Flexible Wearable Devices Research Articles

Stretchable, self-adhesive, and sensitive polyacrylamide-polyvinyl alcohol double network hydrogels for flexible wearable sensors

With the development of artificial intelligence, flexible hydrogel sensors have received widespread attention in fields such as biomedicine, environmental monitoring, and information identification. However, current hydrogels face challenges such as low conductivity, sensitivity, and stability. In this study, we developed a novel PAHS double network hydrogel composed of polyvinyl alcohol (PVA), acrylamide (AM), acrylic acid (AA), chitosan quaternary ammonium salt (HACC-102), and sodium citrate (SC). This hydrogel features a double network structure, combining a chemically cross-linked network of PAMAAC as the soft network to complement the hard network formed by PVA. This unique structure results in an impressive elongation at break of 1140 % and a maximum stress of 490 kPa, indicating superior mechanical properties. The inclusion of SC significantly enhances the hydrogel's conductivity, reaching 0.63 S/m, a 4.8-fold improvement. The hydrogel exhibits a gauge factor of 1.60 under 0 %-100 % strain and 1.97 within the 100 %-400 % strain range. It also demonstrates a quick response time of 155 ms and 160 ms under 1 % strain, ensuring rapid and stable signal transmission. Moreover, the PAHS hydrogel sensor demonstrates quick response capabilities and high sensitivity in monitoring human movements and high precision in handwriting recognition. With its simple preparation method, excellent mechanical properties, and stable signal transmission, the PAHS hydrogel presents a viable strategy for future applications in flexible wearable devices.

Electrifying waste textiles: Transforming fabric scraps into high-performance triboelectric nanogenerators for biomechanical energy harvesting

Textiles are an integral part of daily life globally, but their widespread use leads to significant waste generation. Repurposing these discarded fabrics for energy harvesting offers a sustainable solution to both energy demand and textile waste management. In this study, Textile-based Triboelectric Nanogenerators (T-TENGs) were developed using recycled cloth as tribopositive layers and polyvinyl chloride (PVC) film as the tribonegative layer, with aluminum foil tape serving as electrodes. Five different recycled textiles were evaluated, and Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS) analysis revealed a correlation between yarn structure and carbon content, leading to enhanced triboelectric performance. Silk-based TENG (S-TENG) demonstrated the highest output, with 320.76 V and 8.73 µA, while exhibiting stable performance over 10,000 cycles. Practical applications were explored by integrating T-TENGs into shoe insoles for energy harvesting during walking and jumping, with rayon-based TENG generating up to 208.52 V on a PVC coil mat. This work highlights the dual benefits of waste reduction and sustainable energy applications, making a compelling case for advanced technologies where recycled textiles function as frictional materials to harvest mechanical energy from human motion and convert it into electrical energy for use in flexible sensors and wearable devices.

Carbon nanotube reinforced ionic liquid dual network conductive hydrogels: Leveraging the potential of biomacromolecule sodium alginate for flexible strain sensors

The rapid evolution of multifunctional wearable smart devices has significantly expanded their applications in human-computer interaction and motion health monitoring. Central to these devices are flexible sensors, which require high stretchability, durability, self-adhesion, and sensitivity. Biomacromolecules have attracted attention in sensor design for their biocompatibility, biodegradability, and unique mechanical properties. This study employs a “one-pot” method to integrate ionic liquids and multi-walled carbon nanotubes into a dual-network hydrogel framework, utilizing tannic acid, sodium alginate, acrylamide, and 2-acrylamido-2-methylpropane sulfonic acid. Tannic acid and sodium alginate, natural biomacromolecules, form a robust physical cross-linking network, while P(AM-AMPS) creates a chemical cross-linking network. Ionic liquids enhance carbon nanotube dispersion, resulting in a hybrid hydrogel with remarkable tensile strength (0.12 MPa), adhesive properties (0.039 MPa), and sensing performance (GF 0.12 for 40 %–100 % strain, GF 0.24 for 100 %–250 % strain). This hydrogel effectively monitors large joint movements (fingers, wrists, knees) and subtle biological activities like swallowing and vocalization. Integrating natural biomacromolecules into this composite hydrogel sensor not only enhances the functionality and biocompatibility of flexible wearable devices but also paves the way for innovations in biomedicine and bioelectronics.

Low-Temperature Fabrication of Stable Black-Phase CsPbI3 Perovskite Flexible Photodetectors Toward Wearable Health Monitoring.

Flexible wearable optoelectronic devices fabricated from organic-inorganic hybrid perovskites significantly accelerate the development of portable energy, biomedicine, and sensing fields, but their poor thermal stability hinders further applications. Conversely, all-inorganic perovskites possess excellent thermal stability, but black-phase all-inorganic perovskite film usually requires high-temperature annealing steps, which increases energy consumption and is not conducive to the fabrication of flexible wearable devices. In this work, an unprecedented low-temperature fabrication of stable black-phase CsPbI3 perovskite films is demonstrated by the in situ hydrolysis reaction of diphenylphosphinic chloride additive. The released diphenyl phosphate and chloride ions during the hydrolysis reaction significantly lower the phase transition temperature and effectively passivate the defects in the perovskite films, yielding high-performance photodetectors with a responsivity of 42.1A W-1 and a detectivity of 1.3 × 1014Jones. Furthermore, high-fidelity image and photoplethysmography sensors are demonstrated based on the fabricated flexible wearable photodetectors. This work provides a new perspective for the low-temperature fabrication of large-area all-inorganic perovskite flexible optoelectronic devices.

Double network hydrogel confined MXene/Liquid metal by dynamic hydrogen bond for high-performance wearable sensors

Double-network (DN) hydrogels, which integrate long-chain and short-chain molecular structures, exhibit significant potential in wearable sensors due to their exemplary mechanical properties. However, conventional hydrogels often suffer from poor conductivity and low sensitivity. In this study, we develop a novel DN hydrogel (comprising polyacrylamide/sodium alginate) that encapsulates MXene (Ti3C2Tx) and liquid metal (LM). This configuration leverages the conductive properties of two-dimensional MXene and zero-dimensional liquid metal embedded within the DN hydrogel networks. The Ti3C2Tx MXene sheets enhance the binding capacity with LM by serving as a conductive bridge, thereby improving interfacial interactions and reducing hysteresis. The resultant strain sensor, fabricated from this composite DN hydrogel, achieves high sensitivity (gauge factor up to 12.84), a broad detection range (0 %-1500 %), rapid response time (262 ms), and excellent repeatability and stability. We have integrated this strain sensor into wearable flexible devices, such as contact lenses, enabling real-time monitoring of human physiological pressures.

Radiation synthesis of high conductivity hydrogel based on tragacanth gum/poly (ionic liquids) for multimodal sensors and supercapacitor

Natural polymer-based hydrogels have found extensive use in flexible sensing, energy storage, and other fields because of their environmental sustainability and biocompatibility. Nonetheless, numerous challenges persist in the development of hydrogels with outstanding conductivity solely from natural polymers. Herein, we have successfully synthesized hydrogels based on natural polymer (tragacanth gum) and ionic liquids (1-vinyl-3-ethylimidazolium bromide) using a convenient and efficient one-step ionizing radiation method (TG/PIL hydrogels). The TG/PIL hydrogels exhibit high ionic conductivity (7.1 S m−1 at 25 °C), and can be used for multimodal sensors, including strain and temperature sensors. It has exceptional capabilities in monitoring human motor behavior, capturing subtle facial expressions and pulses beat. TG/PIL hydrogel can also accurately sense changes in the temperature of the external environment, and have significant thermal sensitivity within the range of 40 to 60 °C (−3.22 % /°C). Furthermore, the high conductivity of TG/PIL hydrogels enables them to exhibit outstanding performance in supercapacitor electrolytes, it has good stability in a certain load bearing range, temperature range, folding angle range. This work offers a straightforward technique for creating a multimodal hydrogel sensor, with promising applications in flexible wearable devices, energy storage, and beyond.

Construction of high strength PVA/cellulose conductive hydrogels based on sodium citrate/aluminium chloride dual ions regulation

Hydrogels used for flexible sensing usually require a good balance between mechanical strength and conductivity. In this study, polyvinyl alcohol/carboxymethyl cellulose/cellulose nanofibers (PVA/CMC/CNF) hydrogels with multi-hierarchical structures were firstly prepared by adjusting the CNF content. Then, PVA/CMC/CNF-xM with excellent mechanical properties and conductivity were prepared by cyclic freezing-thawing and sodium citrate/aluminium chloride (Na3Cit/AlCl3) dual ions salt equilibrium methods. Results showed that at an ion concentration of 3 mol/L, PVA/CMC/CNF-3 M hydrogel exhibited a tensile strength, elongation at break and conductivity of 3.41 MPa, 1271 % and 0.35 S/m, respectively. The structural evolution of PVA/CMC/CNF-xM conductive hydrogels were studied, and the results indicated that Cit3− formed numerous intermolecular hydrogen bonds, while Al3+ could strongly coordinate with carboxyl and hydroxyl groups between the polysaccharide chains. Meanwhile, PVA/CMC/CNF-3 M possessed a low strain detection limit of 1 %, making it not only can be used for human motion monitoring, but also information encoding and transmission. This work may provide a facile approach for preparing high-strength and conductive hydrogels, which can be applied in flexible wearable electronic devices and information transmission.

High-strength polyvinyl alcohol/gelatin/LiCl dual-network conductive hydrogel for multifunctional sensors and supercapacitors

The synthesis of conductive hydrogels with high mechanical strength, toughness, optimal fracture growth rate and the capability to detect diverse human body movements poses a significant challenge in the realm of flexible electronics. In this study, a one-pot technique utilized effectively to fabricate conductive materials by doping LiCl into a mixture of polyvinyl alcohol (PVA) and gelatin. The PVA/gelatin/LiCl0.3(PGL) conductive hydrogel demonstrates exceptional robustness, flexibility, and resistance to deformation, enabling the monitoring of various physiological signals such as temperature and humidity. Additionally, the PGL demonstrates exceptional elongation properties (up to 1111.32 %), high lifting capacity (up to 25 kg), resistance to deformation, and sustained stability of peak signals even after 300 cycles at 50 % strain. The hydrogel electrolyte exhibits a conductivity of 2.114 S/m at 25 °C and a specific capacitance of up to 48.75 F/g, along with favorable mechanical and electrochemical characteristics. These findings suggest that the PVA/gelatin/LiCl0.3 hydrogel supercapacitor (PGLSC) conductive hydrogel shows significant potential for integration into flexible electronics and wearable technology devices.

Incorporation of polyvinyl alcohol in bacterial cellulose/polypyrrole flexible conductive films to enhance the mechanical and conductive performance

The integration of polypyrrole (PPy) into bacterial cellulose (BC) has provided significant conductivity and cost benefits. However, this combination has led to a reduction in mechanical properties, particularly in terms of elongation at break and tensile strength. This study investigated the enhancement of BC/PPy composite films by incorporating polyvinyl alcohol (PVA). The resulting BC/PPy/PVA films demonstrated improvements in flexibility, tensile strength and thermal stability. Specifically, with 7 % PVA, the flexible films exhibited remarkable enhancements: tensile strength increased from 11.01 MPa (for BC/PPy) to 25.27 MPa and elongation at break rose from 5.81 % to 11.54 %. Additionally, the electrical conductivity of the BC/PPy/PVA films with a resistance of 38.5 Ω, surpassed that of the BC/PPy films. Furthermore, the equilibrium swelling water absorption rates of BC/PPy and BC/PPy/PVA films were 30.6 % and 81.4 %, respectively, with corresponding resistances of 530 Ω and 540 Ω. The variation in resistance between the dry and swollen states of the BC/PPy/PVA flexible conductive film resulted in differences in the brightness of the small light bulb. These findings highlighted the synergistic effects of PVA within the BC/PPy matrix, presenting a promising avenue for developing high-performance conductive materials suitable for flexible electronics and wearable devices.

Stretchable carbon Nanotube/Ecoflex conductive elastomer films toward multifunctional wearable electronics

Conductive elastomer films (CEFs) have found widespread applications toward flexible electronics and wearable devices. Silicones are particularly advantageous for fabricating high-performance CEFs. However, processing silicone/conductive filler composite prepolymers is challenging because of their high viscosity, especially when incorporating nanofillers like carbon nanotubes (CNTs) that can greatly increase the viscosity of silicone prepolymers. Herein, we present a simple, facile and versatile hexane-assisted air-spraying method for fabricating silicone-based CEFs from Ecoflex and CNTs. This method allows for processing CNT/Ecoflex prepolymers with CNT concentrations of 0.5 ∼ 5 wt%, resulting in stretchable CEFs with controllable structural, mechanical and electrical properties. We leveraged the distinct advantages of CNT/Ecoflex CEFs, including the excellent mechanoelectrical sensitivity, workable strain range and elasticity of the CNT1/Ecoflex group, and the high conductivity and low mechanoelectrical sensitivity of the CNT5/Ecoflex group. These properties enable their multifunctional applications as wearable and waterproof resistive strain sensors, multilayered capacitive pressure sensors, and wearable electric heaters. This work provides valuable insights into the development of high-performance and multifunctional silicone-based composite materials for advanced wearable electronics.

A photocurable ultra-tough eutectic gel with coordination crosslinking used for wearable sensors and TENG flexible electrodes

Flexible polymer molecular chains are of great interest to researchers in the field of flexible wearable electronics due to their unrivaled flexibility and ductility. However, achieving high toughness, high elasticity, environmental stability and easy machining of flexible electronic materials remains a challenge. In this study, we present a photocurable eutectic gel mainly composed of DMAPS, AAc, Zr4+ and deep eutectic solvents (DES). Due to the strong coordination between zirconium ions and polymer networks, the gel can be endowed with excellent properties, such as excellent tensile strength (1.14 MPa), low hysteresis (5 kJ·m−3) and good adhesion (above 40 kPa). DES can not only give the gel good electrical conductivity, but also maintain the stability of the gel to ensure that the leakage or volatilization of the solvent will not occur in use. The properties mentioned above allow the gel to achieve a sensitivity factor of up to 1.7 over a small strain range, demonstrating its potential applications in the field of flexible strain sensors and triboelectric flexible electrode materials. What’s more, the gel can be used to prepare complex geometric shapes with high precision through digital light processing (DLP) based 3D printing technology. Therefore, this work introduces a novel approach for the development of highly stable flexible wearable devices, which has the potential to expand the applications of eutectic gels.

Enhanced performance of flexible BiFeO3 ferroelectric memory with Mica substrate via SrTiO3 buffer layer

BiFeO3 (BFO) application in flexible wearable devices is garnering interest because of its unique ferroelectric and magnetic properties. However, the integration of high-quality BFO films onto flexible substrates presents significant technical challenges. Here, we successfully fabricated high-quality BFO films on mica substrates by using pulsed laser deposition, and report the fatigue characteristics of BFO films on flexible substrates for the first time. The results demonstrated that, after 108 bipolar switching cycles, the polarization only degraded by 0.28%, indicating superior fatigue characteristics compared to previously reported BFO films. Additionally, the device ferroelectric properties remained largely unchanged, with a bending radius of 3.5 mm. The fabricated flexible Pt/BFO/La0.65Sr0.35MnO3(LSMO)/SrTiO3(STO)/mica non-volatile memory devices exhibited mechanical flexibility and fatigue resistance. These findings not only highlight the potential of flexible BFO films for wearable electronic devices and flexible memory devices, they also provide valuable insight for the future development of high-performance flexible ferroelectric materials.

Fabrication and High Dielectric Properties of Sandwich-Structured Ba0.6Sr0.4TiO3/Polyvinylidene Fluoride Layered Composites with Multiscale Parallel Interfaces.

Ba0.6Sr0.4TiO3/polyvinylidene fluoride (BST/PVDF) dielectric functional composites have been widely used in flexible wearable devices, capacitors, and energy storage devices. In addition to the ceramic phase type, polymer matrix type, composition, and interfacial connectivity of BST/PVDF composite materials, their morphology also significantly influences their electrical characteristics. Therefore, herein, sandwich-structured BST/PVDF layered composites were designed and prepared via tape-casting processing using different types of BST fillers [i.e., formless zero-dimensional (0D)-BST, rod-like one-dimensional (1D)-BST, and plate-like two-dimensional (2D)-BST]. The microstructures and electrical characteristics of sandwich-structured BST/PVDF composites were studied in relation to the BST morphology. The effects of the internal mechanisms of different interfacial models on the breakdown strength of BST/PVDF composites were discussed. According to our findings, unlike the 0D-BST and 1D-BST powders, 2D-BST powders form multiscale parallel interfaces in sandwich-structured composites due to their unique lamella-like morphology, which enhances the breakdown strength of sandwich-structured composites. Sandwich-structured BST/PVDF composites containing 2D-BST powders exhibit good electrical characteristics with an energy storage density of 19.71 J/cm3, an energy storage efficiency of 85.3%, a dielectric constant of 30.4 (1 kHz), a dielectric loss of 0.036 (1 kHz), and a dielectric tunability of 93.2%. This study provides a method for preparing functional composites with high dielectric tunability and high energy storage characteristics.

Synthesis of Ag Nanowires with High Aspect Ratio for Highly Sensitive Flexible Strain Sensor

AbstractIn recent years, the research and development of various flexible wearable devices have rapidly advanced due to the continuous introduction of flexible electronic product. These devices have found applications in health monitoring, cardiovascularcare, internal and external workload, and more.Flexible sensors, being a vital component of flexible devices, determine the functionalities and performance capabilities of the devices. Metal nanomaterials, especially silver nanowires, are widely used in flexible sensors in past research due to their great advantages in flexibility and sensitivity. In this work, Silver nanowires with aspect ratios of 600, 1000, and 1400 were synthesized by adjusting the synergy of Br‐ and Cl‐, along with other process parameters, leading to an improved production efficiency of silver nanowires.The stability of the conductive network is enhanced when the aspect ratio of silver nanowires is 1000 and 1400.The sensor demonstrated high sensitivity at high strain, along with an extended strain range.Moreover, it was observed that increasing the aspect ratio of silver nanowires led to a more stable conductive network, thus enhancing the sensor's stability with over 10000 stretching cycles. under the appropriate deposition density, silver nanowires with aspect ratio of 600 have high sensitivity to low strain, and silver nanowires with aspect ratio of 1400 have high sensitivity to high strain, up to 247.3. Introducing microstructures on the surface of PDMS resulted in an increased maximum sensitivity of the sensor with decreasing microstructure size, reaching a maximum sensitivity of 322.2.

Soft wearable electronics for evaluation of biological tissue mechanics

Flexible wearable devices designed to evaluate the biomechanical properties of deep tissues not only facilitate continuous and effective monitoring in basic performance but also exhibit significant potential in broader disease assessments. Recent advancements are highlighted in the structural and principled design of platforms capable of capturing various biomechanical signals. These advancements have led to enhanced testing capabilities concerning spatial scales and resolution modes at different depths. This review discusses the engineering of soft wearable devices for the biomechanical evaluation of deep tissue signals. It encompasses different measurement modes, device design and fabrication methods, integrated circuit (IC) integration schemes, and the characteristics of measurement depth and accuracy. The core discussion focuses on platform development, targeting different monitoring sites and platform structure design, ranging from linear strain gauges and conformal stretchable sensors to complex three-dimensional (3D) circuit-integrated stretchable arrays. We further explore various technologies associated with different measurement mechanisms and engineering designs, as well as the penetration depth and spatial resolution of these wearable sensors. The practical applications of these technologies are evident in the monitoring of deep tissue signals and changes in tissue characteristics. The results suggest that wearable biomechanical sensing systems hold substantial promise for applications in healthcare and research.

Linear strain gradient-regulated bifurcation of circular bilayer plates

Bilayer structures with controllable self-folding capability have found applications in a variety of cutting-edge fields such as flexible electrics, wearable devices and soft robotics. The folding of bilayer structures occurs when the mismatch strain between the two layers exceeds the bifurcation threshold, resulting in a deformation transition from an axisymmetric to a folded state. Previous efforts have predominantly focused on bilayer structures with uniform and/or anisotropic strain distributions. However, the role of non-uniform in-plane strain distributions in regulating the bifurcation of bilayer structures has not been fully understood. In this study, the effects of linear in-plane strain gradients on the bifurcation of circular bilayer plates, both with and without geometric mismatch, are systematically investigated by combining theoretical analysis, finite element simulations and experiments. Our results reveal that both the mismatch strain gradient and the geometric mismatch between the two layers play crucial roles in regulating bifurcation. Notably, linear mismatch strain gradients with larger strain at the center delay bifurcation, while those with larger strain along the edge promote bifurcation. This work offers new insights into the design of controllable self-folding bilayer structures, which is of great significance for advanced applications.

High-Conductivity and Ultrastretchable Self-Healing Hydrogels for Flexible Zinc-Ion Batteries.

Aqueous zinc-ion batteries are promising candidates for flexible energy storage devices due to their safety, economic efficiency, and environmental friendliness. However, the uncontrollable dendrite growth and side reactions at the zinc anode hinder their commercial application. Herein, we designed and synthesized a dual network self-healing hydrogel electrolyte with zwitterionic groups (PAM-PAAS-QCS), which can be used for the large deformations of flexible devices due to its excellent stretchability (ε = 5100%). The incorporation of zwitterionic groups into the PAM-PAAS-QCS hydrogel electrolyte endows it with high ionic conductivity (33.61 mS/cm), a wide electrochemical stability window, and the ability to suppress zinc dendrite formation and side reactions. Besides, the Zn//Zn symmetric cell with PAM-PAAS-QCS can stably plate and strip zinc for 1500 h at 0.5 mA/cm2, and the Zn//Polyaniline full cell retains 82.4% of its capacity after 1500 cycles at 1 A/g. Additionally, flexible batteries based on both the original and self-healed PAM-PAAS-QCS hydrogel electrolytes demonstrate good cycling stability and stable charge-discharge performance under various bending conditions. This self-healing hydrogel electrolyte with excellent stretchability and high ionic conductivity is expected to pave the way for the development of high-performance flexible energy storage and wearable devices.

Self-Powered Handwritten Letter Recognition Based on a Masked Triboelectric Nanogenerator for Intelligent Personal Protective Equipment.

As one of the most important ways of human-machine interfaces, the touchpad has excellent input convenience. Input devices for extreme environments require simpler structures and diverse inputs to ensure information inputs. This paper proposed a self-powered flexible input panel with single-channel output for the input recognition of all 26 letters, and a paper mask was implemented to cover the triboelectric nanogenerator (TENG) board and obtain more complicated electrical signal features. Based on the change of the triboelectric output of the mask, neural network models with different combinations of layers were designed and optimized, and the highest recognition rate of 88.7% for all letters and 100% recognition accuracy for some letters were achieved among the five testers. For letters with low recognition rates, a specific writing specification was further proposed to improve the accuracy of model recognition. These results facilitate the application of the proposed input panel as a flexible wearable device and personal protective equipment for extreme environments including chemical, biological, radiological, nuclear (CBRN) scenarios or aerospace.

Exfoliated MoS2 anchored on graphene oxide nanosheets for enhancing thermoelectric properties of single-walled carbon nanotubes

Carbon nanotubes-based thermoelectric materials with high electrical conductivity (σ) and excellent mechanical properties have promising applications in flexible wearable devices. Two-dimensional transition metal sulfide MoS2 has been used to enhance the thermoelectric properties of carbon nanotubes due to its high Seebeck coefficient (S). However, MoS2 nanosheets are prone to agglomeration due to their high specific surface area, which causes lower doping efficiency. In this work, MoS2@GO hybrids are successfully fabricated using a hydrothermal in-situ growth method to anchor exfoliated MoS2 on graphene oxide (GO) nanosheets, and MoS2@GO hybrids significantly enhance the interfacial interaction between MoS2 and single-walled carbon nanotubes (SWCNT), improve the carrier mobility, lead to a simultaneous enhancement of the S and the σ. The maximum S value of MoS2@GO/SWCNT is 42.3 ± 0.2 μV K-1, the σ is 1173.2 ± 45.6 S cm-1, and an optimum power factor (PF) of 208.8 ± 8.5 μW m-1 K-2 is obtained at room temperature, which reaches 261.3 ± 10.2 μW m-1 K-2 at 385 K. For application demonstration, a thermoelectric device is assembled by connecting six pairs of p-type MoS2@GO/SWCNT and n-type copper sheets in series, which demonstrates an open-circuit voltage of 17.4 mV and an output power of 2.1 μW under a temperature difference of 50 K. Therefore, this study enriches the design and synthesis strategy of exfoliated MoS2 and provides a new approach for the development of high-performance SWCNT-based thermoelectric materials, which has important potential applications in the field of wearable electronics.

Multifunctional conductive organohydrogel with environmental tolerance for flexible wearable devices

Multifunctional conductive organohydrogel with environmental tolerance for flexible wearable devices