PubMed1994
Freeze drying provides a valuable tool to the formulation scientist by permitting dehydration of heat-sensitive drugs and biologicals at low temperature. The final product is quickly and easily reconstituted, and the process is compatible with aseptic operations. Freezing is a critical step, since the microstructure established by the freezing process usually represents the microstructure of the dried product. The product must be frozen to a low enough temperature to be completely solidified. If the solute crystallizes during freezing, this temperature is the eutectic temperature. If the solute remains substantially amorphous with freezing, the relevant temperature is the collapse temperature. Understanding the physical form of the solute--crystalline or amorphous--after freezing can be important from the standpoint of drying characteristics, appearance of the final product, and even product stability during storage. Supercooling is a significant factor in freezing of formulations intended for freeze drying--prior to both primary and secondary (eutectic) crystallization. The driving force for freeze drying is the difference in vapor pressure of ice between the sublimation zone and the condenser. Because the vapor pressure of ice increases sharply with increased product temperature, it is important from the standpoint of process efficiency to maintain product temperature as high as possible during primary drying without damaging the product. The upper limit of product temperature during primary drying again depends on the physical form of the solute. Exceeding either the eutectic temperature (crystalline solute) or the collapse temperature (amorphous solute) results in loss of the desirable properties of a freeze dried product. Freeze drying is a coupled heat and mass transfer process, where either heat transfer or mass transfer may be rate limiting with respect to the overall drying rate. Heat transfer is often the rate-limiting transfer operation because of the high heat of sublimation of ice and the inefficiency of heat transfer. Conduction is the primary mechanism of heat transfer, as opposed to convection or thermal radiation. The rate-limiting resistance to heat transfer is usually the interfacial, or contact, resistance caused by poor contact between materials--the heated shelf, metal trays, and the bottom surface of glass vials. Since the thermal conductivity of a gas is directly proportional to pressure in the free molecular flow regime, the chamber pressure during primary drying is an important determinant of the overall heat transfer rate. As a result, the drying rate for a heat transfer-limited process increases sharply with chamber pressure up to a pressure where free molecular flow conditions no longer apply.(ABSTRACT TRUNCATED AT 400 WORDS)