Flexible Wearable Sensors Research Articles

Evaluating head impact intensities and accelerations using flexible wearable sensors for traumatic brain injury assessment

In response to the significant health threat posed by Traumatic Brain Injury (TBI) resulting from sudden collisions on the head, there is an urgent need for timely assessment and evaluation. With the wide applications of wearable devices in health monitoring, here we report a flexible and wearable smart headband based on a polyvinylidene fluoride (PVDF) sensor to evaluate the impact force and acceleration for TBI assessment. Based on the electromechanical coupling characteristics of PVDF, a lumped parameters model of an equivalent circuit was obtained by a system identification method, which accurately transforms the output signal of the PVDF sensor to the impact force for TBI assessment of focal brain damage. Besides, a drop tower was used to impose different impact intensity levels on a dummy head wearing the smart headband; the results (MSE< 0.018) of transformed impact force show the universal applicability of the model. Moreover, the correlation analysis results show that the impact force has a positive correlation (R>85 %) with impact acceleration at the head center of gravity, which enables the TBI assessment of diffuse brain damage. This smart headband can be used for real-time monitoring of human head impact and has excellent potential for application in medical and sports devices.

Flexible Piezoelectric Sensor Based on Two-Dimensional Topological Network of PVDF/DA Composite Nanofiber Membrane

Owing to the robust scalability, ease of control and substantial industrial applications, the utilization of electrospinning technology to produce piezoelectric nanofiber materials demonstrates a significant potential in the development of wearable products including flexible wearable sensors. However, it is unfortunate that the attainment of high-performance piezoelectric materials through this method remains a challenging task. Herein, a high-performance composite nanofiber membrane with a coherent and uniformly dispersed two-dimensional network topology composed of polyvinylidene fluoride (PVDF)/dopamine (DA) nanofiber membranes and ultrafine PVDF/DA nanofibers was successfully fabricated by the electrospinning technique. Based on the evidence obtained from simulations, experimental and theoretical results, it was confirmed that the unique structure of the nanofiber membrane significantly enhances the piezoelectric performance. The present PVDF/DA composite nanofibers demonstrated a remarkable piezoelectric performance such as a wide response range (1.5–40 N), high sensitivity to weak forces (0–4 N, 7.29 V N−1), and outstanding operational durability. Furthermore, the potential application of the present PVDF/DA membrane as a flexible wearable sensor for monitoring human motion and subtle physiological signals has also been validated. This work not only introduces a novel strategy for the application of electrospun nanofibers in sensors but also provides new insights into high-performance piezoelectric materials.Graphical

A Super-Adhesive and Electromagnetically Shielded Flexible Chip-Selectable and Selectable Wearable HD-sEMG Sensor

A Super-Adhesive and Electromagnetically Shielded Flexible Chip-Selectable and Selectable Wearable HD-sEMG Sensor

A wearable flexible triboelectric nanogenerator for bio-mechanical energy harvesting and badminton monitoring

Recently, textile materials used for wearable flexible sensors have received much attention. Wearable textile based triboelectric nanogenerator (TENG) not only has unique advantages in mechanical energy harvesting, but also has application value in the direction of motion sensing. Here, we proposed a non-woven fabric triboelectric nanogenerator (NW-TENG) for mechanical energy harvesting and badminton monitoring. The non-woven fabric play the role of positive triboelectric, and the fluffy fiber structure endows NW-TENG with a sensitive response to pressure. The pressure sensing sensitivity of NW-TENG sensor can reach 1.22 V N−1 (Pressure range: 0–7 N) and 0.18 V N−1 (Pressure range: 8 N–55 N). Furthermore, the NW-TENG can be installed on the body joints of badminton players for analyzing joint movements, thereby achieving data-driven badminton training and facilitating the evaluation of training effectiveness. This research provide a new path to promote TENG to the badminton monitoring field.

Stretchable, self-adhesion and durable polyacrylamide/polyvinylalcohol dual-network hydrogel for flexible supercapacitor and wearable sensor

Conductive hydrogels show great promise in the field of flexible electronics, but traditional hydrogels have some problems such as poor environmental tolerance, being easy to damage and not being durable. In this work, polyacrylamide (PAM) was used as the main network, and polyvinylalcohol (PVA) was added to construct a dual network hydrogel. Simultaneously, LiTFSI was added to improve hydrogel ionic conductivity, and multiple interionic and intermolecular interactions formed within the gel, significantly improving the gel's mechanical properties and durability. The area specific capacitance of the hydrogel-based supercapacitor is 383.4 mF cm−2, and it still maintains a high cycle life of 90.35 % after 10,000 cycles. The hydrogel-based strain sensor shows fatigue resistance, rapid recovery after 400 stretches, high sensitivity (GF = 3.83 at 300–400 %), and the ability to detect human motion. Therefore, the multi-functional properties of PAM/PVA/LiTFSI hydrogels are expected to play a demonstration role in a new generation of flexible electronic products.

A BaTiO3@polyacrylonitrile/poly(vinylidene fluoride) nanofibrous composite membrane with high piezoelectricity based on the central combination design method and cross-electrospinning technology

The rapid development of piezoelectric sensors has been studied extensively, owing to their good flexibility, wearability, high sensitivity and low cost. However, some inorganic materials with good piezoelectricity cannot make sensors flexible, and the organic materials with good flexibility have a weak output electrical signal and low strength. In order to explore and optimize the preparation technology of piezoelectric sensors, a BaTiO3@polyacrylonitrile (PAN)/poly(vinylidene fluoride) (PVDF) nanofibrous composite membrane (NCM) was prepared by cross-electrospinning technology and the central combination design (CCD) method. The morphology, structure, hydrophobicity, mechanical properties and piezoelectricity of the BaTiO3@PAN/PVDF NCMs were investigated. The BaTiO3@PAN/PVDF NCMs had the better hydrophobicity and mechanical properties compared with the pure PAN/PVDF NCM. The 5BaTiO3@PAN/PVDF NCM designed by CCD had a more uniform fiber diameter, and a more stable output voltage with a 46% improvement. With the help of cross-electrospinning technology and the CCD method, the NCM will be outstanding for the development of fabricating flexible wearable piezoelectric sensors.

Highly Stretchable and Multimodal MXene/CNTs/TPU Flexible Resistive Sensor with Hierarchical Structure Inspired by Annual Ring for Hand Rehabilitation.

With the advent of the intelligent age and people's higher pursuit of health, wearable sensors with functions of health monitoring and assisting physical rehabilitation are increasingly favored by consumers. Wherein, highly stretchable flexible sensors show promising potential, but the unstable conductivity under large strains remains a great challenge to develop flexible wearable sensors with both a wide work range and strain insensitivity. Based on this, a MXene/CNTs/TPU flexible resistive sensor (MCT/FRS) with hierarchical structure inspired by the annual ring was proposed. Benefiting from the bioinspired structure with tightly warped inner layers and deformable spring structure outside, the MCT/FRSs enable stable sensing over a wide working range of up to 700% under the stretching mode, as well as superior durability (7500 cycles). It also possessed linear and adjustable piezoresistive properties under the compression mode. Finally, the sensor was not only successfully employed for monitoring various human movements but also was utilized to assist hand rehabilitation in patients with Guillain-Barré syndrome in both stretching and compression modes. This work provides promising and attractive solutions for flexible wearable devices and intelligent medical care.

Multifunctional fiber-shaped flexible wearable strain sensor with high sensitivity and wide sensing range for detecting autonomous driving technology in automobiles

Flexible strain sensors have developed rapidly and shown enormous application prospects in human motion detection, intelligent robots, and other fields in recent years. However, developing satisfactory strain sensors that meet the requirements of high sensitivity, wide detection range and prominent durability remains a significant challenge. Herein, we developed a flexible and wearable multifunctional fiber-shaped carbon black (CB) and thermoplastic polyurethane (TPU) strain sensor (defined as CTSS) by combining wet spinning and ultrasound. The CTSS possesses characteristics of lightweight, wide sensing range (>390 %), high sensitivity (up to 1414), short response and recovery time (200 ms), ultralow detection limit (0.005 % strain) and good durability. These advantages endow the CTSS with good capability to monitor complicated human activities efficiently (e.g., joint bending, facial expression and breathing). CTSS can also clearly distinguish different bending states. It was then used to monitor the steering of a car in real-time, proving its tremendous potential as high performance flexible electronics device. CTSS is then encapsulated with Ecoflex (E-CTSS) as a single packaged fiber, exhibiting good triboelectric nanogenerator (TENG) performance, with a maximum output voltage beyond 12V for a single packaged fiber, and can maintain stable output. Indicating that E-CTSS has self powered sensing capability.

Wearable sensors for monitoring vital signals in sports and health: progress and perspective

Purpose This paper aims to concentrate on recent innovations in flexible wearable sensor technology tailored for monitoring vital signals within the contexts of wearable sensors and systems developed specifically for monitoring health and fitness metrics. Design/methodology/approach In recent decades, wearable sensors for monitoring vital signals in sports and health have advanced greatly. Vital signals include electrocardiogram, electroencephalogram, electromyography, inertial data, body motions, cardiac rate and bodily fluids like blood and sweating, making them a good choice for sensing devices. Findings This report reviewed reputable journal articles on wearable sensors for vital signal monitoring, focusing on multimode and integrated multi-dimensional capabilities like structure, accuracy and nature of the devices, which may offer a more versatile and comprehensive solution. Originality/value The paper provides essential information on the present obstacles and challenges in this domain and provide a glimpse into the future directions of wearable sensors for the detection of these crucial signals. Importantly, it is evident that the integration of modern fabricating techniques, stretchable electronic devices, the Internet of Things and the application of artificial intelligence algorithms has significantly improved the capacity to efficiently monitor and leverage these signals for human health monitoring, including disease prediction.

A wearable sensor device based on screen-printed chip with biofuel cell-driven electrochromic display for noninvasive monitoring of glucose concentration

Wearable flexible sensor devices have the characteristics of lightweight and miniaturization. Currently, power supply and detection components limit the portability of wearable flexible sensor devices. Meanwhile, conventional liquid electrolytes are unsuitable for the integration of sensing devices. To address these constraints, wearable biofuel cells and flexible electrochromic displays have been introduced, which can improve integration with other devices, safety, and color-coded display data. Meanwhile, electrode chips prepared through screen printing technology can further improve portability. In this work, a wearable sensor device with screen-printed chips was constructed and used for non-invasive detection of glucose. Agarose gel electrolytes doped with PDA-CNTs were prepared, and the mechanical strength and moisture retention were significantly improved compared with traditional gel electrolytes. Glucose in interstitial fluid was non-invasive extracted to the skin surface using reverse iontophoresis. As a biofuel for wearable biofuel cells, glucose drives self-powered sensor and electrochromic display to produce color change, allowing for visually measurement of glucose levels in body fluids. Accurate detection results can be visualized by reading the RGB value with a cell phone.

Polyethyleneimine Controlled Impregnation of Silver Nanowires on Electrospun Ceramic Sponges

AbstractElectrospun sponges with electrical conductivity are promising materials for electrodes with applications in pressure sensors, flexible wearable sensors, and supercapacitors. However, electrospun sponges display poor electric conductivity, which can be increased by the addition of silver nanowire (AgNW). The resulting electric conductivity may depend strongly on the distribution of the AgNW in and on the sponges. A straightforward assembly method for an elastic conductive sponge, composed of 3D structures and metal nanowires, using a synergistic strategy which involves modified impregnation and deposition processes assisted by polyethyleneimine (PEI) impregnation, is presented. The resulting changes in the material properties are also explored, and it is under specific conditions, that percolation, leading to significantly enhanced electrical conductivity in the sponges, is achieved.

Highly sensitive on-skin flexible Sb2O3/Sb thin film pH sensor for real-time sweat sensing

This study presents a significant advancement in the application of a freestanding potentiometric pH sensor patch made of PDMS, incorporating a sensing electrode of Sb2O3/Sb thin film and a reference electrode of AgIO3/Ag thin film. The pH sensing electrodes of Sb2O3/Sb/Au/Ni and AgIO3/Ag/Au/Ni thin films were deposited over the PDMS/cellulose acetate/Si substrate using the ECR/RF sputtering technique. The pH sensor prepared on the PDMS/cellulose acetate/Si substrate displayed excellent sensitivity (−58.24 to −60.84 mV/pH, n = 4) and a robust linear response (R2 ∼ 0.99) in a wider pH range (1.68–9.18). The flexible on-skin wearable pH sensors were obtained by peeling off the pH sensor-assisted PDMS sheet from the Si substrate, which exhibited a remarkable sensitivity of -54.29 mV/pH and a linear pH response (R2 ∼ 0.99). Real-time sweat sensing experiments confirmed the accuracy of the developed flexible on-skin wearable pH sensors in monitoring pH levels at different body parts.

Graphene Aerogel‐Based Pressure Sensors with Gradient Structure for Subtle Physiological Signals and Motion Monitoring

AbstractHigh‐performance flexible wearable pressure sensors require the simple preparation method coupled with a well‐designed structure. Graphene aerogel has garnered significant attention as a flexible pressure sensor due to its exceptional features such as high porosity, favorable compressibility, and fine conductivity. However, it remains challenging to precisely control the 3D structure of the graphene aerogel within the sensors, which limits the detection sensitivity and application range. Here, a freeze‐assisted transfer printing strategy (FATPS) is proposed for fabricating a high‐performance graphene aerogel‐based sensor with the gradient structures. Compared to sensors prepared using mold methods, the sensitivity of the sensor produced through FATPS can be enhanced by nearly tenfold. Furthermore, the gradient structure empowers the graphene pressure sensor with excellent reliability and rapid response time (50 ms) within the pressure range of 0.1–50 kPa. The sensor exhibits the capability to monitor and recognize various physiological activities, including pulse, vocalization, coughing, swallowing, and finger bending. It is expected that the sensor has remarkable potential applications in medical diagnosis, disability aid, and athletic injury prevention and so on.

Flexible vanillin-polyacrylate/chitosan/mesoporous nanosilica-MXene composite film with self-healing ability towards dual-mode sensors

Manufacturing flexible sensors with prominent mechanical properties, multifunctional sensing abilities, and remarkable self-healing capabilities remains a difficult task. In this study, a novel vanillin-modified polyacrylate (VPA), which is capable of forming green dynamic covalent crosslinking with chitosan (CS), was synthesized. The synthesized VPA was combined with mesoporous silica-modified MXene (AMS-MXene) and covalently cross-linked simultaneously with CS, resulting in the formation of a flexible composite conductive film designed for dual-mode sensors. Due to the multidimensional structure formed by the mesoporous silica and MXene layers, the resulting composite film is not only suitable for strain sensing but also excels in gas response sensing. Most importantly, the composite films demonstrate a remarkable self-healing capability through reversible dynamic covalent bonds, specifically Schiff base bonds, coupled with multiple hydrogen bonding interactions with AMS-MXene. This robust self-repair functionality remains effective even at a low temperature of 30 °C. Additionally, the synergistic antibacterial effect exerted by vanillin and CS in the film can endow the composite sensor with excellent antimicrobial properties. This multifunctional composite film holds tremendous potential for applications in green flexible wearable sensors. Furthermore, it can show diverse applications in a wide variety of fields, driving advances in wearable technology and human health monitoring.

Non-Woven Fabric Thermal-Conductive Triboelectric Nanogenerator via Compositing Zirconium Boride.

With the vigorous development of the Internet of Things, 5G technology, and artificial intelligence, flexible wearable sensors have received great attention. As a simple and low-cost power supply in wearable sensors, the triboelectric nanogenerator (TENG) has a wide range of applications in the field of flexible electronics. However, most polymers are thermally poor conductors (less than 0.1 W/(m·K)), resulting in insufficient heat dissipation performance and limiting the development of TENG. In this study, a high-performance non-woven fabric TENG with strong thermal conductivity (0.26 W/m·K) was achieved by introducing ZrB2 into the polyurethane (PU) matrix. The excellent output performance with an open circuit voltage (Voc) of 347.6 V, a short circuit current (Isc) of 3.61 μA, and an accumulated charge of 142.4 nC endows it with good sensitivity. The electrospun PU/ZrB2 composites exhibit excellent sensing performance to detect body movements in situ, such as pressing, clapping, running, and walking. Moreover, the generated power can light up 224 LED bulbs as a demonstration of self-powering ability.

Wearable and Recyclable Water-Toleration Sensor Derived from Lipoic Acid.

Flexible wearable sensors recently have made significant progress in human motion detection and health monitoring. However, most sensors still face challenges in terms of single detection targets, single application environments, and non-recyclability. Lipoic acid (LA) shows a great application prospect in soft materials due to its unique properties. Herein, ionic conducting elastomers (ICEs) based on polymerizable deep eutectic solvents consisting of LA and choline chloride are prepared. In addition to the good mechanical strength, high transparency, ionic conductivity, and self-healing efficiency, the ICEs exhibit swelling-strengthening behavior and enhanced adhesion strength in underwater environments due to the moisture-induced association of poly(LA) hydrophobic chains, thus making it possible for underwater sensing applications, such as underwater communication. As a strain sensor, it exhibits highly sensitive strain response with repeatability and durability, enabling the monitoring of both large and fine human motions, including joint movements, facial expressions, and pulse waves. Furthermore, due to the enhancement of ion mobility at higher temperatures, it also possesses excellent temperature-sensing performance. Notably, the ICEs can be fully recycled and reused as a new strain/temperature sensor through heating. This study provides a novel strategy for enhancing the mechanical strength of poly(LA) and the fabrication of multifunctional sensors.

Human Nervous System Inspired Modified Graphene Nanoplatelets/Cellulose Nanofibers‐Based Wearable Sensors with Superior Thermal Management and Electromagnetic Interference Shielding

AbstractWearable sensing technologies have witnessed rapid development in recent years due to their accessibility, functionality, and affordability. However, heat accumulation and electromagnetic interference in electronic components adversely affect the sensing performance and seriously damage human health. Herein, cellulose nanofibers (CNFs)‐based composites with high thermal conductivity (TC) and excellent electromagnetic interference (EMI) shielding performance are prepared using CNFs as templates followed by coating with tannic acid non‐covalent and 3‐aminopropyltriethoxysilane covalent co‐modified graphene nanoplatelets (denoted as mGNPs) through a simple electrostatic self‐assembly method. The subsequent hot‐pressing process yield order and layer mGNPs in CNFs‐based composites with mGNPs distributed along the orientation and in close contact with CNFs, a fashion similar to the human nervous system. The resulting CNFs‐based composites reveal a high TC of 136.2 W/(m·K) and a superior EMI shielding effectiveness of 105 dB. Thus, they are used as wearable sensors based on the triboelectric effect to monitor human health in real‐time, as well as express emotion through Morse code. In sum, the proposed strategy provides an avenue to prolong the service life of flexible wearable sensors and ensure their safe use, promising for future wisdom in healthcare and smart robotics.

A polypyrrole-dopamine/poly(vinyl alcohol) anisotropic hydrogel for strain sensor and bioelectrodes

Flexible hydrogel-based wearable sensors are of interest for monitoring human movement and electrophysiological signal detection. The high mechanical properties, high conductivity, and sensing sensitivity are the main performance parameters. Inspired by the structure of human muscles, ligaments, and other tissues, a polypyrrole-dopamine/poly(vinyl alcohol) (DA-PPy/PVA) hydrogel with a bionic anisotropic structure is prepared in this study using a directed freeze–thaw strategy. The introduction of DA-PPy nanoparticles can significantly improve the mechanics, adhesion, and conductivity due to the high water dispersibility. Moreover, they can act as physical cross-linking sites to further improve the mechanical properties. The anisotropic structure would influence many properties (mechanical, conductivity, resistance to crack extension, etc.) of DA-PPy/PVA hydrogel. In addition, the prepared DA-PPy/PVA hydrogel exhibits a wide range of strain responses, moreover, its relative resistance change (ΔR/R0) strain curve maintains a line linear within 400% strain. The oriented structure would influence the sensing sensitivity, the gauge factor of anisotropic DA-PPy/PVA in parallel direction is 3.00. In addition, the anisotropic DA-PPy/PVA can effectively motor the physiological electrical signals as a bioelectrode. For example, the acquired electrocardiogram (ECG) and electromyography (EMG) signals not only are clearer than those of the commercial electrodes but show a higher signal-to-noise ratio. It is conceivable that the bionic anisotropic DA-PPy/PVA hydrogels provide a feasible material to construct flexible wearable sensors for monitoring human movement and electrophysiological signals.

A Hydrogel Electrolyte toward a Flexible Zinc-Ion Battery and Multifunctional Health Monitoring Electronics.

The compact design of an environmentally adaptive battery and effectors forms the foundation for wearable electronics capable of time-resolved, long-term signal monitoring. Herein, we present a one-body strategy that utilizes a hydrogel as the ionic conductive medium for both flexible aqueous zinc-ion batteries and wearable strain sensors. The poly(vinyl alcohol) hydrogel network incorporates nano-SiO2 and cellulose nanofibers (referred to as PSC) in an ethylene glycol/water mixed solvent, balancing the mechanical properties (tensile strength of 6 MPa) and ionic diffusivity at -20 °C (2 orders of magnitude higher than 2 M ZnCl2 electrolyte). Meanwhile, cathode lattice breathing during the solvated Zn2+ intercalation and dendritic Zn protrusion at the anode interface are mitigated. Besides the robust cyclability of the Zn∥PSC∥V2O5 prototype within a wide temperature range (from -20 to 80 °C), this microdevice seamlessly integrates a zinc-ion battery with a strain sensor, enabling precise monitoring of the muscle response during dynamic body movement. By employing transmission-mode operando XRD, the self-powered sensor accurately documents the real-time phasic evolution of the layered cathode and synchronized strain change induced by Zn deposition, which presents a feasible solution of health monitoring by the miniaturized electronics.

Bioinspired breathable biodegradable bioelastomer-based flexible wearable electronics for high-sensitivity human-interactive sensing

Flexible wearable electronic sensors are regarded to facilitate revolutionary changes in human's daily life for their great promise in multifunctional artificial electronic skins, personalized healthcare monitoring, and smart human–machine interaction. However, there remains great challenges to simultaneously achieve high sensitivity, broad sensing range and robust cyclic stability for high-performance personal diagnostic healthcare monitoring, excellent breathability for comfortable wearing, as well as environmentally friendly biodegradability for reducing electronic wastes. Herein, bioinspired by three-dimensional network structure of ant nest with high specific surface area, interconnected channels, and robust ventilation, a wearable, breathable, and biodegradable electronic sensor is prepared from facile face-to-face assembly of breathable biodegradable MXene-coated poly(1,8-octanediol-co-polycaprolactone citrate) elastomer sponge (MXene@POPLC), and an MXene ink-printed interdigitated electrode-coated breathable electrospun film. The as-obtained flexible electronic sensor exhibits high sensitivity (up to 246.3 kPa−1), broad sensing range (up to 333.3 kPa), reliable cycling stability (∼20,000 cycles), fast response (10 ms)/recovery time, robust breathability, excellent biocompatibility, facile biodegradability, and wireless wearable human–machine interfacing. Therefore, this line of research work opens up a novel avenue for the preparation of multifunctional wearable, breathable, and biodegradable electronics to hold great promise in smart healthcare monitoring, intelligent human–machine interaction, smart personal protection and next-generation artificial skin.