How is flexible electronics advancing neuroscience research?

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.

Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.

The fabrication of flexible and stretchable electronics is a critical requirement for the successful application of wearable healthcare devices. Although such flexible electronics have been commonly fabricated by microelectromechanical system (MEMS) technologies, they require a specialised equipment for vacuum deposition, photolithography, and wet and dry etching. A photolithography-free simple patterning method using a desktop plotter cutter has been proposed; however, the metal formation and electrode opening still rely on the MEMS technology. To address this issue, we demonstrate a simple, rapid, cost-effective, and a complete microfabrication process for flexible and stretchable sensor platforms encompassing conductor formation and patterning to encapsulate and open sensing windows, which only require an economic plotter cutter and readily available supplies. Despite its simplicity, the proposed process could stably create microscale features of 200 μm wide conductor lines and 1 mm window openings, which are in the useful range for various wearable applications. The feasibility of the simple fabrication of multi-functional sensors for various physiological monitoring applications was successfully demonstrated in electrochemical (glucose), electrical (electrocardiogram), mechanical (strain), and thermal (body temperature) modalities.

Electrical and mechanical integration approaches are essential for emerging hybrid electronics that must robustly bond rigid electrical components with flexible circuits and substrates. However, flexible polymeric substrates and circuits cannot withstand the high temperatures used in traditional electronic processing. This constraint requires new strategies to create flexible materials that simultaneously achieve high electrical conductivity, strong adhesion, and processibility at low temperature. Here, an electrically conductive adhesive is introduced that is flexible, electrically conductive (up to 3.25×105 S m−1) without sintering or high temperature post‐processing, and strongly adhesive to various materials common to flexible and stretchable circuits (fracture energy 350 <Gc < 700 J m−2). This is achieved through a multiphase soft composite consisting of an elastomeric and adhesive epoxy network with dispersed liquid metal droplets that are bridged by silver flakes, which form a flexible and conductive percolated network. These inks can be processed through masked deposition and direct ink writing at room temperature. This enables soft conductive wiring and robust integration of rigid components onto flexible substrates to create hybrid electronics for emerging applications in soft electronics, soft robotics, and multifunctional systems.

Cold spray-based rapid and scalable production of printed flexible electronics

Recently, advances in nano-material researches have opened the door for various transparent conductive materials, which include carbon nanotube, graphene, Ag and Cu nanowire, and printable metal grids. Among them, Ag nanowires are particularly interesting to synthesize because bulk Ag exhibits the highest electrical conductivity among all metals. Here we reviewed recently-published research works introducing various devices from organic light emitting diode to tactile sensing devices, all of which are employing AgNW for a conducting material. They proposed methods to enhance the stretchability and reversibility of the transparent electrodes, and apply them to make various flexible and stretchable electronics. It is expected that Ag nanowires are applicable to a wide range of high-performance, low-cost, stretchable electronic devices.

Flexible electronics have been of great interest in the past few decades for their wide-ranging applications in health monitoring, human-machine interaction, artificial intelligence, and biomedical engineering. Currently, transfer printing is a popular technology for flexible electronics manufacturing. However, typical sacrificial intermediate layer-based transfer printing through chemical reactions results in a series of challenges, such as time consumption and interface incompatibility. In this paper, we have developed a time-saving, wafer-recyclable, eco-friendly, and multiscale transfer printing method by using a stable transferable photoresist. Demonstration of photoresist with various, high-resolution, and multiscale patterns from the donor substrate of silicon wafer to different flexible polymer substrates without any damage is conducted using the as-developed dry transfer printing process. Notably, by utilizing the photoresist patterns as conformal masks and combining them with physical vapor deposition and dry lift-off processes, we have achieved in situ fabrication of metal patterns on flexible substrates. Furthermore, a mechanical experiment has been conducted to demonstrate the mechanism of photoresist transfer printing and dry lift-off processes. Finally, we demonstrated the application of in situ fabricated electrode devices for collecting electromyography and electrocardiogram signals. Compared to commercially available hydrogel electrodes, our electrodes exhibited higher sensitivity, greater stability, and the ability to achieve long-term health monitoring.

An acoustic matching layer is an essential component of an ultrasound transducer to achieve maximum ultrasound transmission efficiency. Here, we develop a flexible printed circuit board (FPCB) with a composite structure consisting of multiple polyimide and copper layers and demonstrate it as a novel acoustic matching layer. With a flexible substrate and robust ACF bonding, the FPCB not only serves as an acoustic matching layer between piezoelectric elements and the surrounding medium but also as a ground for the electrical connection between the transducer array elements and the folded substrate. A 1D linear ultrasound transducer array with the FPCB matching layer exhibits larger output pressure, wider -3dB bandwidth, and higher ultrasound beam intensity compared to that of an ultrasound transducer array with the alumina/epoxy matching layer, which is one of the most commonly applied composite matching layers. The enhanced transmission performance verifies that the proposed FPCB is an excellent matching layer for 1D linear ultrasound transducer arrays.

This chapter presents an overview of material properties, design strategies, and typical applications of liquid metal-enabled (LM-enabled) flexible and stretchable electronics. It focuses on the core manufacturing material: room-temperature liquid metal, mainly represented by gallium and its alloys with the properties of excellent resistivity, enormous bendability, low adhesion, and large surface tension. Along with the development of material science and technology, LM-enabled flexible electronics are quickly shaping the field of flexible electronic circuits and allied machines, serving as a basic way of quickly making functional devices. The chapter discusses the developments in LM soft device applications, which include but are not limited to strain sensors, stretchable coils, e-skin, wearable bioelectronics, and conformable bioelectronics. It examines the drawbacks and challenges of LM material properties and flexible fabrication technologies, and sets goals to achieve LM intelligent flexible future.

Wire bonding is still popular and leading interconnect technologies due to its reliability, reputation for versatility and performance. Increased demand for disposal medical sensors, miniaturization for portable electronic equipment and specialty parts which are small in dimension and light weight area are at utmost importance, which lead to the chip-on-flex technology to become more attractive technology for above application. Thermo-sonic Aluminum (Al) wedge-wedge bond is preferred [1] compare to Gold (Au) bonding in most commercial chip-on-flex applications for chip to substrate interconnection due softening polymers at flexible substrate when heating at 180°C~200°C. This softening of polymers during heating absorbs the ultrasonic energy which caused failure in ball wire bond formation on the pad/wedge, hence reducing the wire bonding yield. In this paper, we shall discuss the challenges face during Au ball bonding using polyimide flexible printed circuit board (FPCB), which has Copper as the single conductor layer and a polyimide layer. The SoC die consist of the temperature sensor, a MEMs pressure sensor, a power management unit (PMU) with an oxygen sensor together with the IC interface (Signal readout interface). The bonding temperature selection, to minimize ultrasonic energy absorption by the FPCB, the Au wire type choice used for this for successful ball bonding/wedge and the wirebonding optimization parameters involved will also be evaluated and covered. A successfully wirebonding onto the polyimide FPCB with ball shear and wire pull measurement that meet the specification will also be shown and discussed.

Mechanically flexible, printed, and stretchable electronics are gaining momentum. While rapid advancement is well underway at the device and circuit levels, researchers have yet to envision the system design in a flexible form. This paper introduces the concept of <i>System-on-Polymer (SoP)</i> based on flexible hybrid electronics (FHE) to combine the advantages of flexible electronics and traditional silicon technology. First, we formally define flexibility as a new design metric in addition to existing power, performance, and area metrics. Then, we present a novel optimization approach to place rigid components onto a flexible substrate while minimizing the loss in flexibility. We show that the optimal placement leads to as much as 5.7 <inline-formula><tex-math notation="LaTeX"> $\times$</tex-math></inline-formula> enhancement in flexibility compared to a naïve placement. We confirm the accuracy of our models and optimization framework using a finite element method (FEM) simulator. Finally, we demonstrate the SoP concept using a concrete hardware prototype, and discuss the major challenges in the architecture and design of SoPs.

In recent years, the field of electronics has witnessed a revolutionary transformation with the emergence of nanoelectronics and related technologies. This abstract delves into the realm of Nanoscale electronic devices and applications, Quantum electronics and quantum computing, Organic and flexible electronics, the smart devices and Internet of Things (IoT). Nanoscale electronic devices have paved the way for groundbreaking advancements in various domains. As electronic components continue to shrink to nanoscale dimensions, novel materials and structures are being explored to enhance device performance. Nanoscale transistors, such as carbon nanotubes and nanowires, have shown the potential to overcome the limitations of conventional silicon transistors in terms of power consumption and switching speed, unlocking new possibilities in electronics and computing. These nanoscale components have the potential to lead to faster, more energy-efficient devices with applications ranging from portable gadgets to high-performance computing systems. Quantum electronics represents the next frontier in electronics research, where quantum effects are harnessed to develop transformative technologies. Quantum computing, in particular, has garnered immense interest due to its potential to revolutionize computation. Quantum computers leverage quantum bits or qubits, which can exist in multiple states simultaneously. It also allow them to perform certain computations exponentially faster than classical computers. Quantum algorithms have the potential to tackle complex problems in cryptography, optimization, and materials science, pushing the boundaries of what is currently achievable in traditional computing paradigms. Organic and flexible electronics have opened up new avenues for seamless integration of electronics into various products and applications. Organic materials, such as organic semiconductors and conductive polymers, offer the advantage of low-cost processing and the ability to be printed on flexible substrates. This allows for the creation of flexible displays, wearable electronics, and even electronic skins that can sense and interact with their surroundings. Organic and flexible electronics are enabling the development of lightweight and conformable devices with applications in healthcare, consumer electronics, and the Internet of Things (IoT). Speaking of the IoT, the interconnectivity of smart devices has become a transformative force in modern life. The IoTdeals with a vast network of devices, sensors, and actuators that communicate with each other and also with the cloud, collecting and exchanging data to enable intelligent decision-making and automation. Nanoscale electronic devices play a crucial role in the IoT ecosystem, as miniaturized sensors and low-power processors are essential for creating smart, energy-efficient devices. The IoT has applications in various sectors, including smart homes, healthcare, agriculture, transportation, and industrial automation. This abstract highlights the exciting prospects brought forth by Nanoelectronics and Emerging Technologies, including nanoscale electronic devices and applications, quantum electronics and quantum computing, organic and flexible electronics, and the IoT and smart devices. As these technologies continue to evolve, they are expected to reshape industries, enhance our quality of life, and drive further innovation in the electronics domain. However, it is crucial to address the challenges posed by these emerging technologies, such as security and privacy concerns, ethical considerations, and environmental impacts, to ensure that they are deployed responsibly and sustainably in our increasingly interconnected world.

Large-area graphene has emerged as a promising material for use in flexible and transparent electronics due to its flexibility and optical and electronic properties. The anchoring of transition metal nanoparticles on large-area single-layer graphene is still a challenge. Here, we report an in situ preparation of carbon nano-onion-encapsulated Fe nanoparticles on rebar graphene, which we term rivet graphene. The hybrid film, which allows for polymer-free transfer and is strong enough to float on water with no added supports, exhibits high optical transparency, excellent electric conductivity, and good hole/electron mobility under certain tensile/compressive strains. The results of contact resistance and transfer length indicate that the current in the rivet graphene transistor does not just flow at the contact edge. Carbon nano-onions encapsulating Fe nanoparticles on the surface enhance the injection of charge between rivet graphene and the metal electrode. The anchoring of Fe nanoparticles encapsulated by carbon nano-onions on rebar graphene will provide additional avenues for applications of nanocarbon-based films in transparent and flexible electronics.

Integrating oxide thin films on flexible substrates is a critical step toward future applications of multifunctional oxides for flexible electronics and spintronic devices. As a demonstration, multifunctional La0.67Sr0.33MnO3 (LSMO) thin films have been deposited on flexible mica substrates. The crystallinity and microstructure of the films have been characterized to show the good epitaxial quality of the films. The LSMO thin films on mica present excellent ferromagnetic and magnetoresistance properties (such as saturation magnetization Ms of 125-400 emu/cm3 at 10 K and a high MR value of ∼45% at 5 K under 1 T for the 50 mTorr deposited sample), which is even better than the ones on conventional rigid single-crystal oxide substrates. More interestingly, no deterioration of the properties is observed under mechanically bending condition, which demonstrates the good mechanical stretchability and property stability of the LSMO thin films on mica. The demonstration of functional oxides integrated on flexible mica substrates paves a route toward future flexible spintronics and electronics.

Organic thin‐film transistors (OTFTs) exhibit great potential applications in flexible logic circuits, sensors, nonvolatile memory, etc. The vertical phase separation (VPS) structure, deriving from the blends of small molecules and polymers, can produce a high‐quality crystalline semiconducting layer and an intrinsic and trap‐free semiconductor/insulator interface. OTFTs with a VPS structure provide a promising route to accelerate their commercialization due to these desirable features including excellent device performance, good uniformity and reproducibility, environmental and mechanical stability, low‐cost and large‐area fabrication. Additionally, VPS OTFTs also exhibit a good balance between electronical properties and stretchability for intrinsic stretchable OTFTs. Herein, a comprehensive review is given on VPS structure for flexible and stretchable OTFTs. First, the evolution history of VPS structures and its formation mechanism are summarized. Based on the understanding of the VPS structure, the optimization strategies of VPS structures for high‐performance OTFTs are presented. Subsequently, the large‐area fabrication techniques of VPS structures, as well as the potential applications in flexible and stretchable OTFTs, are discussed. Finally, conclusions and outlook are provided for the further development of VPS structure in OTFTs. This review would promote the understanding and development of VPS structures in OTFTs, accelerating its applications in flexible and stretchable electronics.