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Stretchable electronic conductors are vital components in soft robotics and flexible electronics. One method for producing these is combining conductive filler with a nonconductive elastomer. These composites commonly exhibit significant piezoresistivity. This work examines various mechanisms that may underlie this effect. These composites are generally analyzed through percolation theory, which describes the nonlinear relationship between filler volume fraction and conductivity. However, it is unclear whether percolation theory can explain their piezoresistivity or whether mechanisms such as rearrangement of the conductive network under deformation must be considered. This work compares volumetric change in the context of percolation theory against network rearrangement to examine the relative significance of these factors in determining piezoresistivity. Digital image correlation is utilized to investigate volumetric changes in carbon‐black silicone composites and finds that the typical assumption of incompressibility is reasonable, suggesting that volumetric changes alone cannot account for the behavior. A computational model is also developed, which implies that network rearrangement is likely a more significant factor and that interparticle interactions are crucial in understanding this effect. It was found that the most realistic modeling results were achieved only when both rigid and attractive interparticle interactions were accounted for in the model. [ABSTRACT FROM AUTHOR]

The miniaturization of microelectromechanical systems (MEMS) physical sensors is driven by global connectivity needs and is closely linked to emerging digital technologies and the Internet of Things. Strong technical advantages of miniaturization such as improved sensitivity, functionality, and power consumption are accompanied by significant economic benefits due to semiconductor manufacturing. Hence, the trend to produce smaller sensors and their driving force resemble very much those of the miniaturization of integrated circuits (ICs) as described by Moore's law. In this respect, with its IC-, and MEMS-compatibility, and scalability, the silicon nanowire is frequently employed in frontier research as the sensor building block replacing conventional sensors. The integration of the silicon nanowire with MEMS has thus generated a multiscale hybrid architecture, where the silicon nanowire serves as the piezoresistive transducer and MEMS provide an interface with external forces, such as inertial or magnetic. This approach has been reported for almost all physical sensor types over the last decade. These sensors are reviewed here with detailed classification. In each case, associated technological challenges and comparisons with conventional counterparts are provided. Future directions and opportunities are highlighted. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The microstructural changes caused by the addition of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide to polydimethylsiloxane (PDMS) elastomer composites filled with carbon black (CB) are analyzed to explain the electrical, mechanical, rheological, and optical properties of IL‐containing precursors and composites. Swelling experiments and optical analysis indicate a limited solubility of the IL in the PDMS matrix that reduces the cross‐linking density of PDMS both globally and locally, which reduces the Young's moduli of the composites. A rheological analysis of the precursor mixture shows that the IL reduces the strength of carbon–carbon and carbon–PDMS interactions, thus lowering the filler–matrix coupling and increasing the elongation at break. Electromechanical testing reveals a combination of reversible and irreversible piezoresistive responses that is consistent with the presence of IL at microscopic carbon–carbon interfaces, where it enables re‐established electrical connections after stress release but reduces the absolute conductivity. [ABSTRACT FROM AUTHOR]

For thermoplastic material extrusion 3D‐printing functional filaments with electrically conductive, piezoresistive, piezoelectric, capacitive or magnetic properties are developed. The functional filaments result in piezoresisitve static/quasi‐static sensors; however, these 3D‐printed sensors are manufactured in several processes, for example, the creation of highly conductive paths using embedded copper wires or by silver inking. The multi‐process 3D printing of sensors presents an obstacle to smart functional 3D‐printed structures where the sensory element is in situ printed at the location and orientation of use, including the electrical paths. This research introduces a three axial piezoresistive accelerometer where the whole sensor, including highly conductive electrical paths, is printed in the same process of thermoplastic material extrusion. The structural components of the sensor are printed with a non‐conductive polylactide material, the sensory element with a conductive material that has a relatively high resistivity, and the electrical paths with a conductive material that has a relatively low resistivity. As discussed in the manuscript with single‐process sensors, the functional tuning of sensors is as easy as changing the design. The design and manufacturing solutions researched for the triaxial accelerometer can be applied to other 3D‐printed sensors, for example, force sensors and opens up the possibility of a single‐process 3D‐printed smart structure. [ABSTRACT FROM AUTHOR]

Stretchable and soft piezoresistive composites are appealing for application to tactile sensors, artificial skin, and wearable electronics. The ability of the composites to deform the geometries when they are strained can allow the electrical behavior of the composites to be manipulated. Although rigid metal and semiconductor inclusions have been utilized to create piezoresistive composites, they limit the degree of mechanical deformation. Here, liquid metal (gallium, melting point ≈ 29.7 °C) inclusion into elastomeric foam substrate with 3D open cell morphologies is utilized. Gallium is a fluidic conductor, thus it is possible to infiltrate the liquid metal into the 3D interconnected pore, resulting in soft, stretchable, and shape reconfigurable conductive composites that can change shape and function in response to external stimuli. Applying strain can enable deformation of the liquid metal, generating changes of electrical resistance. Interestingly, it is found that this piezoresistivity of the composite can be positively and negatively manipulated by adjusting the geometries of the liquid metal in the foam. Furthermore, the liquid metal in the elastomeric foam can be reversibly actuated by applying compressive force, resulting in manipulation of the restorative electrical activity of the composites. [ABSTRACT FROM AUTHOR]

Structural health monitoring (SHM) using self‐sensing cement‐based materials has been reported before, where nano‐fillers have been incorporated in cementitious matrices as functional sensing elements. A percolation threshold is always required in order for conductive nano‐fillers modified concrete to be useful for SHM. Nonetheless, the best pressure/strain sensitivity results achieved for any self‐sensing cementitious matrix are <0.01 MPa−1. In this work, we introduce for the first‐time novel partially reduced graphene oxide based electronic textile (e‐textile) embedded in plain and as well as in polymer‐binder‐modified cementitious matrix for SHM applications. These e‐textile embedded cementitious composites are independent of any percolation threshold due to the interconnected fabric inside the host matrix. The piezo‐resistive response was measured by applying direct and cyclic compressive loads (ranging from 0.10 to 3.90 MPa). A pressure sensitivity of 1.50 MPa−1 and an ultra‐high gauge factor of 2000 was obtained for the system of the self‐sensing cementitious structure with embedded e‐textiles. The sensitivity of this new system with embedded e‐textile is an order of magnitude higher than the state‐of‐the‐art nanoparticle based self‐sensing cementitious composites. The composites showed mechanical stability and functional durability over long‐term cyclic compression tests of 1000 cycles. Additionally, a two time‐constant model was used to validate the experimental results on decay response of the e‐textile embedded composites. [ABSTRACT FROM AUTHOR]

Piezoresistive strain sensing of multiwalled carbon nanotube (CNT)‐reinforced polymer nanocomposites considering the effect of agglomeration is experimentally investigated. Thermoplastic polyurethane is used as composite matrix with melt mixing method for fabricating the nanocomposite. The electrical resistance of the nanocomposite specimens is measured by a designed electrical circuit in conductivity and piezoresistivity tests. The electrical resistance of the nanocomposite decreases by a nonlinear relation when CNT is added up to 1.5 wt% and has an inverse behavior after this weight fraction. Comparison between scanning electron microscope images of the nanocomposite specimen fracture surfaces shows that the CNT agglomeration is the reason for decreasing the nanocomposite electrical properties. Also, decrease of the mechanical properties of the nanocomposite in higher filler content than 1.5 wt%, confirms the agglomeration effects. In addition, it is illustrated that the CNT agglomeration decreases strain sensing in the nanocomposite. Moreover, different piezoresistive behaviors of the nanocomposite in low and large strains are explained, according to the piezoresistivity mechanisms. Change of the piezoresistivity mechanism type, causes a nonlinear growth of the nanocomposite electrical resistance, in large strain. [ABSTRACT FROM AUTHOR]

Electrically conductive, polymeric materials that maintain their conductivity even when under significant mechanical deformation are needed for actuator electrodes, conformable electromagnetic shielding, stretchable tactile sensors, and flexible energy storage. The challenge for these materials is that the percolated, electrically conductive networks tend to separate even at low strains, leading to significant piezoresistance. Herein, deformable conductors are fabricated by spray‐coating a nitrile substrate with a graphene–elastomer solution. The electrical resistance of the coatings shows a decrease after thousands of bending cycles and a slight increase after repeated folding‐unfolding events. The deformable conductors double their electrical resistance at 12% strain and are washable without changing their electrical properties. The conductivity–strain behavior is modeled by considering the nanofiller separation upon deformation. To boost the conductivity at higher strains, the production process is adapted by stretching the nitrile substrate before spraying, after which it is released. This adaption meant that the electrical resistance doubles at 25% strain. The electrical resistance is found sufficiently low to give a 1.9 dB µm−1 shielding in the 8–12 GHz electromagnetic band. The physical and electrical properties, including the electro magnetic screening, of the flexible conductors, are found to deteriorate upon cycling but can be recovered through reheating the coating. [ABSTRACT FROM AUTHOR]

Strain sensing capability of carbon nanotubes (CNTs) doped epoxy under tensile and flexural tests were broadly studied, while the piezoresistive behavior of nanocomposites in precracked specimen, that is, in fracture toughness tests, have not been addressed deeply in the literature. Therefore, in the current paper, both the strain and crack sensing capabilities of single wall carbon nanotubes‐double wall carbon nanotubes (SWCNTs‐DWCNTs) doped epoxy subjected to tensile and mode I fracture tests were investigated. Different CNT loadings including 0.5 and 0.75 wt% were used to compare the effect of various states of CNTs dispersion, including uniformly dispersed CNTs and aggregates, on electromechanical properties. Results showed that specimens loaded at 0.5 wt% of CNTs possessed proper tensile strength, fracture toughness, piezoresistivity, and sensitivity. A nonlinear trend in normalized resistance change with respect to strain was noticed under tensile test resulting from tunneling effect which induced nonlinearity in the electrical signals. During fracture tests, prior to crack propagation, different trends in piezoresistivity as a function of displacement were observed, depending on the CNT loading, that is, linear and nonlinear behavior at CNT content of 0.5 and 0.75 wt%, respectively. The nanocomposite could simultaneously monitor damage initiation and extension with the onset of crack growth by showing abrupt increase in normalized resistance, and two different failure modes can be distinguished including abrupt and crack extensions. It was concluded that any abrupt jump in normalized resistance could be used as an indicator for damage initiation in the specimen. [ABSTRACT FROM AUTHOR]

Carbon nanotube, owing to its excellent mechanical and electrical properties, is promising to be used in polymer composites towards both structural and functional applications. In this paper, one type of multi‐walled carbon nanotubes (MWNTs)/polystyrene‐b‐(ethylene‐co‐butylene)‐b‐polystyrene (SEBS) composite was fabricated. MWNTs were first surface functionalized with SEBS before blended with polymer matrix. The surface modification of MWNTs was characterized by various techniques including X‐ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis (TGA). Results indicated that MAH (maleic anhydride)‐g‐SEBS was successfully grafted onto MWNTs with grafting ratio 8.7 wt%. The SEBS composites containing functionalized MWNTs exhibited excellent mechanical and electrical properties, evidenced by tensile and electrical properties tests, owing to enhanced interfacial interaction between the SEBS‐g‐MWNTs and the SEBS matrix. SEBS‐g‐MWNTs/SEBS was also explored in terms of piezoresistivity. The composites exhibited excellent piezoresistivity after the percolation threshold (1.0 wt%) and good repeatability during multiple cycles. The rubbery behavior coupled with electrical properties make this type of composite promising in advanced soft electronics. [ABSTRACT FROM AUTHOR]

Summary: The intrinsic vulnerability of masonry structures to seismic events makes structural health monitoring of the utmost importance for the conservation of the built heritage. The development of piezoresistive bricks, also termed smart bricks, is an innovative technology recently proposed by the authors for the monitoring of such structures. Smart bricks exhibit measurable variations in their electrical properties when subjected to external loads or, alternatively, strain self‐sensing capabilities. Therefore, the deployment of a network of smart bricks into a masonry structure confers self‐diagnostic properties to the host structure. In this light, this paper presents a theoretical investigation on the application of smart bricks to full‐scale masonry structures for seismic assessment. This includes the study of the convenience of providing electrical isolation conditions to the sensors, as well as the effectiveness of smart bricks when installed into either new constructions or in pre‐existing structures. Secondly, numerical results are presented on the seismic analysis of a three‐dimensional masonry building equipped with a network of smart bricks. Finally, in order to map the strain field throughout the structure exploiting the outputs of a limited number of sensors, an interpolation‐based strain reconstruction approach is proposed. [ABSTRACT FROM AUTHOR]

Abstract: The recent interest of electronic skin (e‐skin) has pushed the research toward the development of flexible sensors, namely, for pressure detection. Several mechanisms can be used to transduce pressure into electrical signals, but piezoresistivity presents advantages due to its simplicity. The microstructuration of the films composing these sensors is a common strategy to improve their sensitivity. As an alternative to conventional and expensive photolithography techniques and low customizable techniques based on natural molds, a novel strategy for the microstructuration of polydimethylsiloxane (PDMS) films is proposed, based on molds fabricated by laser engraving. After design optimization of these microstructured films, which relies on microcones, piezoresistive sensors with a limit of detection of 15 Pa and a sensitivity of −2.5 kPa−1 in the low‐pressure regime are obtained. These sensors are used with success on the detection of the blood pressure wave at the wrist, thus exhibiting a great potential for health applications. [ABSTRACT FROM AUTHOR]

Abstract: Flexible force sensors based on graphene/polymer nanocomposites have attracted tremendous attention owing to their remarkable sensitivity and ease of fabrication. In the present study, nanocomposites consisting of graphene as conducting fillers in a polyimide matrix are prepared, and an electrical breakdown method is used to endow the nanocomposite with a high piezoresistivity. Electromechanical tests and theoretical models confirm that the piezoresistivity of the nanocomposite originates from the cracks in the carbonized polymers induced by electrical breakdown. The fabricated force sensor exhibits a broad working range of 0–495 kPa, ultrafast response (2.12 ms), and excellent stability (>2000 compression–release cycles). In addition, the sensitivities of the force sensor can be tuned by varying the applied current in the electrical breakdown process. Thus, the electrical breakdown method allows highly facile fabrication of piezoresistive force sensors with tunable sensitivities. [ABSTRACT FROM AUTHOR]

Previous and current research on piezoresistivity of polymer composites filled with carbon nanostructures is reviewed. The review covers the use of the coupled electro‐mechanical response of these materials to self‐sense their strain and damage during mechanical loading. The mechanisms yielding changes in electrical resistance upon mechanical loading in polymer composites filled with carbon nanostructures are first discussed. Published knowledge is then summarized, starting with framework literature on carbon black and graphite and then moving to more recent research on carbon nanotubes, exfoliated graphite, and few‐layer graphene sheets. Piezoresistive studies of polymer nanocomposites with aligned carbon fillers are also reviewed. It is aimed that this review contributes in collecting, organizing, and summarizing the knowledge, foundations, and state of the art on the piezoresistive response of polymer composites filled with different carbon allotropes, providing new perspectives and advancing towards the fast development of smart self‐sensing carbon filled nanocomposites. [ABSTRACT FROM AUTHOR]

The design and fabrication of various types of flexible, portable, and foldable devices have received immense interest owing to the remarkable potential in impacting peoples' lives including real‐time health monitoring, point‐of‐care diagnosis, and athletic training. In this work, the authors present 3D graphite as the key sensing element of polymer composite strain sensors that offers ultrahigh sensitivity and durability in the detection of fine motions. The graphite–polymer sensors in this work provide high bending sensitivities that are reproducible within 3% signal shift after 11 000 bending cycles and exhibit gauge factors of 100 and 52 at tensile strains of 80% and 100%, respectively. The sensing mechanism is modeled, and correlated with experimental studies. The high strain sensitivity compared to graphene based devices is analyzed and understood with respect to levels of defect in materials. Such graphite–polymer sensors are able to detect fine features of human pulses, respiration rates, and throat vibration in real time and are also applied in the detection of posture correctness of musical instrument learners for the first time. [ABSTRACT FROM AUTHOR]

Due to the growing demand for tactile sensors, the possibility of detecting an external uniaxial pressure by the piezoresistive measuring of a conductive filler/elastomer composite was investigated. A series of piezoresistive models are discussed. Novel highly sensitive piezoresistive foams with excellent elasticity were fabricated using vapor-grown carbon fiber ( VGCF), two-component silicone elastomer and a new type of thermally expandable micro beads foaming agent to overcome the disadvantages of the silicone elastomer in the utilization of a tactile sensor. Deformations of the foams caused by uniaxial pressure were observed using scanning electron microscopy from cross-sections. Effects of the VGCF and the foaming agent on the piezoresistivitiy were investigated. The piezoresistive mechanisms of the foams are discussed according to the measurements, and good fit was found between the theoretical calculations and the experimental piezoresistivity measurements. It is found that the addition of the micro beads foaming agent can improve the piezoresistivity of the VGCF/silicone foam and increase the sensitivity and repeatability for its application in a tactile sensor. © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

ABSTRACT Tensile piezoresistive properties of multiwall carbon nanotube (MWCNT)/segmented polyurethane (SPU) composites comprising 15, 30, and 50 wt % rigid segment (RS) contents and 2, 4, and 6 wt % MWCNT contents are investigated. The physicochemical properties of such composites are used to better understand their mechanical and piezoresistive behavior. Infrared spectra shows that for 15 and 30 wt % RS composites the addition of MWCNTs promotes a more structured RS domain which increases the phase separation, while for 50 wt % RS composites the MWCNTs disrupt the RS domains of the polymer with a high phase separation. Overall, MWCNT content has less effect on the phase separation than RS content. The composites with 6 wt % MWCNT content reached electrical conductivities of the order of ∼10−1 S/m using 15 and 50 wt % RS polymers. Upon deformation, composites with 15 wt % RS and 4 wt % MWCNT achieved changes in electrical resistance of the order of 5000 times their unstrained value, which are outstanding values that can be exploited for applications such as human motion detection. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44448. [ABSTRACT FROM AUTHOR]

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, skinmountable, and wearable strain sensors have potential in diverse applications while several grand challenges have to be still overcome. [ABSTRACT FROM AUTHOR]

Functional materials with excellent electrical and magnetic properties have a potential application in most high-tech fields. Here, superparamagnetic graphene-Fe3O4 (G-F) nanosheet hybrids are prepared by a facile one-step hydrothermal process, which will beneficial to well combine with various polymer matrix to get binary functional composites. Polydimethylsiloxane (PDMS) nanocomposites filled with G-F nanosheets are prepared by a three rolling method. The microstructures of G-F nanosheets as well as the electrical, piezoresistive, and magnetic properties of the G-F/PDMS nanocomposites are investigated. Slightly above the percolation threshold ( fc = 3.4 wt%), the G-F/PDMS nanocomposites with 3.65 wt% of G-F nanosheets show a high conductivity, which is about 10 orders of magnitudes higher than that of pure PDMS, showing an obvious insulator-semiconductor transition. Moreover, the resistance of the nanocomposites with G-F filler near the percolation threshold exponentially increases with the uniaxial pressure. The normalized resistance ( R/ R0) can reach up to 870 at 0.88 MPa with 3.65 wt% G-F nanosheets loading. The G-F/PDMS nanocomposites also show superparamagnetism with saturation magnetization values of 0.21 emu g−1. These results suggest that the G-F/PDMS nanocomposites in this work provide a new route to fabricate difunctional polymer-based nanocomposites with a potential application as flexible touch sensors. [ABSTRACT FROM AUTHOR]

The development of electronic skin (e-skin) is of great importance in human-like robotics, healthcare, wearable electronics, and medical applications. In this paper, a bioinspired e-skin design of hierarchical micro- and nano-structured ZnO nanowire (NW) arrays in an interlocked geometry is suggested for the sensitive detection of both static and dynamic tactile stimuli through piezoresistive and piezoelectric transduction modes, respectively. The interlocked hierarchical structures enable a stress-sensitive variation in the contact area between the interlocked ZnO NWs and also the efficient bending of ZnO NWs, which allow the sensitive detection of both static and dynamic tactile stimuli. The flexible e-skin in a piezoresistive mode shows a high pressure sensitivity (−6.8 kPa−1) and an ultrafast response time (<5 ms), which enables the detection of minute static pressure (0.6 Pa), vibration level (0.1 m s−2), and sound pressure (≈57 dB). The flexible e-skin in a piezoelectric mode is also demonstrated to be able to detect fast dynamic stimuli such as high frequency vibrations (≈250 Hz). The flexible e-skins with both piezoresistive and piezoelectric sensing capabilities may find applications requiring both static and dynamic tactile perceptions such as robotic hands for dexterous manipulations and various healthcare monitoring devices. [ABSTRACT FROM AUTHOR]

Im Rahmen dieser Arbeit wurden selbstüberwachende Probekörper aus Zement mit mehrlagigen Kohlenstoffnanoröhren (multiwall carbon nanotubes MWCNT) hergestellt und einer zyklischen Belastung unterzogen. Der elastische Widerstand und die Piezoresistivität der Probekörper wurden bei unterschiedlicher Temperatur und verschiedenen Wassergehalten untersucht. Die Ergebnisse zeigen, dass mit steigender Temperatur der elektrische Widerstand und die Piezoresistivität sinken. Mit erhöhtem Wassergehalt verringert sich der elektrische Widerstand, während die Piezoresistivität erst zu‐ und dann abnimmt. Die Hauptursache für die Änderung des spezifischen Widerstandes und der Piezoresistivität ist die Veränderung der Tunnelleitung unter verschiedenen Bedingungen. Der Zusammenhang zwischen Wassergehalt, Temperatur und der Resistivitätsänderung konnte mit einer Regressionsfunktion bei einem korrigierten Bestimmtheitsmaß R2 von 0,92 dargestellt werden. Carbon nanotube‐cement composite for self‐sensing concrete pavement Self‐sensing materials could sense their own strain and damages without sensors. Smart concrete pavement made with cement based self‐sensing materials could achieve in‐situ monitoring for both structural integrity and traffic characteristics. This paper studied the electrical resistivity and piezoresistivity of cement with multiwall carbon nanotubes (MWCNTs) under cyclic loading. Effects of temperature and water content on both resistivity and piezoresistivity were investigated. Test results show that the resistivity decreased as the water content increased. Differently, the piezoresistivity and change in resistance first increased and then decreased. Temperature had significant effect on resistivity and piezoresistivity. As the temperature increased, resistivity decreased but the piezoresistivity increased. The major cause of the change in resistivity and piezoresistivity was the change of tunneling conduction under different conditions. The relationship between water content, temperature and the resistivity could be accurately represented with a regression function with an R2 of 0.92. Therefore, the data collected by self‐sensing materials must be calibrated for the water content and temperature before interpretation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Copper nanowires (CuNWs) are highly conductive yet low‐cost nanostructures for constructing piezoresistive pressure sensors. Rational design and controllable fabrication are essential steps toward high‐performance CuNW‐based sensors, and the oxidation tendency of CuNWs must be inhibited. Conducting and flexible CuNW@graphene (CuNW@G) core–shell aerogels are fabricated with tunable microstructures via a simple assembly followed by thermal annealing, in which the aerogel bulk density, surface wettability, Young's modulus, and electrical conductivity can be adjusted accordingly by varying the initial CuNW dispersion concentration. In particular, these core–shell aerogels convert from hydrophobic to superhydrophobic at higher CuNW concentration with reduced adhesion to water. In the range of investigated CuNW dispersion concentration, the chemical constituent and structure of as‐produced graphene shell is nearly the same, as is the aerogel's antioxidation stability. Based on the piezoresistivity of the core–shell aerogels, flexible pressure sensors operated at a bias voltage of 0.1 V are demonstrated, which can detect compressive stress as low as 640 Pa and exhibit fast response time less than 16 ms. Furthermore, polydimethylsiloxane‐encapsulated CuNW@G aerogels show an enhanced response and higher reversibility. A general and scalable strategy for developing aerogel‐typed piezoresistive pressure sensors with stable performance and low cost is presented. [ABSTRACT FROM AUTHOR]

Highly sensitive tactile sensors with long‐term stability and low power consumption are one of the key components for flexible electronics. Here, for the first time, the fabrication of VO2 nanomembrane tactile sensors by epitaxial liftoff from ZnO sacrificial layer is reported. The wafer‐scale nanomembranes inherit the structural and electrical properties of the as‐grown films, and the wet transfer generates negligible influence on the quality of VO2. Most importantly, giant electrical responses to external strains are found due to the release of substrate clamping, and a high gauge factor up to ≈1100 is derived. Furthermore, the electrical properties show no deterioration after repeatedly bending the nanomembranes for 10 000 times at a radius of 1 cm. The VO2 nanomembrane sensors are utilized to monitor the radial artery pulse, and totally reproducible waveforms with ultrahigh sensitivity to the tactile stimuli are observed. Moreover, the power dissipation of the VO2 tactile sensors can be lowered down to the picowatt level, allowing for the future construction of self‐powered sensing systems together with nanogenerators. This study provides a substantial step toward large‐scale preparation of oxide nanomembranes and therefore paves a promising way for flexible oxide electronics. [ABSTRACT FROM AUTHOR]

The back cover page of the journal is presented.

Flexible electronic skins (e‐skins) with static and dynamic tactile sensing capabilities are demonstrated on page 2841 by H. Ko and co‐workers based on the interlocked geometry of hierarchical polydimethylsiloxane (PDMS) micopillar arrays decorated with ZnO nanowire forests. While the e‐skin in a piezoresistive mode enables a static pressure detection with high sensitivity, the piezoelectric e‐skin mode permits the dynamic sensing of high frequency vibrations. [ABSTRACT FROM AUTHOR]