The key benefit of hollow fiber bioreactor (HFBx) technology is the high cell density attained (>108 cells/mL) through the use of continuous media perfusion to feed cells and remove toxic wastes. These bioreactors consist of semi-permeable hollow fibers in parallel array housed within a polycarbonate shell (Figure 1).
There are two compartments within an HFBx: the intracapillary (IC) space within the hollow fibers and the extracapillary (EC) space surrounding the hollow fibers. The total volume within the polycarbonate shell that surrounds the fibers (EC space) depends on the size of the HFBx.
Typically, cells are seeded into the bioreactor EC space through a port on the top of the HFBx. Fresh medium is continually pumped through the lumen of the fibers. As the cell population expands, this medium feed rate is steadily increased to satisfy nutrient needs and prevent a buildup of metabolic waste. Depending on the average pore size of the semi-permeable hollow fiber membrane, larger MW components such as secreted protein (>10 KD) or viruses will be restricted from crossing into the IC space. Thus, concentrated product is typically harvested from the EC space at a steady rate for the duration of the culture.
Hollow fibers can be constructed from cellulosic, polysulfone, polypropylene, or polyethylene materials, thus allowing a choice depending on the characteristics needed for optimal protein or virus production. The vast majority of HFBx utilize cellulosic fibers for uniform cell expansion and production of mammalian cell-secreted products. Biovest International developed the first commercial-scale HFBx system in the mid 1980s, and ever since the most common application for this technology has been the large-scale production of monoclonal antibodies.
Hollow fiber bioreactors provide several fundamental advantages not found in other reactor systems currently used for virus production:
- growth of anchorage-dependent and independent cells at physiologic densities (>108 cells/mL) for extended periods of time (months),
- continual perfusion to promote normal cell function and 3-D interactions that cells normally experience in vivo, allowing for a more natural virus infection cycle,
- nutrient supply (sugars, amino acids, oxygen, etc.) at least as fast as cellular consumption requires, and removal of metabolic waste products (lactic acid, ammonia, etc.) at a rate that prevents toxic buildup,
- removal or dilution of harmful cytokines and virus-coded host shut-off proteins to avoid premature apoptosis,
- reduced labor requirements and user interactions, and
- virus containment in a small footprint for adaptation to BSL-2 and BSL-3 facilities.