Hi! I just have posted my living hinge advice, but then saw, that I had answered to 2013, so I guess it might be good to say it again, up to date:
I have printed a layer of 0.4 mm soft PLA, then changed to normal PLA, continuing with two objects, between them a gap, and on both sides of the gap a 45 degree slant. The gaps of my test objects were from 0.2 up to 0.5 mm wide, which all worked well (I guess, 0.5 mm lasts longer, and 0.2 mm provides less play, so maybe 0.35 mm might be the optimum). Due to the slants, you can bend them, apart from 180 degrees to one side, also 90 degrees to the other.
Using soft PLA for the ground layer, I needed a printer (Prusa portal) with the drive before the nozzle. The Prusa Mini with the Bowden supply I had used first, obstructed. Also, next time I will use a structured plate, because the soft PLA stuck so well to the smooth plate I had used, that I had to use a knife to detach it, and a flake of varnish came off the plate. But the connection between the soft and the normal PLA lasted.
I think, this is a good way of making prototypes of something you later want to injection mold with PP. But they also seem to last quite many bending cycles as they are.
Best regards, Elmue