Ultralow-Cost, Highly Sensitive, and Flexible Pressure Sensors Based on Carbon Black and Airlaid Paper for Wearable Electronics

Rapid-response, low-detection-limit, positive-negative air pressure sensing: GaN chips integrated with hydrophobic PDMS films

Despite the importance of positive and negative pressure sensing in numerous domains, the availability of a single sensing unit adept at handling this dual task remains highly limited. This study introduces a compact optical device capable of swiftly and precisely detecting positive and negative pressures ranging from -35 kPa to 35 kPa. The GaN chip, which serves as a core component of the device, is monolithically integrated with light-emitting and light-detecting elements. By combining a deformable PDMS film coated with a hydrophobic layer, the chip can respond to changes in optical reflectance induced by pressure fluctuations. The integrated sensing device has low detection limits of 4.3 Pa and -7.8 Pa and fast response times of 0.14 s and 0.22 s for positive and negative pressure variations, respectively. The device also demonstrates adaptability in capturing distinct human breathing patterns. The proposed device, characterized by its compactness, responsiveness, and ease of operation, holds promise for a variety of pressure-sensing applications.

In situ fabrication of porous polymer films embedded with perovskite nanocrystals for flexible superhydrophobic piezoresistive sensors

The application of pressure sensors based on perovskite in high-humidity environments is limited by the effect of water on their stability. Endowing sensors with superhydrophobicity is an effective strategy to overcome the issue. In this work, MAPbBr3/Polyvinylidene Fluoride-TFSI composite was prepared by a one-step in-situ strategy to form a flexible superhydrophobic pressure sensor, which exhibited a contact angle of 150.25°. The obtained sensor exhibited a sensitivity of 0.916 in 1 kPa, a detection limit of 0.2 Pa, a precision of 0.1 Pa, and a response/recovery of ∼100 ms, along with good thermal stability. Through density functional theory calculations, it is revealed that the formation of the porosity is attributed to the interaction between the polymer and EMIM TFSI, which further leads to superhydrophobicity. And, the perovskite structure is easy to change under pressure, affecting the carrier transport and electrical signals output, which explains the sensing mechanism. In addition, the sensor performed well in monitoring facial expression, pulse, respiration, finger bending, and wind speed ranging from 1 m/s to 6 m/s. With both the Linear Regression and the Random Forest algorithm, the sensor can monitor the wind speed with an R2 greater than 0.977 in 60 tests.

Nanoengineering Ultrathin Flexible Pressure Sensors with Superior Sensitivity and Wide Range via Nanocomposite Structures

Flexible pressure sensors have attracted great interest due to their bendable, stretchable, and lightweight characteristics compared to rigid pressure sensors. However, the contradictions among sensitivity, detection limit, thickness, and detection range restrict the performance of flexible pressure sensors and the scope of their applications, especially for scenarios requiring conformal fitting, such as rough surfaces such as the human skin. This paper proposes a novel flexible pressure sensor by combining the nanoengineering strategy and nanocomposite structures. The nanoengineering strategy utilizes the bending deformation of nanofilm instead of the compression of the active layer to achieve super high sensitivity and low detection limit; meanwhile, the nanocomposite structures introduce distributed microbumps that delay the adhesion of nanofilm to enlarge the detection range. As a result, this device not only ensures an ultrathin thickness of 1.6 μm and a high sensitivity of 84.29 kPa-1 but also offers a large detection range of 20 kPa and an ultralow detection limit of 0.07 Pa. Owing to the ultrathin thickness as well as high performance, this device promotes applications in detecting fingertip pressure, flexible mechanical gripping, and so on, and demonstrates significant potential in wearable electronics, human-machine interaction, health monitoring, and tactile perception. This device offers a strategy to resolve the conflicts among thickness, sensitivity, detection limit, and detection range; therefore, it will advance the development of flexible pressure sensors and contribute to the community and other related research fields.

Laser patterned graphene pressure sensor with adjustable sensitivity in an ultrawide response range

Flexible pressure sensors have attracted wide attention because of their applications in wearable electronic, human-computer interface, and healthcare. However, it is still a challenge to design a pressure sensor with adjustable sensitivity in an ultrawide response range to satisfy the requirements of different application scenarios. Here, a laser patterned graphene pressure sensor (LPGPS) is proposed with adjustable sensitivity in an ultrawide response range based on the pre-stretched kirigami structure. Due to the out-of-plane deformation of the pre-stretched kirigami structure, the sensitivity can be easily tuned by simply modifying the pre-stretched level. As a result, it exhibits a maximum sensitivity of 0.243 kPa-1, an ultrawide range up to 1600 kPa, a low detection limit (6 Pa), a short response time (42 ms), and excellent stability with high pressure of 1200 kPa over 500 cycles. Benefiting from its high sensitivity and ultrawide response range, the proposed sensor can be applied to detect physiological and kinematic signals under different pressure intensities. Additionally, taking advantage of laser programmable patterning, it can be easily configured into an array to determine the pressure distribution. Therefore, LPGPS with adjustable sensitivity in an ultrawide response range has potential application in wearable electronic devices.

Highly Sensitive Pressure Sensor Based on Elastic Conductive Microspheres

Elastic pressure sensors play a crucial role in the digital economy, such as in health care systems and human-machine interfacing. However, the low sensitivity of these sensors restricts their further development and wider application prospects. This issue can be resolved by introducing microstructures in flexible pressure-sensitive materials as a common method to improve their sensitivity. However, complex processes limit such strategies. Herein, a cost-effective and simple process was developed for manufacturing surface microstructures of flexible pressure-sensitive films. The strategy involved the combination of MXene-single-walled carbon nanotubes (SWCNT) with mass-produced Polydimethylsiloxane (PDMS) microspheres to form advanced microstructures. Next, the conductive silica gel films with pitted microstructures were obtained through a 3D-printed mold as flexible electrodes, and assembled into flexible resistive pressure sensors. The sensor exhibited a sensitivity reaching 2.6 kPa-1 with a short response time of 56 ms and a detection limit of 5.1 Pa. The sensor also displayed good cyclic stability and time stability, offering promising features for human health monitoring applications.

Flexible Piezoresistive Sensors from Polydimethylsiloxane Films with Ridge-like Surface Structures

Developing flexible sensors and actuators is of paramount importance for wearable devices and systems. In this research, we developed a simple and facile technique to construct flexible piezoresistive sensors from polydimethylsiloxane films with ridge-like surface structures and laser-induced porous graphene. Using a replication strategy, we prepared the ridge-like surface structures from sandpapers. The piezoresistive sensors exhibit excellent sensitivity with a response time of less than 50 ms and long-term cyclic stability under mechanical loading. The smallest weight they can sense is ~96 mg. We demonstrated applications of the piezoresistive sensors in the sensing of bio-related activities, including muscle contraction, finger flexion, wrist flexion, elbow bending, knee bending, swallowing, respiration, sounds, and pulses.

Polyaniline/poly (2-acrylamido-2-methyl-1-propanesulfonic acid) modified cellulose as promising material for sensors design

A material based on cellulose coated with polyaniline/poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (Cell/PANI-PAMPSA) was synthesized in a simple way starting from cellulose fibres, aniline and using PAMPSA as dopant. The morphology, mechanical properties, thermal stability, and electrical conductivity were investigated by means of several complementary techniques. The obtained results highlight the excellent features of the Cell/PANI-PAMPSA composite with respect to the Cell/PANI one. Based on the promising performance of this material, novel device functions and wearable applications have been tested. We focused on its possible single use as: i) humidity sensors and ii) disposable biomedical sensors to provide immediate diagnostic services as close to the patient as possible for heart rate or respiration activity monitoring. To our knowledge, this is the first time that Cell/PANI-PAMPSA system has been used for such applications.

Health Monitoring System from Pyralux Copper-Clad Laminate Film and Random Forest Algorithm

Sensor technologies have been core features for various wearable electronic products for decades. Their functions are expected to continue to play an essential role in future generations of wearable products. For example, trends in industrial, military, and security applications include smartwatches used for monitoring medical indicators, hearing devices with integrated sensor options, and electronic skins. However, many studies have focused on a specific area of the system, such as manufacturing processes, data analysis, or actual testing. This has led to challenges regarding the reliability, accuracy, or connectivity of components in the same wearable system. There is an urgent need for studies that consider the whole system to maximize the efficiency of soft sensors. This study proposes a method to fabricate a resistive pressure sensor with high sensitivity, resilience, and good strain tolerance for recognizing human motion or body signals. Herein, the sensor electrodes are shaped on a thin Pyralux film. A layer of microfiber polyesters, coated with carbon nanotubes, is used as the bearing and pressure sensing layer. Our sensor shows superior capabilities in respiratory monitoring. More specifically, the sensor can work in high-humidity environments, even when immersed in water-this is always a big challenge for conventional sensors. In addition, the embedded random forest model, built for the application to recognize restoration signals with high accuracy (up to 92%), helps to provide a better overview when placing flexible sensors in a practical system.

Modulating the Configurations of "Gel-Type" Soft Silicone Rubber for Electro-Mechanical Energy Generation Behavior in Wearable Electronics

Electro-mechanical configurations can be piezo-electric transducers, triboelectric generators, electromagnetic induction, or hybrid systems. Our present study aims at developing energy generation through the piezoelectric principle. Gel-type soft SR with Shore A hardness below 30 was used as a versatile material for an elastomeric substrate. Also, multi-wall carbon nanotube (MWCNT), and diatomaceous earth (DE) were used as reinforcing fillers. This "gel-type" soft SR has crosslinking polymer networks with silicone encapsulated within its structure. Mechanical properties such as modulus or stretchability are of utmost importance for such devices based on "gel-type" soft. From the experiments, some of the mechanical aspect's values are summarized. For example, the stretchability was 99% (control) and changes to 127% (3 phr, MWCNT), 76% (20 phr DE), and 103% (20 phr hybrid). From electro-mechanical tests, the output voltage was 0.21 mV (control) and changed to 0.26 mV (3 phr, MWCNT), 0.19 mV (20 phr DE), and 0.29 mV (20 phr hybrid). Moreover, from real-time biomechanical human motion tests in "gel-type" soft-based composites, a relationship among output voltage from machine to human motions was established. Overall, these configurations make them promising against traditional portable devices such as batteries for small power applications such as mobile phones.

Nanoengineering Ultrathin Flexible Pressure Sensor with Superior Sensitivity and Perfect Conformability

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

High-Sensitivity Wearable Sensor Based On a MXene Nanochannel Self-Adhesive Hydrogel

To address the shortcomings of traditional filler-based wearable hydrogels, a new type of nanochannel hydrogel sensor is fabricated in this work through a combination of the unique structure of electrospun fiber textile and the properties of a double network hydrogel. Unlike the traditional Ti₃C₂T_x MXene-based hydrogels, the continuously distributed Ti₃C₂T_x MXene in the nanochannels of the hydrogel forms a tightly interconnected structure similar to the neuron network. As a result, they have more free space to flip and perform micromovements, which allows one to significantly increase the electrical conductivity and sensitivity of the hydrogel. According to the findings, the Ti₃C₂T_x MXene nanochannel hydrogel has excellent mechanical properties as well as self-adhesion and antifreezing characteristics. The hydrogel sensor successfully detects different human motions and physiological signals (e.g., low pulse signals) with high stability and sensitivity. Therefore, the proposed Ti₃C₂T_x MXene-based hydrogel with a unique structure and properties is very promising in the field of flexible wearable devices.

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.

Printable All-Paper Pressure Sensors with High Sensitivity and Wide Sensing Range

With the rapid development of flexible electronics, a large amount of electronic waste is becoming a global concern. Because of the biodegradable and environment-friendly properties, cellulose paper as flexible substrates is an alternative pathway to effectively address the electronic pollution. Recently, paper-based piezoresistive pressure sensors with a simple structure and easy signal detection have been widely used in health monitoring, soft robots, and so forth. However, the low sensitivity and narrow working range of paper-based sensors limit their practical applications. Here, an all paper-based piezoresistive pressure sensor is successfully constructed by assembling a bottom electrode with a screen-printed interdigital Cu electrode on paper and a top sensing electrode. The top electrode is simply fabricated using a one-step impregnation method to coat a thin poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer on air-laid paper. The constructed all-paper sensor displays a maximum sensitivity of 768.07 kPa^-1, a wide detection range (up to 250 kPa), and excellent cycle stability (5000 cycles). Furthermore, the sensor can clearly respond from low pressure (such as wrist pulse) to high pressure (finger tapping). The outstanding performance can be attributed to the surface and interface design of rough and fiber-structured paper and the high conductivity of copper and PEDOT:PSS. Finally, based on the printing technology, array sensors are fabricated to identify spatial pressure distributions, demonstrating the capability of low-cost and large-area fabrication for the practical production applications. This printable all-paper sensor with excellent sensing performance exhibits great potential for use in new-generation green and portable electronics.

Flexible and Stretchable Carbon-Based Sensors and Actuators for Soft Robots

In recent years, the emergence of low-dimensional carbon-based materials, such as carbon dots, carbon nanotubes, and graphene, together with the advances in materials science, have greatly enriched the variety of flexible and stretchable electronic devices. Compared with conventional rigid devices, these soft robotic sensors and actuators exhibit remarkable advantages in terms of their biocompatibility, portability, power efficiency, and wearability, thus creating myriad possibilities of novel wearable and implantable tactile sensors, as well as micro-/nano-soft actuation systems. Interestingly, not only are carbon-based materials ideal constituents for photodetectors, gas, thermal, triboelectric sensors due to their geometry and extraordinary sensitivity to various external stimuli, but they also provide significantly more precise manipulation of the actuators than conventional centimeter-scale pneumatic and hydraulic robotic actuators, at a molecular level. In this review, we summarize recent progress on state-of-the-art flexible and stretchable carbon-based sensors and actuators that have creatively added to the development of biomedicine, nanoscience, materials science, as well as soft robotics. In the end, we propose the future potential of carbon-based materials for biomedical and soft robotic applications.

Carbon Black/Multi-Walled Carbon Nanotube-Based, Highly Sensitive, Flexible Pressure Sensor

Flexible piezoresistive pressure sensors have promising applications in wearable devices, artificial intelligence, and other fields. However, developing low-cost and high-performance pressure sensors still poses a great challenge. Herein, we utilize low-cost carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) mixed in porous polydimethylsiloxane to assemble a flexible piezoresistive pressure sensor combined with interdigitated electrodes. Simultaneously, the COMSOL Multiphysics simulation analysis was performed to predict the sensing behavior of the pressure sensor, which was verified by experiments; the preparation of the pressure sensor was guided according to the prediction. Additionally, we studied the effects of the mixed conductive filler's weight ratio, the shape of the interdigital electrode, and the line width and spacing of the interdigital electrode on the performance of the sensor. Based on the interaction of the 3D porous structure and the synergistic conductive network of CB/MWCNTs, the prepared pressure sensor exhibits a high sensitivity of 3.57 kPa^-1 (∼21 kPa), a wide detection range of 0-275 kPa, fast response time (96 ms), fast recovery time (198 ms), good durability (about 3000 cycles), and good flexibility. Moreover, the fabricated sensor can monitor and recognize human activities (such as finger bending and mouse clicking), indicating that it has great potential in flexible wearable devices and other fields. It is worth noting that the preparation process of the entire pressure sensor was simple, low cost, and environmentally friendly, which provides a certain basis for industrial and commercial applications.

Wearable Pressure Sensor Array with Layer-by-Layer Assembled MXene Nanosheets/Ag Nanoflowers for Motion Monitoring and Human-Machine Interfaces

Recently, wearable sensors and electronic skin systems have become prevalent, which can be employed to detect the movement status and physiological signals of wearers. Here, a pressure sensor composed of mesh-like micro-convex structure polydimethylsiloxane (PDMS), MXene nanosheet/Ag nanoflower (AgNF) films, and flexible interdigital electrodes was designed by layer-by-layer (LBL) assembly. The unique microstructure of PDMS effectively increases the contact area and improves sensitivity. Moreover, AgNFs were introduced into the MXene as a "bridge," and the synergistic effect of the two further enhanced the performance of the sensor. The pressure sensor has high sensitivity (191.3 kPa^-1), good stability (18,000 cycles), fast response/recovery time (80 ms/90 ms), and low detection limit (8 Pa), so it can be used for all-round monitoring of the human body. Sensing arrays were integrated with a wireless transmitter as an intelligent artificial electronic skin for spatial pressure mapping and human-computer interaction sensing. Moreover, we develop a smart glove by a simple method, combining it with a 3D model for wireless accurate detection of hand poses. This provides ideas for hand somatosensory detection technology, leading to health monitoring, intelligent rehabilitation training, and personalized medicine.

Sustainable Carbon Derived from Sulfur-Free Lignins for Functional Electrical and Electrochemical Devices

Technical lignins, kraft, soda, lignoboost, and hydrolysis lignins were used for the production of carbon particles at different carbonization temperatures, 1000 °C and 1400 °C. The results showed that the lignin source and carbonization temperature significantly influenced the carbon quality and microstructure of the carbon particles. Soda lignin carbonized up to 1400 °C showed higher degree of graphitization and exhibited the highest electrical conductivity of 335 S·m^-1, which makes it suitable for applications, such as electromagnetic interference shielding and conductive composite based structural energy storage devices. The obtained carbon particles also showed high surface area and hierarchical pore structure. Kraft lignin carbonized up to 1400 °C gives the highest BET surface area of 646 m² g^-1, which makes it a good candidate for electrode materials in energy storage applications. The energy storage application has been validated in a three-electrode set up device, and a specific capacitance of 97.2 F g^-1 was obtained at a current density of 0.1 A g^-1 while an energy density of 1.1 Wh kg^-1 was observed at a power density of 50 W kg^-1. These unique characteristics demonstrated the potential of kraft lignin-based carbon particles for electrochemical energy storage applications.

Interface Engineering in Chip-Scale GaN Optical Devices for Near-Hysteresis-Free Hydraulic Pressure Sensing

In this work, a compact, near-hysteresis-free hydraulic pressure sensor is presented through interface engineering in a GaN chip-scale optical device. The sensor consists of a monolithic GaN-on-sapphire device responsible for light emission and detection and a multilevel microstructured polydimethylsiloxane (PDMS) film prepared through a low-cost molding process using sandpaper as a template. The micro-patterned PDMS film functions as a pressure-sensing medium to effectively modulate the reflectance properties at the sapphire interface during pressure loading and unloading. The interface engineering endows the GaN optical device with near-hysteresis-free performance over a wide pressure range of up to 0-800 kPa. Verified by a series of experimental measurements on its dynamic responses, the tiny hydraulic sensor exhibits superior performance in hysteresis, stability, repeatability, and response time, indicating its considerable potential for a broad range of practical applications.

Fabrication of Janus-type nanocomposites from cellulose nanocrystals for self-healing hydrogels' flexible sensors

Janus bio-nanomaterials have great application potential in functional solid surfactants, probes and flexible sensors. In this manuscript, the sustainable Janus cellulose nanocrystals-type (CNCs-type) nanomaterials were prepared by Pickering emulsion template method. The asymmetric functionalism of Janus nanorods was realized by asymmetrically grafting polypyrrole (PPy) and polydopamine (PDA) onto different sides of CNCs (Janus CNCs-PPy /PDA (JCNs)). JCNs was successfully applied to self-healing nanocomposite hydrogels and further applied to the development of flexible sensors. The self-healing efficiency of nanocomposite hydrogels was 87.2%, and the stress and strain reached 3.50 MPa and 453.45%, respectively. It is worth noting that flexible sensors have been widely used in the field of wearable electronic sensing for real-time monitoring of human movement due to their high sensitivity (gauge factor (GF) = 9.9) and fast response time (260 ms).

Gait Cycle Monitoring System Based on Flexiforce Sensors

Medical technology companies have focused on gait analysis and monitoring for several years due to their importance in the diagnosis of various movement abnormalities. Studying pressure distribution on the foot is very important for the detection of abnormalities, unwanted symptoms, and consequences. This paper aims to design a wearable, low-cost, and real-time gait cycle monitoring system, based on a Flexiforce sensor. In the proposed design, eight force sensors were attached to the insole to estimate the pressure distribution on the foot. Pressure distribution monitoring helps in the estimation of foot disorders and assists in the design of medical shoes for manipulating pressure into the right positions. Sensors were connected to an appropriate microcontroller for real-time monitoring. MATLAB was used to visualize and simulate the real-time plantar pressure variation through static and dynamic states. The obtained experimental results show that the system was stable in both static and dynamic measurements, which could be used to estimate the pressure distribution on the foot.

External field alignment of nickel-coated carbon fiber/PDMS composite for biological monitoring with high sensitivity

Flexible, pressure-sensitive composites can be prepared through the inclusion of electrically conductive particles as functional fillers into an elastomeric polymer matrix, which have been used for the applications of wearable devices for health monitoring and electronic skins. A key challenge associated with these composites is developing anisotropic pressure sensitivity while retaining their flexibility (or low filler content). Herein, we demonstrate a simple and scalable method for aligning anisotropic nickel-coated carbon fibers (NiCF) along with the thickness direction of a polymer matrix by applying a magnetic field. The aligning mechanisms and kinetics of NiCF in the polydimethylsiloxane (PDMS) precursor are revealed by in situ optical microscopy images while a magnetic field is applied. The aligned nickel-coated carbon fibers in the polymer effectively endow the composite films excellent pressure-sensitive performance. The pressure sensitivity of NiCF/PDMS composite films has been systematically studied and can be used for biological monitoring. We believe that this magnetic field assisted processing strategy provides a promising material solution for manufacturing fiber embedded polymer composites with enhanced pressure sensitivity, which is essential for future wearable health monitoring electronics and electronic skin.

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

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

Flexible pressure sensors via engineering microstructures for wearable human-machine interaction and health monitoring applications

Flexible pressure sensors capable of transducing pressure stimuli into electrical signals have drawn extensive attention owing to their potential applications for human-machine interaction and healthcare monitoring. To meet these application demands, engineering microstructures in the pressure sensors are an efficient way to improve key sensing performances, such as sensitivity, linear sensing range, response time, hysteresis, and durability. In this review, we provide an overview of the recent advances in the fabrication and application of high-performance flexible pressure sensors via engineering microstructures. The implementation mechanisms and fabrication strategies of microstructures including micropatterned, porous, fiber-network, and multiple microstructures are systematically summarized. The applications of flexible pressure sensors with microstructures in the fields of wearable human-machine interaction, and ex vivo and in vivo healthcare monitoring are comprehensively discussed. Finally, the outlook and challenges in the future improvement of flexible pressure sensors toward practical applications are presented.

Surface-microstructured cellulose films toward sensitive pressure sensors and efficient triboelectric nanogenerators

To achieve environmental sustainability, cellulose-based functional materials have been extensively used in advanced electronic devices, such as pressure sensor and triboelectric nanogenerator (TENG). Here, we fabricate the surface-microstructured cellulose films (M-CFs) by facile regeneration and hot pressing combined with screen mesh templating. Through simple carbonization, the M-CFs are further converted into the surface-microstructured carbonized cellulose films (M-CCFs) with a good conductivity but maintain the original array concave-pits on surface. These constructed microstructures, which are tunable via controlling the screen mesh's aperture, endow the assembled electronics with adjustable and improved working performance. The pressure sensors with M-CCFs as active materials exhibit an enhanced sensitivity in a wide working range and promising potentials for applications in motions detection and healthcare. The TENGs with M-CFs as tribo-positive friction layers demonstrate higher electrical output and an efficient energy harvesting. Our work provides novel insights into the design and construction of cellulose-based functional films for eco-friendly advanced applications.

A New Class of Electronic Devices Based on Flexible Porous Substrates

With the advent of the Internet of Things era, the connection between electronic devices and humans is getting closer and closer. New-concept electronic devices including e-skins, nanogenerators, brain-machine interfaces, and implantable medical devices, can work on or inside human bodies, calling for wearing comfort, super flexibility, biodegradability, and stability under complex deformations. However, conventional electronics based on metal and plastic substrates cannot effectively meet these new application requirements. Therefore, a series of advanced electronic devices based on flexible porous substrates (e.g., paper, fabric, electrospun nanofibers, wood, and elastic polymer sponge) is being developed to address these challenges by virtue of their superior biocompatibility, breathability, deformability, and robustness. The porous structure of these substrates can not only improve device performance but also enable new functions, but due to their wide variety, choosing the right porous substrate is crucial for preparing high-performance electronics for specific applications. Herein, the properties of different flexible porous substrates are summarized and their basic principles of design, manufacture, and use are highlighted. Subsequently, various functionalization methods of these porous substrates are briefly introduced and compared. Then, the latest advances in flexible porous substrate-based electronics are demonstrated. Finally, the remaining challenges and future directions are discussed.

Wearable Pressure Sensors Based on MXene/Tissue Papers for Wireless Human Health Monitoring

Though the widely available, low-cost, and disposable papers have been explored in flexible paper-based pressure sensors, it is still difficult for them to simultaneously achieve ultrahigh sensitivity, low limit and broad range of detection, and high-pressure resolution. Herein, we demonstrate a novel flexible paper-based pressure sensing platform that features the MXene-coated tissue paper (MTP) sandwiched between a polyimide encapsulation layer and a printing paper with interdigital electrodes. After replacing the polyimide with weighing paper in the MTP pressure sensor, the silver interdigital electrodes can be recycled through incineration. The resulting pressure sensor with polyimide or paper encapsulation exhibits a high sensitivity of 509.5 or 344.0 kPa^-1, a low limit (∼1 Pa) and a broad range (100 kPa) of detection, and outstanding stability over 10 000 loading/unloading cycles. With ultrahigh sensitivity over a wide pressure range, the flexible pressure sensor can monitor various physiological signals and human movements. Configuring the pressure sensors into an array layout results in a smart artificial electronic skin to recognize the spatial pressure distribution. The flexible pressure sensor can also be integrated with signal processing and wireless communication modules on a face mask as a remote respiration monitoring system to wirelessly detect various respiration conditions and respiratory abnormalities for early self-identification of opioid overdose, pulmonary fibrosis, and other cardiopulmonary diseases.

Shape-Designable and Reconfigurable All-Paper Sensor through the Sandwich Architecture for Pressure/Proximity Detection

All-paper sensors that are capable of free cutting and folding maximize the merits of papers, which fully utilize the unique potential of papers in cost effectiveness, flexibility, disposability, biodegradability, and a flexible design. However, most of the paper sensors have applied metals as the electrodes and polyimide/polydimethylsiloxane as the encapsulation/sensitive layers, limiting the advantages of the paper sensor. In this work, an all-paper, shape-designable, and reconfigurable capacitive pressure/proximity sensor is fabricated with multilayered tissue paper as the dielectric and polypyrrole printer paper as the electrode/encapsulation. Without the restriction of heterogeneous materials, the all-paper components enable the sensors' flexible shape design for freely cuttable and foldable 2D and 3D sensors including a 2D braille keyboard and even allow reconfiguration from a 3D box sensor to a 3D candy sensor. The all-paper sensor presents superior pressure-sensing performance (0.96 kPa^-1 at <1.76 kPa and 0.09 kPa^-1 at 1.76-22 kPa) and proximity-sensing ability. The sensing mechanism of the sensor is directly revealed from tissue paper changes using in situ 3D microscopy and dielectric measurement experiments. These results provide inspiration for realizing shape-designable and reconfigurable 3D sensors and fully demonstrate the application potential in omnidirectional perception, stretchable sensors, and green electronics.

Wearable, Washable, and Highly Sensitive Piezoresistive Pressure Sensor Based on a 3D Sponge Network for Real-Time Monitoring Human Body Activities

Wearable pressure sensors are highly desirable for monitoring human health and realizing a nice human-machine interaction. Herein, a chitosan/MXene/polyurethane-sponge/polyvinyl alcohol (CS/MXene/PU sponge/PVA)-based 3D pressure sensor is developed to simultaneously achieve wearability, washability, and high sensitivity in a wide region. In the force-sensitive layer of the sensor, MXene and CS are fully attached to the PU sponge to ensure that the composite sponge has remarkable conductivity and washability. Benefiting from the highly resistive PVA-nanowire spacer, the initial current of the sensor is reduced significantly so that the sensor exhibits extremely high sensitivity (84.9 kPa^-1 for the less than 5 kPa region and 140.6 kPa^-1 for the 5-22 kPa region). Moreover, the sensor has an excellent fast response time of 200 ms and a short recovery time of 30 ms, as well as non-attenuating durability over 5000 cycles. With the high sensitivity in a wide range, the sensor is capable of detecting multiple human and animal activities in real time, ranging from the large pressure of joint activities to a subtle pressure of pulse. Furthermore, the sensor also demonstrates the potential application in measuring pressure distribution. Overall, such a multifunctional pressure sensor can supply a new platform for the design and development of wearable health-monitoring equipment and an efficient human-machine interface.

Review of Materials and Fabrication Methods for Flexible Nano and Micro-Scale Physical and Chemical Property Sensors

The use of flexible sensors has tripled over the last decade due to the increased demand in various fields including health monitoring, food packaging, electronic skins and soft robotics. Flexible sensors have the ability to be bent and stretched during use and can still maintain their electrical and mechanical properties. This gives them an advantage over rigid sensors that lose their sensitivity when subject to bending. Advancements in 3D printing have enabled the development of tailored flexible sensors. Various additive manufacturing methods are being used to develop these sensors including inkjet printing, aerosol jet printing, fused deposition modelling, direct ink writing, selective laser melting and others. Hydrogels have gained much attention in the literature due to their self-healing and shape transforming. Self-healing enables the sensor to recover from damages such as cracks and cuts incurred during use, and this enables the sensor to have a longer operating life and stability. Various polymers are used as substrates on which the sensing material is placed. Polymers including polydimethylsiloxane, Poly(N-isopropylacrylamide) and polyvinyl acetate are extensively used in flexible sensors. The most widely used nanomaterials in flexible sensors are carbon and silver due to their excellent electrical properties. This review gives an overview of various types of flexible sensors (including temperature, pressure and chemical sensors), paying particular attention to the application areas and the corresponding characteristics/properties of interest required for such. Current advances/trends in the field including 3D printing, novel nanomaterials and responsive polymers, and self-healable sensors and wearables will also be discussed in more detail.

Application of Simulated Arms with Real-Time Pressure Monitor in Casting and Splinting by Physiological Sensors

In the real condition, the small sensor found it difficult to detect the position of the pressure sore because of casting displacement clinically. The large sensor will detect the incorrect pressure value due to wrinkles without close to arm. Hence, we developed a simulated arm with physiological sensors combined with an APP and a cloud storage system to detect skin pressure in real time when applying a short arm cast or splint. The participants can apply a short arm cast or splint on the simulative arm and the pressure in the cast or splint could be immediately displaced on the mobile application. The difference of pressure values from six pressure detection points of the simulated arm between the intern and the attending physician with 20-year working experience were 22.8%, −7.3%, 25.0%, 8.6%, 38.2%, 49.6%, respectively. It showed that the difference of pressure values in two farthest points, such as radius stab and ulnar styloid, was maximal. The pressures on the skin surface of the short arm cast were within acceptable range. Doctors would obtain reliable reference data and instantly understand the tightness of the swathed cast which would enable them to adjust it at any time to avoid complications.

Advanced Flexible Skin-Like Pressure and Strain Sensors for Human Health Monitoring

Recently, owing to their excellent flexibility and adaptability, skin-like pressure and strain sensors integrated with the human body have the potential for great prospects in healthcare. This review mainly focuses on the representative advances of the flexible pressure and strain sensors for health monitoring in recent years. The review consists of five sections. Firstly, we give a brief introduction of flexible skin-like sensors and their primary demands, and we comprehensively outline the two categories of design strategies for flexible sensors. Secondly, combining the typical sensor structures and their applications in human body monitoring, we summarize the recent development of flexible pressure sensors based on perceptual mechanism, the sensing component, elastic substrate, sensitivity and detection range. Thirdly, the main structure principles and performance characteristic parameters of noteworthy flexible strain sensors are summed up, namely the sensing mechanism, sensitive element, substrate, gauge factor, stretchability, and representative applications for human monitoring. Furthermore, the representations of flexible sensors with the favorable biocompatibility and self-driven properties are introduced. Finally, in conclusion, besides continuously researching how to enhance the flexibility and sensitivity of flexible sensors, their biocompatibility, versatility and durability should also be given sufficient attention, especially for implantable bioelectronics. In addition, the discussion emphasizes the challenges and opportunities of the above highlighted characteristics of novel flexible skin-like sensors.

Biodegradable Flexible Electronic Device with Controlled Drug Release for Cancer Treatment

Nowadays, controllable drug release is a vitally important strategy for cancer treatment and usually realized using implanting biocompatible devices. However, these devices need to be removed by another surgery after the function fails, which brings the risks of inflammation or potential death. In this article, a biodegradable flexible electronic device with controllable drug (paclitaxel) release was proposed for cancer treatment. The device is powered by an external alternating magnetic field to generate internal resistance heat and promote drug release loaded on the substrate. Moreover, the device temperature can even reach to 65 °C, which was sufficient for controllable drug release. This device also has similar mechanical properties to human tissues and can autonomously degrade due to the structure design of the circuit and degradable compositions. Finally, it is confirmed that the device has a good inhibitory effect on the proliferation of breast cancer cells (MCF-7) and could be completely degraded in vitro. Thus, its great biodegradability and conformity can relieve patients of second operation, and the device proposed in this paper provides a promising solution to complete conquest of cancer in situ.

A flexible and highly sensitive pressure sensor based on three-dimensional electrospun carbon nanofibers

High-performance flexible pressure sensors with high sensitivity are important components of the systems for healthcare monitoring, human–machine interaction, and electronic skin. Herein, a flexible and highly sensitive pressure sensor composed of ferrosoferric oxide (Fe₃O₄)/carbon nanofibers (FeOCN) was fabricated using three-dimensional electrospinning and further heat treatment methods. The obtained pressure sensor demonstrates a wide working range (0–4.9 kPa) and a high sensitivity of 0.545 kPa⁻¹ as well as an ultralow detection limit of 6 Pa. Additionally, the pressure sensor exhibits a rapid response time, good stability, high hydrophobicity, and excellent flexibility. These merits endow the pressure sensor with the ability to precisely detect wrist pulse, phonation, breathing, and finger bending in real-time. Therefore, the FeOCN pressure sensor presents a promising application in real-time healthcare monitoring.

A three-dimensional electrospun carbon nanofiber network was used to measure press strains with high sensitivity.

Triboelectric nanogenerators for human-health care

Since the world's first triboelectric nanogenerator (TENG) was proposed in 2012, numerous TENG-based devices and equipment have sprung up in various fields. In particular, TENG has great potential in the field of human-health care due to its small size, self-powered and low cost. With the continuous deepening of TENG research, its structure, function and technical concept are becoming more and more abundant. In order to summarize the progress and development status of TENG in health care, based on the different types of applications subdirection, this paper reviews the TENG-based research work of this field in recent eight years. The characteristics of various types of TENG-based applications and their corresponding technologies are introduced and analyzed, under the comparison of their structure and performance. This review is dedicated to provide reference and inspiration for the future development and innovation of TENG for health care.

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

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

An Ultrahigh Sensitive Paper-Based Pressure Sensor with Intelligent Thermotherapy for Skin-Integrated Electronics

Porous microstructure pressure sensors that are highly sensitive, reliable, low-cost, and environment-friendly have aroused wide attention in intelligent biomedical diagnostics, human–machine interactions, and soft robots. Here, an all-tissue-based piezoresistive pressure sensor with ultrahigh sensitivity and reliability based on the bottom interdigitated tissue electrode and the top bridge of a microporous tissue/carbon nanotube composite was proposed. Such pressure sensors exhibited ultrahigh sensitivity (≈1911.4 kPa⁻¹), fast response time (<5 ms), low fatigue of over 2000 loading/unloading cycles, and robust environmental degradability. These enabled sensors can not only monitor the critical physiological signals of the human body but also realize electrothermal conversion at a specific voltage, which enhances the possibility of creating wearable thermotherapy electronics for protecting against rheumatoid arthritis and cervical spondylosis. Furthermore, the sensor successfully transmitted wireless signals to smartphones via Bluetooth, indicating its potential as reliable skin-integrated electronics. This work provides a highly feasible strategy for promoting high-performance wearable thermotherapy electronics for the next-generation artificial skin.

Transparent and Stretchable Graphene Electrode by Intercalation Doping for Epidermal Electrophysiology

Epidermal electronics is regarded as the next-generation technology, and graphene is a promising electrode, which is a key building block of such devices. However, graphene has a tendency to crack at small strains with a rapidly increased resistance upon stretching. Here, to enable graphene applicable in epidermal electronics, we designed a novel graphene structure that is molybdenum chloride (MoCl₅)-intercalated few-layer graphene (Mo-FLG) fabricated in a confined environment. In the case of bilayer graphene (BLG), MoCl₅-intercalated bilayer graphene (Mo-BLG) exhibited a low sheet resistance of 40 Ω/square (sq) at a transmittance of 80%. Due to the self-barrier doping effect, the sheet resistance increased to only 60 Ω/sq after exposing to the atmosphere over 1 month. Transferred onto elastomer substrates, Mo-BLG can work as an electrode up to 80% strain and maintain a high conductivity that is durable over 2000 cycles at 30% strain. This mechano-electrostability is attributed to the special intercalated structure where the intercalated dopants act as lubricants to weaken the layer-layer interaction and allow a certain degree of sliding, as well as electrical crack-connectors to bridge the cracked domains at a high strain. Mo-BLG can be applied as epidermal electrodes to monitor electrophysiological signals such as electrocardiogram (ECG), electrooculogram (EOG), electroencephalography (EEG), and surface electromyogram (sEMG) with high signal-to-noise ratios (SNRs) comparable to commercial Ag/AgCl electrode. This is the first demonstration of epidermal electrodes based on intercalation-doped graphene applied in health monitoring, shedding light on the future development of graphene-based epidermal electronics.

Locally Controlled Sensing Properties of Stretchable Pressure Sensors Enabled by Micro-Patterned Piezoresistive Device Architecture

For wearable health monitoring systems and soft robotics, stretchable/flexible pressure sensors have continuously drawn attention owing to a wide range of potential applications such as the detection of human physiological and activity signals, and electronic skin (e-skin). Here, we demonstrated a highly stretchable pressure sensor using silver nanowires (AgNWs) and photo-patternable polyurethane acrylate (PUA). In particular, the characteristics of the pressure sensors could be moderately controlled through a micro-patterned hole structure in the PUA spacer and size-designs of the patterned hole area. With the structural-tuning strategies, adequate control of the site-specific sensitivity in the range of 47~83 kPa-1 and in the sensing range from 0.1 to 20 kPa was achieved. Moreover, stacked AgNW/PUA/AgNW (APA) structural designed pressure sensors with mixed hole sizes of 10/200 µm and spacer thickness of 800 µm exhibited high sensitivity (~171.5 kPa-1) in the pressure sensing range of 0~20 kPa, fast response (100~110 ms), and high stretchability (40%). From the results, we envision that the effective structural-tuning strategy capable of controlling the sensing properties of the APA pressure sensor would be employed in a large-area stretchable pressure sensor system, which needs site-specific sensing properties, providing monolithic implementation by simply arranging appropriate micro-patterned hole architectures.

Flexible paper based piezo-resistive sensor functionalised by 2D-WSe2 nanosheets

High-performance electronics demand extremely sensitive piezo-resistive sensors with important features such as low-fabrication cost, easy implementation, low power consumption and high-pressure sensitivity over broad pressure range. Herein, we report a flexible piezo-resistive paper-based device functionalised by WSe₂ nanosheets. An efficient and low-cost fabrication strategy using Whatman filter paper and tissue paper is adopted for versatile sensing applications. The WSe₂ nanosheets were synthesized by high-yield and size-controlled liquid phase exfoliation technique. The flexible WSe₂ nanosheets-paper sensor shows excellent response in broad pressure range of 1 Pa-100 kPa with exceptionally high sensitivity of 29.24 kPa^-1, current responsivity of 70 and response time of 100 ms. The pressure sensor is also employed to recognize the pressure generated due to finger tapping. Encouragingly, the piezo-resistive sensors can also sense extremely small pressure differences of about 1.4 Pa generated by water drops.

Enclosed paper-based analytical devices: Concept, variety, and outlook

Paper-based analytical devices possess desirable properties such as low cost, convenient production, and rapid output. These advantages over conventional analytical devices have attracted tremendous attention in recent years, and an abundance of fabrication techniques have been achieved with different designs. Related approaches are adopted by scientists and engineers from different research fields to create practical devices tailored for various applications. Among a diverse selection of strategies, paper-based analytical devices featuring enclosed channels can protect its contents from environmental harm, which is helpful in designing paper-based devices aimed toward practical use. However, superior properties of enclosed device designs have often been neglected when a paper-based platform is selected, and related discussion is still lacking in the field. To fill this empty space in the relevant literature, important issues are highlighted and recent research achievements are included in this article, which should have implication for scientists interested in sensing technology, analytical chemistry, material science, and miniaturized devices. For the convenience of reader's understanding, this article provides a general introduction to the basic properties and concepts of paper-based analytical devices. Firstly, commonly used fabrication strategies and detection methods are mentioned, with an in-depth emphasis on paper-based devices with enclosed channels, including breakthroughs in device types, thoughts on novel fabrication, and practical application examples. Subsequently, other important topics related to enclosed paper-based device design are summarized, and future challenges and opportunities in the field are also discussed.

Multiresponsive MXene (Ti3C2Tx)-Decorated Textiles for Wearable Thermal Management and Human Motion Monitoring

Over the past few years, wearable electronics and smart textiles have seen tremendous growth in both academia and industries. However, it is still a challenge to prepare robust, flexible, wearable, and multiresponsive textile electronics. A newly blooming two-dimensional (2D) transition-metal carbide/nitride (MXene) is regarded as an ideal active material to build multifunctional electronics due to its intriguing properties. Herein, a hydrophobic and multifunctional textile composite (Si-MAP) was prepared by decoration of conductive MXene nanosheets onto air-laid paper, followed by wrapping with poly(dimethylsiloxane) (PDMS). These obtained smart textiles exhibited excellent electronic/photonic/mechanical triresponsive properties: Si-MAPs could reach high equilibrium temperatures (104.9 and 118.7 °C) under quite low power illumination (1.25 W cm^-2) and working voltage (4 V). The Si-MAP pressure sensor exhibited high sensitivity and rapid response time (30-40 ms), which can capture a wide range of human movements. Moreover, the thin PDMS layer not only rendered the textile composites hydrophobic but also improved the stability and adaptation for daily use. Remarkably, the hydrophobic Si-MAPs have maintained the advantages of breathability and washability, which make them suitable for wearing. Thus, this smart Si-MAP textile provides a reference for the study of the next generation of light, portable, and wearable textile-based electronic devices.

Paper-Based Sensors for Gas, Humidity, and Strain Detections: A Review

Paper, as a flexible, low-cost, lightweight, tailorable, environmental-friendly, degradable, and renewable material, is emerging in electronic devices. Especially, many kinds of paper-based (PB) sensors have been reported for wearable applications in recent years. Among them, the PB gas, humidity, and strain sensors are widely studied for monitoring gas, humidity, and strain from the human body and the environment. However, gas, humidity, and strain often coexist and interact, and the paper itself is hydrophilic and flexible, resulting in that it is still challenging to develop high-performance PB sensors specialized for gas, humidity, and strain detections. Therefore, it is necessary to summarize and discuss them systematically. In this review, we focus on summarizing the state-of-art studies of the PB gas, humidity, and strain sensors. Specifically, the fabrications (electrodes and sensing materials) and applications of PB gas, humidity, and strain sensors are summarized and discussed. The current challenges and the potential trends of PB sensors for gas, humidity, and strain detections are also outlined. This review not only can help readers to understand the development status of the PB gas, humidity, and strain sensors but also is helpful for readers to find out and solve the problems in this field through comparative reading.

Synergistic Optimization toward the Sensitivity and Linearity of Flexible Pressure Sensor via Double Conductive Layer and Porous Microdome Array

Recently, wearable pressure sensors have attracted considerable interest in various fields such as healthcare monitoring, intelligent robots, etc. Although artificial structures or conductive materials have been well developed, the trade-off between sensitivity and linearity of pressure sensors is yet to be fully resolved by a traditional approach. Herein, from theoretical analysis to experimental design, we present the novel CPDMS/AgNWs double conductive layer (DCL) to synergistically optimize the sensitivity and linearity of piezoresistive pressure sensors. The facilely fabricated solid microdome array (SDA) is first employed as the elastomer to clarify the unrevealed working mechanism of DCL. Attributed to the synergistic effect of DCL, the DCL/SDA based sensor exhibits ultrahigh sensitivity (up to 3788.29 kPa^-1) in an obviously broadened linearity range (0-6 kPa). We also demonstrated that the synergistic effect of DCL can be regulated with use of porous microdome array (PDA) to further optimize the sensing property. The linearity range can be improved up to 70 kPa while preserving the high sensitivity of 924.37 kPa^-1 based on the interlocked PDA structure (IPDA), which is rarely reported in previous studies. The optimized sensitivity and linearity allow the competitive DCL/IPDA based sensor as a reliable platform to monitor kinds of physiological signals covering from low pressures (e.g., artery pulses), medium pressures (e.g., muscle expansions), to high pressures (e.g., body motions). We believe that the methodology along with the robust sensor can be of great potential for reliable healthcare monitoring and wearable electronic applications in the future.

A Flexible and Low-Cost Tactile Sensor Produced by Screen Printing of Carbon Black/PVA Composite on Cellulose Paper

This study presents the design and fabrication of a flexible tactile sensor printed on a cellulose paper substrate using a carbon black (CB) – filled polyvinyl alcohol (PVA) polymer matrix as ink material. In the design, electrodes are obtained by screen printing of CB/PVA composite on dielectric cellulose paper. The screen-printing method is preferred for fabrication because of its simplicity and low manufacturing cost. The tactile sensor is formed by overlapping two ink-printed sheets. Electrical properties are investigated under compressive and tensile strains. The results indicate that the tactile sensor configuration and materials can be used for piezoresistive, capacitive, and also impedance sensors. The same tactile sensor structure is also examined using a commercial carbon-based ink for performance comparison. The comparative study indicates that CB/PVA ink screen-printed on paper demonstrates superior sensitivity for capacitive sensing with low hysteresis, as well as low response and recovery times. The piezoresistive-sensing properties of CB/PVA on cellulose paper show a gauge factor (GF) of 10.68, which is also very promising when conventional metal strain gauges are considered. CB/PVA screen-printed on cellulose paper features impedance-sensing properties and is also sensitive to the measurement frequency. Therefore, the response type of the sensor can be altered with the frequency.

Foot Plantar Pressure Measurement System Using Highly Sensitive Crack-Based Sensor

Measuring the foot plantar pressure has the potential to be an important tool in many areas such as enhancing sports performance, diagnosing diseases, and rehabilitation. In general, the plantar pressure sensor should have robustness, durability, and high repeatability, as it should measure the pressure due to body weight. Here, we present a novel insole foot plantar pressure sensor using a highly sensitive crack-based strain sensor. The sensor is made of elastomer, stainless steel, a crack-based sensor, and a 3D-printed frame. Insoles are made of elastomer with Shore A 40, which is used as part of the sensor, to distribute the load to the sensor. The 3D-printed frame and stainless steel prevent breakage of the crack-based sensor and enable elastic behavior. The sensor response is highly repeatable and shows excellent durability even after 20,000 cycles. We show that the insole pressure sensor can be used as a real-time monitoring system using the pressure visualization program.