I love this idea. I think it is a good idea to start with Berlin- a city that is very focused on sustainability and is, relative to other cities of the world, far along in green efforts. Each city I think would need it's own customized sponge city solution- and I think it would start with looking at the soil make up of the surrounding area. Nature is the ultimate designer - so it would be interesting to try to mimic the layers of soil found in nature with the materials used in the city. I would think in a city like San Francisco, there may be some issue of how the material would stand to common natural disasters , like earthquakes or fires. As climate change progresses- we see more intense and frequent natural disasters- I think it would be interesting to explore if this material be something that you could pitch to local government to be a better alternative to current materials due to potential $$ saved.
In the spirit of circular innovation- it would also be interesting to explore if there is a way to loop water collected back into homes or perhaps to city gardens to further the sustainability concept.