Visco elastic memory foam was first developed in the 70s in conjunction with the NASA Space Program to help astronauts absorb enormous G-forces at liftoff. Most foam manufacturers mention this in their marketing to increase the credibility of their toppers and beds. After all, if NASA uses memory foam, it must make an excellent mattress. But this isn't exactly the truth.
Visco memory foam is made up of billions of tiny open cells tightly packed together. When you lie down on the memory foam mattress or bed, these cells fully compress, spreading the air to adjoining cells and making the foam mold to your exact body shape. When you move, the memory foam simply takes up air again.
Over years of use, any visco elastic foam mattress, be it Tempurpedic or other, no longer regains its exact, original shape. A sag in the center of the bed becomes a permanent feature. The question is, does this happen in a few years or will you enjoy 10 plus years of sleep before you must buy a new bed or mattress?
High density visco foam like the ones used in Tempur-Pedic sleep systems is also more expensive to make because more raw material called resin, rather than air, goes into making the visco elastic foam. This is one of the things which explains why a Tempurpedic mattress is so expensive. Alternatives include magnetic mattress and pads.