Penetrating Sterilization

James Agalloco, Agalloco & Associates, Penetrating Sterilization

Introduction

The process of sterilization is critical to the safety of a myriad of pharmaceutical products and to ensure the absence of microbial contamination in the finished product. The targeted objects include liquids,ⁱ powders, and surfaces that are predominantly comprised of stainless steel, glass and polymers. The processes used to sterilize these items are varied and often specific to a particular item. The prevalent methods include moist heat, dry heat, gas, radiation and increasingly, vaporized hydrogen peroxide. This treatise will explore sterilization on a basic level to examine how the process and materials interact and how sterility is confirmed.

Surface Cleanliness 

Often overlooked, but critically important to process effectiveness is the target surface itself. The surface should be free of particle and microbial residues before being subjected to the process. This point is stressed in hospital practices where the reuse of surgical instruments is universal.

“Cleaning is the removal of foreign material (e.g., soil, and organic material) from objects and is normally accomplished using water with detergents or enzymatic products. Thorough cleaning is required before high-level disinfection and sterilization because inorganic and organic materials that remain on the surfaces of instruments interfere with the effectiveness of these processes.”¹

In the pharmaceutical world, this mandates cleaning validation for equipment and materials to be sterilized with limits for both chemical residuals and bioburden.ⁱⁱ In accordance with CGMP principles all material surfaces introduced or present in the system should have defined pre-sterilization bioburden limits. Increasingly critical product contact parts, e.g., filling sets including fill needles, are being supplied pre-sterilized for single-use to reduce the microbial contamination potential. The control of chemical residues and microbial contamination on surfaces to be sterilized is a CGMP requirement.^2,3

The Importance of Bioburden

Often overlooked, the most important microorganisms in sterilization are those in the pre-sterilization bioburden. They are the microorganisms present in routine processing, and it is their destruction that is critical for patient safety. It is the resistance and population of the bioburden that must be used to assess process efficacy.^4,5 So, while much is written about the destruction of biological indicators, their response to the process is informational rather than critical. The biological indicator serves to measure the process, not define it. The bioburden is the true concern, and practitioners need to pay greater attention to it in the development, validation and maintenance of sterilization processes. It must be recognized that information regarding the pre-sterilization bioburden is often very limited; its population is assessed only intermittently and its resistance to sterilization largely unknown.

Impermeable Surfaces

Solid items are impermeable to the sterilizing agent. Except for radiation sterilization,ⁱⁱⁱ the sterilizing effect is limited to the surfaces directly in contact with the sterilizing agent. There is no penetration of moist heat, sterilizing gas, or vapor hydrogen peroxide beyond the immediate surface. This apparent flaw may be ignored, as, while these interior surfaces are not reachable by the sterilizing medium, they are never exposed to the product or the patient. 

The presence of scratches and crevices on these surfaces are inevitable, but the point of controlling the pre-sterilization bioburden is to ensure that the process is effective in its destruction regardless of their presence.^iv Where the item’s surface is in direct product contact limits on bioburden population and resistance are necessary to ensure the required Probability of a Non-Sterile Unit is realized.⁴ For other surfaces, maintaining effective bioburden control is considered sufficient to ensure process efficacy.

Porous Surfaces

There are, of course, items with substantial internal surfaces that must be sterilized. There are two major categories: rigid and semi-rigid components and tubing intended for direct product handling in processing, formulation and filling, and sponge-like materials used as filters, wipes and spill recovery.

• Material Handling Assemblies / Product Contact Parts – these system can be quite complex, and the sterilizing process must access all interior surfaces. For other than radiation sterilization this typically requires air removal using pre-vacuum (multiple pulses may be needed). For moist heat condensate removal must also be accommodated. As noted above, pre-sterilized single-use disposable systems are increasingly prevalent to avoid the inherent cleaning and sterilization complications. There are no simple solutions to the sterilization of these assemblies regardless of the sterilizing process chosen.
• Sterilizing filters – while seemingly simple, the challenges these present to sterilization can be substantial. The author has had experience with the moist heat sterilization of membrane filters, and the difficulties are real.⁶ Inoculation of the filter membrane with spore challenges highlighted the problems with both steam penetration and condensate removal, something spore strips could not reveal. 
• Sponges, fabric and polymeric wipes present unique challenges. The diversity of materials and their resistance to sterilization is rarely known. Sterilization validation as expected of these has been minimally investigated as they are not in direct product contact, reducing the criticality of their preparation. The means for their sterilization are as varied as the materials themselves. 

Sterile Wrapping

Most items to be sterilized are wrapped to protect the sterile surfaces during transfer, storage and handling prior to opening at the point of use. The sophistication of these barriers varies depending upon the item, its intended use and the typical practices employed for sterilization. 

• Medical Devices/Pre-Sterilized Components – the sterilization of medical devices by other than radiation is accomplished in containers and pouches with a permeable lid or cover for sterilant ingress and egress. Increasingly popular are ready-to-use containers and stoppers supplied sterile. These commonly require sterilizers incorporating pre- and post-vacuum phases to improve performance and efficacy.^vi These items are critical to patient safety and their packaging and sterilization procedures closely monitored.
• Commercial Packaging Systems – there is a wide range of commercially available sterilizable containers available for use. These can be as simple as multi-component bags with press to seal closures, to rigid metal containers relying on mechanical vents or permeable sections for sterilant entry and exit. The packaging materials themselves are well controlled; however, end user practices for item introduction and load assembly are more variable. 
• Hand Wrapped Items – the simplest packing systems rely on sterilization paper, plastic or cloth sheets secured with indicator tape or string. These lack reproducibility and robustness given their total dependence on personnel practice during packaging.

The diversity of methods used for wrapping materials for sterilization results in differing emphasis in sterilization process design and execution. Medical devices rely on rigorous product and package design, and sterilization cycle development supported by ongoing operational controls. Commercial packaging systems balance supplier controls with end user practices for success. Hand wrapped items present the greatest challenge due to their inherent variability. The goal with all wrapping systems is constant: protection of the materials pre- and post-sterilization. 

“An ideal sterilization wrap would successfully address barrier effectiveness, penetrability (i.e., allows sterilant to penetrate), aeration (e.g., allows ETO to dissipate), ease of use, drapeability, flexibility, puncture resistance, tear strength, toxicity, odor, waste disposal, linting, cost, and transparency. Unacceptable packaging for use with ETO (e.g., foil, polyvinylchloride, and polyvinylidene chlorine [kitchen-type transparent wrap] or hydrogen peroxide gas plasma (e.g., linens and paper) should not be used to wrap medical items.”¹

Sterility/Sterility Assurance

The goal in every sterilization process is the complete destruction of the bioburden present, an absolute requirement. While sterility of an item cannot be established, it can be estimated through the cycle development and validation exercise. The minimum expectation is a Probability of a Non-Sterile Unit (Sterility Assurance Level) of not greater than 1 unit in 1,000,000 units. While the requirement is clear, the means vary widely. Terminal sterilization might be the only process where knowledge of the pre-sterilization bioburden population and resistance (mandated for parametric release) allows for calculation of the PNSU as indicated in USP.⁴ The validation of all other processes relies on diverse and less definitive evidence to fulfill the sterility assurance claim.

• Sterilization of porous itemsvii
  • The destruction of a biological indicator with assumed higher resistance and greater population than the bioburden.
  • Execution of a standard cycle (>121.1°C for 15 minutes).
  • Delivery of a minimum F0 of X minutes.⁷
• Radiation
  • Delivery of a minimum dose derived from an initial bioburden assessment coupled with periodic dose audits.
• Gas Sterilization
  • Destruction of a biological indicator with assumed higher resistance and greater population than the bioburden in a half-cycle approach.
• Vapor Processes
  • The destruction of a biological indicator with assumed higher resistance and greater population than the bioburden. Cycles are extended beyond biological indicator death.

None of these validation methods provides a definitive PNSU value. They largely rely on the destruction of a resistant biological indicator often with the exposure period and/or lethal conditions increased to provide greater confidence in the results

Sterilization/Bio-Decontamination

Bio-decontamination is a term used by a supplier of vaporized H₂O₂ systems to differentiate between their deep vacuum systems (primarily for medical device sterilization) and their ambient pressure systems (for more diverse applications). There are individuals who suggest that bio-decontamination is something less than sterilization. This belies the more than 30 years of experience of ambient pressure systems for the sterilization of materials, product contact parts and enclosure surfaces. Relying on a poorly defined marketing term to refute robust data is inappropriate. This may be motivated more by efforts to defend systems unable to reliably kill microorganisms than anything else.

Conclusion

Sterilization is critical to patient safety and application is as diverse as the materials and systems which must be treated. There are no ‘universal’ answers in sterilization. There is no ‘perfect’ sterilization method suitable for every need. 

• Moist heat is typically incompatible with typical biologics,
• PTFE and Nylon cannot be sterilized by radiation.
• Ambient pressure H₂O₂ systems are poorly suited for packaged medical devices
• ClO₂, NO^₂ and H₂O₂ are incompatible with cellulose materials

Sterility must always be balanced with utility. The various methods exist because they provide unique solutions that deliver lethality while maintaining the essential quality attributes of the materials being sterilized where a different method might not be usable. Each method has its limitations whether in air removal, penetration of any protective wrap and removal of residual sterilant. Each sterilization method has its unique aspects and applications, advantages and disadvantages, and comparing one to another serves no purpose.

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