Memory foam (visco-elastic foam) is an oil-based material, made from polyurethane with additional chemicals increasing its viscosity and density. It consists of tiny molecules – cells, that under pressure release air and thus creating a unique molding feel. In absence of pressure, cells refill with air and bounce back to its original shape – hence the name memory foam.
Quality of memory foam is measured by its density - pounds per cubic foot (lbs/ft3). Memory foam is typically denser and heavier than other polyurethane foams. Firmness is measured by a foam's indentation force deflection (IFD) rating but it is not a comprehensive measurement for determining the mattress firmness or softness. It needs to be compared relative to foam density.
It was designed by NASA in late 1960s to protect astronauts against extreme pressure at take off. It absorbs and disperses energy and reacts to weight and temperature. With these qualities it is more popular than ever with mattress manufacturers but it is becoming a go to material in other industries (shoe insoles, health care, sporting goods).