A Guide to Nitrogen Dioxide Gas Sterilisation
Ethylene oxide (EO) and gamma radiation account for the majority of sterilisation cycles performed during the medical device manufacturing process. But while EO has the best overall material compatibility, it comes with the need for increased safety measures and facilities modifications due to the explosive and carcinogenic nature of the sterilant.
USA-based specialist sterilisation equipment supplier Noxilizer has developed a novel alternative sterilisation technique, nitrogen dioxide (NO2) gas sterilisation. A guide to the method, which the company claims is a cost effective alternative that can be carried out at room temperature, is presented below by Noxilizer's technical applications manager Evan Goulet, PhD, and senior vice president, research and development David Opie, PhD.
Some pros and cons of EO and gamma sterilisation
Contract EO sterilisation services often require manufacturers to accumulate and ship inventory that can spend as much as a month in transit and being sterilised. Gamma radiation has some materials compatibility issues due to the oxidation and cross-linking of polymers such as Teflon and polypropylene. NO2 provides rapid and effective microbial inactivation of a broad spectrum of organisms, with Geobacillus stearothermophilus being the most resistant (same as steam and hydrogen peroxide). With NO2 gas, microbial inactivation proceeds in a log-linear fashion, providing a predictable model around which cycle development can be based.
Some benefits of NO2
Noxilizer’s NO2 sterilisation process gives designers and manufacturers an alternative method that can reduce costs and turnaround time. The physical and chemical properties of NO2 gas provide benefits that can lead to cost savings. NO2 boils at room temperature, and the concentrations used in sterilisation are typically less than 1% of the saturated vapour pressure. This allows NO2 gas to readily penetrate primary packaging and complex geometries. It also aids in rapid removal of sterilant from the product and packaging. There is no need to precondition product prior to sterilisation, nor is there a lengthy aeration phase after the cycle. Product is aerated as part of the exposure cycle, which is typically between 60 and 90 minutes in duration. Ready aeration of NO2 from sterile product means that residual concentration is low, and the residuals are non-toxic and non-carcinogenic. Therefore, product can be handled and returned to inventory immediately after sterilisation.
Compatible plastics
Many medical device polymers are compatible with the NO2 sterilisation process, but there are a few that are incompatible. However, for each incompatible material, there exist design alternatives that may be employed. Polyurethanes are generally incompatible with NO2 sterilisation, but thermoplastic elastomers with similar properties may be substituted. Nylon and polyacetal (Delrin) are also incompatible, but polyesters or polyetherimide may provide suitable alternatives, respectively. Cellulosic materials such as paper and cardboard are incompatible as well. NO2 sterilisation is performed on products in primary packaging, prior to final packaging in boxes. The NO2 sterilization process is compatible with many current sterile barrier packaging options including Tyvek-Mylar pouches and tubs with Tyvek lids to allow gas access to the product.
Contract NO2 sterilisation and equipment purchase
Noxilizer offers contract sterilisation services with fast turnaround time. Noxilizer also sells NO2 sterilisation systems that allow manufacturers to bring sterilisation in-house in order to eliminate transportation and inventory carrying costs. The RTS 360 NO2 Sterilizer is capable of sterilising approximately one pallet of product per 8-hour shift. It is a standalone unit that can be vented directly to the manufacturing environment, as the NO2 is completely scrubbed from the exhaust. The scrubber media is a solid, non-hazardous material. Noxilizer is an ISO 13485 registered company.