Flexible Ferroelectric Sensors with Ultrahigh Pressure Sensitivity and Linear Response over Exceptionally Broad Pressure Range

Artificial organic afferent nerves enable closed-loop tactile feedback for intelligent robot

The emulation of tactile sensory nerves to achieve advanced sensory functions in robotics with artificial intelligence is of great interest. However, such devices remain bulky and lack reliable competence to functionalize further synaptic devices with proprioceptive feedback. Here, we report an artificial organic afferent nerve with low operating bias (-0.6 V) achieved by integrating a pressure-activated organic electrochemical synaptic transistor and artificial mechanoreceptors. The dendritic integration function for neurorobotics is achieved to perceive directional movement of object, further reducing the control complexity by exploiting the distributed and parallel networks. An intelligent robot assembled with artificial afferent nerve, coupled with a closed-loop feedback program is demonstrated to rapidly implement slip recognition and prevention actions upon occurrence of object slippage. The spatiotemporal features of tactile patterns are well differentiated with a high recognition accuracy after processing spike-encoded signals with deep learning model. This work represents a breakthrough in mimicking synaptic behaviors, which is essential for next-generation intelligent neurorobotics and low-power biomimetic electronics.

Blade-Coated Porous 3D Carbon Composite Electrodes Coupled with Multiscale Interfaces for Highly Sensitive All-Paper Pressure Sensors

Flexible and wearable pressure sensors hold immense promise for health monitoring, covering disease detection and postoperative rehabilitation. Developing pressure sensors with high sensitivity, wide detection range, and cost-effectiveness is paramount. By leveraging paper for its sustainability, biocompatibility, and inherent porous structure, herein, a solution-processed all-paper resistive pressure sensor is designed with outstanding performance. A ternary composite paste, comprising a compressible 3D carbon skeleton, conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), and cohesive carbon nanotubes, is blade-coated on paper and naturally dried to form the porous composite electrode with hierachical micro- and nano-structured surface. Combined with screen-printed Cu electrodes in submillimeter finger widths on rough paper, this creates a multiscale hierarchical contact interface between electrodes, significantly enhancing sensitivity (1014 kPa-1) and expanding the detection range (up to 300 kPa) of as-resulted all-paper pressure sensor with low detection limit and power consumption. Its versatility ranges from subtle wrist pulses, robust finger taps, to large-area spatial force detection, highlighting its intricate submillimeter-micrometer-nanometer hierarchical interface and nanometer porosity in the composite electrode. Ultimately, this all-paper resistive pressure sensor, with its superior sensing capabilities, large-scale fabrication potential, and cost-effectiveness, paves the way for next-generation wearable electronics, ushering in an era of advanced, sustainable technological solutions.

Breaking the Saturation of Sensitivity for Ultrawide Range Flexible Pressure Sensors by Soft-Strain Effect

The flexible pressure sensors with a broad pressure range and unsaturated sensitivity are highly desired in practical applications. However, pressure sensors by piezoresistive effect are always limited by the compressibility of sensing layers, resulting in a theoretically decreasing sensitivity of less than 100%. Here, a unique strategy is proposed that utilizes the strain effect, simultaneously achieving a trade-off between a wider pressure detection range and unsaturated sensitivity. Ascribed to the strain effect of sensing layers induced by interlaced microdomes, the sensors possess an increased sensitivity (5.22-70 MPa-1) over an ultrawide pressure range (45 Pa-4.1 MPa), a high-pressure resolution (5 Pa), fast response/recovery time (30/45 ms), and a robust response under a high-pressure loading of 3.5 MPa for more than 5000 cycles. These superior sensing performances allow the sensor to monitor large pressure. The flexible pressure sensor array can assist doctors in restoring the neutral mechanical axis, tracking knee flexion angles, and extracting gait features. Moreover, the flexible sensing array can be integrated into the joint motion surveillance system to map the balance medial-lateral contact forces on the metal compartments in real time, demonstrating the potential for further development into precise medical human-machine interfaces during total knee replacement surgery.

Highly efficient recognition of similar objects based on ionic robotic tactile sensors

Tactile sensing provides robots the ability of object recognition, fine operation, natural interaction, etc. However, in the actual scenario, robotic tactile recognition of similar objects still faces difficulties such as low efficiency and accuracy, resulting from a lack of high-performance sensors and intelligent recognition algorithms. In this paper, a flexible sensor combining a pyramidal microstructure with a gradient conformal ionic gel coating was demonstrated, exhibiting excellent signal-to-noise ratio (48 dB), low detection limit (1 Pa), high sensitivity (92.96 kPa-1), fast response time (55 ms), and outstanding stability over 15,000 compression-release cycles. Furthermore, a Pressure-Slip Dual-Branch Convolutional Neural Network (PSNet) architecture was proposed to separately extract hardness and texture features and perform feature fusion. In tactile experiments on different kinds of leaves, a recognition rate of 97.16% was achieved, and surpassed that of human hands recognition (72.5%). These researches showed the great potential in a broad application in bionic robots, intelligent prostheses, and precise human-computer interaction.

Data-driven inverse design of flexible pressure sensors

Artificial skins or flexible pressure sensors that mimic human cutaneous mechanoreceptors transduce tactile stimuli to quantitative electrical signals. Conventional trial-and-error designs for such devices follow a forward structure-to-property routine, which is usually time-consuming and determines one possible solution in one run. Data-driven inverse design can precisely target desired functions while showing far higher productivity, however, it is still absent for flexible pressure sensors because of the difficulties in acquiring a large amount of data. Here, we report a property-to-structure inverse design of flexible pressure sensors, exhibiting a significantly greater efficiency than the conventional routine. We use a reduced-order model that analytically constrains the design scope and an iterative "jumping-selection" method together with a surrogate model that enhances data screening. As an exemplary scenario, hundreds of solutions that overcome the intrinsic signal saturation have been predicted by the inverse method, validating for a variety of material systems. The success in property design on multiple indicators demonstrates that the proposed inverse design is an efficient and powerful tool to target multifarious applications of flexible pressure sensors, which can potentially advance the fields of intelligent robots, advanced healthcare, and human-machine interfaces.

An Artificial Universal Tactile Nociceptor Based on 2D Polymer Film Memristor Arrays with Tunable Resistance Switching Behaviors

Nociceptor is an important receptor in the organism's sensory system; it can perceive harmful stimuli and send signals to the brain in order to protect the body in time. The injury degree of nociceptor can be divided into three stages: self-healing injury, treatable injury, and permanent injury. However, the current studies on nociceptor simulation are limited to the self-healing stage due to the limitation of the untunable resistance switching behavior of memristors. In this study, we constructed Al/2DPTPAK+TAPB/Ag memristor arrays with adjustable memory behaviors to emulate the nociceptor of biological neural network of all three stages. For this purpose, a PDMS/AgNWs/ITO/PET pressure sensor was assembled to mimic the tactile perception of the skin. The memristor arrays can not only simulate all the response of nociceptor, i.e., the threshold, relaxation, no adaptation, and sensitization with the self-healing injury, but can also simulate the treatable injury and the permanent injury. These behaviors are both demonstrated with a single memristor and in the form of pattern mapping of the memristor array.

Photostrictive Actuators Based on Freestanding Ferroelectric Membranes

Complex oxides offer a wide range of functional properties, and recent advances in the fabrication of freestanding membranes of these oxides are adding new mechanical degrees of freedom to this already rich functional ecosystem. Here, photoactuation is demonstrated in freestanding thin film resonators of ferroelectric Barium Titanate (BaTiO3) and paraelectric Strontium Titanate (SrTiO3). The free-standing films, transferred onto perforated supports, act as nano-drums, oscillating at their natural resonance frequency when illuminated by a frequency-modulated laser. The light-induced deflections in the ferroelectric BaTiO3 membranes are two orders of magnitude larger than in the paraelectric SrTiO3 ones. Time-resolved X-ray micro-diffraction under illumination and temperature-dependent holographic interferometry provide combined evidence for the photostrictive strain in BaTiO3 originating from a partial screening of ferroelectric polarization by photo-excited carriers, which decreases the tetragonality of the unit cell. These findings showcase the potential of photostrictive freestanding ferroelectric films as wireless actuators operated by light.

AI-Assisted Insole Sensing System for Multifunctional Plantar-Healthcare Applications

The pervasive global issue of population aging has led to a growing demand for health monitoring, while the advent of electronic wearable devices has greatly alleviated the strain on the industry. However, these devices come with inherent limitations, such as electromagnetic radiation, complex structures, and high prices. Herein, a Solaris silicone rubber-integrated PMMA polymer optical fiber (S-POF) intelligent insole sensing system has been developed for remote, portable, cost-effective, and real-time gait monitoring. The system is capable of sensitively converting the pressure of key points on the sole into changes in light intensity with correlation coefficients of 0.995, 0.952, and 0.910. The S-POF sensing structure demonstrates excellent durability with a 4.8% variation in output after 10,000 cycles and provides stable feedback for bending angles. It also exhibits water resistance and temperature resistance within a certain range. Its multichannel multiplexing framework allows a smartphone to monitor multiple S-POF channels simultaneously, meeting the requirements of convenience for daily care. Also, the system can efficiently and accurately provide parameters such as pressure, step cadence, and pressure distribution, enabling the analysis of gait phases and patterns with errors of only 4.16% and 6.25% for the stance phase (STP) and the swing phase (SWP), respectively. Likewise, after comparing various AI models, an S-POF channel-based gait pattern recognition technique has been proposed with a high accuracy of up to 96.87%. Such experimental results demonstrate that the system is promising to further promote the development of rehabilitation and healthcare.

Organic-inorganic hybrid piezotronic bipolar junction transistor for pressure sensing

With the rapid development of the Internet of Things (IoTs), wearable sensors are playing an increasingly important role in daily monitoring of personal health and wellness. The signal-to-noise-ratio has become the most critical performance factor to consider. To enhance it, on the one hand, good sensing materials/devices have been employed; on the other hand, signal amplification and noise reduction circuits have been used. However, most of these devices and circuits work in an active sampling mode, requiring frequent data acquisition and hence, entailing high-power consumption. In this scenario, a flexible and wearable event-triggered sensor with embedded signal amplification without an external power supply is of great interest. Here, we report a flexible two-terminal piezotronic n-p-n bipolar junction transistor (PBJT) that acts as an autonomous and highly sensitive, current- and/or voltage-mediated pressure sensor. The PBJT is formed by two back-to-back piezotronic diodes which are defined as emitter-base and collector-base diodes. Upon force exertion on the emitter side, as a result of the piezoelectric effect, the emitter-base diode is forward biased while the collector-base diode is reverse biased. Due to the inherent BJT amplification effect, the PBJT achieves record-high sensitivities of 139.7 kPa-1 (current-based) and 88.66 kPa-1 (voltage-based) in sensing mode. The PBJT also has a fast response time of <110 ms under exertion of dynamic stimuli ranging from a flying butterfly to a gentle finger touch. Therefore, the PBJT advances the state of the art not only in terms of sensitivity but also in regard to being self-driven and autonomous, making it promising for pressure sensing and other IoT applications.

The past 10 years of molecular ferroelectrics: structures, design, and properties

Ferroelectricity, which has diverse important applications such as memory elements, capacitors, and sensors, was first discovered in a molecular compound, Rochelle salt, in 1920 by Valasek. Owing to their superiorities of lightweight, biocompatibility, structural tunability, mechanical flexibility, etc., the past decade has witnessed the renaissance of molecular ferroelectrics as promising complementary materials to commercial inorganic ferroelectrics. Thus, on the 100th anniversary of ferroelectricity, it is an opportune time to look into the future, specifically into how to push the boundaries of material design in molecular ferroelectric systems and finally overcome the hurdles to their commercialization. Herein, we present a comprehensive and accessible review of the appealing development of molecular ferroelectrics over the past 10 years, with an emphasis on their structural diversity, chemical design, exceptional properties, and potential applications. We believe that it will inspire intense, combined research efforts to enrich the family of high-performance molecular ferroelectrics and attract widespread interest from physicists and chemists to better understand the structure-function relationships governing improved applied functional device engineering.

Efficient Fabrication of Bioinspired Flexible Pressure Sensors via Electrohydrodynamic Jet Printing Method

Bioinspired microdevices have made significant strides in various applications including human motion and health detection. However, facile and highly efficient fabrication approach of flexible pressure sensors remains a great challenge. Herein, inspired by the gecko's foot structure, a flexible pressure sensor with microdomes structure is fabricated by tip-assisted on-demand electrohydrodynamic jet (EHD-jet) printing method. Ascribed to the interlocking electrodes with microdome structure, 3D deformation rates are substantially enlarged. When the microdromes structure is under pressure, the resistivity of carbon nanotubes film coated on the surface of microdomes structure will change remarkably. By using the combined effect of assisted tip and ring focusing electrode, the influence and constraints on microstructure fabrication caused by substrate material and morphology are minimized. The desired uniform structures can be adjusted rapidly by changing the printing parameters and liquid properties. High length-height ratio (0.64) of microdomes enhances sensitivity, with minimum detection limit is 2 Pa and response time is 40 ms. Finally, the bionic flexible sensor indicated excellent performance in capable of detecting pressure, sound vibrations and human motion. This work presents a new method for high-efficiency fabrication micro-nano patterns for flexible sensors inspired, which could be used in wearable tech and health monitoring.

Research on high sensitivity piezoresistive sensor based on structural design

With the popularity of smart terminals, wearable electronic devices have shown great market prospects, especially high-sensitivity pressure sensors, which can monitor micro-stimuli and high-precision dynamic external stimuli, and will have an important impact on future functional development. Compressible flexible sensors have attracted wide attention due to their simple sensing mechanism and the advantages of light weight and convenience. Sensors with high sensitivity are very sensitive to pressure and can detect resistance/current changes under pressure, which has been widely studied. On this basis, this review focuses on analyzing the performance impact of device structure design strategies on high sensitivity pressure sensors. The design of structures can be divided into interface microstructures and three-dimensional framework structures. The preparation methods of various structures are introduced in detail, and the current research status and future development challenges are summarized.

Ultrawide linear range, high sensitivity, and large-area pressure sensor arrays enabled by pneumatic spraying broccoli-like microstructures

Large-area pressure sensor arrays with a wide linear response range and high sensitivity are beneficial to map the inhomogeneous interface pressure, which is significant in practical applications. Here, we demonstrate a pneumatic spraying method to prepare large-area microstructure films (PSMF) for high performance pressure sensor arrays. The sprayed surface morphology is designable by controlling the spraying parameters. It is worth noting that the constructed "broccoli" like morphology with a swollen top and shrunken bottom inspired a new mechanism to enlarge the linear response range by decreasing the series resistance with pressure increasing. At the same time, the pneumatic sprayed "broccoli" has a rough surface due to droplet stacking, which reduces the initial current effectively. Hence, the sensor achieves a 10 000 kPa ultrawide linear response range with a high sensitivity (98.71 kPa-1), and low detection (5 Pa). The prepared sensor has a small static response error (4.4%) and 5000 cycle full-range dynamic response durability. Finally, the constructed sensor arrays can distinguish the pressure distribution in different ranges clearly, which indicates a great potential in health care, motion detection, and the tire industry.

Facile Multiple Graded Wrinkle Construction Strategy for Vastly Boosting the Sensing Performance of Ionic Skins

The construction of surface microstructures (e.g., micropyramids and wrinkles) has been proven as the most effective means to boost the sensitivity of ionic skins (I-skins). However, the single-scale micronano patterns constructed by the common fabrication strategy generally lead to a limited pressure-response range. Here, a convenient repeated stretching/coordinating/releasing strategy is developed to controllably construct multiple graded wrinkles on the polyelectrolyte hydrogel-based I-skins for increasing their sensitivity over a broad pressure range. We find that the small wrinkles allow for high sensitivity yet small pressure detection range, while the large wrinkles can reduce structural stiffening to generate large pressure-response range but incur limited sensitivity. The multiple graded wrinkles can combine the merits of both the small and large wrinkles to simultaneously improve the sensitivity and broaden the pressure-response range. In particular, the sensing performance of multiple-wrinkle-based I-skins substantially outperforms the superposition of the sensing performance of different single-wrinkle-based I-skins. As a proof of concept, the triple-wrinkle-based I-skins can provide an extremely high sensitivity of 17,309 kPa-1 and an ultrawide pressure detection range of 0.38 Pa to 372 kPa. The approach and insight contribute to the future development of I-skins with a broader pressure-response range and higher sensitivity.

Mass-Produced Skin-Inspired Piezoresistive Sensing Array with Interlocking Interface for Object Recognition

E-skins, capable of responding to mechanical stimuli, hold significant potential in the field of robot haptics. However, it is a challenge to obtain e-skins with both high sensitivity and mechanical stability. Here, we present a bioinspired piezoresistive sensor with hierarchical structures based on polyaniline/polystyrene core-shell nanoparticles polymerized on air-laid paper. The combination of laser-etched reusable templates and sensitive materials that can be rapidly synthesized enables large-scale production. Benefiting from the substantially enlarged deformation of the hierarchical structure, the developed piezoresistive electronics exhibit a decent sensitivity of 21.67 kPa-1 and a subtle detection limit of 3.4 Pa. Moreover, an isolation layer is introduced to enhance the interface stability of the e-skin, with a fracture limit of 66.34 N/m. Furthermore, the e-skin can be seamlessly integrated onto gloves without any detachment issues. With the assistance of deep learning, it achieves a 98% accuracy rate in object recognition. We anticipate that this strategy will render e-skin with more robust interfaces and heightened sensing capabilities, offering a favorable pathway for large-scale production.

Underwater Gesture Recognition Meta-Gloves for Marine Immersive Communication

Rapid advancements in immersive communications and artificial intelligence have created a pressing demand for high-performance tactile sensing gloves capable of delivering high sensitivity and a wide sensing range. Unfortunately, existing tactile sensing gloves fall short in terms of user comfort and are ill-suited for underwater applications. To address these limitations, we propose a flexible hand gesture recognition glove (GRG) that contains high-performance micropillar tactile sensors (MPTSs) inspired by the flexible tube foot of a starfish. The as-prepared flexible sensors offer a wide working range (5 Pa to 450 kPa), superfast response time (23 ms), reliable repeatability (∼10000 cycles), and a low limit of detection. Furthermore, these MPTSs are waterproof, which makes them well-suited for underwater applications. By integrating the high-performance MPTSs with a machine learning algorithm, the proposed GRG system achieves intelligent recognition of 16 hand gestures under water, which significantly extends real-time and effective communication capabilities for divers. The GRG system holds tremendous potential for a wide range of applications in the field of underwater communications.

Fully printed minimum port flexible interdigital electrode sensor arrays

Screen-printed interdigital electrode-based flexible pressure sensor arrays play a crucial role in human-computer interaction and health monitoring due to their simplicity of fabrication. However, the long-standing challenge of how to reduce the number of electrical output ports of interdigital electrodes to facilitate integration with back-end circuits is still commonly ignored. Here, we propose a screen-printing strategy to avoid wire cross-planes for rapid fabrication of flexible pressure sensor arrays. By innovatively introducing an insulating ink to realize electrical insulation and three-dimensional interconnection of wire crossings, the improved sensor array (4 × 4) successfully reduces the number of output ports from 17 to 8. In addition, we further constructed microstructures on the laser-etched electrode surfaces and the sensitive layer, which enabled the sensor to achieve a sensitivity as high as 17 567.5 kPa-1 in the range of 0-50 kPa. Moreover, we integrated the sensors with back-end circuits for the precise detection of tactile and physiological information. This provides a reliable method for preparing high-performance flexible sensor arrays and large-scale integration of microsensors.

Flexible Pressure Sensors in Human-Machine Interface Applications

Flexible sensors are highly flexible, malleable, and capable of adapting todifferent shapes, surfaces, and environments, which opens a wide range ofpotential applications in the field of human-machine interface (HMI). Inparticular, flexible pressure sensors as a crucial member of the flexiblesensor family, are widely used in wearable devices, health monitoringinstruments, robots and other fields because they can achieve accuratemeasurement and convert the pressure into electrical signals. The mostintuitive feeling that flexible sensors bring to people is the change ofhuman-machine interface interaction, from the previous rigid interaction suchas keyboard and mouse to flexible interaction such as smart gloves, more inline with people's natural control habits. Many advanced flexible pressuresensors have emerged through extensive research and development, and to adaptto various fields of application. Researchers have been seeking to enhanceperformance of flexible pressure sensors through improving materials, sensingmechanisms, fabrication methods, and microstructures. This paper reviews the flexible pressure sensors in HMI in recent years, mainlyincluding the following aspects: current cutting-edge flexible pressuresensors; sensing mechanisms, substrate materials and active materials; sensorfabrication, performances, and their optimization methods; the flexiblepressure sensors for various HMI applications and their prospects.

A Soft Bioelectronic Patch for Simultaneous Respiratory and Cardiovascular Monitoring

Sleep is critical to maintaining physical and mental health. Measuring physiological parameters to quantify sleep quality without uncomfortable user experience remains highly desired but a challenge. Here, this work develops a soft bioelectronic patch to perform simultaneous respiration and cardiovascular monitoring during sleep in a wearable and non-invasive manner. The soft bioelectronic patch system is mainly composed of a pressure sensor, a flexible printed circuit for signal processing, and a soft thermoplastic urethane mold for assembling different functional modules. The soft bioelectronic patch holds a sensitivity of >0.12 V kPa-1 and a remarkable low-frequency response from 0.5 to 15 Hz. It is demonstrated to continuously monitor respiration and heartbeat during the whole night, which could be harnessed for sleep monitoring and obstructive sleep apnea-hypopnea syndrome diagnosis. The reported soft bioelectronic patch represents a simple and convenient platform technology for sleep study.

Large Enhancement of Polarization in a Layered Hybrid Perovskite Ferroelectric Semiconductor via Molecular Engineering

Switchable spontaneous polarization is the vital property of ferroelectrics, which leads to other key physical properties such as piezoelectricity, pyroelectricity, and nonlinear optical effects, etc. Recently, organic-inorganic hybrid perovskites with 2D layered structure have become an emerging branch of ferroelectric materials. However, most of the 2D hybrid ferroelectrics own relatively low polarizations (<15 µC cm-2 ). Here, a strategy to enhance the polarization of these hybrid perovskites by using ortho-, meta-, para-halogen substitution is developed. Based on (benzylammonium)2 PbCl4 (BZACL), the para-chlorine substituted (4-chlorobenzylammonium)2 PbCl4 (4-CBZACL) ferroelectric semiconductor shows a large spontaneous polarization (23.3 µC cm-2 ), which is 79% larger than the polarization of BZACL. This large enhancement of polarization is successfully explained via ab initio calculations. The study provides a convenient and efficient strategy to promote the ferroelectric property in the hybrid perovskite family.

Soft Sensors and Actuators for Wearable Human-Machine Interfaces

Haptic human-machine interfaces (HHMIs) combine tactile sensation and haptic feedback to allow humans to interact closely with machines and robots, providing immersive experiences and convenient lifestyles. Significant progress has been made in developing wearable sensors that accurately detect physical and electrophysiological stimuli with improved softness, functionality, reliability, and selectivity. In addition, soft actuating systems have been developed to provide high-quality haptic feedback by precisely controlling force, displacement, frequency, and spatial resolution. In this Review, we discuss the latest technological advances of soft sensors and actuators for the demonstration of wearable HHMIs. We particularly focus on highlighting material and structural approaches that enable desired sensing and feedback properties necessary for effective wearable HHMIs. Furthermore, promising practical applications of current HHMI technology in various areas such as the metaverse, robotics, and user-interactive devices are discussed in detail. Finally, this Review further concludes by discussing the outlook for next-generation HHMI technology.

Noninvasive Blood Pressure Monitoring via a Flexible and Wearable Piezoresistive Sensor

In the present global context, continuous blood pressure (BP) monitoring is paramount in addressing the global mortality rates attributed to hypertension. Achieving precise insights into the human cardiovascular system necessitates accurate measurement of BP, and the accuracy depends on the faithful recording of oscillations or pulsations. This task ultimately depends on the caliber of the pressure sensor embedded in the BP device. In this context, we have fabricated a flexible resistive pressure sensor based on reduced graphene oxide (rGO) and a polydimethylsiloxane (PDMS) sponge that is highly flexible and sensitive. The designed device operates effectively with a minimal bias voltage of 500 mV, at which point it showed its maximum relative change in current, reaching approximately 25%. Additionally, the sensing device showed a notable change in resistance values, exhibiting almost 100% change in resistance when subjected to a pressure of 400 mmHg and high sensitivity of 0.27 mmHg-1. After promising outcomes were obtained during static pressure measurement, the sensor was used for BP monitoring in humans. The sensor accurately traced the oscillometric waveform (OMW) for distinct systolic blood pressure (SBP) and diastolic blood pressure (DBP) combinations to cover a range of medical situations, including hypotension, standard or normal, and hypertension. The values of SBP, DBP, and MAP were derived from the sensor's output using the MAA technique, and the errors in these values concerning the simulator and the traditional BP monitor follow the universal AAMI/ESH/ISO protocols. Bland-Altman (B&A) correlation and scatter plots were used to compare the sensor's results and further validate the proposed sensor. The sensor showed the mean and standard deviation error in the SBP, DBP, and MBP of -0.2 ± 5.9, -0.5 ± 7, and -0.9 ± 4.7 mmHg when compared with the noninvasive blood pressure (NIBP) simulator. The pulse rate (PR) was also calculated from the same OMW for the specified value of 80 beats per minute (bpm) given by the simulator and reported a mean PR value of ∼81 bpm, close to the reference value. The findings show that the flexible resistive sensing device can accurately measure BP and replace the existing sensors of BP devices.

Biomimetic Wearable Sensors: Emerging Combination of Intelligence and Electronics

Owing to the advancement of interdisciplinary concepts, for example, wearable electronics, bioelectronics, and intelligent sensing, during the microelectronics industrial revolution, nowadays, extensively mature wearable sensing devices have become new favorites in the noninvasive human healthcare industry. The combination of wearable sensing devices with bionics is driving frontier developments in various fields, such as personalized medical monitoring and flexible electronics, due to the superior biocompatibilities and diverse sensing mechanisms. It is noticed that the integration of desired functions into wearable device materials can be realized by grafting biomimetic intelligence. Therefore, herein, the mechanism by which biomimetic materials satisfy and further enhance system functionality is reviewed. Next, wearable artificial sensory systems that integrate biomimetic sensing into portable sensing devices are introduced, which have received significant attention from the industry owing to their novel sensing approaches and portabilities. To address the limitations encountered by important signal and data units in biomimetic wearable sensing systems, two paths forward are identified and current challenges and opportunities are presented in this field. In summary, this review provides a further comprehensive understanding of the development of biomimetic wearable sensing devices from both breadth and depth perspectives, offering valuable guidance for future research and application expansion of these devices.

Mechanically Robust and Linearly Sensitive Soft Piezoresistive Pressure Sensor for a Wearable Human-Robot Interaction System

Soft piezoresistive pressure sensors play an underpinning role in enabling a plethora of future Internet of Things (IoT) applications such as human-robot interaction (HRI) technologies, wearable devices, and metaverse ecosystems. Despite significant attempts to enhance the performance of these sensors, existing sensors still fall short of achieving high strain tolerance and linearity simultaneously. Herein, we present a low-cost, facile, and scalable approach to fabricating a highly strain-tolerant and linearly sensitive soft piezoresistive pressure sensor. Our design utilizes thin nanocracked gold films (NC-GFs) deposited on poly(dimethylsiloxane) (PDMS) as electrodes of the sensor. The large mismatch stress between gold (Au) and PDMS induces the formation of secondary wrinkles along the pyramidal-structured electrode under pressure; these wrinkles function as protuberances on the electrode and enable exceptional linear sensitivity of 4.2 kPa-1 over a wide pressure range. Additionally, our pressure sensor can maintain its performance even after severe mechanical deformations, including repeated stretching up to 30% strain, due to the outstanding strain tolerance of NC-GF. Our sensor's impressive sensing performance and mechanical robustness make it suitable for diverse IoT applications, as demonstrated by its use in wearable pulse monitoring devices and human-robot interaction systems.

A Dual-Mode Pressure and Temperature Sensor

The emerging field of flexible tactile sensing systems, equipped with multi-physical tactile sensing capabilities, holds vast potential across diverse domains such as medical monitoring, robotics, and human-computer interaction. In response to the prevailing challenges associated with the limited integration and sensitivity of flexible tactile sensors, this paper introduces a versatile tactile sensing system capable of concurrently monitoring temperature and pressure. The temperature sensor employs carbon nanotube/graphene conductive paste as its sensitive material, while the pressure sensor integrates an ionic gel containing boron nitride as its sensitive layer. Through the application of cost-effective screen printing technology, we have successfully manufactured a flexible dual-mode sensor with exceptional performance, featuring high sensitivity (804.27 kPa-1), a broad response range (50 kPa), rapid response time (17 ms), and relaxation time (34 ms), alongside exceptional durability over 5000 cycles. Furthermore, the resistance temperature coefficient of the sensor within the temperature range of 12.5 °C to 93.7 °C is -0.17% °C-1. The designed flexible dual-mode tactile sensing system enables the real-time detection of pressure and temperature information, presenting an innovative approach to electronic skin with multi-physical tactile sensing capabilities.

Strongly Enhanced Polarization in a Ferroelectric Crystal by Conduction-Proton Flow

Ion conductors comprising noncentrosymmetric frameworks have emerged as new functional materials. However, strongly correlated polarity functionality and ion transport have not been achieved. Herein, we report a ferroelectric proton conductor, K2MnN(CN)4·H2O (1·H2O), exhibiting the strong correlation between its polar skeleton and conductive ions that generate anomalous ferroelectricity via the proton-bias phenomenon. The application of an electric field of ±1 kV/cm (0.1 Hz) on 1·H2O at 298 K produced the ferroelectricity (polarization = 1.5 × 104 μC/cm2), which was enhanced by the ferroelectric-skeleton-trapped conductive protons. Furthermore, the strong polarity-proton transport coupling of 1·H2O induced a proton-rectification-like directional ion-conductive behavior that could be adjusted by the magnitude and direction of DC electric fields. Moreover, 1·H2O exhibited reversible polarity switching between the polar 1·H2O and its dehydrated form, 1, with a centrosymmetric structure comprising an order-disorder-type transition of the nitrido-bridged chains.

One Novel Hybrid Flexible Piezoresistive/Piezoelectric Double-Mode Sensor Design for Water Leakage Monitoring

Water leakage is a significant issue in infrastructure, such as submarine tunnels, which can lead to major disasters and property losses. Therefore, it is of great significance to develop a water leakage detection sensor with simple preparation process, low cost, and small limitation of applicable location. In this study, a novel hybrid flexible piezoresistive/piezoelectric double-mode sensor with a sandpaper negative microstructure is proposed. A unique dual-path perception structure is designed that can simultaneously and independently detect two signals of water leakage frequency and water leakage volume. The piezoresistive layer is formed by polydimethylsiloxane (PDMS) coated with multiwalled carbon nanotubes (MWCNTs), which is molded by sandpaper molding. By sensing the deformation caused by the swelling of superabsorbent polymers (SAPs), the water leakage volume can be detected as low as 0.5 mL. The piezoelectric layer is a polyvinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE) film prepared by the spin-coating method, and the water leakage frequency (0.5-4 Hz) is detected by direct contact with water droplets. This work also studied the performance of the double-mode sensor under low temperature and seawater leakage conditions and further verified its reliability in different environments. The design of the new hybrid flexible piezoresistive/piezoelectric double-mode sensor provides a new possibility for water leakage monitoring, such as in submarine tunnels.

Outstanding Synergy of Sensitivity and Linear Range Enabled by Multigradient Architectures

Flexible pressure sensors are devices that mimic the sensory capabilities of natural human skin and enable robots to perceive external stimuli. One of the main challenges is maintaining high sensitivity over a broad linear pressure range due to poor structural compressibility. Here, we report a flexible pressure sensor with an ultrahigh sensitivity of 153.3 kPa-1 and linear response over an unprecedentedly broad pressure range from 0.0005 to 1300 kPa based on interdigital-shaped, multigradient architectures, featuring modulus, conductivity, and microstructure gradients. Such multigradient architectures and interdigital-shaped configurations enable effective stress transfer and conductivity regulation, evading the pressure sensitivity-linear range trade-off dilemma. Together with high pressure resolution, high frequency response, and good reproducibility over the ultrabroad linear range, proof-of-concept applications such as acoustic wave detection, high-resolution pressure measurement, and healthcare monitoring in diverse scenarios are demonstrated.

Recent advances in smart wearable sensors as electronic skin

Flexible and multifunctional electronic devices and soft robots inspired by human organs, such as skin, have many applications. However, the emergence of electronic skins (e-skins) or textiles in biomedical engineering has made a great revolution in a myriad of people's lives who suffer from different types of diseases and problems in which their skin and muscles lose their appropriate functions. In this review, recent advances in the sensory function of the e-skins are described. Furthermore, we have categorized them from the sensory function perspective and highlighted their advantages and limitations. The categories are tactile sensors (including capacitive, piezoresistive, piezoelectric, triboelectric, and optical), temperature, and multi-sensors. In addition, we summarized the most recent advancements in sensors and their particular features. The role of material selection and structure in sensory function and other features of the e-skins are also discussed. Finally, current challenges and future prospects of these systems towards advanced biomedical applications are elaborated.

Rationally designed cellulose hydrogel for an ultrasensitive pressure sensor

Flexible pressure sensors with high sensitivity are required in fields such as human-machine interactions, electronic skin, and health tracking. In this work, we reported cellulose ion-conductive hydrogel (ICH) rationally designed from both nano and micron perspectives for ultrasensitive pressure sensors, via a zero-waste approach, without involving soft components. By introducing low molecular weight cellulose and using the idea of a rough surface, the piezocapacitive sensitivity of the ICH was increased from 0.04 kPa-1 to 89.81 kPa-1 in increments of 2245, which also has a high degree of transparency, excellent durability, and good electrical transmission. Moreover, the ICH demonstrated great potential as sensors and arrays practicable in various industries, including medical treatment and motion recognition. The design is also applicable for piezoresistive tactile sensors, which realize enhanced sensitivity. This affordable, effective, and environmentally friendly technology definitely offers novel perspectives and the potential to enhance the functionality of flexible pressure sensors.

Ultra-Tough Waterborne Polyurethane-Based Graft-Copolymerized Piezoresistive Composite Designed for Rehabilitation Training Monitoring Pressure Sensors

Effective training is crucial for patients who need rehabilitation for achieving optimal recovery and reducing complications. Herein, a wireless rehabilitation training monitoring band with a highly sensitive pressure sensor is proposed and designed. It utilizes polyaniline@waterborne polyurethane (PANI@WPU) as a piezoresistive composite material, which is prepared via the in situ grafting polymerization of PANI on the WPU surface. WPU is designed and synthesized with tunable glass transition temperatures ranging from -60 to 0 °C. Dipentaerythritol (Di-PE) and ureidopyrimidinone (UPy) groups are introduced, endowing the material with good tensile strength (14.2 MPa), toughness (62 MJ-1 m-3 ), and great elasticity (low permanent deformation: 2%). Di-PE and UPy enhance the mechanical properties of WPU by increasing the cross-linking density and crystallinity. Combining the toughness of WPU and the high-density microstructure derived by hot embossing technology, the pressure sensor exhibits high sensitivity (168.1 kPa-1 ), fast response time (32 ms), and excellent stability (10 000 cycles with 3.5% decay). In addition, the rehabilitation training monitoring band is equipped with a wireless Bluetooth module, which can be easily applied to monitor the rehabilitation training effect of patients using an applet. Therefore, this work has the potential to significantly broaden the application of WPU-based pressure sensors for rehabilitation monitoring.

Monitoring and analysis of cardiovascular pulse waveforms using flexible capacitive and piezoresistive pressure sensors and machine learning perspective

The growing interest in flexible electronics for physiological monitoring, particularly using flexible pressure sensors for cardiovascular pulse waveforms monitoring, has potential applications in cuffless blood pressure measurement and early diagnosis of cardiovascular disease. High sensitivity, fast response time, good pressure resolution and a high signal-to-noise ratio are essential for effective pulse waveform detection. This review focuses on flexible capacitive and piezoresistive pressure sensors, which have seen significant enhancements due to their simple operation, superior performance, wide range of materials, and easy fabrication. The comparison of sensing methods for acquiring pulse waveforms from the wrist artery, device integration configurations, high-quality pulse waveforms collection, and performance analysis of capacitive and piezoresistive sensors are discussed. The review also covers the use of machine learning for analyzing pulse waveforms for cardiovascular disease diagnosis and cuff-less blood pressure monitoring. Lastly, it provides perspectives on current challenges and further advancements in the field.

Revealing the Polarizations of Molecular Ferroelectrics via SHG Polarimetry at the Nanoscale

Multifarious molecular ferroelectrics with multipolar axial characteristics have emerged in recent years, enriching the scenarios for energy harvesting, sensing, and information processing. The increased polar axes have enhanced the urgency of distinguishing different polarization states in material design, mechanism exploration, etc. However, conventional methods hardly meet the requirements of in situ, fast, microscale, contactless, and nondestructive features due to their inherent limitations. Herein, SHG polarimetry is introduced to probe the multioriented polarizations on a nanosized multiaxial molecular ferroelectric, i.e., TMCM-CdCl3 nanoplates, as an example. Combined with the analysis of the second-order susceptibility tensor, SHG polarimetry could serve as an effective method to detect the polarization orders and domain distributions of molecular ferroelectrics. Profiting from the full-optical feature, SHG polarimetry can even be performed on samples covered by transparent mediums, 2D materials, or thin metal electrodes. Our research might spark further fundamental studies and expand the application boundaries of next-generation ferroelectric materials.

A pH-responsive polymer-coated CaO2as oxygen-generating nanoparticlein situfor enhanced chemo-photodynamic synergistic therapy against tumors

Photodynamic therapy (PDT) has emerged as an efficient strategy for tumor treatment. However, Insufficient amounts of inherent hypoxia and intrinsic hydrogen peroxide (H2O2) in the tumor microenvironment severely constrained PDT, as oxygen is the critical substrate for photosensitivity reaction. Here, a pH-responsive H2O2and O2self-supplying hybrid nanoparticle was designed. Through, the calcium peroxide (CaO2) as carriers loading a chemotherapeutic drug a photosensitizer 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) and doxorubicin (DOX), was covered with polyacrylic acid (PAA) to build up a feature material DOX-TAPP-CaO2@OA@PAA (denoted as DTCOP) through the reverse microemulsion method. In the acidic tumor microenvironment conditions exposing the water-sensitive CaO2nanocore to generate hydrogen peroxide (H2O2) and O2, the self-supplied O2alleviates hypoxia to enhance the PDT, and releasing DOX and TAPP. Synthetic characterization shows that the succeeded synthesized Nanocarriers could effectively carry DOX and TAPP to the tumor site and release O2at the low pH of TME. And the experimental results demonstrated that this interpose exogenous oxygen strategy is efficient at inhibition of tumor growth bothin vitroandin vivo. The nanocomposite exhibits excellent biocompatibility and the ability to inhibit tumor growth and has significant potential for the treatment of hypoxic tumors.

Modulus difference-induced embedding strategy to construct iontronic pressure sensor with high sensitivity and wide linear response range

Sensitivity and linearity are two crucial indices to assess the sensing capability of pressure sensors; unfortunately, the two mutually exclusive parameters usually result in limited applications. Although a series of microengineering strategies including micropatterned, multilayered, and porous approach have been provided in detail, the conflict between the two parameters still continues. Here, we present an efficient strategy to resolve this contradiction via modulus difference-induced embedding deformation. Both the microscopic observation and finite element simulation results confirm the embedding deformation behavior ascribed to the elastic modulus difference between soft electrode and rigid microstructures. The iontronic pressure sensor with high sensitivity (35 kPa-1) and wide linear response range (0-250 kPa) is further fabricated and demonstrates the potential applications in monitoring of high-fidelity pulse waveforms and human motion. This work provides an alternative strategy to guide targeted design of all-around and comprehensive pressure sensor.

Emerging Trends in Soft Electronics: Integrating Machine Intelligence with Soft Acoustic/Vibration Sensors

In the last decade, soft acoustic/vibration sensors have gained tremendous research interest due to their unique ability to detect broadband acoustic/vibration stimuli, potentializing futuristic applications including voice biometrics, voice-controlled human-machine-interfaces, electronic skin, and skin-mountable healthcare devices. Importantly, to benefit most from these sensors, it is inevitable to use machine learning (ML) to process their output signals; with ML, a more accurate and efficient interpretation of original data is possible. This paper is dedicated to offering an overview of recent advances empowering the development of soft acoustic/vibration sensors and their signal processing using ML. First, the key performance parameters of the sensors are discussed. Second, popular transduction mechanisms for the sensors are addressed, followed by an in-depth overview of each type, covering materials used, structural designs, and sensing performances. Third, potential applications of the sensors are elaborated and fourth, a thorough discussion on ML is conducted, exploring different types of ML, specific ML algorithms suitable for processing acoustic/vibration signals, and current trends in ML-assisted applications. Finally, the challenges and potential opportunities in soft acoustic/vibration sensor and ML research are revealed to offer new insights into future prospects in these fields.

A nanonewton-scale biomimetic mechanosensor

Biomimetic mechanosensors have profound implications for various areas, including health care, prosthetics, human‒machine interfaces, and robotics. As one of the most important parameters, the sensitivity of mechanosensors is intrinsically determined by the detection resolution to mechanical force. In this manuscript, we expand the force detection resolution of current biomimetic mechanosensors from the micronewton to nanonewton scale. We develop a nanocrack-based electronic whisker-type mechanosensor that has a detection resolution of 72.2 nN. We achieve the perception of subtle mechanical stimuli, such as tiny objects and airflow, and the recognition of surface morphology down to a 30 nm height, which is the finest resolution ever reported in biomimetic mechanosensors. More importantly, we explore the use of this mechanosensor in wearable devices for sensing gravity field orientation with respect to the body, which has not been previously achieved by these types of sensors. We develop a wearable smart system for sensing the body's posture and movements, which can be used for remote monitoring of falls in elderly people. In summary, the proposed device offers great advantages for not only improving sensing ability but also expanding functions and thus can be used in many fields not currently served by mechanosensors.

Biomimetic Flexible Sensors and Their Applications in Human Health Detection

Bionic flexible sensors are a new type of biosensor with high sensitivity, selectivity, stability, and reliability to achieve detection in complex natural and physiological environments. They provide efficient, energy-saving and convenient applications in medical monitoring and diagnosis, environmental monitoring, and detection and identification. Combining sensor devices with flexible substrates to imitate flexible structures in living organisms, thus enabling the detection of various physiological signals, has become a hot topic of interest. In the field of human health detection, the application of bionic flexible sensors is flourishing and will evolve into patient-centric diagnosis and treatment in the future of healthcare. In this review, we provide an up-to-date overview of bionic flexible devices for human health detection applications and a comprehensive summary of the research progress and potential of flexible sensors. First, we evaluate the working mechanisms of different classes of bionic flexible sensors, describing the selection and fabrication of bionic flexible materials and their excellent electrochemical properties; then, we introduce some interesting applications for monitoring physical, electrophysiological, chemical, and biological signals according to more segmented health fields (e.g., medical diagnosis, rehabilitation assistance, and sports monitoring). We conclude with a summary of the advantages of current results and the challenges and possible future developments.

A Review of Epidermal Flexible Pressure Sensing Arrays

In recent years, flexible pressure sensing arrays applied in medical monitoring, human-machine interaction, and the Internet of Things have received a lot of attention for their excellent performance. Epidermal sensing arrays can enable the sensing of physiological information, pressure, and other information such as haptics, providing new avenues for the development of wearable devices. This paper reviews the recent research progress on epidermal flexible pressure sensing arrays. Firstly, the fantastic performance materials currently used to prepare flexible pressure sensing arrays are outlined in terms of substrate layer, electrode layer, and sensitive layer. In addition, the general fabrication processes of the materials are summarized, including three-dimensional (3D) printing, screen printing, and laser engraving. Subsequently, the electrode layer structures and sensitive layer microstructures used to further improve the performance design of sensing arrays are discussed based on the limitations of the materials. Furthermore, we present recent advances in the application of fantastic-performance epidermal flexible pressure sensing arrays and their integration with back-end circuits. Finally, the potential challenges and development prospects of flexible pressure sensing arrays are discussed in a comprehensive manner.

Ferroelectricity in Niobium Oxide Dihalides NbOX2 (X = Cl, I): A Macroscopic- to Microscopic-Scale Study

2D materials with ferroelectric and piezoelectric properties are of interest for energy harvesting, memory storage and electromechanical systems. Here, we present a systematic study of the ferroelectric properties in NbOX₂ (X = Cl, I) across different spatial scales. The in-plane ferroelectricity in NbOX₂ was investigated using transport and piezoresponse force microscopy (PFM) measurements, where it was observed that NbOCl₂ has a stronger ferroelectric order than NbOI₂. A high local field, exerted by both PFM and scanning tunneling microscopy (STM) tips, was found to induce 1D collinear ferroelectric strips in NbOCl₂. STM imaging reveals the unreconstructed atomic structures of NbOX₂ surfaces, and scanning tunneling spectroscopy was used to probe the electronic states induced at defect (vacancy) sites.

Highly degenerate 2D ferroelectricity in pore decorated covalent/metal organic frameworks

Two-dimensional (2D) ferroelectricity, a fundamental concept in low-dimensional physics, serves as the basis of non-volatile information storage and various electronic devices. Conventional 2D ferroelectric (FE) materials are usually two-fold degenerate, meaning that they can only store two logical states. In order to break such limitation, a new concept of highly degenerate ferroelectricity with multiple FE states (more than 2) coexisting in a single 2D material is proposed. This is obtained through the asymmetrical decoration of porous covalent/metal organic frameworks (COFs/MOFs). Using first-principles calculations and Monte Carlo (MC) simulations, Li-decorated 2D Cr(pyz)₂ is systematically explored as a prototype of highly degenerate 2D FE materials. We show that 2D FE Li_0.5Cr(pyz)₂ and LiCr(pyz)₂ are four-fold and eight-fold degenerate, respectively, with sizable spontaneous electric polarization that can be switched across low transition barriers. In particular, the coupling between neighbouring electric dipoles in LiCr(pyz)₂ induces novel ferroelectricity-controlled ferroelastic transition and direction-controllable hole transport channels. Moreover, three-fold and six-fold degenerate ferroelectricity is also demonstrated in P-decorated g-C₃N₄ and Ru-decorated C₂N, respectively. Our work presents a general route to obtain highly degenerate 2D ferroelectricity, which goes beyond the two-state paradigm of traditional 2D FE materials and substantially broadens the applications of 2D FE compounds.

Transparent and Flexible Graphene Pressure Sensor with Self-Assembled Topological Crystalline Ionic Gel

This study demonstrates transparent and flexible capacitive pressure sensors using a high-k ionic gel composed of an insulating polymer (poly(vinylidene fluoride-co-trifluoroethylene-co-chlorofluoroethylene), P(VDF-TrFE-CFE)) blended with an ionic liquid (IL; 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide, [EMI][TFSA]). The thermal melt recrystallization of the P(VDF-TrFE-CFE):[EMI][TFSA] blend films develops the characteristic topological semicrystalline surface of the films, making them highly sensitive to pressure. Using optically transparent and mechanically flexible graphene electrodes, a novel pressure sensor is realized with the topological ionic gel. The sensor exhibits a sufficiently large air dielectric gap between graphene and the topological ionic gel, resulting in a large variation in capacitance before and after the application of various pressures owing to the pressure-sensitive reduction of the air gap. The developed graphene pressure sensor exhibits a high sensitivity of 10.14 kPa^-1 at 20 kPa, rapid response times of <30 ms, and durable device operation with 4000 repeated ON/OFF cycles. Furthermore, broad-range detections from lightweight objects to human motion are successfully achieved, demonstrating that the developed pressure sensor with a self-assembled crystalline topology is potentially suitable for a variety of cost-effective wearable applications.

Flexible Sensors Array Based on Frosted Microstructured Ecoflex Film and TPU Nanofibers for Epidermal Pulse Wave Monitoring

Recent advances in flexible pressure sensors have fueled increasing attention as promising technologies with which to realize human epidermal pulse wave monitoring for the early diagnosis and prevention of cardiovascular diseases. However, strict requirements of a single sensor on the arterial position make it difficult to meet the practical application scenarios. Herein, based on three single-electrode sensors with small area, a 3 × 1 flexible pressure sensor array was developed to enable measurement of epidermal pulse waves at different local positions of radial artery. The designed single sensor holds an area of 6 × 6 mm², which mainly consists of frosted microstructured Ecoflex film and thermoplastic polyurethane (TPU) nanofibers. The Ecoflex film was formed by spinning Ecoflex solution onto a sandpaper surface. Micropatterned TPU nanofibers were prepared on a fluorinated ethylene propylene (FEP) film surface using the electrospinning method. The combination of frosted microstructure and nanofibers provides an increase in the contact separation of the tribopair, which is of great benefit for improving sensor performance. Due to this structure design, the single small-area sensor was characterized by pressure sensitivity of 0.14 V/kPa, a response time of 22 ms, a wide frequency band ranging from 1 to 23 Hz, and stability up to 7000 cycles. Given this output performance, the fabricated sensor can detect subtle physiological signals (e.g., respiration, ballistocardiogram, and heartbeat) and body movement. More importantly, the sensor can be utilized in capturing human epidermal pulse waves with rich details, and the consistency of each cycle in the same measurement is as high as 0.9987. The 3 × 1 flexible sensor array is employed to acquire pulse waves at different local positions of the radial artery. In addition, the time domain parameters including pulse wave transmission time (PTT) and pulse wave velocity (PWV) can be obtained successfully, which holds promising potential in pulse-based cardiovascular system status monitoring.

Recent progress in flexible pressure sensors based on multiple microstructures: from design to application

Flexible pressure sensors (FPSs) have been widely studied in the fields of wearable medical monitoring and human-machine interaction due to their high flexibility, light weight, sensitivity, and easy integration. To better meet these application requirements, key sensing properties such as sensitivity, linear sensing range, pressure detection limits, response/recovery time, and durability need to be effectively improved. Therefore, researchers have extensively and profoundly researched and innovated on the structure of sensors, and various microstructures have been designed and applied to effectively improve the sensing performance of sensors. Compared with single microstructures, multiple microstructures (MMSs) (including hierarchical, multi-layered and hybrid microstructures) can improve the sensing performance of sensors to a greater extent. This paper reviews the recent research progress in the design and application of FPSs with MMSs and systematically summarizes the types, sensing mechanisms, and preparation methods of MMSs. In addition, we summarize the applications of FPSs with MMSs in the fields of human motion detection, health monitoring, and human-computer interaction. Finally, we provide an outlook on the prospects and challenges for the development of FPSs.

Machine Learning-Enhanced Flexible Mechanical Sensing

To realize a hyperconnected smart society with high productivity, advances in flexible sensing technology are highly needed. Nowadays, flexible sensing technology has witnessed improvements in both the hardware performances of sensor devices and the data processing capabilities of the device's software. Significant research efforts have been devoted to improving materials, sensing mechanism, and configurations of flexible sensing systems in a quest to fulfill the requirements of future technology. Meanwhile, advanced data analysis methods are being developed to extract useful information from increasingly complicated data collected by a single sensor or network of sensors. Machine learning (ML) as an important branch of artificial intelligence can efficiently handle such complex data, which can be multi-dimensional and multi-faceted, thus providing a powerful tool for easy interpretation of sensing data. In this review, the fundamental working mechanisms and common types of flexible mechanical sensors are firstly presented. Then how ML-assisted data interpretation improves the applications of flexible mechanical sensors and other closely-related sensors in various areas is elaborated, which includes health monitoring, human-machine interfaces, object/surface recognition, pressure prediction, and human posture/motion identification. Finally, the advantages, challenges, and future perspectives associated with the fusion of flexible mechanical sensing technology and ML algorithms are discussed. These will give significant insights to enable the advancement of next-generation artificial flexible mechanical sensing.

Recent Progress of Tactile and Force Sensors for Human-Machine Interaction

Human-Machine Interface (HMI) plays a key role in the interaction between people and machines, which allows people to easily and intuitively control the machine and immersively experience the virtual world of the meta-universe by virtual reality/augmented reality (VR/AR) technology. Currently, wearable skin-integrated tactile and force sensors are widely used in immersive human-machine interactions due to their ultra-thin, ultra-soft, conformal characteristics. In this paper, the recent progress of tactile and force sensors used in HMI are reviewed, including piezoresistive, capacitive, piezoelectric, triboelectric, and other sensors. Then, this paper discusses how to improve the performance of tactile and force sensors for HMI. Next, this paper summarizes the HMI for dexterous robotic manipulation and VR/AR applications. Finally, this paper summarizes and proposes the future development trend of HMI.

Intentional Blocking Based Photoelectric Soft Pressure Sensor with High Sensitivity and Stability

Soft pressure sensors have recently attracted considerable attention because of their applications in human-machine interface, soft robotics, and prosthetics. However, there remain some challenges in achieving satisfactory performance (e.g., high sensitivity, wide sensing range, high stability) for soft pressure sensors. This article reports an intentional blocking based photoelectric pressure sensor. Two different blocking methods are investigated: the single-row-pyramid blocking and the double-row-pyramid blocking. The sensor has a simple structure, which is made of a light-emitting diode, photosensitive element, and silicone sensor shell. Experiments demonstrate that the sensor has a high sensitivity (the maximum sensitivity is 48.07 kPa^-1, and the minimum measurement pressure is 0.8 Pa), large pressure-sensing range (the sensing range is up to 120 kPa), superior stability (a drift about 0.4% over 12,130 repetitive cycles at 0-80 kPa), low drift (< ±0.2% in different 3-day testing), negligible hysteresis, and high signal-to-noise ratio (over 55 dB). By mounting the pressure sensor at the end of a robotic arm, the robot can detect subtle collisions (such as touching a balloon through a pinpoint). In addition, this article fabricates a tactile glove based on the proposed pressure sensor and shows the application of this glove for music playing and object weighing. This study provides a new structure for photoelectric sensors to increase sensitivity and also provides a more convenient way to fabricate photoelectric pressure sensors.

Experimental Band Structure of Pb(Zr,Ti)O3 : Mechanism of Ferroelectric Stabilization

Pb(Zr,Ti)O₃ (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr_0.2 Ti_0.8 )O₃ using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO₃ and La_x SrMn_{1- x} O₃ substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.

Flexible Pressure Sensor with an Excellent Linear Response in a Broad Detection Range for Human Motion Monitoring

Pressure sensing is highly demanding in wearable devices, robotics, and artificial intelligence, whereas it is still a big challenge to develop a pressure sensor with an excellent linear response in a broad detection range. Herein, a flexible and porous carbon nanotube (CNT)/carbon black (CB)/carbonyl iron powder (CIP)/silicone composite is proposed by a simple strategy of mixing, curing, and washing. Due to the porous structure induced by the sacrifice of sugar particles, an excellent linear response (R² = 0.999) is achieved for the composite sensor by manipulating the contributions of contact resistance and tunnel resistance to the sensing performance via the alternation of CB and CNT contents. Moreover, the porous structure donates the composite sensor a low compressive modulus at a low pressure level, while the CIPs introduced lead to a high compressive modulus at a high pressure level with the assistance of an external magnetic field. As a result, the sensor produced has a wide linear response range of 80 Pa to 220 kPa, much wider than most of the linear response pressure sensors reported previously. The wide detection range is demonstrated by cyclic pressure tests in the frequency range of 0.1-5 Hz, durability tests, and monitoring human or robot motions including breathing, walking, lifting, and boxing, etc. Taking the advantages of low cost, high sensitivity, and excellent linear response in a wide pressure range, the current composite sensor is promising for precise monitoring of human motions and delicate controlling of robots.

A flexible pressure sensor with highly customizable sensitivity and linearity via positive design of microhierarchical structures with a hyperelastic model

The tactile pressure sensor is of great significance in flexible electronics, but sensitivity customization over the required working range with high linearity still remains a critical challenge. Despite numerous efforts to achieve high sensitivity and a wide working range, most sensitive microstructures tend to be obtained only by inverting naturally existing templates without rational design based on fundamental contact principles or models for piezoresistive pressure sensors. Here, a positive design strategy with a hyperelastic model and a Hertzian contact model for comparison was proposed to develop a flexible pressure sensor with highly customizable linear sensitivity and linearity, in which the microstructure distribution was precalculated according to the desired requirement prior to fabrication. As a proof of concept, three flexible pressure sensors exhibited sensitivities of 0.7, 1.0, and 1.3 kPa^{- 1} over a linear region of up to 200 kPa, with a low sensitivity error (<5%) and high linearity (~0.99), as expected. Based on the superior electromechanical performance of these sensors, potential applications in physiological signal recognition are demonstrated as well, and such a strategy could shed more light on demand-oriented scenarios, including designable working ranges and linear sensitivity for next-generation wearable devices.