Despite fluoropolymers having an extremely low coefficient of friction, the material can wear quite easily. By adding a 'filler' to the fluoropolymer you can change the wear resistance and conductivity, without greatly affecting the lubricious surface.
Carbon and graphite fillers offer better wear resistance and thermal conductivity; it does also increase the anti-static properties.
Anti-static has been designed with much smaller coal particles than with the standard carbon or graphite fillers and so the material becomes conductive, making it the best solution for anti-static applications.
Ceramic increases the wear resistance.
Bismuth Trioxide is specifically used for medical applications. Once this filler has been added to the fluoropolymer, the material is then visible to X-Ray - This is also sometimes referred to as radio-opaque tubing. This can be applied as a solid filler or as a stripe along the length of the tube.