Why lyophilization is done

This quick reconstitution is particularly important in the case of emergency vaccines and antibodies, which need to be administered as soon as possible. Every formulation has different freeze-drying characteristics and, therefore, different processing requirements. To ensure cycles are both robust and efficient, they should be tailor-made for each formulation.

Failure to do so can lead to inconsistent dryness across samples, reduced stability during storage, and reduced activity on rehydration. Lyophilization is a complex drying process that involves removing the solvent from a material by sublimation. Sublimation is achieved through varying the temperature and pressure of the material so that the solvent does not pass through the liquid stage, but moves directly from the solid phase to the gas phase see Figure 1.

Lyophilization takes place in three main stages: freezing, primary drying, and secondary drying. Each stage has its own challenges. Figure 1: During freeze drying the temperature and pressure are controlled so that the frozen solvent moves directly from the solid to the gas phase without passing through the liquid phase.

The material is frozen. The rate of freezing, and the final temperature to which the material is lowered, both have a significant impact on the quality of the final product.

The rate at which the temperature is lowered affects the structure of the ice matrix, which has an impact on the ease of flow of the sublimated vapor out of the sample. Annealing, a technique of raising and then lowering the temperature of a frozen material, can be used to encourage crystallization or to provoke a more favorable ice structure.

In delicate materials such as proteins, there is a risk of damage from ice crystal growth. In general, the faster the rate of freezing, the larger the ice crystals formed and the greater the risk of damage.

A slower freezing cycle will result in smaller crystals that cause less damage, but the resulting structure will cause a greater impediment to the flow of vapor and therefore slow the drying process. During the freezing stage, it is vital that the material is cooled below its critical temperature T crit to ensure it is fully frozen.

Every formulation has a different T crit that is affected by the combination and proportions of the elements within it, such as the solvent, excipients, and the active ingredient. It is vital that the critical temperature is determined for every different formulation.

Knowing the T crit not only makes it easy to ensure that the T crit is achieved during freezing, but also means that energy is not wasted by taking the temperature lower than required.

Methods for determining T crit are discussed below. Primary drying. The frozen material is initially dried by sublimation. During primary drying the pressure of the drying chamber is reduced to a very low level, while the temperature is raised slightly to allow the solvents to sublime.

Throughout this stage the temperature must be kept below the critical temperature T crit so that the material does not melt or its structure collapse.

One of the effects of sublimation is cooling of the product, which slows the process of drying. To counter this cooling and provide energy to drive the sublimation process, heat is added through the freeze-dryer shelf. The energy transfer during primary drying must be balanced so that sufficient heat is used to encourage sublimation without risking collapse. Collapse is the most serious processing defect in freeze drying, resulting in reduced shelf life, reduced stability, decreased product activity, and poor reconstitution see Figure 2.

Figure 2: A selection of vials containing the same freeze-dried material. The fill depth of all four vials was identical before processing. Although not required, an end-point detection system makes the sometimes days-long freeze-drying method easier to manage by keeping you informed of when your sample has finished drying. There are three stages in the lyophilization process: Pre-freezing, primary drying, and secondary drying.

The pre-freezing stage is the most important stage of the freeze-drying process. In this stage, sample material will need to be cooled to at least the temperature of the melting point for that sample. This ensures the sample will be completely frozen and can then undergo sublimation. The rate at which your sample freezes will affect the size of the ice crystals that form. If not done properly, it can impact the speed of reconstitution, length of the freeze-drying process and integrity and stability of your sample.

Larger ice crystals facilitate faster and more efficient lyophilization because water molecules are able to move more freely out of the sample during sublimation. For samples like food or tissues, large crystals can break the cell walls and damage your sample. In these situations, it is best for freezing to be done quickly through flash freezing, creating smaller ice crystals. Primary drying begins when you start your freeze dryer and vacuum pump.

With the low pressure environment, evaporative cooling of the sample begins, allowing for energy in the form of heat to speed the freeze-drying process. This stage can take several days, depending on the sample type and heat input. For laboratories that are using their freeze-drying equipment for sample preparation and resuspension, primary drying is where the run would end. For long term preservation of the sample, the run would continue on to secondary drying.

In the secondary drying phase, water molecules that are bound to the sample are released. This is an important thing to remember, not all lyophilizers are created the same. These lyophilizers have what is known as a spool piece in between the product chamber and the condenser. That spool piece is like a net. I will rotate you around to another lyophilizer that I have with the side panel removed.

What we see here, we see a little bit, this neck piece right here between the chamber and the condenser. That is the spool piece. That is important to remember because some lyophilizers do not have a spool piece. They may just have this product chamber, and right next to that product chamber where the shelves are placed is the condenser, meaning really cold coils. Those coils can influence the temperature of your product. That is neither a good nor bad, but it is something to be aware of when developing your process and transferring it.

Other lyophilizers still may not have a neck, but just this wall between the chamber and the condenser, with a plate that raises and falls depending on the stage of the process.

Something else that we need to discuss is how do we cool these vials? Where does this cold temperature come from? These shelves are hollow. They have a cooling fluid, or a heat transfer fluid, that rotates and flows through them.

Something else that is different between different lyophilizers is how that fluid flows. On some shelves it flows in a serpentine pattern, up and down. Why do we care? We care because that definitely will determine how our heat is distributed on shelf. When we have a full shelf full of vials, the vials on the inside part of the shelf, the inner portion, will be much cooler than those that are on the very edge. What comes into effect is the wall temperature, door temperature, how wide these channels are, and how well they cover the entire shelf.

That is something we need to be aware of. When we fill vials, we fill them on a tray, here is a manual operation in our lab. We have vials filled onto a tray, all stoppers are partially seated. You will notice a bunch of wires. These wires lead to vials that are equipped with thermocouples, so that we can monitor the temperature of our product during the process. Here is a vial with a thermocouple placed inside it.

What we try to do, since these thermocouples are point sensors, we try to align that point as close as we can in the center of the vial in the center bottom. We do that because as ice is removed, it is removed from the top down. The bottom is going to be the coldest, and that can provide us a measure of when our primary drying cycle is complete. It is not the best way to measure, but it is a possible measurement. It is also a way to determine how close we are to that failure point temperature for a product.

You will notice that I have a thermocouple placed in the front, the middle, and center. Different people place them with different methods.

The coldest area is going to be in the center, edge areas will tell you how warm it might be— the warmest temperature you might experience during the process. How do we place these in the lyophilizer? On a tray, the tray has a ring around it, so we place it in the lyophilizer and slide this top portion forward as we push.

Now the bottom of the tray and the vials make direct contact with the shelf. We can then plug the thermocouples into the different ports. This allows us to monitor the product temperature throughout the process. There are other types of thermocouples that we need to be aware of, or temperature monitoring systems.

This one happens to be from Tempress and you will see it has a large, not really that large, but a glass bottom to it. That bottom contains a crystal that vibrates. And that vibration or oscillation will directly translate into the temperature of our product.