EtO Sterilization: Principles of Process Design

By following a structured method, process engineers can design and validate safe and efficacious EtO steilization cycles.

Paul J. Sordellini, Susan Edel Satter, and Vincent A. Caputo

Among the sterilization technologies currently available to the medical device industry, 100% ethylene oxide (EtO) gas remains one of the most popular. Validated EtO processes can be run in sterilizers ranging from BIER vessels of a few cubic feet to industrial-sized vessels exceeding 4500 cu ft. Typically, the EtO process can be broken down into four basic phases, each of which needs careful planning to ensure a safe and efficacious process. The four phases are: (1) air removal, (2) steam injection and conditioning dwell, (3) EtO injection and gas dwell, and (4) gas purge and air in bleed.

Mid-infrared gas spectrometer measures EtO and water vapor during sterilization. Photo courtesy Spectros Instruments, Inc. (Whitinsville, MA).

This article is intended to guide the reader through the components of each phase of two hypothetical 100% EtO with nitrogen processes. The following assumptions were made for the purpose of explaining the rationale behind the design of the cycles and the options available:

• Water vapor and process nitrogen are the only inert gases considered in the flammability calculations performed during the air-elimination and gas-purge phases.
• There is no stratification of process gases.
• All process gases are presumed to behave as ideal gases.
• Preconditioning and aeration are performed externally to the sterilizer.
• Atmospheric and barometric pressure are constant, with atmospheric pressure at 14.7 psia.

An effective EtO process can be properly designed for almost every type of medical device and permeable packaging configuration, provided that all variables are assessed through thorough process design and development. It is here, in fact, that one notices how EtO processes possess a greater number of variables in comparison with other sterilization technologies. However, by following a structured method that systematically examines and considers each of these variables, the process engineer can design, validate, and routinely sterilize with a safe and efficacious process.

The critical parameters of an EtO sterilization cycle are typically given as temperature, pressure, humidity, EtO concentration, and gas dwell time. However, the process engineer must also identify and evaluate relationships that may exist between any given process parameter(s), the product being sterilized, and the equipment used.

The sterilization process must consistently deliver all critical process parameters to each and every component contained within the load, to a degree that will ensure a 10^-6 sterility assurance level (SAL) without causing any deleterious effect to the product or its sterile barrier packaging. In addition, this process must occur under controlled conditions that will protect the sterilization personnel monitoring the operation, the equipment employed, and ultimately the end-user.

Each product component contained in the load must be examined for the following characteristics: natural bioburden, physical configuration, raw-material composition, sensitivity to both negative and positive pressure changes, maximum heat tolerance, and chemical reaction to water vapor and ethylene oxide. For example, surgical sutures may present an extreme sensitivity to what are often considered even moderate temperature levels. Other materials, especially those containing salts, may react strongly with EtO to form ethylene chlorohydrin (ECH), a residual chemical produced during the EtO process. Some materials may bind, through a positive reaction, large quantities of EtO molecules, presenting the problem of excessively high postprocess levels of EtO and ethylene glycol (EG), another process residual.¹ Those components presenting the greatest challenge to the process—due either to physical configuration (obstruction of gas permeation) or high bioburden (natural fibers, for example)—should be selected for the microbiological challenge. Other product sensitivities should also be noted, as they will determine maximum ramp rates and set points employed in the cycle. For the validation of the process, a reference load must be selected that will represent the most difficult combination to heat, humidify, sterilize, and aerate.

Each level of packaging, from master cartons to the unit package (the primary sterile barrier), must be examined and evaluated for its ability to allow heat, moisture, and sterilant to permeate. Gas delivery to and permeation within the product, in addition to aeration of the gas from the product, are all important considerations. Data obtained from fractional studies can provide the basis for the calculation of the dwell times for the conditioning and gas exposure. The process engineer must be cautious of excessively long gas-exposure dwells or high gas concentrations, as they can result in the need for long multiple evacuations and/or aeration times that will delay product release. The objective is to decide whether to adopt a cycle using a long gas-exposure time with low EtO concentration or one with brief gas exposure and a high EtO concentration. Naturally, if gas is easily aerated from the product, production times are improved by a short exposure to a high concentration of sterilant.

Before a preliminary cycle plan can be drafted, the process engineer must have a thorough knowledge of the process equipment, including the minimum and maximum operating ranges of the preconditioning facility, the sterilizer and ancillary equipment, and the aeration facility. The sterilizer control system must be able to perform all evacuations and gas injections (nitrogen, steam, EtO, and air) at steady, preprogrammed rates. Accurately calibrated proportional valves facilitate the delivery of these rates. The objective is to perform each process ramp at gradual (linear) rates.

Both in the first part of the sterilization cycle (air removal) and in the final stage (sterilant removal), the safety of the facility and personnel are paramount issues. During the air-removal phase, the sterilizer is evacuated and then backfilled with nitrogen. After each vacuum/nitrogen sequence, a calculated amount of air is displaced. Depending on the depth of each vacuum and the final pressure achieved by the nitrogen addition, the process engineer must determine the minimum number of sequences necessary to bring the air content of the sterilizer atmosphere to a composition at which there is insufficient oxygen left to pilot a combustible reaction. EtO is flammable and can ignite in the presence of static electricity.² It is, therefore, essential to know, prior to EtO injection, the volume percentage of air (%vol_air) left in the chamber before deciding upon the maximum amount of sterilant to be used. Later, when the volume percentages of air, of the inert gases (%vol_steam and %vol_nitrogen), and of EtO (%vol_EtO) are known, they can be plotted on a flammability chart to confirm the nonflammability of the cycle.³

Following gas contact, the EtO must be displaced from the load and removed from the chamber. In planning this segment of the cycle, the same routine—post vacuums followed by nitrogen flushes—is followed. A volumetric calculation of the percentage of EtO left in the sterilizer after each vacuum/nitrogen sequence will determine when the level of EtO has been brought down to an acceptable level. Usually, after the final evacuation is performed, the sterilizer is backfilled with ambient air instead of nitrogen. In the final stage of the cycle, the sterilizer rear exhaust is activated while fresh air is allowed into the sterilizer either through a dedicated vent or by partially opening the door. Sufficient time must be allotted to flushing the sterilizer headspace so that the EtO concentration is brought to a safe level before the sterilizer is unloaded. Some workers wear industrial respirators with catalytic filter canisters rated for atmospheres containing not more than 50 ppm of EtO.

Flammability is not the only factor that determines the number of evacuations. In most cases, increasing the number of evacuations will also lower the EtO residuals left on the product, thus decreasing the amount of time the load must be quarantined for aeration. Although in this case "more is better," limitations are imposed by product and packaging tolerances as well as by equipment demands. A greater number of evacuations will subject the load to increased physical stress, which, when combined with EtO, heat, and humidity, could have a negative effect on product and packaging constructions such as, for example, glues or seals. Time spent for additional postvacuums also reduces the overall productivity of the sterilizer, which can affect facility profitability.

AIR REMOVAL

Before 100% EtO can be introduced into the sterilizer, the original air content (%vol_air = 100% of the initial sterilizer atmosphere) must be displaced and substituted with an inert gas such as nitrogen (N₂). The physical parameters for the air-removal phase are determined by the tolerances of the most sensitive products or packaging (e.g., nonpermeable foil pouches or sealed cavities). If data are not available from the respective component or product manufacturers, they can be generated by conducting preliminary studies during which samples are exposed to different ramp rates (i.e., change in pressure per unit time) and vacuum set points. Each set of samples is then tested (both product and packaging) for conformity to original manufacturing specifications until the fastest permissible ramp rate and deepest acceptable evacuation set point are determined and recorded in the sterilization process design history record.

The initial pressure inside the sterilizer at the moment the door is closed is equivalent to atmospheric pressure (14.7 psia at sea level). The first evacuation will remove a quantifiable amount of air. For example, an initial evacuation from P_initial = 14.7 psia to a depth of P_final = 7.35 psia will eliminate 50% of the original air content in the sterilizer. While the volume percentage of air is still 100%, the partial pressure of air is reduced in direct proportion to the pressure change:

The sterilizer is programmed to backfill with nitrogen to a set point of 14.7 psia. The resulting sterilizer atmosphere is now 50% air and 50% nitrogen. After this first vacuum/nitrogen sequence, the volume percentages of air and nitrogen are represented as:

After the second vacuum/nitrogen sequence, the amount of air in the sterilizer reduces again by half (%vol_air = 25%), while the nitrogen increases by half (%vol_nitrogen = 75%). This sequence is repeated as many times as necessary, until the %vol_air is reduced to a safe level.

Ethylene oxide requires oxygen to ignite. The term safe level is intended to mean that the air originally contained in the sterilizer at the beginning of the process has been reduced to the point that there remains insufficient oxygen to allow a reaction to occur should a source of ignition be available.⁴ Here it should be easy for the reader to see the relationship between the final set point of each vacuum/nitrogen sequence and the total number of sequences that will be required to render the cycle safe.

In selecting the ramp rates and set points for the vacuum/nitrogen sequences of an EtO process, there are various options. In general, a deep vacuum set point is preferred because it allows the air-removal process to be completed more efficiently. As stated earlier, determination of a maximum vacuum set point is a function of product/packaging tolerance as well as equipment limitations. Once the maximum ramp rate tolerances are determined for the vacuum and nitrogen sequences, the process engineer must decide what rate is best for the given product configuration. While the air-removal phase ensures that the sterilizer atmosphere is almost void of air, a consequence of these purges is loss of product moisture. The goal of process design is to displace air as efficiently as possible while minimizing load desiccation.

AIR REMOVAL

Deeper vacuums can complete air removal with fewer sequences. When dealing with a vacuum-resistant product, one vacuum from atmospheric (14.7 psia) to 2.0 psia, followed by a nitrogen backfill to 14.7 psia, will quickly reduce the %vol_air to 13.6% (Table I). Products that can withstand this rate and depth of vacuum will usually tolerate an equally rapid nitrogen injection. Fast ramp rates for nitrogen backfilling also minimize product-level moisture loss.

Process Phase	Cycle Segment	Set Point (in. HgA)	Set Point (psia)	Ramp Rate (psi/min)	Segment Time (min)	Cumulative Cycle Time (min)
Cycle start		29.9	14.7	N/A	N/A	0.0
Air removal	Evacuation 1	4.1	2.0	1.00	12.7	12.7
	Nitrogen purge 1	29.9	14.7	1.00	12.7	25.4
	Evacuation 2	4.1	2.0	1.00	12.7	38.1
Humidification	Steam injection	6.3	3.1	0.02	55.0	93.1
EtO inject	EtO injection	17.9	8.8	0.20	28.5	121.6
	Nitrogen overlay	29.5	14.5	0.20	28.5	150.1
	Gas contact	29.5	14.5	0.00	480.0	630.1
EtO removal	Postevacuation #1	4.1	2.0	1.00	12.5	642.6
	Nitrogen flush #1	29.9	14.7	1.00	12.7	655.3
	Postevacuation #2	4.1	2.0	1.00	12.7	668.0
	Nitrogen flush #2	29.9	14.7	1.00	12.7	680.7
	Postevacuation #3	4.1	2.0	1.00	12.7	693.4
	Final air inbleed	29.9	14.7	1.00	12.7	706.1
	Total cycle time (min)				706.1	706.1

In the case of a vacuum-sensitive product, such as a kit containing multiple devices, the air-removal phase could require multiple slow vacuums—down to 7.0 psia, for example. The first vacuum/nitrogen sequence will only bring the %vol_air from 100% down to 47.62% (Table II). In this case, the process engineer must consider that the desiccating effect inherent in this process is further amplified. Multiple vacuum/nitrogen injections coupled with slow ramp rates mean that there is more time for moisture to be driven out of the load by the induced pressure gradient. Water becomes more volatile as temperature is increased and pressure is decreased. In these circumstances, one must begin the steam-injection phase as soon as possible in order to replace some of the moisture lost during the multiple slow-vacuum phases.

Process Phase	Cycle Segment	Set Point (in.HgA)	Set Point (psia)	Ramp Rate (psi/min)	Segment Time (min)	Cumulative Cycle Time (min)
Cycle start		29.9	14.7	N/A	N/A	0.0
Air removal	Evacuation 1	14.3	7.0	0.25	30.8	30.8
	Nitrogen purge 1	29.9	14.7	0.25	30.8	61.6
	Evacuation 2	14.3	7.0	0.25	30.8	92.4
	Nitrogen purge 2	29.9	14.7	0.25	30.8	123.2
	Evacuation 3	14.3	7.0	0.25	30.8	154.0
	Nitrogen purge 3	29.9	14.7	0.25	30.8	184.8
	Evacuation 4	14.3	7.0	0.25	30.8	215.6
	Nitrogen purge 4	29.9	14.7	0.25	30.8	246.4
	Evacuation 5	14.3	7.0	0.25	30.8	277.2
Humidification	Steam injection	16.5	8.1	0.02	55.0	332.2
EtO inject	EtO injection	28.1	13.8	0.20	28.5	360.7
	Nitrogen overlay	29.5	14.5	0.20	3.5	364.2
	Gas contact	29.5	14.5	0.00	480.0	844.2
EtO removal	Postevacuation #1	14.3	7.0	0.25	30.0	874.2
	Nitrogen flush #1	29.9	14.7	0.25	30.8	905.0
	Postevacuation #2	14.3	7.0	0.25	30.8	935.8
	Nitrogen flush #2	29.9	14.7	0.25	30.8	966.6
	Postevacuation #3	14.3	7.0	0.25	30.8	997.4
	Nitrogen flush #3	29.9	14.7	0.25	30.8	1028.2
	Postevacuation #4	14.3	7.0	0.25	30.8	1059.0
	Nitrogen flush #4	29.9	14.7	0.25	30.8	1089.8
	Postevacuation #5	14.3	7.0	0.30	30.8	1120.6
	Final air inbleed	29.9	14.7	0.25	30.8	1151.4
	Total cycle time (min)				1151.4	1151.4

Process Phase	Cycle Segment	Partial Pressure Air (psi)	Partial Pressure Inerts (psi) (nitrogen and steam)	Partial Pressure EtO (psi)	Total Partial Pressures (psia)
Cycle start		14.70	0.00	0.00	14.70
Air removal	Evacuation 1	7.00	0.00	0.00	7.00
	Nitrogen purge 1	7.70	0.00	14.70	47.62
	Evacuation 2	3.33	3.67	0.00	7.00
	Nitrogen purge 2	3.33	11.37	0.00	14.70
	Evacuation 3	1.59	5.41	0.00	7.00
	Nitrogen purge 3	1.59	13.11	0.00	14.70
	Evacuation 4	0.76	6.24	0.00	7.00
	Nitrogen purge 4	0.76	13.94	0.00	14.70
	Evacuation 5	0.36	6.64	0.00	7.00
Humidification	Steam injection	0.36	7.74	0.00	8.10
EtO inject	EtO injection	0.36	7.74	5.70	13.80
	Nitrogen overlay	0.36	8.44	5.70	14.50
	Gas contact	0.36	8.44	5.70	14.50
EtO removal	Postevacuation #1	0.17	4.07	2.75	7.00
	Nitrogen flush #1	0.17	11.77	2.75	14.70
	Postevacuation #2	0.08	5.61	1.31	7.00
	Nitrogen flush #2	0.08	13.31	1.31	14.70
	Postevacuation #3	0.04	6.34	0.62	7.00
	Nitrogen flush #3	0.04	14.04	0.62	14.70
	Postevacuation #4	0.02	6.68	0.30	7.00
	Nitrogen flush #4	0.02	14.38	0.30	14.70
	Postevacuation #5	0.01	6.85	0.14	7.00
	Final air inbleed	7.71	6.85	0.14	14.70
	Total cycle time (min)

Process Phase	Cycle Segment	%Volume Air	%Volume Inerts (nitrogen and steam)	%Volume EtO	%Volume Total
Cycle start		100.00	0.00	0.00	100.00
Air removal	Evacuation 1	100.00	0.00	0.00	100.00
	Nitrogen purge 1	52.38	0.00	100.00	7.00
	Evacuation 2	47.62	52.38	0.00	100.00
	Nitrogen purge 2	22.68	77.32	0.00	100.00
	Evacuation 3	22.68	77.32	0.00	100.00
	Nitrogen purge 3	10.80	89.20	0.00	100.00
	Evacuation 4	10.80	89.20	0.00	100.00
	Nitrogen purge 4	5.14	94.86	0.00	100.00
	Evacuation 5	5.14	94.86	0.00	100.00
Humidification	Steam injection	4.44	95.56	0.00	100.00
EtO inject	EtO injection	2.61	56.09	41.30	100.00
	Nitrogen overlay	2.48	58.21	39.31	100.00
	Gas contact	2.48	58.21	39.31	100.00
EtO removal	Postevacuation #1	2.48	58.21	39.31	100.00
	Nitrogen flush #1	1.18	80.10	18.72	100.00
	Postevacuation #2	1.18	80.10	18.72	100.00
	Nitrogen flush #2	0.56	90.52	8.91	100.00
	Postevacuation #3	0.56	90.52	8.91	100.00
	Nitrogen flush #3	0.27	95.49	4.24	100.00
	Postevacuation #4	0.27	95.49	4.24	100.00
	Nitrogen flush #4	0.13	97.85	2.02	100.00
	Postevacuation #5	0.13	97.85	2.02	100.00
	Final air inbleed	52.44	46.60	0.96	100.00
	Total cycle time (min)

Table II. Process calculations for cycle 2—shallow-vacuum type.

A comparison of the two cycles illustrated in Tables I and II shows that the deep-vacuum cycle completes the air-elimination phase and arrives at the start of steam injection in 38.1 minutes, whereas the shallow-vacuum cycle requires 277.2 minutes.

STEAM INJECTION AND CONDITIONING DWELL

Product loads are typically preconditioned in a room or chamber in which heat and humidity are controlled to predetermined levels until the load reaches equilibrium with the surrounding conditions. This process occurs prior to moving the load into the sterilizer. When it is transferred to the sterilizer and subjected to the air-removal phase, the load can lose a significant amount of moisture, which must be replaced before introducing EtO. This is accomplished by adding steam under vacuum until the chamber atmosphere is humidified—normally to a range of 40—80% relative humidity (%RH)—and then holding the humidity stable for a period of time (static conditioning dwell). Sterilizer relative humidity is the ratio of the water-vapor pressure in the headspace to the saturation vapor pressure at the same temperature and pressure.

The final pressure set point of steam addition depends on the desired level of relative humidity, which is determined during process validation. The steam-injection rate depends on the physical characteristics of the product and packaging. Heat-resistant products in breathable pouches can withstand rapid steam-injection rates, whereas more-delicate products will require slower rates. The process engineer must monitor the level of relative humidity in the sterilizer during the phase of conditioning dwell. RH inside a sterilizer can be measured in three ways: by pressure- and temperature-based calculation,⁵ by dew-point calculation, or by direct headspace analysis (gas chromatograph or infrared analyzer, electronic RH sensor).

The first method is used to calculate the relative humidity achieved by the static steam-injection phases in Cycle 1 and Cycle 2, using the following formula:

where %RH = percent relative humidity, Pt = final steam-injection pressure, Pi = conditioning vacuum final pressure, Tdc = temperature at the end of steam injection, and Pss = pressure of saturated steam, at Tdc, as reported by the saturated steam table.⁶

The second method, known as dew-point calculation, is also based on process data:

%RH = 100x(exp[4098x(D — T)/((D + 237.3)x(T + 237.3))]),

where %RH = relative humidity, D = dew point in °C, and T = temperature in °C.

The process engineer must consider a number of factors when designing the steam-injection and conditioning-dwell phases. For example, the higher the injection set point (targeted relative humidity level), the greater the physical force (pressure and RH gradient) available to drive the moisture across each layer of packaging. Because the addition of steam will add heat to the sterilizer and product load, one must be aware of the expected temperature of the load prior to its entry into the sterilizer, the approximate amount of load heat and humidity that will be lost during the vacuum/nitrogen sequences, and the amount of heat and humidity the load will absorb from the steam-injection phase. The amount of heat transferred to the load is directly related to the steam-injection rate and the pressure differential. In other words, a faster steam-injection rate coupled with a greater pressure differential will result in a greater heat transfer to the load. In the case of a heat-resistant product, this is a desirable effect, since heat and humidity are conducive to sterilization. However, because heat-sensitive products may be susceptible to damage, a lower, more gradual steam addition may be required.

Once steam injection is complete, the load dwells for a period of time, allowing it to equilibrate to the new conditions inside the sterilizer (e.g., %RH and temperature). Prolonged steam dwell times (>60 minutes) are conducive to achieving a more uniform spread of temperature and humidity across the load.

Greater steam-injection differentials and long conditioning dwell times may cause the primary packaging to absorb excess moisture, creating both physical problems (softening of packaging materials and seals) and chemical problems (higher EtO, EG, and ECH levels) on postprocessed product. Factors that influence steam-injection rates include:

• Complex devices containing different types of components and materials.
• Dense product load configurations.
• Devices with nonvented components.
• Physical obstruction of moisture penetration by multiple layers of packaging or product containers that have limited permeable surface area.

Under any of these conditions, a slow steam-injection phase together with a long conditioning dwell time (>2 hours) may be selected to allow the load to achieve a higher and more-uniform level of temperature and humidity.

Ultimately, the time required for conditioning dwell can be empirically determined through process development and then validated. Placing relative humidity sensors directly inside the primary packaging will allow the humidification of the product load to be profiled.

Product loads can also be conditioned dynamically, using two methods that require deep-vacuum processes. After the sterilizer is evacuated to 0.5 psia or less, steam is introduced and evacuated. The first method alternates the addition of steam and the evacuation of the sterilizer to create a pulse/purge action. Steam is pulsed into the sterilizer and then purged by the vacuum pump. During each purge, both steam and air are removed. Eventually, the volume percentage and partial pressure of air inside the sterilizer are reduced to zero, and the load is allowed to achieve a uniform level of both temperature and humidity.

The second method is different in that it activates both the steam-injection and vacuum-pump systems simultaneously. A laminar flow of steam is injected into one side of the sterilizer and evacuated simultaneously from the opposite side. The sterilizer is kept under deep vacuum, and the rate of steam entering it is balanced—through the use of proportional valves—to equal the rate of evacuation. Ideally, one sterilizer volume per minute of steam should be injected into and evacuated from the sterilizer.⁷ This second method also requires certain sterilizer equipment upgrades. A detailed technical information report (TIR No. 15—1997), which has been published by the Association for the Advancement of Medical Instrumentation (AAMI), provides formulas and instructions for performing the calculations necessary to determine relative humidity during either method of dynamic conditioning.

ETO INJECTION AND GAS DWELL

Liquid EtO is first heated to form a gas phase inside a volatilizer connected directly to the sterilizer with a valve assembly. The volatilized EtO is usually injected into the sterilizer through the recirculation header for uniform distribution. The decision to use a particular EtO concentration is based on examination of the product and packaging as well as flammability concerns. Microbial lethality depends on the delivery, to the product load, of all elements of an EtO cycle, with gas concentration being one of the factors that directly affects lethality. Therefore, higher EtO concentrations will allow the load to achieve its target SAL in less time.

The rate of EtO injection depends on the nature of the product and the equipment. Products that absorb EtO and chemically react rapidly may require fast EtO injection rates in order to avoid excessive EtO absorption. The ability of the product load to withstand rapid ramp rates will be a determining factor. On the other hand, a slower ramp rate may be a better choice for product loads consisting of certain devices. For example, a load consisting of large kits that contain many different kinds of devices may require a high concentration of sterilant that it absorbs slowly from the sterilizer headspace. By using a slower injection rate, allowances can be made for load absorption of EtO during injection, which will ultimately increase the total amount of sterilant injected into the sterilizer.

For EtO addition, the single-charge method is preferred, for safety reasons. In this method, EtO is added after conditioning dwell until a preprogrammed set point is achieved; there is no further sterilant addition for the remainder of the cycle. The set point can either be based on final pressure or on a calculated EtO concentration value.⁸

However, there are cases of high load absorbency in which EtO depletion becomes a concern. In these instances, a "makeup" method is used: as the load absorbs gas, the sterilizer headspace pressure drops below set point. EtO can then be added to the sterilizer to restore the headspace pressure to its original set point. The number of makeup injections may vary, with some systems allowing the addition of makeup gas every time the sterilizer pressure drops while others allow only a preprogrammed number of makeups. The process engineer must be aware that each time an EtO makeup is performed, the partial pressure of EtO increases and it becomes impossible to calculate the new concentration—which raises major safety concerns. Safe conditions can be maintained by installing a gas analyzer (chromatograph or spectrometer) that periodically samples the sterilizer atmosphere and determines EtO concentration.

The EtO injection temperature should be regulated by controlling both the operating temperature of the volatilizer and the gas flow rate. The temperature of the EtO entering the sterilizer should be at or above the sterilizer process temperature. This is especially important to protect those parts of the load closest to the EtO injection ports. Low injection temperatures can cause parts of the load to cool down, thus interfering with microbial inactivation. Conversely, EtO that has been volatilized to a high temperature can rapidly desiccate the outer layers of a pallet, which can adversely affect the integrity of the process.⁹ Sterility failures, inconsistent sublethal results, and product damage are possible as a result of localized heating.

Following EtO injection, an inert gas overlay (N₂ ) can be added to further reduce the volume percentages of both the sterilant and the residual air inside the sterilizer. The data in both Tables I and II show that the simple addition of the inert overlay prior to sterilant dwell will reduce volume percentages of air and EtO. The greatest effect, however, is in the deep-vacuum cycle (Table I). When an inert overlay is applied, neither the EtO concentration nor the relative humidity is altered. The sterilizer atmosphere is rendered safer, and increasing the pressure in the sterilizer headspace will create a greater headspace/product pressure gradient, thus assisting sterilant penetration into the load.

In both types of cycles, the nitrogen overlay is programmed to end at 14.5 psia—or 0.2 psi below atmospheric pressure which, in this exercise, is assumed to be 14.7 psia. The objective is to keep the sterilizer under slight negative pressure during times in which there is EtO inside. Under negative pressure, the only consequence in case of a leak (such as a broken valve or gasket) would be to draw ambient air into the sterilizer, preventing EtO from escaping into the employee work area.

It is at this point that the process engineer, having thus far planned the cycle on paper, must perform the necessary safety calculations. Organizing the volumetric data for each phase of the process will encompass plotting the volume percentages of air, inert gases, and EtO on an EtO-specific flammability chart to determine if the cycle is within the chart's area of flammability at any given time. Should the proposed cycle be flammable, additional vacuum/nitrogen sequences should be added during the air-removal phase, and the cycle should be reprojected on the flammability chart.¹⁰ Although designing only nonflammable cycles is an obvious safety choice, it must be stated that there is no industry regulation preventing the use of cycles which, when plotted on a flammability chart, enter into the flammable region of the chart. Certain cycles, for example, may enter the flammable region temporarily during EtO injection, only to reenter the nonflammable region following application of the nitrogen overlay; other cycles may lie in the flammable region throughout gas contact. (All decisions regarding the use of flammable cycles are the responsibility of the facility management team, which should review state and local regulations as well as insurance policy requirements when determining operating policy.) At completion of the inert blanket application (i.e., start of gas contact), the process engineer should again plot the volume percentages of air, inert gases, and EtO to reconfirm the nonflammability of the cycle.

The phase of gas dwell—also referred to as sterilant dwell—is a time-related event. Following EtO injection and the application of the inert overlay, the ambient environment in the sterilizer is maintained within a constant temperature range for a period of time determined during process development. In planning the parameters of gas dwell, the process engineer should examine the anticipated temperature of the product load as it begins the gas-dwell phase. The most efficient technique in planning the process- and load-temperature specifications for the gas dwell is to raise the product load temperature as close as possible to the temperature set point targeted for the gas-dwell phase. The closer the product temperature is to the process temperature, the less heat is required and the more stable the cycle becomes. Preconditioning is the first opportunity to add required heat to the product load; the next is during steam injection and conditioning. Thus, by strategically selecting temperature and RH set points; injection ramp rates; and dwell times for preconditioning, steam injection, and conditioning, the product load can be gradually brought to the targeted gas-contact temperature set point. For example, an efficient process plan could begin by bringing the product load to 110°F during preconditioning, planning the characteristics of steam injection and conditioning so as to raise the product load temperature to 120°F, and then programming in a temperature set point of 120—125°F for gas dwell.

In some facilities, the sterilizer is used for all three phases—preconditioning, sterilization, and aeration. In this case, it is easier for the user to set and maintain one optimal process temperature throughout the cycle. This is an old practice, however, and is not often used anymore except when dynamic environmental conditioning can be used to condition the load and steam distillation for sterilant removal.

GAS PURGE AND AIR INBLEED

Subsequent to gas contact, the main objective of the process is to displace the EtO from the sterilizer headspace until the level of EtO falls below the lower explosion limit (LEL) established for EtO (3% or 30,000 ppm). The procedure for gas removal follows the same principle that was presented in the earlier air-removal phase. A series of postvacuums, each followed by a nitrogen backfill, are programmed. A maximum vacuum-depth set point must be established for each specific product family. During the gas-dwell phase, the combination of heat, humidity, and EtO has ample opportunity to soften some packaging and product materials such as glues and adhesives. Therefore, the process engineer must be careful in selecting the appropriate set points and ramp rates for this final stage in the process.

Following each vacuum/nitrogen sequence, the volume percentages of air, inert gases, and EtO must be calculated and recorded. The process must be designed so that, by the last vacuum, the sterilizer headspace level of EtO is below 3%. As Tables I and II show, process efficiency in removing EtO is augmented by using deeper vacuum set points. In cycle 1, a single postvacuum/nitrogen flush from 14.5 to 2.0 psia reduces the %vol_EtO from 39.3 to 5.35% in 25.2 minutes. In cycle 2, one shallow postvacuum from 14.5 to 7.0 psia only reduces the %vol_EtO from 39.3 to 18.7% in 60.8 minutes. To protect facility personnel and equipment, the processes are planned to continue evacuating and purging with nitrogen until the level of EtO is brought below 3%. This is achieved in the deep-vacuum cycle 1 after two nitrogen flushes and 50.6 minutes and in the shallow-vacuum cycle 2 after four flushes and 245.6 minutes.

In either cycle, once the EtO level is reduced to less than 3%, a final postvacuum is performed, followed by a phase in which ambient air is drawn into the sterilizer. The rate at which the ambient air is allowed to bleed into the sterilizer should be controlled at the same rate as that programmed for the nitrogen flushes. The air should be filtered to ensure that debris is not also drawn into the sterilizer.

In the two examples of EtO processes presented in the tables, the number of postvacuum/nitrogen sequences was based exclusively on the need to reduce the sterilizer headspace EtO level to less than 3% in order to add air and unload the vessel. However, depending on the product configuration and facility equipment, an EtO process engineer might plan this part of the cycle differently.

When EtO processes are performed inside an empty vessel—as is the case during equipment-qualification studies—the dynamics of air and gas addition and removal can be accurately predicted through calculations. Once a product load is placed inside, however, the dynamics of gas removal change dramatically. Basically, the EtO contained within a fully loaded sterilizer can be categorized as follows:

• Headspace EtO, which occupies the vapor space surrounding the product load.
• Packaging EtO, which occupies the vapor space within every layer of packaging (master cartons, shelf cartons, unit packages, and so forth).
• Absorbed EtO, which is bound to the humidity of the load through weak hydrogen bonds.
• Adsorbed EtO, which adheres directly to the product itself through chemical bonds whose strength depends on the particular product's materials of construction.¹¹

An EtO process that is optimized for the maximum reduction of all four types of poststerilization EtO will offer maximum worker protection, reduced aeration time, and increased plant productivity. The first type, headspace EtO, is readily eliminated during the postvacuum phase, since there is nothing to stop the gas from being drawn out by the vacuum pump. The second type, packaging EtO, is easily and effectively reduced by increasing the depth of each vacuum, programming a slower vacuum ramp rate, and increasing the overall number of postvacuum/nitrogen sequences in the process. EtO residing in the vapor space of the packaging must cross several layers of porous packaging in order to reach the sterilizer headspace, where it is removed by the vacuum pump system. The key variable in this step is time. When the vacuum rate is reduced, one allows more time for the packaging EtO to cross all packaging layers. Additional postvacuum/nitrogen sequences—beyond the number necessary for safety reasons—may be added to more effectively reduce the packaging EtO residual levels.

To reduce the remaining two types of EtO—absorbed and adsorbed—the process parameters for temperature, postvacuum set point, and vacuum hold time must be considered. The optimum postvacuum technique is to raise the temperature of the sterilizer, program the deepest vacuum set point permitted by the load, and then dwell (or "soak") at those pressure and temperature conditions for a brief time. During this postvacuum dwell period, the physical conditions inside the sterilizer are conducive to the release of absorbed and adsorbed humidity, which will facilitate the dissipation of absorbed and adsorbed EtO. During a postvacuum dwell, EtO will desorb from the load and occupy the sterilizer headspace. This flow of residual EtO from the load to the headspace will continue only until an equilibrium is reached in which the headspace concentration of EtO approaches the concentration of EtO contained in the load. Therefore, it is necessary to periodically backfill the sterilizer with nitrogen and perform an additional postvacuum dwell. The optimal number of postvacuums and dwells, and the time programmed for each one, depends on the particular load characteristics and can be determined through process development studies and related EtO-residual product test data.

Another important factor that must be considered when planning an EtO cycle is the constraint placed on process design by the facility's emission-control system. Except for facilities that use less than 2000 lb of EtO per year, federal law requires that the output of the sterilizer vacuum pump and the sterilizer rear exhaust vent (or "backvent") be connected to an emission-control device such as a wet scrubber or catalytic oxidizer.¹²

Wet scrubbers function by diffusing the vacuum-pump stream into acidified water; the acid catalyzes the reaction of water and EtO to generate ethylene glycol. Dispersion tubes, with a specified capacity for gas-stream flow, aid in dissolving the EtO component into the water. The process engineer therefore needs to know the maximum vacuum-pump rate that can be tolerated without damaging the dispersion tubes of the scrubber. Packed towers, another form of wet scrubbing, can guarantee certain efficiencies only if the maximum flow rate of gas entering the unit is in accordance with the manufacturer's tolerances. With either wet-scrubbing technology, an excessively high vacuum rate may damage the control equipment and reduce emission-control efficiency.

Catalytic oxidizers receive the gas stream from the sterilizer vacuum pump, dilute it with 22 parts of air until the concentration of EtO in the stream is below the LEL, heat the diluted gas stream to approximately 300°F or greater, and then pass the stream over catalytic beds to convert the EtO to carbon dioxide and water vapor.¹³ Catalytic oxidizers are sized according to each specific facility: upon installation, there is a preset maximum EtO flow rate that the oxidizer can safely handle. Consequently, every postvacuum must be designed so that the rate at which EtO is pumped from the sterilizer to the oxidizer does not exceed the capacity of the oxidizer. Because many oxidizers employ a natural gas—fired heating system, improperly designed EtO processes that channel streams with excessively high concentrations of EtO to the oxidizer can result in explosions within the oxidizer.

AMBIENT-AIR FLUSH

Before a sterilizer can be unloaded, two potentially dangerous situations must be eliminated by flushing the sterilizer vapor space with ambient air. (This ambient-air flush is not illustrated for the cycle shown in Tables I and II.) Performed at atmospheric pressure, an air flush involves admitting fresh air into one end of the sterilizer through a valved intake or by partially opening the door while a sterilizer exhaust fan is activated. The air sweeps across the load and exits through the exhaust vent, removing residual EtO from the sterilizer vapor space. As detailed on the tables of process calculations for both the deep and shallow cycles, the sterilizer atmosphere at the end of the final air inbleed is not flammable but does contain enough EtO to present a human health hazard. In addition, despite the air inbleeds, neither cycle will have restored the sterilizer atmosphere to 100% air, and the resulting residual EtO and lack of sufficient oxygen poses a danger to personnel.¹⁴ The process engineer must therefore plan to perform an ambient-air flush following the final air inbleed. The minimum required time for the air flush will depend on the size of the sterilizer and the capacity of the exhaust unit. The stronger the unit, the quicker the sterilizer vapor space is cleared of EtO and restored to a breathable status. Once the minimum required time has elapsed, the air flush may even continue throughout the unloading process.

ULTIMATE SAFETY THROUGH DIRECT ANALYSIS

A critical factor to consider is that, in most sterilization systems in which the sterilizer rear exhaust empties into a catalytic oxidizer, the gas stream vented through the sterilizer rear exhaust must be engineered to contain less than 3% EtO by volume. This gas stream is not diluted with air but is vented directly to the oxidizer. Therefore, if the sterilizer rear exhaust is activated when there is more than 3% EtO inside the vessel, a fire hazard or potentially explosive situation could be created. It is absolutely critical that the sterilizer headspace level of EtO be less than 3% before activation of the rear exhaust.

Since the 1994 adoption of ANSI/AAMI/ISO 1135, there has been discussion within the medical device manufacturing industry regarding the need to install analytical hardware to perform direct on-line analysis of the sterilizer headspace content. Parametric release of EtO-sterilized medical supplies is possible provided that, during select portions of the process, the headspace levels of EtO and the RH are accurately determined through direct analysis. Recently, however, the need to install such direct analyzers has been prompted by major safety concerns. EPA requirements for EtO emission control have caused many facilities to install catalytic oxidizers capable of reducing EtO emissions to the atmosphere by 99%.¹²

The volumetric calculations described in this article allow the process engineer to predict the changing composition of the sterilizer atmosphere. However, these calculations are only accurate under ideal conditions—that is, an empty sterilizer. Once a sterilizer is loaded with product, the elements of packaging EtO and of absorbed and adsorbed EtO alter the behavior of the process in such a way that it is impossible to indirectly predict (through paperwork calculation) the composition of the sterilizer atmosphere. The only way to know with certainty—before activating the sterilizer rear exhaust—that the sterilizer atmosphere contains less than 3% EtO is through direct sampling and chemical analyses such as chromatographic and spectroscopic technologies. Interlocking the gas analyzer with the rear exhaust control system can prevent the activation of the rear exhaust fan when the EtO level inside the sterilizer is greater than 3%.¹⁵

CONCLUSION

Before actual experimentation can begin, the physical parameters of a 100% EtO sterilization process require careful planning. Sterilization engineering begins with a conscientious examination of each and every physical step of the process. The phase of process design input must simultaneously include all process elements (temperature, ramp rates, EtO concentration, and relative humidity) and balance them against limitations imposed by the product, packaging, and equipment. Although calculations can be used to verify the safety of an EtO process inside an empty sterilizer, the dynamics will change once the sterilizer is loaded, and the safety of the process design should be verified through direct EtO monitoring. The principles of cycle design will directly affect the efficiency and profitability of the operation, the safety of sterilization personnel and integrity of equipment, and, ultimately, the well-being of the end- user of the EtO-sterilized medical device. It is imperative that these principles be adopted into employee training programs and understood by everyone managing the EtO sterilization process.

ACKNOWLEDGMENTS

The authors would like to express their gratitude to Stephen A. Conviser of AlliedSignal, Inc., Morristown, NJ, for reviewing the volumetric calculations and process data presented in this article.

REFERENCES

1. Matthews IP, Gibson C, and Samuel AH, "Enhancement of the Kinetics of the Aeration of Ethylene Oxide Sterilized Polymers Using Microwave Radiation," J Biomed Mat Res, 23:143—156, 1989.

2. The reader is advised to consult NFPA 77, Recommended Practice on Static Electricity, National Fire Protection Agency, Quincy, MA, 1993.

3. 1984 Flammability Data on EtO-N₂-Air Mixtures at 1 Atmosphere, Danbury, CT, Union Carbide, 1984.

4. Regardless of the amount of air displaced from the sterilizer, EtO will autoignite at 804°F, thus producing its own supply of oxygen. The reader is advised to consult the material safety data sheet for EtO.

5. Relative humidity can be calculated by using the saturated steam tables. The required cycle data are the pressure rise due to steam injection and the process temperature. The reader is advised to consult the appropriate EtO sterilization TIR from the Association for the Advancement of Medical Instrumentation (AAMI) for step-by-step instructions.

6. ASME Steam Tables, New York, American Society of Mechanical Engineers, 1983.

7. Ethylene Oxide Sterilization Equipment, Process Considerations, and Pertinent Calculations, AAMI TIR No. 15-1997, Arlington, VA, 1998.

8. EtO concentration can be calculated using the ideal gas law of PV = nRT. The required cycle data are the pressure rise due to sterilant addition and the process temperature. The reader is advised to consult the appropriate ethylene oxide sterilization TIR from AAMI for step-by-step instructions.

9. Morrissey RF, "Cycle Development—Physical Parameters," in Proceedings of the Seventh Annual AAMI/FDA Conference on Medical Device Regulation, Arlington, VA, AAMI, pp 18—20, 1980.

10. The data for the EtO processes illustrated in this article were plotted on a flammability chart, courtesy of AlliedSignal, Inc. (Morristown, NJ). The flammability chart used was developed specifically for 100% ethylene oxide with nitrogen processes, operating at or less than atmospheric pressure and not exceeding 45°C. Once this process exceeds atmospheric pressure and/or 45°C, the area of flammability on the chart increases.

11. Buonicore AJ, Desai PR, and Magone MA, "Post-Evacuation Cycle Modifications to Reduce Ethylene Oxide Residual Levels and Worker Exposure," in Proceedings of the Medical Design and Manufacturing East '84 Conference and Exposition, Santa Monica, CA, Canon Communications, 1984.

12. "Environmental Protection Agency—National Emission Standards for Hazardous Air Pollutants for Ethylene Oxide Commercial Sterilization and Fumigation Operations," Code of Federal Regulations, 40 CFR 9 and 63.

13. Ethylene Oxide, Material Safety Data Sheet, Danbury, CT, Union Carbide Industrial Gases, Linde Div., April 1991.

14. The reader is advised to consult the safety bulletin published by the Compressed Gas Association (Arlington, VA): CGA SB-2, Oxygen Deficient Atmospheres, which is also recommended for employee training.

15. Regarding catalytic oxidizers and EPA regulations, a safety committee has been formed within the Ethylene Oxide Sterilization Association (Washington, DC) for the purpose of eventually publishing a document to be titled "EOSA Safety Considerations for Ethylene Oxide Sterilization." The reader is advised to consult this document once it becomes available.

Paul J. Sordellini and Vincent A. Caputo, of Quality Solutions, Inc., Annandale, NJ, and Susan Edel Satter, of Satter & Associates, Inc., Boulder, CO, are consultants serving the medical device manufacturing industry.