Manufacturing Of Flexible Electronics Research Articles

Wafer-Recyclable, Eco-Friendly, and Multiscale Dry Transfer Printing by Transferable Photoresist for Flexible Epidermal Electronics.

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.

Femtosecond Laser Direct Writing of Flexible Electronic Devices: A Mini Review.

By virtue of its narrow pulse width and high peak power, the femtosecond pulsed laser can achieve high-precision material modification, material additive or subtractive, and other forms of processing. With additional good material adaptability and process compatibility, femtosecond laser-induced application has achieved significant progress in flexible electronics in recent years. These advancements in the femtosecond laser fabrication of flexible electronic devices are comprehensively summarized here. This review first briefly introduces the physical mechanism and characteristics of the femtosecond laser fabrication of various electronic microdevices. It then focuses on effective methods of improving processing efficiency, resolution, and size. It further highlights the typical progress of applications, including flexible energy storage devices, nanogenerators, flexible sensors, and detectors, etc. Finally, it discusses the development tendency of ultrashort pulse laser processing. This review should facilitate the precision manufacturing of flexible electronics using a femtosecond laser.

Room-Temperature Liquid Metals for Flexible Electronic Devices.

Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.

Nonlinear dynamics of fractional viscoelastic PET membranes with linearly varying density

In this paper, the nonlinear dynamics of fractional viscoelastic polyethylene terephthalate (PET) membranes with linearly varying density are elaborated. The viscoelasticities of the PET membranes are characterized with the fractional Kelvin–Voigt model, and the density distribution is considered a linear fluctuation in the lateral direction. The geometrically nonlinear formulation is established with the von-Karman theory and the Hamilton principle. The Bubnov–Galerkin method is used to solve the resulting fractional differential equations, and a refined numerical scheme combining the finite difference with a discrete Caputo-type fractional derivative approximation is developed. Comparison examples are provided to substantiate the accuracy and effectiveness of the present method. A detailed parametric analysis is performed to illuminate the effects of the fractional order, density coefficient and various system parameters on the time response of the viscoelastic PET membranes. This study paves the way for fractional modelling and vibrational analysis of viscoelastic substrates in flexible electronics manufacturing.

A Review of Manufacturing Methods for Flexible Devices and Energy Storage Devices.

Given the advancements in modern living standards and technological development, conventional smart devices have proven inadequate in meeting the demands for a high-quality lifestyle. Therefore, a revolution is necessary to overcome this impasse and facilitate the emergence of flexible electronics. Specifically, there is a growing focus on health detection, necessitating advanced flexible preparation technology for biosensor-based smart wearable devices. Nowadays, numerous flexible products are available on the market, such as electronic devices with flexible connections, bendable LED light arrays, and flexible radio frequency electronic tags for storing information. The manufacturing process of these devices is relatively straightforward, and their integration is uncomplicated. However, their functionality remains limited. Further research is necessary for the development of more intricate applications, such as intelligent wearables and energy storage systems. Taking smart wear as an example, it is worth noting that the current mainstream products on the market primarily consist of bracelet-type health testing equipment. They exhibit limited flexibility and can only be worn on the wrist for measurement purposes, which greatly limits their application diversity. Flexible energy storage and flexible display also face the same problem, so there is still a lot of room for development in the field of flexible electronics manufacturing. In this review, we provide a brief overview of the developmental history of flexible devices, systematically summarizing representative preparation methods and typical applications, identifying challenges, proposing solutions, and offering prospects for future development.

Amphiphilic Silver Nanoparticles for Inkjet-Printable Conductive Inks.

The large-scale manufacturing of flexible electronics is nowadays based on inkjet printing technology using specially formulated conductive inks, but achieving adequate wetting of different surfaces remains a challenge. In this work, the development of a silver nanoparticle-based functional ink for printing on flexible paper and plastic substrates is demonstrated. Amphiphilic silver nanoparticles with narrow particle size distribution and good dispersibility were prepared via a two-step wet chemical synthesis procedure. First, silver nanoparticles capped with poly(acrylic acid) were prepared, followed by an amidation reaction with 3-morpholynopropylamine (MPA) to increase their lipophilicity. Density functional theory (DFT) calculations were performed to study the interactions between the particles and the dispersion medium in detail. The amphiphilic nanoparticles were dispersed in solvents of different polarity and their physicochemical and rheological properties were determined. A stable ink containing 10 wt% amphiphilic silver nanoparticles was formulated and inkjet-printed on different surfaces, followed by intense pulsed light (IPL) sintering. Low sheet resistances of 3.85 Ω sq-1, 0.57 Ω sq-1 and 19.7 Ω sq-1 were obtained for the paper, coated poly(ethylene terephthalate) (PET) and uncoated polyimide (PI) flexible substrates, respectively. Application of the nanoparticle ink for printed electronics was demonstrated via a simple flexible LED circuit.

Localized Surface Hydrophilicity Tailoring of Polyimide Film for Flexible Electronics Manufacturing Using an Atmospheric Pressure Ar/H2O Microplasma Jet.

The poor hydrophilicity of polyimide (PI) films limits their applications in flexible electronics, such as in wearable and implantable bio-MEMS devices. In this paper, an atmospheric pressure Ar/H2O microplasma jet (μAPPJ) with a nozzle diameter of 100 μm was utilized to site-selectively tune the surface hydrophilicity of a PI film. The electrical and optical characteristics of the μAPPJ were firstly investigated, and the results showed that multi-spikes occurred during the plasma discharge and that diverse reactive species, such as O atoms and OH radicals, were generated in the plasma plume. The physical and chemical properties of pristine and microplasma-modified PI surfaces were characterized by the water contact angle (WCA), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The wettability of the PI surface was significantly enhanced after microplasma modification, and the WCA could be adjusted by varying the applied voltage, water vapor content, plasma treatment time and storage time. The AFM images indicated that the surface roughness increased after the plasma treatment, which partially contributed to an improvement in the surface hydrophilicity. The XPS results showed a reduction in the C content and an increase in the O content, and abundant hydrophilic polar oxygen-containing functional groups were also grafted onto the PI film surface. Finally, the interaction mechanism between the PI molecular chains and the microplasma is discussed. The breaking of C-N and C-O bonds and the grafting of OH radicals were the key pathways to dominate the reaction process.

Roll-to-roll manufacture of flexible thin-film thermoelectric generators using flexography with vacuum vapour deposition

Exploring processes for industrial large-scale manufacture of low-cost electronic devices is timely for the upcoming era of the Internet of Things. In this study, a roll-to-roll process using selective metallisation (a combination of flexographic printing of an oil liquid-mask and physical vapour deposition: sputtering and evaporation) is explored for the large-scale manufacture of flexible electronics, with a wearable thermoelectric generator as an exemplar device. Two post-treatments (electron beam and infrared) are trialled to remove the residual oil after selective metallisation. Both are effective for the removal of oil, however, the oil used in selective metallisation degrades the functional thermoelectric material severely, causing a dramatic increase in the internal resistance of the device. This is attributable to (1) the doping and oxidation issues in the semiconductor film; (2) the existence of oil at the interface between the two coatings. The fabricated thermoelectric generator with post-treatments shows a good voltage output comparable to a standard device fabricated using shadow mask, providing an option for portable devices requiring a voltage source. This study reports an industrially compatible roll-to-roll manufacturing process for large-scale manufacture of flexible electronics. Further studies are under way to optimise the device performance.

Functional Nanomaterial-Based Flexible Electronics

With the rapid development of society, the growing interest in flexible electronics has led to remarkable progress in recent advances in the manufacture of flexible electronics [...]

One‐Step Soft Templated Selective Milling‐Based Circuit Patterning for Eco‐Friendly and High‐Throughput Manufacturing of Flexible Electronics

AbstractOver the past decades, the booming development of flexible electronics in various fields, e.g., Internet of Things and epidermal electronics, requires the support of a reliable manufacturing approach, where circuit patterning is the core. Although the existing circuit patterning methods represented by photochemical etching and direct printing process have shown good performance, they still cannot satisfy today's demands on environmental friendliness and cost‐efficiency concurrently. Here, by regulating the milling force distribution on the flexible workpiece with a grooved soft template, this work presents a milling‐based patterning technique that is compatible with roll‐to‐roll technology, to achieve continuous selective removal of conductive material in one step and hence directly pattern electrical circuits on aluminum clad laminates. Without introducing any chemicals, this new approach generates no emission but recyclable aluminum scraps. Moreover, the process shows a considerable processing speed of 30 m min−1, and costs only commercial aluminum clad laminates. Examples of device fabrication include a radio frequency identification tag and a temperature sensor. Ultimately, this work can bring an eco‐friendly, economic and high‐throughput approach to flexible electronics manufacturing.

Enhanced thermally conductive TPU/graphene filaments for 3D printing produced by melt compounding

AbstractConducting polymers are a potential solution to the issue of wide‐range manufacture of flexible electronics, since many polymers are suitable to advanced fabrication methods, such as fused deposition modeling (FDM) 3D printing. Therefore, it is proposed in this work a ductile thermoplastic polyurethane (TPU)/graphene nanocomposite with enhanced thermal conductivity that would be suitable for additive manufacturing processes. The nanocomposites were produced by melt compounding technique, which does not require the use of toxic solvents and is characteristic of being used by the thermoplastic industry. The filaments were then analyzed in terms of graphene dispersion, microstructural changes, mechanical behavior, and thermal conductivity. It was verified that graphene does not have a good interaction with the TPU matrix, presenting voids at the interface and forming agglomerates at higher contents. However, it was able to affect the TPU's hard segments organization, leading to a more ordered structure up to 1.0 wt%. Consequently, the thermal conductivity was enhanced up to the graphene concentration by about 14% in relation to neat TPU. Additionally, even though the system presented a poor interface, no mechanical properties were lost, that is, the composite remained ductile even after graphene insertion, which is extremely interesting for additive manufacturing processes.

Thermal conductivity and mechanical durability of graphene composite films containing polymer-filled connected multilayer graphene patterns

Due to its high thermal conductivity, graphene has received much attention as a thermal interface material (TIM) for dissipating heat, which would otherwise be accumulated in heat sources, to heatsinks. However, the weak interlayer force induces tearing of graphene under repeated deformation; hence, the application of graphene as a TIM in the manufacturing of flexible electronics has been limited. To overcome this hurdle, the graphene composite (GC) films, in which thermoplastic polymers were infiltrated into connected multilayer graphene (MLG) patterns, were fabricated in this study. While the connected MLG patterns attained high in-plane thermal conductivity (κx), the polymers prevented tearing of the graphene. To investigate the effect of the graphene content on the κx of the GC films, the area of MLG patterns was carefully adjusted by coating a graphene solution through metal masks with various opening sizes. The κx of the GC-4 film was calculated as 53 W/m·K, which was slightly changed after 10,000 folding test cycles with a 1.5-mm bending radius.

Recent development in silver-based ink for flexible electronics

Silver based flexible printed electronics offer the possibility to design low-cost environmentally friendly products through versatile manufacturing procedures. In this regard, conductive silver inks are commonly printed on flexible substrates because in contrast to their macroscale analogues, the silver nanoparticles (AgNPs) present different salient physical and chemical properties. However, an extensive application of silver inks is often limited by factors such as a complicated ink preparation process, conflict between ink stability and nanoparticle content, overall ink formulation and selection of suitable sintering techniques. Nevertheless, advances in nanotechnology and printing techniques have opened new rooms to improve the synthesis, stability, processing, and application of silver inks. This review article presents the progress in AgNPs processing, Ag-conductive ink formulation and important printing techniques, especially as it relates to the manufacture of flexible electronics. Furthermore, the significance of the sintering process and the importance of various kinds of sintering methods for improving ink quality and adhesion to substrates are discussed. In addition, some of the notable applications of printed flexible electronics are highlighted.

3D printing highly stretchable conductors for flexible electronics with low signal hysteresis

ABSTRACT Previous studies on UV-curable elastomer for digital light processing (DLP) 3D printing flexible electronics mostly focused on higher stretchability and healability, thereby ignoring rapid recoverability after a stretch which is very important for the real-time transmission of electrical signals in flexible electronics. Herein, a UV-curable elastomer is developed with a novel dynamic hierarchical crosslinking network that can be stretched up to over 800% and rapid recoverability at 300% strain. We proposed a hydrogel-elastomer assembling process to simplify the manufacturing process for stretchable conductors. The stretchable conductors served as a strain sensor exhibiting excellent stability and repeatability after 10 cycles at 20–50% strain with low signal drift and hysteresis, Moreover, the strain sensor can be utilised in recognising the different bending angles of the finger, making DLP printing of conductive elastomer a viable strategy for the rapid manufacturing of flexible electronics.

Scalable Thermal Masking Technique for Postprocessing of Flexible Electronics

Flexible electronics have found a wide variety of applications, especially in biomedical engineering, where their intrinsic safety and conformability provide for configurations of sensors and actuators that can accommodate complex anatomies and movement. Current scalable techniques in the manufacture of flexible electronics are limited by the challenges of working with novel, intrinsically stretchable materials, or using cost‐intensive clean room fabrication techniques needed to create strain accommodating geometries. Recently, a technique that allows for scalable postprocessing of flexible printed circuit boards (flex PCBs) to convert them to stretchable configurations is described. Herein, a mechanism for understanding this process based on a thermal masking phenomenon is described. Thermal simulation provides an understanding of the phenomenon and various aspects of the process. Copper traces are modeled along with Kapton insulating layer on ANSYS workbench and a Gaussian waveform is introduced to simulate a laser beam. An inhouse laser cutter is used to calibrate the simulations and an accurate predictive model is used to determine working conditions, resolution, and other parameters that affect the standardized usage of this process for various thicknesses of copper and Kapton that can be produced by most mass manufacturers. This process provides a path for cost‐effective manufacture of stretchable electronics.

Devising Materials Manufacturing Toward Lab‐to‐Fab Translation of Flexible Electronics

Flexible electronics have witnessed exciting progress in academia over the past decade, but most of the research outcomes have yet to be translated into products or gain much market share. For mass production and commercialization, industrial adoption of newly developed functional materials and fabrication techniques is a prerequisite. However, due to the disparate features of academic laboratories and industrial plants, translating materials and manufacturing technologies from labs to fabs is notoriously difficult. Therefore, herein, key challenges in the materials manufacturing of flexible electronics are identified and discussed for its lab-to-fab translation, along the four stages in product manufacturing: design, materials supply, processing, and integration. Perspectives on industry-oriented strategies to overcome some of these obstacles are also proposed. Priorities for action are outlined, including standardization, iteration between basic and applied research, and adoption of smart manufacturing. With concerted efforts from academia and industry, flexible electronics will bring a bigger impact to society as promised.

Web Tension Estimation by Local Contact Force Measurement in Roll-to-Roll Manufacturing

Web tension and its distribution are key factors in the operation of web systems, the roll to roll (R2R) manufacturing of flexible electronics brings even higher requirements for it. To effectively get the web tension at any position on the R2R process line, a new web tension estimation method by measuring the local contact force is investigated in this paper, which can not only estimate the web tension, but also the web tension distribution. This study attempts to derive the approximate fitting formula between the local contact force and the web tension. Firstly, the schematic of web tension estimation method by local contact force measurement is presented; Then the mass spring model is used to derive the governing equations, which is discretized and solved by Improved Euler method. And approximate fitting formula of the local contact force with tension is derived based on the numerical results. Finally, to validate the fitting formula, the experiments are carried out to detect the local contact force and estimate both the web tension and its distribution.

Facile large-area uniform photolithography of membrane diffractive lens based on vacuum assisted self contact method

Optical polyimide (PI) membrane is a promising substrate material for diffractive lens applied in future large-aperture space based imaging system because of its light weight, environmental adaptability and deployable feature. In this letter, we put forward a facile large-area uniform photolithography technique using vacuum assisted self contact method to fabricate large-aperture membrane diffractive lens. We fabricated a φ 400 mm aperture membrane off-axis 2-levels Fresnel Zone Lens (FZL) based on the method and achieved uniformly distributed photoresist morphology as well as over 36.6% average diffraction efficiency in full aperture. The results demonstrated that vacuum assisted self contact method effectively eliminates considerable air gaps caused by unevenness of large area photomask and substrate, thus facilitates uniform light field distribution in photoresist. This work provides reference to fabrication techniques of large aperture membrane diffractive lens, and offers feasible methods for future large area flexible electronics manufacturing.

Optimization of TiO2 Mesoporous Photoanodes Prepared by Inkjet Printing and Low-Temperature Plasma Processing

This study describes and evaluates a low-temperature method for ambient-air plasma fabrication of hybrid titania/silica nanocomposite mesoporous layers that generate and transport electrons. The mesoporous layers were prepared using wet coating with a dispersion consisting of prefabricated titania nanoparticles and polysiloxane binder, subsequently inkjet-printed and plasma-processed by diffuse coplanar surface barrier discharge, to form an almost inorganic titania/silica coating of the high specific surface. The study demonstrates approaches to optimization of the coating, using various TiO2 nanoparticles, and of the plasma processing, together with the economic significance of printing and plasma processing—an approach compatible with processes envisaged for the manufacture of flexible electronics.

激光剥离技术在柔性电子制造领域的应用研究进展

激光剥离技术在柔性电子制造领域的应用研究进展