Dear Gavbrum, have you considered FilaFlex filament? This is similar to NinjaFlex and has similar properties to silicone in that it is tough and flexible. We have stocks of the filament and by late next week we will have a printer specifically designed to print it. When we ordered our flexible filament stock, they sent us a watch printed in the material, so I can even send you a photo of it.
Yes. It has a certificate for use in medical situations, visit the Recreus web site to download it. The material seems really tough and the bond between layers is strong, so I would expect it to be good for these applications. The problem with the material is that it is difficult to print using a standard print head, that is why we have invested in a custom made machine designed to use it.