Vacuum H2O2 gas sterilisation in lyophilisers: a future alternative?

Lyophiliser sterilisation is a common demand of the pharmaceutical industry. This is done by steam by nearly 100% of all applications on the market. During this process, pressurised, saturated, clean steam at temperatures >121.1 °C for 15–30 min is applied to the system. However, an alternative to steam sterilisation could be H₂O₂ (hydrogen peroxide) gas sterilisation, which is operated under vacuum conditions.

First, the area to be sterilised (chamber, condenser, all associated pipes) is evacuated to a certain value (approximately 0.02 mbar) and then H₂O₂ gas is transferred into the evacuated area. Because of the high vacuum, only a small amount of H₂O₂ gas is required to cover the entire surface area, including difficult‑to‑reach parts (as an estimate, the design calculation uses 10 mL/m³, which will be reduced to real requirements during cycle development). The final pressure during sterilisation will be approximately 10 mbar. This approach can be repeated for process safety reasons.

This piece compares steam sterilisation with H₂O₂ gas sterilisation.

Advantages of classical steam sterilisation
It is a well‑known and accepted method with a proven track record. The requirements are clearly defined and the authorities usually approve the processes without question.

In most facilities, sufficient amounts of steam are available and its quality is compliant with the requirements. Lyophilisers often use steam as a media for defrosting (CIP water can also be used). If a steam connection is already in place, it is probably best to sterilise with steam.

Advantages of H₂O₂ gas sterilisation
Daily routine costs are much lower. The amount of steam and energy required for recooling are much higher compared with lower vacuum and thus H₂O₂ gas sterilisation. The amount required also depends on the equipment (e.g., 50 mL H₂O₂ per cycle for an average lyophiliser). The cost ratio between H₂O₂ gas and steam is approximately 1:4.

Both methods have a sterility assurance level (SAL) 10⁻⁶. High vacuum in the entire system ensures all dead legs will be sterile (ISO 13408‑4‑compliant) and, unlike steam sterilisation processes, there is no risk of remaining water condensate.

Because of the gentle H₂O₂ gas sterilisation process at ambient temperature, stress to the vessels, diaphragms, joints and filter is reduced. In addition, the low concentration of H₂O₂ gas applies no stress to non‑stainless steel components. The ambient temperatures mean less insulation material and related insulation work are required.

There is no process‑related overpressure in the system and, therefore, no pressure vessel is required, which avoids costs for pressure vessel calculation, approval and periodic inspection. The absence of high pressure and high temperatures avoids unexpected events, such as leaks, and raises the production availability.

Existing steam pipework systems within the facilities are often limited because of capacity. This requires interlocks with other steam consuming systems to avoid potential lack of steam. The risk of rouging out (i.e., dark iron deposits), which can be associated with steam supply, can be eliminated.

Comparable aspects for both methods
The risk to humans and equipment is comparable. For steam it is necessary to provide several measures to reduce the risk, such as pressure vessel, design of chamber, interlocks and safety valves, but proven technology and experience considers these risks as insignificant. The same applies to vacuum H₂O₂ gas sterilisation. As the system is under vacuum during the process, the gas cannot escape and, therefore, poses no risk to personnel. Even if the used amount of H₂O₂ gas does come into surrounding areas, the concentration can be neglected because of the small amount of used gas. In addition, it is advisable to install H₂O₂ gas monitoring in the machine room and to check for residual H₂O₂ gas within the chamber at the end of a cycle.

The purpose of validation is to verify that the SIP system design, process design, design of all facilities, equipment and materials used meet requirements for the intended use. Both methods require validation and the same acceptance criteria (SAL 10⁻⁶). Therefore, the test approach and the amount of related testing is comparable. The main difference is the determination of critical areas. Steam SIP uses temperature mapping to verify that all potential cold spots reach the required temperature of >121.1 °C.

H₂O₂ gas uses temperature mapping to identify the warmest positions within the system after the previous CIP cycle.

H₂O₂ gas performs best within the 15–50 °C temperature range. Mapping, therefore, supports process development while defining worst case position, which will later be biological indicator (BI) tested.

Conclusion
Saturated steam will remain the method of choice for the sterilisation of lyophilisers for years to come — not because it is the better method, but rather because it is a commonly accepted method by users and, of course, the authorities, which is probably the leading reason.

The use of H₂O₂ gas sterilisation will be more often used as a retrofit in existing lyophilisers. In the near future, this method will have niche application for new equipment; for example, facilities where insufficient amounts of clean steam is available. However, if we assume energy costs will continue to rise and more environmental directives will come into play, H₂O₂ gas sterilisation could offer a serious alternative to steam sterilisation.

Based on a contribution by Mike Guttzeit, Team Leader, Validation, from GEA Pharma Systems (Germany).