Photoplethysmography (PPG) and Pulse Oximetry (SpO2) are carried out by a single sensor which is used to measure reflectance of two LEDs of different wavelengths. SpO2 is found by measuring the reflectance of 650 nm light in comparison with 940 nm light. We are currently using the Maxim Semiconductors MAX30100 to reduce the size, cost. The sensor has an integrated amplifier, Analog to Digital Converter (ADC) and inbuilt LED's and LED drives (940nm,650nm). The price takes the cake for this part (3.31$ for a qty of 100)

Neat eh? Guess we have the Smart watch revolution for this. To get started I planned to originally get the MAX30100 Click board from Microe but it was too big for what I had in mind. I eventually bumped into a cheap small MAX30100 breakout board.

Oxullo interscans made a neat library acquiring some data from the MAX30100, After giving it a whirl. I got it ported to the Particle photon you can get it here.
I had it compared against a Nellcor NBP-290 commercial medical grade pulse ox sensor.

The data was staggeringly accurate, Fantastic what we can accomplish in a few years as an industry.