Aerodynamic Droplet Stream Expansion for the Production of Spray Freeze-Dried Powders

Abstract

In spray freeze-srying (SFD), a solution is sprayed into a refrigerant medium, frozen, and subsequently sublimation dried, which allows the production of flowable lyophilized powders. SFD allows commonly freeze-dried active pharmaceutical ingredients (e.g., proteins and peptides) to be delivered using new applications such as needle-free injection and nasal or pulmonary drug delivery. In this study, a droplet stream was injected into a vortex of cold gas in order to reduce the risk of droplet collisions and therefore droplet growth before congelation, which adversely affects the particle size distribution. Droplets with initial diameters of about 40–50 μm were frozen quickly in a swirl tube at temperatures around −75°C and volumetric gas flow rates between 17 and 34 L/min. Preliminary studies that were focused on the evaluation of spray cone footprints were performed prior to SFD. A 2³ factorial design with a model solution of mannitol (1.5% m/V) and maltodextrin (1.5% m/V) was used to create flowable, low density (0.01–0.03 g/cm³) spherical lyophilisate powders. Mean particle diameter sizes of the highly porous particles ranged between 49.8 ± 6.6 and 88.3 ± 5.5 μm. Under optimal conditions, the mean particle size was reduced from 160 to 50 μm (decrease of volume by 96%) compared to non-expanded streams, whereas the SPAN value did not change significantly. This method is suitable for the production of lyophilized powders with small particle sizes and narrow particle size distributions, which is highly interesting for needle-free injection or nasal delivery of proteins and peptides.