Best served cold: The world of freeze drying microscopy

Dr Duncan Stacey, Linkam Scientific Instruments gives an expert insight into the world of lyophilisation and freeze drying microscopy.

Lyophilisation, or freeze drying, is a process that is already widely used in the pharmaceutical industry. The basic process is to remove solvent, usually water, from a drug formulation to provide a stable, controlled form of the drug that can be easily transported without special storage conditions. Freeze drying also extends the product shelf life. The end result of the lyophilisation process should be a dry and stable product that can be stored and transported in normal, ambient condition and can be easily reconstituted to a liquid product. The route to understanding this process can be complex, time consuming and expensive.

The development of the ideal freeze drying protocol or recipe is one of the key areas of research when moving a new compound from the drug development phase to production. Traditionally the development of the lyophilisation process has used large freeze dryers. This requires a lot of time and cost as the compounds are typically of high value and the large freeze drying cabinets are expensive to run.

In order to minimise time and cost in developing the freeze drying process, Freeze Drying Microscopy (FDM) has become a widely used technique. FDM enables the direct visualisation of the freeze drying process using a specially optimised microscope and thermal stage.  FDM can be used early in formulation to determine how the product will react to different thermal conditions. Waste is minimised as an FDM typically uses extremely small volumes of the product.

There are three main stages in the freeze drying process: The initial freezing followed by primary and secondary drying. During stage one, the formulation is cooled below its eutectic point (this is the lowest temperature at which solid and liquid phases can coexist). The primary drying phase is where the water is removed by sublimation under vacuum. In the final, secondary drying phase, unfrozen water is removed by raising the temperature of the product.

In the freeze drying process, it is important to understand the temperature and pressure required for each of the freezing and drying stages. One of the critical points is the collapse temperature (Tc) (Figure 1). This is the temperature at which the structure of the formulated product weakens and is no longer able to support itself. At this point, drying is incomplete and can lead to an unstable product when reconstituted and result in a poor looking product.

FDM combines light microscopy techniques such as phase contrast and polarised light with a freeze drying stage where the temperature and pressure can be accurately controlled and monitored (Figure 2). It is now possible to quickly and accurately determine collapse (Tc) and eutectic temperature (Te), whilst also investigating the freeze dried structure of complex samples. Both stage pressure and temperature are programmed to simulate industrial procedures and determine the ideal drying parameters. Imaging is automated so images of the sample can be directly correlated to the temperature and pressure conditions in the stage (Figure 3).

FDM should be seen as the first stage in the development of a freeze drying process. It provides invaluable information about how a formulation reacts to the process while minimising cost and time making it an extremely efficient technique in the lab.

The next stage is to scale up the results to test how the product reacts when freeze dried within an industry-standard vial. Until recently, this has required moving to a bench top or an industrial scale freeze drying cabinet. These still require a relatively large number of vials and thus a larger quantity of expensive product has to be used. These cabinets are costly to run and may be difficult to access on if they are also being used for production. A new development tool has recently been launched which is slots neatly into the development process between the freeze drying microscope and the full scale freeze drying cabinet. The Freeze Drying Vial System (FDVS) incorporates a chamber with the capability to house up to seven industry-standard lyophilisation vials (Figure 4/5). By incorporating vials, the FDVS works with larger sample volumes using enough material to better simulate large scale industrial processes while minimising sample wastage.

The FDVS incorporates an optical system with real time imaging so the sublimation front can be followed and any structural changes in the sample monitored as lyophilisation occurs. The vials can be stoppered while still in the chamber once the freeze drying process is complete. These can then be removed for further analysis of the freeze dried product.

The combination of a freeze drying microscope with the new FDVS provides important parts of the toolkit required by lyophilisation labs from formulation to production.