Knowing about Sensor Film Kit
It's called a force sensing resistor. It's basically a film of small conductive particles mixed into a non-conductive polymer matrix on top of a conductive substrate. The more force on the film, the more conductive particles can touch the layer underneath, reducing the resistance. Sensitronics owns the original patent for the technology and their site shows how they're constructed, though you'll probably have to dig up the patent to find out exactly what's in the force sensing layer.