TPU filament usage for dental application?
There are a Ton of issues with using this type of material. First off there is no way anything that comes out of your FDM printer will be sanitary enough to meet the requirements for medical applications. Your nozzle might even be contain Lead. Last you would need this printer to be cleaned to clean room standards. As well as stored and operated in one..

Second I have not heard of a flexible material that meets FDA requirements. (which is not the same as does not) You have to worry about even the additives they add for color. Or straight up contamination of the material. 

I do want to mention that there are people who use SLA printers for dental items. I cannot say how SLA would meet food or medical grade requirements. My guess is a lack of regulation. 

So a bad idea I would say...