My Synthetic Sable brushes use modestly springy synthetic filaments. They are my 2nd best selling brush. They are very responsive and a delight to work with, and are excellent for water based paint and ink media plus solvent based media as long is the media is not tooooo viscous. They are great for medium body acrylics and enamels.
The synthetic filaments are both textured and taper to make a hair-fine tip. because they are synthetic they are largely impervious to wear.