Syntheses of New Multisubstituted 1-Acyloxyindole Compounds.

The syntheses of novel 1-acyloxyindole compounds 1 and the investigations on reaction pathways are presented. Nitro ketoester substrate 2, obtained in a two-step synthetic process, underwent reduction, intramolecular addition, nucleophilic 1,5-addition, and acylation to afford 1-acyloxyindoles 1 in one pot. Based on the systematic studies, we established the optimized reaction conditions for 1 focusing on the final acylation step of the intermediate 1-hydroxyindole 8. With the optimized conditions, we succeeded in synthesizing 21 examples of new 1-acyloxyindole derivatives 1 in modest yields (Y = 24 - 35%). Among the 1-acyloxyindole compounds, 1-acetoxyindole compounds 1x were generally unstable, and their yields were relatively lower than the other 1-acyloxyindoles. We expect that a bulkier alkyl or aromatic group on R2 could stabilize the 1-acyloxyindole compounds. Significantly, one-pot reactions of a four-step sequence successfully generated compounds 1 that are all new and might be difficult to be synthesized otherwise.

An Efficient Method for Selective Syntheses of Sodium Selenide and Dialkyl Selenides.

The studies on the selective synthesis of dialkyl selenide compounds 1 were presented. Overcoming the complexity and difficulty of selenides (R-Se-R) and/or multiselenides (R-Sen-R; n ≥ 2), we aimed to optimize the reaction condition for the tolerable preparation of sodium selenide (Na2Se) by reducing Se with NaBH4, and then to achieve selective syntheses of dialkyl selenides 1 by subsequently treating the obtained sodium selenide with alkyl halides (RX). Consequently, various dialkyl selenides 1 were efficiently synthesized in good-to-moderate yields. The investigations on reaction pathways and solvent studies were also described.

A Flexible Loudspeaker Using the Movement of Liquid Metal Induced by Electrochemically Controlled Interfacial Tension.

A flexible liquid metal loudspeaker (LML) is demonstrated consisting of a gallium-based eutectic liquid metal (Galinstan) and basic aqueous electrolyte (NaOH(aq) ). The LML is driven by liquid metal motion induced by the electrochemically controlled interfacial tension of the Galinstan in NaOH(aq) electrolyte under an applied alternating current (AC) voltage. The fabricated LML produces sound waves in the human audible frequency band with a sound pressure level of ≈40-50 dB at 1 cm from the device and exhibits mechanical stability under bending deformation with a bending radius of 3 mm. Various sounds can be generated with the LML from a single tone to piano notes and human voices. To understand the underlying mechanism of sound generation by the LML, motion analyses, sound measurements, and electrical characterization are conducted at various frequencies. For the first time, this work suggests a new type of liquid metal-based electrochemically driven sound generator in the field of flexible acoustic devices that can be applied to future wearable electronics.

Dynamically Stretchable Supercapacitor for Powering an Integrated Biosensor in an All-in-One Textile System.

Textile-based electronics have attracted much attention as they can perfectly combine the functionality of wearable devices with the soft and comfortable properties of flexible textile fibers. In this work, we report a dynamically stretchable high-performance supercapacitor for powering an integrated sensor in an all-in-one textile system to detect various biosignals. The supercapacitor fabricated with MWCNT/MoO3 nanocomposite electrodes and nonaqueous gel electrolyte, along the course direction of the fabric, exhibits stable and high electrochemical performance under dynamic and static deformation, including stretching in real time, regardless of the strain rate. The strain sensor created along the wale direction of the fabric shows a high sensitivity of 46.3 under an applied strain up to 60%, a fast response time of 50 ms, and high stability over 10 000 cycles of stretching/releasing. Finally, the supercapacitor and strain sensor are integrated into an all-in-one textile system via liquid-metal interconnections, and the sensor is powered by the stored energy in the supercapacitor. This system sewed into cloth successfully detects strain due to joint movement and the wrist pulse. This work demonstrates the high feasibility of utilizing the fabricated stretchable all-in-one textile system for real-time health monitoring in everyday wearable devices.

Stretchable, Skin-Attachable Electronics with Integrated Energy Storage Devices for Biosignal Monitoring.

The demand for novel electronics that can monitor human health, for example, the physical conditions of individuals, during daily life using different techniques from those used in traditional clinic diagnostic facilities is increasing. These novel electronics include stretchable sensor devices that allow various biosignals to be directly measured on human skin without restricting routine activity. The thin, skin-like characteristics of these devices enable stable operation under various deformations, such as stretching, pressing, and rubbing, experienced while attached to skin. The mechanically engineered design of these devices also minimizes the inconvenience caused by long-term wear owing to conformal lamination on the skin. The final form of a skin-attachable device must be an integrated platform with an independent and complete system containing all components on a single, thin, lightweight, stretchable substrate. To fabricate fully integrated devices, various aspects, such as material design for deformable interconnection, fabrication of high-performance active devices, miniaturization, and dense arrangement of component devices, should be considered. In particular, a power supply system is critical and must be combined in an electromechanically stable and efficient manner with all devices, including sensors. Additionally, the biosignals obtained by these sensors should be wirelessly transmitted to external electronic devices for free daily activity. This Account covers recent progress in developing fully integrated, stretchable, skin-attachable devices by presenting our strategies to achieve this goal. First, we introduce several integration methods used in this field to build stretchable systems with a special focus on the utilization of liquid gallium alloy. The unique characteristics and patterning process of liquid metal are summarized. Second, various skin-attachable sensors, including strain, pressure, with enhanced sensitivity and mechanical properties are discussed along with their applications for biosignal monitoring. Dual mode sensors that simultaneously detect temperature and pressure signals without interference are also introduced. Third, we emphasize supercapacitors as promising, efficient energy storage devices for power management systems in wearable devices. Supercapacitors for skin-attachable applications should have a high performance, such as high operation voltage, high energy and power densities, cyclic and air stability and water resistance. For this, strategies to select novel materials for electrode, electrolyte, and encapsulation are suggested. Several approaches to fabricate stretchable supercapacitor systems are also presented. Finally, we introduce recent examples of skin-attachable, stretchable electronics that integrate sensors, power management devices, and wireless data transfer functions on a single elastomer substrate. Conventional wireless technologies, such as near-field communications (NFC) and Bluetooth, are incorporated in miniaturized features on the devices. To date, much research has been performed in this field, but there are still many technologies to develop. The performance of individual devices and mass fabrication techniques should be enhanced. We expect that future electronic devices with fully integrated functions will include advanced human-machine interaction capabilities and expand the overall abilities of the human body.

Highly Durable and Flexible Transparent Electrode for Flexible Optoelectronic Applications.

A highly-durable, highly-flexible transparent electrode (FTE) is developed by applying a composite made of a thin metal grid and a doped conducting polymer onto a colorless polyimide-coated NOA63 substrate. The proposed FTE exhibits a transparency of 90.7% at 550 nm including the substrate and a sheet resistance of 30.3 Ω/sq and can withstand both moderately high-temperature annealing (∼180 °C) and acidic solution (70 °C, pH 0.3) processes without performance degradation. The fabricated FTE yielded good mechanical stability under 10 000 cycles of bending deformations at a bending radius less than 1 mm without degradation of electrical conductivity. The high durability of the proposed FTE allows for the fabrication of flexible energy harvesting devices requiring harsh conditions, such as highly flexible perovskite solar cells (FPSCs) with a steady-state power conversion efficiency (PCE) of 12.7%. Notably, 93% of the original PCE is maintained after 2000 bending cycles at an extremely small bending radius of 1.5 mm. The FPSCs installed on curved surfaces of commercial devices drive them under various environments. The applicability of the proposed FTE is further confirmed via the fabrication of a flexible perovskite light-emitting diode. The proposed FTE demonstrates great potential for applications in the field of flexible optoelectronic devices.

Highly Conductive, Stretchable, and Transparent PEDOT:PSS Electrodes Fabricated with Triblock Copolymer Additives and Acid Treatment.

Here, we report on a highly conductive, stretchable, and transparent electrode of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fabricated via modification with triblock copolymer, poly(ethylene glycol)- block-poly(propylene glycol)- block-poly(ethylene glycol) (PEO20-PPO70-PEO20, Pluronic P123), and post-treatment with sulfuric acid. The fabricated electrode exhibits high transparency (89%), high electrical conductivity (∼1700 S/cm), and minimal change in resistance (∼4%) under repetitive stretch-release cycles at 40% tensile strain after stabilization. P123 acts as a secondary dopant and plasticizer, resulting in enhanced electrical conductivity and stretchability of PEDOT:PSS. Furthermore, after sulfuric acid post-treatment, P123 helps the electrode to maintain its stretchability. A successful demonstration of the stretchable interconnection was shown by stretching the P123-modified PEDOT:PSS electrodes, which were connected with light-emitting diodes (LEDs) in series. Finally, a stretchable and transparent touch sensor consisting of our fabricated electrodes and an LED array and stretchable semitransparent supercapacitor were presented, suggesting a great potential of our electrodes in the application to various deformable devices.

Skin-Attachable, Stretchable Electrochemical Sweat Sensor for Glucose and pH Detection.

As part of increased efforts to develop wearable healthcare devices for monitoring and managing physiological and metabolic information, stretchable electrochemical sweat sensors have been investigated. In this study, we report on the fabrication of a stretchable and skin-attachable electrochemical sensor for detecting glucose and pH in sweat. A patterned stretchable electrode was fabricated via layer-by-layer deposition of carbon nanotubes (CNTs) on top of patterned Au nanosheets (AuNS) prepared by filtration onto stretchable substrate. For the detection of glucose and pH, CoWO4/CNT and polyaniline/CNT nanocomposites were coated onto the CNT-AuNS electrodes, respectively. A reference electrode was prepared via chlorination of silver nanowires. Encapsulation of the stretchable sensor with sticky silbione led to a skin-attachable sweat sensor. Our sensor showed high performance with sensitivities of 10.89 µA mM-1 cm-2 and 71.44 mV pH-1 for glucose and pH, respectively, with mechanical stability up to 30% stretching and air stability for 10 days. The sensor also showed good adhesion even to wet skin, allowing the detection of glucose and pH in sweat from running while being attached onto the skin. This work suggests the application of our stretchable and skin-attachable electrochemical sensor to health management as a high-performance healthcare wearable device.

Fabrication of High-Sensitivity Skin-Attachable Temperature Sensors with Bioinspired Microstructured Adhesive.

In this study, we demonstrate the fabrication of a highly sensitive flexible temperature sensor with a bioinspired octopus-mimicking adhesive. A resistor-type temperature sensor consisting of a composite of poly(N-isopropylacrylamide) (pNIPAM)-temperature sensitive hydrogel, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and carbon nanotubes exhibits a very high thermal sensitivity of 2.6%·°C-1 between 25 and 40 °C so that the change in skin temperature of 0.5 °C can be accurately detected. At the same time, the polydimethylsiloxane adhesive layer of octopus-mimicking rim structure coated with pNIPAM is fabricated through the formation of a single mold by utilizing undercut phenomenon in photolithography. The fabricated sensor shows stable and reproducible detection of skin temperature under repeated attachment/detachment cycles onto skin without any skin irritation for a long time. This work suggests a high potential application of our skin-attachable temperature sensor to wearable devices for medical and health-care monitoring.

Fully implantable, battery-free wireless optoelectronic devices for spinal optogenetics.

The advent of optogenetic tools has allowed unprecedented insights into the organization of neuronal networks. Although recently developed technologies have enabled implementation of optogenetics for studies of brain function in freely moving, untethered animals, wireless powering and device durability pose challenges in studies of spinal cord circuits where dynamic, multidimensional motions against hard and soft surrounding tissues can lead to device degradation. We demonstrate here a fully implantable optoelectronic device powered by near-field wireless communication technology, with a thin and flexible open architecture that provides excellent mechanical durability, robust sealing against biofluid penetration and fidelity in wireless activation, thereby allowing for long-term optical stimulation of the spinal cord without constraint on the natural behaviors of the animals. The system consists of a double-layer, rectangular-shaped magnetic coil antenna connected to a microscale inorganic light-emitting diode (µ-ILED) on a thin, flexible probe that can be implanted just above the dura of the mouse spinal cord for effective stimulation of light-sensitive proteins expressed in neurons in the dorsal horn. Wireless optogenetic activation of TRPV1-ChR2 afferents with spinal µ-ILEDs causes nocifensive behaviors and robust real-time place aversion with sustained operation in animals over periods of several weeks to months. The relatively low-cost electronics required for control of the systems, together with the biocompatibility and robust operation of these devices will allow broad application of optogenetics in future studies of spinal circuits, as well as various peripheral targets, in awake, freely moving and untethered animals, where existing approaches have limited utility.

A skin-integrated transparent and stretchable strain sensor with interactive color-changing electrochromic displays.

In this study, we report on the development of a stretchable, transparent, and skin-attachable strain sensor integrated with a flexible electrochromic device as a human skin-inspired interactive color-changing system. The strain sensor consists of a spin-coated conductive nanocomposite film of poly(vinyl alcohol)/multi-walled carbon nanotube/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) on a polydimethylsiloxane substrate. The sensor exhibits excellent performance of high sensitivity, high durability, fast response, and high transparency. An electrochromic device (ECD) made of electrochemically synthesized polyaniline nanofibers and V2O5 on an indium-tin-oxide-coated polyethylene terephthalate film experiences a change in color from yellow to dark blue on application of voltage. The strain sensor and ECD are integrated on skin via an Arduino circuit for an interactive color change with the variation of the applied strain, which enables a real-time visual display of body motion. This integrated system demonstrates high potential for use in interactive wearable devices, military applications, and smart robots.

Flexible Near-Field Wireless Optoelectronics as Subdermal Implants for Broad Applications in Optogenetics.

In vivo optogenetics provides unique, powerful capabilities in the dissection of neural circuits implicated in neuropsychiatric disorders. Conventional hardware for such studies, however, physically tethers the experimental animal to an external light source, limiting the range of possible experiments. Emerging wireless options offer important capabilities that avoid some of these limitations, but the current size, bulk, weight, and wireless area of coverage is often disadvantageous. Here, we present a simple but powerful setup based on wireless, near-field power transfer and miniaturized, thin, flexible optoelectronic implants, for complete optical control in a variety of behavioral paradigms. The devices combine subdermal magnetic coil antennas connected to microscale, injectable light-emitting diodes (LEDs), with the ability to operate at wavelengths ranging from UV to blue, green-yellow, and red. An external loop antenna allows robust, straightforward application in a multitude of behavioral apparatuses. The result is a readily mass-producible, user-friendly technology with broad potential for optogenetics applications.

Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin.

A stretchable polyaniline nanofiber temperature sensor array with an active matrix consisting of single-walled carbon nanotube thin-film transistors is demonstrated. The integrated temperature sensor array gives mechanical stability under biaxial stretching of 30%, and the resultant spatial temperature mapping does not show any mechanical or electrical degradation.

Body-Attachable and Stretchable Multisensors Integrated with Wirelessly Rechargeable Energy Storage Devices.

A stretchable multisensor system is successfully demonstrated with an integrated energy-storage device, an array of microsupercapacitors that can be repeatedly charged via a wireless radio-frequency power receiver on the same stretchable polymer substrate. The integrated devices are interconnected by a liquid-metal interconnection and operate stably without noticeable performance degradation under strain due to the skin attachment, and a uniaxial strain up to 50%.

Stretchable Array of Highly Sensitive Pressure Sensors Consisting of Polyaniline Nanofibers and Au-Coated Polydimethylsiloxane Micropillars.

We report on the facile fabrication of a stretchable array of highly sensitive pressure sensors. The proposed pressure sensor consists of the top layer of Au-deposited polydimethylsiloxane (PDMS) micropillars and the bottom layer of conductive polyaniline nanofibers on a polyethylene terephthalate substrate. The sensors are operated by the changes in contact resistance between Au-coated micropillars and polyaniline according to the varying pressure. The fabricated pressure sensor exhibits a sensitivity of 2.0 kPa(-1) in the pressure range below 0.22 kPa, a low detection limit of 15 Pa, a fast response time of 50 ms, and high stability over 10000 cycles of pressure loading/unloading with a low operating voltage of 1.0 V. The sensor is also capable of noninvasively detecting human-pulse waveforms from carotid and radial artery. A 5 × 5 array of the pressure sensors on the deformable substrate, which consists of PDMS islands for sensors and the mixed thin film of PDMS and Ecoflex with embedded liquid metal interconnections, shows stable sensing of pressure under biaxial stretching by 15%. The strain distribution obtained by the finite element method confirms that the maximum strain applied to the pressure sensor in the strain-suppressed region is less than 0.04% under a 15% biaxial strain of the unit module. This work demonstrates the potential application of our proposed stretchable pressure sensor array for wearable and artificial electronic skin devices.

Stretchable Loudspeaker using Liquid Metal Microchannel.

Considering the various applications of wearable and bio-implantable devices, it is desirable to realize stretchable acoustic devices for body-attached applications such as sensing biological signals, hearing aids, and notification of information via sound. In this study, we demonstrate the facile fabrication of a Stretchable Acoustic Device (SAD) using liquid metal coil of Galinstan where the SAD is operated by the electromagnetic interaction between the liquid metal coil and a Neodymium (Nd) magnet. To fabricate a liquid metal coil, Galinstan was injected into a micro-patterned elastomer channel. This fabricated SAD was operated simultaneously as a loudspeaker and a microphone. Measurements of the frequency response confirmed that the SAD was mechanically stable under both 50% uniaxial and 30% biaxial strains. Furthermore, 2000 repetitive applications of a 50% uniaxial strain did not induce any noticeable degradation of the sound pressure. Both voice and the beeping sound of an alarm clock were successfully recorded and played back through our SAD while it was attached to the wrist under repeated deformation. These results demonstrate the high potential of the fabricated SAD using Galinstan voice coil in various research fields including stretchable, wearable, and bio-implantable acoustic devices.