Considerations on Re-Use of Sterilizing-Grade Filters - Pharmaceutical Technology

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Considerations on Re-Use of Sterilizing-Grade Filters
The author examines re-use of hydrophilic- or hydrophobic-membrane sterilizing-grade filters in liquid sterilizing applications.

Pharmaceutical Technology

Limitations of integrity tests

Filter membranes are often thought of as multiple cylindrical capillaries, in which bacterial retention is governed solely by size exclusion of incident bacteria that are larger than the largest pores in the membrane. Under such a model, bubble point-type tests can indicate the presence of excessively large pores or defects (i.e., holes, seal bypass) in the membrane and forward flow tests can provide a quantitative measurement of flow that demonstrates the absence of excessively large pores or defects.

Retention of bacteria through microporous membranes, however, is not solely a function of size exclusion by cylindrical pores smaller than the incident bacteria. Other properties of membranes can contribute to retention such as the shape and tortuosity of the porous structure, thickness of the membrane (i.e., length of the flow path through the membrane pores from upstream to downstream), and adsorptive forces, which may occur between the bacteria and the walls of any pores large enough for bacteria insertion. Neither forward-flow nor bubble point-type tests are fully capable of detecting changes to these secondary retention factors. Degradation of these conditions typically does not occur in a compatible validated membrane manufacturing process and the limitation of integrity tests to detect deviations in these retention variables can be underappreciated.

Figure 1: Diagrammatic cross-section of a pleated membrane filter showing locations of partial thickness membrane degradation at pleat crests incurred during inadequate cleaning and resterilization. (ALL FIGURES ARE COURTESY OF THE AUTHOR.)
Failure analysis of the re-used pleated filter cartridge ultimately identified the root cause of the filter penetration. The damage incurred during the multiple re-use cycles was manifested by chemical degradation of the membrane, resulting in a thinning that was localized at the pleat crests of the filter cartridge, as shown in Figure 1.

Localized chemical damage and thinning at pleat crests is typically indicative of partial drying of the filter whereby fluid components capable of chemically attacking the membrane under hot steam conditions are concentrated at the pleat crests during evaporation from those points. In this case, the partial degradation of the membrane's thickness was attributed to exposure to hot caustic during the SIP resterilization phase of the re-use cycle. The presence of residual caustic prior to SIP was attributed to insufficient rinse-out of the caustic cleaning agent, whereby subsequent evaporation of water from the cartridge before resterilization caused increased concentration of caustic at the pleat crests during the SIP resterilization phase of the re-use cycle. The elevated temperature from the steam on the residual concentrated caustic at the pleat crests then caused accelerated chemical degradation of the membrane face surface at the aforementioned locations.

The compromise in membrane thickness in these localized regions was sufficient to enable bacterial penetration. However, because the damage did not go all the way through the membrane (no holes or oversized pores), and the thin areas were limited to a very small total area at the pleat crests, neither the bubble point-type test nor the forward-flow test measurements exceeded their pass/fail limits.

Figure 2: Membrane damage during re-use may be detectable or nondetectable by forward flow (FF) or bubble point-type (BP) filter integrity tests. (ALL FIGURES ARE COURTESY OF THE AUTHOR.)
As illustrated in Figure 2, the bubble point-type test can only detect full-thickness hole defects. Forward-flow diffusion-type tests can provide values that relate, in principle, to membrane thickness, but they were unable to detect the limited thin areas isolated at some of the pleat crests in this case. The thin areas were not detected because they did not elevate the forward flow in excess of the flow test limit.


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