THE FREEZE-DRYING CYCLE
Lyophilization is a complex drying process that involves removing the solvent from a material by sublimation. Sublimation
is achieved through varying the temperature and pressure of the material so that the solvent does not pass through the liquid
stage, but moves directly from the solid phase to the gas phase (see Figure 1). Lyophilization takes place in three main stages: freezing, primary drying, and secondary drying. Each stage has its own
Figure 1: During freeze drying the temperature and pressure are controlled so that the frozen solvent moves directly from
the solid to the gas phase without passing through the liquid phase.
The material is frozen. The rate of freezing, and the final temperature to which the material is lowered, both have a significant
impact on the quality of the final product. The rate at which the temperature is lowered affects the structure of the ice
matrix, which has an impact on the ease of flow of the sublimated vapor out of the sample. Annealing, a technique of raising
and then lowering the temperature of a frozen material, can be used to encourage crystallization or to provoke a more favorable
In delicate materials such as proteins, there is a risk of damage from ice crystal growth. In general, the faster the rate
of freezing, the larger the ice crystals formed and the greater the risk of damage. A slower freezing cycle will result in
smaller crystals that cause less damage, but the resulting structure will cause a greater impediment to the flow of vapor
and therefore slow the drying process.
During the freezing stage, it is vital that the material is cooled below its critical temperature (Tcrit) to ensure it is fully frozen. Every formulation has a different Tcrit that is affected by the combination and proportions of the elements within it, such as the solvent, excipients, and the active
ingredient. It is vital that the critical temperature is determined for every different formulation. Knowing the Tcrit not only makes it easy to ensure that the Tcrit is achieved during freezing, but also means that energy is not wasted by taking the temperature lower than required. Methods
for determining Tcrit are discussed below.
The frozen material is initially dried by sublimation. During primary drying the pressure of the drying chamber is reduced
to a very low level, while the temperature is raised slightly to allow the solvents to sublime. Throughout this stage the
temperature must be kept below the critical temperature (Tcrit) so that the material does not melt or its structure collapse.
One of the effects of sublimation is cooling of the product, which slows the process of drying. The rate of sublimation can
decrease by as much as 13% for each unnecessary 1'C decrease in temperature (1). To counter this cooling and provide energy
to drive the sublimation process, heat is added through the freeze-dryer shelf. The energy transfer during primary drying
must be balanced so that sufficient heat is used to encourage sublimation without risking collapse.
Collapse is the most serious processing defect in freeze drying, resulting in reduced shelf life, reduced stability, decreased
product activity, and poor reconstitution (see Figure 2).
Figure 2: A selection of vials containing the same freeze-dried material. The fill depth of all four vials was identical before
processing. The three vials to the right have all undergone serious process defects.
Secondary drying is a desorption process that removes any solvent that is left chemically bound in the material after primary
drying. The moisture level at the beginning of this stage may be around 5–10%, with a final moisture content of typically less than 5%.
To facilitate the desorption process, the temperature is raised and the pressure reduced to a minimum (see Figure 3). This is the slowest phase of the lyophilization process. Depending on the final moisture level required, it could last
several days. Therefore, any increases in efficiency can have a significant impact on manufacturing throughput.
Figure 3: A simplified freeze-drying chart, showing the variations in temperature and pressure throughout the lyophilization