Highly sensitive flexible strain sensor based on carbon nanotube/styrene butadiene styrene@ thermoplastic polyurethane fiber with a double percolated structure

The combination of a high sensitivity and a wide strain detection range in conductive polymer composites-based flexible strain sensors is still challenging to achieve. Herein, a double-percolation structural fiber strain sensor based on carbon nanotubes (CNT)/styrene butadiene styrene (SBS)@thermoplastic polyurethane (TPU) composite was fabricated by a simple melt mixing and fused filament fabrication strategy, in which the CNT/SBS and TPU were the conductive and insulating phases, respectively. Compared with the sensor without the double percolated structure, the CNT/SBS@TPU sensor achieved a lower percolation threshold (from 2.0 to 0.5 wt%, a reduction of 75%), and better electrical and sensing performance. It is shown that the strain detection range of the CNT/SBS@TPU sensor increases with increasing CNT loading. An opposite trend was observed for the sensitivity. The 1%-CNT/SBS@TPU sensor exhibited a high conductivity (1.08 × 10⁻³ S/m), high sensitivity (gauge factor of 2.65 × 10⁶ at 92% strain), wide strain detection range (0.2%–92% strain), high degree of linearity (R² = 0.954 at 0–10% strain), broad monitoring frequencies (0.05–0.5 Hz), and excellent stability (2000 cycles). Moreover, the CNT/SBS@TPU sensor was shown to successfully monitor a range of human physiological activities and to be capable of tactile perception and weight distribution sensing.     

基于银/石墨烯纳米复合材料的柔性压力传感器——材料制备和微纳结构构建

柔性压力传感器是柔性可穿戴设备的核心部件,在医疗保健、运动健身、安全生产等领域极具应用潜力。二维材料石墨烯具有高载流子迁移率、超大比表面积和超高机械强度,被视为制备柔性压力传感器的优良敏感材料。然而石墨烯碎片引入晶界或堆叠等缺陷,用纯石墨烯制备柔性压力传感器存在灵敏度低、稳定性差、响应范围窄等问题。将零维银纳米颗粒或一维银纳米线与石墨烯构建复合材料,可有效跨越缺陷或搭接相邻片层,起到“桥梁”作用,石墨烯片层平铺到纳米银导电网络之间,起到“补丁”作用。本文综述了用于柔性电阻式压力传感器的银/石墨烯复合材料制备方法和工艺,并介绍了不同微纳结构的传感器构建方法。     

二维导电材料/柔性聚合物复合材料基可穿戴压阻式应变传感器的研究进展

可穿戴应变传感器在人体运动检测、健康监测、可穿戴电子设备和柔性电子皮肤等新兴领域具有极大的应用前景。近年来,由二维（2D）导电材料和柔性聚合物基体组成的可穿戴压阻式应变传感器具有较高的灵敏度、良好的拉伸性和柔韧性、优异的耐久性、可调的应变传感性和易加工等特点,受到广泛关注。基于此,本文对基于2D导电材料/柔性聚合物复合材料（2D-CPC）的可穿戴压阻式应变传感器的类型、传感机理、性能指标、影响因素及应用等进行了综述,并对其未来发展趋势进行了展望。     

石墨烯/聚合物泡沫压阻式应变传感器研究进展

基于石墨烯和聚合物的三维多孔结构的导电聚合物基复合材料（CPCs）具有轻量化、高灵敏度、宽应变检测范围、低成本和可扩展性等优点，已成为可穿戴柔性应变传感器的理想选择。首先，总结了柔性压阻式泡沫应变传感器的裂纹扩展机制、重叠-断开机制和隧穿效应机制；其次，介绍了3种具有多孔结构的石墨烯/聚合物柔性应变传感器的构筑工艺，包括基于聚合物泡沫、基于石墨烯/聚合物混合分散液、基于石墨烯泡沫的方法；然后，综述了通过上述3种工艺制备的柔性多孔应变传感器的传感性能，并列举了其在人体运动监测领域中的应用实例；最后，对基于石墨烯和聚合物的柔性多孔应变传感器面临的挑战和发展前景进行了展望。     

Flexible Sandwich Structural Strain Sensor Based on Silver Nanowires Decorated with Self‐Healing Substrate

Flexible and stretchable conducting composites that can sense stress or strain are needed for several emerging fields including human motion detection and personalized health monitoring. Silver nanowires (AgNWs) have already been used as conductive networks. However, once a traditional polymer is broken, the conductive network is subsequently destroyed. Integrating high pressure sensitivity and repeatable self‐healing capability into flexible strain sensors represents new advances for high performance strain sensing. Herein, superflexible 3D architectures are fabricated by sandwiching a layer of AgNWs decorated self‐healing polymer between two layers of polydimethylsiloxane, which exhibit good stability, self‐healability, and stretchability. For better mechanical properties, the self‐healing polymer is reinforced with carbon fibers (CFs). The sensors based on self‐healing polymer and AgNWs conductive network show high conductivity and excellent ability to repair both mechanical and electrical damage. They can detect different human motions accurately such as bending and recovering of the forearm and shank, the changes of palm, fist, and fingers. The fracture tensile stress of the reinforced self‐healing polymer (9 wt% CFs) is increased to 10.3 MPa with the elongation at break of 8%. The stretch/release responses under static and dynamic loads of the sensor have a high sensitivity, large sensing range, excellent reliability, and remarkable stability.     

Polyaniline/Poly(acrylamide‐co‐sodium acrylate) Porous Conductive Hydrogels with High Stretchability by Freeze‐Thaw‐Shrink Treatment for Flexible Electrodes

With the development of alternatives to traditional fossil energy and the rise of wearable technology, flexible energy storage devices have attracted great attention. In this paper, a polyaniline/poly(acrylamide‐sodium acrylate copolymer) hydrogel (PASH) with high flexibility and excellent electrochemical properties for flexible electrodes is fabricated by freeze‐thaw‐shrink treatment of a highly water‐absorptive hydrogel, together with in‐situ polymerization of aniline at a low aniline concentration (0.1 mol L^?1). The PASH exhibits a conductivity of 4.05 S m^?1 and an elongation at break of 1245%. The freeze‐thaw‐shrink treatment greatly improves the electrochemical performance and stability of the conductive PASH. The area specific capacitance of PASH reaches 849 mF cm^?2 and the capacitance maintains 89% after 1000 galvanostatic charge–discharge cycles. All the raw materials are conventional industrialized materials and no additional templating agent is needed during the entire synthesis process. This study provides a cost‐efficient approach for the fabrication of conductive polymer hydrogels, which has a broad application prospect in flexible energy storage electronic devices.     

Stretchable and Designable Textile Pattern Strain Sensors Based on Graphene Decorated Conductive Nylon Filaments

Flexible sensors with stretchable and wearable characteristics have boosted wide interest in human motion detection and physiological signal monitoring. However, the majority of current sensors suffer from the lack of seamlessly integrated with clothes substrates, hindering their applications as “real” wearable devices. Herein, a facile low‐cost and scalable continuous capillary dip coating route is employed to deposit graphene inks onto nylon filaments to obtain graphene decorated nylon conductive filaments. The filaments exhibit noticeable promotion in electrical conductivity with remarkable laundry durability, and the electrical conductivity of our filaments could be up to 6.43 and 2.78 S m^?1 before and after washing 10 times, respectively. Two kinds of conventional textile formation techniques, sewing and knitting, are utilized to form various textile pattern strain sensors from the conductive filaments as the building blocks, such as the linear‐type, knitted‐loop‐type, snail‐coil‐type sewed sensors and the tubular knitted fabric sensors respectively. The textile sensors with different patterns exhibited various sensing response, the knitted‐loop‐type sensor could reach the maximum strain 45.69% while the linear‐type one arrives at 13.64%. In addition, the above strain sensors exhibit high sensitivity and repeatability when monitoring the limb movement and human breathing.     

Highly Stretchable,Tough, and Conductive Ag@Cu Nanocomposite Hydrogels for Flexible Wearable Sensors and Bionic Electronic Skins

Flexible conductive materials and flexible electronic devices are driving the development of the next generation of cutting-edge wearable electronics. However, the existing hydrogel-based flexible conductive materials have limited tensile capacity, low toughness, and poor anti-fatigue performance, resulting in narrow sensing area and insufficient durability. In this paper, a conductive nanocomposite hydrogel with high ductility, toughness, and fatigue resistance is prepared by combining silver coated copper (Ag@Cu) nanoparticles with gelatin followed by one-step immersion in sodium sulfate (Na₂SO₄) solution. The salting-out of gelatin in Na₂SO₄ solution greatly improve the mechanical properties of this gelatin-based hydrogel. The uniform distribution of Ag@Cu nanoparticles inside the whole hydrogel endow the composite hydrogel with excellent electrical conductivity (1.35 S m⁻¹). In addition, it displayed high and stable tensile strain sensitivity over a wide strain range (gauge factor = 2.08). Therefore, the Ag@Cu-Gel hydrogel is sensitive and stable enough to be successfully utilized as flexible wearable sensor for detecting human motion signals in real time, such as bending of human joints, swallowing, and throat vocalization. Furthermore, this hydrogel is also suitable for application as electronic skin for bionic robots. The above results demonstrate the promising application of Ag@Cu-Gel hydrogel for wearable electronics.     

Carbonized electrospun polyacrylonitrile nanofibers as highly sensitive sensors in structural health monitoring of composite structures

Electrospun polyacrylonitrile (PAN) nanofibers were stabilized at 280°C for 1 h in an ambient condition, and then carbonized at 850°C in inert argon gas for additional 1 h in order to fabricate highly pure carbonous nanofibers for the development of highly sensitive sensors in structural health monitoring (SHM) of composite aircraft and wind turbines. This study manifests the real‐time strain response of the carbonized PAN nanofibers under various tensile loadings. The prepared carbon nanofibers were placed on top of the carbon fiber pre‐preg composite as a single layer. Using a hand lay‐up method, and then co‐cured with the pre‐preg composites in a vacuum oven following the curing cycle of the composite. The electric wires were connected to the top surface of the composite panels where the cohesively bonded conductive nanofibers were placed prior to the tensile and compression loadings in the grips of the tensile unit. The test results clearly showed that the carbonized electrospun PAN nanofibers on the carbon fiber composites were remarkably performed well. Even the small strain rates (e.g., 0.020% strain) on the composite panels were easily detected through voltage and resistance changes of the panels. The change in voltage can be mainly attributed to the breakage/deformation of the conductive network of the carbonized PAN nanofibers under the loadings. The primary goal of the present study is to develop a cost‐effective, lightweight, and flexible strain sensor for the SHM of composite aircraft and wind turbines. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43235.     

Simple route for the preparation of graphene/poly(styrene‐b‐butadiene‐b‐styrene) nanocomposite films with enhanced electrical conductivity and hydrophobicity

This paper reports a simple route for the preparation of graphene/poly(styrene‐b‐butadiene‐b‐styrene) (SBS) nanocomposite films employing a vacuum filtration method. Graphene is exfoliated well by an electrochemical procedure and homogeneously dispersed in the polymer matrix. The prepared nanocomposite films were characterized by XRD, Fourier transform IR (FTIR) spectroscopy, X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy, AFM and SEM. Morphological studies showed that graphene formed a smooth coating over the surface of SBS. The increase in graphene concentration induces the wrinkling of graphene sheets at the composite surface which causes a further increase in surface roughness. The FTIR, Raman and XPS spectra of graphene/SBS nanocomposite films indicate the strong interactions between graphene and the polymer matrix. According to the XRD patterns, introducing SBS into graphene did not modify the graphene structure additionally, i.e. the crystal lattice parameters do not depend on SBS content in graphene/SBS nanocomposite films. The graphene/SBS nanocomposite films also exhibited better hydrophobicity due to the increased surface roughness and lower sheet resistivity (reduced 10 times) compared to exfoliated graphene. © 2018 Society of Chemical Industry     

Modified resistivity–strain behavior through the incorporation of metallic particles in conductive polymer composite fibers containing carbon nanotubes

Eutectic metal particles and carbon nanotubes are incorporated into a thermoplastic polyurethane matrix through a simple but efficient method, melt compounding, to tune the resistivity–strain behavior of conductive polymer composite (CPC) fibers. Such a combination of conductive fillers is rarely used for CPCs in the literature. To characterize the strain‐sensing properties of these fibers, both linear and dynamic strain loadings are carried out. It is noted that a higher metal content in the fibers results in higher strain sensitivity. These strain‐sensing results are discussed through a morphological study combined with a model based on the classic tunneling model of Simmons. It is suggested that a high tunneling barrier height is preferred in order to achieve higher strain sensitivity. Copyright © 2012 Society of Chemical Industry     

Microfluidic‐Spinning‐Directed Conductive Fibers toward Flexible Micro‐Supercapacitors

The large‐scale fabrication of the flexible fiber‐shaped micro‐supercapacitors has received major attention from both industrial and academic researchers. Herein, conductive and robust polyaniline‐wrapped multiwall carbon tubes reduced graphene oxide/thermoplastic polyurethane (PANI/MCNTs‐rGO/TPU) composite fibers are successfully fabricated on a large scale via the combination of facile microfluidic‐spinning process and in situ polymerization of aniline. Initially, MCNTs‐rGO/TPU fibers are formed in a T‐shape microfluidic chip, relying on the fast material diffusion and exchange in the microfluidic channel. Then, PANI/MCNTs‐rGO/TPU hybrid fibers are synthesized through an in situ chemical oxidative polymerization of aniline. With the assistance of polyaniline, these PANI/MCNTs‐rGO/TPU hybrid fibers exhibit enhanced electrochemical properties in comparison with pure MCNTs‐rGO/TPU fibers, especially in high specific capacitance, which is dramatically increased from 42.1 to 155.5 mF cm^?2. Moreover, the PANI/MCNTs‐rGO/TPU hybrid fibers can endure various blending stresses, contributing to its outperforming flexibility and weavability. The best of the excellent electrochemical and mechanical properties of these conductive fibers is made to construct the flexible supercapacitors and various complicated functional fabrics.     

Highly flexible fabric strain sensor based on graphene nanoplatelet–polyaniline nanocomposites for human gesture recognition

Flexible and wearable smart fabrics are becoming increasingly popular in healthcare and motion monitoring because of their potential applications in flexible and stretchable electronics. The integration of ordinary fabric with conductive fillers provides the fabric with new and intriguing functions, such as sensation. In this study, a low‐cost and efficient manner was used to fabricate a highly reliable conductive composite on fabric as an effective sensing material for gesture recognition. A strain sensor was fabricated by the incorporation of the highly conductive polyaniline (PANI) polymer, graphene nanoplatelets (GNPs), and a handful of silicon rubber (SR) onto elastic Lycra fabric via a spin‐coating method. We demonstrated that the fabric strain sensor was able to detect and monitor the bending angle of a human finger. By means of the covered structure of the PANI and GNPs, the composite fabric could bear a 40% maximum strain and possess the pleasant characteristic of stretching and bending. The gauge factor of the fabric strain sensor reached 67.3; this was an improvement of approximately four times compared to sensors without PANI microparticles. Finally, the superior performance of our strain sensor through the integration of five strain sensors on a glove for the motion detection of fingers was demonstrated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45340.     

Cable‐Shaped Lithium–Sulfur Batteries Based on Nitrogen‐Doped Carbon/Carbon Nanotube Composite Yarns

The research on flexible and wearable devices has attracted extensive attention in the last few years. Lithium–sulfur (Li‐S) batteries are regarded as a promising option because of their high theoretical capacity and energy density. Here, cable‐shaped Li‐S batteries are developed based on a nitrogen‐doped carbon/carbon nanotube/sulfur (NCNT/S) composite cathode and lithium metal anode. The carbon nanotube (CNT) yarns with high conductivity and an appropriate amount of doped nitrogen are synthesized by wet‐spinning followed by a carbonization process, and further act as a self‐supported conductive backbone for the active material. The NCNT/S yarns exhibit a high initial capacitance of 1001 mAh g^?1 and excellent cyclic stability with 87% capacity retention after 200 cycles at 0.5 C. Furthermore, the assembled cable‐shaped Li‐S batteries by NCNT/S yarns present good ability to light up the LEDs for more than 8 h under normal and bending states at various angles, indicating that the cable‐shaped Li‐S batteries could be a prospective candidate for application in wearable electronics.     

Conductive and electroactive composite paper reinforced by coating of polyaniline on lignocelluloses fibers

Direct use of lignocelluloses fibers as substrate for fabrication of conductive, electroactive, biodegradable, and low‐cost electrode materials are in demand for high‐tech applications of ion‐exchange and energy storage devices. This article presents the preparation and characterizations of conductive and electroactive lignocelluloses‐polyaniline (cellulose/PANI) composite paper. Lignocelluloses fibers were directly collected from the stem of self‐growing plant, Typha Angusitfolia, and subsequently coated with the conductive and electroactive layer of PANI through chemical synthesis. Individual PANI‐coated lignocelluloses fibers were converted into sheet and further characterized with Scanning Electron Microscopy, Fourier Transform Infrared, Thermogravimetric Analysis, electronic conductivity, and Cyclic Voltammetry. Cellulose/PANI composite paper revealed superior thermal characteristics and used as a working electrode in three different electrolytes for ion‐exchange properties. Conductive composite paper (CCP) showed the charge storage capacity of ～52 C/g at scan rate of 5 mV/s in 2M HCl solution. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42293.     

Mechanical and piezo‐resistive properties of styrene–butadiene–styrene copolymer covalently modified with graphene/styrene–butadiene–styrene composites

As novel piezoelectric materials, carbon‐reinforced polymer composites exhibit excellent piezoelectric properties and flexibility. In this study, we used a styrene–butadiene–styrene triblock copolymer covalently grafted with graphene (SBS‐g‐RGO) to prepare SBS‐g‐RGO/styrene–butadiene–styrene (SBS) composites to enhance the organic solubility of graphene sheets and its dispersion in composites. Once exfoliated from natural graphite, graphene oxide was chemically modified with 1,6‐hexanediamine to functionalize with amino groups (GO–NH₂), and this was followed by reduction with hydrazine [amine‐functionalized graphene oxide (RGO–NH₂)]. SBS‐g‐RGO was finally obtained by the reaction of RGO–NH₂ and maleic anhydride grafted SBS. After that, X‐ray diffraction, X‐ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and other methods were applied to characterize SBS‐g‐RGO. The results indicate that the SBS molecules were grafted onto the graphene sheets by covalent bonds, and SBS‐g‐RGO was dispersed well. In addition, the mechanical and electrical conductivity properties of the SBS‐g‐RGO/SBS composites showed significant improvements because of the excellent interfacial interactions and homogeneous dispersion of SBS‐g‐RGO in SBS. Moreover, the composites exhibited remarkable piezo resistivity under vertical compression and great repeatability after 10 compression cycles; thus, the composites have the potential to be applied in sensor production. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46568.     

Magnetite‐functionalized graphene/poly(styrene‐butadiene‐styrene) composites: thermal and mechanical properties

Advanced polymer composites containing organic–inorganic fillers are gaining increasing attention due to their multifunctional applications. In this work, poly(styrene‐butadiene‐styrene) (SBS) composites containing magnetite‐functionalized graphene (FG) were prepared by a dissolution ? dispersion ? precipitation solution method. Evidently, through morphology studies, amounts of FG were well distributed in the SBS matrix. Improvements in neat SBS properties with respect to FG loading in terms of thermal stability, creep recovery and mechanical properties are presented. As expected, the addition of FG improved the thermal stability and mechanical properties of the composites. The yield strength and Young's modulus of the SBS increased by 66% and 146% at 5 wt% filler loading which can be attributed to the reinforcing nature of FG. Similarly, an increase in the storage and loss modulus of the composites showed a reinforcement effect of the filler even at low concentration. The results also showed the significant role of FG in improving the creep and recovery performance of the SBS copolymer. Creep deformation decreased with filler loading but increased with temperature. © 2017 Society of Chemical Industry     

A highly flexible,electrically conductive,and mechanically robust graphene/epoxy composite film for its self-damage detection

Advanced functional composites have attracted a great attention for fabricating flexible devices. In this article, the GnP/epoxy composite film was prepared by mixing graphene platelets (GnPs) with epoxy through sonication process. The morphology, mechanical properties, and electrical conductivity of the prepared composites were investigated. As the GnP contents increased from 2.5 to 7.5 vol%, the composites showed an increase in strain sensitivity with the rapid decrease in the strain gauge to 4.4. Additionally, when dynamic movement of the flexible film was performed, at bending and twist angle of 135° and 180°, respectively, steady increase in both resistance changes were detected and compared. The electrical resistance of the flexible was measured over a temperature range of 20–95°C, an increase in temperature lead to a linearly equivalent increase in resistance. The composites can also detect slight pressure changes at 2 kPa compression force with rapid decrease of resistance. Additionally, fatigue test was performed with stable, sensitive, and no distinguishable reading under 2,000 stretching cycles. The composite film exhibits an excellent self-sensing responds when fracture occurred. Thus, the obtained highly flexible, conductive, and mechanical robust composite sensor can be applied as advanced composites sensors for health monitoring.     

A Flexible and Multipurpose Piezoresistive Strain Sensor Based on Carbonized Phenol Formaldehyde Foam for Human Motion Monitoring

High‐performance flexible strain sensors are extensively studied for various applications including healthcare, robots, and human–computer interaction. In most of the reported research, the fabrication of these sensors involves conductive polymer composites containing expensive metallic or carbon nanomaterials. In this study, commercial phenol formaldehyde foam (PFF) is carbonized by a simple high‐temperature pyrolysis treatment and encapsulated by polydimethylsiloxane (PDMS) to fabricate a flexible and multipurpose piezoresistive strain sensor. The as‐fabricated PDMS‐cPFF strain sensor is capable of detecting various strain modes, including tension, compression, and three‐point bending. Furthermore, the sensor exhibits a high sensitivity with a gauge factor (GF) of ?20.5 under tension and stable signal responses in a frequency range of 0.01–0.5 Hz. The sensor is also capable of accurately monitoring a subtle bending strain of 0.05%. In addition, the sensor shows excellent durability in cyclic loading/unloading tests up to 1000 cycles. The applications of this strain sensor in both large‐ (finger bending and neck movement) and small‐scale human motion monitoring (facial micro‐expression and phonation) are demonstrated, showing its potential for applications in wearable electronics. This work also offers an alternative route to reuse waste thermosetting resins which would otherwise be difficult to recycle.     

Self‐Powered Flexible Sensor Based on the Graphene Modified P(VDF‐TrFE) Electrospun Fibers for Pressure Detection

Polymer P(VDF‐TrFE) has been extensively applied in modern flexible electronics, such as nanogenerators and pressure sensors. In this study, a repolarization method is proposed to exploit the piezoelectric properties of the P(VDF‐TrFE) electrospinning film modified by the reduced graphene oxide (rGO). Then, the repolarized composite film is applied as the self‐powered flexible pressure sensor. Notably, the piezoelectric output voltage and current of the repolarized composite film are up to 1.5 V and 0.125 µA, respectively. Typically, the piezoelectric voltage of the composite film is three times as high as that of the pure spinning film. Meanwhile, this composite film also exhibits piezoresistive effect, which is ascribed to the 3D network structure of the electrospun nanofibers. In addition, the highest piezoresistive sensitivity of the pressure sensor is 0.072 kPa^?1. To sum up, the pressure sensor fabricated in this study allows to simultaneously detect the static and dynamic pressure loads, which thereby has great application potentials in electronic skins (e‐skins) for human motion monitoring, such as motion state and finger bending.