With all the talk about vaccine production these days, I thought it would be beneficial to some readers to learn a little bit more about lyophilization – a process commonly used in vaccine production to stabilize the product for transport.
So what is lyophilization and how does it work? Well simply put, it is freeze drying. Lyophilization is a process for removing water from a product to preserve perishable materials, to extend shelf life, or just make the material more convenient for transport. Lyophilization works by freezing the material, then reducing the pressure and adding heat to allow the frozen water in the material to transfer out or sublimate. Lyophilization has three primary stages: the freezing phase, the primary drying or sublimation phase, and the secondary drying or adsorption phase.
During the first phase, there are several ways that freezing can be done. Freezing can be done in a freezer, a chilled bath (shell freezer) or on a shelf in the freeze dryer. Cooling the material below its triple point (the temperature and pressure at which water can exist in equilibrium in the liquid, solid, and gaseous states – which is 0.01°C) ensures that sublimation, rather than melting will occur. This preserves its physical form. Lyophilization is easiest to accomplish using large ice crystals, which can be produced by slow freezing or annealing. However, with biological materials, when crystals are too large they may break the cell walls, and that leads to less-than-ideal freeze drying results. To prevent this, the freezing is done rapidly. For materials that tend to precipitate, annealing can be used. This process involves fast freezing, then raising the product temperature to allow the crystals to grow.
In the lyophilization’s second phase, the primary drying (sublimation) is done. This is accomplished by lowering the pressure and adding heat to the material in order for it to sublimate. The vacuum speeds sublimation and the cold condenser provides a surface for the water vapor to adhere and solidify. The condenser also protects the vacuum pump from the water vapor. About 95 percent of the water in the material is removed in this phase. The primary drying can be a slow process as too much heat can alter the structure of the material.
The secondary drying (absorption) phase is the final phase of lyophilization during which the ionically bound water molecules are removed. By raising the temperature higher than in the primary drying phase, the bonds are broken between the material and the water molecules. Freeze dried materials retain a porous structure. After the lyophilization process is complete, the vacuum can be broken with an inert gas before the material is sealed. Most materials can be dried to 1-5 percent residual moisture.
And while anyone that works with lyophilization on a daily basis will tell you, there is much more to mastering it than these basic steps, I hope that this summary provides those getting acquainted with the lyophilization process enough of an education to ask the right questions.