Filter integrity testing is critically important in biopharmaceutical production processes. However, when large numbers of filters need to be tested daily, it can be a time-consuming and costly business. Here brings you an wonderful introduction for paralling test when met such situations.

Sterilising grade venting filters are used to avoid the potential risk of air-borne germs contaminating containers such as fermenters or cell-culture vessels. Sterilising grade filtration can only be guaranteed when the filters used are tested for integrity. Therefore, those relatively small venting filters are often tested by the bubble point test. Depending on the program settings and the actual bubble point. it can easily take 10-20 min to perform one such test on the filters. When large quantities of filters are used and tested every day, any improvement regarding the required time would be beneficial as soon as it is guaranteed that there is no increasing risk of overlooking defective filters.

Bubble point tests

Small venting filters with a pore size of 0.2 μm and an effective filtration area of 20 cm² type were used for this investigation to perform bubble point tests using the parallel method. In order to prove their ability for detecting a single defective filter within an array of ten filters in a parallel arrangement, one filter of pore size 0.2 μm was replaced with a filter with pore size 0.45 μm. The theory behind this set up was that, although the 0.45 μm filter was perfectly intact and within specification for 0.45 μm filters, it should not pass the integrity test run on bubble point limit values for 0.2 μm filters. If this theory applied, the question would then be to determine if it is possible for an automatic integrity tester to reliably detect the out-of-specification filter between the 0.2 μm standard filters.

An essential prerequisite for performing bubble point tests is that the membrane is wetted completely with a suitable wetting liquid. Because the PTFE membrane of the filters is extremely hydrophobic and not spontaneously wettable by water, a 60% mixture of isopropyl alcohol (IPA)-water-mixture was used for wetting the membranes. In order to flush each filter with 20 ml of 60% IPA, a disposable syringe was used to apply the wetting solution in the direction of filtration. Because the testing procedure forced wetting liquid out of the pores, any filters used for repeated testing were re-wetted in the same manner to avoid any effect on the test results.

In the first step, the filters were wetted and then tested individually using the from Sartorius Stedim Biotech. The test results were documented electronically and additionally printed out on paper. In the second step, the same filters were mounted on the Midisart manifold containing an array of ten filters and the manifold was subjected to a parallel bubble point test using the same test unit. The results were documented.

In order to show that it is possible to detect a single defective filter among nine others, one Midisart 2000 filter with pore size 0.2 μm (min BP = 1.4 bar) was replaced with a Midisart 2000 filter with pore size of 0.45 μm (min BP = 0.9 bar). Then, the complete manifold with nine intact plus one assumedly ‘defective’ filter was tested. Although the 0.45 μm filter was intact, it should theoretically be identified as defective within the specification for the 0.2 μm filters (BP < BPmin).

The following test parameters were set:
Stabilisation time: 5 min
Min. BP: 1.4 bar
Max. BP: 1.8 bar

Before testing all filters separately the test series was started with a leakage test to prove the validity of the complete test setup. For this purpose, all valves of the test manifold were closed and a pressure hold test (P = 2 bar, t = 5 min.) was run with the integrity tester Sartocheck 4. The pressure was kept stable during the test without any signs of leakage.

Results

An average bubble point of 1.69±0.025 bar was found for the 0.2 μm Midisart filter, while the 0.45 μm version had a significantly lower BP of 1.23 bar. This meant that the 0.45 μm filter failed the test as soon as the BP limit (BPmin) for the 0.2 μm filter was used as test criterion.

The parallel test of the ten ‘intact’ Midisarts resulted in aall of 1.57 bar. This value correlates well with the BP values measured for the individual filters. The test was classified as ‘test passed’ and therefore all ten filters showed integrity.

In the next step where one ‘intact’ Midisart was replaced with the ‘defective’ filter (0.45 μm) and the array of the ten filters (nine ‘intact’ plus one ‘defective’ with a low BP) was subjected to a BP test at the identical test parameter settings, the test resulted in a BP of 1.23 bar and the test was classified as ‘test failed’. The BP value of all ten filters was clearly defined by the filter with the lowest BP (1.23 bar) which was the so-called ‘defective’ one. This meant that the ‘defective’ filter was successfully detected when ten filters were measured in parallel.

In order to confirm this result, the defective filter was pneumatically decoupled by closing the connected valve and the remaining nine intact Midisarts were retested after re-wetting. This test resulted in a BP of 1.57 bar, i.e. it was identical to the test result obtained when all ten ‘intact’ filters were tested.