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This review briefly introduces the degradation mechanisms according to the RH and summarizes various approaches to stabilize halide perovskites. An outlook for research directions of halide perovskites is also suggested.

Metal halide perovskite nanocrystals (PeNCs) with a controlled quantum size effect have received intense interest for potential applications in optoelectronics and photonics. Here, we present a simple and innovative strategy to precisely tune the photoluminescence color of PeNCs by simply printing perovskite precursor solutions on cellulose papers. Depending on the volume of the printed precursor solutions, the PeNCs are autonomously grown into three discrete sizes, and their relative size population is controlled; accordingly, not only the number of multiple PL peaks but also their relative intensities can be precisely tuned. This autonomous size control is obtained through the efflorescence, which is advection of salt ions toward the surface of a porous medium during solvent evaporation and also through the confined crystal growth in the hierarchical structure of cellulose fibers. The infiltrated PeNCs are environmentally stable against moisture (for 3 months in air at 70% relative humidity) and strong light exposure by hydrophobic surface treatment. This study also demonstrates invisible encryption and highly secured unclonable anticounterfeiting patterns on deformable cellulose substrates and banknotes.

Tremendous efforts have been devoted to wearable mechanical sensors to meet growing needs in healthcare sensors and electronic skins.

Drug-induced cardiotoxicity is regarded as a major hurdle in the early stages of drug development. Although there are various methods for preclinical cardiotoxicity tests, they cannot completely predict the cardiotoxic potential of a compound due to the lack of physiological relevance. Recently, three-dimensional engineered heart tissue (EHT) has been used to investigate cardiac muscle functions as well as pharmacological effects by exhibiting physiological auxotonic contractions. However, there is still no adequate platform for continuous monitoring to test acute and chronic pharmacological effects in vitro. Here, we firstly introduce a biohybrid 3D printing method for fabricating a tissue-sensor platform, composed of a bi-pillar-grafted strain gauge sensor and EHT. Two pillars were three-dimensionally printed as grafts onto a strain gauge-embedded substrate to promote the EHT contractility and guide the self-assembly of the EHTs along with the strain gauge. In addition, the integration of a wireless multi-channel electronic system allowed for continuous monitoring of the EHT contractile force by the tissue-sensor platform and, ultimately, for the observation of the acute and chronic drug effects of cardiotoxicants. In summary, we expect biohybrid 3D printing technology to be a potential fabrication method to provide a next-generation tissue-sensor platform for an effective drug development process. This article is protected by copyright. All rights reserved.

Metal thin films with a specific orientation play vital roles in electronics, catalysts, and epitaxial templates. Although oriented metal films have been produced in the recent years, ultrathin oriented metal films (

While various crystalline carbon allotropes, including graphene, have been actively investigated, amorphous carbon (a-C) thin films have received relatively little attention. The a-C is a disordered form of carbon bonding with a broad range of the CC bond length and bond angle. Although accurate structural analysis and theoretical approaches are still insufficient, reproducible structure-property relationships have been accumulated. As the a-C thin film is now adapted as a hardmask in the semiconductor industry and new properties are reported continuously, expectations are growing that it can be practically used as active materials beyond as a simple sacrificial layer. In this perspective review article, after a brief introduction to the synthesis and properties of the a-C thin films, their potential practical applications are proposed, including hardmasks, extreme ultraviolet (EUV) pellicles, diffusion barriers, deformable electrodes and interconnects, sensors, active layers, electrodes for energy, micro-supercapacitors, batteries, nanogenerators, electromagnetic interference (EMI) shielding, and nanomembranes. The article ends with a discussion on the technological challenges in a-C thin films.

Feeling temperature and touch The range of receptors in our skin make it possible to sense when we are touching an object and also gives us a general sense of the temperature of that object. Achieving this in an artificial skin-like material has been a challenge because most of the approaches for sensing touch are themselves temperature sensitive. You et al. studied the ion relaxation dynamics in a conductive elastomeric film (see the Perspective by Liu). They show that the ion relaxation time can be used as a strain-insensitive intrinsic variable for detecting temperature and the capacitance can be used as a temperature-insensitive extrinsic variable for sensing the strain, thus decoupling the two so that their signals do not interfere with each other. Science , this issue p. 961 ; see also p. 910

With emerging interest in foldable and stretchable displays, the need to develop transparent deformable electrode and interconnection is increasing. Even though metal films have been standard electrodes in conventional electronic devices due to their high conductivity and well-established process, they have never been used for transparent deformable electrodes. We present highly conductive transparent deformable Au film electrodes and use them to fabricate a foldable perovskite light-emitting diode (PeLED) and a biaxially stretchable alternating current electroluminescence (ACEL) display. We exhibit the formation of an ultrathin (6 nm) continuous Au film on an anisotropic conductive ultrathin film (ACUF) of amorphous carbon. The ultrathin Au film was first formed on an ACUF-coated Si wafer (4 in. scale) through metal evaporation and transferred to the polymer substrates by a simple and effective water-assisted delamination process. Then, a hybrid electrode (ACUF/ACUF/Au) was produced as the transparent deformable electrode. Complicated interconnections could be created by metal deposition through a mask. The electrical conductance of the hybrid electrode was not affected by the crack formation in the Au film during electrode folding, crumpling, and stretching. We reveal the reason why the hybrid electrode can maintain such excellent electrical stability under deformation.

A triboelectric nanogenerator (TENG) is an attractive alternative source of electric power generation; however, its direct use is difficult due to the spike-like alternating current (AC) output. Th...

Although ion gels are attractive sensing materials for deformable epidermal sensors or implantable devices, their sensing performances are highly affected by environmental humidity change, so that their sensing reliability cannot be secured. This study proposes a new concept of maintaining the high-precision temperature sensing performance of highly deformable ion gel sensors. In this approach, a hydrophobic ion gel sensing layer is kept water-saturated by attaching a hydrogel layer, rather than attempting to completely block water penetration. This study performs experimental and theoretical investigation on water concentration in the ion gel, using the analysis of mass transportation at the interface of the ion gel and the hydrogel. By using the charge relaxation time of the ionic molecules, the temperature sensor is not affected by environmental humidity in the extreme range of humidity (30%-100%). This study demonstrates a highly deformable on-skin temperature sensor which shows the same performance either in water or dry state and while exercising with large strains (ε = 50%).

In fast and transient somatosensory processing, the relative timing of the selected spikes is more important than the spike frequency because the ensemble of the first spikes in the spike trains encodes the dynamic tactile information. Here, inspired by the functional effectiveness of the selected spikes, we propose an artificial dynamic sensory system based on position-encoded spike spectrum. We use a mixed ion-electron conductor to generate a potential spike signal. We design artificial receptors that have different ion relaxation times (τ); thus, a sequence of the spikes from the receptors creates a spike spectrum, providing the spatial information (position and motion trace) and the temporal information (speed and dynamic contact area). The artificial receptors can be incorporated by as much as 132/square centimeters by using only two global signal addressing lines for sensor operation. Structural simplicity of the device opens the possibility of scalable fabrication with dense receptor integration. With computational decoding of the position-encoded spike spectrum, the artificial sensory system can recognize complicated dynamic motions in real time. The high-resolution spatiotemporal tactile perception in the ionic artificial sensory system enables the real-time dynamic robotic manipulation.

On-skin healthcare patch-type devices have great technological challenges in monitoring full-day activities and wearing for multiple days without detachment. These challenges can be overcome when the sensor is air permeable but waterproof. This study presents a light-weight, highly stable, and stretchable Au electrode that is fabricated by sputtering on an imidized nanofiber mat. The contact surface of the electrode is hydro-wetting and the outer surface of the electrode is hydrophobic, so the porous electrode simultaneously has excellent sweat permeability and waterproofing capabilities. The electrode is applied to the electrocardiogram sensor for monitoring the cardiac signals for five consecutive days without detaching while doing various full-day activities such as relaxing, exercising, showering, and sleeping. This study suggests a modular setup of the electrodes and the cardiac signal processing unit for activating the device when cardiac monitoring is required.

Transparent flexible transistor array requests large-area fabrication, high integration, high manufacturing throughput, inexpensive process, uniformity in transistor performance, and reproducibility. This study suggests a facile and reliable approach to meet the requirements. We use the Al-coated polymer nanofiber patterns obtained by electrohydrodynamic (EHD) printing as a photomask. We use the lithography and deposition to produce highly aligned nanolines (NLs) of metals, insulators, and semiconductors on large substrates. With these NLs, we demonstrate a highly integrated NL field-effect transistor (NL-FET) array (10

Pyrolyzed carbon (Py-C) film, a member of graphene family, has not been paid significant attention despite its potential advantages in synthesis procedure and opto-electro-mechanical properties. This academic indifference is due to the lack of a low-cost synthesis method for Py-C ultrathin films whose properties are comparable to those of graphene. In this study, we proposed the direct synthesis of Py-C ultrathin films on various target substrates. We produced a hydrogenated amorphous carbon (a-C:H) ultrathin film in a microwave oven and used it as a transferable precursor film for conversion into the Py-C ultrathin film. The thickness of the Py-C ultrathin film was controlled in the range of 0.7–12 nm. The Py-C ultrathin film has opto-electro-mechanical properties comparable to those of graphene, work function (4.57 eV) similar to that of graphene, high transparency with a relatively low sheet resistance (83% at 1.1 kΩ sq−1), excellent flexibility, stable electrical resistance upon folding, anti-oxidation, and chemical protection. This Py-C ultrathin film is expected to find various practical applications as an alternative to graphene. As an example, we demonstrated a highly flexible pixelated display fabricated using the Py-C ultrathin film as an electrode for alternating current electroluminescent (ACEL) devices.

Although branched WO3 nanostructures have been investigated for electrochromic devices and catalytic electrodes, a detailed study on their structural evolution mechanism has rarely been carried out. In this work, we have demonstrated the microwave-assisted synthesis of branched WO3 nanotrees consisting of [001]-oriented WO3 needle-like structures growing radially from the surface of WO3 nanohelixes. The mechanism for the evolution of WO3 nanotrees with the preferred orientation was systematically investigated in terms of the reaction temperature and the distribution of the electric field applied by microwave irradiation. In addition, it was shown that the WO3 nanotrees can be easily converted into WS2/WO3 hierarchical nanotrees by a simple sulfurization process, and used as an efficient catalytic electrode for the hydrogen evolution reaction.

Colloidal particles with a spherical shape and diameters in the range of 0.01−1 μm have been a subject of extensive research, with applications in areas such as photonics, electronics, catalysis, drug delivery, and medicine. For most of these applications, it is of critical importance to achieve monodispersity for the size while expanding the diversity in terms of structure and composition. The uniformity in size allows one to establish rigorous correlations between this parameter and the physicochemical properties of the colloidal particles while ensuring experimental repeatability and measurement accuracy. On the other hand, the diversity in structure and composition offers additional handles for tailoring the properties. By switching from the conventional plain, solid structure to a core-shell, hollow, porous, or Janus structure, it offers immediate advantages and creates new opportunities, especially in the context of self-assembly, encapsulation, and controlled release. As for composition, monodispersed colloidal spheres were traditionally limited to amorphous materials such as polystyrene and silica. For metals and semiconducting materials, which are more valuable to applications in photonics, electronics, and catalysis, they tend to crystallize and thus grow anisotropically into non-spherical shapes, especially when their sizes pass 0.1 μm. Taken together, it is no wonder why chemical synthesis of monodispersed colloidal spheres has been a constant theme of research in areas such as colloidal science, materials chemistry, materials science, and soft matter. In this account, we summarize our efforts over the past two decades in developing solution-phase methods for the facile synthesis of colloidal spheres that are uniform in size, together with a broad range of compositions (including metals and semiconductors) and structures (e.g., solid, core-shell, hollow, porous, and Janus, among others). We start with the synthesis of monodispersed colloidal spheres made of semiconductors, metals with low melting points, and precious metals. Through chemical reactions, these colloidal spheres can be transformed into core-shell or hollow structures with new compositions and properties. Next, we discuss the synthesis of colloidal spheres with a Janus structure while taking a pseudo-spherical shape. Specifically, metal-polymer hybrid particles comprised of one metal nanoparticle partially embedded in the surface of a polymer sphere can be produced through precipitation polymerization in the presence of metal seed. With these Janus particles serving as templates, other types of Janus structures such as hollow spheres with a single hole in the surface can be obtained via site-selected deposition. Alternatively, such hollow spheres can be fabricated through a physical transformation process that involves swelling of polymer spheres, followed by freeze-drying. All these synthesis and transformation processes are solution-based, offering flexibility and potential for large-scale production. At the end, we highlight some of the applications enabled by these colloidal spheres, including fabrication of photonic devices, encapsulation, and controlled release for nanomedicine.

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

Microparticle aligned-stretchable anisotropic conductive film enables high-resolution stretchable circuit lines to interconnect., In stretchable electronics, high-resolution stretchable interfacing at a mild temperature is considered as a great challenge and has not been achieved yet. This study presents a stretchable anisotropic conductive film (S-ACF) that can electrically connect high-resolution stretchable circuit lines to other electrodes whether they are rigid, flexible, or stretchable. The key concepts of this study are (i) high-resolution (~50 μm) but low–contact resistance (0.2 ohm in 0.25 mm2) interfacing by periodically embedding conductive microparticles in thermoplastic film, (ii) low-temperature interfacing through the formation of chemical bonds between the S-ACF and the substrates, (iii) economical interfacing by selectively patterning the S-ACF, and (iv) direct interfacing of chips by using the adhesion of the thermoplastic matrix. We integrate light-emitting diodes on the patterned S-ACF and demonstrate stable light operation at large biaxial areal stretching (εxy = 70%).

Full advantage of stretchable electronic devices can be taken when utilizing an intrinsically stretchable power source. High-performance stretchable supercapacitors with a simple structure and solid-state operation are good power sources for stretchable electronics. This study suggests a new type of intrinsically stretchable, printable, electroactive ink consisting of 1T-MoS

Significance Multimodal imaging based on optics and ultrasound can provide guide images and complementary structural and functional information, thus improving the accuracy of medical diagnosis and treatment monitoring. However, because conventional ultrasound transducers are opaque, in multimodal imaging with optics, the optical devices must be placed off-axis from the ultrasound transducer. This off-axis arrangement is prone to misalignment, adds complexity and bulk to the system, and can result in a low signal-to-noise-ratio. Here, we present a transparent ultrasound transducer at the heart of a quadruple fusion imaging system that seamlessly integrates ultrasound imaging, photoacoustic imaging, optical coherence tomography, and fluorescence imaging, and we demonstrate the system’s use in imaging responses to both ophthalmologic injuries and oncologic diseases., Ultrasound and optical imagers are used widely in a variety of biological and medical applications. In particular, multimodal implementations combining light and sound have been actively investigated to improve imaging quality. However, the integration of optical sensors with opaque ultrasound transducers suffers from low signal-to-noise ratios, high complexity, and bulky form factors, significantly limiting its applications. Here, we demonstrate a quadruple fusion imaging system using a spherically focused transparent ultrasound transducer that enables seamless integration of ultrasound imaging with photoacoustic imaging, optical coherence tomography, and fluorescence imaging. As a first application, we comprehensively monitored multiparametric responses to chemical and suture injuries in rats’ eyes in vivo, such as corneal neovascularization, structural changes, cataracts, and inflammation. As a second application, we successfully performed multimodal imaging of tumors in vivo, visualizing melanomas without using labels and visualizing 4T1 mammary carcinomas using PEGylated gold nanorods. We strongly believe that the seamlessly integrated multimodal system can be used not only in ophthalmology and oncology but also in other healthcare applications with broad impact and interest.

Although various stretchable optoelectronic devices have been reported, omni-directionally stretchable transparent circuit lines have been a great challenge. Cracks are engineered and fabricated to be highly conductive patterned metal circuit lines in which gold (Au) grids are embedded. Au is deposited selectively in the cracks to form a grid without any junction between the grid lines. Since each grid line is expandable under stretching, the circuit lines are stretchable in all the directions. This study shows that a thin coating of aluminum on the oxide surface enables precise control of the cracks (crack density, crack depth) in the oxide layer. High optical transparency and high stretchability can be achieved simultaneously by controlling the grid density in the circuit line. Light-emitting diodes are integrated directly on the circuit lines and stable operation is demonstrated under 100% stretching.

Multi-modal imaging technique has significantly spotlighted since it can provide a variety of information by combining the complementary merits of several single-modal imaging. In particular, in order to compensate for each shortcoming and improve image quality, an integrated optical and ultrasonic imaging system is being actively researched. However, the non-transparency of the ultrasound transducer made it difficult to integrate the optical and ultrasound imaging system. In previous study, we introduced the transparent ultrasound transducer (TUT) and dual-modal photoacoustic imaging (PAI)/ultrasound imaging (USI) system using the TUT. In this study, we present the multi-modal imaging system integrated with PA, US and optical coherence tomography (OCT). OCT has the advantage of acquiring anatomical information at optical resolution under subsurface and transparent media. To explore the usefulness of the multi-modal imaging system, we have successfully performed in vivo animal experiments: 1) eye imaging experiments and 2) subcutaneous melanoma imaging. In PAI, blood vessels and melanoma are clearly visualized. In OCT, the morphological information in shallow depth are observed in detail. In USI, the melanoma boundary and surrounding tissues are clearly confirmed. These results show that TUT based multi-modal imaging system can serve as a comprehensive in various applications.

We present a multimodal imaging system that seamlessly integrates ultrasound imaging, photoacoustic imaging, and optical coherence tomography using a transparent ultrasonic transducer. We demonstrate the system’s use in imaging responses to mouse body in vivo.

Stretchable electronics has emerged over the past decade and is now expected to bring form factor‐free innovation in the next‐generation electronic devices. Stretchable devices have evolved with the synthesis of new soft materials and new device architectures that require significant deformability while maintaining the high device performance of the conventional rigid devices. As the mismatch in the mechanical stiffness between materials, layers, and device units is the major challenge for stretchable electronics, interface control in varying scales determines the device characteristics and the level of stretchability. This article reviews the recent advances in interface control for stretchable electronic devices. It summarizes the design principles and covers the representative approaches for solving the technological issues related to interfaces at different scales: i) nano‐ and microscale interfaces between materials, ii) mesoscale interfaces between layers or microstructures, and iii) macroscale interfaces between unit devices, substrates, or electrical connections. The last section discusses the current issues and future challenges of the interfaces for stretchable devices., Stretchable electronic devices have diverse interfaces that need to be considered in material design and device fabrication. This article reviews representative approaches to solve the technological issues in stretchable electronic devices, focusing on the interfacial design in different scales (nano/microscale, mesoscale, macroscale). It also discusses the technological advances required for future stretchable device development.

The tactile properties of objects are important for robotic dexterous manipulation. An increasing number of attempts have recently been made to enable tactile information processing in robotic hand via tactile sensors. However, it remains relatively unexplored how to build tactile information processing models. In this study, we aimed to develop a spiking neural network (SNN) based on neural information processing mechanisms in sensory afferents. The SNN processes electrical signals collected from tactile sensor arrays attached to the gripper of the robotic hand while grasping objects with different shapes. We converted each of 42 -channel sensor signals from 2 arrays of 21 sensors into a spike train using the Izhikevich model, which was then fed to the SNN. The synaptic weights of the SNN were learned by the Hebbian learning through pair-based spike timing- dependent plasticity (STDP) algorithm. In addition, we implemented lateral inhibition of the second-layer neurons based on unsupervised learning similar to the one used in self-organizing maps, resulting in a winner-takes-all network. By this unsupervised learning, SNN could learn to discriminate the shape of objects via tactile sensing. In particular, it demonstrated object shape recognition with 100% accuracy. The proposed model could be useful for robots manipulating objects with tactile senses.

Human cutaneous tactile receptors are deformable, and can distinguish touch, strain, relative moving distance, and relative moving velocity. In addition, the tactile potential is self-activated when external stimulation is exerted and the potential is transmitted to the nerve system, resembling the wake-up function in electronic devices. In this study, we mimic such characteristics of the human tactile receptors. We designed a stretchable triboelectric nanogenerator (TENG) for the stimuli-responsive potential generator. The TENG device has a multilayer structure independently recognizing lateral strain by the sliding mode, touch by the contact mode, the relative moving distance, and the relative moving velocity. In addition, the device design allows simultaneous sensing of strain and touch without signal interference. The self-triggered potentials generated by various body motions such as touching, joint bending, and the combinations turn on a sleeping microcontroller unit (MCU) and are used as the distinct motion signals. This study demonstrates a wearable low-power remote tactile interface that controls the 3D movements of a mobile device (drone) by the body motions.

There is an ongoing drive to search for low-temperature processed, stable and efficient electron transport layers (ETLs) for perovskite solar cells (PSCs). Herein, we report, for the first time, the use of a MoS2 thin layer as the ETL for PSCs. MoS2 transparent thin films are directly synthesized on the glass/fluorine doped tin oxide (FTO) substrate by using microwave irradiation. The electrical characteristics of the MoS2 thin film are measured and compared with state-of-the-art efficient electron transporting materials like TiO2 and SnO2. The perovskite solar cells fabricated with the device structure, glass/FTO/MoS2/perovskite/po-spiro-OMeTAD/Au, exhibit a power conversion efficiency (PCE) of 13.1%, which is close to the PCEs obtained from compact TiO2 and SnO2 ETL based PSCs. Good transparency in the visible region (400–900 nm), high electrical conductivity and better charge transfer properties as well as low-temperature synthesis make the MoS2 thin film useful for energy harvesting and other optoelectronic device applications.

Post-transition metal dichalcogenides (P-TMDC) are a promising material group for their unique electronic properties and possibilities for high performance transistor and photodetector devices. As a layered p-type semiconductor of this group, silicon telluride (Si2Te3) is an emerging material due to its wide band gap, near-infrared photoluminescence, optical tuning, and resistive switching properties. However, the synthesis and corresponding study on the growth mechanism of Si2Te3 have rarely been reported. In this work, we synthesized Si2Te3 plates on Si substrates using Au particles as a catalyst. The Au particles enabled the liquid phase reaction with Si and Te due to the eutectic alloy formation of Au–Si and Au–Te. This study presents a series of growth mechanisms: (i) Si2Te3 nuclei formed from the eutectic alloy particles, (ii) formation of Te-rich rods overgrown from the Si2Te3 nuclei, (iii) conversion of the Te-rich rods to Si2Te3 rods, and (iv) transformation of the Si2Te3 rods into horizontal Si2Te3 plates. This study provides an understanding of the reaction and growth mechanism of Si2Te3 plates.

As industrial needs for healthcare sensors, electronic skin, and flexible/stretchable displays increase, interest in stretchable materials is increasing as well. In recent years, the studies on stretchable materials have spread to various pivot components, such as electrodes, circuits, substrates, semiconductors, dielectric layers, membranes, and active nanocomposite films. The block copolymer (BC) elastomers have been playing considerable role in the development of stretchable materials. Since BCs are soft elastomers based on physical cross-links, they show differences in physical properties from normal elastomers formed with chemical cross-linking. BC elastomers does not require additional chemical cross-linking procedure, so they can be easily processed after dissolved in various solvents. Their viscoelasticity and thermoplasticity enable the BCs to become moldable and sticky. Although their unique physical properties may serve as disadvantages in some cases, they have been actively applied to create various stretchable electronic materials and their uses are expected to be enlarged more than ever. In this Account, we summarize recent successful applications of BCs for the stretchable electronic devices and discuss the possibility of further uses and the challenges to be addressed for practical uses. Studies on BC-based stretchable materials have focused initially on the fabrication process of stretchable conductors; mixing conductive fillers physically with BCs, infiltrating BCs in a conductive filler layer, and converting metal precursors into metal nanoparticles inside BCs. When conductive fillers with high aspect ratios, such as nanowires or nanosheets are used, the fillers can be infiltrated by the BCs after deposited. Since the contacts between the fillers are maintained during the infiltration process, even thin composite films possess high conductivity and stretchability. The metal precursor solution printing is suggested as a promising approach because it is compatible with traditional printing techniques without clogging the nozzles and allows high filler loading efficiency. When using a BC as a substrate, it is advisable to use a BC/PDMS double layer because of viscoelastic and thermoplastic properties of BCs. If BC/PDMS double layer is used with much thicker PDMS layer instead of viscoelastic BC alone, the double layer substrate can show a perfect elastomeric behavior, and the advantages of the BC substrate are preserved. Additionally, the use of conventional manufacturing techniques is important for commercialization of the stretchable devices. BC substrates having preformed microfibril network on their surfaces facilitate the fabrication of high-resolution circuitry by directly depositing metals through a mask on the substrate. Recent successes of fabricating stretchable organic transistors were obtained based on in situ phase separation of polymer semiconductors to form nanofibril bundles on the surface of a BC substrate. They have led to the achievement of high resolution transistor array printed in large area. BCs are expected to expand their applicability, including stretchable batteries, since they make it feasible to fabricate various hybrid nanocomposites, pore size-controlled membranes, and microstructured surfaces. However, it is necessary to secure long-term stability under heat, solvent, and UV; in addition, there is a need for the synthesis of functional BCs for use in stretchable implanted biomedical devices.

The triboelectric nanogenerator (TENG) has been investigated intensively during the last decade in terms of power improvement and application as sensors, however, its voltage is usually too high and spike-like to be used directly as a power source for electronic devices. Although there was an effort to adjust the voltage through the circuit design, the complexity and the bulkiness of the circuit board make it difficult. Here, we propose a simple approach to change the spike-like voltage profiles to square-like profiles and adjust the output voltage to be suitable range for electronics. We used printed ion gel electrolyte patterns as capacitors and investigated the effects of dimension of the capacitor, connection types (serial, parallel) of multiple capacitors, and electrochemical conditions. The voltage profile of the TENG-ion gel system was modulated by the contact frequency applied to the TENG, the contact area of the ion gel with the electrode, the type of ions in the electrolyte, reduction/oxidation reaction in the gel, and the connection type (parallel, series) between ion gel patterns. We successfully demonstrated the light emission of a large number of printed electrochemiluminescence patterns.

Flexible tactile sensors have been intensively studied for healthcare and electronic skin devices. Currently, a sensing material, electrode, and substrate are manufactured as one set by depositing the sensing material on the electrode. For this reason, when another electrode or substrate is used in the sensor or when different sensor characteristics are required, a new sensing material must be developed and the fabrication conditions should be changed. This study proposes a novel method of manufacturing a pressure sensing material like a cartridge film. The cartridge film is made by filling the holes of a stencil film (one MP in each hole) with conductive microparticles (MPs). Using the cartridge film, the sensing material can be cut-and-pasted on electrodes and transferred to other electrodes for reuse. This study analyzes the electrical responses of the sensors made of the cartridge film on the basis of the Hertzian contact theory, and also correlates the sensing performance of the sensors with the cond...

The main approach to enhancing the electrical output performance of triboelectric nanogenerators (TENGs) has been focused to increase of triboelectric charge generation. However, there have been few studies on achieving effective electrostatic induction and conserving the triboelectric charges. This study reports that an interlayer containing deep charge traps of large trap density can conserve the surface charges for long period of time and increase the surface potential that can be obtained. This study suggests polydimethylsiloxane (PDMS) added between a charge generation layer and an electrode as an effective material candidate for the interlayer. The PDMS interlayer greatly enhanced the output power density of TENGs (20.8 W/m2 by gentle tapping), which is 173-fold increase compared to TENGs without the interlayer. Surprisingly, the PDMS interlayer resulted in triboelectric performance even between identical surfaces, which is owing to the enhanced charge conservation by the interlayer. This study demonstrates a high-performance stretchable single-electrode TENG (S-TENG) which shows stable high performance at 50% uniaxial strain during repeated stretch cycles. The results in this study provide insight to material design for achieving high-performance stretchable self-powered electronic systems.

The controlled synthesis of large-area, atomically thin molybdenum and tungsten ditelluride (MoTe2 and WTe2) crystals is crucial for their emerging applications based on the attractive electronic properties. However, the solution phase synthesis of high-quality and large-area MoTe2 or WTe2 ultrathin films have not been achieved yet. In this study, we synthesized for the first time, large-area atomically thin MoTe2 and WTe2 films in solution phase, through rapid crystal formation directly on a conducting substrate. For the synthesis, we developed a new Te precursor. The crystal growth involves an in situ chemical transformation from Te nanoparticles into MoTe2 or WTe2 thin films. The synthesis enables precise control of the number of atomic layers over a large area, from a monolayer to multilayers. Micropatterned MoTe2 thin films are also readily synthesized in situ using the same process. The photodetector made of 3-layer semiconducting MoTe2 thin films exhibits high photoresponsivity (Rλ) over a broad sp...

With increasing demands on drug delivery via a transdermal route, there is a therapeutic and regulatory need for on-demand dosage control. Ideally, on-demand dose control would be based on a low-cost, scalable mechanical mechanism without the requirement for ancillary equipment. In this study, we report a touch-actuated transdermal delivery (TATD) patch which provides quantitative permeation control by the degree of mechanical pressing. The patch contains a refillable drug solution reservoir, strain sensor, and drug chamber with an array of microneedles. Mathematical functions are used to predict the normal force applied to the drug reservoir, drug solution released into the drug chamber, and amount of the permeated drug. The final relationship between permeation level and normal force is expressed as a simple equation, which allows for the precise control of drug permeation via external mechanical stimulation. This relationship is demonstrated by image analysis of the permeated drug through animal skin tissue. The TATD patch offers a suitable platform for on-demand control of therapeutic delivery in wearable healthcare systems.

Photoacoustic (PA) imaging is a high-resolution biomedical imaging modality, which can be used to visualize biological tissues located beyond the limited penetration depth of existing optical imaging techniques. An optical wavelength of 1064 nm is of great interest in PA imaging due to low intrinsic absorption at this wavelength. Reduced absorption implies an increased depth of imaging, which enables several new clinical applications such as bladder imaging, gastrointestinal (GI) imaging, and sentinel lymph node (SLN) imaging. In addition, a 1064 nm Nd:YAG laser system enables a high power, cost-effective, and compact laser-based PA imaging system. However, at this wavelength, due to low intrinsic contrast, high absorption exogenous PA contrast agents are necessary for imaging. To this end, we present new Bi2Se3 nanoplates as PA contrast agents at 1064 nm wavelength for PA imaging. We successfully synthesized Bi2Se3 nanoplates and they exhibited relatively strong PA signals at 1064 nm. We confirmed the increased imaging depth of penetration by imaging the Bi2Se3-containing tube located 4.6 cm deep in biological tissues. We present in vivo PA imaging of the bladder, GI tract, and SLN in mice using a Bi2Se3 contrast agent establishing the clinical feasibility of these agents with a clinical photoacoustic/ultrasound imaging system. Our results confirm that Bi2Se3 nanoplates are promising PA contrast agents at 1064 nm that offer a high optical absorbance in the second NIR region providing a high contrast imaging and increased depth of penetration.

The working mechanisms of gel-based electrogenerated chemiluminescence (ECL) devices were revealed using systematic experiments and simulations., Electro-generated chemiluminescence (ECL) has attracted increasing attention as a new platform for light-emitting devices; in particular, the use of mechanically stretchable ECL gels opens up the opportunity to achieve deformable displays. The movements of radical ions under an external electric field include short-range diffusion near the electrodes and long-distance migration between the electrodes. So far, only the diffusion of radical ions has been considered as the operating principle behind ECL. In this study, electrochemical and optical analysis was performed systematically to investigate the role of ion migration in ECL devices. This study reveals that long-distance migration of radical ions can be a key variable in ECL at low frequencies and that this effect depends on the type of ion species and the operating conditions (e.g. voltage and frequency). We also report that the emissions from the two electrodes are not identical, and the emission behaviors are different in the optimal operating conditions for the red, green, and blue ECL emissions.