Rapid, safe, and effective sterilization is of the utmost importance when it comes to protecting the public in general and hospital patients in particular. Today, public health institutions face unprecedented challenges due to the advent of heat sensitive reusable medical tools and due to the appearance of heat resistant microorganisms such as prion, the protein which causes Creutzfeldt-Jacob disease, more commonly known as "mad cow disease". Conventional sterilization methods such as autoclaving and Ethylene Oxide (EtO) are inadequate in these cases. Non-equilibrium "cold" plasmas have recently been shown to be a very promising alternative, potentially capable of overcoming the above-mentioned challenges. In addition to not damaging the articles to be sterilized, cold plasmas proved to be very effective due to the synergistic effects of free radicals and UV photons, which interact with the cells of microorganisms on the atomic and molecular levels.
Sterilization is an act or process, physical or chemical, which destroys or eliminates all forms of life, especially microorganisms. Conventional sterilization techniques, such as those using autoclaves, ovens, and chemicals like ethylene oxide (EtO), rely on irreversible metabolic inactivation or on breakdown of vital structural components of the microorganism. This does not present a problem in cases where material preservation is not an issue.
However, in cases where it is imperative not to damage the materials to be sterilized, conventional methods are either not suitable at all or offer very impractical and/or tedious and time-consuming solutions. This situation led to a drive to develop new techniques as effective as established ones, but with added superior characteristics such as short processing times, non-toxicity, and medium preservation. Amongst these new methods, nonequilibrium atmospheric pressure plasmas have been shown to present a great promise.
Plasma sterilization operates differently because of its specific active agents, which are ultraviolet (UV) photons and radicals. An advantage of the plasma method is the possibility, under appropriate conditions, of achieving such a process at relatively low temperatures (=50 °C), preserving the integrity of polymer-based instruments, which cannot be subjected to autoclaves and ovens. Furthermore, plasma sterilization is safe, both for the operator and the patient, in contrast to EtO.
Hydrogen peroxide gas plasma sterilization is a low-temperature, low-moisture sterilization process that is rapid enough to provide high throughput. Charged particles may play a very significant role in the rupture of the outer membrane of bacterial cells. They showed that the electrostatic force caused by charge accumulation on the outer surface of the cells' membrane could overcome the tensile strength of the membrane and cause its rupture. There are no toxic residuals; therefore, no aeration is required. The primary byproducts of the process are water vapor and oxygen. As a consequence, the cycle time for processing can be relatively short. Several new improvements in hydrogen peroxide gas plasma sterilization technology have reduced cycle time from 74 minutes to 55 minutes, allowing more instruments to be processed.
Hydrogen peroxide gas plasma is used worldwide for terminal sterilization of medical equipment. Sterilization occurs in a low-moisture environment at a temperature less than 50°C. It is suited for sterilizing heat-and moisture-sensitive items, delicate instruments, and instruments with sharp edges. Recent technological advances have resulted in a hydrogen peroxide gas plasma sterilization system that has improved robustness and a shorter cycle.
Next generation of low-temperature hydrogen peroxide gas plasma sterilization systems have demonstrated an improved load tolerance and faster kill in diffusion-restricted areas as opposed to previous-generation hydrogen peroxide gas plasma sterilizers. The systems provide terminal sterilization within a time, shorter than the typical 14 hours required to produce sterile instruments using EO. The short cycle time also means that a single unit can sterilize, not merely disinfect, numerous loads per day.
Low temperature, atmospheric pressure plasmas have been shown to possess very effective germicidal characteristics. Their relatively simple and inexpensive designs, as well as their non-toxic nature, give them the potential to replace conventional sterilization methods in the near future. This is a most welcome technology in the healthcare arena where re-usable; heat sensitive medical tools are becoming more and more prevalent.
(The author is biomedical engineer, MGM'S New Bombay Hospital)