Plasma SterilizationApplications of plasmas for sterilization in medical, food processing, ventilating, and air conditioning industries

OutlineWhat is sterilization?Current sterilization meansSolution: Plasma SterilizationHow it worksDisadvantagesMethods 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: HeatTypes: dry and moist heatMedium is exposed to moist heat (steam) generated by an autoclave, or dry heat in a heaterPressures: 103 kPaTemperatures: 120-140 oCSteam transfers sufficient heat to microorganisms to inflict demiseExposure time ~ 30 minutesCan cause permanent damage,and alter material propertiessignificantly

Current Sterilization Means: ChemicalEtO, H2O2, O3, bleach most commonly usedApplications when heat is damaging to the mediumDamages fiber optics, electronics, some plasticsIntroduces toxicity

Current Sterilization Means: IrradiationTypes: Gamma radiation, Bremsstrahlung, X RaysMedium is subjected to radiation radiochemical and radionucleic reactions cellular deathDisadvantages:EmbrittlementChain ScissionCross LinkingCostly

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 SummaryA plasma is a quasi-neutral collection of electrons, positive ions, and neutrals capable of collective behaviorPositive ions = free radicalsPlasma sterilization operates synergistically via three mechanisms:Free radicals interactionsUV/VUV radiative effectsVolatilizationDead microorganisms = sterilization

Plasma Sterilization Mechanics: IR VolatilizationIR 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 processDamaged DNA/RNA microbial death by 4 mechanisms:Apoptosis nucleases become hardwired to shrink and cause cell to commit suicide. Caused by DNA/RNA damageAutophagy Formation of double membrane vacuoles in cytoplasm separation of mitochondria and ribosomes protein production stopped cell deathNecrosis Murder by cell swellingMitotic 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 reactionsO*, OH* highly reactive ~ 10-9 s

Hydrogen Abstraction & Double Bond Cleavage

Plasma Sterilization Mechanics: IR (Nucleic Acid View), UV RadiationUV/VUV radiation causesformation of thymine dimers in DNA, inhibiting bacterial replication.Base damageStrand breaks

Plasma Sterilization Mechanics: IR (Nucleic Acid View), Charged ParticlesCharged 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 SterilizationWeak penetrating power of the plasma species. Complications arise in:Presence of organic residuePackaging materialComplex geometriesBulk sterilization of many devicesSolutions: Introduce preferentially targetting UV/VUV radiation of proper wavelength

Methods of Plasma SterilizationDielectric 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 WDielectric layers allow for plasma discharge to reach material surface

Inductively Coupled Plasmas (ICP)Plasma generated via coils oppositely faced, 13.56 MHz RF sourceMagnetic flux perpendicular to substrate E field envelopes volume of chamberRoughing 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 pressureOxygen formed by interactions at the exit

Microwave (MW) PlasmasGas enters through an inletInteracts with incoming microwaves from a waveguidekW magnetron power supply

Sterilization Efficacy

Sterilization EfficacyMW plasma most effective ( ~ s)All methods < 10 min treatment time (much less than conventional methods!)

Parameters effecting Sterilization and Plasmas

Parameters contdPressure: volatilization rate, EEDF, residence time of active speciesPower: increased power increased electron density. Thermolabile concerns.Frequency: determines EEDFQuantity: loading effectMicrobiological layer: inhibits free radical reaction, requires volatilizationGeometry: complex geometries impede reaction ratesPackaging: plasmas have low penetrability efficacy low

ConclusionsPlasmas accomplish sterilization on order of minutes, or even secondsMedium preservationNo toxicity introducedEconomical

Questions?