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Plasma sterilization
Meeting VUMC 17-03-2009
Dr. Ariël de Graaf, [email protected]
Contents
• Sterilization applications
• Common sterilization methods
• Plasma sterilization
• What is a plasma?
• Plasma sterilization agents
• Advantages & drawbacks plasma sterilization
• Commercial plasma sterilization systems
• Experiments endoscope sterilization
• Conclusions
Sterilization applications
• Medical
• Surgical tools
• In vivo diagnostic tools (e.g. endoscopes)
• Packaging material (i.e. bottles, sterile bags)
• Human tissue (e.g. surgeon’s hands)
• Surgery room (e.g. tables, floors, etc.)
• Food processing
• Bacteria on surface of foods (e.g. fruits and meats)
• Toxic fungal contamination (e.g. in seeds, grains and spices)
• Food packaging and conservation
• Biological warfare defence
• Anthrax spores and smallpox virus
• Nerve and blister agents
Sterilization applications
• Public health
• Water disinfection
• Air purification (e.g. against “sick building syndrome”)
• Processing industry of natural materials (e.g. Bacillus spores
in raw cotton and fabric manufacture)
• Space
• Decontamination of life-searching probes on planetary
missions
Common sterilization methods
• Sterilization by heat (autoclave)
• Moist or dry heat (120-170°C, 20-60 min.)
• Not applicable for temperature-sensitive materials
• Sterilization by reactive gases
• Mostly used are ethylene oxide, formaldehyde, ozone and
hydrogen-peroxide
• Concerns about toxicity and carcinogenicity of residual gas
• Sterilization by membrane filtration
• For liquids sensitive to heat, chemical interaction or radiation
• Pore size: < 0.2 µm for bacteria, < 20 nm for viruses
• Prions (proteinaceous infectious particles) cannot be removed
• Recycling (i.e. cleaning) of filters is cumbersome
Other sterilization methods
• Sterilization by radiation
• UV light: applicable for UV resistant surfaces and UV
transparent objects
• Gamma rays: good shielding of hazardous radioactive
radiation is necessary
• X-rays: long exposure times required
• Electromagnetic fields: microwave, RF
• Sterilization by electron beams
• Plasma sterilization
• Plasma-assisted sterilization: germicidal liquids and gases
(such as H2O2 and aldehydes) in combination with plasma
• Plasma-based sterilization: gases that become biocidal only
when a plasma is ignited (e.g. air, He/air, He/O2, O2/N2)
What is a plasma?
• Plasma is an ionized gas consisting of free electrons, ions,
reactive atoms, neutral molecules and photons
H2O (s)
H2O (l)
H2O (g)
Solid
Liquid
Gas
H,H2,H+,e-,H2H2O O,O ,O ,O-,O 2
3
2
Plasma
Add energy
• The plasma state can be reached by supplying sufficient energy
(heat or electric power) to a gas or mixture of gases
• Plasmas can be operated both at low and atmospheric pressure
What is a plasma?
eH 2O
+
H
Plasma
O2
e-
H-
O
OH-
H2
OH
+ eO
H 2O
H
H 2O
+
H 2 O2
O+
H2-
O2 -
O3 (ozone)
H+
photon UV/VIS
OH+
Plasma sterilization agents
• Heat
• Sample will receive more heat in direct exposure mode as
compared to remote exposure
• However, temperature increase due to plasma is usually too
low (<100 °C) to have a significant effect
• UV radiation
• In the plasma UV photons are generated (wavelength
depends on plasma species)
• 200-300 nm radiation causes damage to the cell’s DNA and
RNA, inhibiting the bacteria to replicate properly
• Charged particles
• Plasma contains both ions and electrons
• Charging of cell membrane may cause disruption
Plasma sterilization agents
• Reactive species
• Dissociation of molecules and plasma chemistry leads to
reactive species (depending on plasma composition)
• Air plasma: O, O2*, O3, OH, NO, NO2, etc.
• Radicals chemically attack the walls, coats, and membranes
of cells of microorganisms
• Radicals may even remove killed bacteria and their debris
(particularly pyrogens)
Advantages plasma sterilization
• Plasma is more reactive than normal gas or liquid phase
chemistry due to presence of free radicals
• Plasma combines a number of important sterilization agents
• UV light
• Reactive species
• Electric fields and charge
• In some occasions also heat
• This will lead to lower sterilization times as compared to standard
techniques (several seconds instead of minutes to hours)
• Complete removal of organic material due to plasma reactivity!
• Low sample temperatures for temperature-sensitive tools
• No solvents  no toxic residue or hazardous emission
• Hazardous radiation is avoided
• Operation could be very cheap (when operating on air)
Drawbacks plasma sterilization
• Reactive plasma species may also attack instrument materials
• Material degradation due to UV exposure
• Plasma can not enter autoclave bags or non-permeable materials
 plasma must be created from the inside
• Plasma does not enter very narrow crevices and fissures
 requires new plasma techniques
Commercial plasma sterilization systems
• Based on H2O2 plasma
• H2O2 sterilization followed by plasma treatment
• Sterilization of materials sensitive to heat and
humidity
• Gentler cleaning than steam or peracetic acid
• No toxic residue or hazardous emission
• Sterrad, Stericool, Steriplas
• Based on peracetic acid (C2H4O3) plasma
• Plazlyte (not sold anymore due to medical incident)
Commercial plasma sterilization systems
• Plasma-label for food conservation (“PlasEt”
by JE PlasmaConsult)
• Plasma created inside packaging via antenna
on label
• Few minutes plasma generation
• Ozone generation
• Label can be equipped with sensors
Feasibility study – Endoscope sterilization
Without plasma
• Endoscopes have several narrow (down to 0.5mm diameter)
channels for insertion of instruments
• Sterilization of these narrow channels is cumbersome
• Idea: create a plasma inside the narrow channels  plasma will
remove bacteria
25 cm
First experiments with PTFE model system
Two concepts
Sandwich
Axial wire
Experimental results
Number of living spores after
treatment [#]
• Removal of Geobacillus Stearophillus spores
• 97% spores removed within 5 minutes
• Settings not yet optimized
1,0E+07
1,0E+06
1,0E+05
1,0E+04
0
2
4
6
8
10
Treatment time [min]
12
14
16
Experimental results
• Removal of Geobacillus Stearophillus spores and Enterococcus
hirea (ATCC 8043) vegetative cells
Geobacillus
ATCC 8043
1,00E+07
untreated
axial wire
1,00E+06
1,00E+05
sandwich
CFU
1,00E+04
1,00E+03
axial wire
1,00E+02
1,00E+01
1,00E+00
1
2
3
4
5
6
7
8
9
10
11
Conclusions
• There is sufficient evidence that plasma sterilization works, and
that dead bacteria and pyrogenic debris are removed as well
• Plasma allows sterilization of heat- and humidity-sensitive
materials without toxic residue or hazardous emission
• Most important sterilizing agents have been identified
• Possibly a combination of techniques (e.g. liquid, UV and plasma
treatment) will be the best choice
• Sterilization of crevices and fissures with plasma requires more
research, but seems not impossible
• The resistance of the sterilized materials to plasma exposure
should also be examined