How important is filter testing in the pharma industry?
Filtration in the pharmaceutical industry covers a wide range of applications from powder processing to liquid purification,
such as saline or dextrose solutions. In the case of the former, the advantage of good filtration processes mainly lies in
recovering valuable drugs, which can cost in excess of $1 million per kilogram. The advantage here is purely commercial.
For intravenous products, however, good filtration is a matter of life and death; a simple internet search on recalled drugs
reveals particulate contaminants from the clearly visible, such as glass fragments, wood fibres and mould, to the invisible,
and potentially lethal, bacteriological contamination.
The primary test of a filter begins with the filter medium itself to ensure it is fit for purpose. Thereafter the filter system
must be tested in situ. Failure can occur in the filter medium or in the sealing arrangement when assembled.
One of the simplest methods of comparing filter media is to test porosity either by air or water flow under prescribed conditions.
Lower flow rates or higher back pressures indicate better filter performance.
The flow rate method is refined in the so-called bubble point test. Here, the filter is saturated with a liquid and then a
gas (usually air) is pressurised from below. As the pressure increases, the air finds the single largest pore, which is blown
out, forming a bubble on the surface: the bubble point. The Washburn equation is then applied to convert the applied pressure
into the diameter of the pore.
Porometry takes the bubble point one step further by continuing the pressurisation of the filter beyond the bubble point.
Successively smaller pores are blown clear by the air pressure until the smallest pore is finally evacuated. The applied pressure
versus flow rate profile can then be interpolated to provide complete pore size distribution.