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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.

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.

We fabricate the graphene/h-BN stacked mechanical resonator and investigate the gate voltage dependence of the resonance property of the graphene/h-BN mechanical resonator. The mode splitting on the resonance of the stacked resonator is found at a specific gate voltage region, where the frequency difference of two peaks corresponding to two modes showed the square of gate voltage dependence. This implies that the mode splitting observed is that the electrostatic attraction on the graphene layer influences the interlayer interaction of graphene and h-BN. The energy transfer between the two resonance states was also observed.

In wearable electronics, to acquire stability and simultaneously preserve stretchability, sensitivity, and scalability is of high significance yet challenging for practical device applications. In this work, a kirigami-structured graphene–polymer hybrid nanocomposite is proposed for strain sensors by a laser direct writing technique on a polyimide sheet. Using such kirigami structures, the strain in the sensor material can be drastically reduced in the circumstances of high stretching conditions. To protect the device, ecoflex polymer is applied as the passivation layer. Depending on the applications, ecoflex grid-wrapped and film-encapsulated devices are realized with high stretchability (>200% strain) and sensitivity (>80% resistance change at 60% strain), respectively. Significantly, this sensor platform presents almost no performance degradation even after >60 000 stretching cycle tests due to less strain within the sensor. Furthermore, as proof-of-concept for human-interactive applications, motion detection and respiration monitoring with a breathing period of 3–4 s for a comparatively long duration (>2 hours) are successfully demonstrated. These facile, scalable, highly stable and stretchable strain sensors afford a new route towards next-generation reliable skin-inspired wearable electronics.

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.

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.

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/HNb

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.

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.

This study proposes a wearable and flexible heart pulse sensor detected from a wrist using the tactile pressure sensor film. In general, it is difficult to monitor heart pulse stably under movement of wrist due to the change of position and pressure of the sensors on a wrist. To address this issue, soft pneumatic pump and actuator are integrated to fix the sensor on a wrist even under movement of body. Fundamental properties of pneumatic pump and tactile pressure sensor are studied in addition to the heart pulse monitoring.

This study proposes an out-of-plane electronic whisker (e-whisker) array that integrates a three-dimension whisker structure using polydimethylsiloxane (PDMS) and nanocarbon-based strain sensors. The strain sensor is fabricated by a laser-induced graphene from polyimide film and transfer process from polyimide to the PDMS. Fundamental characteristics of an out-of-plane electronic whisker such as sensitivity and response time are studied as well as the demonstration of e-whisker array. By considering the device structure, multiple directional force detection is realized for the first time. Based on the properties of the sensors, simple and low-cost three-dimensional device fabrication process works well for the e-whisker application.

Home-use, wearable healthcare devices may enable patients to collect various types of medical data during daily activities. Electrocardiographic data are vitally important. To be practical, monitoring devices must be wearable, comfortable, and stable, even during exercise. This study develops a breathable, stretchable sensor sheet by employing a kirigami structure, and we examine the size dependence of electrocardiographic sensors. Because the kirigami film has many holes, sweat readily passes through the sensor from the skin to the environment. For comfort, in addition to breathability, electrocardiographic sensor size is minimized. The limitation of the size is studied in relation to the signal-to-noise ratio of electrocardiographic signals, even under exercise. We found that the optimal size of the sensor is ∼200 mm2 and the distance between electrodes is 1.5 cm. Finally, long-term wireless electrocardiographic monitoring is demonstrated using data transmission to a smart phone app during different activities.

Flexible chemical sensors could be used to create wearable devices that continuously monitor a variety of health conditions through the analysis of a person’s sweat. Current wearable pH sensors for sweat monitoring have a limited pH detection sensitivity at room temperature of about 59 millivolts per pH unit, which is determined by the Nernst equation. Here we show that a sensitive pH sensor can be created using a flexible charge-coupled device (CCD) and integrated with a temperature sensor. Our CCD-based pH sensor can, through accumulation cycles of electron charge transfer, achieve a sensitivity of around 240 millivolts per pH unit, which is roughly four times larger than the Nernst theoretical limit. Furthermore, the integrated flexible temperature sensor can be simultaneously used to compensate for the temperature dependence of the pH detection and to monitor skin temperature. As a proof of concept, we demonstrate that our CCD-based chemical sensor can be used to monitor the sweat pH and skin temperature of a person in real time. A pH sensor made from a flexible charge-coupled device, and integrated with a temperature sensor, can be used to monitor the sweat pH and skin temperature of a person in real time.

We demonstrate the control of resonance characteristics of a drum-type graphene mechanical resonator in a nonlinear oscillation regime using the photothermal effect, which is induced by a standing wave of light between graphene and a substrate. Unlike the resonance characteristics of a conventional Duffing-type nonlinearity, those of the nonlinear oscillation regime are modulated by the standing wave of light with a contribution of the scattered light of an actuation laser, despite a slight variation of amplitude. Numerical calculations conducted with a combination of equations of heat and motion with the Duffing-type nonlinearity explain this modulation: the photothermal effect delays the modulation of graphene stress or tension.

Real-time daily healthcare monitoring may increase the chances of predicting and diagnosing diseases in their early stages which, currently, occurs most frequently during medical check-ups. Next-generation noninvasive healthcare devices, such as flexible multifunctional sensor sheets designed to be worn on skin, are considered to be highly suitable candidates for continuous real-time health monitoring. For healthcare applications, acquiring data on the chemical state of the body, alongside physical characteristics such as body temperature and activity, are extremely important for predicting and identifying potential health conditions. To record these data, in this study, we developed a wearable, flexible sweat chemical sensor sheet for pH measurement, consisting of an ion-sensitive field-effect transistor (ISFET) integrated with a flexible temperature sensor: we intend to use this device as the foundation of a fully integrated, wearable healthcare patch in the future. After characterizing the performance, mechanical flexibility, and stability of the sensor, real-time measurements of sweat pH and skin temperature are successfully conducted through skin contact. This flexible integrated device has the potential to be developed into a chemical sensor for sweat for applications in healthcare and sports.

The internet of things concept has promoted research on human-interactive electronics for wearable devices and robotic applications. One interesting application is wallpaper to monitor a room environment and to act as an electronic message board. To demonstrate the potenital of electronic wallpaper (e-wallpaper), this study prepares a flexible nonvolatile floating gate random access memory (FGRAM) array integrated with a tactile touch sensor array as the message board. Additionally, a temperature sensor array is also laminated to monitor the room temperature. Besides the mechanical flexibility, the fundamental properties of flexible FGRAMs, including the tunneling dielectric thickness, program voltage, and program time dependencies, are characterized. Finally, e-wallpaper is demonstrated as a first proof-of-concept to show a possible platform for future macroscale flexible electronics.

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.

This study reports a highly sensitivity wearable flexible pH sensor using a charge-couple device (CCD) structure. Multi-cycle charge transfer and accumulation depending on pH value realize higher sensitivity than that of the Nernst theory (i.e. 59 mV/pH at room temperature (R.T.)). One difficulty to form the CCD structure on a flexible film is the semiconductor junction due to thermal budget issue for doping injection into semiconductor films. To address this, Schottky junction controlled by an external gate bias is proposed. By optimizing the structure and fabrication process, flexible CCD-based pH sensor is successfully demonstrated. In addition to the study of its fundamental characteristics, real-time sweat pH monitoring is carried out as the first proof-of-concept by attaching the device on an arm. The results show that this flexible pH sensor would be helpful to detect a biological information from sweat.

For comfortable and convenient human life, human-interactive and communicative robot has attractive attention from industrial to home-use applications. One of human-friendly robotic structures is pneumatic balloon type soft robots using mainly silicone rubber operated by compressed air. In this study, we develop a tactile pressure sensor array integrated with pneumatic soft robotic hand to monitor tactile pressure like a human skin. The important advance of this study is to integrate flexible sensors embedded in soft robotic hand without sacrificing softness and flexibility.

We investigate the persistent photoconductivity effects on MoS 2 mechanical resonator toward high performance of optically tuned nano-electro-mechanical systems. After the laser irradiation, the resonance frequency of the resonator shifted downward over than 20 minutes, which is induced by not photothermal effects but persistent photoconductivity effects. In our case, the origin of persistent photoconductivity is most likely due to the trapped photogenerated holes at defects or contaminations on MoS 2 . The trapped holes induce softening effects on electrostatic force between mechanical resonator and the substrate, which decreases resonance frequency.

Two-dimensional atomic layer materials with optically transparent materials such as h-BN are expected to be high performance nano-electro-mechanical resonators (NEMR). Elimination of the photothermal effect is very crucial for accurate force sensing using NEMR. Thus, the optically transparent material such as h-BN is one of strong candidates for this purpose. Here, we investigate the photothermal effect on the mechanical resonance properties of the h-BN-NEMR driven by electrostatically using the dielectric effect.

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.

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.

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.

We investigate light-induced persistent resonance frequency shift on MoS2 mechanical resonators towards optically tunable nano-electro-mechanical systems with optical memory function. After the termination of light irradiation, the resonance frequency shifts downwards and the downshift is maintained for more than 20 min. This behavior is induced by trapped photogenerated holes at defects or contamination on MoS2 similar to the persistent photoconductivity. The trapped holes screen the electrostatic force acting on MoS2, which results in the reduction of the tension of MoS2. The downshift can be eliminated by applying the excess voltage pulse to MoS2 to induce Joule heating.

One of the major challenges toward widespread application of flexible tactile pressure sensors lies in the distinction between tactile pressure stimuli and contributions from substrate bending. To this point, this issue has been a bottleneck in further advancement of this class of devices. To address this issue, we demonstrate a flexible tactile pressure sensor sheet with extremely high selectivity between tactile pressure and substrate bending: these devices are designed with strain engineering with the goal of incorporation into artificial skin applications. In addition to characterizing the mechanical and electrical performance of the sensor, a first proof of concept for tactile pressure mapping at several mechanical flex states is conducted, demonstrating that these devices are highly compliant tactile pressure sensors well-suited to robotic skin and prosthesis applications.

Flexible electronics have great potential as the next class of macroscale multi-sensing devices capable of collecting a variety of information from diverse surfaces. A platform to integrate different electric components on a flexible substrate for macroscale electronics without increasing device fabrication costs needs to be explored. To address this requirement, we demonstrate an all-solution-processed tactile touch memory flexible device integrated with a NiO ReRAM, a tactile touch sensor, and resistors. The function of the solution-processed NiO ReRAM is attributed to a threshold switching mechanism due to the charge trap in the NiO film and the trap-controlled space-charge-limited current. In terms of device operations, the NiO ReRAM is mechanically stable with an on/off current ratio more than 103 while bending the substrate with a radius up to 6.1 mm. As a proof-of-concept for device applications, a tactile touch memory device, which has functions of writing and erasing tactile touch information by applying a tactile touch and SET/RESET voltage, respectively, is successfully operated. The results are promising to advance all-solution-based macroscale flexible electronics.

Flapping‐wing unmanned aerial vehicles have potential advantages, such as consuming lower energy by leveraging the force of wind. Since the flapping movements of the soft wings contain information about the wind, measuring the movement of each part of the wings allows these vehicles to distinguish the direction of the wind. To confirm this prediction, herein, the detection of wind flow from the flapping‐wing motion of a bird robot using an integrated flexible strain sensor on its wing and a physical reservoir computing analysis is presented. In the presence of different wind directions, the movement of the flapping‐wings is measured using flexible strain sensors, and the current wind direction is detected by capitalizing on the intrinsic wing dynamics. As a result, it is found that the detection accuracy using our embedded flexible strain sensors is significantly high, showing a similar level of accuracy with a high‐speed camera recorded from the fixed position in the environment. The results indicate that flapping‐wing unmanned aerial vehicles can recognize wind direction by exploiting the natural dynamics of their wings.

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.

We demonstrate manipulation of nonlinear vibration of graphene mechanical resonator (G-MR) optically by photothermal effects of laser. Different photothermal effects are induced by combining of scattering light and different standing waves of light, which have different effects on nonlinear vibration. Experimental results indicate that nonlinearity is suppressed or promoted for each photothermal effects without almost changing its amplitude. These changes cannot be explained by conventional nonlinear vibration that the nonlinearity increases with increasing amplitude. To reveal the principle of the modulation, we proposed novel vibration model including photothermal effects in nonlinear vibration. Numerical calculation from the model well fits experimental results and revealed the principle. We believe that these technics of controlling nonlinear vibration open the further applications of G-MR.

Transition metal dichalcogenide such as MoS 2 is expected as high performance nano-electro-mechanical devices due to their unique electrical, optical and mechanical properties. One can expect that the combination of these properties are efficient to develop the novel functional devices. Here, we demonstrates the amplitude control of resonance characteristics of a cantilevered MoS 2 on the planar substrate, which is actuated by electrostatically. The AC and DC bias voltage dependences of the vibration amplitudes are well explained by the simplified model under linear elastic regime. Moreover, we demonstrate the optical manipulation of the vibration amplitude at the resonance. Under the irradiation of strongly absorbed light by MoS 2 , the vibration amplitude is successfully manipulated by the laser intensity change.