Sensitive Strain Sensor Research Articles

Subject-adaptive Loose-fitting Smart Garment Platform for Human Activity Recognition

The ability to recognize and detect changes in human posture is important in a wide range of applications such as health care and human–computer interaction. Achieving this goal using loose-fit garments instrumented with sensors is particularly challenging, due to the complex interaction between garments and human body. Herein we present a method to detect and recognize human posture with casual loose-fitting smart garments integrated with highly sensitive, stretchable, optical transparent, and low-cost strain sensors. By attaching these sensors to an off-the-shelf casual jacket, we developed a smart loose-fitting sensing garment that enables posture recognition using a deep learning model, domain-adaptive Convolutional Neural Networks–Long Short-Term Memory (CNN-LSTM). This deep learning model overcame the noise and variation due to the complex interaction between loose-fitting garments and human body. Considering that users’ labeled data are usually not available in the training stage, an additional domain discriminator path on the conventional CNN-LSTM model has been introduced to further improve the adaptability. To evaluate the potential of this loose-fitting smart garment, three case studies were conducted under realistic conditions: recognitions of human activities, stationary postures with random hand movements and slouch. Our results demonstrate the potential of the proposed smart garment system for practical applications.

Ultra‐Robust and Sensitive Flexible Strain Sensor for Real‐Time and Wearable Sign Language Translation

AbstractFlexible strain sensors with high sensitivity and high mechanical robustness are highly desirable for their accurate and long‐term reliable service in wearable human‐machine interfaces. However, the current application of flexible strain sensors has to face a trade‐off between high sensitivity and high mechanical robustness. The most representative examples are micro/nano crack‐based sensors and serpentine meander‐based sensors. The former one typically shows high sensitivity but limited robustness, while the latter is on the contrary. Herein, ultra‐robust and sensitive flexible strain sensors are developed by crack‐like pathway customization and ingenious modulation of low/high‐resistance regions on a serpentine meander structure. The sensors show high cyclic stability (10 000 cycles), strong tolerance to harsh environments, high gauge factor (>1000) comparable with that of the crack‐based sensor, and fast response time (<58 ms). Finally, the sensors are integrated into a wearable sign language translation system, which is wireless, low‐cost, and lightweight. Recognition rates of over 98% are demonstrated for the translation of 21 sign languages with the assistance of machine learning. This system facilitates achieving barrier‐free communication between signers and nonsigners and offers broad application prospects in gesture interaction.

Super-Stretchable Resistive Strain Sensor Based on Ecoflex–Gold Nanocomposites

Superstretchable resistive strain sensors can reversibly undergo severe deformations in response to different mechanical stimuli, leading to a variation in their ohmic resistance. They are finding an increasing number of applications in wearable electronics, biomedical devices, and soft robotics. The development of fabrication process of reliable, highly sensitive, and biocompatible superstretchable strain sensors remains a challenge, especially in view of their integration with medical devices. Here, we demonstrate the fabrication of a high-strain resistive sensor based on Ecoflex elastomeric films and conductive Au clusters, obtained by supersonic cluster beam deposition. An extensive characterization of the electromechanical behavior of the sensor shows stable operation up to 4500 working cycles, with a maximum strain of 500% and a gauge factor of 124. Different mechanical stimuli, such as elongation and indentation, can be detected, and the easy coupling of the sensor with an inflatable balloon-type urinary catheter is demonstrated. Aiming at shedding light on the observed electromechanical mechanisms, the nanostructured morphology of the metal–polymer hybrid layer was investigated by low-vacuum scanning electron microscopy.

Ultra-High Sensitivity and Temperature-Insensitive Optical Fiber Strain Sensor Based on Dual Air Cavities.

This study proposed an all-fiber Fabry-Perot interferometer (FPI) strain sensor with two miniature bubble cavities. The device was fabricated by writing two axial, mutually close short-line structures via femtosecond laser pulse illumination to induce a refractive index modified area in the core of a single-mode fiber (SMF). Subsequently, the gap between the two short lines was discharged with a fusion splicer, resulting in the formation of two adjacent bubbles simultaneously in a standard SMF. When measured directly, the strain sensitivity of dual air cavities is 2.4 pm/με, the same as that of a single bubble. The measurement range for a single bubble is 802.14 µε, while the measurement range for a double bubble is 1734.15 µε. Analysis of the envelope shows that the device possesses a strain sensitivity of up to 32.3 pm/με, which is 13.5 times higher than that of a single air cavity. Moreover, with a maximum temperature sensitivity of only 0.91 pm/°C, the temperature cross sensitivity could be neglected. As the device is based on the internal structure inside the optical fiber, its robustness could be guarantee. The device is simple to prepare, highly sensitive, and has wide application prospects in the field of strain measurement.

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

Highly Sensitive and Color Adaptable Polyurethane‐Based Strain Sensors with Embedded Silver Nanowires

AbstractWearable strain sensors are expected to meet the requirements of high sensitivity, facile fabrication, and high aesthetics to realize practical applications. However, reported sensitive strain sensors fail to exhibit multicolor due to high light absorption. Herein, a flexible strain sensor with high sensitivity and color‐exhibition ability is fabricated by half‐embedding silver nanowires (AgNWs) into dyeable thermoplastic polyurethane (TPU) films through thermal annealing. A sheet resistance of 11.5 Ω sq−1 and a transmittance above 79.9% at 550 nm is obtained for intact AgNWs/TPU films. A gauge factor of 34 797 is achieved at a tensile strain of 9%, indicating the high sensitivity for monitoring tiny movement changes. Besides, the high transparency allows color exhibition after dying. The multicolor AgNWs/TPU films are further rolled into rod‐shaped yarns and weaved with commercial yarns to form wearable colorful electronics.

Dual-Nozzle Extrusion Printing of Highly Sensitive and Durable Strain Sensor Based on MWCNT/Ecoflex Composites

Flexible strain sensors have made great progress and been applied tentatively in recent years. However, there are still many challenges and problems hindering their widespread application, including incompatible high sensitivity and wide detection range, poor durability, and time-consuming fabrication procedures. In this work, a dual-nozzle-assisted one-step strategy for the fabrication of integrated flexible strain sensors is proposed. During the fabrication, Ecoflex and its composite multi-walled carbon nanotube (MWCNT)/Ecoflex were extruded and thermally cured from the two nozzles, respectively. Taking advantage of the same base resin, the strain-sensitive and the insulating components in the fabricated flexible strain sensors are chemically bonded firmly and form an integrated device. The fabricated sensor exhibits good performance in terms of high gauge factor (GF = 165.3), good stability, low hysteresis, fast response time (60 ms), wide detection range (0%–300% strain), and ultralow detection limit (0.1% strain), showing promising applications in health monitoring and human–computer interaction.

Piezoresistive Free-standing Microfiber Strain Sensor for High-resolution Battery Thickness Monitoring.

Highly sensitive microfiber strain sensors are promising for the detection of mechanical deformations in applications where limited space is available. In particular for in situ battery thickness monitoring where high resolution and low detection limit are key requirements. Herein, the realization of a highly sensitive strain sensor for in situ lithium-ion (Li-ion) battery thickness monitoring is presented. The compliant fiber-shaped sensor is fabricated by an upscalable wet-spinning method employing a composite of microspherical core-shell conductive particles embedded in an elastomer. The electrical resistance of the sensor changes under applied strain, exhibiting a high strain sensitivity and extremely low strain detection limit of 0.00005 with high durability of 10 000 cycles. To demonstrate the accuracy and ease of applicability of this sensor, the real-time thickness change of a Li-ion battery pouch cell is monitored during the charge and discharge cycles. This work introduces a promising approach with the least material complexity for soft microfiber strain gauges.

Highly Sensitive Strain Sensor Fabricated by Direct Laser Writing on Lignin Paper with Strain Engineering

Paper‐based strain sensors (PSS) have broad prospects in disposable products due to their low cost and easy degradation as environmentally friendly materials. Herein, a strain sensor made of a laser‐induced carbonization electrode is created by direct laser writing with filter paper. The conductivity and gauge factor (GF) of this strain sensor are improved by adding lignin and applying strain engineering. This enables the sensor to simultaneously satisfy high sensitivity (GF ≈ 408 and 91) for weak tension and compression strain, respectively, and with long‐term reliability. The tensile strain GF factors of up to 201 are possible, even with a weak tensile strain of 0.00088%. Furthermore, the paper‐based sensor for monitoring physiological activities like finger gestures, pulsing, swallowing, and eye blinking is demonstrated. The facile fabrication and superior performance of PSS fabricated by direct laser writing with strain engineering may pave the way for promising applications of flexible, portable, and wearable electronic devices.

Highly Stretchable, Self‐Healable and Self‐Adhesive Double‐Network Eutectogel Based on Gellan Gum and Polymerizable Deep Eutectic Solvent for Strain Sensing

AbstractRecently, eutectogels have gained significant attention as promising materials for wearable devices. However, developing eutectogels with integrated stretchability, good toughness, self‐healing, and self‐adhesive properties through non‐covalent scaffold assembly is a major challenge. In this study, a fully physically crosslinked double‐network (DN) eutectogel was fabricated by exploiting the high solubility and gelation of gellan gum in a deep eutectic solvent (DES) to form the first physically crosslinked network and combining it with supramolecular poly (2‐hydroxyethyl acrylate‐choline chloride, HEA‐ChCl). The resulting gel showed high stretchability (1835 %), toughness (7.65 MJ m−3), desirable ionic conductivity (0.41 mS cm−1), adhesion, and high transparency. Assembling the gel into strain sensors allows precise monitoring of human motion (finger, wrist, elbow, and knee). The results of this study suggest that gellan gum/poly (ChCl‐HEA) DN eutectogels have significant potential as highly sensitive and reliable strain sensors for wearable technology applications.

Ultrastretchable, Multihealable, and Highly Sensitive Strain Sensor Based on a Double Cross-Linked MXene Hydrogel

The ability of a flexible strain sensor to directly adapt the complicated human biological motion or combined gestures and remotely control the artificial intelligence robotics could benefit the wearable electronics such as intelligent robotics and patient healthcare. However, it is a challenge for the flexible strain sensor to simultaneously achieve high sensing performances and stretchability and long sustainability under various deformation stress or damage. Herein, a dual-cross-linked poly(acrylic acid-stearyl methacrylate)/MXene [P(AA-SMA)M] hydrogel with enhanced mechanical stretchability and self-healability is fabricated by importing reversible coordination and hydrophobic interaction into polymer networks. As a result, the hydrogel film not only exhibits high tensile strength (525 kPa) and stretchability (∼2600%) but also achieves repetitive healable property with 843% elongation even after the 20th broken/self-healing cycle. More importantly, the resultant strain sensor delivers a low detection limit, wide sensing range, fast response time, and repeatability of 1000 cycles even after repeated self-healing. So, the sensor can monitor subtle human motions and recognize different handwriting and gestures, which reveals potential applications toward health-care devices, flexible electronics, and human-machine interfacing.

Strain-induced orientation facilitates the fabrication of highly stretchable and tough xylan-based hydrogel for strain sensors

Stretchable and tough polysaccharide-based functional hydrogels have gained popularity for various applications. However, it still remains a great challenge to simultaneously own satisfactory stretchability and toughness, particularly when incorporating renewable xylan to offer sustainability. Herein, we describe a novel stretchable and tough xylan-based conductive hydrogel utilizing the natural feature of rosin derivative. The effect of different compositions on the mechanical properties and the physicochemical properties of corresponding xylan-based hydrogels were systematically investigated. Owing to the multiple non-covalent interactions among different components to dissipate energies and the strain-induced orientation of rosin derivative during the stretching, the highest tensile strength, strain, and toughness of xylan-based hydrogels could reach 0.34 MPa, 2098.4 %, and 3.79 ± 0.95 MJ/m3, respectively. Furthermore, by incorporating MXene as the conductive fillers, the strength and toughness of hydrogels were further enhanced to 0.51 MPa and 5.95 ± 1.19 MJ/m3. Finally, the synthesized xylan-based hydrogels were able to serve as a reliable and sensitive strain sensor to monitor the movements of human beings. This study provides new insights to develop stretchable and tough conductive xylan-based hydrogel, especially utilizing the natural feature of bio-based resources.

Flexible Strain Sensor Enabled by Carbon Nanotubes‐Decorated Electrospun TPU Membrane for Human Motion Monitoring

AbstractHigh‐performance flexible strain sensors are gaining more and more attention with their bespoken detection range, excellent sensing performance, and good stability, which are highly desired in wearable electronics. Herein, a thermoplastic polyurethane elastomer (TPU) fibrous membrane is prepared as a flexible substrate by electrostatic spinning technology, then a coating of polydopamine is formed through fast synthesizing the dopamine on TPU fibrous membrane surface and loaded with carbon nanotubes (CNTs) to develop an extremely sensitive flexible strain sensor. The flexible sensor prepared by TPU fibrous membrane coated with polydopamine layer has an outstanding sensibility under the pulling force (Gauge Factor of 10 528.53 with 200% strain), rapid reaction time (188–221 ms), wide sensing range (up to 200%), good stability, and durability. The theoretical studies reveal the underlying cause for the high sensitivity and the inherent relationship between the amount of conducting routes and the length between adjacent conducting fillers in the sensor. The demonstration of the device shows a promising application to sense human motion at various locations of the body, with the accurate and stable electrical signal output generated at corresponding motion.

Silicon flexoelectronic transistors

It is extraordinarily challenging to implement adaptive and seamless interactions between mechanical triggering and current silicon technology for tunable electronics, human-machine interfaces, and micro/nanoelectromechanical systems. Here, we report Si flexoelectronic transistors (SFTs) that can innovatively convert applied mechanical actuations into electrical control signals and achieve directly electromechanical function. Using the strain gradient-induced flexoelectric polarization field in Si as a "gate," the metal-semiconductor interfacial Schottky barriers' heights and the channel width of SFT can be substantially modulated, resulting in tunable electronic transports with specific characteristics. Such SFTs and corresponding perception system can not only create a high strain sensitivity but also identify where the mechanical force is applied. These findings provide an in-depth understanding about the mechanism of interface gating and channel width gating in flexoelectronics and develop highly sensitive silicon-based strain sensors, which has great potential to construct the next-generation silicon electromechanical nanodevices and nanosystems.

Hydroxypropyl methyl cellulose reinforced conducting polymer hydrogels with ultra-stretchability and low hysteresis as highly sensitive strain sensors for wearable health monitoring

Conducting polymer hydrogels have emerged as promising materials to fabricate highly sensitive strain sensors. However, due to weak bindings between conducting polymer and gel network, they usually suffer from limited stretchability and large hysteresis, failing to achieve wide-range strain sensing. Herein, we combine hydroxypropyl methyl cellulose (HPMC), poly (3,4-ethylenedioxythiophene):poly (styrene sulfonic acid) (PEDOT: PSS) with chemically cross-linked polyacrylamide (PAM) to prepare a conducting polymer hydrogel for strain sensors. Owing to abundant hydrogen bonds between HPMC, PEDOT:PSS and PAM chains, this conducting polymer hydrogel exhibits high tensile strength (166 kPa), ultra-stretchability (>1600 %) and low hysteresis (<10 % at 1000 % cyclic tensile strain). The resultant hydrogel strain sensor shows ultra-high sensitivity, wide strain sensing ranges of 2–1600 %, and excellent durability and reproducibility. Finally, this strain sensor can be used as wearable sensor to monitor vigorous human movement and fine physiological activity, and services as bioelectrodes for electrocardiograph and electromyography monitoring. This work provides new horizons to design conducting polymer hydrogels for advanced sensing devices.

Multifunctional two-way shape memory RGO/ethylene-vinyl acetate composite yarns for electro-driven actuators and high sensitivity strain sensors

Shape memory polymer fibers (SMPFs) coated with conductive layers are attracting increasing attention in soft robots and wearable electronics. However, the thickness and poor stretchability of the conductive layer hinder its applications. Here, the multifunctional two-way shape memory reduced graphene oxide (RGO)/ethylene–vinyl acetate copolymer (EVA) composite yarn is designed following the “swelling-ultrasonicating” and one-step twisting procedures. These RGO/EVA composite fiber conductive paths connected with each other by constructing the helical conductive network. The RGO/EVA composite yarn presents excellent electro-thermal conversion performance and outstanding load-bearing capability, enabling controlled electro-driven reversible strain (more than 10% reversible strain) and high energy density (72.5 J/kg). Additionally, the composite yarn has great stretchability (130% strain), high strain sensing sensitivity (gauge factor of 7.3), large linear working range (up to 70% strain). This work provides an innovative manufacturing strategy for the RGO/EVA composite yarn with two-way shape memory, electro-driven actuation, and strain sensing performance.

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.

Tuneable Piezoresistance of Graphene‐Based 2D:2D Nanocomposite Networks

AbstractPiezoresistive nanocomposites are an important class of materials that allow the production of very sensitive strain sensors. Herein, a new class of piezoresistive nanocomposites prepared by mixing different types of 2D nanosheets is explored. In this way, three distinct types of nanocomposite are produced by mixing conducting and insulating nanosheets (graphene, Gr and boron nitride, BN), conducting and semiconducting nanosheets (graphene and tungsten diselenide, WSe2 or tungsten disulfide, WS2) as well as mixing two different types of conducting nanosheets (graphene and silver, Ag). For each nanocomposite type, a different dependence of composite conductivity on filler volume fraction is observed although all behaviors can be fully described by percolation theory. In addition, each composite type shows different piezoresistive properties. Interestingly, while the conductor insulator composites show the standard monotonic relationship between gauge factor and conductivity, both conductor:semi‐conductor and conductor:conductor composites show very unusual behavior, in each case displaying a peak engage factor at the percolation threshold. In each case, percolation theory is used to develop simple equations for gauge factor as a function of both volume fraction and conductivity that fully describes all experimental data. This work expands the understanding of piezoresistive nanocomposites and provides a platform for the engineering of high‐performance strain sensors.

Creation and Analysis of a Respiratory Sensor Using the Screen-Printing Method and the Arduino Platform.

The aim of this paper is to present novel highly sensitive and stretchable strain sensors using data analysis to report on human live parameters using the Arduino embedded system as a proof of concept in developing new and innovative solutions for health care. The article introduces the solution of textile sensor origination with electrical resistance measurement using the mobile Arduino microcontroller in the designed/elaborated textile printed sensor. The textile sensor was developed by the screen printing technique based on the water dispersion of carbon nanotubes during printing composition. By stretching and squeezing the T-shirt during breathing, the electrical resistances of the printed sensor were changed. The measured resistance corresponded to the number of breaths of the person wearing the T-shirt. The microcontroller calculated the number of breaths as a number of electrical resistance peaks, which then led to monitoring human live parameters.

Fabrication and characterization of highly sensitive flexible strain sensor based on biodegradable gelatin nanocomposites and double strain layered structures with crack for gesture recognition

Flexible sensors have attracted extensive attention in the field of human-computer interaction. However, it is still a challenging task to realize accuracy gesture recognition with flexible sensor, which requires sensor not only have high sensitivity, but also have appropriate strain detection range. Here, a high gauge factor flexible sensor (gauge factor ∼ 1296 under 12–20 % strain) based on crack structure is reported. The sensor is made of a biodegradable and stretchable gelatin composite combined with fabric bases, with good repeatability (6000 cycles) and a fast response (60 ms). Because of the double-layer structure, it has a suitable detection range (20 % strain). The sensor is manufactured by a screen-printing process, and it has been used to make data gloves and has realized 9 gestures recognition with machine learning algorithm (99.6 % accuracy). In general, this study offers a wearable gestures recognition scheme through the proposed sensor.