Flexible Electronic Sensors Research Articles

Covalent Adaptable Networks for Sustainable Soft Electronic Sensors and Actuators

AbstractThe flexible electronics market is rapidly growing, driven by demand in the healthcare, automotive, and consumer industries. While these technologies enhance the daily lives, the electronic waste (e‐waste) generated in tandem poses significant environmental challenges. Covalent adaptable networks (CANs) are crosslinked polymers bearing dynamic covalent bonds (DCBs). These exchangeable bonds endow CANs not only with the high mechanical and chemical stability of thermosets but also with the stimuli‐activated reprocessability of thermoplastics. CAN‐derived electronics offer a solution to alleviate e‐waste generation since the DCBs allow for the self‐healing and recycling of these devices. Additionally, the ability of CANs to respond to stimuli is highly desirable for smart materials, especially flexible electronics. In this review, how CANs play the role of conformable substrates and dielectrics, and also as flexible electronic connectors, sensors, and actuators is comprehensively cataloged. Furthermore, the advantages gained by utilizing CANs in electronic sensors and actuators is highlighted, as well as provide the insights into design strategies to address the challenges of creating high‐performance sustainable soft electronics.

Highly sensitivity and wide-range flexible humidity sensor based on LiCl/cellulose nanofiber membrane by one-step electrospinning

Cellulose is considered an ideal material for flexible electronic humidity sensors due to its green renewable nature, rich surface groups and strong hydrophilicity. However, the traditional cellulose-based humidity sensor cannot simultaneously have wide detection range, high sensitivity and fast response time, which seriously hinder their development and application. Here, an ultrathin Lithium chloride (LiCl)/cellulose nanofiber membrane as moisture sensitive materials was prepared by one-step electrospinning and used to assemble a high-performance humidity sensor. N, N-Dimethylacetamide/Lithium chloride (DMAc/LiCl) solvent system was used to dissolve the cellulose, and DMAc volatilized into the air while LiCl remained in the cellulose nanofibers during the electrospinning process. LiCl as a highly moisture-sensitive inorganic salt has strong hydrophilicity and enhances the moisture sensing detection limit of cellulose fiber (especially under low humidity conditions), thus giving the cellulose moisture sensor excellent sensitivity (up to 4191 %) and a wide detection range (5–98 % RH). The nanometer size of cellulose fibers, the extremely low thickness and large pores of cellulose membranes accelerate the mass exchange of moisture between the air and the membrane, thus giving cellulose humidity sensor a fast response/recovery time (99/110 s) and low hysteresis (2.9 %). Moreover, the performances of humidity sensors didn’t degrade even after being subjected to long-term application (>30 days), high/low temperatures (73.6/0.1 ℃) and thousands of bending cycles. The proposed LiCl/cellulose nanofiber humidity sensor brings innovative solutions for the design of cellulose based sensor with great potential for use in non-contact humidity detection, respiratory monitoring and sleep apnea detection applications.

Hybrid Strategies for Enhancing the Multifunctionality of Smart Dynamic Molecular Crystal Materials.

Dynamic molecular crystals are an emerging class of smart engineering materials that possess unique ability to convert external energy into mechanical motion. Moreover, they have being considered as strong candidates for dynamic elements in applications such as flexible electronic devices, artificial muscles, sensors, and soft robots. However, the inherent defects of molecular crystals like brittleness, short-life and fatigue, have significantly impeded their practical applications. Inspired by the concept of "the whole is greater than the sum of its parts" in the field of biology, building stimuli-response composites materials can be regarded as one of the ways to break through the current limitations of dynamic molecular crystals. Moreover, the hybrid materials can exhibit new functionalities that cannot be achieved by a single object. In this review, the focus was placed on the analysis and discussion of various hybrid strategies and options, as well as the functionalities of hybrid dynamic molecular crystal materials and the important practical applications of composite materials, with the introduction of photomechanical molecular crystals and flexible molecular crystals as a starting point.Moreover, the efficiency, limitations, and advantages of different hybrid methods were compared and discussed.

High-performance conductive double-network hydrogel base on sodium carboxymethyl cellulose for multifunctional wearable sensors

Sodium carboxymethyl cellulose showed great potential in wearable intelligent electronic devices due to its low price and good biocompatibility. This research aimed to develop a novel conductive hydrogel with stretchable, self-healing, self-adhesive, antibacterial, 3D printable properties, for the development of multifunctional flexible electronic materials based on sodium carboxymethyl cellulose. A multifunctional conductive hydrogel based on sodium carboxymethyl cellulose (SCMC) was synthesized by simple polymerization of SCMC, acrylic acid (AA) and alkaline calcium bentonite (AC-Bt). The multifunctional hydrogels (PAA-SCMC) possess excellent mechanical property (stress: 0.25 MPa; strain: 1675.0 %), Young's modulus (75.6 kPa), and conductivity (2.25 S/m). The multifunctional PAA-SCMC hydrogels serve as strain sensors (Gauge Factor (GF) = 12.68), temperature sensors (temperature coefficient of resistance (TCR) = −0.887 % °C at 20 °C–60 °C), sweat sensors, and pressure sensors. Importantly, the obtained hydrogels exhibited exceptional self-healing capability, self-adhesive properties, antimicrobial properties and 3D printability. The printed hydrogel has good mechanical properties, conductivity and antibacterial properties. Moreover, the hydrogel sensor possessed prominent sensitivity and cyclic stability to accurately monitor human motion, emotional changes, physiological signals in real time, and a hydrogel-based flexible touch keyboard was also fabricated to recognize writing trajectories. Overall, this study provided novel insights into the simple and efficient synthesis and sustainable manufacturing of environmentally friendly multifunctional flexible electronic skin sensors.

MXene/Bacterial Cellulose Hybrid Materials for Sustainable Soft Electronics.

This work evaluated bacterial cellulose (BC) as a possible biodegradable soft electronics substrate in comparison to polyethylene terephthalate (PET), while also focusing on evaluating hybrid MXene/BC material as potential flexible electronic sensor. Material characterization studies revealed that the BC material structure consists of nanofibers with diameters ranging from 70 to 140 nm, stacked layer-by-layer. BC samples produced are sensitive to post-treatment with isopropanol resulting in a change of structural and mechanical properties. The viscoelastic properties of the BC substrates have been studied experimentally in comparison with the PET film. Aged BC substrate showcased similar viscoelastic properties stability, while exhibiting better properties above 70 °C, with total storage modulus change of -15% and loss modulus change of 21%. MXenes prepared using the Minimally Intensive Layer Delamination (MILD) method were screen-printed onto BC substrates and PET films to form MXene/BC (MX/BC) and MXene/PET (MX/PET) devices. The electrical properties results showcased different resistive behavior on both BC and PET substrate samples with different impedance moduli. MX/PET presented lower sheet resistance of around 156 Ω·sq-1, while MX/BC was 2733 Ω·sq-1. Finally, the MX/BC and MX/PET devices were subjected to repeatable quasi-static load tests and the piezoresistive sensing behavior of the devices has been reported.

Humidity Sensing Using Polymers: A Critical Review of Current Technologies and Emerging Trends

In the post-pandemic era, human demand for a healthy lifestyle and a smart society has surged, leading to vibrant growth in the field of flexible electronic sensor technology for health monitoring. Flexible polymer humidity sensors are not only capable of the real-time monitoring of human respiration and skin moisture information but also serve as a non-contact human–machine interaction method. In addition, the development of moist-electric generation technology is expected to break free from the traditional reliance of flexible electronic devices on power equipment, which is of significant importance for the miniaturization, reliability, and environmentally friendly development of flexible devices. Currently, flexible polymer humidity sensors are playing a significant role in the field of wearable electronic devices and thus have attracted considerable attention. This review begins by introducing the structural types and working principles of various humidity sensors, including the types of capacitive, impedance/resistive, frequency-based, fiber optic, and voltage-based sensors. It mainly focuses on the latest research advancements in flexible polymer humidity sensors, particularly in the modification of humidity-sensitive materials, sensor fabrication, and hygrosensitivity mechanisms. Studies on material composites including different types of polymers, polymers combined with porous nanostructured materials, polymers combined with metal oxides, and two-dimensional materials are reviewed, along with a comparative summary of the fabrication and performance mechanisms of related devices. This paper concludes with a discussion on the current challenges and opportunities faced by flexible polymer humidity sensors, providing new research perspectives for their future development.

A flexible, antifreezing, and long-term stable cellulose ionic conductive hydrogel via one-step preparation for flexible electronic sensors

Ionic conductive hydrogels have attracted great attention due to their good flexibility and conductivity in flexible electronic devices. However, because of the icing and water loss problems, the compatibility issue between the mechanical properties and conductivity of hydrogel electrolytes over a wide temperature range remains extremely challenging to achieve. Although, antifreezing/water-retaining additives could alleviate these problems, the reduced performance and complex preparation methods seriously limit their development. In this work, a simple strategy without additives was provided to prepare an ionic conductive cellulose hydrogel (ICH) in one step through molten salt hydrate. The hydrogel featured controllable mechanical properties (0.19 MPa - 0.67 MPa), high ionic conductivity (78.96 mS/cm), excellent freezing resistance (−80 °C). More importantly, due to the existing metal salts component, the ICH exhibited long-term stability in water-retention ability (75.6 %, after 90 days) and ionic conductivity (85 %, after 90 days) over a wide working temperature range (−80 °C to 40 °C). Benefiting from these advantages, the ICH exhibited excellent electromechanical performance in human movement detection and movement direction identification, indicating a promising apply for flexible electronic device.

High-Performance Sensing Platform Based on Morphology/Lattice Collaborative Control of Femtosecond-Laser-Induced MXene-Composited Graphene.

Flexible sensors based on laser-induced graphene (LIG) are widely used in wearable personal devices, with the morphology and lattice arrangement of LIG the key factors affecting their performance in various applications. In this study, femtosecond-laser-induced MXene-composited graphene (LIMG) is used to improve the electrical conductivity of graphene by incorporating MXene, a 2D material with a high concentration of free electrons, into the LIG structure. By combining pump-probe detection, laser-induced breakdown spectroscopy (LIBS), and density functional theory (DFT) calculations, the morphogenesis and lattice structuring principles of LIMG is explored, with the results indicating that MXene materials are successfully embedded in the graphene lattice, altering both their morphology and electrical properties. The structural sparsity and electrical conductivity of LIMG composites (up to 3187 Sm-1) are significantly enhanced compared to those of LIG. Based on these findings, LIMG has been used in wearable electronics. LIMG electrodes are used to detect uric acid, with a minimum detection limit of 2.48µM. Additionally, LIMG-based pressure and bending sensors have been successfully used to monitor human limb movement and pulse. The direct in situ femtosecond laser patterning synthesis of LIMG has significant implications for developing flexible wearable electronic sensors.

Flexible and anisotropically conductive film by assembly of silicone rubber and cobalt-coated glass fiber composites

In this study, we prepared electromagnetic cobalt-coated glass fiber (Co@GF) composites via an electroless plating method. Subsequently, a conductive sandwich flexible film consisting of Co@GF composites and liquid silicone rubber (RTV-2) was successfully formed using the tape casting method at room temperature. Based on the perfect coating and excellent electrical conductivity of the Co@GF composites, the resultant RTV-2/Co@GF/RTV-2 sandwich flexible film showed a low volume resistivity of 0.264 Ω·cm and could stretch to 100% (of 4.40 Ω·cm) without obvious fracture. When a magnetic field was applied during the curing process, the electromagnetic Co@GF composites were aligned automatically in the RTV-2 matrix because of their ferromagnetic nature. The as-prepared film exhibited anisotropy in its electrical performance. The volume resistivity parallel to the magnetic field direction is approximately two times lower than that in the perpendicular direction. The maximum difference in the volume resistivity (ρ∥ = 0.768 Ω·cm and ρ⊥ = 1.549 Ω·cm) was obtained at a magnetic field intensity of 800 mT. In addition, a magnetic field intensity of 100 mT helps improve the electrical conductivity of the as-obtained sandwich film. The anisotropic RTV-2/Co@GF/RTV-2 sandwich flexible film is considered a promising flexible electronic sensor, where discrepant inductive sensitivity is required in orthogonal directions.

Strong adhesive, high conductive and environmentally stable eutectogel based on “Water in deep eutectic Solvent”: Ultrasensitive flexible temperature and strain sensors

Eutectogel is a promising candidate of conductive materials for flexible electronic sensors. However, the existing eutectogel sensors still have some problems such as poor mechanical properties and antifatigue ability, limited ionic conductivity, and strong hygroscopicity. Herein, an eutectogel is fabricated via one-stop photoinitiated copolymerization of acrylic acid, hydroxyethyl acrylate, and highly branched modified polyethyleneimine in “water in deep eutectic solvent”. The optimized eutectogel exhibits strong adhesive strength (132 ± 9 kPa, with porcine skin), ultra elongation (3500 %), high conductivity (2.7 ± 0.1 mS cm−1), long term antidrying and antifreezing (∼30 d) in a wide range of working temperatures (−40 − 60 °C), flame resistance, biocompatability, and excellent resistance ability to tear and crack propagation. The superb flame resistance and sensitive response to temperature offer a potential application in fire alarm. Besides, this gel-based wearable strain sensors can sensitively monitor and differentiate large body joint movements and subtle muscle motions. The highly distinguishable and reproducible signals can be recorded while further utilizes as sensors for detecting human healthcare accurately and continuously in medical mobile electrocardiogram machine for a long time. This work provides a new strategy for constructing eutectogel based wearable devices and electronic devices with excellent performance.

Self-Healing Natural Rubber Based on the Double Cross-Linked Networks for Fabrication of the Flexible Electronic Sensor

Self-Healing Natural Rubber Based on the Double Cross-Linked Networks for Fabrication of the Flexible Electronic Sensor

Highly Stretchable, Transparent, Self-Healing Ion-Conducting Elastomers for Long-Term Reliable Human Motion Detection.

The flexible electronic sensor is a critical component of wearable devices, generally requiring high stretchability, excellent transmittance, conductivity, self-healing capability, and strong adhesion. However, designing ion-conducting elastomers meeting all these requirements simultaneously remains a challenge. In this study, a novel approach is presented to fabricate highly stretchable, transparent, and self-healing ion-conducting elastomers, which are synthesized via photo-polymerization of two polymerizable deep eutectic solvents (PDESs) monomers, i.e., methacrylic acid (MAA)/choline chloride (ChCl) and itaconic acid (IA)/ChCl. The as-prepared ion-conducting elastomers possess outstanding properties, including high transparency, conductivity, and the capability to adhere to various substrates. The elastomers also demonstrate ultra-stretchability (up to 3900%) owing to a combination of covalent cross-linking and noncovalent cross-linking. In addition, the elastomers can recover up to 3250% strain and over 94.5% of their original conductivity after self-healing at room temperature for 5min, indicating remarkable mechanical and conductive self-healing abilities. When utilized as strain sensors to monitor real-time motion of human fingers, wrist, elbow, and knee joints, the elastomers exhibit stable and strong repetitive electrical signals, demonstrating excellent sensing performance for large-scale movements of the human body. It is anticipated that these ion-conducting elastomers will find promising applications in flexible and wearable electronics.

Responsive-Hydrogel Aquabots.

It remains a challenge to produce soft robots that can mimic the responsive adaptability of living organisms. Rather than fabricating soft robots from bulk hydrogels,hydrogels are integrated into the interfacial assembly of aqueous two-phase systems to generate ultra-soft and elastic all-aqueous aquabots that exhibit responsive adaptability, that can shrink on demand and have electrically conductive functions. The adaptive functions of the aquabots provide a new platform to develop minimally invasive surgical devices, targeted drug delivery systems, and flexible electronic sensors and actuators.

Self‐Healing and Toughness Triboelectric Materials Enabled by Dynamic Nanoconfinement Quenching

AbstractSelf‐healing materials that integrate excellent mechanical properties and high healing efficiency meet the requirements of flexible electronic sensors for mechanical flexibility and reliability. In the field of wearable devices, they are of great significance for improving the stability of the equipment and reducing the frequency of replacement. However, the high strength of materials often limits their self‐healing ability. When damage occurs, it will hinder the microstructural adjustment and fluidity of the material at the damaged site, thus negatively affecting the activation and execution of the self‐healing mechanism. In this study, a strength‐toughness and room‐temperature self‐healing triboelectric material is prepared by the dynamic nanoconfinement effect and the quenching effect of ethanol (referred to as the DNCQ strategy). The quenching effect of ethanol improves the aggregation of nanocluster phase, and the constructed nanoconfined network skillfully balances the contradiction between mechanical properties and self‐healing ability. The obtained triboelectric material has high tensile strength (27.1 MPa), toughness (97.9 MJ m−3), and excellent healing efficiency (88.6%). The self‐powered pressure distribution sensing array based on triboelectric materials can accurately reflect the pressure distribution of the object, which has potential application prospects in the field of wearable devices.

Multi-stimuli-responsive conductive and luminescent hydrogels for wearable sensors and information encryption transmission

Flexible electronic sensors with optical functions have promising applications in wearable sensing, information storage, encryption and transmission, but realizing multifunctional applications in a single system is still challenging. Herein, the conductive and luminescent hydrogel in a single system was fabricated by non-covalent crosslinked poly(vinyl alcohol) (PVA) network and covalent crosslinked acrylamide (AAm) and [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA) network, as well as doping Eu(PDA)3 and glycerol (GL). The acquired PVA0.9/P(AAm-co-SBMA0.5)/Eu(PDA)3/GL (P0.9P0.5EG) hydrogel possesses excellent mechanical properties, adhesion, self-healing capability, water retention ability, anti-freezing property, conductivity, self-powered property, and luminescent properties. Importantly, the hydrogel as a flexible sensor has excellent tensile strain sensitivity (GF = 17.8), pressure sensitivity (S = 1.191 kPa−1), quick response, and stable electrical signals output, which can be used for monitoring multiple movements as well as for information transmission with the help of Morse code. Moreover, the unique luminescent properties and multi-stimuli-responsive behavior also make the hydrogel monitor strain variations via detecting the luminescence signal changes. Notably, the particular process of information encryption transmission was further achieved by converting the luminescence signals to electrical signals. This work provides an alternative strategy for designing and preparing hydrogel sensors for multifunctional applications.

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.

A super-hydrophilic graphite directly from lignin enabled by a room-temperature cascade catalytic carbonization

A cost-effective, and low-energy room-temperature cascade catalytic carbonization strategy is demonstrated for converting lignin into graphite with a high yield of 87 %, a high surface potential of −37 eV and super-hydrophilicity. This super-hydrophilic feature endows the lignin-derived graphite to be dispersed in a variety of polar solvents, which is important for its future applications. Encapsulating of liquid metals with the graphite for electrical circuit patterning on flexible substrates is also advocated. These written patterns show superb conductivity of 4.9 × 106 S/m, offering good performance stability and reliability while being repeatedly stretched, folded, twisted, and bent. This will offer new designs for flexible electronic devices, sensors, and biomedical devices.

Flexible Force Sensor Based on a PVA/AgNWs Nanocomposite and Cellulose Acetate.

Nanocomposites are materials of special interest for the development of flexible electronic, optical, and mechanical devices in applications such as transparent conductive electrodes and flexible electronic sensors. These materials take advantage of the electrical, chemical, and mechanical properties of a polymeric matrix, especially in force sensors, as well as the properties of a conductive filler such as silver nanowires (AgNWs). In this work, the fabrication of a force sensor using AgNWs synthesized via the polyol chemical technique is presented. The nanowires were deposited via drop-casting in polyvinyl alcohol (PVA) to form the active (electrode) and resistive (nanocomposite) sensor films, with both films separated by a cellulose acetate substrate. The dimensions of the resulting sensor are 35 mm × 40 mm × 0.1 mm. The sensor shows an applied force ranging from 0 to 3.92 N, with a sensitivity of 0.039 N. The sensor stand-off resistance, exceeding 50 MΩ, indicates a good ability to detect changes in applied force without an external force. Additionally, studies revealed a response time of 10 ms, stabilization of 9 s, and a degree of hysteresis of 1.9%. The voltage response of the sensor under flexion at an angle of 85° was measured, demonstrating its functionality over a prolonged period. The fabricated sensor can be used in applications that require measuring pressure on irregular surfaces or systems with limited space, such as for estimating movement in robot joints.

Shutters-Inspired metal ions coordination hydrogel Strain/Pressure sensor for joint behavior evaluation and flatfeet correction

The metal ions coordination hydrogels (MI-HG) that polymer gels serve as flexible substrates and metal ions as conductive media have emerged as flexible electronic sensor materials. Nevertheless, fabricating MI-HG by a traditional soaking method is difficult to guarantee the good permeation of metal ions, manifesting as the “core–shell” structure of MI-HG, which undoubtedly reduces their mechanical property, conductivity, and sensitivity. Inspired by the shutter opening and closing regulation mechanism, a strategy that completely opening the backbone layers of polymer gels facilitates metal ions permeation thus improving the whole performance of MI-HG, especially the mechanical property is proposed. Herein, poly(vinyl alcohol)(poly(acrylic acid)-polyacrylamide) (PVA(PAA-PAM)) is used as the typical polymer hydrogel (HG), trivalent aluminum ion (Al3+) as the metal salt, and N,N'-methylenebisacrylamide (MBAA) as the important regulation ‘rod’, fabricating a series of aluminum coordination hydrogels (Al-HGx%). The results indicate that the mechanical performances of Al-HG2% are significantly improved. For example, the toughness of Al-HG2% reaches 996.12 KJ/m3, approximately three times that of the Al-HG0% sample (354.45 KJ/m3). The combination of structural characterization as well as finite element analysis, the mechanism for the enhanced mechanical performances based on shutter’s completely open effect is elucidated, the product of which (Al-HG2%) has potential application in flexible strain/pressure sensor for joint behavior evaluation and flatfeet correction.

Resistance‐type strain sensor based on carbon nanofiber/polypyrrole composite membrane with high sensitivity

AbstractCarbon nanofibers have been widely studied as one of the promising materials for fabricating flexible electronic strain sensors. Surface modification of carbon nanofibers by conducting polymers is a simple yet effective approach to constructing strain sensors with good conductivity and sensitivity. In this work, composite nanofiber membranes composed of carbon nanofiber and conducting polymer are developed via in situ polymerization of pyrrole, aniline, and thiophene respectively on the surface of the carbon nanofiber which serves as a substrate. Different composite membranes, CNF@PANI, CNF/Ppy, and CNF@PTh, are successfully manufactured. The conducting polymer‐carbon nanofiber composite membranes exhibit improved conductivity as well as response sensitivity compared to pristine carbon nanofiber membranes. Among different composite membranes being studied, CNF/Ppy demonstrates the highest strain‐sensing performance. It shows good sensitivity, reproducibility, and stability in detecting both subtle (strain = 0.1%) and large (strain = 25%) deformation. The excellent strain‐sensing performance of CNF/Ppy endows the composite membrane with great potential for applications in wearable electronics.Highlights Adding PANI, Ppy, and PTh could improve the conductivity of the CNF sensor. Ppy doping has a good effect on the performance of the CNF strain sensor. CNF/Ppy strain sensor has high sensitivity, stability, and durability. CNF/Ppy sensors can detect deformation caused by human movement. CP‐CNF composite material has good flexibility and mechanical properties.