All of these plugs are solid on one end and hollow on the inside.
0g through 5/8" have a 9mm wearable area. 3/4" through 1" have a 10mm wearable area.
Parylene coating is a process that is widely used in the medical industry to provide silicone with a low-friction surface. What this means is instead of the eyelet having a rubbery surface, it now has a smooth, almost slick finish. It has been determined that this coating effectively eliminates mechanically-induced sensitivities related to the tacky surface texture of uncoated silicone body jewelry. The coating also makes the silicone slightly firmer, which aids in insertion.