasive and high‐fidelity sensing, have attracted the attention of researchers in the field of wearable electronics. However, fabricating E‐tattoos that are capable of self‐healing and sensing multistimuli, similar to the inherent attributes of human skin, is still challenging. Herein, a healable and multifunctional E‐tattoo based on a graphene/silk fibroin/Ca²⁺ (Gr/SF/Ca²⁺) combination is reported. The highly flexible E‐tattoos are prepared through printing or writing using Gr/SF/Ca²⁺ suspension. The graphene flakes distributed in the matrix form an electrically conductive path that is responsive to environmental changes, such as strain, humidity, and temperature variations, endowing the E‐tattoo with high sensitivity to multistimuli. The performance of the E‐tattoo is investigated as a strain, humidity, and temperature sensor that shows high sensitivity, a fast response, and long‐term stability. The E‐tattoo is remarkably healed after damage by water because of the reformation of hydrogen and coordination bonds at the fractured interface. The healing efficiency is 100% in only 0.3 s. Finally, as proof of concept, its applications for monitoring of electrocardiograms, breathing, and temperature are shown. Based on its unique properties and superior performance, the Gr/SF/Ca²⁺ E‐tattoo may be a promising candidate material for epidermal electronics.     

Self‐Powered Electronics by Integration of Flexible Solid‐State Graphene‐Based Supercapacitors with High Performance Perovskite Hybrid Solar Cells

To develop high‐capacitance flexible solid‐state supercapacitors and explore its application in self‐powered electronics is one of ongoing research topics. In this study, self‐stacked solvated graphene (SSG) films are reported that have been prepared by a facile vacuum filtration method as the free‐standing electrode for flexible solid‐state supercapacitors. The highly hydrated SSG films have low mass loading, high flexibility, and high electrical conductivity. The flexible solid‐state supercapacitors based on SSG films exhibit excellent capacitive characteristics with a high gravimetric specific capacitance of 245 F g^?1 and good cycling stability of 10 000 cycles. Furthermore, the flexible solid‐state supercapacitors are integrated with high performance perovskite hybrid solar cells (pero‐HSCs) to build self‐powered electronics. It is found that the solid‐state supercapacitors can be charged by pero‐HSCs and discharged from 0.75 V. These results demonstrate that the self‐powered electronics by integration of the flexible solid‐state supercapacitors with pero‐HSCs have great potential applications in storage of solar energy and in flexible electronics, such as portable and wearable personal devices.