Protein thin films plasma removal: application to biological decontamination

Surgical instruments are intended to come into direct contact with the patients’ tissue and therefore need to be sterilized and decontaminated in order to prevent infections, inflammations and transmission of diseases. In the last years low-pressure plasma discharges have been successfully applied to remove various biomolecules from surfaces. However, the knowledge of the physical-chemical interaction mechanisms between plasma and biomolecules is still rather poor, which is a major limiting factor for the optimization of this type of plasma treatment. In this work an original contribution to the field is presented, either in terms of process development, of physical mechanisms investigation and process diagnostic protocols assessment. Experimental results were obtained with a low pressure double coil planar inductively coupled plasma reactor. Plasma interactions with low bio-contamination levels (less than 5 μg/mm2) typical of biological residuals present after standard hospital sterilization protocols were studied. Removal mechanisms of biological thin films during plasma treatment with oxygen and water vapor containing discharge mixtures were characterized in-situ by means of quartz crystal microbalance (QCM), time resolved mass spectrometry and optical emission spectrometry. A novel QCM measurement technique was developed in this work as a quasi-online diagnostic tool in pulsed plasma operation. After plasma treatment, surfaces analysis techniques (XPS and AFM and profilometry) have been used to investigate ex-situ chemical and morphological changes at the surface of the protein films. Moreover mass removal rates as measured by QCM were found to depend on treatment time, showing a self limiting etching kinetics. Removal rates dynamics has been characterized in different plasma conditions by a set of descriptive parameters and correlated with plasma induced chemical composition changes and morphological modification of the protein film. The interaction mechanism between plasma and protein films have been studied in-situ. In the last years several authors presented experimental investigations devoted to isolate potential agents effective in plasma decontamination (UV, radicals, ions, heat) and to identify possible synergic mechanisms between them, but in most cases particle fluxes have been produced outside plasma environments (beam experiments) or the effect on protein films was studied by physically decoupling the effects of single mechanisms (UV screen, afterglows). In this work experiments were designed to quantitatively measure the fluxes of different potentially sterilizing species in the plasma phase (ions, radicals, UV and heat) and their interaction effects with a model protein film. Particle fluxes have been calculated using data from Langmuir probe, mass spectrometry, optical emission actinometry and infrared pyrometry measurements. Different experiments have been performed using plasma internal parameters (e.g. fluxes) as independent variables for the decontamination treatments, modifying one flux component at time while keeping the others constant the influence of synergetic effects between decontamination agents have been measured. Furthermore the control of the DC bias applied on the sample holder allows changing the energy of the ions (moderate voltages from 10 to 150 V were applied) interacting with the surface. Within the confidence limits of the statistical method implemented for validation, ion assisted chemical etching operating in an ion limited regime proved to be the mechanism which describes more accurately the etching rates for our biological substrates.