Flexible Interconnects for Electronic Textiles

Kirigami-Triggered Spoof Plasmonic Interconnects for Radiofrequency Elastronics

The flexible and conformal interconnects for electronic systems as a potential signal transmission device have great prospects in body-worn or wearable applications. High-efficiency wave propagation and conformal structure deformation around human body at radio communication are still confronted with huge challenges due to the lack of methods to control the wave propagation and achieve the deformable structure simultaneously. Here, inspired by the kirigami technology, a new paradigm to construct spoof plasmonic interconnects (SPIs) that support radiofrequency (RF) surface plasmonic transmission is proposed, together with high elasticity, strong robustness, and multifunction performance. Leveraging the strong field-confinement characteristic of spoof surface plasmons polaritons, the Type-I SPI opens its high-efficiency transmission band after stretching from a simply connected metallic surface. Meanwhile, the broadband transmission of the kirigami-based SPI exhibits strong robustness and excellent stability undergoing complex deformations, i.e., bending, twisting, and stretching. In addition, the prepared Type-II SPI consisting of 2 different subunit cells can achieve band-stop transmission characteristics, with its center frequency dynamically tunable by stretching the buckled structure. Experimental measurements verify the on-off switching performance in kirigami interconnects triggered by stretching. Overcoming the mechanical limitation of rigid structure with kirigami technology, the designer SPIs exhibit high stretchability through out-of-plane structure deformation. Such kirigami-based interconnects can improve the elastic functionality of wearable RF electronics and offer high compatibility to large body motion in future body network systems.

Exploring the electrical robustness of conductive textile fasteners for wearable devices in different human motion conditions

Conventional snap fasteners used in clothing are often used as electrical connectors in e-textile and wearable applications for signal transmission due to their wide availability and ease of use. Nonetheless, limited research exists on the validation of these fasteners, regarding the impact of contact-induced high-amplitude artefacts, especially under motion conditions. In this work, three types of fasteners were used as electromechanical connectors, establishing the interface between a regular sock and an acquisition device. The tested fasteners have different shapes and sizes, as well as have different mechanisms of attachment between the plug and receptacle counterparts. Experimental evaluation was performed under static conditions, slow walking, and rope jumping at a high cadence. The tests were also performed with a test mass of 140 g. Magnetic fasteners presented excellent electromechanical robustness under highly dynamic human movement with and without the additional mass. On the other hand, it was demonstrated that the Spring snap buttons (with a spring-based engaging mechanism) presented a sub-optimal performance under high motion and load conditions, followed by the Prong snap fasteners (without spring), which revealed a high susceptibility to artefacts. Overall, this work provides further evidence on the importance and reliability of clothing fasteners as electrical connectors in wearable systems.

Porous Conductive Textiles for Wearable Electronics

Over the years, researchers have made significant strides in the development of novel flexible/stretchable and conductive materials, enabling the creation of cutting-edge electronic devices for wearable applications. Among these, porous conductive textiles (PCTs) have emerged as an ideal material platform for wearable electronics, owing to their light weight, flexibility, permeability, and wearing comfort. This Review aims to present a comprehensive overview of the progress and state of the art of utilizing PCTs for the design and fabrication of a wide variety of wearable electronic devices and their integrated wearable systems. To begin with, we elucidate how PCTs revolutionize the form factors of wearable electronics. We then discuss the preparation strategies of PCTs, in terms of the raw materials, fabrication processes, and key properties. Afterward, we provide detailed illustrations of how PCTs are used as basic building blocks to design and fabricate a wide variety of intrinsically flexible or stretchable devices, including sensors, actuators, therapeutic devices, energy-harvesting and storage devices, and displays. We further describe the techniques and strategies for wearable electronic systems either by hybridizing conventional off-the-shelf rigid electronic components with PCTs or by integrating multiple fibrous devices made of PCTs. Subsequently, we highlight some important wearable application scenarios in healthcare, sports and training, converging technologies, and professional specialists. At the end of the Review, we discuss the challenges and perspectives on future research directions and give overall conclusions. As the demand for more personalized and interconnected devices continues to grow, PCT-based wearables hold immense potential to redefine the landscape of wearable technology and reshape the way we live, work, and play.

Construction Strategy for Flexible and Breathable SiO2/Al/NFs/PET Composite Fabrics with Dual Shielding against Microwave and Infrared-Thermal Radiations for Wearable Protective Clothing

Microwave and infrared-thermal radiation-compatible shielding fabrics represent an important direction in the development of wearable protective fabrics. Nevertheless, effectively and conveniently integrating compatible shielding functions into fabrics while maintaining breathability and moisture permeability remains a significant challenge. Here, we take hydrophilic PVA-co-PE nanofibrous film-coated PET fabric (NFs/PET) as a flexible substrate and deposit a dielectric/conductive (SiO2/Al) bilayer film via magnetron sputtering. This strategy endows the fabric surface with high electrical conductivity, nanoscale roughness comparable to visible and infrared waves, and a dielectric-metal contact interface possessing localized plasmon resonance and Mie scattering effects. The results demonstrate that the optimized SiO2/Al/NFs/PET composite conductive fabric (referred to as S4-1) possesses favorable X-band electromagnetic interference (EMI) shielding effectiveness (50 dB) as well as excellent long-wave infrared (LWIR) shielding or IR stealth performance (IR emissivity of 0.60). Notably, the S4-1 fabric has a cooling effect of about 12.4 °C for a heat source (80 °C) and an insulating effect of about 17.2 °C for a cold source (-20 °C), showing excellent shielding capability for heat conduction and heat radiations. Moreover, the moisture permeability of the S4-1 fabric is about 300 g/(m2·h), which is better than the requirement concerning moisture permeability for wearable fabrics (≥2500-5000 g/(m2·24 h)), indicating excellent heat and moisture comfort. In short, our fabrics have lightweight, thin, moisture-permeable and excellent shielding performance, which provides novel ideas for the development of wearable multi-band shielding fabrics applied to complex electromagnetic environments.

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.

LEGO-like Assembly of Fibrous Modules for Display Textiles

Due to their promising advantages over classical rigid devices, the development of display textiles has exciting potential for various fields, including sensor technology, healthcare, and communication. To realize display textiles, it is necessary to prepare light-emitting building blocks at the fiber level and then weave or knit them to form the desired textile structures. However, from a practical viewpoint, it is difficult to continuously weave functional fibers containing light-emitting devices using conventional textile technologies. To address this issue, we introduced fibrous modules that can be assembled like LEGO blocks to realize textile displays. A unique feature of this work is that the light-emitting pixels are generated through a simple contact between modular electrochemiluminescent (ECL) fibers. Each fiber is composed of a single metallic wire coated with a gel-type ECL electrolyte that is formed by using a simple dip-coating method in ambient air. The sticky nature of the gel electrolyte enables the construction of light-emitting pixels through the simple physical contact of two or more fiber modules without the need for external pressure or heating. The diversity of this technology offers in terms of fibrous module arrangements and assembly can provide various options for designing the geometries of light-emitting pixels. We have implemented this technique to demonstrate not only a 1 × 1 pixel but also 3 × 3 pixels with an irregular shape. These results demonstrate that the unique strategy for display devices developed in this work provides a feasible approach for various electronic and optical textile applications.

Interconnection Technologies for Flexible Electronics: Materials, Fabrications, and Applications

Flexible electronic devices require metal interconnects to facilitate the flow of electrical signals among the device components, ensuring its proper functionality. There are multiple factors to consider when designing metal interconnects for flexible electronics, including their conductivity, flexibility, reliability, and cost. This article provides an overview of recent endeavors to create flexible electronic devices through different metal interconnect approaches, with a focus on materials and structural aspects. Additionally, the article discusses emerging flexible applications, such as e-textiles and flexible batteries, as essential considerations.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Electronic textiles: New age of wearable technology for healthcare and fitness solutions

Sedentary lifestyles and evolving work environments have created challenges for global health and cause huge burdens on healthcare and fitness systems. Physical immobility and functional losses due to aging are two main reasons for noncommunicable disease mortality. Smart electronic textiles (e-textiles) have attracted considerable attention because of their potential uses in health monitoring, rehabilitation, and training assessment applications. Interactive textiles integrated with electronic devices and algorithms can be used to gather, process, and digitize data on human body motion in real time for purposes such as electrotherapy, improving blood circulation, and promoting wound healing. This review summarizes research advances on e-textiles designed for wearable healthcare and fitness systems. The significance of e-textiles, key applications, and future demand expectations are addressed in this review. Various health conditions and fitness problems and possible solutions involving the use of multifunctional interactive garments are discussed. A brief discussion of essential materials and basic procedures used to fabricate wearable e-textiles are included. Finally, the current challenges, possible solutions, opportunities, and future perspectives in the area of smart textiles are discussed.

Recent progress in flexible pressure sensors based on multiple microstructures: from design to application

Flexible pressure sensors (FPSs) have been widely studied in the fields of wearable medical monitoring and human-machine interaction due to their high flexibility, light weight, sensitivity, and easy integration. To better meet these application requirements, key sensing properties such as sensitivity, linear sensing range, pressure detection limits, response/recovery time, and durability need to be effectively improved. Therefore, researchers have extensively and profoundly researched and innovated on the structure of sensors, and various microstructures have been designed and applied to effectively improve the sensing performance of sensors. Compared with single microstructures, multiple microstructures (MMSs) (including hierarchical, multi-layered and hybrid microstructures) can improve the sensing performance of sensors to a greater extent. This paper reviews the recent research progress in the design and application of FPSs with MMSs and systematically summarizes the types, sensing mechanisms, and preparation methods of MMSs. In addition, we summarize the applications of FPSs with MMSs in the fields of human motion detection, health monitoring, and human-computer interaction. Finally, we provide an outlook on the prospects and challenges for the development of FPSs.

Soft Fiber Electronics Based on Semiconducting Polymer

Fibers, originating from nature and mastered by human, have woven their way throughout the entire history of human civilization. Recent developments in semiconducting polymer materials have further endowed fibers and textiles with various electronic functions, which are attractive in applications such as information interfacing, personalized medicine, and clean energy. Owing to their ability to be easily integrated into daily life, soft fiber electronics based on semiconducting polymers have gained popularity recently for wearable and implantable applications. Herein, we present a review of the previous and current progress in semiconducting polymer-based fiber electronics, particularly focusing on smart-wearable and implantable areas. First, we provide a brief overview of semiconducting polymers from the viewpoint of materials based on the basic concepts and functionality requirements of different devices. Then we analyze the existing applications and associated devices such as information interfaces, healthcare and medicine, and energy conversion and storage. The working principle and performance of semiconducting polymer-based fiber devices are summarized. Furthermore, we focus on the fabrication techniques of fiber devices. Based on the continuous fabrication of one-dimensional fiber and yarn, we introduce two- and three-dimensional fabric fabricating methods. Finally, we review challenges and relevant perspectives and potential solutions to address the related problems.

Recent Advances and Challenges Toward Application of Fibers and Textiles in Integrated Photovoltaic Energy Storage Devices

Flexible microelectronic devices have seen an increasing trend toward development of miniaturized, portable, and integrated devices as wearable electronics which have the requirement for being light weight, small in dimension, and suppleness. Traditional three-dimensional (3D) and two-dimensional (2D) electronics gadgets fail to effectively comply with these necessities owing to their stiffness and large weights. Investigations have come up with a new family of one-dimensional (1D) flexible and fiber-based electronic devices (FBEDs) comprising power storage, energy-scavenging, implantable sensing, and flexible displays gadgets. However, development and manufacturing are still a challenge owing to their small radius, flexibility, low weight, weave ability and integration in textile electronics. This paper will provide a detailed review on the importance of substrates in electronic devices, intrinsic property requirements, fabrication classification and applications in energy harvesting, energy storage and other flexible electronic devices. Fiber- and textile-based electronic devices for bulk/scalable fabrications, encapsulation, and testing are reviewed and presented future research ideas to enhance the commercialization of these fiber-based electronics devices.

Single-arm diagnostic electrocardiography with printed graphene on wearable textiles

Stimulated by the COVID-19 outbreak, the global healthcare industry better acknowledges the necessity of innovating novel methods for remote healthcare monitoring and treating patients outside clinics. Here we report the development of two different types of graphene textile electrodes differentiated by the employed fabrication techniques (i.e., dip-coating and spray printing) and successful demonstration of ergonomic and truly wearable, single-arm diagnostic electrocardiography (SADE) using only 3 electrodes positioned on only 1 arm. The performance of the printed graphene e-textile wearable systems were benchmarked against the "gold standard" silver/silver chloride (Ag/AgCl) "wet" electrodes; achieving excellent correlation up to ∼ 96% and ∼ 98% in ECG recordings (15 s duration) acquired with graphene textiles fabricated by dip-coating and spray printing techniques, respectively. In addition, we successfully implemented automatic detection of heartrate of 8 volunteers (mean value: 74.4 bpm) during 5 min of static and dynamic daily activities and benchmarked their recordings with a standard fingertip photoplethysmography (PPG) device. Heart rate variability (HRV) was calculated, and the root means successive square difference (rMMSD) metric was 30 ms during 5 min of recording. Other cardiac parameters such as R-R interval, QRS complex duration, S-T segment duration, and T-wave duration were also detected and compared to typical chest ECG values.

Stretchable One-Dimensional Conductors for Wearable Applications

Continuous, one-dimensional (1D) stretchable conductors have attracted significant attention for the development of wearables and soft-matter electronics. Through the use of advanced spinning, printing, and textile technologies, 1D stretchable conductors in the forms of fibers, wires, and yarns can be designed and engineered to meet the demanding requirements for different wearable applications. Several crucial parameters, such as microarchitecture, conductivity, stretchability, and scalability, play essential roles in designing and developing wearable devices and intelligent textiles. Methodologies and fabrication processes have successfully realized 1D conductors that are highly conductive, strong, lightweight, stretchable, and conformable and can be readily integrated with common fabrics and soft matter. This review summarizes the latest advances in continuous, 1D stretchable conductors and emphasizes recent developments in materials, methodologies, fabrication processes, and strategies geared toward applications in electrical interconnects, mechanical sensors, actuators, and heaters. This review classifies 1D conductors into three categories on the basis of their electrical responses: (1) rigid 1D conductors, (2) piezoresistive 1D conductors, and (3) resistance-stable 1D conductors. This review also evaluates the present challenges in these areas and presents perspectives for improving the performance of stretchable 1D conductors for wearable textile and flexible electronic applications.

Flexible stretchable electrothermally/photothermally dual-driven heaters from nano-embedded hierarchical CuxS-Coated PET fabrics for all-weather wearable thermal management

The multifunctional photoelectronic devices are recently attracting much more attention due to their potential enlarged applications. The flexible stretchable electrothermally/photothermally dual-driven heaters for all-weather wearable thermal management are presented in this work with nano-embedded hierarchical Cu_xS-coated PET fabrics. Herein, the hierarchical nano-embedded Cu_xS film is fabricated via a simple chemical bath method for high electrical conductivity and highly efficient inelastic collision of electro/photo-generated carriers. The hierarchical nano-embedded Cu_xS morphology produces the low sheet resistance of 1.26 Ω sq^-1 and the super low total heat transfer coefficient of 3.256 × 10^-5 W/^oC·mm², which lead to the high-efficient electro/photo-dual-driven heating effect in the Cu_xS@PET fabrics. The saturated temperature on the as-fabricated flexible wearable heaters reaches up to 172 °C. The thermal conversion devices also bear the excellent stability, reproducibility, stretchability, controllability and corrosion-resistant characteristics. Interestingly, their excellent thermal conversion performance could be achieved by freely exchanging the driving power sources, such as electricity-supplying equipment, 635-nm laser, infrared physiotherapy lamp and solar simulator, which provides a necessary precondition for the all-weather applications of flexible wearable heaters. The as-fabricated electro/photo-dual-driven heaters on the Cu_xS@PET fabrics have the promising applications in wearable electronics, all-weather self-heating facilities, out/in-vivo physiotherapy, and so on.

Low-Temperature Soldering of Surface Mount Devices on Screen-Printed Silver Tracks on Fabrics for Flexible Textile Hybrid Electronics

The combination of flexible-printed substrates and conventional electronics leads to flexible hybrid electronics. When fabrics are used as flexible substrates, two kinds of problems arise. The first type is related to the printing of the tracks of the corresponding circuit. The second one concerns the incorporation of conventional electronic devices, such as integrated circuits, on the textile substrate. Regarding the printing of tracks, this work studies the optimal design parameters of screen-printed silver tracks on textiles focused on printing an electronic circuit on a textile substrate. Several patterns of different widths and gaps between tracks were tested in order to find the best design parameters for some footprint configurations. With respect to the incorporation of devices on textile substrates, the paper analyzes the soldering of surface mount devices on fabric substrates. Due to the substrate's nature, low soldering temperatures must be used to avoid deformations or damage to the substrate caused by the higher temperatures used in conventional soldering. Several solder pastes used for low-temperature soldering are analyzed in terms of joint resistance and shear force application. The results obtained are satisfactory, demonstrating the viability of using flexible hybrid electronics with fabrics. As a practical result, a simple single-layer circuit was implemented to check the results of the research.

Dynamic Equivalent Resistance Model of Knitted Strain Sensor under In-Plane and Three-Dimensional Surfaces Elongation

The dynamic equivalent resistance is a major index that determines the sensing performance of knitted strain sensors, and has the characteristics of in-plane and three-dimensional curved strain sensing. Therefore, in addition to establishing the in-plane equivalent resistance, it is necessary to establish a three-dimensional equivalent resistance model to fully explain the surface sensing performance. This project establishes two equivalent resistance models of knitted strain sensors under in-plane deformation and one equivalent resistance model of three-dimensional curved surface strain. Based on the length of resistance and the geometric topological structure, an in-plane strain macro–micro equivalent resistance model and a topological equivalent resistance model are established, respectively. In addition, a three-dimensional curved surface equivalent resistance model is created based on the volume resistance. By comparing the theoretical model with the experimental data, the results prove that the proposed in-plane and three-dimensional models can be utilized to calculate the resistance change of knitted strain sensors. Length resistance, coil transfer, and curved surface deformation depth are the main factors that affect the equivalent resistance of knitted strain sensors.

Durability Study of Thermal Transfer Printed Textile Electrodes for Wearable Electronic Applications

Textile-based electronics hold great promise because they can endow wearable devices with soft and comfortable characteristics. However, the inherent porosity and fluffiness of fabrics result in high surface roughness, which presents great challenges in the manufacture of high-performance fabric electrodes. In this work, we propose a thermal transfer printing method to address the above challenges, in which electrodes or circuits of silver flake/thermoplastic polyurethane (TPU) composites are prefabricated on a release film by coating and laser engraving and then laminated by hot-pressing to a variety of fabrics and textiles. This universal and scalable production technique enables fabric electrodes to be made without compromising the original wearability, washability, and stretchability of textiles. The prepared fabric electrodes exhibit high conductivity (5.48 × 10⁴ S/cm), high adhesion (≥1750 N/m), good abrasion/washing resistance, high patterning resolution (∼40 μm), and good electromechanical performance up to 50% strain. To demonstrate the potential applications, we developed textile-based radio frequency identification (RFID) tags for remote identification and a large-sized heater for wearable thermotherapy. More importantly, the solvent-free thermal transfer printing technology developed in this paper enables people to DIY interesting flexible electronics on clothes with daily tools, which can promote the commercial application of smart textile-based electronics.

Silk Fiber Multiwalled Carbon Nanotube-Based Micro-/Nanofiber Composite as a Conductive Fiber and a Force Sensor

Silk cocoon fibers (SFs) are natural polymers that are made up of fibroin protein. These natural fibers have higher mechanical stability and good elasticity properties. In this work, we coated multiwalled carbon nanotubes (MWCNTs) on the surface of SFs using a simple stirring technique with vinegar as the medium. This SF-MWCNT micro-/nanofiber composite was prepared without any adhesives. The characterization results revealed that the SF-MWCNT micro-/nanofiber composite exhibited excellent electrical conductivity (995 Ω cm^-1), tensile strength (up to 200% greater elongation), and durability characteristics. In addition, this micro-/nanofiber composite shows a change in resistance from 1450 to 960 Ω cm^-1 for an applied mechanical force of 0.3-1 N kg^-1. Based on our findings, SF-MWCNT micro-/nanofiber composite-based conductive fibers (CFs) and force sensors (FSs) were developed.

E-textile based modular sEMG suit for large area level of effort analysis

We present a novel design for an e-textile based surface electromyography (sEMG) suit that incorporates stretchable conductive textiles as electrodes and interconnects within an athletic compression garment. The fabrication and assembly approach is a facile combination of laser cutting and heat-press lamination that provides for rapid prototyping of designs in a typical research environment without need for any specialized textile or garment manufacturing equipment. The materials used are robust to wear, resilient to the high strains encountered in clothing, and can be machine laundered. The suit produces sEMG signal quality comparable to conventional adhesive electrodes, but with improved comfort, longevity, and reusability. The embedded electronics provide signal conditioning, amplification, digitization, and processing power to convert the raw EMG signals to a level-of-effort estimation for flexion and extension of the elbow and knee joints. The approach we detail herein is also expected to be extensible to a variety of other electrophysiological sensors.

An Overview of Hierarchical Design of Textile-Based Sensor in Wearable Electronics

Smart textiles have recently aroused tremendous interests over the world because of their broad applications in wearable electronics, such as human healthcare, human motion detection, and intelligent robotics. Sensors are the primary components of wearable and flexible electronics, which convert various signals and external stimuli into electrical signals. While traditional electronic sensors based on rigid silicon wafers can hardly conformably attach on the human body, textile materials including fabrics, yarns, and fibers afford promising alternatives due to their characteristics including light weight, flexibility, and breathability. Of fundamental importance are the needs for fabrics simultaneously having high electrical and mechanical performance. This article focused on the hierarchical design of the textile-based flexible sensor from a structure point of view. We first reviewed the selection of newly developed functional materials for textile-based sensors, including metals, conductive polymers, carbon nanomaterials, and other two-dimensional (2D) materials. Then, the hierarchical structure design principles on different levels from microscale to macroscale were discussed in detail. Special emphasis was placed on the microstructure control of fibers, configurational engineering of yarn, and pattern design of fabrics. Finally, the remaining challenges toward industrialization and commercialization that exist to date were presented.

Influence of Sweat on Joint and Sensor Reliability of E-Textiles

This article addresses reliability under the sweat of interconnection techniques for the mounting surface mounted device (SMD) components and fully printed humidity sensors onto conductive stretchable textile ribbons. Samples underwent testing for the effect of ageing by artificial sweat on their electrical resistance using both alkaline and acidic artificial sweat. The best results in terms of electrical resistance change were obtained for samples soldered to the conductive fibers interwoven in the ribbon. However, this method can damage the ribbon due to the high temperature during soldering and significantly reduce the mechanical properties and flexibility of the ribbon, which can lead to a limited service life of samples. On the other hand, adhesive bonding is a very interesting alternative, where the above-mentioned properties are preserved, but there is a significant effect of sweat ageing on electrical resistance. The results of fully printed graphene-based humidity sensors show that, for the intended use of these sensors (i.e., detection of changes in moisture on the human body), usage of the samples is possible, and the samples are sufficiently reliable in the case of sweat degradation. In addition, the response of the sensor to humidity is quite high: 98% at a relative humidity of 98%.

Review on the Integration of Microelectronics for E-Textile

Modern electronic textiles are moving towards flexible wearable textiles, so-called e-textiles that have micro-electronic elements embedded onto the textile fabric that can be used for varied classes of functionalities. There are different methods of integrating rigid microelectronic components into/onto textiles for the development of smart textiles, which include, but are not limited to, physical, mechanical, and chemical approaches. The integration systems must satisfy being flexible, lightweight, stretchable, and washable to offer a superior usability, comfortability, and non-intrusiveness. Furthermore, the resulting wearable garment needs to be breathable. In this review work, three levels of integration of the microelectronics into/onto the textile structures are discussed, the textile-adapted, the textile-integrated, and the textile-based integration. The textile-integrated and the textile-adapted e-textiles have failed to efficiently meet being flexible and washable. To overcome the above problems, researchers studied the integration of microelectronics into/onto textile at fiber or yarn level applying various mechanisms. Hence, a new method of integration, textile-based, has risen to the challenge due to the flexibility and washability advantages of the ultimate product. In general, the aim of this review is to provide a complete overview of the different interconnection methods of electronic components into/onto textile substrate.

Heat Scanning for the Fabrication of Conductive Fibers

Conductive fibers are essential building blocks for implementing various functionalities in a textile platform that is highly conformable to mechanical deformation. In this study, two major techniques were developed to fabricate silver-deposited conductive fibers. First, a droplet-coating method was adopted to coat a nylon fiber with silver nanoparticles (AgNPs) and silver nanowires (AgNWs). While conventional dip coating uses a large ink pool and thus wastes coating materials, droplet-coating uses minimal quantities of silver ink by translating a small ink droplet along the nylon fiber. Secondly, the silver-deposited fiber was annealed by similarly translating a tubular heater along the fiber to induce sintering of the AgNPs and AgNWs. This heat-scanning motion avoids excessive heating and subsequent thermal damage to the nylon fiber. The effects of heat-scanning time and heater power on the fiber conductance were systematically investigated. A conductive fiber with a resistance as low as ~2.8 Ω/cm (0.25 Ω/sq) can be produced. Finally, it was demonstrated that the conductive fibers can be applied in force sensors and flexible interconnectors.

Fabric based printed-distributed battery for wearable e-textiles: a review

Wearable power supply devices and systems are important necessities for the emerging textile electronic applications. Current energy supply devices usually need more space than the device they power, and are often based on rigid and bulky materials, making them difficult to wear. Fabric-based batteries without any rigid electrical components are therefore ideal candidates to solve the problem of powering these devices. Printing technologies have greater potential in manufacturing lightweight and low-cost batteries with high areal capacity and generating high voltages which are crucial for electronic textile (e-textile) applications. In this review, we present various printing techniques, and battery chemistries applied for smart fabrics, and give a comparison between them in terms of their potential to power the next generation of electronic textiles. Series combinations of many of these printed and distributed battery cells, using electrically conducting threads, have demonstrated their ability to power different electronic devices with a specific voltage and current requirements. Therefore, the present review summarizes the chemistries and material components of several flexible and textile-based batteries, and provides an outlook for the future development of fabric-based printed batteries for wearable and electronic textile applications with enhanced level of DC voltage and current for long periods of time.

Permeable Weldable Elastic Fiber Conductors for Wearable Electronics

Elastic fiber conductors are advantageous for applications in wearable electronics due to their small size, light weight, and excellent integration ability. Here, we report the fabrication of elastic fiber conductors with a three-dimensional (3D) porous structure using electrospun thermoplastic elastomer (TPE) microfibers and silver nanoparticles (AgNPs) as the building blocks. With the 3D porous structure, such a fiber is highly permeable to gases and liquids. As such, the performance of the fiber in many applications of wearable electronics (especially wearable sensors and detectors) can be improved significantly. Benefitting from the excellent processability of TPE and dispersibility of AgNPs, the fiber is highly compatible with thermal and solvent welding. In addition, the fiber also possesses super stretchability, high conductivity, and robust endurance to deformation. As a proof-of-concept application, we demonstrate that a rope-shaped capacitor made by plying one pair of such fibers can detect the volume change of artificial sweat with 17-times higher sensitivity than the capacitor using nonporous fibers as electrodes. We further demonstrate that, by integrating two groups of perpendicularly arranged fibers into a monolithic porous mat, sensitive matrix-addressed monitoring of artificial sweat can be realized.

Futuristic Clothes: Electronic Textiles and Wearable Technologies

This review summarizes the recent developments and importance of wearable electronic textiles in the past decade. Wearable electronic textiles are an emerging interdisciplinary research area that requires new design approaches. This challenging interdisciplinary research field brings together specialists in electronics, information technology, microsystems, and textiles to make an innovation in the development of wearable electronic products. Wearable electronic textiles play a key role among various technologies (clothing, communication, information, healthcare monitoring, military, sensors, magnetic shielding, etc.). In this review, applications of wearable electronic textiles are described, including an investigation of their fabrication techniques. This review highlights the basic processes, possible applications, and main materials to build wearable E-textiles and combines the fundamentals of E-textiles for the readers who have different backgrounds. Moreover, reliability, reusability, and efficiency of wearable electronic textiles are discussed together with the opportunities and drawbacks of the wearable E-textiles that are addressed in this review article.

Application Challenges in Fiber and Textile Electronics

Modern electronic devices are moving toward miniaturization and integration with an emerging focus on wearable electronics. Due to their close contact with the human body, wearable electronics have new requirements including low weight, small size, and flexibility. Conventional 3D and 2D electronic devices fail to efficiently meet these requirements due to their rigidity and bulkiness. Hence, a new family of 1D fiber-shaped electronic devices including energy-harvesting devices, energy-storage devices, light-emitting devices, and sensing devices has risen to the challenge due to their small diameter, lightweight, flexibility, and weavability into soft textile electronics. The application challenges faced by fiber and textile electronics from single fiber-shaped devices to continuously scalable fabrication, to encapsulation and testing, and to application mode exploration, are discussed. The evolutionary trends of fiber and textile electronics are then summarized. Finally, future directions required to boost their commercialization are highlighted.

Fabric Circuit Board Connecting to Flexible Sensors or Rigid Components for Wearable Applications

Electronic textiles demand a new family of flexible circuit boards in the construction of fiber or fiber assemblies. This paper presents a stretchable woven fabric circuit board (FCB) with permanent as well as detachable electrical connections to sensors or other wearable electronics components. The woven FCB was created by integrating conductive yarns into an elastic woven fabric. Permanent connection was designed between the conductive tracks and flexible sensors; detachable connection was achieved by the helical structure of conductive yarns wrapping around the rigid component electrode encapsulated within elastomeric layer. The developed FCB, with its connections to flexible sensors or rigid components, is porous, flexible, and capable of stretching to 30% strain. The woven FCB with permanent connection to temperature sensors has a large fatigue life of more than 10,000 cycles while maintaining constant electrical resistance due to crimped configurations of the conductive track in the elastic fabric substrate and stable contact resistance. A prototype of the FCB assembly, with independent light-emitting diodes electrically linked and mechanically supported by the woven FCB, is also demonstrated for wearable applications.

Semiliquid Metal Enabled Highly Conductive Wearable Electronics for Smart Fabrics

Wearable electronics incorporating electronic components into commonly used fabrics can serve as new-generation personalized health-care systems for applications ranging from health-care monitoring to disease treatment. Conventional rigid materials including gold, silver, and copper generally require a complicated fabrication process to be sewn into clothes. At the same time, other high-stretchable nonmetal materials such as conductive polymers generally have a limitation of low electroconductivity, restricting their further applications. Recently, gallium-based liquid metals have exhibited superior advantages in flexible electronics and have presented valuable potential in creative printing technologies. Here, we proposed a novel wearable electronics prepared through roller printing technology based on the adhesion difference of semiliquid metal (Cu-EGaIn, eutectic gallium-indium mixed with copper microparticles) on cotton fabrics and polyvinyl acetate (PVAC) glue. Results have shown that the surface topography and chemical interaction of fabrics and PVAC glue determine the adhesion effect with the Cu-EGaIn mixture. The electric experiments have demonstrated the electromechanical stability of the fabricated lines on fabrics. Further, a series of smart fabrics were developed including an interactive circuit, stretchable light-emitting diode array, and thermal management device with advantages of easy operation, low cost, and large-area fabrication to show practical applications in the method. This strategy may play an important role in the design and fabrication of smart fabrics, contributing to the development of customized health-care systems.

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS₂ ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

Electrically Conductive Coatings for Fiber-Based E-Textiles

With the advent of wearable electronic devices in our daily lives, there is a need for soft, flexible, and conformable devices that can provide electronic capabilities without sacrificing comfort. Electronic textiles (e-textiles) combine electronic capabilities of devices such as sensors, actuators, energy harvesting and storage devices, and communication devices with the comfort and conformability of conventional textiles. An important method to fabricate such devices is by coating conventionally used fibers and yarns with electrically conductive materials to create flexible capacitors, resistors, transistors, batteries, and circuits. Textiles constitute an obvious choice for deployment of such flexible electronic components due to their inherent conformability, strength, and stability. Coating a layer of electrically conducting material onto the textile can impart electronic capabilities to the base material in a facile manner. Such a coating can be done at any of the hierarchical levels of the textile structure, i.e., at the fiber, yarn, or fabric level. This review focuses on various electrically conducting materials and methods used for coating e-textile devices, as well as the different configurations that can be obtained from such coatings, creating a smart textile-based system.

A Stretchable Strain-Insensitive Temperature Sensor Based on Free-Standing Elastomeric Composite Fibers for On-Body Monitoring of Skin Temperature

To realize the potential applications of stretchable sensors in the field of wearable health monitoring, it is essential to develop a stable sensing device with robust electrical and mechanical properties in the present of varying external conditions. Herein, we demonstrate a stretchable temperature sensor with the elimination of strain-induced interference via geometric engineering of the free-standing stretchable fibers (FSSFs) of reduced graphene oxide/polyurethane composite. The FSSFs were formed in serpentine structures and enabled the implementation of a strain-insensitive stretchable temperature sensor. On the basis of the controlled reduction time of graphene oxide, we can modulate the response and thermal index of the device. These results are attributed to the variation in the density of oxygen-containing functional groups in the FSSFs, which affect the hopping charge transport and thermal generation of excess carriers. The FSSF temperature sensor yields increased responsivity (0.8%/°C), stretchability (90%), sensing resolution (0.1 °C), and stability in response to applied stretching (±0.37 °C for strains ranging from 0 to 50%). When the sensor is sewn onto a stretchable bandage and attached to the human body, it can detect the temperature changes of the human skin during different body motions in a continuous and stable manner.