Ruhr University BochumInstitute for Electrical Engineeringand Plasma TechnologyProf. Dr.-Ing. Peter AwakowiczUniversitätsstraße 150 Tel: 0234/32-2306244801 Bochum Web: www.aept.rub.de

PlasmaDecon - Low pressure plasma for sterilization: mechanismsand effectiveness

Benjamin Denis, Marcel Fiebrandt, Katharina Stapelmannsteri@aept.ruhr-uni-bochum.de

1 Motivation

Sterilization and decontamination of medical instru-ments, implants, syringes, bottles, and so on in hos-pitals or surgeries is crucial to prevent infections ofpatients or employees during therapy (nosocomial in-fections). These infections are often caused by bacteria,fungi or proteinaceous residuals on materials in con-tact with the patient. Currently, conventional sterilizati-on methods are based on treating the sample with mo-ist heat at 134◦C (autoclave), high energetic electrons(e-beam), gamma rays (γ-sterilization) or toxicants (e.g.ethylene oxide). These methods have several drawbacksand restrictions. The moist heat of an autoclave modi-fies or destroys the structure of high performance po-lymers used in implants or medical instruments andshortens the lifespan of e.g. drills or scalpels. e-beam,γ-sterilization or other radiation-based processes requi-re high safety measures and are very expensive. Ethyle-ne oxide, as a chemical sterilization process, is bannedfrom use since it is highly toxic, mutagenic, explosive,and has a long degas time from porose materials (e.g.polymers in implants).

Additionally, investigations reveal deficits concerningthe removal of protein residuals form surgical instru-ments after routine sterilization [1]. This poses a highrisk to patients with respect to transmission of severeneurodegenerative diseases like Creutzfeldt-Jakob disea-se.

A promising alternative to conventional sterilizationmethods is the use of plasma for sterilization and de-contamination. As plasma generates UV radiation, ra-

Fig. 1: Scanning electron microscopy backscatterimage of surface contamination on the cuttingface of a pair of Metzenbaum scissors [1]

dical species like atomic oxygen, hydrogen or hydroxyl,and ions bombarding the surface, it is capable of deac-tivating and removing bacteria, fungi or proteinaceousresiduals. The combined stress factors lead to a syn-ergistic effect, enhancing the sterilization performan-ce of each single stress. By varying the gas composi-tion, power, or pressure, the plasma can be tuned tobe most effective. Furthermore, it is possible to con-trol the process temperature, making it applicable forheat-sensitive materials. To make plasma sterilizationsave for use, it is necessary to understand how biolo-

PlasmaDecon - Low pressure plasma for sterilization: mechanisms and effec-tiveness

gical systems are influenced, deactivated, and removedby plasma. To get this far, the plasma itself as well asthe biological systems have to be investigated in detail.

Fig. 2: Scanning electron microscopy images of sporeson stainless steel screws [2]

2 Plasma Diagnostics

To understand the plasma itself, several diagnostic sys-tems measuring the different parameters describing theplasma can be used. e.g. Optical Emission Spectrosco-py (OES), Laser Absorption Spectrometry (LAS), Lang-muir Probe (LP), Multipol Resonance Probe(MRP), orMass Spectrometry (MS). Each system has pros andcons, why it is useful measuring the same parameterswith different methods to achieve reliable results.

Fig. 3: Spectrum of Ar/N2/O2 plasma in the DICP (seesection 3) [4]

Optical emission spectroscopy uses spectrometers toinvestigate the radiation emitted by the excited speciesin the plasma. The amount of radiation is determined

by the different plasma parameters, why it is possibleto investigate the plasma without influencing it (non-invasive diagnostic method). [3] [4]

Laser absorption spectrometry uses laser radiationwith a very well defined wavelength absorbed by onlyone specific molecule or atom. By measuring the ab-sorption of the radiation, determination of the densityof species is possible. As power is coupled into the sys-tem, it is possible to change the plasma by measuringit, why it is crucial to control the laser power very care-fully.

The Langmuir probe system measures the currentin a tiny wire applied with a defined voltage in the plas-ma. The current in the wire is caused by electrons andions in the plasma attracted by the applied potential.By changing the potential, one can measure the elec-tron density as well as the velocity or energy of the elec-trons. With this, determination of the electron energydistribution function (EEDF) is possible, showing howmany electrons have which kinetic energy. [3] [4]

The multipole resonance probe emits electromagne-tic waves into the plasma and measures their absorpti-on. The absorption is caused by electrons and the fre-quency of maximum absorption is connected to the elec-tron density, which can be determined investigating theabsorption peak.

Fig. 4: MRP in an argon plasma in the DICP(Source: www.ei.rub.de)

Mass spectrometry measures the mass of ionized par-ticles. By this, one can determine different species whichdo not emit radiation in the range of the OES or can notbe measured by LAS. Furthermore it can serve to checkthe reliability of the measured densities gained by OESand LAS.

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PlasmaDecon - Low pressure plasma for sterilization: mechanisms and effec-tiveness

With this different diagnostic systems, it is possibleto determine the irradiation and fluxes of radicals ontobiological samples. By investigating the different dama-ges caused by the plasma, a study and understanding ofthe deactivation mechanisms can be achieved.

3 Plasma & Plasma ComponentSources

Plasma SourcesTwo plasma systems are used studying the deactivationmechanisms. The first system is a double inductivelycoupled plasma (DICP) and the second a very high fre-quency capacitively coupled plasma (VHF-CCP).

Fig. 5: DICP with opened top

The DICP has a discharge chamber volume of 40 li-ters made from stainless steel making it possible to ge-nerate very clean plasmas with a minimum of impuri-ties. For this reason, the DICP is a reliable system toanalyse the influence of plasma onto biological samp-les. Compared to the VHF-CCP, the DICP has a higherelectron density and a lower electron temperature whyboth system have a different excitation of species in the

plasma.

The VHF-CCP is designed to meet industrial needs.Its discharge chamber is a 4.5 liters drawer made ofPEEK, a high resistant polymer, making it possible toseal the chamber after treatment and use it as sterilecontainer. As a polymer is used as chamber material,the plasma interacts with it why many impurities are inthe plasma making the diagnostic of densities and plas-ma parameters challenging. To achieve a reliable andfast sterilization method, the plasma process is combi-ned with evaporation and condensation of 2 ml - 4 mlhydrogen peroxide before the plasma process. Ignitionof the plasma afterwards removes hydrogen peroxideas well as biological residuals. Furthermore, the highUV and VUV radiation of a plasma inactivates biologi-cal samples that survived the condensation process.

Fig. 6: Front view of the VHF-CCP

Plasma Component SourceTo study the influence of single components of the plas-ma, another system called UV-Heat-setup exists, to si-mulate hydrogen plasma radiation and sample heating.The hydrogen radiation is simulated by a HamamatsuX2D2 R© Deuterium lamp and the sample can be heatedvia a temperature controlled aluminium sample holder.Furthermore different gases can be used, like oxygen,nitrogen, hydrogen, or argon, to understand their influ-ence onto biological systems in combination with tem-perature or UV and VUV radiation.

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PlasmaDecon - Low pressure plasma for sterilization: mechanisms and effec-tiveness

4 Biological Systems

To investigate the influence of plasma onto biologicalsystems, spores, the most resistant form of bacteria, aretreated with plasma. To understand the mechanismsleading to inactivation of spores, the different compo-nents in bacteria, like proteins, amino-acids, and DNAare also treated. By analyzing the induced damages ofeach component, it is possible to identify the most ef-fective parts of a plasma for inactivation. The preparati-on and analysis of biological systems is done in coope-ration with the Institute of Biology of Microorganismsat the Ruhr-Univercity Bochum and with the Instituteof Aerospace Medicine of the German Aerospace Cen-ter (DLR) in Cologne.

Fig. 7: Influence of hydrogen plasma and its com-ponents onto the protein GapDH [5]

Acknowledgement

This work is supported by the German Research Foun-dation (DFG) with the grant PAK 728 “Low pressureplasma for sterilization: mechanisms and effectiveness(PlasmaDecon)“

Literatur

[1] R.L. Baxter, H.C. Baxter, G.A. Campbell, K. Grant,A. Jones, P. Richardson, G. Whittaker, "Quantitativeanalysis of residual protein contamination on repro-cessed surgical instruments", Journal of hospital in-fection 63, pp. 1329, (2006)

[2] K. Stapelmann, M. Fiebrandt, M. Raguse, P. Awa-kowicz, G. Reitz, R. Moeller, “Utilization of Low-Pressure Plasma to Inactivate Bacterial Spores onStainless Steel Screws", Astrobiology 13, 7, (2013)

[3] N. Bibinov, H. Halfmann, P. Awakowicz, "Determi-nation of the electron energy distribution functionvia optical emission spectroscopy and a LangmuirProbe in an ICP", Plasma Sources Science and Tech-nology 17, (2008)

[4] B. Denis, S. Steves, E. Semmler, N. Bibinov, W. No-vak, P. Awakowicz, "Plasma Sterilization of Pharma-ceutical Products: From Basics to Production", Plas-ma Processes and Polymers, 9, pp. 619-629, (2012)

[5] K. Stapelmann, J. Lackmann, I. Buerger, J. Ban-dow, P. Awakowicz, “A H2 very high frequency capa-citively coupled plasma inactivates glyceraldehyde 3-phosphate dehydrogenase (GapDH) more efficient-ly than UV photons and heat combined", Journal ofPhysics D: Applied Physics, 47, (2014)

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