Flexible capacitive pressure sensor based on interdigital electrodes with porous microneedle arrays for physiological signal monitoring

Flexible pressure sensors have many potential applications in the monitoring of physiological signals because of their good biocompatibil-ity and wearability.However,their relatively low sensitivity,linearity,and stability have hindered their large-scale commercial application.Herein,aflexible capacitive pressure sensor based on an interdigital electrode structure with two porous microneedle arrays(MNAs)is pro-posed.The porous substrate that constitutes the MNA is a mixed product of polydimethylsiloxane and NaHCO3.Due to its porous and interdigital structure,the maximum sensitivity(0.07 kPa-1)of a porous MNA-based pressure sensor was found to be seven times higher than that of an imporous MNA pressure sensor,and it was much greater than that of aflat pressure sensor without a porous MNA structure.Finite-element analysis showed that the interdigital MNA structure can greatly increase the strain and improve the sensitivity of the sen-sor.In addition,the porous MNA-based pressure sensor was found to have good stability over 1500 loading cycles as a result of its bilayer parylene-enhanced conductive electrode structure.Most importantly,it was found that the sensor could accurately monitor the motion of afinger,wrist joint,arm,face,abdomen,eye,and Adam’s apple.Furthermore,preliminary semantic recognition was achieved by monitoring the movement of the Adam’s apple.Finally,multiple pressure sensors were integrated into a 33 array to detect a spatial pressure distribu-×tion.Compared to the sensors reported in previous works,the interdigital electrode structure presented in this work improves sensitivity and stability by modifying the electrode layer rather than the dielectric layer.

Nanocellulose-Assisted Construction of Multifunctional MXene-Based Aerogels with Engineering Biomimetic Texture for Pressure Sensor and Compressible Electrode

Multifunctional architecture with intriguing structural design is highly desired for realizing the promising performances in wearable sensors and flexible energy storage devices.Cellulose nanofiber(CNF)is employed for assisting in building conductive,hyperelastic,and ultralight Ti_(3)C_(2)T_(x)MXene hybrid aerogels with oriented tracheid-like texture.The biomimetic hybrid aerogels are constructed by a facile bidirectional freezing strategy with CNF,carbon nanotube(CNT),and MXene based on synergistic electrostatic interaction and hydrogen bonding.Entangled CNF and CNT“mortars”bonded with MXene“bricks”of the tracheid structure produce good interfacial binding,and superior mechanical strength(up to 80%compressibility and extraordinary fatigue resistance of 1000 cycles at 50%strain).Benefiting from the biomimetic texture,CNF/CNT/MXene aerogel shows ultralow density of 7.48 mg cm^(-3)and excellent electrical conductivity(~2400 S m^(-1)).Used as pressure sensors,such aerogels exhibit appealing sensitivity performance with the linear sensitivity up to 817.3 kPa^(-1),which affords their application in monitoring body surface information and detecting human motion.Furthermore,the aerogels can also act as electrode materials of compressive solid-state supercapacitors that reveal satisfactory electrochemical performance(849.2 mF cm^(-2)at 0.8 mA cm^(-2))and superior long cycle compression performance(88%after 10,000 cycles at a compressive strain of 30%).

A personalized electronic textile for ultrasensitive pressure sensing enabled by biocompatible MXene/ PEDOT:PSS composite

Flexible,breathable,and highly sensitive pressure sensors have increasingly become a focal point of interest due to their pivotal role in healthcare monitoring,advanced electronic skin applications,and disease diagnosis.However,traditional methods,involving elastomer film-based substrates or encapsulation techniques,often fall short due to mechanical mismatches,discomfort,lack of breathability,and limitations in sensing abilities.Consequently,there is a pressing need,yet it remains a significant challenge to create pressure sensors that are not only highly breathable,flexible,and comfortable but also sensitive,durable,and biocompatible.Herein,we present a biocompatible and breathable fabric-based pressure sensor,using nonwoven fabrics as both the sensing electrode(coated with MXene/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate[PEDOT:PSS])and the interdigitated electrode(printed with MXene pattern)via a scalable spray-coating and screen-coating technique.The resultant device exhibits commendable air permeability,biocompatibility,and pressure sensing performance,including a remarkable sensitivity(754.5 kPa^(−1)),rapid response/recovery time(180/110 ms),and robust cycling stability.Furthermore,the integration of PEDOT:PSS plays a crucial role in protecting the MXene nanosheets from oxidation,significantly enhancing the device's long-term durability.These outstanding features make this sensor highly suitable for applications in fullrange human activities detection and disease diagnosis.Our study underscores the promising future of flexible pressure sensors in the realm of intelligent wearable electronics,setting a new benchmark for the industry.

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.

Self-healing Au/PVDF-HFP composite ionic gel for flexible underwater pressure sensor

Ionic gels can be potentially used in wearable devices owing to their high humidity resistance and non-volatility.However,the applicability of existing ionic gel pressure sensors is limited by their low sensitivity.Therefore,it is very import-ant to develop an ionic gel pressure sensor with high sensitivity and a wide pressure detection range without sacrificing mechan-ical stretchability and self-healing ability.Herein,we report an effective strategy for developing pressure sensors based on ion-ic gel composites consisting of high-molecular-weight polymers,ionic liquids,and Au nanoparticles.The resulting capacitive pressure sensors exhibit high pressure sensitivity,fast response,and excellent self-healing properties.The sensors composed of highly hydrophobic polymers and ionic liquids can be used to track underwater movements,demonstrating broad application prospects in human motion state monitoring and underwater mechanical operations.

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.

Advancing the pressure sensing performance of conductive CNT/PDMS composite film by constructing a hierarchical-structured surface

Flexible pressure sensors have attracted wide attention due to their applications to electronic skin,health monitoring,and human-machine interaction.However,the tradeoff between their high sensitivity and wide response range remains a challenge.Inspired by human skin,we select commercial silicon carbide sandpaper as a template to fabricate carbon nanotube(CNT)/polydimethylsiloxane(PDMS)composite film with a hierarchical structured surface(h-CNT/PDMS)through solution blending and blade coating and then assemble the h-CNT/PDMS composite film with interdigitated electrodes and polyurethane(PU)scotch tape to obtain an h-CNT/PDMS-based flexible pressure sensor.Based on in-situ optical images and finite element analysis,the significant compressive contact effect between the hierarchical structured surface of h-CNT/PDMS and the interdigitated electrode leads to enhanced pressure sensitivity and a wider response range(0.1661 kPa^(-1),0.4574 kPa^(-1)and 0.0989 kPa^(-1)in the pressure range of 0–18 kPa,18–133 kPa and 133–300 kPa)compared with planar CNT/PDMS composite film(0.0066 kPa^(-1)in the pressure range of 0–240 kPa).The prepared pressure sensor displays rapid response/recovery time,excellent stability,durability,and stable response to different loading modes(bending and torsion).In addition,our pressure sensor can be utilized to accurately monitor and discriminate various stimuli ranging from human motions to pressure magnitude and spatial distribution.This study supplies important guidance for the fabrication of flexible pressure sensors with superior sensing performance in next-generation wearable electronic devices.

Silver Nanowire Electrodes: Conductivity Improvement Without Post-treatment and Application in Capacitive Pressure Sensors

Transparent electrode based on silver nanowires(Ag NWs) emerges as an outstanding alternative of indium tin oxide film especially for flexible electronics. However, the conductivity of Ag NWs transparent electrode is still dramatically limited by the contact resistance between nanowires at high transmittance. Polyvinylpyrrolidone(PVP) layer adsorbed on the nanowire surface acts as an electrically insulating barrier at wire–wire junctions, and some devastating post-treatment methods are proposed to reduce or eliminate PVP layer, which usually limit the application of the substrates susceptible to heat or pressure and burden the fabrication with high-cost, time-consuming, or inefficient processes. In this work, a simple and rapid pre-treatment washing method was proposed to reduce the thickness of PVP layer from 13.19 to0.96 nm and improve the contact between wires. Ag NW electrodes with sheet resistances of 15.6 and 204 X sq-1have been achieved at transmittances of 90 and 97.5 %, respectively. This method avoided any post-treatments and popularized the application of high-performance Ag NW transparent electrode on more substrates. The improved Ag NWs were successfully employed in a capacitive pressure sensor with high transparency, sensitivity, and reproducibility.

Non-destructive Evaluation of Composite Pressure Vessel by Using FBG Sensors

In recent years, advanced composite structures are used extensively in many industries such as aerospace, aircraft, automobile, pipeline and civil engineering. Reliability and safety are crucial requirements posed by them to the advanced composite structures be- cause of their harsh working conditions. Therefore, as a very important measure, structural health monitoring （SHM） in-service is deft- nitely demanded for ensuring their safe working in-situ. In this paper, fiber Bragg grating （FBG） sensors are surface-mounted on the hoop and in the axial directions of a FRP pressure vessel to monitor the strain status during its pressurization. The experimental results show that the FBG sensors could be used to monitor the strain development and determine the ultimate failure strain of the composite pressure vessel.

State-of-the-art and recent developments in micro/nanoscale pressure sensors for smart wearable devices and health monitoring systems

Small-sized,low-cost,and high-sensitivity sensors are required for pressure-sensing applications because of their critical role in consumer electronics,automotive applications,and industrial environments.Thus,micro/nanoscale pressure sensors based on micro/nanofabrication and micro/nanoelectromechanical system technologies have emerged as a promising class of pressure sensors on account of their remarkable miniaturization and performance.These sensors have recently been developed to feature multifunctionality and applicability to novel scenarios,such as smart wearable devices and health monitoring systems.In this review,we summarize the major sensing principles used in micro/nanoscale pressure sensors and discuss recent progress in the development of four major categories of these sensors,namely,novel material-based,flexible,implantable,and selfpowered pressure sensors.

Application and analysis of functionally graded piezoelectrical rotating cylinder as mechanical sensor subjected to pressure and thermal loads

The exact thermoelastic analysis of a functionally graded piezoelectrical （FGP） rotating cylinder is investigated analytically. The cylinder is subjected to a com- bination of electrical, thermal, and mechanical loads simultaneously. The structure is a simplified model of a rotational sensor or actuator. The basic governing differential equation of the system is obtained by using the energy method. A novel term, named as the additional energy, is introduced to exact the evaluation of the energy functional. The solution to the governing differential equation is presented for two types of boundary conditions including free rotating and rotating cylinders exposed to the inner pressure. The effect of the angular velocity is investigated on the radial distribution of various components. The mentioned structure can be considered as a sensor for measuring the angular velocity of the cylinder subjected to the pressure and temperature. The obtained results indicate that the electrical potential is proportional to the angular velocity.

Morphological Engineering of Sensing Materials for Flexible Pressure Sensors and Artificial Intelligence Applications

As an indispensable branch of wearable electronics,flexible pressure sensors are gaining tremendous attention due to their extensive applications in health monitoring,human-machine interaction,artificial intelligence,the internet of things,and other fields.In recent years,highly flexible and wearable pressure sensors have been developed using various materials/structures and transduction mechanisms.Morphological engineering of sensing materials at the nanometer and micrometer scales is crucial to obtaining superior sensor performance.This review focuses on the rapid development of morphological engineering technologies for flexible pressure sensors.We discuss different architectures and morphological designs of sensing materials to achieve high performance,including high sensitivity,broad working range,stable sensing,low hysteresis,high transparency,and directional or selective sensing.Additionally,the general fabrication techniques are summarized,including self-assembly,patterning,and auxiliary synthesis methods.Furthermore,we present the emerging applications of high-performing microengineered pressure sensors in healthcare,smart homes,digital sports,security monitoring,and machine learning-enabled computational sensing platform.Finally,the potential challenges and prospects for the future developments of pressure sensors are discussed comprehensively.

Pressure effects in AlAs/Inx Ga1-xAs/GaAs resonant tunnelling diodes for application in micromachined sensors

This paper reports the current-voltage characteristics of [001]-oriented AlAs/InxGa1-xAs/GaAs resonant tunnelling diodes （RTDs） as a function of uniaxial external stress applied parallel to the [110] and the [1^-10] orientations, and the output characteristics of the GaAs pressure sensor based on the pressure effect on the RTDs. Under [110] stress, the resonance peak voltages of the RTDs shift to more positive voltages. For [1^-10] stress, the peaks shift toward more negative voltages. The resonance peak voltage is linearly dependent on the [110] and [1^-0] stresses and the linear sensitivities are up to 0.69 mV/MPa, -0.69 mV/MPa respectively. For the pressure sensor, the linear sensitivity is up to 0.37 mV/kPa.

An Optical Pressure Sensor Based on MEMS

An optical fiber pressure sensor has been developed for the measurement in human body.The sensing element is possessed of a membrane structure,which is fabricated by micromachining.The fabrication process includes anisotropic wet etching on the silicon wafer.For the transmitting source and signal light,a multimode optical fiber 50/125 μm (core/clad)in diameter was used.The intensity of the light reflected back into the fiber varies with the membrane deflection,which is s function of pressure.The deflection of the membrane by applied pressure can be mathematically described.

Artificial Tactile Sense Technique for Predicting Beef Tenderness Based on FS Pressure Sensor

We present a rapid system for predicting beef tenderness by mimicking the human tactile sense. The detection system includes a FS pressure sensor, a power supply conversion circuit, a signal amplifier and a box in which the sample is mounted. A sample of raw Longissimus dorsi (LD) muscle is placed in the measuring box; then a rod connected to the pressure sensor is pressed into the beef sample to a given depth; the reaction force of the beef sample is measured and used to predict the tenderness. Sensory evaluation and Warner-Bratzler Shear Force (WBSF) evaluation of samples from the same LD muscle are used for comparison. The new detection system agrees with established procedure 95% of the time, and the time to test a sample is less than 5 minutes.

Self-Assembly 3D Porous Crumpled MXene Spheres as Efficient Gas and Pressure Sensing Material for Transient All-MXene Sensors

Environmentally friendly degradable sensors with both hazardous gases and pressure efficient sensing capabilities are highly desired for various promising applications,including environmental pollution monitoring/prevention,wisdom medical,wearable smart devices,and artificial intelligence.However,the transient gas and pressure sensors based on only identical sensing material that concurrently meets the above detection needs have not been reported.Here,we present transient all-MXene NO_(2) and pressure sensors employing three-dimensional porous crumpled MXene spheres prepared by ultrasonic spray pyrolysis technology as the sensing layer,accompanied with water-soluble polyvinyl alcohol substrates embedded with patterned MXene electrodes.The gas sensor achieves a ppb-level of highly selective NO_(2) sensing,with a response of up to 12.11%at 5 ppm NO_(2) and a detection range of 50 ppb-5 ppm,while the pressure sensor has an extremely wide linear pressure detection range of 0.14-22.22 kPa and fast response time of 34 ms.In parallel,all-MXene NO_(2) and pressure sensors can be rapidly degraded in medical H_(2)O_(2) within 6 h.This work provides a new avenue toward environmental monitoring,human physiological signal monitoring,and recyclable transient electronics.

Highly Sensitive Pseudocapacitive Iontronic Pressure Sensor with Broad Sensing Range

Flexible pressure sensors are unprecedentedly studied on monitoring human physical activities and robotics.Simultaneously,improving the response sensitivity and sensing range of flexible pressure sensors is a great challenge,which hinders the devices’practical application.Targeting this obstacle,we developed a Ti_(3)C_(2)T_(x)-derived iontronic pressure sensor(TIPS)by taking the advantages of the high intercalation pseudocapacitance under high pressure and rationally designed structural configuration.TIPS achieved an ultrahigh sen-sitivity(S_(min)>200 kPa^(−1),S_(max)>45,000 kPa^(−1))in a broad sensing range of over 1.4 MPa and low limit of detection of 20 Pa as well as stable long-term working durability for 10,000 cycles.The practical application of TIPS in physical activity monitoring and flexible robot manifested its versatile potential.This study provides a demonstration for exploring pseudocapacitive materials for building flexible iontronic sensors with ultrahigh sensitivity and sensing range to advance the development of high-performance wearable electronics.

Wireless contactless pressure measurement of an LC passive pressure sensor with a novel antenna for high-temperature applications

In this paper, a novel antenna is proposed for high-temperature testing, which can make the high-temperature pressure characteristics of a wireless passive ceramic pressure sensor demonstrated at up to a temperature of 600℃. The design parameters of the antenna are similar to those of the sensor, which will increase the coupling strength between the sensor and testing antenna. The antenna is fabricated in thick film integrated technology, and the properties of the alumina ceramic and silver ensure the feasibility of the antenna in high-temperature environments. The sensor, coupled with the ceramic antenna, is investigated using a high-temperature pressure testing platform. The experimental measurement results show that the pressure signal in a harsh environment can be detected by the frequency diversity of the sensor.

THICKNESS-SHEAR VIBRATION OF CIRCULAR CRYSTAL PLATE IN CYLINDRICAL SHELL AS PRESSURE SENSOR

Based on the theory for small fields superposed on relatively larger fields in an electroelastic body, a theoretical analysis is performed on a circular plate thicknessshear crystal resonator sealed in a circular cylindrical shell for pressure measurement. A simple expression is obtained for pressure induced frequency shifts in the resonator, which is examined for design optimization. Numerical results show that the frequency shifts depend linearly on the pressure, and that or a smaller thickness ratio of the crystal plate a pressure sensor with a softer outer shell to the outer shell has higher sensitivity.

Dielectric Properties of Capacitive-Type Humidity Sensor Made under Different Pressures

Capacitive humidity sensors were made of nanometer barium titanate.The pellets were prepared under different pressures between 3920N to 7850N force.The capacitance changes in three orders of magnitude in the relative humidity range of 10%～98%,indicating high humidity sensitivity of the sensors.At a certain measuring frequency,the capacitance of the sensors increases as increasing of the preparation pressure,while the sensitivity of the sensors basically remains the same.The frequencies corresponding to the peaks of the dielectric loss of the sensors move to the higher frequency direction as increasing of the relative humidity.At a certain humidity,the frequencies corresponding to the peaks of the dielectric loss move to the higher frequency direction as increasing of the preparation pressure.