Flexible Multifunctional Sensor Research Articles

Muscle-inspired MXene-based conductive hydrogel by magnetic induced for flexible multifunctional sensors

In recent years, conductive hydrogel-based flexible electronic sensor devices have gained widespread attention in the fields of flexible wearable devices, human-machine interaction interfaces, medical monitoring, etc. However, conventional conductive hydrogels struggle to concurrently achieve exceptional mechanical properties and high conductivity, often exhibiting isotropic mechanical performance and sensing characteristics. In contrast, biological tissues typically demonstrate anisotropic mechanical properties and sensing capabilities due to their ordered microstructure. Drawing inspiration from the ordered structures of biological tissues, this study proposed a simple strategy to fabricate high-strength anisotropic MXene-based conductive hydrogels. Initially, a magnetic and conductive two-dimensional nanohybrid material (PDA-Fe3O4-MXene) was synthesized via acid etching and PDA-mediated co-precipitation methods. Secondly, a magnetic field-induced orientation strategy was employed to induce the oriented arrangement of PDA-Fe3O4-MXene within a dual-network polyvinyl alcohol/polyacrylamide (PAAm/PVA) hydrogel. Subsequently, the oriented PDA-Fe3O4-MXene structure was fixed in the dual-network hydrogel by the photopolymerization and freezing-thawing methods. The anisotropic conductive hydrogel exhibited a highly oriented structure, conferring its anisotropic mechanical properties and conductivity. Its mechanical and conductive performances were enhanced in an anisotropic manner, demonstrating outstanding tensile strength (156 KPa) and good conductivity (1.10 mS/cm). Moreover, the conductive hydrogel-based sensors showcased a broad working range (3 %–300 %), rapid response time (290 ms), high sensitivity, and exceptional stability. This work presents a novel approach for constructing flexible electronic devices based on anisotropic hydrogels, offering promising prospects for various applications.

Recyclable and mechanically tough nanocellulose reinforced natural rubber composite conductive elastomers for flexible multifunctional sensor

The development of flexible wearable multifunctional electronics has gained great attention in the field of human motion monitoring. However, developing mechanically tough, highly stretchable, and recyclable composite conductive materials for application in multifunctional sensors remained great challenges. In this work, a mechanically tough, highly stretchable, and recyclable composite conductive elastomer with the dynamic physical-chemical dual-crosslinking network was fabricated by the combination of multiple hydrogen bonds and dynamic ester bonds. To prepare the proposed composite elastomers, the polyaniline-modified carboxylate cellulose nanocrystals (C-CNC@PANI) were used as both conductive filler to yield high conductivity of 15.08 mS/m, and mechanical reinforcement to construct the dynamic dual-crosslinking network with epoxidized natural rubber latex to realize the high mechanical strength (8.65 MPa) and toughness (29.57 MJ/m3). Meanwhile, the construction of dynamic dual-crosslinking network endowed the elastomer with satisfactory recyclability. Based on these features, the composite conductive elastomers were used as strain sensors, and electrode material for assembling flexible and recyclable self-powered sensors for monitoring human motions. Importantly, the composite conductive elastomers maintained reliable sensing and energy harvesting performance even after multiple recycling process. This study provides a new strategy for the preparation of recyclable, mechanically tough composite conductive materials for wearable sensors.

A nanocellulose-based flexible multilayer sensor with high sensitivity to humidity and strain response for detecting human motion and respiration

Biomass-based flexible sensors with excellent mechanical and sensing properties have attracted significant attention. In this study, based on the excellent dispersibility and degradability of nanocellulose crystals, we designed a polyvinyl alcohol/nanocellulose crystals/phytic acid (PCP) composite film with good flexibility and high sensitivity to humidity. A layer of multiwalled carbon nanotubes (MWCNT) and nanocellulose crystals (CNC) was further sandwiched between two PCP layers as a flexible multifunctional sensor (PCPW) to detect human movement and respiration. Phytic acid contains abundant phosphate groups that enhance proton conduction, allowing the PCPW composite film to change its electrical resistance in a sensitive and repeatable manner when the relative humidity was varied between 35 %–93 %. Meanwhile, CNC derived from sisal fibers enhanced the PCPW sensor's conductivity (3.3 S/m) and mechanical properties (elongation at break: 99 %) by improving the dispersion and connectivity of MWCNT. The PCPW sensor displayed a high sensitivity to strain (gauge factor: 49.5) and could monitor both facial expressions (smiling and winking) and the bending of joints. The sensor also generated stable electrical responses during breathing and blowing due to the change in humidity. Therefore, this biodegradable and multifunctional sensor has good application prospects.

Transparent, stretchable, self-healing, and self-adhesive ionogels for flexible multifunctional sensors and encryption systems

Conductive gels are promising soft ionic conductors for stretchable and flexible electronics. However, the integration of multiple functions into one gel remains a great challenge, as the tailoring of these properties is mutually exclusive. In this paper, a simple photopolymerization method is designed to prepare polyacrylic acid/ionic liquid /hydroxypropyl cellulose (PIH) ionogels, which have high transparency (89 %), and superior electrical conductivity (0.379 mS/cm). Owing to the dynamic and reversible nature between the polymer chains, the ionogel possesses good self-healing capability (79.6 %) and self-adhesive properties (242.32–742.76 kPa, typical substrate at room temperature). At the same time, the excellent chemical stability of ionic liquids ensures that PIH ionogels still have good temperature sensitivity (-30 ∼ 110 °C) in extremely harsh environments. Wearable strain sensors based on PIH ionogels can sensitively detect and distinguish body movements, such as limb bending, pronunciation, and pulse. When PIH ionogels are used as a temperature sensor, a cryptographic device with a temperature alarm and an externally stimulated UV switch is designed by using the excellent transparency and temperature response of PIH ionogels. It is believed that the designed ionogel will show great prospects in wearable devices and flexible encryption.

Semi-embedded flexible multifunctional sensor for on-site continuous monitoring of plant microclimate

The excellent stretchability and biocompatibility of flexible sensors have inspired an emerging field of plant wearables that enable intimate contact with plants to monitor growth status and local microclimate in real-time continuously. Flexible plant wearables provide a promising platform for the development of plant phenotypes. In this work, we have fabricated several plant wearables that can harmlessly and continuously monitor leaf temperature/humidity, which are critical parameters for analyzing the physiological status and microclimate of plants. Credited to the unique semi-embedded design and the selected biocompatible materials, we achieve ultrathin sensors with a conductive layer thickness of 7 μm, exhibiting excellent electrical conductivity with 9.43 × 106 S/m, bendability and fatigue resistance with 0.99 % change rate after 10,000 cyclic bending tests, enabling on-site and non-destructive testing. These plant wearables can be used as a non-invasive, high-throughput, low-cost toolbox with excellent potential for phenotypic analysis.

Recent Advances in Flexible Multifunctional Sensors.

Wearable electronics have received extensive attention in human-machine interactions, robotics, and health monitoring. The use of multifunctional sensors that are capable of measuring a variety of mechanical or environmental stimuli can provide new functionalities for wearable electronics. Advancements in material science and system integration technologies have contributed to the development of high-performance flexible multifunctional sensors. This review presents the main approaches, based on functional materials and device structures, to improve sensing parameters, including linearity, detection range, and sensitivity to various stimuli. The details of electrical, biocompatible, and mechanical properties of self-powered sensors and wearable wireless systems are systematically elaborated. Finally, the current challenges and future developmental directions are discussed to offer a guide to fabricate advanced multifunctional sensors.

A novel highly stretchable, freeze-resistant, and recyclable organohydrogel by waterborne polyurethane and DMSO-H2O binary solvent enhanced for multifunctional sensors

Conductive hydrogels have gained significant attention for wearable devices in recent years. However, hydrogel suffers from limitations such as poor stretchability, freezing under cold climates, and drying out in harsh environments, which restrict its practical applications. To address these issues. We synthesize waterborne polyurethane (WPU) and add it to the polyvinyl alcohol (PVA) hydrogel to enhance its mechanical properties. Additionally, DMSO is introduced to the system, aiming to enhance the environmental stability of the hydrogel through its interaction with water. Hence, we prepare a novel organohydrogel by incorporating WPU into PVA in a dimethyl sulfoxide (DMSO)-water (H2O) binary solvent system by one-pot method. The addition of DMSO and WPU enhances the mechanical properties of the organohydrogel, resulting in high mechanical strength (1.27 MPa), excellent stretchability (874%), and superior toughness (3.62 MJ/m3). In addition, the organohydrogel presents reliable freeze resistance and water retention properties due to DMSO interaction with water. More significantly, the organohydrogel can be recycled and reshaped into various shapes through a simple process. The reconstructed organohydrogel still has the original's mechanical properties and sensing capabilities and can continue to be used in flexible electronic devices. Moreover, the organohydrogel is developed for flexible multifunctional sensors, exhibiting high sensitivity (G = 1.89), a rapid response time (141 ms), and exceptional fatigue resistance (100 cycles). Notably, the sensor signals remain stable even after storage in extreme conditions. This study expands the working field of flexible hydrogel sensors and provides a new idea for the sustainable development of next-generation flexible hydrogel sensors.

Research progress of multifunctional flexible proximity sensors

As a non-contact sensor, flexible proximity sensor can sense the proximity of target objects to achieve position recognition, obstacle avoidance, human-computer interaction and other functions. It has important application prospects in environmental detection, human health monitoring, robot electronic skin and other fields. In recent decades, a variety of flexible multifunctional sensors based on proximity sensing have emerged, and how to improve and optimize their performance has become a research hotspot. This review summarizes the classification, performance index and research progress of flexible proximity sensors with different sensing types. In particular, the design principle, structure composition, material selection and manufacturing process of the current flexible proximity sensors are discussed in detail. Then, the performance of proximity sensing, realizable functions and application scenarios are compared among the reviewed research results. Finally, we discuss the challenges faced by flexible proximity sensors, and summarize and prospect the research direction of flexible proximity sensors. This paper aims to provide important reference value for further improving and optimizing the performance of the flexible proximity sensor and developing the next generation of multifunctional wearable devices.

Easy-to-Prepare Flexible Multifunctional Sensors Assembled with Anti-Swelling Hydrogels.

Recent years have witnessed the development of flexible electronic materials. Flexible electronic devices based on hydrogels are promising but face the limitations of having no resistance to swelling and a lack of functional integration. Herein, we fabricated a hydrogel using a solvent replacement strategy and explored it as a flexible electronic material. This hydrogel was obtained by polymerizing 2-hydroxyethyl methacrylate (HEMA) in ethylene glycol and then immersing it in water. The synergistic effect of hydrogen bonding and hydrophobic interactions endows this hydrogel with anti-swelling properties in water, and it also exhibits enhanced mechanical properties and outstanding self-bonding properties. Moreover, the modulus of the hydrogel is tissue-adaptable. These properties allowed the hydrogel to be simply assembled with a liquid metal (LM) to create a series of structurally complex and functionally integrated flexible sensors. The hydrogel was used to assemble resistive and capacitive sensors to sense one-, two-, and three-dimensional strains and finger touches by employing specific structural designs. In addition, a multifunctional flexible sensor integrating strain sensing, temperature sensing, and conductance sensing was assembled via simple multilayer stacking to enable the simultaneous monitoring of underwater motion, water temperature, and water quality. This work demonstrates a simple strategy for assembling functionally integrated flexible electronics, which should open opportunities in next-generation electronic skins and hydrogel machines for various applications, especially underwater applications.

A drum structure triboelectric nanogenerator based on PS/MXene for football training monitoring

Recently, flexible multifunctional sensors have attracted widespread attention from around the world. Here, we propose a novel PS/MXene-based triboelectric nanogenerator (PM-TENG), which has a drum structure design. Through comparative experiments, the optimal ratio of polystyrene (PS) materials was explored. The PS film and nylon film form the triboelectric materials, and copper foil acts as the conducive electrode. From the results, the open-circuit voltage (Voc) and short-circuit current (Isc) of the PM-TENG based on 35 mg/ml content of PS can reach 141 V and 5.9 µA, respectively. In addition, when the resistance value of the external load is 50 MΩ, the PM-TENG exhibits a maximum output power of 123 µW. Due to its excellent flexibility, the PM-TENG can be installed on the shoulders, neck, wrist, elbow, knee, and ankle to achieve all-round motion monitoring of football players. This research can promote the application of TENG sensors in the football monitoring field.

A Flexible Multifunctional Sensor Based on Triboelectric Nanogenerator for Measuring Variable Stiffness of Soft Grippers and Recognizing Item Size

AbstractThe monitoring of the grasping behavior of variable stiffness grippers has emerged as a challenge due to the increasing use of soft grippers in logistics sorting applications. In this research, a flexible multifunctional sensor is developed by combining a liquid metal microchannel structure with a granular single electrode which detects the bending angle and stiffness of the flexible gripper in real‐time. The novel works by integrating triboelectric nanogenerator with particle interference variable stiffness and detects the stiffness of the soft finger. It is found that increasing the loading force on the gripper by a factor of 2.39 does not affect the kinematic characteristics. The size of the grasped object is determined with 96% accuracy by detecting the resistance offered to the stretched liquid metal during the bending of the soft finger. A soft grasping system is built with multifunctional flexible sensors and the real‐time signal feedback during the grasp‐move‐release process of objects of different diameters and weights is demonstrated. Besides providing a theoretical and experimental foundation of variable stiffness and nondestructive grasping of the soft gripper, the anthropomorphism of the soft grasping gripper is implemented by employing a novel sensor.

Silk fibroin/Ag nanowire-based multifunctional sensor for wearable self-powered wireless multi-sensing microsystems

Wearable sensing devices play an increasingly important role in advancing the rapid development of the Internet of Things (IoT) and artificial intelligence (AI), and a smart sensing network based on wide-distributed sensing nodes has become an appealing trend. However, monolithic integration of multiple sensing functions is still a critical issue for sensing nodes. Herein, we present a flexible multifunctional sensor based on a silk fibroin/silver nanowire (SF/AgNW) composite film. Mask spray coating technology is optimized to prepare the functional sensing layer with a cross-linked AgNW network structure inside. The association of the SF/AgNW composite film and the cross-linked AgNW network structure endows the developed sensor with pressure sensing characteristics and temperature sensing characteristics. Furthermore, by in-depth analysis of the coupling law between the input variables and the resistance changes, the sensor is successfully extended to realize the functions of finger proximity detection and human breathing monitoring. Additionally, the sensor is successfully confirmed to possess remarkable mechanical reliability and sensing characteristic stability by a systematic experiment. More importantly, to highlight the practical feasibility of the proposed sensor, an all-in-one self-powered wireless multi-sensing microsystem is constructed based on the developed sensor, a high-performance triboelectric-piezoelectric-electromagnetic hybrid generator (TPEHG), and system circuits. With the TPEHG as the only power source, the microsystem is successfully demonstrated to realize continuous self-powered work and sense environmental variables through the developed sensor. Meanwhile, the obtained sensing data are transmitted to a mobile phone in real time at 1 Hz. The multiple sensing characteristics of the sensor and the autonomous features of the microsystem give them outstanding potential and value in the smart IoT field, especially in constructing a distributed smart sensing network.

Eco-Friendly Bionic Flexible Multifunctional Sensors Based on Biomass–MXene Composites

Eco-Friendly Bionic Flexible Multifunctional Sensors Based on Biomass–MXene Composites

The dual-mode sensing of pressure and temperature based on multilayer structured flexible sensors for intelligent monitoring of human physiological information

Flexible multifunctional sensor with excellent performance via unsophisticated fabrication method is imperatively anticipated for the development of smart human health monitoring systems and wearable electronic devices. Here, we report a multilayer dual-mode flexible sensor (DMFS) by integrating different sensing layers on the two sides of a flexible dielectric substrate for sensing pressure and temperature. The introduction of elastic nanofiber skeleton structure and internal “sea urchin” like conductive materials is the top priority for the DMFS to have excellent sensing performance and durability. As a result, the fabricated DMFS ultimately shows excellent pressure sensitivity of 111.96 kPa−1 within a wide sensing range up to 70 kPa. In the case of temperature stimulus, the DMFS accomplishes a high, linear response to the ambient temperature variation within the range from 20 °C to 50 °C, while maintaining high stability and durability after long-term bending cycles. In addition, DMFS reveals its great potential for non-contact dynamic temperature measurement by being integrated into an intelligent temperature monitoring system. Most importantly, a novel smart human health-monitoring system, which attributes to the coordination and common work of multiple DMFS, has been developed and realizes simultaneous detection of multiple physiological signals of human beings in real-time.

Anisotropic double-network hydrogels integrated superior performance of strength, toughness and conductivity for flexible multi-functional sensors

Conductive hydrogels have attracted extensive attention in the field of flexible electronics due to its inherent outstanding properties. However, how to solve the contradictory relationship among conductivity, strength and toughness of hydrogel was still a huge challenge. Inspired by the muscle architecture, an anisotropic cellulose nanofiber (CNF) based double-network conductive hydrogel containing oriented MXenes was successfully fabricated, in which the asymmetric double network structure contributed to excellent strength and toughness, while the conductivity of the hydrogel was enhanced by the ion nanochannels derived from aligned MXenes. Consequently, the resulting hydrogel integrated superior performance of strength, toughness and conductivity, which were 3.33 MPa, 1106 % and 13.08 S/m respectively, surpassing the most reported hydrogels. Profiting from the superior conductivity and mechanical performance, the anisotropic hydrogel could be assembled into multi-functional flexible electronics, such as a flexible touch panel, a flexible strain/pressure sensor with high sensitivity and stability. Together with the superior compatibility for human–computer interaction, the obtained hydrogel wearable sensor showed a promising application potential for the real-time wireless monitoring of human motion and the real-time control of a robot.

Perylene Diimide (PDI) based Flexible Multifunctional Sensor Design for Personal Healthcare Monitoring‐ A Complementary Approach Involving Experimental and Theoretical Investigations

AbstractThis work demonstrates the Perylene Diimide (PDI) based multifunctional sensor for breath‐sensing and Geriatric Emergency Signalling applications fabricated by an economically viable vacuum filtration deposition technique. The deposition of PDI molecules on the flexible substrate forms a moderately conducting percolating molecular network, exhibiting superior pressure and breath‐sensing performance. The pressure sensor displays a sensitivity of 2.42198 kPa−1 with response and recovery times of ≈191 and ≈166 msec. Also, the sensor displays response and recovery times of ≈211 msec and ≈2.651 sec for breath sensing. The durability of the sensor is evaluated using a pressure‐tapping machine, and the sensor displayed a negligible performance drop over ≈10 000 cycles. The experimental findings are complemented by an ab‐initio calculation‐based comprehensive theoretical analysis of the underlying transduction mechanism for pressure and breath sensing in the context of the percolating molecular network. The theoretical analysis reveals that applied pressure and molecular adsorption significantly influence the molecular conductivity as well as the inter‐molecular quantum mechanical tunneling component, which primarily contributes to the observed increase in conductivity with pressure and exposure to breath. An Android/iOS application is developed to wirelessly receive data via Bluetooth from the sensors connected to a microcontroller.

Wearable and Flexible Multifunctional Sensor Based on Laser-Induced Graphene for the Sports Monitoring System.

The conversion of diverse polymeric substrates into laser-induced graphene (LIG) has recently emerged as a single-step method for the fabrication of patterned graphene-based wearable electronics with a wide range of applications in sensing, actuation, and energy storage. Laser-induced pyrolysis technology has many advantages over traditional graphene design: eco-friendly, designable patterning, roll-to-roll production, and controllable morphology. In this work, we designed wearable and flexible graphene-based strain and pressure sensors by laminating LIG from a commercial polyimide (PI) film. The as-prepared LIG was transferred onto a thin polydimethylsiloxane (PDMS) sheet, interwoven inside an elastic cotton sports fabric with the fabric glue as a wearable sensor. The single LIG/PDMS layer acts as a strain sensor, and a two-layer perpendicular stacking of LIG/PDMS (x and y laser-directed films) is designed for pressure sensing. This newly designed graphene textile (IGT) sensor performs four functions in volleyball sportswear, including volleyball reception detection, finger touch foul detection during blocking the ball from an opponent player, spike force measurements, and player position monitoring. An inexpensive sensor assists athletes in training and helps the coach formulate competition strategies.

Electric properties of the ITO-orange dye-CNT-rubber cell-based flexible multifunctional sensors

Electric properties of the ITO-orange dye-CNT-rubber cell-based flexible multifunctional sensors

Self-powered multifunctional body motion detectors based on highly compressible and stretchable ferroelectrets with an air-filled parallel-tunnel structure

The new kind of ferroelectret electroactive polymers for flexible film sensors represent an attractive opportunity for self-powered wearable devices, which could offer sustainable and convenient health care services for people without the problem of energy supply. However, due to the mechanical heterogeneity, obtaining ferroelectrets with both a high longitudinal and transverse piezoelectric response is still a great challenge, which limits its multifunctional sensing application, especially in body motion tracking. Herein, we develop an air-filled parallel tunnel ferroelectret-based self-powered body motion detectors. Ascribed to the high mechanical compression and tensile properties of the specially designed parallel tunnel void structure and excellent charge storage stability of the fluorinated polyethylene propylene (FEP) electret film, these laminated films exhibit many fascinating merits, including ultrahigh longitudinal piezoelectric response (piezoelectric coefficient of 6200 pC/N, pressure sensitivity of 1950 pC/kPa), wide pressure detection range (0.03–62 kPa), remarkable transverse piezoelectric response (tensile sensitivity of 1600 pC/mm), long-term durability (1,08,0000 cycles), and superior friction force detection capability. This is the first work, to our knowledge, that ever use one kind of piezoelectric mechanism in a single ferroelectret-based sensor to achieve longitudinal stress, transverse stress, and tangential friction force sensing. Furthermore, on the basis of such sensors and machine learning algorithms, we designed an intelligent gesture recognition system for future human-machine interaction applications. Looking forward, our material design methodologies offer an approach to the self-powered flexible multifunctional sensor with great prosperity in household healthcare, smart mobile medical electronics, and humanoid robots.

An ultra-thin transparent multi-functional sensor based on silk hydrogel for health monitoring

With the gradual improvement of people’s health awareness, wearable devices occupy an important position in daily health care and human physiological activity monitoring. As traditional silicon-based electronic products face problems such as interface mismatch, silk fibroin has gradually become an alternative choice for next-generation wearable electronic devices due to its excellent performance. Herein, an ultra-thin transparent and flexible multi-functional sensor based on silk hydrogel with self-patterned microstructure is proposed. The silk hydrogel exhibits superior transparency (>82%) and thin thickness (∼100 μm). Furthermore, the self-patterned microstructure on the silk hydrogel surface is beneficial for the high sensitivity of pressure sensing response (1.6 kPa−1). This device exhibits advantageous performance on temperature (top sensitivity of 6.25% °C−1) and humidity (top sensitivity of 0.16% RH−1) sensing response. It also shows fast response (0.16 s) and durable stability (over 2000 dynamic cycles). Moreover, this device can be applied to monitor human facial expression, joint movements, temperature change, breathing and other health indicators. It is worth mentioning that this multi-functional sensor can monitor the signal of breathing and throat, so it can be applied to the clinical physiological activity monitoring of patients with upper respiratory tract infection. In addition, we also demonstrate a grasping and relaxing experiment of intelligent manipulator to verify the pressure and temperature sensing performance, providing a possibility for its application in the field of prosthetics. According to these advantages, the reported ultra-thin transparent multi-functional sensor based on silk hydrogel has broad prospects in the fields of health monitoring, intelligent prosthetics, and electronic skin, etc.