Biodegradable Natural Pectin‐Based Flexible Multilevel Resistive Switching Memory for Transient Electronics

Transient electronics that can physically vanish in solution can offer opportunities to address the ecological challenges for dealing with the rapidly growing electronic waste. As one important component, it is desirable that memory devices combined with the transient feature can also be developed as secrecy information storage systems besides the above advantage. Resistive switching (RS) memory is one of the most promising technologies for next‐generation memory. Herein, the biocompatible pectin extracted from natural orange peel is introduced to fabricate RS memory devices (Ag/pectin/indium tin oxides (ITO)), which exhibit excellent RS characteristics, such as forming free characteristic, low operating voltages (≈1.1 V), fast switching speed (<70 ns), long retention time (>10⁴ s), and multilevel RS behaviors. The device performance is not degraded after 10⁴ bending cycles, which will be beneficial for flexible memory applications. Additionally, instead of using acid solution, the Ag/pectin/ITO memory device can be dissolved rapidly in deionized water within 10 min thanks to the good solubility arising from ionization of its carboxylic groups, which shows promising application for green electronics. The present biocompatible memory devices based on natural pectin suggest promising material candidates toward enabling high‐density secure information storage systems applications, flexible electronics, and green electronics.     

Bioinspired Flexible Volatile Organic Compounds Sensor Based on Dynamic Surface Wrinkling with Dual‐Signal Response

Living systems can respond to external stimuli by dynamic interface changes. Moreover, natural wrinkle structures allow the surface to switch dynamically and reversibly from flat to rough in response to specific stimuli. Artificial wrinkle structures have been developed for applications such as optical devices, mechanical sensors, and microfluidic devices. However, chemical molecule‐triggered flexible sensors based on dynamic surface wrinkling have not been demonstrated. Inspired by human skin wrinkling, herein, a volatile organic compound (VOC)‐responsive flexible sensor with a switchable dual‐signal response (transparency and resistance) is achieved based on a multilayered Ag nanowire (AgNW)/SiO_x/polydimethylsiloxane (PDMS) film. Wrinkle structures can form dynamically in response to VOC vapors (such as ethanol, toluene, acetone, formaldehyde, and methanol) due to the instability of the multilayer induced by their different swelling capabilities. By controlling the modulus of PDMS and the thickness of the SiO_x layer, tunable sensitivities in resistance and transparency of the device are achieved. Additionally, the proximity mechanism of the solubility parameter is proposed, which explains the high selectivity of the device toward ethanol vapor compared with that of other VOCs well. This stimuli‐responsive sensor exhibits the dynamic visual feedback and the quantitative electrical signal, which provide a novel approach for developing smart flexible electronics.     

Ultrastretchable and Wireless Bioelectronics Based on All‐Hydrogel Microfluidics

Hydrogel bioelectronics that can interface biological tissues and flexible electronics is at the core of the growing field of healthcare monitoring, smart drug systems, and wearable and implantable devices. Here, a simple strategy is demonstrated to prototype all‐hydrogel bioelectronics with embedded arbitrary conductive networks using tough hydrogels and liquid metal. Due to their excellent stretchability, the resultant all‐hydrogel bioelectronics exhibits stable electrochemical properties at large tensile stretch and various modes of deformation. The potential of fabricated all‐hydrogel bioelectronics is demonstrated as wearable strain sensors, cardiac patches, and near‐field communication (NFC) devices for monitoring various physiological conditions wirelessly. The presented simple platform paves the way of implantable hydrogel electronics for Internet‐of‐Things and tissue–machine interfacing applications.     

Plasticizing Silk Protein for On‐Skin Stretchable Electrodes

Soft and stretchable electronic devices are important in wearable and implantable applications because of the high skin conformability. Due to the natural biocompatibility and biodegradability, silk protein is one of the ideal platforms for wearable electronic devices. However, the realization of skin‐conformable electronic devices based on silk has been limited by the mechanical mismatch with skin, and the difficulty in integrating stretchable electronics. Here, silk protein is used as the substrate for soft and stretchable on‐skin electronics. The original high Young's modulus (5–12 GPa) and low stretchability (<20%) are tuned into 0.1–2 MPa and > 400%, respectively. This plasticization is realized by the addition of CaCl₂ and ambient hydration, whose mechanism is further investigated by molecular dynamics simulations. Moreover, highly stretchable (>100%) electrodes are obtained by the thin‐film metallization and the formation of wrinkled structures after ambient hydration. Finally, the plasticized silk electrodes, with the high electrical performance and skin conformability, achieve on‐skin electrophysiological recording comparable to that by commercial gel electrodes. The proposed skin‐conformable electronics based on biomaterials will pave the way for the harmonized integration of electronics into human.     

Stretchable Thin‐Film Electrodes for Flexible Electronics with High Deformability and Stretchability

Flexible and stretchable electronics represent today's cutting‐edge electronic technologies. As the most‐fundamental component of electronics, the thin‐film electrode remains the research frontier due to its key role in the successful development of flexible and stretchable electronic devices. Stretchability, however, is generally more challenging to achieve than flexibility. Stretchable electronic devices demand, above all else, that the thin‐film electrodes have the capacity to absorb a large level of strain (>>1%) without obvious changes in their electrical performance. This article reviews the progress in strategies for obtaining highly stretchable thin‐film electrodes. Applications of stretchable thin‐film electrodes fabricated via these strategies are described. Some perspectives and challenges in this field are also put forward.     

Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

Wearable electronics have revolutionized the way physiological parameters are sensed, detected, and monitored. In recent years, advances in flexible and stretchable hybrid electronics have created emergent properties that enhance the compliance of devices to our skin. With their unobtrusive attributes, skin conformable sensors enable applications toward real-time disease diagnosis and continuous healthcare monitoring. Herein, critical perspectives of flexible hybrid electronics toward the future of digital health monitoring are provided, emphasizing its role in physiological sensing. In particular, the strategies within the sensor composition to render flexibility and stretchability while maintaining excellent sensing performance are considered. Next, novel approaches to the functionalization of the sensor for physical or biochemical stimuli are extensively covered. Subsequently, wearable sensors measuring physical parameters such as strain, pressure, temperature, as well as biological changes in metabolites and electrolytes are reported. Finally, their implications toward early disease detection and monitoring are discussed, concluding with a future perspective into the challenges and opportunities in emerging wearable sensor designs for the next few years.     

Flexible Sensing Electronics for Wearable/Attachable Health Monitoring

Wearable or attachable health monitoring smart systems are considered to be the next generation of personal portable devices for remote medicine practices. Smart flexible sensing electronics are components crucial in endowing health monitoring systems with the capability of real‐time tracking of physiological signals. These signals are closely associated with body conditions, such as heart rate, wrist pulse, body temperature, blood/intraocular pressure and blood/sweat bio‐information. Monitoring such physiological signals provides a convenient and non‐invasive way for disease diagnoses and health assessments. This Review summarizes the recent progress of flexible sensing electronics for their use in wearable/attachable health monitoring systems. Meanwhile, we present an overview of different materials and configurations for flexible sensors, including piezo‐resistive, piezo‐electrical, capacitive, and field effect transistor based devices, and analyze the working principles in monitoring physiological signals. In addition, the future perspectives of wearable healthcare systems and the technical demands on their commercialization are briefly discussed.     

Flexible Transparent Electronic Gas Sensors

Flexible and transparent electronic gas sensors capable of real‐time, sensitive, and selective analysis at room‐temperature, have gained immense popularity in recent years for their potential to be integrated into various smart wearable electronics and display devices. Here, recent advances in flexible transparent sensors constructed from semiconducting oxides, carbon materials, conducting polymers, and their nanocomposites are presented. The sensing material selection, sensor device construction, and sensing mechanism of flexible transparent sensors are discussed in detail. The critical challenges and future development associated with flexible and transparent electronic gas sensors are presented. Smart wearable gas sensors are believed to have great potential in environmental monitoring and noninvasive health monitoring based on disease biomarkers in exhaled gas.     

High Sensitivity,Wearable, Piezoresistive Pressure Sensors Based on Irregular Microhump Structures and Its Applications in Body Motion Sensing

A pressure sensor based on irregular microhump patterns has been proposed and developed. The devices show high sensitivity and broad operating pressure regime while comparing with regular micropattern devices. Finite element analysis (FEA) is utilized to confirm the sensing mechanism and predict the performance of the pressure sensor based on the microhump structures. Silicon carbide sandpaper is employed as the mold to develop polydimethylsiloxane (PDMS) microhump patterns with various sizes. The active layer of the piezoresistive pressure sensor is developed by spin coating PEDOT:PSS on top of the patterned PDMS. The devices show an averaged sensitivity as high as 851 kPa^?1, broad operating pressure range (20 kPa), low operating power (100 nW), and fast response speed (6.7 kHz). Owing to their flexible properties, the devices are applied to human body motion sensing and radial artery pulse. These flexible high sensitivity devices show great potential in the next generation of smart sensors for robotics, real‐time health monitoring, and biomedical applications.     

Flexible Thermoelectric Materials and Generators: Challenges and Innovations

The urgent need for ecofriendly, stable, long‐lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer‐based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic‐based flexible thermoelectrics that have high energy‐conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state‐of‐the‐art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high‐performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.     

Wearable and Ultrasensitive Strain Sensor Based on High‐Quality GaN pn Junction Microwire Arrays

With the rapid growth in wearable electronics sensing devices, flexible sensing devices that monitor the human body have shown great promise in personalized healthcare. In the study, high‐quality GaN pn junction microwire arrays with different aspect ratios and large‐area uniformity are fabricated through an easy, repeatable fabrication process. The piezoelectric coefficient (d₃₃) of GaN pn junction microwire arrays increases from 7.23 to 14.46 pm V^?1 with the increasing of the aspect ratio, which is several times higher than that of GaN bulk material. Furthermore, flexible ultrasensitive strain sensor based on GaN microwires with the highest d₃₃ is demonstrated to achieve the maximum open circuit voltage of 10.4 V, and presents excellent durability with stable output signals over 10 000 cycles with a response time of 50 ms. As a flexible and wearable sensor attached to the human skin, the GaN microwire pn junction arrays with such a high degree of uniformity can precisely monitor subtle human pulse and motions, which show great promise in future personalized healthcare.     

Lithium Titanate Cuboid Arrays Grown on Carbon Fiber Cloth for High‐Rate Flexible Lithium‐Ion Batteries

High‐rate performance flexible lithium‐ion batteries are desirable for the realization of wearable electronics. The flexibility of the electrode in the battery is a key requirement for this technology. In the present work, spinel lithium titanate (Li₄Ti₅O₁₂, LTO) cuboid arrays are grown on flexible carbon fiber cloth (CFC) to fabricate a binder‐free composite electrode (LTO@CFC) for flexible lithium‐ion batteries. Experimental results show that the LTO@CFC electrode exhibits a remarkably high‐rate performance with a capacity of 105.8 mAh g^?1 at 50C and an excellent electrochemical stability against cycling (only 2.2% capacity loss after 1000 cycles at 10C). A flexible full cell fabricated with the LTO@CFC as the anode and LiNi_0.5Mn_1.5O₄ coated on Al foil as the cathode displays a reversible capacity of 109.1 mAh g^?1 at 10C, an excellent stability against cycling and a great mechanical stability against bending. The observed high‐rate performance of the LTO@CFC electrode is due to its unique corn‐like architecture with LTO cuboid arrays (corn kernels) grown on CFC (corn cob). This work presents a new approach to preparing LTO‐based composite electrodes with an architecture favorable for ion and electron transport for flexible energy storage devices.     

Highly Stretchable Metallic Nanowire Networks Reinforced by the Underlying Randomly Distributed Elastic Polymer Nanofibers via Interfacial Adhesion Improvement

On‐skin electronics require conductive, porous, and stretchable materials for a stable operation with minimal invasiveness to the human body. However, porous elastic conductors that simultaneously achieve high conductivity, good stretchability, and durability are rare owing to the lack of proper design for good adhesion between porous elastic polymer and conductive metallic networks. Here, a simple fabrication approach for porous nanomesh‐type elastic conductors is shown by designing a layer‐by‐layer structure of nanofibers/nanowires (NFs/NWs) via interfacial hydrogen bonding. The as‐prepared conductors, consisting of Ag NWs and polyurethane (PU) NFs, simultaneously achieve high conductivity (9190 S cm^?1), high stretchability (310%), and good durability (82% resistance increase after 1000 cycles of deformation at 70% tensile strain). The direct contact between the Ag NWs enables the high conductivity. The synergistic effect of the layer‐by‐layer structure and good adhesion between the Ag NWs and the PU NFs enables good mechanical properties. Furthermore, without any adhesive gel/tape, the conductors can be utilized as breathable strain sensors for precise joint motion monitoring, and as breathable sensing electrodes for continuous electrophysiological signal recording.     

Emerging Trends in 2D TMDs Photodetectors and Piezo-Phototronic Devices

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.     

An On-Skin Electrode with Anti-Epidermal-Surface-Lipid Function Based on a Zwitterionic Polymer Brush

On-skin flexible devices provide a noninvasive approach for continuous and real-time acquisition of biological signals from the skin, which is essential for future chronic disease diagnosis and smart health monitoring. Great progress has been achieved in flexible devices to resolve the mechanical mismatching between conventional rigid devices and human skin. However, common materials used for flexible devices including silicon-based elastomers and various metals exhibit no resistance to epidermal surface lipids (skin oil and grease), which restricts the long-term and household usability. Herein, an on-skin electrode with anti-epidermal-surface-lipid function is reported, which is based on the grafting of a zwitterionic poly(2-methacryl-oyloxyethyl, methacryloyl-oxyethyl, or meth-acryloyloxyethyl phosphorylcholine) (PMPC) brush on top of gold-coated poly(dimethylsiloxane) (Au/PDMS). Such an electrode allows the skin-lipids-fouled surface to be cleaned by simple water rinsing owing to the superhydrophilic zwitterionic groups. As a proof-of-concept, the PMPC-Au/PDMS electrodes are employed for both electrocardiography (ECG) and electromyography (EMG) recording. The electrodes are able to maintain stable skin-electrode impedance and good signal-to noise ratio (SNR) by water rinsing alone. This work provides a material-based solution to improve the long-term reusability of on-skin electronics and offers a unique prospective on developing next generation wearable healthcare devices.     

Hierarchical Synergistic Structure for High Resolution Strain Sensor with Wide Working Range

Strain sensors have been attracting tremendous attention for the promising application of wearable devices in recent years. However, the trade-off between high resolution, high sensitivity, and broad detection range is a great challenge for the application of strain sensors. Herein, a novel design of hierarchical synergistic structure (HSS) of Au micro cracks and carbon black (CB) nanoparticles is reported to overcome this challenge. The strain sensor based on the designed HSS exhibit high sensitivity (GF > 2400), high strain resolution (0.2%) even under large loading strain, broad detection range (>40%), outstanding stability (>12000 cycles), and fast response speed simultaneously. Further, the experiments and simulation results demonstrate that the carbon black layer greatly changed the morphology of Au micro-cracks, forming a hierarchical structure of micro-scale Au cracks and nano-scale carbon black particles, thus enabling synergistic effect and the double conductive network of Au micro-cracks and CB nanoparticles. Based on the excellent performance, the sensor is successfully applied to monitoring tiny signals of the carotid pulse during body movement, which illustrates the great potential in the application of health monitoring, human-machine interface, human motion detection, and electronic skin.     

Materials and Techniques for Implantable Nutrient Sensing Using Flexible Sensors Integrated with Metal–Organic Frameworks

The combination of novel materials with flexible electronic technology may yield new concepts of flexible electronic devices that effectively detect various biological chemicals to facilitate understanding of biological processes and conduct health monitoring. This paper demonstrates single‐ or multichannel implantable flexible sensors that are surface modified with conductive metal–organic frameworks (MOFs) such as copper‐MOF and cobalt‐MOF with large surface area, high porosity, and tunable catalysis capability. The sensors can monitor important nutriments such as ascorbicacid, glycine, l ‐tryptophan (l ‐Trp), and glucose with detection resolutions of 14.97, 0.71, 4.14, and 54.60 × 10^?6 m , respectively. In addition, they offer sensing capability even under extreme deformation and complex surrounding environment with continuous monitoring capability for 20 d due to minimized use of biological active chemicals. Experiments using live cells and animals indicate that the MOF‐modified sensors are biologically safe to cells, and can detect l ‐Trp in blood and interstitial fluid. This work represents the first effort in integrating MOFs with flexible sensors to achieve highly specific and sensitive implantable electrochemical detection and may inspire appearance of more flexible electronic devices with enhanced capability in sensing, energy storage, and catalysis using various properties of MOFs.     

Materials and Designs for Wearable Photodetectors

Photodetectors (PDs), as an indispensable component in electronics, are highly desired to be flexible to meet the trend of next‐generation wearable electronics. Unfortunately, no in‐depth reviews on the design strategies, material exploration, and potential applications of wearable photodetectors are found in literature to date. Thus, this progress report first summarizes the fundamental design principles of turning “hard” photodetectors “soft,” including 2D (polymer and paper substrate‐based devices) and 1D PDs (fiber shaped devices). In short, the flexibility of PDs is realized through elaborate substrate modification, material selection, and device layout. More importantly, this report presents the current progress and specific requirements for wearable PDs according to the application: monitoring, imaging, and optical communication. Challenges and future research directions in these fields are proposed at the end. The purpose of this progress report is not only to shed light on the basic design principles of wearable PDs, but also serve as the roadmap for future exploration in wearable PDs in various applications, including health monitoring and Internet of Things.     

Synthesis of P‐Doped and NiCo‐Hybridized Graphene‐Based Fibers for Flexible Asymmetrical Solid‐State Micro‐Energy Storage Device

Fiber supercapacitors (FSCs) are promising energy storage devices in portable and wearable smart electronics. Currently, a major challenge for FSCs is simultaneously achieving high volumetric energy and power densities. Herein, the microscale fiber electrode is designed by using carbon fibers as substrates and capillary channels as microreactors to space‐confined hydrothermal assembling. As P‐doped graphene oxide/carbon fiber (PGO/CF) and NiCo₂O₄‐based graphene oxide/carbon fiber (NCGO/CF) electrodes are successfully prepared, their unique hybrid structures exhibit a satisfactory electrochemical performance. An all‐solid‐state PGO/CF//NCGO/CF flexible asymmetric fiber supercapacitor (AFSC) based on the PGO/CF as the negative electrode, NCGO/CF hybrid electrode as the positive electrode, and poly(vinyl alcohol)/potassium hydroxide as the electrolyte is successfully assembled. The AFSC device delivers a higher volumetric energy density of 36.77 mW h cm^?3 at a power density of 142.5 mW cm^?3. In addition, a double reference electrode system is adopted to analyze and reduce the IR drop, as well as effectively matching negative and positive electrodes, which is conducive for the optimization and improvement of energy density. For the AFSC device, its better flexibility and electrochemical properties create a promising potential for high‐performance micro‐supercapacitors. Furthermore, the introduction of the double reference electrode system provides an interesting method for the study on the electrochemical performances of two‐electrode systems.     

All-MXene-Printed RF Resonators as Wireless Plant Wearable Sensors for In Situ Ethylene Detection

Printed flexible electronics have emerged as versatile functional components of wearable intelligent devices that bridge the digital information networks with biointerfaces. Recent endeavors in plant wearable sensors provide real-time and in situ insights to study phenotyping traits of crops, whereas monitoring of ethylene, the fundamental phytohormone, remains challenging due to the lack of flexible and scalable manufacturing of plant wearable ethylene sensors. Here the all-MXene-printed flexible radio frequency (RF) resonators are presented as plant wearable sensors for wireless ethylene detection. The facile formation of additive-free MXene ink enables rapid, scalable manufacturing of printed electronics, demonstrating decent printing resolution (2.5% variation), ≈30000 S m⁻¹ conductivity and mechanical robustness. Incorporation of MXene-reduced palladium nanoparticles (MXene@PdNPs) facilitates 1.16% ethylene response at 1 ppm with 0.084 ppm limit of detection. The wireless sensor tags are attached on plant organ surfaces for in situ and continuously profiling of plant ethylene emission to inform the key transition of plant biochemistry, potentially extending the application of printed MXene electronics to enable real-time plant hormone monitoring for precision agriculture and food industrial management.