# Wearable Sensors for Vital Signs Measurement: A Survey
## 1 Introduction  
With the rapid development of science and technology, the process of urbanization is accelerating, and the construction of smart cities is now on the agenda. The construction of smart cities is closely related to transportation, medical treatment, economy, culture, and tourism in the city. With the global spread of coronavirus disease-2019 (COVID-19), people paid particular attention to their health. Thus, the development of the medical field has become of the utmost priority. In traditional medicine, the use of stethoscopes, thermometers and electrocardiographs has played a great role in human health monitoring and diagnosis. However, with the development of sensor technology and Artificial Intelligence (AI) technology, intelligent performance within the medical field has also greatly improved, such as the intelligent monitoring of patients' blood pressure, wearable sensors or implanted medical sensors to monitor vital signs, etc. [1, 2]. Therefore, the application and intelligent development of medical sensors have become the focus of scientific research scholars in related fields. In the process of intelligent development in the medical field, sensors are an effective way to acquire data. For example, in the process of human breathing detection, a thermal element is placed under the nose. During breathing, the temperature change of the airflow enabled from the nose causes the resistance of the thermistor to change, thereby causing the voltage to change; subsequently, the waveform of the breathing state is obtained after amplification and filtering [3]. However, if the sensor only relies on the thermal element, although the device is relatively simple, it is susceptible to the influence of external temperature changes, which may cause misjudgment. Therefore, the more reliablethermistor airflow method can be used. In a medical infusion monitoring system, the capacitive liquid level sensor detection method is used to measure the infusion liquid level [4]. When the height of the liquid level in the infusion bottle changes, the proportion of air contained in the medium between the two plates of the capacitor changes accordingly, which eventually causes the size of the capacitor to also change, thereby reflecting the change in the remaining amount of liquid medicine. Therefore, an infusion-monitoring sensor can be designed with different specifications according to the volume of different infusion bottles to achieve the result of monitoring the medical infusion. Of course, there are many sensors involved in the medical field, such as photoelectric sensors, ultrasonic sensors, etc., which combine massive Machine Type Communication (mMTC) with the medical system network to achieve human health monitoring [5; 6]. In the application process of a medical sensor system, the wearable medical sensor studied here uses mMTC technology to continuously and accurately transmit, in a timely manner, the normal physiological activity information of the human body in the natural environment to the medical data center through the network, which is convenient for doctors to observe the patient's condition. For example, vital sign parameters collected by wearable or implanted sensors are collected by receivers (such as mobile phones and computers), and the collected parameters can be used for preliminary data analysis [7; 8]. Moreover, depending on the analysis results indicating the physiological condition of the testee, and when it is judged that the physiological condition is abnormal, the guardian will be notified. Finally, the records are transmitted to the medical platform data center via 5 th Generation Mobile Communication Technology, where the data can be stored and further analyzed. In addition, large-scale machine communication technology not only reflects the connections in terms of equipment scale, but also has the advantages of efficient network overheads and reliability.