# 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  
The main route of transmission of oxygen in the blood is hemoglobin. Oxygen saturation is determined by oxygenated hemoglobin. First, oxygen enters the lungs when the human body breathes, and then enters the blood. The blood transports oxygen to the various organs of the human body, which is also the percentage of available hemoglobin that carries oxygen. The specific measurement of the content of oxygenated hemoglobin is found using the characteristics of oxygenated hemoglobin. Oxygenated hemoglobin and deoxygenated hemoglobin absorb light of different wavelengths in a specific way. The wavelength of light is very short: the unit of measurement is the nanometer, the wavelength of red light being around 650 nm, and the wavelength of infrared light being around 950 nm [35; 36]. The measurement of blood oxygen saturation uses the characteristics of oxyhemoglobin and deoxyhemoglobin to absorb light of different wavelengths in a specific way. Oxyhemoglobin absorbs more infrared light than red light, and deoxyhemoglobin absorbs more red light than infrared light. Finally, the heart rate blood oxygen sensor analyzes the ratio of oxygenated hemoglobin and deoxyhemoglobin in the arterial blood that absorb two different wavelengths of light. The absorption ratio R of the two wavelengths of light is calculated, and then the blood oxygen saturation is calculated according to the standard model (SpO\({}_{2}\) = HbO\({}_{2}\)/(HbO\({}_{2}\) + Hb) \(\times\) 100%) for calculating SpO\({}_{2}\)[37]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scholar** & **Type of Sensors** & **Component of Sensors** & **Effects** \\ \hline [PERSON] et al. [30] & Gas sensor & MoS\({}_{2}\) & Enhanced sensitivity, ultrafast response time (about 29 s), and excellent recovery of NO\({}_{2}\) (100 ppm) at room temperature. \\ \hline [PERSON] et al. [31] & Resistance sensors & Flexible ammonia (NH\({}_{3}\)) & Perfect response concentration linearity, good reproducibility, excellent selectivity, significant long-term stability, ultra-low detection concentration (16 ppb) and theoretical detection limit (0.274 ppb) and excellent flexibility, no significant response after 500 bending/extension cycles. \\ \hline [PERSON] et al. [32] & Hydrogel sensor & Hydrogel & With superior mechanical feeling and thermal sensitivity, it can realize a flexible touch keyboard for feature recognition and a “heating indicator” for human forehead temperature detection. \\ \hline [PERSON] et al. [33] & Piezoresistive effect sensor & Graphene nanosheets and multi-walled carbon nanotubes & With the increase in GNP content, the hardness and Young’s modulus of the hybrid films decreased, while the thermal conductivity showed the opposite trend. With the increase in solvent adsorption amount, the change of resistance increases linearly. It can be potentially applied to the detection of temperature and liquid leakage. \\ \hline [PERSON] et al. [34] & Thermoelectric effect sensor & Hydrophobic Films and Graphene/Polydimethylsiloxane & Excellent performance in low cost and material identification provides a design concept to meet the challenges of functional electronics. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of research on temperature detection by medical sensors by different scholars. In the process of blood oxygen saturation measurement, many scholars also put forward their own opinions (Table 4). [PERSON] et al. (2018) believe that the pulse oximetry sensor, as a device for monitoring the basic human health, plays a key role, while also having a large consumption. Therefore, an ultra-low-power reflective patch pulse oximeter sensor was proposed, using the design freedom provided by organic technology. Through optical simulation of color-sensitive light propagation in human skin, the proposed monolithic integrated organic pulse oximeter sensor showed successful operation at an average electric power as low as 24 uW.