The removable surface layer of the Nook Pebble Pure mattress employs a patented asymmetrical design and nano-level technology to maximize breathability. Air flows in valleys between pebbles creating an oxygen-rich crib environment while surface fibers with nano-technology prevent liquids from seeping into the mattress for easy-wipe clean up. When it's time for a nursery makeover, just unzip the Pebble Pure Wrap and swap colors. For use ONLY with the Pebble Pure Mattress. MaterialsEucalyptus, Organic Cotton, Zinc, Polyester, Easy-Close Nylon Zipper 28" x 53" x 4"