Flexible circuits (also known as flex circuits and flexible printed circuit boards) are made up of copper routing layers bonded to a thin insulating polymer sheet and covered with a thin polymer coating to protect the conductor circuits.

High-temperature resistance, superior chemical resistance, great radiation and UV resistance, and the ability to regulate impedances in high-density circuit board designs are all features of flex PCBs. 

Ordering a flex PCB is very similar to ordering a standard FR4 PCB. You must specify the size, number of layers, total thickness, surface finish, etc. You also get to specify if you want stiffeners for insertion into a ZIF connector or to help prevent bends under delicate components, or you can specify if you want double sided tape applied to the hole backside or in certain sections (Standard: 3M/Tesa 8853/8854).