The healing touch of a micro-plasma

Development of a low-power electric discharge for fine medical treatment

Dr. ir. E. Stoffels Dr ir W.W. Stoffels

Elementary Processes in Gas Discharges

Department of Physics,

Eindhoven University of Technology,

PO Box 513, 5600 MB Eindhoven.

Tel: (040) 2475753

Fax: (040) 2465060

e-mail: e.stoffels.adamowicz@tue.nl

Abstract

Plasmas are reactive media, which can be generated by electric discharges in gases. Unlike thermal (equilibrium) plasmas, non-equilibrium plasmas produce no heat overload. Non-equilibrium plasmas can perform unique, refined treatment of a solid state surface, and therefore they are indispensable in modern material science, microelectronics and semiconductor technology. They are generated in large-area reactors, at low pressure and high power input. In this project, we propose a novel, biomedical application of non-equilibrium plasmas: fine surgery on living tissues. A suitable plasma source must operate at atmospheric pressure, and at low power input in order to eliminate any thermal or electrical damage to the biological material. It is a physical challenge to develop such a source and to unravel its interactions with tissues. Low-power atmospheric plasmas can be obtained by miniaturisation of large-area discharges; the resulting micro-discharges are used in modern TV display technology. However, to make such plasmas suitable for medical applications, new methods of plasma operation must be introduced.

The impact of non-equilibrium plasmas on organic materials is an entirely new research area. The behaviour of the plasma in the vicinity of liquids, the penetration of active plasmas species (ions, radicals and photons) into the organic matter and the response of the tissues on the cellular level belong to the most intriguing questions.

Due to its unique physical nature, the micro-plasma ("plasma needle") has the potential to become a novel medical technique: it will act on the tissue surface, with high precision and minimum penetration, and provide active species for refined cell modification. Such a plasma is flexible: it could penetrate difficult to reach places like veins, intestines and dental cavities. It can be also confined in catheters. Micro-plasmas may become powerful tools for a variety of fine surgical techniques: removal of unwanted cells/tissues, cure of skin ailments, restoration of bones/tooth enamel, cleaning of dental cavities. In particular, we will investigate plasma-induced modification of artery walls in treatment of cardiovascular diseases (atherosclerosis). This project serves to prepare the physical basis for a new development in the health care.

Research programme

State of the art plasma science

Plasmas occur commonly in nature, assuming the forms of fire, lightning or stars. In laboratory conditions, plasmas can be created by electric discharges in gases. These media contain free electrons and ions, various active species (excited atoms/molecules, radicals) and energetic UV photons. Free electrons gain energy from the electric field and heat up, reaching typically very high temperatures (10.000 – 100.000 K). These electrons perform ionisation of the gas, which is essential for sustaining the plasma. Electrons are responsible for the unique plasma chemistry: not only do they produce positive and negative ions, but also excite/dissociate neutral species, which yields active radicals, excited atoms/molecules and photons. In plasmas operated under atmospheric pressure, electrons often heat the neutral gas to high temperatures. In these thermal plasmas electrons are in thermal equilibrium with other species, typically at a temperature of a few thousands of degrees. However, in some situations electrons are unable to efficiently convey kinetic energy to other particles. This results in non-equilibrium systems, in which electron temperature is much higher than the temperature of ions and neutrals. Non-equilibrium plasmas can be sustained at room temperature, so one can profit from their reactivity and special chemistry without risk of thermal damage.

Non-equilibrium plasmas have a unique impact on solid-state surfaces, so they are ideal for refined material processing. Modern microelectronics and material technology entirely depend on selective plasma etching, cleaning and thin layer deposition. Plasma processing offers the unique facility to produce extremely sophisticated materials, like semiconductor elements with surface structure dimensions below 1 mm, solar cells, various hard coatings, including diamond layers and composite materials. In most applications in material processing, the desired non-equilibrium plasma is obtained by drastically lowering the pressure, to below 0.001 atmosphere. Naturally, these plasmas are not suitable for treatment of biological material, in particular for tissue processing in vivo. Therefore, in spite of great potential, plasma techniques are not yet well-established in biology and medical sciences.

Plasmas for biomedical applications

For medical purposes, plasmas must operate at atmospheric pressures. Atmospheric discharges typically require high voltages and power densities, and are often associated with a risk of electric shock or thermal damage. Plasma techniques are occasionally used for sterilisation and coating of medical components [1], but one is cautious to apply them in vivo. However, it has been already demonstrated that even thermal plasmas can be applied safely on living tissues. In the so-called argon plasma coagulation (APC) technique tissues are exposed to a high-frequency gas discharge in argon, as shown in Figure 1 [2]. In this method there is no contact between the device's electrodes and the tissue, so in spite of high voltages and powers the treatment is safe. The electrodes can be mounted on a well-insulated flexible catheter and applied internally. The result of plasma action is local, superficial coagulation and desiccation of the tissue, which finds many applications in healing of various lesions and ulcers of skin, intestines and larynx. Plasma treatment has many advantages over other wound treatment techniques, like laser or electric coagulation. The plasma operates strictly on the tissue surface (penetration depth is limited), no carbonisation, vaporisation, perforation and haemorrhaging occurs, no special safety precautions are required and the post-operative recovery is generally good. Although it is believed that coagulation is due to the heat produced by the plasma, the action of ionised species on the tissue may contribute to the effectiveness of the treatment.

In the sterilisation and coagulation techniques mentioned above, action of the plasma is not refined. Thermal atmospheric plasmas involved in these methods can perform (local) destruction and/or sterilisation of the tissue, but are incapable of sophisticated surface modification. The latter can be possibly achieved when low-power, cold non-equilibrium plasmas are applied. Since tissue processing with thermal plasmas can be performed harmlessly, the safety of medical treatment using non-equilibrium plasmas need not be questioned. If a suitable plasma source for fine surgery becomes available, plasma technology promises to have the same revolutionary impact on medical sciences as it had on material science.

Figure 1: High-frequency (HF) argon discharge used for superficial coagulation [1]. This electrode configuration (dimensions – about 5 mm) resembles the coaxial geometry discussed further (see Fig. 3b). It consumes about 60 Watts of power. Mounted on a catheter and applied internally, it is used in gastroenterology and endoscopy.

Recently, possibilities have emerged to create low-power, cold non-equilibrium plasmas at atmospheric pressure. At the moment the best known example is a micro-discharge, i.e. a plasma constricted to the size of 10 - 100 mm by using very small electrodes [3]. Although the power density in these plasmas is of the same order as in larger discharges, the total consumed power is very low (in the order of mW) due to a small volume. Moreover, since gas heating in plasmas is a volume effect, micro-discharges remain cold. These micro-plasmas, arranged in an array, are investigated for applications in modern display technology, to obtain improved TV screens. Another possibility of obtaining a non-equilibrium plasma is limiting the plasma operation time to microsecond range (i.e. by pulsing the plasma power supply). In this case electrons in the plasma have enough time to gain energy and create active species, but not to heat the ambient gas. Of course, the combination of both methods can be applied: a pulsed micro-discharge (or micro-discharges' array) is a very promising plasma source for biomedical purposes.

Problems and tasks

The aim of this project is to lay the basis for novel plasma technology in biomedical sciences. Two major issues will be addressed: the physical aspects of generating a micro-discharge and the biological aspects of tissue treatment using non-equilibrium plasmas. The physical task is to fully understand and control a (pulsed) micro-discharge operating at atmospheric pressure and low voltages and powers. These discharges have been barely investigated, so there are many questions related to their stability and the possible range of operating conditions. Since only the active plasma species (radicals, ions and photons) are responsible for the results of treatment, it is essential that their production remains sufficient also at low power levels.

Another barely explored, yet fundamentally challenging class of problems is related to the interaction of plasmas with (living) tissues. Since plasma is essentially a gaseous medium, while tissues contain aqueous solutions, the contact between these objects must be investigated. Much research effort has been devoted to plasma-solid surface interactions, and a large expertise on plasma etching and deposition is present at the TUE. However, the data on the plasma/liquid interface are scarce. Interesting issues are the distribution of the electric field in the vicinity and under the (liquid) surface, the response of charge carriers (positive and negative ions) present in the aqueous solution, and the penetration of the plasma species (ions, electrons, radicals and photons) into the liquid. To obtain a complete picture of plasma-liquid interactions it is important to study the influence of liquids on the gaseous plasma medium: evaporation of the fluid, changes in plasma composition (including densities of active species), and attendant variations in electric plasma parameters.

From the point of view of medical applications, the most crucial question is the actual chemical and biological impact of plasmas on organic matter. When the micro-plasma, or the "plasma needle" becomes available and well controlled, the most difficult task will be tackled: the processing of real tissues. In plasma processing of solid-state material the action of a plasma is versatile. Dependent on the chosen plasma type and conditions, many different results like selective cleaning/etching, surface activation and deposition of thin layers can be achieved. In processing of organic matter and tissue, the additional complexity is introduced because of the hitherto unknown biochemical response of the treated object. Primarily, atmospheric plasmas operated in air will be applied. It is expected that the basic action is mild surface cleaning and etching due to active oxygen radicals. Efficiency and selectivity with respect to various kinds of material (lean muscle, fat, bone, etc.) should be investigated. It is also important to understand the role of various active plasma species. Radicals, ions and (UV) photons will have different impacts on organic molecules.

The above mentioned problems are related to the plasma-tissue interactions at the molecular level: the removal of organic material, and chemical modification of the remaining tissue. The next step is the understanding and classification of various biological implications of plasma treatment. Plasma impact on tissues at the cellular level is an entirely new research area. Here not only the direct results, but also the long-term post-treatment phenomena and eventually the response of the living organism must be monitored. Although at this stage not all plasma-induced effects are easy to predict, several types of cell modification can possibly be achieved. The simplest and most drastic one is the cell destruction, which has been already demonstrated in plasma-assisted coagulation and sterilisation techniques [1,2]. This action can be compared with non-selective etching in solid-state processing, and this was also the first stage of plasma usage in the history of material science. If the analogy between inorganic and organic material processing holds, one can expect much more sophisticated plasma-tissue interactions than the mere cell destruction. At this stage it is essential to establish the "area of action" of the plasma. One has to deal with questions like:

- can plasma treatment be restricted to one cell/layer of cells, without affecting the rest of the tissue?

- can the membrane be locally removed/modified?

- can the cell interior be modified selectively without affecting the membrane?

Resolving these problems is a condition for the refined treatment of living cells in the possible future biomedical plasma technology. Fine modification of the cell membrane can alter its permeability for a given kind of chemicals. It can increase the sticking of other cells at the top of the processed tissue, or otherwise, make it more inert. Modification of the cell nuclei can "sterilise", or deactivate the cell, i.e. prevent the multiplication without killing the cell. Like in the traditional surface processing, a large versatility is expected.

In this project all these possibilities will be investigated thanks to the joint expertise of plasma physicists and medical researchers. Expertise of the group of Prof. F. Ramaekers from the UM, specialised in molecular cell biology is indispensable in the diagnostics of plasma-treated cells and in the interpretation of treatment results. Vital cells can be monitored for prolonged periods of time, and the induction of apoptosis (programmed cell death) or cell cycle changes can be diagnosed by specific assays. More medical expertise will be provided by the medical promoter Prof. Daemen (UM), whose group conducts leading research in cardiovascular biology. A particular problem related to the treatment of cardiovascular diseases will be addressed. This is due to the major medical interest in exploring new possibilities to cure the number one malady of the modern western society. The common disease atherosclerosis [4] affects mainly the coronary arteries. The inner wall of the artery (endothelium) absorbs fatty matters (LDL, low-density lipoproteins) from the blood and transmits it to the smooth muscle layer. This initiates uncontrolled growth of tissue, and further absorption of lipids, leukocytes and debris from the blood (see Fig. 2). Finally, the disorder results in constriction of the artery, which at present can be only mechanically restored. More details on the disease are given in Section 9 (Medical relevance). We propose to use plasma treatment for the deactivation of cells constituting the endothelium, in order to prevent the growth of plaque. The method of testing the plasma applicability to cure atherosclerosis is described below in Section 8.4.

Figure 2: An artery affected by atherosclerosis: left – initial phase (formation of "fatty-streaks", i.e. accumulation of debris under endothelium), right: advanced disorder, in which the debris is covered by a (growing) tissue and the vessel becomes gradually constricted. From R. Ross [4].

Methods

Investigations on the biomedical applicability of micro-plasmas will proceed in four phases: a) the development and characterisation of the source, b) physical description of plasma-liquid interactions, c) resolving biological consequences of the tissue treatment and finally d) clinical implementation of the new technique.

Development and characterisation

In the initial phase several methods of excitation and geometric designs must be tested. Excitation using DC, radio-frequency (RF) or microwave generators yields plasmas with different properties. DC plasmas at atmospheric pressures are rather difficult to tame. These are so-called corona discharges, with relatively high breakdown voltages (kV range) and unpredictable spatio-temporal behaviour. However, short (sub-microsecond) duration of corona discharges and their high chemical activity can make them useful for biomedical applications. Research on corona discharges is presently conducted in the plasma physics group at the TUE, so both the equipment and the expertise are available. RF excitation (e.g. at 13.56 MHz) is a common means of plasma generation, and several types of RF plasmas can be obtained. Small-size RF plasmas can be operated at low voltages and power inputs of only a few Watt. Microwave (in the range of several GHz) excitation will be also investigated. In these plasmas the breakdown voltages are lower than in case of RF, but they produce relatively more heat than RF discharges. All mentioned plasmas will be operated in the pulsed mode with a varying pulse length and duty cycle. Geometric design of the electrode must suit the plasma type as well as the specific medical applications planned in the latter phases. Some basic designs are shown in Figure 3. In principle, an electric discharge is generated between two electrodes. In order to create a micro-plasma, pin-like metal electrodes can be used. A pin-to-pin discharge (Fig. 3a) may prove inconvenient in some applications, because of its rather poor spatial confinement. In this case one can choose for a spatially better constricted coaxial design (Fig. 3b). In the latter geometry only the downstream plasma zone (i.e. plasma species which leave the tube) will be used. The downstream zone is very suitable for medical purposes, because no electric fields are present there. However, the density of active species may be lower than in the active zone (between the electrodes), so it must be verified whether such a source retains sufficient activity for treatment. In case of RF or microwave excitation there are more possibilities. Such plasmas can be created using only one electrode, powered by the generator, while the counter-electrode is virtual: it consists of the grounded surrounding. These plasmas are very flexible. If necessary, they can be confined in catheters (like in Fig. 3c) and used for internal treatment of a living tissue. A special property of an RF/microwave excitation is that it can propagate through a (solid-state) insulator. In this dielectric barrier discharge (Fig. 3d) a layer of dielectric separates the powered metal electrode from the plasma zone. These discharges are characterised by a good stability, low breakdown voltages and very low currents.

Regardless of the type of excitation (DC, AC, RF or microwave), the aim of the initial phase is to miniaturise the plasma dimensions to the sub-millimetre range. Keeping in mind the future medical applications, the constructed object should have the properties of a "plasma needle": it should be compact, with no heavy generator and casing. It must be portable and convenient to operate at any institute/hospital, without special infrastructure required.

Figure 3a: The simplest pin-to-pin discharge configuration. The electrodes are connected to the power supply (+/- in DC excitation, or RF and ground in RF excitation). The dimensions of the plasma zone (the electrode gap) can be reduced to sub-millimetre range.

Figure 3b: A coaxial configuration (longitudinal view): the powered electrode is a pin, and it is inserted in a metal cylinder (counter-electrode). The active discharge, in which the actual electric power is coupled, is confined in the tube. Species produced in the active zone diffuse outside (downstream zone).

Figure 3c: Plasma confined in a catheter. It is most convenient to apply RF excitation to the pin electrode, with a virtual counter-electrode (grounded surrounding). Plasma is flexible and it adapts to the shape of the catheter (typically up to a few millimetres in diameter).

Figure 3d: A dielectric barrier discharge (RF/microwave excitation). The powered electrode is covered by a layer of insulator. The grounded electrode may be virtual.

After construction of various micro-plasmas, the plasma physical phase will commence in order to obtain a good understanding of the source. Several configurations mentioned above will be tested and characterised in terms of basic plasma properties. This will be performed using the wide selection of diagnostic methods, available at the TUE. The simplest diagnostics will involve measurements of plasma voltage and current, in order to establish the range of electric parameters in which the discharge operation is stable. In order to determine the thermal load of the plasma, gas temperatures and heat fluxes will be determined using thermocouples, inserted both in the active plasma and in the downstream zone. This will be performed in collaboration with the plasma physics researchers at the Greifswald University (Germany), who perform similar measurements in their investigation of non-equilibrium atmospheric discharges. Since the functionality of the plasma is conditioned by the concentration of active species, like oxygen radicals, plasma composition will be monitored. This can be performed using mass spectrometry and optical emission spectroscopy. The latter technique, in combination with elaborate collisional-radiative models developed at the TUE, provides valuable information about the electron temperature (energy distribution), densities of excited species and radiative properties of the plasma (emission of UV and visible radiation). Since pulsed discharges are subject to investigation, time-dependent aspects in experiments and modelling will be stressed. Fast recording of the optical emission will require special electronics, like fast digital oscilloscopes and possibly the single photon counting technique. The selected diagnostics and numerical modelling will allow determining features, which are essential for plasma activity.

Physical aspects of plasma-tissue interactions

The next step is the investigation of plasma interactions with liquid surfaces. The constructed micro-plasma source will be brought into contact with various liquids (pure water, aqueous solutions containing salts and/or organic molecules, emulsions with oils, etc.) and eventually with plant and animal tissues. Gas phase monitoring during the processing will involve the plasma techniques mentioned above. In addition, plasma influence on the liquid phase will be investigated. Here on-line measurements of electric conductivity of the treated object can be performed. This yields information about the in-situ variations in the concentration of charge carriers in the liquid phase (tissue), and about its chemical composition. For example, in case of oil-containing emulsions one can study the efficiency and selectivity of etching of fat with respect to other constituents. Apart from on-line monitoring, ex-situ analysis of the processed liquid will be performed using standard chemical methods (e.g. liquid chromatography, available both at the TUE and UM). It is important to detect and identify the products of plasma-liquid interactions, which remain in the liquid phase.

Biological consequences of tissue treatment

In the subsequent biological phase the research will concentrate on unravelling the impact of plasma treatment on the tissue at the cellular level. Most of the expertise and diagnostic methods involved here will be provided by the groups from the UM. Most insight in the behaviour of the cells can be obtained by vital imaging (in-situ) microscopy. The group of Prof. D.W. Slaaf (TUE/UM) specialises in this technique, so the measurements will be performed there with the necessary equipment and assistance from the biomedical specialists. The group of Prof. Ramaekers and Prof. Daemen (UM) will deliver organic samples, like bacteria, tissues, isolated blood vessels and cells in culture, which will be treated using the portable "plasma needle" from the TUE.

Next to standard microscopic monitoring, we will apply fluorescence microscopy, in which the observed object is illuminated by an external light source (or merely by the plasma light) and its fluorescence is recorded. The objects will be stained using vital fluorescent dyes, which have affinity to specific pathways or chemical constituents of the cell. It will serve to visualise selectively various cell components, and to study the behaviour of the cell. Recording the fluorescence of the cells will also allow to study photochemical reactions in the tissue, induced by the plasma radiation (e.g. UV photons).

NWO has awarded a new facility of TPLSM (two-photon lifetime scanning microscopy) to the core unit vital imaging at the UM (prof Ramaekers and Prof. Slaaf). This unit will become available within the next half year. The advantage of TPLSM with respect to conventional fluorescence microscopy is that it analyzes the life time of fluorescence, which allows us to monitor a thin layer (less than 1 mm) of tissue (area up to 200 mm in diameter) under the plasma operation place and to study the changes in various tissue structures as a function of time. With the support of Prof. Ramaekers, valuable information will be obtained about the consequences of plasma treatment at the cellular level: modification of DNA, RNA and specific proteins, cell apoptosis (programmed death) and cell cycle changes, which will be monitored and quantified by flow cytometric methods.

In addition to in-situ microscopic monitoring, a variety of post-treatment tests will be performed at the UM in the group of Prof. Daemen. Immunohistochemistry will be employed for the analysis of the cell genetic material ("southern blot" and "northern blot" techniques) and attendant changes in the protein structure ("western blot"). In this phase of the research it will be verified whether specific parts or molecular components of the cell can be removed or modified selectively by manipulating the plasma conditions, gas composition (e.g. by using air, nitrogen or noble gas discharges) and duration of the treatment. One of the desired effects would be inactivation of the cell nucleus to prevent cell multiplication, without altering other functionality. During the whole biochemical phase of the project, much attention will be paid to the safety aspects of plasma processing in order to eliminate any thermal or electrical damage to the tissue. When the total harmlessness of plasma operation is warranted, the novel plasma device will be prepared for the final test in the clinical conditions.

Clinical implementation

The knowledge on plasma-tissue interactions gained in the physical and biological studies is needed to perform the last, clinical phase of the project. This phase will proceed under strict supervision of the specialists in cardiovascular biology, belonging to the group of Prof. M. Daemen from the UM. We have chosen to tackle a very challenging problem: the cure of atherosclerosis. This is a serious and complex disorder, and if plasma treatment succeeds, it will have a great impact on modern medicine.

The aim of plasma treatment is to deactivate the inner wall of a blood vessel (endothelium). When the endothelium as well as smooth muscle cells are deprived of their ability to split, the undesired tissue growth can be suppressed. Selective removal of fatty matters helps to diminish the plaque, and the plasma-produced nitrogen oxide (NO) has a beneficial influence on the artery wall. Since coronary arteries are difficult to operate, and must be probed only by catheters, one can make use of the flexible nature of the "plasma needle". For this application, the plasma will be confined in the catheter filled with gaseous medium. In order to operate in the watery environment of a blood vessel's interior, a micro-perforated catheter will be inserted into the blood vessel. The walls of this special catheter are provided with openings of 1 – 100 micrometers in diameter. This allows free diffusion of plasma species produced by the discharge in the catheter to the artery wall, but prevents mixing of gaseous and liquid phase. This is an example of the safest, downstream plasma treatment, in which the treated object profits from active plasma species while being separated from the active plasma zone. The artery treatment will be initially performed on isolated blood vessels, and as a conclusion of the project it will be implemented in vivo, using test animals (rats and goats). The long-term consequences of the plasma exposure will be studied by standard methods in pathology, like monitoring the circulation in the arteries using contrast fluids and with X-ray cameras. It is essential to determine whether the plasma treatment improves the structure of the blood vessel wall, reduces plaque formation and prevents new tissue growth, and whether the effects are long lasting.

In the course of the project two PhD students will be assigned. First two phases (development of the source, characterisation of the plasma and studying plasma-liquid interactions) will be performed by a student with physical background, while the biological and chemical consequences of the plasma processing of organic material require a candidate with a biomedical profile.

Potential medical relevance

Twenty years ago plasma technology introduced a breakthrough in the material science. The efforts invested in plasma research produced enormous amounts of knowledge about plasma-material processing. The next, logical development is to utilise and expand this knowledge in a new research area. Plasma technology is now ready to initiate a new direction in medical sciences. This project serves to prepare the physical background for innovative development in health care. Non-equilibrium plasmas are unique objects that combine high reactivity with non-destructive character. The most important feature is that plasmas act specifically on the tissue surface, with a minimal penetration depth, and provide active species (radicals and ions) for specific cell modification. This results in a non-contact medical treatment, which is local, fast, requires no toxic substances, causes no heating damage and produces no debris. Plasma-based tools cannot perform bulk surgery, but they may be extremely useful in therapies, which require refined action on the outer layers of the tissue. Plasma treatment is essentially different from the currently used laser surgery, because laser radiation penetrates into the tissue, causes heating and cell destruction. Due to flexibility of plasmas, which can be easily confined within catheters, minimum-invasive treatment of difficult to reach places, like blood vessels and intestines can be performed. One can think of numerous processes, like removal, (de)activation, and changing the functionality of cells/tissues. This may find applications in the treatment of various skin ailments, hair growth restoration by activation of hair bulbs, etc. Plasma technology also promises a breakthrough in dentistry, as a novel method of cleaning dental cavities. Unlike the traditional mechanical cleaning (drilling), plasma treatment is expected to be painless and far less destructive (no heating and vibrations which irritate the nerve, and no fractures). Due to action of active oxygen radicals from the plasma the decayed organic matter can be selectively removed. Plasmas can easily penetrate cavities (or root channels in root treatment) and act strictly on the surface, so that high-precision cleaning can be performed without excessive removal of (healthy) pulp. Apart from plasma applications in external skin/tooth healing, numerous plasma techniques can be introduced in internal medicine, e.g. to induce cell death in cancer cells. This project concentrates on plasma applications in cardiology, because of its high relevance for human health in the modern society.

Cardiovascular diseases are the major death cause in the United States and Europe. Constriction of arteries due to accumulation of plaque (atherosclerosis) is a common disorder, related to disturbances in the blood circulation system, high blood levels of cholesterol, etc. Advanced atherosclerosis results in insufficient oxygen supply to the cardiac muscle, which leads to its failure (myocardial infarct). Contemporary treatment of atherosclerosis is based on mechanical expansion of the diseased artery, by inflating it with a balloon (so-called Dotter operation). However, this causes even more damage to the artery and enhances the growth of scar tissue. A Dotter operation is in most cases followed by restenosis, or secondary vessel constriction that proceeds even faster than the normal course of plaque accumulation. To slow down the restenosis, a network of thin metallic wires (stent) is pressed into the artery's inner wall during a Dotter treatment. Although stents provide some support, the response of the damaged artery wall to an externally inserted object can also lead to uncontrolled scar tissue production. Clearly, healing of atherosclerosis solely by mechanical methods is of temporary nature, because it does not change the functionality of the sick cells in the artery. The latter can be achieved by plasma modification of these cells, in order to reduce their capability of reproducing. If plasma treatment can prevent restenosis, it will make stents superfluous and guarantee that the circulation improvement after Dotter operations be lasting.

It must be stressed that plasma tools for medical purposes will consume very little power, so they will be absolutely safe and in addition, cheap. They will be made compact in design and convenient to use. This means that plasma surgery can be performed everywhere, and not only by medical specialists. Possibly, plasma tools can be introduced for consumers' daily use, e.g. for skin treatment and small cosmetic operations.

Concluding, biomedical plasma technology is an unexplored area with a great potential in medical sciences. It is expected that more and more plasma-based medical techniques will emerge in the course of the project. Besides, development of a "plasma needle" is of general interest, also outside the medical world. A simple, cheap and convenient tool for fine surface treatment will surely find applications in material processing and electronics.

Infrastructure

Plasma physics group "Elementary Processes in Gas Discharges", led by Prof. dr. ir. G.M.W. Kroesen at the TUE, is specialised in characterisation of plasmas used for fine surface treatment. Therefore, a large expertise on plasma as well as surface diagnostics is present, and various diagnostic tools are operational. For the development of the plasma source a function generator (with variable frequency in radio-frequency range), an RF amplifier and a matching network must be purchased, as well as a microwave generator. Methods for the physical characterisation of the plasma source, mentioned in Section 8.4 are available: mass spectrometer for detection of neutral species and ions, optical multichannel analyser, various monochromators and photomultipliers for spectral analysis of the plasma light, electric probes for plasma voltage/current measurements. Small apparatus like a pH meter, or conductivity meter for studying the influence of plasma on the liquid phase will be bought in addition.

In the group of Prof. Slaaf at the UM, various types of microscopes are present. There is also a facility to perform fluorescence microscopy and, in the near future, TPLSM (two-photon lifetime scanning microscopy). The group of Prof. Ramaekers provides the tissue and cell culture samples, and techniques to analyse them at the cellular level. Prof. Daemen has also access to laboratory techniques for biological/genetic tissue analysis, and the full infrastructure to investigate circulation in arteries, by using catheters and X-ray monitoring.

References

[1] Special issue on Non-thermal Medical/Biological Treatments Using Electromagnetic Fields and Ionised Gases, IEEE Transactions on Plasma Science 28(1) (2000). (plasma cleaning/sterilisation)

[2] J.D. Waye, "Argon plasma coagulator: should everyone have one?", Surgical Technology International VII, p. 1 (1998); M.J. Sessler, H.-D. Becker, I. Flesch, K.-E. Grund, "Therapeutic effects of argon plasma coagulation on small malignant gastrointestinal tumors", J. Cancer. Res. Clin. Oncol. 121, p. 235 (1995);

[3] G.J.M. Hagelaar, "Modelling of microdischarges for display technology", PhD thesis, Eindhoven University of Technology, 2000.

[4] R. Ross, "Atherosclerosis – an inflammatory disease", The New England Journal of Medicine, 340(2), p. 115 (1999).