Lyophilisation, or freeze drying, is an important manufacturing step widely used to increase the stability of pharmaceutical and biological products. Developed during the 1940s, lyophilisation produces a dry product that can be readily reconstituted to its original form by adding water when required (1). It prolongs product shelf life by inhibiting chemical, microbiological and physical degradation pathways that occur in the presence of moisture, particularly where long periods of storage and transit prior to use are involved (1), (2).
The lyophilisation process consists of three main stages--freezing (solidification), primary drying (ice sublimation) and secondary drying (moisture desorption)--and usually takes several days to complete. Multiple vials containing a liquid drug formulation are loaded on temperature-controlled shelves within a sterile chamber and cooled to low temperatures until completely solidified (2). After that, chamber pressure is reduced and shelf temperature is raised to remove the frozen solvent through sublimation. The remaining unfrozen solvent that is chemically bound to the solid product is removed by a desorption process (3). The drying process is concluded by stoppering the vials in the chamber, generally under a subambient pressure of inert gas. The final dry product, called a cake, usually occupies approximately the same volume as the initial liquid fill because of its high porosity (2). "To ensure that high quality products are consistently produced, it is crucial to be able to control and provide repeatability of the lyophilisation cycles," says Joseph Brower, technology manager at IMA Life North America.
The freezing step
The freezing step is one of the most important steps in lyophilisation because it determines the texture of the frozen material and consequently, the final morphological characteristics of the freeze-dried material and its biological activity stability (4). "Proper freezing creates the foundation for efficient and consistent freeze-drying cycles," says T.N. Thompson, president of Millrock Technology, a company that develops freeze drying/lyophilisation systems for laboratory applications and cGMP production.
The three steps in the freezing process are nucleation, crystallisation of the freeze concentrate, and for the maximal freeze concentrate, either freeze separation in eutectic products or concentration in amorphous products. The parameters of the freezing protocol directly affect pore size distribution and pore connectivity of the porous network of the freeze-dried matrix. The ice-crystal morphology determines both mass and heat transfer rates through the dry layer and as a result, freezing parameters have a strong influence on the total duration of the primary and secondary drying steps (4).
The nucleation process
During the freezing phase of a typical freeze-drying cycle, the nucleation process of which the first solid domains are formed occurs randomly in the vials. "In an uncontrolled environment, due to the lack of nucleation sites in pure systems, the formulation solution must be cooled down to temperatures that are significantly lower than the equilibrium freezing point (i.e., supercooled) to initialise formation of ice crystals," Brower explains.
The contents of individual vials often nucleate or begin freezing over a broad range of temperatures, "usually spanning 10-15 [degrees]C below the formulation's thermodynamic freezing point in...