Search

Water-stable proteins may offer a new field of applications in smart materials for buildings and infrastructures where hydraulic reactions are involved. In this study, cement mortars modified through water-soluble silk fibroin (SF) are proposed. Water-soluble SF obtained by redissolving SF films in phosphate buffer solution (PBS) showed the formation of a gel with the β sheet features of silk II. Electrical measurements of SF indicate that calcium ions are primarily involved in the conductivity mechanism. By exploiting the water solubility properties of silk II and Ca2+ ion transport phenomena as well as their trapping effect on water molecules, SF provides piezoresistive and piezocapacitive properties to cement mortars, thus enabling self-sensing of mechanical strain, which is quite attractive in structural health monitoring applications. The SF/cement-based composite introduces a capacitive gauge factor which surpasses the traditional resistive gauge factor reported in the literature by threefold. Cyclic voltammetry measurements demonstrated that the SF/cement mortars possessed memcapacitive behavior for positive potentials near +5 V, which was attributed to an interfacial charge build-up modulated by the SF concentration and the working electrode. Electrical square-biphasic excitation combined with cyclic compressive loads revealed memristive behavior during the unloading stages. These findings, along with the availability and sustainability of SF, pave the way for the design of novel multifunctional materials, particularly for applications in masonry and concrete structures. [ABSTRACT FROM AUTHOR]

Silver nanowires (AgNWs) have emerged as a promising material for the development of wearable strain sensors due to their excellent electrical conductivity, optical transparency, and mechanical flexibility. This review comprehensively explores the recent advancements and trends in AgNW-based strain sensors for human motion detection. We discuss the fundamental aspects of stretchable strain sensors, the synthesis and properties of AgNWs, and the fabrication techniques for AgNW-based sensors, including solution processing, patterning, printing, and hybridization with other nanomaterials. The applications of these sensors in monitoring joint movements, physiological signals, and gesture recognition are highlighted. Furthermore, we address the challenges and future perspectives in this field, focusing on achieving high sensitivity and stretchability simultaneously, long-term stability and durability, scalable and cost-effective manufacturing, and the potential for multi-modal and self-powered sensing. With ongoing advancements in materials science and manufacturing technologies, AgNW-based strain sensors have the potential to revolutionize wearable and flexible electronics for various applications in healthcare, sports, and human-machine interfaces.

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]

Large structures need to be monitored to ensure safety for users and increase their service life through predictive maintenance. Self-sensing cementitious composites have been studied for structural health monitoring applications. By adding carbon nanotubes (CNTs) to a cement matrix, the resulting composite exhibits piezoresistive properties, making it capable of monitoring strain and mechanical stresses. However, due to their porous microstructure, the electrical properties of these cementitious composites are affected by moisture. This work studies the influence of water in the pores of a cement paste on the electrical resistivity, self-sensing capability, and electrification effects of cement pastes with different amounts of CNT. It was found that for pastes with lower amounts of CNT, the presence of water decreased electrical resistivity and increased electrical polarization effects and self-sensing sensitivity to strain, but not to failure. On the other hand, pastes with higher CNT content were less affected by the presence of water in the pores of the cementitious matrix. It was concluded that a minimum amount of CNT, above the percolation region, can minimize the effects of moisture variations on electrical properties. Practical Applications: The construction industry is one of the most traditional in the world. Nevertheless, it has undergone a significant transformation and evolution through the adoption of new materials. Nanotechnology has played a pivotal role in the development of cementitious materials with novel properties, such as self-healing, energy absorption, self-cleaning characteristics, and the ability to monitor deformations, among others. It is within this context that smart cementitious composites emerge, maintaining their pre-existing properties and functionalities while incorporating new ones. Among these, self-sensing cementitious composites are specialized cement-based materials designed to monitor the safety performance of construction structures. When incorporated into structures, they can correlate their electrical properties with the deformations experienced, providing real-time monitoring of the conditions of these buildings. Self-monitoring cementitious composites serve as sensors for monitoring pavements and large structures such as dams, bridges, and viaducts. Their significant advantage lies in being manufactured from the same material as most structures, ensuring excellent compatibility and greater durability compared to other commercial sensors. [ABSTRACT FROM AUTHOR]

In this work, the effect of multi-walled carbon nanotubes (MWCNT1, MWCNT2, and MWCNT3) with different outer diameters and specific surface areas on the mechanical and electrical properties of cement mortar have been investigated. Various concentrations of MWCNTs were used (0.05, 0.10, and 0.15%), the effective dispersion of which was carried out by an Ultrasonic machine (for 40 min with 160 W power and a 24 kHz frequency) using a surfactant. Composites have been processed with a biosilica content of 10% by weight of cement and without it. Compressive strength tests were carried out on days 7 and 28 of curing. The 7-day compressive strength of samples prepared without biosilica increased compared to the result of the control sample (6.4% for MWCNT1, 7.4% for MWCNT2, and 10.8% for MWCNT3), as did those using biosilica (6.7% in the case of MWCNT1, 29.2% for MWCNT2, and 2.1% for MWCNT3). Compressive strength tests of 28-day specimens yielded the following results: 21.7% for MWCNT1, 3.8% for MWCNT2, and 4.2% for MWCNT3 in the absence of biosilica and 8.5%, 12.6%, and 6.3% with biosilica, respectively. The maximum increase in compressive strength was observed in the composites treated with a 0.1% MWCNT concentration, while in the case of 0.05 and 0.15% concentrations, the compressive strengths were relatively low. The MWCNT-reinforced cement matrix obtained electrical properties due to the high electrical conductivity of these particles. The effect of MWCNT concentrations of 0.05, 0.10, and 0.15 wt% on the electrical properties of cement mortar, especially the bulk electrical resistivity and piezoresistive characteristics of cement mortar, was studied in this work. At a concentration of 0.05%, the lowest value of resistivity was obtained, and then it started to increase. The obtained results show that all investigated specimens have piezoresistive properties and that the measurements led to a deviation in fractional change in resistivity. [ABSTRACT FROM AUTHOR]

Strain sensors have attracted great attentions for practical applications such as human-machine interface. It is challenging to model the electrical behavior of elastomeric strain sensors modified with nanomaterials. This study presents a finite element simulation to explore the piezoresistivity of graphene nanoplatelet (GNP)-filled elastomeric nanocomposites. The percolation network model is employed to determine the critical distance necessary for electrical percolation. The finite element percolation network model uses the Euclidian distance between GNPs distributed inside the representative volume element to calculate the resistivity. The influence of various parameters, including GNP alignment direction, aspect ratio, and volume fraction, on the resistivity changes of the nanocomposite is investigated. Results demonstrate that nearly 20% more tunneling distance is allowed for percolation of nanocomposite with 30% larger aspect ratio GNPs. Findings reveal that as the volume fraction of GNPs increases, the critical distance for the electrical percolation decreases significantly.

Cement-based sensors have attracted considerable attention due to their potential for self-sensing applications, particularly in the realms of structural health monitoring and traffic management, as they eliminate the need for external sensors. However, the performance of these sensors can be adversely affected by various environmental factors, with water ingress being a particularly critical concern that can destabilize their piezoresistive sensing capabilities. In this study, hollow glass microspheres (HGM), recognized for their superhydrophobic properties, were incorporated into cement-based sensors to improve their piezoresistive sensing stability under conditions of water ingress. Carbon nanotubes (CNT) were employed as the conductive filler, and the hybrid effects of CNT and HGM were systematically investigated using a range of characterization techniques. The study experimentally assessed the impact of HGM on the compressive strength, electrical resistivity, and water absorption characteristics of the cementitious composites. Furthermore, the enhancement in sensing performance due to HGM was evaluated with respect to sensitivity, linearity, and stability under both monotonic and cyclic loading scenarios. The experimental findings revealed that while HGM initially reduces water absorption, this effect diminishes over prolonged periods of water exposure. Additionally, HGM prevents CNT-based conductive networks from water ingress, thereby improving sensing stability after water exposure. Consequently, the utilization of HGM is anticipated to enhance the sensing stability of cement-based sensors under water ingress conditions.

Abstract The greener alternatives to tactile‐integrated multimodal sensors with self‐powered and self‐healing abilities are highly desirable for all‐in‐one autonomous sensing systems, particularly impressive in diverse application ranges including smart home, healthcare, and e‐skin. The dynamically self‐healable, stretchable piezoresistive sensors, and triboelectric nanogenerators (TENGs) reported herein are constructed by a facile, industrially viable method of grafting imidazolium ions on epoxidized natural rubber (ENR) backbone. Owing to cation‐π and π–π interaction between the percolated carbon nanotubes (CNTs)‐network and the imidazolium ions formed by non‐covalent interactions, the interfacial adhesion between the filler and elastomer is shown to improve considerably. The sensors show high piezoresistive strain sensitivity, reversible ionic network‐assisted self‐healability (efficiency ≈80%) and wide‐ranging detectability for precise monitoring of human movements. Both the healed and pristine sensors feature low hysteresis and stable electrical outputs over a wide strain range (≤200%). While achieving rapid self‐healing efficiency, the substrates are shown to exhibit remarkable robustness for harsh climates owing to significant mechanical toughness. Supported by excellent triboelectric tactile sensitivity (2.12 V N−1), the multifunctional TENG‐enabled sensor yields superior power density (0.16 mW cm−2). Moreover, the TENG module exhibits high force sensitivity and ease of operation that are considered versatile for all‐weather integrated tactile solutions for future technology.

Pressure sensors for high temperatures are extensively applied in a variety of industries processes including aerospace, energy conversion, petrochemicals, automotive and oil wells, real-time pressure detection during equipment operation is a critical concern. Silicon is widely used for piezoresistive sensing with good sensitivity, however, harsh environments with temperatures above 150 °C will lead to plastic deformation as well as current leakage. In this work, AlN/SiCN hybrid thin films systems with ultra-high piezoresistive performance are proposed and enabled by coupling with piezoelectric effect. The gauge factors of the AlN/SiCN sensing elements are from 3260 to 21428 upon different stresses, which are significantly greater than those of other piezoresistive ceramics. The piezoresistive performance of SiCN upon compressive stress is significantly enhanced by incorporation piezoelectric effect of AlN. Moreover, the interfacial strengths of the AlN/SiCN thin films are larger than those of single-layer SiCN, demonstrating that incorporation of AlN layers also improves interfacial strength of SiCN films. This work provides a crucial basis for designing novel piezoresistive sensors. [Display omitted] [ABSTRACT FROM AUTHOR]

This work presents the design, fabrication and characterization of a nanocomposite-polymer (piezoresistive) based pressure sensor for low-pressure measurement. The developed piezoresistive pressure sensor consists of a PDMS diaphragm and piezoresistor sensing element. The sensing element is made of PDMS/Graphite composite, which works as a piezoresistor active element. The ultrasonic mixing followed by degasification, spin coating and curing is used to fabricate the PDMS diaphragm and PDPM/Graphite piezoresistive composite. The characterization of the pure PDMS and PDMS/Graphite composite with different graphite filler concentrations is done before assembling them to fabricate the pressure-sensing device. The PDPM/Graphite composite shows a higher elastic modulus compared to the pure PDMS and the electrical conductivity of the composite also increases with the increment of the graphite filler in it. After the characterization of the constituents of the pressure sensing device, the device is characterized for low-pressure sensing. [ABSTRACT FROM AUTHOR]

Effects of varying Mn and Ni concentrations on the structure and piezoresistive properties of CuMnNi films deposited by magnetron sputtering with a segmented target were investigated. An increase in the Ni content refines the CuNi film grains, inducing an increase in defects such as internal micropores and a decrease in film density. At the same time, the positive piezoresistive coefficient of the film changes to negative. When 17.5 at.% Ni was added, the negative piezoresistive coefficient of the CuNi film was −2.0 × 10−4 GPa−1. The doping of Ni has a weakening effect on the positive piezoresistive effect of the film. Adding Mn into Cu refines the film grains while increasing the film density. The surface roughness of the film decreases with the increase in Mn content. When the Mn content was 16.7 at.%, the piezoresistive coefficient reached the largest recorded value of 23.81 × 10−4 GPa−1, and the film exhibited excellent repeatability in multiple piezoresistive tests. After the CuMn film with 16.7 at.% Mn was annealed at 400 °C for 2 h, the film grains grew slightly and the film residual stress decreased. The optimization of the film structure can reduce the scattering of electrons during transportation. The piezoresistive coefficient of the film was further improved to 35.78 × 10−4 GPa−1. [ABSTRACT FROM AUTHOR]

Development of smart nanocomposites by reinforcing carbon nanomaterials into polymer composite materials has gained significant attention due to their extensive potential applications, especially in the field of monitoring of mechanical, aerospace and infrastructure systems. These smart nanocomposites exhibit excellent electro-mechanical sensing capability. Carbon nanotubes (CNTs) with outstanding electrical, mechanical, and thermal properties can be reinforced into polymer composite materials by appropriate dispersion to produce polymer/CNTs-based smart nanocomposites with excellent sensitivity. Understanding the processing, characterization and properties of the nanocomposites will help in yielding a deeper insight into the effect of incorporation of carbon nanotubes (CNTs) into the matrix of the polymer composites. This study provides a comprehensive overview (primarily in the last 2 decades) of the major influencing factors on the underlying mechanisms involved in the fabrication and performance of nanocomposites which can cause a paradigm shift in instrumentation and sensing for structural health monitoring (SHM). Therefore, this review article presents the recent advancement in the development of CNTs-reinforced polymer nanocomposites-based sensors encompassing the mechanism for electrical conductivity/piezo properties, fabrication, applications for various types of sensing, and the futuristic application for smart sensing through appropriate tailoring of nanocomposites using CNTs. [ABSTRACT FROM AUTHOR]

In this study, hybrid functional fillers were used to create an electrical network that is used to develop self-damage sensing ability within conventional cementitious mixtures. This electrical network was used to examine the self-sensing properties of cementitious composites under different loading scenarios such as compression, splitting tensile, and cyclic loading for three ages, those are 7, 14, and 28. With the help of a single type of functional filler or fiber, the self-sensing capabilities of the majority of previous works are demonstrated. This study incorporates two types of functional fillers for sustainability and low cost: micro-scale carbon fibers (CF) and waste iron (Ir) powder in the form of microparticles. The purpose of the current work is to fill up the subject’s gap using two different types of functional fillers as a hybrid form. Three hybrid proportions of a micro-scale CF (vol% of mixture) and waste iron powder (Ir) (wt% of cementitious materials) are utilized. These are (0.33, 15), (0.67, 10), and (1.00, 5), respectively. Unlike carbon-based materials, polyvinyl alcohol is used as a mechanical reinforcing fiber 2% by volume of the cementitious mixture. Additionally, a control combination without any fillers has been created. The electrical resistivity of the fabricated samples was monitored during various loading applications for every second to confirm their self-sensing capabilities. Regarding the fractional change in electrical resistivity, the self-sensing behavior was superior for mixes containing high dosages either of waste iron powder or CF in the same hybrid matrix. Quite the contrary to that, the piezoresistivity was modest in the middle hybrid ratio.

Water-stable proteins may offer a new field of applications in smart materials for buildings and infrastructures where hydraulic reactions are involved. In this study, cement mortars modified through water-soluble silk fibroin (SF) are proposed. Water-soluble SF obtained by redissolving SF films in phosphate buffer solution (PBS) showed the formation of a gel with the β sheet features of silk II. Electrical measurements of SF indicate that calcium ions are primarily involved in the conductivity mechanism. By exploiting the water solubility properties of silk II and Ca2+ ion transport phenomena as well as their trapping effect on water molecules, SF provides piezoresistive and piezocapacitive properties to cement mortars, thus enabling self-sensing of mechanical strain, which is quite attractive in structural health monitoring applications. The SF/cement-based composite introduces a capacitive gauge factor which surpasses the traditional resistive gauge factor reported in the literature by threefold. Cyclic voltammetry measurements demonstrated that the SF/cement mortars possessed memcapacitive behavior for positive potentials near +5 V, which was attributed to an interfacial charge build-up modulated by the SF concentration and the working electrode. Electrical square-biphasic excitation combined with cyclic compressive loads revealed memristive behavior during the unloading stages. These findings, along with the availability and sustainability of SF, pave the way for the design of novel multifunctional materials, particularly for applications in masonry and concrete structures.

Conductive polymer composites (CPCs), obtained by incorporating conductive fillers into a polymer matrix, are suitable for producing strain sensors for structural health monitoring (SHM) in infrastructure. Here, the effect of the addition of inorganic semiconductor nanoparticles (INPs) to a poly (vinylidene fluoride) (PVDF) composite filled with multi-walled carbon nanotubes (MWCNTs) on the piezoresistive behavior is investigated. INPs with different morphologies and sizes are synthesized by a hydrothermal method. The added inorganic oxide semiconductors showed two distinct morphologies, including different phases. While particles with flower-like plate morphology contain phases of orth-ZnSnO3 and SnO, the cauliflower-like nanoparticles contain these metal oxides and ZnO. The nanoparticles are characterized by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD), and the nanocomposites by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Cyclic tensile testing is applied to determine the strain-sensing behavior of PVDF/1 wt% MWCNT nanocomposites with 0–10 wt% inorganic nanoparticles. Compared to the PVDF/1 wt% MWCNT nanocomposite, the piezoresistive sensitivity is higher after the addition of both types of nanoparticles and increases with their amount. Thereby, nanoparticles with flower-like plate structures improve strain sensing behavior slightly more than nanoparticles with cauliflower-like structures. The thermogravimetric analysis results showed that the morphology of the semiconductor nanoparticles added to the PVDF/MWCNT matrix influences the changes in thermal properties. [ABSTRACT FROM AUTHOR]

In this work, the effect of multi-walled carbon nanotubes (MWCNT1, MWCNT2, and MWCNT3) with different outer diameters and specific surface areas on the mechanical and electrical properties of cement mortar have been investigated. Various concentrations of MWCNTs were used (0.05, 0.10, and 0.15%), the effective dispersion of which was carried out by an Ultrasonic machine (for 40 min with 160 W power and a 24 kHz frequency) using a surfactant. Composites have been processed with a biosilica content of 10% by weight of cement and without it. Compressive strength tests were carried out on days 7 and 28 of curing. The 7-day compressive strength of samples prepared without biosilica increased compared to the result of the control sample (6.4% for MWCNT1, 7.4% for MWCNT2, and 10.8% for MWCNT3), as did those using biosilica (6.7% in the case of MWCNT1, 29.2% for MWCNT2, and 2.1% for MWCNT3). Compressive strength tests of 28-day specimens yielded the following results: 21.7% for MWCNT1, 3.8% for MWCNT2, and 4.2% for MWCNT3 in the absence of biosilica and 8.5%, 12.6%, and 6.3% with biosilica, respectively. The maximum increase in compressive strength was observed in the composites treated with a 0.1% MWCNT concentration, while in the case of 0.05 and 0.15% concentrations, the compressive strengths were relatively low. The MWCNT-reinforced cement matrix obtained electrical properties due to the high electrical conductivity of these particles. The effect of MWCNT concentrations of 0.05, 0.10, and 0.15 wt% on the electrical properties of cement mortar, especially the bulk electrical resistivity and piezoresistive characteristics of cement mortar, was studied in this work. At a concentration of 0.05%, the lowest value of resistivity was obtained, and then it started to increase. The obtained results show that all investigated specimens have piezoresistive properties and that the measurements led to a deviation in fractional change in resistivity.

This study examines the piezoresistive and mechanical behaviors of multiwalled carbon nanotube (MWCNT)/polyamide (PA12) lattice composites processed via selective laser sintering (SLS). Firstly, neat PA12 powder was ballmilled with either 0.3, 0.5, or 1.0 wt% MWCNT loading to obtain nanoengineered powder feedstocks. The feedstocks were used to realize electrically conductive MWCNT/PA12 bulk and honeycomb lattice structures via SLS. The piezoresistive and mechanical characteristics of bulk samples were assessed under tensile, compressive, and flexure loadings while those of honeycombs were investigated under in-plane and out-of-plane compression. The morphological and thermal characterizations were carried out to gain insight into the processing-structure-property relations. The microstructural observations, including in situ μ-CT imaging, revealed that the porosity of the sintered parts significantly increased (≈150%) with increasing MWCNT loading, resulting in reduced elastic stiffness and strength of the sintered composites, despite their slightly higher crystallinity (+6% for 0.5 wt% MWCNT loading). The bulk nanocomposites exhibited excellent piezoresistive characteristics, showing a maximum gauge factor of 31 (in compression), which is significantly higher than the sensitivity factors of extant piezoresistive self-sensing composites. The honeycomb structures showed a maximum specific energy absorption of 3 and 24 J g−1 at 40% relative density under in-plane and out-of-plane compression loading, respectively. It was also demonstrated, that the MWCNT/PA12 honeycombs possess excellent strain sensing characteristics due to their pronounced piezoresistivity, reporting gauge factors up to 25 and 17 for in-plane and out-of-plane loading, respectively.

Veterinary species cilia sense mechanical vibrations and movements in air as well as in water. They allow fish to locate predators and find prey. Cilium participates in detection of sound vibration phonons and determines underwater pathways. In this paper a hybrid software tool packages are used to study, analyze, and determine optimum dimensions of MEMS sensor imitating fish cilia.The availability of powerful Finite-element software such as COMSOL and the new software generations of computer make it possible to investigate the performance of different types of sensor structures and to propose new ones. MEMS Piezoresistive Vector Hydrophone (MPVH) is one of the recent generations of underwater vibration detection and surveillance devices. In this paper basic design structure of MPVH that is found in many research works is modeled and investigated from points of view of sensor parameters, namely sensor sensitivity, frequency response and linearity. A hybrid optimization technique is deduced to attain sensor's best element dimension and performance.

The development of low-cost biodegradable pressure or force sensors based on a carrageenan and iron (III) oxide mix is a promising way to foster the spread of green technologies in sensing applications. The proposed materials are inexpensive and abundant and are available in large quantities in nature. This paper presents the development and experimental study of carrageenan and iron (III)-oxide-based piezoresistive sensor prototypes and provides their main characteristics. The results show that glycerol is required to ensure the elasticity of the material and preserve the material from environmental impact. The composition of the carrageenan-based material containing 1.8% Fe2O3 and 18% glycerol is suitable for measuring the load in the range from 0 N to 500 N with a sensitivity of 0.355 kΩ/N when the active surface area of the sensor is 100 mm2. Developed sensors in the form of flexible film have square resistance dependence to the force/pressure, and due to the soft original material, they face the hysteresis effect and some plastic deformation effect in the initial use stages. This paper contains extensive reference analysis and found a firm background for a new sensor request. The research covers the electric and mechanical properties of the developed sensor and possible future applications. [ABSTRACT FROM AUTHOR]

A three-dimensional theory has been established for the piezoresistivity of carbon nanotube (CNT) polymer composites. Based on the Mori–Tanaka method in meso-mechanics theory and considering quantum tunneling effect between CNTs, an approach to calculate equivalent electrical conductivity of composites was proposed. On this basis, a piezoresistive theory, which incorporates the effect of composites' geometric nonlinearity, was developed for CNT polymer composites. The theory is dependent only on some basic physical parameters of the materials. A finite element formula of the theory for the numerical calculation of piezoresistivity was presented from the analysis of both elastic and electric fields. Numerical simulations demonstrated that the results predicted by the theory were in good agreement with those of the experimental tests. Parameter sensitivity analysis revealed that when both the potential barrier height of the matrix and the initial average separation distance between CNTs increased, the piezoresistivity obviously increased. However, with the increase in aspect ratio and CNT conductivity, the piezoresistivity decreased gradually. A practical engineering application of this theory is also provided. [ABSTRACT FROM AUTHOR]

This paper aims to explore the structural performance of 3D-printed and casted cement-based steel-reinforced concrete beams and one-way slabs incorporating short carbon fibre and activated carbon powder, which have been shown to enhance concrete's flexural strength and reduce its electrical resistivity. The samples are cast and printed in 250 × 325 × 3500 mm beams and 150 × 400 × 3500 mm one-way slabs and mechanical, electrical, and piezoresistivity properties were measured. This length of beams and one-way slabs with rebars have been considered as they can magnify the flexural and cracking behaviour and make them easier to be monitored and analysed. The samples were loaded up to 80% of maximum stress. Crack propagation and strain was assessed using the 2D digital image correlation (DIC) method. The results compared samples under continuously increasing loads between 3D-printed and cast samples. The 3D-printed composites had a better piezoresistive response due to the enhanced anisotropic behaviour. DIC analysis illustrated similar results among different samples, while 3D-printed blocks had lower cracking performance due to the horizontal case fracture in lower stress. [ABSTRACT FROM AUTHOR]

Vat photopolymerization-based additive manufacturing (AM) is critical in improving solutions for wearable sensors. The ability to add nanoparticles to increase the polymer resin's mechanical, electrical, and chemical properties creates a strong proposition for investigating custom nanocomposites for the medical field. This work uses a low-cost biocompatible polymer resin enhanced with multi-walled carbon nanotubes (MWCNTs), and a digital light processing-based AM system to develop accurate strain sensors. These sensors demonstrate the ability to carry a 244% maximum strain while lasting hundreds of cycles without degradation at lower strain ranges. In addition, the printing process allows for detailed prints to be accomplished at a sub-30 micron spatial resolution while also assisting alignment of the MWCNTs in the printing plane. Moreover, high-magnification imagery demonstrates uniform MWCNT dispersion by utilizing planetary shear mixing and identifying MWCNT pullout at fracture locations. Finally, the proposed nanocomposite is used to print customized and wearable strain sensors for finger motion monitoring and can detect different amounts of flexion and extension. The 3D printed nanocomposite sensors demonstrate characteristics that make it a strong candidate for the applications of human kinematics monitoring and sensing. [ABSTRACT FROM AUTHOR]

In pressure sensors, four piezoresistors connected in a Wheatstone bridge are often provided with a voltage varying between 5 and 10 V. Unfortunately, this voltage is a source of drifts created by electric heating. This study focuses on the internal heating and piezoresistive effect of piezoresistors represented by the variation of their resistivity. To do this, we use the finite difference method (FDM) to solve the heat transfer equation, taking into account the conduction in Cartesian coordinates for the variable regime. We examine how the temperature affects the piezoresistivity in these sensors when the potential is applied. In this case, the variation of the temperature has been calculated as a function of applied voltage, as well as for the operating time of the sensor. Furthermore, the evolution of resistivity over time was determined for several geometric properties of the membrane using the mobility model. This was established for different doping levels. Additionally, the change in resistivity due to the application of voltage was evaluated. It was observed that resistivity is greatly affected by the temperature rise produced by the applied voltage when the device is actuated for a prolonged time. Consequently, this results in drifting in the output response of the sensor. [ABSTRACT FROM AUTHOR]

The multifunctional features of pressure sensitivity and energy-absorption capability of flexible electromechanical sensors are desirable for practical applications, such as personal protection, crash mitigation, and protective packaging of sensitive elements. However, there are still challenges in developing ultrasensitive pressure sensors with high energy absorption capability. Herein, a bio-inspired flexible piezoresistive MWCNTs/PDMS composite with a multilayered architecture mimicking the ant tentacle and the pomelo peel is innovatively constructed, which can simultaneously detect physical stimuli in an extensive pressure range and absorb impact energy. The composite architecture is composed of the bottom-layer interlocked tentacle-like conical micropillars and the top-layer pomelo peel-inspired hierarchically enclosed-porous microstructure, which has successfully wrapped air inside to form a solid-gas dual phase structure with enhanced energy absorbing capability and piezoresistivity. The composite architecture was manufactured by a laser-engraving method associated with the controlled solidification process of the composite by regulating the vacuum pumping rate in the pre-curing condition. Owing to the hollow-shaped tentacle-like conical micropillars, the contact area can be dramatically increased within a subtle pressure loading, leading to superb pressure sensitivity of ∼26.1 kPa−1. Simultaneously, the hierarchically enclosed-porous structure with trapped air (gas-phase), acting as an air cushion packaging, imitating the pomelo peel provides the composite architecture with exceptional mechanical energy absorption of ∼2.74 MJ/m3 and an extensive pressure sensing range from 0.1 Pa to 60 kPa. As a result, the as-proposed composite sensor was able to detect mechanical signals such human finger tapping, wrist flexion, and pendulum hammer impact. Numerical simulations also showed that the wrapped air inside the hierarchical enclosed-porous structure improved the mechanical damping properties of the composite architecture. It is envisioned that our finding could contribute to the development of multifunctional materials to meet the growing demands for next-generation electronic systems, inspired by sophisticated architectures from nature.

Effects of varying Mn and Ni concentrations on the structure and piezoresistive properties of CuMnNi films deposited by magnetron sputtering with a segmented target were investigated. An increase in the Ni content refines the CuNi film grains, inducing an increase in defects such as internal micropores and a decrease in film density. At the same time, the positive piezoresistive coefficient of the film changes to negative. When 17.5 at.% Ni was added, the negative piezoresistive coefficient of the CuNi film was −2.0 × 10−4 GPa−1. The doping of Ni has a weakening effect on the positive piezoresistive effect of the film. Adding Mn into Cu refines the film grains while increasing the film density. The surface roughness of the film decreases with the increase in Mn content. When the Mn content was 16.7 at.%, the piezoresistive coefficient reached the largest recorded value of 23.81 × 10−4 GPa−1, and the film exhibited excellent repeatability in multiple piezoresistive tests. After the CuMn film with 16.7 at.% Mn was annealed at 400 °C for 2 h, the film grains grew slightly and the film residual stress decreased. The optimization of the film structure can reduce the scattering of electrons during transportation. The piezoresistive coefficient of the film was further improved to 35.78 × 10−4 GPa−1.

Concrete structures are susceptible to several factors during their life cycles, which can cause various problems. This situation can be avoided in many cases if Structural Health Monitoring (SHM) is used. However, such monitoring is often expensive due to the large number of sensors and the use of data acquisition systems (DAQ). In this context, self-sensing cement composites (SSCCs), monitoring themselves without sensors by incorporating conductive fillers in their composition, provide a better piezoresistive effect. In this research, the Arduino platform and analog to digital converters (ADC) modules were tested, and a prototype of a low-cost DAQ was developed for monitoring SSCCs. The devices were analyzed from the compression tests on the mortar specimens, and the results were compared to measurements obtained from the reference equipment (professional DAQ). The results showed a good sensitivity and an adequate correlation between both devices in some cases. It showed that the Arduino platform has the potential to be used in the experimental monitoring of SSCCs, however it needs to be combined with an ADC module.

The increasing interest in wearable devices for health monitoring, illness prevention, and human motion detection has driven research towards developing novel and cost-effective solutions for highly sensitive flexible sensors. The objective of this work is to develop innovative piezoresistive pressure sensors utilizing two types of 3D porous flexible open-cell foams: Grid and triply periodic minimal surface structures. These foams will be produced through a procedure involving the 3D printing of sacrificial templates, followed by infiltration with various low-viscosity polymers, leaching, and ultimately coating the pores with graphene nanoplatelets (GNPs). Additive manufacturing enables precise control over the shape and dimensions of the structure by manipulating geometric parameters during the design phase. This control extends to the piezoresistive response of the sensors, which is achieved by infiltrating the foams with varying concentrations of a colloidal suspension of GNPs. To examine the morphology of the produced materials, field emission scanning electron microscopy (FE-SEM) is employed, while mechanical and piezoresistive behavior are investigated through quasi-static uniaxial compression tests. The results obtained indicate that the optimized grid-based structure sensors, manufactured using the commercial polymer Solaris, exhibit the highest sensitivity compared to other tested samples. These sensors demonstrate a maximum sensitivity of 0.088 kPa−1 for pressures below 10 kPa, increasing to 0.24 kPa−1 for pressures of 80 kPa. Furthermore, the developed sensors are successfully applied to measure heartbeats both before and after aerobic activity, showcasing their excellent sensitivity within the typical pressure range exerted by the heartbeat, which typically falls between 10 and 20 kPa. [ABSTRACT FROM AUTHOR]

The gauge factor of strain gauges depends on several factors such as the geometry of the gauge and the piezoresistive properties of the materials used. Herein, the piezoresistive response of a strain gauge comprising a network of carbon nanotubes (CNTs) deposited over a polyimide (Kapton) substrate is theoretically and experimentally investigated. Two computational approaches are used, namely, a continuous finite element model and a multiscale model comprising two dimensional scales. The finite element model is a simplified model which simulates the CNT grid as a continuous solid. The multiscale approach utilizes a resistor network model to simulate the change in electrical resistance of the CNT grid with strain and uses such effective electrical resistance as an input of the finite element model. It is found that the gauge factor increases with increased number of end loops, decreasing grid width, and decreasing the end loop height. Both modeling approaches agree with the observed experimental tendencies. The multiscale approach is more realistic and accounts for effects occurring at the micro and nanoscales, such as electron tunneling. The gauge factors predicted by this model were moderately higher than the measured values, given some idealizations of the model. [ABSTRACT FROM AUTHOR]

In general, the electrical property of soft tissues is sensitive to the force applied to their surface. To further study the relationship between the force and the electrical property of soft tissues, this paper attempts to investigate the effect of static and higher-order stresses on electrical properties. Overall, a practical experimental platform is designed to acquire the force information and the electrical property of soft tissues during a contact procedure, which is featured different compression stimuli, such as constant pressing force, constant pressing speed, and step-force compression, etc. Furthermore, the piezoresistive characteristic is innovatively introduced to model the mechanical-electrical properties of soft tissue. Finite Element Modeling (FEM) is adopted to fit the static piezoresistivity of the soft tissue. Finally, experimental studies were performed to demonstrate the effect of stress on the electrical properties and the feasibility of the proposed piezoresistive model to describe soft tissues' mechanical and electrical properties. [ABSTRACT FROM AUTHOR]

Flexible and stretchable electronics have emerged as highly promising technologies for the next generation of electronic devices. These advancements offer numerous advantages, such as flexibility, biocompatibility, bio-integrated circuits, and light weight, enabling new possibilities in diverse applications, including e-textiles, smart lenses, healthcare technologies, smart manufacturing, consumer electronics, and smart wearable devices. In recent years, significant attention has been devoted to flexible and stretchable pressure sensors due to their potential integration with medical and healthcare devices for monitoring human activity and biological signals, such as heartbeat, respiratory rate, blood pressure, blood oxygen saturation, and muscle activity. This review comprehensively covers all aspects of recent developments in flexible and stretchable pressure sensors. It encompasses fundamental principles, force/pressure-sensitive materials, fabrication techniques for low-cost and high-performance pressure sensors, investigations of sensing mechanisms (piezoresistivity, capacitance, piezoelectricity), and state-of-the-art applications. [ABSTRACT FROM AUTHOR]

In this study, we investigated the capacitive effect and the electromagnetic coupling on the measurement chain induced by impact experiments with a gas gun or powder gun. Reduced bandwidth and noise were noticed on experimental signals. Rogowski coil measurements were added on the cables to characterize the electromagnetic coupling. The perturbation currents on the cables were quantified depending on the configuration. The gauge, the transmission line and the conditioning system were modeled. The calculations reproduced the electrical wave arrival time, the transmission line transfer impedance and the conditioning system transfer impedance; and the bandwidth limitation has been displayed. A capacitive effect with the piezoresistive manganin gauge embedded into the sample was identified, depending on the experimental setup. [ABSTRACT FROM AUTHOR]

During this study, the resistivity of electrically conductive structures 3D-printed via fused filament fabrication (FFF) was investigated. Electrical resistivity characterisation was performed on various structural levels of the whole 3D-printed body, starting from the single traxel (3D-printed single track element), continuing with monolayer and multilayer formation, finalising with hybrid structures of a basic nonconductive polymer and an electrically conductive one. Two commercial conductive materials were studied: Proto-Pasta and Koltron G1. It was determined that the geometry and resistivity of a single traxel influenced the resistivity of all subsequent structural elements of the printed body and affected its electrical anisotropy. In addition, the results showed that thermal postprocessing (annealing) affected the resistivity of a standalone extruded fibre (extruded filament through a printer nozzle in freefall) and traxel. The effect of Joule heating and piezoresistive properties of hybrid structures with imprinted conductive elements made from Koltron G1 were investigated. Results revealed good thermal stability within 70 °C and considerable piezoresistive response with a gauge factor of 15–25 at both low 0.1% and medium 1.5% elongations, indicating the potential of such structures for use as a heat element and strain gauge sensor in applications involving stiff materials and low elongations. [ABSTRACT FROM AUTHOR]

Soft multi-axis force sensors are essential in automation and robotic manipulations for dexterous applications. Carbon-based composites are quality candidates for sensing elements of soft multi-axis force sensors due to its piezoresistivity originated from electron tunneling effect. However, low cost and fast-prototyped soft multi-axis force sensors based on tunneling effect of carbon-based composites are still in their infant. This work investigated a soft multi-axis forces sensor utilizing Velostat film strips as the core sensing elements. A unique serpentine structure of Velostat strips has been designed and implemented as piezoresistors for the soft force sensor. The sensitivity of the proposed multi-axis force sensor has been characterized in x, y and z directions. Experimental results show that the sensor can achieve 2.1%/N and 2.2%/N in x and y axis, and 4.2%/N in z axis, respectively. The force sensor has been implemented in dexterous manipulations with a robotic gripper as the proof-of-concept, which demonstrates great potential for multi-axis force sensing applications.

Cement mortars (CM) containing different contents and sizes of multiwalled carbon nanotubes (MWCNTs) were prepared using surfactants in combination with ultrasonic dispersion. The effects of the content, diameter and length of MWCNTs on the electrical conductivity of cement mortar and the self-inductive piezoresistive rate under single and cyclic loading were studied using the four-electrode method. The results show that the conductive percolation thresholds of CMs containing MWCNTs with different length–diameter ratios occur at an approximate MWCNT content of 0.1 wt%. The electrical conductivity of cement mortars with MWCNT diameters of 10–20 nm was the largest for a content of 0.1 wt%, and the values were 112% and 128.3% of the conductivity of the MWCNTs/CM with diameters of 20–40 nm and 40–60 nm, respectively. In addition, the fractional change in resistivity of the cement mortar specimens varied most significantly at a content of 0.75 wt% of MWCNTs; the fractional change in resistivity reached 14.27%. Adding small-diameter MWCNTs to cement mortar can maximise the sensitivity of electrical resistance of cement mortar to compressive stress. [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]

Smart flexible materials with piezoresistive property are increasingly used in the field of sensors. When embedded in structures, they would allow for in situ structural health monitoring and damage assessment of impact loading, such as crash, bird strikes and ballistic impacts; however, this could not be achieved without a deep characterization of the relation between piezoresistivity and mechanical behavior. The aim of this paper is to study the potential use of the piezoresistivity effect of a conductive foam made of a flexible polyurethane matrix filled with activated carbon for integrated structural health monitoring (SHM) and low-energy impact detection. To do so, polyurethane foam filled with activated carbon, namely PUF-AC, is tested under quasi-static compressions and under a dynamic mechanical analyzer (DMA) with in situ measurements of its electrical resistance. A new relation is proposed for describing the evolution of the resistivity versus strain rate showing that a link exists between electrical sensitivity and viscoelasticity. In addition, a first demonstrative experiment of feasibility of an SHM application using piezoresistive foam embedded in a composite sandwich structure is realized by a low-energy impact (2 J) test. [ABSTRACT FROM AUTHOR]

Cementitious composites reinforced with conductive nanoparticles, also known as smart composite-based sensors, are emerging as novel sensors for real-time monitoring of structures. The present study deals with the development of embeddable smart composite-based sensor (called as e SCS), for monitoring the damage evolution in large-scale beam-column (B-C) sub-assemblages. Firstly, a systematic approach has been developed to fabricate the sensors through incorporation of functionalized (-COOH) multi-walled carbon nanotubes (MWCNTs). The optimum dosage of CNTs required for developing sensors with maximum conductivity and piezo-resistivity was ascertained by determining the percolation threshold limit. The best performing e SCS was then successfully embedded at the critical locations of full-scale B-C joint specimens. The embedded sensors are used, for the first time, for damage monitoring of the B-C sub-assemblages subjected to reverse cyclic loadings. From the study, it is found that e SCS with 0.50 wt.% COOH-MWCNT shows excellent piezoresistive behaviour and gauge factors are found to be approximately 730, 457 and 396 for the applied compressive stress ranges of 2–4, 2–6 and 2–8 MPa, respectively. The embedded sensors showed the excellent strain sensing capability under reverse cyclic loading. The present study demonstrates the suitability of embedded e SCS sensors for robust structural health monitoring applications. [ABSTRACT FROM AUTHOR]

Conductive polymer composites (CPCs) have shown potential for structural health monitoring applications based on repeated findings of irreversible transducer electromechanical property change due to fatigue. In this research, a high-fidelity stochastic modeling framework is explored for predicting the electromechanical properties of spherical element-based CPC materials at bulk scales. CPC dogbone specimens are manufactured via casting and their electromechanical properties are characterized via uniaxial tensile testing. Model parameter tuning, demonstrated in previous works, is deployed for improved simulation fidelity. Modeled predictions are found in agreement with experimental results and compared to predictions from a popular analytical model in the literature. [ABSTRACT FROM AUTHOR]

Carbon nanotube polymer composites exhibit piezoresistivity that can be utilized for flexible strain sensors. Piezoresistive modeling is critical for improving sensor performance, but remains a challenging subject due to the complex CNT network microstructure and the various effects occurring at both the micro and nano scales. Tunneling conduction between CNTs is the primary mechanism underlying piezoresistivity in carbon nanotube polymer composites. Existing models have employed a myriad of assumptions on the CNT network kinematics and the tunneling conduction behavior, but their physical basis and effects have not been thoroughly studied. In this paper, a detailed evaluation on these modeling assumptions is presented with the goal of establishing an approach without ad hoc assumptions and improved predictive accuracy. A computational model that is capable of tracking the history of tunneling separation of each CNT junction is first constructed. The effects of CNT extensibility, tunneling cross-section, conduction channels are studied via statistical analysis of the tunneling conduction and comparison of computed piezoresistive response. It is found that CNT inextensibility leads to higher predicted piezoresistivity, the tunneling cross-section area in the Simmons approximation is a highly influential parameter, and the Landauer formula provides a more physically based approach that accounts for CNT properties. [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]

Smart geosynthetics have attracted significant interest for potential applications in geotechnical structures owing to their conductive properties. This paper presents an analysis of the electro-mechanical response of graphene-based geotextiles for application in pavements. Experimental and numerical studies were conducted on a commercially available graphene-coated geotextile to understand its response to induced strains. The material was subjected to tensile loading with the effect of transverse strains also being tested, utilizing Digital Image Correlation (DIC). Experimental results indicated a significant electro-mechanical behaviour in the geotextile, with the relative resistance change being non-linear with applied strain. Existing analytical solutions could not predict the observed behaviour; hence, a multi-physics numerical model was developed using concepts of piezoresistivity to capture the non-linear behaviour. The numerical model was calibrated using the experimental data and then validated with experimental data from tests conducted for different specimen sizes. The model could capture the non-linear electro-mechanical behaviour and the electrical response due to axial and transverse strains in the material. This study is expected to serve as a basis for using graphene-based geotextiles for structural health monitoring (SHM) purposes in geotechnical structures, such as in roads. • Smart geosynthetics have become increasingly popular in pavement health monitoring. • A graphene-based geotextile was assessed for potential road sensing applications. • Experimental testing was conducted to evaluate its electro-mechanical response. • A multi-physics numerical model was developed to capture the observed piezoresistive behaviour. • The model was validated with experimental data obtained for varying specimen sizes. [ABSTRACT FROM AUTHOR]