Methods using steam and disinfecting gases, particularly ethylene oxide, have been widely used for sterilizing medical products ranging from pharmaceutical preparations to surgical instruments.
A sterilizing method must effectively render all microbial organisms non-viable without damage to the article or goods being sterilized or its packaging. However, many disinfecting gases which meet this criteria, such as ethylene oxide, have been recognized to expose workers and the environment to safety hazards. Recent legislation has severely restricted the amount of hazardous gases such as ethylene oxide (a known carcinogen) in the working environment, or the use of any system or method which produces toxic residues or exhaust products. This has created a major crisis in hospitals and other areas of the health industry. Further, although steam sterilization is inexpensive and effective, it is too hot for many applications, particularly those involving sterilization of plastics with low melting points. Irradiation is a cooler process, but the large and expensive facilities required for irradiation are impractical for hospital uses.
The use of plasma in sterilizing methods has been suggested. Plasma is ionized or partially ionized gas which may be generated by the application of electromagnetic energy which may be obtained from different sources. The ionized gas will contact microorganisms on the surfaces of the items to be sterilized and effectively destroy the microorganisms.
Among the variety of gases attempted for use as sterilizing plasmas has been water vapor. However, these prior attempts have been reported as unsuccessful, or as being not particularly effective. Thus, U.S. Pat. No. 4,643,876, issued Feb. 17,1987, inventors Jacobs and Lin, describes a plasma sterilization process and apparatus where the item to be sterilized has plasma generated around the item. Attempts to use a water plasma treatment by itself in this method of sterilization led to no significant sporicidal activity. Similarly, U.S. Pat. No. 3,701,628, issued Oct. 31, 1972, inventors Ashman, describes generating a plasma from sterilizing gas that is introduced into the sterilizing chamber. Plasmas formed of water vapor were reported as not being particularly effective.
U.S. Pat. No. 5,115,166, issued May 19,1992, inventors Campbell and Moulton, describes the use of electrically neutral active species generated in a gas plasma to sterilize articles. In this system, articles to be sterilized are placed in a vacuum chamber which is attached to one or more plasma generators and a vacuum pump. The plasma generators use a source of microwave energy such as a magnetron to create the required electromagnetic field. The energy is conducted to the gas via a rectangular waveguide. The chamber is sealed and the vacuum pump is used to evacuate air from the chamber. Valves are then opened which allow gas to flow from a source container, through the plasma generators, through a gas distribution system, then through the sterilization chamber, and out through the vacuum pump. The gas mixture is usually oxygen, hydrogen, or a mixture of oxygen, hydrogen, and an inert gas. A strong electromagnetic field in the plasma generators interacts with the gases flowing through the plasma generators and generates a glow discharge. A variation of such an apparatus is described in U.S. Pat. No. 5,184,046, issued Feb. 2,1993, inventor Campbell, where the plasma generators incorporate a cylindrical wave guide.
Both these apparatus of Patents '166 and '046 confine the electromagnetic field generating the plasma within the plasma generator. Once the gas flows out of the region with a high electromagnetic field, plasma is no longer created. Since highly reactive plasma components either react or decay quickly, relatively non-reactive components enter the sterilization chamber. Charged particles recombine in the gas distribution system to form electrically neutral components. Thus, the components entering the sterilization chamber include neutral active species which accomplish sterilization.