Electrical signals produced by various bodily activities are commonly used in monitoring and diagnosis [12]. Measured signals are generally
compared with normal reference signals and variations in signals can be useful to provide accurate diagnosis. The most common devices include the electrocardiography (ECG) – electrical activity of different sections of the heart, the electroencephalography (EEG) – electrical activity of the brain and the electromyography (EMG) – electrical activity of the muscles. In general biomedical sensors take signals representing biomedical variables and convert them into what is usually an electrical signal. As such, the biomedical sensor serves as the interface between a biologic and an electronic system and must function in such a way as to not adversely affect either of these systems. In considering biomedical sensors, it is necessary to
consider both sides of the interface: the biologic and the electronic, since both biologic and electronic factors play an important role in sensor performance.

Sensor convert signals of one type of quantity such as hydrostatic fluid pressure into an equivalent signal of another type of quantity, for example, an electrical signal. Sensors of electrical phenomena in the body, usually known as electrodes, play a special role as a result of their diagnostic and therapeutic applications.

There are, however, special problems that are encountered by biomedical sensors that are unique to them. These problems relate to the interface between the sensor and the biologic system being measured.