Ultrasensitive and Highly Stable Resistive Pressure Sensors with Biomaterial-Incorporated Interfacial Layers for Wearable Health-Monitoring and Human-Machine Interfaces

Inductive Paper-Based Flexible Contact Force Sensor Utilizing Natural Micro-Nanostructures of Paper: Simplicity, Economy, and Eco-Friendliness

Inductive contact force sensors, known for their high precision and anti-interference capabilities, hold significant potential applications in fields such as wearable and medical monitoring devices. Most of the current research on inductive contact force sensors employed novel nanomaterials as sensitive elements to enhance their sensitivity and other performance characteristics. However, sensors developed through such methods typically involve complex preparation processes, high costs, and difficulty in biodegradation, which limit their further development. This article introduces a new flexible inductive contact force sensor using paper as a sensitive element. Paper inherently possesses micro- and nanostructures on its surface and interior, enabling it to sensitively convert changes in contact force into changes in displacement, making it suitable for use as the sensor's sensitive element. Additionally, the advantages of paper also include its great flexibility, low cost, wide availability, and biodegradability. Performance testing on this flexible sensor showed good repeatability, hysteresis, sensitivity, and consistency. When used in experiments for monitoring human motion and respiration, this sensor also exhibited great detection performance. The proposed inductive paper-based flexible contact force sensor, with its simple structure, easy manufacturing process, cost-effectiveness, eco-friendliness, and good sensing performance, provides new insights into research for contact force sensors.

Integrated Actuation and Sensing: Toward Intelligent Soft Robots

Soft robotics has received substantial attention due to its remarkable deformability, making it well-suited for a wide range of applications in complex environments, such as medicine, rescue operations, and exploration. Within this domain, the interaction of actuation and sensing is of utmost importance for controlling the movements and functions of soft robots. Nonetheless, current research predominantly focuses on isolated actuation and sensing capabilities, often neglecting the critical integration of these 2 domains to achieve intelligent functionality. In this review, we present a comprehensive survey of fundamental actuation strategies and multimodal actuation while also delving into advancements in proprioceptive and haptic sensing and their fusion. We emphasize the importance of integrating actuation and sensing in soft robotics, presenting 3 integration methodologies, namely, sensor surface integration, sensor internal integration, and closed-loop system integration based on sensor feedback. Furthermore, we highlight the challenges in the field and suggest compelling directions for future research. Through this comprehensive synthesis, we aim to stimulate further curiosity among researchers and contribute to the development of genuinely intelligent soft robots.

Highly Sensitive Paper-Based Force Sensors with Natural Micro-Nanostructure Sensitive Element

Flexible paper-based force sensors have garnered significant attention for their important potential applications in healthcare wearables, portable electronics, etc. However, most studies have only used paper as the flexible substrate for sensors, not fully exploiting the potential of paper's micro-nanostructure for sensing. This article proposes a novel approach where paper serves both as the sensitive element and the flexible substrate of force sensors. Under external mechanical forces, the micro-nanostructure of the conductive-treated paper will change, leading to significant changes in the related electrical output and thus enabling sensing. To demonstrate the feasibility and universality of this new method, the article takes paper-based capacitive pressure sensors and paper-based resistive strain sensors as examples, detailing their fabrication processes, constructing sensing principle models based on the micro-nanostructure of paper materials, and testing their main sensing performance. For the capacitive paper-based pressure sensor, it achieves a high sensitivity of 1.623 kPa-1, a fast response time of 240 ms, and a minimum pressure resolution of 4.1 Pa. As for the resistive paper-based strain sensor, it achieves a high sensitivity of 72 and a fast response time of 300 ms. The proposed new method offers advantages such as high sensitivity, simplicity in the fabrication process, environmental friendliness, and cost-effectiveness, providing new insights into the research of flexible force sensors.

Predicting the Pressure Behavior and Sensing Property of Elastic Pressure Exerting and Sensing Fabrics for Compression Textiles

Using compression textiles to exert an appropriate and steady pressure on human limbs is a primary treatment method in the medical area. Compression pressure is a crucial parameter that determines the treatment efficacy. However, there is a lack of pressure-sensing fabrics that can both apply and measure the pressure of compression textiles, particularly the theoretical study of the prediction of the pressure and sensing performance of such a sensing fabric. In this study, based on the developed elastic pressure-exerting and -sensing fabrics and a setup test protocol simulating the pressure-exerting process, the relationships between the displacement of the press head, resultant fabric extension, and pressure were theoretically explored. Two finite element (FE) models, continuum and discontinuous models, were first established to predict the pressure behavior of elastic pressure-exerting and -sensing fabrics. The simulation results present good agreement with the experimental results wherein the pressure generated increases with the increase of the fabric strain in a nonlinear form. Furthermore, with the above FE models for the relationship between fabric extension and pressure generated, as well as the measured electrical resistance of the sensing fabric, a model for the electrical resistance of the sensing fabric can thus be established. Among pressure-sensing fabrics in three different structures, the sensing fabric in sateen exhibits better pressure prediction accuracy and a faster response to the pressure change. Finally, a series of numerical simulations were conducted to investigate the effects of the press head diameter, the unit cell crimp factor of fabric and the fabric pretension on the fabric extension, the resultant pressure, and electrical resistance change. The simulation results show that the pressure decreases with the increase of the press head diameter. The crimp factor and pretension of the sensing fabric also have a significant effect on the pressure and electrical resistance change generated. This simulation approach provides a new theoretical understanding of the pressure behavior and mechanism of pressure-sensing fabrics for future smart compression textiles.

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.

Nanoengineering Ultrathin Flexible Pressure Sensor with Superior Sensitivity and Perfect Conformability

Flexible pressure sensors play an increasingly important role in a wide range of applications such as human health monitoring, soft robotics, and human-machine interfaces. To achieve a high sensitivity, a conventional approach is introducing microstructures to engineer the internal geometry of the sensor. However, this microengineering strategy requires the sensor's thickness to be typically at hundreds to thousands of microns level, impairing the sensor's conformability on surfaces with microscale roughness like human skin. In this manuscript, a nanoengineering strategy is pioneered that paves a path to resolve the conflicts between sensitivity and conformability. A dual-sacrificial-layer method is initiated that facilitates ease of fabrication and precise assembly of two functional nanomembranes to manufacture the thinnest resistive pressure sensor with a total thickness of ≈850 nm that achieves perfectly conformable contact to human skin. For the first time, the superior deformability of the nanothin electrode layer on a carbon nanotube conductive layer is utilized by the authors to achieve a superior sensitivity (92.11 kPa-1 ) and an ultralow detection limit (<0.8 Pa). This work offers a new strategy that is able to overcome a key bottleneck for current pressure sensors, therefore is of potential to inspire the research community for a new wave of breakthroughs.

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.

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.

An ultrastretchable, high-performance, and crosstalk-free proximity and pressure bimodal sensor based on ionic hydrogel fibers for human-machine interfaces

The traditional human-machine interaction mode of communicating solely with pressure sensors needs modification, especially at a time when COVID-19 is circulating globally. Here, a transparent, stretchable, resilient, and high-performance hydrogel fiber-based bimodal sensor is fabricated by using a polyacrylamide-alginate double network hydrogel, which features high sensitivity (3.17% cm^-1), wide working range (18 cm), fast response/recovery speeds (90/90 ms) and good stability in proximity sensing, and impressive pressure sensing performance, including high sensitivity (0.91 kPa^-1), short response/recovery time (40/40 ms), low detection limit (63 Pa) and good linearity. Moreover, the response switch between proximity/pressure modes is measured and non-interfering dual-mode detection is achieved. Notably, the stretchable bimodal sensor is capable of working under 100% tensile strain without degrading the sensing performance. Specifically, the proximity sensor shows good immunity to the strain, while the pressure sensitivity is even promoted. Furthermore, the sensor is tough enough to work normally after punctures from a knife and strikes from a wrench. Notably, the sensor can be used for gesture recognition and subtle pressure detection, such as small water droplets (10 mg), wrist pulse, etc. A 3 × 3 array is further shown for accurate spatial sensing and location identification, verifying the feasibility of its practical application.

Recent Advances in Touch Sensors for Flexible Wearable Devices

Many modern user interfaces are based on touch, and such sensors are widely used in displays, Internet of Things (IoT) projects, and robotics. From lamps to touchscreens of smartphones, these user interfaces can be found in an array of applications. However, traditional touch sensors are bulky, complicated, inflexible, and difficult-to-wear devices made of stiff materials. The touch screen is gaining further importance with the trend of current IoT technology flexibly and comfortably used on the skin or clothing to affect different aspects of human life. This review presents an updated overview of the recent advances in this area. Exciting advances in various aspects of touch sensing are discussed, with particular focus on materials, manufacturing, enhancements, and applications of flexible wearable sensors. This review further elaborates on the theoretical principles of various types of touch sensors, including resistive, piezoelectric, and capacitive sensors. The traditional and novel hybrid materials and manufacturing technologies of flexible sensors are considered. This review highlights the multidisciplinary applications of flexible touch sensors, such as e-textiles, e-skins, e-control, and e-healthcare. Finally, the obstacles and prospects for future research that are critical to the broader development and adoption of the technology are surveyed.

An Artificial Tactile Neuron Enabling Spiking Representation of Stiffness and Disease Diagnosis

Mechanical properties of biological systems provide useful information about the biochemical status of cells and tissues. Here, an artificial tactile neuron enabling spiking representation of stiffness and spiking neural network (SNN)-based learning for disease diagnosis is reported. An artificial spiking tactile neuron based on an ovonic threshold switch serving as an artificial soma and a piezoresistive sensor as an artificial mechanoreceptor is developed and shown to encode the elastic stiffness of pressed materials into spike frequency evolution patterns. SNN-based learning of ultrasound elastography images abstracted by spike frequency evolution rate enables the classification of malignancy status of breast tumors with a recognition accuracy up to 95.8%. The stiffness-encoding artificial tactile neuron and learning of spiking-represented stiffness patterns hold a great promise for the identification and classification of tumors for disease diagnosis and robot-assisted surgery with low power consumption, low latency, and yet high accuracy.

Design Rules for a Wearable Micro-Fabricated Piezo-Resistive Pressure Sensor

Wearable flexible piezo-resistive pressure sensors hold a wide-ranging potential in human health monitoring, electronic skin, robotic limbs, and other human–machine interfaces. Out of the most successful recent efforts for arterial pulse monitoring are sensors with micro-patterned conductive elastomers. However, a low-current output signal (typically in the range of nano-amperes) and bulky and expensive measurement equipment for useful signal acquisition inhibits their wearability. Herein, through a finite element analysis we establish the design rules for a highly sensitive piezo-resistive pressure sensor with an output that is high enough to be detectable by simple and inexpensive circuits and therefore ensure wearability. We also show that, out of four frequently reported micro-feature shapes in micro-patterned piezo-resistive sensors, the micro-dome and micro-pyramid yield the highest sensitivity. Furthermore, investigations of different conductivity values of micro-patterned elastomers found that coating the elastomer with a conductive material (usually metallic) leads to higher current response when compared to composited conductive elastomers. Finally, the geometric parameters and spatial configurations of micro-pyramid design of piezo-resistive sensors were optimized. The results show that an enhanced sensitivity and higher current output is achieved by the lower spatial density configuration of three micro-features per millimeter length, a smaller feature size of around 100 μm, and a 60–50 degrees pyramid angle.

Extensible and self-recoverable proteinaceous materials derived from scallop byssal thread

Biologically derived and biologically inspired fibers with outstanding mechanical properties have found attractive technical applications across diverse fields. Despite recent advances, few fibers can simultaneously possess high-extensibility and self-recovery properties especially under wet conditions. Here, we report protein-based fibers made from recombinant scallop byssal proteins with outstanding extensibility and self-recovery properties. We initially investigated the mechanical properties of the native byssal thread taken from scallop Chlamys farreri and reveal its high extensibility (327 ± 32%) that outperforms most natural biological fibers. Combining transcriptome and proteomics, we select the most abundant scallop byssal protein type 5-2 (Sbp5-2) in the thread region, and produce a recombinant protein consisting of 7 tandem repeat motifs (rTRM7) of the Sbp5-2 protein. Applying an organic solvent-enabled drawing process, we produce bio-inspired extensible rTRM7 fiber with high-extensibility (234 ± 35%) and self-recovery capability in wet condition, recapitulating the hierarchical structure and mechanical properties of the native scallop byssal thread. We further show that the mechanical properties of rTRM7 fiber are highly regulated by hydrogen bonding and intermolecular crosslinking formed through disulfide bond and metal-carboxyl coordination. With its outstanding mechanical properties, rTRM7 fiber can also be seamlessly integrated with graphene to create motion sensors and electrophysiological signal transmission electrode.

Bio-inspired materials are an intense area of study as researchers try to adapt biomaterials for other applications. Here, the authors report on the processing of protein materials derived from the byssal thread of scallops to create high-extensibility materials with self-recovery under wet conditions.

Bioinspired Stretchable Fiber-Based Sensor toward Intelligent Human-Machine Interactions

Wearable integrated sensing devices with flexible electronic elements exhibit enormous potential in human-machine interfaces (HMI), but they have limitations such as complex structures, poor waterproofness, and electromagnetic interference. Herein, inspired by the profile of Lindernia nummularifolia (LN), a bionic stretchable optical strain (BSOS) sensor composed of an LN-shaped optical fiber incorporated with a stretchable substrate is developed for intelligent HMI. Such a sensor enables large strain and bending angle measurements with temperature self-compensation by the intensity difference of two fiber Bragg gratings' (FBGs') center wavelength. Such configurations enable an excellent tensile strain range of up to 80%, moreover, leading to ultrasensitivity, durability (≥20,000 cycles), and waterproofness. The sensor is also capable of measuring different human activities and achieving HMI control, including immersive virtual reality, robot remote interactive control, and personal hands-free communication. Combined with the machine learning technique, gesture classification can be achieved using muscle activity signals captured from the BSOS sensor, which can be employed to obtain the motion intention of the prosthetic. These merits effectively indicate its potential as a solution for medical care HMI and show promise in smart medical and rehabilitation medicine.

Flexible Cotton Fiber-Based Composite Films with Excellent Bending Stability and Conductivity at Cryogenic Temperature

The commonly used metal thin film or resin-based flexible composites cannot meet the requirement of cryogenic flexible conductive functional devices, which may be used in space exploration, biomedicine, and other science and technology fields facing a very low temperature environment, because of their poor fatigue and anti-bending properties at cryogenic temperature. In this work, a composite based on functionalized cotton fibers is proposed to achieve the application requirement of flexible electrical systems at cryogenic temperature. A conductive composite film with optimized strength and flexibility was obtained by controlling the size distribution of cotton fibers and adjusting the interaction force among the cotton fibers. The obtained composite film could endure over 10,000 times of bending at 77 K (-196 °C), with the resistance changing less than ±5%, indicating its excellent mechanical flexibility and electrical stability at cryogenic temperature. Finally, a demonstration was successfully conducted by applying the composite film as a flexible electrical connection to a robot arm, which worked at 77 K. This work might be a reference significance for the application of flexible conductors from room temperature to cryogenic temperature.

A stretchable, self-healing, okra polysaccharide-based hydrogel for fast-response and ultra-sensitive strain sensors

Self-healing conductive hydrogels have attracted widespread attention as a new generation of smart wearable devices and human motion monitoring sensors. To improve the biocompatibility and degradability of such strain sensors, we report a sensor with a sandwich structure based on a biomucopolysaccharide hydrogel. The sensor was constructed with a stretchable self-healing hydrogel composed of polyvinyl alcohol (PVA), okra polysaccharide (OP), borax, and a conductive layer of silver nanowires. The obtained OP/PVA/borax hydrogel exhibited excellent stretchability (~1073.7%) and self-healing ability (93.6% within 5 min), and the resultant hydrogel-based strain sensor demonstrated high sensitivity (gauge factor = 6.34), short response time (~20 ms), and good working stability. This study provides innovative ideas for the development of biopolysaccharide hydrogels for applications in the field of sensors.

Force-Sensitive Interface Engineering in Flexible Pressure Sensors: A Review

Flexible pressure sensors have received extensive attention in recent years due to their great importance in intelligent electronic devices. In order to improve the sensing performance of flexible pressure sensors, researchers are committed to making improvements in device materials, force-sensitive interfaces, and device structures. This paper focuses on the force-sensitive interface engineering of the device, which listing the main preparation methods of various force-sensitive interface microstructures and describing their respective advantages and disadvantages from the working mechanisms and practical applications of the flexible pressure sensor. What is more, the device structures of the flexible pressure sensor are investigated with the regular and irregular force-sensitive interface and accordingly the influences of different device structures on the performance are discussed. Finally, we not only summarize diverse practical applications of the existing flexible pressure sensors controlled by the force-sensitive interface but also briefly discuss some existing problems and future prospects of how to improve the device performance through the adjustment of the force-sensitive interface.

Flexible and Stable Carbon Nanotube Film Strain Sensors with Self-Derived Integrated Electrodes

The development of flexible and wearable electronic devices has put an increasing demand on electrode systems with seamless connection and high compatibility with the main device, in order to accommodate complex deformation conditions and maintain stable performance. Here, we present a carbon nanotube-integrated electrode (CNTIE) by wet-pulling the ends of a carbon nanotube (CNT) film to form condensed thin fibers that resemble conventional conducting wire electrodes. A flexible strain sensor was constructed consisting of the middle CNT film as the main functional part and the CNTIE as self-derived electrodes, with inherent CNT connection between the two parts. The sensor can be transferred to versatile substrates (e.g., balloon surface) or encapsulated in thermoplastic polymers, exhibiting a large linear response range (up to 1000% in tensile strain), excellent durability and repeatability over 5000 cycles, and the ability to detect small- to large-degree human body motions. In addition, the strain sensor based on the CNTIE hybrid film (MXene/CNT and graphene/CNT) also shows superior linearity and stability at a strain range of 0-800%. Compared with the sensors using traditional silver wire electrodes and separately fabricated CNT fiber electrodes, our CNTIE plays an important role in achieving highly stable performance in the strain cycles. Our self-derived integrated electrodes provide a potential route to solve the incompatibility issues of conventional electrodes and to develop high-performance flexible and wearable systems based on CNTs and other nanomaterials.

Flexible Pressure Sensors Based on the Controlled Buckling of Doped Semiconducting Polymer Nanopillars

Mechanically flexible and electrically conductive nanostructures are highly desired for flexible piezoresistive pressure sensors toward health monitoring or robotic skin applications. The popular approach for these sensors is to combine flexible but insulating polymers as a micro- or nanostructural functional medium and conductive materials covering the polymer surface, which could give rise to many practical issues, for example, durability, compatibility, and complicated processing steps. We herein report a piezoresistive pressure sensor with a functional component of nanopillars of a doped semiconducting polymer, operating at low bias voltage with a sensing mechanism based on controlled buckling. Nanopillars of poly(3-hexylthiophene-2,5-diyl) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane are patterned using anodic aluminum oxide templates. The nanopillars impart reversible current changes in response to the applied pressure over a wide pressure range (0-400 kPa). The sensor exhibits two current response regimes. Below 50 kPa, a strongly nonlinear response is observed, and above 50 kPa, a linear pressure response is demonstrated. Euler buckling theory is used to predict the deformation behavior of the nanopillars under pressure and in turn elucidate the sensing mechanism. Our results demonstrate that the contact area between the nanopillars and the top electrode increases with the application of pressure due to their elastic buckling in a two-regime fashion underlining the two electrical current response regimes of the sensor. Independent finite element modeling and scanning electron microscopy measurements corroborated this sensing mechanism. In contrast to many reported pressure sensors, the controlled elastic buckling of the nanopillars enables the detection of pressure over a wide range with good sensitivity, excellent reproducibility, and cycling stability.

Flexible wearable sensors - an update in view of touch-sensing

Nowadays, much of user interface is based on touch and the touch sensors have been common for displays, Internet of things (IoT) projects, or robotics. They can be found in lamps, touch screens of smartphones, or other wide arrays of applications as well. However, the conventional touch sensors, fabricated from rigid materials, are bulky, inflexible, hard, and hard-to-wear devices. The current IoT trend has made these touch sensors increasingly important when it added in the skin or clothing to affect different aspects of human life flexibly and comfortably. The paper provides an overview of the recent developments in this field. We discuss exciting advances in materials, fabrications, enhancements, and applications of flexible wearable sensors under view of touch-sensing. Therein, the review describes the theoretical principles of touch sensors, including resistive, capacitive, and piezoelectric types. Following that, the conventional and novel materials, as well as manufacturing technologies of flexible sensors are considered to. Especially, this review highlights the multidisciplinary approaches such as e-skins, e-textiles, e-healthcare, and e-control of flexible touch sensors. Finally, we summarize the challenges and opportunities that use is key to widespread development and adoption for future research.

Waterproof, thin, high-performance pressure sensors-hand drawing for underwater wearable applications

Wearable sensors, especially pressure sensors, have become an indispensable part of life when reflecting human interactions and surroundings. However, the difficulties in technology and production-cost still limit their applicability in the field of human monitoring and healthcare. Herein, we propose a fabrication method with flexible, waterproof, thin, and high-performance circuits - based on hand-drawing for pressure sensors. The shape of the sensor is drawn on the pyralux film without assistance from any designing software and the wet-tissues coated by CNTs act as a sensing layer. Such sensor showed a sensitivity (~0.2 kPa^-1) while ensuring thinness (~0.26 mm) and flexibility for touch detection or breathing monitoring. More especially, our sensor is waterproof for underwater wearable applications, which is a drawback of conventional paper-based sensors. Its outstanding capability is demonstrated in a real application when detecting touch actions to control a phone, while the sensor is dipped underwater. In addition, by leveraging machine learning technology, these touch actions were processed and classified to achieve highly accurate monitoring (up to 94%). The available materials, easy fabrication techniques, and machine learning algorithms are expected to bring significant contributions to the development of hand-drawing sensors in the future.

Metamaterials-Enabled Sensing for Human-Machine Interfacing

Our modern lives have been radically revolutionized by mechanical or electric machines that redefine and recreate the way we work, communicate, entertain, and travel. Whether being perceived or not, human-machine interfacing (HMI) technologies have been extensively employed in our daily lives, and only when the machines can sense the ambient through various signals, they can respond to human commands for finishing desired tasks. Metamaterials have offered a great platform to develop the sensing materials and devices from different disciplines with very high accuracy, thus enabling the great potential for HMI applications. For this regard, significant progresses have been achieved in the recent decade, but haven’t been reviewed systematically yet. In the Review, we introduce the working principle, state-of-the-art sensing metamaterials, and the corresponding enabled HMI applications. For practical HMI applications, four kinds of signals are usually used, i.e., light, heat, sound, and force, and therefore the progresses in these four aspects are discussed in particular. Finally, the future directions for the metamaterials-based HMI applications are outlined and discussed.

Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications

In recent years, innovative research has been widely conducted on flexible devices for wearable electronics applications. Many examples of wearable electronics, such as smartwatches and glasses, are already available to consumers. However, strictly speaking, the sensors used in these devices are not flexible. Many studies are underway to address a wider range of wearable electronics and the development of related fields is progressing very rapidly. In particular, there is intense interest in the research field of flexible pressure sensors because they can collect and use information regarding a wide variety of sources. Through the combination of novel materials and fabrication methods, human-machine interfaces, biomedical sensors, and motion detection techniques, it is now possible to produce sensors with a superior level of performance to meet the demands of wearable electronics. In addition, more compact and human-friendly sensors have been invented in recent years, as biodegradable and self-powered sensor systems have been studied. In this review, a comprehensive description of flexible pressure sensors will be covered, and design strategies that meet the needs for applications in wearable electronics will be presented. Moreover, we will cover several fabrication methods to implement these technologies and the corresponding real-world applications.

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.

Neuron-Inspired Self-Healing Composites via Dynamic Construction of Polypyrrole-Decorated Carbon Nanotubes for Smart Physiochemical Sensing

Mimicking human skin's functions to develop intelligent materials have inspired extensive exploration in the design and synthesis of a novel device. However, how to simulate neuron function and integrate highly sensitive, positive perceptions and self-healing into one single material remains a challenge. Here, we prepared a recycled polyurethane (PU) with high tensile strength values (11.37 ± 0.03 MPa), high maximum elongation (1130 ± 11.59%), and high self-healing property (100% for 6 h at 25 °C) and a smart PU composite of polypyrrole-decorated carbon nanotubes with higher sensitivity. The smart composite can not only actively identify physical change such as strain, moisture, and temperature but also proactively detect various chemical environment changes such as acid, alkali, oxidant, and reductant (T: 25-90 °C, ΔR/R₀ values were 0.1-1.6; strain: 10-150%, ΔR/R₀ values were 2.5-27; 0.01-0.1 mol L^-1 oxidant solutions, ΔR/R₀ values were 0.66-0.75; 0.01-0.1 mol L^-1 reductant solutions, ΔR/R₀ values were 0.51-0.65; 0.1-0.5 mol L^-1 acid solutions, ΔR/R₀ values were 0.54-0.58; and 0.1-0.5 mol L^-1 alkali solutions, ΔR/R₀ values were 0.42-0.46). More importantly, the signal values of the smart composite can quickly return to the initial values after eliminating physical and chemical stimuli. The abovementioned features of the smart composite, the high physicochemical response, and significant restorability make it potentially possible to apply it in intelligent chemical manufacturing.

Highly Sensitive, Breathable, and Flexible Pressure Sensor Based on Electrospun Membrane with Assistance of AgNW/TPU as Composite Dielectric Layer

Pressure sensors have been widely used in electronic wearable devices and medical devices to detect tiny physical movements and mechanical deformation. However, it remains a challenge to fabricate desirable, comfortable wearing, and highly sensitive as well as fast responsive sensors to capture human body physiological signs. Here, a new capacitive flexible pressure sensor that is likely to solve this problem was constructed using thermoplastic polyurethane elastomer rubber (TPU) electrospinning nanofiber membranes as a stretchable substrate with the incorporation of silver nanowires (AgNWs) to build a composite dielectric layer. In addition, carbon nanotubes (CNTs) were painted on the TPU membranes as flexible electrodes by screen printing to maintain the flexibility and breathability of the sensors. The flexible pressure sensor could detect tiny body signs; fairly small physical presses and mechanical deformation based on the variation in capacitance due to the synergistic effects of microstructure and easily altered composite permittivity of AgNW/TPU composite dielectric layers. The resultant sensors exhibited high sensitivity (7.24 kPa^-1 within the range of 9.0 × 10^-3 ~ 0.98 kPa), low detection limit (9.24 Pa), and remarkable breathability as well as fast responsiveness (<55 ms). Moreover, both continuously pressing/releasing cycle over 1000 s and bending over 1000 times did not impair the sensitivity, stability, and durability of this flexible pressure sensor. This proposed strategy combining the elastomer nanofiber membrane and AgNW dopant demonstrates a cost-effective and scalable fabrication of capacitive pressure sensors as a promising application in electronic skins and wearable devices.

Large-Area Integrated Triboelectric Sensor Array for Wireless Static and Dynamic Pressure Detection and Mapping

Large-area flexible pressure sensors are of paramount importance for various future applications, such as electronic skin, human-machine interfacing, and health-monitoring devices. Here, a self-powered and large-area integrated triboelectric sensor array (ITSA) based on coupling a triboelectric sensor array and an array chip of CD4066 through a traditional connection is reported. Enabled by a simple and cost-effective fabrication process, the size of the ITSA can be scaled up to 38 × 38 cm² . In addition, unlike previously proposed triboelectric sensors arrays, which can only react to the dynamic interaction, this ITSA is able to detect static and dynamic pressure. Moreover, through integrating the ITSA with a signal processing circuit, a complete wireless sensing system is present. Diverse applications of the system are demonstrated in detail, including detecting pressure, identifying position, tracking trajectory, and recognizing the profile of external contact objects. Thus, the ITSA in this work opens a new route in the direction of large-area, self-powered, and wireless triboelectric sensing systems.

Seeing pressure in color based on integration of highly sensitive pressure sensor and emission tunable light emitting diode

We demonstrate a highly sensitive, low-cost, environmental-friendly pressure sensor derived from a wool-based pressure sensor with wide pressure sensing range using wool bricks embedded with a Ag nano-wires. The easy fabrication and light weight allow portable and wearable device applications. Wth the integration of a light-emitting diode possessing multi-wavelength emission, we illustrate a hybrid multi-functional LED-integrated pressure sensor that is able to convert different applied pressures to light emission with different wavelengths. Due to the high sensitivity of the pressure sensor, the demonstration of acoustic signal detection has also been presented using sound of a metronome and a speaker playing a song. This multi-functional pressure sensor can be implemented to technologies such as smart lighting, health care, visible light communication (VLC), and other internet of things (IoT) applications.

Ultra-Sensitive Piezo-Resistive Sensors Constructed with Reduced Graphene Oxide/Polyolefin Elastomer (RGO/POE) Nanofiber Aerogels

Flexible wearable pressure sensors have received extensive attention in recent years because of the promising application potentials in health management, humanoid robots, and human machine interfaces. Among the many sensory performances, the high sensitivity is an essential requirement for the practical use of flexible sensors. Therefore, numerous research studies are devoted to improving the sensitivity of the flexible pressure sensors. The fiber assemblies are recognized as an ideal substrate for a highly sensitive piezoresistive sensor because its three-dimensional porous structure can be easily compressed and can provide high interconnection possibilities of the conductive component. Moreover, it is expected to achieve high sensitivity by raising the porosity of the fiber assemblies. In this paper, the three-dimensional reduced graphene oxide/polyolefin elastomer (RGO/POE) nanofiber composite aerogels were prepared by chemical reducing the graphene oxide (GO)/POE nanofiber composite aerogels, which were obtained by freeze drying the mixture of the GO aqueous solution and the POE nanofiber suspension. It was found that the volumetric shrinkage of thermoplastic POE nanofibers during the reduction process enhanced the compression mechanical strength of the composite aerogel, while decreasing its sensitivity. Therefore, the composite aerogels with varying POE nanofiber usage were prepared to balance the sensitivity and working pressure range. The results indicated that the composite aerogel with POE nanofiber/RGO proportion of 3:3 was the optimal sample, which exhibits high sensitivity (ca. 223 kPa⁻¹) and working pressure ranging from 0 to 17.7 kPa. In addition, the composite aerogel showed strong stability when it is either compressed with different frequencies or reversibly compressed and released 5000 times.

Toward Imperfection-Insensitive Soft Network Materials for Applications in Stretchable Electronics

Development of stretchable devices with mechanical responses that mimic those of biological tissues/organs is of particular importance for the long-term biointegration, as the discomfort induced by the mechanical mismatch can be minimized. Recent works have established the bioinspired designs of soft network materials that can precisely reproduce the unconventional J-shaped stress-strain curves of human skin at different regions. Existing studies mostly focused on the design, fabrication, and modeling of perfect soft network materials. When utilized as the substrates of biointegrated electronics, the soft network designs, however, often need to incorporate deterministic holes, a type of imperfection, to accommodate hard, inorganic electronic components. Understanding of the effect of hole imperfections on the mechanical properties of soft network materials is thereby essential in practical applications. This paper presents a combined experimental and computational study of the stretchability and elastic modulus of imperfect soft network materials consisting of circular holes with a variety of diameters. Both the size and location of the circular-hole imperfections are shown to have profound influences on the stretchability. Based on these results, design guidelines of imperfection-insensitive network materials are introduced. For the imperfections that result in an evident reduction of stretchability, an effective reinforcement approach is presented by enlarging the width of horseshoe microstructures at strategic locations, which can enhance the stretchability considerably. A stretchable and imperfection-insensitive integrated device with a light-emitting diode embedded in the network material serves a demonstrative application.

Transparent and Flexible Mayan-Pyramid-based Pressure Sensor using Facile-Transferred Indium tin Oxide for Bimodal Sensor Applications

Transparent and conducting flexible electrodes have been successfully developed over the last few decades due to their potential applications in optoelectronics. However, recent developments in smart electronics, such as a direct human-machine interface, health-monitoring devices, motion-tracking sensors, and artificially electronic skin also require materials with multifunctional properties such as transparency, flexibility and good portability. In such devices, there remains room to develop transparent and flexible devices such as pressure sensors or temperature sensors. Herein, we demonstrate a fully transparent and flexible bimodal sensor using indium tin oxide (ITO), which is embedded in a plastic substrate. For the proposed pressure sensor, the embedded ITO is detached from its Mayan-pyramid-structured silicon mold by an environmentally friendly method which utilizes water-soluble sacrificial layers. The Mayan-pyramid-based pressure sensor is capable of six different pressure sensations with excellent sensitivity in the range of 100 Pa-10 kPa, high endurance of 105 cycles, and good pulse detection and tactile sensing data processing capabilities through machine learning (ML) algorithms for different surface textures. A 5 × 5-pixel pressure-temperature-based bimodal sensor array with a zigzag-shaped ITO temperature sensor on top of it is also demonstrated without a noticeable interface effect. This work demonstrates the potential to develop transparent bimodal sensors that can be employed for electronic skin (E-skin) applications.

Wearable Piezoresistive Sensors with Ultrawide Pressure Range and Circuit Compatibility Based on Conductive-Island-Bridging Nanonetworks

Wearable pressure sensors with wide operating pressure ranges and enhanced wearability via seamless integration with circuits can greatly improve the fields of digital healthcare, prosthetic limbs, and human-machine interfaces. Herein, we report an approach based on a conductive-island-bridging nanonetwork to realize wearable resistive pressure sensors that are operative over ultrawide pressure ranges >400 kPa and are circuit-compatible. The sensor has a simple two-layered structure, where nanonetworks of single-walled carbon nanotubes selectively patterned on a surface-modified elastomeric film interface and bridge conductive Au island patterns on printed circuit boards (PCBs). We show that varying the design of the Au islands and the conductivity of the nanonetworks systematically tunes the sensitivity, linearity, and the operation range of the pressure sensor. In addition, introducing microstructured lead contacts into the sensor based on a Au-island-bridging nanonetwork produces a record-high sensitivity of 0.06 kPa^-1 at 400 kPa. Furthermore, the PCB that serves as the bottom layer of the pressure sensor and contains embedded interconnects enables facile integration of the sensor with measurement circuits and wireless communication modules. The developed sensor enables the monitoring of wrist pulse waves. Moreover, an insole-shaped PCB-based pressure-sensing system wirelessly monitors pressure distributions and gait kinetics during walking. Our scheme can be extended to other nanomaterials and flexible PCBs and thus provides a simple yet powerful platform for emerging wearable applications.

Controllable Configuration of Sensing Band in a Pressure Sensor by Lenticular Pattern Deformation on Designated Electrodes

Resistive-type pressure sensor, which are mainly utilized in industry, are easy to manufacture and are not significantly affected by external electromagnetic fields, unlike capacitive type. However, the produce signal is not linear, and it is also difficult to measure a wide range of pressures using such a sensor. Therefore, before being utilized, the extracted nonlinear data from them need to be processed by. A resistive sensor that is capable of measuring a wide range of pressure of up to 4 MPa with constant linearity is presented. Moreover it can selectively control the sensing pressure band, or act as an on/off switch, without the need for any additional computer processing.

Hydrogel-Templated Transfer-Printing of Conductive Nanonetworks for Wearable Sensors on Topographic Flexible Substrates

Transfer-printing enables the assembly of functional nanomaterials on unconventional substrates with a desired layout in a controllable manner. However, transfer-printing to substrates with complex surfaces remains a challenge. Herein, we show that hydrogels serve as effective template material platforms for the assembly and transfer-printing of conductive nanonetwork patterns for flexible sensors on various topographic surfaces in a very simple yet versatile manner. The non-adherence, nanoporous structure, and molding capability of the hydrophilic hydrogel enable the assembly of conductive nanonetwork patterns on the hydrogel surface and transfer of the nanonetworks onto various flexible and topographic substrates. Flexible strain sensors and pressure sensors that monitor finger motions and arterial pulses are successfully demonstrated using the hydrogel-templated approach. The rich chemistry of polymeric networks, facile molding capability, and biocompatibility of hydrogels could be further combined with additive technology for hydrogels and electronic materials for emerging four-dimensional functional materials and soft bioelectronics.

Multilayered Ag NP-PEDOT-Paper Composite Device for Human-Machine Interfacing

Flexible pressure sensors have attracted increasing interest because of their potential applications on wearable sensing devices for human-machine interface connections, but challenges regarding material cost, fabrication robustness, signal transduction, sensitivity improvement, detection range, and operation convenience still need to be overcome. Herein, with a simple, low-cost, and scalable approach, a flexible and wearable pressure-sensing device fabricated by utilizing filter paper as the solid support, poly(3,4-ethylenedioxythiophene) to enhance conductivity, and silver nanoparticles to provide a rougher surface is introduced. Sandwiching and laminating composite material layers with two thermoplastic polypropylene films lead to robust integration of sensing devices, where assembling four layers of composite materials results in the best sensitivity toward applied pressure. This practical pressure-sensing device possessing properties such as high sensitivity of 0.119 kPa^-1, high durability of 2000 operation cycles, and an ultralow energy consumption level of 10^-5 W is a promising candidate for contriving point-of-care wearable electronic devices and applying it to human-machine interface connections.

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.

Non-Invasive Flexible and Stretchable Wearable Sensors With Nano-Based Enhancement for Chronic Disease Care

Advances in flexible and stretchable electronics, functional nanomaterials, and micro/nano manufacturing have been made in recent years. These advances have accelerated the development of wearable sensors. Wearable sensors, with excellent flexibility, stretchability, durability, and sensitivity, have attractive application prospects in the next generation of personal devices for chronic disease care. Flexible and stretchable wearable sensors play an important role in endowing chronic disease care systems with the capability of long-term and real-time tracking of biomedical signals. These signals are closely associated with human body chronic conditions, such as heart rate, wrist/neck pulse, blood pressure, body temperature, and biofluids information. Monitoring these signals with wearable sensors provides a convenient and non-invasive way for chronic disease diagnoses and health monitoring. In this review, the applications of wearable sensors in chronic disease care are introduced. In addition, this review exploits a comprehensive investigation of requirements for flexibility and stretchability, and methods of nano-based enhancement. Furthermore, recent progress in wearable sensors-including pressure, strain, electrophysiological, electrochemical, temperature, and multifunctional sensors-is presented. Finally, opening research challenges and future directions of flexible and stretchable sensors are discussed.

Self-Powered Wearable Pressure Sensors with Enhanced Piezoelectric Properties of Aligned P(VDF-TrFE)/MWCNT Composites for Monitoring Human Physiological and Muscle Motion Signs

Self-powered operation, flexibility, excellent mechanical properties, and ultra-high sensitivity are highly desired properties for pressure sensors in human health monitoring and anthropomorphic robotic systems. Piezoelectric pressure sensors, with enhanced electromechanical performance to effectively distinguish multiple mechanical stimuli (including pressing, stretching, bending, and twisting), have attracted interest to precisely acquire the weak signals of the human body. In this work, we prepared a poly(vinylidene fluoride-trifluoroethylene)/ multi-walled carbon nanotube (P(VDF-TrFE)/MWCNT) composite by an electrospinning process and stretched it to achieve alignment of the polymer chains. The composite membrane demonstrated excellent piezoelectricy, favorable mechanical strength, and high sensitivity. The piezoelectric coefficient d33 value was approximately 50 pm/V, the Young's modulus was ~0.986 GPa, and the sensitivity was ~540 mV/N. The resulting composite membrane was employed as a piezoelectric pressure sensor to monitor small physiological signals including pulse, breath, and small motions of muscle and joints such as swallowing, chewing, and finger and wrist movements. Moderate doping with carbon nanotubes had a positive impact on the formation of the β phase of the piezoelectric device, and the piezoelectric pressure sensor has the potential for application in health care systems and smart wearable devices.

Hydrodynamic Layer-by-Layer Assembly of Transferable Enzymatic Conductive Nanonetworks for Enzyme-Sticker-Based Contact Printing of Electrochemical Biosensors

Realizing high-performance electrochemical biosensors in a simple contact-printing-based approach significantly increases the applicability of integrated flexible biosensors. Herein, an enzyme-sticker-based approach that enables flexible and multielectrochemical sensors via simple contact-transfer printing is reported. The enzyme sticker consists of an enzymatic conductive network film and a polymeric support. The enzyme-incorporated nanostructured conductive network showing an efficient electrical coupling was assembled via the hydrodynamic layer-by-layer assembly of redox enzymes, polyelectrolytes, single-walled carbon nanotubes, and a biological glue material, M13 phage. The enzymatic conductive network on a polymeric membrane support was facilely wet contact-transfer printed onto integrated electrode systems by exploiting varying degrees of hydrophilicity displayed by the enzymatic electronic film, polymeric support, and receiving electrodes of the sensor system. The glucose sensors fabricated using the enzyme sticker detected glucose at a concentration of as low as 35 μM and showed high selectivity and stability. Furthermore, a flexible dual-sensor array capable of detecting both glucose and lactate was demonstrated using the versatile enzyme sticker concept. This work presents a new route toward assembling and integrating hybrid nanomaterials with efficient electrochemical coupling for high-performance biosensors and health-monitoring devices as well as for emerging bioelectronics and electrochemical devices.