Advancements in transfer printing techniques for flexible electronics:adjusting interfaces and promoting versatility

The burgeoning interest in flexible electronics necessitates the creation of patterning technology specifically tailored for flexible substrates and complex surface morphologies.Among a variety of patterning techniques,transfer printing emerges as one of the most efficient,cost-effective,and scalable methods.It boasts the ability for high-throughput fabrication of 0–3D micro-and nano-structures on flexible substrates,working in tandem with traditional lithography methods.This review highlights the critical issue of transfer printing:the flawless transfer of devices during the pick-up and printing process.We encapsulate recent advancements in numerous transfer printing techniques,with a particular emphasis on strategies to control adhesion forces at the substrate/device/stamp interfaces.These strategies are employed to meet the requirements of competing fractures for successful pick-up and print processes.The mechanism,advantages,disadvantages,and typical applications of each transfer printing technique will be thoroughly discussed.The conclusion section provides design guidelines and probes potential directions for future advancements.

Novel fabrication techniques for ultra-thin silicon based flexible electronics

Flexible electronics offer a multitude of advantages,such as flexibility,lightweight property,portability,and high durability.These unique properties allow for seamless applications to curved and soft surfaces,leading to extensive utilization across a wide range of fields in consumer electronics.These applications,for example,span integrated circuits,solar cells,batteries,wearable devices,bio-implants,soft robotics,and biomimetic applications.Recently,flexible electronic devices have been developed using a variety of materials such as organic,carbon-based,and inorganic semiconducting materials.Silicon(Si)owing to its mature fabrication process,excellent electrical,optical,thermal properties,and cost efficiency,remains a compelling material choice for flexible electronics.Consequently,the research on ultra-thin Si in the context of flexible electronics is studied rigorously nowadays.The thinning of Si is crucially important for flexible electronics as it reduces its bending stiffness and the resultant bending strain,thereby enhancing flexibility while preserving its exceptional properties.This review provides a comprehensive overview of the recent efforts in the fabrication techniques for forming ultra-thin Si using top-down and bottom-up approaches and explores their utilization in flexible electronics and their applications.

Research on the Application of Flexible Electronics Technology in Wearable Devices

This study employs theoretical analysis to explore the application prospects of flexible electronics technology in wearable devices. The research first reviews the development history and theoretical foundations of flexible electronics technology, including materials science, electronic engineering, and human-computer interaction theory. Through systematic analysis, the study evaluates the theoretical potential of flexible displays, flexible sensors, and flexible energy storage devices in wearable technology. The research finds that flexible electronics technology can significantly improve the comfort, functionality, and durability of wearable devices. Theoretical analysis indicates that flexible sensors have unique advantages in physiological monitoring and human-computer interaction, while flexible displays and batteries may revolutionize the form and usage patterns of wearable devices. However, the study also points out theoretical challenges faced by flexible electronics technology, such as material stability and feasibility of large-scale manufacturing. To address these challenges, the research proposes an interdisciplinary research framework, emphasizing the synergistic innovation of materials science, electronic engineering, and ergonomics. Finally, the study envisions the theoretical prospects of integrating flexible electronics with other emerging technologies, providing directions for future research.

Flexible electronics and optoelectronics of 2D van der Waals materials

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries,including healthcare and wellness,electronic skins,the automotive industry,and foldable or rollable displays.Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface.Two-dimensional van der Waals materials(2D VdWMs)have exceptional multifunctional properties,including large specific area,dangling-bond-free interface,plane-to-plane van der Waals interactions,and excellent mechanical,electrical,and optical properties.Thus,2D VdWMs have considerable application potential in functional intelligent flexible devices.To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures,new designs and configurations of electronics and optoelectronics have emerged.However,these new designs and configurations do not consider lattice mismatch and process incompatibility issues.In this review,we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly.Moreover,we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics.

Polymer-Sandwich Ultra-Thin Silicon(100) Platform for Flexible Electronics

As a potential flexible substrate for flexible electronics, a polymer-sandwiched ultra-thin silicon platform is stud- ied. SU-8 photoresist coated on the silicon membrane improves its flexibility as shown by an ANSYS simulation. Using the plasma enhanced chemical vapor deposited Si02/Si3N4 composite film as an etching mask, a 4＂ silicon- （100） wafer is thinned to 26[tm without rupture in a 30wt.% KOH solution. The thinned wafer is coated on both sides with 20 pm of SU-8 photoresist and is cut into strips. Then the strips are bent by a caliper to measure its bending radius. A sector model of bending deformation is adopted to estimate the radius of curvature. The determined minimal bending radius of the polymer-sandwiched ultra-thin silicon layer is no more than 3.3mm. The fabrication process of this sandwich structure can be used as a post-fabrication process for high performance flexible electronics.

Third-order polynomial model for analyzing stickup state laminated structure in flexible electronics

Laminated hard-soft integrated structures play a significant role in the fabrication and development of flexible electronics devices. Flexible electronics have advantageous characteristics such as soft and light-weight, can be folded,twisted, flipped inside-out, or be pasted onto other surfaces of arbitrary shapes. In this paper, an analytical model is presented to study the mechanics of laminated hard-soft structures in flexible electronics under a stickup state. Thirdorder polynomials are used to describe the displacement field,and the principle of virtual work is adopted to derive the governing equations and boundary conditions. The normal strain and the shear stress along the thickness direction in the bimaterial region are obtained analytically, which agree well with the results from finite element analysis. The analytical model can be used to analyze stickup state laminated structures, and can serve as a valuable reference for the failure prediction and optimal design of flexible electronics in the future.

In situ reduction strategy towards high conductivity,anti-freezing and super-stretchable rGO based hydrogel for diverse flexible electronics

Hydrogels electrolytes with flexibility and high conductivity have been widely used in kinds of flexible electronics.However,hydrogels always suffer from the inevitable freezing of water at subzero temperatures,which results in the sacrificing of their electrical properties.Herein,an anti-freezing,flexible hydrogel based on in situ reduction of graphene oxide(GO)and laponite has been developed as electrolyte for high performance supercapacitor and sensitive sensors.The crosslinked GO and laponite in polyacrylamide(PAM)resulted in an enhanced mechanical property,while the in-situ reduction of GO in the hydrogel enhanced the conductivity and diminishes the aggregated of GO.These features guarantee a reliable electro signal as sensor and a high performance of the supercapacitor.Besides,in the process of preparation of reduced graphene oxide(rGO)hydrogel,the addition of ethylene glycol(EG)and KOH,endows the hydrogel antifreeze properties.This anti-freezing electrolyte can be stretched to a strain of 1600%and maintained a specific capacitance of 37.38 F·g^(-1) at-20℃.In addition,the photothermal conversion character of rGO in the hydrogel,endows it’s the potential application in wound healing.The overall merits of the hydrogel will open up a new avenue for sensitive sensor and energy storage device in practical applications.

Boosting flexible electronics with integration of two-dimensional materials

Flexible electronics has emerged as a continuously growing field of study.Two-dimensional(2D)materials often act as conductors and electrodes in elec-tronic devices,holding significant promise in the design of high-performance,flexible electronics.Numerous studies have focused on harnessing the potential of these materials for the development of such devices.However,to date,the incorporation of 2D materials in flexible electronics has rarely been summa-rized or reviewed.Consequently,there is an urgent need to develop compre-hensive reviews for rapid updates on this evolving landscape.This review covers progress in complex material architectures based on 2D materials,including interfaces,heterostructures,and 2D/polymer composites.Addition-ally,it explores flexible and wearable energy storage and conversion,display and touch technologies,and biomedical applications,together with integrated design solutions.Although the pursuit of high-performance and high-sensitivity instruments remains a primary objective,the integrated design of flexible electronics with 2D materials also warrants consideration.By combin-ing multiple functionalities into a singular device,augmented by machine learning and algorithms,we can potentially surpass the performance of existing wearable technologies.Finally,we briefly discuss the future trajectory of this burgeoning field.This review discusses the recent advancements in flex-ible sensors made from 2D materials and their applications in integrated archi-tecture and device design.

Recent progress in printing flexible electronics: A review

Miniaturization and flexibility are becoming the trend in the development of electronic products. These key features are driving new methods in the manufacturing of such products. Printed electronics technology is a novel additive manufacturing technique that uses active inks to print onto a diverse set of substrates, realizing large-area, low-cost, flexible and green manufacturing of electronic products. These advantageous properties make it extremely compatible with flexible electronics fabrication and extend as far as offering revolutionary methods in the production of flexible electronic devices. In this paper, the details of a printing process system are introduced, including the materials that can be employed as inks, common substrates, and the most recently reported printing strategies. An assessment of future setbacks and developments of printed flexible electronics is also presented.

Electrohydrodynamic printing for high resolution patterning of flexible electronics toward industrial applications

Electrohydrodynamic(EHD)printing technique,which deposits micro/nanostructures through high electric force,has recently attracted significant research interest owing to their fascinating characteristics in high resolution(<1μm),wide material applicability(ink viscosity 1–10000 cps),tunable printing modes(electrospray,electrospinning,and EHD jet printing),and compatibility with flexible/wearable applications.Since the laboratory level of the EHD printed electronics'resolution and efficiency is gradually approaching the commercial application level,an urgent need for developing EHD technique from laboratory into industrialization have been put forward.Herein,we first discuss the EHD printing technique,including the ink design,droplet formation,and key technologies for promoting printing efficiency/accuracy.Then we summarize the recent progress of EHD printing in fabrication of displays,organic field-effect transistors(OFETs),transparent electrodes,and sensors and actuators.Finally,a brief summary and the outlook for future research effort are presented.

Multifunctional Flexible Humidity Sensor Systems Towards Noncontact Wearable Electronics

In the past decade,the global industry and research attentions on intelligent skin-like electronics have boosted their applications in diverse fields including human healthcare,Internet of Things,human–machine interfaces,artificial intelligence and soft robotics.Among them,flexible humidity sensors play a vital role in noncontact measurements relying on the unique property of rapid response to humidity change.This work presents an overview of recent advances in flexible humidity sensors using various active functional materials for contactless monitoring.Four categories of humidity sensors are highlighted based on resistive,capacitive,impedance-type and voltage-type working mechanisms.Furthermore,typical strategies including chemical doping,structural design and Joule heating are introduced to enhance the performance of humidity sensors.Drawing on the noncontact perception capability,human/plant healthcare management,human-machine interactions as well as integrated humidity sensor-based feedback systems are presented.The burgeoning innovations in this research field will benefit human society,especially during the COVID-19 epidemic,where cross-infection should be averted and contactless sensation is highly desired.

Two-dimensional MXenes:New frontier of wearable and flexible electronics

Wearable electronics offer incredible benefits in mobile healthcare monitoring,sensing,portable energy harvesting and storage,human-machine interactions,etc.,due to the evolution of rigid electronics structure to flexible and stretchable devices.Lately,transition metal carbides and nitrides(MXenes)are highly regarded as a group of thriving two-dimensional nanomaterials and extraordinary building blocks for emerging flexible electronics platforms because of their excellent electrical conductivity,enriched surface functionalities,and large surface area.This article reviews the most recent developments in MXene-enabled flexible electronics for wearable electronics.Several MXeneenabled electronic devices designed on a nanometric scale are highlighted by drawing attention to widely developed nonstructural attributes,including 3D configured devices,textile and planer substrates,bioinspired structures,and printed materials.Furthermore,the unique progress of these nanodevices is highlighted by representative applications in healthcare,energy,electromagnetic interference(EMI)shielding,and humanoid control of machines.The emerging prospects of MXene nanomaterials as a key frontier in nextgeneration wearable electronics are envisioned and the design challenges of these electronic systems are also discussed,followed by proposed solutions.

Recent advances in designing conductive hydrogels for flexible electronics

Flexible electronics have emerged as an exciting research area in recent years,serving as ideal interfaces bridging biological systems and conventional electronic devices.Flexible electronics can not only collect physiological signals for human health monitoring but also enrich our daily life with multifunctional smart materials and devices.Conductive hydrogels(CHs)have become promising candidates for the fabrication of flexible electronics owing to their biocompatibility,adjustable mechanical flexibility,good conductivity,and multiple stimuli-responsive properties.To achieve on-demand mechanical properties such as stretchability,compressibility,and elasticity,the rational design of polymer networks via modulating chemical and physical intermolecular interactions is required.Moreover,the type of conductive components(eg,electron-conductive materials,ions)and the incorporation method also play an important role in the conductivity of CHs.Electron-CHs usually possess excellent conductivity,while ion-CHs are generally transparent and can generate ion gradients within the hydrogel matrices.This mini review focuses on the recent advances in the design of CHs,introducing various design strategies for electron-CHs and ion-CHs employed in flexible electronics and highlighting their versatile applications such as biosensors,batteries,supercapacitors,nanogenerators,actuators,touch panels,and displays.

Semi-liquid metal and adhesion-selection enabled rolling and transfer(SMART) printing:A general method towards fast fabrication of flexible electronics

Recent breakthrough in eutectic gallium-indium alloy has revealed its great potential in modern electronic engineering. Here, we established a general method towards super-fast fabrication of flexible electronics via semi-liquid metal and adhesion-selection enabled rolling and transfer (SMART) printing on various substrates. Based on the semiliquid metal and its adhesion-difference on specifically designed target materials, we demonstrated that the rolling and transfer printing method could serve to rapidly manufacture a wide variety of complicated patterns with high resolution and large size. The process is much faster than most of the currently existing electronic fabrication strategies including liquid metal printing ever developed, and the cost either in time or consumption rate is rather low. As illustrated, a series of functional flexible and stretchable electronics such as multiple layer and large area circuits were fabricated to show their superior merit in combination with electrical conductivity and deformability. In addition, it was also demonstrated that the electronics fabricated in this way exhibited good repeatablity. A most noteworthy advantage is that all the fabrication processes could be highly automatic in the sense that user-friendly machines can thus be developed. This method paves a practical way for super-fast soft electronics manufacture and is expected to play an important role in the coming industry and consumer electronics.

A transparent, conducting tape for flexible electronics

Transparent electrodes are essential components for optoelectronic devices such as displays and thin-film solar cells. Traditionally, the deposition of transparent conducting layers and the sealing of the device are separate steps. Here we report on a highly transparent, conductive, and flexible ＂tape＂, which can be obtained by transferring silver nanowire networks to conventional transparent tape. We utilized the viscidity of the tape to reduce the junction resistance between silver nanowires and further protect the nanowires from corrosion, oxidation and mechanical damage. By this simple method, we obtained a flexible tape with high transparency （-90% at 550 nm wavelength） and low sheet resistance （approaching 22 Ω.sq^-1）. The transparent tape can be attached and stuck firmly on complex surfaces, making the surface highly conductive. We demonstrated the use of the tape as both a conducting layer and a sealing layer for flexible electronics applications including in-situ temperature monitoring and electrochromic devices.

Ultra-antifreeze,ultra-stretchable,transparent,and conductive hydrogel for multi-functional flexible electronics as strain sensor and triboelectric nanogenerator

Conductive hydrogels have become one of the most promising candidates for flexible electronics due to their excellent mechanical flexibility,durability of deformation,and good electrical conductivity.However,in real applications,severe environments occur frequently,such as extremely cold weather.General hydrogels always lack anti-freeze and anti-dehydration abilities.Consequently,the functions of electronic devices based on traditional hydrogels will quickly fail in extreme environments.Therefore,the development of environmentally robust hydrogels that can withstand extremely low temperatures,overcome dehydration,and ensure the stable operation of electronic devices has become increasingly important.Here,we report a kind of graphene oxide(GO)incorporated polyvinyl alcohol-polyacrylamide(PVA-PAAm)double network hydrogel(GPPDhydrogel)which shows excellent anti-freeze ability.The GPPD-hydrogel exhibits not only good flexibility and ultra-high stretchability up to 2,000%,but ensures a high sensitivity when used as the strain sensor at−50°C.More importantly,when serving as the electrode of a sandwich-structural triboelectric nanogenerator(TENG),the GPPD-hydrogel endows the TENG high and stable output performances even under−80°C.Besides,the GPPD-hydrogel is demonstrated long-lasting moisture retention over 100 days.The GPPD-hydrogel provides a reliable and promising candidate for the new generation of wearable electronics.

Recent progress of flexible electronics by 2D transition metal dichalcogenides

Flexible electronics is the research field with interdisciplinary crossing and integration.It shows the promising advantages of novel device configurations,low-cost and low-power consumption due to their flexible and soft characteristics.Atomic layered two-dimensional(2D)materials especially transition metal dichalcogenides,have triggered great interest in ultra-thin 2D flexible electronic devices and optoelectronic devices because of their direct and tunable bandgaps,excellent electrical,optical,mechanical,and thermal properties.This review aims to provide the recent progress in 2D TMDs and their applications in flexible electronics.The fundamental electrical properties and mechanical properties of materials,flexible device configurations,and their performance in transistors,sensors,and photodetectors are thoroughly discussed.At last,some perspectives are given on the open challenges and prospects for 2D TMDs flexible electronic devices and new device opportunities.

An overview of healthcare monitoring by flexible electronics

Flexible electronics integrated with stretchable/bendable structures and various microsensors that monitor the temperature,pressure, sweat, bioelectricity, body hydration, etc., have a wide range of applications in the human healthcare sector. The science underlying this technology draws from many research areas, such as information technology, materials science, and structural mechanics, to efficiently and accurately monitor technology for various signals. In this paper, we make a classification and comb to the designs, materials, structures and functions of numerous flexible electronics for signal monitoring in the human healthcare sector. Some perspectives in this field are discussed in the concluding remarks.

Flexible electronics based on one-dimensional and two-dimensional hybrid nanomaterials

Research on flexible or wearable electronics has been grown remarkably due to the advent of nanomaterials,such as metal nanowires,graphene,or transition metal dichalcogenides.Although each nanomaterial has mechanical and electrical characteristics that can be applied into flexible electronics,the limitations of each nanomaterial are also clear.In order to overcome the limitations of these nanomaterials,research on the hybrid structures of nanomaterials has been extensively conducted.In this study,we introduce the properties of one-dimensional nanomaterials,twodimensional nanomaterials,and their hybrid nanomaterials.And then,we provide information concerning various flexible electronics based on these nanomaterials.

Flexible electronics manufacturing technology and equipment

Flexible electronics such as mechanically compliant displays,sensors and solar cells,have important applications in the fields of energy,national defence and biomedicine,etc.Various types of flexible electronics have been proposed or developed by the improvements in structural designs,material properties and device integrations.However,the manufacturing of flexible electronics receives little attention,which limits its mass production and industrialization.The increasing demands on the size,functionality,resolution ratio and reliability of flexible electronics bring several significant challenges in their manufacturing processes.This work aims to report the state-of-art technologies and applications of flexible electronics manufacturing.Three key technologies including electrohydrodynamic direct-writing,flip chip and automatic optical inspection are highlighted.The mechanism and developments of these technologies are discussed in detail.Based on these technologies,the present work develops three kinds of manufacturing equipment,i.e.,inkjet printing manufacturing equipment,robotized additive manufacturing equipment,and roll-to-roll manufacturing equipment.The advanced manufacturing processes,equipment and systems for flexible electronics pave the way for applications of new displays,smart sensing skins and epidermal electronics,etc.By reviewing the developments of flexible electronics manufacturing technology and equipment,it can be found that the existing advances greatly promote the applications and commercialization of flexible electronics.Since flexible electronics manufacturing contains many multi-disciplinary problems,the current investigations are confronted with great challenges.Therefore,further developments of the reviewed manufacturing technology and equipment are necessary to break the current limitations of manufacturing resolution,efficiency and reliability.