Sensitive Strain Sensor Research Articles

High stretchable and tough xylan-g-gelatin hydrogel via the synergy of chemical cross-linking and salting out for strain sensors

Stretchable and tough hydrogels have been extensively used in tissue engineering scaffolds and flexible electronics. However, it is still a significant challenge to prepare hydrogels with both tensile strength and toughness by utilizing xylan, which is abundant in nature. Herein, we present a novel hydrogel of carboxymethyl xylan(CMX) graft gelatin (G) and doped with conductive hydroxyl carbon nanotubes (OCNT). CMX and G are combined through amide bonding as well as intermolecular hydrogen bonding to form a semi-interpenetrating hydrogel network. The hydrogel was further subjected to salting-out treatment, which induced the aggregation of the CMX-g-G molecular chain and the formation of chain bundles to toughen the hydrogel, the tensile strain, tensile stress, and toughness of CMX-g-G hydrogels were 1.547 MPa, 324 %, and 2.31 MJ m−3, respectively. In addition, OCNT was used as a conductive filler to impart electrical conductivity and further improve the mechanical properties of CMX-g-G/OCNT hydrogel, and a tensile strength of 1.62 MPa was obtained. Thus, the synthesized CMX-g-G/OCNT hydrogel can be used as a reliable and sensitive strain sensor for monitoring human activity. This study opens up new horizons for the preparation of xylan-based high-performance hydrogels.

Helical TPU/Ag@K2Ti4O9 fibers with shape memory performance for highly stretchable and sensitive strain sensors

Even though wearable electronics based on e-textiles have been attracting increasing attention, their unsatisfactory durability has significantly impeded their practical applications. In this regard, thermoplastic polyurethane (TPU)/Ag@K2Ti4O9 composite fibers with excellent mechanical properties and conductivity along with outstanding durability have been successfully prepared via wet-spinning in this work. However, it still has some shortcomings, such as a high modulus mismatch with skin, and slow resistance recovery after stretching when applied to human sensing. Therefore, helical fibers have been further developed, which are sturdy and do not bring any uncomfortably wearable electronics (the fracture stress is greater than 0.4 N and an elongation of 50% can be achieved under only 0.01 N with a 40% increased resistance). The resultant helical composite fiber has stable sensing performance under 100% strain, with a gauge factor (GF) up to 0.8–1.5. In particular, it can quickly recover its original length and resistance in hot water or under an infrared lamp due to the thermal shrinkage effect of PU when it breaks under destructive overstretch (greater than 300% strain). This durable, repairable, and cost-effective helical fiber can provide new insights into the design and promotion of fibrous human-computer interaction components.

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

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

A Flexible and Stretchable MXene/Waterborne Polyurethane Composite-Coated Fiber Strain Sensor for Wearable Motion and Healthcare Monitoring.

Fiber-based flexible sensors have promising application potential in human motion and healthcare monitoring, owing to their merits of being lightweight, flexible, and easy to process. Now, high-performance elastic fiber-based strain sensors with high sensitivity, a large working range, and excellent durability are in great demand. Herein, we have easily and quickly prepared a highly sensitive and durable fiber-based strain sensor by dip coating a highly stretchable polyurethane (PU) elastic fiber in an MXene/waterborne polyurethane (WPU) dispersion solution. Benefiting from the electrostatic repulsion force between the negatively charged WPU and MXene sheets in the mixed solution, very homogeneous and stable MXene/WPU dispersion was successfully obtained, and the interconnected conducting networks were correspondingly formed in a coated MXene/WPU shell layer, which makes the as-prepared strain sensor exhibit a gauge factor of over 960, a large sensing range of over 90%, and a detection limit as low as 0.5% strain. As elastic fiber and mixed solution have the same polymer constitute, and tight bonding of the MXene/WPU conductive composite on PU fibers was achieved, enabling the as-prepared strain sensor to endure over 2500 stretching-releasing cycles and thus show good durability. Full-scale human motion detection was also performed by the strain sensor, and a body posture monitoring, analysis, and correction prototype system were developed via embedding the fiber-based strain sensors into sweaters, strongly indicating great application prospects in exercise, sports, and healthcare.

Highly tensile and sensitive strain sensors with micro–nano topology optimization

A novel fiber strain sensor was proposed with micro–nano topology optimization, which was achieved through a simple, cost-effective and scalable method and obtained a wide detection range (0–650%) and high sensitivity (GFmax = 976.89).

Encapsulated stretchable amphibious strain sensors.

Soft and stretchable strain sensors have found wide applications in health monitoring, motion tracking, and robotic sensing. There is a growing demand for strain sensors in amphibious environments, such as implantable sensors, wearable sensors for swimmers/divers, and underwater robotic sensors. However, developing a sensitive, stretchable, and robust amphibious strain sensor remains challenging. This work presents an encapsulated stretchable amphibious strain sensor. The conductive layer, made of silver nanowires embedded below the surface of polydimethylsiloxane, was sandwiched by two layers of thermoplastic polyurethane. Periodic sharp cuts were introduced to change the direction of flow from across the sensor to along the conductive path defined by the opening cracks. The crack advancing and opening is controlled by a unique combination of weak/strong interfaces within the sandwich structure. The cut design and the interfacial interactions between the layers were investigated. The strain sensor exhibited a high gauge factor up to 289, a linear sensing response, a fast response time (53 ms), excellent robustness against over-strain, and stability after 16 000 loading cycles and 20 days in an aqueous saline solution. The functionality of this amphibious strain sensor was demonstrated by tracking the motion of a robotic fish, undertaking language recognition underwater, and monitoring the blood pressure of a porcine aorta. This illustrates the promising potential for this strain sensor for both underwater use and surgically implantable applications.

Highly Sensitive Strain and Temperature Sensors Based on Vernier Effect in Cascaded Mach–Zehnder Interferometer and Sagnac Interferometer

Highly Sensitive Strain and Temperature Sensors Based on Vernier Effect in Cascaded Mach–Zehnder Interferometer and Sagnac Interferometer

Wearable strain sensors: design shapes, fabrication, encapsulation and performance evaluation methods

Highly durable, stretchable, sensitive and biocompatible wearable strain sensors are crucial for healthcare, sports, and robotics applications.

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

Highly sensitive strain sensors with ultra-low detection limit based on pre-defined serpentine cracks

In this study, we introduce a novel resistive strain sensor with high sensitivity, excellent linearity, and an ultra-low detection limit by simply modulating ohmic contact within pre-defined conductive cracks.

Highly sensitive strain sensors based on PVA hydrogels with a conductive surface layer of graphene

Highly sensitive strain sensors based on PVA hydrogels with a conductive surface layer of graphene

A high-sensitivity strain sensor based on the core-offset fiber with a micro air bubble

A highly sensitive optic-fiber strain sensor based on the air-microbubble Fabry–Perot interferometer (FPI) formed in a core-offset tapered fiber is proposed. By drawing tapers on the staggered spliced single-mode fibers, micro air bubbles with an average wall thickness of 2.6 μm are formed. The experimental results show that the sensor has a high strain sensitivity of 17.24 pm/με and good linear response in the range of 0–1500 με. A numerical simulation model is developed to study the axial strain response, and the results show that the core-offset structure increases the strain sensitivity by 46 %. In addition, the sensor has a temperature sensitivity of 3.43 pm/°C and a cross-sensitivity with strain as low as 0.199 με/°C over the range of 50–350 °C.

Bending sensor using ionic liquid crystal elastomers as solid electrolyte of organic electrochemical transistors

ABSTRACT Organic Electrochemical Transistors (OECTs) have the potential to enable fully flexible, low-cost electronics. Here, we demonstrate that OECTs can be combined with ionic liquid crystal elastomers (iLCEs) to realise highly sensitive bending sensors. The transfer curve of the iLCE-based OECT shows opposite variation with upward and downward bending representing a directional sensitive strain sensor. The change in the drain current is 4 orders of magnitude larger than the flexo-ionic current. This high sensitivity is due to the OECT that amplifies the flexo-ionic current. iLCE-based OECTs therefore present a new class of curvature sensors that may find applications in soft robotics and biosensing.

Highly stretchable, robust, sensitive and wearable strain sensors based on mesh-structured conductive hydrogels

With the rapid development of wearable electronic devices, flexible sensors, as the core components of wearable devices, have received extensive attention. Manufacturing of the strain sensors with excellent mechanical properties, wide working range, and high sensitivity is crucial for the development of wearable devices. Herein, for the first time we proposed a strategy based on mesh-structured conductive hydrogel to sensitively detect the body movement in a large workable strain ranges, and demonstrated the facile fabrication procedure of the sensors by using a direct ink writing 3D printing technique. Low cost conductive hydrogels were prepared by a simple one-pot method of combining dispersant-free carbon nanocolloids with easily synthesized PVA/TA/PAM. The printing of complex mesh-structured hydrogels was optimized with high fidelity. The strain sensor based on the mesh-structured hydrogel exhibited excellent mechanical properties, of which the maximum failure stress of the honeycomb was 1.28 MPa and the maximum failure tensile strain was 704 %, amazing sensitivity (GF = 32.95 for the tensile strain range of 3.5 %-5%, GF = 21.5 for the tensile strain range of 100 %-120 %), wide detection range, good stability and durability (for 500 tensile cycles, the ΔR/R0 value remained unchanged). In addition, the working mechanism during the stretching process of mesh-structured hydrogels was revealed by combining finite element simulation with microscopic morphology observation. At last, the applications of the mesh-structured hydrogel-based sensors to accurately and reliably detect both subtle physiological signals and large joint movements were soundly demonstrated, endowing the ability of achieving all-round detection of human motion. Our work could provide a universe strategy for fabricating highly ventilated, stretchable, robust, sensitive and wearable strain sensors, which would greatly expand the applications of strain sensors in wearable devices.

A self-healing polypyrrole/oxidized natural rubber conductive composite as a building block for highly sensitive strain sensors

Wearable flexible strain sensors have attracted significant attention in human movements due to their high sensitivity, broad strain range and long-term stability. However, the fragility of the sensors limits their widespread applications. Here, a self-healing rubber-based conductive composite with excellent electrical conductivity and high sensitivity are prepared by in-situ polymerization of pyrrole in oxidized deproteinized natural rubber latex. Based on the hydrogen bonds between oxidized deproteinized natural rubber, polypyrrole and polyethylene glycol monolauryl ether, the composites show efficient self-healing behavior (η = 91.1%) under 50 °C for 24 h. The conductive composites containing 5 wt% polypyrrole perform excellent conductivity (3.5 S/m), fast response time (180 ms), low strain detection limit (1%), and long-term reliability (over 1500 cycles at 40% strain). The gauge factors are 3.2 at low strain (<60%) and up to 477.6 at large strain (>430%). Furthermore, the rubber-based conductive composites are fabricated as sensors to accurately monitor a variety of human joint movements (finger, wrist, elbow, and knee bending) and facial expressions (pouting, and winking).

Ionic Polymer-Metal Composites: From Material Engineering to Flexible Applications.

ConspectusIonic polymer-metal composites (IPMCs) are one kind of artificial muscles that can realize energy conversions in response to external stimulus with merits of lightweight, scalability, quick response, and flexibility and have been treated as an important platform in artificial intelligence, such as bionic robotics, smart sensors, and micro-electromechanical systems. It is well-known that IPMC devices are mainly composed of one electrolyte layer laminated with symmetric electrode layers and realize energy conversion based on ion migration and redistribution inside the devices. However, several critical issues have greatly impeded the practical applications of IPMC devices, including metal electrode cracks, metal-polymer interface detachment, and water loss in the electrolyte. In the past decade, our group and collaborators have made attempts to address the mentioned critical issues with the purpose of accelerating practical applications of IPMC devices. First, in order to address the metal electrode cracks, we have developed various electrode materials to replace the metal electrode material, such as black phosphorus and graphdiyne. These materials display superior electrical and mechanical properties with enhanced material stability without cracking. Second, to address metal-polymer interface detachment, we have designed robust interfaces for IMPC devices with vertical array structures. The as-prepared interfaces present high ionic conductivity with excellent mechanical stability under bending states. As a result, the IPMC devices deliver high working stability exceeding a million cycles under air conditions. Third, in order to avoid water loss in the electrolyte, especially at ambient conditions, we have developed ionogel electrolytes containing highly stable ionic liquids as active ion sources. The ionogel electrolytes effectively prevent water loss in conventional water-containing electrolytes, just like Nafion electrolytes and greatly improve the working stability of IPMC devices. After addressing the key issues of IPMC devices, we finally obtained many high-performing IPMC devices and explored various intelligent applications of them. For instance, we have demonstrated the smart functions of IPMC devices as sensitive strain sensors, such as sign language recognition, handwriting detection, and human muscle monitoring. In addition, we have developed bionic flying robots with a vibration frequency as high as 30 Hz with the aid of high-performance IPMC actuators. A medical catheter based on IPMC actuators has been put forward by our group and can realize multiple degrees of freedom deformation under a low driving voltage of 2.5 V, which presents great potential for application in medical instruments. Lastly, a perspective on critical challenges and future research directions on IPMC materials and devices is highlighted for accelerating practical application.

A highly sensitive and anti-freezing conductive strain sensor based on polypyrrole/cellulose nanofiber crosslinked polyvinyl alcohol hydrogel for human motion detection

Electro-conductive hydrogels emerge as a stretchable conductive materials with diverse applications in the synthesis of flexible strain sensors. However, the high-water content and low cross-links density cause them to be mechanically destroyed and freeze at subzero temperatures, limiting their practical applications. Herein, we report a one-pot strategy by co-incorporating cellulose nanofiber (CNF), Poly pyrrole (PPy) and glycerol with polyvinyl alcohol (PVA) to prepare hydrogel. The addition of PPy endowed the hydrogel with good conductivity (∼0.034 S/m) compared to the no PPy@CNF group (∼0.0095 S/m), the conductivity was increased by 257.9 %. The hydrogel exhibits comparable ionic conductivity at −18 °C as it does at room temperature. It's attributed to the glycerol as a cryoprotectant and the formation of hydrated [Zn(H2O)n]2+ ions via strong interaction between Zn2+ and water molecules. Moreover, the cellulose nanofiber intrinsically assembled into unique hierarchical structures allow for strong hydrogen bonds between adjacent cellulose and PPy polymer chains, greatly improve the mechanical strength (stress∼0.65 MPa, strain∼301 %) and excellent viscoelasticity (G'max ∼ 82.7 KPa). This novel PPy@CNF-PVA hydrogel exhibits extremely high Gauge factor (GF) of 2.84 and shows excellent sensitivity, repeatability and stability. Therefore, the hydrogel can serve as reliable and stable strain sensor which shows excellent responsiveness in human activities monitoration.

Ultrafast Self-Healing, Superstretchable, and Ultra-Strong Polymer Cluster-Based Adhesive Based on Aromatic Acid Cross-Linkers for Excellent Hydrogel Strain Sensors.

Synthetic hydrogel strain sensors rarely exhibit a comprehensive combination of mechanical properties such as ultra-stretchability, ultrafast self-healing, and high sensitivity. Herein, seven small molecule enhanced mechanical behaviors of polymer-cluster based hydrogels are demonstrated. The oxidized polyethyleneimine/polymeric acrylic acid (ohPEI/PAA) hydrogels with aromatic formic acids as supramolecular cross-linkers are prepared by simultaneous formation of ohPEI polymer clusters and PAA upon the addition of ammonium persulfate. The optimized hydrogel adhesive exhibits comprehensive excellent properties, such as high extensibility (up to 12298%), real-time mechanical self-healing capability (<1s, 93% efficiency), high uniformity, underwater adhesivity, and water-sealing ability. The proper binding strength of hydrogel and skin (47kPa) allows the hydrogel to be utilized as highly sensitive (gauge factor:16.08), highly conductive (2.58mScm-1), and underwater strain sensors. Specially, the adhesive strength of the adhesive to wood after dehydration is extremely high, reaching up to 29.59MPa. Additionally, when glycerol is introduced, the obtained gel maintains the physical properties even at harsh-temperature conditions (-40to 80 °C). It presents that multiple and hierarchical non-covalent interactions including multiple hydrogen bonding interactions, π-π stacking, electrostatic interactions, and dipole-dipole interactions of polymer clusters, allow for the energy dissipation and contribute to the excellent performance of the hydrogel.

Highly sensitive strain sensors based on dispensing technology for human–machine interaction

Flexible strain sensors have stable and sensitive sensing performance under deformation conditions such as pressing, bending, and stretching. However, the preparation process of high-performance strain sensors is still very complex, which also limits the application and production of sensors. At the same time, most sensors are unstable and inefficient, so they cannot meet people’s expectations for high sensitivity and stability. In order to solve the above problems, this paper proposes a resistive strain sensor based on dispensing technology, with carbon black and polyurethane mixture as printing ink. Then, a sensor-sensitive layer with a right-angle serpentine structure is printed directly by air pressure extrusion. The sensor can detect changes at 0.1% strain and withstand 2400 tensile cycles while maintaining a sensitivity of 28.07 in the range of 0%–40%. In addition, the sensor can accurately and stably reflect the changes in different joints of the human body. At the same time, the data glove based on the strain sensor shows great application potential in the fields of gesture recognition and human–machine interaction.

Fibrous Conductive Metallogels with Hybrid Electron/Ion Networks for Boosted Extreme Sensitivity and High Linearity Strain Sensor.

Fibrous strain sensing materials with both high sensitivity and high linearity are of significant importance for wearable sensors, yet they still face great challenges. Herein, a photo-spun reaction encapsulation strategy is proposed for the continuous fabrication of fibrous strain sensor materials (AMGF) with a core-sheath structure. Metallogels (MOGs) formed by bacterial cellulose (BC) nanofibers and Ag nanoparticles (AgNPs), and thermoplastic elastomers (TPE) are employed as the core and sheath, respectively. The in situ ultraviolet light reduction of Ag+ ensured AgNPs to maintain the interconnections between the BC nanofibers and form electron conductive networks (0.31 S m-1 ). Under applied strain, the BC nanofibers experience separation, bringing AMGF a high sensitivity (gauge factor 4.36). The concentration of free ions in the MOGs uniformly varies with applied deformation, endowing AMGF with high linearity and a goodness-of-fit of 0.98. The sheath TPE provided AMGF sensor with stable working life (>10000 s). Furthermore, the AMGF sensors are demonstrated to monitor complex deformations of the dummy joints in real-time as a wearable sensor. Therefore, the fibrous hybrid conductive network fibers fabricated via the photo-spun reaction encapsulation strategy provide a new route for addressing the challenge of achieving both high sensitivity and high linearity.