Be it known that we, Waldemar G. deRijk and John

A. Tesk, have invented a new and useful improvement in an

Apparatus For Sterilizing Objects of which the following is a specification.

BACKGROUND OF THE INVENTION

The invention disclosed herein is an apparatus for igniting sustaining a gas plasma used to sterilize

Objects placed therein. DESCRIPTION OF THE PRIOR ART

When it is desired to sterilize an object, such as a dental or medical instrument, several technigues have been employed in the past. Normally these techniques encompass either chemical sterilization and/or the physical destruction or inactivation of the microorganisms. Ideally the process should not only eliminate all icrobial spores and viruses from the surface of the object but should have no negative effect on the material properties of the object or instrument being sterilized. .

Chemical sterilization has proven to be difficult to achieve and tends to breed resistant strains of organisms. In particular viruses have been very difficult to deactivate. As a result, we have found the physical destruction or inactivation of microorganisms to be the more desirable method for sterilization both from an ecological and an environmental point of view.

Gas plasmas, either cold gas or glow discharge, have been used for micro-ashing microorganisms. Typically a high power radio frequency (RF) source is used for the radiation source for the plasma. This RF plasma is established at the FCC industrial frequency 12.5 MHz with a wavelength of about 22 meters.

Although, RF plasma sterilization has proven effective

SUBSTITUTESHE

for exposed surfaces of the instruments little or no penetration of crevious or roughened surface textures is obtained by these techniques.

Microwave radiation by itself has also been attempted as a means of sterilization by using a microwave oven as a source of dry heat. Limited success has been achieved by this method as most microorganisms are not destroyed by the radiation. Further, only non-metallic materials are treatable with microwaves as the potential for arcing of metal could damage the instrument or the microwave's magnetron.

In contrast to the foregoing, our invention makes use of a conventional microwave oven and modifies it by employing a chamber in the oven's compartment. This chamber is maintained near the vacuum level and is in fluid communication with a vacuum pump. An object or instrument to be sterilized is placed in the chamber and its oven; a vacuum drawn to the desired level; and the microwave oven activated. Within a short period of time the microorganisms are either destroyed or rendered inactive. This generally short exposure time for metallic instruments minimizes the effects of surface heating thereon and does so without arcing taking place within the chamber. In addition, our invention has the added advantage of being useable with a wide variety of instruments made of different materials (e.g. metals, polymers) at a very economical cost.

SUMMARY OF THE INVENTION An apparatus to sterilize objects, such as instruments, by use of microwaves and a gas plasma ignited and sustained by the microwaves. A chamber containing the object to be sterilized is in fluid communication with a vacuum forming source and subjected to a source of microwaves. Normally the gas pressure in

the chamber is maintained between 1 to 1,000 micrometers Hg. A typical operating pressure would be about 100 micrometers Hg. Microwave radiation is supplied to this chamber and its associated instrument by a RF generator for a predetermined short period of time. The chamber may be made of a transparent or translucent material and is connected by a non-metallic conduit to the vacuum forming source passing through a radiation sealed orifice. DESCRIPTION OF THE DRAWINGS

The figure illustrates the preferred embodiment of our invention with the door to the microwave oven partially open.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown, the preferred embodiment of our invention has a transparent container 1 or chamber (typically 350 ml to 1500ml in volume) with a removal top which acts to form the desiccator and to hold the object to be sterilized. The top has a rim with a grease fitting and also has a connection to allow gas to move to and from the container via a flexible fluid conduit 12. This chamber, as shown, sits on the floor 2 of the conventional microwave oven unit 3. This microwave unit has both power level and time controls. A radiation tight seal is formed at the wall of the oven where the conduit 12 or tube extents therethrough by making the path optically blocked through offset battles which cause the conduit to take a meandering path.

One or more vacuum gauges (4 and 5) may be used to ascertain gas pressure readings. . The gauge 5 essentially tells a user the pressure in or near the container 1 by an electrical signal supplied thereto by the thermocouple vacuum gauge 9. A second thermocouple vacuum gauge 11 provides a second electrical signal to

SUBSTITUTESHEET

the gauge 4 to indicate the pressure reading at its connection to pump 8. Although our preferral embodiment employs this second gauge (4) to the pump it is not necessary for the operation of our invention. The pump 8 depicted is a conventional rotary vacuum pump. It is connected through the shut off value 7 assembly to the fluid conduit 12. A second valve 6 acts as a bleeder valve to control the flow of air or gas into the sterilizing chamber via the same conduit. It should be apparent that the pump 8 and its conduits, gauges, and valves could be enclosed and placed in a housing adjacent to the microwave for compaction and appearance purposes.

The material used for container 1 is limited to materials that are microwave transparent or translucent and have a very low absorption coefficient and are not highly microwave reflective. Initial tests were conducted on nylon, polyethylene, glass (Pyrex) , polycarbonate and neoprene rubber and silicone rubber. Polyethylene and Pyrex showed that least amount of heating. The Pyrex glass and polycarbonate containers were transparent and as a result allowed the gas plasma to be clearly visible. The Pyrex container is our preferred material. In one instance the gas plasma in the container 1 was drawn by the pump to about 50 micrometers Hg pressure. We contemplate the normal operating range for the gas plasma pressure to be from about 1-1,000 micrometers Hg for the container. Depending on the object to be sterilized the microwave oven will be supplying radiation to the instrument in the

2450 MHz frequency for about 15 seconds to 5 minutes. Ignition of the plasma depends on the make of oven but is typically 3 to 5 seconds. In one test dental instruments

(mirror and an endodontic explorer) were subjected to

radiation in the plasma for one minute without any arcs being observed. Exposure at full microwave power oven a short period of time was more effective in achieving sterilization than using less power over a longer time frame, (e.g. half power at double time resulted in no sterilization of a spore strip whereas full power at single time achieved sterilization) .

Our invention is designed to destroy or deactivate microorganisms including fungi, algae, bacteria, viruses, and spores. We postulate that the heating effect is secondary in their effectiveness and that the direct ionization in the plasma is what destroys the DNA molecules of the microorganism. Both ambient air and oxygen in the chamber yield similar results in our tests. Visually the gas plasma, consisting, of fast moving electrons and slower moving ions, changes from an initial pink glow to a white glow in about 10 seconds when the microwave was at full power.

Sequentially, the valve 6 is open and ambient air or some other gas goes into chamber 1. This valve in then closed and the pump 8 operated to draw the gas from the container 1. Valve 7 is at first closed and then opened as the pump draws the gas through it to where it exists from the pump to the surrounding air remote from the oven. With the oven door closed the microwave is operated at full power for a predetermined length of time previously determined experimentally depending on the particular instrument or object to be sterilized. It should be apparent that our invention is not limited to the sterilization of dental and medical instruments. For example, it can be used to sterilize clinical supplies, barber and beautician instruments,- and polymeric materials (such as denture acrylics) . The mass

SUBSTITUTESHEET

of our experiments were limited to the dental art but we by no means confined thereto. What we sought to do in these experiments were to improve the current high pressure steam (autoclave) method and if autoclaving is not used, the alcohol or quartemary anonium compound disinfection process commonly used today. We believe our invention is not only more effective for sterilizing unwanted microorganisms but is also provides a more convenient instrument as for storage, all at a modest cost without altering the material properties of the objects or instruments. For example, with our invention cutting edges are not blunted or corrosion of the instruments does not occur as may happen in autoclaving.

None of the mentioned variations to the described preferred embodiment should be used to limit or change the scope and spirit of our invention which is limited only by the claims that follow.