Polymer-Nanocomposite Based Pressure Sensors and Energy Harvesting Devices for Wearable Applications

Abstract

Recently, wearable devices have been highlighted with the development of portable electronic devices and internet of things technologies. Unlike typical electronics, wearable devices need to form conformal and intimate contact with human and should be comfortable to wear or carry. Therefore, these devices must be thin, lightweight and flexible or stretchable. Furthermore, for the true commercialization of wearable electronics, it is essential to implement sensing technologies that enable friendly interaction with the user and develop a new power supply that can solve the problem of the use time and the volume from the existing battery. This thesis primarily aims to demonstrate the pressure sensors and energy harvesting devices suitable for advanced wearable electronics. It consists of two major categories, one of which is a flexible pressure sensor with high sensitivity, and other is a highly efficient, transparent and flexible energy harvesting device. First, pressure sensors that can measure the strength of the contact have attracted much attention due to their great potential applications from touch panel to health monitoring systems. Especially, the sensitivity of pressure sensors is one of the most important performance factors for better emulating human skin and more precisely capturing human motions. However, existing researches have limitation of the low sensitivity or the complex and costly fabrication process. In this thesis, a low-cost flexible pressure sensor based on the porous elastomer film contributing to the high sensitivity is developed. The elastomer film embedding uniformly dispersed micro-pores is found to have the low elastic modulus and the high deformability. A pressure sensor based on a porous elastomer film exhibits the high sensitivity in the low pressure region and the fast response. Second, energy harvesting systems have emerged as a prominent research area and continue to grow at rapid pace due to the capability of replacing or supplementing a battery. Among them, triboelectric nanogenerators (TENGs) converting mechanical energy sources from surroundings into electricity have been extensively investigated due to the numerous advantages such as the cost-effectiveness, the fabrication simplicity, the robustness, the small volume, and the high efficiency. However, in the conventional TENGs, it was difficult to implement a flexible device because of metal layers. Although the researches replacing metals to flexible electrodes have been proposed, they have the limitation of a low output power. In this thesis, the research on the performance improvement of a transparent and flexible TENG through the conducting polymer and silver nanowires composite layer is described. A conducting polymer layer with high electronegativity contributes to the improvement of output power of TENG. And its characteristics of high transparency and flexibility will make it suitable for advanced wearable applications. Furthermore, the increase of the conductivity and the surface roughness through the introduction of silver nanowires is found to further improve the output performance of TENG. The proposed TENG exhibits the highest output performance among the conventional transparent and flexible TENGs. In summary, this thesis focuses on the performance improvement of pressure sensors and TENGs, which are core technologies of advanced wearable applications, through the utilization of polymer-nanocomposites such as the porous elastomer film and the nano-structured conducting polymer. The proposed approaches will provide a viable and effective framework for developing advanced wearable applications such as self-powered pressure sensor with human-friendly interactions.