What is a Sterility Assurance Level (SAL) and what SAL is required for medical devices?

Sterility Assurance Level is a probabilistic measure of sterilization effectiveness, expressed as the probability of a single viable microorganism surviving the sterilization process on a given device — an SAL of 10⁻⁶ means a one-in-one-million probability of a nonsterile unit, which is the standard requirement for medical devices intended to be sterile. This is a probabilistic standard rather than an absolute guarantee, because sterilization processes are validated statistically (typically using biological indicators — standardized populations of highly resistant bacterial spores deliberately exposed alongside real product) rather than by testing every single unit for sterility, which would itself require destructive testing that would defeat the purpose. A slightly less stringent SAL of 10⁻³ is sometimes accepted for devices that only contact intact skin, reflecting the lower infection risk of that contact category compared to devices contacting sterile body tissue or the bloodstream.

Why can't all devices simply be steam sterilized, since it is the cheapest and fastest method?

Steam sterilization (autoclaving) requires exposing the device to saturated steam at high temperature (typically 121-134°C) and pressure for a validated cycle time, which is fast, inexpensive, and leaves no toxic chemical residue — but that same heat and moisture is destructive to many materials used in modern medical devices. Most electronic components, many polymers (especially those with lower glass transition or melting temperatures), certain adhesives, and moisture-sensitive materials cannot survive autoclave conditions without degrading or losing functional performance. This is precisely why ethylene oxide (a low-temperature gas process) and gamma irradiation (which requires no significant heat at all) exist as alternative sterilization methods — device material selection and sterilization method selection have to be considered together from early in development, since choosing a heat-sensitive material commits the device to a non-steam sterilization pathway with its own tradeoffs.

What are EtO residuals and why does the sterilization method create an additional post-process requirement?

Ethylene oxide is itself a toxic, carcinogenic, and flammable gas, and after the sterilization cycle, residual EtO gas and its reaction byproducts (ethylene chlorohydrin and ethylene glycol, formed from EtO reacting with residual moisture or chloride) remain absorbed within the device and its packaging, particularly in porous materials like foams and certain polymers. Regulatory standards (ISO 10993-7) set strict limits on allowable residual EtO and byproduct levels a patient can be exposed to, which means every EtO-sterilized device must go through a validated aeration (out-gassing) period — often ranging from hours to several days depending on material and packaging — before it can be released for use, adding meaningful cycle time and warehouse/logistics burden that gamma irradiation and steam sterilization, which leave no chemical residue of this kind, do not require.

Engineering·9 min read·August 7, 2026

🧪 Sterilization Methods and Validation: EtO, Gamma, and Steam

An engineering guide to medical device sterilization — ethylene oxide (EtO), gamma irradiation, and steam (autoclave) sterilization, how each method affects material and device design choices, and the sterility assurance level validation framework.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 7, 2026

Sterilization Method Selection Is a Materials and Design Decision, Not Just a Manufacturing Step

Sterilization is the process of eliminating or inactivating all forms of microbial life on a device to a specified statistical confidence level (the Sterility Assurance Level, or SAL, covered in this article's FAQ). While sterilization itself happens at the end of manufacturing, the choice of sterilization method has to be made early in device design, because each of the three dominant methods — ethylene oxide, gamma irradiation, and steam — imposes real constraints on material selection, packaging design, and even electronic component choice. A device designed without considering its intended sterilization method risks discovering late in development that a chosen material, adhesive, or electronic component cannot survive the process, forcing an expensive design or material substitution.

Ethylene Oxide (EtO) Sterilization

Ethylene oxide is a low-temperature (typically 30-60°C) gas sterilization process that penetrates packaging and complex device geometries effectively, making it the dominant method for devices containing electronics, heat-sensitive polymers, or complex internal geometries that steam or radiation might not adequately reach or that heat would damage. EtO works by alkylating microbial DNA and proteins, disrupting cellular reproduction and metabolism. Its major practical drawback, beyond the toxic residual and required aeration period covered in the FAQ, is cycle time — a full EtO sterilization and aeration cycle can take several days from start to release, substantially longer than gamma or steam. EtO is also a regulated environmental and occupational hazard requiring specialized facility engineering controls, which has led to increasing regulatory scrutiny and, in some regions, capacity constraints on EtO sterilization facilities in recent years — a real supply-chain consideration for devices dependent on this method.

Gamma Irradiation Sterilization

Gamma irradiation uses high-energy photons, typically from a Cobalt-60 radioactive source, to directly damage microbial DNA, achieving sterilization without significant heat and without leaving any chemical residue — devices can generally be released for use immediately after a validated dose is confirmed, without an aeration hold period. Gamma is well suited to high-volume, single-use disposable devices (syringes, IV sets, many implants) where its capital-intensive but high-throughput processing economics work well at scale. Its principal design constraint is that ionizing radiation can degrade certain polymers — some materials become brittle, discolor, or lose mechanical properties after gamma exposure (UHMWPE, the polyethylene bearing material used in joint implants, is a well-documented example requiring careful process control, since gamma-induced oxidative degradation was historically linked to accelerated wear in early irradiated implant components) — so material gamma-compatibility testing is a required part of material selection for any gamma-sterilized device.

Steam Sterilization (Autoclaving)

Steam sterilization exposes devices to saturated steam under pressure at temperatures typically between 121°C and 134°C for a validated hold time, killing microorganisms through moist heat protein denaturation and coagulation. It is the fastest, least expensive, and most environmentally benign of the three major methods, with no toxic residual and no radiation licensing requirements, which makes it the preferred choice whenever device materials can tolerate the thermal and moisture exposure — most metal, glass, and heat-stable polymer instruments and some reusable surgical devices. Its fundamental constraint, discussed in the FAQ, is that many modern device materials — most electronics, many common polymers, and certain adhesives — cannot survive the combination of high heat and moisture without degrading, which excludes it as an option for a large and growing share of modern electronics-containing and polymer-heavy medical devices.

Method	Temperature	Cycle Time	Material Constraints	Typical Devices
Ethylene Oxide (EtO)	30-60°C	Hours to days (incl. aeration)	Broadly compatible; toxic residual requires aeration	Electronics-containing devices, complex geometries, catheters
Gamma Irradiation	Ambient (minimal heat)	Hours; no aeration hold	Some polymers degrade/embrittle under radiation dose	High-volume disposables, syringes, IV sets, many implants
Steam (Autoclave)	121-134°C	Minutes to ~1 hour	Heat/moisture-sensitive materials excluded	Reusable metal/glass instruments, heat-stable devices

Sterilization Validation

Regardless of method, a sterilization process cannot simply be assumed effective — it must be formally validated per relevant ISO standards (ISO 11135 for EtO, ISO 11137 for radiation, ISO 17665 for steam), typically through a combination of bioburden testing (quantifying the actual microbial load naturally present on unsterilized product, since required sterilization dose or exposure scales with starting bioburden) and challenge testing with biological indicators — standardized, highly resistant bacterial spore populations deliberately exposed alongside real product to confirm the process reliably achieves a lethality margin well beyond what the natural bioburden alone would require. This validated process, once established, must be periodically requalified and is subject to change control — any modification to the device, its packaging, or the manufacturing process that could plausibly affect bioburden or sterilant penetration requires re-evaluating whether the original sterilization validation remains valid, another example of how medical device engineering treats process changes as requiring documented, risk-assessed re-verification rather than an informal "it should still be fine" judgment call.

Topics covered

medical device sterilizationethylene oxide sterilizationgamma irradiation sterilizationsteam sterilization autoclavesterility assurance level SALsterilization validationbioburden testingEtO residuals