Multidirectional Strain Research Articles

Artificial neural network assisted omnidirectional strain sensors for human motion perception

Flexible resistance strain sensors gain tremendous attentions in emerging fields such as wearable sensing, electronic skins and human–machine interfaces due to its excellent stretchability and sensitivity. However, due to the coupling of strain along both principal and perpendicular axes, conventional uniaxial strain sensors face challenges in recognizing multi-directional strain which naturally occurs in practical applications. In this work, we present the omnidirectional strain sensors comprised of multiple-stacked oriented conductive layers and harness the artificial neural network (ANN) to assist recognizing strain information with high accuracy. Each conductive layer consists of a patterned multi-walled carbon nanotubes mixture coated on a soft substrate, exhibiting remarkable sensitivity with a gauge factor of 4.18 along the principal direction. When centrally aligning two or three layers along different directions, the assembled structures can monitor the multi-directional electronic resistance changes under deformations, enabling the recognition of both strain direction and amplitude. The ANN model used three-dimensional resistance data as input to perform a multi-class classification task, with strain amplitude and direction as the outputs. The hidden layers employed the ReLU activation function, while the output layer utilized the SoftMax activation function. The model was compiled with the sparse_categorical_crossentropy loss function and optimized using the Adam optimizer. Its performance was evaluated based on prediction accuracy. The strain prediction accuracy is above 98 % with the assistance of an ANN model. The triple-layer configuration can perceive the human wrist motions such as rotation and bending, exhibiting high potential in applications such as multidimensional wearable electronics.

Multi-directional deformation and hydraulic conductivity of expansive soils subjected to freeze-thaw cycles from three distinct initial saturation levels

Infrastructure construction in seasonally frozen regions, covering 23% of total land, faces challenges from freeze-thaw (F-T) induced damages. Expansive soils, as an important problematic soil undergo major hydromechanical properties changes influenced by F-T cycles. Sand-bentonite mixtures are extensively used for constructing earthen hydraulic barriers in cold regions. This study investigates the influence of F-T cycles on multi-directional strains and anisotropic hydraulic conductivity of different sand-bentonite mixtures prepared at optimum water content and experienced three distinct saturation levels. Results indicate that saturation level and bentonite content significantly influence volumetric strain under F-T cycles. The simultaneous effect of ice lens formation, expanding micro-voids, and suction generated by freezing processes cause different volumetric behaviors at varying saturation degrees. The dry specimen exhibits no strain under F-T cycles, while optimum and saturated specimens experienced final volumetric strains of 1.02% and 3.03%, respectively. Notably, during freezing, the specimen at optimum water content shrank, while the saturated specimen expanded. Increasing bentonite content from 40% to 70% developed freezing-induced shrinkage, from 1.73% to 4.72%, with higher thaw strain attributed to increased specimen plasticity. Also, dimensional variations revealed the cross-anisotropic nature of specimens, highlighting direct influence of water content on the shrinkage ratio. F-T cycles also increased hydraulic conductivity along both orthogonal directions by two orders of magnitude, while the anisotropy ratio decreased by about 3 after 9 F-T cycles, indicating altered pore structures. F-T cycles induce reduced swelling potential and compressibility over subsequent cycles. Microstructural observations also confirmed the F-T effects on the enhancement of porosity.

Multi-Directional Strain Measurement in Fiber-Reinforced Plastic Based on Birefringence of Embedded Fiber Bragg Grating.

Embedded fiber Bragg gratings are increasingly applied for in-situ strain measurement in fiber-reinforced plastics, integral to high-end aerospace equipment. Existing research primarily focuses on in-plane strain measurement, limited by the fact that fiber Bragg gratings are mainly sensitive to axial strain. However, out-of-plane strain measurement is equally important for comprehending structural deformation. The birefringence of fiber Bragg gratings shows promise for addressing this problem; yet, the strain transfer relationship between composites and optical fibers, along with the decoupling method for multi-directional strains, remains inadequately explored. This study introduces an innovative method for multi-directional strain measurement in fiber-reinforced plastics using the birefringence of a single-fiber Bragg grating. The strain transfer relationship between composites and embedded optical fibers was derived based on Kollar's analytical model, leading to the development of a multi-directional strain decoupling methodology. This method was experimentally validated on carbon fiber/polyetherimide laminates under thermo-mechanical loading. Its reliability was confirmed by comparing experimental results and finite element simulations. These findings significantly broaden the application scenarios of fiber Bragg gratings, advancing the in-situ measurement technology crucial for the next generation of high-end aerospace equipment.

3D Printing-Induced Hierarchically Aligned Nanocomposites With Exceptional Multidirectional Strain Sensing Performance.

High-performance sensors capable of detecting multidirectional strains are indispensable to understand the complex motions involved in flexible electronics. Conventional isotropic strain sensors can only measure uniaxial deformations or single stimuli, hindering their practical application fields. The answer to such challenge resides in the construction of engineered anisotropic sensing structures. Herein, a hierarchically aligned carbon nanofiber (CNF)/polydimethylsiloxane nanocomposite strain sensor is developed by one-step 3D printing. The precisely controlled printing path and shear flow bring about highly aligned nanocomposite filaments at macroscale and orientated CNF network within each filament at microscale. The periodically orientated nanocomposite filaments along with the inner aligned CNF network successfully control the strain distribution and the appearance of microcracks, giving rise to anisotropic structural response to external deformations. The synergetic effect of the multiscale structural design leads to distinguishable gauge factors of 164 and 0.5 for applied loadings along and transverse to the alignment direction, leading to an exceptional selectivity of 3.77. The real-world applications of the hierarchically aligned sensors in multiaxial movement detector and posture-correction device are further demonstrated. The above findings propose new ideas for manufacturing nanocomposites with engineered anisotropic structure and properties, verifying promising applications in emerging wearable electronics and soft robotics.

Sensing with Thermally Reduced Graphene Oxide under Repeated Large Multi-Directional Strain.

This paper presents a recent investigation into the electromechanical behavior of thermally reduced graphene oxide (rGO) as a strain sensor undergoing repeated large mechanical strains up to 20.72%, with electrical signal output measurement in multiple directions relative to the applied strain. Strain is one the most basic and most common stimuli sensed. rGO can be synthesized from abundant materials, can survive exposure to large strains (up to 20.72%), can be synthesized directly on structures with relative ease, and provides high sensitivity, with gauge factors up to 200 regularly reported. In this investigation, a suspension of graphene oxide flakes was deposited onto Polydimethylsiloxane (PDMS) substrates and thermally reduced to create macroscopic rGO-strain sensors. Electrical resistance parallel to the direction of applied tension (x^) demonstrated linear behavior (similar to the piezoresistive behavior of solid materials under strain) up to strains around 7.5%, beyond which nonlinear resistive behavior (similar to percolative electrical behavior) was observed. Cyclic tensile testing results suggested that some residual micro-cracks remained in place after relaxation from the first cycle of tensile loading. A linear fit across the range of strains investigated produced a gauge factor of 91.50(Ω/Ω)/(m/m), though it was observed that the behavior at high strains was clearly nonlinear. Hysteresis testing showed high consistency in the electromechanical response of the sensor between loading and unloading within cycles as well as increased consistency in the pattern of the response between different cycles starting from cycle 2.

High-Performance Multidirectional Flexible Strain Sensor for Human Motion and Health Monitoring.

Multidirectional strain sensors are pivotal for wearable electronic devices and human-computer interaction. In this investigation, we translocate carbon/graphene (CB/Gr) conductive nanocomposites onto an Ecoflex flexible substrate via a facile technique encompassing reverse molding and spraying, culminating in the fabrication of a 45° strain rosette-shaped multidirectional flexible strain sensor. The sensor distinguishes itself with extraordinary performance characteristics, including high sensitivity (boasting a gauge factor of 35), an extensive strain range from 0 to 100%, exceptional linearity, a rapid response time of merely 200 ms, remarkable stability, and outstanding durability, effortlessly withstanding over 5000 stretch-release cycles. The sensor exhibits its exceptional capability to discern intricate movements, particularly in detecting human hand and neck motions. The sensor's remarkable comprehensive performance and strain direction recognition ability underscore its significant potential for diverse applications, notably in human-computer interaction, human motion monitoring, and health monitoring.

Improvement of formability of Mg-2.2Gd-0.4Zr alloy by forming multi-type textures through the transverse gradient extrusion process

Mg-2.2Gd-0.4Zr sheets were prepared using transverse gradient extrusion (TGE) technology to improve their formability. The effects of different TGE processes (inclination angles of the die are 15°, 30°, 37°, 45° and 49°) on the microstructure, texture, mechanical properties, and formability of extruded sheets were researched. Furthermore, the flow behaviors of Mg-2.2Gd-0.4Zr sheets during extrusion were analyzed by the finite element simulation method. For comparison, Mg-2.2Gd-0.4Zr sheets were processed by conventional extrusion (CE) process. Results showed that the texture of TGE sheets was effectively altered. With the increase of the TGE dies tilt angle, the deviation of the basal pole in the center region of the extruded sheet towards the extrusion direction (ED) increased, while in the 1/4 edge region the basal pole deviated towards the transverse direction (TD) along the ED. The multi-type textures formed in TGE sheets are mainly attributed to the extra flow velocity along the TD and the flow gradient along the ED during extrusion. High elongation of 45.6% and low yield stress of 98.9 MPa are obtained for TGE-45 sheets. In addition, the Erichsen value (IE) of the TGE-45 sheet is 5.46 mm, which is about 47% higher than the 3.72 mm of the CE Mg-2.2Gd-0.4Zr sheet. The improved formability is due to the effective coordination of multi-directional strains during the Erichsen test for the TGE sheets.

Anisotropic V-Groove/Wrinkle Hierarchical Arrays for Multidirectional Strain Sensors with High Sensitivity and Exceptional Selectivity.

Flexible strain sensors have been continuously optimized and widely used in various fields such as health monitoring, motion detection, and human-machine interfaces. There is a higher demand for sensors that can sensitively identify both the strain amplitude and direction in real-time to adapt to complex human movements. This study proposes a flexible strain sensor construction strategy based on V-groove/wrinkle hierarchical structures via a facile and scalable prestretching approach. A gold film is sputtered on a V-groove structure soft substrate under a vertical biaxial prestrain. When the strain is released, a variety of wondrous V-groove/wrinkle hierarchical structures are formed. The microstructure and the properties of the resulting sensor can be controlled by adjusting the prestrain, which has obvious anisotropic response characteristics and exhibits high sensitivity (maximum gauge factor up to 20,727.46) and a wide sensing range (up to 51%). In addition, the resulting multidirectional sensor based on double-sided microstructures has an exceptional directional selectivity of 67.39, at an advanced level among all stretchable multidirectional strain sensors reported so far. The sensor can detect human motion signals and distinguish motion patterns, proving its great potential in the field of human motion detection and laying a foundation for high-performance wearable devices.

Multi-directional strain sensor based on carbon nanotube array for human motion monitoring and gesture recognition

Flexible strain sensors have enormous application demands in health monitoring, soft robotics, and artificial skin. Traditional flexible strain sensors mainly focus on identifying strain amplitude. However, to determine complex movements, it is necessary for sensors to recognize not only the amplitude but also the direction of strain. Here, we propose a multi-directional strain sensor constructed from three flexible strain sensor units, which are based on a carbon nanotube (CNT) array, and configured in a right-angled triangle arrangement. Each strain sensor unit exhibited a strain detection range of over 90%, durability of over 5000 cycles, and fast response time (<150 ms). The multi-directional strain sensor can accurately identify the amplitude and direction (up to 180°) of the strain simultaneously. By optimizing the height and the size of the CNT array strips within the strain sensor unit, the preferred parameters for the sensor unit were determined. Further, by incorporating a neural network algorithm, the multi-directional strain sensor achieved synchronous detection of strain amplitude and direction with an accuracy rate exceeding 98%. Therefore, this work provides a new strategy for the design and construction of multi-directional strain sensors, underscoring their broad application potential in the fields of human motion monitoring and gesture recognition.

Fiber Bragg Gratings Sensor Strain-Optic Behavior with Different Polymeric Coatings Subjected to Transverse Strain.

This research work is based on a previous study by the authors that characterized the behavior of FBG sensors with a polyimide coating in a structural monitoring system. Sensors applied to structural health monitoring are affected by the presence of simultaneous multidirectional strains. The previous study observed the influence of the transverse strain (εy) while keeping the longitudinal strain constant (εx), where the x direction is the direction of the optical fiber. The present study develops an experimental methodology consisting of a biaxial test plan on cruciform specimens with three embedded FBG sensors coated with polyimide, acrylate, and ORMOCER®. Applying the Strain-Optic Theory as a reference, a comparison of the experimental values obtained with the different coatings was studied. This experimental work made it possible to study the influence of the transverse strain (εy) on the longitudinal measurements of each FBGS and the influence of the coating material. Finally, the calibration procedure was defined as well as K (strain sensitivity factor) for each sensor.

Three-dimensional surface strain sensor based on PDMS/LIG composite film with adjustable electromechanical performance

Flexible strain sensors play a crucial role in health monitoring, smart wearable devices, and human–machine interaction. Three-dimensional surface evaluation methods for strain sensors offer advantages by being closer to actual strain, featuring a larger working range, and being more suitable for multidirectional strain. In this study, a three-dimensional (3D) surface strain sensor based on polydimethylsiloxane/laser-induced graphene (PDMS/LIG) composite films has been developed. The electromechanical properties of this sensor, encompassing 3D strain range and sensitivity, can be adjusted by manipulating laser parameters and LIG patterns. The key to attaining these specific characteristics lies in the intentional design of crack types and orientations on the sensor's surface. Remarkably, the line-vertical (LV) sensor exhibits outstanding sensitivity with a GF of 211.3. The line-parallel (LP) sensor achieves a GF of 115.1. Additionally, it demonstrates a stretching range of 25% and maintains stable performance over an extensive number of strain/release test cycles (more than 3000 cycles). With these advantages, the 3D strain sensor can not only be applied in human activity monitoring but also monitoring pressure within microchannels in microfluidic chips, suggesting promising applications in the health and medical fields.

Highly aligned electrospun film with wave-like structure for multidirectional strain and visual sensing

Recently, flexible strain sensors have attracted great attention on account of the application potential in medical treatment, flexible prosthetics and human–machine interaction. Nevertheless, traditional uniaxial strain sensors cannot detect movement in different directions, limiting the applications in sensing technologies. In addition, it is still a significant challenge to simultaneously acquire visualized and high-performance strain sensors. Here, a highly flexible sensor with visual sensing function was fabricated by spraying silver nanowires (AgNWs)/carbon nanotubes (CNTs) ink on the aligned thermoplastic polyurethane (TPU) electrospun mat. The sensing performances of AgNWs/CNTs aligned TPU fluorescent mats (ACUM-F) are individually explored in vertical and parallel directions. Herein, the ACUM-F in vertical can exhibit high sensitivity (gauge factor (GF) up to 244.3), wide working range (0.1 ∼ 380 % strain), fast response/recovery time (80 ms/80 ms), good sensing durability and reproducibility (1000 stretching/releasing test cycles). Furthermore, ACUM-F is assembled as strain sensors to monitor various human body movements, which can achieve sign language interpretation through finger gesture monitoring, demonstrating its great potential in human–machine interaction. The direction-aware application of novel multidirectional sensors with visual sensing functions may bring inspiration for a new era of flexible electronics development.

Self-Powered Multidirectional Strain Sensor for Electronic Skin Based on Coiled Carbon Nanotube Yarns

Self-Powered Multidirectional Strain Sensor for Electronic Skin Based on Coiled Carbon Nanotube Yarns

Multidirectional myocardial function in bicuspid aortic valve stenosis patients: a three-dimensional speckle tracking analysis.

The impact of aortic stenosis (AS) severity on multidirectional myocardial function in patients with bicuspid aortic valve (BAV) remains unclear, despite the recognized presence of early left ventricular longitudinal myocardial dysfunction in BAV patients with normal valve function. The aim of the study was to evaluate the multidirectional myocardial functions of BAV patients. A total of 86 BAV patients (age 46.71 ± 13.62 years, 69.4% men) with normally functioning (BAV-nf), mild AS, moderate AS, and severe AS with preserved left ventricular ejection fraction (LVEF ≥ 52%) were included. 30 healthy volunteers were recruited as the control group. Multidirectional strain and volume analysis were performed by three-dimensional speckle tracking echocardiography(3D-STE). Global longitudinal strain (GLS), and global radial strain (GRS) were reduced in BAV-nf patients compared with the controls. With each categorical of AS severity from BAV-nf to severe AS, there was an associated progressive impairment of GLS and GRS (all P < 0.001). Global circumferential strain (GCS) did not show a significant decrease from BAV-nf to mild AS but began to decrease from moderate AS. Multiple linear regressions indicated that indexed aortic valve area (AVA/BSA), as a measure of AS severity, was an independent determinant of GLS, GCS and GRS. Left ventricular longitudinal myocardial reduction is observed even in patients with well-functioning bicuspid aortic valves. With each categorical increase in the grade of AS severity from normally functioning to severe aortic stenosis, there was an associated progressive impairment of longitudinal myocardial function. Furthermore, circumferential myocardial function was starting damaged from moderate AS. AVA/BSA was independently associated with multidirectional myocardial function injuries.

Highly sensitive multidirectional strain sensors based on laser-induced graphene for human activity monitoring

Highly sensitive multidirectional strain sensors based on laser-induced graphene for human activity monitoring

Next-generation wearable sensors: toward multi-directional strain sensing in sensory integration platforms

Strain sensors capable of recognizing the direction of mechanical stimuli are a key contributor to the development of wearable sensory platforms.

Flexible liquid metal-based microfluidic strain sensors with fractal-designed microchannels for monitoring human motion and physiological signals

With the rapid advancement of wearable electronics, there is an increasing demand for high-performance flexible strain sensors. In this work, a flexible strain sensor based on liquid metal (LM)-integrated into a microfluidic device is developed with Peano-type fractal structure design. Compared with the microfluidic sensors with straight and wavy microchannels, the sensor with Peano-shaped channels shows lower hysteresis and improved stretchability. Furthermore, the increase of the fractal order can further improve the sensing performances. The third-order Peano sensor exhibits excellent mechanical and electrical properties, including high tensile capability (490.3%), minimal hysteresis (DH = 0.86%), ultra-low detection limit (0.1%), low overshoot, rapid response time (117 ms), as well as good stability and durability. By adding two independent and perpendicular straight channels to the Peano sensing unit, the feasibility of multi-directional strain recognition is demonstrated. To further improve the sensitivity of the Peano-shaped sensor, a multi-layer Peano sensor is developed, exhibiting remarkably enhanced sensitivity while maintaining low hysteresis. Overall, the developed LM-based microfluidic strain sensors enrolling Peano fractal geometry hold high potential for various wearable electronics applications.

Cartilage‐Inspired Multidirectional Strain Sensor with High Elasticity and Anisotropy Based on Segmented Embedded Strategy

AbstractFlexible, stretchable, and sensitive multidirectional sensing systems that can decouple different mechanical inputs and identify multidirectional signals are crucial for dynamic human signal perception and intelligent human–computer interaction. Most reported multidirectional sensors are suitable for discriminating in‐plane deformation directions, and the sensing materials are difficult to balance between stretchability and mechanical strength. Here, a segmented embedded structure strategy inspired by the interlaced structure of cartilage is proposed. This strategy combines soft and hard materials in a topological and zipper‐shear chain manner and balances the performance of reinforced composites with flexibility and high toughness. In the case of segmented embedded hydrogels (SEHs), a wearable multidirectional sensing system that can decouple and identify planar strain/pressure is constructed. The multidirectional sensing system exploits the inherent anisotropy and layered structure design of composites to decouple the sensing functions. Supported by machine learning algorithms, the high accuracy demonstration of the multidirectional sensors in typical multidirectional motion joint posture monitoring and recognition confirms their potential in practical applications such as personal health sensing and human–computer interaction.

Anisotropic and Highly Sensitive Flexible Strain Sensors Based on Carbon Nanotubes and Iron Nanowires for Human-Computer Interaction Systems.

Flexible strain sensors for multi-directional strain detection are crucial in complicated hman-computer interaction (HCI) applications. However, enhancing the anisotropy and sensitivity of the sensors for multi-directional detection in a simple and effective method remains a significant issue. Therefore, this study proposes a flexible strain sensor with anisotropy and high sensitivity based on a high-aspect-ratio V-groove array and a hybrid conductive network of iron nanowires and carbon nanotubes (Fe NWs/CNTs). The sensor exhibits significant anisotropy, with a difference in strain detection sensitivity of up to 35.92 times between two mutually perpendicular directions. Furthermore, the dynamic performance of the sensor shows a good response rate, ranging from 223 ms to 333 ms. The sensor maintains stability and consistent performance even after undergoing 1000 testing cycles. Additionally, the constructed flexible strain sensor is tested using the remote control application of a trolley, demonstrating its high potential for usage in practical HCI systems. This research offers a significant competitive advantage in the development of flexible strain sensors in the field of HCI.

Flexible sensor based on carbonized multilayer facial tissue with anisotropic structure: Towards multiaxial detection

Due to the multiaxial property of mechanical stimuli from human motion, flexible sensors capable of multiaxial detection have enjoyed steady growth of interest. Utilizing the distinctive anisotropic multilayer structure of carbonized multilayer facial tissue (CMFT), we successfully prepared flexible CMFT sensors capable of anisotropic strain sensing and pressure detection. Moreover, an integrated sensor was developed through orthogonally stacking two CMFT sensors, realizing the detection of mechanical stimuli on three orthogonal axes (multidirectional strain sensing in the XY plane and pressure sensing along the vertical Z-axis). Finally, the applications of the integrated sensor for monitoring complex waist movement as well as heel falling and lifting were demonstrated, indicating the great potential of CMFT sensors in human motion detection.