Greetings From UCLA ECE 202A Healthy Alexa

PPG and Tissue Oxygenation Board Development &

Data Collection and Processing

• MAX30101 the chip used for heart rate and PPG readings. The library developed by the Maxim integrated was not very well developed. Trough one Mbed source code the working library was implemented. However, the readings are not appealing (figure 2).

• The heart rate sensor on the Hexiwear was interesting enough for me to try to make the sensor myself and improve it further since by looking at the data and the drift, I am not satisfying that this sensor is reaching its full potential. Curious about the fact that some smartwatches use the green light, but some use the red light was a bugging question. Additionally, being constrained to put the device tightly on hand to perform the measurements and with the readings to be very different, there is a potential to develop a wearable system to not only measure the blood oxygenation, but also to measure the tissue oxygenation, and with future directions that we will introduce, it can be made into a smart system to detect were on the body it is placed and adjust the measurements.

• PPG (photoplethysmogram) - uses optical sensors to measure changes in volume within an organ or whole body [16, 17] (figure 1A).

• Modified Beer-Lambert’s law helped calculating light attenuation (A) which accounts for scattering (G) But given that scattering remains constant, we can eliminate G by using different light attenuation (aka optical densities) and subtracting to find the difference.

• LEDs are placed at distances away from the photodetector to measure different depths in the tissue. Then, light is shined towards the blood vessel and photodetector looks at a light intensity from blood vessel: “Light from the diode enters the tissue, is reflected by deeper structures, and 'backlights' superficial blood vessels. Then, as superficial blood vessels fill with more blood, they absorb more light returning from the deeper tissues, and photodetector light intensity diminishes” [18]. 

• “Of the two or more wavelengths measured by the pulse oximeter, traditionally only the infrared signal (approximately 940 nm) is presented. The information from this wavelength is displayed because it is more stable over time, especially when compared to the red signal (660 nm), which is more susceptible to changes in the oxygen saturation.” [19]

• “The shorter wavelengths of light are strongly absorbed by melanin. Water absorbs light in the ultraviolet and longer IR regime; however, red and near-IR light pass easily.” -- most absorption occurs in blue region [20]

• Green-yellow (500-600 nm) - good for superficial blood flow, “exhibits large modulation depth”, “A green LED has much greater absorptivity for both oxyhemoglobin and deoxyhemoglobin compared to infrared light.” (but also green light is easily absorbed by body) (figure 3).

PPG and Tissue Oxygenation Board Development &

Data Collection and Processing

​​

• “Hemoglobin absorbs green light better than red and green light penetrates tissue to a deeper level than blue light”

• Red-near-infrared (650 - 1100 nm) - good for deep tissue blood flow, transparent to melanin [21]

• The most suitable LED wavelengths will be in the range between 650nm- 700nm
  (minimal absorption of HbO2) and 850nm – 930nm (minimal absorption of HHb) (figure 4).

• There are two models to measure PPG:

1. Transmissive (through the finger)

2. Reflective

• PPG sensors

  • “use electrical signals derived from light reflected due to changes in blood flow” [23]

  • “require a relatively long settling time due to the need for measuring the amount of ambient light and calculating the compensation needed for canceling its effect.”

  • “may also require compensation of motion artifacts to produce a reading,” so employing signal processing techniques such is a 5th order low pass filter, moving average, and denoising with multiple photodetector readings helped me with getting clean output signals (figures 5, 6). It is also important to measure the signal on areas with maximal PPG signals (figure 7).

• AC & DC components of the readings represent the following:

  • AC - pulsating component based on pulse

  • DC - static component created “mostly by the absorption of light by surrounding tissue”, then an auto-centering algorithm is used to eliminate this (figure 8).

• Additionally, to confirm the readings, we found that research on reflection PPG to measure blood flow changes in human patellar bone (31) shows a linear correlation between change in pulsatile pressure & change in PPG signal.