## 2 Sensors
### State-of-the-Art Research on BSN Status
#### 2.1.2 Main Researched Sensors  
With the development of different applications, more and more sensors are appearing in the BSN field. The most commonly used sensors are shown in Table 1. Several typical BSN sensors are introduced below. \begin{table}
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(1) ECG Sensors  
ECG sensors are used for electrocardiograph signal monitoring, which is the main way to diagnose heart disease in electronic healthcare systems. ECG signals reflect the change of current intensity on the skin caused by the contractile activity of the heart over time, which can easily be recorded using non-invasive electrodes on the chest or limbs of body [40]. They can be represented as a pattern of cyclically occurring ECG waveforms with different frequency contents: QRS complex, P and T waves. ECG waves indicate the overall rhythm of the heart and weaknesses in different parts of the heart muscles, and they can be measured to diagnose abnormal heart rhythms. Among ECG waveforms, the QRS complex reflects the electrical activity within the heart during ventricular contraction and provides much information about the state of heart [41]. ECG sensor nodes transmit data in a wireless way, which contributes to better wearability. Generally, the nodes are so small that they can easily be embedded in fabric or plaster. According to the types of electrodes, ECG sensors can be divided into three categories: sensors with wet electrodes, sensors with dry electrodes, and sensors with non-contact electrodes. Wet electrodes were used for ECG monitoring first, but now are rarely used in BSNs, as this kind of electrode may cause skin irritation and signal degradation due to dehydration. As an alternative, dry electrodes have become more and more popular, but they still contact the skin directly. In addition, dry electrodes which don't have the benefit of conductive gel, are much more sensitive to the conditions of the skin and are highly susceptible to motion artifacts (MAs). In Reference [42], a kind of wireless sensors with non-contact electrodes is presented. The sensor consists of a set of capacitive electrodes manufactured on a standard printed circuit board that can work within fabric or other insulation. In contrast to wet and dry contact sensors, non-contact sensors do not require any direct contact with the body, so they are completely insensitive to skin conditions. Nevertheless, the denoising ability requirement is relatively high for non-contact sensors. Experiments show that insulating layers between non-contact sensors and skin may obscure the features of smaller signals, such as P-waves, if the layers are too thick to maintain signal quality. Moreover, insulating layers may cause noise too, which must be weakened or removed by more effective denoising algorithms. (2) Accelerometers  
Accelerometers are used for measuring the acceleration of components in an inertial three-dimensional coordinate system. They play an important role in human energy expenditure detection and behavioral recognition, because of their small size, relatively low cost, as well as the convenience of integrating them into existing sensor network platforms [22]. One purpose of utilizing accelerometers is to detect human energy expenditure. Accelerometers can obtain physical activity frequency, motion intensity, and other information if placed on the human body. Energy expenditure can be inferred by a series of algorithms based on the information accelerometers get. Compared with other methods utilizing passometers or cardiotachometers alone, this method has higher accuracy and has become a trend in energy expenditure detection [43]. At present, the most accurate energy consumption instrument is called the LivePod [44], which employs triaxial accelerometers to identify the action of a human body in any direction by an intelligent fuzzy algorithm. It can achieve an accuracy of 97% using a high-precision personalized model for energy consumption calculation.