U.S. patent number 3,753,651 [Application Number 05/067,337] was granted by the patent office on 1973-08-21 for method and apparatus for surface sterilization.
This patent grant is currently assigned to Wave Energy Systems Inc.. Invention is credited to Raymond M. G. Boucher.
United States Patent |
3,753,651 |
Boucher |
August 21, 1973 |
METHOD AND APPARATUS FOR SURFACE STERILIZATION
Abstract
A method and apparatus for the rapid sterilization of a
contaminated surface which involves the use of microwave energy
fields combined with a humid atmosphere having a relative humidity
of at least 50 percent. The material to be sterilized is placed
into a self-sealed container at least partially transparent to
microwaves. Said container is then introduced into an oven cavity.
The moist atmosphere is confined inside the container walls.
Through both thermal and non-thermal effects, surface
decontamination by electromagnetic radiation takes place in a
matter of minutes. The apparatus of the invention eliminates
potential oven walls contamination and is entirely safe from the
radiation view point. It can be operated by unskilled
personnel.
Inventors: |
Boucher; Raymond M. G. (New
York, NY) |
Assignee: |
Wave Energy Systems Inc. (New
York, NY)
|
Family
ID: |
22075334 |
Appl.
No.: |
05/067,337 |
Filed: |
August 27, 1970 |
Current U.S.
Class: |
422/21; 422/22;
219/756; 219/738 |
Current CPC
Class: |
A61L
2/12 (20130101) |
Current International
Class: |
A61L
2/08 (20060101); A61L 2/12 (20060101); A61l
001/00 (); A61l 003/00 () |
Field of
Search: |
;21/54,102,91,92,93,DIG.4 ;99/217,219,221,253 ;219/10.55 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Richman; Barry S.
Claims
We claim:
1. The method of destroying surface deposited micro-organisms
including bacteria pathogens, viruses and spores like organisms
which comprises:
providing a gas-tight container made of a material at least
partially transparent to microwave energy;
placing a contaminated material into said container and sealing
said container;
placing said container and the material confined therewithin into a
microwave oven having top, bottom and opposite side walls defining
a processing cavity therein;
creating a microwave energy field inside said cavity;
irradiating said container and contaminated material with said
microwave energy so that the contaminated material remains at a
temperature lower than saturated steam at atmospheric pressure;
and using a source of moisture separate from said contaminated
material to create at least 50 percent relative humidity inside
said container during irradiation, said microwave energy in
conjunction with said at least 50 percent relative humidity
destroying said microorganisms in a shorter time and at a lower
temperature than heretofore obtained and without contaminating said
oven walls.
2. The invention of claim 1, wherein the microwave energy field has
a frequency of from about 100 MHz to about 300,000 MHz, and an
average electromagnetic density greater than about .01
watt/cm.sup.3.
3. The invention of claim 1 wherein the processing time is greater
than 2 seconds.
4. The invention of claim 3 wherein the electromagnetic field is a
continuous wave emission.
5. The invention of claim 3 wherein the electromagnetic field is a
pulsed wave emission having a repetition rate of the order of
between one per nanosecond and one per minute.
6. The invention of claim 1, including the step of introducing a
chemical having bactericidal or sporicidal effects into said sealed
container.
7. The invention of claim 6, including the step of introducing a
gas sterilant into said sealed container.
8. The invention of claim 6, including the step of introducing a
sterilant vapor or aerosol into said sealed container.
9. The invention of claim 6, wherein said chemical is a liquid and
including the step of utilizing said microwave energy to evaporate
said cehmical and thus produce said at least 50% relative humidity
inside said sealed container.
10. The invention of claim 6, wherein the chemical sterilant is an
organic compound selected from the group consisting of ethylene
oxide, propylene oxide, methylbromide, chloropicrin,
epichlorohydrin, ethylene immine, glutaraldehyde, formaldehyde,
glycidaldehyde, and peracetic acid.
11. Apparatus for surface decontamination of contaminated articles
comprising:
a microwave oven having top, bottom and opposite side walls
defining a processing cavity therein;
means connected with said microwave oven for generating a microwave
electromagnetic energy field inside said processing cavity;
a reusable gas-tight container for said articles and made of a
material impermeable to moisture and at least partially transparent
to microwave energy, said container having a readily removable top
and a bottom and means for sealing said top to said bottom with
said articles sealed inside said container, a receptacle for said
separate source of moisture located within said container;
access means in said microwave oven for introducing into and
removing said container from said processing cavity, and means in
said container other than said articles for producing at least 50
percent relative humidity inside said container during irradiation
of said articles with said microwave energy;
said at least 50% relative humidity and said microwave energy
decontaminating said articles in a shorter time and at a lower
temperature than heretofore obtained and without contaminating said
oven walls.
12. The invention of claim 11, wherein the means for generating
microwave energy includes a source of microwave energy, a wave
guide connected between said source and said cavity, a means in
said cavity in the path of the microwave energy entering said
cavity from said wave guide to spread said microwave energy
throughout said cavity, said oven walls made of metallic material
to reflect microwave energy, and said access means comprising a
door in one oven wall constructed to seal the processing cavity and
keep radiation through said door to less than one milliwatt per
square centimeter at a distance of two inches from said door.
13. The invention of claim 12, wherein said generating means
comprises a source of continuous waves.
14. The invention of claim 12, wherein said generating means
comprises a source of pulse waves.
15. Apparatus for surface decontamination of instruments or other
materials comprising a housing, a cavity within said housing, a
container in said cavity for said articles or other materials and
made of a material at least partially transparent to microwave
energy, means for sealing said container with said articles or
other material therein, means connected with said cavity for
generating a microwave electromagnetic energy field inside said
cavity and said container, means in said container for producing at
least 50 percent relative humidity inside said container during
irradiation, said container having means associated therewith for
selectively establishing communication between the interior of the
container and the exterior of the container for introducing a
chemical having bactericidal, or sporicidal characteristics into
said container, means in said cavity for spreading the microwave
energy field in all directions throughout the entire cavity and
container, access door means in said housing for introducing and
removing said container into and from said cavity, means to
automatically expose the contaminated instruments or other
materials to microwave energy for a period at least longer than two
seconds and means to maintain the level of radiation through said
housing to outside thereof during the radiation of said articles
and other materials below 1 milliwatt per square centimeter at a
distance of 2 inches from said housing.
16. The invention of claim 15, wherein the chemical sterilant is an
organic compound selected from the group consisting of ethylene
oxide, propylene oxide, methylbromide, chloropicrin,
epichlorohydrin, ethylene immine, glutaraldehyde, formaldehyde,
glycidaldehyde, and peracetic acid.
Description
This invention relates to a method and apparatus for surface
sterilization of laboratory, medical and dental instruments in a
moist atmosphere at lower temperature than those presently used and
in a shorter period of time by means of electromagnetic radiation
in the microwaves range.
In the past, several attempts have been made to use microwaves for
the destruction of microorganisms. We shall recall for instance the
experiments conducted for pasteurization of raw milk (M.A.K. Hamid
and col., Journal of Microwave Power, 4-4, 1969), pasteurization of
baked goods (R.B. Decareau and col., Journal of Microwave Power,
3-3,1968), irradiation of potato waste water (M.A.K. Hamid and
col., Journal of Microwave Power, 5-1, 1970) and the destruction of
larvae and insects in grain (M.A.K. Hamid and col., Journal of
Microwave Power, 4-1, 1969). Unfortunately serious limitations
(short exposure time) were always placed on these methods which
dealt with the continuous processing of a dynamic stream of liquids
pr products. The efficiency of microwaves sterilization is
essentially a function of both the density of electromagnetic
energy and exposure time. It depends also on a large extent upon
the physical characteristics of the irradiated material (moisture
content, dielectric loss factor, etc.) Therefore, to successfully
develop an economical method for the destruction of microorganisms
one must be able to use a reasonable exposure time (at least
several minutes) which will in turn enable to operate with
relatively inexpensive power sources commercially available in the
kilowatt range.
A laboratory type sterilizer would fulfill such requirements since
it will be a safe and relatively uncomplicated instrument which
will easily compete with other systems such as dry heat or steam
sterilizers; which both require longer exposure time (at least an
hour) to destroy thermal resistant spores (B. Subtilis var. niger.,
B. Stearothermophilus, C. Botulinum, etc.) It is, however, to be
understood that the object of the present invention deals
exclusively with surface sterilization. In other words, we speak of
the destruction of bacteria, spores, viruses, etc. deposited at the
interface of solids. "In depth" sterilization of certain solids by
microwave (thermal effect) may also be successful in some
particular instances, but the present invention does not intend to
cover such special cases.
Although scattered information found in the scientific literature
mentioned examples of fast microwave sterilization of various
contaminated surfaces, no commercial system has yet been developed
or built for this purpose. During our experimentation, we
discovered that the main reason for this failure was due to the
fact that few scientists maintained the right humidity at the level
of the irradiated microorganisms. It is only through a combination
of proper humidifcation with the thermal and non-thermal effects of
microwaves irradiation that reproducible and satisfactory results
can be obtained with a wide variety of species including
thermoresistant spores.
It is, therefore, an object of this invention to provide a method
to surface-sterilize, in a matter of minutes, laboratory, medical,
dental tools, instruments and other goods.
It is also an object of the present invention to surface-sterilize
at a far lower temperature than those encountered today in dry heat
and steam sterilizers, thus allowing the decontamination of
numerous laboratory instruments or components made of plastic or
low melting point materials.
It is a further object of this invention to surface-sterilize heat
sensitive laboratory equipment, products and components which
otherwise would be damaged by dry heat or steam processing.
It is a further object of this invention to quickly
surface-sterilize all types of surfaces without any need to later
decontaminate part or totality of the sterilizer between runs.
It is a further object of this invention to provide an apparatus
which will be automatic, completely safe and would allow fast
surface-sterilization in batches by nonskilled personnel.
It is a further object of this invention to provide a microwave
apparatus for surface-sterilization which will eliminate any risk
of arcing or metal instruments piting during processing.
It is a further object of this invention to provide a microwave
apparatus with a cavity into which interchangeable loaded
containers, transparent to wave energy, will be inserted. Said
containers being filled with both the material to be sterilized and
the liquid to be evaporated to achieve a faster destruction of the
microbiological contaminants.
Other objects, advantages, features and uses will be apparent
during the course of the following discussion. To aid in the
understanding of the present invention, we shall briefly discuss
the mechanisms by which microwave energy destroys microorganisms at
gas/solid interfaces.
As is well known, microwave energy is coherent electromagnetic
energy. By this we mean that it is ordered. In other words, we can
readily identify its characteristics and can control it with
precision. Thermal energy, on the other hand, has random,
disordered, characteristics which are not so easily controlled.
Although the term microwave, in general, may cover a rather wide
range of frequencies (from 100 MHz up to several hundred thousands
MHz) the present invention mainly contemplates the use of
frequencies between 100 and 23,000 MHz.
Any biological material irradiated by microwaves is submitted to
two different effects: the first of a thermal and the second of a
non-thermal nature. Let us start with the purely thermal effect
which is in general more widely known (often referred to as
"microwave heating").
The mechanism through which microwave heating occurs at the
above-mentioned frequencies is based upon the dipole moment, or
"polarization" of the molecules of the irradiated substance. When
the polar molecules (absorbed water in cellular organisms for
instance) are subjected to a strong alternating field, their rapid
reorientations within the field create some kind of internal
friction resulting in heat. In a more precise sense, one could say
that heat is produced through the conversion of the potential
energy of polarization into random energy. It is important to note
that with microwave heating, no contact with the substance itself
is required. In other words, the transfer of energy takes place
directly without the necessity of an intermediate medium such as a
hot surface or a high temperature air stream. Energy transfer
occurs wherever the field penetrates. By a proper choice of the
materials used in the construction of the components of our
instruments-sterilizer it is then possible to produce heat
exclusively inside the irradiated microorganisms while keeping the
walls and all the other components and interfaces of the processing
chamber cool.
The advantages of this approach over conventional, dry heat
sterilizers based mainly on heat conduction and convection
phenomena are numerous. Microwave heating eliminates the inherent
inefficiency of transferring heat from an external source to the
processed load. Since microwave energy can be switched on to full
power levels and off again by simply flipping a switch, the time
lags associated with thermal processes are not present either. As
an example, we shall recall that an efficient dry heat sterilizer
(Dri-Clave Bulletin, Westbury, L. I. 1956) will require a warm up
time of 15 minutes to reach the operational temperature of
320.degree.F.
Also of great importance is the fact that all the risks associated
with the presence of high temperature radiating elements (dry heat
sterilizers) in contact with potentially explosive gas mixtures,
are eliminated. This is indeed a tremendous advantage in operating
rooms or clinical laboratories where various exotic gases
(anesthesic, oxygen, etc.) are often in use.
In first approximation, the amount of power that can be delivered
to a standard unit of volume of water containing microorganisms is
proportional to the product e tan Sf E.sup.2 where e is the
permittivity (the amount of electric field that is produced by the
molecules for a given applied field), tan S is the loss tangent of
the material (proportional to the conductivity), f is the frequency
and E is the electrical field strength. Since the product e tan S
will vary for each microorganisms species, different irradiation
times will be needed for sterilization through a purely thermal
effect when using a fixed output power at a fixed frequency.
Let us now review the present knowledge of the mechanisms of heat
sterilization since it may shed light on some peculiar aspects of
microwave sterilization phenomena.
It has long been known (Chick, 1908-1910) that thermal death of
bacterial cells and spores is logarithmic. There are, however,
numerous exceptions (Reynolds and LIchtenstein, 1952) which exhibit
sigmoidal curves. This led to the adoption of a theory (Charm,
1958) called "The Distribution of Resistance" which among other
things pointed out the existence of non-uniform heat resistant
spores.
When applying heat to a microorganisms population (spores for
instance) it seems to be well understood that deviation from the
linear logarithmic nature of the survival curve is generally due to
two basic factors: (1) The presence of a hump or "lag" in the
initial portion of the survival curve is due to heat "activation;"
(2) The presence of a tailing of the final portion is due to the
presence of thermoresistant variants in the population.
An energy of "activation" is generally necessary to initiate a
chemical or biological process, in the case of spores, it is the
energy necessary to release spores from their dormant state to
begin their germination process. There is also an activation energy
requirement to inactivate (lethal effect) microorganisms. Heat
activation and inactivation both obey first order kinetics (Busta
and Ordal, 1963), in combination and in that order. The moment a
spore becomes activated, it is subjected to the inactivation law. A
mathematical approach to this complex problem has been attempted
recently (Shull, Cargo and Ernst, 1963) and seems to have given
satisfactory results with Bacillus Stearothermophilus spores.
The effect of heat on microorganisms is in general regarded as the
result of enzyme inactivation, proteins denaturation, or both. This
is, in other words, the integer of several complex phenomena. Among
the various factors to consider during heat sterilization, one must
mention the widely different thermal resistance of spores. A
species of bacterial spore highly resistant to moist heat is not
necessarily highly resistant to dry heat and vice versa. For
instance, to inactivate 100,000 spores of B. Stearothermophilus in
saturated steam at 121.degree.C requires 12 minutes but 1,000,000
spores of B. Subtilis var. niger are inactivated in less than one
minute at the same temperature. However, B. Subtilis var. niger is
much more resistant to dry heat than B. Stearothermophilus under
identical experimental conditions.
The type of heat flux (dry or moist), the manner into which the
heat is generated or penetrates through the microorganisms
(convection, conduction, radiation, etc.) are therefore extremely
important. This could explain the fast killing rates observed with
microwave energy which acts at once at molecular level and creates
a coherent state of molecular turbulence throughout the entire
irradiated mass.
In short, for an equivalent amount of thermal energy released
through the same spore species various death rates will be observed
according to the nature of the heating process. It is also true
that the same heating process applied to different species may give
different survival curves.
Another reason which could explain the efficiency of microwaves
heating in spores destruction is the influence of molecular
controlled agitation on ions. In 1957, Amahada and Ordal indicated
that differences in degree and rate of thermal destruction might be
associated with the loss of cations which enhance the thermal
resistance of spores. DPA (pyridine-2, 6 dicarboxylic acid) which
has been found in all bacterial endospores (Wooley and Collier,
1965) has been recognized with calcium as one of the main agents in
thermoresistance. Both calcium and DPA may respond more effectively
to friction heat caused by microwave energy and thus explain the
faster killing rate observed with this kind of heat source.
Hereabove we have stressed the modifications produced in
microorganisms by heat and the special role played by microwave
induced heat. It is also important to consider the non-thermal
effects of microwaves during sterilization.
Although very difficult to study separately, the non-thermal
effects of microwaves (and electro-magnetic radiation in general)
have been established beyond doubt and thoroughly investigated by
several authors (See C.M. Olsen, Journal Micro. Power, 1-2 , 45-56,
1966). For instance, microspores of Fusarium solani f. phaseoli in
water suspension (Baker and Fuller, 1965) were irradiated at 2450
MHz and compared with a treatment in water at a slightly higher
temperature. Spores germination data showed that thermal treatment
curves were guite conventional in shape but microwave treatment
spores germinated on an "all or nothing" basis. These data
suggested that microwaves may affect a metabolic system distinct
from that of thermal energy.
In another experiment C. M. Olsen irradiated at 2450 MHz three
bread mold fungi during 2 minutes with a maximum end temperature of
65.degree.C. When Penicillium sp. spores on the microwave treated
bread were recovered and plated, counts showed there were only 0.1
colonies per plate while samples of unirradiated bread (same final
temperature achieved by thermal means) yielded 1486 colonies per
plate.
During studies in soil sterilization at 915,2450 and 5800 MHz
several fungi were irradiated and the maximum sterilization
temperature was compared to the thermal death point. The latter, by
definition, is the lowest temperature at which a suspension of
bacteria is killed in 10 minutes. It was found, for instance, that
a relatively large soil fungus, Rhizoctonia solani, was killed at a
temperature about 10.degree.C below that of its normal thermal
death point. Verticillium albo-atrum, another soil fungus with
extremely small spores, was killed at about 3.degree.C below its
lethal temperature. Non-spore forming bacteria, were in general
killed by microwave energy at points as much as 10.degree.C below
thermal death point.
The non-thermal effects of microwaves on micro-organisms can be due
to several overlapping phenomena such as: high speed molecular
oscillations which produce chemical bonds breaking, accelerated
diffusion of ions through membranes, electrical charges
modification at interfaces or P.sub.h modification. Regarding
chemical bond cleavage (C.M. Olsen, 1965) it has been shown that a
few seconds irradiation at 2450 MHz of 0.1 N solutions of NaOH will
produce hydrogen peroxide. The first 5 second exposure yielded
about 0.01% H.sub.2 O.sub.2 and each subsequent exposure yielded an
additional amount. The temperature at the end of three 10 second
(i.e. 30 seconds) exposures was about 100.degree.C. When similar
samples wree treated in a water bath to the same temperature, no
hydrogen peroxide was detected with the UV absorption
technique.
This, among other things, shows that the result of checmical bonds
breakage can also be the production of new chameicals with
sporicidal or bactericidal characteristics.
All the above described phenomena (dehydration and non-thermal
effects) play a role in microwave surface sterilization and
according to the type of irradiated microorganism and substrate,
their contribution varies in importance. We, however, discovered
that in all cases the sterilization rate was greatly improved when
the original water content of the micrroorganisms was increased.
This had already been noticed by other for the industrial
processing of fungal spores. C.M. Olsen, for instance, mentioned
that when Penicillium sp. spores were exposed to 40 percent
relative humidity for 15 minutes prior to a 30 second microwave
treatment (1350 watt output cavity), a 90 percent greater kill was
recorded over that of microwave treated dry spores. This indeed
makes sense from the theoretical view point (Alderton and Snell,
1963) since as previously stated ion enchange phenomena in liquid
phase are one of the keys to thermoresistance.
It is also important to recall that microwave sterilization is a
function of both the electrical energy density of the field and the
exposure time. Most of the attempts made in the past to assess
microwave sterilization economics were geared to the use of
continuous irradiation processes (irradiated tunnel with conveyor
belt system) at industrial scale. This indeed limited rather
sharply the practical exposure time.
The method object of our invention does not have such limitations
since it will compete with other established techniques (dry heat
and steam sterilizers) which necessitate at least thirty minutes of
residence time. In FIG. 1 we show the trend of three typical spores
kill curves as a function of time. The curve No. 1 (dotted line)
represents the microwave kill curve without prehumidifying of the
microorganisms. Curve No. 2 shows under the same experimental
condition the percent of kills with humidifying. Sigmoidal curves
No. 3 and No. 4 correspond to dry heat and steam sterilization.
The method of surface sterilization, object of the present
invention, consists of a combination of microwaves irradiation in
an oven type cavity within an atmosphere partially saturated (at
least 50 percent R.H.) or super saturated with water or a saline
solution. The water or saline solution can be present as a vapor or
as an aerosol dispersion (i.e., liquid droplets smaller than 10
microns). Slightly saline solutions (0.1 to 0.5 molal NaCl
solution) are particularly suited since their high dielectric loss
factor (between 200,000 and 400,000.10.sup.4) enables a quick
evaporation in the microwave field. The water or saline solution to
be evaporated is placed inside a plastic or paper container
transparent to microwave energy. The instruments, tools or material
to be sterilized are also placed inside the container and are
therefore in direct contact with the moist atmosphere. The
container is self-sealing with a lid and retains the vapor or
aerosol phase during the sterilization. It therefore avoids any
contact between moist air and cavity oven walls. This new method
provides a solution to the problem of the contamination of the
microwaves cavity walls. The use of an open container with water,
or any other suitable liquid, placed directly with the instruments
inside the microwave oven cavity would obviously create a severe
contamination problem since some of the liquid in contact with
living microorganisms during the irradiation could be reentrained
or projected against the cool cavity walls where they would survive
due to an insufficient dose of radiation. This would necessitate a
tedious and lengthy decontamination of the cavity after each
sterilizing run.
To the contrary the method object of the present invention does not
necessitate such decontamination of the cavity walls since the
container is introduced inside the microwave oven as a sealed
gas-tight unit. The container can be made of disposable hard paper
or plastic transparent to microwave energy. The shape of the
container is such that it provides room for both the material to be
processed and the humidifying solution. The volume of the
humidifying solution is calculated to correspond after evaporation
to partial (at least 50 percent RH) or complete saturation of the
container volume without its load. When the microwaves cavity is
turned "on" the electro-magnetic energy penetrates through the
container walls, it then evaporates the water which "conditions"
the microorganisms. At the same time, electro-magnetic waves
proceed with their thermal and non thermal effects. The whole
operation results in a considerable gain of time (often a matter of
minutes) when compared to the dry heat or steam sterilizing
methods. If using a molded container made of reusable plastic, the
container can indeed be sterilized separately at the end of each
operation.
Another advantage of the method hereabove described lies in the
fact that intense microwave fields in the presence of metal objects
(instruments, etc.) can produce localized arcing which will pit the
metal surface. In the presence of a moist atmosphere this important
drawback is practically eliminated. The invention, contemplates the
use of either a continuous wave emission or a pulsed wave emission
in an oven type cavity. In the latter case, the average power
requirements will be decreased while the lethal effects on
microorganisms will be enhanced due to sharp variations in the
electrical field gradient (A. P. Wehner, Int. Journ. Biometer, Vol.
7 No. 3, 277-82, 1964).
Having described our microwave sterilization technique in a moist
atmosphere, we shall now describe, by way of a non-limiting
example, one embodiment of the apparatus of the present invention,
as shown in the accompanying drawings.
FIG. 2 is a simplified cross-sectional front view of a preferred
form of apparatus of the invention taken along line b--b' as seen
in FIG. 3.
FIG. 3 is a simplified cross-sectional view of the apparatus of
FIG. 2 taken along the line a--a' in FIG. 2 with the front door in
an open position. (Position used to insert or remove the container
before and after sterilization).
As can be seen from FIG. 2, the apparatus consists of a metal
housing 1 quite similar to those used today in microwave ovens.
Located within the housing are the main components of the microwave
system. They comprise the magnetron 2 which, with the help of the
transformer, rectifier and magnetic field circuit (all contained in
the power pack 3), converts the 60 cycle AC current from the line 4
into microwave energy. The high power beam of microwave energy is
contained in a wave guide 5 and directed against the blades 6 of a
fan 7 which rotates at a slow RPM. The fan, often called stirrer,
reflects the power beam bouncing it off the walls, ceiling, back
and bottom of the oven cavity 8. At the bottom of the oven cavity 8
one can see a Pyrex glass plate 9 transparent to microwaves, which
is suspended approximately one inch above the metal bottom of the
processing cavity. The instruments or materials to be
surface-sterilized 10 are placed inside a gas-tight sealed
container 11 which is positioned in the oven cavity 8 and rests
upon the glass plate 9. The container 11 can be made of any
material transparent to microwave energy: plastic polypropylene,
polyethylene, polystyrene, teflon, etc.), paper or special glass
composition. The container 11 shown in FIG. 2 is of
parallelepipedic shape with an upper lid 12 also made of microwave
transparent material. The container can be designed in any form and
shape as long as the microwave energy will penetrate through it
from all directions when placed inside the oven cavity 8. As can be
seen, the container 11 of FIG. 2 contains three trays 13 which
support the workload (instruments placed in a perforated basket for
instance). The trays are perforated to let the moisture penetrate
throughout the entire processing area. The water or solution to be
evaporated or aerosolized is placed in a plastic or disposable
paper cup 14 inside the container.
FIG. 3 is a cross-sectional side view of the apparatus at the plane
of the stirrer. We show in FIG. 3 the drop-down door 15 in an open
position. To decrease radiation leaks the door is equipped with a
quarter wave choke seal 16 in addition to an absorber seal 17. The
choke cavity is filled with polypropylene for instance while the
secondary seal is made of vinyl loaded with carbon black. The door
is equipped with a viewing screen 18 made of perforated stainless
steel (diameter holes 0.0265 inch). Transmission through this
viewing screen is approximately -70 db. In some cases added
protection is given to the door by the addition of a glass or
optically transparent plastic plate 19. This means that when the
door is closed the maximum of radiation leakage in the immediate
vicinity of the door is well below the 1 mW/cm.sup.2 specified by
the US Public Law, 90-602. Of course, a unit as shown in FIG. 2 and
3 has other standard safety features such as door interlock
switches which are not shown for the sake of clarity since they are
the kind of standard feature well known to the man of the art. Also
not shown for the same reason are other common features such as
timer, oven and dial lights, start switches, stirrer shield and
blower used to cool the magnetron tube.
It is obvious that within the scope of our invention any gas can be
used to fill the gas-tight container. This means that according to
the type of microorganisms one can also take advantage of the
chemical effects of gas sterilants, vapors or aerosols, such as
ethylene oxide, propylene oxide, formaldehyde, methylbromide,
chloropicrin, epichlorohydrin, ethyleneimine, glycidaldehyde,
peracetic acid, glutaraldehyde solution or any other chemical with
known bactericidal or sporicidal characteristics.
In the case of using such chemicals, precautions must be taken to
operate during a period of time short enough to avoid any
irradiated part to reach a temperature close to the ignition or
explosive point. Mixtures of gases which decrease the flammability
or explosive point are recommended. As an example, we can refer to
the use of carbon dioxide or fluorinated hydrocarbons mixed with
ethylene oxide.
There are indeed several ways to introduce sterilant gases into a
container such as represented in FIG. 2. As an example, we show in
FIG. 2 two stopcocks or valves 20 which when open would enable
introducing the sterilant gas with or without the help of
vacuum.
Without departing from the framework of the present invention it
must be well understood that, according to the results derived, the
present invention can be applied to variable volumes of different
gases at different temperatures or at multiple pressures, and that,
still without departing from the scope of the invention, the
structural details of the described apparatus, the dimensions and
the shapes of their members (such as the shape of the container)
and their arrangement (position of trays inside the container) may
be modified, and that certain members may be replaced by other
equivalent means (Magnetron replaced by klystron or amplitron
tubes.)
In order to illustrate the possibilities of the invention more
concretely by a precise example, but without limiting the scope of
the invention, following is an example of a sterilizing
operation.
The tests were conducted with a Magnetron type apparatus whose
cavity had the following dimensions: height 8 1/2 inch, width 15
1/2 inch, depth 13 inch. The Magnetron emitted at a nominal
frequency of 2540 MHz (.+-.25 MHz). The electrical energy input to
the
TABLE 1
50 Spordex strips (B. Subtilis) and 25 control strips are used in
each experiment. Microwave Frequency: 2540MHz; Energy Density:
0.023 watt/cm.sup.3
Incu- Number of Irra- Number of Con- Experimental bation diated
Test Tubes trol Test Tubes Conditions time in With Growth with
Growth hours Microwave with 24 0 25 moist air 10 minutes
irradiation 48 0 25 72 0 25 168 0 25 Microwaves without 24 4 25
moist air 10 minutes irradiation 168 5 25 Dry-heat at 160.degree.C
24 9 25 15 minutes 168 10 25 Dry-heat at 160.degree.C 24 0 25 30
minutes 168 0 25
Magnetron tube was approximately 1200 watts. Under loading
conditions the average amount of microwave energy radiated in the
cavity was 650 watts. (Minumum 605 watts -- Maximum 675 watts)
Inside the cavity we placed a teflon container whose volume was
approximately half a cubic foot. In the container was also placed a
cup filled with 25 cm.sup.3 of water. The container had two
perforated trays on which the samples were placed among a load of
instruments and gauze. The samples consisted of fifty SPORDEX
Bacterial spore strips from the American Sterilizer Co. The
Bacillus Subtilis population of each strip was said to average
100,000. Since each package of SPORDEX contained three strips, two
were used for our test and the third one was always kept as a
reference.
The microwave surface-sterilizer was turned on and kept running for
ten minutes. The spore strips were then removed and under sterile
conditions were individually placed in labelled test tubes, each
containing 25 cm.sup.3 of sterile fluid thioglycollate medium. The
control strips left unsterilized, were also placed into test tubes
containing the same medium.
Table 1 gives the results of our tests for various incubation
times. A second series of tests was then conducted under identical
conditions without any humidification (no water in the cup inside
the teflon container). Results of such tests are also given in
Table 1. A third series of tests was also conducted, for comparison
purposes, with the same load and number of spores strips inside a
DRI-CLAVE (model 75) dry heat sterilizer. The dry heat sterilizer
was run at 320.degree.F (160.degree.C) for 15 and 30 minutes.
Data from Table 1 clearly show the advantage of the object of our
invention. They also show the tremendous reduction in processing
time when comparing our method with dry heat sterilization.
The teachings of the invention may be practiced within the
following parameters:
a. Microwave Energy --
Nominal frequency: 100 MHz to 300,000 MHz
Average radiated energy level inside the cavity (i.e. total power
output divided by cavity volume): Higher than 0.01
watt/cm.sup.3.
Type of wave: Continuous pr pulsed emission at a repetition rate
between one per nanosecond and one per minute.
b. Humidity in processing chamber: Higher than 50 percent relative
humidity at the gas temperature during processing.
c. Any gas in the container at a temperature safely below its
ignition or explosive point.
While the invention has been described by means of specific
examples and in a sepcific embodiment, it is not limited thereto,
for obvious modifications will occur to those skilled in the art
without departing from the spirit or scope of the invention.
* * * * *