A Biodegradable and Stretchable Protein‐Based Sensor as Artificial Electronic Skin for Human Motion Detection

Due to the natural biodegradability and biocompatibility, silk fibroin (SF) is one of the ideal platforms for on‐skin and implantable electronic devices. However, the development of SF‐based electronics is still at a preliminary stage due to the SF film intrinsic brittleness as well as the solubility in water, which prevent the fabrication of SF‐based electronics through traditional techniques. In this article, a flexible and stretchable silver nanofibers (Ag NFs)/SF based electrode is synthesized through water‐free procedures, which demonstrates outstanding performance, i.e., low sheet resistance (10.5 Ω sq^?1), high transmittance (>90%), excellent stability even after bending cycles >2200 times, and good extensibility (>60% stretching). In addition, on the basis of such advanced (Ag NFs)/SF electrode, a flexible and tactile sensor is further fabricated, which can simultaneously detect pressure and strain signals with a large monitoring window (35 Pa–700 kPa). Besides, this sensor is air‐permeable and inflammation‐free, so that it can be directly laminated onto human skins for long‐term health monitoring. Considering the biodegradable and skin‐comfortable features, this sensor may become promising to find potential applications in on‐skin or implantable health‐monitoring devices.     

An All‐Freeze‐Casting Strategy to Design Typographical Supercapacitors with Integrated Architectures

The emergence of flexible and wearable electronics has raised the demand for flexible supercapacitors with accurate sizes and aesthetic shapes. Here, a strategy is developed to prepare flexible all‐in‐one integrated supercapacitors by combining all‐freeze‐casting with typography technique. The continuous seamless connection of all‐in‐one supercapacitor devices enhances the load and/or electron transfer capacity and avoids displacing and detaching between their neighboring components at bending status. Therefore, such a unique structure of all‐in‐one integrated devices is beneficial for retaining stable electrochemical performance at different bending levels. More importantly, the sizes and aesthetic shapes of integrated supercapacitors could be controlled by the designed molds, like type matrices of typography. The molds could be assembled together and typeset randomly, achieving the controllable construction and series and/or parallel connection of several supercapacitor devices. The preparation of flexible integrated supercapacitors will pave the way for assembling programmable all‐in‐one energy storage devices into highly flexible electronics.     

Flexible Zn‐Ion Batteries: Recent Progresses and Challenges

To keep pace with the increasing pursuit of portable and wearable electronics, it is urgent to develop advanced flexible power supplies. In this context, Zn‐ion batteries (ZIBs) have garnered increasing attention as favorable energy storage devices for flexible electronics, owing to the high capacity, low cost, abundant resources, high safety, and eco‐friendliness. Extensive efforts have been devoted to developing flexible ZIBs in the last few years. This work summarizes the recent achievements in the design, fabrication, and characterization of flexible ZIBs. Representative structures, such as sandwich and cable type, are particularly highlighted. Special emphasis is put on the novel design of electrolyte and electrode, which aims to endow reliable flexibility to the fabricated ZIBs. Moreover, current challenges and future opportunities for the development of high‐performance flexible ZIBs are also outlined.     

All‐Solution‐Processed Metal‐Oxide‐Free Flexible Organic Solar Cells with Over 10% Efficiency

All‐solution‐processing at low temperatures is important and desirable for making printed photovoltaic devices and also offers the possibility of a safe and cost‐effective fabrication environment for the devices. Herein, an all‐solution‐processed flexible organic solar cell (OSC) using poly(3,4‐ethylenedioxythiophene):poly‐(styrenesulfonate) electrodes is reported. The all‐solution‐processed flexible devices yield the highest power conversion efficiency of 10.12% with high fill factor of over 70%, which is the highest value for metal‐oxide‐free flexible OSCs reported so far. The enhanced performance is attributed to the newly developed gentle acid treatment at room temperature that enables a high‐performance PEDOT:PSS/plastic underlying substrate with a matched work function (≈4.91 eV), and the interface engineering that endows the devices with better interface contacts and improved hole mobility. Furthermore, the flexible devices exhibit an excellent mechanical flexibility, as indicated by a high retention (≈94%) of the initial efficiency after 1000 bending cycles. This work provides a simple route to fabricate high‐performance all‐solution‐processed flexible OSCs, which is important for the development of printing, blading, and roll‐to‐roll technologies.     

A Voltage‐Boosting Strategy Enabling a Low‐Frequency,Flexible Electromagnetic Wave Absorption Device

Nowadays, low‐frequency electromagnetic interference (<2.0 GHz) remains a key core issue that plagues the effective attenuation performance of conventional absorption devices prepared via the component‐morphology method (Strategy I). According to theoretical calculations, one fundamental solution is to develop a material that possesses a high ε′ but lower ε″. Thus, it is attempted to control the dielectric values via applying an external electrical field, which inducts changes in the macrostructure toward a performance improvement (Strategy II). A sandwich‐structured flexible electronic absorption device is designed using a carbon film electrode to conduct an external current. Simultaneously, an absorption layer that is highly responsive to an external voltage is selected via Strategy I. Relying on the synergistic effects from Strategies I and II, this device demonstrates an absorption value of more than 85% at 1.5–2.0 GHz with an applied voltage of 16 V while reducing the thickness to ≈5 mm. In addition, the device also shows a good absorption property at 25–150 °C. The method of utilizing an external voltage to break the intrinsic dielectric feature by modifying a traditional electronic absorption device is demonstrated for the first time and has great significance in solving the low‐frequency electromagnetic interference issue.     

Inkjet‐Printed High‐Performance Flexible Micro‐Supercapacitors with Porous Nanofiber‐Like Electrode Structures

Flexible planar micro‐supercapacitors (MSCs) with unique loose and porous nanofiber‐like electrode structures are fabricated by combining electrochemical deposition with inkjet printing. Benefiting from the resulting porous nanofiber‐like structures, the areal capacitance of the inkjet‐printed flexible planar MSCs is obviously enhanced to 46.6 mF cm^?2, which is among the highest values ever reported for MSCs. The complicated fabrication process is successfully averted as compared with previously reported best‐performing planar MSCs. Besides excellent electrochemical performance, the resultant MSCs also show superior mechanical flexibility. The as‐fabricated MSCs can be highly bent to 180° 1000 times with the capacitance retention still up to 86.8%. Intriguingly, because of the remarkable patterning capability of inkjet printing, various modular MSCs in serial and in parallel can be directly and facilely inkjet‐printed without using external metal interconnects and tedious procedures. As a consequence, the electrochemical performance can be largely enhanced to better meet the demands of practical applications. Additionally, flexible serial MSCs with exquisite and aesthetic patterns are also inkjet‐printed, showing great potential in fashionable wearable electronics. The results suggest a feasible strategy for the facile and cost‐effective fabrication of high‐performance flexible MSCs via inkjet printing.     

Flexible and Stretchable Energy Storage: Recent Advances and Future Perspectives

Energy‐storage technologies such as lithium‐ion batteries and supercapacitors have become fundamental building blocks in modern society. Recently, the emerging direction toward the ever‐growing market of flexible and wearable electronics has nourished progress in building multifunctional energy‐storage systems that can be bent, folded, crumpled, and stretched while maintaining their electrochemical functions under deformation. Here, recent progress and well‐developed strategies in research designed to accomplish flexible and stretchable lithium‐ion batteries and supercapacitors are reviewed. The challenges of developing novel materials and configurations with tailored features, and in designing simple and large‐scaled manufacturing methods that can be widely utilized are considered. Furthermore, the perspectives and opportunities for this emerging field of materials science and engineering are also discussed.     

Bioinspired,Spine‐Like,Flexible, Rechargeable Lithium‐Ion Batteries with High Energy Density

The rapid development of flexible and wearable electronics proposes the persistent requirements of high‐performance flexible batteries. Much progress has been achieved recently, but how to obtain remarkable flexibility and high energy density simultaneously remains a great challenge. Here, a facile and scalable approach to fabricate spine‐like flexible lithium‐ion batteries is reported. A thick, rigid segment to store energy through winding the electrodes corresponds to the vertebra of animals, while a thin, unwound, and flexible part acts as marrow to interconnect all vertebra‐like stacks together, providing excellent flexibility for the whole battery. As the volume of the rigid electrode part is significantly larger than the flexible interconnection, the energy density of such a flexible battery can be over 85% of that in conventional packing. A nonoptimized flexible cell with an energy density of 242 Wh L^?1 is demonstrated with packaging considered, which is 86.1% of a standard prismatic cell using the same components. The cell also successfully survives a harsh dynamic mechanical load test due to this rational bioinspired design. Mechanical simulation results uncover the underlying mechanism: the maximum strain in the reported design (≈0.08%) is markedly smaller than traditional stacked cells (≈1.1%). This new approach offers great promise for applications in flexible devices.     

Phase‐Engineered Type‐II Multimetal–Selenide Heterostructures toward Low‐Power Consumption,Flexible, Transparent,and Wide‐Spectrum Photoresponse Photodetectors

Phase‐engineered type‐II metal–selenide heterostructures are demonstrated by directly selenizing indium‐tin oxide to form multimetal selenides in a single step. The utilization of a plasma system to assist the selenization facilitates a low‐temperature process, which results in large‐area films with high uniformity. Compared to single‐metal–selenide‐based photodetectors, the multimetal–selenide photodetectors exhibit obviously improved performance, which can be attributed to the Schottky contact at the interface for tuning the carrier transport, as well as the type‐II heterostructure that is beneficial for the separation of the electron–hole pairs. The multimetal–selenide photodetectors exhibit a response to light over a broad spectrum from UV to visible light with a high responsivity of 0.8 A W^?1 and an on/off current ratio of up to 10². Interestingly, all‐transparent photodetectors are successfully produced in this work. Moreover, the possibility of fabricating devices on flexible substrates is also demonstrated with sustainable performance, high strain tolerance, and high durability during bending tests.