WHAT IS LYOPHILIZATION OR CRYODESSICATION

 What is Lyophilization or cryodessication 

Lyophilization or cryodessication  is also known as Freeze drying. Lyophilization is a process in which solvent (usually water) is sublimated from the product after feezing in a vacuum allowing the ice to change directly from solid  state to vapour  state without passing through a liquid phase.

In this process low temperature is maintained to minimize the thermal degradation of the drug compound.

This drying process can be divided into Freezing, a primary and a secondary phase.

Freezing

It Involves following steps

1. Freezing the product solution to a temperature below its eutectic temperature
2. Decrease the shelf temperature to -50oc.
3. Low temperature and low atmospheric pressure are maintained
4. Formation of ice crystals occurs. Freons are used as refrigerant in this process
5. The rate of ice crystallization define the freezing process and efficiency of primary drying.

Primary Phase (sublimation)

The drug solution is filled into vials and placed within a drying chamber which is temperature-controlled. There the solution is frozen according to physicochemical principles as the shelf temperature is lowered to below freezing. The shelf temperature is subsequently increased but maintained below the freezing point. Solvent (usually water) is sublimated under vaccum in a chamber.

50–80% of solvent is extracted in this phase of  drying process .

Secondary phase.(desorption).

Goal of secondary drying is to minimize the moisture content of the product.

During the secondary drying phase, the remaining solvent is removed at Higher temperature but still below freezing temperature.

During freezing, supercooling is done to encourage crystallization of the drug compound.

The extent to which the compound is supercooled depends on the

1.    nature of the compound,
2.   the temperature program of the shelf,
3.   the heat-transfer properties of the container, and
4.   the presence of particulates in the solution.

The degree of supercooling determines the size of the solvent crystal and, subsequently, the size of the channel formed during primary drying.

The extent of supercooling affects

1.  Rate of sublimation,
2.  Rate of secondary drying,
3.  Surface area of the finished powder.

So, it is most important to select a cooling temperature  to achieve the desired objective(s).

The lyophilized product is milled. The particle size of milled lyophilized powders generally has a broader distribution than spray-dried powder,

Keywords :

Lyophilization or cryodessication , Freeze drying ,eutectic temperature,shelf temperature ,sublimation,desorption,supercooling

Authored by :
S.M.Imroz , M.Pharm(Pharmaceutics) .,

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