Health and safety guide

There is a lot of discussion about the safety of 3d printed mouthpieces. When you intend to print your own mouthpiece and intend to use it frequently during a long time. It is wise to make the right choice considering the material and to adapt a proper cleaning procedure.

Biocompatibility

It is important that you product is biocompatible. This means that a product is safe in contact with the human body. This means it is non-toxic and does not cause allergic reactions. Biocompatibility is required for medical implants an also important for mouthpieces for musical instruments. Another important characteristic is biodegradability. This has to do with the environmental impact. This is described in the environmental guide.

Materials

Hard rubber

Conventional mouthpieces are made from hard rubber. 
Ebonite is created by vulcanizing natural rubber over a long period of time. As a result, it contains 25% to 80% sulfur. The name refers to its use as a substitute for ebony.
The strength properties are achieved with a sulfur content of 30% to 35%. 
Hard rubber oxidizes under the influence of daylight. When combined with moisture, sulfates form on the surface, causing it to discolor brown and acquire a sour, sulfurous taste.
Sulfates are found in all kinds of natural products. Too much sulfate can cause irritation. Therefore, it is recommended to clean browned mouthpieces. This can be done by polishing the exterior.

3D printed resin 

SLA or resin printers is a form of 3D printing technology used for creating parts in a layer by layer fashion using photochemical processes by which light causes chemical monomers and oligomers to cross-link together to form polymers. With SLA it is possible to maken very accurate mouthpieces. However uncured resins are highly toxic, and in practice, resin printed products will often contain minimal amounts of uncured resin. These can cause allergic reactions by the users.

3D printed filament

FDM is a 3D printing process that uses a continuous filament of a thermoplastic material to create parts in a layer by layer fashion. 3D-printed mouthpieces made of ABS or PLA are made from food-safe materials. They are therefore biocompatible after production. This requirement does limit the use of certain colors and additives.   Therefore, it is important to pay attention to the filament's certification.
Due to the nature of the FDM printing process, small gaps can develop between the layers. In theory this could potentially allow saliva to enter these gaps, creating a breeding ground for bacteria. This is independent of the filament material. 
As a remedy, mouthpieces are coated to prevent saliva from entering the mouthpiece. Every mouthpiece, regardless of the material used, is susceptible to the development of bacteria. Therefore it is interesting to read the article from Stephan Howard on this subject.

In summary, studies show that all mouthpieces contain viable bacterial colonies after playing. It also appears that 100% sterilizing a mouthpiece after playing is not easy. The article recommends cleaning the mouthpiece after playing with at least lukewarm water and dish soap, common disinfectants, or alcohol. He cites this old but very interesting article:
W.G. Walter & Dorothy Chaffey (.pdf file)  "Bacteriological and Cleaning Studies on the Mouthpieces of Musical Instruments": Department of Botany and Bacteriology, Montana State College, Bozeman, Montana - 1958