U.S. patent application number 10/200564 was filed with the patent office on 2003-01-23 for sterilization system employing low frequency plasma.
Invention is credited to Agamohamadi, Mitch, Platt, Robert C. JR..
Application Number | 20030015415 10/200564 |
Document ID | / |
Family ID | 24716561 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015415 |
Kind Code |
A1 |
Platt, Robert C. JR. ; et
al. |
January 23, 2003 |
Sterilization system employing low frequency plasma
Abstract
A method and system for sterilizing an article is provided that
includes use of a low frequency (LF) gas discharge plasma. The
method includes placing the article in a vacuum chamber and
evacuating the vacuum chamber to a predetermined pressure. Gas or
vapor species are introduced into the vacuum chamber, and a low
frequency plasma is generated within the vacuum chamber, the low
frequency plasma having a frequency of from 0 to approximately 200
kHz. The low frequency plasma is maintained for a time period
sufficient to substantially remove gas or vapor species from the
article. The sterilization system includes a vacuum chamber coupled
to a vacuum pump and a vent, a first electrode, and a second
electrode. The sterilization system further includes a second
region within the vacuum chamber, the second region including a
region between the first and second electrodes, and a first region
within the vacuum chamber, the first region being in fluid
communication with the second region. The sterilization system
further includes a source of reactive agent species coupled to the
vacuum chamber, a process control monitor, and a low frequency
power module including components adapted to apply a low frequency
voltage between the first electrode and second electrode to
generate a low frequency plasma in the vacuum chamber, the low
frequency voltage having a frequency of from 0 to approximately 200
kHz.
Inventors: |
Platt, Robert C. JR.;
(Laguna Niguel, CA) ; Agamohamadi, Mitch; (Orange,
CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
24716561 |
Appl. No.: |
10/200564 |
Filed: |
July 19, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10200564 |
Jul 19, 2002 |
|
|
|
09676919 |
Oct 2, 2000 |
|
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6458321 |
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Current U.S.
Class: |
204/164 ; 422/23;
422/29 |
Current CPC
Class: |
A61L 2/14 20130101; A61L
2202/122 20130101; A61L 2/208 20130101 |
Class at
Publication: |
204/164 ; 422/29;
422/23 |
International
Class: |
A61L 002/14; A61L
002/00; A61L 009/00 |
Claims
What is claimed is:
1. A method of sterilization of an article, the method comprising:
placing the article in a first region; introducing gas or vapor
species into a second region; generating a plasma by applying an
applied electric field in the second region, the second region in
fluid communication with the first region, the first region having
an electric field weaker than the applied electric field in the
second region, the applied electric field having a frequency of
less than 10 kHz; and maintaining the plasma for a time period
sufficient to substantially remove gas or vapor species from the
article.
2. The method as defined in claim 1, wherein the gas or vapor
species comprises hydrogen peroxide.
3. The method as defined in claim 2, wherein an amount of hydrogen
peroxide remaining on the article after maintaining the plasma is
less than approximately 8000 ppm.
4. The method as defined in claim 1, wherein the applied electric
field is applied in the second region by applying a voltage between
two electrodes.
5. The method as defined in claim 1, wherein the voltage has a
frequency from 0 to approximately 1 kHz.
6. The method as defined in claim 1, wherein the voltage has a
frequency from 0 to approximately 400 Hz.
7. The method as defined in claim 1, wherein the plasma has a
plasma decay time and the applied electric field has a half-period
greater than said plasma decay time.
8. A system for sterilizing an article, the system comprising: a
first electrode; a second electrode; a first region comprising a
region between the first and second electrodes; a second region in
fluid communication with the first region, the second region
adapted to receive the article; a source of reactive agent species,
the source in fluid communication with the first region; a process
control module; and a power module comprising components adapted to
apply a voltage between the first electrode and second electrode so
as to generate a plasma in the first region, the voltage having a
frequency of less than 10 kHz.
9. The system as described in claim 8, wherein the reactive agent
species comprises hydrogen peroxide.
10. The system as described in claim 8, wherein the power module
comprises a power controller, a flyback current shunt element, a
current monitor, a voltage monitor, an inductor, a capacitor, and a
power control module.
11. The system as described in claim 10, wherein the power control
module is coupled to the process control module and the power
controller, and the power controller is adapted to adjust the duty
cycle of the voltage applied between the first and second
electrodes in response to the power control module.
12. The system as described in claim 10, wherein the power control
module is coupled to the process control module and the power
controller, and the power controller is adapted to adjust the
amplitude of the voltage applied between the first and second
electrodes in response to the power control module.
13. The system as described in claim 10, wherein the inductor and
capacitor comprise an LC circuit connected in series with the power
controller.
14. The system as described in claim 13, wherein an inductance and
a capacitance of the LC circuit are chosen to match a resonant
frequency of the LC circuit to the frequency of the voltage applied
between the first and second electrodes.
15. The system as described in claim 8, wherein the voltage is
above a threshold voltage required to ignite a plasma.
16. The system as described in claim 8, wherein the voltage has a
frequency from 0 to approximately 1 kHz.
17. The system as described in claim 8, wherein the voltage has a
frequency from 0 to approximately 400 Hz.
18. The system as defined in claim 8, wherein the plasma has a
plasma decay time and the voltage has a half-period greater than
said plasma decay time.
19. The system as described in claim 8, wherein the power module
further comprises a switching module that in response to an input
voltage provides an output voltage with a frequency different from
the frequency of the input voltage.
Description
CLAIM OF PRIORITY
[0001] This application is a continuation of U.S. Utility patent
application Ser. No. 09/676,919, filed Oct. 2, 2000, the disclosure
of which is hereby incorporated in its entirety by reference
herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to systems and methods for sterilizing
articles that include the use of a gas discharge plasma.
[0004] 2. Description of the Related Art
[0005] Plasmas produced using radio frequency (RF) generators in
particular have proven to be valuable tools in processes for the
sterilization of medical devices. For example, in U.S. Pat. Nos.
4,643,876 and 4,756,882, which are incorporated by reference
herein, Jacobs, et al. disclose using hydrogen peroxide as a
precursor in a low temperature sterilization system that employs RF
plasma. The combination of hydrogen peroxide vapor and a RF plasma
provides an efficient method of sterilizing medical devices without
using or leaving highly toxic materials or forming toxic
by-products. Similarly, Jacob, U.S. Pat. No. 5,302,343, and
Griffiths, et al., U.S. Pat. No. 5,512,244, teach the use of RF
plasmas in a sterilization process.
[0006] However, there are problems associated with the use of an RF
plasma in a sterilization process. The RF plasma may leave residual
hydrogen peroxide on the sterilized article. The residual amount of
hydrogen peroxide remaining on the sterilized article depends upon
the RF power applied to the article, the amount of time exposed to
the RF plasma, and the material of the article. For example, while
some plastics (e.g., polyurethane) absorb hydrogen peroxide, other
materials (e.g., Teflon) absorb relatively little, thereby yielding
less residual hydrogen peroxide after sterilization.
[0007] In addition, inherent inefficiencies in the energy
conversion from the low frequency (e.g., 60 Hz) line voltage to the
RF (e.g., approximately 1 MHz-1 GHz) voltage used to generate the
RF plasma limit the power efficiency of such systems to typically
less than 50%. Energy efficiency is further reduced by typically
5-20% by virtue of the losses from the required impedance matching
network between the RF generator and the load. Such low energy
efficiency significantly increases the cost per watt applied to the
sterilized articles. The required instrumentation for using RF
electrical energy (e.g., RF generator, impedance matching network,
monitoring circuitry) is expensive, which also increases the cost
per watt applied to the sterilized articles.
SUMMARY OF THE INVENTION
[0008] One aspect of the present invention is a method of
sterilization of an article. The method comprises placing the
article in a vacuum chamber and evacuating the vacuum chamber to a
predetermined pressure. Gas or vapor species are introduced into
the, vacuum chamber, and a low frequency plasma is generated within
the vacuum chamber, the low frequency plasma having a frequency of
from 0 to approximately 200 kHz. The low frequency plasma is
maintained for a time period sufficient to substantially remove gas
or vapor species from the article.
[0009] Another aspect of the present invention is a method of
sterilization of an article. The method comprises placing the
article in a vacuum chamber and evacuating the vacuum chamber to a
predetermined pressure. A low frequency plasma is generated within
the vacuum chamber, the low frequency plasma having a frequency of
from 0 to approximately 200 kHz. The low frequency plasma is
maintained for a time period sufficient to heat the article to aid
the evaporation and removal of water and other absorbed gases from
the vacuum chamber and the article.
[0010] Another aspect of the present invention is a system for
sterilizing an article. This system comprises a vacuum chamber
coupled to a vacuum pump and a vent, a first electrode, and a
second electrode. The system further comprises a second region
within the vacuum chamber, the second region comprising a region
between the first and second electrodes. The system further
comprises a first region within the vacuum chamber, the first
region being in fluid communication with the second region. The
system further comprises a source of fluid coupled to the vacuum
chamber, a process control monitor, and a low frequency power
module comprising components adapted to apply a low frequency
voltage between the first electrode and second electrode to
generate a low frequency plasma in the vacuum chamber, the low
frequency voltage having a frequency of from 0 to approximately 200
kHz.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 schematically illustrates a preferred embodiment of a
sterilization system compatible with the present invention.
[0012] FIG. 2A schematically illustrates a preferred embodiment of
a cylindrically-shaped electrode with open ends and perforated
sides.
[0013] FIG. 2B schematically illustrates an alternative embodiment
of a cylindrically-shaped electrode with open ends and louvered
sides.
[0014] FIG. 2C schematically illustrates an alternative embodiment
of a cylindrically-shaped electrode with open ends and solid
sides.
[0015] FIG. 2D schematically illustrates an alternative embodiment
of an electrode comprising one or more colinear
cylindrically-shaped segments with open ends and solid sides.
[0016] FIG. 2E schematically illustrates an alternative embodiment
of an electrode with a partial cylindrical shape, open ends, and
solid sides.
[0017] FIG. 2F schematically illustrates an alternative embodiment
of a cylindrically-symmetric and longitudinally-asymmetric
electrode with open ends and solid sides.
[0018] FIG. 2G schematically illustrates an alternative embodiment
of one or more asymmetric electrodes with open ends and solid
sides.
[0019] FIG. 2H schematically illustrates an alternative embodiment
of an electrode system with a first electrode that is
cylindrically-shaped with open ends and solid sides, and a second
electrode comprising a wire substantially colinear with the first
electrode.
[0020] FIG. 2I schematically illustrates an alternative embodiment
of a generally square or rectangular electrode within a generally
square or rectangular vacuum chamber.
[0021] FIG. 3, which is broken into FIGS. 3a and 3b, schematically
illustrates an embodiment of a low frequency power module
compatible with the phase angle control method of the present
invention.
[0022] FIG. 4, which is broken into FIGS. 4a and 4b, schematically
illustrates an embodiment of a low frequency power module
compatible with the amplitude control method of the present
invention.
[0023] FIG. 5A schematically illustrates the phase angle control
method of controlling the low frequency power applied to the
plasma.
[0024] FIG. 5B schematically illustrates the amplitude control
method of controlling the low frequency power applied to the
plasma.
[0025] FIG. 6 schematically illustrates a preferred embodiment of a
method of sterilization compatible with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] Production of gas discharge plasmas using low frequency (LF)
voltages avoids the various problems inherent in the state of the
art sterilization devices and processes which form and use plasmas
produced by radio frequency (RF) voltages. First, LF plasma
processing leaves less residual reactive species remaining on the
sterilized articles than does RF plasma processing. Second,
generation of the LF plasma is highly energy efficient because
little or no frequency conversion from the line voltage is needed.
For example, by using no frequency conversion with a line voltage
frequency of 60 Hz, the energy efficiency of the sterilization
system can reach approximately 85-95%. Use of LF voltages also does
not require an impedance matching network, thereby avoiding the
associated energy losses. Third, due to the simplified
instrumentation and higher energy efficiency of LF generation, the
cost per watt applied to the sterilized articles using LF plasmas
can be as low as one-tenth the cost per watt of using RF plasmas.
Fourth, the simplified instrumentation used for generating LF
plasmas has proven to be more reliable and robust, and requiring
less complicated diagnostic instrumentation.
[0027] FIG. 1 schematically illustrates one preferred embodiment of
the present invention comprising a sterilization system 10. The
sterilization system 10 comprises a vacuum chamber 12, a vacuum
pump 14, a vacuum pump line 15, a vacuum pump valve 16, a reactive
agent source 18, a reactive agent line 19, a reactive agent valve
20, a low frequency (LF) power module 22, an LF voltage conduit 24,
a vent 26, a vent line 27, a vent valve 28, a process control
module 30, an electrode 32, and a reactive agent monitor 34.
Persons skilled in the art recognize that other embodiments
comprising sterilization systems of different configurations than
that illustrated in FIG. 1 are compatible with the present
invention.
[0028] In the preferred embodiment of the present invention, the
articles (not shown in FIG. 1) to be sterilized are packaged in
various commonly employed packaging materials used for sterilized
products. The preferred materials are spunbonded polyethylene
packaging material commonly available under the trademark "TYVEK"
or composites of "TYVEK" with a polyethylene terephthalate
packaging material commonly available under the trademark "MYLAR".
Other similar packaging materials may also be employed such as
polypropylene. Paper packaging materials may also be used. With
paper packaging, longer processing times may be required to achieve
sterilization because of possible interactions of the reactive
agent with paper.
[0029] The vacuum chamber 12 of the preferred embodiment is
sufficiently gas-tight to support a vacuum of approximately less
than 40 Pa (0.3 Torr). Coupled to the vacuum chamber 12 is a
pressure monitor (not shown) which is also coupled to the process
control module to provide a measure of the total pressure within
the vacuum chamber. Also coupled to the vacuum chamber 12 is the
reactive agent monitor 34 which is capable of detecting the amount
of the reactive agent in the vacuum chamber 12. In the exemplary
embodiment of the present invention, the reactive agent is hydrogen
peroxide, and the reactive agent monitor 34 measures the absorption
of ultraviolet radiation at a wavelength characteristic of hydrogen
peroxide. Other methods of reactive agent detection compatible with
the present invention include, but are not limited to, pressure
measurement, near infrared absorption, and dew point measurement.
The reactive agent monitor 34 is also coupled to the process
control module 30 to communicate the detected amount of the
reactive agent to the process control module 30.
[0030] In the preferred embodiment of the present invention, inside
and electrically isolated from the vacuum chamber 12 is the
electrode 32, which is electrically conductive and perforated to
enhance fluid communication between the gas and plasma species on
each side of the electrode 32. The electrode 32 of the preferred
embodiment generally conforms to the inner surface of the vacuum
chamber 12, spaced approximately one to two inches from the wall of
the vacuum chamber 12, thereby defining a gap region between the
vacuum chamber 12 and the electrode 32. The electrode 32 is coupled
to the LF power module 22 via the LF voltage conduit 24. In the
preferred embodiment, with the vacuum chamber 12 connected to
electrical ground via a bypass capacitor and shunt resistor,
application of an LF voltage between the vacuum chamber 12 and the
electrode 32 creates an LF electric field which is stronger in a
second region 31 which includes the gap region and the vicinity of
the edges of the electrode 32. The LF electric field is weaker in a
first region 33 where the sterilized articles are placed.
Generally, in other embodiments, the LF electric field can be
generated by applying an LF voltage between the electrode 32 and a
second electrode in the vacuum chamber 12. In such embodiments, the
second region 31 includes the gap region between the two
electrodes, and the vicinity of the edges of one or both of the
electrodes. The preferred embodiment in which the vacuum chamber 12
serves as the second electrode is one of the many different ways to
generate the gas plasma.
[0031] In the preferred embodiment illustrated in FIG. 2A, a
cylindrically-shaped electrode 32 provides fluid communication
between the gas and plasma on each side of the electrode 32 through
the open ends of the electrode 32 as well as through the
perforations in the side of the electrode 32. These open ends and
perforations permit gaseous and plasma species to freely travel
between the second region 31 between the electrode 32 and the walls
of the vacuum chamber 12 and the first region 33 where the
sterilized articles are placed. Similarly, as illustrated in FIGS.
2B-2I, other configurations of the electrode 32 provide fluid
communication between the second region 31 and the first region 33.
FIG. 2B schematically illustrates a cylindrically-shaped electrode
32 with open ends and louvered openings along its sides. FIG. 2C
schematically illustrates a cylindrically-shaped electrode 32 with
open ends and solid sides. FIG. 2D schematically illustrates an
electrode 32 comprising a series of colinear cylindrically-shaped
segments with open ends and solid sides. FIG. 2E schematically
illustrates an electrode 32 with a partial cylindrical shape, open
ends and solid sides. FIG. 2F schematically illustrates a
cylindrically-symmetric and longitudinally-asymmetric electrode 32
with open ends and solid sides. FIG. 2G schematically illustrates
an asymmetric electrode 32 with open ends and solid sides. More
than one electrode can be used to generate the plasma. FIG. 2H
schematically illustrates an electrode system with a first
electrode 32 that is cylindrically-shaped with open ends and solid
sides, and a second electrode 32' comprising a wire substantially
colinear with the first electrode 32. The LF voltage is applied
between the first electrode 32 and the second electrode 32'. In
this embodiment, the second region 31 is the region between the
first electrode 32 and the second electrode 32', and the first
region 33 is between the first electrode 32 and the vacuum chamber
12. FIG. 21 schematically illustrates a generally square or
rectangular electrode within a generally square or rectangular
vacuum chamber. The various configurations for generally
cylindrical electrodes schematically illustrated in FIGS. 2A-2H can
also be applied to the generally square or rectangular electrode of
FIG. 21. Each of these embodiments of the electrode 32 provide
fluid communication between the second region 31 and the first
region 33.
[0032] The vacuum pump 14 of the preferred embodiment is coupled to
the vacuum chamber 12 via the vacuum pump line 15 and the vacuum
valve 16. Both the vacuum pump 14 and the vacuum pump valve 16 are
coupled to, and controlled by, the process control module 30. By
opening the vacuum valve 16, gases within the vacuum chamber 12 are
pumped out of the vacuum chamber 12 through the vacuum pump line 15
by the vacuum pump 14. In certain embodiments, the vacuum valve 16
is capable of being opened to variable degrees to adjust and
control the pressure in the vacuum chamber 12.
[0033] The reactive agent source 18 of the preferred embodiment is
a source of fluid coupled to the vacuum chamber 12 via the reactive
agent line 19 and the reactive agent valve 20. The reactive agent
valve 20 is coupled to, and controlled by, the process control
module 30. The reactive agent source 18 of the preferred embodiment
comprises reactive agent species. In the preferred embodiment, the
reactive agent species comprises a germicide which is a sterilant
or a disinfectant, such as hydrogen peroxide. In addition, the
germicide supplied by the reactive agent source 18 can be in gas or
vapor form. By opening the reactive agent valve 20, reactive agent
atoms and molecules from the reactive agent source 18 can be
transported into the vacuum chamber 12 via the reactive agent line
19. In certain embodiments, the reactive agent valve 20 is capable
of being opened to variable degrees to adjust the pressure of the
reactive agent in the vacuum chamber 12. In the exemplary
embodiment of the present invention, the reactive agent species of
the reactive agent source 18 comprising hydrogen peroxide
molecules.
[0034] The vent 26 of the preferred embodiment is coupled to the
vacuum chamber 12 via the vent line 27 and the vent valve 28. The
vent valve 28 is coupled to, and controlled by, the process control
module 30. By opening the vent valve 28, vent gas is vented into
the vacuum chamber 12 via the vent line 27. In certain embodiments,
the vent valve 28 is capable of being opened to variable degrees to
adjust the pressure of the air in the vacuum chamber 12. In the
exemplary embodiment of the present invention, the vent 26 is a
High Efficiency Particulate-filtered Air (HEPA) vent which provides
filtered air as the vent gas. Other vent gases compatible with the
present invention include, but are not limited to, dry nitrogen,
and argon.
[0035] The process control module 30 is coupled to various
components of the sterilization system 10 to control the
sterilization system 10. In an exemplary embodiment of the present
invention, the process control module 30 is a microprocessor
configured to provide control signals to the various other
components in response to the various signals received from other
components.
[0036] The LF power module 22 of the preferred embodiment is
coupled to the electrode 32 via the LF voltage conduit 24, and is
coupled to, and controlled by, the process control module 30. The
LF power module 22 is adapted to apply a low frequency voltage
between the electrode 32 and the vacuum chamber 12 so as to
generate a low frequency plasma in the vacuum chamber 12. FIG. 3,
which is broken into FIGS. 3a and 3b, schematically illustrates an
embodiment of the LF power module 22 compatible with the phase
angle control method of controlling the low frequency power applied
to the plasma. As illustrated in FIG. 3, the LF power module 22
comprises an over-power relay 40, a pair of metal oxide varistors
42, a step-up transformer 50, a flyback current shunt element 62,
an inductor 64, a capacitor 66, and a LF power feedback control
system 70. The LF power feedback control system 70 illustrated in
FIG. 3 comprises a power controller 60, a current monitor 80, a
voltage monitor 90, and a power monitor 100 coupled to the current
monitor 80 and the voltage monitor 90. Line voltage (typically
200-240 VAC, 50/60 Hz) is provided to the step-up transformer 50
via the closed over-power relay 40 which is coupled to the LF power
feedback control system 70. For other frequencies, the LF power
module 22 may also include a switching module to provide lower
frequencies or frequencies up to a few hundred kHz.
[0037] In the embodiment illustrated in FIG. 3, the metal oxide
varistors (MOVs) 42 are used to suppress transient voltage
impulses. Each MOV 42 is a multiple-junction solid-state device
capable of withstanding large magnitude impulses with a low amount
of let-through voltage. The MOVs 42 serve as fast acting "variable
resistors" with a low impedance at higher-than-normal voltages and
a high impedance at normal voltages. MOVs are manufactured for
specific voltage configurations and for a variety of impulse
magnitudes. Persons skilled in the art are able to select MOVs 42
consistent with the present invention.
[0038] The output voltage of the step-up transformer 50 is
preferably between approximately 100 and 1000 V.sub.rms, more
preferably between approximately 200 and 500 V.sub.rms, and most
preferably between approximately 250 and 450 V.sub.rms. The output
voltage of the step-up transformer 50 is transmitted to the power
controller 60, which provides the LF voltage to the electrode 32
and vacuum chamber 12 via the flyback current shunt element 62, the
inductor 64, the capacitor 66, and the LF power feedback control
system 70. The flyback current shunt element 62 provides a path for
fly-back current and to tune the circuit, and in the preferred
embodiment the flyback current shunt element 62 is a load resistor
of approximately 1500 ohms. In other embodiments, the flyback
current shunt element 62 can be a snubber. The inductance of the
inductor 64 is chosen to limit noise spikes in the LF current, and
is typically approximately 500 mH. The capacitance of the capacitor
66 is chosen to maximize the efficiency of power transfer to the LF
plasma by matching the resonant frequency of the series LC circuit
to the frequency of the applied LF voltage. For a 60 Hz voltage and
an inductance of 500 mH, a capacitance of approximately 13.6 .mu.F
provides the resonant condition for which the impedance of the
series LC circuit is approximately zero, thereby maximizing the
transmitted LF power. Persons skilled in the art are able to select
appropriate values for these components depending on the frequency
of the applied LF voltage in a manner compatible with the present
invention.
[0039] FIG. 4, which is broken into FIGS. 4a and 4b, schematically
illustrates an embodiment of the LF power module 22 compatible with
the amplitude control method of controlling the low frequency power
applied to the plasma. As illustrated in FIG. 4, the LF power
module 22 comprises an over-power relay 40, a pair of metal oxide
varistors 42, a step-up transformer 55, and a LF power feedback
control system 70. The LF power feedback control system 70
illustrated in FIG. 4 comprises a high voltage (HV) DC power supply
51, a voltage-controlled oscillator (VCO) 52, a voltage-controlled
amplifier (VCA) 53, a HV operational amplifier 54, a current
monitor 80, a voltage monitor 90, and a power monitor 100 coupled
to the current monitor 80 and the voltage monitor 90. Line voltage
is provided to the HV DC power supply 51 via the closed over-power
relay 40 which is coupled to the LF power feedback control system
70. The output of the HV DC power supply 51 is preferably between
approximately 100 and 1000 VDC, more preferably between
approximately 200 and 500 VDC, and most preferably between
approximately 250 and 450 VDC.
[0040] In the embodiment illustrated in FIG. 4, the VCO 52
generates a sinewave output with a constant amplitude and fixed low
frequency from 0 to 1 MHz, the low frequency selected by supplying
an appropriate set-point voltage to the VCO 52. Alternative
embodiments can utilize other waveforms, e.g., triangular or square
waveforms. The LF output of the VCO 52 is supplied to the VCA 53,
which serves as a power controller to maintain a substantially
stable average power applied to the low frequency plasma. In
response to a feedback signal from the power control module 110,
the VCA 53 amplifies the LF output of the VCO 52 to generate an
amplified LF voltage with an amplitude between approximately 0 and
12 VAC. The amplified LF voltage from the VCA 53 is supplied to the
HV operational amplifier 54 which in response generates a high
voltage LF output with an amplitude determined by the amplitude of
the amplified LF voltage from the VCA 53. Appropriate HV
operational amplifiers are commercially available (e.g., Apex
Microtechnology, Tuscon, Ariz., part number PA93), and persons
skilled in the art are able to select a HV operational amplifier
compatible with the present invention. Typically, the amplitude of
the high voltage LF output from the HV operational amplifier 54 is
approximately 100 to 150 VAC. In order to generate larger amplitude
LF voltages to be applied to the plasma, the high voltage LF output
from the HV operational amplifier 54 can be further amplified by
the step-up transformer 55, as illustrated in FIG. 4.
Alternatively, the step-up transformer 55 may be omitted if the HV
operational amplifier 54 is capable of generating a high voltage LF
output with the desired amplitude to be applied to the plasma.
[0041] In both the phase angle control embodiment illustrated in
FIG. 3 and the amplitude control embodiment illustrated in FIG. 4,
the LF power feedback control system 70 of the LF power module 22
further comprises a power control module 110 coupled to the power
monitor 100, which is coupled to the current monitor 80 and voltage
monitor 90. The current monitor 80 measures the LF current through
the electrode 32 and the vacuum chamber 12. In the preferred
embodiment of the present invention, the current monitor 80
includes a current sensor 82 which provides a voltage output
indicative of the measured real-time, cycle-by-cycle LF current, a
first converter 84 which produces a DC voltage in response to the
RMS of the voltage output of the current sensor 82, and a first
voltage amplifier 86 which amplifies the DC voltage from the first
converter 84 to produce a real-time current signal. In addition,
the current monitor 80 also includes an over-current detector 88,
which monitors the DC voltage from the first converter 84 in
real-time and sends an error signal to the power control module 110
if the LF current exceeds a pre-set value, caused for example by a
short circuit between the electrode 32 and the vacuum chamber 12.
Under such an occurrence, the LF voltage is turned off momentarily.
This occurrence can result in a few cycles being lost, however the
power is stabilized so that the average power is not affected by
more than a predetermined tolerance.
[0042] The voltage monitor 90 measures the LF voltage between the
electrode 32 and the vacuum chamber 12. In the preferred embodiment
of the present invention, the voltage monitor 90 includes a
step-down transformer 92 which produces a voltage output indicative
of the measured real-time, cycle-by-cycle LF voltage, a second
converter 94 which produces a DC voltage in response to the RMS of
the voltage output of the step-down transformer 92, and a second
voltage amplifier 96 which amplifies the DC voltage from the second
converter 94 to produce a real-time voltage signal.
[0043] In the preferred embodiment, the power monitor 100 further
comprises a multiplier that receives the DC voltages from the
current monitor 80 and the voltage monitor 90, and multiplies these
two voltages to produce a real-time power signal proportional to
the LF power applied to the plasma between the electrode 32 and the
vacuum chamber 12, the real-time power signal being generated in
response to the real-time current and real-time voltage signals,
and transmitted to the power control module 110. In other
embodiments, the power monitor 100 monitors the power applied to
the plasma by utilizing a signal indicative of the real-time
impedance of the plasma with either the real-time current or
real-time voltage signals. In still other embodiments, the power
monitor 100 monitors the power applied to the plasma by utilizing
other real-time signals which indirectly indicate the power applied
to the plasma; e.g., a real-time signal proportional to the
brightness of the glow discharge generated by the plasma. Persons
skilled in the art can select an appropriate power monitor 100
compatible with the present invention.
[0044] The power control module 110 of the preferred embodiment
includes a fault detector, such as an over-power detector 112 which
monitors the real-time power signal from the power monitor 100 and
opens the over-power relay 40 if the LF power exceeds a pre-set
value, thereby extinguishing the LF plasma. After such an
occurrence, the control of restart can be given to the user or to
software. The power control module 110 of the preferred embodiment
further comprises an additional fault detector, such as a thermal
switch 114 which detects overheating, and a power control processor
120.
[0045] In the preferred embodiment, the power control processor 120
controls and monitors the status of the LF power feedback control
system 70. The power control processor 120 is coupled to a user
interface 122 which provides user input regarding a selected power
magnitude setting and a selected power on/off setting. The power
control processor 120 is also coupled to the power monitor 100, the
thermal switch 114, and the over-current detector 88. In the
preferred embodiment, the power magnitude setting can be selected
from two power levels, 800 W and 600 W. When the power is turned
on, the preferred embodiment of the power control processor 120
ensures that a "soft start" condition is maintained in which the
inrush current is minimized. In addition, the user interface 122
receives signals from the power control processor 120 indicative of
the status of the sterilization system 10, which is communicated to
the user.
[0046] In the phase angle control embodiment illustrated in FIG. 3,
the power control processor 120 is also coupled to the power
controller 60. In this embodiment, the power control processor 120
transmits a signal to the power controller 60 in response to
signals from the user interface 122, power monitor 100,
over-current detector 88, and thermal switch 114 in order to
maintain a substantially stable LF power applied to the LF plasma
while avoiding error conditions. In the amplitude control
embodiment illustrated in FIG. 4, the power control processor 120
is coupled to the VCA 53. In this embodiment, the power control
processor 120 transmits a signal to the VCA 53 in response to
signals from the user interface 122, power monitor 100,
over-current detector 88, and thermal switch 114 in order to
maintain a substantially stable LF power applied to the LF plasma
while avoiding error conditions. In both embodiments illustrated in
FIG. 3 and FIG. 4, the power control processor 120 typically
maintains the LF power applied to the LF plasma within a tolerance
of approximately 0-10% of the specified power level.
[0047] Note that not all of the components listed and described in
FIG. 3 and FIG. 4 are required to practice the present invention,
since FIG. 3 and FIG. 4 merely illustrate particular embodiments of
the LF power module 22. These components include components for
automation, safety, regulatory, efficiency, and convenience
purposes. Other embodiments compatible with the present invention
can eliminate some or all of these components, or can include
additional components.
[0048] In response to the signal from the power control processor
120, the power controller 60 of the embodiment illustrated in FIG.
3 controls the LF power applied between the electrode 32 and the
vacuum chamber 12 by utilizing phase angle control. Under phase
angle control, the duty cycle of the LF power is modified by
zeroing the voltage and current applied between the electrode 32
and the vacuum chamber 12 for a portion A of the cycle period. Such
phase angle control is often used to maintain constant power from
electric heaters or furnaces. FIG. 5A schematically illustrates the
voltage and current for a 100% duty cycle (i.e., .DELTA.=0) and for
a reduced duty cycle (i.e., .DELTA..noteq.0). During normal
operations, the power controller 60 maintains a constant LF power
applied to the plasma by actively adjusting the duty cycle of the
LF power in response to the feedback real-time signal received from
the power control module 110 in response to the measured LF power.
When a fault event is detected by the over-current detector 88 or
thermal switch 114, the power control processor 120 reduces the LF
power by reducing the duty cycle of the LF power, and it transmits
a signal to the user interface 122 to provide notification of the
fault event. Persons skilled in the art are able to select
appropriate circuitry to modify the duty cycle of the LF power
consistent with the present invention.
[0049] Alternatively, the LF power can be controlled by utilizing
amplitude control, as in the embodiment illustrated in FIG. 4.
Under amplitude control, the LF power is modified by adjusting the
amplitude of the voltage and current applied between the electrode
32 and the vacuum chamber 12. FIG. 5B schematically illustrates the
voltage and current corresponding to a first LF power setting and a
second LF power setting less than the first LF power setting.
During normal operations, the VCA 53 maintains a constant LF power
applied to the plasma by actively adjusting the amplitude of the LF
power in response to the feedback real-time signal received from
the power control module 110 in response to the measured LF power.
Persons skilled in the art are able to select appropriate circuitry
to modify the amplitude of the LF power consistent with the present
invention.
[0050] The electronics for RF sterilizers are complicated by the
need of such systems to attempt to closely match the output
impedance of the RF generator with the plasma impedance at all
times in order to maximize power efficiency and to avoid damage to
the RF generator. Plasma impedance varies widely during plasma
formation, being very high until the plasma is fully formed, and
very low thereafter. When first igniting a plasma, the RF generator
cannot match the high plasma impedance which exists prior to the
full formation of the plasma, so a large fraction of the power
output is reflected back to the RF generator. RF generators have
protection systems which typically limit the RF generator output
during periods of high reflected power to avoid damage. However, to
ignite the plasma, the voltage output of the RF generator must
exceed the threshold voltage required for plasma ignition. The
threshold voltage is dependent on the chamber pressure, reactive
agent, and other operating parameters and is approximately 300
V.sub.rms. In an RF system, once ignition has been achieved, and
the plasma impedance is thereby reduced, the magnitude of the
applied RF voltage must be reduced to a sustaining voltage, e.g.,
approximately 140 V.sub.rms, to avoid excessive power delivery.
Because the higher RF voltages required for plasma ignition produce
excessively high reflected power before full plasma formation, RF
generators require complicated safeguards to prevent damage during
the plasma ignition stage.
[0051] Conversely, the complexity and rate of ignition failures are
significantly reduced for LF sterilizers since the LF sterilizers
may operate using applied voltages above the threshold voltage and
have much less restrictive output impedance matching requirements.
During the times at which the applied LF voltage equals zero, as
seen in FIG. 5A, the LF plasma is extinguished and there is no LF
plasma in the vacuum chamber. The LF plasma must then be re-ignited
twice each cycle. By only operating in one voltage regime, LF
sterilizers have simpler and more reliable electrical systems than
do RF sterilizers. These electrical systems are easier to service
and diagnose, thereby reducing the costs associated with repair. In
addition, the higher peak plasma densities resulting from LF
sterilizers likely result in increased dissociative recombination
on the articles, thereby reducing the amount of residual reactive
species remaining on the articles after the sterilization
procedure.
[0052] FIG. 6 schematically illustrates a preferred method of
sterilization using the apparatus schematically illustrated in FIG.
1. The sterilization process shown in FIG. 6 is exemplary, and
persons skilled in the art recognize that other processes are also
compatible with the present invention. The preferred process begins
by sealing 200 the article to be sterilized into the vacuum chamber
12. The vacuum chamber is then evacuated 210 by engaging the vacuum
pump 14 and the vacuum valve 16 under the control of the process
control module 30. The vacuum chamber 12 is preferably evacuated to
a pressure of less than approximately 660 Pa (5 Torr), more
preferably between approximately 25 to 270 Pa (0.2 to 2 Torr), and
most preferably between approximately 40 to 200 Pa (0.3 to 1.5
Torr).
[0053] In an exemplary process, upon reaching a desired pressure in
the vacuum chamber 12, the process control module 30 signals the LF
power module 22 to energize the electrode 32 within the vacuum
chamber 12. By applying a LF voltage to the electrode 32, the LF
power module 22 ionizes the residual gases in the vacuum chamber
12, thereby creating 220 a gas discharge LF plasma inside the
vacuum chamber 12. This gas discharge LF plasma is formed from the
residual gases in the vacuum chamber 12, which are primarily air
and water vapor. Because this gas discharge LF plasma is created
220 before the reactive agent is injected into the vacuum chamber
12, this gas discharge LF plasma is typically called the
"pre-injection" plasma. The vacuum valve 14 is controllably opened
and closed to maintain a preset vacuum pressure during the
pre-injection plasma step 220. The pre-injection plasma heats the
surfaces inside the vacuum chamber 12, including the articles,
thereby aiding the evaporation and removal of condensed water and
other absorbed gases from the vacuum chamber 12 and the articles. A
similar pre-injection plasma is described by Spencer, et al. in
U.S. Pat. Nos. 5,656,238 and 6,060,019, which are incorporated by
reference herein. In an exemplary process, the pre-injection plasma
is turned off after approximately 0 to 60 minutes. Other
embodiments that are compatible with the present invention do not
include the creation of the pre-injection plasma, or use multiple
pre-injection plasmas. In still other embodiments, the vacuum
chamber 12 can be vented after the articles are exposed to the
pre-injection plasma.
[0054] In the preferred process, upon reaching a desired chamber
pressure, the vacuum valve 16 is closed, and the reactive agent
valve 20 is opened under the control of the process control module
30, thereby injecting 230 reactive agent from the reactive agent
source 18 into the vacuum chamber 12 via the reactive agent line
19. In the preferred embodiment, the reactive agent comprises
hydrogen peroxide, which is injected in the form of a liquid which
is then vaporized. The injected liquid contains preferably from
about 3% to 60% by weight of hydrogen peroxide, more preferably
from about 20% to 60% by weight of hydrogen peroxide, and most
preferably from about 40% to 60% by weight of hydrogen peroxide.
The concentration of hydrogen peroxide vapor in the vacuum chamber
12 may range from 0.125 to 20 mg of hydrogen peroxide per liter of
chamber volume. The higher concentrations of hydrogen peroxide will
result in shorter sterilization times. Air or inert gas such as
argon, helium, nitrogen, neon, or xenon may be added to the chamber
with the hydrogen peroxide to maintain the pressure in the vacuum
chamber 12 at the desired level. This injection 230 of reactive
agent may occur as one or more separate injections.
[0055] Due to this injection 230 of reactive agent, the chamber
pressure of the preferred process rises to approximately 2000 Pa
(15 Torr) or more. After approximately 6 minutes into the injection
stage 230, the reactive agent is permitted to diffuse 240
completely and evenly throughout the vacuum chamber 12. After
approximately 1-45 minutes of diffusing 240, the reactive agent is
substantially in equilibrium inside the vacuum chamber 12. This
diffusing 240 allows the reactive species to diffuse through the
packaging material of the articles, and come into close proximity,
if not contact, with the surfaces of the articles, thereby
sterilizing the articles. In other embodiments, the diffusion of
the reactive agent can be immediately followed by a vent of the
vacuum chamber 12.
[0056] The vacuum chamber 12 is then partially evacuated 250 by
pumping out a fraction of the reactive agent from the vacuum
chamber 12 by controllably opening the vacuum valve 16 under the
control of the process control module 30. Once the vacuum pressure
within the vacuum chamber 12 has reached the desired pressure, the
vacuum valve 16 is controllably adjusted to maintain the desired
pressure, and the process control module 30 signals the LF power
module 22 to energize the electrode 32 within the vacuum chamber
12. In the preferred embodiment in which the reactive agent
comprises hydrogen peroxide, the pressure of the hydrogen peroxide
in the vacuum chamber 12 is preferably less than approximately 670
Pa (5 Torr), more preferably between approximately 25 and 270 Pa
(0.2 to 2 Torr), and most preferably between approximately 40 and
200 Pa (0.3 to 1.5 Torr). By applying a LF voltage to the electrode
32, the LF power module 22 generates 260 a reactive agent LF plasma
inside the vacuum chamber 12 by ionizing the reactive agent. The
article is exposed to the reactive agent LF plasma for a controlled
period of time. In the preferred embodiment, an additional cycle
275 is performed. Other embodiments may omit this additional cycle
275, or may include further cycles.
[0057] In both RF and LF plasmas, the components of the reactive
agent plasma include dissociation species of the reactive agent and
molecules of the reactive agent in excited electronic or
vibrational states. For example, where the reactive agent comprises
hydrogen peroxide as in the preferred embodiment, the reactive
agent plasma likely includes charged particles such as electrons,
ions, various free radicals (e.g., OH, O.sub.2H), and neutral
particles such as ground state H.sub.2O.sub.2 molecules and excited
H.sub.2O.sub.2 molecules. Along with the ultraviolet radiation
produced in the reactive agent plasma, these reactive agent species
have the potential to kill spores and other microorganisms.
[0058] Once created, the charged particles of the reactive agent
plasma are accelerated by the electric fields created in the vacuum
chamber 12. Because of the fluid communication between the second
region 31 and the first region 33, some fraction of the charged
particles created in the second region 31 are accelerated to pass
from the second region 31 to the first region 33 which contains the
articles.
[0059] Charged particles passing from the second region 31 to the
first region 33 have their trajectories and energies affected by
the electric potential differential of the sheath regions between
the plasma and the walls of the vacuum chamber 12 and the electrode
32. These sheath regions are created by all electron-ion plasmas in
contact with material walls, due to charged particles impinging
from the plasma onto the walls. Electrons, with their smaller mass
and hence greater mobility, are lost from the plasma to the wall
before the much heavier and less mobile ions, thereby creating an
excessive negative charge density surrounding the walls and a
corresponding voltage differential which equalizes the loss rates
of the electrons and the ions. This voltage differential, or sheath
voltage, accelerates electrons away from the wall surface, and
accelerates positive ions toward the wall surface.
[0060] The sheath voltage varies for different plasma types,
compositions, and methods of production. For RF plasmas, the sheath
voltage is typically 40%-80% of the RF voltage applied to the
electrode 32. For example, for a root-mean-squared (RMS) RF voltage
of 140 V.sub.rms applied to the electrode 32 once the RF plasma is
established, the corresponding sheath voltage is approximately
55-110 V.sub.rms. An ion entering the sheath region surrounding the
electrode 32 will then be accelerated to an energy of 55-110 eV.
This acceleration of positive ions by the sheath voltage is the
basic principle behind semiconductor processing by RF plasmas.
[0061] As described above, for the LF plasmas of the preferred
embodiment of the present invention, the voltage applied to the
electrode 32 may be equal to or greater than the ignition threshold
voltage, which is typically 300 V.sub.rms. In addition, for LF
plasmas, the sheath voltage is typically a higher percentage of the
applied voltage than for RF plasmas, so the sheath voltage of the
preferred embodiment of the present invention is then much higher
than the sheath voltage for an RF plasma system. This higher sheath
voltage thereby accelerates the charged particles of the LF plasma
to much higher energies. Therefore, because the charged particles
are accelerated to higher energies, the charged particles of the LF
plasma of the preferred embodiment travel farther and interact more
with the articles than do the charged particles of RF plasma
sterilizers.
[0062] Since the LF electric field changes polarity twice each
cycle, the direction of the electric field acceleration on the
charged particles reverses twice each cycle. For charged particles
in the second region 31, this oscillation of the direction of the
acceleration results in an oscillation of the position of the
charged particles. However, because of the fluid communication
between the second region 31 and the first region 33, some fraction
of the charged particles are able to pass to the first region 33
containing the articles from the second region 31 before the
direction of the electric field acceleration reverses.
[0063] The fraction of the charged particles created in the
reactive agent LF plasma which enter the first region 33 is a
function of the frequency of the applied electric field. The
charged particles have two components to their motion--random
thermal speed and drift motion due to the applied electric field.
The thermal speed, measured by the temperature, is the larger of
the two (typically approximately 10.sup.7-10.sup.8 cm/sec for
electrons), but it does not cause the charged particles to flow in
any particular direction. Conversely, the drift speed is directed
along the electric field, resulting in bulk flow of charged
particles in the direction of the applied electric field. The
magnitude of the drift speed is approximately proportional to the
magnitude of the applied electric field, and inversely proportional
to the mass of the charged particle. In addition, the magnitude of
the drift speed is dependent on the gas species and chamber
pressure. For example, for typical operating parameters of gas
discharge plasma sterilizers, including an average electric field
magnitude of approximately 1 volt/cm, the drift speed for an
electron formed in a gas discharge plasma is typically
approximately 106 cm/sec.
[0064] A charged particle enters the first region 33 containing the
articles only if it reaches the first region 33 before the polarity
of the applied electric field changes, which would reverse the
acceleration of the charged particle away from the electrode 32.
For example, for an applied RF electric field with a frequency of
13.56 MHz, the period of the electric field is approximately
7.4.times.10.sup.-8 sec, so an electron only moves a distance of
approximately 3.7.times.10.sup.-3 cm during the half-cycle or
half-period before the direction of the electric field changes and
the electron is accelerated away from the electrode 32. Due to
their much larger masses, ions move much less than do electrons.
Where the second region 31 between the vacuum chamber 12 and the
electrode 32 is approximately 2.54 cm wide, as in the preferred
embodiment, only a fraction of the charged particles created by an
RF plasma would actually reach the first region 33 containing the
articles.
[0065] Conversely, for an applied LF electric field with a
frequency of 60 Hz, the period of the electric field is
approximately 16.7.times.10.sup.-3 sec, so an electron can move
approximately 8.35.times.10.sup.3 cm before it is accelerated away
from the electrode 32. Therefore, the use of LF voltages to create
the plasma in the sterilization system 10 of the preferred
embodiment results in more activity in the first region 33, as
compared to a plasma generated using RF voltages. This higher
activity in LF sterilizers likely contributes to the increased
efficiency for the removal of residual reactive species from the
sterilized articles as compared to RF sterilizers.
[0066] The plasma decay time, defined as a characteristic time for
the plasma to be neutralized after power is no longer applied,
provides an approximate demarcation between the LF and RF regimes.
The plasma decay time is not known precisely, but it is estimated
to be approximately 10.sup.-4-10.sup.-3 sec for the plasma
densities used in sterilizer systems, such as the preferred
embodiment of the present invention. This plasma decay time
corresponds to the time a charged particle exists before it is
neutralized by a collision with a surface or another plasma
constituent, and is dependent on the plasma species generated and
the geometries of the various components of the sterilization
system 10. As described above, the LF regime is characterized by a
plasma which is extinguished and re-ignited twice each cycle, i.e.,
the half-period of the applied LF voltage is greater than the
plasma decay time. Therefore, the sterilization system 10 is
continually run at an applied voltage above the ignition threshold
voltage of the plasma in order to re-ignite the plasma. The
estimated approximate range of plasma decay times of
10.sup.-4-10.sup.-3 sec for many of the plasmas compatible with the
present invention then translates to an upper limit on the low
frequency regime of approximately 1-10 kHz. However, under certain
circumstances, higher frequencies can be tolerated.
[0067] Alternatively, the upper limit of the low frequency regime
may be defined as the frequency at which the electron drift speed
is too slow for an electron to traverse the 2.54-cm-wide second
region 31 during a half-period of the applied LF voltage. Under
typical operating geometries, this upper limit of the low frequency
regime would be approximately 200 kHz. For other geometries, the
upper limit of the low frequency regime can be correspondingly
different.
[0068] In the preferred embodiment of the present invention, the
frequency of the LF voltage applied to the plasma is preferably
from 0 to approximately 200 kHz, more preferably from 0 to
approximately 10 kHz, still more preferably from 0 to approximately
1 kHz, and even more preferably from 0 to approximately 400 Hz.
When selecting the frequency of the LF voltage applied to the
plasma, the frequency is most preferably selected to have a
half-period greater than the plasma decay time of the plasma.
[0069] In the preferred method, the LF power module 22 remains
energized for approximately 2-15 minutes, during which the plasma
removes excess residual reactive species present on surfaces within
the vacuum chamber 12, including on the articles. There is a brief
rise of the vacuum pressure upon generating 260 the plasma,
however, the majority of the residual removal step 270 is conducted
at an approximately constant vacuum pressure of 50 to 70 Pa (0.4 to
0.5 Torr). The residual removal step 270 is ended by the process
control module 30, which turns off the LF power module 22, thereby
quenching the plasma.
[0070] After the residual removal step 270, the vacuum chamber 12
is vented 280 by the process control module 30 which opens the vent
valve 28, thereby letting in vent gas from the vent 26 through the
vent line 27 and the vent valve 28. In the preferred process, the
vacuum chamber 12 is then evacuated 290 to a pressure of
approximately 40 to 105 Pa (0.3 to 0.8 Torr) to remove any
remaining reactive agent which may be present in the vacuum chamber
12. The vacuum chamber 12 is then vented again 300 to atmospheric
pressure, and the sterilized articles are then removed 310 from the
vacuum chamber 12.
[0071] The LF plasma provides a reduction of the amount of residual
reactive agent molecules remaining on the articles after the
sterilization procedure is complete. Where the reactive agent
comprises hydrogen peroxide, the amount of residual hydrogen
peroxide remaining on the sterilized articles is preferably less
than approximately 8000 ppm, more preferably less than
approximately 5000 ppm, and most preferably less than approximately
3000 ppm. In a comparison of the amount of residual hydrogen
peroxide remaining after a LF plasma sterilization as compared to a
RF plasma sterilization, nine polyurethane test samples were
exposed to hydrogen peroxide during a simulated sterilization cycle
in both a LF sterilizer and a RF sterilizer. Each sample was
prepared by washing with Manuklenz.RTM. and drying prior to
sterilization to avoid any cross contamination. The nine samples
were then distributed uniformly across the top shelf of a standard
industrial rack.
[0072] A full LF sterilization cycle, which matched nearly exactly
the conditions of a standard RF sterilizer cycle, was used to
perform the comparison. The full LF sterilization cycle included a
20-minute exposure to a pre-injection plasma, a first 6-minute
hydrogen peroxide injection, a vent to atmosphere, a 2-minute
diffusion, a first 2-minute post-injection plasma, a second
6-minute hydrogen peroxide injection, a vent to atmosphere, a
2-minute diffusion, a second 2-minute post-injection plasma, and a
vent to atmosphere. Two full LF sterilization cycles were performed
and compared to two full RF sterilization cycles. As seen in Table
1, all parameters other than the post-injection plasma power were
maintained as constant as possible from run to run.
1 TABLE 1 LF Run 1 LF Run 2 RF Run 1 RF Run 2 Pre-injection 727 W
779 W 751 W 752 W plasma power First post- 783 W 874 W 757 W 756 W
injection plasma power Second post- 755 W 893 W 758 W 758 W
injection plasma power Chamber temp. 45.degree. C. nom. 45.degree.
C. nom. 45.degree. C. nom. 45.degree. C. nom. Injection
65-75.degree. C. 65-75.degree. C. 65-75.degree. C. 65-75.degree. C.
system temp. H.sub.2O.sub.2 17 mg/l 17 mg/l 17 mg/l 17 mg/l
concentration Chamber 50 Pa 50 Pa 50 Pa 50 Pa pressure during (0.4
Torr) (0.4 Torr) (0.4 Torr) (0.4 Torr) plasma
[0073] Variations in the pre-plasma power were .+-.3.5%, so the
sample temperature was approximately constant from run to run. The
samples were then removed and the residual analysis was
performed.
[0074] The LF sterilizer used to generate the LF plasma was
operated at 60 Hz, and with an inductor of 500 mH and a capacitor
of 13.6 .mu.F. LF plasma power was determined by multiplying the
voltage across the LF plasma by the current, then averaging on an
oscilloscope. The fluctuation level of the LF power was
approximately 10%. Table 2 illustrates the results of the
comparison.
2 TABLE 2 LF Run 1 LF Run 2 RF Run 1 RF Run 2 Average post- 769 W
884 W 757 W 757 W injection plasma power H.sub.2O.sub.2 1973 .+-.
144 1864 .+-. 75 2682 .+-. 317 2510 .+-. 203 residuals (ppm)
[0075] Exposure to a LF post-injection plasma reduced the residual
reactive species more effectively than did exposure to a RF
post-injection plasma of comparable power. LF Run 1 had
approximately 23% less residual hydrogen peroxide than either RF
Run 1 or RF Run 2, even though all had approximately the same
post-injection plasma power. The LF processes therefore resulted in
less residual hydrogen peroxide than did the corresponding RF
process.
[0076] The comparison of the two LF sterilization cycles
illustrates that increased plasma power results in a reduction of
the hydrogen peroxide residuals. Furthermore, the variation between
samples, as indicated by the standard deviation of the residual
measurements, was significantly reduced in the LF process, thereby
indicating an increased uniformity as compared to the RF
process.
[0077] Although described above in connection with particular
embodiments of the present invention, it should be understood the
descriptions of the embodiments are illustrative of the invention
and are not intended to be limiting. Various modifications and
applications may occur to those skilled in the art without
departing from the true spirit and scope of the invention as
defined in the appended claims.
* * * * *