Flexible and Stretchable Capacitive Sensors with Different Microstructures

Self-Perceptional Soft Robotics by a Dielectric Elastomer

Soft robotics has been a rapidly growing field in recent decades due to its advantages of softness, deformability, and adaptability to various environments. However, the separation of perception and actuation in soft robot research hinders its progress toward compactness and flexibility. To address this limitation, we propose the use of a dielectric elastomer actuator (DEA), which exhibits both an actuation capability and perception stability. Specifically, we developed a DEA array to localize the 3D spatial position of objects. Subsequently, we integrate the actuation and sensing properties of DEA into soft robots to achieve self-perception. We have developed a system that integrates actuation and sensing and have proposed two modes to achieve this integration. Furthermore, we demonstrated the feasibility of this system for soft robots. When the robots detect an obstacle or an approaching object, they can swiftly respond by avoiding or escaping the obstacle. By eliminating the need for separate perception and motion considerations, self-perceptional soft robots can achieve an enhanced response performance and enable applications in a more compact and flexible field.

Electronic Skin for Health Monitoring Systems: Properties, Functions, and Applications

Electronic skin (e-skin), a skin-like wearable electronic device, holds great promise in the fields of telemedicine and personalized healthcare because of its good flexibility, biocompatibility, skin conformability, and sensing performance. E-skin can monitor various health indicators of the human body in real time and over the long term, including physical indicators (exercise, respiration, blood pressure, etc.) and chemical indicators (saliva, sweat, urine, etc.). In recent years, the development of various materials, analysis, and manufacturing technologies has promoted significant development of e-skin, laying the foundation for the application of next-generation wearable medical technologies and devices. Herein, the properties required for e-skin health monitoring devices to achieve long-term and precise monitoring and summarize several detectable indicators in the health monitoring field are discussed. Subsequently, the applications of integrated e-skin health monitoring systems are reviewed. Finally, current challenges and future development directions in this field are discussed. This review is expected to generate great interest and inspiration for the development and improvement of e-skin and health monitoring systems.

A Rational Design of Bio-Derived Disulfide CANs for Wearable Capacitive Pressure Sensor

Classic approaches to integrate flexible capacitive sensor performance are to on-demand microstructuring dielectric layers and to adjust dielectric material compositions via the introduction of insoluble carbon additives (to increase sensitivity) or dynamic interactions (to achieve self-healing). However, the sensor's enhanced performances often come with increased material complexity, discouraging its circular economy. Herein, a new intrinsic self-healable, closed-loop recyclable dielectric layer material, a fully nature-derived dynamic covalent poly(disulfide) decorated with rich H bonding and metal-catechol complexations is introduced. The polymer network possesses a mechanically ductile character with an Arrhenius-type temperature-dependent viscoelasticity. The assembled capacitive pressure sensor is able to achieve a sensitivity of up to 9.26 kPa-1, fast response/recovery time of 32/24 ms, and can deliver consistent signals of continuous consecutive cycles even after being self-healed or closed-loop recycled for real-time detection of human motions. This is expected to be of high interest for current capacitive sensing research to move toward a life-like, high performance, and circular economy direction.

A Review of Recent Advancements in Sensor-Integrated Medical Tools

A medical tool is a general instrument intended for use in the prevention, diagnosis, and treatment of diseases in humans or other animals. Nowadays, sensors are widely employed in medical tools to analyze or quantify disease-related parameters for the diagnosis and monitoring of patients' diseases. Recent explosive advancements in sensor technologies have extended the integration and application of sensors in medical tools by providing more versatile in vivo sensing capabilities. These unique sensing capabilities, especially for medical tools for surgery or medical treatment, are getting more attention owing to the rapid growth of minimally invasive surgery. In this review, recent advancements in sensor-integrated medical tools are presented, and their necessity, use, and examples are comprehensively introduced. Specifically, medical tools often utilized for medical surgery or treatment, for example, medical needles, catheters, robotic surgery, sutures, endoscopes, and tubes, are covered, and in-depth discussions about the working mechanism used for each sensor-integrated medical tool are provided.

A terahertz meta-sensor array for 2D strain mapping

Large-scale stretchable strain sensor arrays capable of mapping two-dimensional strain distributions have gained interest for applications as wearable devices and relating to the Internet of Things. However, existing strain sensor arrays are usually unable to achieve accurate directional recognition and experience a trade-off between high sensing resolution and large area detection. Here, based on classical Mie resonance, we report a flexible meta-sensor array that can detect the in-plane direction and magnitude of preloaded strains by referencing a dynamically transmitted terahertz (THz) signal. By building a one-to-one correspondence between the intrinsic electrical/magnetic dipole resonance frequency and the horizontal/perpendicular tension level, arbitrary strain information across the meta-sensor array is accurately detected and quantified using a THz scanning setup. Particularly, with a simple preparation process of micro template-assisted assembly, this meta-sensor array offers ultrahigh sensor density (~11.1 cm-2) and has been seamlessly extended to a record-breaking size (110 × 130 mm2), demonstrating its promise in real-life applications.

A Highly Sensitive and Stretchable Core-Shell Fiber Sensor for Gesture Recognition and Surface Pressure Distribution Monitoring

This work reports a highly-strain flexible fiber sensor with a core-shell structure utilizes a unique swelling diffusion technique to infiltrate carbon nanotubes (CNTs) into the surface layer of Ecoflex fibers. Compared with traditional blended Ecoflex/CNTs fibers, this manufacturing process ensures that the sensor maintains the mechanical properties (923% strain) of the Ecoflex fiber while also improving sensitivity (gauge factor is up to 3716). By adjusting the penetration time during fabrication, the sensor can be customized for different uses. As an application demonstration, the fiber sensor is integrated into the glove to develop a wearable gesture language recognition system with high sensitivity and precision. Additionally, the authors successfully monitor the pressure distribution on the curved surface of a soccer ball by winding the fiber sensor along the ball's surface.

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.

Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors

With the commercialization of first-generation flexible mobiles and displays in the late 2010s, humanity has stepped into the age of flexible electronics. Inevitably, soft multifunctional sensors, as essential components of next-generation flexible electronics, have attracted tremendous research interest like never before. This review is dedicated to offering an overview of the latest emerging trends in soft multifunctional sensors and their accordant future research and development (R&D) directions for the coming decade. First, key characteristics and the predominant target stimuli for soft multifunctional sensors are highlighted. Second, important selection criteria for soft multifunctional sensors are introduced. Next, emerging materials/structures and trends for soft multifunctional sensors are identified. Specifically, the future R&D directions of these sensors are envisaged based on their emerging trends, namely i) decoupling of multiple stimuli, ii) data processing, iii) skin conformability, and iv) energy sources. Finally, the challenges and potential opportunities for these sensors in future are discussed, offering new insights into prospects in the fast-emerging technology.

A Microphase-Separated Design toward an All-Round Ionic Hydrogel with Discriminable and Anti-Disturbance Multisensory Functions

Stretchable ionic hydrogels with superior all-round properties that can detect multimodal sensations with excellent discriminability and robustness against external disturbances are highly required for artificial electronic skinapplications. However, some critical material parameters exhibit intrinsic tradeoffs with each other for most ionic hydrogels. Here, a microphase-separated hydrogel is demonstrated by combining three strategies: (1) using of a low crosslinker/monomer ratio to obtain highly entangled polymer chains as the first network; (2) the introduction of zwitterions into the first network; (3) the synthesis of an ultrasoft polyelectrolyte as the second network. This all-round elastic ionic hydrogel exhibits a low Young's modulus (< 60 kPa), large stretchability (> 900%), high resilience (> 95%), unique strain-stiffening behavior, excellent fatigue tolerance, high ionic conductivity (> 2.0 S m⁻1), and anti-freezing capability, which have not been achieved before. These properties allow the ionic hydrogel to operate as a stretchable multimodal sensor that can detect and decouple multiple stimuli (temperature, pressure, and proximity) with excellent discriminability, high sensitivity, and strong sensing-robustness against strains or temperature perturbations. The ionic hydrogel sensor exhibits great potential for intelligent electronic skin applications such as reliable health monitoring and accurate object identification.

Bimodal Intelligent Electronic Skin Based on Proximity and Tactile Interaction for Pressure and Configuration Perception

The flexible bimodal e-skin exhibits significant promise for integration into the next iteration of human-computer interactions, owing to the integration of tactile and proximity perception. However, those challenges, such as low tactile sensitivity, complex fabrication processes, and incompatibility with bimodal interactions, have restricted the widespread adoption of bimodal e-skin. Herein, a bimodal capacitive e-skin capable of simultaneous tactile and proximity sensing has been developed. The entire process eliminates intricate fabrication techniques, employing DLP-3D printing for the electrode layers and sacrificial templating for the dielectric layers, conferring high tactile sensitivity (1.672 kPa-1) and rapid response capability (∼30 ms) to the bimodal e-skin. Moreover, exploiting the "fringing electric field" effect inherent in parallel-plate capacitors has facilitated touchless sensing, thereby enabling static distance recognition and dynamic gesture recognition of varying materials. Interestingly, an e-skin sensing array was created to identify the positions and pressure levels of various objects of different masses. Furthermore, with the aid of machine learning techniques, an artificial neural network has been established to possess intelligent object recognition capabilities, facilitating the identification, classification, and training of various object configurations. The advantages of the bimodal e-skin render it highly promising for extensive applications in the field of next-generation human-machine interaction.

Highly Sensitive Iontronic Pressure Sensor with Side-by-Side Package Based on Alveoli and Arch Structure

Flexible pressure sensors play a significant role in wearable devices and electronic skin. Iontronic pressure sensors with high sensitivity, wide measurement range, and high resolution can meet requirements. Based on the significant deformation characteristics of alveoli to improve compressibility, and the ability of the arch to disperse vertical pressure into horizontal thrust to increase contact area, a graded hollow ball arch (GHBA) microstructure is proposed, greatly improving sensitivity. The fabrication of GHBA ingeniously employs a double-sided structure. One side uses mold casting to create convex structures, while the other utilizes the evaporation of moisture during the curing process to form concave structures. At the same time, a novel side-by-side package structure is proposed, ensuring pressure on flexible substrate is maximally transferred to the GHBA microstructure. Within the range of 0.2 Pa-300 kPa, the iontronic pressure sensor achieves a maximum sensitivity of 10 420.8 kPa-1 , pressure resolution of 0.1% under the pressure of 100 kPa, and rapid response/recovery time of 40/35 ms. In wearable devices, it is capable of monitoring dumbbell curl exercises and wirelessly correcting sitting positions. In electronic skin, it can non-contactly detect the location of the wind source and achieve object classification prediction when combined with the CNN model.

Capacitive Pressure Sensor Combining Dual Dielectric Layers with Integrated Composite Electrode for Wearable Healthcare Monitoring

Foot activity can reflect numerous physiological abnormalities in the human body, making gait a valuable metric in health monitoring. Research on flexible sensors for gait monitoring has focused on high sensitivity, wide working range, fast response, and low detection limit, but challenges remain in areas such as elasticity, antibacterial activity, user-friendliness, and long-term stability. In this study, we have developed a novel capacitive pressure sensor that offers an ultralow detection limit of 1 Pa, wide detection ranges from 1 Pa to 2 MPa, a high sensitivity of 0.091 kPa-1, a fast response time of 71 ms, and exceptional stability over 6000 cycles. This sensor not only has the ability of accurately discriminating mechanical stimuli but also meets the requirements of elasticity, antibacterial activity, wearable comfort, and long-term stability for gait monitoring. The fabrication method of a dual dielectric layer and integrated composite electrode is simple, cost-effective, stable, and amenable to mass production. Thereinto, the introduction of a dual dielectric layer, based on an optimized electrospinning network and micropillar array, has significantly improved the sensitivity, detection range, elasticity, and antibacterial performance of the sensor. The integrated flexible electrodes are made by template method using composite materials of carbon nanotubes (CNTs), two-dimensional titanium carbide Ti3C2Tx (MXene), and polydimethylsiloxane (PDMS), offering synergistic advantages in terms of conductivity, stability, sensitivity, and practicality. Additionally, we designed a smart insole that integrates the as-prepared sensors with a miniature instrument as a wearable platform for gait monitoring and disease warning. The developed sensor and wearable platform offer a cutting-edge solution for monitoring human activity and detecting diseases in a noninvasive manner, paving the way for future wearable devices and personalized healthcare technologies.

High Signal-to-Noise Ratio Event-Driven MEMS Motion Sensing

Two solutions for improving MEMS triboelectric vibration sensors performance in contact-separation mode are reported experimentally and analytically. Triboelectric sensors have mostly been studied in the mesoscale. The gap variation between the electrodes induces a potential difference that represents the external vibration. Miniaturizing the device limits the sensor output because of the limited gap. This work offers a warped MEMS diaphragm constrained on its edges. The dome-shaped structure provides one order of magnitude larger displacement after contact-separation than standard designs resulting in one order of magnitude greater voltage and signal-to-noise-ratio. Second, micro triboelectric sensors do not operate unless the external vibration is sufficiently forceful to initiate contact between layers. The proposed constraints on the edge of the diaphragm provide friction during periodic motion and generate charges. The combination of the warped diaphragm and boundary constraints instead of serpentine springs increases the charge density and voltage generation. The mechanical properties and electrical output are thoroughly investigated including nonlinearity, sensitivity, and signal-to-noise ratio. A sensitivity of 250 mV g-1 and signal-to-noise-ratio of 32 dB is provided by the presented device at resonance, which is very promising for event-driven motion sensors because it does not require signal conditioning and therefore simplifies the sensing circuitry.

Ultraconformable Capacitive Strain Sensor Utilizing Network Structure of Single-Walled Carbon Nanotubes for Wireless Body Sensing

Capture and real-time recording of precise body movements using strain sensors provide personal information for healthcare monitoring and management. To acquire this information, a sensor that conforms to curved irregular surfaces, including biological tissue, is desired to record complex body movements while acting like a second skin to avoid interference with the movements. In this study, we developed a thin-film-type capacitive strain sensor that is flexible and stretchable on the surface of a living body. We fabricated conductive polymeric ultrathin films ("nanosheets") comprising polystyrene-block-polybutadiene (SB) elastomers and single-walled carbon nanotubes (SWCNTs) (i.e., SWCNT-SB nanosheets) via gravure coating; the SWCNT-SB-coated nanosheets were used as the flexible electrode in a capacitive strain sensor. The dielectric (DE) layer was then prepared using the silicone elastomer Ecoflex 00-30 because its Young's modulus is comparable to that of the epidermis. The normalized capacitance changes (ΔC/C0) in the sensor increased with increasing tensile strain over a range from 0-100%, indicating that the proposed sensor can measure the strain of biological movements, including those of skin and blood vessels. To improve sensor conformability further, the effect of sensor thickness on the gauge factor (GF) was investigated using thinner DE layers by focusing on their flexural rigidity. As a result, the GF increased from 0.64 to 1.13 as the DE layer thickness decreased from 260 to 40 μm. Finally, we evaluated the fabricated sensor's signal stability and mechanical durability, including during wireless sensing when applied to human skin and a vascular model. The ΔC/C0 values varied in response to the bending motion of a finger, dilation of a blood vessel, and the swallowing movement of the throat. These results indicate that our capacitive strain sensor is conformable and functional on biological tissue to enable monitoring of dynamic biological movements (e.g., pulse rate and arterial dilation) without wearer discomfort.

Synergistic advancements in high-performance flexible capacitive pressure sensors: structural modifications, AI integration, and diverse applications

The development of flexible pressure sensors for monitoring human motion and physiological signals has attracted extensive scientific research. However, achieving low monitoring limits, a wide detection range, large bending stresses, and excellent mechanical stability simultaneously remains a serious challenge. With the aim of developing a high-performance capacitive pressure sensor (CPS), this paper introduces the successful preparation of a single-walled carbon nanotube (SWNT)/polydimethylsiloxane (S-PDMS) composite dielectric with a foam-like structure (high permittivity and low elasticity modulus) and MXene/SWNT (S-MXene) composite film electrodes with a micro-crumpled structure. The above structurally modified CPS (SMCPS) demonstrated an excellent response output during pressure loading, achieving a wide pressure detection range (up to 700 kPa), a low detection limit (16.55 Pa), fast response/recovery characteristics (48/60 ms), enhanced sensitivity across a wide pressure range, long-term stability under repeated heavy loading and unloading (40 kPa, >2000 cycles), and reliable performance under various temperature and humidity conditions. The SMCPS demonstrated a precise and stable capacitive response in monitoring subtle physiological signals and detecting motion, owing to its unique electrode structure. The flexible device was integrated with an Internet of Things module to create a smart glove system that enables real-time tracking of dynamic gestures. This system demonstrates exceptional performance in gesture recognition and prediction with artificial intelligence analysis, highlighting the potential of the SMCPS in human-machine interface applications.

Highly Sensitive Fiber Crossbar Sensors Enabled by Second-Order Synergistic Effect of Air Capacitance and Equipotential Body

Fiber crossbars, an emerging electronic device, have become the most promising basic unit for advanced smart textiles. The demand for highly sensitive fiber crossbar sensors (FCSs) in wearable electronics is increased. However, the unique structure of FCSs presents challenges in replicating existing sensitivity enhancement strategies. Aiming at the sensitivity of fiber crossbar sensors, a second-order synergistic strategy is proposed that combines air capacitance and equipotential bodies, resulting in a remarkable sensitivity enhancement of over 20 times for FCSs. This strategy offers a promising avenue for the design and fabrication of FCSs that do not depend on intricate microstructures. Furthermore, the integrative structure of core-sheath fibers ensures a robust interface, leading to a low hysteresis of only 2.33% and exceptional stability. The outstanding capacitive response performance of FCSs allows them to effectively capture weak signals such as pulses and sounds. This capability opens up possibilities for the application of FCSs in personalized health management, as demonstrated by wireless monitoring systems based on pulse signals.

Nanocarbon-based sensors for the structural health monitoring of smart biocomposites

Structural health monitoring (SHM) is a critical aspect of ensuring the safety and durability of smart biocomposite materials used as multifunctional materials. Smart biocomposites are composed of renewable or biodegradable materials and have emerged as eco-friendly alternatives of traditional non-biodegradable glass fiber-based composite materials. Although biocomposites exhibit fascinating properties and many desirable traits, real-time and early stage SHM is the most challenging issue to enable their long-term use. Smart biocomposites are integrated with sensors for in situ identification of the progress of damage and composite failure. The sensitivity of such smart biocomposites is a key functionality, which can be tuned by the introduction of an appropriate filler. In particular, nanocarbons hold promising potential to be incorporated in SHM applications of biocomposites. This review focused on the potential applications of nanocarbons in SHM of biocomposites. The aspects related to fabrication techniques and working mechanism of sensors are comprehensively discussed. Furthermore, their unique mechanical and electrical properties and sustainable nature ensure seamless integration into biocomposites, allowing for real-time monitoring without compromising the material's properties. These sensors offer multi-parameter sensing capabilities, such as strain, pressure, humidity, temperature, and chemical exposure, allowing a comprehensive assessment of biocomposite health. Additionally, their durability and longevity in harsh conditions, along with wireless connectivity options, provide cost-effective and sustainable SHM solutions. As research in this field advances, ongoing efforts seek to enhance the sensitivity and selectivity of these sensors, optimizing their performance for real-world applications. This review highlights the significant advances, ongoing efforts to enhance the sensitivity and selectivity, and performance optimization of nanocarbon-based sensors along with their working mechanism in the field of SHM for smart biocomposites. The key challenges and future research perspectives facing the conversion of nanocarbons to smart biocomposites are also displayed.

The application of impantable sensors in the musculoskeletal system: a review

As the population ages and the incidence of traumatic events rises, there is a growing trend toward the implantation of devices to replace damaged or degenerated tissues in the body. In orthopedic applications, some implants are equipped with sensors to measure internal data and monitor the status of the implant. In recent years, several multi-functional implants have been developed that the clinician can externally control using a smart device. Experts anticipate that these versatile implants could pave the way for the next-generation of technological advancements. This paper provides an introduction to implantable sensors and is structured into three parts. The first section categorizes existing implantable sensors based on their working principles and provides detailed illustrations with examples. The second section introduces the most common materials used in implantable sensors, divided into rigid and flexible materials according to their properties. The third section is the focal point of this article, with implantable orthopedic sensors being classified as joint, spine, or fracture, based on different practical scenarios. The aim of this review is to introduce various implantable orthopedic sensors, compare their different characteristics, and outline the future direction of their development and application.

Boosting the Sensitivity and Hysteresis of a Gel Polymer Electrolyte by Embedding SiO2 Nanoparticles and PVP for Humidity Applications

Enhancing sensitivity and hysteresis in capacitance humidity sensors is vital for precise, reliable, and consistent humidity control. This study explores this concern by incorporating polyvinylpyrrolidone (PVP) and SiO2 nanoparticles into a polyvinyl alcohol (PVA)-based ionic liquid gel polymer electrolyte (ILGPE), studying two capacitor types: ILGPE and SiO2 composite ILGPE (CILGPE) capacitors. These novel electrolytes use ammonium acetate as a plasticiser, 1-butyl-3-methylimidazolium bromide as an ionic liquid, SiO2 nanoparticles as a composite, and PVA and PVP as host polymers. Capacitors were characterised and modelled using impedance spectroscopy (IS), providing an electrophysical insight into their working principle. Sensitivity and hysteresis were evaluated within a 20-90% relative humidity (RH) range at 25 °C. The SiO2 CILGPE capacitor with PVP presented superior sensitivity and hysteresis, revealing the beneficial combination of SiO2 nanoparticles and PVP. These benefits are due to the creation of pathways that facilitate water molecule diffusion and crystallinity reduction in PVA-ILGPE. In particular, at 10 kHz, it demonstrates a calibrated capacitance sensitivity of 2660 pF/%RH and a hysteresis of 3.28 %RH. This optimised capacitor outperforms some previous humidity capacitive sensors in sensitivity while exhibiting low hysteresis.

Fiber-Based Noncontact Sensor with Stretchability for Underwater Wearable Sensing and VR Applications

The rapid development of artificial intelligent wearable devices has led to an increasing need for seamless information exchange between humans, machines, and virtual spaces, often relying on touch sensors as the primary interaction medium. Additionally, the demand for underwater detection technologies is on the rise owing to the prevalent wet and submerged environment. Here, a fiber-based capacitive sensor with superior stretchability and hydrophobicity is proposed, designed to cater to noncontact and underwater applications. The sensor is constructed using bacterial cellulose (BC)@BC/carbon nanotubes (CNTs) (BBT) helical fiber as the matrix and methyltrimethoxysilane (MTMS) as the hydrophobic modified agent, forming a hydrophobic silylated BC@BC/CNT (SBBT) helical fiber by the chemical vapor deposition (CVD) technique. These fibers exhibit an impressive contact angle of 132.8°. The SBBT helicalfiber-based capacitive sensor presents capabilities for both noncontact and underwater sensing, which exhibits a significant capacitance change of -0.27 (at a distance of 0.5 cm). We have achieved interactive control between real space and virtual space through intelligent data analysis technology with minimal interference from the presence of water. This work has laid a solid foundation of noncontact sensing with attributes such as degradability, stretchability, and hydrophobicity. Moreover, it offers promising solutions for barrier-free communication in virtual reality (VR) and underwater applications, providing avenues for smart human-machine interfaces for submerged use.

Tension-Induced Localized Wrinkling in a Patched Thin Film Supported by an Elastomer

The wrinkling behavior of thin films has received great attention for their applications in developing various wrinkle-based novel technologies. Herein, a new wrinkling system: tension-induced wrinkling in an elastomer-supported patched thin film (TW-P&SF) is investigated by using PDMS-supported patched polyimide thin films with different thicknesses and varied length/width ratios. Different from the well-studied compression-induced wrinkling in an elastomer-supported thin film (CW-SF) and tension-induced wrinkling in an edge-clamped free-standing thin film (TW-FF), in the system of TW-P&SF, the wrinkles are localized near the edge of the film with a finite length that follows a center-symmetric distribution. It was found that the wrinkle length lmax and the wrinkle period λ scale with the film thickness h as λ ∼ h0.86 and lmax ∼ h-0.79. With the assistance of the two-dimensional shear lag model and scaling analysis, the underlying mechanism for wrinkle localization is clarified. Furthermore, the promise of the TW-P&SF-enabled wrinkle-based method as a new method for thin film mechanical characterization is demonstrated.

Microstructured Polyfluoroacrylate Elastomeric Dielectric Layer for Highly Stretchable Wide-Range Capacitive Pressure Sensors

Capacitive pressure sensors capable of replicating human tactile senses have garnered tremendous attention. Introducing microstructures into the dielectric layer is an effective approach to improve the sensitivity of the sensors. However, most reported processes to fabricate microstructured dielectric layers are complicated and time-consuming and usually have adverse effects on the mechanical properties. Herein, we report a mechanically strong and highly stretchable dielectric layer fabricated from a microstructured fluorinated elastomer with a high dielectric constant (5.8 at 1000 Hz) via a simple and low-cost thermal decomposition process. Capacitive pressure sensors based on this microstructured fluorinated elastomer dielectric layer and soft ionotronic electrodes illustrate an impressing stretchability (>300%), a high pressure sensitivity (17 MPa-1), a wide detection range (70 Pa-800 kPa), and a fast response time (below 300 ms). Moreover, the multipixel capacitive pressure sensors sensing array maintains the unique spatial tactile sensing performance even under significant tensile deformation. It is believed that our microstructured fluorinated elastomer dielectric layer might find wide applications in stretchable ionotronic devices.

Engineering the Dielectric Constants of Polymers: From Molecular to Mesoscopic Scales

Polymers are essential components of modern-day materials and are widely used in various fields. The dielectric constant, a key physical parameter, plays a fundamental role in the light-, electricity-, and magnetism-related applications of polymers, such as dielectric and electrical insulation, battery and photovoltaic fabrication, sensing and electrical contact, and signal transmission and communication. Over the past few decades, numerous efforts have been devoted to engineering the intrinsic dielectric constant of polymers, particularly by tailoring the induced and orientational polarization modes and ferroelectric domain engineering. Investigations into these methods have guided the rational design and on-demand preparation of polymers with desired dielectric constants. This review article exhaustively summarizes the dielectric constant engineering of polymers from molecular to mesoscopic scales, with emphasis on application-driven design and on-demand polymer synthesis rooted in polymer chemistry principles. Additionally, it explores the key polymer applications that can benefit from dielectric constant regulation and outlines the future prospects of this field.

Evaluation of blood pressure using a flexible and wearable capacitive pressure sensor

In recent times, the high demand for flexible and wearable pressure sensors to monitor human health, particularly for patients afflicted with hypertension or high blood pressure (BP), has captured the keen interest of researchers. Capacitance-based flexible sensing devices offer real-time metrics regarding vital physiological parameters of the human body, such as BP and pulse rate (PR), thereby enabling the identification of cardiovascular complications. In this regard, we have developed a capacitive pressure sensor using polydimethylsiloxane (PDMS) and deionized water (DIW) and improved its key parameters by adding baking powder to PDMS-DIW. The sensor demonstrated excellent performance in static pressure measurements with a sensitivity of 0.021 Pa-1, detection limit of 1 Pa, and response time of 100 ms. We further investigated its application in human BP monitoring. The sensor successfully captured the oscillometric waveform (OMW) for all 160 participants and demonstrated excellent performance in accurately measuring BP, meeting all criteria outlined as the universal standard when compared with the reference devices: OMRON BP device and the gold-standard mercury-based sphygmomanometer. Furthermore, the sensor accurately provided the PR and agreed well with the reference BP device. Therefore, the developed BP sensor can be a viable alternative to replace the pressure sensors in existing BP devices.

Highly Sensitive Capacitive Fiber Pressure Sensors Enabled by Electrode and Dielectric Layer Regulation

Capacitive pressure sensors play an important role in the field of flexible electronics. Despite significant advances in two-dimensional (2D) soft pressure sensors, one-dimensional (1D) fiber electronics are still struggling. Due to differences in structure, the theoretical research of 2D sensors has difficulty guiding the design of 1D sensors. The multiple response factors of 1D sensors and the capacitive response mechanism have not been explored. Fiber sensors urgently need a tailor-made theoretical research and development path. In this regard, we established a fiber pressure-sensing platform using a coaxial wet spinning process. Aiming at the two problems of the soft electrode modulus and dielectric layer thickness, the conclusions are drawn from three aspects: model analysis, experimental verification, and formula derivation. It makes up some theoretical blanks of capacitive fiber pressure sensors. Through the self-regulation of these two factors without a complex structural design, the sensitivity can be significantly improved. This provides a great reference for the design and development of fiber pressure sensors. Besides, taking advantage of the scalability and easy integration of 1D electronics, multipoint sensors prepared by fibers have verified their application potential in health monitoring, human-machine interface, and motion behavior analysis.

High-Performance Dual-Responsive Sensing Skin Enabled by Bioinspired Transduction of Coplanar Square-Loop Electrodes

Advancements in intelligent robots and human-machine interaction necessitate a shift in artificial skins toward multimodal perception. Dual-responsive skins that can detect proximity and pressure information are significant to establishing continuous sensing of interaction processes and extending interactive application scenarios. To address the current limitations of inadequate dual-mode performance, such as limited proximal response change and low tactile sensitivity, this paper presents a bioinspired complementary gradient architecture-enabled (CGA) transduction design and a high-performance dual-responsive skin based on coplanar square-loop electrodes. Through systematic investigation into the transduction of various electrode configurations, comparative results reveal the remarkable potential of coplanar electrodes to deliver superior dual-mode performance without compromise. Simulations and experiments prove that the proposed CGA response mechanism can capture local interface deformation and overall compression signals, further enhancing response sensitivity. The final developed artificial skin is sensitive to external pressure and the approach of objects simultaneously, exhibiting a long detection distance (∼40 mm), a high proximity response (>0.4), and outstanding touch sensitivity (0.131 kPa-1). Furthermore, we demonstrate proof-of-concept applications for the proposed sensing skin in a dual-mode teleoperation interface and adaptive grasping interactions.

Superelastic Cellulose Sub-Micron Fibers/Carbon Black Aerogel for Highly Sensitive Pressure Sensing

Superelastic aerogels with rapid response and recovery times, as well as exceptional shape recovery performance even from large deformation, are in high demand for wearable sensor applications. In this study, a novel conductive and superelastic cellulose-based aerogel is successfully developed. The aerogel incorporates networks of cellulose sub-micron fibers and carbon black (SMF/CB) nanoparticles, achieved through a combination of dual ice templating assembly and electrostatic assembly methods. The incorporation of assembled cellulose sub-micron fibers imparts remarkable superelasticity to the aerogel, enabling it to retain 94.6% of its original height even after undergoing 10 000 compression/recovery cycles. Furthermore, the electrostatically assembled CB nanoparticles contribute to exceptional electrical conductivity in the cellulose-based aerogel. This combination of electrical conductivity and superelasticity results in an impressive response time of 7.7 ms and a recovery time of 12.8 ms for the SMF/CB aerogel, surpassing many of the aerogel sensors reported in previous studies. As a proof of concept, the SMF/CB aerogel is utilized to construct a pressure sensor and a sensing array, which exhibit exceptional responsiveness to both minor and substantial human motions, indicating its significant potential for applications in human health monitoring and human-machine interaction.

Wireless, Smart Hemostasis Device with All-Soft Sensing System for Quantitative and Real-Time Pressure Evaluation

The properly applied pressure between the skin and hemostasis devices is an essential parameter for preventing bleeding and postoperative complications after a transradial procedure. However, this parameter is usually controlled based on the subjective judgment of doctors, which might cause insufficient hemostatic effect or thrombosis. Here this study develops a compact and wireless sensing system for continuously monitoring the pressure applied on the radial artery and wrist skin in clinical practice. A liquid metal (LM)-based all-soft pressure sensor is fabricated to enable conformal attachment between the device and skin even under large deformation conditions. The linear sensitivity of 0.007 kPa-1 among the wide pressure range of 0-100 kPa is achieved and the real-time detection data can be wirelessly transmitted to mobile clients as a reference pressure value. With these devices, detailed pressure data can be collected, analyzed, and stored for medical assistance as well as to improve surgery quality.

Bioinspired Engineering of Fillable Gradient Structure into Flexible Capacitive Pressure Sensor Toward Ultra-High Sensitivity and Wide Working Range

Tactile sensing is required for electronic skin and intelligent robots to function properly. However, the dielectric layer's poor structural compressibility in conventional pressure sensors results in a limited pressure sensing range and low sensitivity. To solve this issue, a flexible pressure sensor with a crocodile-inspired fillable gradient structure is provided. The fillable gradient structure and grooves in the pressure sensor accommodate the deformed microstructure that permits the enhancement of the media layer compressibility via COMSOL finite element simulation and optimization. The pressure sensor exhibits a high sensitivity of up to 0.97 k Pa-1 (0-4 kPa), a wide pressure detection range (7 Pa-380 kPa), and outstanding repeatability. The sensor can detect Morse code, robotic grabbing, and human motion monitoring. As a result, flexible sensors with a bionic fillable gradient structure pave the way for wearable devices and offer a novel method for achieving highly precise tactile perception.

Patterning Techniques in Coplanar Micro/Nano Capacitive Sensors

Rapid technological advancements have led to increased demands for sensors. Hence, high performance suitable for next-generation technology is required. As sensing technology has numerous applications, various materials and patterning methods are used for sensor fabrication. This affects the characteristics and performance of sensors, and research centered specifically on these patterns is necessary for high integration and high performance of these devices. In this paper, we review the patterning techniques used in recently reported sensors, specifically the most widely used capacitive sensors, and their impact on sensor performance. Moreover, we introduce a method for increasing sensor performance through three-dimensional (3D) structures.

3D printed microstructured ultra-sensitive pressure sensors based on microgel-reinforced double network hydrogels for biomechanical applications

Hydrogel-based wearable flexible pressure sensors have great promise in human health and motion monitoring. However, it remains a great challenge to significantly improve the toughness, sensitivity and stability of hydrogel sensors. Here, we demonstrate the fabrication of hierarchically structured hydrogel sensors by 3D printing microgel-reinforced double network (MRDN) hydrogels to achieve both very high sensitivity and mechanical toughness. Polyelectrolyte microgels are used as building blocks, which are interpenetrated with a second network, to construct super tough hydrogels. The obtained hydrogels show a tensile strength of 1.61 MPa, and a fracture toughness of 5.08 MJ m-3 with high water content. The MRDN hydrogel precursors exhibit reversible gel-sol transitions, and serve as ideal inks for 3D printing microstructured sensor arrays with high fidelity and precision. The microstructured hydrogel sensors show an ultra-high sensitivity of 0.925 kPa-1, more than 50 times that of plain hydrogel sensors. The hydrogel sensors are assembled as an array onto a shoe-pad to monitor foot biomechanics during gaiting. Moreover, a sensor array with a well-arranged spatial distribution of sensor pixels with different microstructures and sensitivities is fabricated to track the trajectory of a crawling tortoise. Such hydrogel sensors have promising application in flexible wearable electronic devices.

Flexible tactile sensors with biomimetic microstructures: Mechanisms, fabrication, and applications

In recent years, flexible devices have gained rapid development with great potential in daily life. As the core component of wearable devices, flexible tactile sensors are prized for their excellent properties such as lightweight, stretchable and foldable. Consequently, numerous high-performance sensors have been developed, along with an array of innovative fabrication processes. It has been recognized that the improvement of the single performance index for flexible tactile sensors is not enough for practical sensing applications. Therefore, balancing and optimization of overall performance of the sensor are extensively anticipated. Furthermore, new functional characteristics are required for practical applications, such as freeze resistance, corrosion resistance, self-cleaning, and degradability. From a bionic perspective, the overall performance of a sensor can be optimized by constructing bionic microstructures which can deliver additional functional features. This review briefly summarizes the latest developments in bionic microstructures for different types of tactile sensors and critically analyzes the sensing performance of fabricated flexible tactile sensors. Based on this, the application prospects of bionic microstructure-based tactile sensors in human detection and human-machine interaction devices are introduced.

Flexible electronics for cardiovascular healthcare monitoring

Cardiovascular diseases (CVDs) are one of the most urgent threats to humans worldwide, which are responsible for almost one-third of global mortality. Over the last decade, research on flexible electronics for monitoring and treatment of CVDs has attracted tremendous attention. In contrast to conventional medical instruments in hospitals that are usually bulky, hard to move, monofunctional, and time-consuming, flexible electronics are capable of continuous, noninvasive, real-time, and portable monitoring. Notable progress has been made in this emerging field, and thus a number of significant achievements and concomitant research prospects deserve attention for practical implementation. Here, we comprehensively review the latest progress of flexible electronics for CVDs, focusing on new functions provided by flexible electronics. First, the characteristics of CVDs and flexible electronics and the foundation of their combination are briefly reviewed. Then, four representative applications of flexible electronics for CVDs are elaborated: blood pressure (BP) monitoring, electrocardiogram (ECG) monitoring, echocardiogram monitoring, and direct epicardium monitoring. Their operational principles, progress, merits and demerits, and future efforts are discussed. Finally, the remaining challenges and opportunities for flexible electronics for cardiovascular healthcare are outlined.

Flexible Capacitive Sensors Based on Liquid Crystal Elastomer

The disordered transformation of the ordered aligned polar liquid crystal molecules in liquid crystal elastomers (LCEs) under the influence of an external field imbues them with the unique property of thermally reversible shape memory, making them highly valuable for various applications, particularly in actuators. In this study, we examined the high dielectric constant exhibited by the orientation polarization of polar liquid crystal molecules in RM257-LCE films, which holds significant potential for developing flexible capacitive sensors. By manipulating the flexibility of the molecular chain network and introducing hydrogen bonds and metal ions into the main chain, we were able to enhance the relative dielectric constant of LCEs to an impressive value of 62 (at 100 Hz), which is approximately 23 times higher than for polydimethylsiloxane (PDMS). This elevated dielectric constant displays a noteworthy positive temperature coefficient within a specific temperature range, starting from room temperature and extending to the clearing point. Using this property, we fabricated highly sensitive capacitive, flexible temperature sensors. Moreover, we successfully engineered a flexible pressure sensor with an excellent pressure-sensing range of 0-2 MPa by combining the porous structure of the prepared LCEs with mushroom electrodes. Additionally, the sensor showcases a remarkable capacitance recovery time of 0.8 s at 90 °C. These outstanding features collectively contribute to the excellent pressure-sensing characteristics of our sensor. The findings of this study offer valuable insights and serve as a reference for the design of innovative flexible sensors, enabling advancements in sensor technology.

A liquid-free conducting ionoelastomer for 3D printable multifunctional self-healing electronic skin with tactile sensing capabilities

Conductive elastomers with both softness and conductivity are widely used in the field of flexible electronics. Nonetheless, conductive elastomers typically exhibit prominent problems such as solvent volatilization and leakage, and poor mechanical and conductive properties, which limit their applications in electronic skin (e-skin). In this work, a liquid-free conductive ionogel (LFCIg) with excellent performance was fabricated by utilizing the innovative double network design approach based on a deep eutectic solvent (DES). The double-network LFCIg is cross-linked by dynamic non-covalent bonds, which exhibit excellent mechanical properties (2100% strain while sustaining a fracture strength of 1.23 MPa) and >90% self-healing efficiency, and a superb electrical conductivity of 23.3 mS m-1 and 3D printability. Moreover, the conductive elastomer based on LFCIg has been developed into a stretchable strain sensor that achieves accurate response recognition, classification, and identification of different robot gestures. More impressively, an e-skin with tactile sensing functions is produced by in situ 3D printing of sensor arrays on flexible electrodes to detect light weight objects and recognize the resulting spatial pressure variations. Collectively, the results demonstrate that the designed LFCIg has unparalleled advantages and presents wide application potential in flexible robotics, e-skin and physiological signal monitoring.

Stretchable and Self-Adhesive Humidity-Sensing Patch for Multiplexed Non-Contact Sensing

The slippage of moisture-sensitive materials from substrates during bending or stretching is a common issue that causes baseline drift and even failure of the flexible humidity sensors, which are essential components of wearable electronic devices. In this study, we report a stretchable, self-adhesive, and transparent humidity-sensing electronic patch comprising liquid metal particle electrodes with a stretchable serpentine structure and a humidity-sensing layer made of Ti3C2Tx MXene/carboxymethyl cellulose. This patch is constructed on a soft-hard integrated heterostructure substrate and demonstrates stable humidity-sensitive response performance at 100% tensile strain, along with autonomous adhesion to human skin. Additionally, it exhibits maximum response (1145.4%) at 90% relative humidity (RH), fast response and recovery time (1.4/5.9 s), elevated sensitivity (64.63%/% RH), and preserved humidity sensing under deformation, as well as easy scalability for multiplexed detection. We further illustrate the patch's potential applications in healthcare and environmental monitoring through a non-contact security door control system and wind monitor system. Our proposed strain-isolation strategy can be extended to other rigid conductive materials and stretchable substrates, providing a feasible mechanism for producing stretchable electronic skin patches.

Flexible BaTiO3-PDMS Capacitive Pressure Sensor of High Sensitivity with Gradient Micro-Structure by Laser Engraving and Molding

The significant potential of flexible sensors in various fields such as human health, soft robotics, human-machine interaction, and electronic skin has garnered considerable attention. Capacitive pressure sensor is popular given their mechanical flexibility, high sensitivity, and signal stability. Enhancing the performance of capacitive sensors can be achieved through the utilization of gradient structures and high dielectric constant media. This study introduced a novel dielectric layer, employing the BaTiO3-PDMS material with a gradient micro-cones architecture (GMCA). The capacitive sensor was constructed by incorporating a dielectric layer GMCA, which was fabricated using laser engraved acrylic (PMMA) molds and flexible copper-foil/polyimide-tape electrodes. To examine its functionality, the prepared sensor was subjected to a pressure range of 0-50 KPa. Consequently, this sensor exhibited a remarkable sensitivity of up to 1.69 KPa-1 within the pressure range of 0-50 KPa, while maintaining high pressure-resolution across the entire pressure spectrum. Additionally, the pressure sensor demonstrated a rapid response time of 50 ms, low hysteresis of 0.81%, recovery time of 160 ms, and excellent cycling stability over 1000 cycles. The findings indicated that the GMCA pressure sensor, which utilized a gradient structure and BaTiO3-PDMS material, exhibited notable sensitivity and a broad linear pressure range. These results underscore the adaptability and viability of this technology, thereby facilitating enhanced flexibility in pressure sensors and fostering advancements in laser manufacturing and flexible devices for a wider array of potential applications.

Highly Sensitive Fiber Pressure Sensors over a Wide Pressure Range Enabled by Resistive-Capacitive Hybrid Response

Soft capacitive pressure sensors with high performance are becoming increasingly in demand in the emerging flexible wearable field. While capacitive fiber pressure sensors have achieved high sensitivity, their sensitivity range is limited to low-pressure levels. As fiber sensors typically require preloading and fixation, this narrow range of high sensitivity poses a challenge for practical applications. To overcome this limitation, the study proposes resistive-capacitive hybrid response fiber pressure sensors (HFPSs) with three-layer core-sheath structures. The trigger and sensitivity enhancement mechanisms of the hybrid response are determined through model analysis and experimental verification. By adjustment of the sensitivity enhancement range of the hybrid response, the sensitivity attenuation of HFPSs is alleviated significantly. The obtained results demonstrate that HFPSs have excellent characteristics such as fast response, low hysteresis, wide response frequency, small signal drift, and good durability. The hybrid response enhances the practical sensitivity of HFPSs for various applications. With enhanced sensitivity, HFPSs can effectively monitor pulse signals at preloads ranging from 0 to 22.7 kPa. This wide range of preloads improves the fault tolerance of pulse monitoring and expands the potential application scenarios of fiber pressure sensors.

Adaptive Memory of a Neuromorphic Transistor with Multi-Sensory Signal Fusion

One of the ultimate goals of artificial intelligence is to achieve the capability of memory evolution and adaptability to changing environments, which is termed adaptive memory. To realize adaptive memory in artificial neuromorphic devices, artificial synapses with multi-sensing capability are required to collect and analyze various sensory cues from the external changing environments. However, due to the lack of platforms for mediating multiple sensory signals, most artificial synapses have been mainly limited to unimodal or bimodal sensory devices. Herein, we present a multi-modal artificial sensory synapse (MASS) based on an organic synapse to realize sensory fusion and adaptive memory. The MASS receives optical, electrical, and pressure information and in turn generates typical synaptic behaviors, mimicking the multi-sensory neurons in the brain. Sophisticated synaptic behaviors, such as Pavlovian dogs, writing/erasing, signal accumulation, and offset, were emulated to demonstrate the joint efforts of bimodal sensory cues. Moreover, associative memory can be formed in the device and be subsequently reshaped by signals from a third perception, mimicking modification of the memory and cognition when encountering a new environment. Our MASS provides a step toward next-generation artificial neural networks with an adaptive memory capability.

Flexible Microstructured Capacitive Pressure Sensors Using Laser Engraving and Graphitization from Natural Wood

In recent years, laser engraving has received widespread attention as a convenient, efficient, and programmable method which has enabled high-quality porous graphene to be obtained from various precursors. Laser engraving is often used to fabricate the dielectric layer with a microstructure for capacitive pressure sensors; however, the usual choice of electrodes remains poorly flexible metal electrodes, which greatly limit the overall flexibility of the sensors. In this work, we propose a flexible capacitive pressure sensor made entirely of thermoplastic polyurethane (TPU) and laser-induced graphene (LIG) derived from wood. The capacitive pressure sensor consisted of a flexible LIG/TPU electrode (LTE), an LIG/TPU electrode with a microhole array, and a dielectric layer of TPU with microcone array molded from a laser-engraved hole array on wood, which provided high sensitivity (0.11 kPa^-1), an ultrawide pressure detection range (20 Pa to 1.4 MPa), a fast response (~300 ms), and good stability (>4000 cycles, at 0-35 kPa). We believe that our research makes a significant contribution to the literature, because the easy availability of the materials derived from wood and the overall consistent flexibility meet the requirements of flexible electronic devices.

Stretchable, Adhesive, and Bioinspired Visual Electronic Skin with Strain/Temperature/Pressure Multimodal Non-Interference Sensing

It is highly desirable to construct a single-multimodal sensor that could synchronously perceive multiple stimuli without interference. Here, we propose an adhesive multifunctional chromotropic electronic skin (MCES) that can respond to and distinguish three different stimuli of stain, temperature, and pressure within the two-terminal sensing unit. The mutually discriminating "three-in-one" device converts strain into capacitance and pressure into voltage signals for a tactile stimulus response and produces visual color changes against temperature. In this MCES system, the interdigital capacitor sensor shows high linearity (R² = 0.998), and temperature sensing is realized via reversible multicolor switching bioinspired by the chameleon, showing attractive potential in visualization interaction. Notably, the energy-harvesting triboelectric nanogenerator in MCES can not only detect pressure incentive but also identify objective material species. Looking forward, these findings promise for multimodal sensor technology with reduced complexity and production costs that are highly anticipated in soft robotics, prosthetics, and human-machine interaction applications.

A 3D Composited Flexible Sensor Based on Percolative Nanoparticle Arrays to Discriminate Coupled Pressure and Strain

Flexible mechanical sensors based on nanomaterials operate on a deformation-response mechanism, making it challenging to discern different types of mechanical stimuli such as pressure and strain. Therefore, these sensors are susceptible to significant mechanical interference. Here, we introduce a multifunctional flexible sensor capable of discriminating coupled pressure and strain without cross-interference. Our design involves an elastic cantilever fixed on the pillar of the flexible main substrate, creating a three-dimensional (3D) substrate, and two percolative nanoparticle (NP) arrays are deposited on the cantilever and main substrate, respectively, as the sensing materials. The 3D flexible substrate could confine pressure/strain loading exclusively on the cantilever or main substrate, resulting in independent responses of the two nanoparticle arrays with no cross-interference. Benefitting from the quantum transport in nanoparticle arrays, our sensors demonstrate an exceptional sensitivity, enabling discrimination of subtle strains down to 1.34 × 10^-4. Furthermore, the suspended cantilever with one movable end can enhance the pressure perception of the NP array, exhibiting a high sensitivity of -0.223 kPa^-1 and an ultrahigh resolution of 4.24 Pa. This flexible sensor with multifunctional design will provide inspiration for the development of flexible mechanical sensors and the advancement of decoupling strategies.

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.

Dielectric Elastomers with Liquid Metal and Polydopamine-Coated Graphene Oxide Inclusions

Suspending microscale droplets of liquid metals like eutectic gallium-indium (EGaIn) in polydimethylsiloxane (PDMS) has been shown to dramatically enhance electrical permittivity without sacrificing the elasticity of the host PDMS matrix. However, increasing the dielectric constant of EGaIn-PDMS composites beyond previously reported values requires high EGaIn loading fractions (>50% by volume) that can result in substantial increases in density and loss of material integrity. In this work, we enhance permittivity without further increasing EGaIn loading by incorporating polydopamine (PDA)-coated graphene oxide (GO) and partially reduced GO. In particular, we show that the combination of EGaIn and PDA-GO within a PDMS matrix results in an elastomer composite with a high dielectric constant (∼10-57), a low dissipation factor (∼0.01), and rubber-like compliance and elasticity.

Scalable multi-dimensional topological deformation actuators for active object identification

Rarely are bionic robots capable of rapid multi-dimensional deformation and object identification in the same way as animals and plants. This study proposes a topological deformation actuator for bionic robots based on pre-expanded polyethylene and large flake MXene, inspired by the octopus predation behavior. This unusual, large-area topological deformation actuator (easily reaching 800 cm² but is not constrained to this size) prepared by large-scale blow molding and continuous scrape coating exhibits different distribution states of molecular chains at low and high temperatures, causing the actuator's deformation direction to change axially. With its multi-dimensional topological deformation and self-powered active object identification capabilities, the actuator can capture objects like an octopus. The contact electrification effect assists the actuator to identify the type and size of the target object during this multi-dimensional topological deformation that is controllable and designable. This work demonstrates the direct conversion of light energy into contact electrical signals, introducing a new route for the practicality and scaling of bionic robots.

Technology Roadmap for Flexible Sensors

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

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.

Highly Sensitive and Flexible Capacitive Pressure Sensors Based on Vertical Graphene and Micro-Pyramidal Dielectric Layer

Many practical applications require flexible high-sensitivity pressure sensors. However, such sensors are difficult to achieve using conventional materials. Engineering the morphology of the electrodes and the topography of the dielectrics has been demonstrated to be effective in boosting the sensing performance of capacitive pressure sensors. In this study, a flexible capacitive pressure sensor with high sensitivity was fabricated by using three-dimensional vertical graphene (VG) as the electrode and micro-pyramidal polydimethylsiloxane (PDMS) as the dielectric layer. The engineering of the VG morphology, size, and interval of the micro-pyramids in the PDMS dielectric layer significantly boosted the sensor sensitivity. As a result, the sensors demonstrated an exceptional sensitivity of up to 6.04 kPa^-1 in the pressure range of 0-1 kPa, and 0.69 kPa^-1 under 1-10 kPa. Finite element analysis revealed that the micro-pyramid structure in the dielectric layer generated a significant deformation effect under pressure, thereby ameliorating the sensing properties. Finally, the sensor was used to monitor finger joint movement, knee motion, facial expression, and pressure distribution. The results indicate that the sensor exhibits great potential in various applications, including human motion detection and human-machine interaction.

Capacitive Sensors with Hybrid Dielectric Structures and High Sensitivity over a Wide Pressure Range for Monitoring Biosignals

Various dielectrics with porous structures or high dielectric constants have been designed to improve the sensitivity of capacitive pressure sensors (CPSs), but this strategy has only been effective for the low-pressure range. Here, a hierarchical gradient hybrid dielectric, composed of low-permittivity (low-k) polydimethylsiloxane (PDMS) foam with low Young's modulus (low-E) and high-permittivity (high-k) MWCNT/PDMS foam with high Young's modulus (high-E), is designed to develop a CPS for monitoring biosignals over a wide force range. The foam-like structure with hybrid permittivity (low-k + high-k) is facilitated to improve the sensitivity, while the hierarchical structure with gradient Young's modulus (low-E + high-E) contributes to broadening the pressure sensing range. With the hierarchical gradient hybrid structure, the flexible pressure sensor achieves an enhanced sensitivity of 2.155 kPa^-1, a wide pressure range (up to 500 kPa), a minimum detection limit (50 Pa), and an excellent durability (>2500 cycles). As a demonstration, a venous thrombosis simulation and smart insole system are established to monitor venous blood clots and plantar pressures, respectively, which reveal potential applications in wearable medicine, sports health prediction, athlete training, and sports equipment design.