Origami.

The simplest way would be a flat, thin piece of triangular metal or plastic that folds several times along its length (along pre-machined lines), to form a somewhat stiff skewer. It will be more of a thrust than a cut-and thrust sword, think small sword or even rapier. The actual tip is manufactured from a hardened steel. The hilt provides a small mechanism that does the folding when drawn. 

Such a weapon will be significantly weaker, structurally, than a solid sword, especially against blows against one of the flat sides of the material. It's conceivable that such a sword could be drawn and resheathed many times before material fatigue sets in, but it won't survive many fights.