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Tuesday, 8 January 2019

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 ยต 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............ 

Tuesday, 1 January 2019

Basics of freeze drying or lyophilization in pharmaceutical industry


Hello Pharma people,

Let us know about basic principles of Pharmaceutical freeze drying.

Freeze Drying, or lyophilization as it is referred to in the Pharmaceutical and Diagnostic Industries, is a dehydration technique, which enables liquid or slurry products, which have previously been frozen to be dried under a vacuum.

      Lyophilization and freeze drying are the process that were used inter changeably depending on the industry and location where the drying is taking place.
      Lyophilization is the process we use to remove water from a formulation at low temperatures (prevents thermal degradation) through a process of sublimation and the removal of bound water molecules through the process of desorption.
Basic Lyophilizer  System Components:
      Vacuum Pump: Vacuum is the drying force for change in pressure which is the driving force for freeze drying. A high vacuum and low pressure system to be created inside the chamber. 
      Temperature Controlled Shelves: Shelves with thermal fluid to control, monitor temperature.
      Condenser (External or Internal): A coil or a plate that is very cold and lower than product temperature kept some where away from the product. The water generated during freeze drying process is migrated down the condenser and collected.
      Compressible Shelves: To load the vials and also to full stopper the lyophilized vials
      Temperature Monitoring Devices: sensors (RTD, thermocouples, thermistors)
      Vacuum Monitoring Devices (Capacitance manometers, Pirani guage)
      Bleed Valve: To control chamber pressure, for very precise pressure/vacuum level
      Data Recording Device


The simplified diagram of freeze dryer.


      Why do we lyophilize (freeze-dry)?
We don’t want to freeze dry but we have to………
Products are not stable (<10% degradation) in the solution state at controlled room temperature for at least 2 years.
      Water
Without water, living processes cease to function or go dormant
     Water can however induce damage (Hydrolysis)
     Spoilage and growth promotion of organisms
•The effects of water can be
               -Immobilized by freezing
               -Eliminated by drying
•The effects of water can be
              -Slowed down by High salt content
            - Slowed down by High sugar content
Merits of lyophilization
+ Compatible with aseptic operations
+ Drying takes place at low temperatures compared to conventional drying: Minimizes chemical decomposition
+ Filling vials as liquid allows more precise fill weight control and avoids cross-contamination /containment problems
Limitations of Freeze-Drying
     Drug may not be stable as a freeze-dried solid. Example: Many cephalosporins
     Many biological molecules are damaged by the stresses associated with freezing, freeze-drying, or both
     Not all materials can be freeze-dried to form a pharmaceutically acceptable cake
     Cost?- A bit expensive
What Can Be Freeze-Dried?
Non Biologicals –Reactive Chemicals (Small Molecules)
     Non Living Bio Products such as Vaccines
     Enzymes
     Hormones
     Vitamins
     Blood Products
     Antibodies
     Tissues for Surgery
     Foods
     Living Organisms –Seed Cultures
     Miscellaneous –Museum Specimens/Taxidermy
      What Can’t Be Freeze-Dried?
x Oil rich products
x Sugar rich products
x Products that form an impervious skin
x High salt containing products (Tg’ suppressing substances)


Principle of lyophilization


  •     · At normal temperature (around 25°C) and 1 atmospheric  pressure, an aqueous solution contains dissolved solutes in water and the whole formulation exists as liquid.
         The liquid state of  aqueous formulation can be altered by increasing the temperature to 100°C to form a vapor and decreasing the temperature to 0°C to form an Ice.
         At normal temperature (around 25°C) and 1 atmospheric  pressure, an aqueous solution contains dissolved solutes in water and the whole formulation exists as liquid.
         The liquid state of  aqueous formulation can be altered by increasing the temperature to 100°C to form a vapor and decreasing the temperature to 0°C to form an Ice.
         There is an alternative way to alter the liquid state of water i.e., by decreasing the pressure from 1 atmosphere (increasing the vacuum) at the same temperature.
         water coexist as all the three phases  (Solid, liquid, vapor) in equilibrium at 0°C temperature and 4.5 mm of Hg (4.58mTorr) pressure . This point is called Triple point of water.
         At triple point, at below atmospheric pressure, water is converted to solid without passing through the liquid phase, it is called as sublimation.
      
Sublimation
-          Sublimation is when a solid (ice) changes directly to a vapor without first going through a liquid (water) phase.
-          Low pressures are required for sublimation to take place.
-          Sublimation is a phase change and heat energy must be added to the frozen product for it to occur.
-          Sublimation in the freeze drying process can be described simply as:
FREEZE - The product is completely frozen, usually in a vial, flask or tray.
VACUUM - The product is then placed under a deep vacuum, well below the triple point of water.
DRY – Heat energy is then added to the product causing the ice to sublime.
“Ice sublimes in an attempt to achieve vapor equilibrium. When chamber pressure = ice vapor pressure, sublimation stops.“

Steps of Lyophilization
The steps required to lyophilize a product in a batch process can be summarized as follows:
1)      Pretreatment / Formulation
2)      Loading / Container (Bulk, Flask, Vials)
3)      Freezing (Thermal Treatment) at atmospheric pressure
4)      Primary Drying (Sublimation) under vacuum
5)      Secondary Drying (Desorption) under vacuum
6)      Backfill & Stoppering (for product in vials) under partial vacuum
7)      Removal of Dried Product from Freeze Dryer
Pharmaceutically Elegant Product looks like below.


 
The concept also available at https://youtu.be/aL_IjzHRP3k

Bye folks.. Will dig deep in next sessions..

urs,
Teja Ponduri