Nanocomposite Based Strain Sensors Research Articles

Lithographically patternable SU-8/Graphene nanocomposite based strain sensors for soft-MEMS applications

Abstract This paper presents an optimized, lithographically patternable SU-8/Graphene nanocomposite based piezoresistive strain sensor for localized, high-precision assessment, which marks a significant advancement in the field of soft-MEMS based technologies. The fabrication process involves the photolithography of a SU-8/Graphene nanocomposite with a minimum resolution of 50µm, resulting in a material with excellent electrical conductivity and mechanical properties. Specifically, a 3% SU-8/Graphene composition was chosen to exceed the percolation threshold, enabling substantial changes in the resistance %ΔR/R and facilitating photopatternability. The sensor exhibited exceptional performance characteristics, including a rapid response time of 0.1 seconds and a wide bending range from 0o to 60o. Notably, it demonstrated a remarkable %ΔR/R of 19.21, indicating its superior sensing capability. Such high sensitivity is crucial for applications that require precise, localized measurements, such as biomedical engineering, sports science, and smart healthcare.

Nacre-Inspired MXene Nanocomposite-based Strain Sensor with Ultrahigh Sensitivity in a Small Strain Range for Parkinson's Disease Diagnosis.

Nowadays, chronic diseases are the primary threat to public health and are getting younger. By taking the advantages of continuousness, convenience, and real-time response, wearable strain sensors have been given great attention to diagnose chronic diseases via analyzing the patient's health state. However, most physiological signals, such as limb tremor of Parkinson's disease, microexpression, and slight joint movement, are tiny and difficult to be detected. Therefore, the development of strain sensors characterized with ultrahigh sensitivity in a small strain range (ε < 10%) is urgent. Inspired by nacre's hierarchical structure, we have fabricated nacre-mimetic nanocomposites with "brick-and-mortar" architecture by employing polyacrylamide (PAM) and Ti3C2Tx MXene nanosheets through a layer-by-layer (LBL) spin-coating technique. The resultant nanocomposite-based strain sensor exhibits ultrahigh sensitivity in a small strain range (GF = 296.8, ε < 10%), attributed to the bioinspired hierarchical structure and hydrogen bond-enhanced interfacial interactions. In addition, a high reliability, broad working sensing range (453%), short response time (183 ms), skin-like tensile stress (7.2 MPa), and excellent durability (2000 cycles) are also achieved. Due to the ultrahigh sensitivity within a small strain, the reported strain sensor can accurately diagnose and distinguish Parkinson's disease symptoms, including thumb pill-rolling tremor, masked face (microexpression), intermittent shaking of the head, and limb cogwheel motion. This work provides new insights to design strain sensors with high sensitivity for monitoring tiny signals and for disease diagnosis.

Highly Sensitive and Wearable ZnO–Graphene Nanocomposite-Based Strain Sensors for Human Motion Detection

Flexible electronic sensors have garnered considerable interest in wearable health-monitoring devices and electronic skin. In this work, a simple method for the fabrication of flexible zinc oxide-graphene nanoplatelets (ZnO-GNP) nanocomposite based strain sensors has been proposed. The sensing element was deposited on polydimethylsiloxane (PDMS) substrate by a facile spin and peel strategy to yield flexible sandwiched sensors. Effect of varying the blending ratio of the constituents on the electromechanical responses of the sensors was studied. The sensors exhibited high stretchability, good sensitivity, high reversibility and superior stability under tensile and bending loads. The flexible nanocomposite sensors were able to detect extensive human movements such as bending of the elbow, wrist and finger and also subtle motions such as eye blinking, wrist pulse and phonation. Owing to the facile and economical fabrication method, high sensitivity and good reversibility, the ZnO-GNP sensors have high prospect of application in wearable health monitoring devices, robotics and various forms of human-machine interface.

Ultrasensitive Strain Sensor Utilizing a AgF-AgNW Hybrid Nanocomposite for Breath Monitoring and Pulmonary Function Analysis.

Breath monitoring and pulmonary function analysis have been the prime focus of wearable smart sensors owing to the COVID-19 outbreak. Currently used lung function meters in hospitals are prone to spread the virus and can result in the transmission of the disease. Herein, we have reported the first-ever wearable patch-type strain sensor for enabling real-time lung function measurements (such as forced volume capacity (FVC) and forced expiratory volume (FEV) along with breath monitoring), which can avoid the spread of the virus. The noninvasive and highly sensitive strain sensor utilizes the synergistic effect of two-dimensional (2D) silver flakes (AgFs) and one-dimensional (1D) silver nanowires (AgNWs), where AgFs create multiple electron transmission paths and AgNWs generate percolation networks in the nanocomposite. The nanocomposite-based strain sensor possesses a high optimized conductivity of 7721 Sm-1 (and a maximum conductivity of 83,836 Sm-1), excellent stretchability (>1000%), and ultrasensitivity (GFs of 35 and 87 when stretched 0-20 and 20-50%, respectively), thus enabling reliable detection of small strains produced by the body during breathing and other motions. The sensor patching site was optimized to accurately discriminate between normal breathing, quick breathing, and deep breathing and analyze numerous pulmonary functions, including the respiratory rate, peak flow, FVC, and FEV. Finally, the observed measurements for different pulmonary functions were compared with a commercial peak flow meter and a spirometer, and a high correlation was observed, which highlights the practical feasibility of continuous respiratory monitoring and pulmonary function analysis.

Evaluation of conductivity and piezo-impedance response of VACNTs/PDMS nanocomposite-based strain sensors under small deformations

The vertically aligned multi-walled carbon nanotubes (VACNTs) / polydimethylsiloxane (PDMS) nanocomposite-based strain sensors presented in this study show different behavior depending on catalyst concentrations for VACNT growth. Under static tensile load, the sensor with lower catalyst concentration shows a high gauge factor (GF~1400), whereupon tunneling effect is the mechanism that dictates the sensitivity. For higher concentrations, the GF decreases (GF~40) and shows an ohmic conduction. Morphological examinations showed VACNTs are homogeneously and randomly distributed as clusters with CNT bridging in the PDMS polymer matrix. Based on dielectric impedance (DI) and direct current (DC) electrical analysis, it was possible to identify that cut-off frequency (fc) increases with VACNTs concentration. Cut-off frequency can also define high sensitivity VACNT sensors with lower VACNT density. When compared with dispersed MWCNT sensors, VACNTs have a reduced fc due to the larger internal resistance variation associated with the high tunneling effect. This effect associated with the high sensitivity of the VACNT/PDMs sensors makes it a key factor to understand the mechanisms responsible for increasing the sensitivity and manufacturing of high GF nanocomposite stretchable sensors.

Self-healing flexible strain sensors based on dynamically cross-linked conductive nanocomposites

Self-healing, flexible, robust and adaptable strain sensors with intelligent skin-like features are greatly promising for future wearable devices. In this work, we demonstrate a self-healing flexible strain sensor with stretchability, robust mechanical strength, and high sensing sensitivity, which can monitor human motions. The sensor is constructed by the dynamically cross-linked conductive nanocomposite based on imine and quadruple hydrogen bonds. Due to the reversible interactions, the nanocomposites exhibit excellent self-healing performances with the healing efficiency up to 95%. The interfacial compatibility between the nanofillers and polymer networks is enhanced through the supramolecular interactions. Based on piezoresistive effects, the change of resistance for the nanocomposite-based strain sensor can be obviously observed under various deformation including stretching, bending, and twisting. Meanwhile, the gauge factor of the sensor reaches 46, showing high sensing sensitivity. Benefited from the self-healing feature, the sensor can withstand mechanical damage and restore the function of detecting human motions, enabling reliability and stability in practical applications, which shows great potential for developing smart wearable devices, healthcare monitoring, and human-machine interfaces.

Characterization of MWCNTs-polystyrene nanocomposite based strain sensor

The usage of nano phase materials for strain sensing applications has attracted attention due to their unique electromechanical properties. The nanocomposite as piezo-resistive films provides an alternative for the realization of strain sensors with high sensitivity than the conventional sensors based on metal and semiconductor strain gauges. In this work, polymer based nano-composite with carbon nanotubes as filler were developed. The multi-walled carbon nanotubes/polystyrene (MWCNTs/PS) nano-composite films were prepared with different wt.% of CNTs using solution mixing method. Field emission scanning electron microscopy technique was carried out to investigate the morphology and dispersion of CNTs in the nano-composite sample. Fourier transform infrared spectroscopy technique was employed to characterize the bonds present in the prepared nano-composite. The electrical response of the composite films was recorded in the form of current-voltage (I-V) characteristics using source meter. The electromechanical response of the nano-composite films with different wt.% of filler CNTs was recorded by applying uni-axial tensile load. The electromechanical responses were then analyzed to obtain gauge factor for the strain sensitivity. The highest gauge factor of 133 was recorded during tensile testing of the nano-composite with 3 wt.% of CNTs fillers.

Waterproof Flexible Polymer-Functionalized Graphene-Based Piezoresistive Strain Sensor for Structural Health Monitoring and Wearable Devices.

In recent times, flexible piezoresistive polymer nanocomposite-based strain sensors are in high demand in wearable devices and various new age applications. In the polymer nanocomposite-based strain sensor, the dispersion of conductive nanofiller remains challenging due to the competing requirements of homogenized dispersion of nanofillers in the polymer matrix and retaining of the inherent characteristics of nanofillers. In the present work, waterproof and flexible poly(vinylidene difluoride) (PVDF) with a polymer-functionalized hydrogen-exfoliated graphene (HEG)-based piezoresistive strain sensor is developed and demonstrated. The novelty of the work is the incorporation of polystyrene sulfonate sodium salt (PSS) polymer-functionalized HEG in a PVDF-based flexible piezoresistive strain sensor. The PSS-HEG provides stable dispersion in the hydrophobic PVDF polymer matrix without sacrificing its inherent characteristics. The electrical conductivity of the PVDF/PSS-HEG-based strain sensor is 0.3 S cm–1, which is two orders of magnitude higher than the PVDF/HEG-based strain sensor. Besides, near the percolation region, the PVDF/PSS-HEG shows a maximum gauge factor of 10, which is about two times higher than the PVDF/HEG-based flexible strain sensor and 5-fold higher than the commercially available metallic strain gauge. The enhancement in the gauge factor is due to the stable dispersion of PSS-HEG in the PVDF matrix and electron conjugation caused by the adherence of negatively charged sulfonate functional groups on the HEG. The developed waterproof flexible strain sensor is demonstrated using portable wireless interfacing device for various applications. This work shows that the waterproof flexible PVDF/PSS-HEG-based strain sensor can be a potential alternative to the commercially available metallic strain gauge.

Nuomici-Inspired Universal Strategy for Boosting Piezoresistive Sensitivity and Elasticity of Polymer Nanocomposite-Based Strain Sensors.

Electrically conductive polymer composites (CPCs) are potential alternatives to conventional strain gauges due to their tunable sensitivity and strain ranges. Currently, to achieve very high piezoresistive sensitivity in thermoplastic-based CPCs with Gauge factors GF above 20 at low tensile strains (ε ≤ 5%) is a big challenge, but critical for structural health monitoring application in infrastructures. Here, inspired by the unique structures of a famous Chinese food, nuomici, we coat carbon nanotubes (CNTs) onto sticky acrylic rubber (AR) granules (ARG) to form nuomici-like CNT@ARG composite granules, which are employed as unique conductive filler to fabricate highly piezoresistive and flexible CPCs based on poly(vinylidene fluoride) (PVDF). This strategy of localizing CNTs densely on the surface of touching rubbery particles resulted in a much more sensitive elastic conductive network built by the CNT@AR composite and showed a big gain effect. The resultant PVDF/CNT@AR nanocomposites (AR content ranging from 0 to 10 wt %) show extremely high piezoresistive sensitivity at low strain, depending on the AR content. In particular, the GF value of PVDF with 1.5 wt % CNT@10 wt % AR is 41 at 5% strain, which is more than one magnitude higher than that (ca. 3) of traditional PVDF/CNT nanocomposite sensors. Moreover, the elongation at break increases by about 60% with the addition of 1.5 wt CNT@10 wt % AR. This study introduces a universal effective strategy for tailoring the mechanical properties and strain sensitivity of conductive network in CPCs, which is critical for the fabrication of high-performance strain sensors.

Electrical impedance analysis of carbon nanotube/epoxy nanocomposite-based piezoresistive strain sensors under uniaxial cyclic static tensile loading

Carbon nanotubes-based nanocomposites have gained a great amount of attraction and play a key role in the realization of strain sensors owing to their remarkable physical properties. In this study, the piezoresistivity of multi-walled carbon nanotubes (MWCNTs)/epoxy-based nanocomposite-based strain sensor under static tensile load is examined using electrochemical impedance spectroscopy. Morphological examinations show that MWCNTs are randomly and homogeneously distributed in the epoxy polymer matrix. A simplified resistance constant phase element model is proposed and validated by impedance spectrum to fit the impedance spectra and the equivalent circuit parameters are extracted under uniaxial static load. Impedance results suggest that depending on the frequency regions, the sensor exhibits different responses under loading. Moreover, the proposed sensor gives high sensitivity, linearity and low hysteresis under cyclic quasi-static loading and unloading that makes the sensor a promising candidate for practical strain sensor applications.

Graphene-based nanocomposite strain sensor response to ultrasonic guided waves

Certain traditional sensors like lead zirconate titanate (PZT) wafers and ultrasonic probes can respond to extremely weak disturbances such as ultrasonic guided waves (UGWs). However, their further development for applications to meet increasing engineering demands is limited on account of their hardness, brittleness, and complex manufacturing processes. Now, emerging nanotechnology ushers in a brand-new world for nanocomposite-based strain sensors, endowing them with higher flexibility, better surface compatibility and easier fabrication. Yet there are few reports of composites which can be used to perceive high frequency UGWs with an ultralow magnitude. Here, we present a novel graphene-based nanocomposite possessing strong sensitivity for sensing ultrasonic waves by virtue of a neoteric sensing mechanism − the tunneling effect. By designing and optimizing the microstructure of the conductive network in the nanocomposite sensor, we successfully capture ultrasonic waves with high signal-to-noise ratio in a broad frequency range up to 1 MHz. With the feature of high sensitivity and rapid response times, the graphene-based nanocomposite becomes a promising candidate for structural health monitoring in developing prospective applications.

Development of Nanocomposite-Based Strain Sensor with Piezoelectric and Piezoresistive Properties.

Sensors provide an interface between mechanical systems and the physical world. With the move towards Industry 4.0 and cyber-physical systems, demands for cost-effective sensors are rapidly increasing. Conventional sensors used for monitoring manufacturing processes are often bulky and need complex processes. In this study, a novel high-sensitive nanocomposite-based sensor is developed for measuring strain. The developed sensor is comprised of polyvinylidene fluoride (PVDF) as a piezoelectric polymer matrix, and embedded carbon nanotube (CNT) nanoparticles creating a conductive network. Exhibiting both piezoelectric and piezoresistive properties, the developed sensors are capable of strain measurement over a wide frequency band, including static and dynamic measurements. The piezoresistive and piezoelectric properties are fused to improve the overall sensitivity and frequency bandwidth of the sensor. To simulate the sensor, a 3D random walk model and a 2D finite element (FE) model are used to predict the electrical resistivity and the piezoelectric characteristics of the sensor, respectively. The developed models are verified with the experimental results. The developed nanocomposite sensors were employed for strain measurement of a cantilever beam under static load, impulse excitation, free and forced vibrations, collecting both piezoelectric and piezoresistive properties measurements. The obtained signals were fused and compared with those of a reference sensor. The results show that the sensor is capable of strain measurement in the range of 0–10 kHz, indicating its effectiveness at measuring both static and high frequency signals which is an important feature of the sensor.

Effect of processing conditions on the electrical resistance of MWCNT/epoxy nanocomposite based strain sensors

This paper presents fabrication methods for some Multi Walled Carbon Nano Tube (MWCNT)/epoxy composites. These nanocomposite are characterized with respect to temperature and strain. The effect of parameters such as sonication time, percolation threshold, film thickness, use of substrate and formation of electric contact have been observed. These nanocomposites are fabricated: (i) with mould without substrate, (ii) without mould on poly ethylene terephthalate substrate, (iii) without mould without substrate and (iv) without mould without substrate with embedded electrode wire. Field Emission Scanning Electron Microscope of these samples is done to measure the dispersion and agglomerates formation of CNT bundles into polymer matrices. Piezoresistive characterization of these samples with respect to temperature and strain is performed. This paper gives a novel strategy for fabricating a highly sensitive MWCNT/epoxy strain sensor with excellent repeatability.

Polymer Nanocomposite-Based Strain Sensors with Tailored Processability and Improved Device Integration

Because of its easy processability and elastomeric properties, the triblock copolymer styrene–butadiene–styrene (SBS) is an excellent matrix for the development of piezoresistive polymer composites, mostly for larger strain composites. Piezoresistive sensors based in SBS and conductive nanofillers were processed by scalable methods, namely, extrusion and spray printing, allowing the measurement of large deformations up to 20% of strain with low mechanical hysteresis in loading–unloading cycles. The carbon nanotube (CNT) reinforcement increases the mechanical properties (maximum stress and strain) and provides electrical properties to the composites. Extruded and spray-printed CNT/SBS composites show a piezoresistive sensibility (gauge factor) up to 4 and 2, respectively. Their percolation threshold is near 6 and 1 wt %, for extruded and spray-printed methods, respectively. The excellent piezoresistive reproducibility, processability, and easy integration in structures and devices show the suitability of t...

Lowering Internal Friction of 0D–1D–2D Ternary Nanocomposite‐Based Strain Sensor by Fullerene to Boost the Sensing Performance

AbstractThe development of strain sensors with both large strain range (&gt;50%) and high gauge factor (&gt;100) is a grand challenge. High sensitivity requires material to perform considerable structural deformation under tiny strain, whereas high stretchability demands structural connection or morphological integrity for materials upon large deformation, yet both features are hard to be achieved in one thin film. A new 0D–1D–2D ternary nanocomposite–based strain sensor is developed that possesses high sensitivity in broad working strain range (gauge factor 2392.9 at 62%), low hysteresis, good linearity, and long‐term durability. The skin‐mountable strain sensor, fabricated through one‐step screen‐printing process, is made of 1D silver nanowire offering high electrical conductivity, 2D graphene oxide offering brittle layered structure, and 0D fullerene offering lubricity. The fullerene constitutes a critical component that lowers the friction between graphene oxide–based layers and facilitates the sliding between adjacent layers without hurting the brittle nature of the nanocomposite film. When stretching, layer slippage induced by fullerene can accommodate partial applied stress and boost the strain, while cracks originating and propagating in the brittle nanocomposite film ensure large resistance change over the whole working strain range. Such high comprehensive performance renders the strain sensor applicable to full‐spectrum human motion detection.

Broadband dynamic responses of flexible carbon black/poly (vinylidene fluoride) nanocomposites: A sensitivity study

Nanocomposites fabricated based on a compound of carbon nanofiller and polymer matrix exhibit extraordinary piezoresistive performance, and the capacity of nanocomposite sensors in measuring structural dynamic strains has been widely demonstrated. However, the sensitivity of existing nanocomposite strain sensors is limited, particularly in aspects of sensing precision and response frequency. This study focuses on sensitivity investigations of flexible nanocomposite sensors fabricated using a compound of carbon black (CB) fillers and polyvinylidene fluoride (PVDF) matrix, which has shown promising potential in perceiving extremely weak strain signals within a considerably broad range of response frequencies.As a highly important indication of sensitivity, sensing precision was characterized in a quantitative manner. Broadband spectrum analysis was conducted as an effective way to examine sensor performance in perceiving minimal disturbances in dynamic strains. Disturbances were generated in two experiments, by introducing local material and geometric variations into the inspected structures, in terms of mass attachment and structural damage. The effectiveness of the nanocomposites in structural damage detection was demonstrated. Moreover, the experimental results indicate the promising potential of nanocomposite-based strain sensors for applications such as advanced bioelectronics, ultrasonic inspection, and in-situ structural health monitoring.

Preparation and Property Research of Strain Sensor Based on PDMS and Silver Nanomaterials

Based on the advantages and broad applications of stretchable strain sensors, this study reports a simple method to fabricate a highly sensitive strain sensor with Ag nanomaterials-polydimethylsiloxane (AgNMs-PDMS) to create a synergic conductive network and a sandwich-structure. Three Ag nanomaterial samples were synthesized by controlling the concentrations of the FeCl3 solution and reaction time via the heat polyols thermal method. The AgNMs network’s elastomer nanocomposite-based strain sensors show strong piezoresistivity with a high gauge factor of 547.8 and stretchability from 0.81% to 7.26%. The application of our high-performance strain sensors was demonstrated by the inducting finger of the motion detection. These highly sensitive sensors conform to the current trends of flexible electronics and have prospects for broad application.

Computational analysis of metallic nanowire-elastomer nanocomposite based strain sensors

Possessing a strong piezoresistivity, nanocomposites of metal nanowires and elastomer have been studied extensively for its use in highly flexible, stretchable, and sensitive sensors. In this work, we analyze the working mechanism and performance of a nanocomposite based stretchable strain sensor by calculating the conductivity of the nanowire percolation network as a function of strain. We reveal that the nonlinear piezoresistivity is attributed to the topological change of percolation network, which leads to a bottleneck in the electric path. We find that, due to enhanced percolation, the linearity of the sensor improves with increasing aspect ratio or volume fraction of the nanowires at the expense of decreasing gauge factor. In addition, we show that a wide range of gauge factors (from negative to positive) can be obtained by changing the orientation distribution of nanowires. Our study suggests a way to intelligently design nanocomposite-based piezoresistive sensors for flexible and wearable devices.

Automation Highlights from the Literature

Automation Highlights from the Literature

Highly Stretchable and Sensitive Strain Sensor Based on Silver Nanowire–Elastomer Nanocomposite

The demand for flexible and wearable electronic devices is increasing due to their facile interaction with human body. Flexible, stretchable and wearable sensors can be easily mounted on clothing or directly attached onto the body. Especially, highly stretchable and sensitive strain sensors are needed for the human motion detection. Here, we report highly flexible, stretchable and sensitive strain sensors based on the nanocomposite of silver nanowire (AgNW) network and PDMS elastomer in the form of the sandwich structure (i.e., AgNW thin film embedded between two layers of PDMS). The AgNW network-elastomer nanocomposite based strain sensors show strong piezoresistivity with tunable gauge factors in the ranges of 2 to 14 and a high stretchability up to 70%. We demonstrate the applicability of our high performance strain sensors by fabricating a glove integrated with five strain sensors for the motion detection of fingers and control of an avatar in the virtual environment.