Sterilization Validation for Vial Closures: Gamma and EO
Sterilization validation is the documented process that proves a sterilization method reliably renders a product sterile to a defined sterility assurance level, and for ready-to-use vial seals and closures it is what lets a supplier ship components that a drug maker can use directly on an aseptic fill line. The two methods most used for vial closures are gamma irradiation, validated to ISO 11137, and ethylene oxide (EO), validated to ISO 11135. Both are validated against a target sterility assurance level (SAL) of 10⁻⁶, and both build on a bioburden baseline established under ISO 11737. Validated sterilisation is what underpins a ready-to-use (RTU) seal.
This guide explains what sterilization validation means, the standards behind gamma and EO, the role of bioburden and SAL, and how it ties to ready-to-use seals.
Key takeaways
- Sterilization validation proves a process reliably achieves sterility, to a defined sterility assurance level, through documented development, validation, and routine control.
- The target for terminally sterilised health care products is a sterility assurance level (SAL) of 10⁻⁶: a probability of no more than one viable microorganism in one million items.
- Gamma irradiation is validated to ISO 11137 (radiation sterilisation); ethylene oxide is validated to ISO 11135 (EO sterilisation).
- Bioburden, the population of microorganisms on the product before sterilisation, is determined under ISO 11737 and drives dose setting and routine monitoring.
- For gamma, dose-setting methods (such as the VDmax approach, often anchored at 25 kGy) establish the dose that achieves the SAL; for EO, validation covers the cycle and residual limits.
- Ready-to-use (RTU) seals are supplied pre-sterilised by a validated method and double-bagged, so the drug maker can introduce them into the aseptic area without sterilising them in-house.
What is sterilization validation?
Sterilization validation is the set of documented studies that establish, and then keep confirming, that a sterilization process consistently produces sterile product to a specified sterility assurance level. It is not a single test but a lifecycle: process development, qualification, and routine control, with revalidation after changes.
Sterility cannot be proven by testing every item, since testing is destructive and a single test can never demonstrate the absence of all microorganisms. Instead, sterilisation is treated as a validated process: you characterise the product’s microbial load, establish a dose or cycle that reduces the probability of a surviving organism to an agreed level, qualify the equipment and process, and then monitor routinely so the validated state holds. The agreed level is the SAL, and for terminally sterilised products it is conventionally 10⁻⁶.
Gamma irradiation: ISO 11137
Gamma irradiation sterilises by exposing product to ionising radiation, and it is validated to ISO 11137, the standard for radiation sterilisation of health care products. Gamma penetrates packaging well, so it suits pre-packed, double-bagged components, and it leaves no chemical residue.
ISO 11137 is structured in parts covering the requirements for development, validation, and routine control of a radiation sterilisation process; the methods for establishing the sterilisation dose; and the dosimetric aspects that measure and control the delivered dose. In practice, validation determines the dose that achieves the target SAL for the product’s bioburden. Dose-setting approaches include methods that verify a substantiated dose, such as the VDmax method often anchored at a familiar dose like 25 kGy, rather than testing an arbitrary dose. Because radiation dose is measured by dosimetry, routine control is a matter of confirming the delivered dose stays within the validated range.
Ethylene oxide: ISO 11135
Ethylene oxide (EO) sterilises chemically by exposing product to EO gas, and it is validated to ISO 11135, the standard for EO sterilisation of health care products. EO suits materials that are sensitive to radiation, but because it is a chemical process it introduces residues that must be removed and controlled.
ISO 11135 sets the requirements for the development, validation, and routine control of an EO sterilisation cycle. An EO cycle involves preconditioning, gas exposure under controlled temperature and humidity, and aeration to remove residual gas. Validation covers the cycle parameters and confirms the SAL is met, and it also addresses EO and related residuals, which are held within defined limits so the sterilised component is safe for its intended use. EO is a lower-temperature option than steam, which makes it usable for heat-sensitive and some radiation-sensitive materials.
Gamma vs EO for vial closures
Gamma and EO both reach a 10⁻⁶ SAL; they differ in mechanism, residues, and material compatibility, so the choice depends on the component and the customer’s requirement.
| Aspect | Gamma (ISO 11137) | Ethylene oxide (ISO 11135) |
|---|---|---|
| Mechanism | Ionising radiation | Chemical (EO gas) |
| Residues | None | EO residuals, controlled and aerated out |
| Penetration | High; good for double-bagged packs | Requires gas access and aeration |
| Temperature | Ambient | Low, controlled temperature and humidity |
| Suits | Pre-packed components; materials tolerant of radiation | Radiation-sensitive or heat-sensitive materials |
For a vial seal supplied ready-to-use, gamma is common because the seals can be double-bagged first and sterilised in the pack, with no residue to remove. EO remains an option where a material or customer specification calls for it. In both cases the point of validation is the same: documented evidence that the chosen process reliably reaches the target SAL.
The role of bioburden and SAL
Bioburden is the microbial load on the product before sterilisation, and the sterility assurance level is the probability target the process must reach; validation connects the two. Bioburden is determined under ISO 11737-1, which measures the population of microorganisms on the product, while ISO 11737-2 covers the tests of sterility used within the validation. The measured bioburden feeds the dose-setting or cycle-development work: the more, or more resistant, the initial load, the more process is needed to reach the SAL.
The SAL of 10⁻⁶ means the process is validated to leave no more than a one-in-a-million probability of a viable microorganism on a sterilised item. Because bioburden can drift with materials, environment, and handling, it is monitored routinely, and a rise triggers investigation. This is why a clean, controlled manufacturing environment matters before sterilisation: lower and more stable bioburden makes the validated process robust.
How this works in practice at Autofits
Autofits supplies its vial seals in Ready-to-Use (RTU) form, sterilised by gamma irradiation or ethylene oxide and double-bagged so the customer can bring them into an aseptic area without sterilising them in-house. Before sterilisation, the RTU seals are assembled in an ISO Class 8 cleanroom from double-lacquered aluminium and washed by a fully automated process to control particles and keep bioburden low. RTU seals carry a two-year shelf life before capping. The FlipTop Optima flip-off seals, tear-off and tear-down aluminium seals, and aluminium pilfer-proof (ROPP) caps are produced under an ISO 15378:2017 quality system, alongside ISO 9001:2015 and ISO 14001:2015 certification, a Drug Master File (DMF 18100), and CFDA registration (B20200000501), in a 75,000 sq ft facility in Nashik, India. Ready-to-use, validated sterilisation supports the container closure integrity that a sterile product depends on, and fits the sterile-manufacturing expectations of WHO TRS 961 and EU GMP Annex 1.
Frequently asked questions
What is sterilization validation?
Sterilization validation is the documented process that establishes, and keeps confirming, that a sterilisation method reliably renders product sterile to a defined sterility assurance level. It spans development, qualification, and routine control, with revalidation after changes, because sterility cannot be proven by testing every item.
What is a sterility assurance level of 10⁻⁶?
A sterility assurance level (SAL) of 10⁻⁶ is the conventional target for terminally sterilised health care products. It means the process is validated to leave no more than a one-in-a-million probability that a single item still carries a viable microorganism. It is a probability target, achieved through a validated process, not a per-item test result.
Which standards apply to gamma and EO sterilization?
Gamma (radiation) sterilisation is validated to ISO 11137, which covers development, validation, routine control, dose setting, and dosimetry. Ethylene oxide sterilisation is validated to ISO 11135, which covers the development, validation, and routine control of the EO cycle, including residual limits. Bioburden and sterility testing within validation follow ISO 11737.
What is bioburden and why does it matter?
Bioburden is the population of microorganisms on a product before sterilisation, determined under ISO 11737-1. It drives the sterilisation dose or cycle needed to reach the target SAL and is monitored routinely, because a rise in bioburden can undermine the validated process. Keeping manufacturing clean lowers and stabilises bioburden.
How does sterilization validation relate to ready-to-use seals?
A ready-to-use (RTU) seal is supplied pre-sterilised by a validated method, typically gamma or EO, and double-bagged, so the drug maker can introduce it into the aseptic area without sterilising it in-house. The validation is what gives the customer documented assurance that the RTU seals meet the target sterility assurance level.
Related reading
- USP <1207>: container closure integrity
- CCIT: container closure integrity testing
- EU GMP Annex 1: manufacture of sterile medicinal products
- WHO TRS 961: WHO GMP annexes
- FlipTop Optima flip-off seals
Sources
- ISO: ISO 11137-1, Sterilization of health care products, Radiation, Part 1: Requirements for development, validation and routine control (https://www.iso.org/standard/33952.html)
- ISO: ISO 11135:2014, Sterilization of health-care products, Ethylene oxide, Requirements for development, validation and routine control (https://www.iso.org/standard/56137.html)
- ISO: ISO 11737-1, Sterilization of health care products, Microbiological methods, Part 1: Determination of a population of microorganisms on products (https://www.iso.org/standard/66451.html)
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*Last updated: 2026-07-06. This article is general technical information, not legal or compliance advice; confirm current regulatory text and requirements with ISO and the applicable regulator.*
