Plasma Sterilization
Applications of plasmas for
sterilization in medical, food
processing, ventilating, and air
conditioning industries
Outline
What is sterilization?
 Current sterilization means
 Solution: Plasma Sterilization
 How it works
 Disadvantages
 Methods of Plasma Sterilization

What is Sterilization?

Sterilization is any process or procedure
designed to entirely eliminate microorganisms
from a material or medium
Current Sterilization Means: Heat







Types: dry and moist heat
Medium is exposed to moist heat (steam) generated
by an autoclave, or dry heat in a heater
Pressures: 103 kPa
Temperatures: 120-140 oC
Steam transfers sufficient heat to
microorganisms to inflict demise
Exposure time ~ 30 minutes
Can cause permanent damage,
and alter material properties
significantly
Current Sterilization Means: Chemical




EtO, H2O2, O3, bleach most
commonly used
Applications when heat is
damaging to the medium
Damages fiber optics,
electronics, some plastics
Introduces toxicity
Current Sterilization Means: Irradiation



Types: Gamma radiation, Bremsstrahlung, X Rays
Medium is subjected to radiation  radiochemical
and radionucleic reactions  cellular death
Disadvantages:
 Embrittlement
 Chain Scission
 Cross Linking
 Costly
Current Sterilization Means: Plasmas?
Yes.
 Plasmas are currently employed in many
industries to accomplish both highly effective,
and delicate sterilization.
 Not future technology! Plasmas are used today!
 But, how do they work?

Plasma Sterilization in Summary



A plasma is a quasi-neutral collection of electrons,
positive ions, and neutrals capable of collective
behavior
Positive ions = free radicals
Plasma sterilization operates synergistically via three
mechanisms:
 Free radicals interactions
 UV/VUV radiative effects
 Volatilization

Dead microorganisms = sterilization
Plasma Sterilization Mechanics: IR Volatilization


IR is able to vaporize microbiological matter, causing
physical destruction of spores.
Charged particles react with cellular chemical bonds of
microbiological layer to form gaseous compounds 
volatile compounds.
Plasma Sterilization Mechanics: Ionizing Radiation (IR)


IR (UV/VUV radiation) can damage DNA/RNA, chemical
cellular bonds, and induces free radicals to perpetuate the
process
Damaged DNA/RNA  microbial death by 4 mechanisms:




Apoptosis – nucleases become hardwired to shrink and cause cell to
commit suicide. Caused by DNA/RNA damage
Autophagy – Formation of double membrane vacuoles in cytoplasm 
separation of mitochondria and ribosomes  protein production
stopped  cell death
Necrosis – Murder by cell swelling
Mitotic Catastrophe – radiation causes mis-segregation of chromosomes,
leading to Apoptosis.
Plasma Sterilization Mechanics: IR (Cellular View)

IR impacts the cell, three outcomes can result.
Plasma Sterilization Mechanics: IR (Chemical View)
Free radicals O* and OH* play crucial role in
microorganism destruction by way of chemical
reactions
 O*, OH* highly reactive ~ 10-9 s

Hydrogen Abstraction & Double Bond Cleavage
Plasma Sterilization Mechanics: IR (Nucleic Acid View), UV Radiation

UV/VUV radiation causes
 formation of thymine dimers in DNA, inhibiting bacterial
replication.
 Base damage
 Strand breaks
Plasma Sterilization Mechanics: IR (Nucleic Acid View), Charged Particles


Charged species in the plasma can damage DNA if formed in
the vicinity of chromatin.
RSH act as radical scavengers
Quantifying Sterilization Efficacy


i.e. Time required for the microbial population to be reduced to one decimal
Disadvantages of Plasma Sterilization

Weak penetrating power of the plasma species.
Complications arise in:
 Presence
of organic residue
 Packaging material
 Complex geometries
 Bulk sterilization of many devices

Solutions: Introduce preferentially targetting
UV/VUV radiation of proper wavelength
Methods of Plasma Sterilization
Dielectric Discharge Barrier (DBD)
 Inductively Coupled Plasmas (ICP)
 Atmospheric Pressure Plasma Jet (AAPJ)
 Microwave (MW) Plasmas

Dielectric Discharge Barrier (DBD)


High AC voltage (1.2 kV), atmospheric pressure, 200-300 W
Dielectric layers allow for plasma discharge to reach material surface
Inductively Coupled Plasmas (ICP)



Plasma generated via coils oppositely faced, 13.56 MHz RF source
Magnetic flux perpendicular to substrate  E field envelopes volume of
chamber
Roughing pump needed
Atmospheric Pressure Plasma Jet (AAPJ)



RF coupled capacitive discharge  neutral, cold effluent with
high concentrations of reactive species and UV/VUV radiation.
Atmospheric pressure
Oxygen formed by interactions at the exit
Microwave (MW) Plasmas



Gas enters through an inlet
Interacts with incoming microwaves from a waveguide
kW magnetron power supply
Sterilization Efficacy
Sterilization Efficacy
MW plasma most effective ( ~ s)
 All methods < 10 min treatment time (much less
than conventional methods!)

Parameters effecting Sterilization and Plasmas
Parameters cont’d







Pressure: volatilization rate, EEDF, residence time of active species
Power: increased power  increased electron density. Thermolabile
concerns.
Frequency: determines EEDF
Quantity: loading effect
Microbiological layer: inhibits free radical reaction, requires volatilization
Geometry: complex geometries impede reaction rates
Packaging: plasmas have low penetrability  efficacy low
Conclusions
Plasmas accomplish sterilization on order of
minutes, or even seconds
 Medium preservation
 No toxicity introduced
 Economical

Questions?