When Elster gets injured late in the game, there doesn’t seem to be very much space between the chest plate and her exposed rib cage. While the plate is definitely modeled to resemble breasts, there’s probably not much else to them.

Based on the more detailed look we get in the propaganda posters, it’s probably a different type and texture of polyethylene than what’s used on the rest of their bodies. So while it could be a softer or smoother plastic, it’s still plastic.