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Piezoresistive fibers with large working factors remain of great interest for strain sensing applications involving large strains, yet difficult to achieve. Here, we produced strain-sensitive fibers with large working factors by dip-coating nanocomposite piezoresistive inks on surface-modified polyether block amide (PEBA) fibers. Surface modification of neat PEBA fibers was carried out with polydopamine (PDA) while nanocomposite conductive inks consisted of styrene-ethylene-butylene-styrene (SEBS) elastomer and carbon black (CB). As such, the deposition of piezoresistive coatings was enabled through nonconventional hydrogen-bonding interactions. The resultant fibers demonstrated well-defined piezoresistive linear relationships, which increased with CB filler loading in SEBS. In addition, gauge factors decreased with increasing CB mass fractions from ∼15 to ∼7. Furthermore, we used the fatigue theory to predict the endurance limit (

With unparalleled sensitivities, nanocomposites are believed to be key components in future bodily sensor and healthcare devices. However, there is a lack in understanding of how repeated strain cycles effect their electromechanical performance and what measures can be taken to accommodate changes in measurement using modelling and signal processing. Here, the author examines published cyclic data from a wide range of nanocomposite strain sensors. From the datasets, the author reports a near universal scaling in electromechanical signal with cycle number (C) as a result of the Mullin’s effect. Using a modified model based on Basquin’s law of fatigue, for all nanocomposites, signal was found to following a nearly identical C-0.1 power law scaling with cycle number. Using the presented model, the author demonstrated that a critical conditioning cycle number for a nanocomposite at which a steady state signal occurs, known as the endurance limit, can be predicted. Endurance limit was reported to be highly dependent on the scaling exponent noted in the cyclic data.

Electrically conductive composite materials are highlighted as a potential tech path toward future flexible devices for wearable health technologies. To be commercially viable, these materials must not only be mechanically soft, highly sensitive to deformation, and report a sustainable signal but also utilize manufacturing methods that facilitate large-scale production. An ideal candidate for these envisioned technologies is the viscous, electromechanically sensitive composite material g-putty. Inks based on g-putty here are shown to transform a commercial polymer foam into a sensitive strain sensing material through a simple, scalable soaking procedure. Foam composites reported here have sensitives as high as ∼20 in terms of compressive strain and ∼0.4 kPa

Electrically conductive nanocomposites are an exciting ever expanding area of research that has yielded many new technologies for wearable health devices. Acting as strain sensing materials, they have paved the way towards real time medical diagnostic tools that may very well lead to a golden age of healthcare. Currently, the goal in research is to create a material that simultaneously has both a large gauge factor G and sensing range. However, a weakness in the area of electromechanical research is the lack of standardisation in the reporting of the figure of merit, i.e. G, and the need for new metrics to give researchers a more complete view of the research landscape of resistive type sensors. A paradigm shift in the way in which data is reported is required, to push research in the right direction and to facilitate achieving research goals. Here, we report a standardised method for reporting strain sensing performance and the introduction of the working factor W and the Young's modulus Y of a material as two new material criteria. Using this new method, we can now for the first time define the benchmarks for an optimum sensing material, G > 7, W > 1, Y < 300 kPa, using limits set by standard commercial materials and the human body. Using extrapolated data from 200 publications normalised to this standard method, we can review what composite types meet these benchmark limits, what governs composite performances, the literary trends in composites and individual nanomaterial performance and the future prospects of research., 7 Figures

While nanocomposite electromechanical sensors are expected to display reasonable conductivity and high sensitivity, little consideration is given to eliminating hysteresis and strain rate/frequency dependence from their response. For example, while G-putty, a composite of graphene and polysiloxane, has very high electromechanical sensitivity, its extreme viscoelasticity renders it completely unsuitable for real sensors due to hysteretic and rate-/frequency-dependent effects. Here it is shown that G-putty can be converted to an ink and printed into patterned thin films on elastic substrates. A partial graphene-polymer phase segregation during printing increases the thin-film conductivity by ×106 compared to bulk, while the mechanical effects of the substrate largely suppress hysteresis and completely remove strain rate and frequency dependence. This allows the fabrication of practical, high-gauge-factor, wearable sensors for pulse measurements as well as patterned sensors for low-signal vibration sensing.

Electrically conductive polymer-based nanocomposites have displayed excellent performances when applied as strain sensors for bodily monitoring. Among the most common forms of these systems is the mixed-phased nanocomposite. Through a simple model that combines electromechanical and percolation theory, it is reported here that in comparative systems, two-dimensional nanofillers exhibit a larger electromechanical response than their one-dimensional counterparts. For both nanofiller types, the electromechanical response was found to increase greatly with the aspect ratio and when shifting from an isotropic to anisotropic system. Furthermore, nanocomposites with bulk dimensions intrinsically outperform thinner systems theorized to be based on ink-printing production methods.

The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result. Herein, we show that two-dimensional titanium carbide or carbonitride nanosheets, known as MXenes, can be used as a conductive binder for silicon electrodes produced by a simple and scalable slurry-casting technique without the need of any other additives. The nanosheets form a continuous metallic network, enable fast charge transport and provide good mechanical reinforcement for the thick electrode (up to 450 µm). Consequently, very high areal capacity anodes (up to 23.3 mAh cm−2) have been demonstrated., Developing thick electrodes could enable high-energy-density Li-ion batteries, however, above a critical thickness, the mass transport issues become dominating. Here the authors show that MXene can serve as a conductive binder leading to thick silicon anodes (up to 450 µm) with high areal capacity.

Here we describe using nanosheets of both graphene and boron nitride, produced by liquid phase exfoliation, as fillers in composite fibres. The fibres were prepared by coagulation spinning using polyvinylalcohol as a matrix. We obtained good quality fibres with diameter and nanosheet volume fraction which could be controlled via the ratio of nanosheet to polymer injection rates. The mechanical stiffness (modulus, Y) and strength, σ B , increased relatively slowly with volume fraction (dY/dV f ≤ 160 GPa and dσ B /dV f ≤ 0.8 GPa). However, both stiffness and strength continued increasing with nanosheet content to loading levels of ∼20vol%, after which the properties fell off. Such relatively high loading levels result in impressive mechanical properties with stiffness and strength of up to 30 GPa and 260 MPa observed. In addition, we found the graphene-filled fibres to be electrically conducting with conductivities of up to 3 S/m.

The use of graphene-based nanocomposites as electromechanical sensors has been broadly explored in recent times with a number of papers describing porous, foam-like composites. However, there are no reported foam-based materials that are capable of large dynamic compressive load measurements and very few studies on composite impact sensing. In this work, we describe a simple method of infusing commercially-available foams with pristine graphene to form conductive composites, which we refer to as G-foam. Displaying a strain-dependent electrical response, G-foam was found to be a reasonably effective pressure sensing material. More interestingly, G-foam is a sensitive impact-sensing material. Through the addition of various amounts of polymer filler, the mechanical properties of the composites can be tuned leading to the controllable variation of the impact sensing range. We have developed a simple model which quantitatively explains all our impact sensing data.

Integrated sensors for bodily measurements require a sensing material that is highly conductive, flexible, thin and sensitive. It is important that these materials are non-invasive in application but robust in nature to allow for effective, continuous measurement. Herein, we report a comparative study of two simple, scalable methods to produce silver nanowire (AgNW) polyurethane (PU) composite materials: layer-by-layer (LBL) and mixed filtration. Both types of composites formed were ultrathin (∼50 μm) and highly conductive (104 S m−1), with the LBL method ultimately found to be superior due to its low percolation threshold. Electrical resistance of the LBL composites was found to vary with strain, making these materials suitable for strain sensing. LBL composites displayed a working strain up to ∼250% and a high gauge factor (G), with values of G ∼70 reported. The sensors reported here were ∼109-times more conductive and ∼104-times thinner than their carbon-based composite sensor counterparts with similar gauge factor. This made the strain sensors presented here among one of the most flexible, highly sensitive, thinnest, conductive materials in literature. We demonstrated that with these properties, the LBL composites formed were ideal for bodily motion detection.

Monitoring of human bodily motion requires wearable sensors that can detect position, velocity and acceleration. They should be cheap, lightweight, mechanically compliant and display reasonable sensitivity at high strains and strain rates. No reported material has simultaneously demonstrated all the above requirements. Here we describe a simple method to infuse liquid-exfoliated graphene into natural rubber to create conducting composites. These materials are excellent strain sensors displaying 10(4)-fold increases in resistance and working at strains exceeding 800%. The sensitivity is reasonably high, with gauge factors of up to 35 observed. More importantly, these sensors can effectively track dynamic strain, working well at vibration frequencies of at least 160 Hz. At 60 Hz, we could monitor strains of at least 6% at strain rates exceeding 6000%/s. We have used these composites as bodily motion sensors, effectively monitoring joint and muscle motion as well and breathing and pulse.

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals.

Despite its widespread use in nanocomposites, the effect of embedding graphene in highly viscoelastic polymer matrices is not well understood. We added graphene to a lightly cross-linked polysilicone, often encountered as Silly Putty, changing its electromechanical properties substantially. The resulting nanocomposites display unusual electromechanical behavior, such as postdeformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain. These phenomena are associated with the mobility of the nanosheets in the low-viscosity polymer matrix. By considering both the connectivity and mobility of the nanosheets, we developed a quantitative model that completely describes the electromechanical properties. These nanocomposites are sensitive electromechanical sensors with gauge factors >500 that can measure pulse, blood pressure, and even the impact associated with the footsteps of a small spider.

Few-layer black phosphorus (BP) is a new two-dimensional material which is of great interest for applications, mainly in electronics. However, its lack of environmental stability severely limits its synthesis and processing. Here we demonstrate that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions in solvents such as N-cyclohexyl-2-pyrrolidone (CHP). Nanosheets are surprisingly stable in CHP, probably due to the solvation shell protecting the nanosheets from reacting with water or oxygen. Experiments, supported by simulations, show reactions to occur only at the nanosheet edge, with the rate and extent of the reaction dependent on the water/oxygen content. We demonstrate that liquid-exfoliated BP nanosheets are potentially useful in a range of applications from ultrafast saturable absorbers to gas sensors to fillers for composite reinforcement., While phosphorene is an exciting new 2D material, techniques to produce it in large quantities in a stable, processable form are lacking. Here, the authors achieve this using liquid phase exfoliation and demonstrate the resultant nanosheets to be useful in a number of applications.

International audience; 2D metal chalcogenide (MC) nanosheets (NS) have displayed high capacities as lithium‐ion battery (LiB) anodes. Nevertheless, their complicated synthesis routes coupled with low electronic conductivity greatly limit them as promising LiB electrode material. Here, this work reports a facile single‐walled carbon nanotube (SWCNT) percolating strategy for efficiently maximizing the electrochemical performances of gallium chalcogenide (GaX, X = S or Se). Multiscaled flexible GaX NS/SWCNT heterostructures with abundant voids for Li+ diffusion are fabricated by embedding the liquid‐exfoliated GaX NS matrix within a SWCNT‐percolated network; the latter improves the electron transport and ion diffusion kinetics as well as maintains the mechanical flexibility. Consequently, high capacities (i.e., 838 mAh g−1 per gallium (II) sulfide (GaS) NS/SWCNT mass and 1107 mAh g−1 per GaS mass; the latter is close to the theoretical value) and good rate capabilities are achieved, which can be majorly attributed to the alloying processes of disordered Ga formed after the first irreversible GaX conversion reaction, as monitored by in situ X‐ray diffraction. The presented approach, colloidal solution processing of SWCNT and liquid‐exfoliated MC NS to produce flexible paper‐based electrode, could be generalized for wearable energy storage devices with promising performances.

We have developed methods to disperse and partially size separate NbSe(3) nanowires in aqueous surfactant solutions. These dispersions can easily be formed into thin films. Optical and electrical studies show these films to display sheet resistances and transmittances ranging from (460 Ω/□, 22%) to (12 kΩ/□, 79%) depending on thickness. For thicker films, we measured the transparent conducting figure of merit to be σ(DC, B)/σ(Op) = 0.32, similar to graphene networks. Thickness measurements gave individual values of σ(Op) = 17,800 S m(-1) and σ(DC, B) = 5700 S m(-1). Films thinner than ∼ 70 nm displayed reduced DC conductivity due to percolative effects.

Mixtures of 1D carbon nanotubes and 2D nanosheets are important in electrochemical applications where the nanosheets are the active material, while the nanotubes provide electrical conductivity and...

Nanocomposite strain sensors, particularly those consisting of polymer-graphene composites, are increasingly common and are of great interest in the area of wearable sensors. In such sensors, application of strain yields an increase in resistance due to the effect of deformation on interparticle junctions. Typically, widening of interparticle separation is thought to increase the junction resistance by reducing the probability of tunnelling between conducting particles. However, an alternative approach would be to use piezoresistive fillers, where an applied strain modifies the intrinsic filler resistance and so the overall composite resistance. Such an approach would broaden sensing capabilities, as using negative piezoresistive fillers could yield strain-induced resistance reductions rather than the usual resistance increases. Here, we introduce nanocomposites based on polyethylene oxide (PEO) filled with MoS2 nanosheets. Doping of the MoS2 by the PEO yields nanocomposites which are conductive enough to act as sensors, while efficient stress transfer leads to nanosheet deformation in response to an external strain. The intrinsic negative piezoresistance of the MoS2 leads to a reduction of the composite resistance on the application of small tensile strains. However, at higher strain the resistance grows due to increases in junction resistance. MoS2-PEO composite gauge factors are approximately -25 but fall to -12 for WS2-PEO composites and roughly -2 for PEO filled with MoSe2 or WSe2. We develop a simple model, which describes all these observations. Finally, we show that these composites can be used as dynamic strain sensors.

Liquid phase exfoliation has become an important method for the production of large quantities of two-dimensional (2D) nanosheets. This method is versatile, having been used to produce dozens of different 2D materials in a range of stabilizing liquids. The resultant liquid-suspended nanosheets have been characterized in great detail and have been processed into a number of structures for a wide range of applications. This has led to a growing number of researchers adopting this method. As a result, best practice in terms of experimental procedure has evolved rapidly over recent years. As experimental complexity has increased, it has become more and more difficult to discuss the rational behind a chosen experimental procedure in full detail using standard “Methods” sections due to the frequent use of procedures developed in related prior reports. This can make it difficult to reproduce complex procedures and acts as a barrier to new researchers entering the field. To address this shortcoming, here we descr...

Increasing the energy storage capability of lithium-ion batteries necessitates maximization of their areal capacity. This requires thick electrodes performing at near-theoretical specific capacity. However, achievable electrode thicknesses are restricted by mechanical instabilities, with high-thickness performance limited by the attainable electrode conductivity. Here we show that forming a segregated network composite of carbon nanotubes with a range of lithium storage materials (for example, silicon, graphite and metal oxide particles) suppresses mechanical instabilities by toughening the composite, allowing the fabrication of high-performance electrodes with thicknesses of up to 800 μm. Such composite electrodes display conductivities up to 1 × 104 S m−1 and low charge-transfer resistances, allowing fast charge-delivery and enabling near-theoretical specific capacities, even for thick electrodes. The combination of high thickness and specific capacity leads to areal capacities of up to 45 and 30 mAh cm−2 for anodes and cathodes, respectively. Combining optimized composite anodes and cathodes yields full cells with state-of-the-art areal capacities (29 mAh cm−2) and specific/volumetric energies (480 Wh kg−1 and 1,600 Wh l−1). While thicker battery electrodes are in high demand to maximize energy density, mechanical instability is a major hurdle in their fabrication. Here the authors report that segregated carbon nanotube networks enable thick, high-capacity electrodes for a range of materials including Si and NMC.

Robust strain gauges are fabricated by growing PtSealt;subagt;2alt;/subagt; layers directly on top of flexible polyimide foils. These PtSealt;subagt;2alt;/subagt; layers are grown by low-temperature, thermally-assisted conversion of predeposited Pt layers. Under applied flexure the PtSealt;subagt;2alt;/subagt; layers show a decrease in electrical resistance signifying a negative gauge factor. The influence of the growth temperature and film thickness on the electromechanical properties of the PtSealt;subagt;2alt;/subagt; layers is investigated. The best-performing strain gauges fabricated have a superior gauge factor to that of commercial metal-based strain gauges. Notably, the strain gauges offer good cyclability and are very robust, surviving repeated peel tests and immersion in water. Furthermore, preliminary results indicate that the stain gauges also show potential for high-frequency operation. This host of advantageous properties, combined with the possibility of further optimization and channel patterning, indicate that PtSealt;subagt;2alt;/subagt; grown directly on polyimide holds great promise for future applications.