Destructive testing is usually performed by the manufacturer on a statistical sample of each batch of membrane and production equipment produced. At the same time, the manufacturer conducts non-destructive testing on each sterilizing grade filter before sale to ensure its integrity.
The product delivered to the end user is integrity tested, but after the product is released from the manufacturing company, there is a possibility that it will be damaged during the shipping/supply to the end user, so pre-use and post-use integrity is best performed by the end user testing to ensure product integrity.
1) Destructive filter integrity check:
Bacterial challenge assays are used as destructive assays. In this assay, a culture medium solution containing bacteria is introduced upstream of the filter and ensures that zero organisms are recovered after the filter. This test is called destructive, because in this test the microbial culture is “intentionally” loaded and cannot be used anymore, ie the filter has to be destroyed.
2) Non-destructive testing:
Such detection occurs on the user side before and after the filter is applied. There are three types of non-destructive testing:
i) Bubble point test
ii) Diffusion flow test
iii) Water Intrusion Test or Water Flow Test
Bubble point test
The bubble point is the amount of pressure, when applied to the upstream side of the filter, that overcomes the water tension of the largest pore in the system and forces water through the membrane pores while air flows freely through the pores. This method is most commonly used in small filtration devices and can be used with both hydrophobic and hydrophilic filters.
The method can be performed manually or using automated equipment that measures the minimum bubble point value. This test is based on the fact that liquid is held in the pores of the filter by surface tension and capillary forces, and it is used to measure the pore diameter by measuring the minimum pressure required to force the liquid out of the membrane pores.
The bubble point is expressed as:
BP= 4*k*γ*cosѲ/d
Where: k = shape correction factor,
γ = surface tension
Ѳ = contact angle
d = aperture
The simple operation procedure of bubble point test is as follows:
– Wet the filter with a suitable liquid.
– Pressurize the system to 80% of the expected bubble point pressure in the product instructions.
– Slowly increase the pressure until a rapid sustained bubbling is observed at the outlet.
– The point at which bubbly flow is observed is the bubble point of the filter.
Manual detection:
This test can be done manually, as it is done automatically, where a pressure gauge is used to test the case for a pressure check.
– Manually apply 80% of test pressure to the upstream side of the filter and allow to stabilize.
– The outlet is placed in water so constant bubbling can be observed.
– Slowly increase the pressure until a bubble flow is observed at the outlet.
– The point at which bubbly flow is observed is the bubble point of the filter.
The test results are recorded as pressure measurements (bubble point in mbar). The minimum bubble point is provided by the manufacturer and must be used during assay development, while the maximum bubble point is set by the user during assay development.
For example, for a filter, if the manufacturer provides a minimum bubble point of 1000 mbar, then this is the pass criterion for the filter.
Diffusion flow test
The Diffusion Flow Test applies a constant air pressure on the upstream side of a liquid-filled filter and measures the volume of air that diffuses through the filter over time. At different gas pressures, gas molecules migrate through the water-filled pores of the wetted membrane according to Fick’s law of diffusion.
The gas diffusion flow rate is proportional to the pressure difference (upstream and downstream sides of the filter) and the total surface area of the filter. Diffusion flow test pressures are below the minimum bubble point because the gas dissolved in the liquid is trapped in the pores and begins to diffuse out.
The test method can be carried out manually or using automated equipment. The simple operation procedure of the diffusion flow test is as follows:
– Wet the filter with a suitable liquid.
– Slowly increase the pressure on the upstream side of the filter to the manufacturer’s recommended test pressure.
– Let the system balance.
– In automated equipment, the gas diffusion is measured by the equipment, while manually it is necessary to measure the gas flow at the outlet in one minute with an inverted graduated cylinder or a flow meter.
The test result is measured by the diffusion volume in ml.
Pressure hold test/pressure decay/pressure drop test:
This assay is a “variant” of the diffusive flow assay. In this test, high-precision gauges are used to monitor changes in upstream pressure due to gas diffusion through the filter. Pressure hold values depend on diffuse flow and upstream volume. It can be calculated using the following equation,
Pressure hold (Δp) = D*T*Pa/Vh
Δp = pressure drop (bar or psi),
D = diffusion rate (ml/min),
T = time (min),
Pa = atmospheric pressure (1 atm or 14.7 psi),
Vh = upstream volume of the device (ml).
Water Intrusion Test
This method is used for hydrophobic filters. In this test, the filter is fixed in the housing and filled with ambient temperature water (WFI). Allow to stabilize so air bubbles can be removed and water forms a layer on the membrane surface. A test pressure is applied on the upstream side of the filter and the pressure drop due to water vapor passing through the membrane is measured.
The advantages of this method are:
– This assay is becoming more popular as it no longer requires solvent-based wetting.
– Since the membrane does not wet out during the test, the filter can be used after draining and with a minimum drying time, reducing filter downtime.
– It can be performed on a filter in a sterilization system since there is no operation on the downstream side of the filter.
Factors Affecting Filter Integrity Testing
1) Temperature: Temperature plays a major role in the integrity test, so the temperature of the area, test gas (air or nitrogen), filter temperature, filter housing temperature, wetting agent or test liquid temperature should be ambient temperature during the test. Since the water intrusion test relies on the evaporative flow of water across the membrane under test, any increase in temperature will result in a high evaporation rate, resulting in a higher water flow being measured.
The gas used for the test should be at normal temperature, neither heating up nor cooling down during the process, which will cause distortion of the results. Using the ideal gas law as a rule of thumb, a uniform temperature change of 1°C inside the sample during the diffusion measurement phase can affect the diffusion detection result by approximately 25%.
2) Detection gas: The detection gas should be clean, dry and at ambient temperature. When performing diffusion flow testing, the type of gas being tested must be considered. If no gas is specified, the maximum permissible diffusion value of the filter element is usually given for air as the detection gas.
Since nitrogen (N2) is less soluble in water than air, the diffusion rate for a given filter will be reduced. If nitrogen is used instead of compressed air, the maximum permissible diffusivity must be modified, nitrogen max diff = air max diff * 0.82.
3) Wetting Agent/Test Fluid: The same wetting agent mentioned by the manufacturer in the product specification should be used. In water intrusion test, any material that can change the surface tension of water will affect the test results, such as any contaminated material, natural oil, conductivity of water, because the evaporation of water is higher at lower conductivity, at the bubble point Different wetting agents can give erroneous results in the test and in the diffusion flow test due to changes in surface tension. The filter wetting procedure and wetting time also play an important role in the test results.
4) Testing procedures: Different filters have different testing procedures, and the parameters of the testing procedures should be set according to the manufacturer’s recommendations.
5) Filter housing: The filter housing should be filter specific, housing design and housing volume will affect the results.