The Serta Express Perfect Sleeper Firm King Mattress features a substantial layer of premium gel memory foam supported by a durable foam core. This gives you cradling, head-to-toe comfort thatâ€™s packaged to easily fit through your front door. Gel infused into premium memory foam creates a cradling, pressure-relieving comfort that helps reduce heat at the sleep surface. SertaÂ® Foam Core This dense, durable foam core was designed for all-over body support and long-lasting comfort. Adjustable Friendly.