I am trying to find a polymer that can be expanded against pressure, as a result of piezoelectric or other mechanism; which can be manipulated and is expandable after applying some pressure between the fingers.
PVDF = poly vinylidene fluoride. This from wikipedia.
PVC also supposedly.
I doubt either is as efficient as some of the ceramic materials. None of the materials will expand very much without hazardous levels of voltage for an object being held.