Fluids that would be damaged by heat (such as those containing proteins like large molecule drug products, but also wine and beer) irradiation or chemical sterilization, can be only sterilized by Microfiltration using membrane filters. This method is commonly used for heat labile pharmaceuticals and protein solutions in medicinal drug processing. Commonly filter with pore size 0.2 µm (microfiltration) will effectively remove microorganisms. In processing of Biologics viruses must be removed or inactivated. Nanofilter with smaller pore size of 20 -50 nm (nanofiltration) are used. The smaller the pore size the lower the flow rate. To achieve higher total throughput or avoid premature blockage, pre-filters might be used to protect small pore membrane filters. In some studies it has been shown that Prions can be removed or reduced by filtration.
Membrane filters used in production processes are commonly made from materials such as mixester cellulose or polyethersulfone (PES). The filtration equipment and the filters themselves may be purchased as pre-sterilized disposable units in sealed packaging, or must be sterilized by the user, generally by autoclaving at a temperature that does not damage the fragile filter membranes. To ensure proper functioning of the filter, the membrane filters are integrity tested post-use and in occasions pre-use. The non-destructive integrity test assures the filter is undamaged, it also is a regulatory requirement enforced by agencies like FDA, EMA etc. For best results, final or terminal pharmaceutical sterile filtration is performed in cleanroom classes A.
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