For a project, I need a transparent ball that is rigid, but under application of a voltage changes it's size, like a self-inflating balloon. Because it's rigid, however, and thus would break when expanded manually, I need to find a material that expands itself when the voltage is applied. I though about heating and about piezoelectric materials, but the changes are always too small, staying within a few milimeters of the original size. I've heard of electroactive polymers, but those aren't rigid. Shape memory polymers also seem to be an option, but I don't really know how well they work, and where to get them, etc. So, if anyone knows a material that can expand this way, or if someone finds a material that expands well enough when heated, please respond.