A 2.5 V In-Plane Flexi-Pseudocapacitor with Unprecedented Energy and Cycling Efficiency for All-Weather Applications

As electronic devices for aviation, space, and satellite applications become more sophisticated, built-in energy storage devices also require a wider temperature spectrum. Herein, an all-climate operational, energy and power-dense, flexible, in-plane symmetric pseudocapacitor is demonstrated with utmost operational safety and long cycle life. The device is constructed with interdigital-patterned laser-scribed carbon-supported electrodeposited V5O12·6H2O as a binder-free electrode and a novel high-voltage anti-freezing water-in-salt-hybrid electrolyte. The anti-freezing electrolyte can operate over a wide temperature range of -40-60 °C while offering a stable potential window of ≈2.5 V. The device undergoes rigorous testing under diverse environmental conditions, including rapid and regular temperature and mechanical transition over multiple cycles. Additionally, detailed theoretical simulation studies are performed to understand the interfacial interactions with the active material as well as the local behavior of the anti-freeze electrolyte at different temperatures. As a result, the all-weather pseudocapacitor at 1 A g-1 shows a high areal capacitance of 234.7 mF cm-2 at room temperature and maintains a high capacitance of 129.8 mF cm-2 even at -40 °C. Besides, the cell operates very reliably for over 80 950 cycles with a capacitance of 25.7 mF cm-2 at 10 A g-1 and exhibits excellent flexibility and bendability under different stress conditions.

Analysis of a Flexible Photoconductor, Manufactured with Organic Semiconductor Films

This work presents the evaluation of the electrical behavior of a flexible photoconductor with a planar heterojunction architecture made up of organic semiconductor films deposited by high vacuum evaporation. The heterojunction was characterized in its morphology and mechanical properties by scanning electron microscopy and atomic force microscopy. The electrical characterization was carried out through the approximations of ohmic and SCLC (Space-Charge Limited Current) behaviors using experimental J-V (current density-voltage) curves at different voltages and under different light conditions. The optimization of the photoconductor was carried out through annealing and accelerated lighting processes. With these treatments, the Knoop Hardness of the flexible photoconductor has reached a value of 8 with a tensile strength of 5.7 MPa. The ohmic and SCLC approximations demonstrate that the unannealed device has an ohmic behavior, whereas the annealed device has an SCLC behavior, and after the optimization process, an ohmic behavior and a maximum current density of 0.34 mA/mm2 were obtained under blue light. The approximations of the device's electron mobility (μn) and free carrier density (n0) were performed under different light conditions, and the electrical activation energy and electrical gap were obtained for the flexible organic device, resulting in appropriate properties for these applications.

Adaptive Gap-Tunable Surface-Enhanced Raman Spectroscopy

Gap plasmon (GP) resonance in static surface-enhanced Raman spectroscopy (SERS) structures is generally too narrow and not tunable. Here, we present an adaptive gap-tunable SERS device to selectively enhance and modulate different vibrational modes via active flexible Au nanogaps, with adaptive optical control. The tunability of GP resonance is up to ∼1200 cm-1 by engineering gap width, facilitated by mechanical bending of a polyethylene terephthalate substrate. We confirm that the tuned GP resonance selectively enhances different Raman spectral regions of the molecules. Additionally, we dynamically control the SERS intensity through the wavefront shaping of excitation beams. Furthermore, we demonstrate simulation results, exhibiting the mechanical and optical properties of a one-dimensional flexible nanogap and their advantage in high-speed biomedical sensing. Our work provides a unique approach for observing and controlling the enhanced chemical responses with dynamic tunability.

Covalently-Bonded Laminar Assembly of Van der Waals Semiconductors with Polymers: Toward High-Performance Flexible Devices

Van der Waals semiconductors (vdWS) offer superior mechanical and electrical properties and are promising for flexible microelectronics when combined with polymer substrates. However, the self-passivated vdWS surfaces and their weak adhesion to polymers tend to cause interfacial sliding and wrinkling, and thus, are still challenging the reliability of vdWS-based flexible devices. Here, an effective covalent vdWS-polymer lamination method with high stretch tolerance and excellent electronic performance is reported. Using molybdenum disulfide (MoS2 )and polydimethylsiloxane (PDMS) as a case study, gold-chalcogen bonding and mercapto silane bridges are leveraged. The resulting composite structures exhibit more uniform and stronger interfacial adhesion. This enhanced coupling also enables the observation of a theoretically predicted tension-induced band structure transition in MoS2 . Moreover, no obvious degradation in the devices' structural and electrical properties is identified after numerous mechanical cycle tests. This high-quality lamination enhances the reliability of vdWS-based flexible microelectronics, accelerating their practical applications in biomedical research and consumer electronics.

Gallium-Based Liquid Metals in Rechargeable Batteries: From Properties to Applications

Gallium-based (Ga-based) liquid metals have attracted considerable interest due to their low melting points, enabling them to feature both liquid properties and metallic properties at room temperature. In light of this, Ga-based liquid metals also possess excellent deformability, high electrical and thermal conductivity, superior metal affinity, and unique self-limited surface oxide, making them popular functional materials in energy storage. This provides a possibility to construct high-performance rechargeable batteries that are deformable, free of dendrite growth, and so on. This review primarily starts with the property of Ga-based liquid metal, and then focuses on the potential applications in rechargeable batteries by exploiting these advantages, aiming to construct the correlation between properties and structures. The glorious applications contain interface protection, self-healing electrode construction, thermal management, and flexible batteries. Finally, the opportunities and obstacles for the applications of liquid metal in batteries are presented.

Single-crystalline GaN microdisk arrays grown on graphene for flexible micro-LED application

We report the growth of single-crystalline GaN microdisk arrays on graphene and their application in flexible light-emitting diodes (LEDs). Graphene layers were directly grown on c-sapphire substrates using chemical vapor deposition and employed as substrates for GaN growth. Position-controlled GaN microdisks were laterally overgrown on the graphene layers with a micro-patterned SiO2mask using metal-organic vapor-phase epitaxy. The as-grown GaN microdisks exhibited excellent single crystallinity with a uniform in-plane orientation. Furthermore, we fabricated flexible micro-LEDs by achieving heteroepitaxial growth of n-GaN, InxGa1-xN/GaN multiple quantum wells, and p-GaN layers on graphene-coated sapphire substrates. The GaN micro-LED arrays were successfully transferred onto bendable substrates and displayed strong blue light emission under room illumination, demonstrating their potential for integration into flexible optoelectronic devices. .

A BTO/PVDF/PDMS Piezoelectric Tangential and Normal Force Sensor Inspired by a Wind Chime

There is a growing demand for flexible pressure sensors in environmental monitoring and human-robot interaction robotics. A flexible and susceptible sensor can discriminate multidirectional pressure, thus effectively detecting signals of small environmental changes and providing solutions for personalized medicine. This paper proposes a multidimensional force detection sensor inspired by a wind chime structure with a three-dimensional force structure to detect and analyze normal and shear forces in real time. The force-sensing structure of the sensor consists of an upper and lower membrane on a polydimethylsiloxane substrate and four surrounding cylinders. A piezoelectric hemisphere is made of BTO/PVDF/PDMS composite material. The sensor columns in the wind chime structure surround the piezoelectric layer in the middle. When pressure is applied externally, the sensor columns are connected to the piezoelectric layer with a light touch. The piezoelectric hemisphere generates a voltage signal. Due to the particular structure of the sensor, it can accurately capture multidimensional forces and identify the direction of the external force by analyzing the position of the sensor and the output voltage amplitude. The development of such sensors shows excellent potential for self-powered wearable sensors, human-computer interaction, electronic skin, and soft robotics applications.

Direct Current Treatment Tuning Crystallinity Leading to High-Performance p-Type Sb2Te3 Flexible Thin Films

With the development of wearable electronics, inorganic flexible thin films (f-TFs) with high thermoelectric performance have attracted increasing research interest. To further enhance the thermoelectric performance of p-type inorganic Sb2Te3-based f-TFs, we employed direct current treatment to tune the crystallinity by rationally tuning the direct current treatment time. Correspondingly, a high electrical conductivity of >845 S cm-1 and a moderate Seebeck coefficient of >110 μV K-1 within the entire measurement temperature range have been simultaneously achieved. Consequently, a high power factor of 12.84 μW cm-1 K-2 at 423 K has been realized in the as-prepared p-type Sb2Te3 f-TF treated by a direct current of 5 A for 4 min. A flexible thermoelectric device has been further assembled to demonstrate the power-generating capacity. This study indicates that the direct current treatment is an effective method to improve the thermoelectric performance of Sb2Te3 f-TFs.

Fabrication of Flexible PDMS Films with Micro-Convex Structure for Light Extraction from Organic Light-Emitting Diodes

In this study, we demonstrated organic light-emitting diodes (OLEDs) outcoupling with a flexible polydimethylsiloxane (PDMS) film with a micro-convex structure using the breath figure (BF) method. We can easily control the micro-convex pattern by adjusting the concentration of polystyrene and the humidity during the BF process. As process conditions to fabricate the micro-convex structure, polymer concentrations of 10, 20, 40, and 80 mg/mL and 60, 70, and 80% relative humidity were used. To evaluate the optical properties, we analyzed the transmission, diffusion, and electroluminescence with or without the micro-convex structure on the OLEDs. The shape and density of the micro-convex structure are related to its optical properties and outcoupling and we have experimentally demonstrated this. By applying a micro-convex structure, it achieved up to a 42% improvement in the external quantum efficiency compared to bare OLEDs (without any light extraction film). We expect the fabricated flexible light extraction film to be effective for outcoupling and applicable to flexible devices.

Nanoengineering Approach toward High Power Factor Ag2Se/Se Composite Films for Flexible Thermoelectric Generators

Herein, a flexible Ag₂Se/Se composite film with a high power factor has been fabricated on a nylon membrane. The film has a high density and contains well-crystallized Ag₂Se grains and embedded Se nanoinclusions, which exhibits not only excellent flexibility but also a comparably large room-temperature power factor and Seebeck coefficient of up to 2023 μW m^-1 K^-2 and -155 μV K^-1, respectively. The high Seebeck coefficient is ascribed to the energy-filtering effect as caused by the Se/Ag₂Se heterointerface. The assembled flexible thermoelectric generator (4-leg) exhibits a maximum output power of 1135 nW and a power density of up to 16.4 W m^-2 when the applied temperature difference is 30 K. This work offers a feasible method to design high-performance and low-cost flexible thermoelectric generators used for wearable electronics.

Centimeter-Transferable III-Nitride Membrane Enabled by Interfacial Adhesion Control for a Flexible Photosensitive Device

Flexible III-nitride-based optoelectronic devices are crucial for the next-generation foldable/wearable lighting sterilization and sensor working in the ultraviolet (UV) region. However, the strong bonding effect at the epitaxial interface of III-nitride and bare sapphire substrate makes it difficult for epilayer separation and flexible applications. Although the emerging van der Waals epitaxy (vdWE) with graphene insertion layer offers a feasible route for weakening the interfacial adhesion, the intact centimeter-transferable III-nitride membrane still remains challenging. The spontaneous delamination occurs due to the too weak interfacial adhesion of pure vdW force, and on the contrary, the structural damage of graphene by high-temperature hydrogen etching during the III-nitride growth might also cause separation failure. Up to now, the efficient control of vdWE interfacial adhesion is still an on-going research hotspot. Herein, we demonstrate the interfacial adhesion control of III-nitride vdWE by utilizing graded high-temperature nitridation treatment of the graphene insertion layer, which generates defects and N doping in different levels. The corresponding epitaxial modes of pure-vdWE, quasi-vdWE, and mixed epitaxy are achieved according to the interfacial adhesion difference. It reveals that the quasi-vdWE enabled by small graphene defects and proper N doping triggers the low formation energy for AlN nucleation; meanwhile, the proper interfacial adhesion ensures the growth integrality and intact separation of III-nitride membrane in the centimeter scale. The UV resin-assisted bonding technique is proposed for the successful transfer of III-nitride onto a flexible substrate. The flexible photodetector is fabricated by using a graphene monolayer as the photocarrier transport channel, and it achieves a high device yield of 90%, retaining ∼60% of its initial performance after 250 bending cycles. This work offers the promising strategy for controlling vdWE interfacial adhesion, and the separable and transferable III-nitride membrane lays the foundation for advances of future UV foldable and wearable devices.

Stretchable and Skin-Mountable Temperature Sensor Array Using Reduction-Controlled Graphene Oxide for Dermatological Thermography

Since thermometry of human skin is critical information that provides important aspects of human health and physiology, accurate and continuous temperature measurement is required for the observation of physical abnormalities. However, conventional thermometers are uncomfortable because of their bulky and heavy features. In this work, we fabricated a thin, stretchable array-type temperature sensor using graphene-based materials. Furthermore, we controlled the degree of graphene oxide reduction and enhanced the temperature sensitivity. The sensor exhibited an excellent sensitivity of 2.085% °C^-1. The overall device was designed in a wavy meander shape to provide stretchability for the device so that precise detection of skin temperature could be performed. Furthermore, polyimide film was coated to secure the chemical and mechanical stabilities of the device. The array-type sensor enabled spatial heat mapping with high resolution. Finally, we introduced some practical applications of skin temperature sensing, suggesting the possibility of skin thermography and healthcare monitoring.

An Ultrathin Flexible Programmable Spin Logic Device Based on Spin-Orbit Torque

Flexible electronic devices have shown increasingly promising value facilitating our daily lives. However, flexible spintronic devices remain in their infancy. Here, this research demonstrates a type of nonvolatile, low power dissipation, and programmable flexible spin logic device, which is based on the spin-orbit torque in polyimide (PI)/Ta/Pt/Co/Pt heterostructures fabricated via capillary-assisted electrochemical delamination. The magnetization switching ratio is shown to be about 50% for the flexible device and does not change after 100 cycles of bending, indicating the device has stable performance. By designing the path of pulse current, five Boolean logic gates AND, NAND, NOT, NOR, and OR can be realized in an integrated two-element device. Moreover, such peeling-off devices can be successfully transferred to almost any substrate, such as paper and human skin, and maintain high performance. The flexible PI/Ta/Pt/Co/Pt spin logic device serves as logic-in-memory architecture and can be used in wearable electronics.

Highly Flexible Triboelectric Nanogenerator Using Porous Carbon Nanotube Composites

The rapid development of portable and wearable electronic devices has led researchers to actively study triboelectric nanogenerators (TENGs) that can provide self-powering capabilities. In this study, we propose a highly flexible and stretchable sponge-type TENG, named flexible conductive sponge triboelectric nanogenerator (FCS-TENG), which consists of a porous structure manufactured by inserting carbon nanotubes (CNTs) into silicon rubber using sugar particles. Nanocomposite fabrication processes, such as template-directed CVD and ice freeze casting methods for fabricating porous structures, are very complex and costly. However, the nanocomposite manufacturing process of flexible conductive sponge triboelectric nanogenerators is simple and inexpensive. In the tribo-negative CNT/silicone rubber nanocomposite, the CNTs act as electrodes, increasing the contact area between the two triboelectric materials, increasing the charge density, and improving charge transfer between the two phases. Measurements of the performance of flexible conductive sponge triboelectric nanogenerators using an oscilloscope and a linear motor, under a driving force of 2-7 N, show that it generates an output voltage of up to 1120 V and a current of 25.6 µA. In addition, by using different weight percentages of carbon nanotubes (CNTs), it is shown that the output power increases with the weight percentage of carbon nanotubes (CNTs). The flexible conductive sponge triboelectric nanogenerator not only exhibits good performance and mechanical robustness but can also be directly used in light-emitting diodes connected in series. Furthermore, its output remains extremely stable even after 1000 bending cycles in an ambient environment. In sum, the results demonstrate that flexible conductive sponge triboelectric nanogenerators can effectively power small electronics and contribute to large-scale energy harvesting.

Origami-Type Flexible Thermoelectric Generator Fabricated by Self-Folding

The flexibility of thermoelectric generators (TEGs) is important for low-contact thermal resistance to curved heat sources. However, approaches that depend on soft materials, which are used in most existing studies, have the problem of low performance in terms of the substrate's thermal conductivity and the thermoelectric conversion efficiency of the thermoelectric (TE) elements. In this study, we propose a method to fabricate "Origami-TEG", a TEG with an origami structure that enables both flexibility and the usage of high-performance rigid materials by self-folding. By applying the principle of the linkage mechanism to self-folding, we realized a fabrication process in which the TE element-mounting process and the active-material-addition process were separated in time. The fabricated origami-TEG showed similar internal resistance and maximum output power when attached to heat sources with flat and curved surfaces. Furthermore, it exhibited high-performance stability against both stretching and bending deformations.

Study of Pressure Distribution in Floor Tiles with Printed P(VDF:TrFE) Sensors for Smart Surface Applications

Pressure sensors integrated in surfaces, such as the floor, can enable movement, event, and object detection with relatively little effort and without raising privacy concerns, such as video surveillance. Usually, this requires a distributed array of sensor pixels, whose design must be optimized according to the expected use case to reduce implementation costs while providing sufficient sensitivity. In this work, we present an unobtrusive smart floor concept based on floor tiles equipped with a printed piezoelectric sensor matrix. The sensor element adds less than 130 µm in thickness to the floor tile and offers a pressure sensitivity of 36 pC/N for a 1 cm² pixel size. A floor model was established to simulate how the localized pressure excitation acting on the floor spreads into the sensor layer, where the error is only 1.5%. The model is valuable for optimizing the pixel density and arrangement for event and object detection while considering the smart floor implementation in buildings. Finally, a demonstration, including wireless connection to the computer, is presented, showing the viability of the tile to detect finger touch or movement of a metallic rod.

Wide-Range Humidity-Temperature Hybrid Flexible Sensor Based on Strontium Titanate and Poly 3,4 Ethylenedioxythiophene Polystyrene Sulfonate for Wearable 3D-Printed Mask Applications

In this paper, we report a fast, linear wide-range hybrid flexible sensor based on a novel composite of strontium titanate (SrTiO₃) and poly 3,4 ethylenedioxythiophene polystyrene sulfonate (PEDOT: PSS) as a sensing layer. Inter-digitate electrodes (IDEs) were printed for humidity monitoring (finger: 250 µm; spacing: 140 µm; length: 8 mm) whilst a meander-based pattern was printed for the temperature measurement (meander thickness: 180 µm; spacing: 400 µm) on each side of the PET substrate using silver ink. Moreover, active layers with different concentration ratios were coated on the electrodes using a spray coating technique. The as-developed sensor showed an excellent performance, with a humidity measurement range of (10-90% RH) and temperature measurement range of (25-90 °C) with a fast response (humidity: 5 s; temperature: 4.2 s) and recovery time (humidity: 8 s; temperature: 4.4 s). The reliability of the sensor during mechanical bending of up to 5.5 mm was validated with a reliable performance. The sensor was also used in real-world applications to measure human respiration. For this, a suggested sensor-based autonomous wireless node was included in a 3D-printed mask. The manufactured sensor was an excellent contender for wearable and environmental applications because of its exceptional performance, which allowed for the simultaneous measurement of both quantities by a single sensing device.

Multifunctional Flexible Ionic Skin with Dual-Modal Output Based on Fibrous Structure

Flexible wearable electronic devices with multiple sensing functions that simulate human skin in all aspects have become a popular research topic. However, the current expensive and time-consuming means of integration and the complex decoupling process are hampering the further development of multifunctional sensors. Here, an ultraflexible ionic fiber membrane (IFM) prepared by a simple electrospinning technique is reported, which exhibits pressure and humidity sensing properties. With the help of different electrode structures, the IFM-based multifunctional sensor achieved pressure and humidity detection with different sensing mechanisms. Pressure sensing with high sensitivity (49.7 kPa^-1 at 0-30 kPa) and wide detection range (0-220 kPa) was indicated by the capacitive signal. Humidity sensing with high linearity (1.086% per percent relative humidity (RH)) in the range 15%-90% RH was indicated by the resistance signal. In particular, the multimodal output of capacitance/resistance corresponding to pressure/humidity in this study directly addresses the problem of accurately distinguishing the two stimuli. Furthermore, we have demonstrated that the impact between pressure and humidity is negligible when measured simultaneously and independently. Because of the excellent pressure/humidity sensing performance, we have fabricated a smart bracelet and mask for pulse, skin moisture, and breathe monitoring, which indicates the promising future of multifunctional flexible sensors based on IFM in the healthcare field.

High Power Factor of Ag2Se/Ag/Nylon Composite Films for Wearable Thermoelectric Devices

A flexible thermoelectric device has been considered as a competitive candidate for powering wearable electronics. Here, we fabricated an n-type Ag₂Se/Ag composite film on a flexible nylon substrate using vacuum-assisted filtration and a combination of cold and hot pressing. By optimising the Ag/Se ratio and the sequential addition and reaction time of AA, an excellent power factor of 2277.3 μW∙m^-1 K^-2 (corresponding to a ZT of ~0.71) at room temperature was achieved. In addition, the Ag₂Se/Ag composite film exhibits remarkable flexibility, with only 4% loss and 10% loss in electrical conductivity after being bent around a rod of 4 mm radius for 1000 cycles and 2000 cycles, respectively. A seven-leg flexible thermoelectric device assembled with the optimised film demonstrates a voltage of 19 mV and a maximum power output of 3.48 μW (corresponding power density of 35.5 W m^-2) at a temperature difference of 30 K. This study provides a potential path to design improved flexible TE devices.

Flexible Self-Powered Weak Light Detectors Based on ZnO/CsPbBr3/γ-CuI Heterojunctions

Halide perovskites (HPs) with marvelous optical and electrical properties are regarded as one of the competitive candidates for building next-generation photodetectors (PDs). However, combining their excellent properties with satisfactory long-term robustness is still challenging, ultimately limiting the practical applications of HP-based PDs. Herein, a high vacuum deposition system is employed to fabricate flexible self-powered PDs with a ZnO/CsPbBr₃/γ-CuI structure, which shows excellent stability and outstanding performance in weak light detection. Benefiting from the improved crystallinity and optimized device structure, a high detectivity of 8.1 × 10¹³ Jones and a rapid response speed (rise/decay time of 3.9/1.8 μs) are obtained in this self-powered device. Furthermore, the unencapsulated device exhibits intriguing environmental stability and mechanical flexibility. The photocurrent remains unchanged after 7000 s of continuous operation or 100 bending cycles. Furthermore, a 15 × 15 PD array is fabricated as an image sensor. A high contrast image of the target object can be obtained owing to the high sensitivity and uniformity of the self-powered PDs. These results demonstrate the feasibility and practicality of the ZnO/CsPbBr₃/γ-CuI heterojunction for applications in weak light detection and image formation.

Large-area vertically aligned 2D MoS2layers on TEMPO-cellulose nanofibers for biodegradable transient gas sensors

Crystallographically anisotropic two-dimensional (2D) molybdenum disulfide (MoS₂) with vertically aligned (VA) layers is attractive for electrochemical sensing owing to its surface-enriched dangling bonds coupled with extremely large mechanical deformability. In this study, we explored VA-2D MoS₂layers integrated on cellulose nanofibers (CNFs) for detecting various volatile organic compound gases. Sensor devices employing VA-2D MoS₂/CNFs exhibited excellent sensitivities for the tested gases of ethanol, methanol, ammonia, and acetone; e.g. a high response rate up to 83.39% for 100 ppm ethanol, significantly outperforming previously reported sensors employing horizontally aligned 2D MoS₂layers. Furthermore, VA-2D MoS₂/CNFs were identified to be completely dissolvable in buffer solutions such as phosphate-buffered saline solution and baking soda buffer solution without releasing toxic chemicals. This unusual combination of high sensitivity and excellent biodegradability inherent to VA-2D MoS₂/CNFs offers unprecedented opportunities for exploring mechanically reconfigurable sensor technologies with bio-compatible transient characteristics.

Flexible and Highly Conductive Textiles Induced by Click Chemistry for Sensitive Motion and Humidity Monitoring

To date, multifunctional sensors have aroused widespread concerns owing to their vital roles in the healthcare area. However, there are still significant challenges in the fabrication of functionalized integrated devices. In this work, hydrophobic-hydrophilic patterns are constructed on polyester-spandex-blended knitted fabric surface by the chemical click method, enabling accurate deposition of functionalized materials for sensitive and stable motion and humidity sensing. Representatively, a conductive silver nanowire (Ag NW) network was deliberately deposited on only the designated hydrophilic fabric surface to realize accurate, repeatable, and stable motion sensing. Such a Ag NWs sensor recorded a low electrical resistance (below 60 Ω), stable resistance cycling response (over 2000 cycles), and fast response time to humidity (0.46 s) during the sensing evaluation. In addition to experimental sensing, real human motions, such as mouth-opening and joint-flexing (wrist and neck), could also be detected using the same sensor. Similar promising outputs were also obtained over the humidity sensor fabricated over the same chemical click method, except the sensing material was replaced with polydopamine-modified carboxylated carbon nanotubes. The resultant sensor exhibits excellent sensitivity to not only experimentally adjusted environment humidity but also to the moisture content of breath and skin during daily activities. On top of all these, both sensors were fabricated over highly flexible fabric that offers high wearability, promising great application potential in the field of healthcare monitoring.

Magnetically Levitated Flexible Vibration Sensors with Surficial Micropyramid Arrays for Magnetism Enhancement

Magnetically levitated vibration sensors possess wide frequency response ranges and high sensitivity. Compared with springs and cantilevers, the levitated magnet suffers no mechanical abrasion, allowing minimized mechanical fatigue after prolonged exposure to vibration. However, magnetic levitated sensors are mostly based on fully rigid components, which are difficult to match the soft and curvilinear surface of the biological tissues and machines. Here, an innovative vibration sensor based on magnetic levitation has been developed. The proposed sensor contains two parallel magnetic membranes, one of which is levitated by magnetic force and connected to a specially designed sensor package. The surfaces of the membranes are modified with micropyramid arrays to enhance the magnetism and integrated with flexible coil arrays to maximize the changes in magnetic flux during vibration. The sensor exhibits a wide frequency response ranging from 1 Hz to 20 kHz and high sensitivity of 0.82 mV/μm at an operating frequency of 120 Hz. Various applications have been demonstrated through bone-conducted speech acquisition, sound recording, human motion detection, and machine condition evaluation. The sensor is one of the first flexible vibration sensors based on magnetic levitation. Its innovative levitated sensing structures may inspire development of novel flexible sensors with soft mechanical moving structures for force and displacement sensing in healthcare and industrial monitoring.

Graphene-Based Optically Transparent Metasurface Capable of Dual-Polarized Modulation for Electromagnetic Stealth

Microwave stealth technology with optical transparency is of great significance for solar-powered aircrafts (e.g., satellites or unmanned aerial vehicles) in increasingly complex electromagnetic environments. By coating them with optically transparent absorbing materials or devices, these large-sized solar panels could avoid detection by radar while maintaining highly efficient collection of solar energy. However, conventional microwave-absorbing materials/devices for solar panels suffer from bulky volume and fixed stealth performance that significantly hinders their practicality or multifunctionality. Particularly, dynamic modulation of microwave absorption for dual polarization remains a challenge. In this paper, we propose the design, fabrication, and characterization of an optically transparent and dynamically tunable microwave-absorbing metasurface that enables dual modulations (amplitude and frequency) independently for two orthogonal linearly polarized excitations. The tunability of the proposed metasurface is guaranteed by an elaborately designed anisotropic meta-atom composed of a patterned graphene structure whose electromagnetic responses for different polarizations can be dynamically and independently controlled via bias voltages. The dual tunability in such a graphene-based absorbing metasurface is experimentally measured, which agrees well with those numerical results. We further build an equivalent lumped circuit model to analyze the physical relation between the tunable sheet resistance of graphene and the polarization-independent modulations of the metasurface. Taking into account the advantages of optical transparency and flexibility, the proposed microwave-absorbing metasurface significantly enhances the multitasking stealth performance in complex scenarios and has the potential for advanced solar energy devices.

Large Curvature Self-Folding Method of a Thick Metal Layer for Hinged Origami/Kirigami Stretchable Electronic Devices

A self-folding method that can fold a thick (~10 μm) metal layer with a large curvature (>1 mm−1) and is resistant to repetitive folding deformation is proposed. Given the successful usage of hinged origami/kirigami structures forms in deployable structures, they show strong potential for application in stretchable electronic devices. There are, however, two key difficulties in applying origami/kirigami methods to stretchable electronic devices. The first is that a thick metal layer used as the conductive layer of electronic devices is too hard for self-folding as it is. Secondly, a thick metal layer breaks on repetitive folding deformation at a large curvature. To overcome these difficulties, this paper proposes a self-folding method using hinges on a thick metal layer by applying a meander structure. Such a structure can be folded at a large curvature even by weak driving forces (such as those produced by self-folding) and has mechanical resistance to repetitive folding deformation due to the local torsional deformation of the meander structure. To verify the method, the large curvature self-folding of thick metal layers and their mechanical resistance to repetitive folding deformation is experimentally demonstrated. In addition, an origami/kirigami hybrid stretchable electronic device with light-emitting diodes (LEDs) is fabricated using a double-tiling structure called the perforated extruded Miura-ori.

Flow-Enhanced Flexible Microcomb Printing of Organic Solar Cells

Scalable and roll-to-roll compatible processing methods have become pressing needs to transfer organic solar cells (OSCs) to realistic energy sources. Herein a new fabrication method of flexible microcomb printing is proposed. The microcomb is based on a PET sheet micromachined into comb teeth by a laser marker. A computational fluid mechanics simulation shows that the fluid flow around the microcomb teeth induces high shear as well as extensional strain rates, which enhance the molecular alignment and lateral mass transport. The PTQ10:Y6-BO OSCs printed by the flexible microcomb demonstrate a substantially increased degree of crystallinity and phase separation with a suitable domain size. Devices printed by the flexible microcomb in air achieve PCEs of up to 15.93%, higher than those of control devices spin-coated in the N₂ glovebox. The flexibility of the PET film makes the microcomb teeth contact directly with the substrate without a suspended liquid meniscus, thus facilitating printing on soft or curved substrates. Printing of flexible OSCs and large-area devices are demonstrated. The flexible OSCs exhibit PCEs of up to 13.62%, which is the highest for flexible OSCs made by scalable printing techniques to date. These results make flexible microcomb printing a feasible and promising strategy toward the manufacture of efficient OSCs.

Bendable Polycrystalline and Magnetic CoFe2O4 Membranes by Chemical Methods

The preparation and manipulation of crystalline yet bendable functional complex oxide membranes has been a long-standing issue for a myriad of applications, in particular, for flexible electronics. Here, we investigate the viability to prepare magnetic and crystalline CoFe₂O₄ (CFO) membranes by means of the Sr₃Al₂O₆ (SAO) sacrificial layer approach using chemical deposition techniques. Meticulous chemical and structural study of the SAO surface and SAO/CFO interface properties have allowed us to identify the formation of an amorphous SAO capping layer and carbonates upon air exposure, which dictate the crystalline quality of the subsequent CFO film growth. Vacuum annealing at 800 °C of SAO films promotes the elimination of the surface carbonates and the reconstruction of the SAO surface crystallinity. Ex-situ atomic layer deposition of CFO films at 250 °C on air-exposed SAO offers the opportunity to avoid high-temperature growth while achieving polycrystalline CFO films that can be successfully transferred to a polymer support preserving the magnetic properties under bending. Float on and transfer provides an alternative route to prepare freestanding and wrinkle-free CFO membrane films. The advances and challenges presented in this work are expected to help increase the capabilities to grow different oxide compositions and heterostructures of freestanding films and their range of functional properties.

Stable Doping of Single-Walled Carbon Nanotubes for Flexible Transparent Conductive Films

Possessing excellent electronic and mechanical properties and great stability, single-walled carbon nanotubes (SWCNTs) are exceptionally attractive in fabricating flexible transparent conductive films. Doping is a key step to further enhance the conductivity of the SWCNT films and the reliable doping is highly needed. We developed a feasible strategy that uses solid acids such as phosphotungstic acid (PTA) to dope the SWCNT films stably relying on the nonvolatility of the dopants. The sheet resistance of the films was reduced to around a half of the original value meanwhile with no obvious change in transmittance. The doping effect maintained during a 700 days' observation. The excellent flexibility of the PTA-doped films was demonstrated by a bending test of 1000 cycles, during which the sheet resistance and transmittance was basically unaffected. The blue shifts of G band in the Raman spectra and the increase of work function measured by the Kelvin probe force microscopy both reveal the p-type doping of the films by PTA. The strong acidity of PTA plays a key role in the doping effect by increasing the redox potential of the ambient O₂ and thus the Fermi level of the SWCNTs is brought down. The great feasibility and robustness of our doping strategy are desirable in the practical application of SWCNT-based flexible transparent conductive films. This strategy can be extended to the p-type doping of various CNT-based assemblies (such as sponges and forests) as well as other material families, expanding the application spectrum of polyacids.

Practical Demonstration of Deep-Ultraviolet Detection with Wearable and Self-Powered Halide Perovskite-Based Photodetector

Flexible and self-powered photodetectors (PDs) have become one of the most popular topics, attracting researchers in the field of optoelectronic applications. In this study, for the first time, we demonstrate partial discharge detection in a practical environment with a prepared flexible device. Poly(vinylidene fluoride) (PVDF) is utilized as a highly transparent material in the UVC region, to create a flexible substrate with the antihumidity property. A detector that uses a mixed-halide perovskite (FAPbI₃)_1-x(MAPbBr₃)_x as the photoactive material is constructed in a vertical structure on the as-prepared hydrophobic PVDF substrate. The fabricated device exhibits good performance with a fast response speed (t_rise = 82 ms, t_fall = 64 ms) and a high detectivity of 7.21 × 10¹⁰ Jones at zero bias under 254 nm UV illumination, along with superior mechanical flexibility at various bending angles. Additionally, the air-exposure stability and reproducibility of the as-prepared device exhibit almost the original performance after 6 weeks of storage. For practical applications, we demonstrate a facile and sensitive detection for UVC leakage from a germicidal lamp and simulated a partial discharge system using our PD without energy consumption. These results indicate that this new approach may be useful and convenient for the detection of the partial discharge as well as for several practical applications.

Interconnecting 3D Conductive Networks with Nanostructured Iron/Iron Oxide Enables a High-Performance Flexible Battery

Aqueous Ni/Fe alkaline batteries with features of low cost and high safety show great potential for application in portable and wearable electronics. However, the poor kinetics of the Fe-based anode greatly limits the large-scale applications of Ni/Fe batteries. Herein, we report an interconnected 3D conductive network with carbon-coated nanostructured iron/iron oxide (3D-Fe/Fe₂O₃@C) as an efficient anode for a flexible Ni/Fe battery. A hydrogel precursor is used to molecularly link and confine Fe³⁺ to spatial networks, resulting in a uniform dispersion of Fe/Fe₂O₃-heterostructured nanoparticles. Theoretical investigations reveal regulated potential loss and improved delocalized carrier density as a result of carbon coating and the mixed metal/metal oxide structure. In addition to these merits, due to the regulated wettability and electroactive surface areas, the 3D-Fe/Fe₂O₃@C anode with a high mass loading delivers an extraordinary areal capacity of 3.07 mA h cm^-2, as well as the boosted rate capability and Coulombic efficiency. When coupled with the NiCo₂O₄ cathode, the flexible quasi-solid-state Ni/Fe battery exhibits an admirable energy density of 15.53 mW h cm^-3 and a maximum power density of 761.91 W h cm^-3. The good stability after 20,000 cycles and severe mechanical deformations of the as-fabricated Ni/Fe battery imply it as a promising flexible energy storage device for practical applications.

Flexible and Stable Carbon Nanotube Film Strain Sensors with Self-Derived Integrated Electrodes

The development of flexible and wearable electronic devices has put an increasing demand on electrode systems with seamless connection and high compatibility with the main device, in order to accommodate complex deformation conditions and maintain stable performance. Here, we present a carbon nanotube-integrated electrode (CNTIE) by wet-pulling the ends of a carbon nanotube (CNT) film to form condensed thin fibers that resemble conventional conducting wire electrodes. A flexible strain sensor was constructed consisting of the middle CNT film as the main functional part and the CNTIE as self-derived electrodes, with inherent CNT connection between the two parts. The sensor can be transferred to versatile substrates (e.g., balloon surface) or encapsulated in thermoplastic polymers, exhibiting a large linear response range (up to 1000% in tensile strain), excellent durability and repeatability over 5000 cycles, and the ability to detect small- to large-degree human body motions. In addition, the strain sensor based on the CNTIE hybrid film (MXene/CNT and graphene/CNT) also shows superior linearity and stability at a strain range of 0-800%. Compared with the sensors using traditional silver wire electrodes and separately fabricated CNT fiber electrodes, our CNTIE plays an important role in achieving highly stable performance in the strain cycles. Our self-derived integrated electrodes provide a potential route to solve the incompatibility issues of conventional electrodes and to develop high-performance flexible and wearable systems based on CNTs and other nanomaterials.

Giant Wrinkles on the Surface of Epitaxial BaTiO3 Thin Films with Drastic Shrinkage during Transfer from a MgO(100) Single-Crystal Substrate to a Flexible Polyethylene Terephthalate Sheet

The transfer of ferroelectric and piezoelectric BaTiO₃ epitaxial thin films from an original MgO(100) single-crystal substrate to a polyethylene terephthalate (PET) sheet has been studied to fabricate flexible epitaxial functional oxides. The outline of our previous transfer process is as follows: the epitaxial BaTiO₃ thin films were deposited on the MgO(100). Then, the surface of the BaTiO₃ was adhered onto a PET sheet. Finally, only the MgO(100) substrate was dissolved in a phosphoric aqueous solution, which resulted in the transfer of the epitaxial BaTiO₃ thin film from the MgO(100) to a PET sheet. To establish this transfer process, our aim was to prevent any damage, such as cracks and exfoliation, during the transfer of the epitaxial functional oxides. We found that a Pt buffer layer with a ductile nature was effective for improving the quality of transferred epitaxial BaTiO₃ thin films. Moreover, the epitaxial BaTiO₃ thin films showed a drastic shrinkage of ca. 10%. The surfaces of the shrunk, epitaxial BaTiO₃ thin films showed giant wrinkles with a micrometer-order amplitude and a 10-μm-order periodicity without any damage. The epitaxial BaTiO₃ thin films with giant wrinkles, accompanied by drastic shrinkage, are similar to the thin films that are coated on a pre-stretched elastomer, which is one of the fabrication processes of stretchable devices.

Continuous Wave Laser Nanowelding Process of Ag Nanowires on Flexible Polymer Substrates

In this paper we present the laser nanowelding process of silver nanowires (AgNWs) deposited on flexible polymer substrates by continuous wave (CW) lasers. CW lasers are cost-effective and can provide moderate power density, somewhere between nanosecond pulsed lasers and flash lamps, which is just enough to perform the nanowelding process efficiently and does not damage the nanowires on the polymer substrates. Here, an Nd:YAG CW laser (wavelength 532 nm) was used to perform the nanowelding of AgNWs on polyethylene terephthalate (PET) substrates. Key process parameters such as laser power, scan speed, and number of scans were studied and optimized, and mechanisms of observed phenomena are discussed. Our best result demonstrates a sheet resistance of 12 ohm/squ with a transmittance at λ = 550 nm of 92% for AgNW films on PET substrates. A transparent resistive heater was made, and IR pictures were taken to show the high uniformity of the CW laser nanowelded AgNW film. Our findings show that highly effective and efficient nanowelding can be achieved without the need of expensive pulse lasers or light sources, which may contribute to lower the cost of mass producing AgNWs on flexible substrates.

High-Performance Washable PM2.5 Filter Fabricated with Laser-Induced Graphene

This study demonstrates a novel application of laser-induced graphene (LIG) as a reusable conductive particulate matter (PM) filter. Four types of LIG-based filters were fabricated based on the laser-induced pyrolysis of thin polyimide (PI) sheets, each pyrolyzed on either a single side or both sides, with or without densification. The LIG filters exhibited a high removal efficiency while maintaining minimal pressure drop compared to a commercial fiberglass filter. The densified LIG (dLIG) filters displayed a higher PM_2.5 removal efficiency (>99.86%) than regular LIG filters. The dLIG filters also exhibited excellent durability when tested for washability by ultrasonication in tap water. After being cleaned and left to dry, the structures of the dLIG filters were well-maintained; their filtration efficiencies were also well-maintained (less than a 7% change in PM_2.5 removal efficiency), and their resistances only marginally increased (less than a 7% increase after five uses). These results demonstrate the robustness and reusability of the dLIG filters and the accessibility of their cleaning (not requiring aggressive cleaning agents). These promising features will enable the application of LIG in economical, scalable, and high-performance air cleaning.

GaOx@GaN Nanowire Arrays on Flexible Graphite Paper with Tunable Persistent Photoconductivity

Flexible optoelectronic synaptic devices that functionally imitate the neural behavior with tunable optoelectronic characteristics are crucial to the development of advanced bioinspired neural networks. In this work, amorphous oxide-decorated GaN nanowire arrays (GaO_x@GaN NWAs) are prepared on flexible graphite paper. A GaO_x@GaN NWA-based flexible device has tunable persistent photoconductivity (PPC) and shows a conversible fast/slow decay process (SDP). Photoconductivity can be modulated by single or double light pulses with different illumination powers and biases. PPC gives rise to the high-performance SDP such as a long decay time of 2.3 × 10⁵ s. The modulation mechanism is proposed and discussed. Our results reveal an innovative and efficient strategy to produce decorated NWAs on a flexible substrate with tunable optoelectronic properties and exhibit potential for flexible neuromorphic system applications.

Layer exchange synthesis of multilayer graphene

Low-temperature synthesis of multilayer graphene (MLG) on arbitrary substrates is the key to incorporating MLG-based functional thin films, including transparent electrodes, low-resistance wiring, heat spreaders, and battery anodes in advanced electronic devices. This paper reviews the synthesis of MLG via the layer exchange (LE) phenomenon between carbon and metal from its mechanism to the possibility of device applications. The mechanism of LE is completely different from that of conventional MLG precipitation methods using metals, and the resulting MLG exhibits unique features. Modulation of metal species and growth conditions enables synthesis of high-quality MLG over a wide range of growth temperatures (350 °C-1000 °C) and MLG thicknesses (5-500 nm). Device applications are discussed based on the high electrical conductivity (2700 S cm^-1) of MLG and anode operation in Li-ion batteries. Finally, we discuss the future challenges of LE for MLG and its application to flexible devices.

Application of Titanium-Carbide MXene-Based Transparent Conducting Electrodes in Flexible Smart Windows

Among various available materials used in transparent and flexible devices, MXenes are attracting attention as a brand-new candidate in this category. Ti₃C₂Tx MXene as a 2D material has exceptional properties, making it a potential material having numerous applications in different areas. Because of its high conductivity, it can be used in transparent conducting electrodes (TCEs). In this study, the MXenes etched by highly concentrated acid at 50 °C,were spin-coated on polyethylene terephthalate (PET) film and annealed at moderate temperatures up to 170 °C. The adhesion of MXene to PET was found to be remarkably improved by annealing. These TCEs exhibited a sheet resistance of ∼424 Ω/sq. and transmittance of ∼87%. The aging stability of MXene-coated PET films against oxidation under ambient conditions was studied up to 28 days and resistance change was found ∼30% during this period. The flexibility test showed low bending resistance change (∼1.5%) at 1000th cycle and cumulative resistance change of ∼20% at a bending radius of ∼3.9 mm after 1000 cycles. These transparent, flexible, and conducting electrodes were used to fabricate polymer dispersed liquid crystal (PDLC)-based flexible smart windows. The smart windows fabricated by curing PDLC mixture sandwiched between the MXene electrodes were also found flexible in ON/OFF states. The MXene-based flexible smart windows resulted in good opacity in the OFF state and high transparency in the ON state, exhibiting low threshold voltage <10 V and high transmittance ∼80% at 60 V. The flexible smart windows operated normally even at ∼4 mm bending radius.

Polydopamine nanofilms for high-performance paper-based electrochemical devices

Since the discovery of polydopamine (PDA), there has been a lot of progress on using this substance to functionalize many different surfaces. However, little attention has been given to prepare functionalized surfaces for the preparation of flexible electrochemical paper-based devices. After fabricating the electrodes on paper substrates, we formed PDA on the surface of the working electrode using a chemical polymerization route. PDA nanofilms on carbon were characterized by contact angle (CA) experiments, X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy (topography and electrical measurements) and electrochemical techniques. We observed that PDA introduces chemical functionalities (RNH₂ and RC═O) that decrease the CA of the electrode. Moreover, PDA nanofilms did not block the heterogeneous electron transfer. In fact, we observed one of the highest standard heterogeneous rate constants (k_s ) for electrochemical paper-based electrodes (2.5 ± 0.1) × 10^-3  cm s^-1 , which is an essential parameter to obtain larger currents. In addition, our results suggest that carbonyl functionalities are ascribed for the redox activity of the nanofilms. As a proof-of-concept, the electrooxidation of nicotinamide adenine dinucleotide showed remarkable features, such as, lower oxidation potential, electrocatalytic peak currents more than 30 times higher when compared to unmodified paper-based electrodes and electrocatalytic rate constant (k_obs ) of (8.2 ± 0.6) × 10²  L mol^-1  s^-1 .

Solar Seawater Distillation by Flexible and Fully Passive Multistage Membrane Distillation

Solar-assisted distillation is considered promising to solve the freshwater supply for off-grid communities. In this work, a passive and flexible multistage membrane distillation (F-MSMD) device is devised to produce freshwater via solar distillation with the latent heat of vapor condensation being recycled to enhance its energy efficiency. By designing a siphon effect, source water is continuously wicked into the evaporation layer and the concentrated brine flows out of the device before reaching saturation, which successfully solves the otherwise challenge of salt accumulation inside the device. To achieve such siphon flow, the recycled paper is prepared from spent copy paper and used as the evaporation layer for efficient water delivery owing to its large pore size and high hydrophilicity. An eight-stage F-MSMD device exhibits a stable clean water production rate at 3.61 kg m^-2 h^-1 in the newly designed siphon-flow mode. This work provides a green route for designing a solar-assisted distillation device.

Bifacial Color-Tunable Electroluminescent Devices

Alternate current electroluminescent (ACEL) devices provide a range of interesting properties, such as facile large-area processability, mechanical flexibility, and outstanding resilience, when compared with other large-area light-emitting technologies. To widen the scope of possible applications for ACEL devices, color tunability and white light emission are desirable. Here, we introduce a novel three-terminal device architecture based on two monolithically stacked ACEL devices (e.g., orange and blue) that allows for color tunability via independent operation of the subdevices. The tandem devices comprise semitransparent bottom and top electrodes based on networks of silver nanowires, which endow the tandem ACEL device with bifacial Janus-type emission. We provide a detailed analysis of the sources of optical losses in single and tandem ACEL devices. Our novel device concept enables novel facets of applications for ACEL in signage and lighting.

Large Low-Field Magnetoresistance (LFMR) Effect in Free-Standing La0.7Sr0.3MnO3 Films

The realization of a large low-field magnetoresistance (LFMR) effect in free-standing magnetic oxide films is a crucial goal toward promoting the development of flexible, low power consumption, and nonvolatile memory devices for information storage. La_0.7Sr_0.3MnO₃ (LSMO) is an ideal material for spintronic devices due to its excellent magnetic and electronic properties. However, it is difficult to achieve both a large LFMR effect and high flexibility in LSMO films due to the lack of research on LFMR-related mechanisms and the strict LSMO growth conditions, which require rigid substrates. Here, we induced a large LFMR effect in an LSMO/mica heterostructure by utilizing a disorder-related spin-polarized tunneling effect and developed a simple transfer method to obtain free-standing LSMO films for the first time. Electrical and magnetic characterizations of these free-standing LSMO films revealed that all of the principal properties of LSMO were sustained under compressive and tensile conditions. Notably, the magnetoresistance of the processed LSMO film reached up to 16% under an ultrasmall magnetic field (0.1 T), which is 80 times that of a traditional LSMO film. As a demonstration, a stable nonvolatile multivalue storage function in flexible LSMO films was successfully achieved. Our work may pave the way for future wearable resistive memory device applications.

Flexible Epsilon Iron Oxide Thin Films

Metastable ε-Fe₂O₃ is a unique phase of iron oxide, which exhibits a giant coercivity field. In this work, we grew epitaxial ε-Fe₂O₃ films on flexible two-dimensional muscovite substrates via quasi van der Waals epitaxy. It turns out that twinning and interface energies have been playing essential roles in stabilizing metastable ε-Fe₂O₃ on mica substrates. Moreover, the weak interfacial bonding between ε-Fe₂O₃ and mica is expected to relieve the substrate clamping effect ubiquitously encountered in films epitaxially grown on rigid substrates, such as SrTiO₃. It is anticipated that these flexible ε-Fe₂O₃ thin films can serve as a platform for exploring possible interesting emergent physical properties and eventually be integrated as flexible functional devices.

Design of a Kirigami Structure with a Large Uniform Deformation Region

We designed a kirigami structure with a particular shape at both ends to provide a large uniform deformation region when stretched. When a kirigami structure is stretched, non-deformation regions, where the regions' cuts do not open, and non-uniform deformation regions, where the regions' cuts are not uniformly deformed, are produced. The extent of the non-deformation and non-uniform deformation regions increases in proportion to the number of cut cycles in the width direction n _w this reduces the percentage of the uniform deformation region. We propose a method that increases the uniform deformation region in a kirigami structure by deforming the shape of the ends from a rectangle to a trapezoid when stretched. The proposed kirigami structure has separation lines at both ends that separate cuts in the width direction, and the position of contacts at both ends are moved to the center. The proposed kirigami structure has a large uniform deformation region, even when n _w is large, as evidenced by calculating the area of open cuts under stretching. The product of our study realizes a stretchable electro device with a large area, which maintains the position of evenly mounted functional elements when stretched.

Highly Conductive and Flexible Dopamine-Graphene Hybrid Electronic Textile Yarn for Sensitive and Selective NO2 Detection

Graphene-based electronic textile (e-textile) gas sensors have been developed for detecting hazardous NO₂ gas. For the e-textile gas sensor, electrical conductivity is a critical factor because it directly affects its sensitivity. To obtain a highly conductive e-textile, biomolecules have been used for gluing the graphene to the textile surface, though there remain areas to improve, such as poor conductivity and flexibility. Herein, we have developed a dopamine-graphene hybrid electronic textile yarn (DGY) where the dopamine is used as a bio-inspired adhesive to attach graphene to the surface of yarns. The DGY shows improved electrical conductivity (∼40 times) compared to conventional graphene-based e-textile yarns with no glue. Moreover, it exhibited improved sensing performance in terms of short response time (∼2 min), high sensitivity (0.02 μA/ppm), and selectivity toward NO₂. The mechanical flexibility and durability of the DGY were examined through a 1000-cycle bending test. For a practical application, the DGY was attempted to detect the NO_x emitted from vehicles, including gasoline, diesel, and fuel cell electric vehicles. Our results demonstrated that the DGYs-as a graphene-based e-textile gas sensor for detecting NO₂-are simple to fabricate, cheap, disposable, and mechanically stable.

Textile Based Electrochromic Cells Prepared with PEDOT: PSS and Gelled Electrolyte

Electrochromic devices can act as passive displays. They change their color when a low voltage is applied. Flexible and bendable hybrid textile-film electrochromic devices with poly-3,4-ethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) were prepared on polyethylene polyethylene terephthalate (PEPES) membranes using a spray coating technique. The electrolyte consisted of a gelatin glycerol mixture as host matrix and calcium chloride. Titanium dioxide was used as an ion storage layer and a carbon containing dispersion was used for the counter electrode on a polyester rip-stop fabric. The sheet resistance of PEDOT:PSS on PEPES was 500 Ohm/sq. A 5 × 5 electrochromic matrix with individually addressable pixels was successfully designed and assembled. The switching time of the pixels was 2 s at a voltage of 2.0 V directly after assembling. The use of titanium dioxide as ion storage also increased the contrast of the dark-blue reduced electrochromic layer. Coloration was not self-sustaining. The PEDOT:PSS layer needed a constant low voltage of at least 0.5 V to sustain in the dark-blue reduced state. The switching time increased with time. After 12 months the switching time was ~4 s at a voltage of 2.8 V. The addition of glycerol into the electrolyte extended the lifetime of a non-encapsulated textile electrochromic cell, because moisture is retained in the electrolyte. Charge carriers can be transported into and out of the electrochromic layer.

High-Performance n-Type Carbon Nanotubes Doped by Oxidation of Neighboring Sb2Te3 for a Flexible Thermoelectric Generator

Flexible thermoelectric devices can be potentially used for flexible cooling and energy harvesting from various heat sources such as the human body. However, the development of flexible thermoelectric materials with excellent thermoelectric performance is still very challenging. In this study, a simple solution process is proposed for the preparation of flexible inorganic/carbon nanotube hybrid films with record power factors among those of the reported flexible n-type thermoelectric materials. The hybrid films fabricated by bar-coating a carbon nanotube-dispersed Sb₂Te₃ solution exhibit n-type power factors of up to 2440 ± 267 μV m^-1 K^-2 at room temperature. The dissolved Sb₂Te₃ recrystallizes on the carbon nanotube surfaces and form hybrid solids. The ultrahigh power factor may be originated from the effective n-doping of carbon nanotubes by the oxidation of neighboring Sb₂Te₃. Using the thermoelectric hybrid film, a multilayer stacked thermoelectric generator is fabricated. The flexible device with a thermal contact area of 3.8 cm² exhibits an output power of up to 11.3 μW at a vertical ΔT of 7.5 K. This study paves the way for the realization of flexible thermoelectric devices with various device geometries.

Bication-Mediated Quasi-2D Halide Perovskites for High-Performance Flexible Photodetectors: From Ruddlesden-Popper Type to Dion-Jacobson Type

Quasi-2D halide perovskites, especially the Ruddlesden-Popper perovskites (RPPs), have attracted great attention because of their promising properties for optoelectronics; however, there are still serious drawbacks, such as inefficient charge transport, poor stability, and unsatisfactory mechanical flexibility, restricting further utilization in advanced technologies. Herein, high-quality quasi-2D halide perovskite thin films are successfully synthesized with the introduction of the unique bication ethylenediammonium (EDA) via a one-step spin-coating method. This bication EDA, with short alkyl chain length, can not only substitute the typically bulky and weakly van der Waals-interacted organic bilayer spacer cations forming the novel Dion-Jacobson phase to enhance the mechanical flexibility of the quasi-2D perovskite (e.g., EDA(MA)_n-1Pb_nI_3n+1; MA = CH₃NH₃⁺) but also serve as a normal cation to achieve the more intact films (e.g., (iBA)₂(MA)_3-2x(EDA)_xPb₄I₁₃). When fabricated into photodetectors, these optimized EDA-based perovskites deliver an excellent responsivity of 125 mA/W and a fast response time down to 380 μs under 532 nm irradiation. More importantly, the device with the Dion-Jacobson phase perovskite can be bent down to a radius of 2 mm and processed with 10,000 cycles of the bending test without any noticeable performance degradation because of its superior structure to RPPs. Besides, these films do not exhibit any material deterioration after ambient storage for 30 days. All these performance parameters are already comparable or even better than those of the state-of-the-art RPPs recently reported. This work provides valuable design guidelines of the quasi-2D perovskites to obtain high-performance flexible photodetectors for next-generation optoelectronics.

Flexible Bottom-Gated Organic Field-Effect Transistors Utilizing Stamped Polymer Layers from the Surface of Water

The facile sequential deposition of functional organic thin films by solution processes is critical for the development of a variety of high-performance organic devices without restriction in terms of materials and processes. Herein, we propose a simple fabrication process that entails stacking multiple layers of functional polymers to fabricate organic field-effect transistors (OFETs). The process involves stamping organic semiconducting layers formed on the surface of water onto a commonly used polymeric dielectric layer. Our scheme makes it possible to independently optimize organic semiconductor films by controlling the solvent evaporation time during the process of film formation on the surface of water. This approach eliminates the need to be concerned about any interference with adjacent layers. Utilizing this process, the fabrication of high-performance bottom-gated OFETs is demonstrated on a glass and a flexible substrate. The OFETs consist of a vertically stacked diketopyrrolopyrrole-based polymer semiconducting layer on the poly(methyl methacrylate) film with a maximum hole mobility of 0.85 cm²/V s.

Sacrificial Water-Soluble BaO Layer for Fabricating Free-Standing Piezoelectric Membranes

The use of a sacrificial water-soluble BaO layer was investigated for the purpose of fabricating free-standing single-crystalline BaTiO₃ membranes for vibrational energy harvesting applications. An epitaxial BaTiO₃/SrTiO₃/BaO heterostructure was deposited on a SrTiO₃(001) substrate by pulsed laser deposition. The sacrificial BaO layer was dissolved by immersing the heterostructure in water. A single-crystalline BaTiO₃ membrane was thereby released from the SrTiO₃(001) substrate and transferred onto a glass or polyethylene terephthalate (PET) substrate coated with a polydimethylsiloxane (PDMS) adhesion layer. A 200 nm-thick BaTiO₃ membrane was sandwiched between two flexible PDMS/ITO/PET electrode layers, forming a vibrational energy harvester test device that could operate at an average electrical power output of over 1 μW. This membrane fabrication process is applicable to various flexible structures and materials in vibrational energy harvesting or dynamic strain-sensing applications.

Flexible Electrode by Hydrographic Printing for Surface Electromyography Monitoring

Surface electromyography (sEMG) monitoring has recently inspired new applications in the field of patient diagnose, rehabilitation therapy, man–machine–interface and prosthesis control. However, conventional wet electrodes for sEMG recording cannot fully satisfy the requirements of these applications because they are based on rigid metals and conductive gels that cause signal quality attenuation, motion artifact and skin allergy. In this study, a novel flexible dry electrode is presented for sEMG monitoring. The electrode is fabricated by screen-printing a silver–eutectic gallium–indium system over a transfer tattoo paper, which is then hydrographically printed on 3D surface or human skin. Peano curve in open-network pattern is adopted to enhance the mechanics of the electrode. Hydrographic printing enables the electrode to attach to skin intimately and conformably, meanwhile assures better mechanical and electrical properties and therefore improves the signal quality and long-term wearability of the electrode. By recording sEMG signal of biceps under three kinds of movement with comparison to conventional wet electrode, the feasibility of the presented flexible dry electrode for sEMG monitoring was proved.