# Piezoelectric Sensor with a Helical Structure on the Thread Core
## 1 Introduction  
Traditionally, clothing has played a major role in civilization because it helps people adapt to changing environments comfortably [1; 2]. Clothing has been designed to assist the daily activities of humans with various features such as warming up the body in cold weather, protecting the body from external threats, and providing surplus room to carry necessary items [3; 4; 5; 6].  
Recent advances in electronics, wearable devices, and the Internet of things have led to the emergence of smart clothing [7; 8; 9; 10; 11]. Various types of smart clothing have been reported with the advantages of detecting the surrounding environment and protecting the human body simultaneously [12; 13]. Early smart fibers for smart clothing mostly comprised bulky and complex electronic devices [14; 15]. The recent ones have thread-type conductive fibers with good sensitivity and flexibility [16; 17; 18; 19; 20; 21]. Smart fiber technology can be adapted to various applications such as human motion detection, real-time fitness monitoring, and human mobility pattern research [22; 23; 24; 25; 26; 27; 28; 29; 30].  
Herein, we fabricate a textile-based piezoelectric sensor using polyvinylidene fluoride (PVDF), which is flexible, lightweight, and sensitive [31; 32]. Specifically, the textile structure is implemented by curling PVDF with threads in a helical form. Through the helicoidal geometry, the combined sensing structure shows high stability and durability under repetitive mechanical deformation [33; 34]. In addition, by using piezoelectric properties coupled with the interaction between the electrical charge and mechanical strain [35], the mechanical deformation of the sensor can be measured using the electrical response of the sensor while maintaining flexibility. Figure 1 illustrates the schematic of the textile-based piezoelectric sensor. Previous methods of fabricating textile-based piezoelectric sensors have been reported by utilizing heat-spun piezoelectric yarns [36; 37]. Unlike from the previous studies, we focus on the helical combination of a film-type piezoelectric material and thread core that enables implementing fibrous characteristics with good sensing stability and durability [33].  
Energy harvesters [34; 38] and sensors [34] have been reported by using piezoelectric materials combined with threads in a helical structure. Unlike the previous studies, we focus on the sensing characteristic of the helical piezoelectric sensor. Specifically, we compare the repeatability and sensing characteristics of the sensor and its expanded textile form to suggest the feasibility of the proposed sensor for biometric applications.  
The main contributions of this paper are as follows: (i) we present the detailed manufacturing process of the textile-based piezoelectric sensor with a helical structure, (ii) analyze the electrical properties of the sensor, and (iii) demonstrate the smart textile by weaving the piezoelectric sensor.  
The remainder of this paper is organized as follows. Section 2 presents the manufacturing process of the sensor. Section 3 describes the experimental setup used to measure the electrical response of the piezoelectric sensor according to the applied flexural loading. Section 4 analyzes the sensing results of the piezoelectric sensor and smart textile. Section 5 discusses the possible applications of the proposed sensor. The conclusions are summarized in Section 6.