Carbon nanotubes reinforced hydrogel as flexible strain sensor with high stretchability and mechanically toughness

Mussel-inspired cellulose nanofiber/poly(vinyl alcohol) hydrogels with robustness, self-adhesion and antimicrobial activity for strain sensors

Flexible strain sensors fabricated using carbon-based nanomaterials: A review

A flexible and sensitive strain sensor with three-dimensional reticular structure using biomass Juncus effusus for monitoring human motions

Fabrication method of flexible strain sensors with CNTs and solvents

Flexible and sensitive strain sensors can be utilized as wearable sensors and electronic devices in a wide range of applications, such as personal health monitoring, sports performance, and electronic skin. This paper presents the fabrication of a highly flexible and sensitive strain sensor by 3D printing an electrically conductive polydimethylsiloxane (PDMS)/multi-wall carbon nanotube (MWNT) nanocomposite on a PDMS substrate. To maximize the sensor’s gauge factor, the effects of MWNT concentration on the strain sensing function in nanocomposites are evaluated. Critical 3D printing and curing parameters, such as 3D printing nozzle diameter and nanocomposites curing temperature, are explored to achieve the highest piezoresistive response, showing that utilizing a smaller deposition nozzle size and higher curing temperature can result in a higher gauge factor. The optimized 3D printed nanocomposite sensor’s sensitivity is characterized under cyclic tensile loads at different maximum strains and loading rates. A linear piezoresistive response is observed up to 70% strain with an average gauge factor of 12, pointing to the sensor’s potential as a flexible strain sensor. In addition, the sensing function is almost independent of the applied load rate. The fabricated sensors are attached to a glove and used as a wearable sensor by detecting human finger and wrist motion. The results indicate that this 3D printed functional nanocomposite shows promise in a broad range of applications, including wearable and skin mounted sensors.

Flexible strain sensors play a key role in the field of human–machine interaction because of their ability to sense deformation. To meet the needs of mass production of wearable electronic devices, it is very necessary to develop a strain sensor with excellent sensing performance and low-cost mass fabrication. In this article, a fabrication process of a flexible resistive strain sensor, which is perfectly matched by the combined method with dip-coating and water bath, is proposed. A layer of conductive polymers containing carbon black/multiwall carbon nanotubes/thermoplastic polyurethane (CB/MWCNTs/TPU) was quickly and evenly wrapped on the surface of the core-spun elastic yarn. Combining the synergistic conductive effect of the filler and the high elasticity of the yarn matrix, the designed flexible strain sensor has high sensitivity (gauge factor (GF) to 68), low detection limit, wide strain range, fast response time, fast recovery time, and high stability. The designed strain sensor exhibits outstanding sensing performance and wearability, which can be used for real-time monitoring of human joint bending and breathing state. Furthermore, a wireless operating system based on wearable data gloves has been developed to realize the remote synchronous movement of the human hand/the bionic manipulator. The results indicate that the proposed strain sensor has great potential in human–machine interaction and its fabrication technology provides guidance for the sensor in mass production.

Superhydrophobic flexible strain sensors have great application value in the fields of personal health monitoring, human motion detection, and soft robotics due to their good flexibility and high sensitivity. However, complicated preparation processes and costly processing procedures have limited their development. To overcome these limitations, in this work we develop a facile and low-cost method for fabricating superhydrophobic flexible strain sensor via spraying carbon black (CB) nanoparticles dispersed in a thermoplastic elastomer (SEBS) solution on a polydimethylsiloxane (PDMS) flexible substrate. The prepared strain sensor had a large water contact angle of 153 ± 2.83° and a small rolling angle of 8.5 ± 1.04°, and exhibited excellent self-cleaning property. Due to the excellent superhydrophobicity, aqueous acid, salt, and alkali could quickly roll off the flexible strain sensor. In addition, the sensor showed excellent sensitivity (gauge factor (GF) of 5.4–7.35), wide sensing ranges (stretching: over 70%), good linearity (three linear regions), low hysteresis (hysteresis error of 4.8%), and a stable response over 100 stretching-releasing cycles. Moreover, the sensor was also capable of effectively detecting human motion signals like finger bending and wrist bending, showing promising application prospects in wearable electronic devices, personalized health monitoring, etc.

Flexible, stretchable and wearable electronic devices are becoming moredemanding due to their facile interaction with human body. These devices can beeasily mount on clothing or directly attached onto skin. Among wearableelectronic devices, highly stretchable and sensitive strain sensors have drawn lotsof attention for monitoring human body motions. There are number ofrequirements for a high performance strain sensor including high sensitivity (i.e.,gauge factor (GF)), high stretchability, fast response, high stability and lowfabrication cost. Personal health monitoring, sport performance monitoring andhuman motion capturing for entertainment systems are the most popularapplications for a high performance strain senor.In this paper, we present a cost effective simple strain sensor fabricated throughsilver electroless metallization of commercial polyester fabric. To avoid usingSnCl2 or PdCl2 activation solutions, a silica-like layer was formed on polyesterfabric using an acetone based aminopropyltrimethoxysilane (APTMS) solutionfollowed by UV irradiation. Once the anchoring sites for later silanization weredeveloped, a toluene based solution of (3-Mercaptopropyl)trimethoxysilane(MPTMS) were used to activate the polyester fabric. Silver electrolessmetallization was carried out on activated polyester using silver nitrate salt andglucose as reducing agent. Scanning electron microscopy (SEM) and X-raydiffraction spectroscopy (XRD) were used to characterize the Ag-coated polyester.A thin silver coating (<1 μm) with uniform morphology and high purity wasdeposited on polyester. The conductive fabric was cut into strips of 5mmthickness for stretch-conductivity measurements. A micro-tensile machine withan in-situ confocal microscope was coupled with a digital multimeter (2-pointprobing) to investigate strain dependence electrical resistivity. In-situ confocalimaging was used to investigate the micromechanical mechanisms responsible forchanges in resistivity during stretching. A linear decrease in electrical resistancewas detected by stetching untill it reaches to a platu. The results of stretch–conductivitymeasurements suggest the potential application of Ag/Polyester forstrain sensors. Figure 1

Flexible strain sensors have a wide range of applications in biomedical science, aerospace industry, portable devices, precise manufacturing, etc. However, the manufacturing processes of most flexible strain sensors previously reported have usually required high manufacturing costs and harsh experimental conditions. Besides, research interests are often focused on improving a single attribute parameter while ignoring others. This work aims to propose a simple method of manufacturing flexible graphene-based strain sensors with high sensitivity and fast response. Firstly, oxygen plasma treats the substrate to improve the interfacial interaction between graphene and the substrate, thereby improving device performance. The graphene solution is then sprayed using a soft PET mask to define a pattern for making the sensitive layer. This flexible strain sensor exhibits high sensitivity (gauge factor ~100 at 1% strain), fast response (response time: 400–700 μs), good stability (1000 cycles), and low overshoot (<5%) as well. Those processes used are compatible with a variety of complexly curved substrates and is expected to broaden the application of flexible strain sensors.

Wearable strain sensors have been widely used for human activity monitoring. Most reported strain sensors have mainly focused on material engineering, high stretchability and large gauge factors. Few works have focused on strain sensor’s robustness and reliability, including low hysteresis, good long-term stability, good electrode material stability, and low coupling effects under multi-input signals, which are the factors that limit practical strain sensor applications. To develop a high-performance strain sensor, we propose a flexible capacitive sensor structure with three-dimensional (3D) interdigital electrodes fabricated by vertically aligned carbon nanotubes. Compared with a traditional resistive strain sensor and a capacitive strain sensor with vertical sandwich electrodes, a strain sensor with horizontal parallel interdigital electrodes can benefit from low cross talk in terms of the normal force and improve substrate transparency. Additionally, embedding 3D electrodes into the substrate improves ultrahigh robustness with a low-pressure coupling effect under normal force. Moreover, compared with other reported works, the electrode variation under strain is small (less than 1.6%), which means that the perturbation of inert properties on device performance is small. Finally, the fabricated strain sensor achieves an ultralow hysteresis (0.35%), excellent pressure-insensitive performance (less than 0.8%), fast response (60 ms), good long-term stability, and good transparency. As an application example, a flexible strain sensor was successfully demonstrated as a wearable device for the precise monitoring of different types of human activities, including bending of the finger, knee, elbow, wrist, and neck with large strain signals and small strain signals generated by a mouth-opening activity. This excellent performance indicates that the flexible strain sensor is a promising candidate for human motion detection, soft robotics, and medical care.

The rapid development of flexible electronics greatly promotes personalized health monitoring and thereupon spawns diverse wearable electronic devices. As a key functional component of flexible electronics, flexible strain sensor receives tremendous research effort, and is widely applied to various areas such as electronic skin, human–machine interaction, and structural health monitoring. Carbon‐based materials including carbon nanotubes (CNTs), carbon black, and graphene are employed as functional materials in this paper to be mixed with the elastic polydimethylsiloxane (PDMS) matrix via the solution blending method. A bridge‐island structure is designed due to its unique structure and dispersion ability, enabling the derived sensor with both good stretchability of 100% and sensitivity of 8.99. The results offer a potential strategy for addressing the difficulties of sensitivity‐stretchability tradeoff originating from the flexible strain sensors prepared with single carbon‐based filler material and other similar strain sensors. Moreover, the realization of an intercommunication system between the flexible strain sensor and a mobile phone via NRF52805 Bluetooth communication chips can monitor various body motions in real time, demonstrating a promising prospect for building remote health monitoring and management systems.

High-performance flexible strain sensors with synergistic and outstanding thermal regulation function are poised to make a significant impact on next-generation multifunctional sensors. However, it has long been intractable to optimize the sensing performance and high thermal conductivity simultaneously. Herein, a novel flexible sandwich-like strain sensor with advanced thermal regulation capability was prepared by assembling electrospun thermoplastic polyurethane (TPU) fibrous membrane, MXene layer, and TPU/boron nitride nanosheet (BNNS) composite films. The as-prepared sensor demonstrates a wide strain working range (∼100% strain), an ultrahigh gauge factor (2080.9), and a satisfactory reliability. Meanwhile, benefiting from the uniform dispersion and promising orientation of BNNSs in TPU composites, the sensor possesses a high thermal conductivity of 1.5 W·m-1·K-1, guaranteeing wearer comfort. Additionally, the unique structure endows the sensor with high stretchability, breathability, biocompatibility, and tunable electromagnetic interference shielding performances. Furthermore, an integrated wireless motion monitoring device based on this sensor is rationally designed. It exhibits a fast response time, a wide recognition range, and the ability to maintain skin temperature during prolonged physical activity. These encouraging findings provide a new and feasible approach to designing high-performance and versatile flexible strain sensors with broad applications in advanced wearable technology.

Wearable flexible electronic strain sensor devices have gained significant attention in recent years due to their potential for detecting human motion in various scenarios. However, the development of strain sensors with high sensitivity across a wide range of strains remains a major challenge. We present herein a novel strain sensor based on a graded structure thermoplastic polyurethane (TPU)/carbon nanotube (CNT) composite yarn with significantly enhanced mechanical performance imparted by the designed structure. The twisted CNTs/TPU spiral yarn demonstrated a fracture elongation of up to 1066% while maintaining charge conductivity under high‐strain conditions. Moreover, it exhibited sensitive changes in resistance versus tensile strain, excellent repeatability, and stability. As a strain sensor, it achieved a gauge factor (GF) of 67.2 within a strain range below 50%, reaching 51.7 in a strain range exceeding 150%. With a fast response time of 0.12 s, it enabled accurate identification of movements in different body parts. These findings highlight the broad application potential of the designed spiral yarn strain sensor in areas such as human motion monitoring and human–computer interaction.Highlights Prepares a flexible strain sensor with graded‐structure (CNT‐fiber‐yarn). Shows high sensitivity and a wide strain response range Reveals the conductive model of strain sensor.

In the past decade, flexible strain sensors have attracted much attention in the fields of health care, soft robots and other flexible electronics due to their unique flexibility, high stability, and strong mechanical properties. To further meet the requirements of the excellent performance for electronic equipment, carbon-based conductive sensitive materials have become one of the first choice for the preparation of flexible strain sensors due to their excellent electrical conductivity, mechanical properties, and high compatibility. Herein, based on different strain behaviors, this paper analyzes the working mechanism of tensile and compressive strain sensors, focusing on the latest research progress of carbon-based conductive materials in strain sensors with different dimensions. The applications of carbon-based sensitive materials with multifunctional strain sensing in the areas of physiological information detection, human motion, human–machine interaction, and visual display have also been summarized. Furthermore, it has been attempted to discuss the current challenges of carbon-based strain sensors as well as the prospect of flexible strain sensors. This review is aimed to provide appropriate references for further exploitation of multi-functional flexible carbon-based strain sensors.

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