Highly Sensitive and Ultrastable Skin Sensors for Biopressure and Bioforce Measurements Based on Hierarchical Microstructures

PVA/PANI-DBSA Nanomesh Tactile Sensor for Force Feedback

Touch serves as an important medium for human-environment interaction. The piezoresistive tactile sensor has attracted much attention due to its convenient technology, simple principle, and convenient signal acquisition and analysis. In this paper, conductive beads-on-string polyvinyl alcohol (PVA)/polyaniline doped with dodecyl benzene sulfonic acid (PANI-DBSA) nanofibers were fabricated via the electrospinning technique. Due to the special nanostructure of PVA-coated PANI-DBSA, the tactile sensor presented a wide measuring range of 12 Pa-121 kPa and appreciable sensitivity of 8.576 kPa-1 at 12 Pa~484 Pa. In addition, the response time and recovery time of the sensor were approximately 500 ms, demonstrating promising prospects in the field of tactile sensing for active upper limb prostheses.

Three-Dimensional Touch Device with Two Terminals

A crossbar array is an essential element that determines the operating position and simplifies the structure of devices. However, in the crossbar array, wiring numerous electrodes to address many positions poses significant challenges. In this study, a method is proposed that utilizes only two electrodes to determine multiple positions. The method significantly simplifies the wiring and device fabrication process. Instead of defining the node location of the crossbar, it is experimentally demonstrated that the x-y-z coordinates can be determined from i) the resistance change as a function of distance, ii) the resistance variation influenced by the electrode composition, and iii) capacitance fluctuation resulting from changes in the dielectric thickness. By employing two-terminal transparent electrodes, a fully functional 3D touch device is successfully fabricated, introducing a groundbreaking approach to simplify input device architectures.

Skin-Inspired Highly Sensitive Tactile Sensors with Ultrahigh Resolution over a Broad Sensing Range

Flexible tactile sensors with high sensitivity, a broad pressure detection range, and high resolution are highly desired for the applications of health monitoring, robots, and the human-machine interface. However, it is still challenging to realize a tactile sensor with high sensitivity and resolution over a wide detection range. Herein, to solve the abovementioned problem, we demonstrate a universal route to develop a highly sensitive tactile sensor with high resolution and a wide pressure range. The tactile sensor is composed of two layers of microstructured flexible electrodes with high modulus and conductive cotton fabric with low modulus. By optimizing the sensing films, the fabricated tactile sensor shows a high sensitivity of 8.9 × 10⁴ kPa^-1 from 2 Pa to 250 kPa because of the high structural compressibility and stress adaptation of the multilayered composite films. Meanwhile, a fast response speed of 18 ms, an ultrahigh resolution of 100 Pa over 100 kPa, and excellent durability over 20 000 loading/unloading cycles are demonstrated. Moreover, a 6 × 6 tactile sensor array is fabricated and shows promising potential application in electronic skin (e-skin). Therefore, employing multilayered composite films for tactile sensors is a novel strategy to achieve high-performance tactile perception in real-time health monitoring and artificial intelligence.

Recent progress in flexible micro-pressure sensors for wearable health monitoring

In recent years, flexible micro-pressure sensors have been used widely in wearable health monitoring applications due to their excellent flexibility, stretchability, non-invasiveness, comfort wearing and real-time detection. According to the working mechanism of the flexible micro-pressure sensor, it can be classified as piezoresistive, piezoelectric, capacitive and triboelectric types. Herein, an overview of flexible micro-pressure sensors for wearable health monitoring is presented. The physiological signaling and body motions contain a lot of health status information. Thus, this review focuses on the applications of flexible micro-pressure sensors in these fields. Additionally, the contents of sensing mechanism, sensing materials and performance of flexible micro-pressure sensors are introduced in detail. Finally, we predict the future research directions of the flexible micro-pressure sensors, and discuss the challenges in practical applications.

Emerging Functional Polymer Composites for Tactile Sensing

In recent years, extensive research has been conducted on the development of high-performance flexible tactile sensors, pursuing the next generation of highly intelligent electronics with diverse potential applications in self-powered wearable sensors, human-machine interactions, electronic skin, and soft robotics. Among the most promising materials that have emerged in this context are functional polymer composites (FPCs), which exhibit exceptional mechanical and electrical properties, enabling them to be excellent candidates for tactile sensors. Herein, this review provides a comprehensive overview of recent advances in FPCs-based tactile sensors, including the fundamental principle, the necessary property parameter, the unique device structure, and the fabrication process of different types of tactile sensors. Examples of FPCs are elaborated with a focus on miniaturization, self-healing, self-cleaning, integration, biodegradation, and neural control. Furthermore, the applications of FPC-based tactile sensors in tactile perception, human-machine interaction, and healthcare are further described. Finally, the existing limitations and technical challenges for FPCs-based tactile sensors are briefly discussed, offering potential avenues for the development of electronic products.

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.

Ultra-Sensitive and Wide Sensing-Range Flexible Pressure Sensors Based on the Carbon Nanotube Film/Stress-Induced Square Frustum Structure

Flexible pressure sensors have attracted much attention due to their significant potentials in E-skin, artificial intelligence, and medical health monitoring. However, it still remains challenging to achieve high sensitivity and wide sensing range simultaneously, which greatly limit practical applications for flexible sensors. Inspired by the surface stress-induced structure of mimosa, we propose a novel flexible sensor based on the carbon nanotube paper film (CNTF) and stress-induced square frustum structure (SSFS) and demonstrated their excellent sensing performances. Based on interdigital electrodes and uniform CNTF consisting of fibers with large specific surface area, rich conductive paths are formed for enhanced resistance variation. Besides, both experiments and modeling are conducted to verify the synergistic effect of substrates with diverse stiffnesses and SSFS. The SSFS of polydimethylsiloxane transfer small pressure to the CNTF, resulting in sensitive responses with a broad resistance variation. The sensor achieves an ultrahigh sensitivity (2027.5 kPa^-1) and a wide pressure range (0.0003-200 kPa). Therefore, it can not only detect human signals such as pulse, vocal cord vibration, wrist flexion, and foot pressure but also be integrated onto car tires to monitor vehicle statuses. These fascinating features endow the sensors with great potentials for future health monitoring, human-computer interaction, and virtual reality.

Conjugated Polymer-Based Nanocomposites for Pressure Sensors

Flexible sensors are the essential foundations of pressure sensing, microcomputer sensing systems, and wearable devices. The flexible tactile sensor can sense stimuli by converting external forces into electrical signals. The electrical signals are transmitted to a computer processing system for analysis, realizing real-time health monitoring and human motion detection. According to the working mechanism, tactile sensors are mainly divided into four types-piezoresistive, capacitive, piezoelectric, and triboelectric tactile sensors. Conventional silicon-based tactile sensors are often inadequate for flexible electronics due to their limited mechanical flexibility. In comparison, polymeric nanocomposites are flexible and stretchable, which makes them excellent candidates for flexible and wearable tactile sensors. Among the promising polymers, conjugated polymers (CPs), due to their unique chemical structures and electronic properties that contribute to their high electrical and mechanical conductivity, show great potential for flexible sensors and wearable devices. In this paper, we first introduce the parameters of pressure sensors. Then, we describe the operating principles of resistive, capacitive, piezoelectric, and triboelectric sensors, and review the pressure sensors based on conjugated polymer nanocomposites that were reported in recent years. After that, we introduce the performance characteristics of flexible sensors, regarding their applications in healthcare, human motion monitoring, electronic skin, wearable devices, and artificial intelligence. In addition, we summarize and compare the performances of conjugated polymer nanocomposite-based pressure sensors that were reported in recent years. Finally, we summarize the challenges and future directions of conjugated polymer nanocomposite-based sensors.

Interlinked Microcone Resistive Sensors Based on Self-Assembly Carbon Nanotubes Film for Monitoring of Signals

Flexible pressure sensors still face difficulties achieving a constantly adaptable micronanostructure of substrate materials. Interlinked microcone resistive sensors were fabricated by polydimethylsiloxane (PDMS) nanocone array. PDMS nanocone array was achieved by the second transferring tapered polymethyl methacrylate (PMMA) structure. In addition, self-assembly 2D carbon nanotubes (CNTs) networks as a conducting layer were prepared by a low-cost, dependable, and ultrafast Langmuir−Blodgett (LB) process. In addition, the self-assembled two-dimensional carbon nanotubes (CNTs) network as a conductive layer can change the internal resistance due to pressure. The results showed that the interlinked sensor with a nanocone structure can detect the external pressure by the change of resistivity and had a sensitive resistance change in the low pressure (<200 Pa), good stability through 2800 cycles, and a detection limit of 10 kPa. Based on these properties, the electric signals were tested, including swallowing throat, finger bending, finger pressing, and paper folding. The simulation model of the sensors with different structural parameters under external pressure was established. With the advantages of high sensitivity, stability, and wide detection range, this sensor shows great potential for monitoring human motion and can be used in wearable devices.

Highly Compressible and Sensitive Flexible Piezoresistive Pressure Sensor Based on MWCNTs/Ti3C2Tx MXene @ Melamine Foam for Human Gesture Monitoring and Recognition

Flexible sensing devices provide a convenient and effective solution for real-time human motion monitoring, but achieving efficient and low-cost assembly of pressure sensors with high performance remains a considerable challenge. Herein, a highly compressible and sensitive flexible foam-shaped piezoresistive pressure sensor was prepared by sequential fixing multiwalled carbon nanotubes and Ti3C2Tx MXene on the skeleton of melamine foam. Due to the porous skeleton of the melamine foam and the extraordinary electrical properties of the conductive fillers, the obtained MWCNTs/Ti3C2Tx MXene @ melamine foam device features high sensitivity of 0.339 kPa−1, a wide working range up to 180 kPa, a desirable response time and excellent cyclic stability. The sensing mechanism of the composite foam device is attributed to the change in the conductive pathways between adjacent porous skeletons. The proposed sensor can be used successfully to monitor human gestures in real-time, such as finger bending and tilting, scrolling the mouse and stretching fingers. By combining with the decision tree algorithm, the sensor can unambiguously classify different Arabic numeral gestures with an average recognition accuracy of 98.9%. Therefore, our fabricated foam-shaped sensor may have great potential as next-generation wearable electronics to accurately acquire and recognize human gesture signals in various practical applications.

Multi-height micropyramids based pressure sensor with tunable sensing properties for robotics and step tracking applications

Precise sensing of pressure is essential for various mechanical and electrical systems. The recent emergence of flexible pressure sensors has enabled novel applications, such as human–machine interfaces, soft robotics, and wearable devices. Specifically, the piezoresistive sensing scheme is widely adapted for flexible pressure sensors as it is simple and exhibits outstanding measurement sensitivity and stability. The sensing properties of piezoresistive pressure sensors mainly depends on the materials and contact morphologies at the interface. This paper proposes a flexible pressure sensor based on multi-height microstructures in which the measurement sensitivity and detection range are tunable. Such tunability is due to the sequential contact of micropyramids with different heights. The multi-height micropyramid structured PDMS layer with stamp-coated multi-walled carbon nanotubes (MWCNTs) acts as a conductive active layer and a gold interdigitated electrode (IDE) patterned polyimide (PI) layer works as the bottom electrode. The fabricated sensor exhibits a sensitivity of 0.19 kPa−1, a fast response speed of 20 ms, and a detection range of up to 100 kPa. The sensor is applied to a robotic gripper for object recognition and integrated into a shoe to track walking motions.

Self-Powered Multifunctional Electronic Skin Based on Carbon Nanotubes/Poly(dimethylsiloxane) for Health Monitoring

Flexible and multifunctional electronic skin (e-skin) has received remarkable attention for its potential applications in health monitoring, human-machine interface, and artificial sensory nerves. However, conventional multifunctional e-skins require complex material systems, sophisticated fabrication, and external power supplies, leading to increased preparation cost and duration, thus hindering their large-scale utilization. Herein, a self-powered multifunctional e-skin system with properties of pressure, temperature, underwater sensing, and photothermal heating is designed based on carbon nanotubes/poly(dimethylsiloxane) (CNT/PDMS) acting as both the multifunctional sensing layer and the cathode of the power supply. Our micropyramidal structured e-skin exhibits outstanding pressure sensitivity (1.51 × 10³ kPa^-1) over a wide sensing range (2.5-255.7 kPa) and maintains ultralong-term durability (>20 000 cycles). It can also provide personalized photothermal therapy at an adjustable temperature (40-110 °C) and heating area under near-infrared irradiation due to the photothermal effect of CNTs, with the temperature being detected synchronously by current signals. Additionally, the hydrophobicity of the CNT/PDMS film endows our device with underwater sensing capability. Furthermore, practical healthcare applications have been demonstrated with reliable signal quality and stability, such as daily activities and underwater movements/temperature monitoring, SOS Morse code communication, and human-machine interface. This work could provide insight on developing simple, stable, and wearable healthcare devices with self-power supply and multifunction.

Wearable Pressure Sensors for Pulse Wave Monitoring

Cardiovascular diseases remain the leading cause of death worldwide. The rapid development of flexible sensing technologies and wearable pressure sensors have attracted keen research interest and have been widely used for long-term and real-time cardiovascular status monitoring. Owing to compelling characteristics, including light weight, wearing comfort, and high sensitivity to pulse pressures, physiological pulse waveforms can be precisely and continuously monitored by flexible pressure sensors for wearable health monitoring. Herein, an overview of wearable pressure sensors for human pulse wave monitoring is presented, with a focus on the transduction mechanism, microengineering structures, and related applications in pulse wave monitoring and cardiovascular condition assessment. The conceptualizations and methods for the acquisition of physiological and pathological information related to the cardiovascular system are outlined. The biomechanics of arterial pulse waves and the working mechanism of various wearable pressure sensors, including triboelectric, piezoelectric, magnetoelastic, piezoresistive, capacitive, and optical sensors, are also subject to systematic debate. Exemple applications of pulse wave measurement based on microengineering structured devices are then summarized. Finally, a discussion of the opportunities and challenges that wearable pressure sensors face, as well as their potential as a wearable intelligent system for personalized healthcare is given in conclusion.

Flexible Piezoresistive Pressure Sensor Based on Electrospun Rough Polyurethane Nanofibers Film for Human Motion Monitoring

Flexible piezoresistive pressure sensors have been attracted a lot of attention due to their simple mechanism, easy fabrication, and convenient signal acquisition and analysis. Herein, a new flexible piezoresistive sensor based on microstructured electrospun rough polyurethane (PU) nanofibers film is assembled. The microstructured PU film with tiny bumps is prepared in one step via electrospinning technology, which imparts a microstructured rough upper surface and a smooth lower surface. With this unique microstructure, we have made it possible for PU/Ag films to serve as sensing layers for piezoresistive sensors by introducing a silver conductive layer on the surface of electrospun PU film. The fabricated piezoresistive pressure sensor delivers high sensitivity (10.53 kPa⁻¹ in the range of 0–5 kPa and 0.97 kPa⁻¹ in the range of 6–15 kPa), fast response time (60 ms), fast recovery time (30 ms), and long-time stability (over 10,000 cycles). This study presents a fabrication strategy to prepare the microstructured PU nanofiber film using electrospinning technology directly, and the as-developed sensor shows promise in applications such as wrist pulse measurement, finger movement monitoring, etc., proving its great potential for monitoring human activities.

Neural Network-Enabled Flexible Pressure and Temperature Sensor with Honeycomb-like Architecture for Voice Recognition

Flexible pressure sensors have been studied as wearable voice-recognition devices to be utilized in human-machine interaction. However, the development of highly sensitive, skin-attachable, and comfortable sensing devices to achieve clear voice detection remains a considerable challenge. Herein, we present a wearable and flexible pressure and temperature sensor with a sensitive response to vibration, which can accurately recognize the human voice by combing with the artificial neural network. The device consists of a polyethylene terephthalate (PET) printed with a silver electrode, a filament-microstructured polydimethylsiloxane (PDMS) film embedded with single-walled carbon nanotubes and a polyimide (PI) film sputtered with a patterned Ti/Pt thermistor strip. The developed pressure sensor exhibited a pressure sensitivity of 0.398 kPa−1 in the low-pressure regime, and the fabricated temperature sensor shows a desirable temperature coefficient of resistance of 0.13% ∘C in the range of 25 ∘C to 105 ∘C. Through training and testing the neural network model with the waveform data of the sensor obtained from human pronunciation, the vocal fold vibrations of different words can be successfully recognized, and the total recognition accuracy rate can reach 93.4%. Our results suggest that the fabricated sensor has substantial potential for application in the human-computer interface fields, such as voice control, vocal healthcare monitoring, and voice authentication.

Highly Sensitive Flexible Tactile Sensors in Wide Sensing Range Enabled by Hierarchical Topography of Biaxially Strained and Capillary-Densified Carbon Nanotube Bundles

Flexible tactile sensors with high sensitivity have received considerable attention for their use in wearable electronics, human-machine interfaces, and health-monitoring devices. Although various micro/nanostructured materials are introduced for high-performance tactile sensors, simultaneously obtaining high sensitivity and a wide sensing range remains challenging. Here, a resistive tactile sensor is presented based on the hierarchical topography of carbon nanotubes (CNTs) prepared by a low-cost and straightforward manufacturing process. The 3D hierarchical structure of the CNTs over large areas is formed by transferring vertically aligned CNT bundles to a prestrained elastomer substrate and subsequently densifying them through capillary forming, providing a monotonic increase in the contact area as applied pressure. The deformable and hierarchical structure of CNTs allows the sensor to exhibit a wide sensing range (0-100 kPa), high sensitivity (141.72 kPa^-1 ), and low detection limit (10 Pa). Additionally, the capillary-formed CNT structure results in increased durability of the sensor over repeated pressures. Based on these advantages, meaningful applications of tactile sensors, such as object recognition gloves and multidirectional force perceptions, are successfully realized. Given the scalable fabrication method, 3D hierarchically structured CNTs provide an essential step toward next-generation wearable devices.

Graded Microstructured Flexible Pressure Sensors with High Sensitivity and an Ultrabroad Pressure Range for Epidermal Pulse Monitoring

Precisely detecting epidermal pulse waves with pressure sensors is crucial for pulse-based personalized health-monitoring technologies. However, developing a pressure sensor that simultaneously demonstrates high sensitivity and an ultrabroad pressure range and a convenient fabrication process for large-scale production is a considerable challenge. Herein, by utilizing a commercial conductive fabric (CF) and a silica gel film, we develop a high-performance pressure sensor (HPPS) for the monitoring of human physiological signals. Based on convenient turnover formwork technology, the silica gel film was fabricated by replicating the microstructure of the sandpaper surface. This microstructure and the plain weave structure on the CF surface together provide a sharp increase in the contact-separation area and structural compressibility, which are beneficial for the enhancement of output performance. Made of these two materials, the graded microstructured HPPS holds high sensitivity (4.5 mV/Pa), an ultrabroad pressure range (0-30 kPa), a wide working frequency bandwidth (up to 35 Hz), decent stability (>50,000 cycles), and a simple fabrication process that is suitable for large-scale production. Given these noticeable features, the developed HPPS not only succeeds in precisely detecting subtle pulse waves on various positions of different people but can also objectively capture changes in cardiovascular parameters caused by exercise training at different intensities in real time. These findings exhibit the enormous potential application of HPPS in tracking an individual's health status and comprehensively evaluating exercise intensity.

One-Step Synthesis of Microdome Patterns for Microstructured Pressure Sensors with Ultra-High Sensing Performance

Pressure sensors usually suffer from a trade-off between sensitivity and the linear sensing range, which may be improved by manipulating the geometric microstructure of active sensing materials via the molding strategy, standard photolithography technique, and so on. However, these conventional microengineering techniques require specialized equipment, which are extremely complicated, high-cost, and time-consuming to manufacture. Herein, a mold-free, scalable, low-cost, and environment-friendly one-step thermofoaming strategy is proposed to fabricate surface morphology-tunable microdome-patterned composites (MPCs). The microstructured pressure sensor is then prepared by coating the MPCs with highly conductive graphene. Remarkably, the as-prepared pressure sensor presents a better overall sensing performance compared to the previous pressure sensors prepared using complicated microengineering methods. Moreover, an electromechanical response model and finite-element analysis are used to clarify the sensing mechanisms of the present microstructured pressure sensor. Furthermore, several successful application demonstrations are conducted under various pressure levels. Considering the advantages of the one-step fabrication strategy over conventional surface microengineering techniques and the high performance of the microstructured pressure sensor, the present pressure sensor has promising potential applications in health monitoring, tactile sensation, wearable devices, etc.

A porous PDMS pulsewave sensor with haircell structures for water vapor transmission rate and signal-to-noise ratio enhancement

We present a porous polydimethylsiloxane (PDMS) pulsewave sensor with haircell structures that improves both water vapor transmission rate (WVTR) and signal-to-noise ratio (SNR). The conventional planar PDMS pulsewave sensors have the problems of low WVTR and low SNR for real-time and long-term pulsewave monitoring. In order to improve WVTR, we fabricated a porous PDMS layer with the thickness of 40 μm and high porosity of 45% by crystallizing and dissolving citric acid powders in PDMS. On the porous PDMS layer, we form haircell structures to increase the skin contact area, thus enhancing SNR. The porous PDMS pulsewave sensor with haircell structures achieved an enhanced WVTR of 486.17 g^-1 d^-1 m^-2 and an SNR of 22.89, respectively, 72% and 757% higher than those of the conventional PDMS pulsewave sensors without haircell structures. Furthermore, the enhanced WVTR is 13% higher than the human skin sweat rate of 432 g^-1 d^-1 m^-2. The present pulsewave sensor shows strong potential for applications in real-time and long-term pulsewave monitoring with the lower skin irritation and the enhanced SNR.

Recent Progress in Flexible Microstructural Pressure Sensors toward Human-Machine Interaction and Healthcare Applications

With the rapid growth of artificial intelligence, wearable electronic devices have caught intensive research interest recently. Flexible sensors, as the significant part of them, have become the focus of research. Particularly, flexible microstructural pressure sensors (FMPSs) have attracted extensive attention because of their controllable shape, small size, and high sensitivity. Microstructures are of great significance to improve the sensitivity and response time of FMPSs. The FMPSs present great application prospects in medical health, human-machine interaction, electronic products, and so on. In this review, a series of microstructures (e.g., wave, pillar, and pyramid shapes) which have been elaborately designed to effectively enhance the sensing performance of FMPSs are introduced in detail. Various fabrication strategies of these FMPSs are comprehensively summarized, including template (e.g., silica, anodic aluminum oxide, and bionic patterns), pre-stressing, and magnetic field regulation methods. In addition, the materials (e.g., carbon, polymer, and piezoelectric materials) used to prepare FMPSs are also discussed. Moreover, the potential applications of FMPSs in human-machine interaction and healthcare fields are emphasized as well. Finally, the advantages and latest development of FMPSs are further highlighted, and the challenges and potential prospects of high-performance FMPSs are outlined.

Robust Flexible Pressure Sensors Made from Conductive Micropyramids for Manipulation Tasks

Flexible pressure sensors that can robustly mimic the function of slow-adapting type I (SA-I) mechanoreceptors are essential for realizing human-like object manipulation in artificial intelligent (AI) robots or amputees. Here, we report a straightforward approach to highly sensitive and robust flexible pressure sensors with fast response time and low operating voltage based on conductive micropyramids made of polydimethylsiloxane/carbon nanotube composites. Both numerical simulations and experimental studies show that the pressure-sensing properties of the devices can be systematically tuned by the spatial arrangement of micropyramids. In particular, by tailoring the ratio between the spacing and the pyramidal base length, the optimal pressure sensors can be achieved with a combination of high sensitivity in both low-pressure (<10 kPa) and medium-pressure (10-100 kPa) regimes, rapid response, high mechanical robustness, low operating voltage, and low power consumption, along with linear response and low hysteresis in the medium-pressure regimes. The optimized pressure sensor is further used for constructing a wearable pressure-sensing system that can convert the amplitude of pressure to wirelessly transmittable frequency signals (spikes) with nearly linear response, closely mimicking SA-I mechanoreceptors. Furthermore, we demonstrate that the high uniformity and scalability of the pressure sensors enable large-area pressure-sensing arrays for spatially resolved pressure mapping.

Highly Sensitive Flexible Iontronic Pressure Sensor for Fingertip Pulse Monitoring

The pulse is a key biomedical signal containing various human physiological and pathological information highly related to cardiovascular diseases. Pulse signals are often collected from the radial artery based on Traditional Chinese Medicine, or by using flexible pressure sensors. However, the wrist wrapped with a flexible pressure sensor exhibits unstable signals under hand motion because of the concave surface of the wrist. By contrast, fingertips have a convex surface and therefore show great promises in stable and long-term pulse monitoring. Despite the promising potential, the fingertip pulse signal is weak, calling for highly sensitive detecting devices. Here, a highly sensitive and flexible iontronic pressure sensor with a linear sensitivity of 13.5 kPa^-1 , a swift response, and remarkable stability over 5000 loading/unloading cycles is developed. This sensor enables stable and high-resolution detection of pulse waveform under both static condition and finger motion. Fingertip pulse waveforms from subjects of different genders, age, and health conditions are collected and analyzed, suggesting that fingertip pulse information is highly similar to that of the radial artery. This work justifies that fingertip is an ideal platform for pulse signals monitoring, which would be a competitive alternative to existing complex health monitoring systems.

Transduction Mechanisms, Micro-Structuring Techniques, and Applications of Electronic Skin Pressure Sensors: A Review of Recent Advances

Electronic skin (e-skin), which is an electronic surrogate of human skin, aims to recreate the multifunctionality of skin by using sensing units to detect multiple stimuli, while keeping key features of skin such as low thickness, stretchability, flexibility, and conformability. One of the most important stimuli to be detected is pressure due to its relevance in a plethora of applications, from health monitoring to functional prosthesis, robotics, and human-machine-interfaces (HMI). The performance of these e-skin pressure sensors is tailored, typically through micro-structuring techniques (such as photolithography, unconventional molds, incorporation of naturally micro-structured materials, laser engraving, amongst others) to achieve high sensitivities (commonly above 1 kPa⁻¹), which is mostly relevant for health monitoring applications, or to extend the linearity of the behavior over a larger pressure range (from few Pa to 100 kPa), an important feature for functional prosthesis. Hence, this review intends to give a generalized view over the most relevant highlights in the development and micro-structuring of e-skin pressure sensors, while contributing to update the field with the most recent research. A special emphasis is devoted to the most employed pressure transduction mechanisms, namely capacitance, piezoelectricity, piezoresistivity, and triboelectricity, as well as to materials and novel techniques more recently explored to innovate the field and bring it a step closer to general adoption by society.

Flexible Piezoresistive Sensors with Wide-Range Pressure Measurements Based on a Graded Nest-like Architecture

Flexible pressure sensors present great potential in the application of human health monitoring, tactile function of prosthesis, and electronic skin for robotics. These applications require different trade-off between the sensitivity and sensing range, therefore, it is imperative to develop range-specific sensitivities in a single sensor. In this paper, a bioinspired strategy for a resistive pressure sensor using a graded porous material is proposed to measure pressures from several pascals to megapascals. Its fabrication is based on an easily accessible template method. The nest-architecture-based wide-range pressure sensor exhibits adequate sensitivity under an extensive pressure regime (20 Pa to 1.2 MPa). In addition, with rational structural design and subtle engineering of the material properties, the sensor achieves remarkable mechanical stability. To prove the concept, sensors were attached on a bicycle wheel to monitor the tire-pavement pressure and on human skin to detect biosignals such as venous and arterial blood pressure pulses.

A Low-Cost, Flexible Pressure Capacitor Sensor Using Polyurethane for Wireless Vehicle Detection

Detection of vehicles on the road can contribute to the establishment of an intelligent transportation management system to allow smooth transportation and the reduction of road accidents. Thus far, an efficient and low-cost polymer flexible pressure sensor for vehicle detection is lacking. This paper presents a flexible sensor for vehicle sensing and demonstrates a wireless system for monitoring vehicles on the road. A vehicle sensor was fabricated by sandwiching a polyurethane material between aluminum top/bottom electrodes. The sensing mechanism was based on changes in capacitance due to variation in the distance between the two electrodes at an applied external pressure. A clear response against a pressure load of 0.65 Mpa was observed, which is the same pressure as that of the car tire area in contact with the road. Significantly, the sensor was easy to embed on the road line due to its mechanical flexibility and large size. A field test was carried out by embedding the sensor on the road and crossing the sensor with a car. Moreover, the signal displayed on the tablet indicated that the sensing system can be used for wireless detection of the axle, speed, or weight of the vehicle on the road. The findings suggest that the flexible pressure sensor is a promising tool for use as a low-cost vehicle detector in future intelligent transportation management.

Using Artificial Skin Devices as Skin Replacements: Insights into Superficial Treatment

Artificial skin devices are able to mimic the flexibility and sensory perception abilities of the skin. They have thus garnered attention in the biomedical field as potential skin replacements. This Review delves into issues pertaining to these skin-deep devices. It first elaborates on the roles that these devices have to fulfill as skin replacements, and identify strategies that are used to achieve such functionality. Following which, a comparison is done between the current state of these skin-deep devices and that of natural skin. Finally, an outlook on artificial skin devices is presented, which discusses how complementary technologies can create skin enhancements, and what challenges face such devices.

Multilevel Microstructured Flexible Pressure Sensors with Ultrahigh Sensitivity and Ultrawide Pressure Range for Versatile Electronic Skins

Flexible pressure sensors as electronic skins have attracted wide attention to their potential applications for healthcare and intelligent robotics. However, the tradeoff between their sensitivity and pressure range restricts their practical applications in various healthcare fields. Herein, a cost-effective flexible pressure sensor with an ultrahigh sensitivity over an ultrawide pressure-range is developed by combining a sandpaper-molded multilevel microstructured polydimethylsiloxane and a reduced oxide graphene film. The unique multilevel microstructure via a two-step sandpaper-molding method leads to an ultrahigh sensitivity (2.5-1051 kPa^-1 ) and can detect subtle and large pressure over an ultrawide range (0.01-400 kPa), which covers the overall pressure regime in daily life. Sharp increases in the contact area and additional contact sites caused by the multilevel microstructures jointly contribute to such unprecedented performance, which is confirmed by in situ observation of the gap variations and the contact states of the sensor under different pressures. Examples of the flexible pressure sensors are shown in potential applications involving the detection of various human physiological signals, such as breathing rate, vocal-cord vibration, heart rate, wrist pulse, and foot plantar pressure. Another object manipulation application is also demonstrated, where the material shows its great potential as electronic skin intelligent robotics and prosthetic limbs.

Recent Advances in Flexible and Wearable Pressure Sensors Based on Piezoresistive 3D Monolithic Conductive Sponges

High-performance flexible strain and pressure sensors are important components of the systems for human motion detection, human-machine interaction, soft robotics, electronic skin, etc., which are envisioned as the key technologies for applications in future human healthcare monitoring and artificial intelligence. In recent years, highly flexible and wearable strain/pressure sensors have been developed based on various materials/structures and transduction mechanisms. Piezoresistive three-dimensional (3D) monolithic conductive sponge, the resistance of which changes upon external pressure or stimuli, has emerged as a forefront material for flexible and wearable pressure sensor due to its excellent sensor performance, facile fabrication, and simple circuit integration. This review focuses on the rapid development of the piezoresistive pressure sensors based on 3D conductive sponges. Various piezoresistive conductive sponges are categorized into four different types and their material and structural characteristics are summarized. Methods for preparation of the 3D conductive sponges are reviewed, followed by examples of device performance and selected applications. The review concludes with a critical reflection of the current status and challenges. Prospects of the 3D conductive sponge for flexible and wearable pressure sensor are discussed.