U.S. patent application number 10/164838 was filed with the patent office on 2004-01-01 for system and method of applying energetic ions for sterilization.
Invention is credited to Schmidt, John A..
Application Number | 20040001773 10/164838 |
Document ID | / |
Family ID | 26871572 |
Filed Date | 2004-01-01 |
United States Patent
Application |
20040001773 |
Kind Code |
A1 |
Schmidt, John A. |
January 1, 2004 |
SYSTEM AND METHOD OF APPLYING ENERGETIC IONS FOR STERILIZATION
Abstract
A method of sterilization of a container is provided whereby a
cold plasma is caused to be disposed near a surface to be
sterilized, and the cold plasma is then subjected to a pulsed
voltage differential for producing energized ions in the plasma.
Those energized ions then operate to achieve spore destruction on
the surface to be sterilized. Further, a system for sterilization
of a container which includes a conductive or non-conductive
container, a cold plasma in proximity to the container, and a high
voltage source for delivering a pulsed voltage differential between
an electrode and the container and across the cold plasma, is
provided.
Inventors: |
Schmidt, John A.;
(Princeton, NJ) |
Correspondence
Address: |
WOLFF & SAMSON, P.C.
ONE BOLAND DRIVE
WEST ORANGE
NJ
07052
US
|
Family ID: |
26871572 |
Appl. No.: |
10/164838 |
Filed: |
June 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10164838 |
Jun 7, 2002 |
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09760513 |
Jan 12, 2001 |
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6403029 |
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60175785 |
Jan 12, 2000 |
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Current U.S.
Class: |
422/22 ;
422/1 |
Current CPC
Class: |
A61L 2/14 20130101; A23L
3/26 20130101; B65B 55/04 20130101 |
Class at
Publication: |
422/22 ;
422/1 |
International
Class: |
A61L 002/00 |
Claims
1. A method for sterilization of container surfaces comprising the
steps of: evacuating said container to a desired pressure;
injecting a working gas into said container at said desired
pressure; causing a plasma discharge to be initiated in said
working gas; pulsing a voltage potential across said plasma
discharge operative to produce energetic ions in said plasma; and
accelerating said energetic ions toward said container surfaces
using the pulsed voltage potential; wherein said energetic ions
effect a sterilization of said container by destruction of
microorganisms on surfaces of said container.
2. The method of claim 1, wherein the step of pulsing the voltage
potential comprises applying a voltage potential for a short period
of time.
3. The method of claim 2, wherein the short period of time
comprises 1-50 microseconds.
4. The method for sterilization of claim 1, wherein said plasma
discharge is initiated by a glow discharge technique.
5. The method for sterilization of claim 1, wherein said plasma
discharge is initiated by an RF signal.
6. The method of sterilization of claim 1, wherein said energetic
ions are deposited on said container surface by a capacitive
displacement current.
7. The method for sterilization of claim 1, wherein said applied
voltage is of a magnitude to impart an ion energy on the order of
50 keV.
8. A method for sterilization of interior surfaces of a container
comprising the steps of: forming a plasma discharge in a working
gas disposed within said container; pulsing a voltage potential
between said plasma discharge and an interior surface of said
container, said voltage potential being operative to produce
energetic ions in said plasma; and accelerating said energetic ions
toward the interior surfaces using the voltage potential; wherein
said energetic ions effect a sterilization of said container
surfaces by destruction of microorganisms on said surface
9. The method of claim 8, wherein the step of pulsing the voltage
potential comprises applying a voltage potential for a short period
of time.
10. The method of claim 9, wherein the short period of time
comprises 1-50 microseconds.
11. The method for sterilization of claim 8, wherein said plasma
discharge is initiated by a glow discharge technique.
12. The method for sterilization of claim 8, wherein said plasma
discharge is initiated by an RF signal.
13. The method for sterilization of claim 8, wherein said applied
voltage is of a magnitude to impart an ion energy on the order of
50 keV.
14. An apparatus for sterilization of a container comprising: means
for evacuating said container to a desired pressure; means for
injecting a working gas into said container at said desired
pressure; a first electrode introduced through an aperture of said
container and protruding into an interior portion thereof; a second
electrode established at a surface of said container; and a
modulated power supply connected between said first and said second
electrode and operative to provide a pulsed voltage potential
between said electrodes having a duration of 1-50 microseconds;
wherein a plasma discharge is caused to be initiated in said
injected working gas and said pulsed voltage potential between said
electrodes produces energetic ions in said plasma, said energetic
ions being accelerated toward the surfaces of the container and
effecting sterilization of said container by destruction of
microorganisms on surfaces of said container.
15. The sterilization apparatus of claim 14, wherein a portion of
said container structure is constituted as said second
electrode.
16. The sterilization apparatus of claim 14, wherein said second
electrode is disposed outside of said container and the energetic
ions are deposited on the surface by a capacitive displacement
current.
17. The sterilization apparatus of claim 14, wherein a polarization
of said potential difference between said first and said second
electrode is established to attract ions to an interior surface of
said container.
18. The sterilization apparatus of claim 14, wherein said second
electrode is constituted of a porous material and established
proximate to interior surfaces of said container.
19. The sterilization apparatus of claim 14, wherein said potential
difference between said first and said second electrode is
established at a magnitude to impart an ion energy on the order of
50 keV.
Description
RELATED APPLICATIONS
[0001] The present invention is a continuation-in-part application
of U.S. patent application Ser. No. 09/760,513 filed Jan. 12, 2001,
now U.S. Pat. No. 6,403,029, issued Jun. 11, 2002, which is related
to and claims the benefit of U.S. Provisional Patent Application
Serial No. 60/175,785, filed Jan. 12, 2000, both applications of
which are assigned to the same assignee and incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The invention relates to sterilization processing and
particularly to the use of energetic ions for the sterilization of
surfaces.
BACKGROUND OF THE INVENTION
[0003] In the field of food processing, as well as in other fields,
sterilization to protect against danger from harmful microorganisms
is a critical concern. For the food industry, the sterilization of
containers for food products is particularly important, and
improvements in container sterilization processes can be expected
to have a large economic impact. In the current art, sterilization
of an object may be carried out by subjecting the object to heated
steam pressure, to permeation by a gas such as hydrogen peroxide or
ethylene oxide, and to ionizing radiation, such as a
gamma-rays.
[0004] While steam-pressure sterilization can be effective, plastic
packaging which will withstand the requisite temperature is more
expensive than similar packaging which does not have to withstand
the high-temperature. The penetration depth of high-energy
electromagnetic radiation (e.g., gamma rays) is roughly six orders
of magnitude greater than the size of the microorganism to be
destroyed. Accordingly, high-energy radiation is effective for slow
volume sterilization but inefficient for rapid surface
sterilization. This inefficiency is manifested in long time scales
for surface sterilization. Finally, while UV radiation has the
right penetration depths for surface sterilization, sufficient
intensities are difficult to achieve for providing the desired
destruction rate.
SUMMARY OF THE INVENTION
[0005] Accordingly, it is an object of the invention to provide a
more efficient sterilization process for food-product containers
and the like. To that end, the method of the invention operates to
cause a cold plasma to be disposed near a surface to be sterilized,
and that cold plasma is then subjected to a pulsed voltage
differential for producing energized ions in the plasma that are
directed toward the surface. Those energized ions then operate to
achieve spore destruction on a surface to be sterilized.
[0006] The cold plasma discharge with the parameters needed for the
approach of the invention should be easily produced by a range of
established techniques. The power requirements for the high voltage
pulses are very modest.
[0007] A series of pulses of total duration in the range of one
millisecond should be sufficient for the doses required. With
pulses of this duration the charge buildup on insulating surfaces
should be well within a usable range. If larger doses are required,
additional pulsing can be used. The heating of the container
surface is minimal for the doses required.
BRIEF DESCRIPTION OF FIGURES
[0008] FIG. 1 provides a schematic depiction of the application of
the method of the invention for a container to be sterilized.
DETAILED DESCRIPTION
[0009] An improved methodology is disclosed herein for
sterilization of containers used for packaging contents requiring a
sterile environment. In the preferred embodiment of the invention,
the containers of interest are those used for packaging food
products and the invention will be described in respect to such
food-product containers. It should be understood, however, that the
method of the invention is also applicable to the sterilization of
containers for other products requiring sterile packaging, such as
medical cosmetic and pharmaceutical products.
[0010] A primary object of sterilization for product packaging is,
of course, the destruction of microorganisms that might otherwise
contaminate the product so packaged. Among the more difficult
microorganisms to eliminate during sterilization are bacterial
spore. For the purpose of illustrating the operation and principle
of the method of the invention, the focus of the description
hereafter will be the destruction of such spore. However, the
sterilization method of the invention is expected to be effective
on the full range of microorganisms which may be encountered and
all such applications are intended to be within the scope of the
claimed invention.
[0011] Techniques for spore destruction can be divided into two
categories: (1) techniques that destroy the spore shell to get to
the spore center, and (2) techniques that directly impact the spore
center. At their present state of development the former techniques
take minutes or longer for spore destruction. This is too slow for
production line applications. The latter techniques include heat
and electromagnetic radiation. As explained in the Background
section, both of these techniques suffer from significant
limitations in respect to the sterilization of product packaging
containers. The methodology of the invention overcomes the
limitations of the prior art by directly impacting the spore center
with high energy ions.
[0012] A. Description of the Preferred Embodiment
[0013] As a predicate to the description of the preferred
embodiment, it is to be noted that light ions (e.g., hydrogen)
having energies in the 20-70 keV or greater range have penetration
depths comparable to spore sizes. This short range results in very
high damage coefficients. Modest ion fluences (e.g.
3.multidot.10.sup.-8 coul cm.sup.-2) in this energy range will
provide damage in the Mrad range required for spore destruction.
[The destruction of bacterial spores by energized ions is further
explained by Russell, The Destruction of Bacterial Spores, Academic
Press, 1982, p. 121.]
[0014] According to the method of the invention, a cold plasma is
disposed near a surface to be sterilized and subjected to pulsed
voltage differential with the surface that produces energized ions
in the plasma directed toward the surface. Those energized ions
then operate in a known manner to achieve spore destruction on the
surface to be sterilized.
[0015] A range of approaches (e.g. rf or glow discharge) can be
used to create low density (e.g. 10.sup.8 cm.sup.-3) cold plasmas
near the surface to be sterilized. The surface to be sterilized is
then pulsed to the required voltage (e.g. 50 kV) for a series of
short periods (e.g. 1-50 microseconds), and the resulting ion
deposition will be in the range required. If the surface to be
sterilized is a conductor, that surface may be constituted as one
of the electrodes for the pulsed voltage potential across the
plasma. If, on the other hand, the surface to be sterilized is an
insulator (e.g. plastic) it can be backed by a conductor and the
capacitive displacement current will support the charge densities
required. In the event that a greater charge is required, the
backing conductor can be pulsed with a greater number of pulse
repetitions with a period between pulses to allow the low-density
low-voltage plasma to discharge the insulator surface. Glow
discharge cleaning with the cold plasma discharge could be used to
help clean the surface of coatings over the microbes if
desired.
[0016] FIG. 1 provides a schematic illustration of the application
of the method of the invention to a generic container. The
container (A) is evacuated to a desired pressure (illustratively,
in the 0.1-100 milliTorr range). A gas feed is used to inject a
working gas (e.g. hydrogen) into the container in this pressure
range. A plasma discharge is initiated in the container with known
techniques, such as glow discharge or rf. If a glow discharge is
used, the electrode (B) could be segmented to act as an anode and
cathode, or a low frequency voltage could be induced between the
electrode and the conforming conductor (C) outside the container.
If high frequency rf is used to initiate plasma discharge, the
launcher could be any of a number of known configurations,
including an external coil, the indicated electrodes or alternately
driven segments of the external conductor. After the low density
discharge has been initiated, a high voltage is driven between the
internal electrode (B) and the conforming conductor (C) external to
the container, by a power supply with modulator (D). Note that
conforming conductor (C) may also be placed at or near the inside
surface of the container, but in that case the conductor should be
made porous in order to permit ions attracted thereto to pass
through and reach the container surface. The polarity of the
voltage will be established to attract ions to the container. The
energetic ions will damage the microorganisms on the container
surface.
[0017] In a further embodiment adapted for use with small
containers having open mouths or where difficulty is experienced or
expected in producing the low temperature discharge in a small
space in the container, a more planer discharge could be generated
above and into a group of containers and the ions accelerated into
the container surfaces by the process described above.
[0018] B. Illustrative Operating Characteristics for Invention
[0019] For the purpose of illustrating the operation of the
invention, it is assumed that one megaRad is required to assure
spore destruction. It should be understood, however, that a
somewhat higher dose may be required for some microorganisms. One
Rad is one esu per cc of charge (i.e. 3.3.multidot.10.sup.-10
coul.multidot.cm.sup.-3.multidot.Rad.sup.-1- ). Therefore
3.3.multidot.10.sup.-10 coul.multidot.cm.sup.-3.multidot.Rad.-
sup.-1.times.1.multidot.10.sup.6 Rad=3.3.multidot.10.sup.-4
Coul.multidot.cm.sup.-3 will be the required dose. If the
penetration depth is one micron, then the surface charge will be
1.multidot.10.sup.-4 cm.times.3.3.multidot.10.sup.-4
Coul.multidot.cm.sup.-3=3.3.multidot.10.s- up.-8
Coul.multidot.cm.sup.-2. If each high energy ion produces
approximately 100 ion electron pairs, the surface charge required
is in the 10.sup.-9 Coul.multidot.cm.sup.-2 range. For 50 keV ions
the energy density will be only 1.65.multidot.10.sup.-3
joul.multidot.cm.sup.-2.
[0020] It is noted that the penetration depth of energetic ions
into microbes has some uncertainty because data is not available
for microbes. However, data is available for materials that are
sufficiently similar to support the validity of the process. The
penetration depth in materials is strongly energy dependent and
increases with increasing energy. The penetration depth decreases
with increasing atomic number of the ions used. For this reason
light ions, such as hydrogen, are preferable as a working gas. 50
keV hydrogen ions should be sufficient to penetrate about one
micron for the analog material. In addition, it is believed likely
that spore material is somewhat easier to penetrate.
[0021] As a test case, one can assume a cold plasma discharge with
a hydrogen density of 1.multidot.10.sup.8 cm.sup.-3 and a
temperature of 2 eV. The ion sound speed (c.sub.s) will be about
1.4.multidot.10.sup.6 cm/sec. The current density to an electrode
will be about:
j=enc.sub.s=1.6.multidot.10.sup.-19 coul.times.1.multidot.10.sup.8
cm.sup.-3.times.1.4.multidot.10.sup.6 cm/sec=2.multidot.10.sup.-5
A.multidot.cm.sup.-2
[0022] To achieve 3.3.multidot.10.sup.-8 Coul.cm.sup.-2 will
require a total pulse duration of about 1.5 msec.
[0023] The capacitance of a container surface with a thickness of
approximately 0.5 mm and a dielectric constant of about 2 will
be:
0.5 .mu.F.multidot.m.sup.-2
[0024] The voltage increase of the surface with a charge density of
3.3.multidot.10.sup.-8 Coul.multidot.cm.sup.-2 will be about 700
volts. This voltage drop is small compared to the total voltage
drop of approximately 50 kV as required. If increased charge
densities are required, then more pulses can be used as outlined
above.
[0025] Surface heating by the sterilization method of the invention
is expected to be minimal. The solution to the heat diffusion
equation for a step function heat deposition is an error function
of the variable x(Kt).sup.-1/2, where (K) is the thermal diffusion
coefficient. For simplicity, the penetration depth (h) is estimated
as h (Kt).sup.1/2 for (K) approximately
4.multidot.10.sup.-3.multidot.cm.sup.2.multidot.sec.sup- .-1. The
temperature increase can be estimated as T u/hC where u is the
energy flux and C is the heat capacity of the surface. With u
1.5.multidot.10.sup.-3 J/cm.sup.2 and C 2
J.multidot.cm.sup.-3.multidot.K- .sup.-1 the temperature increase
for a 2-millisecond pulse is about 1/4 centigrade. Thus, there is
considerable temperature margin.
[0026] Conclusion
[0027] A novel method and apparatus for container sterilization has
been described using energetic ions to penetrate and destroy
microorganisms with a range of sizes. In particular these ions
should be able to penetrate spore coatings for spore diameters up
to and greater than a micron. For microorganisms without coatings
these ions are expected to be effective in destroying sizes that
are quite large. If conditions are encountered where the
microorganisms are too large to be destroyed by energetic ions with
a realizable energy, this method of the invention can be used in
conjunction with other sterilization techniques.
[0028] Although the methodology of the invention, and illustrative
applications of that methodology, have been described in detail, it
should be understood that various changes, alterations and
substitutions may be made therein without departing from the spirit
and scope of the invention as defined by the appended claims.
* * * * *