Paul Dvorak | Founding Editor | Medical Design & Outsourcing
The FDA’s expectation is that validating a sterilization process follows the traditional validation and verification model, commonly referred to as “V&V.” An installation qualification verifies that the sterilization equipment has been installed to manufacturer’s specification, while the operational qualification verifies that the machinery is working as intended. The final step in a validation process is a performance qualification, which validates that products exposed to the sterilization process meet the required sterility assurance level.
“That’s it in a nutshell,” said Paul Littley, a consulting manager at Nelson Laboratories. “When we talk about V&V, the installation qualification, more of a verification, is making sure the equipment is installed correctly, for example electrical and plumbing is correctly installed.”
“An operation qualification ensures the chamber is functional for a generic process,” said Littley, noting that operation and performance qualifications are a bit more in-depth. “Once we get to performance qualification, you are looking for repeatability and reproducibility of the cycle for a particular process challenge.”
The common approach for ethylene oxide sterilization (EO sterilization) validation is referred to as the “Overkill Approach,” in which a biological indicator, placed in the most resistant location of the device, is deactivated or killed in one-half the time of the routine sterilization cycle. The half cycle’s lethality must be demonstrated a minimum of three times to be considered adequate or validated. Additionally, a full cycle is performed to demonstrate that the defined cycle is capable of maintaining the process parameters for the entirety of the routine production cycle.
The equipment and process
EO chambers are basically pressure vessels which work under sub-atmospheric conditions (i.e., below atmospheric pressure). Some EO sterilization processes which use a gas blend, such as carbon dioxide, are performed at a positive pressure, but most proesses are 100% EO and are sub-atmospheric due to flamibility concerns.
“The chambers will pull a particular vacuum, commonly referred to as deep or shallow vacuum. A deep vacuum is typically below six psia, pounds per square inch absolute,” said Littley.
Most sterilization processes are batch operations. Some of the smaller test chambers are in the 9-ft³ range, but larger industrial units handle 30 pallets, a full load for a tractor trailer.
EO sterilization proceeds through a three-phase process. The first phase, preconditioning, conditions the load to an even temperature and relative humidity, often for 24 hours at 115° F and 65% RH.
“This does several things. Because the product is in packages, the heat and humidity condition the packaging material so the gas can penetrate easier,” he said. “The actual sterile barrier is typically Tyvek, a breathable layer that provides a difficult pathway for microorganisms to enter, yet it lets EO and other gases through. EO’s penetration is unique in that it is a small molecule which can diffuse through plastics quite readily.”
The second purpose is to condition the microorganisms on the device, as well as the Biological Indicator (BI) used to measure the process, for gassing. Humidity helps condition the cellular wall and reduces the organism’s resistance to sterilization. The BI contains a minimum of one million organisms of the EO sterilization indicator organism Bacillus atrophaeus, a spore-forming organism.
“These spore-forming organisms exist in two states. One is a dormant state in which they are called endospores. The organism forms a shell-like structure to protect itself until a nutrient-rich environment becomes available, at which time it can sporulate, grow and reproduce,” said Littley. “When the organism is in this second state of growth and reproduction, we call it the vegetative state. Endospores are so durable some have been pulled off of ancient mummies making them at least 4,000 years old. Spore-forming microorganisms are used as the indicator because they are difficult to kill.”
After the pre-conditioning phase, operators quickly load the product into the sterilization chamber so as to not lose heat from preconditioning.
“Once in the chamber, the system pulls a vacuum to evacuate the air. Typical cycles pull deep vacuums down to 1 psia. EO in air raises an issue, because it is flammable at about 10% oxygen concentration, so it is important to remove the air from the chamber and the product,” he said. “Depending on the cycle and product, there might be a few nitrogen washes during the initial stages to flush the residual oxygen. This is more common in shallow vacuums of 6 psia or greater. Next the EO gas is injected and it is absorbed into the nooks and crannies of the devices, referred to as restricted pathways or mated surfaces which are void of air.”
From there, the process goes into a dwell state, typically 3 to 5 hours, of static conditions in which the gas is allowed to penetrate deep into the load and devices.
“When the dwell period ends, it is important to exhaust the EO out of the load and devices due to the flammability as well as because it is a carcinogen,” said Littley.
To remove most of the EO gas before opening the chamber, the load is washed with an inert gas such as nitrogen or steam, until most of the EO is out. Once below flammable concentrations, air is typically used for additional washes. Finally, the product goes into a heated aeration chamber where it dwells at approximately 110° to 115° F with air circulation, to further reduce the residual EO absorbed into the product.
In general, sterilization validation is required to demonstrate that the devices are free of viable microorganisms. As a first step in the validation process, the resistance of the device must be determined. According to Littley, there are two ways to verify the resistance.
“One is direct innoculation of the product, where a BI is placed in the most difficult-to-sterilize location of the device, such as restricted pathways, mated surfaces or long narrow lumens,” Littley said. “In theory, the process should kill a given population of that organism. The biological indicator, with a minimum of one million organisms on a paper strip, is our ‘meter stick’ for determining the sterility-assurance level. Naked, by themselves, the organisms are not difficult to kill. But placing the BI inside a device relatively compares to how other microbes may or might not survive during EO exposure.”
Another approach to validating the process is through a process-challenge device (PCD), which provides an additional avenue for products which are expensive or not easily accessible. In these cases, the manufacturer would prefer not to destroy a $5,000 product, for example.
Instead, an easy-to-handle, inexpensive PCD, like a coupon, is qualified for use in the sterilization validation to simulate the product. Common PCD configuration might be a sealed a polybag, sealed tubing, or 5 to 10cc capped non-vented syringe, with a BI placed inside. The average cost of a PCD is in the $10 to $15 range. The PCD is then used in the validation process instead of the inoculated device. If the PCD can be deactivated or sterilized, then a device of lesser resistance would also be considered sterile.
How often to revalidate a product?
Revalidation would not always be necessary for a design change. For example, if a design change increases an inner diameter, making a pathway easier for a gas to infiltrate, a requalification would not be required.
“But if the pathway or inner diameters are reduced in size and made more restrictive, that would call for a revalidation. Also, as production ramps up and loads become larger or perhaps more dense, or if the packaging significantly changes, that would also require a revalidation,” said Littley.
However, once the validation process is defined and the device and sterilization process does not change, there is no reason to revalidate.
Nevertheless, annual requalification of the sterilization process is required by ISO and the FDA. This can be a pen-and-paper-change assessment or actual testing.
“During requalification it is not uncommon for the device manufacturer to reach out to the contract sterilization facility and ask if anything has changed with the process, paying special attention to unscheduled maintenance or equipment changes,” Littley said. “At minimum, there should be some level of testing every two years, say a half cycle with a full cycle to be conservative.”