Lyophilization
Lyophilization is a low-temperature drying process that removes water from a product by sublimation.​
Lyophilization (or freeze-drying) of a product in aqueous solution consists of eliminating the water it contains. This elimination occurs through the progressive transformation of water from the solid phase (frozen state) to the vapour phase, without passing through the liquid state. This process is also referred to as "sublimation".
Lyophilization is in fact a low-temperature drying process carried out in 3 main stages: freezing, sublimation and finally desorption.
Discover our lyophilization equipments >
What are the advantages of lyophilization and its applications?
Lyophilization of a product ensures its stability at room temperature, thus enabling ease of storage and transport.
As the process operates at low temperature, the integrity of the product is maintained, as is the preservation of sterility.
It can be applied to antibiotics (penicillins, acyclovir, vancomycin), hormone therapy (menotropin, levothyroxine), blood factors (factor VIII, antithrombin, fibrinogen) and anti-infectives (minocycline, teicoplanin, metronidazole).
In gene therapy, biological products can also be lyophilized: proteins, genetic material (RNA, DNA) and monoclonal antibodies.
Lyophilization cycles
Freezing
Pre-treatment
Sublimation
Primary drying
Desorption
Secondary drying
Stoppering
Final packaging
Freezing
Given that the substances to be processed are in a liquid solution state, it is necessary, prior to sublimation, to carry out total solidification by freezing down to a sufficiently low temperature (eutectic temperature).
​
Sublimation
Sublimation (or primary desiccation) of the water (solvent to be removed) can only occur if the pressure P generated by the water molecules present in the surrounding atmosphere is lower than the pressure T generated by the water molecules contained in the product. This latter pressure T is directly dependent on the temperature of the product — the lower the temperature, the lower the pressure.
In order to maintain the partial pressure at the lowest possible value, it is essential to remove water molecules as they are extracted from the product. This molecular transfer is achieved through natural attraction followed by blocking via "trapping".
Water molecules thus travel from a point of "high" pressure (the product) to a point of "low" pressure (the trap/condenser).
In addition to the refrigeration unit used to cool the product (via the shelves), a freeze-dryer also incorporates a refrigeration unit dedicated to cooling the condenser.
During sublimation, the temperature of the product must remain below the total solidification temperature (2 to 5°C below).
During sublimation, the flow of water molecules is subject to multiple collisions with air molecules present at atmospheric pressure. This flow is therefore impeded, causing a significant slowdown in the process. To facilitate the removal of air molecules from the chamber, a vacuum pumping unit (vacuum pump) is preferably used, which then has the effect of facilitating the transfer of water molecules from the product to the trap.
This flow is therefore impeded, causing a significant slowdown in the process. To facilitate the removal of air molecules from the chamber, the use of a vacuum pumping unit (vacuum pump) is preferred, which then has the effect of facilitating the transfer of water molecules from the product to the trap.
​
Desorption
During sublimation, the water molecules "released" in vapour form must pass through a layer that is already "dry" (and therefore easily re-hydratable). A portion will be arrested within this layer by adsorption.
Once the last traces of ice have been sublimated, the moisture concentration in the resulting product is too high due to the large number of molecules that have been adsorbed into the dry structure. At the end of sublimation, the temperature of the product gradually rises, tending towards that of the shelves. From this point onwards, and provided that the pressure in the chamber is sufficiently low, the desorption stage (or secondary desiccation) begins, with the aim of eliminating the last traces of water (adsorbed) contained in the product.
​
Stoppering
Once residual moisture has reached the desired low level, the product takes on the appearance of a highly porous material — the pores being the "cavities" that were occupied by water when the product was in solution form. This porosity makes the product a "sponge" that can rapidly bind molecules of substances harmful to its preservation, such as oxygen, which can cause degradation through oxidation. In order to meet the preservation objective, it is therefore most often necessary to "condition" the product — and to do so within the freeze-dryer chamber itself.
The composition of a freeze-dryer
The chamber: A stainless steel metal enclosure designed to withstand the pressure conditions specific to lyophilization cycles (from atmospheric pressure down to a few millionths thereof). Can be cleaned and sterilised in place via CIP/SIP.
The shelves: They support the product and act as heat exchangers, providing the energy required for freezing and then sublimation. Made of hollow stainless steel to allow the heat transfer fluid to circulate, and rigid to withstand the mechanical stresses to which they are subjected.
The trap: The name given to the condenser for the water vapour sublimated from the product. A cold wall of plates or tubes, through which the cooling medium circulates on the inside, and on the outside of which the water molecules (sublimated from the product) condense. Depending on the machine, the cooling medium used is either a refrigerant liquid (Freon, Forane…) or a cooled liquid (e.g. silicone oil).
The machinery: Vacuum pump, refrigeration unit, etc.