During gap freezing, vials are placed on a metal tray, which is separated from the shelf surface with a small air gap that eliminates significant conductive heat transfer from the shelf to the bottom of the vial. The purpose of this freezing approach is to reduce the lyophilization cycle time of various amorphous formulations by nearly isothermal freezing. Such isothermal freezing promotes the formation of large ice crystals, and thus large pores throughout the cake, which subsequently accelerates the primary drying rate. The nucleation temperature using gap freezing, for the experimental conditions tested, was in the range of -1°C to -6°C, much higher than the range of -10°C to -14°C found using conventional shelf freezing. Isothermal freezing becomes effective when the gap is greater than 3 mm. The pore sizes and cake resistance during primary drying for various formulations were determined using the pore diffusion model developed by Kuu et al. (Pharm Dev Technol, 2011, 16(4): 343-357). Reductions in primary drying time were 42% (for 10% sucrose), 45% (for 10% trehalose), and 33% (for 5% sucrose).
Keywords: algorithm; crystallization; diffusion; freeze-drying; lyophilization; mathematical model; porosity; transport.
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