Spirally Wrapped Carbon Nanotube Microelectrodes for Fiber Optoelectronic Devices beyond Geometrical Limitations toward Smart Wearable E-Textile Applications

Strain-Driven Negative Resistance Switching of Conductive Fibers with Adjustable Sensitivity for Wearable Healthcare Monitoring Systems with Near-Zero Standby Power

Recently, one of the primary concerns in e-textile-based healthcare monitoring systems for chronic illness patients has been reducing wasted power consumption, as the system should be always-on to capture diverse biochemical and physiological characteristics. However, the general conductive fibers, a major component of the existing wearable monitoring systems, have a positive gauge-factor (GF) that increases electrical resistance when stretched, so that the systems have no choice but to consume power continuously. Herein, a twisted conductive-fiber-based negatively responsive switch-type (NRS) strain-sensor with an extremely high negative GF (resistance change ratio ≈ 3.9 × 108 ) that can significantly increase its conductivity from insulating to conducting properties is developed. To this end, a precision cracking technology is devised, which could induce a difference in the Young's modulus of the encapsulated layer on the fiber through selective ultraviolet-irradiation treatment. Owing to this technology, the NRS strain-sensors can allow for effective regulation of the mutual contact resistance under tensile strain while maintaining superior durability for over 5000 stretching cycles. For further practical demonstrations, three healthcare monitoring systems (E-fitness pants, smart-masks, and posture correction T-shirts) with near-zero standby power are also developed, which opens up advancements in electronic textiles by expanding the utilization range of fiber strain-sensors.

Fibriform Organic Electrochemical Diodes with Rectifying, Complementary Logic and Transient Voltage Suppression Functions for Wearable E-Textile Embedded Circuits

In this study, a fibriform electrochemical diode capable of performing rectifying, complementary logic and device protection functions for future e-textile circuit systems is fabricated. The diode was fabricated using a simple twisted assembly of metal/polymer semiconductor/ion gel coaxial microfibers and conducting microfiber electrodes. The fibriform diode exhibited a prominent asymmetrical current flow with a rectification ratio of over 10², and its performance was retained after repeated bending deformations and washings. Fundamental studies on the electrochemical interactions of polymer semiconductors with ions reveal that the Faradaic current generated in polymer semiconductors by electrochemical reactions results in an abrupt current increase under a forward bias, in which the threshold voltages of the device are determined by the oxidation or reduction potential of the polymer semiconductor. Textile-embedded full-wave rectifiers and logic gate circuits were implemented by simply integrating the fibriform diodes, exhibiting AC-to-DC signal conversion and logic operation functions, respectively. It was also confirmed that the proposed fibriform diode can suppress transient voltages and thus protect a low-voltage operational wearable e-textile circuit.

Engineering of Hydrogenated (6,0) Single-Walled Carbon Nanotube under Applied Uniaxial Stress: A DFT-1/2 and Molecular Dynamics Study

Herein, we systematically studied the electronic, optical, and mechanical properties of a hydrogenated (6,0) single-walled carbon nanotube [(6,0) h-SWCNT] under applied uniaxial stress from first-principles density functional theory (DFT) and molecular dynamics (MD) simulation. We have applied the uniaxial stress range from -18 to 22 GPa on the (6,0) h-SWCNT (- sign indicates compressive and + indicates tensile stress) along the tube axes. Our system was found to be an indirect semiconductor (Γ-Δ), with a band gap value of ∼0.77 eV within the linear combination of atomic orbitals (LCAO) method using a GGA-1/2 exchange-correlation approximation. The band gap for (6,0) h-SWCNT significantly varies with the application of stress. The indirect to direct band gap transition was observed under compressive stress (-14 GPa). The strained (6,0) h-SWCNT showed a strong optical absorption in the infrared region. Application of external stress enhanced the optically active region from infrared to Vis with maximum intensity within the Vis-IR region, making it a promising candidate for optoelectronic devices. Ab initio molecular dynamics (AIMD) simulation has been used to study the elastic properties of the (6,0) h-SWCNT which has a strong influence under applied stress.

New Opportunities for Organic Semiconducting Polymers in Biomedical Applications

The life expectancy of humans has been significantly elevated due to advancements in medical knowledge and skills over the past few decades. Although a lot of knowledge and skills are disseminated to the general public, electronic devices that quantitatively diagnose one’s own body condition still require specialized semiconductor devices which are huge and not portable. In this regard, semiconductor materials that are lightweight and have low power consumption and high performance should be developed with low cost for mass production. Organic semiconductors are one of the promising materials in biomedical applications due to their functionalities, solution-processability and excellent mechanical properties in terms of flexibility. In this review, we discuss organic semiconductor materials that are widely utilized in biomedical devices. Some advantageous and unique properties of organic semiconductors compared to inorganic semiconductors are reviewed. By critically assessing the fabrication process and device structures in organic-based biomedical devices, the potential merits and future aspects of the organic biomedical devices are pinpointed compared to inorganic devices.

Structure and Dynamics of Adsorbed Dopamine on Solvated Carbon Nanotubes and in a CNT Groove

Advanced carbon microelectrodes, including many carbon-nanotube (CNT)-based electrodes, are being developed for the in vivo detection of neurotransmitters such as dopamine (DA). Our prior simulations of DA and dopamine-o-quinone (DOQ) on pristine, flat graphene showed rapid surface diffusion for all adsorbed species, but it is not known how CNT surfaces affect dopamine adsorption and surface diffusivity. In this work, we use molecular dynamics simulations to investigate the adsorbed structures and surface diffusion dynamics of DA and DOQ on CNTs of varying curvature and helicity. In addition, we study DA dynamics in a groove between two aligned CNTs to model the spatial constraints at the junctions within CNT assemblies. We find that the adsorbate diffusion on a solvated CNT surface depends upon curvature. However, this effect cannot be attributed to changes in the surface energy roughness because the lateral distributions of the molecular adsorbates are similar across curvatures, diffusivities on zigzag and armchair CNTs are indistinguishable, and the curvature dependence disappears in the absence of solvent. Instead, adsorbate diffusivities correlate with the vertical placement of the adsorbate’s moieties, its tilt angle, its orientation along the CNT axis, and the number of waters in its first hydration shell, all of which will influence its effective hydrodynamic radius. Finally, DA diffuses into and remains in the groove between a pair of aligned and solvated CNTs, enhancing diffusivity along the CNT axis. These first studies of surface diffusion on a CNT electrode surface are important for understanding the changes in diffusion dynamics of dopamine on nanostructured carbon electrode surfaces.

High-Performance and Reliable White Organic Light-Emitting Fibers for Truly Wearable Textile Displays

Light-emitting fibers have been intensively developed for the realization of textile displays and various lighting applications, as promising free-form electronics with outstanding interconnectivity. These advances in the fiber displays have been made possible by the successful implementation of the core technologies of conventional displays, including high optoelectronic performance and essential elements, in the fiber form-factor. However, although white organic light-emitting diodes (WOLEDs), as a fundamental core technology of displays, are essential for realizing full-color displays and solid-state lighting, fiber-based WOLEDs are still challenging due to structural issues and the lack of approaches to implementing WOLEDs on fiber. Herein, the first fiber WOLED is reported, exhibiting high optoelectronic performance and a reliable color index, comparable to those of conventional planar WOLEDs. As key features, it is found that WOLEDs can be successfully introduced on a cylindrical fiber using a dip-coatable single white-emission layer based on simulation and optimization of the white spectra. Furthermore, to ensure durability from usage, the fiber WOLED is encapsulated by an Al2 O3 /elastomer bilayer, showing stable operation under repetitive bending and pressure, and in water. This pioneering work is believed to provide building blocks for realizing complete textile display technologies by complementing the lack of the core technology.

Recent Advances in Materials and Flexible Sensors for Arrhythmia Detection

Arrhythmias are one of the leading causes of death in the United States, and their early detection is essential for patient wellness. However, traditional arrhythmia diagnosis by expert evaluation from intermittent clinical examinations is time-consuming and often lacks quantitative data. Modern wearable sensors and machine learning algorithms have attempted to alleviate this problem by providing continuous monitoring and real-time arrhythmia detection. However, current devices are still largely limited by the fundamental mismatch between skin and sensor, giving way to motion artifacts. Additionally, the desirable qualities of flexibility, robustness, breathability, adhesiveness, stretchability, and durability cannot all be met at once. Flexible sensors have improved upon the current clinical arrhythmia detection methods by following the topography of skin and reducing the natural interface mismatch between cardiac monitoring sensors and human skin. Flexible bioelectric, optoelectronic, ultrasonic, and mechanoelectrical sensors have been demonstrated to provide essential information about heart-rate variability, which is crucial in detecting and classifying arrhythmias. In this review, we analyze the current trends in flexible wearable sensors for cardiac monitoring and the efficacy of these devices for arrhythmia detection.

Smart E-Textile Systems: A Review for Healthcare Applications

E-textiles is a new hybrid field developed with the help of the integration of electronic components into our daily usage of textile products. These wearable e-textiles provide user-defined applications as well as normal textile clothing. The medical field is one of the major leading areas where these new hybrid products are being implemented, and relatively mature products can be observed in the laboratory as well as in commercial markets. These products are developed for continuous patient monitoring in large-scale hospital centers as well as for customized patient requirements. Meanwhile, these products are also being used for complex medical treatments and the replacement of conventional methods. This review manuscript contains a basic overview of e-textile systems, their components, applications, and usages in the field of medical innovations. E-textile systems, integrated into customized products for medical needs, are discussed with their proposed properties and limitations. Finally, some recommendations to enhance the e-textile system’s integration into the medical field are argued.

Applications of Carbon Nanotubes in the Internet of Things Era

The post-Moore's era has boosted the progress in carbon nanotube-based transistors. Indeed, the 5G communication and cloud computing stimulate the research in applications of carbon nanotubes in electronic devices. In this perspective, we deliver the readers with the latest trends in carbon nanotube research, including high-frequency transistors, biomedical sensors and actuators, brain-machine interfaces, and flexible logic devices and energy storages. Future opportunities are given for calling on scientists and engineers into the emerging topics.

Recent Advances in Fiber-Shaped Electronic Devices for Wearable Applications

Fiber electronics is a key research area for realizing wearable microelectronic devices. Significant progress has been made in recent years in developing the geometry and composition of electronic fibers. In this review, we present that recent progress in the architecture and electrical properties of electronic fibers, including their fabrication methods. We intensively investigate the structural designs of fiber-shaped devices: coaxial, twisted, three-dimensional layer-by-layer, and woven structures. In addition, we introduce remarkable applications of fiber-shaped devices for energy harvesting/storage, sensing, and light-emitting devices. Electronic fibers offer high potential for use in next-generation electronics, such as electronic textiles and smart integrated textile systems, which require excellent deformability and high operational reliability.

Mechanically and electrically durable, stretchable electronic textiles for robust wearable electronics

A monolithic integration of high-performance soft electronic modules into various fabric materials has enabled a paradigm shift in wearable textile electronics. However, the current textile electronics have struggled against fatigue under repetitive deformation due to the absence of materials and structural design strategies for imparting electrical and mechanical robustness to individual fibers. Here, we report a mechanically and electrically durable, stretchable electronic textile (MED-ET) enabled by a precisely controlled diffusion of tough self-healing stretchable inks into fibers and an adoption of the kirigami-inspired design. Remarkably, the conductive percolative pathways in the fabric of MED-ET even under a harshly deformed environment were stably maintained due to an electrical recovery phenomenon which originates from the spontaneous rearrangement of Ag flakes in the self-healing polymer matrix. Specifically, such a unique property enabled damage-resistant performance when repetitive deformation and scratch were applied. In addition, the kirigami-inspired design was capable of efficiently dissipating the accumulated stress in the conductive fabric during stretching, thereby providing high stretchability (a tensile strain of 300%) without any mechanical fracture or electrical malfunction. Finally, we successfully demonstrate various electronic textile applications such as stretchable micro-light-emitting diodes (Micro-LED), electromyogram (EMG) monitoring and all-fabric thermoelectric devices (F-TEG).

Stretchable MED-ET was fabricated by a soaking process of self-healing stretchable Ag ink. Conductive pathways in MED-ET under a damaged environment were stably maintained due to an electrical recovery phenomenon which enables a robust device system.

Roll-to-roll layer-by-layer assembly bark-shaped carbon nanotube/Ti3C2Tx MXene textiles for wearable electronics

Smart wearable electronics have drawn increasing attention for their potential applications in personal thermal management, human health monitoring, portable energy conversion/storage, electronic skin and so on. However, it is still a critical challenge to fabricate the multifunctional textiles with tunable morphology and performance while performing well in flexibility, air permeability, wearing comfortability. Herein, we develop a novel roll-to-roll layer-by-layer assembly strategy to construct bark-shaped carbon nanotube (CNT)/Ti₃C₂T_x MXene composite film on the fiber surface. The fabricated bark-shaped CNT/MXene decorated fabrics (CMFs) exhibit good flexibility, air permeability and electrical conductivity (sheet resistance, 6.6 Ω/□). In addition, the CMFs demonstrate good electrothermal performance (70.9 °C, 5 V), electromagnetic interference (EMI) shielding performance (EMI shielding effectiveness, 30.0 dB under X-Brand), and high sensitivity as the flexible piezoresistive sensors for monitoring the human motions. Importantly, our CMFs show distinctive EMI shielding mechanism, where a great proportion of incident electromagnetic microwaves are reflected by the bark-shaped CNT/MXene films owing to the multi-interface scattering effects. This work may provide a new strategy for the fabrication of multifunctional textile-based electronics and pave the way for smart wearable electronics.

Systematic Review on Human Skin-Compatible Wearable Photoplethysmography Sensors

The rapid advances in human-friendly and wearable photoplethysmography (PPG) sensors have facilitated the continuous and real-time monitoring of physiological conditions, enabling self-health care without being restricted by location. In this paper, we focus on state-of-the-art skin-compatible PPG sensors and strategies to obtain accurate and stable sensing of biological signals adhered to human skin along with light-absorbing semiconducting materials that are classified as silicone, inorganic, and organic absorbers. The challenges of skin-compatible PPG-based monitoring technologies and their further improvements are also discussed. We expect that such technological developments will accelerate accurate diagnostic evaluation with the aid of the biomedical electronic devices.