Thursday, November 15, 2012

We Don’t Use Hydrogen Peroxide, We Have a Sterrad® Plasma Sterilizer

We often hear refrains similar to that of the title when we discuss hydrogen peroxide monitoring with customers. Thirty years ago, most hospitals used ethylene oxide for their low temperature gas sterilization. In the 1990s, Advanced Sterilization Products (ASP), a J&J company, launched the Sterrad line of sterilizers. This product has been remarkably successful and is now the dominant low temperature sterilizer in the US, and many other countries around the world.

A sterilized product is one that is free of all microbial life, including the highly resilient sporoidal bacteria. Obviously, any chemical gas or vapor that can achieve sterilization poses a risk to anyone exposed to it. Hydrogen peroxide for example has an OSHA permissible exposure limit (PEL) of 1 ppm (8 hr time weighted average), the same as ethylene oxide. The Sterrad line of sterilizers generally has a good safety record, but as with any complex equipment malfunctions can occur, engineering controls can fail and user error can result in potential exposures. Therefore, ChemDAQ recommends gas monitoring for all gas and vapor sterilant chemicals.

The Sterrad sterilizers function by reducing the pressure, introducing hydrogen peroxide vapor (made by evaporating liquid hydrogen peroxide solution) which performs the primary sterilization step, and then the sterilizers activates its radio frequency coils to convert the hydrogen peroxide vapor into a plasma which eliminates residual hydrogen peroxide. The total cycle time for the 100S is about 55 minutes. [100 S data sheet].

As the old saying goes, ‘Time is Money’ and so there is a need for shorter cycle times. Chemical sterilization is achieved by exposing the articles to be sterilized to high concentrations of reactive gases or vapors. Shorter exposure times require higher concentrations of these gases. The NX series of sterilizers takes the 59% solution of hydrogen peroxide used in the 100S and internally concentrates it about 90%. Sterilizing with this solution allows cycle times of only about 28 minutes.

Most matter (solid, liquid, gas) is electrically neutral, but plasma, sometimes described as the fourth sate of matter, is different. Plasmas are found in stars and flames and even fluorescent lights etc., and consist of atoms with at least some of their electrons stripped off forming an ionic gas with lots of high energy, very reactive radicals. While the plasma contributes to sterilization the surfaces of an object, the radicals are so reactive that they would have very limited penetrating power into crevices etc. since they would react with the first surfaces they reach. Therefore the main sterilizing agent in the Sterrad sterilizers is the hydrogen peroxide vapor.

The other main hydrogen peroxide sterilizer used in US hospitals is the V-PRO from Steris Corporation. The Amsco® V-PRO sterilizer forms hydrogen peroxide vapor from a solution of 59% hydrogen peroxide, and also offers a 28 minutes cycle time. The V-PRO generally functions similarly to the Sterrads, though both manufacturers may take exception to this description; but it does not use a plasma to remove the left over hydrogen peroxide at the end of the sterilization cycle. Instead, the hydrogen peroxide is destroyed by passing it through a catalyst that converts it to oxygen and water. The Sterrads have a similar catalyst on their exhaust to remove any hydrogen peroxide vapor surviving the plasma treatment

In 1990 Amsco (a forebear of today’s Steris Corporation) sued Surgikos Inc (ASP was founded as a division of Surgikos) for patent infringement claiming that the Sterrad plasma process was merely icing on the cake and therefore Surgikos were infringing the Steris patent claiming hydrogen peroxide sterilization. [US patent 4,169,123]. Surgikos argued that the plasma process was an essential part of their process, as describe in their own patent [US patent 4,643,876]; and managed to convince the court that they were right. The cynic might claim that this success was more attributable to the skill of Surgikos’s attorneys that hard science; but patent was not infringed as the rest is Sterrad history.

While the technologists may argue whether the plasma phase really increases sterility, one fact is sure, having a plasma phase greatly helps marketing. What a ‘cool’ name: a Plasma Sterilizer!

In summary, both the Sterrad sterilizers and the V-Pro sterilizers use hydrogen peroxide as the sterilant chemical. The plasma phase may contribute to the sterilization, but even ASP say it is primarily there to remove residual hydrogen peroxide vapor. By their very nature, all chemical sterilants are potentially harmful to anyone exposed to them. Even though both Sterrad and V-Pro sterilizers and designed and built to the highest standards, leaks can occur and therefore all chemical sterilizers should be monitored for leaks. Like a fire alarm, major leaks rarely happen, but if they do, you will be glad you had a ChemDAQ monitor there. Amsco® is a registered trademark of Steris Corporation; Sterrad® is a registered trademark of Advanced Sterilization Products.

1 comment:

  1. A recent publication on role of the plasma in a hydrogen peroxide plasma sterilizer concluded that the sterilization was achieved by the hydrogen peroxide vapor before the plasma phase.

    K. TAMAZAWA, Y. TAMAZAWA, and H. SHIMAUCHI, in their Poster titled "Evaluation of Plasma Sterilization Mechanism and H2O2 Residues"; at the International Association of Dental Research General Session & Exhibition (June 28 – July 1, 2006), Brisbane; available from (retrieved 1/29/2013)