CA2001461A1 - Apparatus for sterilizing objects - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus for sterilizing objects, such as dental and medical instruments, utilizing microwaves and a gas plasma in a chamber in which the object is placed. The gas in the chamber is maintained at a pressure of between 1 to 1,000 micrometers Hg and upon radiation the gas plasma is ignited ant sustained in the chamber. A pump maintains the desired pressure and a microwave oven provides the source of radiation to the object at about 2450 MHz. Sterilization of objects takes place after the objects are exposed for a predetermined time interval to the radiation in the gas plasma.

Description

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SPECIFICAT'ON

~ IT KNO~ that we, ~1al~emar G. deP.ijk and Johr A.
Tesk, have invented a new and useful improvement in an Apparatus For Sterilizing Objects of wnich the 'ollowing is a specification.
BACKGP~OUND OF THE NVENTION
The invention disclosed herein is an apparatus for igniting and sustaining a gas plasma used to sterilize objects placed 'herein.
DESCRIPTION OF THE ~RIOR APT
tihen it is desired to sterilize ar object, such as a dental or medical instrument, several techni~ues have been employed in the past. Normally these techni~ues encompass either chemical sterilization and/or the physical destruction or lractivation o4 the microorganisms. $deally the process should not onlv eliminate all microbial 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 rterilization has proven to be difficult to ;
achleve and tenas to breed resistant s~.rains oÇ 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 eco'ogical 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 '' ''-.' ~

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1 ' frequency 12.5 MHz with a wavelength of about 22 meters.
Although, RF plasma sterilization has proven e''ective for exposed surfaces of the in~truments 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 thLs 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 mlcrowave oven and modifies it by employing a chamber in the oven' 3 compartment. This charber is malntained near the vacuum level ar.d is in fluid communication with a vacuum pump. An object or instrument to be sterilized is placed in the chamber and its ovent a vacuum drawn to the desired level; and the microwave oven activated. Within a short period of time the microorganism~ are either destroyed or rendered inactive. This genorally short exposure time for metallic instruments minimizes the effects of surface heating thereon and does so without arc~ng taklng place within the chambar. Jn addition, our invention has the added advantage o being useable with a wide variety of instruments made of different materials (e.g. metals, polymers) at a very economical cost.
SUMMARY O~ T~E INVEttTION
An ap~aratus to sterilize objectg, 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 fluld communication wlth a vaccum forming source and subjected to a source of mlcrowaves. Normally the gas pres~ure in the chamber ~' '; '~

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is maintained between 1 to 1,000 micrometers ~g. A typical operating pressure would be about 100 micrometers Hg. Microwave radiation is supplied tn this chamber and its associated lnstrument by a RF generator for a predetermined short period o tlme. 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.
DESCRIPT~ON OF T~ DRAW~NGS
The figure illustrates the preferred embodiment of our invention wlth the door to the microwave oven partially open.
DESCRIPTION OF T~E PREFERRED EMBOD~MENT
As shown, the preferred embodiment of our invention has a transparent container 1 or chamber (t~pica 1~ 350 ml to 1500ml ln volume) with a removal top which acts to ~orm the desiccator and to hold the ob~ect to be sterilized. The top has a rim with a grease fitting and also has a connection to a;low gas to move to and ~rom 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 's formed at the wall of the oven where the condult 12 or tube extents therethrough by maXing the path optically blocked thrnugh offset battles which cause the conduit to ta~.e a meandering path.
One or more vacuum gauges ~4 and 5) may be used to ascertaln gas pressure readings. The gauge 5 essentially tells a user the pressure in or near the container 1 by an electr;cal slgnal supplied there~o by the thermocouple vacuum gauge 9. A second thermocouple vacuum gauge 11 provides a second electrical signal to 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.

_ 4 _ 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 ant placed ~n a houslng ad~acent to the microwave for compaction and appearance purpo~es. ',' :' The materlal used for container 1 is limited to materla;is that are microwavs transparent or translucent and have a very low absorption coefficient and are not hlghly microwave re~lective. Initial tests were conducted on nylon, polyethylene, glass ~Pyrext, polycarbonate and neoprene rubber and silicone rubber. Polyethylene and Pyrex showed that least amount of heatlng. The Pyrex glass and polycarbonate containers were transparent and as a result allowed the gas plasma to be clearly vi~ible. 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 plasm's pressure to be from about 1-1,000 micrometers Hg for the container. Depending on the object to be sterill~ed the microwave oven will be supplving radiation to the instrur,~ent 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. Tn 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 20~1461 in no sterilization of a spore qtrip whereaq full power at single time achieved sterilization).
Our invention is designed to destroy or deactivate microorganisms including fungi, algae, bacteria, viruses, and ~pores. We postulate that the heating effect is secondary ln thoir effectiveness and that the direct ionization in the plasma Ls what destroys the DNA molecules of the microorganism. 30th ambient air and oxygen in the chamber yield similar results in our tests. Visually the gas plasma, conslsting of fast moving electron~ and slower movlng lons, changes from an lnitial plnk glow to a white glow in about 10 seconds when the microwave was at ~ull power.
Sequentlally, 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 oparated to draw the gas from the containor 1. Valve 7 i~ at irJt closed and then opened a~ the pump draws the gas . :.
through lt to where it exists from the pump to the surrounding air remote from the oven. Nith the oven door closed the microwave is operated at ~ull power ~or a predetermined length of tLme prevlously determlned experimentally depending on the particular .... ..
instrument or ob~ect to be sterillzed.
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 aenture ~
acrylics). The mass of oux experiments were limited to the dental ~ ;
art but we by ~o means confined thereto. What we sought to do in these experiments were to lmprove the current high pressure steam ~autoclave) method and if autoclaving is not used, the alcohol or quarternary anonium compound disinfection proces~ co~monly use today. We believe our~invention is not only more ffec sterilizing unwanted microorganisms but is also provides a mor ; :
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convenient instrument as for ~torage, all at a modest cost without alterinq the material properties of the objects or instruments.
For example, with our invention cutting edges are not blunted or corro~ion of the instruments doe~ not occur as may happen in autoclaving.
None of the mentioned variation~ to the described preferred embodiment should be used to limit or change the scope and ~pirit of our invention which is limited only by the clalm~
that follow.

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Claims (7)

1. An apparatus for sterilizing objects comprising: a chamber into which the subject to be sterilized is placed;
a pump means in fluid communication with said chamber for removing gases therein and for maintaining a pressure in the chamber between 1 to 1,000 micrometers of Hg; and a controlled microwave source of radiation for radiating the object in the chamber for a predetermined period of time when the pressure is maintained in said range to thereby form a gas plasma and to sterilize the object.

2. The apparatus of claim 2 wherein:
The microwave source of radiation is a microwave oven with an inner doored cavity operating at about 2450 MHz with the chamber being located within said cavity;
said chamber and door being transparent to allow the visual observation of an object to be sterilized when placed therein.

3. The apparatus of claim 1 wherein: fluid communication between the chamber and pump means is maintained by a conduit assembly extending therebetween; said assembly comprising means sealed to the ambient air at its connection to the chamber and a valve for controlling the entrance of gas into the chamber.

4. The appartus of claim 3 comprising additionally: a pressure indicator connected to said conduit assembly to indicate the gas pressure in the chamber; and means for controlling the amount and duration of microwave radiation supplied from said source to the object.

5. The apparatus of claim 4 wherein: the microwave source of radiation is a doored microwave oven with power level and time controls; and said chamber is a separate container within the microwave oven connected to the pump means by a radiation scaled connection, said pump being located outside of the microwave oven.

6. The apparatus of claim 5 wherein the chamber has means for allowing the object to be placed therein and removed therefrom; and said pump means maintains the pressure at about 100 micrometer Hg within the chamber while the object is subjected to about 2450 MHz radiation.

7. The apparatus of claim 6 wherein the chamber and the door of the microwave oven are transparent to provide for the visual observation of the object and gas plasma formed in the chamber.