Siliconisation, And The Trade-Off It Buys

Untreated butyl rubber against untreated glass is tacky, and the tack scales with how clean and dry both surfaces are. Stoppers clump in hoppers, fail to seat evenly, and drag on the glass neck. Silicone applied as an emulsion and then baked at elevated temperature gives the elastomer a slippery, low-friction skin that resolves all three problems and adds nothing that has to be removed later.

On the glass side the reasoning is different. Borosilicate glass presents silanol groups and a net negative surface charge at neutral pH, and peptides, particularly small amphipathic ones, adsorb onto it. A siliconized inner surface lowers the available binding sites. For dilute solutions in the low microgram per millilitre range, adsorptive loss onto untreated Type I glass has been reported at 10 to 40% of the dissolved peptide depending on sequence, surface-area-to-volume ratio and contact time, measured by comparing solution concentration before and after contact by RP-HPLC.

So siliconisation is not cosmetic. It changes a real and quantified loss. The question is what it costs.

How the film is applied and how much is there

The quantities are small in absolute terms and large relative to what a sensitive measurement can see. A milligram of silicone on a stopper face, partially released into 2 mL of solution, sits in the same concentration band as the peptide itself in a dilute preparation. That is why silicone is characterised as an extractable and a leachable in packaging assessments rather than dismissed as a trace, and why the measurement chosen has to be one that can attribute the material to the container rather than to the contents.

Two of those rows deserve emphasis. Total silicon by ICP-MS is the honest measurement, because it does not care where the silicone came from. And flow imaging microscopy is the only routine technique that separates silicone oil droplets from protein or peptide aggregates, because both show up as sub-visible particles of similar size and only their morphology differs: droplets are spherical and translucent, aggregates are irregular.

The failure mode: silicone droplets counted as aggregate

The characteristic problem with siliconized containers is a misattributed particle count. A sub-visible particle measurement made by light obscuration alone reports size and number and nothing about identity. A siliconized vial or stopper that is shedding oil produces a rising particle count that reads exactly like an aggregating peptide, and the two lead to opposite conclusions: one is a container issue with no bearing on the molecule, the other is a molecular stability issue.

The separation requires either flow imaging, which resolves morphology, or elemental analysis for silicon on the filtered particulate. Without one of those, a particle count from a siliconized container is ambiguous by construction. Shaking or transport makes this worse, because mechanical agitation both sheds more silicone and promotes genuine aggregation, so the two signals rise together.

When the trade-off runs the other way

For lyophilized material the adsorption argument for siliconizing the glass largely evaporates, because there is no solution in contact with the wall during storage. What remains is the stopper-side case, which is about processing rather than contents, and that case is strong enough that most pharmaceutical stoppers are siliconized whether or not the glass is.

• Siliconisation earns its place where a dilute solution sits in glass for a long time, or where the surface-area-to-volume ratio is high, as in small-volume containers.
• It costs the most where sub-visible particle counting is a release or stability endpoint, because it degrades the interpretability of that endpoint.
• Alternatives exist: silicone-free coated elastomers with fluoropolymer laminate faces, and cyclic olefin polymer containers, both of which avoid the oil entirely and bring their own trade-offs in barrier properties and cost.
• Whether a vial is siliconized is not visible. A faint film cannot be seen or felt reliably, so it is a documentation question rather than an inspection one.

The elastomer laminate route is worth a sentence of detail because it is often confused with siliconisation. A fluoropolymer-laminated stopper has a barrier film bonded to the face that contacts the contents, which reduces extractables generally, but the laminate does not address machinability, so such stoppers are frequently still lightly siliconized on the non-contact surfaces.

What Aurum knows about the containers it uses

Specify or verify siliconisation on the vials and stoppers holding the material we list is not among the specifications Aurum publishes, and Silicon determination by ICP-MS or sub-visible particle counting on any lot is not among the specifications Aurum publishes. We cannot state whether a given container is siliconized, baked-on or emulsion-coated, or how much silicone it may release into a solution.

This sits alongside a limit already stated elsewhere in our documentation: we do not perform or claim sterility or endotoxin testing. Silicone extractables belong in the same category. They are real, measurable, and outside what we measure, and where a research question turns on container-derived silicone, our records do not answer it.

Common questions

Is silicone in the vial an impurity?

It is an extractable or leachable, which is a different category from a peptide-related impurity. It is not detected by RP-HPLC at 214 nm and it is not part of an area-percent purity figure.

Can a siliconized vial be identified by looking at it?

Not reliably. A baked film of a few micrograms per square centimetre is optically invisible. Some heavily coated containers show a faint sheen or water beading, but absence of those signs proves nothing.

Does washing remove the film?

A baked-on film is chemically bonded and largely resists aqueous washing. An emulsion film is more readily displaced, which is the same property that makes it shed into contents.

Why is silicone used at all if it migrates?

Because the alternative on the stopper side is rubber seizing against glass, which produces seating failures and container closure integrity problems. A quantified, characterised extractable is generally preferred over an unpredictable seal.

Does siliconisation affect a lyophilized cake?

There is no prolonged liquid contact during storage, so adsorption is not the issue. The film can still influence how a solution wets the wall once one is present, and it remains a source of extractables from that point on.