It is common practice to freeze protein bulks for long-term storage, allowing product life to be extended up to several years.
For high-volume products such as antibodies, the batch size can be hundreds of liters. Freezing such large quantities in a
timely manner has generally been handled using "freeze–thaw vessels", which are jacketed metal tanks with a cooling surface
provided by internal coil and fin assemblies. With the increasing cost of high-grade steel alloys, it is desirable to explore
disposable technology alternatives to costly freeze–thaw vessels. The following is a report on the design and testing of a
prototype large-scale bag freezing system developed at Genentech.
Interest in disposable technologies for long-term frozen bulk storage has been on the rise. This is partly because of the
high capital cost of large freeze–thaw vessels, the scarcity of higher alloys as a material of construction, differences in
pressure vessel fabrication standards, and requirements from country to country. Disposable containers are typically available
in presterilized and custom fitted configuration, alleviating the need for clean-in-place and steam-in-place steps required
for traditional freeze–thaw vessels.
Polymeric containers with capacity of up to 20 L can be frozen in conventional freezers, though many days may be required
before the entire volume reaches the desired temperature. The Celsius, a commercial system offered by Integrated Biosystem
Inc. (a division of Sartorius Stedim Biotech, Goettigen, Germany), allows rapid freezing (several hours) of material in flexible
containers at up to 16.5 L per container with the ability to process up to six containers at once. However, the system requires
specialized equipment in addition to the freeze–thaw skid and is not operationally optimal for large batch sizes.
The lack of a commercial large-scale disposable container system that could meet our needs prompted us to develop the technology
in-house. The following reports our efforts at designing and testing the first prototype of a large scale freeze–thaw system
based on disposable bags that would complement our current frozen bulk storage strategy. We discuss our rationale and design
methodology, and present experimental results from the first at-scale prototype.
Design Constraints and Initial Concepts
Disposable bags used as liquid bulk storage containers are readily available in volumes of 1,000 L and above. The technology
is proven and, because of its high structural flexibility, is relatively resilient against mild impact from blunt objects.
Unfortunately, these soft-wall containers are much less forgiving when the contents are frozen. Light bumps may compromise
the integrity of the bag, and the puncture may not be discovered until the contents are thawed. Such difficulties must be
taken into account when designing a freezable bag system. Our aim was to develop a system that is compatible with facilities
already geared toward handling commercially available freeze–thaw tanks. We had four main design constraints:
1 Each individual container must hold at least 100 L. There is potential for large bulk sizes, possibly in excess of 1,000 L
per batch, and the use of many small containers would cause increased handling, tracking, and testing.
2 The system must be compatible with current commercially available freeze–thaw skids. The system also must be able to function
under the same conditions with operational times, equivalent (within a few hours) to those of current commercial freeze–thaw
3 The system must be compatible with the bulk transportation and storage infrastructure used by commercial freeze–thaw vessels.
The footprint and vertical dimensions must not be larger than those of existing 300-L freeze tanks in order to accommodate
storage freezers, air shipping containers, and entrances to existing facilities.
4 The system must cost less than current commercial freeze–thaw tanks.