The most current demands on freeze dryers are validation and the ability to be 21 CFR Part 11compliant. These factors are a significant part of the cost of pharmaceutical processing freeze dryers.
For validation, a full component catalogue must be supplied. An installation qualification, operational qualification (IQOQ) document is generated that outlines the proper validation process, and a factory acceptance test (FAT) and site acceptance test (SAT) are implemented to verify that the system is supplied as ordered and performs within the required specifications.
The most common oversight is the concept that "all freeze dryers are the same". The choice of components, materials, construction and instrumentation create a wide variety in cost versus performance.
Older systems tend to have undersized compressors/condensers, as well as restrictions between the product chamber and the condenser, which limits the rate of freeze drying and often causes the freezedrying process to be extended. Today, freeze dryers are much better designed to accommodate the maximum load that may be placed in the system and the freeze dryer is not the limit to the process. For example, compressor reliability has significantly improved during the last 15 years. In small freeze dryers, the use of scroll compressors has virtually eliminated the failures common with reciprocating compressors.
As there are so many possible variations in size and features, advanced freeze dryers are 'built to order' where the end user works with the manufacturer to obtain a system suitable for their application requirements.
Innovations in freeze dryer technology
Thermal analysis and freeze drying microscopes have helped improve the understanding of the critical temperature — the temperature at which the product may collapse or melt-back — of the product being freeze dried. This knowledge provides the information required to produce a robust and efficient freeze drying cycle.
Classic freeze drying control is open loop, where the shelf temperature and chamber pressure are controlled based on a predetermined profile. It is assumed that the temperature of the product stays below its critical temperature, and the result is a reproducible — but very conservative and long — freezedrying cycle.
Closed loop control of the shelf temperature is required to both prevent collapse and minimise the length of the freezedrying cycle. The latest control systems use critical temperature information to dynamically control shelf temperature, which both protects against collapse and meltback whilst optimising the freezedrying cycle.
Methods that use an average measurement of the product in the chamber, such as calculated via pressure rise testing, adjust the temperature of the shelves a few times throughout the first half of the primary drying process. This process is limited to the first half of primary drying and only provides a conservative protocol. However, it is not optimised and does not take into account variations inside the chamber.
The latest control systems take into account both average and specific measurements to ensure there is no melt back and constantly control the shelf temperature throughout the entire cycle to produce a userselectable conservative or aggressive protocol.
In the future, more advanced closed loop control systems will be available that offer improved process control. Today, most applicable measuring instruments, such as tunable diode laser absorption spectroscopy, near infrared and mass spectrometry, are expensive and provide only marginal process improvement, which means they are not economically feasible for process control. As instrumentation and techniques advance, they will be incorporated into real-time process control systems.