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 tubing 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.