In the guest lecture by John Meador, a device was described which uses an LED emitter and a pair of detectors, each mounted looking out the side of a 1mm thick catheter. This same technique can be applied to the measurement of skin color. There are many different applications for chemical sensors, and many techniques which are to be applied for any given application. We might have spent the whole quarter looking at this field in detail, but there wasn't time. In general, chemical sensing devices do not compare favorably to biologically-developed detectors.
Devices generally suffer from a lack of sensitivity, selectivity, and speed.
For some applications, the signals of interest are large and easy to detect. For others, it is very tough going. For research in this area, such difficulties represent job security.