The respiration monitor electronics are built into a clear ultra-thin custom retainer. This retainer keeps the sensors and other electronics correctly positioned and also acts as a protective barrier between the electronics and the sensitive skin inside the user's mouth. The first step in creating this retainer is a negative impression of the upper teeth using dental alginate. 

Materials and workspace

The finished negative impression

The fragile alginate negative impression is used as a mold to create a tough heat resistant positive impression. The positive impression will be used to form the specialized dental thermoplastic sheets into ultra-thin ultra-clear retainers. I use Bondo because I had some in my apartment and it is very easy to use. Heat resistant casting resins or silicone rubbers could probably also be used though they will have to be pretty tough.

Use a thin rod ( I used a 1/8" diameter barbecue skewer ) to poke out any air pockets deep down in the individual teeth cavities. Major air pockets anywhere in the impression will cause problems later when it is heated to 230 degrees C. I did two batches to Bondo, the first to fill the teeth and the second to create a structural base. Bondo adheres to itself very well, especially if it hasn't cured yet.

The finished positive impression. I used a Dremel cutting tool to trim down the impression as much as possible. This is important because the dental thermoform sheets must be able to curve around the impression as if it were a real part of your body. Gaps and holes are inevitable with this crude process. I use epoxy to fill in any obvious air pockets.