Concept of Freezing in lyophilization

Concept of Freezing in lyophilization-I 

Hello Pharma people,

Let us know about basic principles of Pharmaceutical freeze drying II.

In the previous post, we have seen overview of freeze drying in pharmaceutical industry.

The freeze-drying process typically consists of 3 different phases.

· Freezing phase: The principal dehydration step. Most of the solvent (typically water) is separated from the solutes to form ice.

·Primary drying phase: removal of (Crystalline) ice by sublimation. Longest phase in the lyophilization process, optimization has a great impact on process economics.

·Secondary drying phase: removal of unfrozen water by diffusion and desorption.

In this post, we will have a look at role of freezing in lyophilization cycle: 

In pharma circles, there is a famous love story between freezing and lyophilization. 

     If you can freeze it, you can freeze dry it.

Is freezing really matters in lyophilization, then as a lyo lover i would say yes.

Come on folks, lets dig deep into the concept of freezing.

Freezing:

After preparation of bulk solution (Formulation), will filtered through 0.22 µ filters (we call it as Aseptic filtration). The aseptically filtered solution will be filled into the vials and half stoppered in clean room under controlled environment (Grade A). The half-stoppered vials transferred into shelves of lyophilizer and once the loading of all the vials is completed, lyophilization cycle will be commenced.

what might happen during cooling: 

 We will start with a solution in liquid state and as we go down the temperature, cooling is initiated. 

due to cooling, the material is hardened by low temperatures.

During this very critical period all fluids present become solid bodies, either crystalline, amorphous, or glass. 

Most often, water gives rise to a complex ice network, but it might also be embedded in glassy structures or remain more or less firmly bound within the interstitial structures. 

Solutes do concentrate and might finally crystallize out. 

At the same time, the volumetric expansion of the system might induce powerful mechanical stresses that combine with the osmotic shock given by the increasing concentration of interstitial fluids.

In diluted solutions, which is a current case in freeze-drying of pharmaceuticals, ice can develop in the course of cooling either as a well-defined front moving upward in the liquid from the cold supporting shelf or at the same time in the whole mass of a supercooled fluid.

In the first case, there might be some cryoconcentration of the product, which provokes a solute gradient from bottom to top resulting in an increasing solid concentration in the upper layer. The
result is, most often, the occurrence of a thin film, rather compact, at the surface of the dry plug at the end of the process, which might create problems at the reconstitution step and impedes the water vapor (mass) transfer in the course of drying.

 In the second case, when nucleation starts up all at once throughout the liquid the structure of the frozen mass is more homogenous, and this may lead to a more finely porous dry cake, but if the degree of supercooling is important and the ice development is pretty fast it may result also in the rupture of the vial.

Before proceeding further, let have a look at nucleation.

We will start with a solution in liquid state and as we go down the temperature, cooling is initiated. Pure water cooled below the freezing point can remain a super cooled liquid until it is disturbed. Later nucleation will happen and proceed further.

Nucleation is a process where the molecules in a liquid start to gather into tiny clusters, arranging in a way that will define the crystal structure of the solid. It involves two steps.

Primary nucleation: when first ice crystals form distinguishes between homogenous and heterogeneous (Primary) nucleation.

Secondary nucleation: Nucleation that moves with a velocity of mm/s until the equilibrium freezing point is achieved.

As the pharmaceutical freeze drying involves a clean room environment, Ice formation is attributed to condensation freezing when water condenses at supercooled temperatures (T < 273 K) and ice nucleation spontaneously occurs without further cooling of the condensed water. The mechanism of ice nucleation involves, below steps. 

There are two types of nucleation:

Heterogeneous nucleation, which occurs when ice begins to form around a nucleation site, such as a physical disturbance, an impurity (such as salt) in the liquid or an irregularity in a container. Since biological samples are never pure water, they always experience heterogeneous nucleation. Nucleus is container (vial surface) or foreign particle (dust in lab, thermocouple) which reduces free energy for the formation of a nucleus. 

Homogenous nucleation, which occurs when ice forms without any predefined nucleation site. Pure water will freeze at approximately -39°C in the absence of nucleation sites. In practice, though, homogenous nucleation is not often seen because of the rarity of completely pure water. Grouping of water molecule in microscopic clusters (nucleus= water molecules). Never occurs in a freeze-drying process just a text book example.

Why water wont freeze at 0°C:

There are several factors that prevent an aqueous solution from freezing at 0°C:

•Freezing Point Depression (if added solutes)
•Supercooling (the process of lowering of temperature of a liquid below its freezing point without it becoming an ice).

Need to be aware of these during cycle devel1opment as they can impact product stability and how the product behaves in the freeze-dryer.

Freezing point depression: As pure water crystallizes at 0°C, but due to alteration of freezing point of water by added solutes. It results from a change in the “escaping tendency” or vapor pressure due to an added additional species. At the triple point of water, the molecules have the same tendency to go from the solid phase, to the liquid phase, to the vapor phase. Vapor Pressure is a Colligative Property–Depends chiefly on the number of rather than the nature of the constituents. Added salts can depress the freezing point of ice and significantly reduce glass transition temperatures. Rule of thumb is keep added salts to a minimum and try to avoid divalent and higher species. Salts increase the amount of unfrozen water. Salts can delay, or sometimes prevent crystallization of other components. Tonicity should be adjusted with mannitol or glycine if possible.

Super cooling:

The degree of super cooling is the temperature difference between the equilibrium freezing point and the actual nucleation temperature at which ice begins to form.

• Lab scale: typically, in the range of -5°C to -15°C (Particulate contamination)
• Production scale: Can be as low as -40°C (sterile environment)

The degree of super cooling may change with cooling rate (low freezing rates, higher super cooling).

 Ice crystal morphology:

The morphology of ice is a unique function of the nucleation temperature.
Low super cooling results in dendritic crystals.
High super cooling yields crystal filaments.
Aqueous solutions usually produce hexagonal, dendritic, and dispersed spherulitic (dependent on freezing temperature, solute and concentration)

Cakes with better connected ice crystal morphology with more direct vapor flow paths toward the top of the cake have higher primary drying rates.

Bye folks will keep posting.

Teja ponduri signing off............