Solution-Processed Flexible Temperature Sensor Array for Highly Resolved Spatial Temperature and Tactile Mapping Using ESN-Based Data Interpolation.

High-performance flexible temperature sensors are crucial in various technological applications, such as monitoring environmental conditions and human healthcare. The ideal characteristics of these sensors for stable temperature monitoring include scalability, mechanical flexibility, and high sensitivity. Moreover, simplicity and low power consumption will be essential for temperature sensor arrays in future integrated systems. This study introduces a solution-based approach for creating a V2O5 nanowire network temperature sensor on a flexible film. Through optimization of the fabrication conditions, the sensor exhibits remarkable performance, sustaining long-term stability (>110 h) with minimal hysteresis and excellent sensitivity (∼-1.5%/°C). In addition, this study employs machine learning techniques for data interpolation among sensors, thereby enhancing the spatial resolution of temperature measurements and adding tactile mapping without increasing the sensor count. Introducing this methodology results in an improved understanding of temperature variations, advancing the capabilities of flexible-sensor arrays for various applications.

Neuromorphic antennal sensory system.

Insect antennae facilitate the nuanced detection of vibrations and deflections, and the non-contact perception of magnetic or chemical stimuli, capabilities not found in mammalian skin. Here, we report a neuromorphic antennal sensory system that emulates the structural, functional, and neuronal characteristics of ant antennae. Our system comprises electronic antennae sensor with three-dimensional flexible structures that detects tactile and magnetic stimuli. The integration of artificial synaptic devices adsorbed with solution-processable MoS2 nanoflakes enables synaptic processing of sensory information. By emulating the architecture of receptor-neuron pathway, our system realizes hardware-level, spatiotemporal perception of tactile contact, surface pattern, and magnetic field (detection limits: 1.3 mN, 50 µm, 9.4 mT). Vibrotactile-perception tasks involving profile and texture classifications were accomplished with high accuracy (> 90%), surpassing human performance in "blind" tactile explorations. Magneto-perception tasks including magnetic navigation and touchless interaction were successfully completed. Our work represents a milestone for neuromorphic sensory systems and biomimetic perceptual intelligence.

Unusual Selective Monitoring of N,N-Dimethylformamide in a Two-Dimensional Material Field-Effect Transistor.

N,N-Dimethylformamide (DMF) is an essential solvent in industries and pharmaceutics. Its market size range was estimated to be 2 billion U.S. dollars in 2022. Monitoring DMF in solution environments in real time is significant because of its toxicity. However, DMF is not a redox-active molecule; therefore, selective monitoring of DMF in solutions, especially in polar aqueous solutions, in real time is extremely difficult. In this paper, we propose a selective DMF sensor using a molybdenum disulfide (MoS2) field-effect transistor (FET). The sensor responds to DMF molecules but not to similar molecules of formamide, N,N-diethylformamide, and N,N-dimethylacetamide. The plausible atomic mechanism is the oxygen substitution sites on MoS2, on which the DMF molecule shows an exceptional orientation. The thin structure of MoS2-FET can be incorporated into a microfluidic chamber, which leads to DMF monitoring in real time by exchanging solutions subsequently. The designed device shows DMF monitoring in NaCl ionic solutions from 1 to 200 µL/mL. This work proposes the concept of selectively monitoring redox-inactive molecules based on the nonideal atomic affinity site on the surface of two-dimensional semiconductors.

A wearable, flexible sensor for real-time, home monitoring of sleep apnea.

A flexible sensor that can be attached to the body to collect vital data wirelessly enables real-time human healthcare management. One potential application for home-use healthcare devices is monitoring of sleep conditions to diagnose sleep apnea syndrome. Such data are not readily gathered using conventional tools, owing to the bulk and cost of instrumentation. In order to monitor respiration at home, it is necessary to improve sensing performance and long-term stability of the sensors without sacrificing wearability and comfortability. To build a platform for wireless home-use respiration monitoring, this study develops a mask-borne flexible humidity sensor using ZnIn2S4 nanosheets as a humidity-sensitive material with high sensitivity and stability for more than 150 h. As proof-of-concept, long-term wireless respiration monitoring is demonstrated during sleep to identify symptoms of sleep apnea in wearers.

A Multitasking Flexible Sensor via Reservoir Computing.

Natural disasters are reported globally, and one source of severe damage to cities is flooding caused by locally heavy rain. Sharing of local weather information can save lives. However, it is difficult to collect local weather information in real-time because such data collection requires bulky, expensive sensors. For local, real-time monitoring of heavy rain and wind, a sensor system should be simple and low-cost so that it can be attached to a variety of surfaces, including roofs, vehicles, and umbrellas. To develop simple, low-cost multitasking sensors located on nonplanar surfaces, a flexible rain sensor to monitor waterdrop volume and wind velocity is devised. To monitor both simultaneously, a laser-induced graphene-based superhydrophobic conductive film is introduced. Using the superhydrophobic surface, water dynamics are measured when waterdrops collide with the sensor surface, and obtained time-series data are processed using "reservoir computing" to extract the volume and velocity from a single sensor as multitasking electronics. As a proof-of-concept, it is shown that the sensor measures continuous, long-term volume and wind-change dynamics. The results demonstrate feasibility of multitasking electronics with reservoir computing to reduce sensor integration complexity with low power consumption for both sensor and signal processing.

Single-layered assembly of vanadium pentoxide nanowires on graphene for nanowire-based lithography technique.

Graphene nanoribbon (GNR)-based materials are a promising device material because of their potential high carrier mobility and atomically thin structure. Various approaches have been reported for preparing the GNR-based materials, from bottom-up chemical synthetic procedures to top-down fabrication techniques using lithography of graphene. However, it is still difficult to prepare a large-scale GNR-based material. Here, we develop a procedure to prepare a large-scale GNR network using networked single-layer inorganic nanowires. Vanadium pentoxide (V2O5) nanowires were assembled on graphene with an interfacial layer of a cationic polymer via electrostatic interaction. A large-scale nanowire network can be prepared on graphene and is stable enough for applying an oxygen plasma. Using plasma etching, a networked graphene structure can be generated. Removing the nanowires results in a networked flat structure whose both surface morphology and Raman spectrum indicate a GNR networked structure. The field-effect device indicates the semiconducting character of the GNR networked structure. This work would be useful for fabricating a large-scale GNR-based material as a platform for GNR junctions for physics and electronic circuits.

Controlling the thermal conductivity of multilayer graphene by strain.

Straintronics is a new concept to enhance electronic device performances by strain for next-generation information sensors and energy-saving technologies. The lattice deformation in graphene can modulate the thermal conductivity because phonons are the main heat carriers. However, the device fabrication process affects graphene's heat transport properties due to its high stretchability. This study experimentally investigates the change in the thermal conductivity when biaxial tensile strain is applied to graphene. To eliminate non-strain factors, two mechanisms are considered: pressure-induced and electrostatic attraction-induced strain. Raman spectroscopy and atomic force microscopy precisely estimate the strain. The thermal conductivity of graphene decreases by approximately 70% with a strain of only 0.1%. Such thermal conductivity controllability paves the way for applying graphene as high-efficiency thermal switches and diodes in future thermal management devices.

Wireless and Flexible Skin Moisture and Temperature Sensor Sheets toward the Study of Thermoregulator Center.

A disorder in the thermoregulator center in a human body leads to some potential diseases such as fever and hyperthyroidism. To predict these diseases early, monitoring the health condition of the human body due to the influence of thermoregulation disorders is important. Although extensive works are performed on sweat-rate detection by constructing microfluidic channels, skin-moisture evaporation before sweating remains unknown. This work proposes a wireless and flexible sensor sheet to investigate the thermoregulatory responses of different people under cold stimulation and exercise by measuring the temperature and moisture variations on the finger skin. An integrated flexible sensor system consists of a ZnIn2 S4 nanosheet-based humidity sensor and carbon nanotube/SnO2 temperature sensor. The results exhibit distinct thermoregulation abilities of five volunteers. Interestingly, the sudden increase in finger moisture that results from the excitation by the sympathetic nerve is observed during the cold-stimulus test. Although further studies are required to predict the potential diseases resulted from thermoregulation disorders in human body, this study provides a possibility of continuous and real-time monitoring of thermoregulatory activities via skin moisture and temperature detection using a flexible sensor sheet.

Multimodal Wearable Sensor Sheet for Health-Related Chemical and Physical Monitoring.

Continuous multiple data health monitoring has high potential to detect abnormal conditions or early stages of diseases in the future. To monitor a continuous small vital signal, one of the promising architectures is an attachable flexible multimodal sensor system, which can detect multiple health conditions from the skin surface. Recent breakthroughs have realized continuous sweat chemicals or physical conditions using flexible sensors. However, multimodal sensor integration to monitor chemical and physical information simultaneously and precisely is still a challenge. In this study, we present a multimodal wearable sensor sheet, which allows us to monitor sweat glucose, electrocardiograms, and skin temperature. Furthermore, to prevent the accumulation of glucose on the sensor surface for precise monitoring, a fluidic channel is also integrated to refresh the sweat from the sensor surface, resulting in the precise measurement of chemical substances in real time. This multimodal and flexible sensor platform takes a significant step toward realizing wearable healthcare applications to diagnose the early stages of diseases in advance.

A Wearable Body Condition Sensor System with Wireless Feedback Alarm Functions.

Emerging feedback systems based on tracking body conditions can save human lives. In particular, vulnerable populations such as disabled people, elderly, and infants often require special care. For example, the high global mortality of infants primarily owing to sudden infant death syndrome while sleeping makes request for extraordinary attentions in neonatal intensive care units or daily lives. Here, a versatile laser-induced graphene (LIG)-based integrated flexible sensor system, which can wirelessly monitor the sleeping postures, respiration rate, and diaper moisture with feedback alarm notifications, is reported. A tilt sensor based on confining a liquid metal droplet inside a cavity can track at least 18 slanting orientations. A rapid and scalable laser direct writing method realizes LIG patterning in both the in-plane and out-of-plane configurations as well as the formation of nonstick conductive structures to the liquid metal. By rationally merging the LIG-based tilt, strain, and humidity sensors on a thin flexible film, the multimodal sensor device is applied to a diaper as a real-time feedback tracking system of the sleeping posture, respiration, and wetness toward secure and comfortable lives. User-friendly interfaces, which incorporate alarming functions, provide timely feedback for caregivers tending to vulnerable populations with limited self-care capabilities.

Highly stable Pd/HNb3O8-based flexible humidity sensor for perdurable wireless wearable applications.

Real-time, daily health monitoring can provide large amounts of patient data, which may greatly improve the likelihood of diagnosing health conditions at an early stage. One potential sensor is a flexible humidity sensor to monitor moisture and humidity information such as dehydration. However, achieving a durable functional nanomaterial-based flexible humidity sensor remains a challenge due to partial desorption of water molecules during the recovery process, especially at high humidities. In this work, we demonstrate a highly stable resistive-type Pd/HNb3O8 humidity sensor, which exhibits a perdurable performance for over 100 h of cycle tests under a 90% relative humidity (RH) without significant performance degradation. One notable advantage of the Pd/HNb3O8 humidity sensor is its ability to regulate hydroniums due to the strong reducibility of H atoms dissociated on the Pd surface. This feature realizes a high stability even at a high humidity (99.9% RH). Using this superior performance, the Pd/HNb3O8 humidity sensor realizes wireless monitoring of the changes in the fingertip humidity of an adult under different physiological states, demonstrating a facile and reliable path for dehydration diagnosis.

Multimodal Plant Healthcare Flexible Sensor System.

The rising global human population and increased environmental stresses require a higher plant productivity while balancing the ecosystem using advanced nanoelectronic technologies. Although multifunctional wearable devices have played distinct roles in human healthcare monitoring and disease diagnosis, probing potential physiological health issues in plants poses a formidable challenge due to their biological complexity. Herein an integrated multimodal flexible sensor system is proposed for plant growth management using stacked ZnIn2S4(ZIS) nanosheets as the kernel sensing media. The proposed ZIS-based flexible sensor can not only perceive light illumination at a fast response (∼4 ms) but also monitor the humidity with a perdurable steady performance that has yet to be reported elsewhere. First-principles calculations reveal that the tunneling effect dominates the current model associated with humidity response. This finding guides the investigation on the plant stomatal functions by measuring plant transpiration. Significantly, dehydration conditions are visually recorded during a monitoring period (>15 days). This work may contribute to plant-machine biointerfaces to precisely manage plant health status and judiciously utilize limited resources.

Very Thin, Macroscale, Flexible, Tactile Pressure Sensor Sheet.

In artificial intelligence and deep learning applications, data collection from a variety of objects is of great interest. One way to support such data collection is to use very thin, mechanically flexible sensor sheets, which can cover an object without altering the original shape. This study proposes a thin, macroscale, flexible, tactile pressure sensor array fabricated by a simple process for economical device applications. Using laser-induced graphene, a transfer process, and a printing method, a relatively stable, reliable, macroscale, thin (∼300 µm), flexible, tactile pressure sensor is realized. The detectable pressure range is about tens to hundreds of kPa. Then, as a proof-of-concept, the uniformity, sensitivity, repeatability, object mapping, finger pressure distribution, and pressure mapping are demonstrated under bending conditions. Although many flexible, tactile pressure sensors have been reported, the proposed structure has the potential for macroscale, thin, flexible, tactile pressure sensor sheets because of the simple and easy fabrication process.

Photoactivation of Strong Photoluminescence in Superacid-Treated Monolayer Molybdenum Disulfide.

To advance the development of atomically thin optoelectronics using two-dimensional (2D) materials, engineering strong luminescence with a physicochemical basis is crucial. Semiconducting monolayer transition-metal dichalcogenides (TMDCs) are candidates for this, but their quantum yield (QY) is known to be poor. Recently, a molecular superacid treatment of bis(trifluoromethane)sulfonimide (TFSI) generated unambiguously bright monolayer TMDCs and a high QY. However, this method is highly dependent on the processing conditions and therefore has not been generalized. Here, we shed light on environmental factors to activate the photoluminescence (PL) intensity of the TFSI-treated monolayer MoS2, with a factor of more than 2 orders of magnitude greater than the original by photoactivation. The method is useful for both mechanically exfoliated and chemically deposited samples. The existence of photoirradiation larger than the band gap demonstrates enhancement of the PL of MoS2; on the other hand, activation by thermal annealing, as demonstrated in the previous report, is less effective for enhancing the PL intensity. The photoactivated monolayer MoS2 shows a long lifetime of ∼1.35 ns, more than a 30-fold improvement over the original as exfoliated. The consistent realization of the bright monolayer MoS2 reveals that air exposure is an essential factor in the process. TFSI treatment in a N2 environment was not effective for achieving a strong PL, even after the photoactivation. These findings can serve as a basis for engineering the bright atomically thin materials for 2D optoelectronics.

Wrist flexible heart pulse sensor integrated with a soft pump and a pneumatic balloon membrane.

To monitor health and diagnose disease in the early stage, future healthcare standards will likely include the continuous monitoring of various vital data. One approach to collect such information is a wearable and flexible device, which detects information from the skin surface. An important dataset is heart pulse information. Herein a method to monitor the detailed pulse signal from a wrist stably and reliably is proposed. Specifically, a soft pneumatic balloon operated by a soft pump applies the appropriate pressure over a tactile sensor onto the radial artery of the wrist to detect detailed heart pulse waves. The soft pump, pneumatic balloon, and flexible tactile pressure sensor are characterized as a fundamental study. Additionally, a proof-of-concept of this integrated device platform is demonstrated by monitoring the heart pulse from a wrist with and without the soft pump functions.

Highly Precise Multifunctional Thermal Management-Based Flexible Sensing Sheets.

Elaborate manipulation of heat transfer renders proper operation of diverse thermal-related technologies. However, accurate implementation of thermal-based or transduction sensing on a thin flexible film over unusual surfaces remains challenging. Herein, efficient thermal management realizes highly accurate flexible multifunctional sensor sheets using a low thermal conductive medium as a thermal barrier. An approximately 50-fold enhancement in the thermal sensing accuracy, which is nearly independent of the changes in the external surroundings, is achieved. Such rational control of heat convection and conduction allows to not only dynamically monitor air flow, but also sight the large-scale air flow distribution on curved surfaces using a flexible thermal flow sensor array. Additionally, accurate wearable skin temperature monitoring independent of the sudden surrounding variations is achieved. This work addresses the formidable challenge of untethered heat transfer induced imprecise thermal related sensing, which universally exists in skin-inspired Internet of Things (IoT) applications.

Toward Flexible Surface-Enhanced Raman Scattering (SERS) Sensors for Point-of-Care Diagnostics.

Surface-enhanced Raman scattering (SERS) spectroscopy provides a noninvasive and highly sensitive route for fingerprint and label-free detection of a wide range of molecules. Recently, flexible SERS has attracted increasingly tremendous research interest due to its unique advantages compared to rigid substrate-based SERS. Here, the latest advances in flexible substrate-based SERS diagnostic devices are investigated in-depth. First, the intriguing prospect of point-of-care diagnostics is briefly described, followed by an introduction to the cutting-edge SERS technique. Then, the focus is moved from conventional rigid substrate-based SERS to the emerging flexible SERS technique. The main part of this report highlights the recent three categories of flexible SERS substrates, including actively tunable SERS, swab-sampling strategy, and the in situ SERS detection approach. Furthermore, other promising means of flexible SERS are also introduced. The flexible SERS substrates with low-cost, batch-fabrication, and easy-to-operate characteristics can be integrated into portable Raman spectroscopes for point-of-care diagnostics, which are conceivable to penetrate global markets and households as next-generation wearable sensors in the near future.

Graphene and Carbon Nanotube Heterojunction Transistors with Individual Gate Control.

Heterogeneously integrated nanomaterial devices show interesting characteristics for transistors and sensors due to their band diagram or steep material junctions. If these junctions and band alignments can be tuned by an electrical input bias, the device platform not only could be expanded but also could be used to explore fundamental characteristics. However, most reports on hetero-nanomaterial junctions use a global back-gate voltage, which makes it difficult to control band alignment at an interface. To explore device junctions, this study reports a laterally integrated heterojunction of graphene and a carbon nanotube (CNT) network film with individual gate electrodes to tune the band alignment corresponding to the Fermi level shift of graphene in contact with the semiconducting CNT network film. By developing the fabrication process, multiple gate structures are designed to apply a gate bias to CNTs and graphene separately. The threshold voltage shift of the CNT transistor depends on the gate voltage of graphene. Based on the thermionic emission theory, the barrier height between graphene and CNTs for both the conduction and valence band sides varies from 70 to 85 meV, with a linear change as a function of the applied gate voltage to graphene. Although the current Fermi level shift is small, this device platform may realize the exploration of fundamental properties and device concepts.

Flexible Electronics toward Wearable Sensing.

Wearable sensors play a crucial role in realizing personalized medicine, as they can continuously collect data from the human body to capture meaningful health status changes in time for preventive intervention. However, motion artifacts and mechanical mismatches between conventional rigid electronic materials and soft skin often lead to substantial sensor errors during epidermal measurement. Because of its unique properties such as high flexibility and conformability, flexible electronics enables a natural interaction between electronics and the human body. In this Account, we summarize our recent studies on the design of flexible electronic devices and systems for physical and chemical monitoring. Material innovation, sensor design, device fabrication, system integration, and human studies employed toward continuous and noninvasive wearable sensing are discussed. A flexible electronic device typically contains several key components, including the substrate, the active layer, and the interface layer. The inorganic-nanomaterials-based active layer (prepared by a physical transfer or solution process) is shown to have good physicochemical properties, electron/hole mobility, and mechanical strength. Flexible electronics based on the printed and transferred active materials has shown great promise for physical sensing. For example, integrating a nanowire transistor array for the active matrix and a conductive pressure-sensitive rubber enables tactile pressure mapping; tactile-pressure-sensitive e-skin and organic light-emitting diodes can be integrated for instantaneous pressure visualization. Such printed sensors have been applied as wearable patches to monitor skin temperature, electrocardiograms, and human activities. In addition, liquid metals could serve as an attractive candidate for flexible electronics because of their excellent conductivity, flexibility, and stretchability. Liquid-metal-enabled electronics (based on liquid-liquid heterojunctions and embedded microchannels) have been utilized to monitor a wide range of physiological parameters (e.g., pulse and temperature). Despite the rapid growth in wearable sensing technologies, there is an urgent need for the development of flexible devices that can capture molecular data from the human body to retrieve more insightful health information. We have developed a wearable and flexible sweat-sensing platform toward real-time multiplexed perspiration analysis. An integrated iontophoresis module on a wearable sweat sensor could enable autonomous and programmed sweat extraction. A microfluidics-based sensing system was demonstrated for sweat sampling, sensing, and sweat rate analysis. Roll-to-roll gravure printing allows for mass production of high-performance flexible chemical sensors at low cost. These wearable and flexible sweat sensors have shown great promise in dehydration monitoring, cystic fibrosis diagnosis, drug monitoring, and noninvasive glucose monitoring. Future work in this field should focus on designing robust wearable sensing systems to accurately collect data from the human body and on large-scale human studies to determine how the measured physical and chemical information relates to the individual's specific health conditions. Further research in these directions, along with the large sets of data collected via these wearable and flexible sensing technologies, will have a significant impact on future personalized healthcare.

All-Solution-Based Heterogeneous Material Formation for p-n Junction Diodes.

All-solution-based devices have potential as the next class of macroscale and multifunctional electronics on versatile amorphous substrates. Different methods and materials have been studied to control the formation of p-type and n-type semiconducting materials because forming active materials for transistors and sensors remains a challenge. This study proposes an approach for solution-based devices in which a p-n junction diode is fabricated using a solution-based InZnO thin film for the n-type semiconductor and a carbon nanotube network film for the p-type semiconductor. Additionally, the barrier height (∼160 meV) is extracted and a p-n junction diode on a plastic film is demonstrated. Although the performance requires further improvements by modifying the interfaces, this printing method may be an interesting approach for all-printed electronics, which can replace conventional Si electronics.