Cluster Fiber is a very resilient and long lasting polyester made into tiny "soft pellet" size balls or clusters of fiber that blend together. It can rearrange it-self inside the pillow making it similar to down and feather. It is very, very durable and is used primarily in bed pillows where durability and comfort are a must. It is soft and bouncy and it is one of the most durable fibers on the market today. Feel: soft, bouncy, the most durable poly fiber we have, good for long lasting bed pillows. Very similar in Feel to the Perma Soft, but a little more durable. CLUSTER Polyester Travel Pillow: Filled with Cluster Poly hypoallergenic fiber. The cover is a 220 thread count, white down proof ticking.