Highly sensitive and wide-detection range pressure sensor constructed on a hierarchical-structured conductive fabric as a human-machine interface

Smart Driving Hardware Augmentation by Flexible Piezoresistive Sensor Matrices with Grafted-on Anticreep Composites

Signal drift and hysteresis of flexible piezoresistive sensors pose significant challenges against the widespread applications in emerging fields such as electronic skin, wearable equipment for metaverse and human-AI (artificial intelligence) interfaces. To address the creep and relaxation issues associated with pressure-sensitive materials, a highly stable piezoresistive composite is proposed, using polyamide-imide (PAI) fibers as the matrix and in situ grafted-polymerized polyaniline (PANI) as the semi-conducting layer. The PAI with large rigid fluorenylidene groups exhibits a high glass transition temperature of 372 °C (PAI 5-5), which results in an extremely long relaxation time at room temperature and consequently offers outstanding anti-creep/relaxation performances. The enhancement of PAI-PANI interfacial bonding through in situ grafting improves the sensor reliably. The sensor presents high linear sensitivity of 35.3 kPa-1 over a pressure range of 0.2-20 kPa, outstanding repeatability, and excellent dynamic stability with only a 3.8% signal deviation through ≈10 000 cycles. Real-time visualization of pressure distribution is realized by sensor matrices, which demonstrate the capability of tactile gesture recognition on both flat and curved surfaces. The recognition of sitting postures is achieved by two 12 × 12 matrices facilitated by machine learning, which prompts the potential for the augmentation of smart driving.

High sensing performance flexible nanocomposite sensor with a hybrid nanostructure constructed via nanoscale confined motion of nanofibers and nanoplatelets

The swing process between the construction and destruction of hybrid nanostructures in conductive nanocomposites under an external stimulation plays a pivotal role in their sensing performance and is directly related to the nanoscale motion of the corresponding hybrid nanoparticles. When one-dimensional (1D) nanofibers and two-dimensional (2D) nanoplatelets were selectively distributed in thin cell walls via supercritical CO2 foaming, the confined nanoscale motion of 1D nanofibers and 2D nanoplatelets in the stretching process, including hybrid nanoparticle rotation and separation, was precisely regulated based on the hybrid nanoparticles' Monte Carlo theoretical modelling. Correspondingly, an optimized complex hybrid nanostructure with a suitable nanoparticle content, hybrid ratio and geometry was proposed to achieve a high gauge factor of 4469. The flexible nanocomposite sensor with a designated hybrid nanostructure that shows high sensing performance was then tested for different signals, and it shows great potential in the application of monitoring human body motions.

Efficient Fabrication of TPU/MXene/Tungsten Disulfide Fibers with Ultra-Fast Response for Human Respiratory Pattern Recognition and Disease Diagnosis via Deep Learning

Flexible wearable pressure sensors have received increasing attention as the potential application of flexible wearable devices in human health monitoring and artificial intelligence. However, the complex and expensive process of the conductive filler has limited its practical production and application on a large scale to a certain extent. This study presents a kind of piezoresistive sensor by sinking nonwoven fabrics (NWFs) into tungsten disulfide (WS2) and Ti3C2Tx MXene solutions. With the advantages of a simple production process and practicality, it is conducive to the realization of large-scale production. The assembled flexible pressure sensor exhibits high sensitivity (45.81 kPa-1), wide detection range (0-410 kPa), fast response/recovery time (18/36 ms), and excellent stability and long-term durability (up to 5000 test cycles). Because of the high elastic modulus of MXene and the synergistic effect between WS2 and MXene, the detection range and sensitivity of the piezoresistive pressure sensor are greatly improved, realizing the stable detection of human motion status in all directions. Meanwhile, its high sensitivity at low pressure allows the sensor to accurately detect weak signals such as weak airflow and wrist pulses. In addition, combining the sensor with deep-learning makes it easy to recognize human respiratory patterns with high accuracy, demonstrating its potential impact in the fields of ergonomics and low-cost flexible electronics.

Recent Development of Mechanical Stimuli Detectable Sensors, Their Future, and Challenges: A Review

By virtue of their wide applications in transportation, healthcare, smart home, and security, development of sensors detecting mechanical stimuli, which are many force types (pressure, shear, bending, tensile, and flexure) is an attractive research direction for promoting the advancement of science and technology. Sensing capabilities of various force types based on structural design, which combine unique structure and materials, have emerged as a highly promising field due to their various industrial applications in wearable devices, artificial skin, and Internet of Things (IoT). In this review, we focus on various sensors detecting one or two mechanical stimuli and their structure, materials, and applications. In addition, for multiforce sensing, sensing mechanism are discussed regarding responses in external stimuli such as piezoresistive, piezoelectric, and capacitance phenomena. Lastly, the prospects and challenges of sensors for multiforce sensing are discussed and summarized, along with research that has emerged.

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.

Nanocellulose-based sensors in medical/clinical applications: The state-of-the-art review

In recent years, the considerable importance of healthcare and the indispensable appeal of curative issues, particularly the diagnosis of diseases, have propelled the invention of sensing platforms. With the development of nanotechnology, the integration of nanomaterials in such platforms has been much focused on, boosting their functionality in many fields. In this direction, there has been rapid growth in the utilisation of nanocellulose in sensors with medical applications. Indeed, this natural nanomaterial benefits from striking features, such as biocompatibility, cytocompatibility and low toxicity, as well as unprecedented physical and chemical properties. In this review, different classifications of nanocellulose-based sensors (biosensors, chemical and physical sensors), alongside some subcategories manufactured for health monitoring, stand out. Moreover, the types of nanocellulose and their roles in such sensors are discussed.

Plasmonic Bridge Sensor Enabled by Carbon Nanotubes and Au-Ag Nano-Rambutan for Multifunctional Detection of Biomechanics and Bio/Chemical Molecules

Wearable, noninvasive, and simultaneous sensing of subtle strains and eccrine molecules on human body is essential for future health monitoring and personalized medicine. However, there is a huge chasm between biomechanics and bio/chemical molecule detections. Here, a wearable plasmonic bridge sensor with multiple abilities to monitor subtle strains and molecules is developed. Hollow Au-Ag nano-rambutans and carbon nanotubes (CNTs) are adsorbed in the nonwoven fabrics (NWFs) conjointly, where the gap between the conducting network of CNTs is bridged by the Au-Ag nano-rambutans during the subtle strain sensing, and the detection sensitivity for stress is improved at least 1 order of magnitude compared to that with the only CNTs. In order to acquire the accurate human action recognition, a machine learning algorithm (support vector machines) based on output biomechanics data is designed. The average accuracy of our plasmonic bridge sensor reaches 89.0% for human action recognition. Moreover, due to the hollow structure and high nanoroughness, the single Au-Ag nano-rambutan particle has strong localized surface plasmon resonance effect and high surface-enhanced Raman scattering (SERS) activity. Based on their unique SERS spectra introduced by the hollow Au-Ag nano-rambutan adsorbed in the NWFs, noninvasive extraction and "fingerprint" recognition of bio/chemical molecules could be realized during the wearable sensing. In sum, the NWFs/CNTs/Au-Ag sensor bridges the barrier between the bodily strain detection and molecule recognition during the wearable sensing. Such integrated and multifunctional sensing strategy for universal biomechanics and bio/chemical molecules means to assess human health to be of importance.

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.

Soft CNT-Polymer Composites for High Pressure Sensors

Carbon−polymer composite-based pressure sensors have many attractive features, including low cost, easy integration, and facile fabrication. Previous studies on carbon−polymer composite sensors focused on very high sensitivities for low pressure ranges (10 s of kPa), which saturate quickly at higher pressures and thus are ill-suited to measure the high pressure ranges found in various applications, including those in underwater (>1 atm, 101 kPa) and industrial environments. Current sensors designed for high pressure environments are often difficult to fabricate, expensive, and, similarly to their low-pressure counterparts, have a narrow sensing range. To address these issues, this work reports the design, synthesis, characterization, and analysis of high-pressure TPU-MWCNT based composite sensors, which detect pressures from 0.5 MPa (4.9 atm) to over 10 MPa (98.7 atm). In this study, the typical approach to improve sensitivity by increasing conductive additive concentration was found to decrease sensor performance at elevated pressures. It is shown that a better approach to elevated pressure sensitivity is to increase sensor response range by decreasing the MWCNT weight percentage, which improves sensing range and resolution. Such sensors can be useful for measuring high pressures in many industrial (e.g., manipulator feedback), automotive (e.g., damping elements, bushings), and underwater (e.g., depth sensors) applications.

Flexible microstructured pressure sensors: design, fabrication and applications

In recent years, flexible pressure sensors have caused widespread concern for their extensive applications in human activity and health monitoring, robotics and prosthesis, as well as human-machine interface. Flexible pressure sensors in these applications are required to have a high sensitivity, large detective limit, linear response, fast response time, and mechanical stability. The mechanisms of capacitive, piezoresistive, and piezoelectric pressure sensors and the strategies to improve their performance are introduced. Sensing layers with microstructures have shown capability to significantly improve the performances of pressure sensors. Various fabrication methods for these structures are reviewed in terms of their pros and cons. Besides, the interference caused by environmental stimuli and internal stress from different directions leads to the infidelity of the signal transmission. Therefore, the anti-interference ability of flexible pressure sensors is highly desired. Several potential applications for flexible pressure sensors are also briefly discussed. Last, we conclude the future challenges for facilely fabricating flexible pressure sensors with high performance and anti-interference ability.

All-Nanofiber Network Structure for Ultrasensitive Piezoresistive Pressure Sensors

Sensing materials with fiber structures are excellent candidates for the fabrication of flexible pressure sensors due to their large specific surface area and abundant contact points. Here, an ultrathin, flexible piezoresistive pressure sensor that consists of a multilayer nanofiber network structure prepared via a simple electrospinning technique is reported. The ultrathin sensitive layer is composite nanofiber films composed of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) and polyamide 6 (PEDOT:PSS/PA6) prepared by simultaneous electrospinning. PEDOT:PSS conductive fibers and PA6 elastic fibers are interwoven to form a multilayer network structure that can achieve ultrahigh sensitivity by forming a wealth of contact points during loading. In particular, gold-deposited PA6 fibers as upper and lower flexible electrodes can effectively increase the initial resistance. Due to this special fiber electrode structure, the sensor is able to generate a large electrical signal variability when subjected to a weak external force. The devices with different sensing properties can be obtained by controlling the electrospinning time. The sensor based on the PEDOT:PSS/PA6 nanofiber network has high sensitivity (6554.6 kPa^-1 at 0-1.4 kPa), fast response time (53 ms), and wide detection range (0-60 kPa). Significantly, the device maintains ultrahigh sensitivity when cyclically loaded over 10,000 cycles at 5 kPa, which makes it have great prospects for applications in human health monitoring and motion monitoring.

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

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

Advanced Nanocellulose-Based Composites for Flexible Functional Energy Storage Devices

With the increasing demand for wearable electronics (such as smartwatch equipment, wearable health monitoring systems, and human-robot interface units), flexible energy storage systems with eco-friendly, low-cost, multifunctional characteristics, and high electrochemical performances are imperative to be constructed. Nanocellulose with sustainable natural abundance, superb properties, and unique structures has emerged as a promising nanomaterial, which shows significant potential for fabricating functional energy storage systems. This review is intended to provide novel perspectives on the combination of nanocellulose with other electrochemical materials to design and fabricate nanocellulose-based flexible composites for advanced energy storage devices. First, the unique structural characteristics and properties of nanocellulose are briefly introduced. Second, the structure-property-application relationships of these composites are addressed to optimize their performances from the perspective of processing technologies and micro/nano-interface structure. Next, the recent specific applications of nanocellulose-based composites, ranging from flexible lithium-ion batteries and electrochemical supercapacitors to emerging electrochemical energy storage devices, such as lithium-sulfur batteries, sodium-ion batteries, and zinc-ion batteries, are comprehensively discussed. Finally, the current challenges and future developments in nanocellulose-based composites for the next generation of flexible energy storage systems are proposed.

Binder-free printed PEDOT wearable sensors on everyday fabrics using oxidative chemical vapor deposition

Polymeric sensors on fabrics have vast potential toward the development of versatile applications, particularly when the ready-made wearable or fabric can be directly coated. However, traditional coating approaches, such as solution-based methods, have limitations in achieving uniform and thin films because of the poor surface wettability of fabrics. Herein, to realize a uniform poly(3,4-ethylenedioxythiophene) (PEDOT) layer on various everyday fabrics, we use oxidative chemical vapor deposition (oCVD). The oCVD technique is a unique method capable of forming patterned polymer films with controllable thicknesses while maintaining the inherent advantages of fabrics, such as exceptional mechanical stability and breathability. Utilizing the superior characteristics of oCVD PEDOT, we succeed in fabricating blood pressure– and respiratory rate–monitoring sensors by directly depositing and patterning PEDOT on commercially available disposable gloves and masks, respectively. Those results are expected to pave efficient and facile ways for skin-compatible and affordable sensors for personal health care monitoring.