Freeze drying also known as lyophilization is an important process in sample preparation and for the preservation and storage of biologicals, pharmaceuticals and foods. Of the various methods of dehydration, freeze drying is especially suited for substances that are heat sensitive. Other than food processing (e.g., coffee, whole dinners), freeze drying has been extensively used in the development of pharmaceuticals (e.g., antibiotics) and preservation of biologicals (e.g., proteins, plasma, viruses and cell lines).
Freeze drying is a process whereby water or other solvent is removed from frozen material by converting the frozen water directly into vapor without the intermediate formation of liquid water. The basis for this sublimation process involves the absorption of heat by the frozen sample in order to vaporize the ice; the use of a vacuum pump to enhance the removal of water vapor from the surface of the sample; the transfer of water vapor to a collector; and the removal of heat by the collector in order to condense the water vapor. In essence, the freeze dry process is a balance between the heat absorbed by the sample to vaporize the ice and the heat removed from the collector to convert the water vapor into ice.
Why Freeze Dry?
Pharmaceutical companies often use freeze-drying to increase the shelf life of products, such as vaccines and other injectables. By removing the water from the material and sealing the material in a vial, the material can be easily stored, shipped, and later reconstituted to its original form for injection.
The advantages of lyophilization include:
- Ease of processing a liquid, which simplifies aseptic handling
- Enhanced stability of a dry powder
- Removal of water without excessive heating of the product
- Enhanced product stability in a dry state
- Rapid and easy dissolution of reconstituted product
Freeze Drying Process
Freeze drying is mainly used to remove the water from sensitive products, mostly of biological origin, without damaging them, so they can be preserved easily, in a permanently storable state and be reconstituted simply by adding water. Examples of freeze dried products are: antibiotics, bacteria, serum, vaccines, diagnostic medications, protein-containing and biotechnological products, cells and tissues, and chemicals.
The product to be dried is frozen under atmospheric pressure. Then, in an initial drying phase referred to as primary drying, the water (in form of ice) is removed by sublimation; in the second phase, called secondary drying, it is removed by desorption. Freeze drying is carried out under vacuum. The conditions under which the process takes place will determine the quality of the freeze dried product. Some important aspects to be considered during the freeze drying process are as follows:
Freezing
Freezing is a process used to transform the basic product by abstracting heat to create a state that is suitable for sublimation drying. When an aqueous product is cooled down, at first crystal nuclei are formed. The surrounding water will be taken up around these nucleation sites, resulting in crystals of different sizes and shapes. Freezing speed, composition of the basic product, water content, viscosity of the liquid, and the presence of non-crystallizing substance are all decisive factors in determining the crystal shape and size and in influencing the following sublimation process. Large crystals leave a relatively open lattice after sublimation, while small ice crystals leave narrow spaces in the dried product slowing down the removal of water vapour.
Primary Drying
At the beginning of the primary drying phase, sublimation of the ice takes place at the surface. As the process continues, the subliming surface withdraws into the product, and the evolving vapour must be conducted through the previously dried outer layers. This means that the drying process depends on the speed of vapour transfer and removal as well as on the necessary heat of sublimation. The heat required for sublimation is supplied to the product by convection and thermal conduction and in a small part by thermal radiation. Apart from heat transfer by thermal conduction and radiation, it is most important that the heat transfer by convection is optimized. It must be taken into account, however, that due to the reduction of pressure in the drying chamber, convection will practically cease at a pressure below 10 mbar. This is why, as a function of the required sublimation temperature, the pressure in the drying chamber is adjusted during primary drying to the highest permissible value.
The sublimation heat is not needed at the product surface, but at the boundary of the ice core that is withdrawing into the centre of the product as drying proceeds. Whilst the flow of water vapour is from within the product to the outside, the transfer of heat must be accomplished in the opposite direction from the outside to the inside. Due to the low thermal conductivity of the dried product layers, the temperature gradient required for heat transfer steadily increases. To avoid damage to the product, the maximum admissible temperature for the dried product must not be exceeded. On the other hand, care must be taken to maintain the required sublimation temperature throughout drying, keep the heat supply to the ice-core boundary in equilibrium with the heat requirement at that particular location, and avoid any overheating of the sublimation zone. The primary drying phase continues until all the ice contained in the product has been sublimated.
The freezing point of pure water is 0°C. Any other substances dissolved in the water will lower the freezing point; where inorganic salts are present it may be considerably lower. If a weak solution is frozen, at first pure ice will be separated, thereby increasing the concentration of dissolved substance in the residual solution making its freezing point lower still. The effect of such freezing concentration on the product is different from case to case and has to be taken into account when selecting the most appropriate freezing technique.
The most suitable freezing technique for a specific product should be determined and its parameters ascertained prior to sublimation drying. The freezing behaviour of the product may be investigated, for instance, using the resistance-measurement method.
Two different freezing methods are chiefly used for pharmaceutical products:
1. Freezing by contact with cooled surface.
2. Rotation or dynamic freezing in a coolant bath.
The first method is a static freezing technique where a versatile freeze dryer must be capable of adjusting the freezing rate to the specific product and should allow control of the freezing speed. A final temperature of -50°C will in many cases be sufficient to meet all requirements.
The second method is used wherever larger quantities of a liquid product are to be frozen and dried in flasks or large bottles.
The appropriate freezing technique will also be chosen to produce a layer thickness of the frozen product that is favourable for sublimation drying, i.e. not only uniform but also as thin as possible to achieve a short drying time.
Secondary Drying
In the secondary or final drying phase the aim is to reduce the residual moisture content in the product as much as necessary to ensure the product is in a permanently storable state. The water bound by adsorption at the internal surface of the product has to be removed. To achieve this, it is often necessary to overcome the capillary forces of the water, and freeze drying plant must therefore be designed to give a high pressure gradient during the secondary drying phase, as in most cases it is not possible to raise the product temperature without damaging the product. The secondary drying process must be precisely controlled so that any over drying of the product will be safely avoided.
After-Treatment
This section refers to the manner in which the dried products, often very hygroscopic owing to their large internal surface, can be protected after drying. If the product is dried in bottles, flasks or vials, it appears logical to close these containers immediately after drying before removal from the plant. For this purpose, special ribbed rubber stoppers are placed in the necks of the bottles or vials before charging the plant and, on termination of drying, are firmly pressed into the necks by a stoppering device. The containers may be sealed under vacuum or under protective gas atmosphere. The choice of method depends on the type of product. The drying chamber is then vented after termination of the drying cycle with dry sterile nitrogen or other inert gas up to atmospheric pressure.
Equipment
Empower Pharmacy uses a 53 square-foot lyophilizer to guarantee our customers a reliable validated cycle capable of producing a wide range of pharmaceutical products. We only employ production rated freeze driers as they offer many advantages over lab rated freeze driers.
Utilizing the knowledge of our engineering and cycle development groups differential scanning calorimetry, freeze dry microscopy analysis of eutectic points, critical temperatures and vapor pressures determine lyophilization conditions. Vials are filled and partially stoppered by way of an automated sterile filling line within an ISO Class 5 cleanroom before being transferred to the lyophilizer.
Clean-in-place (CIP) of the drying and condensing chambers is performed in between each batch using deionized purified water followed up with a steam-in-place (SIP) sterilization cycle. All piping and chamber materials are constructed of ASME 316L stainless steel. Sealing of vials is performed in process under inert gas back-fill with a hydraulic stoppering system. Empower Pharmacy performs full IQ/OQ/PQ validation and records cycle parameters employing a secure CFR 21 compliant data system. Complete project support from formulation through cycle development and scale up production is standard for all our products.