Lyophilization is a process more commonly known as freeze-drying. The word is derived from Greek, and means "made solvent-loving". This process is a way of drying something that minimizes damage to its internal structure. Because it is a relatively complex and expensive form of drying, it is limited to those materials which are sensitive to heat and have delicate structures and substantial value. One of the only substances which cannot be preserved effectively by freeze-drying is mammalian cells, which are too fragile.
The preferred method of preservation in the biotechnology industry, lyophilization is regularly used to preserve vaccines, pharmaceuticals, and other proteins. Freeze-drying is also used to preserve special food products, eliminating the need for refrigeration. Freeze-dried food is eaten by mountain climbers and astronauts. Lyophilization is used by botanists to preserve flower samples indefinitely. Because the process of freeze-drying removes most of the water from the sample, freeze-dried materials become highly absorbent, and merely adding water can restore the sample to something close to its original state.
The energy and equipment costs of lyophilization are around 2-3 times higher than those of other drying methods. The drying cycle is also longer, about 24 hours. First, the temperature of the sample is lowered to near freezing point. Then, the sample is inserted into a vacuum chamber. The more energetic molecules escape, lowering the temperature further, while the extremely low pressure causes water molecules to be drawn out of the sample. Attached to the vacuum chamber is a condenser, which converts the airborne moisture into liquid and siphons it away.
Great care is taken throughout the process to ensure that the structure of the sample remains constant. For instance, the sample could merely be frozen by the vacuum rather than being frozen under atmospheric pressures, but that would cause shrinkage in the sample, damaging its structure irreversibly.
The primary mechanism that allows for freeze-drying is sublimation, whereby ice is directly converted to water vapor, without passing through the intermediary stage of a liquid. Rather than through heating, this is done by removal of pressure so that the ice boils without melting. The result is a sample whose structure is largely preserved, which can be stored at room temperatures and pressures.