This mattress features high density memory foam with a 4.5 lb/sq. ft. density, along with a firm support foam base that keeps dogs from sinking through to the floor. The top 1.5" memory foam and bottom 3.5" support foam create a supportive sleeping surface great for older dogs alike, especially bigger breeds and those with arthritic conditions. The mattress also features a Water-Resistant Nylon Liner which keeps damaging fluids away from the high quality foam. This liner can be especially helpful for owners of dogs with incontinence problems.