Machine Learning-Powered Ultrahigh Controllable and Wearable Magnetoelectric Piezotronic Touching Device.

Recent advancements in nanomaterials have enabled the application of nanotechnology to the development of cutting-edge sensing and actuating devices. For instance, nanostructures' collective and predictable responses to various stimuli can be monitored to determine the physical environment of the nanomaterial, such as temperature or applied pressure. To achieve optimal sensing and actuation capabilities, the nanostructures should be controllable. However, current applications are limited by inherent challenges in controlling nanostructures that counteract many sensing mechanisms that are reliant on their area or spacing. This work presents a technique utilizing the piezo-magnetoelectric properties of nanoparticles to enable strain sensing and actuation in a flexible and wearable patch. The alignment of nanoparticles has been achieved using demagnetization fields with computational simulations confirming device characteristics under various types of deformation followed by experimental demonstrations. The device exhibits favorable piezoelectric performance, hydrophobicity, and body motion-sensing capabilities, as well as machine learning-powered touch-sensing/actuating features.

Monomer-mixed hole transport layers for improving hole injection of quantum dot light-emitting diodes.

Quantum-dot light-emitting diodes (QLEDs) are promising components for next-generation displays and related applications. However, their performance is critically limited by inherent hole-injection barrier caused by deep highest-occupied molecular orbital levels of quantum dots. Herein, we present an effective method for enhancing the performance of QLEDs by incorporating a monomer (TCTA or mCP) into hole-transport layers (HTL). The impact of different monomer concentrations on the characteristics of QLEDs were investigated. The results indicate that sufficient monomer concentrations improve the current efficiency and power efficiency. The increased hole current using monomer-mixed HTL suggests that our method holds considerable potential for high-performance QLEDs.

Photoresponse Analysis of All-Inkjet-Printed Single-Walled Carbon Nanotube Thin-Film Transistors for Flexible Light-Insensitive Transparent Circuit Applications.

We report daylight-stable, transparent, and flexible single-walled carbon nanotube thin-film transistors (SWCNT TFTs) using an all-inkjet printing process. Although most of the previous reports classified SWCNT TFTs as photodetectors, we demonstrated that SWCNT films actually show two different types of photoresponses depending on the power levels of light sources. The electrical characteristics of SWCNT TFTs show no significant change under daily illumination conditions such as halogen lamps and sunlight, while under high-power laser illumination, they change as reported in the previous results. In addition, the low-temperature solution process of the SWCNT with its one-dimensional nature allows us to realize highly transparent and flexible TFTs and logic circuits on plastic substrates. Our result will provide new insights into utilizing SWCNT TFTs for light-insensitive transparent and flexible electronic applications.

Crack-inducing Strain Sensor Array using Inkjet-Printed Silver Thin Film for Underplate and Off-centered Force Sensing Applications.

The change in resistance upon bending in metal films as thick as 1 mm used for underpanel force touch applications is limited by the low sensitivity, thus requiring high-performance readout circuitry. In this paper, we report inkjet-printed silver thin films having crack-inducing underlayers, which further increases the sensitivity of their resistance changes under deformation. This allows for detecting weak vertical forces even through the plates (force-receiving layer), such as 0.4 or 1.2 mm thick polyethylene terephthalate or 0.4 mm thick glass. The underplate sensors will detect a force level as low as 10 gf, which corresponds to the amount of force required for fingerprint recognition. Furthermore, such highly sensitive strain sensors can potentially solve the inaccuracy issue of wearable devices, which can occur when misplaced sensors detect relatively weak biosignals, such as heart rate and blood pressure. The sensor detects the accurate pulse patterns of the wrist artery even though it is off-centered from the artery by 6 mm or larger. The crack-based strain sensor and its usage as a hidden underplate force sensing device will create various wearable and user-machine interface applications.

Risk factors based vessel-specific prediction for stages of coronary artery disease using Bayesian quantile regression machine learning method: Results from the PARADIGM registry.

BACKGROUND AND HYPOTHESIS: The recently introduced Bayesian quantile regression (BQR) machine-learning method enables comprehensive analyzing the relationship among complex clinical variables. We analyzed the relationship between multiple cardiovascular (CV) risk factors and different stages of coronary artery disease (CAD) using the BQR model in a vessel-specific manner. METHODS: From the data of 1,463 patients obtained from the PARADIGM (NCT02803411) registry, we analyzed the lumen diameter stenosis (DS) of the three vessels: left anterior descending (LAD), left circumflex (LCx), and right coronary artery (RCA). Two models for predicting DS and DS changes were developed. Baseline CV risk factors, symptoms, and laboratory test results were used as the inputs. The conditional 10%, 25%, 50%, 75%, and 90% quantile functions of the maximum DS and DS change of the three vessels were estimated using the BQR model. RESULTS: The 90th percentiles of the DS of the three vessels and their maximum DS change were 41%-50% and 5.6%-7.3%, respectively. Typical anginal symptoms were associated with the highest quantile (90%) of DS in the LAD; diabetes with higher quantiles (75% and 90%) of DS in the LCx; dyslipidemia with the highest quantile (90%) of DS in the RCA; and shortness of breath showed some association with the LCx and RCA. Interestingly, High-density lipoprotein cholesterol showed a dynamic association along DS change in the per-patient analysis. CONCLUSIONS: This study demonstrates the clinical utility of the BQR model for evaluating the comprehensive relationship between risk factors and baseline-grade CAD and its progression.

Effect of Back-Channel Surface on Reliability of Solution-Processed In0.51Ga0.15Zn0.34O Thin-Film Transistors with Thin Active Layer.

We have investigated the degradation mechanism of solution-processed indium-gallium-zinc-oxide (IGZO) thin-film transistors. The threshold voltage shift (ΔVth) followed a linear function under negative gate bias stress (NBS), while it showed a stretched-exponential behavior under positive gate bias stress. The slope of ΔVth for stress time was rarely changed with variations below 0.3 mV/s. The thickness of the fabricated IGZO layer (In0.51Ga0.15Zn0.34O) was approximately 10 nm. The Debye length (LD) was larger than IGZO thickness (tIGZO) due to the fully depleted active layer under NBS. Therefore, the degradation phenomenon under NBS was related to the adsorption at back-channel surface. The back-channel surface could be affected by the gate bias under NBS, and the molecules adsorbed at the IGZO layer were positively charged and induced extra electrons by NBS. We verified that the number of positively charged adsorbates had a proportional relationship with the ΔVth based on the two-dimensional technology computer-aided design (TCAD) simulation. Furthermore, we investigated the degradation phenomenon with the ΔVth equation regarding the adsorbates, and the result confirmed that the adsorption process could cause the linear ΔVth. We experimentally confirmed the effect of back-channel surface by comparing the ΔVth between different atmospheric conditions and LD. Consequently, the reaction at the back-channel surface should be considered to develop the metal-oxide semiconductor devices.

Nonpatterned Soft Piezoresistive Films with Filamentous Conduction Paths for Mimicking Multiple-Resolution Receptors of Human Skin.

Soft pressure sensors play key roles as input devices of electronic skin (E-skin) to imitate real human skin. For efficient data acquisition according to stimulus types such as detailed pressure images or macroscopic strength of stimuli, soft pressure sensors can have variable spatial resolution, just like the uneven spatial distribution of pressure-sensing receptors on the human body. However, previous methods on soft pressure sensors cannot achieve such tunability of spatial resolution because their sensor materials and read-out electrodes need to be elaborately patterned for a specific sensor density. Here, we report a universal soft pressure-sensitive platform based on anisotropically self-assembled ferromagnetic particles embedded in elastomer matrices whose spatial resolution can be facilely tuned. Various spatial densities of pressure-sensing receptors of human body parts can be implemented by simply sandwiching the film between soft electrodes with different pitches. Since the anisotropically aligned nickel particles form independent filamentous conductive paths, the pressure sensors show spatial sensing ability without crosstalk, whose spatial resolution up to 100 dpi can be achieved from a single platform. The sensor array shows a wide dynamic range capable of detecting various pressure levels, such as liquid drops (∼30 Pa) and plantar (∼300 kPa) pressures. Our universal soft pressure-sensing platform would be a key enabling technology for actually imitating the receptor systems of human skin in robot and biomedical applications.

Silent Speech Recognition with Strain Sensors and Deep Learning Analysis of Directional Facial Muscle Movement.

Silent communication based on biosignals from facial muscle requires accurate detection of its directional movement and thus optimally positioning minimum numbers of sensors for higher accuracy of speech recognition with a minimal person-to-person variation. So far, previous approaches based on electromyogram or pressure sensors are ineffective in detecting the directional movement of facial muscles. Therefore, in this study, high-performance strain sensors are used for separately detecting x- and y-axis strain. Directional strain distribution data of facial muscle is obtained by applying three-dimensional digital image correlation. Deep learning analysis is utilized for identifying optimal positions of directional strain sensors. The recognition system with four directional strain sensors conformably attached to the face shows silent vowel recognition with 85.24% accuracy and even 76.95% for completely nonobserved subjects. These results show that detection of the directional strain distribution at the optimal facial points will be the key enabling technology for highly accurate silent speech recognition.

Recent progress in strain-engineered elastic platforms for stretchable thin-film devices.

Strain-engineered elastic platforms that can efficiently distribute mechanical stress under deformation offer adjustable mechanical compliance for stretchable electronic systems. By fully exploiting strain-free regions that are favourable for fabricating thin-film devices and interconnecting with reliably stretchable conductors, various electronic systems can be integrated onto stretchable platforms with the assistance of strain engineering strategies. Over the last decade, applications of multifunctional stretchable thin-film devices simultaneously exhibiting superior electrical and mechanical performance have been demonstrated, shedding light on the realization of further reliable human-machine interfaces. This review highlights recent developments in enabling technologies for strain-engineered elastic platforms. In particular, representative approaches to realize strain-engineered substrates and stretchable interconnects in island-bridge configurations are introduced from the perspective of the material homogeneity and structural design of the substrate. State-of-the-art achievements in sophisticated stretchable electronic devices on strain-engineered elastic platforms are also presented, such as stretchable sensors, energy devices, thin-film transistors, and displays, and then, the challenges and outlook are discussed.

Stretchable PPG sensor with light polarization for physical activity-permissible monitoring.

Skin-attachable sensors, which represent the ultimate form of wearable electronic devices that ensure conformal contact with skin, suffer from motion artifact limitations owing to relative changes in position between the sensor and skin during physical activities. In this study, a polarization-selective structure of a skin-conformable photoplethysmographic (PPG) sensor was developed to decrease the amount of scattered light from the epidermis, which is the main cause of motion artifacts. The motion artifacts were suppressed more than 10-fold in comparison with those of rigid sensors. The developed sensor-with two orthogonal polarizers-facilitated successful PPG signal monitoring during wrist angle movements corresponding to high levels of physical activity, enabling continuous monitoring of daily activities, even while exercising for personal health care.

Electronic Skin Based on a Cellulose/Carbon Nanotube Fiber Network for Large-Area 3D Touch and Real-Time 3D Surface Scanning.

Electronic skin (E-skin) based on tactile sensors has great significance in next-generation electronics such as biomedical application and artificial intelligence that requires interaction with humans. To mimic the properties of human skin, high flexibility, excellent sensing capability, and sufficient spatial resolution through high-level sensor integration are required. Here, we report a highly sensitive pressure sensor matrix based on a piezoresistive cellulose/single-walled carbon nanotube-entangled fiber network, which forms its own porous structure enabling a superior pressure sensor with a high sensitivity (9.097 kPa-1), a fast response speed (<2 ms), and orders of magnitude detection range with a detection limit of 1 Pa. Furthermore, the remarkable device expandability based on the ease of patterning and scalability allows easy implementation of a large-area pressure sensor matrix which has 2304 (48 × 48) pixels. Combined with a real-time pressure distribution monitoring system, a flexible 3D touch sensor that simultaneously displays plane coordinates and pressure information and a scanning device that detects the morphology of the soft body 3D surface are successfully demonstrated.

Inkjet-Printing-Based Density Profile Engineering of Single-Walled Carbon Nanotube Networks for Conformable High-On/Off-Performance Thin-Film Transistors.

Random networks of single-walled carbon nanotubes (SWCNTs) offer new-form-factor electronics such as transparent, flexible, and intrinsically stretchable devices. However, the long-standing trade-off between carrier mobility and on/off ratio due to the coexistence of metallic and semiconducting nanotubes has limited the performance of SWCNT-random-network-based thin-film transistors (SWCNT TFTs), hindering their practical circuit-level applications. Methods for high-purity separation between metallic and semiconducting nanotubes have been proposed, but they require high cost and energy and are vulnerable to contamination and nanotube shortening, leading to performance degradation. Alternatively, additional structures have been proposed to reduce the off-state current, but they still compromise carrier mobility and suffer from inevitable expansion in device dimensions. Here, we propose a density-modulated SWCNT network using an inkjet-printing method as a facile approach that can achieve superior carrier mobility and a high on/off ratio simultaneously. By exploiting picoliter-scale drops on demand, we form a low-density channel network near the source and drain junctions and a high-density network at the middle of the channel. The modulated density profile forms a large band gap near the source and drain junctions that efficiently blocks electron injection under the reverse bias and a narrow band gap at the high-density area that facilitates the hole transport under the on-state bias. As a result, the density-modulated SWCNT TFTs show both high carrier mobility (27.02 cm2 V-1 s-1) and a high on/off ratio (>106). We also demonstrate all-inkjet-printed flexible inverter circuits whose gain is doubled by the density-modulated SWCNT TFTs, highlighting the feasibility of our approach for realizing high-performance flexible and conformable electronics.

Stamp-Perforation-Inspired Micronotch for Selectively Tearing Fiber-Bridged Carbon Nanotube Thin Films and Its Applications for Strain Classification.

Cracks typically deteriorate the structural and electrical properties of materials when not properly controlled. A few papers recently reported the controlling methods of crack formation in the brittle materials utilizing the lateral V-notch structure. For ductile materials, however, there have been few papers reporting cracking phenomenon, but full cracking control including predesigned initiation, propagation, and termination has not been reported yet. Therefore, we report a predesigned full cracking control in ductile conductive carbon nanotube (CNT) films by introducing inkjet-printed L-shape micronotch (LMN) structures inspired by directional stamp perforation marks. In spite of the high fracture toughness of CNT films, the LMNs determine locations of initial crack formation and guide crack propagation in a predesigned way. Selective connection of isolated cracks in the CNT film increases its resistance monotonically under tensile strain and thus tremendously well maintains high linearity (adj. R2 value > 0.99) in resistance change over record large strain ranges of 0.01-100%, which enables us to quantitatively classify strain values accurately for previously reported practical body signals for the first time. We believe that our facile printing-based crack control strategy not only provides a comprehensive solution to various stretchable sensor applications but also builds a new milestone for cracking mechanism studies in fracture mechanics.

Standalone real-time health monitoring patch based on a stretchable organic optoelectronic system.

Skin-like health care patches (SHPs) are next-generation health care gadgets that will enable seamless monitoring of biological signals in daily life. Skin-conformable sensors and a stretchable display are critical for the development of standalone SHPs that provide real-time information while alleviating privacy concerns related to wireless data transmission. However, the production of stretchable wearable displays with sufficient pixels to display this information remains challenging. Here, we report a standalone organic SHP that provides real-time heart rate information. The 15-µm-thick SHP comprises a stretchable organic light-emitting diode display and stretchable organic photoplethysmography (PPG) heart rate sensor on all-elastomer substrate and operates stably under 30% strain using a combination of stress relief layers and deformable micro-cracked interconnects that reduce the mechanical stress on the active optoelectronic components. This approach provides a rational strategy for high-resolution stretchable displays, enabling the production of ideal platforms for next-generation wearable health care electronics.

Underwater maneuvering of robotic sheets through buoyancy-mediated active flutter.

Falling leaves flutter from side to side due to passive and intrinsic fluid-body coupling. Exploiting the dynamics of passive fluttering could lead to fresh perspectives for the locomotion and manipulation of thin, planar objects in fluid environments. Here, we show that the time-varying density distribution within a thin, planar body effectively elicits minimal momentum control to reorient the principal flutter axis and propel itself via directional fluttery motions. We validated the principle by developing a swimming leaf with a soft skin that can modulate local buoyancy distributions for active flutter dynamics. To show generality and field applicability, we demonstrated underwater maneuvering and manipulation of adhesive and oil-skimming sheets for environmental remediation. These findings could inspire future intelligent underwater robots and manipulation schemes.

3D printing-based mirrored image component for seamless modular curved-edge displays.

A facile method for designing and fabricating a concave mirror from a 3D printed mold is proposed for a seamless modular curved-edge display. The concave mirror is placed on the seam of modular curved-edge display, thereby reflecting images at the curved-edge area toward the observer direction. By investigating the concave mirror structures based on parametric modeling, we obtain a continuous image in a modular curved-edge display by optically concealing the seam. We also analyze the luminance distribution and the viewing angle of the seamless modular curved-edge display to show the capability of concealing the seam by the concave mirror.

High-performance compliant thermoelectric generators with magnetically self-assembled soft heat conductors for self-powered wearable electronics.

Softening of thermoelectric generators facilitates conformal contact with arbitrary-shaped heat sources, which offers an opportunity to realize self-powered wearable applications. However, existing wearable thermoelectric devices inevitably exhibit reduced thermoelectric conversion efficiency due to the parasitic heat loss in high-thermal-impedance polymer substrates and poor thermal contact arising from rigid interconnects. Here, we propose compliant thermoelectric generators with intrinsically stretchable interconnects and soft heat conductors that achieve high thermoelectric performance and unprecedented conformability simultaneously. The silver-nanowire-based soft electrodes interconnect bismuth-telluride-based thermoelectric legs, effectively absorbing strain energy, which allows our thermoelectric generators to conform perfectly to curved surfaces. Metal particles magnetically self-assembled in elastomeric substrates form soft heat conductors that significantly enhance the heat transfer to the thermoelectric legs, thereby maximizing energy conversion efficiency on three-dimensional heat sources. Moreover, automated additive manufacturing paves the way for realizing self-powered wearable applications comprising hundreds of thermoelectric legs with high customizability under ambient conditions.

Dense Assembly of Finely Patterned Semiconducting Single-Walled Carbon Nanotubes via a Selective Transfer Method of Nanotube-Attracting Layers.

Development of technology for assembled single-walled carbon nanotube (SWCNT) film with the fine resolution is an essential technique for penetrating practical electronic applications. A promising approach is the assembly method by adding a chemical-functionalizing substrate to enhance affinity between the SWCNTs and the substrate. However, the various introduced approaches for solution-based assembly have suffered from low SWCNT deposition selectivity or low SWCNT deposition density. Herein, we demonstrated a facile method for selectively assembling semiconducting SWCNT network on the substrate. The substrate was prepared via a transfer printing of a poly-l-lysine (PLL)-coated poly(dimethylsiloxane) (PDMS) stamp. The thermal-assisted transfer method enabled an ultrafine PLL pattern (≤4 µm) and a high transfer yield (96.5%) by only one-time stamping without a change of the SWCNT-attracting nature. So, semiconducting SWCNTs were deposited on the patterned regions selectively and precisely. The benefit of the patterned semiconducting SWCNTs was lowering leakage current and turn-on voltage in the transfer characteristics by suppressing attachment of unnecessary SWCNT network. They showed excellent electrical performance, a log10(Ion/Ioff) ratio of 4.76, and an average value of linear field-effect mobility of 7.56 cm2/(V s). This research provides a simple but high-quality assembling technique of semiconducting SWCNTs, thereby improving the feasibility of solution-processed SWCNT-TFTs.

Ultraflexible and transparent electroluminescent skin for real-time and super-resolution imaging of pressure distribution.

The ability to image pressure distribution over complex three-dimensional surfaces would significantly augment the potential applications of electronic skin. However, existing methods show poor spatial and temporal fidelity due to their limited pixel density, low sensitivity, or low conformability. Here, we report an ultraflexible and transparent electroluminescent skin that autonomously displays super-resolution images of pressure distribution in real time. The device comprises a transparent pressure-sensing film with a solution-processable cellulose/nanowire nanohybrid network featuring ultrahigh sensor sensitivity (>5000 kPa-1) and a fast response time (<1 ms), and a quantum dot-based electroluminescent film. The two ultrathin films conform to each contact object and transduce spatial pressure into conductivity distribution in a continuous domain, resulting in super-resolution (>1000 dpi) pressure imaging without the need for pixel structures. Our approach provides a new framework for visualizing accurate stimulus distribution with potential applications in skin prosthesis, robotics, and advanced human-machine interfaces.