Graphene-based Strain Sensors Research Articles

Graphene-Based Tunneling Strain Sensors: A Mechanism Study

Graphene-Based Tunneling Strain Sensors: A Mechanism Study

Prototyping a wearable and stretchable graphene-on-PDMS sensor for strain detection on human body physiological and joint movements.

In the era of wearable electronic devices, which are quite popular nowadays, our research is focused on flexible as well as stretchable strain sensors, which are gaining humongous popularity because of recent advances in nanocomposites and their microstructures. Sensors that are stretchable and flexible based on graphene can be a prospective 'gateway' over the considerable biomedical speciality. The scientific community still faces a great problem in developing versatile and user-friendly graphene-based wearable strain sensors that satisfy the prerequisites of susceptible, ample range of sensing, and recoverable structural deformations. In this paper, we report the fabrication, development, detailed experimental analysis and electronic interfacing of a robust but simple PDMS/graphene/PDMS (PGP) multilayer strain sensor by drop casting conductive graphene ink as the sensing material onto a PDMS substrate. Electrochemical exfoliation of graphite leads to the production of abundant, fast and economical graphene. The PGP sensor selective to strain has a broad strain range of ⁓60%, with a maximum gauge factor of 850, detection of human physiological motion and personalized health monitoring, and the versatility to detect stretching with great sensitivity, recovery and repeatability. Additionally, recoverable structural deformation is demonstrated by the PGP strain sensors, and the sensor response is quite rapid for various ranges of frequency disturbances. The structural designation of graphene's overlap and crack structure is responsible for the resistance variations that give rise to the remarkable strain detection properties of this sensor. The comprehensive detection of resistance change resulting from different human body joints and physiological movements demonstrates that the PGP strain sensor is an effective choice for advanced biomedical and therapeutic electronic device utility.

Improved performance of strain sensors constructed from highly crystalline graphene with nanospacer

Graphene shows promise as an alternative material for strain sensors due to its excellent properties and could overcome the limitations of conventional metal sensors. However, current graphene-based strain sensors are fabricated from chemically reduced graphene oxide (rGO) and suffer from low linearity and large hysteresis in the sensor response as well as high initial resistance. These issues are caused by functional groups and defects remaining on the rGO. Herein, highly crystalline rGO is employed for the fabrication of the strain sensor. Porous rGO sponge with low defect density is prepared in bulk scale via the ethanol-associated thermal process at ultra-high temperature. The obtained rGO sensor exhibits improved linearity, low initial resistance, and very small hysteresis owing to the high crystallinity of the rGO. The composite of rGO with nano-diamond, which has the role of a nanospacer to separate the rGO layers, is found to be highly effective in enhancing the sensitivity.

Highly Efficient 3D-Printed Graphene Strain Sensors Using Fused Deposition Modeling with Filament Deposition Techniques

ABSTRACT Graphene is a two-dimensional (2D) material known for its exceptional strength and high electrical conductivity, making it an ideal substance for resistive strain sensors. Recently, fused deposition modeling (FDM) in three-dimensional (3D) printing has gained attractiveness as a promising process due to its ability to produce 3D structured strain sensors by layer-by-layer melting and depositing conductive polymer composites. To ensure reliable strain sensors, comprehending how sensor properties change based on strain direction is crucial. In this study, graphene-based sensors with different slicing angles were successfully fabricated using FDM, enabling systematic study of the effect of strain angles on the performance of graphene-based sensors. The alignment of graphene filaments relative to the direction of applied strain was found to impact the gauge factor (GF) and other important sensor parameters. Our results showed that the 45° pattern exhibited higher sensitivity and stability compared to the 180° pattern, while the GF was greater for the 180° pattern. Additionally, we demonstrated high reliability and linearity through 1000 bending tests. The findings of this study will contribute to the growing body of research on FDM-fabricated graphene-based strain sensors.

Graphene-Based Flexible Strain Sensor Based on PDMS for Strain Detection of Steel Wire Core Conveyor Belt Joints.

Because of their superior performance, flexible strain sensors are used in a wide range of applications, including medicine and health, human-computer interaction, and precision manufacturing. Flexible strain sensors outperform conventional silicon-based sensors in high-strain environments. However, most current studies report complex flexible sensor preparation processes, and research focuses on enhancing and improving one parameter or property of the sensors, ignoring the feasibility of flexible strain sensors for applications in various fields. Since the mechanical properties of flexible sensors can be well combined with rubber conveyor belts, in this work polydimethylsiloxane (PDMS) was used as a flexible substrate by a simple way of multiple drop coating. Graphene-based flexible strain sensor films that can be used for strain detection at the joints of steel cord core conveyor belts were successfully fabricated. The results of the tests show that the sensor has a high sensitivity and can achieve a fast response (response time: 43 ms). Furthermore, the sensor can still capture the conveyor belt strain after withstanding high pressure (1.2-1.4 MPa) and high temperature (150 °C) during the belt vulcanization process. This validates the feasibility of using flexible strain sensors in steel wire core conveyor belts and has some potential for detecting abnormal strains in steel wire core conveyor belt, broadening the application field of flexible sensors.

Transparent single crystal graphene flexible strain sensor with high sensitivity for wearable human motion monitoring

Human-machine interfaces, smart glasses, touch screens and some electronic skins all require highly transparent and flexible strain sensing components. Flexible strain sensors usually use microstructures or porous active materials to improve sensitivity and response speed, resulting in low transparency of the device. Graphene has attracted extensive attention due to its special physical properties, especially excellent flexibility, and high transmittance. However, previous studies found that it is difficult to obtain high gauge factor in graphene. Thus, graphene cannot be used to monitor subtle deformations. Although previous research has shown that the strain sensors based on the combination of graphene and other materials have high sensitivity, but lack of transmittance. In this paper, we report a single-crystal graphene (SCG) strain sensor fabricated by encapsulating single-crystal hexagonal graphene in a flexible substrate. The strain sensor shows excellent performance with a high gauge factor (16.9) because it is not affected by wrinkles, grain boundaries and additional layers. Further, the strain sensor retains the advantage of high light transmittance of graphene. This sensitive SCG strain sensor has been demonstrated to detect a range of human motions, including finger gestures, tiny vibration of different sounds and heartbeats with high performance. This work further confirms the application of graphene in strain sensors, and provides new ideas for the research of graphene-based transparent strain sensors.

Fabrication and verification of vertical graphene-based strain sensor

The purpose of structural health monitoring in civil and mechanical engineering infrastructure is to make a structure safer at a lower cost. Strain, one of the crucial monitoring variables, has an innate relationship with deformation and is sensitive to the degeneration of load-bearing structural elements. In this study, a set of wireless real-time data acquisition and strain monitoring systems based on vertical graphene (VG) were developed. Both the parametric study and the feasibility of the VG-based strain sensor in crack monitoring of concrete structures were investigated. The greatest resistance difference in the cyclic tensile test is 2.99%, and the linearity is 2.4%, proving the newly designed sensor has good repeatability and stability. According to the monotonic tensile test based on a micro-displacement control platform, the relative resistance to change ΔR/R rises as applied strain rises and the repeatability error results in 0.576%. According to the notched concrete beam under the three-point bending test, the VG-based strain sensor's discrepancy from the extensometer's monitoring data is within 11.04%. The findings of this study suggest that the newly developed VG-based strain sensor has the potential for practical application in field strain monitoring, thus reaffirming the feasibility and effectiveness of this novel sensing technology.

Wearable and Flexible Multifunctional Sensor Based on Laser-Induced Graphene for the Sports Monitoring System.

The conversion of diverse polymeric substrates into laser-induced graphene (LIG) has recently emerged as a single-step method for the fabrication of patterned graphene-based wearable electronics with a wide range of applications in sensing, actuation, and energy storage. Laser-induced pyrolysis technology has many advantages over traditional graphene design: eco-friendly, designable patterning, roll-to-roll production, and controllable morphology. In this work, we designed wearable and flexible graphene-based strain and pressure sensors by laminating LIG from a commercial polyimide (PI) film. The as-prepared LIG was transferred onto a thin polydimethylsiloxane (PDMS) sheet, interwoven inside an elastic cotton sports fabric with the fabric glue as a wearable sensor. The single LIG/PDMS layer acts as a strain sensor, and a two-layer perpendicular stacking of LIG/PDMS (x and y laser-directed films) is designed for pressure sensing. This newly designed graphene textile (IGT) sensor performs four functions in volleyball sportswear, including volleyball reception detection, finger touch foul detection during blocking the ball from an opponent player, spike force measurements, and player position monitoring. An inexpensive sensor assists athletes in training and helps the coach formulate competition strategies.

Printable and Wearable Graphene-Based Strain Sensor With High Sensitivity for Human Motion Monitoring

Wearable strain sensors are appealing for monitoring human activities and evaluating human health. However, the development of wearable strain sensors with high sensitivity, large-scale manufacturing ability as well as the using of eco-friendly raw materials is still challenging. In this work, a simple and environmentally friendly high-performance strain sensor based on graphene/thermoplastic polyurethane (TPU) composite is reported. The green solvent dihydrolevoglucosenone (Cyrene) was used to dissolve and disperse TPU and graphene to make conductive ink, and the graphene/TPU composite film was embedded in the polydimethylsiloxane (PDMS) matrix by stencil printing and transfer process. The optimized sensor showed a very low detection limit (strain <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim ~0.1~\unicode{0x2030}$ </tex-math></inline-formula> ), high sensitivity (GF >3905) and broad sensing range (0 - 82.4 %). Moreover, the sensor remains stable under 5,000 times of loading/unloading cycles, with outstanding fatigue resistance and long-term durability. Additionally, a series of human motion monitoring applications were demonstrated, indicating their great potential in practical applications.

Sensitivity enhanced, highly stretchable, and mechanically robust strain sensors based on reduced graphene oxide-aramid nanofibers hybrid fillers

Recently, high-performance flexible strain sensors with remarkable sensing performance and desired mechanical properties have attracted great attention in the field of wearable electronic devices. However, it is still challenging to realize the tuning of sensitivity and sensing range in graphene-based strain sensors. Herein, a sensitivity enhanced, highly stretchable, and mechanically robust strain sensor is proposed based on reduced graphene oxide (RGO)-aramid nanofibers (ANFs) hybrid fillers and natural rubber (NR) matrix. Due to the strong π-π interactions, the RGO-ANFs hybrid fillers were constructed and orderly assembled around the NR particles, resulting in the formation of a unique interconnected conductive network. The resultant RGO-ANFs/NR strain sensors exhibited significantly improved sensitivity (gauge factor up to 870), wide detection range (0 ∼ 226%), expected mechanical properties (tensile strength >6 MPa), excellent stability, repeatability, and durability (1000 cycles), as well as fast feedback (response time of 195 ms). The strain sensor could monitor joint movements and physiological activities in real time at a wide temperature range (-20 ℃ ∼ 60 ℃), demonstrating the prospect as a flexible wearable device that could be applied under various temperature conditions.

Novel Two-Dimensional ABX3 Dirac Materials: Achieving a High-Speed Strain Sensor via a Self-Doping Effect

The pristine semimetal property of two-dimensional (2D) Dirac materials has limited their practical applications in today's electronic devices. Here we report a new type of 2D Dirac material, termed ABX3 (A = F, Cl, Br, or I; B = P or As; X = C or Si) monolayers. We demonstrate that 14 ABX3 monolayers possess good stability and high Fermi velocities. The FPC3, ClPC3, BrPC3, and FAsC3 monolayers exhibit a pristine n-type self-doping Dirac cone due to the interactions of electrons between the A-B units and C6 rings, which is beneficial for realizing high-speed carriers. Interestingly, the ClPSi3 monolayer exhibits remarkable responses to strain because a self-doping Dirac cone can be induced by relatively small in-plane biaxial strains (-5%), and the current-voltage (I-V) curves verified that the response strength is 11.57 times that of the graphene-based strain sensor at a bias of 1.10 V, indicating that the ClPSi3 monolayer could be used as a potential excellent strain sensor.

Graphene as a Piezoresistive Material in Strain Sensing Applications.

High accuracy measurement of mechanical strain is critical and broadly practiced in several application areas including structural health monitoring, industrial process control, manufacturing, avionics and the automotive industry, to name a few. Strain sensors, otherwise known as strain gauges, are fueled by various nanomaterials, among which graphene has attracted great interest in recent years, due to its unique electro-mechanical characteristics. Graphene shows not only exceptional physical properties but also has remarkable mechanical properties, such as piezoresistivity, which makes it a perfect candidate for strain sensing applications. In the present review, we provide an in-depth overview of the latest studies focusing on graphene and its strain sensing mechanism along with various applications. We start by providing a description of the fundamental properties, synthesis techniques and characterization methods of graphene, and then build forward to the discussion of numerous types of graphene-based strain sensors with side-by-side tabular comparison in terms of figures-of-merit, including strain range and sensitivity, otherwise referred to as the gauge factor. We demonstrate the material synthesis, device fabrication and integration challenges for researchers to achieve both wide strain range and high sensitivity in graphene-based strain sensors. Last of all, several applications of graphene-based strain sensors for different purposes are described. All in all, the evolutionary process of graphene-based strain sensors in recent years, as well as the upcoming challenges and future directions for emerging studies are highlighted.

A Practical Strain Sensor Based on Ecoflex/Overlapping Graphene/Ecoflex Sandwich Structures for Vocal Fold Vibration and Body Motion Monitoring

Graphene-based stretchable and flexible strain sensors are one of the promising “bridges” to the biomedical realm. However, enhancing graphene-based wearable strain sensors to meet the demand of high sensitivity, broad sensing range, and recoverable structure deformation simultaneously is still a great challenge. In this work, through structural design, we fabricated a simple Ecoflex/Overlapping Graphene/Ecoflex (EOGE) strain sensor by encapsulating a graphene sensing element on polymer Ecoflex substrates using a drop-casting method. The EOGE strain sensor can detect stretching with high sensitivity, a maximum gauge factor of 715 with a wide strain range up to 57%, and adequate reliability and stability over 1,000 cycles for stretching. Moreover, the EOGE strain sensor shows recoverable structure deformation, and the sensor has a steady response in the frequency disturbance test. The good property of the strain sensor is attributed to the resistance variation induced by the overlap and crack structure of graphene by structural design. The vibrations caused by sound and various body movements have been thoroughly detected, which exhibited that the EOGE strain sensor is a promising candidate for wearable biomedical electronic applications.

Graphene-based temperature, humidity, and strain sensor: A review on progress, characterization, and potential applications during Covid-19 pandemic

Graphene's potential as material for wearable, highly sensitive and robust sensor in various fields of technology has been widely investigated until now in order to capitalize on its unique intrinsic physical and chemical properties. In the wake of Covid-19 pandemic, it has been noticed that there are various potentials roles that can be fulfilled by graphene-based temperature, humidity and strain sensor, whose roles has not been widely explored to date. This paper takes the liberty to mainly highlight the progress layout and characterization technique for graphene-based sensor while including a brief discussion on the possible strategy of sensing data analysis that can be employed to minimize and prevent the risk of Covid-19 infection within a living community. While majority of the reported sensor is still in the in-progress status, its highlighted role in this work may provide a brief idea on how the ongoing research in graphene-based sensor may lead to the future implementation of the device for routine healthcare check-up and diagnostic point-care during and post-pandemic era. On the other hand, the sensitivity and response time data against working temperature, humidity and strain range that are provided could serve as a reference for benchmarking purpose, which certainly would help enthusiast in the development of a graphene-based sensor with a better performance for the future.

Graphene-based strain sensor with sandwich structure and its application in bowel sounds monitoring.

Surgery is one of the primary treatment modalities for gastrointestinal tumors but can lead to postoperative ileus (POI), which can aggravate pain and increase costs. The incidence of POI can be effectively reduced by monitoring bowel sounds to assist doctors in deciding the timing of transoral feeding. In this study, we prepared a flexible strain sensor based on a graphene composite material and tested the feasibility of sensor monitoring of bowel sounds using simultaneous stethoscope and sensor monitoring. We found that the time of hearing the bowel sounds (12.0-12.1 s) corresponded to the time of waveform change monitored by the sensor (12.036 s), and the sound tone magnitude corresponded to the waveform amplitude. This proves that the application of sensors to monitor bowel sounds is feasible, which opens up a new field for the application of graphene sensors and provides a new way for clinicians to judge the condition of the intestine.

Waterproof Graphene-PVDF Wearable Strain Sensors for Movement Detection in Smart Gloves.

In this work, new highly sensitive graphene-based flexible strain sensors are produced. In particular, polyvinylidene fluoride (PVDF) nanocomposite films filled with different amounts of graphene nanoplatelets (GNPs) are produced and their application as wearable sensors for strain and movement detection is assessed. The produced nanocomposite films are morphologically characterized and their waterproofness, electrical and mechanical properties are measured. Furthermore, their electromechanical features are investigated, under both stationary and dynamic conditions. In particular, the strain sensors show a consistent and reproducible response to the applied deformation and a Gauge factor around 30 is measured for the 1% wt loaded PVDF/GNP nanocomposite film when a deformation of 1.5% is applied. The produced specimens are then integrated in commercial gloves, in order to realize sensorized gloves able to detect even small proximal interphalangeal joint movements of the index finger.

Ultrafast laser direct writing of screen-printed graphene-based strain electrodes for sensing glass deformation

This study aimed to develop screen-printed graphene-based strain sensors for sensing the deformation of glass. Graphene electrodes of strain sensors with various grid lengths were fabricated by ultrafast laser direct writing. The experimental results revealed that the laser ablation threshold for graphene films was approximately 0.35 J/cm2. For areal fluences exceeding 7.8 J/cm2, the laser-ablated zone on graphene films served as electrical insulation because the conductive film was completely removed. The optimal laser fabrication parameters for strain sensors were a pulse repetition frequency of 300 kHz, areal fluence of 31.9 J/cm2, scan speed of 500 mm/s, line-scan spacing of 0.01 mm, and overlapping rate of laser spots of 94.4%. Laser-formed electrode structures on graphene films had straight edges and sharp corners. Moreover, no residual film and debris existed on the processing paths. During deformation tests on glass substrates with bending distances of 0–180 μm, the strain sensor with a 6-mm grid length exhibited an excellent linear relationship with the change in resistance. Furthermore, the gauge factor of 550.14 for strain sensors with a 6-mm grid length was larger than that of 392.48 and 308.17 for strain sensors with 8- and 10-mm grid lengths, respectively.

Graphene-based pressure sensor and strain sensor for detecting human activities

Wearable pressure sensors and strain sensors with high sensitivity and large working range are essential for detecting human motions. However, fabricating wearable sensors, which are capable of detecting pressure or strain signals, still remains challenging. Herein, two kinds of novel sensors, pressure sensors and strain sensors, are fabricated using the same materials and similar fabrication processes. The sensors are fabricated by soaking cotton in graphene inks so as to avoid high temperature and toxic chemicals caused by reduction compared to traditional graphene oxide inks. The pressure sensor shows excellent performance with high sensitivity (0.12–0.41 kPa−1) and broad working range (0–20 kPa). The strain sensor also has outstanding sensitivity (gauge factor 22.6–83.7) and large working strain of 27%. These two sensors are further demonstrated to show the ability of detecting human activities such as breathing, wrist pulsing, respiration, etc. The low-cost and scalable fabrication along with the good comprehensive performance of the sensors makes them applicable in wearable electronic devices for human health monitoring and movements.

Mechanically robust, highly sensitive and superior cycling performance nanocomposite strain sensors using 3-nm thick graphene platelets

Flexible, highly sensitive stain sensors prepared from polymer nanocomposites have attracted a great deal of interests, due to the rapid development of robotics, transportation, aerospace and health monitoring. However, these sensors are often limited by unideal mechanical properties, sensitivity and cycling performance. We herein report a facile approach to developing high-cycling performance strain sensors based on a nanocomposite film. The film was prepared by dispersing 3 nm-thick graphene platelets (GnPs) within an epoxy matrix. TEM micrographs confirmed that GnPs were uniformly dispersed in epoxy whilst some were connected to each other, which contributed to the mechanical properties and electrical conductivity of the resulting nanocomposites. At 2.0 vol% GnPs, Young's modulus, fracture toughness and energy release rate of neat epoxy were improved by 93%, 135% and 215%, respectively. Having gauge factors of 1–33, the film sensor demonstrated high sensitivity to tensile strains 0–0.67%. Meanwhile, the film sensor revealed excellent performance over 35,000 cycles due to high fracture toughness and effective dissipation of the heat accumulated during fatigue cycles. Practically, the film sensor also demonstrated effective response to temperature, humidity and damage evolution. This work provides an effective strategy for developing graphene-based strain sensors of excellent mechanical properties, sensitivity and reliability.

High sensitivity and flexible fabric strain sensor based on electrochemical graphene

In this study, electrochemical graphene (ECG) was chosen to be attached to a polyester fabric with a knitted structure as a flexible fabric strain sensor through a cloth dyeing method. According to scanning electron microscopy observation and Raman scattering spectroscopy measurements, ECG was successfully attached to the fabric structure. The electromechanical performance and strain sensing properties of the graphene-based flexible strain sensor were evaluated. The resistance change and gauge factor of different ECG attached as a fabric strain sensor were quantified. The gauge factor of the fabric strain sensor with less ECG is greater than that of the fabric strain sensor with more ECG. The high gauge factor of 47 was achieved in a strain range from 0% to 2%. The fabricated strain sensor exhibited high sensitivity, fast response time and good repeatability. The fabricated ECG fabric strain sensor could be used to detect tiny physiological signals and human motion.