Large‐Area Ultrathin Graphene Films by Single‐Step Marangoni Self‐Assembly for Highly Sensitive Strain Sensing Application

Promoted by the demand for wearable devices, graphene has been proved to be a promising material for potential applications in flexible and highly sensitive strain sensors. However, low sensitivity and complex processing of graphene retard the development toward the practical applications. Here, an environment‐friendly and cost‐effective method to fabricate large‐area ultrathin graphene films is proposed for highly sensitive flexible strain sensor. The assembled graphene films are derived rapidly at the liquid/air interface by Marangoni effect and the area can be scaled up. These graphene‐based strain sensors exhibit extremely high sensitivity with gauge factor of 1037 at 2% strain, which represents the highest value for graphene platelets at this small deformation so far. This simple fabrication for strain sensors with highly sensitive performance of strain sensor makes it a novel approach to applications in electronic skin, wearable sensors, and health monitoring platforms.     

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.     

Flexible and Highly Sensitive Strain Sensors Fabricated by Pencil Drawn for Wearable Monitor

Functional electrical devices have promising potentials in structural health monitoring system, human‐friendly wearable interactive system, smart robotics, and even future multifunctional intelligent room. Here, a low‐cost fabrication strategy to efficiently construct highly sensitive graphite‐based strain sensors by pencil‐trace drawn on flexible printing papers is reported. The strain sensors can be operated at only two batteries voltage of 3 V, and can be applied to variously monitoring microstructural changes and human motions with fast response/relaxation times of 110 ms, a high gauge factor (GF) of 536.6, and high stability >10 000 bending–unbending cycles. Through investigation of service behaviors of the sensors, it is found that the microcracks occur on the surface of the pencil‐trace and have a major influence on the functions of the strain sensors. These performances of the strain sensor attain and even surpass the properties of recent strain sensing devices with subtle design of materials and device architectures. The pen‐on‐paper (PoP) approach may further develop portable, environmentally friendly, and economical lab‐on‐paper applications and offer a valuable method to fabricate other multifunctional devices.     

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.     

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.     

Porous Fibers Composed of Polymer Nanoball Decorated Graphene for Wearable and Highly Sensitive Strain Sensors

Wearable textile strain sensors that can perceive and respond to human stimuli are an essential part of wearable electronics. Yet, the detection of subtle strains on the human body suffers from the low sensitivity of many existing sensors. Generally, the inadequate sensitivity originates from the strong structural integrity of the sensors because tiny external strains cannot trigger enough variation in the conducting network. Inspired by the rolling friction where the interaction is weakened by decreasing interface area, porous fibers made of graphene decorated with nanoballs are prepared via a prolonged phase‐separation process. This novel structure confers the graphene fibers with high gauge factors (51 in 0–5% and 87 in 5–8%), which is almost 10 times larger than the same structures without nanoballs. A low detection limit (0.01% strain) and good durability (over 6000 circles) are obtained. By the virtue of these qualities, these fiber‐based textile sensors can recognize a pulse wave and eyeball movement in real‐time while keeping comfortable wearing sense. Moreover, by weaving such fibers, the electronic fabrics with a specially designed structure can distinguish the multilocation in real time, which shows great potential as wearable electronics.     

Carbonized Silk Nanofiber Membrane for Transparent and Sensitive Electronic Skin

Recent years have witnessed the explosive development of electronic skin. Highly sensitive pressure sensing is one of the primary abilities of electronic skin. To date, most of the reported skin‐like pressure sensors are based on nanomaterials and microstructured polydimethylsiloxane (PDMS) films, limiting their wide practical applications due to the unknown biotoxicity and the redundant fabrication procedure. A cost‐effective, large‐area‐capable, and biocompatible approach for fabrication of high‐performance skin‐like pressure sensors is highly desired. Silk fibroin (SF) is a natural protein that has recently drawn great attention due to its application as the substrate for flexible electronics. Here, the fabrication of skin‐like pressure sensors is demonstrated using SF‐derived active materials. Flexible and conformal pressure sensors can be fabricated using transparent carbonized silk nanofiber membranes (CSilkNM) and unstructured PDMS films through a cost‐effective and large‐scale capable approach. Due to the unique N‐doped carbon nanofiber network structure of CSilkNM, the obtained pressure sensor shows superior performance, including ultrahigh sensitivity (34.47 kPa^?1) for a broad pressure range, an ultralow detection limit (0.8 Pa), rapid response time (<16.7 ms), and high durability (>10 000 cycles). Based on its superior performance, its applications in monitoring human physiological signals, sensing subtle touch, and detecting spatial distribution of pressure are demonstrated.     

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.     

Microflotronics: A Flexible,Transparent, Pressure‐Sensitive Microfluidic Film

There is an increasing demand for sensitive, flexible, and low‐cost pressure sensing solutions for health monitoring, wearable sensing, robotic and prosthetic applications. Here, the first flexible and pressure‐sensitive microfluidic film is reported, referred to as a microflotronic, with high transparency and seamless integratability with the state‐of‐the‐art microelectronics. The microflotronic film represents the initial effort to utilize a continuous microfluidic layer as the sensing elements for large‐area dynamic pressure mapping applications, and meanwhile an ultrahigh sensitivity of 0.45 kPa^?1 has been achieved in a compact, flexible, and transparent packaging. The response time of the device is in the millisecond range, which is at least an order of magnitude faster than that of its conventional flexible solid‐state counterparts. In addition, the fabrication process of the device is fully compatible with the industrial‐scale manufacturing of capacitive touchscreen devices and liquid‐crystal displays. The overall device packaging can be as thin as 200 μm with an optical transparency greater than 80%. Several practical applications were successfully demonstrated, including surface topology mapping and dynamic blood pressure monitoring. The microflotronic devices offer an alternative approach to the solid‐state pressure sensors, by offering an unprecedented sensitivity and ultrafast response time in a completely transparent, flexible and adaptive platform.     

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.     

Multifunctional Smart Textronics with Blow‐Spun Nonwoven Fabrics

Here, the fabrication of nonwoven fabric by blow spinning and its application to smart textronics are demonstrated. The blow‐spinning system is composed of two parallel concentric fluid streams: i) a polymer dissolved in a volatile solvent and ii) compressed air flowing around the polymer solution. During the jetting process with pressurized air, the solvent evaporates, which results in the deposition of nanofibers in the direction of gas flow. Poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVdF‐HFP) dissolved in acetone is blow‐spun onto target substrate. Conductive nonwoven fabric is also fabricated from a blend of single‐walled carbon nanotubes (SWCNTs) and PVdF‐HFP. An all‐fabric capacitive strain sensor is fabricated by vertically stacking the PVdF‐HFP dielectric fabric and the SWCNT/PVdF‐HFP conductive fabric. The resulting sensor shows a high gauge factor of over 130 and excellent mechanical durability. The hierarchical morphology of nanofibers enables the development of superhydrophobic fabric and their electrical and thermal conductivities facilitate the application to a wearable heater and a flexible heat‐dissipation sheet, respectively. Finally, the conductive nonwoven fabric is successfully applied to the detection of various biosignals. The demonstrated facile and cost‐effective fabrication of nonwoven fabric by the blow‐spinning technique provides numerous possibilities for further development of technologies ranging from wearable electronics to textronics.     

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.     

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.     

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.     

Strain Sensors with a High Sensitivity and a Wide Sensing Range Based on a Ti3C2Tx (MXene) Nanoparticle–Nanosheet Hybrid Network

A high sensitivity and large stretchability are desirable for strain sensors in wearable applications. However, these two performance indicators are contradictory, since the former requires a conspicuous structural change under a tiny strain, whereas the latter demands morphological integrity upon a large deformation. Developing strain sensors with both a high sensitivity (gauge factor (GF) > 100) and a broad strain range (>50%) is a considerable challenge. Herein, a unique Ti₃C₂T_x MXene nanoparticle–nanosheet hybrid network is constructed. The migration of nanoparticles leads to a large resistance variation while the wrapping of nanosheet bridges the detached nanoparticles to maintain the connectivity of the conductive pathways in a large strain region. The synergetic motion of nanoparticles and nanosheets endows the hybrid network with splendid electrical–mechanical performance, which is reflected in its high sensitivity (GF > 178.4) over the entire broad range (53%), the super low detection limit (0.025%), and a good cycling durability (over 5000 cycles). Such high performance endows the strain sensor with the capability for full‐range human motion detection.     

Polymer Template Synthesis of Flexible BaTiO3 Crystal Nanofibers

BaTiO₃ crystals are attractive materials due to their high dielectric properties, but they are brittle and inelastic ceramics, which limits their broader applications in emerging fields, such as flexible electronics. A scalable strategy for the fabrication of ultra‐flexible crystalline BaTiO₃ nanofiber (NF) films by a sol–gel electrospinning method, followed by a brief calcination, is reported. It facilitates the formation of perovskite BaTiO₃ crystals with intricate grain boundaries at a low temperatures by growing them within polymer NF templates. The ceramic films have a polymer‐like softness of 50 mN, a large Young's modulus of 61 MPa, and an elastic strain of 0.9%. Moreover, they have a low density of 28 mg cm^?3 and demonstrate superior softness without fracture after deformation. Piezoelectric sensors fabricated based on these films exhibit a high sensitivity of 80 ms with an output voltage of 1.05 V at a pressure of 100 kPa. This approach allows for the large‐scale fabrication of flexible BaTiO₃ crystal NF films.     

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.     

Large‐Area Compliant,Low‐Cost,and Versatile Pressure‐Sensing Platform Based on Microcrack‐Designed Carbon Black@Polyurethane Sponge for Human–Machine Interfacing

It is a challenge to manufacture pressure‐sensing materials that possess flexibility, high sensitivity, large‐area compliance, and capability to detect both tiny and large motions for the development of artificial intelligence products. Herein, a very simple and low‐cost approach is proposed to fabricate versatile pressure sensors based on microcrack‐designed carbon black (CB)@polyurethane (PU) sponges via natural polymer‐mediated water‐based layer‐by‐layer assembly. These sensors are capable of satisfying the requirements of ultrasmall as well as large motion monitoring. The versatility of these sensors benefits from two aspects: microcrack junction sensing mechanism for tiny motion detecting (91 Pa pressure, 0.2% strain) inspired by the spider sensory system and compressive contact of CB@PU conductive backbones for large motion monitoring (16.4 kPa pressure, 60% strain). Furthermore, these sensors exhibit excellent flexibility, fast response times (<20 ms), as well as good reproducibility over 50 000 cycles. This study also demonstrates the versatility of these sensors for various applications, ranging from speech recognition, health monitoring, bodily motion detection to artificial electronic skin. The desirable comprehensive performance of our sensors, which is comparable to the recently reported pressure‐sensing devices, together with their significant advantages of low‐cost, easy fabrication, especially versatility, makes them attractive in the future of artificial intelligence.     

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.     

Enhanced Sensitivity of Capacitive Pressure and Strain Sensor Based on CaCu3Ti4O12 Wrapped Hybrid Sponge for Wearable Applications

Pressure sensors with highly sensitive and flexible characteristics have extensive applications in wearable electronics, soft robotics, human–machine interface, and more. Herein, an effective strategy is explored to enhance the sensitivity of the capacitive pressure sensor by fabricating a dielectric hybrid sponge consisting of calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO), a giant dielectric permittivity material, in polyurethane (PU). An ultrasoft CCTO@PU hybrid sponge is fabricated via dip‐coating the PU sponge into surface‐modified CCTO nanoparticles using 3‐aminopropyl triethoxysilane. The overall results show that the –NH₂ functionalized CCTO attributes proper adhesion of CCTO with the –OCN group of the PU to enhance interfacial polarization leading to a high dielectric permittivity (167.05) and low loss tangent (0.71) beneficial for flexible pressure sensing applications. Moreover, the as‐prepared CCTO@PU hybrid sponge at 30 wt% CCTO concentration exhibits excellent electromechanical properties with an ultralow compression modulus of 27.83 kPa and a high sensitivity of 0.73 kPa^?1 in a low‐pressure regime (<1.6 kPa). Finally, pressure and strain sensing performance is demonstrated for the detection of human activities by mounting the sensor on various parts of the human body. The work reveals a new opportunity for the facile fabrication of high performance CCTO‐based capacitive sensors with multifunctional properties.