Keywords

1 Introduction

A variety of watch-type and glasses-type wearable devices have been widely available to purchase for activity monitorings. However, every commercially available wearable device is based on solid devices for circuits, sensors, and some other components. Ideal wearable devices should be like a cloth or a bandage, so that they can be attached on a human skin or cloth and interacted with humans without awareness during the use of devices. To realize future flexible and wearable electronics, a bottleneck and a challenge are to form mechanically flexible electrical materials uniformly on a macroscale flexible substrate without sacrificing the material performances. Since the flexible wearable devices are probably torn readily and need to keep sanitary clean environments if the devices are attached directly on a human skin, the devices should be disposal, so that the cost is another factor to achieve the flexible devices.

To address these requirements, we have developed macroscale printing methods of inorganic nanomaterials and/or organic materials to form the uniform films for active device components such as sensors and transistors economically on a variety of flexible substrates [e.g. polyethylene (PE), polyethylene terephthalate (PET), silicone rubber, and polyimide (PI)] [1–10]. Inorganic materials are usually mechanically rigid because these are used as a bulk substrate. However, by shrinking to nano-scale size, they are also mechanically flexible [11] at a bending radius at least 1 mm, which is good enough for flexible wearable devices.

In this report, we present some examples about not only wearable devices for human, but also sensor tape for other targets such as a robot and prosthesis. In particular, a fully-printed e-skin, which enables to detect tactile force, friction force, and temperature distributions, is demonstrated by considering a strain engineering [10]. Another application is a flexible and wearable device for a health monitoring, which is like a bandage to attach it on a human skin [7].

2 Multi-functional E-Skin

2.1 Three-Axis Force Sensor

To form a strain sensor using a screen printer, first, silver (Ag) (Asahi Chem., Japan) interconnection was screen-printed on a PET film. Ag film was cured at 130°C, followed by a strain sensor printing. For the strain sensor, a mixture of carbon nanotube (CNT) ink (SWeNT, USA) and Ag nanoparticle (NP) ink (PARU, Korea) was used as a screen print ink with the composition weight ratio of 5:3 as shown in Fig. 1. After printing the ink on a PE film with the alignment of Ag interconnections, the film was cured at 70°C.

Fig. 1.
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Schematic of a screen print process for nanomaterial-based flexible sensor film

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To create stress difference for tactile force and friction force, a three-dimensional prong with 2 mm height and 1 mm diameter was fabricated as we called “fingerprint-like structure”. The fingerprint-like structure was formed by a soft-lithography technique using an acrylic plate mold and polydimethylsiloxane (PDMS) solution. After detaching PDMS-based fingerprint-like structure from the mold, it is laminated to a strain sensor sheet using an adhesive tape. Finally, to allow the strain sensor sheet to have stress distribution freely at tactile and friction forces, polyester film with 5 mm-diameter hole was laminated under the strain sensor sheet as described in Fig. 2.

Fig. 2.
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(a) Schematic of each sensor layer and structure. e-skin device consists of 4 layers of a temperature sensor sheet, a fingerprint-like structure sheet, a strain sensor sheet, and a polyester sheet from top to bottom. (b) Schematic of the detail cross-sectional device of a pixel with device dimensions. Reproduced with permission from ref. [10] (Copyright 2014 American Chemical Society).

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2.2 Temperature Sensor

Temperature sensor was printed on a PET film using a shadow printing method via a polyester hard mask. The ink for temperature sensor was prepared by mixing CNT ink (SWeNT, USA) and a conductive Poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) (Sigma Aldrich, USA) with weight ratio of 1:3. The curing temperature of printed mixed ink was 70°C for more than 1 h. Subsequently, 6 mm diameter holes in PET film were formed by a laser cutter tool to laminate the strain sensor sheet and fingerprint-like structure. Due to good adhesion between PDMS for fingerprint-like structure and PET film, any adhesive tape was not used to laminate together (Fig. 2 for more detail of structures).

2.3 Layer Lamination and 3 × 3 Array E-Skin

To assemble the full functional e-skin, all layers were laminated as shown in Fig. 2. Since all sensors and structures were fabricated on a mechanically flexible substrate such as PE and PET films, e-skin can be readily bent without delamination or cracks of sensors as shown in Fig. 3a. The device has a 3 × 3 array to detect two-dimensional distributions of tactile and friction forces and temperature like a human skin. Four strain sensors with a fingerprint-like structure and one temperature sensor are integrated in a pixel (Fig. 3b).

Fig. 3.
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(a) Photo of multi-functional (tactile force, friction force, and temperature) 3 × 3 array e-skin device. (b) Zoom-up photo of a pixel of the e-skin integrated with four strain sensors and temperature sensor on a membrane with a fingerprint-like structure. Reproduced with permission from ref. [10] (Copyright 2014 American Chemical Society).

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2.4 Mechanism to Distinguish Friction Force from Tactile Force

Strain distribution is key information to observe tactile and friction forces in this device structure. Finite element method (FEM) simulation was conducted as shown in Fig. 4. When a tactile force is applied on top of a fingerprint-like structure, all membrane is depressed, resulting in that the strain/stress distribution is identical (Fig. 4b). On the other hand, when a friction force is applied, strain/stress distribution shows asymmetry around a fingerprint-like structure (Fig. 4c). By utilizing this distribution difference, tactile and friction force can be measured by integrating four strain sensors as shown in Figs. 4b and c. When a tactile pressure is applied, all strain sensors indicate the same stress. However, when a friction force is applied, strain sensor #2 indicates a higher stress than that of #4 in Fig. 4c whereas sensors #1 and #3 indicate the same stress.

Fig. 4.
figure 4

Finite element method simulation. (a) Cross-sectional structural deformation when tactile force (left) and friction force (right) are applied on a fingerprint-like structure. Strain (stress) distribution in the integrated four strain sensor when (b) tactile force and (c) friction force are applied. Reproduced with permission from ref. [10] (Copyright 2014 American Chemical Society).

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2.5 E-Skin Demonstration

As the first proof-of-concept of multi-functional e-skin, 3 × 3 array e-skin was demonstrated by touching the device to apply a tactile force and a friction force. Due to temperature difference between the room (~ 23°C), where the device was measured, and a human skin (~ 30°C), the temperature sensor also shows touch information. Figure 5 exhibits two-dimensional strain/stress and temperature distributions, and they clearly show that the multi-functional e-skin can successfully detect tactile, friction, and temperature distribution from economically fabricated fully printed sensor sheet (e-skin). Due to the high sensitivity of temperature sensor (~ 0.8 %/°C), radiation heat can be also detected, so that the pixel under a finger without touching the device physically indicates a temperature difference as shown in Figs. 5a and b. This suggests that the device is very similar function to one of human skin.

Fig. 5.
figure 5

Tactile, friction, and temperature distributions when a human finger touches on an e-skin device. Two-dimensional mapping results when (a) a tactile force and (b) a friction force are applied. Reproduced with permission from ref. [10] (Copyright 2014 American Chemical Society).

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3 Wearable Smart Bandage

Next, a flexible and wearable device is introduced to show a variety of possibilities for macroscale and low-cost electronics. Especially, a flexible health monitoring device should open a next class of electronics that can be attached onto a human skin without awareness of human for a comfortable, convenient, and secure human life. As an example, smart bandage that can monitor temperature and deliver drug is demonstrated in this report.

3.1 Fabrication and Device Structure

The screen printing technique was mainly used to pattern Ag electrodes as explained in Fig. 1. First, Ag interconnection was printed on a Kapton substrate to form a wireless coil, a capacitive touch sensor, and electrodes for temperature sensor. After curing the Ag film at 130°C for 30 min, a temperature sensor with the same condition as explained above for e-skin was printed with the alignment of Ag electrodes. For a drug delivery pump, soft lithography technique was used to make a semi-sphere structure and microchannel using PDMS. After making an ejection hole of drug in the Kapton substrate, PDMS drug delivery pump was bonded on the Kapton substrate. Figure 6a exhibits more detail of the smart bandage structure. Since all devices were fabricated on a flexible Kapton substrate and studied strain distribution, the device is mechanically flexible without any delamination of devices and cracks, it can be attached on a human body or any other surfaces as shown in Fig. 6b.

Fig. 6.
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(a) Schematic of the first proof-of-concept smart bandage integrated with a drug delivery pump, a wireless coil, a touch sensor, and a temperature sensor. (b) Photo of a fully printed smart bandage device on a Kapton substrate. Reproduced with permission from ref. [7] (Copyright 2014, John Wiley and Sons).

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3.2 Drug Delivery Pump

Flexible PDMS-based drug delivery pump using a soft-lithography technique was bonded on a Kapton substrate integrated with a temperature sensor, a touch sensor, and a wireless coil as explained in Fig. 6. Due to a permanent bonding between PDMS and Kapton substrate, the drug delivery pump can be flexible and wearabe without any delamination (Fig. 7a). By applying a pressure onto the pump structure, dyed-water can be readily ejected through the ejection hole as shown in Fig. 7b. The threshold pressure to eject the liquid from the pump and ejection rate are ~ 3.3 kPa and ~ 35 nL/kPa, respective. That threshold pressure is like a gentle touch of a human, so that child and old-aged person can also operate this easily.

Fig. 7.
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(a) Photo of smart bandage attached on a human wrist. (b) Photos of before and after drug delivery (red-dyed water) from a pump to an absorbent gause. Reproduced with permission from ref. [7] (Copyright 2014, John Wiley and Sons)

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Due to wearable and flexible devices, the device should be non-invasive devices to prevent the controversy about medical ethic for the use of this device. To realize non-invasive drug delivery into a human, a diffusion type drug without using any needle should be appropriate for this application. However, the diffusion type drug is still limited to use for medical application. It is required to improve and develop the techniques and drugs for the future practical uses.

3.3 Temperature and Wireless Touch Sensors

As a demonstration of printed flexible sensor, real-time measurements of integrated temperature sensor and wireless touch sensor were conducted by touching the device with a human finger. Figure 8 shows the results of human sensing. For the wireless detection, 42 MHz and 3 V transmission signal was used. The wireless coil shows the attenuation of transmission signal with ~ 1.2 mV/mm (3.3 %/mm) at 2 V and 42 MHz as a function of coil distance. As the first demonstration of real-time monitoring, the coil distance was only ~ 0.5 mm to observe clear sensing results. Figure 8 depicts that the integrated sensor with wireless coil can clearly measure information of human touch, suggesting that the device can be applied to human active monitoring.

Fig. 8.
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Real-time temperature and wireless touch detections. Reproduced with permission from ref. [7] (Copyright 2014, John Wiley and Sons).

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3.4 Real-Time Human Skin Temperature Monitoring

Finally, real-time human skin temperature monitoring under some activities were demonstrated as the first proof-of-concept of a wearable device. To record the temperature information, a portable data logger was connected to the temperature sensor, and 5 V battery was used. The device was attached to an arm. Figure 9 exhibits the skin temperature during having a lunch with a spicy food and a short-time exercise. Since the normal human skin temperature is ~ 29-31°C, lunch with spicy food makes human skin temperature higher while a short-time exercise doesn’t make a skin temperature change. Based on these results, we confirmed that the wearable temperature sensor can observe the skin temperature based on human activity with ~ 0.1°C resolutions.

Fig. 9.
figure 9

Real-time human skin temperature monitoring during lunch with a spicy soup and a short-time exercise. The smart bandage was attached on an arm. Reproduced with permission from ref. [7] (Copyright 2014, John Wiley and Sons).

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4 Summary

To realize future wearable electronics and sensor sheets for a variety of applications, flexible devices such as multi-functional e-skin and a smart bandage were introduced in this report. Those demonstrated devices are fabricated by a fully printing method, which allows us to realize macroscale flexible devices economically. The demonstrations described here are only a few examples. Currently, tremendous efforts are conducted to achieve high performance, multi-function, and low-cost wearable and flexible devices [1–15]. These contributions should open a next class of electronics.