Flexible Wearable Devices Research Articles

High-tear resistant gels crosslinked by DA@CNC for 3D printing flexible wearable devices

Photocurable gels have broad application prospects in biomedicine, bionics, flexible wearable devices and other fields. However, there are still some problems in the current photocurable gels, such as notch sensitivity, that is, poor tear resistance. In this study, we provided a photocurable gel with excellent tear resistance. The gel prepolymer is mainly composed of hydroxymethylacrylamide (NAM) and cellulose nanocrystals (CNC) modified with dopamine hydrochloride (DA), referred to as DA@CNC. After photocuring, the prepared gels show excellent mechanical properties such as tear resistance, elasticity and toughness. The introduction of DA@CNC not only endows gels with a large amount of energy dissipation through hydrogen bond crosslinking, but also effectively resists crack expansion as a nano-sized reinforcing phase, which greatly improves the tear resistance of the gels. Even at a 40 % gap, the elongation at break of the gel can still reach 1445 %. In addition, the DA can endow the gel with good electrical conductivity and excellent sensitivity (GF = 23.8). Some flexible wearable devices like finger sleeve and wristband can be customized by photocurable 3D printing using the gel with high toughness. This high-performance gel has great application potential in flexible wearable devices.

Enhanced Thermoelectric Properties of Stable n-Type Ferrocene Derivatives-Doped Polyethylenimine/Single-Walled Carbon Nanotube Composite Films.

Preparing stable n-type flexible single-walled carbon nanotube (SWCNT)-based thermoelectric films with high thermoelectric (TE) performance is desirable for self-powering wearable electronics but remains a challenge. Here, the interface regulation and thermoelectric enhancement mechanism of ferrocene derivatives on polyethylenimine/single-walled carbon nanotube (PEI/SWCNT) composite films have been explored by doping ferrocene derivatives (f-Fc-OH) into PEI/SWCNT films. The results show that the introduction of f-Fc-OH leads to the formation of "thorn" structures on the surfaces of SWCNT bundles via hydrophilic and hydrophobic interactions, the generated energy-filtering effect improves the thermoelectric properties of the PEI/SWCNT film, and the f-Fc-OH-doped PEI/SWCNT (f-Fc-OH/PEI/SWCNT) achieves the highest room-temperature power factor of 182.22 ± 8.60 μW m-1 K-2 with a Seebeck coefficient of -64.28 ± 0.96 μV K-1 and the corresponding ZT value of 4.69 × 10-3. The Seebeck coefficient retention ratio of the f-Fc-OH/PEI/SWCNT nearly remained 68% after being exposed to air for 3672 h, while the PEI/SWCNT film changed from n-type to p-type after being exposed to air for about 432 h. In addition, the temperature-dependent thermoelectric properties show that the f-Fc-OH/PEI/SWCNT achieves a high power factor of 334.57 μW m-1 K-2 at 353 K. Finally, a flexible TE module consisting of seven pairs of p-n junctions is assembled using the optimum composite film, which produces an open-circuit voltage of 42 mV and a maximum output power of 4.32 μW at a temperature gradient of 60 K. Therefore, this work provides guidance for preparing stable n-type SWCNT-based composite films with enhanced thermoelectric properties, which have potential applications in flexible generators and wearable electronic devices.

Research progress on key technologies of flexible wearable electronic devices

Flexible wearable device is an innovative technology product, using soft, lightweight material design, can fit the curve of the human body, providing a comfortable wearing experience. These devices typically integrate a variety of sensors, batteries, and connectivity technologies to enable biometric monitoring, motion tracking, communication, and other intelligent functions. Traditional electronic products tend to give a "stiff" impression that they cannot withstand bending, twisting and stretching. In recent years, under the background of the integration of electronic technology and modern material science technology, the emerging flexible electronic wearable device has become a research hotspot with its more flexible material, more sensitive and accurate signal transmission, and more extensive monitoring range. The development of flexible electronic technology has greatly improved the user's wearing experience and is more suitable for the free movement of the human body. This paper summarizes the basic principle and classification of key technologies of wearable electronic devices, and introduces the research progress and research trend.

Environmentally stable, Adhesive, Self-healing, Stretchable and Transparent Gelatin based Eutectogels for Self-Powered Humidity Sensors

Intrinsic environmental and stability limitations of hydrogels have inhibited their practical applications as a flexible wearable device due to water evaporation or freezing in complex environments such as low temperatures and arid environments. In this work, a multifunctional gelatin based ionic conductive eutectogel with double network structure is designed via ternary deep eutectic solvent (DES) (acrylic acid (AA), choline chloride (ChCl) and ethylene glycol (EG)). In this system, the introduction of ethylene glycol (EG) can be used to dissolve gelatin. The resulting DESG eutectogel exhibited excellent adhesion, mechanical robustness, anti-freeze, anti-drying, and self-healing. Interestingly, the DESG gels showed high humidity sensitivity in a wide humidity detection range (11 %–83 %), which can be assembled as a self-powdered humidity sensor to monitor human mouth and nose breathing. This work is expected to bring new prospect to construct high performance humidity sensors using gelatin based humidity-responsive materials for a wide range of potential applications in respiratory diagnostics, sleep monitoring, electronic skin and wearable electronics.

Imidazole-stabilized gold nanoclusters as photocatalytic initiator and crosslinker to fabricate functional hydrogel

Photopolymerization is the most widely used method to fabricate hydrogels, which hold great potentials in various fields including biomedical applications, electronic skin and flexible wearable devices. Herein, gold nanoclusters (AuNCs) were employed as both efficient photo-initiators and crosslinkers to prepare polyacrylamide (PAM) hydrogels with good mechanical properties. Firstly, an imidazole-based monomer, ImPAA, was designed and synthesized, serving as an reducing agent and ligand to prepare imidazole-stabilized AuNCs (AuNC@ImPAA) in water. Under white light irradiation, AuNC@ImPAA efficiently initiated the radical polymerization of acrylamide monomers in water via a photocatalytic mechanism. The abundant vinyl groups on the surface of AuNC@ImPAA enabled its role as a multiple covalent crosslinker, endowing the resulting hydrogel with impressive mechanical properties. The maximum tensile strength and fracture strain was 287 kPa and 591 %, respectively. Moreover, triggered by UV light, the embedded AuNC@ImPAA within the hydrogel aggregated and converted into gold nanoparticles (AuNPs), yielding an AuNP/PAM hydrogel. This hybrid AuNP/PAM hydrogel exhibited a good photothermal conversion performance, with an efficiency of 14.14 % under 660 nm laser irradiation, and its potential as a photothermal actuator was demonstrated. Thus, we provide a facile light-mediated strategy to fabricate functional hydrogel.

Flexible and robust polypyrrole/cross-linked collagen sponge with collagen aggregates as building blocks for piezoresistive sensing

The polypyrrole/cross-linked collagen sponge (PPy/CCS) was synthesized through in-situ polymerization of PPy nanoparticles onto the collagen skeleton composed of mesoscopic collagen aggregates (CAs) as building blocks, which was employed as a sensing material. The structure and morphology of PPy/CCS were characterized using scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermal analysis (TA). The conductivity and mechanical properties of PPy/CCS can be regulated by adjusting the polymerization conditions. The PPy/CCS treated with glycerin aqueous solution exhibits exceptional recovery performance under 10,000 compression stress cycles, with a short response time of 420 ms and recovery time of 400 ms, along with high sensitivity of 1.13 kPa−1 within a control pressure range of 0–43 kPa. The assembled PPy/CCS-based sensors enable monitoring of human movement behavior and achieving multi-touch control, thereby demonstrating significant potential in flexible wearable devices and human–machine interaction.

Microporous tungsten oxide spheres coupled with Ti3C2Txnanosheets for high-volumetric capacitance supercapacitors.

In the contemporary landscape of technological advancements, the burgeoning demand for portable electronics and flexible wearable devices has necessitated the development of energy storage systems with superior volumetric performance. Tungsten oxide (WO3), known for its high density and theoretical capacitance, is a promising electrode material for supercapacitors. However, low conductivity and poor cycling stability are still the key bottlenecks for its application. Herein, a novel composite comprising hollow porous WO3 spheres (HPWS) derived by template method was electrostatic self-assembled on the surface of the Ti3C2Tx nanosheets. The resulting electrodes exhibited ultra-high volumetric capacitance of 1930 F cm-3 at 1 A g-1 and rate capability of 46% at 50 A g-1, attributed to enhanced ion accessibility from microporous structure and electron transport from conductive network of Ti3C2Tx even at a high packing density of 3.86 g cm-3. Utilizing HPWS/Ti3C2Tx as the negative electrode and porous carbon as the positive electrode, the assembled asymmetric supercapacitor achieved an energy density of 31 Wh kg-1 at a power density of 650 W kg-1 with over 107% capacitance retention after 5000 cycles. This work provides a promising approach for developing next-generation supercapacitors with ultra-high volumetric capacitance.

Wearable broadband MoS2 photodetector for dual heart rate and UV detection powered by PDMS-MXene TENG

Wearable electronic devices capable of monitoring health parameters and environmental factors are tremendously desirable. Environmental energy harvesting as a power source is crucial to replace traditional batteries. This study aims to contribute to the development of sustainable and efficient wearable devices for health monitoring and environmental sensing. In this study, a self-powered wearable MoS2-based photodetector, integrated with a PDMS-MXene-based triboelectric nanogenerator (TENG) is presented. The TENG generates isometric AC voltage using a diode, and it is used to charge a capacitor for DC voltage output. The photodetector, with a responsivity of 1.25 × 105A/W, specific detectivity of 2×1013 Jones, millisecond response time, enhanced responsivity of 1.1×1010 V/W and specific detectivity of 9.5×1016 Jones while coupled with TENG, successfully measured heart rate using a 660 nm LED and compared well with a commercial pulse meter. Additionally, UV illumination was detected using a 395 nm LED in the AC mode of the TENG. The study’s results support the advancement of flexible, integrated, and sustainable wearable devices, offering solutions to challenges posed by current battery-powered devices.

Wearable Device with High Thermoelectric Performance and Long-Lasting Usability Based on Gel-Thermocells for Body Heat Harvesting.

Utilizing the thermogalvanic effect, flexible thermoelectric materials present a compelling avenue for converting heat into electricity, especially in the context of wearable electronics. However, prolonged usage is hampered by the limitation imposed on the thermoelectric device's operational time due to the evaporation of moisture. Deep eutectic solvents (DESs) offer a promising solution for low-moisture gel fabrication. In this study, a bacterial cellulose (BC)/polyacrylic acid (PAA)/guanidinium chloride (GdmCl) gel is synthesized by incorporating BC into the DES. High-performance n-type and p-type thermocells (TECs) are developed by introducing Fe(ClO4)2/3 and K3/4Fe(CN)6, respectively. BC enhances the mechanical properties through the construction of an interpenetrating network structure. The coordination of carboxyl groups on PAA with Fe3+ and the crystallization induced by Gdm+ with [Fe(CN)6]4- remarkably improve the thermoelectric performance, achieving a Seebeck coefficient (S) of 2.4mVK-1 and ion conductivity (σ) of 1.4 Sm-1 for the n-type TEC, and ‒2.8mVK-1 and 1.9 Sm-1 for the p-type TEC. A flexible wearable thermoelectric device is fabricated with a S of 82mVK-1 and it maintains a stable output over one month. This research broadens the application scope of DESs in the thermoelectric field and offers promising strategies for long-lasting wearable energy solutions.

Triboelectric Nanogenerator Made with Stretchable, Antibacterial Hydrogel Electrodes for Biomechanical Sensing.

Triboelectric nanogenerators (TENGs) have attracted widespread attention as a promising candidate for energy harvesting due to their flexibility and high power density. To meet diverse application scenarios, a highly stretchable (349%), conductive (1.87 S m-1), and antibacterial electrode composed of carbon quantum dots/LiCl/agar-polyacrylamide (CQDs/LiCl/agar-PAAm) dual-network (DN) hydrogel is developed for wearable TENGs. Notably, the concentration of agar alters the pore spacing and pore size of the DN hydrogel, thereby impacting the network cross-linking density and the migration of conductive ions (Li+ and Cl-). This variation further affects the mechanical strength and conductivity of the hydrogel electrode, thus modulating the mechanical stability and electrical output performance of the TENGs. With the optimal agar content, the tensile strength and conductivity of the hydrogel electrode increase by 211 and 719%, respectively. This enhancement ensures the stable output of TENGs during continuous operation (6000 cycles), with open-circuit voltage, short-circuit current, and transferred charge increasing by 200, 530, and 155%, respectively. Additionally, doping with CQDs enables the hydrogel electrode to effectively inhibit the Gram-negative bacterium Escherichia coli. Finally, the TENGs are utilized as a self-power smart ring for efficient and concise information transmission via Morse code. Consequently, this study introduces a creative approach for designing and implementing multifunctional, flexible wearable devices.

Tailoring Mechanical and Optical Properties of Sputtered SiNx/BN Coatings.

The swift evolution of contemporary electronics products, such as flexible screens and wearable electronic devices, highlights the significance of flexible protective coatings, which combine superior mechanical and optical properties. Even though the recently developed polymer protective coatings can satisfy requirements for flexibility and transparency, their intrinsic nature often results in a hardness below 1 GPa, rendering them susceptible to scratches. On the other hand, traditional inorganic coatings, known for their high hardness and transparency, fall short of meeting flexibility requirements. In the present study, a SiNx/BN periodical nanolayered coatings (PNCs) structure has been tailored to achieve high mechanical durability, transparency, and flexibility. In SiNx/BN PNCs, the optical and mechanical properties of the single-layer SiNx film are crucial to the overall performance of the PNCs. Therefore, pulse direct current (DC) magnetron sputtering was optimized first to enhance the ionization efficiency of Si and N, thereby promoting their reaction and diminishing the presence of elemental silicon in SiNx. The effects of the pulse frequency and duty cycle on SiNx were evaluated. Additionally, the influence of the thickness ratio and modulation periods on the overall performance of the SiNx/BN PNCs was investigated. As a result, a SiNx/BN coating with sapphire-grade hardness, almost no optical absorption in the visible-near-infrared (vis-NIR) range, high wear resistance, and exceptional flexibility was demonstrated.

Hydrotalcite-Enhanced Tough and Strong Hydrogel Endowed by Coordination and Electrostatic Interactions for Both Strain and Pressure Sensors.

Polymer hydrogels have a wide range of applications in the field of flexible wearable devices from the perspective of easy commercialization and environmental compatibility. However, traditional hydrogels often fail to achieve adequate mechanical strength and performance such as toughness, resilience, and ionic conductivity. Herein, a significant enhancement of tensile strength in 2 orders of magnitude (from 36 kPa to 1.5 MPa) is obtained by the introduction of hydrotalcite into polymer network via multiple, multilevel, and strong interactions of strengthened interface interactions, and the enhancement effect is superior to most of known records. Meanwhile, the enhanced conductivity may be rationally attributed to effective channels of hydrotalcite for ion transport. As a result, high toughness (9.5 MJ/m3), stretchability (1520%), excellent resilience (100% rebound of 400% strain), high conductivity (2.6 mS/cm), and low-temperature resistance are successfully achieved. The work shows an efficient approach to design desired ultratough and multifunctional hydrogels.

Ultrastretchable, Ultralow Hysteresis, High-Toughness Hydrogel Strain Sensor for Pressure Recognition with Deep Learning.

Hydrogel, as a promising material for a wide range of applications, has demonstrated considerable potential for use in flexible wearable devices and engineering technologies. However, simultaneously realizing the ultrastretchability, low hysteresis, and high toughness of hydrogels is still a great challenge. Here, we present a dual physically cross-linked polyacrylamide (PAM)/sodium hyaluronate (HA)/montmorillonite (MMT) hydrogel. The introduction of HA increases the degree of chain entanglement, and the addition of MMT acts as a stress dissipation center and cross-linking agent, resulting in a hydrogel with high toughness and low hysteretic properties. This hydrogel synthesized by a simple strategy exhibited ultrahigh stretchability (3165%), high breaking stress (228 kPa), high toughness (4.149 MJ/m3), and ultralow hysteresis (≈2% at 100% of strain). The fabricated hydrogel flexible strain sensors possessed fast response and recovery times (62.5:75 ms), a wide strain detection range (2000%), a strain detection limit of 1%, and excellent cycling stability over 500 cycles. Furthermore, the hydrogel flexible strain sensor can be used for human motion monitoring, gesture recognition, and pressure recognition assisted by deep learning algorithms, showing enormous promise for applications.

High elongation, low hysteresis, fatigue resistant gel electrolyte for supercapacitor and strain sensor

With the arrival of the era of the Internet of Everything and the popularization of flexible wearable devices, energy supply devices serving flexible wearable devices have also become a research hotspot. Hydrogels are gaining ground in the field of flexible energy supply devices, especially supercapacitors, which have attracted wide attention due to the advantages of fast charging/discharging, high power density, and stability. However, flexible supercapacitor inevitably undergoes varying degrees of deformation during use, and their service life decreases dramatically with the prolongation of use. Therefore, the fabrication of a flexible supercapacitor that can be assembled from hydrogel electrolyte and is fatigue-resistant and stable has become a promising research direction. In this study, we constructed a gel electrolyte with high stretchability, low hysteresis, high stability, and fatigue resistance by designing a dual network structure, and assembled it into a supercapacitor. The device has many excellent features, providing a surface capacitance of 100 mF/cm2, a power density of 0.19 mW/cm2, and an energy density of 425 μWh/cm2 at a frequency of 1 mA/cm2. More importantly, it also exhibits superb stability when subjected to varying degrees of bending and compression. In addition, the gel electrolyte can detect small changes of the human body and quickly and sensitively convert motion signals into electrical signals, showing great potential as a flexible wearable device. This study provides an effective strategy for designing gel electrolytes with low hysteresis and high stability for wider application in flexible electronic devices.

Flexible and robust polyaniline/cross-linked collagen sponge with fibrils network structure as a piezoresistive sensing material

The polyaniline/cross-linked collagen sponge (PANI/CCS) was synthesized by polymerizing PANI onto the collagen skeleton using mesoscopic collagen fibrils (CFs) as building blocks, serving as a piezoresistive sensing material. The structure and morphology of PANI/CCS were characterized using scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermal analysis (TA). The mechanical properties of PANI/CCS could be controlled by adjusting the CFs content and polymerization conditions. PANI/CCS treated with pure water exhibited exceptional compressive elasticity under 1000 compression cycles, demonstrating a wide strain range (0–85 %), rapid response time (200 ms), recovery time (90 ms), and high sensitivity (6.72 at 40–50 % strain). The treatment of the ionic liquid further improved the elasticity and the strain sensing range (0–95 %). The presence of PANI nanoparticles and mesoscopic collagen fibrils imparted antibacterial properties, stability in solvents, and biodegradability to PANI/CCS. Utilizing PANI/CCS as a piezoresistive sensing material enabled monitoring human movement behavior through the assembled sensor, showing significant potential for flexible wearable devices.

Regulation of thermal migration channel in cellulose hydrogel to enhance thermopower

Ionic thermoelectric material is gaining increasing attention, and many attempts have been devoted to obtain huge ionic thermopower, which remains a big challenge. Herein, the idea of “ion thermal migration channel” was propose, and a high ionic thermopower was achieved by regulating the thermal migration channel in cellulose hydrogel system. It is found that the channels size as well as the channel-ion interaction played dominant roles. Initially, the channel size in cellulose/KCl hydrogel were optimized to enlarge the channel-Cl− interactions via counter-ion condensation effect, which improved the thermopower from 2.45 mV·K−1 to 8.13 mV·K−1. Then, sodium alginate as polyanion was doped and multi-valance ion cross-linking was carried out, to further enhance the channel-Cl− interaction, and a high thermopower of 22.09 mV·K−1 was realized. In addition, the cellulose based ionic thermoelectric hydrogel displayed viable basis for energy harvesting and temperature sensing as wearable flexible devices. The present work not only provides a systematic strategy for the thermopower regulation, but also supplies an applicable approach for the preparation of sustainable thermoelectric materials with biomass resources, demonstrating great potential in sustainability and green energy.

Recent advances in self-healing hydrogel composites for flexible wearable electronic devices

Recent advances in self-healing hydrogel composites for flexible wearable electronic devices

Design, application, and recycling of zinc alginate/guar gum hydrogel-based fibers

Extreme cold events are quite common, highlighting the urgent need for flexible wearable electronic devices capable of diagnosing human health in low-temperature environments. Using a wet spinning strategy, we successfully developed sodium zinc alginate/guar gum(SZA/GG) hydrogel fibers with excellent environmental resistance, antimicrobial properties, and electrical conductivity. Building on this, we developed a flexible wearable sensing device that operates stably at low temperatures and exhibits a sensitivity of 0.585 within the range of −20 °C to −40 °C, demonstrating excellent response performance. When evaluating the physical state of outdoor athletes, the amplitude and fluctuation range of electrical resistance provide valuable information about the monitored environment and the risk of frostbite for the individual. However, like any device, it eventually reaches its usage limit. To address the issue of recycling hydrogel fiber waste, we propose recycling and carbonizing the discarded devices to use as a biomass carbon source for fabricating button-type supercapacitors. After 10,000 charge-discharge cycles, the capacitance retention rate reached 92.53 %, demonstrating the potential of these supercapacitors and offering a new approach to reducing resource waste.

Advanced hydrogel electrolyte with enhanced interfacial adhesion and low-temperature resistant for flexible zinc-ion batteries

Flexible zinc-ion batteries (ZIBs) have great potential in wearable devices due to their excellent mechanical properties, safety and high energy density. However, the poor adhesion of the electrodes in bending, folding and low-temperature environments remains a challenge. In this study, we developed a dual-network hydrogel electrolyte (C-PAM@TA) by incorporating tannic acid (TA) into a cross-linked polyacrylamide (PAM) network. The C-PAM@TA hydrogel electrolyte exhibits strong interfacial adhesion, high ion conductivity, and low-temperature frost resistance. Through experimental and theoretical analyses, we demonstrate that TA helps regulate anion/cation transport channels, weakens the solvation of zinc ions and enables stable and uniform plating/stripping behavior of the zinc metal anode. As a result, the Zn//Zn symmetric cell employing the C-PAM@TA hydrogel electrolyte exhibits outstanding cycling stability (2000 h) at 0.5 mA cm−2. Furthermore, the assembled Zn//C-PAM@TA//fcc@PANI flexible ZIBs can undergo 7000 cycle stability tests, withstand 4500 bending tests and operate normally in a low-temperature environment of −18 ℃. This study successfully develops high-performance, low-temperature resistant flexible ZIBs, providing valuable insights for the design of multifunctional flexible energy storage and 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.