Note: Descriptions are shown in the official language in which they were submitted.
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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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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.
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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
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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
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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
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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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