Wataru Honda

Fully Printed, Highly Sensitive Multifunctional Artificial Electronic Whisker Arrays Integrated with Strain and Temperature Sensors

Mammalian-mimicking functional electrical devices have tremendous potential in robotics, wearable and health monitoring systems, and human interfaces. The keys to achieve these devices are (1) highly sensitive sensors, (2) economically fabricated macroscale devices on flexible substrates, and (3) multifunctions beyond mammalian functions. Although highly sensitive artificial electronic devices have been reported, none have been fabricated using cost-effective macroscale printing methods and demonstrate multifunctionalities of artificial electronics. Herein we report fully printed high-sensitivity multifunctional artificial electronic whiskers (e-whisker) integrated with strain and temperature sensors using printable nanocomposite inks. Importantly, changing the composition ratio tunes the sensitivity of strain. Additionally, the printed temperature sensor array can be incorporated with the strain sensor array beyond mammalian whisker functionalities. The sensitivity for the strain sensor is impressively high (∼59%/Pa), which is the best sensitivity reported to date (>7× improvement). As the proof-of-concept for a truly printable multifunctional artificial e-whisker array, two- and three-dimensional space and temperature distribution mapping are demonstrated. This fully printable flexible sensor array should be applicable to a wide range of low-cost macroscale electrical applications.

Toward Flexible and Wearable Human‐Interactive Health‐Monitoring Devices

This Progress Report introduces flexible wearable health-monitoring devices that interact with a person by detecting from and stimulating the body. Interactive health-monitoring devices should be highly flexible and attach to the body without awareness like a bandage. This type of wearable health-monitoring device will realize a new class of electronics, which will be applicable not only to health monitoring, but also to other electrical devices. However, to realize wearable health-monitoring devices, many obstacles must be overcome to economically form the active electrical components on a flexible substrate using macroscale fabrication processes. In particular, health-monitoring sensors and curing functions need to be integrated. Here recent developments and advancements toward flexible health-monitoring devices are presented, including conceptual designs of human-interactive devices.

Highly selective flexible tactile strain and temperature sensors against substrate bending for an artificial skin

Flexible devices can conformally cover any surfaces and interact with different stimuli such as human touch. Although a flexible tactile sensor has been reported as an artificial skin application, distinguishing between a tactile force and strain due to substrate bending remains challenging. Here we report a highly selective tactile force sensor against bending on the basis of strain engineering by fabricating a cantilever structure. The proposed device achieves a 4–23 times improvement in selectivity compared to conventional pressure sensitive rubber. As a proof-of-concept for e-skin, an array composed of highly selective tactile force sensors and temperature sensors is successfully demonstrated to imitate human skin.

Printed multifunctional flexible device with an integrated motion sensor for health care monitoring

Printable, multifunctional, flexible, health monitoring detachable patch sheets with human motion detection capability. Real-time health care monitoring may enable prediction and prevention of disease or improve treatment by diagnosing illnesses in the early stages. Wearable, comfortable, sensing devices are required to allow continuous monitoring of a person’s health; other important considerations for this technology are device flexibility, low-cost components and processing, and multifunctionality. To address these criteria, we present a flexible, multifunctional printed health care sensor equipped with a three-axis acceleration sensor to monitor physical movement and motion. Because the device is designed to be attached directly onto the skin, it has a modular design with two detachable components: One device component is nondisposable, whereas the other one is disposable and designed to be worn in contact with the skin. The design of this disposable sensing sheet takes into account hygiene concerns and low-cost materials and fabrication methods as well as features integrated, printed sensors to monitor for temperature, acceleration, electrocardiograms, and a kirigami structure, which allows for stretching on skin. The reusable component of the device contains more expensive device components, features an ultraviolet light sensor that is controlled by carbon nanotube thin-film transistors, and has a mechanically flexible and stable liquid metal contact for connection to the disposable sensing sheet. After characterizing the electrical properties of the transistors and flexible sensors, we demonstrate a proof-of-concept device that is capable of health care monitoring combined with detection of physical activity, showing that this device provides an excellent platform for the development of commercially viable, wearable health care monitors.

High‐Performance, Mechanically Flexible, and Vertically Integrated 3D Carbon Nanotube and InGaZnO Complementary Circuits with a Temperature Sensor

A vertically integrated inorganic-based flexible complementary metal-oxide-semiconductor (CMOS) inverter with a temperature sensor with a high inverter gain of ≈50 and a low power consumption of <7 nW mm(-1) is demonstrated using a layer-by-layer assembly process. In addition, the negligible influence of the mechanical flexibility on the performance of the CMOS inverter and the temperature dependence of the CMOS inverter characteristics are discussed.

Mechanically Flexible and High-Performance CMOS Logic Circuits

Low-power flexible logic circuits are key components required by the next generation of flexible electronic devices. For stable device operation, such components require a high degree of mechanical flexibility and reliability. Here, the mechanical properties of low-power flexible complementary metal–oxide–semiconductor (CMOS) logic circuits including inverter, NAND, and NOR are investigated. To fabricate CMOS circuits on flexible polyimide substrates, carbon nanotube (CNT) network films are used for p-type transistors, whereas amorphous InGaZnO films are used for the n-type transistors. The power consumption and voltage gain of CMOS inverters are <500 pW/mm at Vin = 0 V (<7.5 nW/mm at Vin = 5 V) and >45, respectively. Importantly, bending of the substrate is not found to cause significant changes in the device characteristics. This is also observed to be the case for more complex flexible NAND and NOR logic circuits for bending states with a curvature radius of 2.6 mm. The mechanical stability of these CMOS logic circuits makes them ideal candidates for use in flexible integrated devices.

Printed wearable temperature sensor for health monitoring

We developed a printed high sensitive temperature sensor by synthesizing poly (3, 4-ethlenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) and carbon nanotubes (CNTs) composition ink on a flexible substrate. There are two advantages of this flexible temperature sensor, which are (1) high sensitivity (~0.6%/°C) compared to other reported flexible temperature sensors, (2) fully-printable low-cost process on macroscale flexible substrates due to solution-based process. The key point to achieve the high sensitivity was a mixture of PEDOT:PSS and CNTs for the temperature sensor material. Based on the analyses, the mechanism of this high sensitivity is most likely due to electron hopping at the interface of PEDOT:PSS and CNTs. In addition to the sensor analyses, real-time human skin temperature was monitored by attaching on a human skin like a bandage as a proof-of-concept, and it was successfully conducted to observe small skin temperature change during activities.

Printable flexible tactile pressure and temperature sensors with high selectivity against bending

Flexible electronics are of great interest in a future electric device such as artificial electronic devices. Especially, artificial electronic skin (e-skin) is widely studied by developing a tactile pressure sensor on a flexible substrate. However, conventional flexible tactile sensors also detect the bending of substrate without applying a tactile pressure, and that is the one of bottlenecks to realize stable operation of a flexible device. This study demonstrates the high selectivity of tactile pressure and temperature sensors against bending based on strain engineering. To achieve high selectivity, a cantilever type strain sensor in a flexible substrate is developed. In addition, the temperature sensor is also mechanically stable. It should be worth to note that these sensors are fabricated by a fully printing method using a screen printer. This finding and demonstration eventually allow us to apply the flexible devices on versatile surfaces with accurate sensing.

Inorganic material-based flexible CMOS circuit and optical sensor

A mechanically flexible complementary metal-oxide-semiconductor (CMOS) inverter and optical sensor are developed by integrating p-type carbon nanotubes (CNTs) network thin-film transistors (TFTs) and n-type InGaZnO TFTs on a polyimide substrate. Although a flexible CMOS circuitry has been reported [1], mechanical reliability and its sensor application have yet to be demonstrated. Here, we demonstrate a flexible CMOS inverter and an optical sensor using CNT and InGaZnO with relatively high field-effect mobility. Furthermore, we experimentally confirmed that these devices are mechanically stable by comparing electrical properties under flat and bending states.

Fully printed, large-scale, high sensitive strain sensor array for stress monitoring of infrastructures

We demonstrate a macroscale sensor sheet by fabricating the fully printed, large-scale, and high sensitive strain sensor array on mechanically flexible substrates. This sensor sheet can conformally cover any surfaces for the application of real-time infrastructure stress monitoring as the first proof-of-concept. To realize this concept, a screen printing method is proposed to use by developing an ink for strain sensor. Printed strain sensor array exhibits impressively high sensitivity, and successfully detects two-dimensional strain distribution of small deformation <;10μm.