Diseases related to your heart and blood vessels are called Cardiovascular diseases (CVD), and major risk factor for CVD is Hypertension or High Blood pressure. It means, if your blood pressure is not normal, you have a high probability of having a CVD. Hence measuring your blood pressure regularly is very important.
When your heart pumps blood, a pressure wave originating from the heart traverse along the arterial walls. As these pressure waves propagate, the volume of blood in these artery pulsates and a PPG sensor records these pulsations using light. Researchers extract features from these pulsations to estimate blood pressure and one of the most common features often used is Pulse Transit Time aka PTT. (FYI: Pulse Transit Time is the time taken by the pressure wave to traverse the arterial network).
We measure PTT by calculating the time shift between PPG signals recorded from two different arterial locations. To prevent motion artifact in the recorded data, sensors are clipped or wrapped on to the body surface. These sensors compress the artery beneath the skin and this compression pressure is referred to as contact pressure. We observed that the contact pressure of the PPG sensor changes PTT significantly, and the PTT versus contact pressure curve has a U shape with a maximum variation of 22 ms. These variations in PTT cannot be ignored as it translates to a BP error of 11 mmHg. Please refer to our paper for details.