Flexible printed circuits are useful when the size and shape of your invention is integral to its overall design. Very many consumer electronics gadgets have them inside: in mobile phones, cameras, etc. Tips for Building a Flex Circuit article tells that because of the unique characteristics that set them apart, an electronics engineer must consider additional factors when designing these circuits: the degree of bending, the tightness of the bend, how the bend will be formed, and how frequently the circuit bends. Tips for Building a Flex Circuit article tells the key factors to consider when building a flex circuit board.
Basics of Flex Circuit Design article tells that reliability, designers must create flex circuits that are neither too thick nor bend too much. The neutral-bend axis is a key concept in flexible-circuit design: tension or compression on the outer layers of the circuit depend on their distance from this neutral-bend axis and the radius of the bend. A circuit should flex no more than necessary to handle the goals of the design. In general, 90° is considered the maximum angle any circuit should bend. Avoid unnecessary thickness. Tight bends conductor traces boost the risk of circuit damage. Small conductors of less than 10 mils can tolerate compression better than stretching. If you use surface-mount (SMT) components, be aware of their special requirements. Avoid “discontinuities” in the bend area. Copper plating is less ductile than rolled, annealed copper, making it more susceptible to cracking when flexed.
Hackaday.com lists many flex circuit projects that could be worth to check out or DIY hardware hackers. They prove that flex circuits can be successfully build even on home laboratories.