Sensors science and engineering is relevant to virtually all aspects of life including safety, security, surveillance, monitoring, and awareness in general. Sensors are central to industrial applications being used for process control, monitoring, and safety. Sensors are also central to medicine being used for diagnostics, monitoring, critical care, and public health. Sensors have been around for quite some time in various forms. The first thermostat came to market in 1883, and many consider this the first modern, manmade sensor. Hertzman and colleagues first introduced the term “photoplethysmography” in the 1930s and suggested that it represented the volumetric changes (“plethysmo” means “enlargement” in Greek) in the dermal vasculature.
Photoplethysmography (PPG) sensor is a non-invasive technology used to detect volumetric changes in blood in peripheral circulation at the surface of the skin. PPG sensor can be divided into 4 groups: single-point contact PPG, multi-point contact PPG, close/contact IPPG and noncontact IPPG. In this paper, I will concenrate on noncontact PPG sensor which is a wide range of commercially available medical devices for measuring oxygen saturation, blood pressure and cardiac output, assessing autonomic function and also detecting peripheral vascular disease.
PPG makes uses of low-intensity infrared (IR) light. When light travels through biological tissues it is absorbed by bones, skin pigments and both venous and arterial blood. Since light is more strongly absorbed by blood than the surrounding tissues, the changes in blood flow can be detected by PPG sensors as changes in the intensity of light. The voltage signal from PPG is proportional to the quantity of blood flowing through the blood vessels.
Even small changes in blood volume can be detected using this method, though it cannot be used to quantify the amount of blood. The PPG waveform comprises a pulsatile (‘AC’) physiological waveform attributed to cardiac synchronous changes in the blood volume with each heartbeat, and is superimposed on a slowly varying (‘DC’) baseline with various lower frequency components attributed to respiration, sympathetic nervous system activity and thermoregulation. (See Annex 1a, b) The design enables significant miniaturization of traditional oscillometric devices without the need for occlusive circumferential pressures. PPG sensor converts input into output unit of mmHg.
A significant aging portion among world population is causing rapid rise in healthcare costs. The advancement of PPG sensor technology has been contributing its role to tackle such problems by eliminating hospitalization, monitoring physiological parameters continuously and so on. During the implementation of the wearable BP portable devices, PPG sensor technology has encountered 1. technical challenges more:
Currently, wearable devices require high power consumption. There is a need to extend battery life or consume lower power to determine health information.
A PPG sensor can only monitor the dynamic change of the blood volume at one site/point per probe. Multi-sensor PPG systems have been developed to monitor blood volume changes at different sites simultaneously. However, multiple sensing locations significantly constrain the daily activities of the subject. Moreover, the use of multiple sensors will make the subject uncomfortable and constrain the feasibility for everyday use.
Monitoring sensor suffers from motion artifacts because the system shares the ground with the human body of which the ground is floating and sensitive to motion. Since the artifacts from the floating ground are common mode inputs to the front-end amplifier, the artifacts can be easily rejected at the output of the amplifier. Wrist-worn devices should be taken more special attention in terms of motion artifacts because the wrist is one of the most frequent-moving organs in the human body.
Different skin tones absorb light differently, and each skin tone will thus be characterized by a different absorption spectrogram. This means that the intensity and wavelength of light that is captured by the PPG sensor will depend on the skin tone of the person wearing the sensor. For example, darker skin absorbs more green light.
The system will not be marketable or acceptable for users if the device is good but very expensive. The device, HeartGuide released on 8th of January 2019 costs $499. For those who are in the poverty, it is not affordable.
It is sometimes difficult to compare one device to another. It would be useful to create a way to set metrics or validate devices (without needing to get FDA or government certification). The medical industry is generally biased towards maximizing performance and cost, but in the lowcost consumer health market, it is necessary to determine the appropriate level of performance.
Since the PPG sensor was introduced in 1930s, it has passed almost 80 years. During this period many scientists did research and improvement on PPG sensor. In more recent decades the desire for small, reliable, low-cost and simple-to-use non-invasive (cardiovascular) assessment techniques are key factors that have helped re-establish photoplethysmography. Advances in opto-electronics and clinical instrumentation have also signi?cantly contributed to its advancement. The developments in semiconductor technology, i.e. light emitting diodes (LED), photodiodes and phototransistors, have made considerable improvements in the size, sensitivity, reliability and reproducibility of PPG probe design. A major advance in the clinical use of a PPG-based technology came with the introduction of the pulse oximeter as a non-invasive method for monitoring patients’ arterial oxygen saturation. There have also been considerable developments from computer-based digital signal processing to smart phone via bluetooth and pulse wave analysis.
To sum up from the above PPG sensor is in the mature level of the “S” curve.
The Global Optical Sensor market was valued at USD 1,493.9 million in 2018, recording a CAGR of 16.0%, over the forecast period of 2019-2024.
Currently, there are two new competing wearable devices, Heartguide by Omron Corp (Kyoto, Japan) and Blood pressure monitor by Echolabs (Englewoor, CO, USA) on the market.
The Heartguide has an extra-stiff band that actually inflates to take an oscillometric measurement in the same way as a normal doctor’s blood pressure cuff. The watch takes manual readings and spot heart rate measurements but can also be programmed to take night readings to test for hypertension and the risk of stroke while sleeping. The watch last 10 to 14 days on a single charge, which gives convenience to elderly patients.
The technology of Blood pressure monitor leverages spectroscopy to analyze blood, using transmitters to send light and other electromagnetic frequencies to the skin and measuring the light that reflects back. The reflectance is not only from skin. When an electromagnetic wave is beamed into the tissue, the light is reflected from every element it hits: the arteries, skin, bone, and muscle. This system is said to focus on accuracy. Both of those technology have pros and cons.
In health and wellness sector, major players of the optical sensors are ROHM Co. Ltd, Rockwell Automation Inc, SICK AG, Vishay Intertechnology Inc and Honeywell International Inc.
Technology is invented day by day to meet customers’ need in every field. In the fast movement life society, this technology not only save our time, but also keep us from becoming a victim of the “Silent Killer”! (Stroke and Heart attack). Among competitions of companies to introduce new technologies (products), customers and whole society get benefits of obtaining precise measurement of blood pressure easily, preventing stroke and giving chance to be human wellbeing for ages. Once you investing in home monitor can take away a lot of stress and convenience. Weareble BP monitor will be big investment not only for those who need to monitor his/her blood pressure continuously but also hospitals.
PPG sensor is the one of the basic technology in its mature cycle to make wearable devices for any size of company because of it low cost of utility.
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