Flexible Filament or Resin Safe for Use in Mouth

[OvernightMadness]

Hi!
I'm new to this and educating myself quickly over the last few weeks. I want to prototype an idea I have for something that would fit in a person's mouth. I'd need it to be somewhat flexible or rubber-like but obviously also biocompatible (non-toxic, non-latex, non-bpa etc). I'm not sure what people who print mouhtguards make them out of now and I haven't yet found a lot of specific info online. I turn to you.

I'm not yet sure what kind of machine I'm going to be working with. It seems most companies that do dental work are using SLA machines but I'll go a cheaper route if I can initially. There's going to be a lot of iterating for a while. So I'm going to determine the most appropriate and easy to use/modify end-material and work backwards to the printer type and ultimate purchase decision. Is there a flaw in that logic?

Thank you for any and all input you may have!
Philip