The liquid silicone they use in TJ is like hyaluronic acids, in that the filler itself is the added volume. I think it\'s mainly more popular in a predominantly gay sub culture known as \"juicing.\" There are a few yahoo groups devoted to it. It\'s used in large quantities and doesn\'t feel or look natural as the material is to viscous.
With PMMA the volume comes from collagen forming around the beads. So the new tissue isn\'t what is initially injected, but a reaction to what is injected.