# Wearable Sensors for Vital Signs Measurement: A Survey
## 2 Classification of Wearable Medical Sensors
### Classification and Application of Wearable Medical Sensors
#### 2.2.4 Wearable Medical Sensors for Detecting Blood Oxygen Saturation
Therefore, it was demonstrated that organic devices not only have the advantages of appearance in such applications, but also have great potential as supporters of all-day wearable health monitoring systems [38]. [PERSON] et al. (2018) proposed a flexible printed sensor array composed of organic light-emitting diodes and organic photodiodes. The array sensors reflected light from tissue to determine blood oxygen saturation. It was finally verified that the sensor measured the blood oxygen saturation of the forehead with an average error of 1.1%, and created a two-dimensional oxygenation map of the adult forearm under the pressure cuff-induced ischemia. In addition, a mathematical model was proposed for the detection of blood oxygen saturation to determine oxygenation in the presence and absence of pulsating arterial blood signals. Therefore, the mechanical flexibility, oxygenation mapping capability, and the ability to place sensors in different positions of the reflective oximeter array proposed in this study make it promising for medical sensing applications, such as real-time chronic disease monitoring and tissue, organ, and postoperative recovery management of wounds [39]. [PERSON] et al. (2019) used medical sensors generated by photoplethysmography (PPG) to detect blood oxygen saturation for outpatient care and general health monitoring. They found that this type of sensor device can collect pulse oximeter signals and can obtain accurate and continuous blood pressure measurements from mobile and wearable devices [40]. [PERSON] et al. (2020) evaluated the diagnostic accuracy of dental pulse oximeters, thermal tests, and electric endodontic testers with custom sensor holders in assessing actual pulp status. They also assessed the oxygen saturation level of control healthy teeth, non-living teeth and irreversible pulitis teeth. Finally, a randomized controlled trial found that the customized scaffold used in this study helped to provide an accurate response to the pulp vitality test. At the same time, under different pulp conditions, the diagnostic accuracy of the dental pulse oximeter was higher, followed by cold and hot pulp testers, with electric pulp testers having the lowest diagnostic accuracy [41]. [PERSON] et al. (2020) used sensors to record blood volume changes and used PPG to extract heart rate and other physiological parameters to inform users of activity, sleep, and health [42]. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Scholars**} & **Sleep Monitoring by Wearable** & \multirow{2}{*}{**Achievements**} \\  & **Medical Sensors** & \\ \hline [PERSON] et al. (2019) [43] & Wearable in ear electroencephalogram (ear EEG) was used for night sleep monitoring & It was feasible for in-ear sensor to monitor night sleep outside the sleep laboratory, which \\  & & reduced the technical difficulty related to PSG. \\ \hline [PERSON] et al. (2020) [44] & A system based on bed vibration sensor was proposed & Accurately monitored physiological parameters during sleep, such as HR, RR, body movement, and sleep posture. \\ \hline [PERSON] et al. (2020) [45] & A wearable multi-biological signal wireless interface for sleep analysis was designed & The correlation of four sleep stages was 74\%. \\ \hline [PERSON] et al. (2021) [46] & A non-synchronous sensor data analysis (USDA) model was introduced & Responsive healthcare solutions using asynchronous WS-data helped achieve greater efficiency and reduce the \\  & & Complexity, when evaluating healthcare system performance. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Review of research on sleep monitoring by wearable medical sensor.