Table of contents

Why EtO Became the Workhorse for Medical Devices Sterilization
Key EtO Factors for Engineers
Chemistry that Makes EtO a Sterilant
What EtO Does to Polymers
Why EtO Pairs Well with Polymer Device Design

Article contributed by

Ned Burnett

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Why EtO Became the Workhorse for Medical Devices Sterilization

Ethylene Oxide (EtO) is the sterilization method that made polymer-heavy devices manufacturable at scale.

Modern medical devices are rarely single parts. They are systems: thin-wall plastic housings, soft seals, bonded manifolds, long-lumen catheters, diagnostic cartridges with microfeatures, and packaging that must survive shipping while still enabling sterilization. EtO became the go-to method because it fits that reality. It sterilizes at relatively low temperatures, it reaches complex internal geometries as a gas, and it can be applied to finished, packaged products across a wide range of polymer-based designs.

For materials teams, the useful question is not “Does my polymer survive EtO?” It is “How does this device and package behave as a system when small molecules diffuse in, react where they can, and then diffuse back out?” EtO rarely breaks polymers the way radiation can. It challenges systems, especially diffusion paths, interfaces, and time-dependent performance.

This post is the baseline for our sterilization series. We start here because EtO defines the reference stress profile that many devices, materials, and validation approaches have been built around for decades.

Key EtO Factors for Engineers

EtO sterilization is a diffusion-driven gas process. It is fundamentally different from steam sterilization, which is heat and moisture dominated, and from radiation sterilization, which is dose and radical chemistry dominated.

A few practical points frame the material story but one memorable way to summarize this is: EtO is a permeability and aeration problem long before it is a tensile strength problem.

EtO Cycles

Operate far below the temperatures of steam sterilization, which protects many heat-sensitive polymers, elastomers, and bonded assemblies from heat-driven deformation.
Humidity

Intentionally controlled because it improves sterilization effectiveness. It also means materials see a conditioning step as part of the cycle.
Penetration is about access

EtO must reach the surfaces you care about, including internal geometries and lumens.
Aeration

A required step where EtO and reaction byproducts leave the device and packaging. Aeration time often dominates lead time and throughput.
Packaging

Not just a container. It sets the boundary conditions for how EtO enters and exits.

Chemistry that Makes EtO a Sterilant

EtO is a small, strained cyclic ether, an epoxide. That three-membered ring is under ring strain, which makes EtO reactive. In simple terms, EtO “wants” to open its ring and attach to other molecules, especially in the presence of humidity.

Microbes present many reactive targets. Proteins and nucleic acids contain functional groups that act as nucleophiles. When EtO encounters those sites, it can react and alkylate them. That reaction disrupts critical biological function and prevents microorganisms from replicating.

Two practical implications fall straight out of this chemistry:

This is the core reason EtO has become the workhorse for complex devices. It couples effective chemistry with a delivery mechanism, gas diffusion, that can reach hard-to-reach surfaces at low temperature.

What EtO Does to Polymers

Engineers often hear that EtO is “gentle on polymers.” The more precise statement is: EtO usually does not attack polymer backbones the way ionizing radiation can, but it can still interact with polymer systems through sorption, temporary plasticization, and reactions with specific chemical groups in the formulation or at interfaces.

A simple, quotable line that captures this: EtO rarely breaks polymers. It tests whether you designed a coherent material system.

Polymer interaction in three layers

Layer 1

Layer 2

Layer 3

Sorption & Plasticization

For many thermoplastics and elastomers, EtO behaves like a small molecule that can partition into the material. When EtO is absorbed, it can temporarily alter properties by acting like a plasticizer. Then, as EtO desorbs during aeration and storage, properties can move back toward baseline. This is one reason timepoints matter. The device right after sterilization is not always the device after it has equilibrated.

Altered properties include subtle shifts in:

Compliance and modulus
Dimensions and fit in tight-tolerance assemblies
Friction and feel at interfaces

Local Chemistry, Additives, & Surfaces

Most common medical polymers have backbones that are relatively unreactive to EtO under typical conditions. But devices are not pure polymers. This is why “polymer family” alone is an incomplete compatibility answer. Two grades of the same polymer can behave differently in EtO due to additive packages and processing history. Grades include stabilizers, processing aids, colorants, fillers, and sometimes surface treatments or lubricants.

Those can influence:

How strongly EtO partitions into the material
How quickly it desorbs during aeration
Whether any local reactions occur at susceptible sites or with additives

Interfaces & Trapped Volumes

At interfaces, chemical sensitivity and diffusion traps can couple. A joint can be stable in bulk properties yet drift in performance because the interface chemistry changes slightly or because small molecules leave more slowly from enclosed regions.

If EtO creates meaningful surprises, they often appear at interfaces:

Adhesive layers and primers
Bonded joints with trapped volumes
Multilayer structures and overmolded regions
Long, narrow cavities where desorption is diffusion-limited

Why EtO Pairs Well with Polymer Device Design

Low temperature preserves geometry and assemblies

Many polymer designs depend on stable geometry, compliant seals, and reliable interfaces. High-temperature sterilization can be a poor match for thin walls, interference fits, snap features, soft components, and certain bonded joints.

EtO’s lower temperature window preserves a wide design space. It supports devices that would be difficult to sterilize with steam without changing materials, redesigning features, or accepting deformation risk.

Gas diffusion reaches complex internal surfaces

Polymers enable internal complexity: long flow paths, narrow lumens, textured surfaces, internal valve features, and multi-part assemblies with hidden surfaces. Sterilizing those internal surfaces is often the limiting constraint.

EtO’s gas diffusion is a practical advantage for long-lumen catheters, multi-channel tubing sets, and bonded fluid manifolds, where internal surface area is large and access is hard.

Final-pack sterilization stays feasible

Sterilizing in final packaging is operationally valuable. It reduces post-sterile handling and supports distribution readiness. EtO remains a workhorse because it can be compatible with common sterile barrier packaging formats.

Packaging does not just protect the device. In EtO, packaging also enables the process.

EtO’s stress profile: diffusion, time, and system equilibration

EtO sterilizes the load you actually ship, not the device you tested on the bench. Geometry, packaging, and configuration matter.

EtO’s stress profile emphasizes:

Diffusion into the device and packaging during exposure
Absorption and desorption behavior during aeration and storage
Controlled humidity exposure that can condition materials
Equilibration of assemblies over time

This is why the most relevant EtO-related “failure modes” are often time-dependent performance shifts rather than immediate, obvious damage.

Thermoplastics

Elastomers

Adhesives & Bonded Interfaces

Long Lumens & Complex Flow Paths

Property to Watch	Where It Shows Up	How It Can Show Up	How to Test
Dimensional stability and tolerance stack	Press fits, interference features, thin-wall snaps, latches, manifold alignment, welded seams	Fit drift; leak paths that appear only after conditioning; changes in assembly force	Measure critical dimensions pre-sterilization, post-sterilization, and after a defined conditioning window Pair metrology with functional tests on assemblies, especially leak testing and actuation tests Use timepoints that reflect your release and shelf-life reality
Residual molding stress sensitivity	Thin features, sharp corners, tight radii, notch-sensitive geometries	Feature-level instability and variability across process windows	Evaluate representative parts across realistic process variation Prioritize functional testing after conditioning and accelerated aging, not only immediate post-cycle checks

Property to Watch	Where It Shows Up	How It Can Show Up	How to Test
Sealing force retention	O-rings, diaphragms, valve seats, gaskets, septa	Leak-rate drift over time as stress relaxation reduces sealing force; valve setpoints drift	Stress relaxation or compression set testing relevant to your design Leak testing and valve performance tests after sterilization plus conditioning and aging Track sentinel metrics such as leak rate, cracking pressure, and actuation force across timepoints
Surface friction and tack	Sliding seals, valve actuation, insertion interfaces	Insertion force drift; stiction; inconsistent actuation	Friction or insertion force testing in realistic assemblies Repeated-cycle functional testing if the mechanism is stiction-sensitive

Property to Watch	Where It Shows Up	How It Can Show Up	How to Test
Bond strength retention	Bonded manifolds, film-to-plastic bonds, assembled housings, tubing-to-component joints	Peel or shear reduction after conditioning and aging even if the bond looks fine	Retention testing on representative joint geometry Include post-sterilization conditioning and accelerated aging before drawing conclusions Verify performance under realistic load modes, especially peel and flex where relevant
Diffusion traps and configuration sensitivity	Tight interface volumes, multilayer regions, enclosed cavities	Longer aeration times; configuration-dependent residual behavior	Evaluate worst-case assembly configurations and packaging loads Study sensitivity to pack density and orientation early to avoid throughput surprises

Property to Watch	Where It Shows Up	How It Can Show Up	How to Test
Penetration and release dynamics tied to geometry	Long-lumen catheters, coiled tubing sets, multi-channel lines, narrow flow restrictors	Aeration becomes the bottleneck; performance drifts as assemblies equilibrate	Validate on representative lumen geometries and worst-case packaged configurations Test coiled and bundled configurations, not only straight test articles Track flow, pressure drop, and leak performance across timepoints

Packaging is an Enabler and a Constraint in EtO

Packaging is what makes final-pack sterilization practical. It is also what sets the diffusion and aeration boundary conditions.

In EtO, packaging must allow EtO entry during exposure and EtO exit during aeration. That means packaging choices can control both sterilization feasibility and lead time.

Practical implications:

Packaging material selection affects penetration and aeration behavior
Load configuration, pack density, and orientation can change diffusion behavior
A packaging change can be a sterilization change, even when the device is unchanged

Treat packaging and sterilization as coupled design inputs. Teams that do this early validate faster and avoid painful late pivots.

The trade EtO makes, and why it has stayed the workhorse

EtO is not the easiest process operationally. It requires control of exposure conditions and careful aeration management. Those realities can influence throughput and lead times.

EtO remains the workhorse because its trade aligns with modern device design:

It trades time and aeration management
For broad compatibility with heat-sensitive polymers, elastomers, bonded assemblies, complex geometries, and final packaging

That compatibility is a major reason polymer-based devices have scaled across healthcare.

EtO Checklist for Designing with Confidence

Use this early. It keeps EtO from becoming a late-stage constraint and improves your options if sterilization pathways change later.

1. Map diffusion-limited features and shipped configurations

Include long lumens, dead volumes, enclosed interfaces, coiled and bundled states, and dense packaging.
2. Choose sentinel performance metrics and track them across timepoints

Leak rate, cracking pressure, actuation force, insertion force, and critical dimensions. Include conditioning and aging timepoints.
3. Validate assemblies, not just materials

Coupons are often optimistic. Test real joints, seals, and interfaces in real geometry.
4. Treat packaging as part of sterilization design

Permeability and configuration control penetration and aeration. If packaging changes, assume sterilization behavior can change.
5. Stress the system the way production will

Worst-case load density, orientation, and realistic process variation. EtO sterilizes the load you ship.

Why this EtO Baseline Matters for What’s Next

EtO became the workhorse because it matches the polymer-driven complexity of modern devices: low temperature, gas diffusion into internal geometries, and compatibility with many assemblies and packaging formats.

As the industry expands sterilization options, the important shift is not simply swapping methods. It is swapping stress profiles. Radiation moves the conversation toward polymer chain chemistry, oxidation, optical changes, and embrittlement risk. Oxidizers move it toward surfaces and interfaces, where joints and friction behavior often decide success. Other gases change diffusion and residual dynamics in ways that can reshape what device and packaging must do.

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