Wearable sensors play a crucial role in realizing personalized medicine, as they can continuously collect data from the human body to capture meaningful health status changes in time for preventive intervention. However, motion artifacts and mechanical mismatches between conventional rigid electronic materials and soft skin often lead to substantial sensor errors during epidermal measurement. Because of its unique properties such as high flexibility and conformability, flexible electronics enables a natural interaction between electronics and the human body. In this Account, we summarize our recent studies on the design of flexible electronic devices and systems for physical and chemical monitoring. Material innovation, sensor design, device fabrication, system integration, and human studies employed toward continuous and noninvasive wearable sensing are discussed. A flexible electronic device typically contains several key components, including the substrate, the active layer, and the interface layer. The inorganic-nanomaterials-based active layer (prepared by a physical transfer or solution process) is shown to have good physicochemical properties, electron/hole mobility, and mechanical strength. Flexible electronics based on the printed and transferred active materials has shown great promise for physical sensing. For example, integrating a nanowire transistor array for the active matrix and a conductive pressure-sensitive rubber enables tactile pressure mapping; tactile-pressure-sensitive e-skin and organic light-emitting diodes can be integrated for instantaneous pressure visualization. Such printed sensors have been applied as wearable patches to monitor skin temperature, electrocardiograms, and human activities. In addition, liquid metals could serve as an attractive candidate for flexible electronics because of their excellent conductivity, flexibility, and stretchability. Liquid-metal-enabled electronics (based on liquid-liquid heterojunctions and embedded microchannels) have been utilized to monitor a wide range of physiological parameters (e.g., pulse and temperature). Despite the rapid growth in wearable sensing technologies, there is an urgent need for the development of flexible devices that can capture molecular data from the human body to retrieve more insightful health information. We have developed a wearable and flexible sweat-sensing platform toward real-time multiplexed perspiration analysis. An integrated iontophoresis module on a wearable sweat sensor could enable autonomous and programmed sweat extraction. A microfluidics-based sensing system was demonstrated for sweat sampling, sensing, and sweat rate analysis. Roll-to-roll gravure printing allows for mass production of high-performance flexible chemical sensors at low cost. These wearable and flexible sweat sensors have shown great promise in dehydration monitoring, cystic fibrosis diagnosis, drug monitoring, and noninvasive glucose monitoring. Future work in this field should focus on designing robust wearable sensing systems to accurately collect data from the human body and on large-scale human studies to determine how the measured physical and chemical information relates to the individual's specific health conditions. Further research in these directions, along with the large sets of data collected via these wearable and flexible sensing technologies, will have a significant impact on future personalized healthcare.

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Flexible Electronics toward Wearable Sensing

A method of making a structure having a patterned a base layer and useful in the fabrication of optical and electronic devices including bioelectronic devices includes, in one embodiment, the steps of: a) providing a layer of a radiation-sensitive resin; b) exposing the layer of radiation-sensitive resin to patterned radiation to form a base layer precursor having a first pattern of exposed radiation-sensitive resin and a second pattern of unexposed radiation-sensitive resin; c) providing a layer of fluoropolymer in a third pattern over the base layer precursor to form a first intermediate structure; d) treating the first intermediate structure to form a second intermediate structure; and e) selectively removing either the first or second pattern of resin by contacting the second intermediate structure with a resin developing agent, thereby forming the patterned base layer. The method is capable of providing multilayer articles having almost any shape at high resolution without the need for expensive or damaging mechanical or laser cutting.

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Organic electrochemical transistor

All-optical electrophysiology—spatially resolved simultaneous optical perturbation and measurement of membrane voltage—would open new vistas in neuroscience research. We evolved two archaerhodopsin-based voltage indicators, QuasAr1 and QuasAr2, which show improved brightness and voltage sensitivity, have microsecond response times and produce no photocurrent. We engineered a channelrhodopsin actuator, CheRiff, which shows high light sensitivity and rapid kinetics and is spectrally orthogonal to the QuasArs. A coexpression vector, Optopatch, enabled cross-talk–free genetically targeted all-optical electrophysiology. In cultured rat neurons, we combined Optopatch with patterned optical excitation to probe back-propagating action potentials (APs) in dendritic spines, synaptic transmission, subcellular microsecond-timescale details of AP propagation, and simultaneous firing of many neurons in a network. Optopatch measurements revealed homeostatic tuning of intrinsic excitability in human stem cell–derived neurons. In rat brain slices, Optopatch induced and reported APs and subthreshold events with high signal-to-noise ratios. The Optopatch platform enables high-throughput, spatially resolved electrophysiology without the use of conventional electrodes.

All-optical electrophysiology in mammalian neurons using engineered microbial rhodopsins

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin-film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current-voltage characteristics, known as the "kink" or "double slope," has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field-effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier-mobility-related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm2 V-1 s-1 , respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high-performance p-type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n-type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next-generation organic semiconductors.

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Recent Progress in High-Mobility Organic Transistors: A Reality Check.

The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

Controlling Molecular Doping in Organic Semiconductors

We develop miniature high-voltage sources from polymer solar cells (PSCs) with charge-transporting molybdenum oxide (MoOx) integrated in a serial architecture through sacrificial layer (SL)-assisted patterning. The MoOx layer, being patterned by the lift-off process of the SL of a hydrophobic fluorinated-polymer, as a hole transporting layer plays a critical role on the reduction of the dark current and the increase of a high open circuit voltage of an integrated PSC array. The underlying mechanism lies primarily on the elimination of the lateral charge pathways in the MoOx layer in the presence of the electrode interconnection. Two miniature voltage sources consisting of 20 PSCs and 50 PSCs are demonstrated in the operation of a liquid crystal display and an organic field-effect transistor, respectively. Our SL-assisted integration approach will be directly applicable for implementing the self-power sources made of the PSCs into a wide range of the electronic and optoelectronic devices.

http://iopscience.iop.org/article/10.7567/JJAP.53.042301/pdf

Integrated polymer solar cells in serial architecture with patterned charge-transporting MoOx for miniature high-voltage sources

Organic p-i-n diodes enable the development of highly efficient organic devices such as organic light-emitting diodes. Understanding charge carrier trapping in these diodes is essential to comprehensively describe their electrical behaviors and increase their efficiency further. Here, a new bias stress protocol is developed to study charge trapping and the influence of trapping on molecular doping in organic p-i-n diodes. The results are discussed with the help of a novel analytical model, which is capable of quantifying the density of trapped charges and the doping efficiency from capacitance spectroscopy. We propose that this combined experimental/modeling approach is versatile and can lead to an advanced understanding of trapping in organic electronic devices.

Quantifying charge trapping and molecular doping in organic p-i-n diodes

Despite widespread interest, ultrathin and highly flexible light-emitting devices that can be seamlessly integrated and used for flexible displays, wearables, and as bioimplants remain elusive. Organic light-emitting diodes (OLEDs) with $\mu$m-scale thickness and exceptional flexibility have been demonstrated but show insufficient stability in air and moist environments due to a lack of suitable encapsulation barriers. Here, we demonstrate an efficient and stable OLED with a total thickness of $\approx$12 $\mu$m that can be fully immersed in water or cell nutrient media for weeks without suffering substantial degradation. The active layers of the device are embedded between conformal barriers formed by alternating layers of parylene-C and metal oxides that are deposited through a low temperature chemical vapour process. These barriers also confer stability of the OLED to repeated bending and to extensive postprocessing, e.g. via reactive gas plasmas, organic solvents, and photolithography. This unprecedented robustness opens up a wide range of novel possibilities for ultrathin OLEDs.

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A substrateless, flexible, and water-resistant organic light-emitting diode

The progress of organic field-effect transistors (OFETs) has led to the advent of a new area of printed and/or flexible electronics. In organic transistors and circuits, the interface between a gate insulator (GI) and an organic semiconductor (OS) plays a critical role on the electrical performance together with the functionality, the reliability and the long-term stability. In this review, we describe the basic principles of engineering a variety of the GI/OS interfaces for the development of advanced OFETs from the framework of the surface morphology and the physico-chemical surface interactions. We also discuss the dielectric interface modification and the resultant device performance of the OFETs.

INVITED ARTICLE Tailoring and patterning of dielectric interfaces for the development of advanced organic field-effect transistors

Orientation of transition dipoles can strongly improve the outcoupling efficiency of or-ganic LEDs (OLEDs). We present two new classes of OLED emitters that may achieve horizontal alignment: columnar liquid crystals and carbon nanotubes.

Chang-Min Keum

Papers

Integrated polymer solar cells in serial architecture with patterned charge-transporting MoOx for miniature high-voltage sources

Quantifying charge trapping and molecular doping in organic p-i-n diodes

A substrateless, flexible, and water-resistant organic light-emitting diode

INVITED ARTICLE Tailoring and patterning of dielectric interfaces for the development of advanced organic field-effect transistors

Improved OLED Outcoupling Using Alternative Emitters with Preferred Horizontal Orientation