High-performance Flexible Sensors Research Articles

Laser-Sculptured Hierarchical Spinous Structures for Ultra-High-Sensitivity Iontronic Sensors with a Broad Operation Range.

Tactile pressure sensing over a wide operation range (>1 MPa) is challenging for a variety of applications in fields such as aviation, oceanography, and biomedicine. Recently, innovative strategies have been utilized to improve the performances of tactile sensors using specially designed structures, dielectric layers, and electrodes. Here, a hierarchical structural design based on ionic gel films has been utilized to build iontronic pressure sensors with ultrahigh sensitivities and broad operation ranges. Sculptured patterns made by a controlled CO2 laser scanning process have been produced on polyimide films to achieve two kinds of protrusion structures for high specific surface areas and strength to withstand high pressure. The iontronic sensor has been constructed by adding two screen-printed electrodes of high surface areas to achieve an ultrahigh sensitivity of 2593 kPa-1 and a wide pressure range from 0 Pa to 3.36 MPa. The prototype device also has a fast response and recovery time of 26 and 13 ms, respectively, and an excellent mechanical durability in the endurance test of over 2700 repeated loading and unloading cycles under a pressure of 1 MPa. Several application examples have been demonstrated, including the detection of physiological signals on human volunteers, the feedback control of intelligent robots, the grasping operation of underwater soft grippers, and the environmental wind-speed monitoring. As such, this work demonstrates a versatile and economical methodology to produce high-performance flexible sensors for various potential applications.

Ultrahigh Sensitive Flexible Piezoresistive Sensor with Carbonized Metal–Organic Framework Fe3O4@MIL-100(Fe)

The piezoresistive sensor is an important device for sensing pressure and strain in flexible electronics. The application of composite conductive nanomaterials is helpful to obtain high-performance and multifunctional flexible piezoresistive sensors. Herein, using carbonized Fe3O4@MIL-100(Fe) as a composite conductive nanomaterial, an ultrahigh-sensitivity flexible piezoresistive sensor was achieved by a laser-processed template. Due to the porous Fe3O4@MIL-100(Fe) conductive nanomaterials and microridges on the styrene–isoprene–styrene substrate, the sensitivity of the sensor reaches 1727.46 kPa–1 in the pressure range of 0–50 Pa, which has a significant advantage over the reported sensors. Meanwhile, the sensor exhibits good cycling stability (>1500 cycles), a fast response of 24 ms, an instant recovery of 15 ms, excellent reversibility, and an antielectromagnetic interference capability. For practical applications, the sensor was applied for monitoring body physiological signals such as speech and artery pulses, indicating that the sensor is suitable for health monitoring. Moreover, using carbonized metal–organic frameworks with functional nanoparticles as conductive nanomaterials in the sensing unit provides a feasible way to develop multifunctional flexible piezoresistive sensors.

Functionalized Graphene Fiber Modified With MOF-Derived Rime-Like Hierarchical Nanozyme for Electrochemical Biosensing of H2O2 in Cancer Cells.

The rational design and construction of high-performance flexible electrochemical sensors based on hierarchical nanostructure functionalized microelectrode systems are of vital importance for sensitive in situ and real-time detection of biomolecules released from living cells. Herein, we report a novel and facile strategy to synthesize a new kind of high-performance microelectrode functionalized by dual nanozyme composed of rime-like Cu2(OH)3NO3 wrapped ZnO nanorods assembly [Cu2(OH)3NO3@ZnO], and explore its practical application in electrochemical detection of hydrogen peroxide (H2O2) released from living cells. Benefiting from the merits of the unique hierarchical nanohybrid structure and high catalytic activities, the resultant Cu2(OH)3NO3@ZnO-modified AGF microelectrode shows remarkable electrochemical sensing performance towards H2O2 with a low detection limit of 1 μM and a high sensitivity of 272 μA cm−2 mM−1, as well as good anti-interference capability, long-term stability, and reproducibility. These properties enabled the proposed microelectrode-based electrochemical platform to be applied for in situ amperometric tracking of H2O2 released from different types of human colon cells, thus demonstrating its great prospect as a sensitive cancer cell detection probe for the early diagnosis and management of various cancer diseases.

Magnetically induced robust anisotropic structure of multi-walled carbon nanotubes/Ni for high-performance flexible strain sensor

Multi-walled carbon nanotubes (MWCNTs) are widely used as functional nanomaterials in flexible strain sensing polymer composites. They are expected to exhibit high anisotropy when aligned along a certain direction due to their unique one-dimensional structure, which has a positive effect on enhancing the sensing performance of the original polymer composites. Here, we used a weak curing magnetic field (less than 0.7 T) to induce the self-assembly of MWCNTs in polymer composites to obtain a novel flexible strain sensor with anisotropic structure (MAPC). To further optimize the comprehensive sensing performance, the synergistic effect between MWCNTs and conductive particles was utilized to enhance the sensitivity of MAPC to tensile strain by filling a small amount of nickel nanoparticles (MAPCN). The sensor can measure the tensile strain range up to 720%, with high sensitivity (GF=1768.7, 90%–100% strain), fast response (loading delay time τ1 = 92.2 ms and unloading delay time τ2 = 143.6 ms, 0–10% strain), and high durability (1050 cycles at 10% strain). These important properties allowed MAPCN to accurately monitor human physiological activities, and a gesture recognition system was designed to demonstrate its practical value and potential application prospect in wearable devices.

Freestanding Translucent ZnO-Cellulose Nanocomposite Films for Ultraviolet Sensor Applications.

The rapidly advancing technology of wearable and miniaturized electronics has increased the demand for low-cost high-performance flexible sensors. Herein, the preparation of translucent freestanding films consisting of cellulose nanofibers (CNFs) and ZnO nanoparticles (NPs) via a simple spray coating method is presented. The obtained nanocomposite films were thin (~10 µm) and flexible. The scanning electron microscopy and atomic force microscopy analysis revealed that the nanocomposite film was composed of regions of ZnO NP-modified CNFs and regions of aggregation of ZnO NPs with each other. The electrical conductance of the films was rapidly increased beyond 40 wt.% ZnO and reached up to >50 nA at 60 wt.% ZnO. This was attributed to the increased number of conductive paths formed by the ZnO NPs in the nanocomposite film when a certain threshold was crossed. The ZnO–CNF nanocomposite film exhibited a stable response over on/off cycles of UV light exposure. The responsivity and sensitivity of the nanocomposite film with 60 wt.% ZnO were 36.5 mA/W and 247, respectively. Even when the device was curved (radius of curvature: 3 mm), the response and sensitivity remained high. The developed nanocomposite films are expected to be applied as environmentally friendly flexible UV sensors.

Development of High-Sensitivity Piezoresistive Sensors Based on Highly Breathable Spacer Fabric with TPU/PPy/PDA Coating.

In recent years, the research of flexible sensors has become a hot topic in the field of wearable technology, attracting the attention of many researchers. However, it is still a difficult challenge to prepare low-cost and high-performance flexible sensors by a simple process. Three-dimensional spacer fabric (SF) are the ideal substrate for flexible pressure sensors due to its good compression resilience and high permeability (5747.7 mm/s, approximately 10 times that of cotton). In this paper, Thermoplastic polyurethane/Polypyrrole/Polydopamine/Space Fabric (TPU/PPy/PDA/SF) composite fabrics were prepared in a simple in-situ polymerization method by sequentially coating polydopamine (PDA) and Polypyrrole (PPy) on the surface of SF, followed by spin-coating of different polymers (thermoplastic polyurethane (TPU), polydimethylsiloxane (PDMS) and Ecoflex) on the PPy/PDA/SF surface. The results showed that the TPU/PPy/PDA/SF pressure sensors prepared by spin-coating TPU at 900 rpm at a concentration of 0.3 mol of pyrrole monomer (py) and a polymerization time of 60 min have optimum sensing performance, a wide working range (0–10 kPa), high sensitivity (97.28 kPa−1), fast response (60 ms), good cycling stability (>500 cycles), and real-time motion monitoring of different parts of the body (e.g., arms and knees). The TPU/PPy/PDA/SF piezoresistive sensor with high sensitivity on a highly permeable spacer fabric base developed in this paper has promising applications in the field of health monitoring.

A Fully Flexible Intelligent Thermal Touch Panel Based on Intrinsically Plastic Ag2 S Semiconductor.

Wearable touch panels, a typical flexible electronic device, can recognize and feed back the information of finger touch and movement. Excellent wearable touch panels are required to accurately and quickly monitor the signals of finger movement as well as the capacity of bearing various types of deformation. High-performance thermistor materials are one of the key functional components, but to date, a long-standing bottleneck is that inorganic semiconductors are typically brittle while the electrical properties of organic semiconductors are quite low. Herein, a high-performance flexible temperature sensor is reported by using plastic Ag2 S with ultrahigh temperature coefficient of resistance of -4.7% K-1 and resolution of 0.05 K, and rapid response/recovery time of 0.11/0.11 s. Moreover, the temperature sensor shows excellent durability without performance damage or loss during force stimuli tests. In addition, a fully flexible intelligent touch panel composed of a 16 × 10 Ag2 S-film-based temperature sensor array, as well as a flexible printed circuit board and a deep-learning algorithm is designed for perceiving finger touch signals in real-time, and intelligent feedback of Chinese characters and letters on an app. These results strongly show that high-performance flexible inorganic semiconductors can be widely used in flexible electronics.

High performance flexible and wearable strain sensor based on rGO and PANI modified Lycra cotton e-textile

Intelligent flexible strain sensors based on fabric have caused tremendous attention in the application of wearable devices because of their lightweight and excellent air permeability. It is still a great challenge to achieve excellent stability, high sensitivity, and wide working range. In this work, a high-performance strain sensor with reduced graphene oxide (rGO) reinforced polyaniline (PANI) based on polydopamine (PDA) modified Lycra cotton (LC) electronic textile (e-textile) was fabricated. The strain sensor of PANI/rGO/PDA/LC exhibits high sensitivity with a gauge factor (GF) of 24, a wide strain range of 0.2–50%, and excellent mechanical durability and stability for 1500 stretching/releasing cycles test. Such e-textile is promising to be integrated with other wearable devices to track human motion signals in real-time for human-machine and medical monitoring.

High-Performance Sensors Based on Chinese Ink and Water-Based Glue for Detection of Strain, Temperature, and Humidity

Flexible sensors with high sensitivity and reproducibility have critical applications in various industrial fields. However, current sensors have not yet been able to respond to diverse signal changes in the environment due to their poor stability, weak conductivity, high cost, and time-consuming integration process. Herein, we used dip-coating technology to prepare multifunctional rubber flexible sensors which were based on the Chinese ink carbon nanoparticle (ICN) and water-based glue. The prepared flexible sensors have the ability to sensitively detect temperature, humidity, and strain signals. The conductive ICN can well adhere on the elastic substrate (natural rubber latex glove), which gives the flexible sensors excellent structural stability even after 500 cycles. The strain response and recovery time of the flexible sensors are 132.8 and 133.8 ms, respectively. The flexible sensors can sensitively detect tiny pressure, human body movements, pronunciation, and human facial expression. Moreover, the prepared high-performance flexible sensors can also be used as temperature and humidity sensors, which displayed a good linear relationship between resistance change and the external signals. This work developed a simple, green, high-efficiency, and low-cost flexible sensors, which shows promising potential in wearable electronic devices for detection of strain, humidity, and temperature.

A Printed Flexible Humidity Sensor with High Sensitivity and Fast Response Using a Cellulose Nanofiber/Carbon Black Composite

In the emerging Internet of Things (IoT) society, there is a significant need for low-cost, high-performance flexible humidity sensors in wearable devices. However, commercially available humidity sensors lack flexibility or require expensive and complex fabrication methods, limiting their application and widespread use. We report a high-performance printed flexible humidity sensor using a cellulose nanofiber/carbon black (CNF/CB) composite. The cellulose nanofiber enables excellent dispersion of carbon black, which facilitates the ink preparation and printing process. At the same time, its hydrophilic and porous nature provides high sensitivity and fast response to humidity. Significant resistance changes of 120% were observed in the sensor at humidity ranging from 30% RH to 90% RH, with a fast response time of 10 s and a recovery time of 6 s. Furthermore, the developed sensor also exhibited high-performance uniformity, response stability, and flexibility. A simple humidity detection device was fabricated and successfully applied to monitor human respiration and noncontact fingertip moisture as a proof-of-concept.

High-Performance Flexible Self-Powered Triboelectric Tactile Sensor Based on Chemically Modified Micropatterned Pdms Film

High-Performance Flexible Self-Powered Triboelectric Tactile Sensor Based on Chemically Modified Micropatterned Pdms Film

A high-performance porous flexible composite film sensor for tension monitoring.

Flexible, lightweight sensors with a wide strain-sensing range are showing increasing significance in structural health monitoring compared with conventional hard sensors, which typically have a small strain range, are heavyweight, and have a large volume. In this work, salt particle precipitation and mechanical coating methods are used to fabricate porous graphene nanoplatelet (GNP)/polydimethylsiloxane (PDMS) flexible sensors for tension monitoring in structural health applications. The signal transformation through the Back Propagation (BP) algorithm is integrated to provide monitoring data that are comparable with other sensors. The results reveal that the flexible sensors with a low content of GNPs (0.1–0.25 wt%) possess better flexibility, allowing tensile strains over 200% to be measured. In addition, due to the enhanced deformation capacity of the pore structures, they can achieve high sensitivity (1–1000) under 65% strain, and a fast response time (70 ms) under 10% strain at 60 mm min−1. They also show high performance in the fatigue test (20 000 cycles) under 5% strain, and can effectively respond to bending and torsion. In addition, the sensors show an obvious response to temperature. Overall, the prepared flexible composite sensors in this work have the advantages of a wide strain-sensing range, a full-coverage conductive network, and being lightweight, and show potential for structural health monitoring in the near future.

High-performance strain sensor for detection of human motion and subtle strain by facile fabrication

Flexible strain sensors are significant components for smart wearable electronic devices because of wide applications. However, the design and construction of high-performance strain sensors through facile fabrication remains a challenge. In this study, we designed a simple and high-performance flexible strain sensor based on gold nanoparticles and a polyimide substrate. The fabrication included drop-casting and high-temperature annealing, enhancing the connections between the nanoparticles. The results reveal that the sensor can sensitively detect subtle strains (GF = 19.94). It exhibits excellent linearity (R2 = 0.99) when sensing outward bending strain and low power consumption (<2.5 mW). Furthermore, it can be adhered to human skin to detect human physiological parameters. The strain sensor can detect the angles of the cervical vertebra in different bending states. It can show the distinctions of artery pulse waveforms at different conditions (before and after exercise) and in people (pregnant and unpregnant) with a high precision.

Skin-friendly corrugated multilayer microspherical sensor fabricated with silk fibroin, poly (lactic-co-glycolic acid), polyaniline, and kappa-carrageenan for wide range pressure detection

To enlarge the linear detection range without sacrificing the sensitivity is one of the urgent problems in the development of high-performance piezoresistive flexible sensors. Inspired by a multilayer corrugated board, this study develops a new multilayer microspherical sensor in which conductive core-shell SiO2/Polyaniline (PANI) (PS) microspheres serve as active particles, while insulated silk fibroin (SF)/poly lactic-co-glycolic acid (PLGA) (SP) fibers are used as the support. The size of conductive microspheres attached to the insulated layer is controllable. The multiple layers of assembly endow the flexible sensor with a high sensitivity (0.071 kPa−1) and a wide linear detection (from 10 Pa to 380 kPa) simultaneously. This corrugated sensor also have a fast response time (145 ms) and an excellent durability (over 2000 cycles), and it can be used to detect human joint pressure signals and transmit encrypted information. Moreover, flexible keyboard, safety protection of machinery, as well as object position tracking can be achieved based on this sensor. Most importantly, the sensor encapsulated by biological polysaccharide kappa-carrageenan (KC) is skin-friendly and breathable, and it can be decomposed in 90 °C hot water. In conclusion, this multilayer microspherical sensor presents great potential for flexible wearable devices.

Graphene paper with electrodeposited NiCo2S4 nanoparticles as a novel flexible sensor for simultaneous detection of folic acid and ascorbic acid

We studied the production and electrochemical performance of NiCo2S4 electrodeposited reduced graphene oxide (rGO) paper for use as a freestanding flexible electrode for the simultaneous determination of folic acid (FA) and ascorbic acid (AA). Production of rGO paper was achieved through vacuum filtration of the aqueous dispersion of graphene oxide, followed by chemical reduction in hydrogen iodide solution. NiCo2S4/rGO paper material was fabricated with a simple approach using electrodeposition of hydrothermal synthesized NiCo2S4 nanoparticles directly on rGO paper. The physicochemical properties of as-prepared flexible NiCo2S4/rGO paper were characterized by means of field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray powder diffraction patterns. This freestanding paper electrode demonstrated high sensitivity, a wide linear range, and a low detection limit towards the simultaneous detection of FA and AA. NiCo2S4/rGO paper sensor, possessing easy preparation, low cost, and high flexibility, was tested in real samples with promising recoveries. This study exhibited that the approach of the electrodeposition of bimetallic sulfides on freestanding graphene paper substrates has great potential to develop high-performance flexible electrochemical sensors.

High-performance flexible strain sensors based on biaxially stretched conductive polymer composites with carbon nanotubes immobilized on reduced graphene oxide

In this paper, reduced graphene oxide (rGO), decorated with immobilized carbon nanotubes (CNTs), was introduced into a thermoplastic polyurethane (TPU) matrix to obtain a conductive nanocomposite with 2 wt% nanofillers content. Hot-pressed composite sheets were then biaxially stretched into films to efficiently fabricate flexible strain sensors. The biaxial stretching process was shown to promote secondary dispersion and parallel alignment of the nanofillers, resulting in the sensors exhibited significantly higher sensitivity (GF = 150 for the rGO/TPU4×4 sensor with a stretching ratio of 4 × 4 relative to GF = 3.5 for the rGO/TPU1×1 sensor at 30% strain). Furthermore, due to the synergy of the CNTs and rGO nanoparticles, the rGO/CNT/TPU4×4 sensor had a lower resistivity (5.62 × 104 Ω·m), wider monitoring range (0.3 ∼ 400% strain), and higher stability compared with the rGO/TPU4×4 sensor. The ability of the sensor to recognize body movements and physiological activities was also shown.

3D printed high-performance spider web-like flexible strain sensors with directional strain recognition based on conductive polymer composites

Inspired by the unique structure of spider web, we designed and prepared a spider web-like strain sensor based on multi-walled carbon nanotubes (MWCNT)/graphene nanoplatelets (GNP)/thermoplastic polyurethane (TPU) conductive polymer composites. Fused filament fabrication (FFF) 3D printing was used to meet the growing demand for flexible electronic devices and overcome limitations with the directional sensitivity of traditional strain sensors. The sensor response exhibited high linearity (R2 = 0.98 at 50% strain), a large strain response range (0–300%), high sensitivity (GF > 10000 at 300% strain), short response time (40 ms), low hysteresis, and good stability (1000 cycles). In addition, due to the special structural design, the sensor identified external forces in different directions. This further expands the application range of the sensor. This work provides valuable guidance for the customized fabrication of flexible strain sensors.

Flexible Ti3C2Tx MXene/PANI/Bacterial Cellulose Aerogel for e-Skins and Gas Sensing.

Flexible pressure sensors made of carbon materials have been used in electronic skins (e-skins), whose performance can be enhanced if composite sensing materials are used. Herein, an MXene/polyaniline/bacterial cellulose (MXene/PANI/BC) aerogel sensor has been fabricated through the self-assembly process between the MXene and one-dimensional active material. Combined with fewer-layer or single-layer MXenes, the as-fabricated aerogel could be used as the active layer of the pressure sensor, monitoring tiny motion signals of finger bending, wrist bending, and pulse beating. Bluetooth wireless transmission could also be realized to monitor the real-time spatial pressure distributions on the mobile phone, making the aerogel-based sensor an ideal candidate in e-skins. Meanwhile, the aerogel-based sensor is sensitive toward NH3 due to the unique three-dimensional (3D) structure of the aerogel and the abundant terminal groups (such as -O, -OH, and -F) of the MXene in the system that ensure efficient electronic transfer for the sensing process and create active sites for the absorption with the target gas. This work offers a versatile platform to develop MXenes to fabricate 3D composite aerogels for high-performance flexible multiple sensors.

High performance stretchable fibrous supercapacitors and flexible strain sensors based on CNTs/MXene-TPU hybrid fibers

In order to improve the practicability of fibrous supercapacitors, stretchability is a significant characteristic for the devices, which puts forward high requirements for electrode materials. In this work, highly stretchable and conductive CNTs/MXene-TPU hybrid fiber electrodes with porous structure were prepared by wet spinning technique. The mechanical and electrical properties of the fiber electrodes remained stable during the tensile process when the amount of CNTs/MXene is 10 wt%. Meanwhile, the electrodes also showed a high volumetric specific capacitance of 3.9 F cm−3 even at the scan rate of 1 V s−1. The all-solid-state fibrous supercapacitors based on CNTs/MXene-TPU fiber electrodes also exhibited superior electrochemical performance and can be stretched by more than 50% strain. The specific capacitance of the devices changed with the tensile strain and reached 3.1 F cm−3 while the stain was 50%. Based on these characteristics, the fibrous supercapacitors were further applied to flexible strain sensors, which can convert action signals to electrochemical signals rapidly. The stretchable fibrous supercapacitor can be used not only for energy storage, but also as a strain sensor to monitor human movements, making it very promising for the next generation smart textiles.

Flexible GO/Nb2CT x hybrid films for high-performance piezoresistive sensors

High-performance flexible piezoresistive sensors have attracted more and more scientific attention due to their promising applications in robotics and portable electronic devices. However, most flexible piezoresistive sensors require a fine structure design at the nanoscale, which makes the manufacturing process very complex and expensive. Two-dimensional (2D) transition metal carbides named MXene have been regarded as an important candidate material for piezoresistive sensors due to their good conductivity, large interlayer spacing, and the inherent nature of 2D materials. The interlayer spacing between MXene nanosheets would be varied under an external pressure, resulting in the conductivity modulation. To improve the sensitivity of MXene-based piezoresistive sensors, the interlayer spacing of MXene-based films should be further tuned and optimized. In this contribution, flexible Nb2CT x -based films were developed using a simple filtration method and evaluated for piezoresistive sensors. 2D GO nanosheets were introduced between the Nb2CT x layers to tune the interlayer spacing to enhance piezoresistive performance. The fabricated flexible piezoresistive sensors based on GO/Nb2CT x films possess high sensitivity up to 228 kPa−1 and rapid response time as fast as 101 ms. This study would promote the application of MXenes-based 2D nanomaterials in the field of piezoresistive sensors.