The rapid uptake of single-use vessels for use in bioprocessing applications has made assuring integrity that much more crucial. Although major failures in the seams or large punctures in a vessel should be visible to the naked eye, the real problem lies in the potential presence of microscopic holes. These smaller imperfections are often undetected and can cause not only leaks, but also the ingress of microbial or other contaminants into the vessel, thus ruining the entire batch. It is crucial that each vessel is tested, prior to use, to assure that there are no holes or leaks in either the vessel’s walls, seals, or—importantly—the joints and seals between the valves and tubings that enter and leave the vessel.
For many years, the only nondestructive technique available for detecting leaks was the pressure-decay method, which can typically identify holes that are 250 µm (for a 200-L vessel) or 500 µm (for a 1000-L vessel) and is reasonably effective when used on small vessels. In testing larger vessels, however, the accuracy level exponentially decreases because the vessels’ lack of rigidity means they can relax, therefore continually changing the pressure within the vessel. This can be overcome, to a certain extent, by constraining the vessel between two plates as it is pressurized. Holding the vessel in this manner makes it less likely to relax, which results in a more steady pressure hold and, hence, a greater sensitivity to any loss in pressure. In some instances, using this additional constraint method, defects down to 100 µm can be detected in a 200-L vessel, which is a significant improvement from standard pressure-decay testing limits of approximately 250 µm, but still risks allowing smaller defects to go undetected.
Helium integrity testing
Studies coordinated by ATMI found that microbes proved to be unable to penetrate a vessel defect of 10 µm or smaller, and ATMI developed a solution that can measure defects as small as 10 µm using helium tracer gas (HIT, ATMI). Unlike the pressure-decay method that measures pressure loss in a vessel, this method measures the amount of tracer gas leaking through a defect. Although helium was well established in leak-testing protocols in the automotive, aerospace, and vacuum industries, ATMI’s method was the first use of helium in integrity testing of flexible vessels used in the bioprocessing industry.
To carry out HIT testing, the vessel is placed inside a well-sealed, rigid chamber and connected to a helium inlet valve. The seals ensure no air or other gas can enter or leave while the test is in progress. All the air is then pulled from the chamber with a vacuum until there is a negligible amount of helium in the chamber. A predetermined amount of helium is injected into the vessel. If there are no defects in the vessel, the helium will remain inside it. If there are any defects (e.g., holes or splits), the vacuum will cause helium to escape from the vessel into the chamber. If this happens, the helium is detected using mass spectrometry; the amount of helium detected correlates precisely to defect size.
Detection limits of HIT, however, are not the only advantage over the constrained-plate method. In the real world, a single-use vessel is not just a vessel with a single inlet. It will have a number of different inlet ports, all attached to connectors and tubes, particularly if it is being used as a bioreactor instead of a product-storage container. These ports and connectors are the most vulnerable part of the vessel in terms of leaks, holes, and other defects. To run a constrained-plate pressure-decay test, however, all of these ports and inlets must be removed from the vessel before it can be placed between the plates. Therefore, not only does the constrained pressure-decay method offer lower sensitivity, it also omits testing of the ports and inlets that have shown to be most likely to fail. In contrast, the HIT method allows a fully assembled vessel to be tested. The entire vessel, including the tubing sets, is placed inside the test chamber. If end connectors have porous membranes on the tube ends, the outlets are blocked using a plug or sealing mechanism to allow testing of the joints between the tube and end connector.
In addition to being offered for the preshipping validation of vessels and manifolds manufactured by ATMI, HIT technology is now available to end-users for testing in their own facilities of single-use systems from any supplier. On-site testing enables processors to verify the integrity of vessels at the point-of-use, immediately prior to use, giving a further failsafe to ensure that expensive batches of biopharmaceutical products are not spoiled by microbial contamination or lost through a leak. HIT testing can currently be applied to vessels up to 200 L, with plans to extend this to 2000 L.
—Vishwas Pethe is a R&D scientist, [email protected], and Alex Terentiev, PhD, is R&D and engineering director, both at ATMI LifeSciences.