Every single-use system (SUS) sheds particles. Bags, tubing, connectors, weld points, peristaltic interfaces, filter housings—every polymer surface in contact with a fluid stream releases some quantity of fragments, fibers, or extractable material over the course of normal use. This is not a defect. It is the physics of how polymer materials behave under flow, pressure, and mechanical stress.
The relevant question for a quality program is not whether SUS components shed particles. They do. The question is which of those particles will end up in the final container, in what form, at what size, and with what probability of being detected before release. In our work with sterile manufacturers, the gap we see most often is between the SUS qualification data the manufacturer holds and the inspection performance the final product actually requires.
Why do SUS qualification results not predict final inspection outcomes?
Most SUS qualification programs test particles at the component level using liquid extraction—flushing the component with water or buffer and counting particles in the extract by microscopy or light obscuration. This is necessary work. It is also incomplete, because the particle population in a 2D bag is not the same as the particle population in a 2 mL vial. Container geometry, fluid volume, optical clarity, and inspection lighting all change between the SUS qualification environment and the final inspection environment.
The result is a familiar pattern: SUS components pass their qualification testing; the final fill passes its release inspection, and yet visible particle findings continue to appear in retained samples, stability pulls, or field complaints. The qualification did not predict the inspection outcome because the two measurements were not made on comparable systems.
Does particle shape matter more than particle count?
One of the most consistent findings in our particle characterization work is that shape matters more than counts when it comes to predicting visible particle risk. A long, thin fiber and a compact fragment can have the same nominal size by one measure and behave completely differently under visual inspection.
A long, thin fiber and a compact fragment can carry the same nominal size by one measure and present very differently under visual inspection. Maximum Feret diameter — the longest dimension across a particle — is a useful descriptor, but it does not scale linearly with projected area for elongated shapes. A fiber with a length well over a millimeter can have a projected area equivalent to a much smaller compact particle. Equivalent Circular Diameter (ECD), which reflects projected area directly, gives a more inspection-relevant view of how the particle is likely to behave. Programs that characterize SUS-released particles by count and longest dimension alone, without capturing shape, leave themselves unable to model inspection risk with any precision.
Our recommendation: SUS particle characterization should include shape descriptors as a routine output, not as a special study. The shape distribution of your SUS-released particle population changes the inspection risk profile in ways that count data alone cannot reveal — and that distribution is facility-specific, component-specific, and worth measuring rather than assuming.
Filtration Downstream Changes Everything—Or Nothing
The single most consequential question in SUS particle risk assessment is whether there is sterilizing filtration downstream of the SUS. If yes, most of the particle load released by the SUS is removed before it reaches the final container, and SUS particle shedding becomes a clean-fluid-path concern rather than a final product concern. If no—as is common in late-stage aseptic processing, cell therapy, and any process that introduces SUS components after the last filtration step—then SUS-released particles travel directly into the final product.
This is the question we ask first when a client brings us a SUS particle concern. It determines whether the answer is a component-level investigation or a final-product-level inspection redesign. Programs that have not mapped their SUS components against their filtration architecture cannot answer this question quickly, and they cannot prioritize their qualification effort.
What Gillson Recommends for SUS Particle Programs
A defensible SUS particle control program in our view has four elements:
- A facility-wide map of which SUS components are upstream of sterilizing filtration and which are downstream. This is foundational and most programs do not have it written down.
- Component-level extraction testing that captures both particle count and particle shape, not just count, with documented acquisition methods that can be defended in an audit.
- Predictive translation from extraction data to expected final-container appearance, using realistic assumptions about fluid volume, container format, and inspection conditions.
- Visual inspection of high-risk SUS assemblies before they enter the fluid path, using methods appropriate to the bag-and-tubing geometry rather than vial-based pharmacopeial conditions.
Where Suppliers Help—and Where They Cannot
SUS suppliers have become considerably more transparent about particle release characteristics over the past several years, and their data is a useful starting point. It is not, however, a substitute for facility-specific testing. The particle release behavior of a component depends on how it is installed, how it is flushed, how it is operated, and what fluid is flowing through it. A supplier’s lab data on water flow does not predict your facility’s performance with your product.
Use supplier data to qualify the component as suitable for evaluation. Use your own data to qualify it as suitable for use.
The Bottom Line
SUS components are part of the inspection program, whether your quality system treats them that way or not. Particles released upstream that travel to the final container are an inspection burden the program has to absorb. Particles characterized only by count, not shape, are inspection risk the program cannot accurately model.
Treating SUS particle release as a measurable, characterizable, manageable input—rather than a vague qualification artifact—is what moves a program from compliant to controlled. The work is not glamorous. The payoff is investigations that close in days instead of weeks and inspection results that hold up because the upstream story is already documented.
