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The authors describe a novel approach for the integrity testing of large sterile filter systems such as multiround housings and describe a multipoint diffusion test capable of detecting minor failures.
Integrity testing multiround filter housings is difficult and inaccurate because of the multitude of filter elements involved. The diffusive flow sum of all of the filter elements can make it difficult to detect a single flawed filter or determine the bubble point which is bulk flow point.
The single-point diffusion test reflects the overall diffusion rate of the cartridges being tested, thus increasing the risk for a minor failure (e.g., one single filter element with a loss in integrity) being potentially masked by the overall low diffusion values of the surrounding cartridges. A statistical approach with a reduced maximum allowable diffusion test limit would reduce this potential risk.
The bubble point will reflect the biggest pore-size area. Its accuracy depends on the capacity of the integrity test unit to detect the over-proportional increase of gas flow when passing from the diffusive flow region to the bulk flow region. For a large filtration area this transition phase potentially will be masked because of the overall high diffusion rate. In addition, the bulk flow through possible flaws does not increase exponentially as the test pressure increases, thus making bubble-point determination difficult.
The multipoint diffusion test will create a diffusive flow profile (diffusion versus applied pressure) of the filter system over a broad range of test pressures all the way up to the characteristic bulk flow of the bubble point. The combination of multipoint diffusion test and bubble point leverages the advantages of the combination and increases a higher accuracy of detection.
Some automated multipoint diffusion-test systems allow programming of a specific maximum allowable diffusion value for every pressure step, thus setting a maximum diffusion profile.
The following materials were used: automated integrity-test unit (Sartocheck 4, Sartorius Stedim Biotech GmbH); five pieces of 30-in. PESU 0.2-µm cartridges (Sartopore 2, Sartorius Stedim Biotech GmbH), referred to as A, B, C, D, and E, respectively; one piece of 20-in. PESU 0.2-µm cartridge (Sartopore 2), referred to as F; one piece of 10-in. PESU 0.2-µm cartridge (Sartopore 2) failing cartridge (low bubble point), referred to as G; one piece of 10-in. PESU 0.2-µm cartridge (Sartopore 2) failing cartridge (high diffusion), referred to as H; one multiround (5 x 30 in.) housing; one single-round 30-in. housing; and one single-round 10-in. housing.
All filter cartridges were individually tested three times with a multipoint diffusion test over a range of pressure points. The lowest point was at 2500 mbar (36.25 psi), which is the traditional single-point diffusion test pressure for the filter used. The highest point was 3250 mbar (47.13 psi), which is 50 mbar (0.73 psi) above the specified minimum allowable bubble point of the filter tested. The filter cartridges were thoroughly rinsed with demineralized water at room temperature before each integrity test.
After the diffusion profile curves (from the multipoint diffusion test) and the individual bubble point values were generated, the data were analyzed. The individual cartridge values were used to mathematically simulate different filter combinations in a multiround housing (see Figure 1).
Figure 1: Individual values of cartridges A, B, C, D, F, and H and their sum. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
To verify the mathematically simulated test results, the 30-in. filter elements (i.e., A, B, C, D, and E) with no break in integrity were installed into the multiround housing and tested with the bubble point and multipoint diffusion tests. All cartridges were tested three times and rinsed between each test (see Figure 2).
Figure 2: Multiround housing 5 x 30-in. cartridges A, B, C, D, and E. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
One 30-in. cartridge (cartridge E) was then replaced by a 20-in. cartridge. A 10-in. filter housing, containing the bubble-point failing cartridge, was connected in parallel with the multiround housing, consequently representing a five-round 30-in. setup in terms of membrane surface. The system containing cartridges A, B, C, D, F, and G was tested with bubble-point and multipoint diffusion tests. All cartridges were rinsed between each of the three tests (see Figure 3).
Figure 3: Multiround housing and 10-in. housing, 4 x 30-in. cartridges A, B, C, and D; 1 x 20-in. cartridge F; and 1 x 10-in. cartridge G or H. ( (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
The bubble-point failing cartridge (cartridge G) in the 10-in. housing was then replaced by the high-diffusion cartridge (cartridge H), and the same tests and rinses were performed. Instead of performing a separate single-point diffusion test at 2500 mbar (36.25 psi), the authors considered the diffusion value at the same pressure during the multipoint diffusion test was representative.
Individual test result
The expected difference in diffusion between five 30-in. cartridges (i.e., A, B, C, D, and E) (137.9 mL/min) without loss of integrity and an equivalent setup including the bubble-point failing 10-in. cartridge (i.e., A, B, C, D, F, and G) (139.4 mL/min), at the standard test pressure of 2500 mbar (36.25 psi) was <2 mL/min, thus being not possible to detect with a high degree of confidence.
An equivalent setup including the failing 10-in. high-diffusion cartridge (i.e., A, B, C, D, F, and H) would require the maximum allowable diffusion value to be lowered by more than 40% as compared with the cumulated individual maximum diffusion values to be detected at all. Using such a high safety margin creates a high potential risk of getting false-failure test results with filters that otherwise have no break of integrity.
Based on the typical diffusion profile of a cartridge with no loss of integrity, it was possible to define a maximum allowable diffusion curve, which made it possible to get a clear multipoint-diffusion test failure when the bubble-point failing 10-in. cartridge was included in a multiround setup (cartridges A, B, C, D, F, and G). The same approach showed that it should be possible to get a clear test failure when having the 10-in. high-diffusion cartridge in a multiround set-up (cartridges A, B, C, D, F, and H).
To simulate the expected bubble-point value of various multiround setups the automated integrity test bubble-point test algorithm was programmed in an Excel (Microsoft) spreadsheet, and the diffusion profiles of the individual cartridges were entered. The authors found that it should be possible to get a clear bubble-point test failure involving the 10-in. bubble-point failing cartridge in a multiround setup (cartridges A, B, C, D, F, and G). In addition, the authors found that it should not be possible to get a bubble-point test failure when including the failing 10-in. high-diffusion cartridge in a multiround set-up (cartridges A, B, C, D, F, and H), because no exponential increase would be expected to take place before the minimum bubble point.
Multipoint diffusion limit curve
The diffusion profile of a filter cartridge can be plotted in two distinct areas, with the applied differential pressure on the x-axis and the resulting diffusion on the y-axis.The first area is the so called diffusive flow region, where the diffusion rate can be considered linear as a function of the applied test pressure. In this test-pressure region, the pores are still filled with the wetting liquid.
The second region is the so-called bulk flow region where the applied pressure gradually blows out an increasing number of pores, giving a free-gas flow through the membrane. The flow graph turns exponential.
Between the linear and exponential area is the so-called inflection point or transition phase. The sharpness of the inflection point is directly related to pore-size distribution, pore design, and the size of the filter surface. The smaller the filter surface and the more homogeneous the pore-size distribution, the sharper the inflection point (see Figure 4).
Figure 4: Small filter surface inflection point versus large filter surface inflection point. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
The multipoint diffusion limit curve is based on the diffusive flow and the bulk flow and is defined by three set points. It is a straight line from the first test pressure point, which is the common single-point diffusion-test pressure (in this case 2500 mbar or 36.25 psi), up to a point at 100 mbar before the minimum bubble point (in this case 3100 mbar or 44.95 psi). The last point at 50 mbar after the minimum bubble point (in this case 3250 mbar or 47.15 psi). This much higher maximum allowable diffusion rate allows one cartridge to reach the bubble point at the minimum bubble point without giving a false failed test result (see Figure 5).
Figure 5: Multipoint diffusion test limit curve. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
The maximum allowable diffusion value for the two first pressure steps is calculated taking into account the number of cartridges, their individual maximum allowable diffusion value, the applied test pressure, and a safety margin. The maximum allowable diffusion value for the last test-pressure point is calculated taking into account the same variables as the two previous set points but allowing one cartridge to reach bubble point. This is achieved by adding a cartridge size-related diffusion value representing the over-proportional increase generated by the bulk flow at the bubble point. In the integrity-test unit bubble point test algorithm, this value is called the A2-value (c.f. Functional Design Specification of Sartocheck 3, Sartocheck 3+, Sartocheck 4). The set point equations thus become:
First set point at p1:
Diffmaxp1 = nbDiffmaxindividual@p1 α
Second set point at p2:
Diffmaxp2 = nbDiffmaxindividual@p1 α(p2/p1)
Third set point at p3:
Diffmaxp3 = (nb – 1)Diffmaxindividual@p1 α(p3/p1) + A2
in which nb is the number of cartridges in the multiround housing, nb – 1 is the value of nb minus the cartridge eventually reaching the bubble point, Diffmaxindividual@p1 is the validated maximum allowable diffusion value for the concerned cartridge type (here 54 mL/min), α = (1 – safety margin), p1 is the standard diffusion test pressure (here 2500 mbar or 36.25 psi), p2 is the second set point (here 3100 mbar or 44.95 psi), p3 is the third and last set point (here 3250 mbar or 47.15 psi), and A2 is the cartridge size–related diffusion value (here 240 mL/min).
During out-of-box trials using new 30-in. Sartopore 2 filter cartridges, the typical diffusion value was only slightly higher than half of the maximum allowable diffusion value given by the manufacturer. During the simulation trials, a maximum allowable diffusive flow limit one-third lower than the standard manufacturer's limit created a robust test result without causing false failures. The a-value was thus set to two-thirds, and the programmed multipoint diffusion limit line became:
Max diffusion at p1 = 5 x 54 x (2/3) = 180 mL/min
Max diffusion at p2
5 x 54 x (2/3) x (3100/2500) = 223.2 mL/min
Max diffusion at p3
(5-1) x 54 x (2/3) x (3250/2500) + 240 = 427.2 mL/min
The trials showed that the traditional single-point diffusion test was not accurate enough for multiround housing integrity testing. Subtracting more than 40% from the cumulative individual maximum diffusion values would have detected the failing high-diffusion cartridge (cartridge H), but it also would have increased the risk for getting false-failure test results. Single-point diffusion testing also was incapable of detecting the bubble-point failing cartridge even when the maximum value was reduced by almost 50%.
Figure 6: Multipoint diffusion test on cartridges A, B, C, D, and E. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
The bubble-point test, however, was capable of detecting the bubble-point failing cartridge. If the multiround setup contained the high-diffusion cartridge as the only failing cartridge, the bubble point was incapable of detecting this flawed filter because the high diffusion cartridge did not have a clearly defined bubble-point value below the minimum bubble point. The difference between the expected bubble-point value and the measured bubble point was ≤50 mbar (0.73 psi).
Figure 7: Multipoint diffusion test on cartridges A, B, C, D, F, and G. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
The multipoint diffusion test was capable of detecting the flawed high diffusive flow filter element respectively low bubble-point filter element. The difference between expected diffusion values and measured diffusion values on the multiround housing was less than ±5% (see Figures 6–8).
Figure 8: Multipoint diffusion test on cartridges A,B, C, D, F, and H. (ALL FIGURES ARE COURTESY OF THE AUTHORS.)
The integrity test of sterilizing filters is a fundamental element in performance and retentivity assurance. Only a test procedure capable of detecting minor failures fulfills the requirements for high-quality assurance. No integrity test would be able to detect an infinite number of small membrane failures. Nevertheless, the production release testing of filter manufacturers and the process validation minimize the risk of those shortcomings.
The described trials were performed with cartridges rejected by the manufacturers' release criteria. Such minor failures are unlikely to happen after production release and could thus represent worst-case conditions.
The tests were performed with polyethersulfone (Sartopore 2) 0.2 µm only. Other sterilizing-grade membrane filter cartridges may require different parameters, depending on membrane type, design, and polymer.
Increased safety margins will be required to ensure the same degree of integrity-test reliability for multiround housings larger than 5 x 30 in. Using the simulating approach discussed in this article, one can see that to detect a single failing 10-in. element in an 8 x 30 in. multiround housing, the maximum allowable diffusive flow must be lowered by 37%, instead of 33%
In the future, the authors will perform additional trials using the same type of cartridge to increase the statistical background. Different cartridges will also be tested to determine whether other membrane types and membrane configurations may be tested with the same accuracy.
Pascal Martin* is a project manager at Merial SAS, Lyon, France, tel. +33 4 72 72 39 61, fax +33 4 72 72 34 93, firstname.lastname@example.orgMagnus Stering is head of Application Services South Europe Filtration, Sartorius Stedim Biotech SA, Aubagne, France. Maik W. Jornitz is vice-president of product management FT/FRT, Sartorius Stedim North America Inc., Edgewood, NY. Jens Meyer is a product manager of integrity testing at Sartorius Stedim Biotech GmbH, Goettingen, Germany.
*To whom all correspondence should be addressed.