Smart Technology for Non Invasive Biomedical Sensors to Measure Physiological Parameters

1. Importance Of Bio Signals

Bio signal can be defined as a description of a physiological phenomenon. Commonly used bio signals are Electroencephalogram (EEG), Electrocardiogram (ECG) Electromyogram (EMG) Mechanomyogram (MMG) Electrooculography (EOG), Galvanic skin response (GSR), Magneto encephalogram (MEG). Among these bio signals, ECG and PPG signals are widely used.

A photoplethysmogram (PPG) is an optically obtained using plethysmogram, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption. PPG technique allows us to study heart beat rate thereby increasing the reliability of clinical measurements. It is also used to evaluate the vascular blood flow resistance which is found to be an important physiological parameter for vascular diagnostics. PPG pulse wave analysis helps to study diabetes and arthritis which is unique for each individual and helps to get unique identification as biometric identification. It also helps to study large artery damage & an abnormality in the cardiovascular disease which is one of the common causes of high mortality rate. PPG analysis emphasizes the importance of early evaluation of the diseases (Tamura et.al., 2014).

Electrocardiogram (ECG) signals are used for diagnosis of heart diseases. Figure 1 shows the waveform of ECG signal. The QRS waveforms which are used to indicate electrical activity of heart over a period of time are called Electrocardiography (Kumar 2010).

Figure 1.

ECG waveform

The electrical activity of heart is found using electrodes placed in patient’s body. Normally in a 12 lead ECG, ten electrodes are placed on the patient’s limbs and on the surface of the chest (shown in Figure 2) and the magnitude of electrical activity from twelve leads are recorded over 10 seconds (Lee and Chung 2009, Engel 2010). This non-invasive procedure of obtaining magnitude and direction of electrical activity is called cardiac cycle.

Figure 2.

Placement of electrodes to measure ECG signal

2. Principle Behind Ppg Sensor

The photoplethysmogram sensor consist of an infrared light source (typically a photodiode emitting light at a wavelength of around 900 nm) and a photo detector (phototransistor) as shown in Figure 3. The light source is used to illuminate the tissue (e.g. skin) and a photo detector is used to measure the small variations in light intensity associated with changes in the blood vessels volume. It gives a voltage signal, which is proportional to the amount of blood present in the blood vessels. This type of sensors gives only a relative measurement of the blood volumetric changes and it cannot quantify the amount of blood (Tamura 2014).

Figure 3.

PPG sensor