# Recent Progress in Pressure Sensors for Wearable Electronics: From Design to Applications
## 1 Introduction  
Modern society strives to make the world smart and comfortable. Numerous electronic devices have been actively researched and developed to realize these goals. Through a number of well-developed devices, flexible and wearable devices that change the way we work and play are on the horizon. Electronic devices, such as smartphones and smartwatches, enable an unprecedented amount of information to be easily accessed. Many researchers are immersed in the continuing effort to make lighter and more compact devices and systems. In particular, many studies have been conducted to realize a compact system capable of providing interaction between a human and machine. For human-machine interaction, the role of the sensor is important, and in particular, pressure sensors can acquire substantial information from people, so most research is being conducted on the human/sensor interface. Pressure sensors can measure various information sources acting on the inside and outside of human skin, as well as on the surface of an object or a machine [1; 2; 3; 4; 5; 6; 7; 8]. However, there are many difficulties in applying conventional pressure sensors to wearable electronics because they are traditionally made of stiff materials. Therefore, in order to apply pressure sensors to wearable electronics, such as human-machine interfaces, biomedical sensors, and motion detection, it is important to develop a flexible sensor that is capable of sensitive and consistent performance, even on a curved surface. Thus, the flexibility of the electrode, as well as the sensing layer, should be secured, and various electricalperformances, including the sensitivity, detection limit, response time, and mechanical durability must be guaranteed. Many studies are underway to produce flexible pressure sensors. In particular, there are two main aspects to realize flexible pressure sensors; one is to use inherently flexible materials, and the second is to produce flexibility by forming specially designed structures. Flexible pressure sensors using functional polymers, such as silicone rubber, have received substantial attention. Silicone rubber, like polydimethylsiloxane (PDMS), has excellent flexibility and is particularly suitable for wearable electronics due to its biocompatible nature [9; 10; 11]. Recently, composite materials have been made by mixing BaTiO\({}_{3}\), carbon nanotubes (CNTs), reduced graphene oxide (rGO), carbon black (CB), Ag nanowires (Ag NWs), and Ag nanoparticles (Ag NPs) to improve the electrical performance of silicone rubber [12; 13; 14; 15; 16; 17]. These composite materials show electrical properties superior to bulk materials, which can significantly improve the electrical performance of the sensors. The fabrication of the sensing layer to have a porous structure, or manufacturing it in NW form, is another major way to give the sensor flexibility [18; 19; 20; 21; 22]. The nano-size or micro-size structural form of materials can provide excellent flexibility, even if the materials are naturally brittle. In particular, if a naturally flexible material is produced to have various structures, the flexibility can be significantly modified. A flexible material and specially structured sensing layer not only provide excellent mechanical durability, but also improve electrical performances [23; 24; 25; 26; 27]. In addition to flexible sensors, self-powered sensor systems have recently been actively studied [28; 29; 30; 31; 32; 33; 34; 35]. These self-powered sensor systems do not require an additional power source such as a battery, so they show promise for application in wearable electronics. In particular, triboelectric nanogenerators (TENGs), which were first invented by [PERSON]'s group in 2012 [36], have opened up a new field in self-powered sensor systems, in addition to piezoelectric pressure sensors. TENGs are electronic devices that combine electrostatic induction and the triboelectric effect, and they produce a relatively high voltage output from the contact/separation of two friction layers.