I use the MAX30102, an integrated pulse oximeter and heart rate monitor, as the biosensor. It can also be used to measure body temperature. Since the sensor has to lay very well on the skin, I cast it in 2-component silicone. After some experiments, it is possible to perform the measurements on the temple. Since the integrated LEDs can be switched on and off in real-time via the software and you can also change their brightness, machine learning can be applied.
Since we measure temperature at the temple, we need to convert it to body temperature. This chart from here is very useful for that.
To get a corresponding function, I entered the values in an Excel sheet, created a scatter plot, and chose a second-degree polynomial as a trend line. Excel then outputs the function automatically.