The MAX30110, the MAX86140 and the MAX86141 can be used to measure SpO2 by transmissive means.
There are two methods of pulse oximetry measurement. The first is when certain wavelengths of light are transmitted through, say, a fingertip or earlobe, and the amount of light absorption changes caused by arterial blood flow pulsations is detected and measured on the other side. This is "transmissive" pulse oximetry. The other method of pulse oximetry is called "reflectance". This method has the light transmitter and receiver in the same plane.
While most of Maxim's first generation of pulse oximetry devices are designed with reflective pulse oximetry in mind, the MAX30110 is the better choice for transmissive designs. For Maxim's second generation of pulse oximetry devices with ultra-low power consumption, the MAX86140 and MAX86141 are well-suited for transmissive designs.