Memory foam is actually a rather advanced material, and has only gone down in cost enough to be practical for personal use in the last ten years or so. More specifically, it is an open-cell foam with viscoelastic properties, typically manufactured using polyurethane. If this means nothing to you don’t worry, and read on!
Open Cell Foam
We can think of foam as a fluffy mixture of a solid (polyurethane in this case) and air. If the mixture is made up of many tiny bubbles, like in the sole of a tennis shoe, we can squeeze and compress this foam and the air will never come out as long as the bubbles don’t break. This is called closed cell foam, and is good for tennis shoes which need to protect our feet from harsh impacts again and again, but tends to be a bit stiff for beds.
Open cell foam is still a mixture of a solid and air, but in this case, all the little bubbles have even smaller holes in them! We can see this on the magnified picture on the left. This means we can squeeze and compress the foam a great deal, as the air will rush out just like water from a foam dish scrubber when you squeeze it.
This open-cell foam is soft and conforming to the body, and allows air movement through it which helps keep you cool and comfortable.
Viscoelasticity is what makes makes memory foam have a “memory”.
A spring mattress is what engineers would call elastic. No matter how fast or how slow you put a certain load on the mattress, the spring still compresses the same amount in the end. The spring does not (or at least should not) soften or harden over time.
A memory foam mattress is what engineers would call viscoelastic. This is a combination of elastic properties (like a spring) and viscous properties (like oil or molasses). These viscous properties cause the foam to react differently depending on how fast you put weight on it. If you jump onto a memory foam bed, you can feel the foam slowly start to conform to your body as you sink in. If you get up quickly, there will be a dent in the foam in the shape of your body which will slowly disappear over time.
So who cares? Is a memory foam mattress just an expensive way to leave a cool handprint? Well, there’s more to it. Sleep comfort can be correlated with how pressure varies along the body during sleep. If the pressure changes rapidly with many individual pressure points (like sleeping on gravel, or that one spring that pokes out of the mattress), we’ll be very uncomfortable.
Memory foam does a very good job of reducing these “stress gradients”. Over a short period of time, the foam remolds to reduce these pressure points, something a typical elastic material cannot do. The resulting smooth pressure distribution provides a very comfortable sleeping surface.
Without getting into too much chemistry, polyurethane is a polymer. This means it consists of very long chains of urethane molocules, which then become tangled together like spaghetti to produce a solid. To produce basic polyurethane foam, a reaction occurs where urethane is chained together into these long strings while water reacts with another compound to produce carbon dioxide. This combination of solid and gas gives us polyurethane foam.
One important advantage of polyurethane is that its properties are readily modifiable by adding different compounds to the mix. Typical polyurethane foam is not “memory” foam unless certain additives are incorporated which give it these “memory” effects.