Foam provides stability and even pressure distribution without constricting blood flow. Constructed of 1-1/2" thick visco-elastic "memory foam" that is laminated to a firm urethane foam base. Lessens the risk of decubitus ulcers by distributing pressure evenly without constricting blood circulation. Convex bottom reduces hammocking. Vinyl cover is waterproof\, odor-resistant and helps reduce teh forward sliding. Zippered outer cover is removable for washing. Item weighs 3lbs. (1.4kg) size is 18" x 16" x 3" (46x41x7.6cm).