Microflotronics: A Flexible,Transparent, Pressure‐Sensitive Microfluidic Film

There is an increasing demand for sensitive, flexible, and low‐cost pressure sensing solutions for health monitoring, wearable sensing, robotic and prosthetic applications. Here, the first flexible and pressure‐sensitive microfluidic film is reported, referred to as a microflotronic, with high transparency and seamless integratability with the state‐of‐the‐art microelectronics. The microflotronic film represents the initial effort to utilize a continuous microfluidic layer as the sensing elements for large‐area dynamic pressure mapping applications, and meanwhile an ultrahigh sensitivity of 0.45 kPa^?1 has been achieved in a compact, flexible, and transparent packaging. The response time of the device is in the millisecond range, which is at least an order of magnitude faster than that of its conventional flexible solid‐state counterparts. In addition, the fabrication process of the device is fully compatible with the industrial‐scale manufacturing of capacitive touchscreen devices and liquid‐crystal displays. The overall device packaging can be as thin as 200 μm with an optical transparency greater than 80%. Several practical applications were successfully demonstrated, including surface topology mapping and dynamic blood pressure monitoring. The microflotronic devices offer an alternative approach to the solid‐state pressure sensors, by offering an unprecedented sensitivity and ultrafast response time in a completely transparent, flexible and adaptive platform.     

Extraordinarily Sensitive and Low‐Voltage Operational Cloth‐Based Electronic Skin for Wearable Sensing and Multifunctional Integration Uses: A Tactile‐Induced Insulating‐to‐Conducting Transition

Electronic skin sensing devices are an emerging technology and have substantial demand in vast practical fields including wearable sensing, robotics, and user‐interactive interfaces. In order to imitate or even outperform the capabilities of natural skin, the keen exploration of materials, device structures, and new functions is desired. However, the very high resistance and the inadequate current switching and sensitivity of reported electronic skins hinder to further develop and explore the promising uses of the emerging sensing devices. Here, a novel resistive cloth‐based skin‐like sensor device is reported that possesses unprecedented features including ultrahigh current‐switching behavior of ≈10⁷ and giant high sensitivity of 1.04 × 10⁴–6.57 × 10⁶ kPa^?1 in a low‐pressure region of <3 kPa. Notably, both superior features can be achieved by a very low working voltage of 0.1 V. Taking these remarkable traits, the device not only exhibits excellent sensing abilities to various mechanical forces, meeting various applications required for skin‐like sensors, but also demonstrates a unique competence to facile integration with other functional devices for various purposes with ultrasensitive capabilities. Therefore, the new methodologies presented here enable to greatly enlarge and advance the development of versatile electronic skin applications.     

Flexible Electronics and Materials for Synchronized Stimulation and Monitoring in Multi‐Encephalic Regions

Simultaneous neuron stimulation and biophysiological sensing in multi‐encephalic regions can lead to profound understanding of neural pathways, neurotransmitter transportation, and nutrient metabolism. Here, a flexible electronic device with tentacle‐like channels radiating from a central wireless circuit is presented. The device is constructed by different organic and inorganic materials that have been made into thin‐film or nanoparticle formats. All channels have been equipped with flexible components for distributed and synchronized opto‐electrical stimulation, biopotential sensing, and ion concentration monitoring. They can be implanted into different brain regions through adaptive bending and individually addressed to follow programmable working sequences. Experimental results conducted in vitro and in vivo have demonstrated the capability in generating optical or electrical stimulation, while sensing 16‐channels biopotential and concentration of Ca²⁺, Na⁺, and K⁺ ions in distributed regions. Behavior and immunohistochemistry studies suggest potential applications in regulating brain functions for freely moving animals. In combination with various functional materials, the device can serve as a comprehensive research platform that can be modularized to accommodate different needs for brain studies, offering numerous possibilities and combinations to yield sophisticated neuromodulation and behavior regulation.     

Fabrication of Releasable Single‐Crystal Silicon–Metal Oxide Field‐Effect Devices and Their Deterministic Assembly on Foreign Substrates

A new class of thin, releasable single‐crystal silicon semiconductor device is presented that enables integration of high‐performance electronics on nearly any type of substrate. Fully formed metal oxide–semiconductor field–effect transistors with thermally grown gate oxides and integrated circuits constructed with them demonstrate the ideas in devices mounted on substrates ranging from flexible sheets of plastic, to plates of glass and pieces of aluminum foil. Systematic study of the electrical properties indicates field‐effect mobilities of ≈710 cm² V^?1 s^?1, subthreshold slopes of less than 0.2 V decade^?1 and minimal hysteresis, all with little to no dependence on the properties of the substrate due to bottom silicon surfaces that are passivated with thermal oxide. The schemes reported here require only interconnect metallization to be performed on the final device substrate, which thereby minimizes the need for any specialized processing technology, with important consequences in large‐area electronics for display systems, flexible/stretchable electronics, or other non‐wafer‐based devices.     

Carbon‐Stabilized High‐Capacity Ferroferric Oxide Nanorod Array for Flexible Solid‐State Alkaline Battery–Supercapacitor Hybrid Device with High Environmental Suitability

Iron oxides are promising to be utilized in rechargeable alkaline battery with high capacity upon complete redox reaction (Fe³⁺ Fe⁰). However, their practical application has been hampered by the poor structural stability during cycling, presenting a challenge that is particularly huge when binder‐free electrode is employed. This paper proposes a “carbon shell‐protection” solution and reports on a ferroferric oxide–carbon (Fe₃O₄–C) binder‐free nanorod array anode exhibiting much improved cyclic stability (from only hundreds of times to >5000 times), excellent rate performance, and a high capacity of ≈7776.36 C cm^?3 (≈0.4278 C cm^?2; 247.5 mAh g^?1, 71.4% of the theoretical value) in alkaline electrolyte. Furthermore, by pairing with a capacitive carbon nanotubes (CNTs) film cathode, a unique flexible solid‐state rechargeable alkaline battery‐supercapacitor hybrid device (≈360 μm thickness) is assembled. It delivers high energy and power densities (1.56 mWh cm^?3; 0.48 W cm^?3/≈4.8 s charging), surpassing many recently reported flexible supercapacitors. The highest energy density value even approaches that of Li thin‐film batteries and is about several times that of the commercial 5.5 V/100 mF supercapacitor. In particular, the hybrid device still maintains good electrochemical attributes in cases of substantially bending, high mechanical pressure, and elevated temperature (up to 80 °C), demonstrating high environmental suitability.     

Skin‐Like Oxide Thin‐Film Transistors for Transparent Displays

Flexible transparent display is a promising candidate to visually communicate with each other in the future Internet of Things era. The flexible oxide thin‐film transistors (TFTs) have attracted attention as a component for transparent display by its high performance and high transparency. The critical issue of flexible oxide TFTs for practical display applications, however, is the realization on transparent and flexible substrate without any damage and characteristic degradation. Here, the ultrathin, flexible, and transparent oxide TFTs for skin‐like displays are demonstrated on an ultrathin flexible substrate using an inorganic‐based laser liftoff process. In this way, skin‐like ultrathin oxide TFTs are conformally attached onto various fabrics and human skin surface without any structural damage. Ultrathin flexible transparent oxide TFTs show high optical transparency of 83% and mobility of 40 cm² V^?1 s^?1. The skin‐like oxide TFTs show reliable performance under the electrical/optical stress tests and mechanical bending tests due to advanced device materials and systematic mechanical designs. Moreover, skin‐like oxide logic inverter circuits composed of n‐channel metal oxide semiconductor TFTs on ultrathin, transparent polyethylene terephthalate film have been realized.     

Fabrication of a Large‐Area Al‐Doped ZnO Nanowire Array Photosensor with Enhanced Photoresponse by Straining

The photosensing properties of flexible large‐area nanowire (NW)‐based photosensors are enhanced via in situ Al doping and substrate straining. A method for efficiently making nanodevices incorporating laterally doped NWs is developed and the strain‐dependent photoresponse is investigated. Photosensors are fabricated by directly growing horizontal single‐crystalline Al‐doped ZnO NW arrays across Au microelectrodes patterned on a flexible SiO₂/steel substrate to enhance the transportation of carriers and the junction between NWs and electrodes. The Raman spectrum of the Al:ZnO NWs, which have an average diameter and maximum length of around 40 nm and 6.8 μm, respectively, shows an Al‐related peak at 651 cm^?1. The device shows excellent photosensing properties with a high ultraviolet/visible rejection ratio, as well as extremely high maximum photoresponsivity and sensitivity at a low bias. Increasing the tensile strain from 0 to 5.6% linearly enhances the photoresponsivity from 1.7 to 3.8 AW^?1 at a bias of 1 V, which is attributed to a decrease in the Schottky barrier height resulting from a piezo‐photonic effect. The high‐performance flexible NW device presented here has applications in coupling measurements of light and strain in a flexible photoelectronic nanodevice and can aid in the development of better flexible and integrated photoelectronic systems.     

Engineering MoS2 Nanosheets on Spindle‐Like α‐Fe2O3 as High‐Performance Core–Shell Pseudocapacitive Anodes for Fiber‐Shaped Aqueous Lithium‐Ion Capacitors

Fiber‐shaped aqueous lithium‐ion capacitors (FALICs) featured with high energy and power densities together with outstanding safety characteristics are emerging as promising electrochemical energy‐storage devices for future portable and wearable electronics. However, the lack of high‐capacitance fibrous anodes is a major bottleneck to achieve high performance FALICs. Here, hierarchical MoS₂@α‐Fe₂O₃ core–shell heterostructures consisting of spindle‐shaped α‐Fe₂O₃ cores and MoS₂ nanosheet shells on a carbon nanotube fiber (CNTF) are successfully fabricated. Originating from the unique core/shell architecture and prominent synergetic effects for multi‐components, the resulting MoS₂@α‐Fe₂O₃/CNTF anode delivers a remarkable specific capacitance of 2077.5 mF cm^?2 (554.0 F cm^?3) at 2 mA cm^?2, substantially outperforming most of the previously reported fibrous anode materials. Further density functional theory calculations reveal that the MoS₂@α‐Fe₂O₃ nano‐heterostructure possesses better electrical conductivity and stronger adsorption energy of Li⁺ than those of the individual MoS₂ and α‐Fe₂O₃. By paring with the self‐standing LiCoO₂/CNTF battery‐type cathode, a prototype quasi‐solid‐state FALIC with a maximum operating voltage of 2.0 V is constructed, achieving impressive specific capacitance (253.1 mF cm^?2) and admirable energy density (39.6 mWh cm^?3). Additionally, the newly developed FALICs can be woven into the flexible textile to power wearable electronics. This work presents a novel effective strategy to design high‐performance anode materials for next‐generation wearable ALICs.     

Solution‐Processable,High‐Performance Flexible Electroluminescent Devices Based on High‐k Nanodielectrics

Flexible alternating‐current electroluminescent (ACEL) devices have attracted considerable attention for their ability to produce uniform light emission under bent conditions and have enormous potential for applications in back lighting panels, decorative lighting in automobiles, and panel displays. Nevertheless, flexible ACEL devices generally require a high operating bias, which precludes their implementation in low power devices. Herein, solution‐processed La‐doped barium titanate (BTO:La) nanocuboids (≈150 nm) are presented as high dielectric constant (high‐k) nanodielectrics, which can enhance the dielectric constant of an ACEL device from 2.6 to 21 (at 1 kHz), enabling the fabrication of high‐performance flexible ACEL devices with a lower operating voltage as well as higher brightness (≈57.54 cd m^?2 at 240 V, 1 kHz) than devices using undoped BTO nanodielectrics (≈14.3 cd m^?2 at 240 V, 1 kHz). Furthermore, a uniform brightness across the whole panel surface of the flexible ACEL devices and excellent device reliability are achieved via the use of uniform networks of crossaligned silver nanowires as highly conductive and flexible electrodes. The results offer experimental validation of high‐brightness flexible ACELs using solution‐processed BTO:La nanodielectrics, which constitutes an important milestone toward the implementation of high‐k nanodielectrics in flexible displays.     

High‐Performance Flexible Multilayer MoS2 Transistors on Solution‐Based Polyimide Substrates

Transition metal dichalcogenides (TMDs) layers of molecular thickness, in particular molybdenum disulfide (MoS₂), become increasingly important as active elements for mechanically flexible/stretchable electronics owing to their relatively high carrier mobility, wide bandgap, and mechanical flexibility. Although the superior electronic properties of TMD transistors are usually integrated into rigid silicon wafers or glass substrates, the achievement of similar device performance on flexible substrates remains quite a challenge. The present work successfully addresses this challenge by a novel process architecture consisting of a solution‐based polyimide (PI) flexible substrate in which laser‐welded silver nanowires are embedded, a hybrid organic/inorganic gate insulator, and multilayers of MoS₂. Transistors fabricated according to this process scheme have decent properties: a field‐effect‐mobility as high as 141 cm² V^?1 s^?1 and an I_on/I_off ratio as high as 5 × 10⁵. Furthermore, no apparent degradation in the device properties is observed under systematic cyclic bending tests with bending radii of 10 and 5 mm. Overall electrical and mechanical results provide potentially important applications in the fabrication of versatile areas of flexible integrated circuitry.     

Microchannel‐Confined MXene Based Flexible Piezoresistive Multifunctional Micro‐Force Sensor

Multifunctional micro‐force sensing in one device is an urgent need for the higher integration of the smaller flexible electronic device toward wearable health‐monitoring equipment, intelligent robotics, and efficient human–machine interface. Herein, a novel microchannel‐confined MXene‐based flexible piezoresistive sensor is demonstrated to simultaneously achieve multi‐types micro‐force sensing of pressure, sound, and acceleration. Benefiting from the synergistically confined effect of the fingerprint‐microstructured channel and the accordion‐microstructured MXene materials, the as‐designed sensor remarkably endows a low detection limit of 9 Pa, a high sensitivity of 99.5 kPa^?1, and a fast response time of 4 ms, as well as non‐attenuating durability over 10 000 cycles. Moreover, the fabricated sensor is multifunctionally capable of sensing sounds, micromotion, and acceleration in one device. Evidently, such a multifunctional sensing characteristic can highlight the bright prospect of the microchannel‐confined MXene‐based micro‐force sensor for the higher integration of flexible electronics.     

Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes

The first reduction methodology, compatible with flexible, temperature‐sensitive substrates, for the production of reduced spin‐coated graphene oxide (GO) electrodes is reported. It is based on the use of a laser beam for the in situ, non‐thermal, reduction of spin‐coated GO films on flexible substrates over a large area. The photoreduction process is one‐step, facile, and is rapidly carried out at room temperature in air without affecting the integrity of the graphene lattice or the flexibility of the underlying substrate. Conductive graphene films with a sheet resistance of as low as 700 Ω sq^?1 and transmittance of 44% can be obtained, much higher than can be achieved for flexible layers reduced by chemical means. As a proof of concept of our technique, laser‐reduced GO (LrGO) films are utilized as transparent electrodes in flexible, bulk heterojunction, organic photovoltaic (OPV) devices, replacing the traditional ITO. The devices displayed a power‐conversion efficiency of 1.1%, which is the highest reported so far for OPV device incorporating reduced GO as the transparent electrode. The in situ non‐thermal photoreduction of spin‐coated GO films creates a new way to produce flexible functional graphene electrodes for a variety of electronic applications in a process that carries substantial promise for industrial implementation.     

Liquid‐Gated Transistors Based on Reduced Graphene Oxide for Flexible and Wearable Electronics

Graphene is regarded as the ultimate material for future flexible, high‐performance, and wearable electronics. Herein, a novel, robust, all‐green, highly reliable (yield ≥ 99%), and upscalable technology is reported for wearable applications comprising reduced graphene oxide (rGO) as the electroactive component in liquid‐gated transistors (LGTs). rGO is a formidable material for future flexible and wearable applications due to its easy processability, excellent surface reactivity, and large‐area coverage. A novel protocol is established toward the high‐yield fabrication of flexible rGO LGTs combining high robustness (>1.5 h of continuous operation) with state‐of‐the‐art performances, being similar to those of their rigid counterparts operated under liquid gating, including field‐effect mobility of ≈10^?1 cm² V^?1 s^?1 and transconductance of ≈25 µS. Permeable membranes have been proven crucial to operate flexible LGTs under mechanical stress with reduced amounts of solution (<20 µL). Our rGO LGTs are operated in artificial sweat exploiting two different layouts based on lateral‐flow paper fluidics. These approaches pave the road toward future real‐time tracking of perspiration via a simple and cost‐effective approach. The reported findings contribute to the robust and scalable production of novel graphene‐based flexible devices, whose features fulfill the requirements of wearable electronics.     

Reusable Cellulose‐Based Hydrogel Sticker Film Applied as Gate Dielectric in Paper Electrolyte‐Gated Transistors

A new concept for reusable eco‐friendly hydrogel electrolytes based on cellulose is introduced. The reported electrolytes are designed and engineered through a simple, fast, low‐cost, and eco‐friendly dissolution method of microcrystalline cellulose at low temperature using an aqueous LiOH/urea solvent system. The cellulose solution is combined with carboxymethyl cellulose, followed by the regeneration and simultaneous ion incorporation. The produced free standing cellulose‐based electrolyte films exhibit interesting properties for application in flexible electrochemical devices, such as biosensors or electrolyte‐gated transistors (EGTs), because of their high specific capacitances (4–5 µF cm^?2), transparency, and flexibility. Indium–gallium–zinc‐oxide EGTs on glass with laminated cellulose‐based hydrogel electrolytes (CHEs) as the gate dielectric are produced presenting a low working voltage (<2 V), showing an on–off current ratio (I _on/off) of 10⁶, a subthreshold swing lower than 0.2 V dec^?1, and saturation mobility (μ_Sat) reaching 26 cm² V^?1 s^?1. The flexible CHE‐gated transistors on paper are also demonstrated, which operate at switching frequencies up to 100 Hz. Combining the flexibility of the EGTs on paper with the reusability of the developed CHEs is a breakthrough toward biodegradable advanced functional materials allied with disposable/recyclable and low‐cost electronic devices.     

“Regioselective Deposition” Method to Pattern Silver Electrodes Facilely and Efficiently with High Resolution: Towards All‐Solution‐Processed,High‐Performance,Bottom‐Contacted,Flexible, Polymer‐Based Electronics

“Regioselectivity deposition” method is developed to pattern silver electrodes facilely and efficiently by solution‐process with high resolution (down to 2 μm) on different substrates in A4 paper size. With the help of this method, large‐area, flexible, high‐performance polymer field‐effect transistors based on the silver electrodes and polyimide insulator are fabricated with bottom‐contact configuration by all‐solution processes. The polymer devices exhibit high performance with average field‐effect mobility over 1.0 cm² V^?1 s^?1 (the highest mobility up to 1.5 cm² V^?1 s^?1) and excellent environmental stability and flexibility, indicating the cost effectiveness of this method for practical applications in organic electronics.     

Flexible Sodium‐Ion Pseudocapacitors Based on 3D Na2Ti3O7 Nanosheet Arrays/Carbon Textiles Anodes

Flexible energy storage devices are critical components for emerging flexible and wearable electronics. Improving the electrochemical performance of flexible energy storage devices depends largely on development of novel electrode architectures and new systems. Here, a new class of flexible energy storage device called flexible sodium‐ion pseudocapacitors is developed based on 3D‐flexible Na₂Ti₃O₇ nanosheet arrays/carbon textiles (NTO/CT) as anode and flexible reduced graphene oxide film (GFs) as cathode without metal current collectors or conducting additives. The NTO/CT anode with advanced electrode architectures is fabricated by directly growing Na₂Ti₃O₇ nanosheet arrays on carbon textiles with robust adhesion through a simple hydrothermal process. The flexible GF//NTO/CT configuration achieves a high energy density of 55 Wh kg^?1 and high power density of 3000 W kg^?1. Taking the fully packaged flexible sodium‐ion pseudocapacitors into consideration, the maximum practical volumetric energy density and power density reach up to 1.3 mWh cm^?3 and 70 mW cm^?3, respectively. In addition, the flexible GF//NTO/CT device demonstrates a stable electrochemical performances with almost 100% capacitance retention under harsh mechanical deformation.     

Room‐Temperature Printing of Organic Thin‐Film Transistors with π‐Junction Gold Nanoparticles

Printing semiconductor devices under ambient atmospheric conditions is a promising method for the large‐area, low‐cost fabrication of flexible electronic products. However, processes conducted at temperatures greater than 150 °C are typically used for printed electronics, which prevents the use of common flexible substrates because of the distortion caused by heat. The present report describes a method for the room‐temperature printing of electronics, which allows thin‐film electronic devices to be printed at room temperature without the application of heat. The development of π‐junction gold nanoparticles as the electrode material permits the room‐temperature deposition of a conductive metal layer. Room‐temperature patterning methods are also developed for the Au ink electrodes and an active organic semiconductor layer, which enables the fabrication of organic thin‐film transistors through room‐temperature printing. The transistor devices printed at room temperature exhibit average field‐effect mobilities of 7.9 and 2.5 cm² V^?1 s^?1 on plastic and paper substrates, respectively. These results suggest that this fabrication method is very promising as a core technology for low‐cost and high‐performance printed electronics.     

The Design and Realization of Flexible,Long‐Lived Light‐Emitting Electrochemical Cells

Polymer light‐emitting electrochemical cells (LECs) offer an attractive opportunity for low‐cost production of functional devices in flexible and large‐area configurations, but the critical drawback in comparison to competing light‐emission technologies is a limited operational lifetime. Here, it is demonstrated that it is possible to improve the lifetime by straightforward and motivated means from a typical value of a few hours to more than one month of uninterrupted operation at significant brightness (>100 cd m^?2) and relatively high power conversion efficiency (2 lm W^?1 for orange‐red emission). Specifically, by optimizing the composition of the active material and by employing an appropriate operational protocol, a desired doping structure is designed and detrimental chemical and electrochemical side reactions are identified and minimized. Moreover, the first functional flexible LEC with a similar promising device performance is demonstrated.     

Light‐Emitting Paper

A solution‐based fabrication of flexible and light‐weight light‐emitting devices on paper substrates is reported. Two different types of paper substrates are coated with a surface‐emitting light‐emitting electrochemical cell (LEC) device: a multilayer‐coated specialty paper with an intermediate surface roughness of 0.4 μm and a low‐end and low‐cost copy paper with a large surface roughness of 5 μm. The entire device fabrication is executed using a handheld airbrush, and it is notable that all of the constituent layers are deposited from solution under ambient air. The top‐emitting paper‐LECs are highly flexible, and display a uniform light emission with a luminance of 200 cd m^?2 at a current conversion efficacy of 1.4 cd A^?1.     

Cellulose‐Based Ionogels for Paper Electronics

A new class of biofriendly ionogels produced by gelation of microcellulose thin films with tailored 1‐ethyl‐3‐methylimidazolium methylphosphonate ionic liquids are demonstrated. The cellulose ionogels show promising properties for application in flexible electronics, such as transparency, flexibility, transferability, and high specific capacitances of 5 to 15 μF cm^?2. They can be laminated onto any substrate such as multilayer‐coated paper and act as high capacitance dielectrics for inorganic (spray‐coated ZnO and colloidal ZnO nanorods) and organic (poly[3‐hexylthiophene], P3HT) electrolyte‐gated field‐effect transistors (FETs), that operate at very low voltages (<2 V). Field‐effect mobilities in ionogel‐gated spray‐coated ZnO FETs reach 75 cm² V^?1 s^?1 and a typical increase of mobility with decreasing specific capacitance of the ionogel is observed. Solution‐processed, colloidal ZnO nanorods and laminated cellulose ionogels enable the fabrication of the first electrolyte‐gated, flexible circuits on paper, which operate at bending radii down to 1.1 mm.