Memory foam is a viscoelastic material: it is known for its ability to both absorb and redistribute pressure from a body falling upon it, enabling it to form a conforming cradle that matches your sleeping style – this in turn results in pressure being taken away from the hard points e.g. hips and shoulders for a side sleeper, so that a more natural spinal position can be achieved.
Viscoelastic foam by itself isn’t able to provide total support – pressure relief is more its forte. For a mattress to be both supportive and pressure relieving, the base will need to comprise of a harder, more resilient material such as HD polyfoam.
In terms of composition, viscoelastic foam is closely related to regular polyurethane, with the exception that it uses additional chemicals to achieve its viscoelastic nature. High quality viscoelastic foam – that is to say, with a density surpassing 4 lb./cu.ft., usually retains its conforming ability for much longer than other types of foam.