Shrinkage of spray-freeze-dried microparticles of pure protein for ballistic injection by manipulation of freeze-drying cycle
Spray-freeze-drying was used to produce shrivelled, partially-collapsed microparticles of pure proteins that may be suitable for use in a ballistic injector. Various modifications of the freeze drying cycle were examined for their effects on collapse of the pure protein microparticles. The use of annealing at a shelf temperature of up to +10 °C resulted in no visible particle shrinkage. This was because of the high Tg' of the pure protein. Inclusion of trehalose or sucrose led to particle shrinkage because of the plasticizing effects of the disaccharides on the protein. Only by extending the duration of primary drying from 240 to 2745 min at shelf temperatures in the range −12 to −8 °C were shrivelled, wrinkled particles of bSA and bCA of reduced porosity obtained. Manipulation of the freeze-drying cycle used for SFD can therefore be used to modify particle morphology and increase particle density.