Today, with the technology to produce a variety of synthetic fibers, synthetic brushes are now the choice for gel or many applications, including fine art.
The word nylon is often used to generically represent synthetic fibers. In artist brushes this synthetic fiber is actually polyester. "Taklon" is a trade name commonly used. Each filament is tapered, ending in a fine point. Often the filaments is dyed, to make it softer and more absorbent.