Hollow-Structured Graphene-Silicone-Composite-Based Piezoresistive Sensors: Decoupled Property Tuning and Bending Reliability

Anisotropic V-Groove/Wrinkle Hierarchical Arrays for Multidirectional Strain Sensors with High Sensitivity and Exceptional Selectivity

Flexible strain sensors have been continuously optimized and widely used in various fields such as health monitoring, motion detection, and human-machine interfaces. There is a higher demand for sensors that can sensitively identify both the strain amplitude and direction in real-time to adapt to complex human movements. This study proposes a flexible strain sensor construction strategy based on V-groove/wrinkle hierarchical structures via a facile and scalable prestretching approach. A gold film is sputtered on a V-groove structure soft substrate under a vertical biaxial prestrain. When the strain is released, a variety of wondrous V-groove/wrinkle hierarchical structures are formed. The microstructure and the properties of the resulting sensor can be controlled by adjusting the prestrain, which has obvious anisotropic response characteristics and exhibits high sensitivity (maximum gauge factor up to 20,727.46) and a wide sensing range (up to 51%). In addition, the resulting multidirectional sensor based on double-sided microstructures has an exceptional directional selectivity of 67.39, at an advanced level among all stretchable multidirectional strain sensors reported so far. The sensor can detect human motion signals and distinguish motion patterns, proving its great potential in the field of human motion detection and laying a foundation for high-performance wearable devices.

A Novel Hollow Graphene/Polydimethylsiloxane Composite for Pressure Sensors with High Sensitivity and Superhydrophobicity

Flexible pressure sensors have attracted great interest as they play an important role in various fields such as health monitoring and human-machine interactions. The design of the pressure sensors still faces challenges in achieving a high sensitivity for a wide sensing range, and the interference of water restricts the applications of the sensors. Herein, we developed a graphene-polydimethylsiloxane film combining a hierarchical surface with nanowrinkles on it and a hollow structure. The microstructure design of the composite can be facilely controlled to improve the sensing and hydrophobic performance by tailoring the microsphere building units. Attributed to the irregular surface and hollow structure of the sensing layer, the optimized sensor exhibits a superior sensitivity of 1085 kPa-1 in a 50 kPa linear range. For practical applications, the nanowrinkles on the surface of the microspheres and the polymer coating endow the composite with waterproof properties. Inspired by the dual receptors of the skin, two designed microstructured films can simply integrate into one with double-sided microstructures. The sensing performance and the water-repellence property allow the sensor to detect physiological signals under both ambient and underwater conditions. Furthermore, underwater stimuli detection and communication are demonstrated. This method of fabricating a flexible sensor shows great potential in wearable and robotic fields.

3D Printed MXene-Based Wire Strain Sensors with Enhanced Sensitivity and Anisotropy

Stretchable strain sensors play a crucial role in intelligent wearable systems, serving as the interface between humans and environment by translating mechanical strains into electrical signals. Traditional fiber strain sensors with intrinsic uniform axial strain distribution face challenges in achieving high sensitivity and anisotropy. Moreover, existing micro/nano-structure designs often compromise stretchability and durability. To address these challenges, a novel approach of using 3D printing to fabricate MXene-based flexible sensors with tunable micro and macrostructures.  Poly(tetrafluoroethylene) (PTFE) as a pore-inducing agent is added into 3D printable inks to achieve controllable microstructural modifications. In addition to microstructure tuning, 3D printing is employed for macrostructural design modifications, guided by finite element modeling (FEM) simulations. As a result, the 3D printed sensors exhibit heightened sensitivity and anisotropy, making them suitable for tracking static and dynamic displacement changes. The proposed approach presents an efficient and economically viable solution for standardized large-scale production of advanced wire strain sensors.

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.

New Carbon Materials for Multifunctional Soft Electronics

Soft electronics are garnering significant attention due to their wide-ranging applications in artificial skin, health monitoring, human-machine interaction, artificial intelligence, and the Internet of Things. Various soft physical sensors such as mechanical sensors, temperature sensors, and humidity sensors are the fundamental building blocks for soft electronics. While the fast growth and widespread utilization of electronic devices have elevated life quality, the consequential electromagnetic interference (EMI) and radiation pose potential threats to device precision and human health. Another substantial concern pertains to overheating issues that occur during prolonged operation. Therefore, the design of multifunctional soft electronics exhibiting excellent capabilities in sensing, EMI shielding, and thermal management is of paramount importance. Because of the prominent advantages in chemical stability, electrical and thermal conductivity, and easy functionalization, new carbon materials including carbon nanotubes, graphene and its derivatives, graphdiyne, and sustainable natural-biomass-derived carbon are particularly promising candidates for multifunctional soft electronics. This review summarizes the latest advancements in multifunctional soft electronics based on new carbon materials across a range of performance aspects, mainly focusing on the structure or composite design, and fabrication method on the physical signals monitoring, EMI shielding, and thermal management. Furthermore, the device integration strategies and corresponding intriguing applications are highlighted. Finally, this review presents prospects aimed at overcoming current barriers and advancing the development of state-of-the-art multifunctional soft electronics.

Mushroom-mimetic 3D hierarchical architecture-based e-skin with high sensitivity and a wide sensing range for intelligent perception

Electronic skin (e-skin) is one of the most important components of future wearable electronic devices, whose sensing performances can be improved by constructing micropatterns on its sensitive layer. However, in traditional e-skins it is difficult to balance sensitivity and the pressure sensing range, and most micropatterns are generally prepared by some complex technologies. Herein, mushroom-mimetic micropatterns with 3D hierarchical architecture and an interdigital electrode are facilely prepared. The micropatterned sensitive layer is further developed through spraying carbon nanotube (CNT) dispersion on the thermoplastic polyurethane (TPU) film with mushroom-mimetic micropatterns (denoted as MMTC). Thanks to the "interlocking effect" between mushroom-mimetic micropatterns and the interdigital electrode in the as-prepared MMTC/interdigital electrode e-skin, the e-skin exhibits a high sensitivity (up to 600 kPa-1), a wide pressure sensing range (up to 150 kPa), a short response time (<20 ms) and excellent durability (15 000 cycles). The MMTC/interdigital electrode e-skin is capable of precisely monitoring health conditions via the as-acquired physiological parameters in real time. Moreover, such e-skins can be used to monitor gestures wirelessly, sense the trajectory of pressure stimuli and recognize Morse code under water. This study provides a cost-efficient, facile strategy to design e-skin for future-oriented wearable intelligent systems.

A Selective-Response Hypersensitive Bio-Inspired Strain Sensor Enabled by Hysteresis Effect and Parallel Through-Slits Structures

Flexible strain sensors are promising in sensing minuscule mechanical signals, and thereby widely used in various advanced fields. However, the effective integration of hypersensitivity and highly selective response into one flexible strain sensor remains a huge challenge. Herein, inspired by the hysteresis strategy of the scorpion slit receptor, a bio-inspired flexible strain sensor (BFSS) with parallel through-slit arrays is designed and fabricated. Specifically, BFSS consists of conductive monolayer graphene and viscoelastic styrene-isoprene-styrene block copolymer. Under the synergistic effect of the bio-inspired slit structures and flexible viscoelastic materials, BFSS can achieve both hypersensitivity and highly selective frequency response. Remarkably, the BFSS exhibits a high gage factor of 657.36, and a precise identification of vibration frequencies at a resolution of 0.2 Hz through undergoing different morphological changes to high-frequency vibration and low-frequency vibration. Moreover, the BFSS possesses a wide frequency detection range (103 Hz) and stable durability (1000 cycles). It can sense and recognize vibration signals with different characteristics, including the frequency, amplitude, and waveform. This work, which turns the hysteresis effect into a "treasure," can provide new design ideas for sensors for potential applications including human-computer interaction and health monitoring of mechanical equipment.

Structural Design and DLP 3D Printing Preparation of High Strain Stable Flexible Pressure Sensors

Flexible pressure sensors are crucial force-sensitive devices in wearable electronics, robotics, and other fields due to their stretchability, high sensitivity, and easy integration. However, a limitation of existing pressure sensors is their reduced sensing accuracy when subjected to stretching. This study addresses this issue by adopting finite element simulation optimization, using digital light processing (DLP) 3D printing technology to design and fabricate the force-sensitive structure of flexible pressure sensors. This is the first systematic study of how force-sensitive structures enhance tensile strain stability of flexible resistive pressure sensors. 18 types of force-sensitive structures have been investigated by finite element design, simultaneously, the modulus of the force-sensitive structure is also a critical consideration as it exerts a significant influence on the overall tensile stability of the sensor. Based on simulation results, a well-designed and highly stretch-stable flexible resistive pressure sensor has been fabricated which exhibits a resistance change rate of 0.76% and pressure sensitivity change rate of 0.22% when subjected to strains ranging from no tensile strain to 20% tensile strain, demonstrating extremely low stretching response characteristics. This study presents innovative solutions for designing and fabricating flexible resistive pressure sensors that maintain stable sensing performance even under stretch conditions.

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.

Dual-Layer All-Textile Flexible Pressure Sensor Coupled by Silver Nanowires with Ti3C2-Mxene for Monitoring Athletic Motion during Sports and Transmitting Information

At present, wearable flexible pressure sensors have broad application prospects in fields such as motion monitoring and information transmission. However, it is still a challenge to design flexible pressure sensors with high sensitivity over a large sensing range and simple fabrication. Here, we use a simple "dipping-drying" method to fabricate a fabric-based flexible pressure sensor by coupling silver nanowires (AgNWs) with Ti3C2-MXene. The interaction between MXene and AgNWs helps realize a dual-layer sensing network, achieving good synergistic effects between pressure sensitivity and sensing range. The effects of the material combination and dip-coating sequence on the sensor's performance are systematically studied. The results show that the sensor was impregnated sequentially with AgNWs solution, and the MXene solution has the highest sensitivity (0.168 kPa-1) over a wide range (190 kPa). Meanwhile, it has the advantages of low response hysteresis and detection limit, as well as good linearity and durability. We further demonstrate the application of this sensor in human physiological signal monitoring and motion pattern recognition. It can also encrypt and transmit information according to different pressing states. In addition, the proposed pressure sensor array exhibits spatial resolution detection capabilities, laying the foundation for applications in the fields of motion monitoring and human-computer interaction.

Toward Real-Time Blood Pressure Monitoring via High-Fidelity Iontronic Tonometric Sensors with High Sensitivity and Large Dynamic Ranges

Continuous, noninvasive blood pressure (CNIBP) monitoring provides valuable hemodynamic information that renders detection of the early onset of cardiovascular diseases. Wearable mechano-electric pressure sensors that mount on the skin are promising candidates for monitoring continuous blood pressure (BP) pulse waveforms due to their excellent conformability, simple sensing mechanisms, and convenient signal acquisition. However, it is challenging to acquire high-fidelity BP pulse waveforms since it requires highly sensitive sensors (sensitivity larger than 4 × 10-5 kPa-1 ) that respond linearly with pressure change over a large dynamic range, covering the typical BP range (5-25 kPa). Herein, this work introduces a high-fidelity, iontronic-based tonometric sensor (ITS) with high sensitivity (4.82 kPa-1 ), good linearity (R2 > 0.995), and a large dynamic range (up to 180% output change) over a broad working range (0 to 38 kPa). Additionally, the ITS demonstrates a low limit of detection at 40 Pa, a fast load response time (35 ms) and release time (35 ms), as well as a stable response over 5000 load per release cycles, paving ways for potential applications in human-interface interaction, electronic skins, and robotic haptics. This work further explores the application of the ITS in monitoring real-time, beat-to-beat BP by measuring the brachial and radial pulse waveforms. This work provides a rational design of a wearable pressure sensor with high sensitivity, good linearity, and a large dynamic range for real-time CNIBP monitoring.

Emerging sensing and modeling technologies for wearable and cuffless blood pressure monitoring

Cardiovascular diseases (CVDs) are a leading cause of death worldwide. For early diagnosis, intervention and management of CVDs, it is highly desirable to frequently monitor blood pressure (BP), a vital sign closely related to CVDs, during people's daily life, including sleep time. Towards this end, wearable and cuffless BP extraction methods have been extensively researched in recent years as part of the mobile healthcare initiative. This review focuses on the enabling technologies for wearable and cuffless BP monitoring platforms, covering both the emerging flexible sensor designs and BP extraction algorithms. Based on the signal type, the sensing devices are classified into electrical, optical, and mechanical sensors, and the state-of-the-art material choices, fabrication methods, and performances of each type of sensor are briefly reviewed. In the model part of the review, contemporary algorithmic BP estimation methods for beat-to-beat BP measurements and continuous BP waveform extraction are introduced. Mainstream approaches, such as pulse transit time-based analytical models and machine learning methods, are compared in terms of their input modalities, features, implementation algorithms, and performances. The review sheds light on the interdisciplinary research opportunities to combine the latest innovations in the sensor and signal processing research fields to achieve a new generation of cuffless BP measurement devices with improved wearability, reliability, and accuracy.

Biocompatible and Long-Term Monitoring Strategies of Wearable, Ingestible and Implantable Biosensors: Reform the Next Generation Healthcare

Sensors enable the detection of physiological indicators and pathological markers to assist in the diagnosis, treatment, and long-term monitoring of diseases, in addition to playing an essential role in the observation and evaluation of physiological activities. The development of modern medical activities cannot be separated from the precise detection, reliable acquisition, and intelligent analysis of human body information. Therefore, sensors have become the core of new-generation health technologies along with the Internet of Things (IoTs) and artificial intelligence (AI). Previous research on the sensing of human information has conferred many superior properties on sensors, of which biocompatibility is one of the most important. Recently, biocompatible biosensors have developed rapidly to provide the possibility for the long-term and in-situ monitoring of physiological information. In this review, we summarize the ideal features and engineering realization strategies of three different types of biocompatible biosensors, including wearable, ingestible, and implantable sensors from the level of sensor designing and application. Additionally, the detection targets of the biosensors are further divided into vital life parameters (e.g., body temperature, heart rate, blood pressure, and respiratory rate), biochemical indicators, as well as physical and physiological parameters based on the clinical needs. In this review, starting from the emerging concept of next-generation diagnostics and healthcare technologies, we discuss how biocompatible sensors revolutionize the state-of-art healthcare system unprecedentedly, as well as the challenges and opportunities faced in the future development of biocompatible health sensors.

Conjugated Polymer-Based Nanocomposites for Pressure Sensors

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

Assessment of graphene-based polymers for sustainable wastewater treatment: Development of a soft computing approach

Since graphene possesses distinct electrical and material properties that could improve material performance, there is currently a growing demand for graphene-based electronics and applications. Numerous potential applications for graphene include lightweight and high-strength polymeric composite materials. Due to its structural qualities, which include low thickness and compact 2D dimensions, it has also been recognized as a promising nanomaterial for water-barrier applications. For barrier polymer applications, it is usually applied using two main strategies. The first is the application of graphene, graphene oxide (GO), and reduced graphene oxide (rGO) to polymeric substrates through transfer or coating. In the second method, fully exfoliated GO or rGO is integrated into the material. This study provides an overview of the most recent findings from research on the use of graphene in the context of water-barrier applications. The advantages and current limits of graphene-based composites are compared with those of other nanomaterials utilized for barrier purposes in order to emphasize difficult challenges for future study and prospective applications.

Black phosphorous-based human-machine communication interface

Assistive technology involving auditory feedback is generally utilized by those who are visually impaired or have speech and language difficulties. Therefore, here we concentrate on an auditory human-machine interface that uses audio as a platform for conveying information between visually or speech-disabled users and society. We develop a piezoresistive tactile sensor based on a black phosphorous and polyaniline (BP@PANI) composite by the facile chemical oxidative polymerization of aniline on cotton fabric. Taking advantage of BP's puckered honeycomb lattice structure and superior electrical properties as well as the vast wavy fabric surface, this BP@PANI-based tactile sensor exhibits excellent sensitivity, low-pressure sensitivity, reasonable response time, and good cycle stability. For a real-world application, a prototype device employs six BP@PANI tactile sensors that correspond to braille characters and can convert pressed text into audio on reading or typing to assist visually or speech-disabled persons. Overall, this research offers promising insight into the material candidates and strategies for the development of auditory feedback devices based on layered and 2D materials for human-machine interfaces.

3D Single-Layer-Dominated Graphene Foam for High-Resolution Strain Sensing and Self-Monitoring Shape Memory Composite

Flexible intelligent materials are desired to effectively regulate their own deformation and accurately sense their immediate morphology at the same time. Graphene foam is an attractive material for strain sensing and electrical/thermal performance control due to its outstanding mechanical, electrical, and thermal properties. However, graphene-foam-based materials with both strain sensing and deformation control capabilities are rarely reported. Here, a multiscale design of graphene foam with a single-layer-graphene-dominated microstructure and resilient 3D network architecture, which leads to exceptional strain sensing performance as well as modulation ability of the electrical and thermal conductivity for shape memory polymers, is reported. The graphene foams exhibit a strain detection limit of 0.033%, a rapid response of 53 ms, long-term stability over 10 000 cycles, significant thermoacoustic effect, and great heat-generation and heat-diffusion ability. By combining these advantages, an electro-activated shape-memory composite that is capable of monitoring its own shape state during its morphing process, is demonstrated.

Large-Scale Integrated Flexible Tactile Sensor Array for Sensitive Smart Robotic Touch

In the long pursuit of smart robotics, it has been envisioned to empower robots with human-like senses, especially vision and touch. While tremendous progress has been made in image sensors and computer vision over the past decades, tactile sense abilities are lagging behind due to the lack of large-scale flexible tactile sensor array with high sensitivity, high spatial resolution, and fast response. In this work, we have demonstrated a 64 × 64 flexible tactile sensor array with a record-high spatial resolution of 0.9 mm (equivalently 28.2 pixels per inch) by integrating a high-performance piezoresistive film (PRF) with a large-area active matrix of carbon nanotube thin-film transistors. PRF with self-formed microstructures exhibited high pressure-sensitivity of ∼385 kPa^-1 for multi-walled carbon nanotubes concentration of 6%, while the 14% one exhibited fast response time of ∼3 ms, good linearity, broad detection range beyond 1400 kPa, and excellent cyclability over 3000 cycles. Using this fully integrated tactile sensor array, the footprint maps of an artificial honeybee were clearly identified. Furthermore, we hardware-implemented a smart tactile system by integrating the PRF-based sensor array with a memristor-based computing-in-memory chip to record and recognize handwritten digits and Chinese calligraphy, achieving high classification accuracies of 98.8% and 97.3% in hardware, respectively. The integration of sensor networks with deep learning hardware may enable edge or near-sensor computing with significantly reduced power consumption and latency. Our work could empower the building of large-scale intelligent sensor networks for next-generation smart robotics.

Multimodal Sensors with Decoupled Sensing Mechanisms

Highly sensitive and multimodal sensors have recently emerged for a wide range of applications, including epidermal electronics, robotics, health-monitoring devices and human-machine interfaces. However, cross-sensitivity prevents accurate measurements of the target input signals when a multiple of them are simultaneously present. Therefore, the selection of the multifunctional materials and the design of the sensor structures play a significant role in multimodal sensors with decoupled sensing mechanisms. Hence, this review article introduces varying methods to decouple different input signals for realizing truly multimodal sensors. Early efforts explore different outputs to distinguish the corresponding input signals applied to the sensor in sequence. Next, this study discusses the methods for the suppression of the interference, signal correction, and various decoupling strategies based on different outputs to simultaneously detect multiple inputs. The recent insights into the materials' properties, structure effects, and sensing mechanisms in recognition of different input signals are highlighted. The presence of the various decoupling methods also helps avoid the use of complicated signal processing steps and allows multimodal sensors with high accuracy for applications in bioelectronics, robotics, and human-machine interfaces. Finally, current challenges and potential opportunities are discussed in order to motivate future technological breakthroughs.

Flexible Pressure Sensors with Combined Spraying and Self-Diffusion of Carbon Nanotubes

High-performance wearable sensors are required for applications in medical health and human-machine interaction, but their application has limited owing to the trade-off between sensitivity, pressure range, and durability. Herein, we propose the combined spraying and self-diffusion process of carbon nanotubes (CNTs) to balance and improve these parameters with the CNTs spontaneously diffusing into the film surface before the film curing. The obtained sensor not only achieves high sensitivity (155.54 kPa^-1) and ultrawide pressure detection range (0.1-500 kPa) but also exhibits exceptional durability (over 12,000 pressure cycles at a high pressure of 300 kPa). In addition, the sensor exhibits a fast response (25 ms), good stability, and full flexibility. This process is a general approach that may improve the performance of various types of thin film piezoresistive sensors. Besides, the fabricated sensors can be flexibly scaled into sensor arrays and communicate with smart devices to achieve wireless smart monitoring. At present, the sensor shows broad application prospects in the fields of intelligent medical health and motion sensing.

Digitized Construction of Iontronic Pressure Sensor with Self-Defined Configuration and Widely Regulated Performance

Flexible pressure sensors are essential components for wearable smart devices and intelligent systems. Significant progress has been made in this area, reporting on excellent sensor performance and fascinating sensor functionalities. Nevertheless, geometrical and morphological engineering of pressure sensors is usually neglected, which, however, is significant for practical application. Here, we present a digitized manufacturing methodology to construct a new class of iontronic pressure sensors with optionally defined configurations and widely modulated performance. These pressure sensors are composed of self-defined electrode patterns prepared by a screen printing method and highly tunable pressure-sensitive microstructures fabricated using 3D printed templates. Importantly, the iontronic pressure sensors employ an iontronic capacitive sensing mechanism based on mechanically regulating the electrical double layer at the electrolyte/electrode interfaces. The resultant pressure sensors exhibit high sensitivity (58 kPa^-1), fast response/recovery time (45 ms/75 ms), low detectability (6.64 Pa), and good repeatability (2000 cycles). Moreover, our pressure sensors show remarkable tunability and adaptability in device configuration and performance, which is challenging to achieve via conventional manufacturing processes. The promising applications of these iontronic pressure sensors in monitoring various human physiological activities, fabricating flexible electronic skin, and resolving the force variation during manipulation of an object with a robotic hand are successfully demonstrated.

Highly Sensitive Piezoresistive Pressure Sensor Based on Super-Elastic 3D Buckling Carbon Nanofibers for Human Physiological Signals’ Monitoring

The three-dimensional (3D) carbon nanostructures/foams are commonly used as active materials for the high-performance flexible piezoresistive sensors due to their superior properties. However, the intrinsic brittleness and poor sensing properties of monolithic carbon material still limits its application. Rational design of the microstructure is an attractive approach to achieve piezoresistive material with superior mechanical and sensing properties, simultaneously. Herein, we introduce novel three-dimensional buckling carbon nanofibers (3D BCNFs) that feature a unique serpentine-buckling microstructure. The obtained 3D BCNFs exhibit superior mechanical properties, including super-elasticity (recovery speed up to 950 mm s⁻¹), excellent flexibility (multiple folds), high compressibility (compressed by 90%), and high fatigue resistance (10,000 bending cycles). The pressure sensor fabricated by the 3D BCNFs shows a high sensitivity of 714.4 kPa⁻¹, a fast response time of 23 ms, and a broad measuring range of 120 kPa. The pressure sensor is further applied to monitor the physiological signals of humans, and is capable of detecting the characteristic pulse waves from the radial artery, fingertip artery, and human-breath, respectively.

Wearable RGO/MXene Piezoresistive Pressure Sensors with Hierarchical Microspines for Detecting Human Motion

Flexible piezoresistive pressure sensors may exhibit excellent sensing performances to be applied in wearable electronics, medical diagnosis, and electronic skin. Herein, we report a multi-layer and phased-responsive reduced graphene oxide/MXene-based piezoresistive pressure sensor with hierarchical microspines constructed by sandpaper as the template. Thanks to the multi-level and multi-layer structure, the obtained sensor realized phased response and showed wide detection range (up to 70 kPa), fast response (response/recovery time of 40/80 ms), and excellent working stability (1000 fatigue cycles). Furthermore, the sensor was successfully applied for detecting various human motions including pulse beats, cheek bulging, nodding, finger bending, speech recognition, handwriting, and other pressure signals. Besides, a 6 × 6 sensing matrix integrated by the sensors was able to sensitively perceive the distribution of plane pressure. The findings in this work conceivably stand out as a new strategy to fabricate high-performance piezoresistive pressure sensors in the fields of intelligent healthcare and medical diagnosis, wearable electronic devices, electronic skin, and human-machine interaction.

Fully implantable wireless batteryless vascular electronics with printed soft sensors for multiplex sensing of hemodynamics

The continuous monitoring of hemodynamics attainable with wireless implantable devices would improve the treatment of vascular diseases. However, demanding requirements of size, wireless operation, and compatibility with endovascular procedures have limited the development of vascular electronics. Here, we report an implantable, wireless vascular electronic system, consisting of a multimaterial inductive stent and printed soft sensors capable of real-time monitoring of arterial pressure, pulse rate, and flow without batteries or circuits. Developments in stent design achieve an enhanced wireless platform while matching conventional stent mechanics. The fully printed pressure sensors demonstrate fast response times, high durability, and sensing at small bending radii. The device is monitored via inductive coupling at communication distances notably larger than prior vascular sensors. The wireless electronic system is validated in artery models, while minimally invasive catheter implantation is demonstrated in an in vivo rabbit study. Overall, the vascular system offers an adaptable framework for comprehensive monitoring of hemodynamics.

Crosstalk-Free, High-Resolution Pressure Sensor Arrays Enabled by High-Throughput Laser Manufacturing

Simultaneously achieving high spatial resolution and low crosstalk interference has been a fundamental challenge for flexible pressure sensor arrays. Here the authors present a high-resolution flexible pressure sensor array fabricated through a two-step laser manufacturing process, where individual sensing pixels and their interconnects are sequentially defined by laser-induced graphenization and ablation to minimize crosstalk interferences. The geometry of the interconnects is optimized through theoretical modeling and experimental validation. Characterization results show that the new device design induces a remarkable reduction of the crosstalk coefficient, from -8.21 to -43.63 dB, of the 0.7 mm-resolution sensor arrays, and the crosstalk suppression is particularly beneficial for application scenarios involving pressure sensing on soft surfaces (e.g., human skin and organs). Applications of the sensor array in tactile pattern recognition and minimally-invasive cancer surgery are demonstrated.

Ultrasensitive Pressure Sensor Sponge Using Liquid Metal Modulated Nitrogen-Doped Graphene Nanosheets

Wearable pressure sensors are crucial for real-time monitoring of human activities and biomimetic robot status. Here, the ultrasensitive pressure sensor sponge is prepared by a facile method, realizing ultrasensitive pressure sensing for wearable health monitoring. Since the liquid metal in the sponge-skeleton structure under pressure is conducive to adjust the contact area with nitrogen-doped graphene nanosheets and thus facilitates the charge transfer at the interface, such sensors exhibit a fast response and recovery speed with the response/recovery time 0.41/0.12 s and a comprehensive response range with a sensitivity of up to 476 KPa^-1. Notably, the liquid metal-based spongy pressure sensor can accurately monitor the human body's pulse, the pressure on the skin, throat swallowing, and other activities in real time, demonstrating a broad application prospect. Those results provide a convenient and low-cost way to fabricate easily perceptible pressure sensors, expanded the application potential of liquid metal-based composites for future electronic skin development.

Novel Bending Sensor Based on a Solution-Processed Cu2O Film with High Resolution Covering a Wide Curvature Range

A Cu2O film is prepared on a flexible polyethylene terephthalate substrate for a bending sensor using the spin-spray method, a facile and low-environmental-load solution process. The Cu2O bending sensor shows high sensitivity and high resolution not only over a wide range of curvatures (0 < κ < 0.21 mm-1) but also for very small curvature changes (Δκ = ∼ 0.03 mm-1). The bending response of the sensor exhibited a curvature change of high linearity with a good gauge factor (18.2) owing to the grain-boundary resistance and piezoresistive effects of the fabricated Cu2O film. In addition, the sensor possesses good repeatability, stability, and long-term (>30 days) and mechanical fatigue durability (1000 bending-release cycles). The sensor is capable of detailed monitoring of large- and small-scale human motions, such as finger bending, wrist bending, nodding, mouth opening/closing, and swallowing. In addition, excellent stability and repeatability of the monitoring performance is observed over a wide range of motion angles and speeds. All of these results demonstrate the potential of the flexible bending sensor based on the Cu2O film as a candidate for healthcare monitoring and wearable electronics.

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

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

A bioinspired porous-designed hydrogel@polyurethane sponge piezoresistive sensor for human-machine interfacing

Conductive coating sponge piezoresistive pressure sensors are attracting much attention because of their simple production and convenient signal acquisition. However, manufacturing sponge-structure pressure-sensing materials with high compressibility and wide pressure detection ranges is difficult because of the instability of rigid and brittle conductive coatings at large strains. Herein, a tough conductive hydrogel@polyurethane (PU) sponge with a porous design is prepared via immersion of a polyurethane sponge in a low-cost and biocompatible polyvinyl alcohol (PVA)/glycerin (Gl)/sodium chloride (NaCl) solution. The sensor based on the hydrogel/elastomer sponge composite material exhibits a compressible range of 0-93%, a pressure detection range of 100 Pa-470.2 kPa, and 10 000-cycle stability (80% strain) because of the compressibility, flexibility, and toughness of the porous hydrogel coating. Benefiting from the resistance change mechanism of microporous compression, the sensor also exhibits a wide range of linear resistance changes, and the corresponding sensitivity and gauge factor (GF) are -0.083 kPa^-- (100 Pa-10.0 kPa) and -1.33 (1-60% strain), respectively. Based on its flexibility, compressibility, and wide-ranging linear resistance changes, the proposed sensor has huge potential application in human activity monitoring, electronic skin, and wearable electronic devices.

Beyond Color: The New Carbon Ink

For thousands of years, carbon ink has been used as a black color pigment for writing and painting purposes. However, recent discoveries of nanocarbon materials, including fullerenes, carbon nanotubes, graphene, and their various derivative forms, together with the advances in large-scale synthesis, are enabling a whole new generation of carbon inks that can serve as an intrinsically programmable materials platform for developing advanced functionalities far beyond color. The marriage between these multifunctional nanocarbon inks with modern printing technologies is facilitating and even transforming many applications, including flexible electronics, wearable and implantable sensors, actuators, and autonomous robotics. This review examines recent progress in the reborn field of carbon inks, highlighting their programmability and multifunctionality for applications in flexible electronics and stimuli-responsive devices. Current challenges and opportunities will also be discussed from a materials science perspective towards the advancement of carbon ink for new applications beyond color.

Anisotropic conductive networks for multidimensional sensing

In the past decade, flexible physical sensors have attracted great attention due to their wide applications in many emerging areas including health-monitoring, human-machine interfaces, smart robots, and entertainment. However, conventional sensors are typically designed to respond to a specific stimulus or a deformation along only one single axis, while directional tracking and accurate monitoring of complex multi-axis stimuli is more critical in practical applications. Multidimensional sensors with distinguishable signals for simultaneous detection of complex postures and movements in multiple directions are highly demanded for the development of wearable electronics. Recently, many efforts have been devoted to the design and fabrication of multidimensional sensors that are capable of distinguishing stimuli from different directions accurately. Benefiting from their unique decoupling mechanisms, anisotropic architectures have been proved to be promising structures for multidimensional sensing. This review summarizes the present state and advances of the design and preparation strategies for fabricating multidimensional sensors based on anisotropic conducting networks. The fabrication strategies of different anisotropic structures, the working mechanism of various types of multidimensional sensing and their corresponding unique applications are presented and discussed. The potential challenges faced by multidimensional sensors are revealed to provide an insightful outlook for the future development.

Micro-Nano Processing of Active Layers in Flexible Tactile Sensors via Template Methods: A Review

Template methods are regarded as an important method for micro-nano processing in the active layer of flexible tactile sensors. These template methods use physical/chemical processes to introduce micro-nano structures on the active layer, which improves many properties including sensitivity, response/recovery time, and detection limit. However, since the processing process and applicable conditions of the template method have not yet formed a perfect system, the development and commercialization of flexible tactile sensors based on the template method are still at a relatively slow stage. Despite the above obstacles, advances in microelectronics, materials science, nanoscience, and other disciplines have laid the foundation for various template methods, enabling the continuous development of flexible tactile sensors. Therefore, a comprehensive and systematic review of flexible tactile sensors based on the template method is needed to further promote progress in this field. Here, the unique advantages and shortcomings of various template methods are summarized in detail and discuss the research progress and challenges in this field. It is believed that this review will have a significant impact on many fields of flexible electronics, which is beneficial to promote the cross-integration of multiple fields and accelerate the development of flexible electronic devices.

Highly Sensitive Flexible Tactile Sensor Mimicking the Microstructure Perception Behavior of Human Skin

A 3D printed flexible tactile sensor with graphene-polydimethylsiloxane (PDMS) microspheres for microstructure perception is presented. The structure of the tactile sensor is inspired by the texture of the human finger and is designed to enable the detection of various levels of surface roughness via the processing of tactile signals. The tactile sensor with a unique graphene-PDMS microsphere structure shows excellent comprehensive mechanical properties, including a robust stretching ability (elongation at break of the sensing layer is 70%), excellent sensing ability (short response time of 60 ms), high sensitivity (sensitivity up to 2.4 kPa^-1), and cycle stability (over 2000 loading cycles). In addition, such versatility and sensitivity allow the electronic skin not only to accurately monitor pressure but also to distinguish various surface topographies with microscale differences, and to detect the action of an air fluid.

Ultrafast Flame-Induced Pyrolysis of Poly(dimethylsiloxane) Foam Materials toward Exceptional Superhydrophobic Surfaces and Reliable Mechanical Robustness

Superhydrophobic surfaces are imperative in flexible polymer foams for diverse applications; however, traditional surface coatings on soft skeletons are often fragile and can hardly endure severe deformation, making them unstable and highly susceptible to cyclic loadings. Therefore, it remains a great challenge to balance their mutual exclusiveness of mechanical robustness and surface water repellency on flexible substrates. Herein, we describe how robust superhydrophobic surfaces on soft poly(dimethylsiloxane) (PDMS) foams can be achieved using an extremely simple, ultrafast, and environmentally friendly flame scanning strategy. The ultrafast flame treatment (1-3 s) of PDMS foams produces microwavy and nanosilica rough structures bonded on the soft skeletons, forming robust superhydrophobic surfaces (i.e., water contact angles (WCAs) > 155° and water sliding angles (WSAs) < 5°). The rough surface can be effectively tailored by simply altering the flame scanning speed (2.5-15.0 cm/s) to adjust the thermal pyrolysis of the PDMS molecules. The optimized surfaces display reliable mechanical robustness and excellent water repellency even after 100 cycles of compression of 60% strain, stretching of 100% strain, and bending of 90° and hostile environmental conditions (including acid/salt/alkali conditions, high/low temperatures, UV aging, and harsh cyclic abrasion). Moreover, such flame-induced superhydrophobic surfaces are easily peeled off from ice and can be healable even after severe abrasion cycles. Clearly, the flame scanning strategy provides a facile and versatile approach for fabricating mechanically robust and surface superhydrophobic PDMS foam materials for applications in complex conditions.

Interface Engineering of Flexible Piezoresistive Sensors via Near-Field Electrospinning Processed Spacer Layers

The interface contact between the active material and its neighboring metal electrodes dominates the sensing response of mainstream high-sensitivity piezoresistive pressure sensors. However, the properties of such interface are often difficult to control and preserve owing to the limited strategies to precisely engineer the surface structure and mechanical property of the active material. Here, a top-down fabrication method to create a grid-like polyurethane fiber-based spacer layer at the interface between a piezoresistive layer and its contact electrodes is proposed. The tuning of the period and thickness of the spacer layer is conveniently achieved by a programmable near-field electrospinning process, and the influence of the spacer structure on the sensing performance is systematically investigated. The sensor with the optimized spacer layer shows a widened sensing range (230 kPa) while maintaining a high sensitivity (1.91 kPa^-1 ). Furthermore, the output current fluctuation of the sensors during a 74 000-cycle test is drastically reduced from 14.28% (without a spacer) to 3.63% (with a spacer), demonstrating greatly enhanced long-term reliability. The new near-field electrospinning-based strategy is capable of tuning sensor responses without changing the active material, providing a universal and scalable path to engineer the performances of contact-dominant sensors.

Flexible pressure sensors with high pressure sensitivity and low detection limit using a unique honeycomb-designed polyimide/reduced graphene oxide composite aerogel

It is still a challenge to fabricate flexible pressure sensors that possess high sensitivity, ultralow detection limit, wide sensing range, and fast response for intelligent electronic devices. We here demonstrate superelastic and highly pressure-sensitive polyimide (PI)/reduced graphene oxide (rGO) aerogel sensors with unique honeycomb structure, which were designed and fabricated using a bidirectional freezing technique. This unique honeycomb structure with large aspect ratio is composed of aligned thin lamellar layers and interconnected bridges. The combination of the aligned lamellar layers and the bridges endows the aerogel sensors with high pressure sensitivity (1.33 kPa^-1), ultralow detection limit (3 Pa), broad detection range (80% strain, 59 kPa), fast response time (60 ms), and excellent stability during cycling (over 1000 cycles). Remarkably, the aerogel sensors maintain stable piezoresistive performance at -50 °C, 100 °C, and 200 °C in air, indicating promising potential applications in harsh environments. Owing to the high sensitivity and wide sensing range, the aerogel sensors have been used to detect a full-range of human motion including small-scale motion monitoring (wrist pulse, blowing, puffing) and large-scale movement monitoring (finger bending, elbow bending, walking, running). These advantages make the composite aerogels attractive for high-performance flexible pressure sensors and wearable electronic devices.

Supersonically Sprayed Washable, Wearable, Stretchable, Hydrophobic, and Antibacterial rGO/AgNW Fabric for Multifunctional Sensors and Supercapacitors

Wearable electronic textiles are used in sensors, energy-harvesting devices, healthcare monitoring, human-machine interfaces, and soft robotics to acquire real-time big data for machine learning and artificial intelligence. Wearability is essential while collecting data from a human, who should be able to wear the device with sufficient comfort. In this study, reduced graphene oxide (rGO) and silver nanowires (AgNWs) were supersonically sprayed onto a fabric to ensure good adhesiveness, resulting in a washable, stretchable, and wearable fabric without affecting the performance of the designed features. This rGO/AgNW-decorated fabric can be used to monitor external stimuli such as strain and temperature. In addition, it is used as a heater and as a supercapacitor and features an antibacterial hydrophobic surface that minimizes potential infection from external airborne viruses or virus-containing droplets. Herein, the wearability, stretchability, washability, mechanical durability, temperature-sensing capability, heating ability, wettability, and antibacterial features of this metallized fabric are explored. This multifunctionality is achieved in a single fabric coated with rGO/AgNWs via supersonic spraying.

Facile Fabrication of Flexible Pressure Sensor with Programmable Lattice Structure

Flexible pressure sensors have attracted intense attention because of their widespread applications in electronic skin, human-machine interfaces, and healthcare monitoring. Conductive porous structures are always utilized as active layers to improve the sensor sensitivities. However, flexible pressure sensors derived from traditional foaming techniques have limited structure designability. Besides, random pore distribution causes difference in structure and signal repeatability between different samples even in one batch, therefore limiting the batch production capabilities. Herein, we introduce a structure designable lattice structure pressure sensor (LPS) produced by bottom-up digital light processing (DLP) 3D printing technique, which is capable of efficiently producing 55 high fidelity lattice structure models in 30 min. The LPS shows high sensitivity (1.02 kPa^-1) with superior linearity over a wide pressure range (0.7 Pa to 160 kPa). By adjusting the design parameters such as lattice type and layer thickness, the electrical sensitivities and mechanical properties of LPS can be accurately controlled. In addition, the LPS endures up to 60000 compression cycles (at 10 kPa) without any obvious electrical signal degradation. This benefits from the firm carbon nanotubes (CNTs) coating derived from high-energy ultrasonic probe and the subsequent thermal curing process of UV-heat dual-curing photocurable resin. For practical applications, the LPS is used for real time pulse monitoring, voice recognition and Morse code communication. Furthermore, the LPS is also integrated to make a flexible 4 × 4 sensor arrays for detecting spatial pressure distribution and a flexible insole for foot pressure monitoring.

Extending Porous Silicone Capacitive Pressure Sensor Applications into Athletic and Physiological Monitoring

Porous polymer dielectric materials have been developed to increase the sensitivity of capacitive pressure sensors, so that they might expand capacitive sensor use, and promote the realization of the advantages of this class of sensor in further fields. However, their use has not been demonstrated in physiological monitoring applications such as respiration monitoring and body position detection during sleep; an area in need of unmet medical attention for conditions such as sleep apnea. Here, we develop and characterize a sensor comprised of a poly dimethylsiloxane (PDMS) sponge dielectric layer, and PDMS/carbon black (CB) blend electrode layers, with suitable compliance and sensitivity for integration in mattresses, pillows, and athletic shoe insoles. With relatively high pressure sensitivity (~0.1 kPa^-1) and mechanical robustness, this sensor was able to fulfill a wide variety of roles, including athletic monitoring in an impact mechanics scenario, by recording heel pressure during running and walking, and physiological monitoring, by detecting head position and respiration of a subject lying on a pad and pillow. The sensor detected considerably greater relative signal changes than those reported in recent capacitive sensor studies for heel pressure, and for a comparably minimal, resistive sensor during respiration, in line with its enhanced sensitivity.

A porous self-healing hydrogel with an island-bridge structure for strain and pressure sensors

Conductive hydrogels have attracted widespread attention in wearable electronic devices and human motion detection. However, designing self-healing hydrogels with high conductivity and excellent mechanical properties remains a challenge. In this work, polyvinyl alcohol/carbon nanotubes/graphene (PVA/CNTs/graphene) with an island-bridge hydrogel structure and self-healing properties was designed by merging PVA/CNTs hydrogel and PVA/graphene hydrogel, in which the PVA/graphene hydrogel acts as an "island" and PVA/CNTs hydrogel acts as a "bridge" to bridge the entire conductive network. Hydrogen-bonding between the borate ion and the -OH group of PVA allows the conductive hydrogel to heal without any external stimulation. The PVA/CNTs/graphene hydrogel can be used for both stretchable strain and pressure sensors. The obtained PVA/CNTs/graphene composite hydrogel exhibits excellent electrical conductivity, extreme high elastic strain (up to 900%) and strong mechanical pressure (up to 10 kPa). The strain sensor based on the PVA/CNTs/graphene hydrogel exhibits excellent tensile strain sensitivity (a gauge factor of 152.6 in the strain region of 316-600%) and wide detection working range (1-600%) with high durability and repeatability. The sensor also remains highly sensitive when being used as a pressure sensor (-0.127 kPa-1 at 0-5 kPa). Additionally, the PVA/CNTs/graphene hydrogel-based sensor can detect human motions after multiple cuts and self-healing with excellent stability and repeatability. The PVA/CNTs/graphene hydrogel provides a new idea in the development of wearable electronics, demonstrating the potential of the next generation of wearable electronics.

Bioinspired Gradient Conductivity and Stiffness for Ultrasensitive Electronic Skins

Hierarchical and gradient structures in biological systems with special mechanical properties have inspired innovations in materials design for construction and mechanical applications. Analogous to the control of stress transfer in gradient mechanical structures, the control of electron transfer in gradient electrical structures should enable the development of high-performance electronics. This paper demonstrates a high performance electronic skin (e-skin) via the simultaneous control of tactile stress transfer to an active sensing area and the corresponding electrical current through the gradient structures. The flexible e-skin sensor has extraordinarily high piezoresistive sensitivity at low power and linearity over a broad pressure range based on the conductivity-gradient multilayer on the stiffness-gradient interlocked microdome geometry. While stiffness-gradient interlocked microdome structures allow the efficient transfer and localization of applied stress to the sensing area, the multilayered structure with gradient conductivity enables the efficient regulation of piezoresistance in response to applied pressure by gradual activation of current pathways from outer to inner layers, resulting in a pressure sensitivity of 3.8 × 10⁵ kPa^-1 with linear response over a wide range of up to 100 kPa. In addition, the sensor indicated a rapid response time of 0.016 ms, a low minimum detectable pressure level of 0.025 Pa, a low operating voltage (100 μV), and high durability during 8000 repetitive cycles of pressure application (80 kPa). The high performance of the e-skin sensor enables acoustic wave detection, differentiation of gas characterized by different densities, subtle tactile manipulation of objects, and real-time monitoring of pulse pressure waveform.

Flexible and wearable strain sensor based on electrospun carbon sponge/polydimethylsiloxane composite for human motion detection

Flexible and wearable strain sensors have attracted considerable attention due to their potential applications in human motion detection. In this work, the as-fabricated strain sensor was obtained by encapsulation of electrospun carbon sponge (CS) with polydimethylsiloxane (PDMS). The formation mechanism of the self-assembled sponge has been explored. Meanwhile, the piezoresistive properties and the strain sensing mechanism of the CS/PDMS sensor were investigated. The results showed that the as-fabricated CS/PDMS sensor had high piezoresistive sensibility with a maximum gauge factor up to 130.49, superior stability and fast response to various cyclic loading with a tensile strain from 0% up to 40% and a tensile speed range of 2-18 mm min-1. Finally, all the superior performances endow the sensor with abilities to precisely detect pronunciation, human palm motion, wrist joint motion, elbow joint motion, and finger motion in real-time. These results indicate that the strain sensor based on the CS/PDMS could have promising applications in flexible and wearable devices for human motion detection.

Ultrahigh compressibility and superior elasticity carbon framework derived from shaddock peel for high-performance pressure sensing

Shaddock peel, a crop by-product mainly composed of cellulose, hemicellulose, lignin, and pectin, was developed as a flexible sensitive material for detecting environmental external pressure. Firstly, a natural carbon framework (C-SPF) with high conductivity was prepared using hydrothermal treatment followed by carbonization. Then, the PDMS elastomer was coated on the C-SPF instead of dense filling to convert the brittle C-SPF into elastic porous materials (M-SPF). Benefiting from the large deformation space of the porous framework and the stable interactions between PDMS and C-SPF, M-SPF exhibited ultrahigh coercibility (up to 99.0% strain) and high elasticity (99.4% height retention for 10 000 cycles at 50.0% strain). The M-SPF-based pressure sensor also exhibited a quick response (loading and unloading times were 20 ms and 30 ms), high sensitivity (63.4 kPa⁻¹), wide working range (from 0 to 800 kPa), and stable stress-electric current response (10 000 cycles). These advantages open a door to a variety of applications, such as flexible wearable devices, which demonstrated human physiological signal monitoring. The low cost, simple design and portable use of piezoresistive sensors highlight the potential application of the crop by-product shaddock peel as a high-value material.

The piezoresistive sensor constructed by a PDMS modified Shaddock peel 3D carbon skeleton has an excellent sensing performance, which has promising potential in the field of human health detection.

Fabrication of a Sensitive Strain and Pressure Sensor from Gold Nanoparticle-Assembled 3D-Interconnected Graphene Microchannel-Embedded PDMS

Manufacture of uniform, sensitive, and durable microtextured sensing materials is one of the greatest challenges for pressure sensors and electronic skins. Reported in this article is a gold nanoparticle-assembled, 3D-interconnected, graphene microchannel-embedded PDMS (3D GMC-PDMS) film for strain and pressure sensors. The film consists of porous nickel foam with its inner walls coated by multilayer graphene. Embedding in PDMS with etching removal of the Ni yields a 3D GMC-PDMS. Coating the inner walls with Au nanoparticles yields an Au nanoparticle-assembled 3D GMC-PDMS (AuNPs-GMC-PDMS) film, which is useful as an ultrasensitive pressure and strain sensor. This sensor exhibits a wide detection range (∼50 kPa) and ultrahigh sensitivity of 5.37, 1.56, and 0.5 kPa^-1 in the ranges of <1, 1-10, and 10-50 kPa, respectively. Its lower detection limit is 4.4 Pa, its response time is 20 ms, and its strain factor is up to 15. Comparison of a AuNPs-GMC-PDMS film with a 3D GMC-PDMS film reveals a sensitivity improvement of 40 times in the 0-1 kPa pressure range and a gauge factor of more than 4 times in the 0-30% tensile strain range. The device has broad applications as a traditional or wearable medical sensor.

Sensitivity Improvement of Stretchable Strain Sensors by the Internal and External Structural Designs for Strain Redistribution

Fiber strain sensors that are directly woven into smart textiles play an important role in wearable systems. These sensors require a high sensitivity to detect the subtle strain in practical applications. However, traditional fiber strain sensors with constant diameters undergo homogeneous strain distribution in the axial direction, thereby limiting the sensitivity improvement. Herein, a novel strategy of internal or external structural design is proposed to significantly improve the sensitivity of fiber strain sensors. The fibers are produced with directional increases in diameter (internal design) or polydimethylsiloxane (PDMS) microbeads attached to surfaces (external design) by combining hollow glass tubes used as templates with PDMS drops. The structural modification of the fiber significantly impacts the sensing performance. After optimizing structural parameters, the highest gauge factor reaches 123.1 in the internal-external structure design at 25% strain. A comprehensive analysis reveals that the desirable scheme is the internal structural design, which features a high sensitivity of 110 with a 100% improvement at ∼5-20% strain. Because of the sufficiently robust interface, even at the 800th cycle, fiber sensors still possessed an excellent stable performance. The morphology evolution mechanism indicates that the resistance increase is closely related with the increased peak width and distance, and the appearance of gaps. Based on the finite element modeling simulation, the quantified effective contributions of different strategies positively correlate with the improved sensitivity. The proposed fiber strain sensors, which are woven into the two-dimensional network structure, exhibit an excellent capability for displacement monitoring and facilitate the traffic control of crossroads.