Today, being physically fit has become a way of life for many. Eating healthy foods, sleeping seven to eight hours a night, and exercising daily are the basic building blocks. And with the use of an activity tracking device, we can monitor our daily progress. We can easily track how many steps we take, how many calories we burn, and how well we sleep with the aid of wearable activity trackers. Wearable sensors collect, process, and display a set of personal data to help monitor and manage all aspects of personal health. Wearable devices can measure a wide variety of body functions, including blood pressure, heart rate, and the level of oxygen in our blood. Optoelectronic components play a key role in measuring these attributes.
Heart Rate Monitoring
Heart rate is important in determining and improving your level of personal fitness. Once you know your maximum heart rate, you can calculate your desired target heart rate zone — the level at which your heart is being exercised and conditioned but not overworked. You’ll get the most from your workouts if you’re exercising at the proper exercise intensity. There are two methods used to monitor heart rate: electrocardiogram (EKG or ECG) and photoplethysmogram (PPG).
An EKG measures the electrical activity of your heart. With each beat, an electrical impulse (or “wave”) travels through the heart. This wave causes the muscle to squeeze and pump blood from the heart. While it is an ideal tool in a hospital or your doctor’s office, it is not ideal for monitoring your daily activity. For this we turn to the photoplethysmogram.
A PPG optically detects the blood volume change in tissue. A PPG sensing system consists of a light source and a photodetector. The light source can be either a single green LED, or a red and infrared LED. The PPG sensor emits light and then a photodetector measures the light intensity that is either reflected off the tissue or transmitted through the tissue. The change in light intensity is associated with small variations in blood perfusion of the tissue and provides information on the cardiovascular system; in particular, the pulse rate. Until activity trackers or wearables came along, the most common device to measure heart rate was a pulse oximeter. It is mainly found in the doctor’s office and is based on transmission of light through the finger. Wearables are based on reflection of light off the skin.
Monitoring Blood Oxygen Level
As the name implies, a pulse oximeter performs two functions; one, provides a pulse rate and, two, measures the level of oxygen in the blood stream. Monitoring the oxygen level is important for patients who have a risk of respiratory failure such as asthma, athletes during strenuous workouts, and anyone exercising at high-altitudes. Oximetry is a noninvasive measurement and monitoring technique to estimate oxygen saturation in blood. Oxidized hemoglobin (HbO2), a protein which is bound to the red blood cells, absorbs light differently when compared to reduced hemoglobin (Hb). Hemoglobin has a higher optical absorption in the red region of the spectrum, around 660 nm, compared with oxidized hemoglobin. In the near-infrared region of the spectrum, around 940 nm, the optical absorption by hemoglobin is much lower if compared to oxidized hemoglobin. By measuring the light transmitted through the tissue or reflected back at two different wavelengths, the oxygen saturation of the blood can be determined.
Designing a Sensor for Heart Rate and Blood Oxygen Measurements
As mentioned earlier, wearable readings are based on reflected light. This means that the emitter and photodetector are mounted on the same PCB. It is critical that the photodetector only “see” the reflected light; light from the emitted signal will corrupt the measurement. Designers can use mechanical barriers to optically isolate the emitter from the detector or they can use a FAM package which has built in side-barrier blockage.
The performance characteristics of the photodetector are critical in designing a sensor for heart rate and blood oxygen measurements. Wide viewing angles of 130°, response times of at least 100 ns, linear output current regardless of reflected intensity, and sensitivity from 430 nm to 1100 nm, which is an ideal pairing to the green LED signal of 525 nm or for the dual emitter sensors using red at 660 nm and infrared at 940 nm. Finally, high-efficiency LEDs with narrow spectral bandwidth, high radiant power, 10-ns rise and fall times, and good thermal stability are required for heart rate and blood oxygen measurements.
Article by Elena Poklonskaya
as appeared in THE QUINTESSENCE an EBV Elektronik publication