This article focuses on new designs of sterilizing-grade filters that remove bacteria, toxins, and other contaminants from initial purification steps to final formulation and filling. We discuss these important parameters in filter selection: materials, compatibility, wettability, sterilization, adsorption, and membrane and filter construction.
Materials and Compatibility
The ability to wet a filter completely is important, because the objective is to use the full filtration area. If a significant portion of the membrane is not wetted properly, then the filtration flow can be impaired. In addition, it is necessary to have a fully wetted membrane during the performance of an integrity test. If the filter membrane is not completely wetted, then even a very small non-wetted area of the filter could cause a failure of the integrity test, causing process delays and re-testing.
In terms of wettability of membrane material, Nylon 6,6 is the most easily wetted because it is naturally hydrophilic. PVDF is naturally hydrophobic, and for liquid applications will under go some sort of modification to make the membrane hydrophilic. This version is close to Nylon 6,6 in terms of wettability. Most common PES filters do not wet easily and usually require backpressure to wet completely, making integrity testing more difficult and time consuming. However, recent improvements in PES wetting chemistry have rendered its wettability comparable to PVDF membrane filters.
The purpose of filtration is to remove a particular contaminant or groups of contaminants from the process feed. The filter system should not be adding anything significant (extractables) to the process fluid, nor should it remove a desired product component from the fluid that is being filtered.