Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics.

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Exploring Social Capital Level in Regions with Large and Increasing Wealth Inequality: Lesson from Seoul, South Korea.

While wealth inequality is increasing worldwide, social interaction quality among communities is simultaneously decreasing. Although inequality is primarily correlated with the income gap, inequality is also significantly associated with the housing price gap, especially in South Korea. This study explores the correlation between housing price inequality and social interaction levels in Seoul. For this analysis, the housing price Gini coefficient was utilized through the housing transaction price, and social interaction was measured using the Korea Housing Survey. The results of this study indicate that the social interaction level was low in regions with large housing price inequality. Moreover, the social interaction level was low in regions where housing price inequality increased for 10 years. Furthermore, the negative correlation between housing price inequality and social interaction was significant only in the lower-asset class. The fact that inequality negatively influences social interactions only in the low-asset class is another aspect of inequality.

The buffering effect of social capital for daily mental stress in an unequal society: a lesson from Seoul.

This study attempted to illustrate whether mental health deterioration could be alleviated by high social capital in an environment with high economic inequality. Daily mental stress was employed as a mental health factor when analyzing the association with economic inequality in the Seoul Survey data. Regarding social capital, community trust and altruism were included as cognitive dimensions, and participation and cooperation were included as structural dimensions in each model. The first finding showed a significantly positive relationship between economic inequality and daily stress, meaning that, like other mental health problems, daily mental stress is also high in regions with high economic inequality. Second, the slope of the daily stress increased in respondents with high social trust and participation was alleviated in an economically unequal environment. This indicates that social trust and participation have a buffering effect by moderating the slope of daily stress in societies with high inequality. Third, the buffering effect differs depending on the social capital factor. The buffering effect of trust and participation showed in an unequal environment, while the buffering effect of cooperation showed regardless of the unequal environment. In summary, social capital factors showed the effect of relieving daily mental stress in the relationship with economic inequality. Also, the buffering effect of social capital on mental health may show different aspects for each element.

Stretchable colour-sensitive quantum dot nanocomposites for shape-tunable multiplexed phototransistor arrays.

High-performance photodetecting materials with intrinsic stretchability and colour sensitivity are key requirements for the development of shape-tunable phototransistor arrays. Another challenge is the proper compensation of optical aberrations and noises generated by mechanical deformation and fatigue accumulation in a shape-tunable phototransistor array. Here we report rational material design and device fabrication strategies for an intrinsically stretchable, multispectral and multiplexed 5 × 5 × 3 phototransistor array. Specifically, a unique spatial distribution of size-tuned quantum dots, blended in a semiconducting polymer within an elastomeric matrix, was formed owing to surface energy mismatch, leading to highly efficient charge transfer. Such intrinsically stretchable quantum-dot-based semiconducting nanocomposites enable the shape-tunable and colour-sensitive capabilities of the phototransistor array. We use a deep neural network algorithm for compensating optical aberrations and noises, which aids the precise detection of specific colour patterns (for example, red, green and blue patterns) both under its flat state and hemispherically curved state (radius of curvature of 18.4 mm).

Soft Bioelectronics Based on Nanomaterials.

Recent advances in nanostructured materials and unconventional device designs have transformed the bioelectronics from a rigid and bulky form into a soft and ultrathin form and brought enormous advantages to the bioelectronics. For example, mechanical deformability of the soft bioelectronics and thus its conformal contact onto soft curved organs such as brain, heart, and skin have allowed researchers to measure high-quality biosignals, deliver real-time feedback treatments, and lower long-term side-effects in vivo. Here, we review various materials, fabrication methods, and device strategies for flexible and stretchable electronics, especially focusing on soft biointegrated electronics using nanomaterials and their composites. First, we summarize top-down material processing and bottom-up synthesis methods of various nanomaterials. Next, we discuss state-of-the-art technologies for intrinsically stretchable nanocomposites composed of nanostructured materials incorporated in elastomers or hydrogels. We also briefly discuss unconventional device design strategies for soft bioelectronics. Then individual device components for soft bioelectronics, such as biosensing, data storage, display, therapeutic stimulation, and power supply devices, are introduced. Afterward, representative application examples of the soft bioelectronics are described. A brief summary with a discussion on remaining challenges concludes the review.

Precarious Suicide Behavior According to Housing Price Gap: A Case Study on South Korea.

In 2018, the suicide rate in South Korea was the highest among the Organisation for Economic Co-operation and Development countries, and socioeconomic inequality has intensified. This study analyzes the impact relationship between suicidal impulses and economic inequality in South Korea. This study measures suicidal impulses thoughts National Health Survey Data and economic inequality based on the housing prices gap in the country. The primary analysis results were as follows: First, suicidal impulses were positively associated with the high index of housing price inequality; this correlation has become tight in recent years. Second, it was confirmed that the higher the income level, the higher the correlation between suicidal impulses with the index of housing price inequality. Third, the correlation between housing price inequality with suicidal impulse increased consistently in highly urbanized areas, but the statistical significance was low in non-urban areas.

Highly conductive and elastic nanomembrane for skin electronics.

Skin electronics require stretchable conductors that satisfy metallike conductivity, high stretchability, ultrathin thickness, and facile patternability, but achieving these characteristics simultaneously is challenging. We present a float assembly method to fabricate a nanomembrane that meets all these requirements. The method enables a compact assembly of nanomaterials at the water-oil interface and their partial embedment in an ultrathin elastomer membrane, which can distribute the applied strain in the elastomer membrane and thus lead to a high elasticity even with the high loading of the nanomaterials. Furthermore, the structure allows cold welding and bilayer stacking, resulting in high conductivity. These properties are preserved even after high-resolution patterning by using photolithography. A multifunctional epidermal sensor array can be fabricated with the patterned nanomembranes.

Durable and Fatigue-Resistant Soft Peripheral Neuroprosthetics for In Vivo Bidirectional Signaling.

Soft neuroprosthetics that monitor signals from sensory neurons and deliver motor information can potentially replace damaged nerves. However, achieving long-term stability of devices interfacing peripheral nerves is challenging, since dynamic mechanical deformations in peripheral nerves cause material degradation in devices. Here, a durable and fatigue-resistant soft neuroprosthetic device is reported for bidirectional signaling on peripheral nerves. The neuroprosthetic device is made of a nanocomposite of gold nanoshell (AuNS)-coated silver (Ag) flakes dispersed in a tough, stretchable, and self-healing polymer (SHP). The dynamic self-healing property of the nanocomposite allows the percolation network of AuNS-coated flakes to rebuild after degradation. Therefore, its degraded electrical and mechanical performance by repetitive, irregular, and intense deformations at the device-nerve interface can be spontaneously self-recovered. When the device is implanted on a rat sciatic nerve, stable bidirectional signaling is obtained for over 5 weeks. Neural signals collected from a live walking rat using these neuroprosthetics are analyzed by a deep neural network to predict the joint position precisely. This result demonstrates that durable soft neuroprosthetics can facilitate collection and analysis of large-sized in vivo data for solving challenges in neurological disorders.

Functionalized Elastomers for Intrinsically Soft and Biointegrated Electronics.

Elastomers are suitable materials for constructing a conformal interface with soft and curvilinear biological tissue due to their intrinsically deformable mechanical properties. Intrinsically soft electronic devices whose mechanical properties are comparable to human tissue can be fabricated using suitably functionalized elastomers. This article reviews recent progress in functionalized elastomers and their application to intrinsically soft and biointegrated electronics. Elastomers can be functionalized by adding appropriate fillers, either nanoscale materials or polymers. Conducting or semiconducting elastomers synthesized and/or processed with these materials can be applied to the fabrication of soft biointegrated electronic devices. For facile integration of soft electronics with the human body, additional functionalization strategies can be employed to improve adhesive or autonomous healing properties. Recently, device components for intrinsically soft and biointegrated electronics, including sensors, stimulators, power supply devices, displays, and transistors, have been developed. Herein, representative examples of these fully elastomeric device components are discussed. Finally, the remaining challenges and future outlooks for the field are presented.

Material Design and Fabrication Strategies for Stretchable Metallic Nanocomposites.

Stretchable conductive nanocomposites fabricated by integrating metallic nanomaterials with elastomers have become a vital component of human-friendly electronics, such as wearable and implantable devices, due to their unconventional electrical and mechanical characteristics. Understanding the detailed material design and fabrication strategies to improve the conductivity and stretchability of the nanocomposites is therefore important. This Review discusses the recent technological advances toward high performance stretchable metallic nanocomposites. First, the effect of the filler material design on the conductivity is briefly discussed, followed by various nanocomposite fabrication techniques to achieve high conductivity. Methods for maintaining the initial conductivity over a long period of time are also summarized. Then, strategies on controlled percolation of nanomaterials are highlighted, followed by a discussion regarding the effects of the morphology of the nanocomposite and postfabricated 3D structures on achieving high stretchability. Finally, representative examples of applications of such nanocomposites in biointegrated electronics are provided. A brief outlook concludes this Review.

Material-Based Approaches for the Fabrication of Stretchable Electronics.

Stretchable electronics are mechanically compatible with a variety of objects, especially with the soft curvilinear contours of the human body, enabling human-friendly electronics applications that could not be achieved with conventional rigid electronics. Therefore, extensive research effort has been devoted to the development of stretchable electronics, from research on materials and unit device, to fully integrated systems. In particular, material-processing technologies that encompass the synthesis, assembly, and patterning of intrinsically stretchable electronic materials have been actively investigated and have provided many notable breakthroughs for the advancement of stretchable electronics. Here, the latest studies of such material-based approaches are reviewed, mainly focusing on intrinsically stretchable electronic nanocomposites that generally consist of conducting/semiconducting filler materials inside or on elastomer backbone matrices. Various approaches for fabricating these intrinsically stretchable electronic materials are presented, including the blending of electronic fillers into elastomer matrices, the formation of bi-layered heterogeneous electronic-layer and elastomer support-layer structures, and modifications to polymeric molecular structures in order to impart stretchability. Detailed descriptions of the various conducting/semiconducting composites prepared by each method are provided, along with their electrical/mechanical properties and examples of device applications. To conclude, a brief future outlook is presented.

Solution-processed thin films of semiconducting carbon nanotubes and their application to soft electronics.

Semiconducting single-walled carbon nanotube (SWNT) networks are promising for use as channel materials in field-effect transistors (FETs) in next-generation soft electronics, owing to their high intrinsic carrier mobility, mechanical flexibility, potential for low-cost production, and good processability. In this article, we review the recent progress related to carbon nanotube (CNT) devices in soft electronics by describing the materials and devices, processing methods, and example applications in soft electronic systems. First, solution-processed semiconducting SWNT deposition methods along with doping techniques used to achieve stable complementary metal-oxide-semiconductor devices are discussed. Various strategies for developing high-performance SWNT-based FETs, such as the proper material choices for the gates, dielectrics, and sources/drains of FETs, and methods of improving FET performance, such as hysteresis repression in SWNT-based FETs, are described next. These SWNT-based FETs have been used in flexible, stretchable, and wearable electronic devices to realize functionalities that could not be achieved using conventional silicon-based devices. We conclude this review by discussing the challenges faced by and outlook for CNT-based soft electronics.

Wearable and Implantable Soft Bioelectronics Using Two-Dimensional Materials.

Soft bioelectronics intended for application to wearable and implantable biomedical devices have attracted great attention from material scientists, device engineers, and clinicians because of their extremely soft mechanical properties that match with a variety of human organs and tissues, including the brain, heart, skin, eye, muscles, and neurons, as well as their wide diversity in device designs and biomedical functions that can be finely tuned for each specific case of applications. These unique features of the soft bioelectronics have allowed minimal mechanical and biological damage to organs and tissues integrated with bioelectronic devices and reduced side effects including inflammation, skin irritation, and immune responses even after long-term biointegration. These favorable properties for biointegration have enabled long-term monitoring of key biomedical indicators with high signal-to-noise ratio, reliable diagnosis of the patient's health status, and in situ feedback therapy with high treatment efficacy optimized for the requirements of each specific disease model. These advantageous device functions and performances could be maximized by adopting novel high-quality soft nanomaterials, particularly ultrathin two-dimensional (2D) materials, for soft bioelectronics. Two-dimensional materials are emerging material candidates for the channels and electrodes in electronic devices (semiconductors and conductors, respectively). They can also be applied to various biosensors and therapeutic actuators in soft bioelectronics. The ultrathin vertically layered nanostructure, whose layer number can be controlled in the synthesis step, and the horizontally continuous planar molecular structure, which can be found over a large area, have conferred unique mechanical, electrical, and optical properties upon the 2D materials. The atomically thin nanostructure allows mechanical softness and flexibility and high optical transparency of the device, while the large-area continuous thin film structure allows efficient carrier transport within the 2D plane. In addition, the quantum confinement effect in the atomically thin 2D layers introduces interesting optoelectronic properties and superb photodetecting capabilities. When fabricated as soft bioelectronic devices, these interesting and useful material features of the 2D materials enable unconventional device functions in biological and optical sensing, as well as superb performance in electrical and biochemical therapeutic actuations. In this Account, we first summarize the distinctive characteristics of the 2D materials in terms of the mechanical, optical, chemical, electrical, and biomedical aspects and then present application examples of the 2D materials to soft bioelectronic devices based on each aforementioned unique material properties. Among various kinds of 2D materials, we particularly focus on graphene and MoS2. The advantageous material features of graphene and MoS2 include ultrathin thickness, facile functionalization, large surface-to-volume ratio, biocompatibility, superior photoabsorption, and high transparency, which allow the development of high-performance multifunctional soft bioelectronics, such as a wearable glucose patch, a highly sensitive humidity sensor, an ultrathin tactile sensor, a soft neural probe, a soft retinal prosthesis, a smart endoscope, and a cell culture platform. A brief comparison of their characteristics and performances is also provided. Finally, this Account concludes with a future outlook on next-generation soft bioelectronics based on 2D materials.

Wearable Electrocardiogram Monitor Using Carbon Nanotube Electronics and Color-Tunable Organic Light-Emitting Diodes.

With the rapid advances in wearable electronics, the research on carbon-based and/or organic materials and devices has become increasingly important, owing to their advantages in terms of cost, weight, and mechanical deformability. Here, we report an effective material and device design for an integrative wearable cardiac monitor based on carbon nanotube (CNT) electronics and voltage-dependent color-tunable organic light-emitting diodes (CTOLEDs). A p-MOS inverter based on four CNT transistors allows high amplification and thereby successful acquisition of the electrocardiogram (ECG) signals. In the CTOLEDs, an ultrathin exciton block layer of bis[2-(diphenylphosphino)phenyl]ether oxide is used to manipulate the balance of charges between two adjacent emission layers, bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) and bis(2-phenylquinolyl-N,C(2'))iridium(acetylacetonate), which thereby produces different colors with respect to applied voltages. The ultrathin nature of the fabricated devices supports extreme wearability and conformal integration of the sensor on human skin. The wearable CTOLEDs integrated with CNT electronics are used to display human ECG changes in real-time using tunable colors. These materials and device strategies provide opportunities for next generation wearable health indicators.

Colloidal Synthesis of Uniform-Sized Molybdenum Disulfide Nanosheets for Wafer-Scale Flexible Nonvolatile Memory.

Large-scale colloidal synthesis and integration of uniform-sized molybdenum disulfide (MoS2 ) nanosheets for a flexible resistive random access memory (RRAM) array are presented. RRAM using MoS2 nanosheets shows a ≈10 000 times higher on/off ratio than that based on exfoliated MoS2 . The good uniformity of the MoS2 nanosheets allows wafer-scale system integration of the RRAM array with pressure sensors and quantum-dot light-emitting diodes.

A wearable multiplexed silicon nonvolatile memory array using nanocrystal charge confinement.

Strategies for efficient charge confinement in nanocrystal floating gates to realize high-performance memory devices have been investigated intensively. However, few studies have reported nanoscale experimental validations of charge confinement in closely packed uniform nanocrystals and related device performance characterization. Furthermore, the system-level integration of the resulting devices with wearable silicon electronics has not yet been realized. We introduce a wearable, fully multiplexed silicon nonvolatile memory array with nanocrystal floating gates. The nanocrystal monolayer is assembled over a large area using the Langmuir-Blodgett method. Efficient particle-level charge confinement is verified with the modified atomic force microscopy technique. Uniform nanocrystal charge traps evidently improve the memory window margin and retention performance. Furthermore, the multiplexing of memory devices in conjunction with the amplification of sensor signals based on ultrathin silicon nanomembrane circuits in stretchable layouts enables wearable healthcare applications such as long-term data storage of monitored heart rates.

Stretchable carbon nanotube charge-trap floating-gate memory and logic devices for wearable electronics.

Electronics for wearable applications require soft, flexible, and stretchable materials and designs to overcome the mechanical mismatch between the human body and devices. A key requirement for such wearable electronics is reliable operation with high performance and robustness during various deformations induced by motions. Here, we present materials and device design strategies for the core elements of wearable electronics, such as transistors, charge-trap floating-gate memory units, and various logic gates, with stretchable form factors. The use of semiconducting carbon nanotube networks designed for integration with charge traps and ultrathin dielectric layers meets the performance requirements as well as reliability, proven by detailed material and electrical characterizations using statistics. Serpentine interconnections and neutral mechanical plane layouts further enhance the deformability required for skin-based systems. Repetitive stretching tests and studies in mechanics corroborate the validity of the current approaches.

Large-area formation of self-aligned crystalline domains of organic semiconductors on transistor channels using CONNECT.

The electronic properties of solution-processable small-molecule organic semiconductors (OSCs) have rapidly improved in recent years, rendering them highly promising for various low-cost large-area electronic applications. However, practical applications of organic electronics require patterned and precisely registered OSC films within the transistor channel region with uniform electrical properties over a large area, a task that remains a significant challenge. Here, we present a technique termed "controlled OSC nucleation and extension for circuits" (CONNECT), which uses differential surface energy and solution shearing to simultaneously generate patterned and precisely registered OSC thin films within the channel region and with aligned crystalline domains, resulting in low device-to-device variability. We have fabricated transistor density as high as 840 dpi, with a yield of 99%. We have successfully built various logic gates and a 2-bit half-adder circuit, demonstrating the practical applicability of our technique for large-scale circuit fabrication.

Oxide nanomembrane hybrids with enhanced mechano- and thermo-sensitivity for semitransparent epidermal electronics.

Oxide nanomembrane hybrids with enhanced mechano- and thermo-sensitivity for semitransparent epidermal electronics are developed. The use of nanomaterials (single wall nanotubes and silver nanoparticles) embedded in the oxide nanomembranes significantly enhances mechanical and thermal sensitivities. These mechanical and thermal sensors are utilized in wheelchair control and hypothermia detection, which are useful for patients with strokes.

Large-area assembly of densely aligned single-walled carbon nanotubes using solution shearing and their application to field-effect transistors.

Dense alignment of single-walled carbon nanotubes over a large area is demonstrated using a novel solution-shearing technique. A density of 150-200 single-walled carbon nanotubes per micro-meter is achieved with a current density of 10.08 µA µm(-1) at VDS = -1 V. The on-current density is improved by a factor of 45 over that of random-network single-walled carbon nanotubes.