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BACKGROUND OF THE INVENTION
1. Field
This inventions relates to plasma sterilization, and provides a method for
exposing articles to be sterilized to substantially neutral species of a
plasma in a field free, glowless volume.
2. State of the Art
Modern medical and dental practice require the use of aseptic materials and
devices, many of them meant for repeat use. In order to achieve this
sterilization, processes are needed, at the manufacturer, and also at the
hospitals or dental offices for treatment of reusable materials and
devices.
Typical of materials which are reused in the hospital environment and
require repeated sterilization are major surgical instrument trays, minor
surgical kits, respiratory sets, fiber optics (endoscopes, proctoscopes,
angioscopes, bronchioscopes) and breast pumps. Typical instruments and
devices which are reused in a dental environment and require repeated
sterilization are hand-pieces, dental mirrors, plastic tips, model
impressions and fabrics.
There are a wide variety of medical devices and materials that are to be
supplied from the manufacturer already packaged and sterile. Many of these
devices and materials are disposable. Typical of this group are barrier
packs, head coverups and gowns, gloves, sutures, syringes and catheters.
One major sterilization process in present use is that which employs
ethylene oxide (EtO) gas in combination with Freon-12 (CCl.sub.2 F.sub.2)
at up to three atmospheres of pressure in a special shatter-proof
sterilization chamber. This process, in order to achieve effective asepsis
levels, requires exposure of the materials to the gas for at least one to
three hours followed by a minimum of twelve hours, or longer, aeration
period. The initial gas exposure time is relatively long because the
sterilization is effected by alkylation of amino groups in the
proteinaceous structure of any microorganism. EtO sterilization requires
the attachment of the entire EtO molecule, a polyatomic structure
containing seven atoms to the protein. This is accompanied by the
requirement of hydrogen atom rearrangement on the protein to enable the
attachment of EtO. Because of kinetic space-hindrance factors governing
the attachment of such a bulky molecule, the process needs to be carried
out at high pressure and be extended over a long period of time. It is,
therefore, deemed very inefficient by the industry at large.
Perhaps the chief drawback to this system, however, is its dangerous
toxicity. Ethylene-oxide (EtO) is a highly toxic material dangerous to
humans. It was recently declared a carcinogen as well as a mutagen. It
requires a very thorough aeration process following the exposure of the
medical materials to the gas in order to flush away toxic EtO residues and
other toxic liquid by-products like ethylene glycol and ethylene
chlorohydrin. Unfortunately, it is a characteristic of the gas and the
process that EtO and its toxic by-products tend to remain on the surface
of the materials being treated. Accordingly, longer and longer flush
(aeration) times are required in order to lower the levels of these
residues absorbed on the surface of the materials to a safe operational
value. A typical volume for each batch using this EtO process is 0.2 to 50
cu. ft. within the health and dental care environments.
A number of other approaches for performing sterilization have also been
employed. One such process is high pressure steam autoclaving. However,
this requires high temperature and is not suitable for materials which are
affected by either moisture or high temperature, e.g., corrodible and
sharp-edged metals, plastic-made devices, etc., employed by the hospital
and the dental communities.
Another approach utilizes either x-rays or radioactive sources. The x-ray
approach is difficult and expensive. The use of radioactive sources
requires expensive waste disposal procedures, as well as requiring
radiation safety precautions. The radiation approach also presents
problems because of radiation-induced molecular changes of some materials,
which, for example, may render flexible materials brittle, e.g.,
catheters.
It is therefore a primary object of the present invention to provide a
process and apparatus for dry sterilization of medical and dental devices
and materials, which can be operated efficiently, both with respect to
time and volume and which can be carried out below 70.degree. C.
It is another object of the present invention to provide a safe, nontoxic,
process for the sterilization and surface treatment of medical and dental
devices and materials, a process which does not employ toxic feed gases
and one which does not yield toxic absorbed surface residues and
by-products.
SUMMARY OF THE INVENTION
Broadly speaking in the present invention, sterilization or surface
treatment is achieved by exposing the medical or dental devices and
materials to a highly reducing gas plasma like that generated by gas
discharging molecular hydrogen, or to a highly oxidizing gas plasma, for
example, one containing oxygen. Depending on the specific sterilization
requirements, a mildly oxidizing environment, somewhere between the
environment offered by oxygen and that offered by hydrogen is presented by
gas discharging molecular nitrogen, either in pure state, or in
multicomponent mixtures with hydrogen or oxygen, supplemented by an inert
gas. In such a manner, plasma discharge chemical-physical parameters can
be adjusted to fit almost any practical application of sterilization and
surface treatment.
Such a plasma is generated by creating an electrical discharge in a gaseous
atmosphere maintained at sub-atmospheric or atmospheric pressure, within
which the materials to be sterilized are placed.
Generation of gas plasmas is a very well developed discipline, which has
been specifically employed in semiconductor processing. See, for example,
U.S. Pat. Nos. 3,951,709; 4,028,155; 4,353,777; 4,362,632; 4,505,782 and
RE 30,505 assigned to the present inventor.
In one instance the gas plasma sterilization process of this invention
involves evacuating a chamber to a relatively low pressure after the
devices or materials to be sterilized or treated have been placed within
it.
An oxidizing gaseous atmosphere, as an example, is then provided to the
chamber at a relatively low pressure, typically in the range 10 microns Hg
to 10 torr, corresponding to a continuous gaseous flow rate range of 20 to
3000 standard cc per minute. An electrical discharge is produced within
the chamber by conventional means, such as a microwave cavity or a radio
frequency (RF) excited electrode. Alternatively, RF power in the power
density range 0.0125-0.08 W/cm.sup.3 may be coupled into the gas via a
single electrode disposed within the chamber in a nonsymmetrical
electrical configuration, or via two electrodes contained within the
chamber in an electrically symmetrical configuration. In either case the
material to be sterilized is placed on one of the electrodes, while the
chamber's wall is commonly maintained at ground potential.
The nonsymmetrical arrangement provides the basis for a low plasma
potential mode of operation which is conducive to low sterilization
temperatures and the suppression of otherwise deleterious ion bombardment
and contamination of the devices and materials.
The resultant discharge produces a gas plasma including both excited
electrically charged gaseous species and excited electrically neutral
gaseous species. For example, free radicals of atomic oxygen as well as
excited molecular oxygen are formed in a discharge through molecular
oxygen. These oxygen-bearing active species interact chemically with the
proteinaceous components of the microorganisms residing on the surfaces of
medical or dental devices to be sterilized, thereby denaturing the
proteinaceous molecules and achieving kill rates of microorganisms
equivalent to a probability of microorganism survival of less than one in
a million.
The efficiency of this process is due, in part, to the fact that the
gaseous plasma entities are very reactive and atomically small (usually
monoatomic or diatomic) and therefore exhibit an enhanced ability to
chemically attach themselves to a proteinaceous structure and/or abstract
(remove) hydrogen atoms from it. It was also ascertained that the presence
of low levels of water vapor in the plasma feed gas enhances sterilization
efficiency dramatically. It is believed that accentuation of active
species concentration and/or favorable preconditioning of micro-organisms'
proteinaceous structure occurs in the presence of moisture during the
discharge process. These processes are responsible for the total kill of
the microorganisms. The kinetic space (or steric) restriction for this
type of interaction is at least one thousand times lower than that for EtO
alkylation.
Several specific types of interaction take place. One specific interaction
is hydrogen abstraction from amino groups. Another is rupturing ring
structures, particularly those including nitrogen, or carbon-carbon bond
cleavages. It is important to note that these processes produce only
gaseous effluents, such as water vapor and carbon dioxide, which would not
remain absorbed on the surface of medical devices, but would, instead, be
carried away from such devices with the main gas stream to the pump.
This sterilization process may be used with pre-packaged materials, such as
disposable or reusable devices contained within gas-permeable bags or
pouches. With sealed pouches (e.g., polyethylene/Tyvek packaging), the
barrier wall of the package is pervious to the relatively small active
species of the sterilizing plasma, but impervious to the larger
proteinaceous microorganisms. (Tyvek is a bonded polyolefin produced by
DuPont.)
After evacuation of the chamber, and introduction of the gas or gas
mixture, the gas(es) will permeate the package wall with a dynamic free
exchange of gas(es) from within and from outside the package.
Upon striking a microwave or an RF discharge to form the plasma, and,
depending upon electrical configuration and pressure, the plasma may
actually be created within and outside the package or, alternatively, the
package may be placed in a substantially electrically shielded
(field-free) glowless zone, so that it is subject to predominantly
electrically neutral, rather than electrically charged, active species
which pass through the packaging wall to interact with the surface of the
materials it contains.
In yet a different electrical configuration, the packages containing
devices to be sterilized can be placed on a conveyor belt and swept into
an atmospheric pressure corona discharge gap operated in ambient air. With
this configuration, the discharge electrodes are comprised of a grounded
metal-backed conveyor belt forming the bottom electrode, while the top
electrode is comprised of a metal block with multiple needle-like nozzles
for the dispersion of gas into the discharge gap.
Sterilization with this continuous, in-line, apparatus, is brought about by
either ozone formation, due to presence of discharged oxygen in air, or
due to any other oxidizing gas mixture that can be introduced into the
discharge gap via a plurality of nozzles, which are an integral part of
the top electrode.
This corona discharge will normally operate in the power density range 5-15
W/cm.sup.2 and in the frequency range 10-100 KHz and 13-27 MHz, associated
with gas flows in the range of several standard liters per second.
For example, in order to enable device sterilization by a strongly
oxidizing plasma when employing the process with a polyethylene-based
packaging, it is necessary to provide that oxygen-bearing active species
can permeate through the organic package barrier in the first place, and
that a sufficient number of these species traverse that barrier in order
to effectively kill all microorganisms on a medical or dental device
enclosed within the pouch.
Relevant strongly reducing, oxidizing, mildly oxidizing or mildly reducing
conditions can be obtained by plasma discharging diatomic gases like
hydrogen, oxygen, nitrogen, halogens, or binary mixtures of oxygen and
hydrogen, oxygen and nitrogen (e.g., air), oxygen and inert gases, or the
gaseous combination of oxygen, nitrogen and inert gases like helium or
argon, depending on the particular substances to be sterilized or treated.
The predominance of oxygen in the above mixtures is preferred but not
mandatory. A predominance of nitrogen, for example, will result in mildly
oxidizing conditions, but in somewhat higher process temperatures during
sterilization for a given reaction pressure and power density. The inert
gas fraction can be variable in the range 10 to 95%; the higher the
fraction, the lower the processing temperature for a given pressure and
power density. However, sterilization exposure time increases the higher
the inert gas fraction in the mix. Substitution of argon for helium, for
example, will result in higher sterilization temperatures for a given
pressure and power density. In this case, instability of the gas discharge
operation may set in, requiring a power density increase at a given
pressure, compared to that employed with helium, resulting in higher
process temperatures.
Effective sterilization can also be obtained with a pure reducing hydrogen
plasma or with a plasma discharge through pure inert gases like for
example, helium, argon, and their mixtures, due to their very strong
hydrogen atom abstraction (removal) capabilities from proteinaceous
structures of microorganisms. The addition of pure helium to an argon
sterilizing plasma will enhance the stability of the latter and reduce
overall sterilization temperatures. Hydrogen and its mixtures with either
nitrogen or oxygen, or with both, in the presence or absence of an inert
gas, will show effective sterilization capabilities over a wide range of
concentrations in these mixtures, thereby enhancing sterilization process
flexibility and versatility.
A first objective of facilitating the gaseous permeation through an organic
barrier (e.g., plastic or paper) is accomplished by evacuating the chamber
(containing the loaded pouches) to a base pressure of approximately 20
microns Hg. This rids the pouches of previously entrapped atmospheric air,
and equalizes the pressure inside the pouch to that inside the chamber
(across the organic barrier). The subsequent introduction into the chamber
of an oxygen-containing gas, in a typical situation, will establish an
instantaneous higher pressure inside the chamber (outside the pouch)
relative to that inside the pouch. This pressure gradient across the
pouches' barrier will serve as the initial driving force of gas into the
pouch. At an equilibrated state, an active and ongoing interchange of
molecules across the barrier will take place, attempting at all times to
maintain the same pressure on both sides of the organic barrier. Upon
striking a discharge through this gas, oxygen-bearing active species will
be generated. Typically, these active species will be deactivated in large
amounts by the organic barrier or due to interaction with neighboring
metallic surfaces. This will commonly substantially reduce the
availability of these active species to do the sterilizing job.
In order to accomplish the objective of generating a sufficient number of
reactive species traversing the organic barrier of a package to effect
efficient sterilization cycles, the plasma discharging of gaseous moisture
mixtures proved extremely beneficial. Plasma discharging of various
innocuous gases containing moisture levels in the range 100 to 10,000 ppm
of water vapor enabled the accentuation of active species concentration by
more than a factor of two, thereby substantially shortening sterilization
exposure times. Consequently, in a few system configurations which were
previously characterized by relatively high processing temperatures,
process temperatures were now kept sufficiently low due to the shortened
sterilization cycles. Effective binary moisture mixtures were those
comprised of oxygen, nitrogen, hydrogen and argon. Ternary moisture
mixtures of nitrogen-oxygen and argon--oxygen were somewhat more effective
at similar power densities than moisture mixtures of pure nitrogen or pure
argon. Moisture mixtures containing halogens although very effective, were
too corrosive and toxic. The most effective moisture mixture was that of
oxygen, reducing sterilization cycles by more than a factor of two.
In addition, it was found that the organic barrier of a packaging pouch
could be passivated in such a way as to substantially reduce its take-up
of oxygen-bearing active species needed as a sterilizing agent and one
which must render a final non-toxic medical device, without the formation
of any toxic by-products.
One such passivation method consists of simultaneously introducing into the
chamber a gaseous mixture, which in addition to oxygen-containing gas(es),
also contains selected other gases as set forth below:
1. Organohalogens, based on carbon and/or silicon, attached to any of the
known halogens. Particularly those organic compounds of carbon and/or
silicon that are saturated or unsaturated and contain in their molecular
structures one (1) or two (2) carbon or silicon atoms attached to: a
predominance of fluorine atoms; a predominance of chlorine atoms; a
predominance of bromine or iodine atoms; an equal number of fluorine and
chlorine atoms simultaneously; an equal number of chlorine and bromine
atoms simultaneously; an equal number of fluorine and bromine atoms
simultaneously; an equal number of fluorine and iodine atoms
simultaneously; an equal number of chlorine and iodine atoms
simultaneously. A predominance of fluorine in these compounds includes
structures where all other atoms attached to a carbon or a silicon atom
can be all the other halogens, or only one or two other halogens out of
the four halogens known, in conjunction with other atoms, as for example
hydrogen. The same comments apply to a predominance of chlorine, bromine
and iodine. For the latter, however, the simultaneous presence of bromine
is unlikely to be practical due to a low volatility of the structure, but
the simultaneous presence of fluorine or chlorine, or both, is practical.
It is worth noting that hydrogen-containing organohalogens will have a
tendency to polymerize under plasma conditions, and in some cases, be
flammable in as-received condition.
Most effective sterilizing mixtures of oxygen and an organohalogen are
those where the organohalogen is a mixture of organohalogens in itself,
either based on carbon and/or silicon, where the oxygen fraction is over
70% by volume; yet sterilization will be effected for lower oxygen content
at the expense of excessive halogenation of the surface of the material to
be sterilized, and at the expense of excessive loss of transparency of the
wrapping pouch.
2. Organohalogens in conjunction with either nitrogen or an inert gas like
helium or argon. In these cases, it is considered practical to keep the
fraction of the inert gas in predominance in order to keep the process
temperature as low as possible. Inert gas fractions up to 95% by volume
will be effective in killing microorganisms. The nitrogen fraction is
ideally kept below that of the oxygen fraction.
3. Inorganic halogens, defined as compounds not containing carbon or
silicon, but preferably containing as the central atom or atoms either
hydrogen, nitrogen, sulfur, boron, or phosphorus linked to any of the
known halogens in a similar manner as described for the organohalogens
under item 1 above, or defined as compounds that contain only halogens
without a different central atom, like for example molecular halogens
(e.g., F.sub.2, Cl.sub.2) and the interhalogens which contain two
dissimilar halogen atoms (e.g., Cl-F, I-F, Br-Cl based compounds, etc.).
Also in this case the inorganic halogen maybe, in itself, a mixture of
different inorganic halogens as defined above.
Most effective sterilizing mixtures of oxygen and an inorganic halogen are
those where the oxygen fraction is over 80% by volume; yet sterilization
will be effected for lower oxygen content at the expense of excessive
halogenation of the surface of the material to be sterilized, and at the
expense of excessive loss of transparency of the wrapping pouch.
4. Inorganic halogens in conjunction with either nitrogen or an inert gas
as described in item 2 above.
5. Inorganic oxyhalogenated compounds, not containing carbon or silicon,
but preferably contain either nitrogen, phosphorus, or sulfur, each of
which is simultaneously attached to oxygen and a halogen (e.g., NOCl,
SOCl.sub.2, POCl.sub.3, etc.). More specifically, the nitrogen-oxygen, or
the sulfur-oxygen, or the phosphorus-oxygen entities in the previous
examples are linked to any of the known halogens in a similar manner as
described for the organohalogens under item 1 above. The inorganic
oxyhalogenated fraction may be, in itself, a mixture of different
inorganic oxyhalogenated compounds as defined above.
Most effective sterilizing mixtures of oxygen and an inorganic
oxyhalogenated structure are those where the oxygen fraction is over 70%
by volume; yet effective sterilization will be obtained for lower oxygen
content at the expense of excessive halogenation of the surface to be
sterilized, and at the expense of excessive loss of transparency of the
wrapping pouch.
6. Inorganic oxyhalogenated compounds in conjunction with free nitrogen or
an inert gas as described in item 2 above.
7. Multicomponent mixtures comprised of members in each of the
aforementioned groups. The simultaneous presence of free nitrogen and an
inert gas like helium or argon in any of the above mentioned groups, or in
multicomponent mixtures comprised of members in each of the aforementioned
groups, will also be effective in killing microorganisms. The free
nitrogen fraction should be ideally below that of oxygen in order to
maintain a lower reaction temperature.
More specific and relatively simple multicomponent mixtures that are
effective sterilants as well as effective organic barrier passivation
agents are listed below:
______________________________________
Specific Multicomponent Mixtures Comprised of
Fractions A + B (percent of fraction is by volume)
Fraction A Fraction B
______________________________________
O.sub.2 (92-97%) CF.sub.4 (3-8%)
[O.sub.2 (40%) - He(60%)]
CF.sub.4 (0.25-3%)
[O.sub.2 (8%) - CF.sub.4 (92%)]
He(80%)
[O.sub.2 (17%) - CF.sub.4 (83%)]
He(80%)
[O.sub.2 (83%) - CF.sub.4 (17%)]
He(80%)
[O.sub.2 (92%) - CF.sub.4 (8%)]
He(80%)
______________________________________
Many of the aforementioned gas mixtures are, in themselves, novel chemical
compositions.
The plasma discharge through such a composite mixture will, for example,
create both oxygen-bearing and fluorine, or chlorine-bearing active
species simultaneously. The latter will predominantly be responsible for
passivating the organic barrier, since fluorination or chlorination,
rather than oxidation of the organic barrier is favored thermodynamically.
Therefore, the take-up of fluorine or chlorine-bearing active species by
the organic barrier of the pouch will be preferential. This will leave a
relatively larger fraction of oxygen-bearing active species available for
sterilization, since the latter cannot easily be taken up by a fluorinated
or chlorinated surface.
In addition, sterilization by oxygen-bearing active species may be aided,
for example, by simultaneously discharging an oxygen-containing and
fluorine or chlorine containing gas residing inside the enclosing pouch.
This gas had previously permeated through the organic barrier prior to the
commencement of the discharge. This will create active species that
contain both oxygen and fluorine or chlorine within the pouch directly. As
previously described, the competition for take-up by the organic barrier
(pouch) will be won by the fluorinating or chlorinating species, leaving a
larger net concentration of active species containing oxygen to do an
effective sterilizing job.
However, residual fluorine or chlorine-bearing active species within the
pouch and not taken-up by it will also perform effective surface
sterilization, since they are strongly chemically oxidizing agents. But,
the fraction of fluorine or chlorine-containing gas in the original
composite gaseous mixture, is substantially smaller than the
oxygen-containing component. Thus, a major portion of microorganisms kill
will be attributed to the oxygen-bearing species in the plasma. In either
case, however, the end result is a continuous attack on the proteinaceous
structure of the microorganism resulting in its degradation and
fragmentation into gaseous products. This chemical action by the reactive
plasma is to initially modify (denature) the proteinaceous network of the
microorganism, disrupting its metabolism at a minimum, but more commonly
impeding its reproduction.
DESCRIPTION OF THE DRAWINGS
In the drawing
FIG. 1 is a general diagrammatic illustration of an apparatus suitable for
use in the practice of this invention;
FIG. 2 is a cross sectional view of another apparatus suitable for use in
the practice of this invention;
FIG. 3 is a generally diagrammatic illustration of another apparatus
suitable for use in the practice of this invention;
FIG. 4 is a cross sectional view of another embodiment of a sterilization
chamber for use in the practice of the invention;
FIG. 5 is a side view of the apparatus of FIG. 4; and
FIGS. 6, 7, 8, 9, 10, 11, 12, 13 and 14 are cross sectional and side views
of alternative embodiments.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 is a general diagrammatic illustration of an RF excited discharge
chamber of the type used in the process of this invention. The cylindrical
chamber 11 is formed, in this instance, of glass or quartz and encloses
within it the material 14 to be treated. The chamber is commonly connected
to a mechanical vacuum pump (not shown) that establishes sub-atmospheric
pressure conditions within the chamber. An exciter coil 12 couples RF
energy from RF source 13 to the gas enclosed within the gas tight chamber
creating a plasma therein.
Alternatively, a microwave discharge cavity operating at 2450 MHz may
replace the RF exciter coil to couple power into the gas. With a suitable
selection of a reducing gas, like hydrogen, or an oxidizing gas, such as
oxygen, as a typical example, a discharge may be initiated and maintained
within the chamber. In the gas plasma formed by such a discharge a number
of excited species, both molecular and atomic, are formed. The interaction
of these species with a surface of the device or material to be sterilized
accomplishes the sterilization in the manner described above. The time
duration of the process needed to achieve satisfactory sterilization will
vary with other parameters of the discharge such as gas flow, pressure, RF
or microwave power density, and load size.
In the embodiment illustrated in FIG. 1 the apparatus includes an inner
perforated metallic cylinder 15 mounted generally concentric with the long
axis of the chamber 11, to form within the perforated cylinder a
substantially glowless, field-free zone. The perforated cylinder 15 is
electrically-floating and is cooled by recirculating a suitable coolant
(e.g., a 50--50 mixture of water and ethylene glycol) through cooling
coils 9 wrapped around the cylinder's length, to effect low sterilization
temperatures (<70.degree. C.). Still lower sterilization temperatures
could be effected with two concentric perforated metallic cylinders 15 and
15a, surrounded by cooling coils 9 and 8, respectively, and enclosed by
non-conducting chamber 11, as shown in FIG. 2. Energy coupling into this
chamber is accomplished in a similar manner as described in FIG. 1. In a
few cases, the configurations described in FIGS. 1 and 2 may not require
cooling coils 8 and 9 if the plasma feed gas contains low levels of water
vapor for the enhancement of sterilization efficiency and the reduction of
processing cycle time and temperature.
The resultant glowless and field-free zone within the confines of the
electrically-floating perforated cylinders could be ascribed to electrical
faraday-cage effects, coupled with catalytic deactivation of active
species, which are the precursors of visible emission, on the metallic
surface of the perforated cylinder.
When, as illustrated in FIG. 3, a microwave energy source 18 at for
example, 2540 MHz. is employed in lieu of the RF generator 13, the
perforated metallic cylinder cannot be mounted concentric about the long
axis of the chamber. Instead, the microwave cavity 16 is mounted at one
end of a metallic or non-metallic chamber 11, and a perforated metallic
shield 17 cooled by coolant-recirculating coils 20 may be placed just
beyond it toward the opposite end of the chamber, spanning the entire
diameter cross section of the chamber, thus creating a field-free and
glowless reactive zone immediately below it and away from the microwave
cavity. These arrangements permit material 14 placed within this zone to
be generally isolated from electrically charged species, while allowing
the electrically neutral reactive plasma species, such as, for example,
oxygen radicals, to interact with the surface of the material to be
sterilized. In this manner, sterilization is commonly effected at
substantially lower process temperatures.
Alternatively, the perforated metallic shield 17 may be removed, if
microwave cavity 16 is remotely located from material 14.
Microwave discharges lend themselves to this mode of operation, since the
effectiveness of neutral active species generated in such a discharge
survive substantial distances downstream, and away from, the microwave
cavity itself. This is a direct consequence of the higher population of
electrons in microwave plasmas, and consequently the higher degree of
ionization and dissociation in these discharges. Also, microwave plasma
electric probe measurements indicated plasma potentials nearly equal to
ground potential, thereby practically eliminating energic particle
bombardment during processing. This mode of operation is thus well suited
for low temperature exposure of heat-sensitive devices and material, even
for extended periods of sterilization time.
In the most preferred embodiments, the chamber is formed of a metallic
electrically grounded and water-cooled outer shell with either a single
internal perforated cylindrical shield, as shown in FIG. 1, or perhaps
with two such metallic shields, as shown in FIG. 2, which may be also
purposely cooled, the RF energy being coupled, in this latter
configuration, between the two conducting perforated cylinders. In either
case, conditions for low plasma potentials will prevail, with the
discharge glow being confined to the space between the inner wall of the
chamber and the surface(s) of the perforated cylinder(s), leaving the work
volume defined by the inner perforated cylinder substantially field-free,
void of the plasma glow, and at a relatively low operating temperature.
One such chamber configuration is illustrated in FIGS. 4 and 5. The
cylindrical outer wall 21, typically formed of aluminum or stainless
steel, is maintained at ground potential and serves as the chamber
enclosure. This enclosure may be water-cooled with the aid of cooling
coils 28 wrapped around it. Suitable dimensions for this chamber are a
diameter of 36" and a length of 48". A metallic perforated inner cylinder
23 cooled by cooling coils 19 is mounted on insulating spacers 29 within
the chamber so that it is positioned generally parallel with the long axis
of the outer wall 21 of the chamber and concentric with it. These spacers
may be formed of any suitable non-reactive and insulating type of material
such as ceramic. The cylinder perforations are typically 2.5-4 mm diameter
holes spaced in all directions from one another by approximately 0.5 cm in
a triangulated manner. Longitudinal support rails 27 are fastened to the
inner wall of the perforated cylinder 23 to support a wire basket 25 in
which the materials and devices to be sterilized are placed. A suitable RF
source 22 is coupled between the grounded outer chamber wall 21 and the
perforated inner cylinder 23. Usually this RF source should be capable of
producing an RF output in the range 0.01 to 0.1 W/cm.sup.3 at frequencies
in the 10-100 kilohertz or 13-27 megahertz range.
As illustrated in FIG. 5, an evacuation port 31 at the end of cylinder 21
is connected to a pump (not shown) and provides for suitable evacuation of
the chamber and for continuous gas flow during the sterilization process.
The gas supplied for the discharge is generally flowed through the chamber
by means of perforated diffusion tubes 35. Alternately, gas may be
introduced into the chamber via a gas dispersion device (not shown)
mounted behind chamber door 39 from the inside.
Material to be sterilized may be placed within wire basket 25 resting on
rail 27 through the entry port behind chamber door 39. Chamber door 39 may
be any suitable closure that can be conveniently opened and closed and
left in a sealed position during evacuation and the gas discharge
operation.
FIG. 6 illustrates a second preferred embodiment of the apparatus for
practicing the process of the invention. In this configuration, the outer
chamber wall 21 may be water-cooled by cooling coils 28, is again formed
of metal, such as electrically grounded aluminum or stainless steel, and
is of similar dimensions to that illustrated in FIG. 4. Mounted within the
chamber is an inner concentric cylinder 43 formed of a perforated metal
which may be purposely cooled by cooling coils 30, and is supported on
insulating support struts 46. The spacing between the inner wall of the
chamber and the perforated interior cylinder may range typically from 10
to 17 cm, where the chamber has an I.D. of 36". A second metallic
perforated cylinder 41 is concentrically mounted intermediate between the
inner perforated cylinder 43 and the inner wall of the chamber and may
also be cooled by cooling coils 19. This second perforated cylinder is
supported on insulating struts 47 and is spaced typically 4 to 7 cm away
from the inner perforated cylinder 43. The insulator struts may again be
formed of a ceramic material. Mounted on the interior of the inner
concentric cylinder 43 are support rails 27 for carrying a wire basket
which would contain the materials to be sterilized. Both the outer chamber
wall 21 and the inner perforated cylinder 43 are electrically connected to
point of potential reference (ground). Electrical connections would most
usually be made through ceramic seal feedthroughs 48 and 49. The
intermediate cylinder 41 is electrically connected to one side of the RF
power supply 22, the other side of which is connected to the point of
potential reference.
While a variety of conventional RF sources may be used, the most typical
value for the RF frequency is 13.56 MHz or, alternatively, 10-100 KHz. As
in the embodiment of FIG. 5 longitudinally extending gas diffusion tubes
35 may be employed to provide the gas to the interior of the chamber.
Typically each tube would have holes of diameter between 0.5 and 1.5 mm,
spaced approximately 1" apart along its length. The hole diameters closer
to the gas source would be of the smaller diameter. Alternatively, gas
inlets may be provided behind chamber door 39. As indicated in the
embodiments of FIGS. 4, 5 and 6 the perforated inner cylinders may be
open-ended at both ends or, may be closed with the same perforated stock
as is used to form the cylinder(s). The sterilization chambers shown in
FIGS. 4, 5 and 6 may be connected to a microwave discharge source,
typically operating at 2540 MHz, in lieu of an RF energy source. In this
case, the concentric perforated metallic cylinder(s) may be replaced by a
single perforated shield in accordance with the operational description
given for FIG. 3.
FIG. 7 illustrates a third preferred embodiment of the apparatus for
practicing the process of the invention. In this diagrammatic description
the outer chamber wall 21 is again formed of metal, such as aluminum or
stainless steel, and is of similar dimensions to that illustrated in FIG.
4. Mounted within the chamber are two planar, metallic, electrodes 50 and
51, preferably constructed of aluminum which may be coated with insulating
aluminum oxide. The gap 52 between electrodes 50 and 51, is adjustable by
virtue of the movable bottom electrode 50. Terminals A and B are connected
to the electrodes via an insulating feedthrough 48. The outer end of these
terminals may be connected to an RF source (not shown) in such a way that
when terminal B is connected to a ground potential, terminal A must be
connected to the RF source, or vice versa, providing for an electrical
symmetrical configuration. The work load to be sterilized is placed on
lower electrode 50.
It is important to maintain the distance between the electrodes always
smaller than the distance of the RF-powered electrode's edge to the
grounded chamber's wall. This enables a well defined and intense plasma
glow to be confined to space 52 between the electrodes and prevents
deleterious sparking. The electrode material may also be made of the
perforated stock previously mentioned. However, it is desirable to have
the RF-powered electrode made of solid stock to | | |
