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Memory foam is made from polyurethane with additional chemicals to increase its density. The foam is made up of millions of spherical shaped, open cells. What makes memory foam unique from regular polyurethane foam is that it is temperature and weight sensitive.
Being temperature sensitive, memory foam is firmer in cool temperatures (more viscous) and softer in warm temperatures (more elastic).
Higher density memory foam, such as that used in several Tempur-Pedic mattresses, is particularly temperature sensitive in that it responds to body heat by molding itself to the shape of a person's body within minutes.
This characteristic explains why people often experience a "melting in" feeling when lying on higher density memory foam. This molding ability, however, can also be a disadvantage in that it can make moving on or getting up off the bed somewhat difficult. It can also give the sensation that one is sleeping in "mud" or "quicksand," two words that are sometimes used to describe sleeping on higher density foam.
Temperature sensitivity may also cause the foam to mold and contour to one's body to an extent that limits blood circulation. Consequently, numbness of the arms and hands is reported by about 3% of memory foam mattress owners.
Lower density memory foam is less temperature sensitive than higher density foam and, as a result, retains a more consistent feel and firmness regardless of temperature. (Learn more about higher vs lower density memory foam.)
Memory foam, especially higher density foam, is also weight sensitive. Unlike traditional foam that compresses and returns to its original shape right away, memory foam cells compress completely and spread their air pressure to other cells. This action allows the foam to better conform to a person's shape and, consequently, reduce pressure points.
The example often used to demonstrate the weight sensitivity of memory foam is a hand pressed into it and then removed, leaving a clear impression in the foam. The foam then slowly returns to its original shape. High-density memory foam tends to have a slower response time than low-density foam.
People who don't weigh much (under 110 lbs or so) often say that they don't sink into the foam. As a result, they can't fully benefit from the mattress or find adequate comfort. This problem can occur with all densities, but it is especially an issue with higher density foam.
Memory foam was originally developed by NASA to lower the extreme physical pressure put on astronauts when they experience high g-force.
The foam was never used by NASA in practice. However, Tempur-Pedic developed a modified version of the foam called Tempur® and it was used as mattress material in the medical industry. The foam helped immobile patients find better comfort and improved health.
Eventually, Tempur-pedic began selling their product to the non-medical consumer market.
Soon other foam makers began offering their own versions of memory foam, often at a lower density and price. Memory foam is now used not just for mattresses, but for pillows, footwear, chairs and more.
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