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Viral vaccines, biological samples, and a variety of other materials used daily benefit from freeze-drying.
The lyophilization process removes water from the material before it is packaged and sealed. This process increases the shelf life of pharmaceuticals, making storage and shipping simple. Reconstituting the dehydrated products is often as simple as adding water or another liquid when required.
Outside of longer shelf life, freeze-dried products maintain their appearance and characteristics, making them invaluable when studying heat-sensitive materials, such as microbes, plasma, proteins, pharmaceutical products, and tissue samples.
Additionally, when freeze-drying samples and products, unique glassware, stainless steel trays, and stoppering tray dryers are generally used in conjunction with the freeze-drying instrument to facilitate sample lyophilization. These products are available from many equipment manufacturers working in the life sciences and pharmaceuticals industries.
Lyophilizers work on the principle of sublimation, the process of changing ice, a solid, directly into a vapor without entering a liquid state in between. Freeze-drying, also called cryodesiccation or lyophilization, comprises specific steps requiring vacuum pumps and refrigeration.
Vacuum pumps are used to create an ultimate vacuum of below 0.1 mbar. Unlike most dehydration methods, refrigeration is used to provide the low temperatures (in the ULT range) needed to lyophilize a sample, resulting in a better-dried product.
The pump should be sized according to the lyophilizer. If the pump’s flow rate is too high, it will pull vapor through the freeze dryer’s condenser too quickly, reducing the instrument’s efficiency.
It is also essential to clean the condenser after each run to prevent the sublimation of frozen chemicals into the vacuum pump. Chemical resistance requirements for your freeze dryer vacuum pump depend upon the aggressiveness of your solvent.
The general steps of sublimation are as follows:
There are numerous ways to pretreat samples, but common ones include: concentrating the sample, decreasing the amount of high-vapor-pressure solvents in the sample, adding components or compounds for stability, or simply increasing the surface area the sample occupies.
The freezing, or annealing, stage comes next. First, the sample is cooled to a temperature below its triple point, the lowest point where the solid, liquid, and gas phases are all at equilibrium. By performing this process at this temperature range, sublimation can occur as opposed to melting.
Generally speaking, the goal is to create large ice crystal structures within the sample during this process. This helps to expedite the removal of water vapor once the sublimation process occurs. This can be done by annealing, a method where the temperature is cycled up and down until the structure is appropriately set.
Once the sample is frozen, the drying process can start. The pressure is lowered and the sample has just enough heat applied to it to cause the ice to sublimate. This causes approximately 95% of the water in the sample to sublimate. This process is frequently a slow one, sometimes taking several days because if too much heat is applied the structure of the sample can change.
The secondary drying phase aims to remove any remaining unfrozen water molecules. The temperature is generally raised during this process, but the pressure is frequently also lowered to force out the remaining moisture. Once this process is completed the process is complete and the material gets sealed in whatever container is needed.
The major components of a lyophilizer include:
The refrigeration system cools the condenser located inside the freeze dryer. The refrigeration system can also cool shelves in the product chamber, which helps freeze the product.
Product chambers can be a manifold with attached flasks or a larger chamber with a system of shelves where products are placed.
The vacuum system is made up of a vacuum pump connected to an airtight condenser and attached product chamber.
A lyophilizer’s control systems vary in complexity and usually include temperature and pressure sensing capabilities. More advanced controllers offer programmable steps for freeze drying and include monitoring options, which allow you to monitor the freeze drying process as it’s completed. When Choosing a control system for the freeze dryer, remember to consider the application and freeze dryer’s use. Will it be used in the lab or for production?
The condenser’s function is to collect the vapors sublimating off of the product in the chamber. This happens because the condenser provides a surface for the water vapor to adhere to, where it eventually turns into ice. The sublimated ice accumulates in the condenser and is then manually removed at the end of the freeze drying cycle, also known as a defrost step.
The condenser’s required temperature is dictated by the freezing point and eutectic temperature (or collapse temperature) of the product. The refrigeration system must be able to maintain the temperature of the condenser substantially below the temperature of the product.
Determining the critical collapse temperature of a product is an essential part of establishing and optimizing a freeze drying process.
It represents the maximum temperature, or eutectic point, that the product being freeze-dried can withstand during the primary drying phase, without it melting or becoming damaged. Thermal analysis is a common methods used to determine the eutectic temperature of the product.
Frozen samples and products are typically categorized as either crystalline or amorphous glass in structure. While crystalline products have a well defined eutectic point, amorphous products have a glass transition temperature that makes it much more difficult to freeze dry amorphous samples or products. The eutectic temperature of an amorphous product is generally a few degrees warmer than its glass transition temperature.
The first known application of freeze-drying happened somewhere you might not have expected, the Andes mountains. The indigenous peoples who lived in the area needed to prolong the amount of time their food remains edible. Due to the harshness of the environment, having emergency stores of food was tremendously helpful.
The potatoes they took from the lowland areas of their home were smashed to remove water and then were left to freeze on the mountain at night. This plus their exposure to sunlight during the day completed the cryodessication process giving them shelf-stable rations for the harder months.
It took hundreds of years for this methodology to make its way to the industrial side of the world, having been implemented in 1890 at its earliest.
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