Universal selective transfer printing via micro-vacuum force.

Transfer printing of inorganic thin-film semiconductors has attracted considerable attention to realize high-performance soft electronics on unusual substrates. However, conventional transfer technologies including elastomeric transfer printing, laser-assisted transfer, and electrostatic transfer still have challenging issues such as stamp reusability, additional adhesives, and device damage. Here, a micro-vacuum assisted selective transfer is reported to assemble micro-sized inorganic semiconductors onto unconventional substrates. 20 µm-sized micro-hole arrays are formed via laser-induced etching technology on a glass substrate. The vacuum controllable module, consisting of a laser-drilled glass and hard-polydimethylsiloxane micro-channels, enables selective modulation of micro-vacuum suction force on microchip arrays. Ultrahigh adhesion switchability of 3.364 × 106, accomplished by pressure control during the micro-vacuum transfer procedure, facilitates the pick-up and release of thin-film semiconductors without additional adhesives and chip damage. Heterogeneous integration of III-V materials and silicon is demonstrated by assembling microchips with diverse shapes and sizes from different mother wafers on the same plane. Multiple selective transfers are implemented by independent pressure control of two separate vacuum channels with a high transfer yield of 98.06%. Finally, flexible micro light-emitting diodes and transistors with uniform electrical/optical properties are fabricated via micro-vacuum assisted selective transfer.

Vertically Integrated CMOS Ternary Logic Device with Low Static Power Consumption and High Packing Density.

A ternary logic system to realize the simplest multivalued logic architecture can enhance energy efficiency compared to a binary logic system by reducing the number of transistors and interconnections. For the ternary logic system, a ternary logic device to harness three stable states is needed. In this study, a vertically integrated complementary metal-oxide-semiconductor ternary logic device is demonstrated by monolithically integrating a thin-film transistor (TFT) over a transistor-based threshold switch (TTS). Because the TFT and the TTS have their own source (S), drain (D), and gate (G), there are physically six electrodes. But the hybrid ternary logic device of the TFT over the TTS has only four electrodes: S, D, GTFT, and GTTS like a single MOSFET. It is because the D of the underlying TTS is electrically tied with the S of the superjacent TFT. By combining an on- and off-state of the TFT and the TTS, ternary logic values of low current ("0"-state), middle current ("1"-state), and high current ("2"-state) are realized. Particularly, static power consumption at the "1"-state is decreased by employing the TTS with low off-state leakage current compared to previously reported other ternary logic devices. In addition, a footprint of the ternary logic device with the vertically overlaying structure that has a framework of "one over the other" can be lowered by roughly twice compared to that with the laterally deployed structure that has an organization of "one alongside the other".

3D Neuromorphic Hardware with Single Thin-Film Transistor Synapses Over Single Thin-Body Transistor Neurons by Monolithic Vertical Integration.

Neuromorphic hardware with a spiking neural network (SNN) can significantly enhance the energy efficiency for artificial intelligence (AI) functions owing to its event-driven and spatiotemporally sparse operations. However, an artificial neuron and synapse based on complex complementary metal-oxide-semiconductor (CMOS) circuits limit the scalability and energy efficiency of neuromorphic hardware. In this work, a neuromorphic module is demonstrated composed of synapses over neurons realized by monolithic vertical integration. The synapse at top is a single thin-film transistor (1TFT-synapse) made of poly-crystalline silicon film and the neuron at bottom is another single transistor (1T-neuron) made of single-crystalline silicon. Excimer laser annealing (ELA) is applied to activate dopants for the 1TFT-synapse at the top and rapid thermal annealing (RTA) is applied to do so for the 1T-neuron at the bottom. Internal electro-thermal annealing (ETA) via the generation of Joule heat is also used to enhance the endurance of the 1TFT-synapse without transferring heat to the 1T-neuron at the bottom. As neuromorphic vision sensing, classification of American Sign Language (ASL) is conducted with the fabricated neuromorphic module. Its classification accuracy on ASL is ≈92.3% even after 204 800 update pulses.

Flash-Induced High-Throughput Porous Graphene via Synergistic Photo-Effects for Electromagnetic Interference Shielding.

Porous 2D materials with high conductivity and large surface area have been proposed for potential electromagnetic interference (EMI) shielding materials in future mobility and wearable applications to prevent signal noise, transmission inaccuracy, system malfunction, and health hazards. Here, we report on the synthesis of lightweight and flexible flash-induced porous graphene (FPG) with excellent EMI shielding performance. The broad spectrum of pulsed flashlight induces photo-chemical and photo-thermal reactions in polyimide films, forming 5 × 10 cm2-size porous graphene with a hollow pillar structure in a few milliseconds. The resulting material demonstrated low density (0.0354 g cm-3) and outstanding absolute EMI shielding effectiveness of 1.12 × 105 dB cm2 g-1. The FPG was characterized via thorough material analyses, and its mechanical durability and flexibility were confirmed by a bending cycle test. Finally, the FPG was utilized in drone and wearable applications, showing effective EMI shielding performance for internal/external EMI in a drone radar system and reducing the specific absorption rate in the human body.

Clinical Validation of a Wearable Piezoelectric Blood-Pressure Sensor for Continuous Health Monitoring.

Wearable blood-pressure sensors have recently attracted attention as healthcare devices for continuous non-invasive arterial pressure (CNAP) monitoring. However, the accuracy of wearable blood-pressure (BP) monitoring devices has been controversial due to the low signal quality of sensors, the absence of an accurate transfer function to convert the sensor signals into BP values, and the lack of clinical validation regarding measurement precision. Here, a wearable piezoelectric blood-pressure sensor (WPBPS) is reported, which achieves a high normalized sensitivity (0.062 kPa-1 ), and fast response time (23 ms) for CNAP monitoring. The transfer function of a linear regression model is designed, offering a simple solution to convert the flexible piezoelectric sensor signals into BP values. In order to verify the measurement accuracy of WPBPS, clinical trials are performed on 35 subjects aged from 20 to 80 s after screening. The mean difference between the WPBPS and a commercial sphygmomanometer of 175 BP data pairs is -0.89 ± 6.19 and -0.32 ± 5.28 mmHg for systolic blood pressure (SBP) and diastolic blood pressure (DBP), respectively. By building a WPBPS-embedded wristwatch, the potentially promising use of a convenient, portable, continuous BP monitoring system for cardiovascular disease diagnosis is demonstrated.

Wearable Surface-Lighting Micro-Light-Emitting Diode Patch for Melanogenesis Inhibition.

Wearable light-emitting diode (LED)-based phototherapeutic devices have recently attracted attention as skin care tools for wrinkles, acne, and hyperpigmentation. However, the therapeutic effectiveness and safety of LED stimulators are still controversial due to their inefficient light transfer, high heat generation, and non-uniform spot irradiation. Here, a wearable surface-lighting micro-LED (SµLED) photostimulator is reported for skin care and cosmetic applications. The SµLEDs, consisting of a light diffusion layer (LDL), 900 thin film µLEDs, and polydimethylsiloxane (PDMS), achieve uniform surface-lighting in 2 × 2 cm2 -sized area with 100% emission yields. The SµLEDs maximize photostimulation effectiveness on the skin surface by uniform irradiation, high flexibility, and thermal stability. The SµLED's effect on melanogenesis inhibition is evaluated via in vitro and in vivo experiments to human skin equivalents (HSEs) and mouse dorsal skin, respectively. The anti-melanogenic effect of SµLEDs is confirmed by significantly reduced levels of melanin contents, melan-A, tyrosinase, and microphthalmia-associated transcription factor (MITF), compared to a conventional LED (CLED) stimulator.

Siloxane Hybrid Material-Encapsulated Highly Robust Flexible µLEDs for Biocompatible Lighting Applications.

Flexible micro-light-emitting diodes (f-µLEDs) have been regarded as an attractive light source for the next-generation human-machine interfaces, thanks to their noticeable optoelectronic performances. However, when it comes to their practical utilizations fulfilling industrial standards, there have been unsolved reliability and durability issues of the f-µLEDs, despite previous developments in the high-performance f-µLEDs for various applications. Herein, highly robust flexible µLEDs (f-HµLEDs) with 20 × 20 arrays, which are realized by a siloxane-based organic-inorganic hybrid material (SHM), are reported. The f-HµLEDs are created by combining the f-µLED fabrication process with SHM synthesis procedures (i.e., sol-gel reaction and successive photocuring). The outstanding mechanical, thermal, and environmental stabilities of our f-HµLEDs are confirmed by a host of experimental and theoretical examinations, including a bending fatigue test (105 bending/unbending cycles), a lifetime accelerated stress test (85 °C and 85% relative humidity), and finite element method simulations. Eventually, to demonstrate the potential of our f-HµLEDs for practical applications of flexible displays and/or biomedical devices, their white light emission due to quantum dot-based color conversion of blue light emitted by GaN-based f-HµLEDs is demonstrated, and the biocompatibility of our f-HµLEDs is confirmed via cytotoxicity and cell proliferation tests with muscle, bone, and neuron cell lines. As far as we can tell, this work is the first demonstration of the flexible µLED encapsulation platform based on the SHM, which proved its mechanical, thermal, and environmental stabilities and biocompatibility, enabling us to envisage biomedical and/or flexible display applications using our f-HµLEDs.

Simultaneous emulation of synaptic and intrinsic plasticity using a memristive synapse.

Neuromorphic computing targets the hardware embodiment of neural network, and device implementation of individual neuron and synapse has attracted considerable attention. The emulation of synaptic plasticity has shown promising results after the advent of memristors. However, neuronal intrinsic plasticity, which involves in learning process through interactions with synaptic plasticity, has been rarely demonstrated. Synaptic and intrinsic plasticity occur concomitantly in learning process, suggesting the need of the simultaneous implementation. Here, we report a neurosynaptic device that mimics synaptic and intrinsic plasticity concomitantly in a single cell. Threshold switch and phase change memory are merged in threshold switch-phase change memory device. Neuronal intrinsic plasticity is demonstrated based on bottom threshold switch layer, which resembles the modulation of firing frequency in biological neuron. Synaptic plasticity is also introduced through the nonvolatile switching of top phase change layer. Intrinsic and synaptic plasticity are simultaneously emulated in a single cell to establish the positive feedback between them. A positive feedback learning loop which mimics the retraining process in biological system is implemented in threshold switch-phase change memory array for accelerated training.

A Flash-Induced Robust Cu Electrode on Glass Substrates and Its Application for Thin-Film µLEDs.

A robust Cu conductor on a glass substrate for thin-film µLEDs using the flash-induced chemical/physical interlocking between Cu and glass is reported. During millisecond light irradiation, CuO nanoparticles (NPs) on the display substrate are transformed into a conductive Cu film by reduction and sintering. At the same time, intensive heating at the boundary of CuO NPs and glass chemically induces the formation of an ultrathin Cu2 O interlayer within the Cu/glass interface for strong adhesion. Cu nanointerlocking occurs by transient glass softening and interface fluctuation to increase the contact area. Owing to these flash-induced interfacial interactions, the flash-activated Cu electrode exhibits an adhesion energy of 10 J m-2 , which is five times higher than that of vacuum-deposited Cu. An AlGaInP thin-film vertical µLED (VLED) forms an electrical interconnection with the flash-induced Cu electrode via an ACF bonding process, resulting in a high optical power density of 41 mW mm-2 . The Cu conductor enables reliable VLED operation regardless of harsh thermal stress and moisture infiltration under a high-temperature storage test, temperature humidity test, and thermal shock test. 50 × 50 VLED arrays transferred onto the flash-induced robust Cu electrode show high illumination yield and uniform distribution of forward voltage, peak wavelength, and device temperature.

Biomimetic and flexible piezoelectric mobile acoustic sensors with multiresonant ultrathin structures for machine learning biometrics.

Flexible resonant acoustic sensors have attracted substantial attention as an essential component for intuitive human-machine interaction (HMI) in the future voice user interface (VUI). Several researches have been reported by mimicking the basilar membrane but still have dimensional drawback due to limitation of controlling a multifrequency band and broadening resonant spectrum for full-cover phonetic frequencies. Here, highly sensitive piezoelectric mobile acoustic sensor (PMAS) is demonstrated by exploiting an ultrathin membrane for biomimetic frequency band control. Simulation results prove that resonant bandwidth of a piezoelectric film can be broadened by adopting a lead-zirconate-titanate (PZT) membrane on the ultrathin polymer to cover the entire voice spectrum. Machine learning-based biometric authentication is demonstrated by the integrated acoustic sensor module with an algorithm processor and customized Android app. Last, exceptional error rate reduction in speaker identification is achieved by a PMAS module with a small amount of training data, compared to a conventional microelectromechanical system microphone.

Wireless smart contact lens for diabetic diagnosis and therapy.

A smart contact lens can be used as an excellent interface between the human body and an electronic device for wearable healthcare applications. Despite wide investigations of smart contact lenses for diagnostic applications, there has been no report on electrically controlled drug delivery in combination with real-time biometric analysis. Here, we developed smart contact lenses for both continuous glucose monitoring and treatment of diabetic retinopathy. The smart contact lens device, built on a biocompatible polymer, contains ultrathin, flexible electrical circuits and a microcontroller chip for real-time electrochemical biosensing, on-demand controlled drug delivery, wireless power management, and data communication. In diabetic rabbit models, we could measure tear glucose levels to be validated by the conventional invasive blood glucose tests and trigger drugs to be released from reservoirs for treating diabetic retinopathy. Together, we successfully demonstrated the feasibility of smart contact lenses for noninvasive and continuous diabetic diagnosis and diabetic retinopathy therapy.

Progress in Brain-Compatible Interfaces with Soft Nanomaterials.

Neural interfaces facilitating communication between the brain and machines must be compatible with the soft, curvilinear, and elastic tissues of the brain and yet yield enough power to read and write information across a wide range of brain areas through high-throughput recordings or optogenetics. Biocompatible-material engineering has facilitated the development of brain-compatible neural interfaces to support built-in modulation of neural circuits and neurological disorders. Recent developments in brain-compatible neural interfaces that use soft nanomaterials more suitable for complex neural circuit analysis and modulation are reviewed. Preclinical tests of the compatibility and specificity of these interfaces in animal models are also discussed.

TFT Channel Materials for Display Applications: From Amorphous Silicon to Transition Metal Dichalcogenides.

As the need for super-high-resolution displays with various form factors has increased, it has become necessary to produce high-performance thin-film transistors (TFTs) that enable faster switching and higher current driving of each pixel in the display. Over the past few decades, hydrogenated amorphous silicon (a-Si:H) has been widely utilized as a TFT channel material. More recently, to meet the requirement of new types of displays such as organic light-emitting diode displays, and also to overcome the performance and reliability issues of a-Si:H, low-temperature polycrystalline silicon and amorphous oxide semiconductors have partly replaced a-Si:H channel materials. Basic material properties and device structures of TFTs in commercial displays are explored, and then the potential of atomically thin layered transition metal dichalcogenides as next-generation channel materials is discussed.

Flexible Piezoelectric Acoustic Sensors and Machine Learning for Speech Processing.

Flexible piezoelectric acoustic sensors have been developed to generate multiple sound signals with high sensitivity, shifting the paradigm of future voice technologies. Speech recognition based on advanced acoustic sensors and optimized machine learning software will play an innovative interface for artificial intelligence (AI) services. Collaboration and novel approaches between both smart sensors and speech algorithms should be attempted to realize a hyperconnected society, which can offer personalized services such as biometric authentication, AI secretaries, and home appliances. Here, representative developments in speech recognition are reviewed in terms of flexible piezoelectric materials, self-powered sensors, machine learning algorithms, and speaker recognition.

Modulation of surface physics and chemistry in triboelectric energy harvesting technologies.

Mechanical energy harvesting technology converting mechanical energy wasted in our surroundings to electrical energy has been regarded as one of the critical technologies for self-powered sensor network and Internet of Things (IoT). Although triboelectric energy harvesters based on contact electrification have attracted considerable attention due to their various advantages compared to other technologies, a further improvement of the output performance is still required for practical applications in next-generation IoT devices. In recent years, numerous studies have been carried out to enhance the output power of triboelectric energy harvesters. The previous research approaches for enhancing the triboelectric charges can be classified into three categories: i) materials type, ii) device structure, and iii) surface modification. In this review article, we focus on various mechanisms and methods through the surface modification beyond the limitations of structural parameters and materials, such as surficial texturing/patterning, functionalization, dielectric engineering, surface charge doping and 2D material processing. This perspective study is a cornerstone for establishing next-generation energy applications consisting of triboelectric energy harvesters from portable devices to power industries.

Janus Graphene Liquid Crystalline Fiber with Tunable Properties Enabled by Ultrafast Flash Reduction.

Flash photothermal treatment via Xenon lamp with a broad wavelength spectrum can effectively remove oxygen functionalities and restore sp2 domains at graphitic carbon materials. The chemical composition and relevant structure formation of flash reduced graphene oxide liquid crystal (GOLC) fibers are investigated in accordance with flash irradiation conditions. Owing to the spatial controllability of reduction level via anisotropic flash irradiation, the mechanical properties and electrical conductivity of graphene fibers can be delicately counterbalanced to attain desired properties. High sensitivity humidity sensors can be fabricated from the flash reduced fibers demonstrating notably higher sensitivity over the thermally reduced counterparts. This ultrafast flash reduction holds great promise for multidimensional macroscopic GO based structures, enabling a wide range of potential applications, including textile electronics and wearable sensors.

Flash-Induced Stretchable Cu Conductor via Multiscale-Interfacial Couplings.

Herein, a novel stretchable Cu conductor with excellent conductivity and stretchability is reported via the flash-induced multiscale tuning of Cu and an elastomer interface. Microscale randomly wrinkled Cu (amplitude of ≈5 µm and wavelength of ≈45 µm) is formed on a polymer substrate through a single pulse of a millisecond flash light, enabling the elongation of Cu to exceed 20% regardless of the stretching direction. The nanoscale interlocked interface between the Cu nanoparticles (NPs) and the elastomer increases the adhesion force of Cu, which contributes to a significant improvement of the Cu stability and stretchability under harsh yielding stress. Simultaneously, the flash-induced photoreduction of CuO NPs and subsequent Cu NP welding lead to outstanding conductivity (≈37 kS cm-1) of the buckled elastic electrode. The 3D structure of randomly wrinkled Cu is modeled by finite element analysis simulations to show that the flash-activated stretchable Cu conductors can endure strain over 20% in all directions. Finally, the wrinkled Cu is utilized for wireless near-field communication on the skin of human wrist.

Trichogenic Photostimulation Using Monolithic Flexible Vertical AlGaInP Light-Emitting Diodes.

Alopecia is considered an aesthetic, psychological, and social issue among modern people. Although laser-induced skin stimulation is utilized for depilation treatment, such treatment has significant drawbacks of high energy consumption, huge equipment size, and limited usage in daily life. Here, we present a wearable photostimulator for hair-growth applications using high-performance flexible red vertical light-emitting diodes (f-VLEDs). Flexible microscale LEDs were effectively fabricated by a simple monolithic fabrication process, resulting in high light output (∼30 mW mm-2), low forward voltage (∼2.8 V), and excellent flexibility for wearable biostimulation. Finally, trichogenic stimulation of a hairless mouse was achieved using high-performance red f-VLEDs with high thermal stability, device uniformity, and mechanical durability.