Applications In Wearable Devices Research Articles

Effect of Polymer Encapsulation on the Mechanoluminescence of Mn2+-Doped CaZnOS.

Rare earth and transition metal ion-doped CaZnOS has garnered significant attention for its exceptional mechanoluminescence (ML) performance under mild mechanical stimuli and its capability for multicolor emissions. Since powdered phosphors are not directly usable, they require encapsulation within with polymers to create stable structures. This study investigates Mn2+-doped CaZnOS (CaZnOS:Mn2+) as the ML phosphor, optimizing its performance by varying the Mn2+ content, resulting in bright orange-red emissions from the d-d transitions of the Mn2+ activator. A quantum efficiency of 59.08% was achieved through the self-sensitization of the matrix lattice and energy transfer to the Mn2+ luminescent centers. The enhancement in ML due to Mn2+ doping is attributed to the reduced trap depth and increased trap concentration. Encapsulation with four polymers-PDMS, PU, SIL, and RTV-2-was explored to further optimize ML performance. Among these, PDMS provides the best ML output and sensitivity, owing to its slightly cross-linked structure and good triboelectric properties. The optimized CaZnOS:0.03Mn2+/PDMS composite, featuring excellent flexibility and recoverability, shows great potential for applications in anti-counterfeiting encryption, stress sensors, and wearable devices.

Nacre-like Anisotropic Multifunctional Aramid Nanofiber Composites for Electromagnetic Interference Shielding, Thermal Management, and Strain Sensing.

Developing multifunctional flexible composites with high-performance electromagnetic interference (EMI) shielding, thermal management, and sensing capacity is urgently required but challenging for next-generation smart electronic devices. Herein, novel nacre-like aramid nanofibers (ANFs)-based composite films with an anisotropic layered microstructure were prepared via vacuum-assisted filtration and hot-pressing. The formed 3D conductive skeleton enabled fast electron and phonon transport pathways in the composite films. As a result, the composite films showed a high electrical conductivity of 71.53 S/cm and an outstanding thermal conductivity of 6.4 W/m·K when the mass ratio of ANFs to MXene/AgNWs was 10:8. The excellent electrical properties and multi-layered structure endowed the composite films with superior EMI shielding performance and remarkable Joule heating performance, with a surface temperature of 78.3 °C at a voltage of 2.5 V. Additionally, it was found that the composite films also exhibited excellent mechanical properties and outstanding flame resistance. Moreover, the composite films could be further designed as strain sensors, which show great promise in monitoring real-time signals for human motion. These satisfactory results may open up a new opportunity for EMI shielding, thermal management, and sensing applications in wearable electronic devices.

Ultrasensitive, highly stretchable and multifunctional strain sensors based on scorpion-leg-inspired gradient crack arrays

Wearable flexible strain sensors based on conductive films have shown great application potential in many high-tech fields. They have high requirements for synergistic performances of sensitivity, flexibility, stability and durability. However, overcoming the trade-off between high sensitivity and high stretchability is still a great challenge in this research. Here we develop a facile strategy to prepare high-performance flexible strain sensors based on gradient crack arrays inspired by the gradient slits on scorpion legs. The gradient crack arrays with branched or hierarchical features controllably form in a periodic thickness-gradient metal film prepared by one-step masking technique on a flexible substrate. The cracks are progressively open from the thickest film region to the thinnest one under mechanical strain, behaving a unique sensing mechanism. As a result, the gradient crack-based strain sensors possess excellent comprehensive performances including high sensitivity (up to 21000), wide sensing range (∼70 %), low detection limit (0.02 %), quick response speed (below to 80 ms), excellent stability and durability (up to 15,000 cycles at 5 % strain). As demonstration of applications, the sensors are used to monitor various human activities and to express and encrypt information by bending bidirectionally. This work provides a novel way to fabricate highly sensitive, stretchable, robust, multifunctional, crack-based flexible strain sensors, which would greatly expand the applications in flexible electronics and wearable devices.

Topology optimization design for voltage enhancement in variable structure double-layer flexible thermoelectric devices

Flexible thermoelectric devices possess the capability to generate electricity by harnessing temperature differences, rendering them highly promising for applications in wearable devices and powering the Internet of Things. This paper introduces a topology optimization design approach for double-layer thin-film flexible thermoelectric devices with the aim of enhancing the device's output voltage and improving the efficiency of green energy utilization. The design process took into consideration the coupled effects of mechanical, electrical, and thermal aspects. It not only optimized the force transmission paths of the variable structure device but also accounted for heat dissipation and circuit topology configuration. The study employed a field function model to concurrently optimize the structure of both the substrate and thermoelectric materials in an integrated design, with the primary objective being the maximization of output voltage. The efficacy of this methodology was confirmed through a series of numerical simulations and experiments conducted under various load and operational conditions. The optimized flexible thermoelectric devices exhibited an output voltage increase of more than threefold when compared to flat devices.

Controllable synthesis of TiO2/graphene composites for human voice recognition in strain sensor.

Low-dimensional materials have demonstrated strong potential for use in diverse flexible strain sensors for wearable electronic device applications. However, the limited contact area in the sensing layer, caused by the low specific surface area of typical nanomaterials, hinders the pursuit of high-performance strain-sensor applications. Herein, we report an efficient method for synthesizing TiO2-based nanocomposite materials by directly using industrial raw materials with ultrahigh specific surface areas that can be used for strain sensors. A kinetic study of the self-seeded thermal hydrolysis sulfate process was conducted for the controllable synthesis of pure TiO2 and related TiO2/graphene composites. The hydrolysis readily modified the crystal form and morphology of the prepared TiO2 nanoparticles, and the prepared composite samples possessed a uniform nanoporous structure. Experiments demonstrated that the TiO2/graphene composite can be used in strain sensors with a maximum Gauge factor of 252. In addition, the TiO2/graphene composite-based strain sensor showed high stability by continuously operating over 1,000 loading cycles and aging tests over three months. It also shows that the fabricated strain sensors have the potential for human voice recognition by characterizing letters, words, and musical tones.

Multi-walled carbon nanotube-enhanced polyurethane composite materials and the application in high-performance 3D printed flexible strain sensors

Flexile strain sensors hold vast potential for applications in monitoring human motion, enabling human-machine interaction, and facilitating information transfer, etc. However, the available materials and manufacturing techniques for fabricating flexible strain sensors with high sensitivity and extended sensing range still face significant challenges. By utilizing solution casting and twin-screw extrusion techniques, this study prepared high-performance thermoplastic polyurethane/multi-walled carbon nanotube (TPU/MWCNT) nanocomposite 3D printable filaments, and employed material extrusion 3D printing technology to facilely fabricate flexible strain sensors. The 1-pyrene carboxylic acid (PCA)-modified TPU/MWCNT(3 wt%) nanocomposite exhibited outstanding mechanical properties, with a tensile strength of 24.3 ± 0.5 MPa and a fracture elongation of 625.8 ± 12.3 %. The 3D printed TPU/MWCNT(3 wt%)/PCA flexible strain sensors demonstrated amazing sensitivity (GFmax = 10279.95) within a wide strain range (0–300 % strain). The addition of PCA brings the benefits of improved dispersion of MWCNTs in the TPU composite, as well as enhanced thermal stability, and upgraded mechanical and electrical properties under different tensile strains. Meanwhile, the 3D printed samples displayed remarkable durability and reproducibility. Finally, the flexible strain sensors fabricated from this nanocomposite material could accurately perceive human motion, providing great prospects for wearable device applications.

Transparent and flexible fish-tail shaped antenna for ultra-wideband MIMO systems

Abstract The article presents an innovative transparent, flexible antenna design tailored specifically for ultra-wideband multiple-input multiple-output (MIMO) systems. Using polyethylene terephthalate substrates as the base material, we enhance the antenna’s radiation performance, transparency, and flexibility. Broadband impedance matching is achieved through a hollow ground plane fed by a fish-tail radiator and coplanar waveguide. Additionally, significant MIMO antenna isolation in ultra-wideband scenarios is achieved through the vertical arrangement of units and neutral lines. The MIMO antennas we manufactured exhibit minimum transparencies of 71.1% and 85.9% with and without reflectors, respectively. Measurement results demonstrate that the antenna operates within the 3–20 GHz frequency range, with over 24 dB of isolation, 2 ± 2 dBi of peak gain, and 45% ± 5% of radiation efficiency, suitable for applications in wearable devices, vehicle intelligence, and other fields.

Harnessing the potential of porous carbon derived from chlorinated polyvinyl chloride as an effective gas analyte channel for NO2 organic sensors

Organic electronics fabricated using flexible and lightweight conjugated polymers show substantial potential for use in wearable device applications. Nonetheless, the inherently poor carrier transport characteristics of conjugated polymer semiconductors are a prevalent organic-electronics constraint that limits the sensitivities and response rates of organic gas sensors. In this study, we adhered to the principles of green chemistry and developed high-performance, cost-effective organic gas sensors by introducing stable and conductive porous carbon materials derived from chlorinated polyvinyl chloride (cPVC) into a poly(3-hexylthiophene) (P3HT) matrix. We then investigated how carbonization temperature and the presence of active potassium hydroxide (KOH) in the precursor affect sensing performance. Organic field-effect transistor (OFET) gas sensors manufactured using porous carbon obtained by carbonizing cPVC with KOH at 900 °C (referred to as “K9”) exhibited the best sensing performance, with an eight-fold increase in sensitivity to NO2 gas compared to that of the pristine P3HT-based sensor; it also demonstrated selective NO2-sensing performance in the presence of other oxidizing gases.

Building microcracked structure fibrous cathode coated by Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) for ultra-stable fiber-shaped aqueous zinc ions batteries

Attributing to the advantages of intrinsic safety, high energy density, and good omnidirectional flexibility, fiber-shaped aqueous zinc ions batteries (FAZIBs), serving as energy supply devices, have multitude applications in flexible and wearable electronic devices. However, the detachment of active materials caused by bending stress generated during flexing process limits their practical application severely. To address the above issue, an effective integrated strategy employing microcracked activated cobalt hydroxide [A-Co(OH)2] cathode with protective coating of poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) was proposed in this work to enhance the cyclic and bending performances of FAZIBs. The microcracked A-Co(OH)2 cathode relieves stress concentration under bending conditions, while the PEDOT:PSS coating is responsible to maintain the structural integrity and prevents the detachment of A-Co(OH)2. The FAZIBs based on a gel electrolyte achieved a high energy density (173.5 Wh·kg−1) at a power density 90 W·kg−1 and a bending durability (94.4 % capacity retention after 500 cycles) as a consequence of the synergistic effect of microcracked A-Co(OH)2 cathode and the PEDOT:PSS coating. This work will offer a new approach for devising high-performance FAZIBs and promote the development of highly flexible and stable fiber-shaped batteries.

Wet-Spinning Carbon Nanotube/Shape Memory Polymer Composite Fibers with High Actuation Stress and Predesigned Shape Change.

Actuators based on shape memory polymers and composites incorporating nanomaterial additives have been extensively studied; achieving both high output stress and precise shape change by low-cost, scalable methods is a long-term-desired yet challenging task. Here, conventional polymers (polyurea) and carbon nanotube (CNT) fillers are combined to fabricate reinforced composite fibers with exceptional actuation performance, by a wet-spinning method amenable for continuous production. It is found that a thermal-induced shrinkage step could obtain densified strong fibers, and the presence of CNTs effectively promotes the tensile orientation of polymer molecular chains, leading to much improved mechanical properties. Consequently, the CNT/ polyurea composite fibers exhibit stresses as high as 33MPa within 0.36 s during thermal actuation, and stresses up to 22MPa upon electrical stimulation enabled by the built-in conductive CNT networks. Utilizing the flexible thin fibers, various shape change behavior are also demonstrated including the conversion between different structures/curvatures, and recovery of predefined simple patterns. This high-performance composite fibers, capable of both thermal and electrical actuation and produced by low-cost materials and fabrication process, may find many potential applications in wearable devices, robotics, and biomedical areas.

Design and Preparation of One-Dimensional Cathode By Electrophoretic Deposition Method for Flexible Knitted Batteries

Rechargeable batteries are highly promising energy storage systems that can convert and store electrical energy. Effective energy storage systems should have high energy density, high power, safety, environmentally friendliness, and long cycle life. To meet the aforementioned objectives, researchers have attempted to use different morphologies of nanomaterials as electrodes to improve the electrochemical performance of batteries. Among the materials available, one-dimensional structures have unique properties over traditional ones and extensive applications in wearable devices due to their flexibility [1]. Flexible batteries have several advantages such as mechanical flexibility, good energy storage capabilities, affordability, and environmental safety [2]. They knitted or sewed into textiles, folded into random forms, and wrapped around body parts like the neck or wrist [3]. However, there is little research that covers Li-ion batteries that have a one-dimensional flexible structure.Achieving simultaneous high electronic/ionic conductivity and electrochemical stability is difficult in a homogeneous single-component electrode material. Consequently, the development of precisely defined functional one-dimensional (1D) hetero-nanostructures that effectively integrate the advantages and address the limitations of various electrochemically active materials becomes critical [1].The proposed methodology involves the electrophoretic deposition of lithium iron phosphate on conductive nickel wire, providing a one-dimensional cathode structure. As an anode, nickel-tin is deposited on nickel wire by electrochemical deposition. The deposition method allows greater precision and control over the deposition of active material on the current collector and also has possible applications on different structures such as 1D and 3D. Also, the electrolyte was obtained by layer-by-layer method, by dipping the cathode and anode, separately, in polymers to coat the cathode material as an electrolyte-separator film. The full cell was assembled and the better results will be presented in the meeting. Acknowledgments This research was funded by the Science Committee of the Ministry of Education and Science of the Republic of Kazakhstan (Grant No. АР19679855).

Ultrarobust, Self-Healing Poly(urethane-urea) Elastomer with Superior Tensile Strength and Intrinsic Flame Retardancy Enabled by Coordination Cross-Linking.

Poly(urethane-urea) elastomers (PUUEs) have gained significant attention recently due to their growing demand in electronic skin, wearable electronic devices, and aerospace applications. The practical implementation of these elastomers necessitates many exceptional properties to ensure robust and safe utilization. However, achieving an optimal balance between high mechanical strength, good self-healing at moderate temperatures, and efficient flame retardancy for poly(urethane-urea) elastomers remains a formidable challenge. In this study, we incorporated metal coordination bonds and flame-retarding phosphinate groups into the design of poly(urethane-urea) simultaneously, resulting in a high-strength, self-healing, and flame-retardant elastomer, termed PNPU-2%Zn. Additional supramolecular cross-links and plasticizing effects of phosphinate-endowed PUUEs with relatively remarkable tensile strength (20.9 MPa), high elastic modulus (10.8 MPa), and exceptional self-healing efficiency (above 97%). Besides, PNPU-2%Zn possessed self-extinguishing characteristics with a limiting oxygen index (LOI) of 26.5%. Such an elastomer with superior properties can resist both mechanical fracture and fire hazards, providing insights into the development of robust and high-performance components for applications in wearable electronic devices.

Large Enhancement on Performance of Flexible Cellulose‐Based Piezoelectric Composite Film by Welding CNF and MXene via Growing ZnO to Construct a “Brick‐Rebar‐Mortar” Structure

AbstractCellulose‐based piezoelectric composites have attracted extensive attention in recent years due to their low cost, easy molding, high flexibility, and biocompatibility in the manufacture of piezoelectric nanogenerators (PENG) and sensors. In this work, cellulose nanofibril (CNF)/MXene@ZnO (CM@ZnO) piezoelectric composite films with “brick‐rebar‐mortar” structure are prepared by growing ZnO on the mixed substrate via a high‐efficiency two‐step hydrothermal method. As a hard bridge, ZnO welds CNF and MXene together to construct the “brick‐rebar‐mortar” structure, which greatly improves the piezoelectric performance. Its tensile strength reaches 65.13 ± 3.61 MPa, as well as that the longitudinal piezoelectric constant (d33) attains 14.3 ± 1.3 pC N−1. Under the force of 300 KPa, the open‐circuit voltage (VOC) and short‐circuit current (ISC) of CM@ZnO PENG are as high as 17.15 V and 997 nA, respectively. Under the force of 100 KPa, it can charge a 1.0 µF commercial capacitor to 6.07 V in 400 s. The assembled flexible piezoelectric sensor can accurately collect the pressure or vibration signal caused by bending the wrist (0.59 V) with excellent sensitivity. The performance demonstrates the feasibility of its application in wearable smart devices and provides a reference for new cellulose‐based PENGs.

PEDOT:PSS/XNBR films for low-temperature stretchable electromagnetic interference shielding

Electromagnetic interference shielding materials being sufficiently stretchable at low temperatures have a wider range of applications. Herein, stretchable electromagnetic shielding films for low temperatures applications with segregated conductive networks were produced by a simple method of evaporating carboxylated nitrile butadiene rubber (XNBR) latex containing lithium bis(pentafluoroethylsulfonyl)imide (Li–PFSI) doped poly (3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS). It is proved that the mechanical and electrical properties can be improved by changing the crystallization of PEDOT by doping. For the composite film with 4 wt% PEDOT:PSS and 1.4 mm thickness, electromagnetic interference shielding efficiency can maintain at about 60 dB at 100 % strain and 48 dB at 150 % strain, being the highest among stretchable EMI shields reported. At −10 °C and 100 % strain, shielding efficiency of the composite film with 8 wt% PEDOT:PSS is about 32 dB, indicating that the stretchable film is promising in applications in wearable devices, flexible devices, etc. for EMI shielding at low temperatures.

Fast Responsive and High-Strain Electro-Ionic Soft Actuator Based on the 3D-Structure MXene-EGaIn/MXene Bilayer Composite Electrode.

Electro-ionic soft actuators have garnered significant attention owing to their promising applications in flexible electronics, wearable devices, and soft robotics. However, achieving high actuation performance (large bending strain and fast response time) of these soft actuators under low voltage has been challenging due to issues related to ion diffusion and accumulation. In this study, an electro-ionic soft actuator is fabricated using Ti3C2Tx MXene and eutectic gallium-indium (EGaIn) composite material as the bilayer electrode and methylammonium formate/1-ethyl-3-methylimidazolium tetrafluoroborate/poly(vinylidene fluoride) (MAF-EMIMBF4/PVDF) as the ionic liquid-type electrolyte. The research results indicate that the prepared soft actuator exhibits excellent actuation performance with a peak-to-peak displacement of 35 mm and a bending strain of 0.69% (a peak-to-peak strain of 1.38%) under a low voltage (3 V). The electro-ionic soft actuator shows a wide frequency range (0.1-10 Hz), fast response time (0.35 s), and a rise time of 7.5 s. Furthermore, it demonstrates good cyclic durability, with a retention rate of 92.5% of its performance for 10 000 cycles. These excellent performances are attributed to the 3D structure of the Ti3C2Tx-EGaIn/Ti3C2Tx bilayer composite electrode, as well as the characteristics of the low viscosity, high conductivity, small ion volume, and larger volume difference between cations and anions in MAF ionic liquid. The high-performance electro-ionic soft actuator can be used in various fields such as artificial muscles, tactile devices, and soft robots.

Anti-freezing and conductive hydrogel with water-resistant properties for flexible underwater sensors

Flexible strain sensors based on hydrogels have numerous application prospects. However, the high water content and unsatisfactory water resistance of traditional hydrogel-based sensors limits its application in harsh environment, such as subzero environment and underwater environment. Herein, conductive poly(acrylamide-co-2-hydroxyethyl methacrylate) ionic glycerol-hydrogel was developed through the hydrophobic aggregation interaction and double cross-linking network. The resultant ionic glycerol-hydrogel exhibited excellent anti-freezing properties (-34 °C), water-resistant properties (swelling ratio less than 3 %), ionic conductivity (0.7 S m−1), and strain sensing capabilities (Gauge factor up 1.73). The flexible strain sensors based on ionic glycerol-hydrogel could detect electrical signals at subzero temperatures. Moreover, the hydrogel sensors achieved effective detection and discrimination of Morse codes to transmit underwater information. Demonstrated its potential application in wearable devices.

Synergistic Strategies of Biomolecular Transport Technologies in Transdermal Healthcare Systems.

Transdermal healthcare systems have gained significant attention for their painless and convenient drug administration, as well as their ability to detect biomarkers promptly. However, the skin barrier limits the candidates of biomolecules that can be transported, and reliance on simple diffusion poses a bottleneck for personalized diagnosis and treatment. Consequently, recent advancements in transdermal transport technologies have evolved toward active methods based on external energy sources. Multiple combinations of these technologies have also shown promise for increasing therapeutic effectiveness and diagnostic accuracy as delivery efficiency is maximized. Furthermore, wearable healthcare platforms are being developed in diverse aspects for patient convenience, safety, and on-demand treatment. Herein, a comprehensive overview of active transdermal delivery technologies is provided, highlighting the combination-based diagnostics, therapeutics, and theragnostics, along with the latest trends in platform advancements. This offers insights into the potential applications of next-generation wearable transdermal medical devices for personalized autonomous healthcare.

Self-powered and degradable humidity sensors based on silk nanofibers and its wearable and human–machine interaction applications

Humidity sensors have received increasing attention due to their broad potential application in wearable devices, health monitoring, intelligent control, and other fields. However, traditional humidity sensors are prone to produce various toxic substances to pollute the environment after discarding. Also, they are prone to damage during use, which shortens their service life. Therefore, the development of eco-friendly, degradable, and repairable humidity sensors is of great significance for reducing e-waste and protecting the environment. Here, we started with the selection of materials by using silk nanofiber (SNF) film as the substrate material, copper/aluminum as electrodes, and calcium chloride as the electrolyte, combined with the working mechanism of the primary battery, to construct a self-powered humidity sensor. The humidity sensor exhibits excellent humidity sensing performance and good linearity within the range of 30–90 % relative humidity (RH). A single humidity sensor can output 346 mV at 90 % RH. After being damaged, the humidity sensor can be repaired with the help of water. When the ruptured SNF film encounters water molecules, the water molecules can prompt the hydrogen bonds between the SNFs to rearrange and connect, connecting the broken area. Young’s modulus retention rate of the repaired SNF film is over 85 %. The repaired humidity sensor can also output 328 mV. Additionally, an alkaline solution can completely degrade the humidity sensor in less than 80 min. The proposed humidity sensors have been applied to wearable self-powered wristbands, wireless monitoring systems, and non-contact human–machine interaction systems. Hence, we provide comprehensive research from the material level (SNFs) to the functional device level (the self-powered humidity sensor), and then to the system level (wearable and human–machine interface system). The research will open a new way for the design of the self-powered humidity sensor and have great potential applications in wearable electronics and human–machine interface applications.

MXene-CNC super performing composite films for flexible and degradable electronics

Flexible electronics have gained significant interest due to their potential applications in wearable devices, biomedical sensors, and flexible displays. This study explores the combination of MXene and cellulose nanocrystals (CNC) for the development of flexible, conductive, and degradable materials. CNC and MXene are high-performance nanomaterials with known degradability. CNC can self-assemble into highly ordered layers and act as an ideal structural companion for enabling large surface MXene-based devices with outstanding performance. Three fabrication routes are explored in this work: self-assembly, drop casting, and dip coating. The mechanical and electrical properties of the CNC-MXene composite films were assessed. Tensile testing revealed that dip-coated films in high-loadings of MXene could exhibit enhanced toughness of up to 1.9 MJ‧m−3, higher than pure MXene or CNC. The electrical conductivity of the films varied depending on MXene concentration and drying, exhibiting superior conductivity in self-assembly with higher MXene loading and rapid drying that prevents oxidation. Self-assembled MXene-CNC with a 1:3 MXene-to-CNC ratio maintained high conductivity of 2.2 × 105 S‧m−1. Zeta potential measurements indicate that CNC enhances the stability of MXene dispersion. These findings demonstrate good synergy between CNC and MXenes for applications in flexible, transparent, and environmentally degradable electronic.

Application Progress of Wearable Devices in the Field of Depression Monitoring and Intervention

Depression's high recurrence rate and severe consequences pose significant challenges to public health. To address this issue effectively, this review explores the innovative application of wearable devices in monitoring and intervening in depression, surpassing the limitations of traditional subjective assessments and patient self-reports. The paper systematically analyzes recent studies utilizing wearable devices to monitor physiological and behavioral indicators of depression, categorizing them by different technological types and evaluating their practical effectiveness in early diagnosis and intervention. The findings indicate that wearable devices can continuously monitor physiological indicators and behavioral patterns related to depression, potentially enabling early detection of depressive episodes and supporting timely interventions. Despite challenges such as data privacy and user acceptance, wearable technology holds immense potential in enhancing clinical outcomes in depression treatment.