Flexible Sensor Sheets for Healthcare Applications

Kuniharu Takei

Osaka Prefecture University


Daily and home-use medical and healthcare diagnoses are of great interest due to the worldwide increase of the elder population and the technological progress in integrating multiple sensors. In terms of wearable sensors, continuous real-time health condition monitoring may be a potential target. If continuous monitoring of multiple health conditions can be realized by simply attaching a comfortable noninvasive device onto the skin, preventive medical care, or diagnosis of diseases in the early stage can be realized by detecting trends in the health condition changes. It should be noted that the absolute values of vitals such as body temperature, blood pressure, glucose level, etc., are not so important for this application. Additionally, this concept may be easier to build a robust device market because most applications do not require the approval for medical uses since the exact values corresponding to the diagnosis are not indicated. For wearable healthcare applications, real-time continuous trend changes of the health conditions should be useful information to predict disease, which may be a way to spread flexible healthcare sensor sheets to the market.

Another useful target for healthcare applications is a feedback system between patients and doctors via the internet (Figure 6.1) (Honda et al. 2014). Briefly, patients or users can monitor their real-time health condition changes by tracking trends and changes. If an artificial intelligence (AI) or deep learning concept can be integrated into the wearable sensor system, a drug or curing agent may be delivered to the user automatically based on


Concept of wearable healthcare patches with interactions between a user and a doctor. (Reproduced with permission from Honda et al. (2014). Copyright 2014, John Wiley and Sons.)

the signals. However, this medical action capability will depend on the laws in a country. Regardless of the drug delivery function, the health condition trends provide useful information for doctors and users. Doctor can treat patients based on the condition changes during the patients' daily life. Furthermore, users can make decisions to keep their physical and mental conditions without having to consult medical doctors. This health condition monitoring may have an extremely high impact on future medical and healthcare platforms.

Although this concept and platform are promising for not only academic research but also big markets, almost no practical devices have been commercialized. There are several challenges toward the realization of this platform. The first one is the integration of multiple sensors on a flexible film. The second is the integration of signal processing, a wireless system, and battery without sacrificing mechanical flexibility and increasing the cost. The third one is determining which information is required to monitor and diagnose health conditions.

To decide which kinds of sensors to integrate, a lot of data must be analyzed by developing different combinations of sensors, although some may prove unnecessary for the healthcare applications. Once the above-mentioned challenges are addressed, wearable healthcare devices should create a huge market and change how people check their health conditions. Herein, the progress of flexible sensor sheets attached onto the human skin to monitor health conditions is reviewed.

Flexible Physical Healthcare Device Patches

To address the aforementioned challenges, many studies have investigated flexible and/or stretchable multifunctional sensor sheets (Takei et al. 2015, Xu, Lu, and Takei 2019, Gao et al. 2019). Research has included sensors (Bariya, Nyein, and Javey 2018, Martin et al. 2017,

Yan, Wang, and Lee 2015, Takei et al. 2010, Kim, Lu, Ghaffari, et al. 2011, Kim, Lu, Ma, et al. 2011), circuits (Wang et al. 2018, Someya et al. 2005, Cao et al. 2008, Honda et al. 2015), and power sources (Kim et al. 2016, Hu et al. 2009, Yoon et al. 2008). Here, flexible healthcare sensors are introduced to understand the fundamental characteristics and applications as a flexible sensor sheet based on device developments in Takei's laboratory at Osaka Prefecture University in Japan.

Many groups, including Takei, have proposed noninvasive monitoring of the surface of the skin using attachable bandage-type sensor sheets (Figure 6.2) (Gao et al. 2019, Yamamoto et al. 2016). Takei's group's idea involves a device that consists of at least two layers (Figure 6.2b). The first layer is disposable and the second is reusable.

The first layer is a sheet in direct contact with the skin. This layer is disposable because direct contact with the skin is necessary for precise monitoring, but also creates hygiene concerns. To realize a disposable sheet, a low-cost fabrication process is required. For the disposable sheet, Takei's group printed flexible sensors to make electrocardiogram (ECG) sensor, skin temperature sensor, and three-axis acceleration sensor (motion sensor) on a polyethylene terephthalate (PET) film.

The other sheet is integrated with expensive components such as signal-processing circuits, wireless systems, and battery. Hence, it is reusable due to the high fabrication costs. To minimize device costs, the sheet with the more expensive components should not come into direct contact with the skin. Thus, this sheet is assembled on top of the disposable sheet.


(a) Photo and (b) schematic of a flexible healthcare patch integrated with a three-axis acceleration sensor, skin temperature sensor, ECG sensor, UV sensor, and switching transistors, (c) Photo of the flexible connection region between the reusable and disposable sheets. (Reproduced with permission from Yamamoto et al. (2016). Copyright 2016, American Association for the Advancement of Science.)

Another critical feature is creating an electrical connection between the sheets that does not affect the mechanical flexibility. One approach is to use a human-friendly (biocompatible) liquid metal of eutectic gallium-indium metal alloy (Figure 6.2c). For the details about this material, including its structure and stability, please refer to the literature (Harada et al. 2015).

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