Piezoresistive Strain Sensors Research Articles

Simulation of the role of agglomerations in the tunneling conductivity of polymer/carbon nanotube piezoresistive strain sensors

A 3D Monte Carlo method paired with a percolation model is carried out to predict the electrical resistivity and piezoresistive sensitivity of carbon nanotube (CNT)-polymer piezoresistive sensors. CNTs are randomly dispersed into the insulating layer of a polymer matrix having agglomerated morphologies in some parts. The electrical resistivity of nanocomposite with a defined agglomeration state is determined by considering the tunneling effect between each connected pair of CNTs. The influence of various parameters such as agglomeration radius, percentage, intrinsic properties and geometries of CNTs are investigated. A comparison was made between the analytical results and experimental data. Considering the tunneling behavior in the vicinity of the percolation transition, a good agreement is obtained between the analytical results and experimental data. Monte Carlo simulations indicated that the tunneling resistance with a random distribution of CNTs depends on the level of agglomeration. Results revealed that piezoresistive sensitivity was diminished by larger agglomerations.

Transparent and Stretchable Piezoresistive Strain Sensors with Buckled Indium Tin Oxide Film

AbstractOwing to excellent performance and a simple mechanism, stretchable piezoresistive strain sensors have been applied to human skin for monitoring physical activities, physiological activities, etc. However, it is still a challenge to simultaneously realize highly sensitive and stretchable piezoresistive strain sensors with high optical transparency. This study reports a transparent and stretchable piezoresistive strain sensor with 2D surface buckling by fabricating ultrathin indium tin oxide (ITO) film on the biaxially pre‐stretched polyacrylate (VHB) elastomer followed by pre‐stretch releasing. To the authors' knowledge, semiconductors are applied for a stretchable piezoresistive strain sensor for the first time. Furthermore, this strain sensor exhibits a high sensitivity of 569, a high transparency of 88% and a high biaxial stretchability of 110% at the same time. This device demonstrates the good long‐term stability over 500 stretching–relaxing cycles. The high sensitivity can be mainly attributed to the piezoresistive effect of the semiconductor where carrier mobility and the resulting resistivity can be significantly changed by the strain. The strain sensors attached to human skin are used to monitor many human motions such as chewing, swallowing, breathing, and walking. ITO‐based strain sensors pave the way toward the development of highly stretchable and sensitive wearable electronics.

Multifunctional bacterial cellulose photothermal aerogels with multi-bonded network assisted by carbon nanotube

As one of the important ways to utilize solar energy, it is of great significance to construct composite photothermal materials with high photothermal conversion and multifunctionality for in-depth investigation of their application performance. The nanofiber network and the reactivity of a large number of hydroxyl groups on the surface of environmentally friendly bacterial cellulose (BC) provide a prerequisite for its use as an excellent matrix material. Here, we use bacterial cellulose nanofibers (BCNFs) as the matrix, methyltrimethoxysilane (MTMS) as the crosslinking agent and hydroxylated multi-walled carbon nanotubes (MCNT − OH) as the photothermal absorber and conductive substrate to construct a multifunctional BC photothermal aerogel with multi-bonded network structure. Multiple chemical bond and physical bond interactions are formed between BCNF, MTMS and CNT, and fiber entanglement is combined to make CNT uniformly dispersed. This optimal photothermal aerogel has rich network structure with high hydrophobicity of 147°, compressive strength of 17.62 kPa and high elasticity. It also shows high light absorption of 93.5%, eminent photothermal conversion of 80.4 °C and photothermal stability under 1 kW m−2 irradiation with a low CNT content of 27.1%. Therefore, the photothermal aerogel shows potential of multi-scenario applications in the field of solar-driven high-viscosity crude oil recovery and piezoresistive strain sensors.

Development and testing of a stationary dynamometer using cross-beam-type force-sensing elements for three-axis cutting force measurement in milling operations

This study focused on the development and testing of a stationary dynamometer using force-sensing elements capable of measuring three axes of cutting force during milling operations. The force-sensing element was based on the Maltese cross-beam concept but has been improved and modified to increase sensitivity and reduce interference or cross-talk error. A Finite element analysis was conducted to study strain distribution and determine sensor positions in the force-sensing element. By using four force-sensing elements and several piezoresistive strain sensors, a stationary dynamometer prototype was constructed. A series of calibration tests were performed to evaluate the dynamometer’s sensitivity, linearity, hysteresis, and repeatability over three orthogonal axes. The dynamometer’s dynamic features and functionality for milling applications were examined through modal analysis and real milling tests. Additionally, the end-milling and slot-milling cutting force values were compared to those of a reference dynamometer (Kistler 9129AA). Overall, the experimental results indicated that the proposed stationary dynamometer is a reliable and accurate alternative for measuring cutting force during machining operations and other force measurements.

Electrically Conductive and Highly Stretchable Piezoresistive Polymer Nanocomposites via Oxidative Chemical Vapor Deposition.

Electrically conductive polymer nanocomposites have been the subject of intense research due to their promising potential as piezoresistive biomedical sensors, leveraging their flexibility and biocompatibility. Although intrinsically conductive polymers such as polypyrrole (PPy) and polyaniline have emerged as lucrative candidates, they are extremely limited in their processability by conventional solution-based approaches. In this work, ultrathin nanostructured coatings of doped PPy are realized on polyurethane films of different architectures via oxidative chemical vapor deposition to develop stretchable and flexible resistance-based strain sensors. Holding the substrates perpendicular to the reactant flows facilitates diffusive transport and ensures excellent conformality of the interfacial integrated PPy coatings throughout the 3D porous electrospun fiber mats in a single step. This allows the mechanically robust (stretchability > 400%, with fatigue resistance up to 1000 cycles) nanocomposites to elicit a reversible change of electrical resistance when subjected to consecutive cycles of stretching and releasing. The repeatable performance of the strain sensor is linear due to dimensional changes of the conductive network in the low-strain regime (ε ≤ 50%), while the evolution of nano-cracks leads to an exponential increase, which is observed in the high-strain regime, recording a gauge factor as high as 46 at 202% elongational strain. The stretchable conductive polymer nanocomposites also show biocompatibility toward human dermal fibroblasts, thus providing a promising path for use as piezoresistive strain sensors and finding applications in biomedical applications such as wearable, skin-mountable flexible electronics.

A Mechanized Air-Atomized Spray Coating Method for Fabricating Flexible Nanocomposite Piezoresistive Strain Sensors

Nanocomposite piezoresistive strain sensors have offered new avenues for strain measurement of structures with significant deformations, large geometries or complex contours. The affordable and efficient spray coating technique, especially the air-atomized spray coating method, has considerable potential for batch production of such sensors. However, one of the most popular ways to fabricate sensors is manual spray coating, which not only has a low transfer efficiency, resulting in material waste and environmental pollution, but also makes it challenging to precisely control the homogeneity of sensor coatings, leading to inconsistent quality of existing sprayed sensors. Therefore, a mechanized air-atomized spray coating method is proposed to actualize the stable batch production of sprayed sensors, and spraying parameters are then determined to establish proper process conditions. Carbon black (CB) nanoparticle reinforced polyvinyl pyrrolidone (PVP) nanocomposites are prepared to fabricate CB/PVP strain sensors to validate the proposed method. Compared with the manual spray coating, the proposed method can manufacture CB/PVP coatings with uniform thickness distribution and micro-morphology. Additionally, the mechanical and electrical properties of CB/PVP nanocomposites are subsequently investigated to verify the consistency of coating quality using nanoindentation and four-probe techniques, respectively. Furthermore, the superior sensitivity and large strain range of CB/PVP strain sensors are evaluated by uniaxial tensile testing. In conclusion, the mechanized air-atomized spray coating method is not only time-saving, inexpensive and straightforward to implement, but also features wide applicability due to minor requirements for nanocomposites, thus laying a foundation for batch production and popularization of nanocomposite piezoresistive strain sensors.

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

All Knitted and Integrated Soft Wearable of High Stretchability and Sensitivity for Continuous Monitoring of Human Joint Motion.

E-textiles have recently gained significant traction in the development of soft wearables for healthcare applications. However, there have been limited works on wearable e-textiles with embedded stretchable circuits. Here, stretchable conductive knits with tuneable macroscopic electrical and mechanical properties are developed by varying the yarn combination and the arrangement of stitch types at the meso-scale. Highly extensible piezoresistive strain sensors are designed (>120% strain) with high sensitivity (gauge factor 8.47) and durability (>100,000 cycles), interconnects (>140% strain) and resistors (>250% strain), optimally arranged to form a highly stretchable sensing circuit. The wearable is knitted with a computer numerical control (CNC)knitting machine that offers a cost effective and scalable fabrication method with minimal post-processing. The real-time data from the wearable is transmitted wirelessly using a custom designed circuit board. In this work, an all knitted and fully integrated soft wearable is demonstrated for wireless and continuous real-time sensing of knee joint motion of multiple subjects performing various activities of daily living.

Additive manufacturing of nylon composites with embedded multi-material piezoresistive strain sensors for structural health monitoring

A hybrid additive manufacturing process is proposed to incorporate piezoresistive sensors into a polyamide matrix for real-time structural health monitoring. 3D-printed samples were tested under tensile, flexural, and indentation testing using various sizes, numbers, and locations of the sensors, while the electrical resistance was recorded in real-time. Under uniaxial tensile loading, strength reached 36 MPa, and the resistance values linearly increased by up to 105% at ∼150% of strain. During flexural testing, the sensor had higher resistance when it was located above the supports rather than under the loading roller. The conductive paths were placed near the plate's top, middle and bottom sections to localise the damage during indentation testing. The energy absorbed during indentation testing reached 34–36 J. Resistance readings of sensors with 1 cm width showed better linearity and sensitivity than their wide (10 cm) counterparts. Three 2D sensors were interconnected to make a 3D sensor. The resistance values gradually increased after each strike during repeated indentation, but the overall strength remained similar. In summary, this work demonstrates a novel approach toward customisable 3D-printed piezoresistive sensors for detecting global or local damage.

Isotropic auxetic structure with fixed Poisson's ratio for piezo-resistive strain sensor to detect human body motion

Auxetic materials have different types whose mechanical behaviors are different from each other, and these differences affect their sensitivity. However, in the piezo-resistive strain sensor field, less attention has been paid to this variety of structures. In this article, the isotropic auxetic structure with three axes of symmetry was used to construct the isotropic linear piezo-resistive strain sensor. Structures that have only three or six symmetry axes have isotropic mechanical behavior, and structures with many symmetry axes other than these two cases are not isotropic. Also, to obtain a sensor with a linear sensing performance, the Poisson's ratio of the sensor must be constant during the strain. Most of the structures presented in the previous works have a variable Poisson's ratio along the strain. When the Poisson’s ratio of the structure is constant during the strain, it causes the sensing elements to have the same increased space to separate from each other during the strain, and as a result, the sensor will have a linear performance. A computer numerical control device was utilized to manufacture the sensor mold. Silicon RTV2 as a substrate material, and graphite powder as a sensing element were mixed to provide the sensor constitutive material, and molded into the manufactured die. To evaluate the response of the sensor in different directions, the sensitivity tests were performed in the directions of 0°, 60°, and 120°, which are compatible with each other, and also the suggested sensor has linear sensitivity during the strain.

Synergistically toughened silicone rubber nanocomposites using carbon nanotubes and molybdenum disulfide for stretchable strain sensors

Stretchable rubber nanocomposites have been investigated for detection in piezoresistive strain sensors. Tensile toughness must be improved while maintaining stretchability to realize high-performance, robust, stretchable nanocomposites. We investigate a stretchable and synergistically toughened silicone nanocomposite-based, piezoresistive strain sensor using multiwalled carbon nanotubes (MWCNTs) and molybdenum disulfide (MoS2). One-dimensional MWCNTs mechanically reinforce the silicone rubber matrix, improving tensile strength by 95% but decreasing fracture strain by 27% at 5 phr, providing an electrical percolation network. Adding two-dimensional nanolayers of MoS2 as a single filler increases fracture strain from 205% (unfilled) to 320% (5 phr MoS2), possibly due to cure retardation in the silicone rubber, preventing over-curing. Silicone nanocomposites exhibit improved tensile toughness at 8.46 kJ/m3 for 5-phr MWCNT-MoS2 hybrids due to the synergistic effects of the hybrid filler, a 125% improvement over unfilled rubber. Nanocomposites maintain a low elastic modulus (<0.45 MPa) suitable for stretchable sensors. The nanocomposite utility is demonstrated as body-attachable, piezoresistive strain sensing with a gauge factor of ∼25.95 (100%<Δε<155%), stretchability of up to 155%, and durability of over 5000 cycles. Toughened nanocomposites with MWCNT and MoS2 hybrid fillers could be useful for applications requiring robust stretchability and sensitivity.

Manufacturing of Highly Sensitive Piezoresistive Two-Substances Auxetic Strain Sensor Using Composite Approach

Manufacturing of Highly Sensitive Piezoresistive Two-Substances Auxetic Strain Sensor Using Composite Approach

Thermally Drawn Super‐Elastic Multifunctional Fiber Sensor for Human Movement Monitoring and Joule Heating

AbstractWith the increasing demand of smart electronic textiles, fiber‐shaped sensing devices have rapidly attracted growing interests, in which elastic fiber sensors can monitor a wider range of strains without compromising their flexibility and adaptability with conventional textiles. However, it is still a major challenge to prepare highly sensitive and durable sensing fibers with multifunction. Herein, using a thermally drawn process, highly stretchable elastomeric fibers that are composited with copper particle‐mixed liquid metal electrodes to produce highly elastic piezoresistive strain sensor are fabricated. The composite liquid medal fiber sensor (LMFS) exhibits a strain at break up to 850% and a tensile stress of 6 MPa. When used as a tensile sensor, LMFS can detect large strain levels up to 400%. As a pressure sensor, LMFS can detect an object as light as 0.2 N. The composite fibers can be integrated into a fabric to monitor human motions and Morse code signals. In addition, the fibers can also be applied as an electric heater, the temperature quickly reaches 45 °C at 0.6 V in its original length and 29.3 °C at 0.6 A current with a 250% strain. This work exhibits excellent potential of multifunctional fiber devices in wearable sensing devices.

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.

Wearable and Stretchable SEBS/CB Polymer Conductive Strand as a Piezoresistive Strain Sensor

A wearable and stretchable strain sensor with a gauge factor above 23 was prepared using a simple and effective technique. Conducting nanocomposite strands were prepared from styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer (SEBS) and carbon black (CB) through a solvent-processing method that uses a syringe pump. This novel nanocomposite preparation technique is a straightforward and cost-effective process and is reported in the literature for the first time. The work included two stages: the flexible nanocomposite preparation stage and the piezoresistive sensor stage. Depending on its molecular structure, the thermoelastic polymer SEBS is highly resilient to stress and strain. The main aim of this work is to fabricate a highly flexible and piezoresistive nanocomposite fibre/strand. Among the prepared composites, a composite corresponding to a composition just above the percolation threshold was selected to prepare the strain sensor, which exhibited good flexibility and conductivity and a large piezoresistive effect that was linearly dependent on the applied strain. The prepared nanocomposite sensor was stitched onto a sports T-shirt. Commercially available knee and elbow sleeves were also purchased, and the nanocomposite SEBS/CB strands were sewn separately on the two sleeves. The results showed a high sensitivity of the sensing element in the case of breathing activity (normal breathing, a 35% change, and deep breathing at 135%, respectively). In the case of knee and elbow movements, simultaneous measurements were performed and found that the sensor was able to detect movement cycles during walking.

Flexible strain sensors based on gold nanowire dominoes for human motion detection

Flexible strain sensors have broad application potentials in sports, healthcare and various other fields. However, the large-scale fabrication of strain sensors with both high sensitivity and stretchability simultaneously is still challenging nowadays. Inspired by the game of dominoes, we proposed a strain sensor with a novel mechanism based on gold nanowire (AuNWs) dominoes. Unlike the general piezoresistive strain sensors of micro-crack or percolation mechanisms, AuNWs domino sensors realize the mechanical sensing due to the change of the overlapping area when subjected to external force. The strain sensor performs excellently with a wide detection range(0–250 % strain), an ultrahigh sensitivity (gauge factor of 490 at a strain of 10 %), and high durability and repeatability. The in-situ characterization proved that the overlapped AuNWs dominoes slid and displaced with the increasing strain, and the overlap area decreased before the fracture occurred, thus keeping the effective conducting pathway. Based on its innovative sensing mechanism and superior sensing performance, AuNWs domino sensors effectively suppress conductive path cracks and have the potentials for wearable healthcare devices.

Development of Low Cost, Fast, and Highly Stable Piezoresistive Strain Sensors to Monitor Human Movement

A low-cost, easy-to-fabricate, and highly sensitive piezoresistive strain sensor was designed in this study. A well-known conductive polymer, for example, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), was deposited on the latex-free elastic bands to prepare the strain sensor. The sensitivity of the proposed sensor was about 4.27 over the wide strain measurement range of about 0%–83%. The sensor also exhibited a good hysteresis response; highly stable sensing performance with a relative standard deviation (RSD) value of about 1.32%; good linearity, that is, the correlation coefficient, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}^{{2}}$ </tex-math></inline-formula> , value of about 0.987; and high reproducibility. The proposed strain sensor was installed on different joints of the human body to observe the movement of the human body, and satisfactory results were obtained. In addition, the proposed strain sensing element provided a fast response and recovery time of about 1.74 and 3.73 ms, respectively. Index Terms— Conductive polymer, linearity, piezoresistive, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), stability, strain sensor.

Optics-Free, In Situ Swelling Monitoring of Articular Cartilage with Graphene Strain Sensors.

Articular cartilage derives its load-bearing strength from the mechanical and physiochemical coupling between the collagen network and negatively charged proteoglycans, respectively. Current disease modeling approaches and treatment strategies primarily focus on cartilage stiffness, partly because indentation tests are readily accessible. However, stiffness measurements via indentation alone cannot discriminate between proteoglycan degradation versus collagen degradation, and there is a lack of methods to monitor physiochemical contributors in full-stack tissue. To decouple these contributions, here, we developed a platform that measures tissue swelling in full-depth equine cartilage explants using piezoresistive graphene strain sensors. These piezoresistive strain sensors are embedded within an elastomer bulk and have sufficient sensitivity to resolve minute, real-time changes in swelling. By relying on simple DC resistance measurements over optical techniques, our platform can analyze multiple samples in parallel. Using these devices, we found that cartilage explants under enzymatic digestion showed distinctive swelling responses to a hypotonic challenge and established average equilibrium swelling strains in healthy cartilage (4.6%), cartilage with proteoglycan loss (0.5%), and in cartilage with both collagen and proteoglycan loss (-2.6%). Combined with histology, we decoupled the pathologic swelling responses as originating either from reduced fixed charge density or from loss of intrinsic stiffness of the collagen matrix in the superficial zone. By providing scalable and in situ monitoring of cartilage swelling, our platform could facilitate regenerative medicine approaches aimed at restoring osmotic function in osteoarthritic cartilage or could be used to validate physiologically relevant swelling behavior in synthetic hydrogels.

A triboelectric nanogenerator powered piezoresistive strain sensing technique insensitive to output variations

With the increasing trend of self-powered sensing in the era of Internet of Things (IoTs), triboelectric nanogenerator (TENG) sensors have attracted great attention due to their high efficiency, versatile configuration, and low cost. However, because of their inherent variation of outputs due to changing environmental conditions, the output magnitude of TENGs cannot be used directly as the primary sensing signal in practical scenario. To address this issue, herein a new method of extracting sensing signal is proposed that is independent of TENG output variations. The core idea is to split the output current of TENG into two parallel branches containing a piezoresistive strain sensor and reference resistors, respectively. The ratio of the currents in the two branches can be used to accurately determine the applied strain because the ratio is uniquely dependent on the resistance of the piezoresistive sensor and is insensitive to the TENG output magnitude, thus eliminating the effect of output variation on the sensing performance. As demonstrated in this work, a TENG can be used as the power source to activate piezoresistive sensors without knowing its output characteristics, although higher current outputs are preferred to minimise error from noise. This new method provides a new self-powered sensing solution that is invariant to output vairations due to changing environments.

A Reliable Strain Sensor Based on Bridging GaN Nanowires

A reliable piezoresistive strain sensor (PSS) was fabricated by using the directly bridging GaN nanowires (NWs). The bridging NWs were epitaxially grown over a deep trench on GaN-coated sapphire substrate, so homogeneous, solid, and robust connections between the electrode (GaN coating layer) and NWs can be realized without electrical contact barrier. The sensing properties of the GaN-NW PSS were investigated by measuring the deflection-induced resistance variation of NWs. The GaN-NW PSS demonstrates good repeatability, fast response speed, and a relatively high gauge factor (GF) of ~59. To our knowledge, it is the first time that the piezoresistive effect of GaN NW was investigated.