Here is the HydroSpan description:
HydroSpan 100 is a unique two component, Flexible, 100% solids (contains no V.O.C.), polyurethane elastomeric compound. HydroSpan 100, when fully cured provides a simple method for expanding three dimensionally molded parts. Expanding parts is done by simply molding the part with HydroSpan 100 to from a rubber master. After curing 24 hours at room temperature the molded HydroSpan 100 part is soaked in room temperature water and allowed to expand. Full expansion is 161% larger than the original part. Full expansion of parts can be achieved in 5 to 14 days depending on cross section thickness of the part. Thicker cross sections take longer while thinner cross sections require less time.
The video of polymer being used:
So how is this connected to 3d printing? Imagine if we had similar material for 3d printed objects? You print them out, leave them in water and they grow several times. Hopefully someone will develop this concept :-) You have seen it mentioned here first :-) !