This whole issue should be mentioned in the article, just in case anybody ignorant wants to do this with unsuitable materials.

Nice. The only other risk then would be interaction or contamination between the retainer material and the printed mold during the forming process (high heat), but that's probably grasping at straws.

The SDS for the retainer material in case anyone's interested:

http://dental.keystoneindustries.com/wp-content/uploads/2016...

The toxicology section is entirely devoid of data, but I doubt that means anything since the product is purpose-built for dental use in the first place. Safety Data Sheets are more oriented towards immediate and occupational hazards anyways.

Taulman Nylon 680 FDA would probably be suitable too, for anyone using a more generic 3D printer. :)