Flexible Piezoresistive Strain Sensors Research Articles

Flexible piezo-resistive strain sensors based on silver nanowires and graphene nanoplatelets reinforced polydimethylsiloxane for human motion detection

In this work, flexible sensors consisting of silver nanowires (Ag NWs), graphene nanoplatelets (GNP), and polydimethylsiloxane (PDMS) are fabricated using a straightforward, affordable, and simple solution processing technique. The performance of the flexible devices with structure Ag NWs/GNP/PDMS has been studied, in which Ag NWs/GNP-based ternary conductive hybrid nanocomposites are used as acting sensing layers. The diameter of Ag NWs and flake size of GNPs were determined to be 80–160 nm and 30–190 nm, respectively. The Raman peaks of Ag NWs within the composite are appeared at 392.08, 482.12, 543.7, 679.42 and 854.87 cm−1. The peaks existing at 1317.68, 1565.36 and 2639.75 cm−1 represent the D, G and 2D bands of GNP, respectively. The gauge factor is determined to be 865 within the strain range of 0–71 %. The response and recovery time of the fabricated sensor are calculated to be 101 and 110 ms, respectively. The sensor exhibits a minimum detectable limit of 0.8 %. The sensor shows a highly reproducible response for more than 1150 bending and stretching cycles suggesting long-term durability which may be attributed to the Ag NWs/GNPs conductive networks for significant strengthening the hybridisation with PDMS. Different human activities, viz. finger bending, stretching, mouse/mobile screen scrolling, swallowing through the throat and drinking are monitored by the fabricated Ag NWs/GNP/PDMS-based flexible strain sensor, indicating promising applications in wearable electronics.

Recycling of Flyash: Route toward high-performance, eco-friendly, and cost-effective flexible strain sensor via synergizing multi-walled carbon nanotubes

Along with the widespread utilization of wearable and portable electronics, the rise of sensitive flexible strain sensors with wide detection range, excellent stability and affordable costs emerges as a substantial crux of this domain. Herein, we present a convenient and scalable approach for constructing high-performance flexible piezoresistive strain sensor by rationally assembling Flyash/Multi-carbon nanotubes (MWCNTs) active sensing hybrids. Owing to the unique heterostructure of Flyash/MWCNTs wherein Flyash is anchored with MWCNTs via polydopamine, the active sensing hybrids accommodated by porous polyurethane enables a sophisticated conductive network and large tensile deformability, permitting outstanding gauge factor of 1379, wide strain detection range of 0–100 %, remarkable operation stability and durability. Furthermore, the feasibility of sensor in human motions monitoring is validated. For the first time, the synergy of Flyash and MWCNTs demonstrates effective in advancing the sensing performance of piezoresistive strain sensor, providing a new sight towards high-performance, eco-friendly, and cost-effective flexible strain sensor.

Flexible piezo-resistive strain sensors using all-polydimethylsiloxane based hybrid nanocomposites for wearable electronics.

We report flexible piezo-resistive strain sensors composed of silver nanoparticle (Ag NP), graphene nanoplatelet (GNP), and multi walled carbon nanotube (MWCNT)-based ternary conductive hybrid nanocomposites as an active sensing layer fabricated using a simple solution processing method on flexible polydimethylsiloxane (PDMS) substrates. The electrical characteristics have been studied in PDMS-based flexible devices having three different kinds of structures, namely Ag NPs/MWCNT/PDMS, GNP/PDMS and Ag NPs/GNP/PDMS. The microscopic analysis of the hybrid nanocomposites is undertaken using field emission scanning electron microscopy. The diameter of the CNTs is found to be in the range of 20-40 nm, whereas the length is determined to be 100-800 nm. The average diameter and length of the GNPs are observed to be 30-50 nm and 100-500 nm, respectively. The crystallite size of the silver nanoparticles in the Ag NPs/MWCNT/PDMS and Ag NPs/GNP/PDMS-based nanocomposites is determined to be 22.8 nm and 29.1 nm, respectively. The prepared sample of Ag NPs shows four distinct peaks in the X-ray diffraction pattern, which correspond to the (111), (200), (220), and (311) face-centered cubic (FCC) crystalline planes. Raman spectroscopy is undertaken to study the fundamental physical properties and chemical analysis of the nanocomposites. Ag NPs/GNP/PDMS-based sensors exhibit superior performance in terms of sensitivity, response and recovery time during breathing/unbreathing analysis. The large surface area of the Ag NPs and GNPs promotes uniform distribution of Ag NPs to fill into the porous GNP surface, thereby facilitating high contact area along with better electron transport in the Ag NPs/GNP/PDMS hybrid nanocomposite-based sensors. The gauge factor (GF), response and recovery time of the Ag NPs/GNP/PDMS hybrid nanocomposite-based sensors are determined to be 221, 130 ms and 119 ms, respectively. The ternary conductive nanocomposite-based sensors are free from the drawbacks of binary nanocomposite-based sensors where the high percolation threshold and poor mechanical behaviour lead to the degradation of the device performance.

A flexible piezoresistive strain sensor based on MXene/bacterial cellulose hydrogel with high mechanical strength for real-time monitoring of human motions

A piezoresistive hydrogel sensor composed of bacterial cellulose and MXene nanosheets shows real-time stress sensing abilities at different amplitudes of human motions and maintains its sensing ability underwater.

A flexible piezoresistive strain sensor based on black phosphorus/gold nanocomposites interspersed sponge for motion sensing

A flexible piezoresistive strain sensor based on black phosphorus/gold nanocomposites interspersed sponge for motion sensing

Regulation of sulfur vacancies in vertical nanolamellar MoS2 for ultrathin flexible piezoresistive strain sensors

• The vertical MoS 2 nanosheets prepared by magnetron sputtering displayed a piezoresistive effect, which has not been reported previously. • Contact and separation of channels between the vertical MoS 2 nanosheets under strain, leading to excellent piezoresistive properties. • Sulfur vacancies enhance electron transport of in-plane MoS 2 nanosheets. • The flexible sensors were excellent in human motion and bionic flight monitoring. Magnetron-sputtered MoS 2 has applications in piezoresistive functional materials research owing to its unique nanostructure. However, the controlled incorporation of sulfur vacancies and realization of enhanced piezoresistive performance remain significant challenges. In this work, the direct growth of large-area MoS 2 films with tunable sulfur vacancy concentrations was successfully achieved via magnetron sputtering at various temperatures. Microstructural analysis revealed that the application of strain altered the number of conductive channels between the vertical MoS 2 nanosheets, changing the measured resistance and leading to excellent piezoresistive properties. More importantly, the unsaturated electrons due to the sulfur vacancies increased the in-plane carrier concentration of the MoS 2 nanosheets. A deposition temperature of 50°C afforded the highest concentrations of sulfur vacancies and carriers. These MoS 2 films possessed a carrier concentration of 6.58 × 10 17 cm −3 , which was 40.9% higher than that obtained at 150°C, and displayed superior piezoresistive performance. The films exhibited high gauge factors of 2.66 and 23.22 under tensile and compressive strain of ≤0.29%, respectively. These values were 118% and 323% higher, respectively, than those obtained for films deposited at 150°C. This work provides an effective route for modulating and mass producing MoS 2 -based piezoresistive electronic devices.

Characterization and optimization of 3D-printed, flexible vibration strain sensors with triply periodic minimal surfaces

Flexible piezoresistive strain sensors have promising applications in wearables and soft robotics. For sensing dynamic strains, such as a runner’s gait or a slipping object held by a robotic gripper, the sensor must be capable of vibration strain sensing for a range of amplitudes and frequencies. This article presents the characterization and design optimization of a flexible sensor using Schwarz P-type triply periodic minimal surface (TPMS) structures for vibration strain sensing. Sensors are fabricated using a 3D-printing process, coating a silicone rubber (SR) matrix with graphene nanoplatelets (GNP). Sensors are characterized under uniaxial compressive strain amplitudes from 0% to 10% and frequencies of 10–110 Hz. Frequency and time-domain analyses demonstrate sensor performance and explain the unique deformation mechanisms of the TPMS structure. Low sensor delays of less than 6.3 ms demonstrate its capability for high-frequency sensing. The frequency independence of the sensor is demonstrated, as the mean frequency dependence error is only ±3.89%. In addition, the TPMS sensor was parametrized, and a size optimization was performed on it using a multi-objective adaptive firefly algorithm (MOAFA). The MOAFA converged within 1618 unique function evaluations, a reduction of 85.5% compared to the entire set of feasible solutions. The optimized sensor was fabricated and characterized, displaying a reduced mean frequency dependence error of only ±2.18%.

Review of Flexible Piezoresistive Strain Sensors in Civil Structural Health Monitoring

Owing to the outstanding sensing properties, especially high sensitivity and large stretchability, flexible piezoresistive strain sensors are advantageous for achieving intelligent sensing and have become a popular topic in the field of civil structural health monitoring (SHM). To explore advanced flexible strain sensors for civil SHM, this paper summarizes the recent research progress, achievements and challenges in flexible piezoresistive strain sensors. First, four common piezoresistive mechanisms are introduced theoretically. Sensor materials, including conductive materials, flexible substrates and electrodes, are explained in detail. Second, essential sensing parameters are interpreted and then followed by specific explanations of improvement strategies for the sensor performance in terms of each parameter. Third, applications of flexible piezoresistive strain sensors in the deformation measurement and damage detection of steel structures, concrete structures and fiber-reinforced composite structures are presented. Existing challenges and prospects in the practical application and large-scale production of flexible strain sensors are also reported. Last but not least, strategies for the selection of piezoresistive sensors for civil SHM are explained.

A flexible piezoresistive strain sensor based on laser scribed graphene oxide on polydimethylsiloxane

Flexible strain sensors are an important constituent in soft robotics, health care devices, and in the defence industry. Strain sensors are characterized by their sensitivity (gauge factor-GF) and sensing range. In flexible strain sensors, simultaneously achieving consistency and high sensitivity has always been challenging. A number of materials and their derivatives have been explored to achieve balanced sensitivity with respect to sensing range with limited results. In this work, a low-cost flexible piezoresistive strain sensor has been developed using reduced graphene oxide (rGO) on polydimethylsiloxane (PDMS). The reduction has been performed using laser scribing, which enables the fabrication of arbitrary structures. After lead-out, the devices were again sandwiched in a layer of PDMS to secure the structures before performing their testing using a locally developed testing rig. Compared to previously reported graphene strain sensors, the devices fabricated in this work show relatively high GF with respect to sensing range. The GF calculated for stretching, bending and torsion was 12.1, 3.5, and 90.3 respectively, for the strain range of 0–140%, 0–130%, and 0–11.1%. A hand test was performed for the detection of joint movement. Change of resistance has been observed indicating muscle motion.

Flexible piezoresistive strain sensor based on CNTs–polymer composites: a brief review

Flexible piezoresistive strain sensor based on CNTs–polymer composites: a brief review

Carbon-based aerogel in three-dimensional polyurethane scaffold: The effect of in situ unidirectional aerogel growth on piezoresistive properties

Carbon-based aerogels (CA) with durable piezoresistive properties possess great promise for applications in wearable electronics. However, it is still a great challenge to fabricate high performance aerogel sensors due to the limits of mechanical properties. We recently developed a novel strategy to build aerogel/foam hybrid structure that can effectively enhance the mechanical properties of aerogel-based sensors. Herein, CA/polyurethane foam (PUF) composite was fabricated via in situ unidirectional freeze-drying process starting from a carbon nanotubes (CNTs)/graphene oxide (GO) aqueous dispersion stabilized with chitosan (CS) which was filled into the porosity of an open cell PUF. The resulting composite shows a microscopic anisotropy in the CA with electrical conductivity as high as 10.54 S m−1. The piezoresistive sensitivity of the resulting composite can be simply modulated over a wide range from 1.1 to 3.6 by adequately selecting its initial pre-strain. The CA/PUF composite exhibits excellent compressible resilience at the strain of 50% for at least 100 cycles. In addition, the sensor was successfully applied for detecting various human motions. These unique properties make the realized composite a promising candidate for effective flexible piezoresistive strain sensors in wearable applications.

A new approach for an ultra-thin piezoresistive sensor based on solidified carbon ink film

Conventional flexible piezoresistive strain sensors that use conductive particles polymer composites exhibit thick structures with a low sensitivity to external tension. This paper presents a cost-effective method to fabricate ultra-thin and highly sensitive piezoresistive strain sensors. In our fabrication steps, carbon ink that is mainly composed of carbon black particles is solidified with a drying process to form a “paperlike,” flexible conductive film. Without any surface modification techniques, the carbon ink film is directly placed onto liquid-state PDMS and then bonded after the drying process. Following the rapid prototyping, different performance metrics of the fabricated sensors, including piezoresistivity, gauge factor, temperature dependency, elastic modulus, and repeatability are measured. Specifically, sensors fabricated with this method show a significantly improved gauge factor (~26) compared to similar flexible sensors fabricated by more complicated micro-fabrication methods. The proposed method of fabrication and the corresponding ultra-thin (~45 μm) sensor prototype may benefit the design and mass production of future wearable biomedical and healthcare sensors.

Piezoresistive strain sensing behavior of flexible conductive microporous membrane using acidic ionic liquid

Flexible piezoresistive strain sensors are smart materials used for sensing applications. In the present work, a flexible piezoresistive strain sensor was developed by infiltrating acidic ionic liquid (IL) [BMIM HSO4] in poly (vinylidene fluoride) (PVDF) polymer using a solvent casting technique. The use of IL leads to the generation of pores of uniform shape at lower concentrations (~10 wt%), thus enhancing mechanical flexibility by allowing more space to stretch. Furthermore, the pores contained IL inside, providing extra ions for charge conduction, thus improving the sensitivity of the material. Maximum flexibility and gauge factor of 5.29 were obtained for ratio 5 to 10 wt% of IL. Maximum elasticity was obtained for ratio IL/PVDF (5:95) in comparison to pure PVDF. An increase in dielectric constant and conductivity was observed for samples containing IL. The most important part of IL can be reused and shows biocompatible behavior. The developed system showed a more flexible, durable, and repeatable performance for strain sensing applications for human and structural health monitoring.

Flexible piezoresistive strain sensor based on optimized elastomer-electronic polymer blend

Biocompatible polymer-based sensors have unique advantages over semiconductor-carbon-based, and metal-based piezoresistive sensors because of the flexible nature, high gauge factor, and high bendability of the polymers. In this study, various compositions of biocompatible Poly(vinylidene fluoride-trifluoroethylene- chlorotrifluoroethylene) P(VDF-TrFE-CTFE) (TER)/poly(3,4-ethylene dioxythiophene) (PEDOT):poly(styrene sulfonate) (PSS) elastomer-electronic polymer blends were developed using cost-effective blending method for piezoresistive strain sensor. The TER /PEDOT:PSS polymer blend with an optimized blending ratio of 40/60 produced a gauge factor of around 680 with a high dielectric constant and a high mechanical strain of 221%. The TER/PEDOT:PSS polymer blend shows stable and repeatable response sensing signals up to 6500 bending cycles at a bending strain of 0.009 and checked for the duration of sixty days. The SEM analysis shows the penetration of the PEDOT:PSS particles between the backbone of TER polymer in the optimized TER/PEDOT:PSS polymer blend, which enhances the gauge factor of the TER/PEDOT:PSS blend sensors. Owing to high ductility and high sensitivity, our proposed elastomer-electronic polymer blend with ~0.18 mm thickness will find applications in wearable and electronic skin technology.

Effects of non-covalent interactions on the properties of 3D printed flexible piezoresistive strain sensors of conductive polymer composites

ABSTRACT In this work, flexible piezoresistive strain sensors of carbon nanotubes (CNT)/thermoplastic polyurethane (TPU) conductive polymer composites were prepared using fused filament fabrication (FFF) 3D printing. The constructed porous structure and highly elastic TPU matrix allow the sensors to experience strains up to 30% with great recoverability. The printed sensors exhibited a typical negative piezoresistive effect under compressive strain. The effects of non-covalent modification and loading of CNTs on the mechanical and piezoresistive behavior of sensors were investigated. The introduction of 1-pyrenecarboxylic acid (PCA) increased the dispersibility of CNTs and nanofiller-polymer interfacial interactions, resulting in the excellent sensing performance. A sensor with higher CNT content showed greater sensitivity due to generation of more conductive paths during compression. The gauge factor () of the 3 wt% CNT/PCA/TPU sensor increased by 43 and 35 times compared with the 3 wt% CNT/TPU and the 1.5 wt% CNT/PCA/TPU sensors, respectively, when strained at 26% to 30%. In addition, the printed sensor of 3 wt% CNT/PCA/TPU showed great reproducibility over 1500 loading cycles. Some applications in monitoring external loads and motion of the human body were demonstrated. This work provides important information for the property optimization and customizable fabrication of flexible piezoresistive strain sensors.

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.

Flexible piezoresistive strain sensor with high sensitivity based on carbonised waste thermosetting resin

ABSTRACT In this study, a novel flexible piezoresistive strain sensor was prepared by using polydimethylsiloxane (PDMS) to encapsulate carbonised waste polybenzoxazine (cPBa). Benefiting from the porous conductive framework of cPBa, the infused PDMS endowed the cPBa/PDMS strain sensor with excellent mechanical properties. It was observed that this sensor exhibited both negative and positive piezoresistive effects due to the conductive disconnection/reconnection process of carbon fragments while monitoring compressive deformation. Moreover, it showed a high sensitivity with a gauge factor () of 1954.3 at the strain range of 12–30% and excellent repeatability (up to 1000 cycles). Also, the potential application of this strain sensor in monitoring human body motions was evaluated. This work presents an eco-friendly way to fabricate highly sensitive flexible strain sensors using recycled thermosetting resins.

Modelling of PVDF/CNF Conducting Polymer Nanocomposite

Polymer nanocomposites are highly apricated for the sensor and actuator applications. As they are soft and flexible and can produce higher cyclic loading with good repeatability. But when conductive fillers are woven in the polymer matrix it loses flexibility and enhances the conductivity. Therefore, studying the loading behavior of the nanocomposite becomes important for determining the stability and load bearing capacity of the conducting polymer nanocomposite membranes (CPNC). Therefore, the intent was to design a flexible piezoresistive strain sensor. Finite element analysis (FEA) technique was used for investigating the deformation behavior with a change in stress and strain by applying loads of 0.4 N, 0.6 N, 0.8 N and 1N. Displacement was taken as one parameter for determining the stress intensities in CPNC local regions.

Highly responsive flexible strain sensor using polystyrene nanoparticle doped reduced graphene oxide for human health monitoring

A highly responsive, flexible piezoresistive strain sensor was demonstrated using laser thermal reduced graphene oxide doped with polystyrene nanoparticles. The nanoparticles alter the morphology of the sensing film by separating stacked graphene fragments and creating partially connected conducting channels, which significantly increase the resistance change under strain. Two types of distinctive resistance-changing mechanisms were observed associated with doping of nanoparticles with different sizes. Compared with graphene oxide based strain sensors, the measured gauge factor of the sensor doped with 90 nm nanoparticles can reach up to 250 under small deformation (linear region below 1.05%). Furthermore, the flexible strain sensor was applied for real-time monitoring of human body activities including swallowing process, lower back posture and pulse on neck. The demonstrated results present the potential of a viable sensing material for a number of human health monitoring applications in preventing injuries and detecting certain disease-related disorders.

Flexible piezoresistive strain sensor based on single Sb-doped ZnO nanobelts

Using a two-end bonded Sb-doped ZnO nanobelt on a flexible polystyrene substrate, the decrease of the resistance with increasing compressed strains in the nanobelt has been observed, which is suggested to be attributed to the piezoresistance effect. The longitudinal piezoresistance coefficient of the Sb-doped ZnO nanobelt is about 350. On the basis of this finding, we made a flexible piezoresistive strain sensor in a signature pen, which can be used to detect the corresponding compressed strains when the characters are recorded.