High-performance Flexible Sensors Research Articles

PANI/3D crumpled Ti3C2TX/TiO2 nanocomposites for flexible conductometric NH3 sensors working at room temperature

Gas sensors based on polyaniline (PANI) materials often have some issues of intrinsic poor stability, low sensitivity, and slow response-recovery rate. To solve these issues, the combination of PANI and metal oxide nanoparticles is a common effective strategy. However, the agglomeration of these nanoparticles in the composite is still a major challenge, decreasing NH3 sensing response and stability. Herein, a flexible NH3 sensor based on PANI/Ti3C2TX/TiO2 (PMT) composite was successfully fabricated by ultrasonic spray pyrolysis and in-situ polymerization. The introduced 3D crumpled Ti3C2TX/TiO2 sphere (MT) with TiO2 nanoparticles in-situ uniform growth on the Ti3C2TX effectively prevented nanoparticles agglomeration in PMT. The optimized PMT film sensor displayed outstanding NH3 sensing performance at room temperature, including high response (2.30–10 ppm NH3), low detection limit (20 ppb), good selectivity, and excellent mechanical reliability. The superior sensing properties were attributed to the synergetic effect of unique structure of MT, the improved protonation degree of PANI component, and the formation of heterojunctions. Moreover, the detection of fish spoilage has been realized using this film sensor. The present work provides a new strategy to fabricate high-performance flexible NH3 sensors operated at room temperature, which may hold great promise for application in our daily life.

Superior sensitive, high-tensile flexible fabric film strain sensor

Developing strain sensor with higher sensitivity and larger monitoring range is urgently needed for meeting the increasing potential applications of high-performance sensors in intelligent electronics, motion signal analysis and human monitoring. Herein, via an advanced and efficient method about embedding silver nanowires on electrospun thermoplastic polyurethane fabric film modified with carbon black, a high-performance flexible strain sensor is fabricated. The strain sensor exhibits excellent electrical response abilities of the superior sensitivity (gauge factor >16,000) with strain of 360%, an ultra-wide strain range (0.1% to 200%) and outstanding durability and reliability (>10,000 cycles) under cyclic stretching-recovering operating. Furthermore, mathematical fitting models about variation of resistance (ΔR/R0) and gauge factor upon the applied strains during the stretching process are proposed, which can quantitatively describe and analysis the variation of conductive pathways and inter-particles distance under the external stimuli. It is expected that such fitting models can be used to predict conductive sensing behaviors of flexible strain sensors for actual manufacturing and improvement of products.

Catalytic Modification of Porous Two-Dimensional Ni-MOFs on Portable Electrochemical Paper-Based Sensors for Glucose and Hydrogen Peroxide Detection.

Rapid and accurate detection of changes in glucose (Glu) and hydrogen peroxide (H2O2) concentrations is essential for the predictive diagnosis of diseases. Electrochemical biosensors exhibiting high sensitivity, reliable selectivity, and rapid response provide an advantageous and promising solution. A porous two-dimensional conductive metal-organic framework (cMOF), Ni-HHTP (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene), was prepared by using a one-pot method. Subsequently, it was employed to construct enzyme-free paper-based electrochemical sensors by applying mass-producing screen-printing and inkjet-printing techniques. These sensors effectively determined Glu and H2O2 concentrations, achieving low limits of detection of 1.30 μM and 2.13 μM, and high sensitivities of 5573.21 μA μM-1 cm-2 and 179.85 μA μM-1 cm-2, respectively. More importantly, the Ni-HHTP-based electrochemical sensors showed an ability to analyze real biological samples by successfully distinguishing human serum from artificial sweat samples. This work provides a new perspective for the use of cMOFs in the field of enzyme-free electrochemical sensing, highlighting their potential for future applications in the design and development of new multifunctional and high-performance flexible electronic sensors.

A novel MXene-based high-performance flexible pressure sensor for detection of human motion

Flexible pressure sensors possess superior conformal ability, great flexibility, and strong biocompatibility than conventional silicon-based sensors, thereby widely used in electronic skin, wearable devices, and robotic tactile sensing, among other fields. However, flexible pressure sensors are still limited by many challenges related to extended linearity and high sensitivity. In this paper, MXene with a loose layered structure was employed to fabricate high-performance flexible piezoresistive sensors comprising polydimethylsiloxane (PDMS) film with cylindrical microstructure, multilayer Ti3C2T x -MXene film, and interdigital electrodes. A cylindrical microstructured silicon wafer was designed and processed by deep silicon etching process, and PDMS flexible substrate was obtained by two inversions as the pressure sensing layer. The resulting flexible pressure sensor exhibited excellent performance in terms of excellent sensitivity up to 519 kPa−1 in the large detection range of 0–8 kPa coupled with great linearity, a response time of 62.7 ms, and a recovery time of 62.8 ms. The high sensitivity was associated with the compression of the interlayer spacing of multilayer MXene nanosheets. In addition, a single flexible pressure sensor and integrated array were utilized to detect the human physical signals and quantitative measurements of pressure distributions. Overall, these findings provided experimental verification for the design and manufacturing of highly sensitive and linear flexible pressure sensors.

An Intelligent Glove of Synergistically Enhanced ZnO/PAN-Based Piezoelectric Sensors for Diversified Human–Machine Interaction Applications

Human–machine interaction is now deeply integrated into our daily lives. However, the rigidity and high-power supply of traditional devices limit their further development. Herein, a high-performance flexible piezoelectric sensor (HFPS) based on a novel zinc oxide/polyacrylonitrile/Ecoflex (ZnO/PAN/Ecoflex) composite membrane is proposed. Due to the synergistic piezoelectricity of ZnO and PAN, the output voltage/current of the HFPS is increased by 140%/100% compared to the pure Zno/Ecoflex composite membrane. Furthermore, the fabricated HFPSs also have excellent sensitivity, linearity, stability and flexibility under periodic pressure. On this basis, due to its flexibility, stretchability and battery-free characteristics, a self-powered HFPS-based intelligent glove is proposed to wirelessly control diverse electronic systems through human hand gestures. In the meanwhile, the intelligent glove has been successfully applied to car two-dimensional motion, light bulb control and fan control. With the advantages of simple operation, portability and low power consumption, the glove is expected to provide new application prospects for human–machine interaction systems.

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception.

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human-machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.

3D Extruded Graphene Thermoelectric Threads for Self-Powered Oral Health Monitoring.

Flexible sensors play a crucial role in intelligent electronic devices, while strain-sensing is the fundamental feature for these sensors of different fields. Therefore, developing high-performance flexible strain sensors is essential for building the next generation of smart electronics. Here, a self-powered ultrasensitive strain sensor based on graphene-based thermoelectric composite threads through a simple 3D extrusion method is reported. The optimized thermoelectric composite threads show a large stretchable strain of over 800%. After 1000 cycles of bending, the threads still maintain excellent thermoelectric stability. The thermoelectric effect-induced electricity can realize ultrasensitive strain and temperature detection with high resolution. As wearable devices, the thermoelectric threads can also realize self-powered physiological signals monitoring, including the opening degree of mouth, occlusal frequency, and force of the tooth during the eating process. It provides significant judgment and guidance for promoting oral healthcare and developing good eating habits.

Wrinkled and cracked amorphous carbon film for high-performance flexible strain sensors

Wrinkled and cracked amorphous carbon film for high-performance flexible strain sensors

Ultrasensitive and Wide‐Range Flexible Hydrogen Sensor Based on Pd Nanoparticles Decorated Ultrathin SnO2 Film

AbstractHighly sensitive and reliable hydrogen detection is prerequisite for the large‐scale implementation of green energy hydrogen. It remains a tough challenge to get a highly selective H2 sensor to low concentration H2 gas with low working temperature. In this work, high performance flexible hydrogen sensor using ultrathin SnO2 film decorated by Pd nanoparticles (NPs) on polyimide (PI) was developed based on versatile atomic layer deposition (ALD) and cluster beam deposition (CBD). The influence of Pd NPs loading amount and operating temperature on the performance of hydrogen sensors was studied. The sensors obtain an extremely high response over 20000 to 30 ppm H2 at the relatively low operating temperature of 75–125 °C. The Pd NPs@SnO2/PI sensor achieves a wide detection range of 0.1‐5000 ppm with the limit of detection (LOD) of 100 ppb at 125 °C. A low LOD of 250 ppb can still be achieved at room temperature. The sensors also exhibit outstanding sensing selectivity for hydrogen over interfering gases and excellent mechanical resistance after 2000 bending cycles. The mechanism for improved sensing performance of ALD ultrathin film with CBD‐derived noble metal NPs decoration may be a feasible strategy for the construction of highly sensitive, selective, and flexible hydrogen sensors.

High-performance flexible strain sensors based on silver film wrinkles modulated by liquid PDMS substrates.

Flexible strain sensors based on controllable surface microstructures in film-substrate systems can be extensively applied in high-tech fields such as human-machine interfaces, electronic skins, and soft robots. However, the rigid functional films are susceptible to structural destruction and interfacial failure under large strains or high loading speeds, limiting the stability and durability of the sensors. Here we report on a facile technique to prepare high-performance flexible strain sensors based on controllable wrinkles by depositing silver films on liquid polydimethylsiloxane (PDMS) substrates. The silver atoms can penetrate into the surface of liquid PDMS to form an interlocking layer during deposition, enhancing the interfacial adhesion greatly. After deposition, the liquid PDMS is spontaneously solidified to stabilize the film microstructures. The surface patterns are well modulated by changing film thickness, prepolymer-to-crosslinker ratio of liquid PDMS, and strain value. The flexible strain sensors based on the silver film/liquid PDMS system show high sensitivity (above 4000), wide sensing range (∼80%), quick response speed (∼80 ms), and good stability (above 6000 cycles), and have a broad application prospect in the fields of health monitoring and motion tracking.

Bending-Sensitive Optical Waveguide Sensor With Carbon-Fiber Layer for Monitoring Grip Strength.

With the gradual popularity of wearable devices, the demand for high-performance flexible wearable sensors is also increasing. Flexible sensors based on the optical principle have advantages e.g. anti-electromagnetic interference, antiperspirant, inherent electrical safety, and the potential for biocompatibility. In this study, an optical waveguide sensor integrating a carbon fiber layer, fully constraining stretching deformation, partly constraining pressing deformation, and allowing bending deformation, was proposed. The sensitivity of the proposed sensor is three times higher than that of the sensor without a carbon fiber layer, and good repeatability is maintained. We also attached the proposed sensor to the upper limb to monitor grip force, and the sensor signal showed a good correlation with grip force (the R-squared of the quadratic polynomial fitting was 0.9827) and showed a linear relationship when the grip force was greater than 10N (the R-squared of the linear fitting was 0.9523). The proposed sensor has the potential for applications in recognizing the intention of human movement to help the amputees control the prostheses.

Emerging poly(aniline co-pyrrole) nanocomposites by in-situ polymerized for high-performance flexible ammonia sensor

Emerging poly(aniline co-pyrrole) nanocomposites by in-situ polymerized for high-performance flexible ammonia sensor

Thermosensitive hydrogel-based, high performance flexible sensors for multi-functional e-skins

A thermo-sensitive and conductive hydrogel with a VPTT value of 38 °C is developed. The gel is stretchable, self-adhesive, self-healable, puncture-resistant and can respond to multimodal stimuli including tensile strain, compressive stress and temperature with high sensitivity.

High-performance flexible and stretchable self-powered surface engineered PDMS-TiO2 nanocomposite based humidity sensors driven by triboelectric nanogenerator with full sensing range

Harvesting electrical charge using triboelectrification provides a promising path to realizing uninterrupted self-powered sensors with application in wearable electronics and personal healthcare. This paper reports a high-performance flexible and stretchable self-powered humidity sensor based on triboelectric nanogenerators (TENG) that provides extreme responsivity, excellent repeatability with fast recovery time, and high selectivity. The device relies on a surface-engineered titanium oxide nanoparticle-silicone nanocomposite that generates triboelectric charge density strongly dependent on ambient humidity. The performance of the nanoengineered humidity sensor was systematically investigated at different humidity levels, which exhibits high responsivity (90% change for 75% RH change), a fast response time (1 s for 50% RH), a fast recovery time (3 s for 50% RH), and nearly full sensing range (0–99%). When activated by vertical contact separation at 2 Hz, the device generates a power density as high as 90 mW.m−2 and an open-circuit voltage of 50 V. An analytical model is derived to understand the underlying mechanism. The theoretical data explain the role of dielectric permittivity and surface area on the device operation, and the results are in good agreement with the experiments. The excellent combination of the properties suggests that these flexible TENGs can serve as reliable self-powered motion-driven humidity sensor.

High-performance microcone-array flexible piezoelectric acoustic sensor based on multicomponent lead-free perovskite rods

<h2>Summary</h2> Piezoelectric materials can serve as favorable candidates for future cochlear implants. However, most piezoelectric hearing devices until now have failed to simultaneously provide high sensitivity, desired flexibility, broad frequency selectivity, and biocompatibility. Herein, inspired by human outer ear hair cells, we meet these issues by introducing a microcone patterning array strategy based on novel lead-free, multicomponent rod-shaped niobate piezoelectric materials with a morphotropic phase boundary. The output voltage of the rod-based, flexible piezoelectric acoustic sensor (FPAS) is nearly 300% of that of the isotropic particles. The microcone patterning FPAS with stable performance in harsh environments shows high sensitivity (39.22 mV Pa⁻¹⋅cm⁻²) due to the significant enhancement of sound energy absorption and can record sound signals, recognize audio signals, and realize human-computer interactions. Our work demonstrates that the FPAS holds promise for various applications, such as the Internet of Things (IoT), cochlea implants, wearable acoustic equipment, and human-computer interaction.

High-performance dual-sensitive flexible sensors based on conductive alginate sponge electrode/polyvinylidene fluoride microporous composite film

This work reported a novel dual-sensitive flexible sandwich sensor based on conductive natural biopolymers sponge electrode and polyvinylidene fluoride (PVDF) film with superiorities of high sensitivity, long lifespan and low density. The unique natural biopolymers sponge electrode, consisting of bidisperse magnetic particles and sodium alginate/chitosan (SA/CHI) with physically-crosslinked double-network structure, showed a high response capability to external magnetic fields. Besides, by combining the advantages of SA/CHI sponge electrode (high elasticity and flexibility) and PVDF film (exceptional stiffness and piezoelectricity), the SA/CHI/PVDF composite film also exhibited preferable mechanical strength and strain-dependent electrical property, which can simultaneously satisfy the requirements of high sensitivity detection to external magnetic fields and strains. Specifically, the relative resistance variation of SA/CHI/PVDF-1.00 sensor reached as high as 60.4% under a cycling loading of 240 mT magnetic field. Meanwhile, their electrical responses could also display a significant variation and relatively stable recoverability under periodic stretching, bending or compressing excitations. Afterwards, a potential working mechanism and equivalent circuit model were provided to study the magnetic/mechanic sensitivity of SA/CHI/PVDF sensors. Furthermore, a 4 × 4 SA/CHI/PVDF sensor array was developed to perceive and distinguish both magnetic field and compressive force, which indicated its favorable potential in wearable electronics and soft robotics.

Additive Manufacturing of Stretchable Multi-Walled Carbon Nanotubes/Thermoplastic Polyurethanes Conducting Polymers for Strain Sensing.

With the development of science and technology, flexible sensors play an indispensable role in body monitoring. Rapid prototyping of high-performance flexible sensors has become an important method to develop flexible sensors. The purpose of this study was to develop a flexible resin with multi-walled carbon nanotubes (MWCNTs) for the rapid fabrication of flexible sensors using digital light processing additive manufacturing. In this study, MWCNTs were mixed in thermoplastic polyurethane (TPU) photosensitive resin to prepare polymer-matrix composites, and a flexible strain sensor was prepared using self-developed additive equipment. The results showed that the 1.2 wt% MWCNTs/TPU composite flexible sensor had high gauge factor of 9.988 with a linearity up to 45% strain and high mechanical durability (1000 cycles). Furthermore, the sensor could be used for gesture recognition and monitoring and has good performance. This method is expected to provide a new idea for the rapid personalized forming of flexible sensors.

High performance flexible and antibacterial strain sensor based on silver‑carbon nanotubes coated cellulose/polyurethane nanofibrous membrane: Cellulose as reinforcing polymer blend and polydopamine as compatibilizer

In this study, ethyl cellulose was used as the second-phase polymer blended with polyurethane to make nanofibrous membrane as antibacterial strain sensor. The results indicated that ethyl cellulose could regulate the morphology of polyurethane through strong hydrogen bonding, which observably enhanced the nanofiber uniformity of polyurethane. Furthermore, rigid cellulose also remarkably improved the mechanical strength and thermal stability of the nanofibrous membrane. After being coated with silver nanoparticles and carbon nanotubes assisted by polydopamine (PDA), the membrane with outstanding bacteria inhibition performance exhibited outstanding sensitivity toward external mechanical stretching, as well as real-time motion of human body parts. The conductive composite membrane possessed sensitive and regular resistance feedback to 100 cycles of varied human motions. The cellulose in the nanofiber structure ensured the shape recovery and longtime use stability of the membrane. This study proposed a novel thinking for the construction of high performance strain sensor by rational introduction of rigid polysaccharide into the polymer matrix.

Highly sensitive pH sensor based on flexible polyaniline matrix for synchronal sweat monitoring

The design of a flexible pH sensor with high sensitivity, excellent stability, and low cost has great potential in the applications of the medical care and health fields. In this study, a high-performance flexible pH sensor with a two-electrode configuration was fabricated. This flexible pH sensor integrated the three-dimensional polyaniline (3D PANI) working electrode and Ag/AgCl reference electrode for monitoring the pH value of sweat in real-time. Polyaniline with phytic acid as dopant and crosslinker with a three-dimensional porous matrix structure provided a larger contact area for hydrogen ions in solution and a wider transmission path for charges, which improves the pH response performance of the 3D PANI sensor. The results demonstrated that the 3D PANI sensor exhibited linear super-Nernstian behavior (69.33 mV/pH) in the physiological range (pH 4.00 ∼ 9.00) of sweat, excellent performances in terms of response time (7.75 s), reversibility (3.80 mV), repeatability (0.44 % RSD), drift rate (0.10 mV/h), and temperature drift (0.06 mV/pH/oC). Particularly, the 3D PANI sensor in the bending state showed identical performances to the unbent condition, which makes it a promising candidate in the field of in-situ real-time sweat pH sensors for wearable electronics.

A low-cost 3D flexible pressure sensor with a wide working range prepared from articles for daily use for monitoring human motion

Flexible piezoresistive pressure sensors have attracted extensive attention due to their portability, flexibility, and high sensitivity. The development of low-cost, high-performance flexible piezoresistive sensors is a key issue. We have prepared a three-dimensional (3D) Carbon@MF flexible piezoresistive pressure sensor by dipping melamine foam (MF) in inexpensive ink with amorphous carbon materials. The material cost per unit volume (cm3) is only $0.006. The sensor has excellent sensing performance: wide working range of 150 kPa, high sensitivity of 0.62 kPa−1, superior stability with 2500 cycles of loading, fast response ability (30 ms). The sensor can monitor various human physiological activities such as different breathing modes, pulse, limb bending, and walking speed etc.