Photocapacitance plethysmography (PPG) is a simple optical technique used to detect changes in the volume of blood in the peripheral circulation. This is a low-cost and non-invasive method that can be measured on the surface of the skin. This technology provides valuable information related to our cardiovascular system. The latest developments in technology have revived people's interest in this technology, which is widely used for clinical physiological measurements and monitoring. PPG Sensor 1. Principle

PPG uses low intensity infrared (IR) light. When light passes through biological tissues, it is absorbed by bones, skin pigments, and venous and arterial blood. Due to the stronger absorption of light by blood compared to surrounding tissues, PPG sensors can detect changes in blood flow as changes in light intensity. The voltage signal from PPG is proportional to the amount of blood flowing through the blood vessels. Using this method can even detect small changes in blood volume, although it cannot be used to quantify blood volume. The PPG signal has several components, including changes in arterial blood volume related to cardiac activity, changes in venous blood volume that regulate the PPG signal, DC components that display tissue optical properties, and subtle energy changes in the body. Some of the main factors affecting PPG recording are the measurement position and the contact force between the position and the sensor. Blood flow changes mainly occur in arteries rather than veins.

PPG sensor II. Waveform

PPG displays blood flow changes as waveforms using bar charts or graphs. This waveform has both AC and DC components. The AC component corresponds to the blood volume change synchronized with the heartbeat. The DC component comes from the light signal reflected or transmitted by the tissue and is determined by the tissue structure and venous and arterial blood volume. The DC component shows slight changes with respiration. The fundamental frequency of the AC component varies with heart rate and is superimposed on the DC baseline.

PPG Sensor 3. Product Usage

Medical devices based on PPG technology are widely used in various clinical settings. Specific applications include: clinical physiological monitoring, blood oxygen saturation, blood pressure, cardiac output, heart rate, respiration, vascular assessment, arterial disease, arterial compliance and aging, venous assessment, endothelial function, microvascular flow, vasospasm status, autonomic function monitoring, vasomotor function and temperature regulation, blood pressure and heart rate variability, and upright other cardiovascular variability assessment.

PPG sensor IV, wearable devices

A wearable pulse rate monitor has been developed using this technology. These low-cost and small devices have high-strength green light emitting diodes (LEDs) and photodetectors, which help to reliably monitor pulse rate in a non-invasive manner. The important design requirements for these systems include miniaturization, robustness, and user friendliness. These devices have a sensor that can monitor small changes in the intensity of light reflected through or from tissues. These intensity changes are related to changes in blood flow through tissues and provide important cardiovascular information, such as pulse rate.

PPG sensor 5. Other systems

PPG has been used in other technologies such as telemedicine, PPG imaging technology, and remote monitoring. Researchers used a near-infrared CCD PPG imaging system to study skin blood flow and related rhythms. This study aims to gain new insights into biological tissue perfusion and investigate changes related to wound healing and ulcer formation. Other studies have applied PPG to remote imaging of arterial oxygen saturation (SpO2) distribution within tissues. Such images may be valuable in medical diagnostic research, such as quantifying tissue vitality. In the field of telemedicine, PPG has great prospects in remotely monitoring patient health.