High-performance Flexible Sensors Research Articles

Optically Decoupling Electrochromic Dynamics and In Situ Morphological Evolution of a Single Soft Polyaniline Nanoentity.

Electroresponsive multicolored materials have tremendous potential in flexible electronics and smart wearable devices. Herein, the electrochromic dynamics and in situ morphological evolution of a single soft polyaniline nanoentity can be visualized and decoupled by an opto-electrochemical imaging strategy. The durability, tinting speed, and reversibility down to the single-nanoparticle level are quantified, and the switching of transient intermediate electrochromic states is trapped. The mechanistic studies suggest that the heterogeneity of electrochromic activity is attributed to the nonuniformity of the polymer network interspersed at the nanometric level. Furthermore, the representative Pauli repulsion effect is uncovered from the self-stretching behavior of the conductive state of polyaniline at the oxidized potential. It provides novel insights for advancing high-performance electrochromic devices and flexible strain sensors, which can be dynamically manipulated by external stimuli.

Ultrafast self-healing zwitterionic hydrogels reinforced by carboxymethyl chitosan-oxidized hyaluronic acid and graphene oxide toward high-performance strain sensors

Inspired by the inherent recuperative ability of organisms in nature, researchers have dedicated significant efforts towards developing self-healing hydrogel sensors. Although the works on self-healing hydrogels have made great progress, achieving hydrogel sensors combining with rapid and efficient healing capability, excellent mechanical properties and high sensing sensitivity remains a challenging task. In this study, we proposed a novel approach for fabricating a self-healing conductive zwitterionic hydrogel sensor by adding carboxymethyl chitosan (CMCs) and oxidized hyaluronic acid (OHA) to induce dynamic Schiff base reaction, and nano-reinforced by adding with graphene oxide (GO) nanosheets. This zwitterionic hydrogel exhibited a high tensile strength of 133 kPa and elongation at break of 878 %. By leveraging various dynamic interactions within the system, the hydrogel exhibited ultra-fast and efficient healing property, achieving a self-healing efficiency of 98.9 % within just 15 min. The hydrogel demonstrated exceptional adhesion to diverse substrates especially to glass with a maximum adhesion strength reaching up to 53.7 kPa. The hydrogel exhibited a high gauge factor (GF) value of 23.2, which showed clear and stable monitoring and sensing capabilities across various human movements ranging from swallowing to bending knees. Notably, the hydrogel could also be employed as a Morse code transmitter and flexible tablet. The zwitterionic hydrogel demonstrates its great potential applications in the field of high-performance flexible sensors as well as human-computer interaction.

Empowering soft conductive elastomers with self-reinforcement and remarkable resilience via phase-locking ions.

Endowing soft and long-range stretchable elastomers with exceptional strength, resilience, and ion-conductivity is crucial for high-performance flexible sensors. However, achieving this entails significant challenges due to intrinsic yet mutually exclusive structural factors. In this work, a series of self-reinforcing ion-conductive elastomers (SRICEs) is thus designed to meet the advanced but challenging requirements. The SRICEs behave like a soft/hard dual-phase separated micro-structure, which is optimized through a straightforward preferential assembly strategy (PAS) to ensure that the subsequently introduced ions are locked in the soft phase. Meanwhile, the interaction between ions and soft segments is meticulously tailored to achieve self-reinforcement through strain-induced crystallization. Consequently, an outstanding ultimate strength of approximately ∼51.0 MPa and an exceptional instant resilient efficiency of ∼92.9% are attained. To the best knowledge of the authors, these are the record-high values achieved simultaneously in one ion-conductive elastomer. Furthermore, the resultant toughness of ∼202.4 MJ m-3 is significantly higher, while the modulus of ∼5.0 MPa is lower than that of most reported robust ion-conductive elastomers. This unique combination of properties makes it suitable for advanced flexible applications, e.g. grid-free position recognition sensors. This work provides guidance for designing soft yet robust ion-conductive elastomers and optimizing their mechanical properties.

One-Step Digital Light Processing 3D Printing of Robust, Conductive, Shape-Memory Hydrogel for Customizing High-Performance Soft Devices.

Mechanically robust and electrically conductive hydrogels hold significant promise for flexible device applications. However, conventional fabrication methods such as casting or injection molding meet challenges in delivering hydrogel objects with complex geometric structures and multicustomized functionalities. Herein, a 3D printable hydrogel with excellent mechanical properties and electrical conductivity is implemented via a facile one-step preparation strategy. With vat polymerization 3D printing technology, the hydrogel can be solidified based on a hybrid double-network mechanism involving in situ chemical and physical dual cross-linking. The hydrogel consists of two chemical networks including covalently cross-linked poly(acrylamide-co-acrylic acid) and chitosan, and zirconium ions are induced to form physically cross-linked metal-coordination bonds across both chemical networks. The 3D-printed hydrogel exhibits multiple excellent functionalities including enhanced mechanical properties (680% stretchability, 15.1 MJ/m3 toughness, and 7.30 MPa tensile strength), rapid printing speed (0.7-3 s/100 μm), high transparency (91%), favorable ionic conductivity (0.75 S/m), large strain gauge factor (≥7), and fast solvent transfer induced phase separation (in ∼3 s), which enable the development of high-performance flexible wearable sensors, shape memory actuators, and soft pneumatic robotics. The 3D printable multifunctional hydrogel provides a novel path for customizing next-generation intelligent soft devices.

Multilevel Cu-LIG Tactile Sensing Arrays for 3D Touch Human-Machine Interaction.

High-performance flexible tactile sensors have attracted significant attention in the domains of human-machine interactions. However, the efficient fabrication of sensors with highly sensitive responses over a broad load range still remains a challenge. Here, we propose a one-step laser writing route to construct a distinctive multilevel piezoresistive structure, consisting of Cu nanoparticle-doped graphene protrusions and surrounding porous Cu sheets. This multilevel structure enables the assembled tactile sensors to exhibit superior sensitivity at both low-pressure (1468 kPa-1 at 0-200 kPa) and high-pressure (1345 kPa-1 at 600-800 kPa) stimulations. Its enhancement mechanism for piezoresistive sensing has been investigated. The programmable laser writing process facilitates the development of human-machine interaction devices that recognize multidimensional gestures such as sliding, clicking, and pressing. This advancement serves to promote the development of high-performance interactive sensing technologies.

High-performance flexible sensor with rapidly adjustable sensitivity for wearable electronics

Despite numerous advances in sensor structural design, existing sensors still struggle to achieve rapidly adjustable sensitivity. Here, we utilized a structural design with alternating hard and soft sensor connections, and a lockout device to construct a sensitivity-adjustable sensor (AHS-L). Silicone rubber foams (SF) served as the flexible matrix for fabricating sensors with different hardnesses. Among them, the soft sensor (SRC-ACPF) retained the foam structure, and constructed dual conductive networks both within and on the pore walls, enhancing its resistance sensitivity with a gauge factor (GF) of up to 561.9. In contrast, the SF used in the hard sensor (ASR-PPS) featured a fingerprint-like structure. A conductive network was introduced on the pores and backfilled with liquid silicone rubber to achieve a “soft and hard” distinction from SRC-ACPF. The fingerprint-like structure induced stress concentration during stretching, resulting in high resistance sensitivity for ASR-PPS (GF up to 711.5). This microstructure also amplified contact signals, enabling ASR-PPS to realize intelligent material recognition. Notably, the alternating soft and hard structure of the AHS-L allowed the tensile deformation to be concentrated in the SRC-ACPF. Meanwhile, due to the initial resistance difference, the resistance change in SRC-ACPF has less effect on the overall resistance fluctuation of AHS-L than ASR-PPS. Therefore, a lockout device was introduced to limit the deformation of the SRC-ACPF, allowing the deformation pattern of the AHS-L to be tuned and enabling the sensitivity to be quickly adjusted from low (GF of 3.7 within 0–22% strain, 63.8 within 22–38% strain) to ultra-high (GF of 444.7 within 0–20% strain, 821.2 within 20–46% strain).

Fabricated Nanocomposite ZnO-Loaded PVA/PEDOT:PSS Via Electrospinning for Ammonia Detection

The problem of air pollution has increased dramatically around the world and there are concerns about the detection of hazardous gases present in the atmosphere as they are very harmful to human health. Remarkably, ammonia (NH3) gas is one of the most hazardous environmental pollutants, contributing to chronic lung disorders and respiratory inflammations. Gas sensors play a crucial role in monitoring and detecting toxic gases, ensuring public safety, preventing food spoilage, and monitoring air quality. Typically, a resistive gas sensor comprises an active sensing material whose electrical conductivity is sensitive to gases. Up to now, lots of work have been related to MOS-based gas sensors. However, MOS-based sensors still face some challenges in selectivity, response/recovery times, and stability. Therefore, this study is dedicated to the investigation of Polyvinyl alcohol (PVA)/Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and ZnO-loaded PVA/PEDOT:PSS electrospun composite nanofibers for gas sensing application. The successful integration of ZnO nanoparticles into the composite polymer matrix was confirmed through a combination of structural, morphological, optical, and thermal analyses, revealing a fibrous structure with enhanced morphology. The gas-sensing study demonstrated the superior performance of the ZnO-loaded fibers in NH3 gas detection, showcasing enhanced sensitivity, selectivity, and long-term stability compared to its pristine counterpart (PVA/PEDOT:PSS). Fabricated sensors showed selectivity and 70 % response to 100 ppm NH3, as well long- term stability for up to 12 months. Our findings not only contribute to the understanding of composite sensor materials but also offers promising candidates for the development of high-performance flexible gas sensors. Acknowledgment: This work was supported by the Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP23489498). "Development of advanced polymer-based sensor containing biowaste-derived carbon for detection of NH3" Figure 1

Flexible Piezoelectric Sensor Enhanced by Ordered Piezoelectric Composite Material for Three-Dimensional Force Detection.

With the rapid development of intelligent devices, the demand for high-performance flexible sensors with three-dimensional force perception capabilities has become increasingly prominent. In this study, we utilized dielectrophoresis regulation to achieve an ordered arrangement of lead zirconate titanate particles in piezoelectric composite films, enhancing the pressure sensitivity by approximately 4.06 times compared to randomly distributed composite films. Furthermore, an additional bump structure was constructed to convert three-dimensional forces into different compression states on the sensing units of the film, enabling effective decoupling of three-dimensional forces. Experimental results demonstrated that the three-dimensional piezoelectric force sensor exhibited sensitivity values of 0.2524, 0.1702, and 0.1946 V/N in three directions within the range of 1-9 N. Additionally, the sensor possesses significant advantages such as rapid response (4 ms), good repeatability, and simple manufacturing. These characteristics offer a viable strategy for self-powered wearable devices and human-machine interactions in intelligent devices.

Bending and sensing performances of electro-ionic soft actuators based on carboxylated cellulose nanofibers reinforced with graphene nanoplatelets

Abstract Soft robots not only possess greater degrees of freedom and the capability for continuous transformation, but they also offer exceptionally high safety in human–robot interactions, avoiding harm to the human body. Soft actuators are essential for developing high-performance soft robots, offering significant bending deformation, rapid response times, and prolonged operational capabilities. Herein, we present an ionic electroactive soft actuator based on functional cellulose nanofibers, graphene nanoplatelets, and ionic liquid. The proposed actuator achieved a large displacement about ±8 mm under 2.0 V at 0.1 Hz, with long working stability (98% of initial peak displacement maintained after 1260 cycles of cycling). The human–robot interaction applications of this actuator were explored by simulating human fingers. More importantly, the static and dynamic sensing performances of the actuator were investigated, finding that it generated a sensing voltage of 0.37 V at a vibration displacement of only 1.75 mm. The designed actuator provides a promising approach for developing high-performance soft robots, soft actuators, flexible sensors, and flexible active devices.

Self-Powered Machine-Learning-Assisted Material Identification Enabled by a Thermogalvanic Dual-Network Hydrogel with a High Thermopower.

Wearable devices equipped with high-performance flexible sensors that can identify diverse physical information free from batteries are playing an indispensable role in various fields. However, previous studies on flexible sensors have primarily focused on their elasticity and temperature-sensing capability, with few reports on material identification. In this paper, a thermogalvanic dual-network hydrogel is fabricated with [Fe(CN)6]3-/4- as a redox couple and lithium magnesium silicate, Gdm+ and lithium bromideas key electrolytes to optimize the interconnected porous structure of the gel, which shows excellent mechanical and thermoelectric properties with a thermopower as high as 4.01mV K-1. A self-powered material identification ring is developed based on the temperature-triggered thermoelectric response of the gel in conjunction with machine learning, which can actively infer materials without an external power connection by analyzing the voltage signals correlated with interfacial heat transfer produced upon contact with different materials. The proposed gel ring has important applications for future areas such as human-computer interaction and haptic-associated artificial intelligence.

Lignin sulfonate induced ultrafast fabrication of polypyrrole-based conductive organohydrogel for high performance flexible strain and temperature sensor

The ultrafast preparation of electrically conductive hydrogels to endow high sensing performance and temperature tolerance remains a critical challenge. Herein, lignosulfonate sodium-templated polypyrrole (LS-PPy) nanofillers were rapidly introduced into polyacrylic acid (PAA) hydrogel through ultrafast free radical polymerization in a glycerol/water binary solvent system. The resultant LS-PPy/PAA electrically conductive organohydrogel possesses satisfactory mechanical performance (strength of 56 kPa at a tensile strain of 800 %), strong adhesion, and a desirable low freezing point (−35 °C). Furthermore, this organohydrogel exhibits high strain sensitivity (gauge factor = 2.65), fast response time (∼160 ms), low signal hysteresis, and excellent cyclic stability (over 1200 cycles). And the wearable LS-PPy/PAA organohydrogel sensor could accurately and real-time monitor various intense or subtle human movements, such as joint bending, facial expression and hand writing. Besides, the developed LS-PPy/PAA temperature sensor can respond to environmental temperature variations over a wide range of −20–100 °C. High resolution of 0.5 °C with remarkable sensitivity (−0.80 %/°C and linearity of R2 = 0.99) and repeatability were achieved within 36.5–40 °C, which makes it suitable for human body temperature monitoring. All these results demonstrate the substantial prospective value of the LS-PPy/PAA hydrogel in wearable sensors and other associated fields.

Self-assembled conductive coating and bifacial microstructures Collaboratively Adjusting the sensing performance and high reliability of the flexible pressure sensors

Flexible pressure sensors are being studied intensively for applications such as smart wearables and health monitoring. While ensuring the excellent sensing performance, the reliability (durability and environmental resistance) of the sensors are also key indicators that determine whether they can be used in practice. In this research, a strategy to adjust the sensing performance and high reliability of the flexible pressure sensors by self-assembled conductive coating and bifacial microstructures (BM) is proposed. Polydimethylsiloxane/carbon nanotube films with BM are first modified to reduce the surface contact angle. Then poly(dimethyl diallyl ammonium chloride) (PDAC) and hydroxylated carbon nanotubes/carboxylated cellulose nanofibers/glycerol (CNT-OH/CNF-C/GL) are successively assembled on the BM surface to construct conductive coatings (PCCG). The sensor based on PCCG and BM (the PCCG-BM sensor) shows a high sensitivity (9.97 kPa−1), a wide detection range (0–330 kPa), and a fast response time (59 ms). The PCCG-BM sensor has excellent repeatability, and it still outputs stable current change signals after 240,000 pressure cycles. What’s more, the PCCG-BM sensor is treated with extreme environments such as bending, stretching, washing, and high pressure, and the sensor still maintains excellent performances. This preparation strategy provides a new perspective for the fabrication of high-performance and high-reliability flexible sensors.

Porous dielectric design for high-performance flexible shear sensors

Abstract Flexible sensors, renowned for their exceptional adaptability, are widely utilized in areas such as robotics and healthcare monitoring. Currently, understanding the relationship between the structure and performance of flexible shear sensors has become increasingly important. In this study, the structural characteristics of porous flexible shear sensors and their impact on performance were analyzed using Finite Element Method (FEM). The investigation focused on three factors: porosity, pore size, and the shape of elliptical pores. The findings suggest that augmenting the porosity substantially boosts the shear sensitivity of the sensor, whereas the pore size exerts a relatively minor influence on sensitivity. Furthermore, compared to circular ones, elliptical pores can further increase the shear sensitivity of the sensor, particularly when the ellipse’s major axis is oriented horizontally. These findings are of significant reference value for designing and optimizing high-performance flexible shear sensors.

High-Performance Flexible Strain Sensor Based on Thermoplastic Polyurethane Melt-Blown Nonwoven with Molybdenum Disulfide for Human Motion Monitoring

High-Performance Flexible Strain Sensor Based on Thermoplastic Polyurethane Melt-Blown Nonwoven with Molybdenum Disulfide for Human Motion Monitoring

High-performance flexible humidity sensor based on B-TiO2/ SnS2 nanoflowers for non-contact sensing and respiration detection

The development of flexible humidity sensors is essential for the advancement of wearable devices and electronic skin. However, the process of preparing these sensors is typically complex, and they often suffer from poor stability and a limited sensing range, which fails to meet the requirements for practical applications. Here, we prepared SnS2 with nanoflower morphology by hydrothermal method, and successfully prepared a high-performance flexible humidity sensor with a high response range (20-95 %) by compositing it with B-TiO2 nanoparticles with oxygen defects after sodium borohydride reduction and coating it on a PET flexible substrate. This sensor demonstrates an exceptional response (753.2 at 90 % RH) in high humidity environments and exhibits rapid response (13 s) and recovery times (19 s). Moreover, the sensor accurately detects human breathing patterns and frequencies, making it suitable for non-contact sensing applications. This research offers valuable insights into the simplified preparation and advancement of flexible humidity sensors.

Fabrication of anti-freezing and self-healing hydrogel sensors based on carboxymethyl guar gum and poly(ionic liquid)

As classical soft materials, conductive hydrogels have attracted wide attention in the field of strain sensors due to their unique flexibility and conductivity. However, there are still challenges in developing conductive hydrogels with comprehensive mechanical strength, self-healing ability and sensitive sensing properties. In this paper, a novel PAV/CMGG hydrogel was prepared by a simple one-pot method through the introduction of 1-vinyl-3-butylimidazolium bromide (VBIMBr), acrylic acid (AA), carboxymethyl guar gum (CMGG) and AlCl3. The coordination bond between Al3+ and –COO− groups on PAA and CMGG, the hydrogen bond between PAA and CMGG, and the electrostatic interaction between [VBIM]+ and –COO− endow the hydrogel with good mechanical properties, self-recovery ability, fatigue resistance and great self-healing properties. PAV/CMGG hydrogel had good conductivity of 2.31 S/m which could successfully light up the bulb. The hydrogel as the strain sensor had not only a wide strain sensing capability (strain ranging from 0 to 800 %), but also a high strain sensitivity (gauge factor (GF) = 28.50 for the strain ranging from 600 to 800 %). This study can provide inspiration for the construction of new high-performance flexible sensors.

High-Performance Flexible Sensor with Sensitive Strain/Magnetic Dual-Mode Sensing Characteristics Based on Sodium Alginate and Carboxymethyl Cellulose.

Flexible sensors can measure various stimuli owing to their exceptional flexibility, stretchability, and electrical properties. However, the integration of multiple stimuli into a single sensor for measurement is challenging. To address this issue, the sensor developed in this study utilizes the natural biopolymers sodium alginate and carboxymethyl cellulose to construct a dual interpenetrating network, This results in a flexible porous sponge that exhibits a dual-modal response to strain and magnetic stimulation. The dual-mode flexible sensor achieved a maximum tensile strength of 429 kPa and elongation at break of 24.7%. It also exhibited rapid response times and reliable stability under both strain and magnetic stimuli. The porous foam sensor is intended for use as a wearable electronic device for monitoring joint movements of the body. It provides a swift and stable sensing response to mechanical stimuli arising from joint activities, such as stretching, compression, and bending. Furthermore, the sensor generates opposing response signals to strain and magnetic stimulation, enabling real-time decoupling of different stimuli. This study employed a simple and environmentally friendly manufacturing method for the dual-modal flexible sensor. Because of its remarkable performance, it has significant potential for application in smart wearable electronics and artificial electroskins.

A highly responsive and sensitive flexible strain sensor with conductive cross-linked network by laser direct writing

As a critical component of wearable electronic devices, flexible strain sensors have made significant contributions in the fields of smart healthcare, motion monitoring and communication, garnering escalating research interests. However, the pursuit of ideal materials synergizing high sensitivity with facile and cost-effective manufacturing methodologies remains a persistent challenge, significantly hindering the full realization of the potential of flexible strain sensors. Herein, an innovative hybrid conductive network architecture constituted by multi-walled carbon nanotubes and laser-induced graphene (MWCNTs/LIG) has been explored via a facile, cost-effective, controllable, and environmentally friendly laser direct writing (LDW) technique. Subsequently, the resultant MWCNTs/LIG composite with a cross-linked microstructure has been harnessed for the fabrication of a high-performance strain sensor. Notably, the proposed sensor exhibits remarkable sensitivity with a gauge factor (GF) reaching 192, rapid response and recovery times of 27/20 ms, and favorable durability exceeding 2300 cycles. Furthermore, the developed sensor demonstrates the capability to accurately capture full-range motions in real-time, as well as the ability to transmit information as a portable communication device. This promising discovery offers a new insight for the flourishing development of high-performance flexible strain sensors in the realm of wearable sensing.

Ultrasensitive, highly stretchable and multifunctional strain sensors based on scorpion-leg-inspired gradient crack arrays

Wearable flexible strain sensors based on conductive films have shown great application potential in many high-tech fields. They have high requirements for synergistic performances of sensitivity, flexibility, stability and durability. However, overcoming the trade-off between high sensitivity and high stretchability is still a great challenge in this research. Here we develop a facile strategy to prepare high-performance flexible strain sensors based on gradient crack arrays inspired by the gradient slits on scorpion legs. The gradient crack arrays with branched or hierarchical features controllably form in a periodic thickness-gradient metal film prepared by one-step masking technique on a flexible substrate. The cracks are progressively open from the thickest film region to the thinnest one under mechanical strain, behaving a unique sensing mechanism. As a result, the gradient crack-based strain sensors possess excellent comprehensive performances including high sensitivity (up to 21000), wide sensing range (∼70 %), low detection limit (0.02 %), quick response speed (below to 80 ms), excellent stability and durability (up to 15,000 cycles at 5 % strain). As demonstration of applications, the sensors are used to monitor various human activities and to express and encrypt information by bending bidirectionally. This work provides a novel way to fabricate highly sensitive, stretchable, robust, multifunctional, crack-based flexible strain sensors, which would greatly expand the applications in flexible electronics and wearable devices.

An ultrathin, rapidly fabricated, flexible giant magnetoresistive electronic skin

In recent years, there has been a significant increase in the prevalence of electronic wearables, among which flexible magnetoelectronic skin has emerged as a key component. This technology is part of the rapidly progressing field of flexible wearable electronics, which has facilitated a new human perceptual development known as the magnetic sense. However, the magnetoelectronic skin is limited due to its low sensitivity and substantial field limitations as a wearable electronic device for sensing minor magnetic fields. Additionally, achieving efficient and non-destructive delamination in flexible magnetic sensors remains a significant challenge, hindering their development. In this study, we demonstrate a novel magnetoelectronic touchless interactive device that utilizes a flexible giant magnetoresistive sensor array. The flexible magnetic sensor array was developed through an electrochemical delamination process, and the resultant ultra-thin flexible electronic system possessed both ultra-thin and non-destructive characteristics. The flexible magnetic sensor is capable of achieving a bending angle of up to 90 degrees, maintaining its performance integrity even after multiple repetitive bending cycles. Our study also provides demonstrations of non-contact interaction and pressure sensing. This research is anticipated to significantly contribute to the advancement of high-performance flexible magnetic sensors and catalyze the development of more sophisticated magnetic electronic skins.