Looking for circuit board material that can be dissolved
Researchers from the National Physical Laboratory (NPL), in London, in
  cooperation with partners In2Teck Ltd and Gwent Electronic Materials
  Ltd, have developed a 3D printable circuit board that separates into
  individual components when immersed in hot water.  The goal of the
  ReUSE project was to increase the recyclability of electronic
  assemblies in order to reduce the ever-increasing amount of electronic
  waste.



Source: http://environmentaltestanddesign.com/dissolvable-printed-circuit-board-recycled-with-hot-water/

If that doesn't work, nitric acid will work on just about everything.

Oh, if you wanted to 'roll your own' manufacturing process, you could find a dissolveable material (maybe a some kind of cellulose?) and print on it with on of these PCB conductive ink printers: https://www.voltera.io/

As per Edgar Browns suggestion,  also this idea for dissolving polyimide for flat flex:


  Try a mixture of Methanol:THF=1:1 , but it will take 1-2 days; The
  easiest way to dissolve Kapton - is to use 0.1-0.3M NaOH in water. By
  using alkaline solutions  you can completely decompose the Kapton -
  down to initial monomers.


https://www.researchgate.net/post/can_polyimide_filmskapton_dissolved

NaOH is lye, I don't know in what concentration you would have to have to get kapton to dissolve but that seems like it would be easy to experiment with.