Flexible Circuits are made from materials allowing for a non-planar positioning within the end product. Typical materials are polyimide-based, and can go under trade names such as Kapton (DuPont).
Often flexible circuits are used for applications that have repeated flex cycles, such as cellular phones, doors, etc. A growing application for flexible circuits are their use as a connector in LED-based lighting systems. Often, there are two components: a metal substrate LED PCB; an FR-4 based digital driver PCB. These two devices are often connected with a high profile and sometimes costly connector cable; however, a flexible PCB in its place provides a lower profile, slimmer design, and higher reliability.