Introduction

Biotechnology began with recombinant DNA technology, cell fusion techniques, and protein production techniques. With advances in genomics, numerous biological targets are likely to be identified, paving way for generating a wider array of products. Transgenic technology has also begun to develop in parallel. As technologies and techniques continue to dwell deep into established areas, biotechnology and bioprocessing are bringing about new products every minute.

Hollow-Fiber Membrane

The applications of hollow-fiber membranes include blood purification, purification and desalination of water, artificial organ devices, complex solute mixture separation, and the production of therapeutic and diagnostic proteins. Despite the differences in the application, membrane permeability for water and solutes, primarily hollow-fiber modules share many performance and design features, which include tube and shell arrangement, fibers packed at high density, importance of membrane properties and flow configuration, and the occurrence of the polarization phenomena.

Hollow-Fiber Reactors

Hollow-fiber reactors were first used in the 1970s to immobilize whole cells and enzymes. Since then, whole-cell immobilization has taken over enzyme immobilization due to certain advantages. Some of the advantages include:
(1) intracellular enzymes exhibit greater stability;
(2) expensive enzyme purification process is not required; and
(3) whole cells can catalyze metabolic pathways.

The key advantages of perfused immobilized cell-culture over the traditional culture suspension include:
(1) cell protection from extreme stress;
(2) high productivity;
(3) high cell density;
(4) increased concentrations of products; and
(5) minimal medium requirement.

In commercial terms, cell culture systems have high demands in diagnostic and therapeutic proteins.Examples include: erythropoietin (erythrocyte production stimulator), interferons, human growth hormone, monoclonal antibodies (MAbs), and factor VIII (blood coagulation factor). Recombinant DNA technology plays the roles of an attractive and an effective tool for obtaining numerous products from plant, insects, yeast, or bacterial cells.

Though these cultures require only simple medium for higher growth rates, scientists found out that most proteins are fully expressed only by mammalian cells.

Hollow-Fiber Process

Although, many Hollow-fiber bioreactors (HFBRs) designs have been proposed, scientists still prefer the conventional method that involves entrapping mammalian cells in shell side of a cylindrical cartridge or extra- capillary space (ECS) with medium recirculating through the inter-capillary space (ICS) or fiber lumina. The recycle flow of ICS, low molecular weight exchanges between metabolites and nutrients in the ECS, and analogous blood flow via capillaries provide the cells a stable and strong growth environment. Higher-molecular weight proteins that are required for growth of the cell are supplied at right intervals.

Fresh medium is supplied into the shell via an ECS port while the harvested protein product is collected from other ECS port. The presence of ultrafiltration membranes allows high-molecular weight (high-MW) concentration in the ECS before the protein product is harvest thereby reducing time and the cost of the downstream purification process. The ICS medium is aerated in a hollow-fiber oxygenator to provide oxygen for the cells. The two HFBR ECS ports remain closed during 90 percent of the operation; this is called closed-shell mode (corresponding configuration flow). Secondary flow in ECS is named as Startling flow to honor the physiologists E.H. Starling, who pioneered the study of fluid exchange between blood vessels and body tissues.

Protein Transport

The concept of protein transports dates back to the past century. Starling flow allows the convective accumulation of high-MW proteins and cells in the reactors downstream part. Inoculation of ECS with a cell medium and harvesting product from the ECS is a good example of an open-shell operation. This operation is conducted counter-currently (inoculation) or co-currently (harvesting) pertaining to the ICS flow. The HFBR cartridge is usually inclined at 45 degree to horizontal (ICS recycle flow is directed upward). The aim of this configuration is to reduce cell sedimentation and downstream polarization effects.

Conclusion

The challenges and promises HFBR cell cultivation have given rise to good deal of modeling work. Majority of the models include kinetics of substrate consumption and neglects convective transport in the ECS and in membranes. ECS convective transport plays a vital role in the HFBR operation pertaining to experimental and theoretical studies. Models, which include the osmotic pressure effects on the ECS distribution of protein HFRR cell growth dynamics, and hindered protein transport, have been recently proposed. Hollow- fiber reactors have a much broader relevance in biotechnology and thus provide useful experimental insights.

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