The Serta 4″ Pillow-Top and Memory Foam Mattress Topper turns your mattress into a luxurious sleeping surface with a soft pillow-top and cradling memory foam. Memory foam consists mainly of polyurethane as well as additional chemicals increasing its viscosity and density.It is often referred to as “viscoelastic” polyurethane foam, or low-resilience polyurethane foam ().The foam bubbles or ‘cells’ are open, effectively creating a matrix through which air can move. Higher-density memory foam softens in reaction to body heat, allowing it to mold to Invest in comfortable, restful sleep for your family with mattresses that suit individual sleeping styles and preferred levels of firmness. Overstock.com – Your Online Bedroom Furniture Store! Get 5% in rewards with Club O! – Serta Mattresses .