In general, normal flow filtration operations can be divided into three main categories:
Cell culture media sterilization
One of the first unit operations in any biopharmaceutical operation is the preparation and sterilization of cell culture media. Cell culture media are nutrient-rich, buffered solutions containing amino acids, salts, vitamins, and energy sources (e.g., glucose)—all of which are essential components for the culture of healthy cells. Over the past several decades, formulations have evolved from generic basal media supplemented with animal-derived sera, to more cell-line specific formulations that are serum-free, animal-derived component-free and chemically defined. The sterilization of these media is critical to successful cell growth and protein expression.
There are many characteristics one should look for when selecting a cell culture media filter. The following paragraphs describe some of the most important features.
Sterilizing-grade membranes. The term "sterilizing-grade filter" is defined in the FDA's document Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing — Current Good Manufacturing Practice (1), which describes a sterilizing-grade filter as a filter that is "validated to re-producibly remove viable microorganisms from the process stream, producing a sterile effluent." The validation of sterilizing-grade membranes is commonly performed using the procedure documented in ASTM F838-05, "Standard Test Method for Determining Bacterial Retention of Membrane Filters Utilized for Liquid Filtration" (2). To be labeled "sterilizing-grade," a filter must produce a sterile effluent when challenged with Brevundimonas diminuta (B. diminuta) at a minimum concentration of 107 colony forming units (CFU) per square centimeter (cm2 ) of membrane area. Sterile filters are nearly always constructed of one or more sheets of polymeric membrane, either in pleated or flat-sheet form.