Flexible piezoresistive pressure sensor based on a graphene-carbon nanotube-polydimethylsiloxane composite

Flexible sensors are used widely in wearable devices, specifically flexible piezoresistive sensors, which are common and easy to manipulate.However, fabricating such sensors is expensive and complex, so proposed here is a simple fabrication approach involving a sensor containing microstructures replicated from a sandpaper template onto which polydimethylsiloxane containing a mixture of graphene and carbon nanotubes is spin coated. The surface morphologies of three versions of the sensor made using different grades of sandpaper are observed, and the corresponding pressure sensitivities and linearity and hysteresis characteristics are assessed and analyzed. The results show that the sensor made using 80-mesh sandpaper has the best sensing performance. Its sensitivity is 0.341 kPa-1in the loading range of 0–1.6 kPa, it responds to small external loading of 100 Pa with a resistance change of 10%, its loading and unloading response times are 0.126 and 0.2 s, respectively,and its hysteresis characteristic is ~7%, indicating that the sensor has high sensitivity, fast response, and good stability. Thus, the presented piezoresistive sensor is promising for practical applications in flexible wearable electronics.

Fully sprayed MXene-based high-performance flexible piezoresistive sensor for image recognition

High-performance flexible pressure sensors provide comprehensive tactile perception and are applied in human activity monitoring,soft robotics,medical treatment,and human-computer interface.However,these flexible pressure sensors require extensive nano-architectural design and complicated manufacturing and are timeconsuming.Herein,a highly sensitive,flexible piezoresistive tactile sensor is designed and fabricated,consisting of three main parts:the randomly distributed microstructure on T-ZnOw/PDMS film as a top substrate,multilayer Ti_(3)C_(2)-MXene film as an intermediate conductive filler,and the few-layer Ti_(3)C_(2)-MXene nanosheetbased interdigital electrodes as the bottom substrate.The MXene-based piezoresistive sensor with randomly distributed microstructure exhibits a high sensitivity over a broad pressure range(less than 10 kPa for 175 kPa^(-1))and possesses an out-standing permanence of up to 5000 cycles.Moreover,a 16-pixel sensor array is designed,and its potential applications in visualizing pressure distribution and an example of tactile feedback are demonstrated.This fully sprayed MXene-based pressure sensor,with high sensitivity and excellent durability,can be widely used in,electronic skin,intelligent robots,and many other emerging technologies.

Bioinspired flexible piezoresistive sensor for high-sensitivity detection of broad pressure range

The human skin has the ability to sense tactile touch and a great range of pressures.Therefore,in prosthetic or robotic systems,it is necessary to prepare pressure sensors with high sensitivity in a wide measurement range to provide human-like tactile sensation.Herein,we developed a flexible piezoresistive pressure sensor that is highly sensitive in a broad pressure range by using lotus leaf micropatterned polydimethylsiloxane and multilayer superposition.By superposing four layers of micropatterned constructive substrates,the multilayer piezoresistive pressure sensor achieves a broad pressure range of 312 kPa,a high sensitivity of 2.525 kPa^(−1),a low limit of detection(LOD)of<12 Pa,and a fast response time of 45 ms.Compared with the traditional flexible pressure sensor,the pressure range of this sensor can be increased by at least an order of magnitude.The flexible piezoresistive pressure sensor also shows high robustness:after testing for at least 1000 cycles,it shows no sign of fatigue.More importantly,these sensors can be potentially applied in various human motion detection scenarios,including tiny pulse monitoring,throat vibration detection,and large under-feet pressure sensing.The proposed fabrication strategy may guide the design of other kinds of multifunctional sensors to improve the detection performance.

MXene-based flexible pressure sensor with piezoresistive properties significantly enhanced by atomic layer infiltration

The flexible pressure sensor has been credited for leading performance including higher sensitivity,faster response/recovery,wider detection range and higher mechanical durability,thus driving the development of novel sensing materials enabled by new processing technologies.Using atomic layer infiltration,Pt nanocrystals with dimensions on the order of a few nanometers can be infiltrated into the compressible lamellar structure of Ti3C2Tx MXene,allowing a modulation of its interlayer spacing,electrical conductivity and piezoresistive property.The flexible piezoresistive sensor is further developed from the Pt-infiltrated MXene on a paper substrate.It is demonstrated that Pt infiltration leads to a significant enhancement of the pressure-sensing performance of the sensor,including increase of sensitivity from 0.08 kPa^(-1)to 0.5 kPa^(-1),extension of detection limit from 5 kPa to 9 kPa,decrease of response time from 200 ms to 20 ms,and reduction of recovery time from 230 ms to 50 ms.The mechanical durability of the flexible sensor is also improved,with the piezoresistive performance stable over 1000 cycles of flexure fatigue.The atomic layer infiltration process offers new possibilities for the structure modification of MXene for advanced sensor applications.

Recent progress in graphene-based wearable piezoresistive sensors:From 1D to 3D device geometries

Electronic skin and flexible wearable devices have attracted tremendous attention in the fields of human-machine interaction,energy storage,and intelligent robots.As a prevailing flexible pressure sensor with high performance,the piezoresistive sensor is believed to be one of the fundamental components of intelligent tactile skin.Furthermore,graphene can be used as a building block for highly flexible and wearable piezoresistive sensors owing to its light weight,high electrical conductivity,and excellent mechanical.This review provides a comprehensive summary of recent advances in graphene-based piezoresistive sensors,which we systematically classify as various configurations including one-dimensional fiber,two-dimensional thin film,and threedimensional foam geometries,followed by examples of practical applications for health monitoring,human motion sensing,multifunctional sensing,and system integration.We also present the sensing mechanisms and evaluation parameters of piezoresistive sensors.This review delivers broad insights on existing graphene-based piezoresistive sensors and challenges for the future generation of high-performance,multifunctional sensors in various applications.

Flexible Waterproof Piezoresistive Pressure Sensors with Wide Linear Working Range Based on Conductive Fabrics

Developing flexible sensors with high working performance holds intense interest for diverse applications in leveraging the Internet-of-things(IoT)infrastructures.For flexible piezoresistive sensors,traditionally most efforts are focused on tailoring the sensing materials to enhance the contact resistance variation for improving the sensitivity and working range,and it,however,remains challenging to simultaneously achieve flexible sensor with a linear working range over a high-pressure region(>100 kPa)and keep a reliable sensitivity.Herein,we devised a laserengraved silver-coated fabric as"soft"sensor electrode material to markedly advance the flexible sensor's linear working range to a level of 800 kPa with a high sensitivity of 6.4 kPa^-1 yet a fast response time of only 4 ms as well as long-time durability,which was rarely reported before.The integrated sensor successfully routed the wireless signal of pulse rate to the portable smartphone,further demonstrating its potential as a reliable electronic.Along with the rationally building the electrode instead of merely focusing on sensing materials capable of significantly improving the sensor's performance,we expect that this design concept and sensor system could potentially pave the way for developing more advanced wearable electronics in the future.

Laser direct writing and characterizations of flexible piezoresistive sensors with microstructures

Functional materials with high viscosity and solid materials have received more and more attentions in flexible pressure sensors,which are inadequate in the most used molding method.Herein,laser direct writing(LDW)method is proposed to fabricate flexible piezoresistive sensors with microstructures on PDMS/MWCNTs composites with an 8%MWCNTs mass fraction.By controlling laser energy,microstructures with different geometries can be obtained,which significantly impacts the performances of the sensors.Subsequently,curved microcones with excellent performance are fabricated under parameters of f=40 kHz and v=150 mm·s^(-1).The sensor exhibits continuous multi-linear sensitivity,ultrahigh original sensitivity of 21.80%kPa^(-1),wide detection range of over 20 kPa,response/recovery time of~100 ms and good cycle stability for more than 1000 times.Besides,obvious resistance variation can be observed when tiny pressure(a peanut of 30 Pa)is applied.Finally,the flexible piezoresistive sensor can be applied for LED brightness controlling,pulse detection and voice recognition.

Piezoresistive Response Extraction for Smart Cement-based Composites/Sensors

A kind of piezoresistive response extraction method for smart cement-based composites/sensors was proposed.Two kinds of typical piezoresistive cement-based composites/sensors were fabricated by respectively adding carbon nanotubes and nickel powders as conductive fillers into cement paste or cement mortar.The variation in measured electrical resistance of such cement-based composites/sensors was explored without loading and under repeated compressive loading and impulsive loading.The experimental results indicate that the measured electrical resistance of piezoresistive cement-based composites/sensors exhibits a two-stage variation trend of fast increase and steady increase with measurement time without loading,and an irreversible increase after loading.This results from polarization caused by ionic conduction in these composites/sensors.After reaching a plateau,the measured electrical resistance can be divided into an electrical resistance part and an electrical capacity part.The piezoresistive responses of electrical resistance part in measured electrical resistance to loading can be extracted by eliminating the linear electrical capacity part in measured electrical resistance.

Development of an Ultra-Sensitive and Flexible Piezoresistive Flow Sensor Using Vertical Graphene Nanosheets

This paper suggests development of a flexible,lightweight,and ultra-sensitive piezoresistive flow sensor based on vertical graphene nanosheets(VGNs) with a mazelike structure.The sensor was thoroughly characterized for steady-state and oscillatory water flow monitoring applications.The results demonstrated a high sensitivity(103.91 mV(mm/s)-1) and a very low-velocity detection threshold(1.127 mm s-1) in steady-state flow monitoring.As one of many potential applications,we demonstrated that the proposed VGNs/PDMS flow sensor can closely mimic the vestibular hair cell sensors housed inside the semicircular canals(SCCs).As a proof of concept,magnetic resonance imaging of the human inner ear was conducted to measure the dimensions of the SCCs and to develop a 3D printed lateral semicircular canal(LSCC).The sensor was embedded into the artificial LSCC and tested for various physiological movements.The obtained results indicate that the flow sensor is able to distinguish minute changes in the rotational axis physical geometry,frequency,and amplitude.The success of this study paves the way for extending this technology not only to vestibular organ prosthesis but also to other applications such as blood/urine flow monitoring,intravenous therapy(Ⅳ),water leakage monitoring,and unmanned underwater robots through incorporation of the appropriate packaging of devices.

A button switch inspired duplex hydrogel sensor based on both triboelectric and piezoresistive effects for detecting dynamic and static pressure

The capability to sense complex pressure variations comprehensively is vital for wearable electronics and flexible human–machine interfaces.In this paper,inspired by button switches,a duplex tactile sensor based on the combination of triboelectric and piezoresistive effects is designed and fabricated.Because of its excellent mechanical strength and electrical stability,a double-networked ionic hydrogel is used as both the conductive electrode and elastic current regulator.In addition,micro-pyramidal patterned polydimethylsiloxane(PDMS)acts as both the friction layer and the encapsulation elastomer,thereby boosting the triboelectric output performance significantly.The duplex hydrogel sensor demonstrates comprehensive sensing ability in detecting the whole stimulation process including the dynamic and static pressures.The dynamic stress intensity(10–300 Pa),the action time,and the static variations(increase and decrease)of the pressure can be identified precisely from the dual-channel signals.Combined with a signal processing module,an intelligent visible door lamp is achieved for monitoring the entire“contact–hold–release–separation”state of the external stimulation,which shows great application potential for future smart robot e-skin and flexible electronics.

Sandwich probe temperature sensor based on In_(2)O_(3)-IZO thin film for ultra-high temperatures

High-temperature thin-film thermocouples(TFTCs)have attracted significant attention in the aerospace and steel metallurgy industry.However,previous studies on TFTCs have primarily focused on the two-dimensional planar-type,whose thermal sensitive area has to be perpendicular to the test environment,and therefore affects the thermal fluids pattern or loses accuracy.In order to address this problem,recent studies have developed three-dimensional probe-type TFTCs,which can be set parallel to the test environment.Nevertheless,the probe-type TFTCs are limited by their measurement threshold and poor stability at high temperatures.To address these issues,in this study,we propose a novel probe-type TFTC with a sandwich structure.The sensitive layer is compounded with indium oxide doped zinc oxide and fabricated using screen-printing technology.With the protection of sandwich structure on electrode film,the sensor demonstrates robust high-temperature stability,enabling continuous working at 1200℃ above 5 h with a low drift rate of 2.3℃·h^(−1).This sensor exhibits a high repeatability of 99.3% when measuring a wide range of temperatures,which is beyond the most existing probe-type TFTCs reported in the literature.With its excellent high-temperature performance,this temperature sensor holds immense potentials for enhancing equipment safety in the aerospace engineering and ensuring product quality in the steel metallurgy industry.

The Effect of Temperature and Doping Level on the Characteristics of Piezoresistive Pressure Sensor

Piezoresistive pressure sensors based on silicon have a large thermal drift because of their high sensitivity to temperature. The study of the effect of the temperature and doping level on characteristics of these sensors is essential to define the parameters that cause the output characteristics drift. In this study, we adopted the model of Kanda to determine the effect of the temperature and of doping level on the piezoresistivity of the Silicon monocrystal. This is to represent P(N,T) and for p-type silicon as functions of impurity concentration for different temperatures. This allows us to see the effect of temperature and doping concentration on the output characteristics of the sensor. Finally, we study the geometric influence parameters and doping on these characteristics to optimize the sensor performance. This study allows us to predict the sensor behavior against temperature and to minimize this effect by optimizing the doping concentration.

Piezoresistive Characteristic of Conductive Rubber for Flexible Tactile Sensor

In the research of 2D flexible tactile sensor matrix,pressure-sensitive conductive rubber was developed and tested in which carbon black was used as its conductive phase and silicon rubber as its matrix layer.Experiments were undertaken and the resultant data were used for its piezoresistive characteristics investigation for two kinds of electrode connection configurations,the surface directive connection and embedded connection.It is found that due to the rather strong nonlinearity of the piezoresistive characteristic curves obtained,a higher correlation relationship can be obtained by means of quadratic polynomial fitting.It also showed that the embedded electrode assembling has higher fitting accuracy while the surface directive connection has better mechanical sensitivity.

Enhancing Accuracy of Flexible Piezoresistive Pressure Sensors by Suppressing Seebeck Effect

Flexible piezoresistive pressure sensors can offer convenient detection of mechanical deformations for wearable electronics.Previous studies of flexible piezoresistive pressure sensors focus on the sensitivity but the low-cost and self-powered sensors remain a challenge due to the deviation of resistance signal acquisition caused by thermoelectric voltage.Here,piezoresistive pressure sensors with ultralow Seebeck coefficient of-0.72μV/K based on carbon nanotubes(CNTs)/polyethyleneimine(PEI)/melamine(CPM)sponge are reported.Due to the diminished Seebeck effect,the CPM sponge pressure sensors successfully reduce the deviation to 18.75%and can keep stable sensitivity and resistance change under a very low working voltage and change temperature environment.The stable properties of the sensors make them successful to work for real-time sensing in self-powered wearable electronics.

Practical Approach Design Piezoresistive Pressure Sensor in Circular Diaphragm

This paper presents a methodology for analytical calculation and computational simulation using the finite element method for piezoresistive graphite sensor element on flexible polymer substrate,A4 paper.The computer simulation aims to find the region of greatest mechanical tension and deflection of the circular diaphragm set in the circumference edges.The steps for simulation are geometry definition,mesh generation,inclusion of material physical properties and simulation execution.The mathematical modeling of maximum mechanical stress and deflection is described analytically and computationally.The analytical calculations were compared with the computer simulation and presented a relative percentage error of 3.38%for the maximum deflection.The results show that the piezoresistor should be positioned at the edges of the circular diaphragm to take advantage of maximum mechanical stress by defining the best location for graphite film deposition for sensor device designs and fabrications.

A multiple flame-retardant,early fire-warning,and highly sensitive thread-shaped all-fabric-based piezoresistive sensor

In the artificial intelligence age,multifunctional and intelligent fireproof fabric-based electronics are urgently needed.Herein,a novel thread-shaped all-fabric-based piezoresistive sensor(denoted as TAFPS)with properties such as flame retardancy,firewarning,and piezoresistivity is proposed,which is composed of an inner nickel-plated fabric electrode,a multifunctional double helix fabric,and an external flame-retardant encapsulation fabric.Owing to the multiple flame-retardant properties of glass fiber tubular fabric,aminated carbon nanotubes(ACNTs),and ammonium polyphosphate,the char residue of the TAFPS reaches40.3 wt%at 800℃.In addition,the heat-sensitive effect of ACNTs during combustion causes a rapid decrease in the TAFPS resistance,triggering the fire alarm system within 2 s.Additionally,benefiting from the force-sensitive behavior of the double helix layer and tightly wrapped pattern of the external heat-shrinkable tubular fabric,TAFPS demonstrated a high sensitivity of4.40 kPa^(-1)(0–5.81 k Pa)and good stability for 10000 s.Considering its excellent flame resistance,high sensitivity,and agreeable stability,the developed TAFPS can be integrated into fire suits to monitor the exercise training process and the external fire environment.This work offers a novel approach for fabricating all-fabric-based piezoresistive sensors in the future for fire prevention and fire alarms,with promising applications in fire protection,the Internet of Things,and smart apparel.

High-performance piezoresistive sensors based on transfer-free large-area PdSe_(2) films for human motion and health care monitoring

Two-dimensional transition metal dichalcogenides(TMDs)are needed in highperformance piezoresistive sensors due to their strong strain-induced bandgap modification and thereby large gauge factors.However,integrating a conventional high-temperature chemical vapor deposition(CVD)-grown TMD with a flexible substrate necessitates a transfer process that inevitably degrades the sensing properties of the TMDs and increases the overall fabrication complexity.We present a high-performance piezoresistive strain sensor that employs largearea PdSe_(2) films grown directly on polyimide(PI)substrates via plasma-assisted selenization of a sputtered Pd film.The reliable strain transfer from the substrate to the PdSe_(2) film ensures an outstanding strain-sensing capability of the sensor.Specifically,the sensors have a gauge factor of up to315±2.1,a response time under 25 ms,a detection limit of 8×10^(-6),and an exceptional stability of over 104 loadingunloading cycles.By attaching the sensors to the skin surface,we demonstrate their application for measuring physiological parameters in health care monitoring,including motion,voice,and arterial pulse vibration.Furthermore,using the PdSe_(2) film sensor combined with deep learning technology,we achieved intelligent recognition of artery temperature from arterial pulse signals with only a 2%difference between predicted and actual temperatures.The excellent sensing performance,together with the advantages of low-temperature fabrication and simple device structure,make the PdSe_(2) film sensor promising for wearable electronics and health care sensing systems.

Optimization of microstructure design for enhanced sensing performance in flexible piezoresistive sensors

Flexible piezoresistive strain sensors have received significant attention due to their diverse applications in monitoring human activities and health,as well as in robotics,prosthetics,and human–computer interaction interfaces.Among the various flexible sensor types,those with microstructure designs are considered promising for strain sensing due to their simple structure,high sensitivity,extensive operational range,rapid response time,and robust stability.This review provides a concise overview of recent advancements in flexible piezoresistive sensors based on microstructure design for enhanced strain sensing performance,including the impact of microstructure on sensing mechanisms,classification of microstructure designs,fabrication methods,and practical applications.Initially,this review delves into the analysis of piezoresistive sensor sensing mechanisms and performance parameters,exploring the relationship between microstructure design and performance enhancement.Subsequently,an in-depth discussion is presented,focusing on the primary themes of microstructure design classification,process selection,performance characteristics,and specific applications.This review employs mathematical modeling and hierarchical analysis to emphasize the directionality of different microstructures on performance enhancement and to highlight the performance advantages and applicable features of various microstructure types.In conclusion,this review examines the multifunctionality of flexible piezoresistive sensors based on microstructure design and addresses the challenges that still need to be overcome and improved,such as achieving a wide range of stretchability,high sensitivity,and robust stability.This review summarizes the research directions for enhancing sensing performance through microstructure design,aiming to assist in the advancement of flexible piezoresistive sensors.

Dynamic Calibration of the Cutting Temperature Sensor of NiCr/NiSi Thin-film Thermocouple

In high-speed cutting, natural thermocouple, artificial thermocouple and infrared radiation temperature measurement are usually adopted for measuring cutting temperature, but these methods have difficulty in measuring transient temperature accurately of cutting area on account of low response speed and limited cutting condition. In this paper, NiCr/NiSi thin-film thermocouples（TFTCs） are fabricated according to temperature characteristic of cutting area in high-speed cutting by means of advanced twinned microwave electro cyclotron resonance（MW-ECR） plasma source enhanced radio frequency（RF） reaction non-balance magnetron sputtering technique, and can be used for transient cutting temperature measurement. The time constants of the TFTCs with different thermo-junction film width are measured at four kinds of sampling frequency by using Ultra-CFR short pulsed laser system that established. One-dimensional unsteady heat conduction model is constructed and the dynamic performance is analyzed theoretically. It can be seen from the analysis results that the NiCr/NiSi TFTCs are suitable for measuring transient temperature which varies quickly, the response speed of TFTCs can be obviously improved by reducing the thickness of thin-film, and the area of thermo-junction has little influence on dynamic response time. The dynamic calibration experiments are made on the constructed dynamic calibration system, and the experimental results confirm that sampling frequency should be larger than 50 kHz in dynamic measurement for stable response time, and the shortest response time is 0.042 ms. Measurement methods and devices of cutting heat and cutting temperature measurement are developed and improved by this research, which provide practical methods and instruments in monitoring cutting heat and cutting temperature for research and production in high-speed machining.

Piezoresistive design for electronic skin: from fundamental to emerging applications

There is growing recognition that the developments in piezoresistive devices from personal healthcare to artificial intelli-gence,will emerge as de novo translational success in electronic skin.Here,we review the updates with regard to piezoresistive sensors including basic fundamentals,design and fabrication,and device performance.We also discuss the prosperous advances in piezoresistive sensor application,which offer perspectives for future electronic skin.