Advances in Organic Two-dimensional Materials for X-Ray Detection and Their Applications in Flexible Electronics

Progress and challenges in flexible electrochromic devices

The rapid development of wearable technology has spurred considerable interest in hydrogels, which are hydrophilic three-dimensional polymer networks known for their remarkable flexibility. Nevertheless, their application in flexible electronics has been constrained by inferior mechanical and physical properties. As an emerging flexible material, dual-network hydrogels possess high mechanical strength, self-healing capability, excellent fatigue resistance, and electrical conductivity, showing great potential for use in flexible electronics. This article systematically reviews the design and performance optimization strategies of dual-network hydrogels. It reviews the advancements in their applications in flexible electronic devices, including bodily fluid biomarker sensors, flexible energy storage devices, health monitoring sensors, and physical motion sensors. The potential future challenges and opportunities for dual-network hydrogel materials are also discussed. This review aims to provide a theoretical foundation for developing next-generation dual-network hydrogels for flexible electronics and to promote their practical implementation in this field.

3 - Hybrid and nanocomposite materials for flexible organic electronics applications

In recent years, to meet the greater demand for next generation electronic devices that are transplantable, lightweight and portable, flexible and large-scale integrated electronics attract much more attention have been of interest in both industry and academia. Organic electronics and stretchable inorganic electronics are the two major branches of flexible electronics. With the semiconductive and flexible properties of the organic semiconductor materials, flexible organic electronics have become a mainstay of our technology. Compared to organic electronics, stretchable and flexible inorganic electronics are fabricated via mechanical design with inorganic electronic components on flexible substrates, which have stretchability and flexibility to enable very large deformations without degradation of performance. This review summarizes the recent progress on fabrication strategies, such as hydrodynamic organic nanowire printing and inkjet-assisted nanotransfer printing of flexible organic electronics, and screen printing, soft lithography and transfer printing of flexible inorganic electronics. In addition, this review considers large-scale organic and inorganic flexible electronic systems and the future applications of flexible and stretchable electronics.

Single-crystal silicon and germanium are the basis of the modern semiconductor industry. They exhibit unique mechanical, optical, electrical, and thermal properties when their thicknesses decrease to the nanoscale. Ultra-small thickness provides silicon and germanium flexibility. Compared with organic semiconductors, silicon and germanium have much higher carrier mobility. This makes them ideal components for high-performance devices and gives them great potential in the application of the internet of things, wearable/implantable electronics, and bio-electronics. In this review, we discuss the strategies of “Device-Last Approach and “Device-First Approach for silicon and germanium nanomembrane devices and their applications in flexible electronics. The latest development of transferred nanomembranes and their applications in flexible electronics, as well as the scientific and technique issues to be solved, are specifically discussed.

Electronic skins with distinctive features have attracted remarkable attention from researchers because of their promising applications in flexible electronics. Here, we present novel morphologically conductive hydrogel microfibers with MXene encapsulation by using a multi-injection coflow glass capillary microfluidic chip. The coaxial flows in microchannels together with fast gelation between alginate and calcium ions ensure the formation of hollow straight as well as helical microfibers and guarantee the in situ encapsulation of MXene. The resultant hollow straight and helical MXene hydrogel microfibers were with highly controllable morphologies and package features. Benefiting from the easy manipulation of the microfluidics, the structure compositions and the sizes of MXene hydrogel microfibers could be easily tailored by varying different flow rates. It was demonstrated that these morphologically conductive MXene hydrogel microfibers were with outstanding capabilities of sensitive responses to motion and photothermal stimulations, according to their corresponding resistance changes. Thus, we believe that our morphologically conductive MXene hydrogel microfibers with these excellent features will find important applications in smart flexible electronics especially electronic skins.

Over the last decade, extensive efforts have been made on utilizing advanced materials and structures to improve the properties and functionalities of flexible electronics. While the conventional ways are approaching their natural limits, a revolutionary strategy, namely metamaterials, is emerging toward engineering structural materials to break the existing fetters. Metamaterials exhibit supernatural physical behaviors, in aspects of mechanical, optical, thermal, acoustic, and electronic properties that are inaccessible in natural materials, such as tunable stiffness or Poisson's ratio, manipulating electromagnetic or elastic waves, and topological and programmable morphability. These salient merits motivate metamaterials as a brand-new research direction and have inspired extensive innovative applications in flexible electronics. Here, such a groundbreaking interdisciplinary field is first coined as "flexible metamaterial electronics," focusing on enhancing and innovating functionalities of flexible electronics via the design of metamaterials. Herein, the latest progress and trends in this infant field are reviewed while highlighting their potential value. First, a brief overview starts with introducing the combination of metamaterials and flexible electronics. Then, the developed applications are discussed, such as self-adaptive deformability, ultrahigh sensitivity, and multidisciplinary functionality, followed by the discussion of potential prospects. Finally, the challenges and opportunities facing flexible metamaterial electronics to advance this cutting-edge field are summarized.

Wireless charging delivers power wirelessly across an air gap to recharge electronic devices without requiring direct physical connections. The rapid advancements in wireless charging technologies and the emergence of commercial products have introduced an alternative to overcome the energy limitations of traditional portable, battery‐operated devices. This paper offers an in‐depth review of radio frequency (RF)‐based wireless charging, emphasizing its applications in flexible electronics. We begin with a comprehensive overview of wireless charging methods, followed by a detailed introduction to RF‐based charging principles. We then review the applications of RF‐based charging in wearable devices, biomedical systems, integrated flexible and stretchable electronics, and on‐body wireless data transmission. Wireless charging, RF, flexible electronic, wearable devices.

This is a special issue dedicated to the fascinating fi eld of fl exible electronics, and it stems from one of the Symposia held at the European Materials Research Society (E-MRS) Spring Meeting from May 14 th to 18 th 2012 in Strasbourg (France): Symposium H “ Organic and Hybrid Materials for Flexible Electronics: Properties and Applications ”. The symposium aimed at bringing together key researchers in the above mentioned fi eld in order to stimulate discussions regarding the main challenges still to be faced towards the widespread application of fl exible electronics, with contributions covering both fundamental properties of novel semiconducting organic and hybrid materials and their effi cient integration in functional devices mainly by means of, but not limited to, solutionbased processes. The symposium was organized by Dr. Mario Caironi (Istituto

Super-capacitors, lithium ion batteries, aluminium air batteries, lithium air batteries, lithium sulfur batteries, and zinc-air batteries can be utilized for flexible electronic device applications as their energy storage devices. All of them possess desired features of all-dimension-deformability and weaveability. Also they can be part of bigger picture by integrating with flexible, wearable and energy harvesting devices, helping them to self-power and multi-function. In this paper we have presented materials perspective and requirements of constitutional parts (electrodes, electrolytes) of these energy storage devices. Their electrical, mechanical, environmental constraints, design principles, material, structure and challenges for integration have been discussed. Flexible electronics applications in various fields have also been mentioned.

Stretchable devices significantly expand the scope of applications such as flexible displays and wearable devices/sensors. To enable stretchable wearable electronics, methods for connecting unit devices and developing circuits are required. Previously, research was performed to manufacture circuits with 3D structures or patterns that remain intact when the flexible and stretchable substrates are deformed. A method for drawing a circuit directly on a substrate with a conductive ink pen is proposed, although it is limited by the surface properties of the substrate. Most existing transparent papers are not sufficiently stretchable for flexible and wearable electronics applications. Therefore, a polydimethylsiloxane–cellulose nanocrystals (PDMS–CNC) composite paper is developed that is both highly flexible and stretchable, while maintaining a high transmittance. The versatility of the composite paper is demonstrated as a suitable substrate for flexible devices by patterning with a conductive ink pen. The PDMS–CNC composite paper has an excellent transmittance of ≈70%, and can withstand over 800% tensile strain. The patterned circuits have only minor increase in resistance after a 50% deformation and recovery. The composite paper is a suitable technology for fabricating electrical components and devices for the Internet of Things and wearable and flexible electronics applications.

The fragility of traditional metallic or semi-conductive materials hinders their application in flexible electronics. Low dimensional materials including carbon nanotubes, graphene and metal nanowires own outstanding flexibility and have been wildly used to fabricate flexible devices. Bendable/stretchable substrate is another key component of flexible electronics. Various thin polymer films made of polyethylene terephthalate, polyimide, polydimethylsiloxane et. al. were adopted. However, the air impermeability of these substrates will cause discomfort of humanbeing if applied in wearable electronics. Fiber is an ideal substrate for flexible and wearable electronics due to its excellent flexibility/stretchability, superior breathability, abundant microstructure and low cost. Herein, a series of conductive elastomers and strain sensors were fabricated by combining the low dimensional conductive materials with fiber substrates and regulating the microstructure on the interface. With the help of “twining spring” hierarchical architecture, silver nanowire-double covered yarn (Ag NW-DCY) composite fibers with ultrahigh stretchability were obtained. The conductivity of the composite fibers reached up to 104 S/cm and remained 90% at 2000% tensile strain. Commercial electronic components (LED arrays) were integrated onto a transparent, foldable and stretchable substrate using the composite fibers as stretchable electric wiring, demonstrating the potential application in large-area stretchable electronics. When AgNWs were replaced with graphene, strain sensing fiber with high sensitivity and large working range (100% strain) were fabricated, which enabled the detection of multiple deformation forms, including tensile strain, bending, and torsion. We employ the fibers as wearable sensors, realizing the monitoring of full-range human activities and intricate movement combinations of a robot. Besides, these fibers exhibits fast response, low hysteresis and excellent cycling stability. Another advantage needs to be noted is that these fiber are fabricated by a facial dip coating method, which can be scaled up easily. These smart fibers are of great meaning to the development of flexible and wearable electronics.

Economical and abundant natural biological materials provide a low-cost and scalable approach to develop next-generation flexible and wearable electronics. Herein, a universal strategy of nature-inspired and amine-promoted metallization, namely, NIAPM, is presented to make high-quality metals for electronics fabrication. The introduction of poly(ethyleneimine) (PEI) significantly shortens the time of metallization from >48 h to ≈6 h, and the phenol compounds (TP) from green tea make metals bond tightly on all demonstrated surfaces. The as-made thin metal films of Cu and Ni feature high conductivity (∼1.0 Ω/□), excellent mechanical stability and flexibility even at the bending radius of ∼1 mm. Moreover, NIAPM is compatible with typical lithography techniques for fabricating metallic patterns, showing considerable potential applications in flexible electronics. As a proof-of-concept, two devices based on melamine-templated Cu sponges are first prepared for detecting the change of external pressure with a resistance sensitivity of 18.1 kPa-1 and collecting high-viscosity crude oil, respectively. Then, a high-performance bendable solid supercapacitor is demonstrated using as-prepared Ni metallized fabrics and the activated porous carbon from the recycled waste in NIPAM as flexible electrodes, which possesses comparable areal capacitance of 45.5 F·g-1, and energy density of 7.88 Wh·g-1 at the power density of 35 W·g-1. Therefore, it is anticipated that such a time-saving, cost-effective and whole solution-processed NIAPM strategy can inspire further practical applications in the fields of surface chemistry, material science, flexible and wearable electronics, etc.

Human soft tissues-like PVA/cellulose hydrogels with multifunctional properties towards flexible electronics applications.

Recently, hydrogels have attracted great attention because of their unique properties, including stretchability, self-adhesion, transparency, and biocompatibility. They can transmit electrical signals for potential applications in flexible electronics, human–machine interfaces, sensors, actuators, et al. MXene, a newly emerged two-dimensional (2D) nanomaterial, is an ideal candidate for wearable sensors, benefitting from its surface’s negatively charged hydrophilic nature, biocompatibility, high specific surface area, facile functionalization, and high metallic conductivity. However, stability has been a limiting factor for MXene-based applications, and fabricating MXene into hydrogels has been proven to significantly improve their stability. The unique and complex gel structure and gelation mechanism of MXene hydrogels require intensive research and engineering at nanoscale. Although the application of MXene-based composites in sensors has been widely studied, the preparation methods and applications of MXene-based hydrogels in wearable electronics is relatively rare. Thus, in order to facilitate the effective evolution of MXene hydrogel sensors, the design strategies, preparation methods, and applications of MXene hydrogels for flexible and wearable electronics are comprehensively discussed and summarized in this work.