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.     

Recent Advances in 1D Stretchable Electrodes and Devices for Textile and Wearable Electronics: Materials,Fabrications, and Applications

Research on wearable electronic devices that can be directly integrated into daily textiles or clothes has been explosively grown holding great potential for various practical wearable applications. These wearable electronic devices strongly demand 1D electronic devices that are light–weight, weavable, highly flexible, stretchable, and adaptable to comport to frequent deformations during usage in daily life. To this end, the development of 1D electrodes with high stretchability and electrical performance is fundamentally essential. Herein, the recent process of 1D stretchable electrodes for wearable and textile electronics is described, focusing on representative conductive materials, fabrication techniques for 1D stretchable electrodes with high performance, and designs and applications of various 1D stretchable electronic devices. To conclude, discussions are presented regarding limitations and perspectives of current materials and devices in terms of performance and scientific understanding that should be considered for further advances.     

Soft Elastomers with Ionic Liquid‐Filled Cavities as Strain Isolating Substrates for Wearable Electronics

Managing the mechanical mismatch between hard semiconductor components and soft biological tissues represents a key challenge in the development of advanced forms of wearable electronic devices. An ultralow modulus material or a liquid that surrounds the electronics and resides in a thin elastomeric shell provides a strain‐isolation effect that enhances not only the wearability but also the range of stretchability in suitably designed devices. The results presented here build on these concepts by (1) replacing traditional liquids explored in the past, which have some nonnegligible vapor pressure and finite permeability through the encapsulating elastomers, with ionic liquids to eliminate any possibility for leakage or evaporation, and (2) positioning the liquid between the electronics and the skin, within an enclosed, elastomeric microfluidic space, but not in direct contact with the active elements of the system, to avoid any negative consequences on electronic performance. Combined experimental and theoretical results establish the strain‐isolating effects of this system, and the considerations that dictate mechanical collapse of the fluid‐filled cavity. Examples in skin‐mounted wearable include wireless sensors for measuring temperature and wired systems for recording mechano‐acoustic responses.     

Organic Photodetectors for Next-Generation Wearable Electronics

Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.     

Ultraflexible and Lightweight Bamboo‐Derived Transparent Electrodes for Perovskite Solar Cells

Wearable devices are mainly based on plastic substrates, such as polyethylene terephthalate and polyethylene naphthalate, which causes environmental pollution after use due to the long decomposition periods. This work reports on the fabrication of a biodegradable and biocompatible transparent conductive electrode derived from bamboo for flexible perovskite solar cells. The conductive bioelectrode exhibits extremely flexible and light‐weight properties. After bending 3000 times at a 4 mm curvature radius or even undergoing a crumpling test, it still shows excellent electrical performance and negligible decay. The performance of the bamboo‐based bioelectrode perovskite solar cell exhibits a record power conversion efficiency (PCE) of 11.68%, showing the highest efficiency among all reported biomass‐based perovskite solar cells. It is remarkable that this flexible device has a highly bendable mechanical stability, maintaining over 70% of its original PCE during 1000 bending cycles at a 4 mm curvature radius. This work paves the way for perovskite solar cells toward comfortable and environmentally friendly wearable devices.     

Stretchable,Curvilinear Electronics Based on Inorganic Materials

All commercial forms of electronic/optoelectronic technologies use planar, rigid substrates. Device possibilities that exploit bio‐inspired designs or require intimate integration with the human body demand curvilinear shapes and/or elastic responses to large strain deformations. This article reviews progress in research designed to accomplish these outcomes with established, high‐performance inorganic electronic materials and modest modifications to conventional, planar processing techniques. We outline the most well developed strategies and illustrate their use in demonstrator devices that exploit unique combinations of shape, mechanical properties and electronic performance. We conclude with an outlook on the challenges and opportunities for this emerging area of materials science and engineering.     

EGaIn‐Assisted Room‐Temperature Sintering of Silver Nanoparticles for Stretchable,Inkjet‐Printed,Thin‐Film Electronics

Coating inkjet‐printed traces of silver nanoparticle (AgNP) ink with a thin layer of eutectic gallium indium (EGaIn) increases the electrical conductivity by six‐orders of magnitude and significantly improves tolerance to tensile strain. This enhancement is achieved through a room‐temperature “sintering” process in which the liquid‐phase EGaIn alloy binds the AgNP particles (≈100 nm diameter) to form a continuous conductive trace. Ultrathin and hydrographically transferrable electronics are produced by printing traces with a composition of AgNP‐Ga‐In on a 5 µm‐thick temporary tattoo paper. The printed circuit is flexible enough to remain functional when deformed and can support strains above 80% with modest electromechanical coupling (gauge factor ≈1). These mechanically robust thin‐film circuits are well suited for transfer to highly curved and nondevelopable 3D surfaces as well as skin and other soft deformable substrates. In contrast to other stretchable tattoo‐like electronics, the low‐cost processing steps introduced here eliminate the need for cleanroom fabrication and instead requires only a commercial desktop printer. Most significantly, it enables functionalities like “electronic tattoos” and 3D hydrographic transfer that have not been previously reported with EGaIn or EGaIn‐based biphasic electronics.     

Room Temperature Electrochemical Sintering of Zn Microparticles and Its Use in Printable Conducting Inks for Bioresorbable Electronics

This study describes a conductive ink formulation that exploits electrochemical sintering of Zn microparticles in aqueous solutions at room temperature. This material system has relevance to emerging classes of biologically and environmentally degradable electronic devices. The sintering process involves dissolution of a surface passivation layer of zinc oxide in CH₃COOH/H₂O and subsequent self‐exchange of Zn and Zn²⁺ at the Zn/H₂O interface. The chemical specificity associated with the Zn metal and the CH₃COOH/H₂O solution is critically important, as revealed by studies of other material combinations. The resulting electrochemistry establishes the basis for a remarkably simple procedure for printing highly conductive (3 × 10⁵ S m^?1) features in degradable materials at ambient conditions over large areas, with key advantages over strategies based on liquid phase (fusion) sintering that requires both oxide‐free metal surfaces and high temperature conditions. Demonstrations include printed magnetic loop antennas for near‐field communication devices.     

Mechanically-Guided Structural Designs in Stretchable Inorganic Electronics

Over the past decade, the area of stretchable inorganic electronics has evolved very rapidly, in part because the results have opened up a series of unprecedented applications with broad interest and potential for impact, especially in bio-integrated systems. Low modulus mechanics and the ability to accommodate extreme mechanical deformations, especially high levels of stretching, represent key defining characteristics. Most existing studies exploit structural material designs to achieve these properties, through the integration of hard inorganic electronic components configured into strategic 2D/3D geometries onto patterned soft substrates. The diverse structural geometries developed for stretchable inorganic electronics are summarized, covering the designs of functional devices and soft substrates, with a focus on fundamental principles, design approaches, and system demonstrations. Strategies that allow spatial integration of 3D stretchable device layouts are also highlighted. Finally, perspectives on the remaining challenges and open opportunities are provided.     

Stretchable,Wireless Sensors and Functional Substrates for Epidermal Characterization of Sweat

This paper introduces materials and architectures for ultrathin, stretchable wireless sensors that mount on functional elastomeric substrates for epidermal analysis of biofluids. Measurement of the volume and chemical properties of sweat via dielectric detection and colorimetry demonstrates some capabilities. Here, inductively coupled sensors consisting of LC resonators with capacitive electrodes show systematic responses to sweat collected in microporous substrates. Interrogation occurs through external coils placed in physical proximity to the devices. The substrates allow spontaneous sweat collection through capillary forces, without the need for complex microfluidic handling systems. Furthermore, colorimetric measurement modes are possible in the same system by introducing indicator compounds into the depths of the substrates, for sensing specific components (OH^?, H⁺, Cu⁺, and Fe²⁺) in the sweat. The complete devices offer Young's moduli that are similar to skin, thus allowing highly effective and reliable skin integration without external fixtures. Experimental results demonstrate volumetric measurement of sweat with an accuracy of 0.06 μL/mm² with good stability and low drift. Colorimetric responses to pH and concentrations of various ions provide capabilities relevant to analysis of sweat. Similar materials and device designs can be used in monitoring other body fluids.