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

This research was financially supported by the European Research Council of the European Union (ERC Grant Agreements No. 640012/ABLASE and 321305/EXCITON), by the Scottish Funding Council (through SUPA), by EPSRC (through the CDT Capital Equipment funding stream, EP/L017008/1) and by a European Commission Marie Curie Career Integration Grant (PCIG12-GA-2012-334407). MS acknowledges funding by the European Commission through a Marie Sklodowska-Curie Individual Fellowship (659213). IDWS acknowledges support from a Royal Society Wolfson Research Merit Award.

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Time‐Resolved Studies of Energy Transfer in Thin Films of Green and Red Fluorescent Proteins

Scaling of High‐Performance Organic Permeable Base Transistors

Patterning organic transistors by dry-etching: The double layer lithography

We describe a control scheme of the molecular order and cracks in a solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) film on a polymeric insulator by the anisotropic temperature-gradient in the process of solvent drying. The solvent for TIPS-pentacene sequentially evaporates from the high temperature region to the low temperature region. As a result, the elongated domains of TIPS-pentacene molecules are developed along the temperature-gradient direction. As increasing of temperature-gradient, TIPS-pentacene molecules tend to be more ordered, which facilitates intramolecular charge transport. Consequently, it is found that the mobility increases and reaches about 0.4 cm 2 /Vs in an organic thin-film transistor fabricated with the TIPS-pentacene film prepared at the condition of room temperature −90 °C. Comparing with 0.1 cm 2 /Vs for the case of a conventional solvent drying method, the mobility is enhanced about four-times. On further increasing of temperature-gradient, however, the crack begins to develop at the higher temperature region toward the lower temperature region, and the corresponding mobility decreases drastically. This indicates that there is a trade-off between the enhancement of the molecular order and the generation of the cracks of a solution-processed organic semiconductor prepared by anisotropic solvent drying.

Control of the molecular order and cracks of the 6,13-bis(triisopropylsilylethynyl)-pentacene on a polymeric insulator by anisotropic solvent drying

We report on the enhancement of the field-effect mobility of solution-processed 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) by unidirectional topography (UT) of an inkjet-printed polymer insulator. The UT leads to anisotropic spreading and drying of the TIPS-pentacene droplet and enables to spontaneously develop the ordered structures during the solvent evaporation. The mobility of the UT-dictated TIPS-pentacene film (0.202 ± 0.012 cm2/Vs) is found to increase by more than a factor of two compared to that of the isotropic case (0.090 ± 0.032 cm2/Vs). The structural arrangement of the TIPS-pentacene molecules in relation to the mobility enhancement is described within an anisotropic wetting formalism. Our UT-based approach to the mobility enhancement is easily applicable to different classes of soluble organic field-effect transistors by adjusting the geometrical parameters such as the height, the width, and the periodicity of the UT of an inkjet-printed insulator.

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Topography-guided spreading and drying of 6,13-bis(triisopropylsilylethynyl)-pentacene solution on a polymer insulator for the field-effect mobility enhancement

Chang-Min Keum

Papers

Time‐Resolved Studies of Energy Transfer in Thin Films of Green and Red Fluorescent Proteins

Scaling of High‐Performance Organic Permeable Base Transistors

Patterning organic transistors by dry-etching: The double layer lithography

Control of the molecular order and cracks of the 6,13-bis(triisopropylsilylethynyl)-pentacene on a polymeric insulator by anisotropic solvent drying

Topography-guided spreading and drying of 6,13-bis(triisopropylsilylethynyl)-pentacene solution on a polymer insulator for the field-effect mobility enhancement