Large‐Area All‐Textile Pressure Sensors for Monitoring Human Motion and Physiological Signals

Wearable pressure sensors, which can perceive and respond to environmental stimuli, are essential components of smart textiles. Here, large‐area all‐textile‐based pressure‐sensor arrays are successfully realized on common fabric substrates. The textile sensor unit achieves high sensitivity (14.4 kPa^?1), low detection limit (2 Pa), fast response (≈24 ms), low power consumption (<6 µW), and mechanical stability under harsh deformations. Thanks to these merits, the textile sensor is demonstrated to be able to recognize finger movement, hand gestures, acoustic vibrations, and real‐time pulse wave. Furthermore, large‐area sensor arrays are successfully fabricated on one textile substrate to spatially map tactile stimuli and can be directly incorporated into a fabric garment for stylish designs without sacrifice of comfort, suggesting great potential in smart textiles or wearable electronics.     

All‐Carbon Pressure Sensors with High Performance and Excellent Chemical Resistance

An all‐carbon pressure sensor is designed and fabricated based on reduced graphene oxide (rGO) nanomaterials. By sandwiching one layer of superelastic rGO aerogel between two freestanding high‐conductive rGO thin papers, the sensor works based on the contact resistance at the aerogel–paper interfaces, getting rid of the alien materials such as polymers and metals adopted in traditional sensors. Without the limitation of alien materials, the all‐carbon sensors demonstrate an ultrawide detecting range (0.72 Pa–130 kPa), low energy consumption (≈0.58 µW), ultrahigh sensitivity (349–253 kPa^?1) at low‐pressure regime (<1.4 Pa), fast response time (8 ms at 1 kPa), high stability (10 000 unloading–loading cycles between 0 and 1 kPa), light weight (<10 mg), easily scalable fabrication process, and excellent chemical stability. These merits enable them to detect real‐time human physiological signals and monitor the weights of various droplets of not only water but also hazardous chemical reagents including strong acid, strong alkali, and organic solvents. This shows their great potential applications in real‐time health monitoring, sport performance detecting, harsh environment‐related robotics and industry, and so forth.     

Splashing-Assisted Femtosecond Laser-Activated Metal Deposition for Mold- and Mask-Free Fabrication of Robust Microstructured Electrodes for Flexible Pressure Sensors (Small 24/2023)

Flexible pressure sensors play an indispensable role in flexible electronics. Microstructures on flexible electrodes have been proven to be effective in improving the sensitivity of pressure sensors. However, it remains a challenge to develop such microstructured flexible electrodes in a convenient way. Inspired by splashed particles from laser processing, herein, a method for customizing microstructured flexible electrodes by femtosecond laser-activated metal deposition is proposed. It takes advantage of the catalyzing particles scattered during femtosecond laser ablation and is particularly suitable for moldless, maskless, and low-cost fabrication of microstructured metal layers on polydimethylsiloxane (PDMS). Robust bonding at the PDMS/Cu interface is evidenced by the scotch tape test and the duration test over 10 000 bending cycles. Benefiting from the firm interface, the developed flexible capacitive pressure sensor with microstructured electrodes presents several conspicuous features, including a sensitivity (0.22 kPa⁻¹) 73 times higher than the one using flat Cu electrodes, ultralow detection limit (<1 Pa), rapid response/recovery time (4.2/5.3 ms), and excellent stability. Moreover, the proposed method, inheriting the merits of laser direct writing, is capable of fabricating a pressure sensor array in a maskless manner for spatial pressure mapping.     

High Sensitivity,Wearable, Piezoresistive Pressure Sensors Based on Irregular Microhump Structures and Its Applications in Body Motion Sensing

A pressure sensor based on irregular microhump patterns has been proposed and developed. The devices show high sensitivity and broad operating pressure regime while comparing with regular micropattern devices. Finite element analysis (FEA) is utilized to confirm the sensing mechanism and predict the performance of the pressure sensor based on the microhump structures. Silicon carbide sandpaper is employed as the mold to develop polydimethylsiloxane (PDMS) microhump patterns with various sizes. The active layer of the piezoresistive pressure sensor is developed by spin coating PEDOT:PSS on top of the patterned PDMS. The devices show an averaged sensitivity as high as 851 kPa^?1, broad operating pressure range (20 kPa), low operating power (100 nW), and fast response speed (6.7 kHz). Owing to their flexible properties, the devices are applied to human body motion sensing and radial artery pulse. These flexible high sensitivity devices show great potential in the next generation of smart sensors for robotics, real‐time health monitoring, and biomedical applications.     

High‐Performance Al/PDMS TENG with Novel Complex Morphology of Two‐Height Microneedles Array for High‐Sensitivity Force‐Sensor and Self‐Powered Application

Triboelectric nanogenerators (TENGs) are widely applied to self‐powered devices and force sensors. TENGs consist of the electrode‐layer frequently made of high‐cost conductors (Ag, Au, ITO) and the tribo‐layer of rigid negative‐triboelectricity fluoropolymers (PTFE, FEP). The surface morpholoy is studied for enhancing performance. Here, a high‐performance Al/PDMS‐TENG is proposed with a complex morphology of overlapped deep two‐height microneedles (OL‐DTH‐MN) fabricated by the integrated process of low‐cost CO₂ laser ablation and PDMS casting for self‐powered devices and high‐sensitivity force/pressure sensors. The high open‐circuit voltage and short‐circuit current of the OL‐DTH‐MN‐TENG are 167 V and 129.3 µA. Also, the sensitivity of the force/pressure sensor of the OL‐DTH‐MN‐TENG is very high, 1.03 V N^?1 and about 3.11 V kPa^?1, at an area of 30 cm² that is much higher than the sensitivity of about 0.18–0.414 V N^?1 and 0.013–0.29 V kPa^?1 of conventional TENG sensors. Meanwhile, the high‐performance OL‐DTH‐MN‐TENG not only exhibits the energy storage capability of charging a 0.1 µF capacitor to 2.75 V at 1.19 s, to maximum 3.22 V, but also activates various self‐powered devices including lighting colorful 226 LEDs connected in series, the “2020‐ME‐NCKU” advertising board, a calculator and a temperature sensor. Numerical simulation is also performed to support the experiments.     

Molecule–Graphene Hybrid Materials with Tunable Mechanoresponse: Highly Sensitive Pressure Sensors for Health Monitoring

The development of pressure sensors is crucial for the implementation of electronic skins and for health monitoring integrated into novel wearable devices. Tremendous effort is devoted toward improving their sensitivity, e.g., by employing microstructured electrodes or active materials through cumbersome processes. Here, a radically new type of piezoresistive pressure sensor based on a millefeuille‐like architecture of reduced graphene oxide (rGO) intercalated by covalently tethered molecular pillars holding on‐demand mechanical properties are fabricated. By applying a tiny pressure to the multilayer structure, the electron tunnelling ruling the charge transport between successive rGO sheets yields a colossal decrease in the material's electrical resistance. Significantly, the intrinsic rigidity of the molecular pillars employed enables the fine‐tuning of the sensor's sensitivity, reaching sensitivities as high as 0.82 kPa^?1 in the low pressure region (0–0.6 kPa), with short response times (≈24 ms) and detection limit (7 Pa). The pressure sensors enable efficient heartbeat monitoring and can be easily transformed into a matrix capable of providing a 3D map of the pressure exerted by different objects.     

Highly Morphology‐Controllable and Highly Sensitive Capacitive Tactile Sensor Based on Epidermis‐Dermis‐Inspired Interlocked Asymmetric‐Nanocone Arrays for Detection of Tiny Pressure

The tactile sensor lies at the heart of electronic skin and is of great importance in the development of flexible electronic devices. To date, it still remains a critical challenge to develop a large‐scale capacitive tactile sensor with high sensitivity and controllable morphology in an economical way. Inspired by the interlocked microridges between the epidermis and dermis, herein, a highly sensitive capacitive tactile sensor by creating interlocked asymmetric‐nanocones in poly(vinylidenefluoride‐co‐trifluoroethylene) film is proposed. Particularly, a facile method based on cone‐shaped nanoporous anodized aluminum oxide templates is proposed to cost‐effectively fabricate the highly ordered nanocones in a controllable manner and on a large scale. Finite‐element analysis reveals that under vertical forces, the strain/stress can be highly strengthened and localized at the contact apexes, resulting in an amplified variation of film permittivity and thickness. Benefiting from this, the developed tactile sensor presents several conspicuous features, including the maximum sensitivity (6.583 kPa^?1) in the low pressure region (0–100 Pa), ultralow detection limit (≈3 Pa), rapid response/recovery time (48/36 ms), excellent stability and reproducibility (10 000 cycles). These salient merits enable the sensor to be successfully applied in a variety of applications including sign language gesture detection, spatial pressure mapping, Braille recognition, and physiological signal monitoring.     

Multiscale Hierarchical Design of a Flexible Piezoresistive Pressure Sensor with High Sensitivity and Wide Linearity Range

Flexible piezoresistive pressure sensors have been attracting wide attention for applications in health monitoring and human‐machine interfaces because of their simple device structure and easy‐readout signals. For practical applications, flexible pressure sensors with both high sensitivity and wide linearity range are highly desirable. Herein, a simple and low‐cost method for the fabrication of a flexible piezoresistive pressure sensor with a hierarchical structure over large areas is presented. The piezoresistive pressure sensor consists of arrays of microscale papillae with nanoscale roughness produced by replicating the lotus leaf's surface and spray‐coating of graphene ink. Finite element analysis (FEA) shows that the hierarchical structure governs the deformation behavior and pressure distribution at the contact interface, leading to a quick and steady increase in contact area with loads. As a result, the piezoresistive pressure sensor demonstrates a high sensitivity of 1.2 kPa⁻¹ and a wide linearity range from 0 to 25 kPa. The flexible pressure sensor is applied for sensitive monitoring of small vibrations, including wrist pulse and acoustic waves. Moreover, a piezoresistive pressure sensor array is fabricated for mapping the spatial distribution of pressure. These results highlight the potential applications of the flexible piezoresistive pressure sensor for health monitoring and electronic skin.     

Fingerprint‐Inspired Flexible Tactile Sensor for Accurately Discerning Surface Texture

Inspired by the epidermal–dermal and outer microstructures of the human fingerprint, a novel flexible sensor device is designed to improve haptic perception and surface texture recognition, which is consisted of single‐walled carbon nanotubes, polyethylene, and polydimethylsiloxane with interlocked and outer micropyramid arrays. The sensor shows high pressure sensitivity (?3.26 kPa^?1 in the pressure range of 0?300 Pa), and it can detect the shear force changes induced by the dynamic interaction between the outer micropyramid structure on the sensor and the tested material surface, and the minimum dimension of the microstripe that can be discerned is as low as 15 µm × 15 µm (interval × width). To demonstrate the texture discrimination capability, the sensors are tested for accurately discerning various surface textures, such as the textures of different fabrics, Braille characters, the inverted pyramid patterns, which will have great potential in robot skins and haptic perception, etc.     

Tactile Chemomechanical Transduction Based on an Elastic Microstructured Array to Enhance the Sensitivity of Portable Biosensors

Tactile sensors capable of perceiving biophysical signals such as force, pressure, or strain have attracted extensive interest for versatile applications in electronic skin, noninvasive healthcare, and biomimetic prostheses. Despite these great achievements, they are still incapable of detecting bio/chemical signals that provide even more meaningful and precise health information due to the lack of efficient transduction principles. Herein, a tactile chemomechanical transduction strategy that enables the tactile sensor to perceive bio/chemical signals is proposed. In this methodology, pyramidal tactile sensors are linked with biomarker‐induced gas‐producing reactions, which transduce biomarker signals to electrical signals in real time. The method is advantageous as it enhances electrical signals by more than tenfold based on a triple‐step signal amplification strategy, as compared to traditional electrical biosensors. It also constitutes a portable and general platform capable of quantifying a wide spectrum of targets including carcinoembryonic antigen, interferon‐γ, and adenosine. Such tactile chemomechanical transduction would greatly broaden the application of tactile sensors toward bio/chemical signals perception which can be used in ultrasensitive portable biosensors and chemical‐responsive chemomechanical systems.     

A Highly Aligned Nanowire‐Based Strain Sensor for Ultrasensitive Monitoring of Subtle Human Motion

Achieving highly accurate responses to external stimuli during human motion is a considerable challenge for wearable devices. The present study leverages the intrinsically high surface‐to‐volume ratio as well as the mechanical robustness of nanostructures for obtaining highly‐sensitive detection of motion. To do so, highly‐aligned nanowires covering a large area were prepared by capillarity‐based mechanism. The nanowires exhibit a strain sensor with excellent gauge factor (≈35.8), capable of high responses to various subtle external stimuli (≤200 µm deformation). The wearable strain sensor exhibits also a rapid response rate (≈230 ms), mechanical stability (1000 cycles) and reproducibility, low hysteresis (<8.1%), and low power consumption (<35 µW). Moreover, it achieves a gauge factor almost five times that of microwire‐based sensors. The nanowire‐based strain sensor can be used to monitor and discriminate subtle movements of fingers, wrist, and throat swallowing accurately, enabling such movements to be integrated further into a miniaturized analyzer to create a wearable motion monitoring system for mobile healthcare.     

Ultraminiature and Flexible Sensor Based on Interior Corner Flow for Direct Pressure Sensing in Biofluids

Conventional pressure sensing devices are well developed for either indirect evaluation or internal measuring of fluid pressure over millimeter scale. Whereas, specialized pressure sensors that can directly work in various liquid environments at micrometer scale remain challenging and rarely explored, but are of great importance in many biomedical applications. Here, pressure sensor technology that utilizes capillary action to self‐assemble the pressure‐sensitive element is introduced. Sophisticated control of capillary flow, tunable sensitivity to liquid pressure in various mediums, and multiple transduction modes are realized in a polymer device, which is also flexible (thickness of 8 µm), ultraminiature (effective volume of 18 × 100 × 580 µm³), and transparent, enabling the sensor to work in some extreme situations, such as in narrow inner spaces (e.g., a microchannel of 220 µm in width and 100 µm in height), or on the surface of small objects (e.g., a 380 µm diameter needle). Potential applications of this sensor include disposables for in vivo and short‐term measurements.     

Advanced Wearable Microfluidic Sensors for Healthcare Monitoring

Wearable flexible sensors based on integrated microfluidic networks with multiplex analysis capability are emerging as a new paradigm to assess human health status and show great potential in application fields such as clinical medicine and athletic monitoring. Well‐designed microfluidic sensors can be attached to the skin surface to acquire various pieces of physiological information with high precision, such as sweat loss, information regarding metabolites, and electrolyte balance. Herein, the recent progress of wearable microfluidic sensors for applications in healthcare monitoring is summarized, including analysis principles and microfabrication methods. Finally, the challenges and opportunities for wearable microfluidic sensors in practical applications are discussed.     

In-situ sugar-templated porous elastomer sensor with high sensitivity for wearables

Fabrication of elastic pressure sensors with low cost, high sensitivity, and mechanical durability is important for wearables, electronic skins and soft robotics. Here, we develop high-sensitivity porous elastomeric sensors for piezoresistive and capacitive pressure detection. Specifically, a porous polydimethylsiloxane (PDMS) sponge embedded with conductive fillers of carbon nanotubes (CNTs) or reduced graphene oxide (rGO) was fabricated by an in-situ sugar template strategy. The sensor demonstrates sensitive deformation to applied pressure, exhibiting large and fast response in resistance or capacitance for detection of a wide range of pressure (0‒5 kPa). PDMS, as a high-elasticity framework, enables creation of sensors with high sensitivity, excellent stability, and durability for long-term usage. The highest sensitivities of 22.1 and 68.3 kPa⁻¹ can be attained by devices with 5% CNTs and 4% rGO, respectively. The geometrics of the sponge sensor is tailorable using tableting technology for different applications. The sensors demonstrate finger motion detection and heart-rate monitoring in real-time, as well as a capacitive sensor array for identification of pressure and shape of placed objects, exhibiting good potential for wearables and human-machine interactions.     

Emerging MXene-Based Flexible Tactile Sensors for Health Monitoring and Haptic Perception

Due to their potential applications in physiological monitoring, diagnosis, human prosthetics, haptic perception, and human–machine interaction, flexible tactile sensors have attracted wide research interest in recent years. Thanks to the advances in material engineering, high performance flexible tactile sensors have been obtained. Among the representative pressure sensing materials, 2D layered nanomaterials have many properties that are superior to those of bulk nanomaterials and are more suitable for high performance flexible sensors. As a class of 2D inorganic compounds in materials science, MXene has excellent electrical, mechanical, and biological compatibility. MXene-based composites have proven to be promising candidates for flexible tactile sensors due to their excellent stretchability and metallic conductivity. Therefore, great efforts have been devoted to the development of MXene-based composites for flexible sensor applications. In this paper, the controllable preparation and characterization of MXene are introduced. Then, the recent progresses on fabrication strategies, operating mechanisms, and device performance of MXene composite-based flexible tactile sensors, including flexible piezoresistive sensors, capacitive sensors, piezoelectric sensors, triboelectric sensors are reviewed. After that, the applications of MXene material-based flexible electronics in human motion monitoring, healthcare, prosthetics, and artificial intelligence are discussed. Finally, the challenges and perspectives for MXene-based tactile sensors are summarized.     

A Highly Responsive Organic Image Sensor Based on a Two‐Terminal Organic Photodetector with Photomultiplication

Highly responsive organic image sensors are crucial for medical imaging applications. To enhance the pixelwise photoresponse in an organic image sensor, the integration of an organic photodetector with amplifiers, or the use of a highly responsive organic photodetector without an additional amplifying component, is required. The use of vertically stacked, two‐terminal organic photodetectors with photomultiplication is a promising approach for highly responsive organic image sensors owing to their simple two‐terminal structure and intrinsically large responsivity. However, there are no demonstrations of an imaging sensor array using organic photomultiplication photodetectors. The main obstacle to a sensor array is the weak‐light sensitivity, which is limited by a relatively large dark current. Herein, a highly responsive organic image sensor based on monolithic, vertically stacked two‐terminal pixels is presented. This is achieved using pixels of a vertically stacked diode‐type organic photodetector with photomultiplication. Furthermore, applying an optimized injection electrode and additionally stacked rectifying layers, this two‐terminal device simultaneously demonstrates a high responsivity (>40 A W^?1), low dark current, and high rectification under illumination. An organic image sensor based on this device with an extremely simple architecture exhibits a high pixel photoresponse, demonstrating a weak‐light imaging capability even at 1 µW cm^?2.     

Highly Flexible Graphene Oxide Nanosuspension Liquid‐Based Microfluidic Tactile Sensor

A novel graphene oxide (GO) nanosuspension liquid‐based microfluidic tactile sensor is developed. It comprises a UV ozone‐bonded Ecoflex–polydimethylsiloxane microfluidic assembly filled with GO nanosuspension, which serves as the working fluid of the tactile sensor. This device is highly flexible and able to withstand numerous modes of deformation as well as distinguish various user‐applied mechanical forces it is subjected to, including pressing, stretching, and bending. This tactile sensor is also highly deformable and wearable, and capable of recognizing and differentiating distinct hand muscle‐induced motions, such as finger flexing and fist clenching. Moreover, subtle differences in the handgrip strength derived from the first clenching gesture can be identified based on the electrical response of our device. This work highlights the potential application of the GO nanosuspension liquid‐based flexible microfluidic tactile sensing platform as a wearable diagnostic and prognostic device for real‐time health monitoring. Also importantly, this work can further facilitate the exploration and potential realization of a functional liquid‐state device technology with superior mechanical flexibility and conformability.     

A Biodegradable and Stretchable Protein‐Based Sensor as Artificial Electronic Skin for Human Motion Detection

Due to the natural biodegradability and biocompatibility, silk fibroin (SF) is one of the ideal platforms for on‐skin and implantable electronic devices. However, the development of SF‐based electronics is still at a preliminary stage due to the SF film intrinsic brittleness as well as the solubility in water, which prevent the fabrication of SF‐based electronics through traditional techniques. In this article, a flexible and stretchable silver nanofibers (Ag NFs)/SF based electrode is synthesized through water‐free procedures, which demonstrates outstanding performance, i.e., low sheet resistance (10.5 Ω sq^?1), high transmittance (>90%), excellent stability even after bending cycles >2200 times, and good extensibility (>60% stretching). In addition, on the basis of such advanced (Ag NFs)/SF electrode, a flexible and tactile sensor is further fabricated, which can simultaneously detect pressure and strain signals with a large monitoring window (35 Pa–700 kPa). Besides, this sensor is air‐permeable and inflammation‐free, so that it can be directly laminated onto human skins for long‐term health monitoring. Considering the biodegradable and skin‐comfortable features, this sensor may become promising to find potential applications in on‐skin or implantable health‐monitoring devices.     

Combined differential and static pressure sensor based on a double-bridged structure

A combined differential and static silicon microelectromechanical system pressure sensor based on a double piezoresistive Wheatstone bridge structure is presented. The developed sensor has a conventional (inner) bridge on a micromachined diaphragm and a secondary (outer) bridge on the chip substrate. A novel approach is demonstrated with a combined measurement of outputs from the two bridges, which results in a combined deduction of both differential and static media pressure. Also following this new approach, a significant improvement in differential pressure sensor accuracy is achieved. Output from the two bridges depends linearly on both differential and absolute (relative to atmospheric pressure) media pressure. Furthermore, the sensor stress distributions involved are studied by three-dimensional finite-element (FE) stress analysis. Furthermore, the FE analysis evaluates current and other potential piezoresistor positions on the outer Wheatstone bridge.     

Metal Deposition on a Self‐Generated Microfibril Network to Fabricate Stretchable Tactile Sensors Providing Analog Position Information

Stretchable conductors and sensors have attracted great attention for use in electronic skin and healthcare monitoring. Despite the development of many stretchable conductors, there are still very few studies that utilize the conventional methods making electrodes and circuits used in current industry. A method is proposed to fabricate a stretchable electrode pattern and a stretchable tactile sensor by simply depositing linear metal lines through a mask on a stretchable substrate. A method is developed of a self‐generating microfibril network on the surface of stretchable block copolymer substrates. The formation mechanism of the microfibril network is studied with finite element method simulations. Metals (Au and Ag nanowires) are deposited directly on the substrate through a patterned mask. This study shows that strain‐insensitive circuit and strain‐sensitive sensor can be fabricated in a controlled way by adjusting the thickness of the deposited metal, which makes it easy to fabricate a tactile sensor by metal deposition. Also, by using the characteristic that the sensor has different sensitivity depending on the line pattern width, a novel sensor structure simultaneously providing analog‐type position information and pressure value is proposed.