## 1 Introduction  
Recently, with the development of the mobile Internet and the introduction of the high-performance and low-power System-on-Chip (SoC), wearable devices have been widely applied in various fields such as health care, sports, communication, and virtual reality [1]. It emphasizes that the equipment is \"worn\" on the body and usually has sensors with the function of collecting and detecting information such as physiological signals, acceleration, and location. The sensors used for human-computer interaction are one of the more important types, which have the advantages of portability, comfort, non-invasion, and lower power consumption.  
Gesture recognition technology is a research focus in the field of human-computer interaction. At present, there are two main implementations of this technology, including visual images and sensors [2]. The visual image has been widely studied, but it is limited in the application of wearable devices due to its complex process of installation and usage and high dependence on the environment [3]. Gesture recognition technology based on sensors is often realized by analyzing gestures and location information acquired by sensors such as Inertial Measurement Unit (IMU), magnetometer, and Micro-Electro-Mechanical Systems (MEMS) gyroscope [4]. However, this kind of sensor may not meet the application requirements of complex human-computer interaction scenarios, because it is difficult to identify some slight and complex gestures.  
In 1969, Japanese scientists found that PVDF piezoelectric film had high piezoelectric properties after high-temperature and high-pressure polarization [5]. At present, PVDF piezoelectric film has become a kind of piezoelectric sensing material with excellent performance after many years of development. It has a variety of advantages such as high piezoelectric constant, wide frequency response, low acoustic impedance, high dielectric strength, and excellent mechanical properties [6]. The sensors fabricated by PVDF piezoelectric film are widely applied in biosensors and other fields [7; 8; 9; 10; 11; 12; 13; 14; 15]. [PERSON] et al. [10] designed a touch sensor based on PVDF piezoelectric film, which could help the blind to recognize texts by Braille. The sensor that was based on the PVDF piezoelectric film consisted of two layers of composite material with 12 Braille literacy covered to recognize pressure from fingers. [PERSON] et al. [11] developed a system that could be used for remote pulse measurement based on the excellent sensing characteristics of PVDF piezoelectric film. The system utilized PVDF sensors to measure pulse and provide force and other information. [PERSON] and [PERSON] [12] developed a new type of cardiopulmonary signal sensor consisting of a PVDF piezoelectric film and a pair of conductive fabric sheets to detect the cardiopulmonary signal during sleep. In prior research studies, it can be seen that PVDF piezoelectric film is very sensitive to dynamic stress and suitable for detecting weak physiological signals on the skin surface of the human body.  
In order to meet the application needs of complex human-computer interactions and the recognition of slight gestures, we designed and built a sensor to detect wrist motion signals based on PVDF piezoelectric film in this paper. The innovations of this work include two aspects: a novel structure design and the detection of weak wrist motion signals.