GB2459461A - A non-thermal microwave plasma sterilisation system using automatic tuning - Google Patents

Abstract

This invention relates to a system and method of generating non-thermal plasma using microwave energy and a single or combination of gases for sterilisation of biological tissue and other surfaces where an automatically adjustable tuning mechanism contained inside the applicator or antenna is used to enable the plasma to be effectively struck, maintained, and matched into the treatment tissue or surface to ensure optimal sterilisation effect. A system of introducing controlled levels of moisture into the applicator to assist with the production of hydroxyl radicals is also disclosed. The system comprises: a waveguide antenna structure containing a section of waveguide 280, tuning elements 250, 260, 270, an actuator to control the position of the tuning elements within the waveguide 240, a means of coupling microwave energy into the waveguide 160, 210, 220, and a means of coupling gas into the waveguide 170, 230. The rest of the system consists of a controlled source of modulated microwave energy 10, 20, 30, 40, 50, 60, 70, a means of measuring forward going and reflected microwave energy 80, 90, a means of detecting magnitude and/or phase information 100, a suitable gas or gas mixture, and a means of introducing the gas into the antenna 130, 150, 200, a means controlling the operation of the system, and a user interface unit 120. Apparatus for producing controllable non-thermal plasma comprising a controllable microwave energy source, an antenna, means for automatically matching the microwave energy with antenna structure whereby the control system is integrated into the antenna or hand-piece and is implemented using a combination of analogue components is also claimed.

Description

A NON-THERMAL MICROWAVE PLASMA STERILISATION

SYSTEM USING AUTOMATIC TUNING CONTAINED

WITHIN THE HAND-PIECE OF THE APPLICATOR

FIELD OF INVENTION

This invention relates to a system and method of generating non-thermal plasma using microwave energy and a single or combination of gases for sterilisation of biological tissue and other surfaces where an automatically adjustable tuning mechanism contained inside the applicator or antenna is used to enable the plasma to be effectively struck, maintained, and matched into the treatment surface to ensure optimal sterilisation effect or bacteria kill. A system of introducing controlled levels of moisture into the applicator to assist with the production of hydroxyl radicals is also disclosed.

OVERALL DESCRIPTION OF THE INVENTION

The current invention is concerned with a system or device that uses microwave energy and a single or combination of gases to destroy or treat certain bacteria and/or viruses and/or fungi associated with the human or animal biological system and/or the surrounding environment. In the invention presented here, an automatically controlled tuning mechanism is contained inside the applicator or antenna arrangement used to create the plasma, and the distal end of the plasma applicator is in close contact with the tissue or surface to ensure that the plasma energy is efficiently coupled to the surface being treated to enhance the efficacy of the system. This arrangement also ensures that the microwave energy is efficiently converted into clinically useful plasma energy.

Plasma typically contains charged electrons and ions as well as chemically active species, such as ozone, nitrous oxides, and hydroxyl radicals. Hydroxyl radicals are far more effective at oxidizing pollutants in the air than ozone and are several times more germicidal and fungicidal than chlorine, which makes them very interesting in terms of destroying mold, bacteria and viruses.

Sterilisation is an act or process that destroys or eliminates all form of life, especially micro-organisms. During the process of plasma sterilisation, active agents are produced. These active agents are high intensity ultraviolet photons and free radicals, which are atoms or assemblies of atoms with chemically unpaired electrons. An attractive feature of plasma sterilisation is that it is possible to achieve sterilisation at relatively low temperatures, i.e. around 37°C.

Plasma sterilisation also has the benefit that it is safe to the operator and the patient.

In a room temperature environment, plasma is usually supported by electro-magnetic (EM) fields. Light electrons absorb energy from an electric field and transfer part of this energy to heavy particles in the plasma. If electrons are not given sufficient opportunity to transfer their energy, heavier plasma components remain at much lower temperatures than the electrons. Such plasmas are called non-thermal plasma and their gas temperatures can be as low as room temperature.

Active plasma particles (electrons, ions, radicals, and other chemically active species) and UV radiation may be used to disinfect and sterilise living tissue, biological inserts placed inside living tissue, external surfaces, or surgical instruments.

The closer the plasma source is located with respect to the living tissue (or other surfaces) and the higher the electric field in the plasma, the higher the intensity and efficacy of the non-thermal plasma sterilisation treatment process.

Low temperature atmospheric pressure plasmas may be used to replace conventional sterilisation methods and offer clear advantage over existing means of sterilisation in terms of their non-toxic nature, instant treatment effects, and the ability to produce the plasma at a range of energy levels and in a range of different forms, i.e. plasma from a wand shape antenna structure, plasma from a flat bed type antenna structure.

The current invention makes use of modulated controllable non-ionising microwave radiation and an inert gas, or a mixture of inert gases, to produce atmospheric plasma (or conducting gas). The invention may use a mixture of inert gases, e.g. air and helium, argon and helium.

It has been discovered through practical experimentation that a clinically useful gas mixture to use is helium and compressed air (produced by a small electric compressed air generator). It has also been discovered that a solid state microwave source, developed to produce microwave power up to 300W within the frequency band of between 850MHZ and 925MHZ, with the ability to enable this frequency band to be swept, and the ability to be modulated from less than 1Hz to greater than ioKHz, can be used to produce clinically useful plasma when used with an applicator designed using two co-axial voltage transformers and fed with a helium and compressed air mix. It has also been discovered that an applicator designed to operate at 900MHZ produces optimal plasma when the source frequency is set somewhere between 862MHZ and 866MHz, and the modulation frequency is set somewhere between 10Hz and 1KHz at 20% duty cycle. This work is also reported in our patent application GB0721714.4, filed on 6th November 2007.

This invention may be used to significantly reduce levels of bacteria without whipping it out completely. There are advantages of using the invention in this way to prevent the eradication of natural flora.

The treatment system introduced here makes use of non-ionising radiation generated using a source oscillator to produce a low power microwave frequency signal, and a power amplifier comprising of an arrangement of microwave transistors to amplifr the low power signal to a level that is high enough to provide an electric field to be produced which is required to strike and maintain a plasma using an inert gas (or mixture of inert gases) found to be suitable for the particular intended application, and a tuning arrangement contained within the applicator or hand-piece to enable the plasma to be matched with the treatment surface and to optimise the microwave energy to plasma energy conversion efficiency. The invention makes use of solid state signal amplifiers, but this invention is not limited to using this particular technology to amplify the low power signal.

The invention makes use of at least one signal modulator and a means of controllably adjusting the microwave power level.

In this specification the frequency spectrum used is between iGHz and 6o GHz.

Specific frequencies that have been considered to be useful in our work are: 900MHZ, 2.45GHZ, 3.3GHZ, 5.2GHZ, 10GHz, 14.5GHz and 24GHz.

The system described here enables the dosage of plasma energy delivered into the biological tissue or a particular surface to be accurately quantified.

The system provides automatic compensation of microwave energy mismatch caused by changing the position of the applicator with respect to the treatment surface or the change in the characteristics of the treatment surface or treatment site.

The impedance of the non-conducting gas and that of the conducting gas (plasma) changes significantly and so the system ensures that the energy produced by the microwave generator is efficiently matched to these two impedances without loss in microwave energy due to the power being reflected back towards the generator.

The gases that are of interest for implementation of the current invention are: air, helium, argon, nitrogen, compressed air, and carbon dioxide, but this invention is not limited to using these particular gases. This invention is not limited to using only one inert gas, for example, various concentration of argon, air and helium may be used, i.e. i% air and 99% helium.

The plasma should not harm or damage healthy tissue structures or surfaces. The plasma jet will be directed to the treatment or sterilisation site in such a manner that it targets the bacteria, fungi, or virus of interest. The plasma produced at the distal end of the applicator will be automatically adapted to the environment or changes within the environment it is being used to treat, i.e. it will adapt to changes in the distance between the applicator and the treatment surface or a change occurring within the antenna or applicator structure.

The invention overcomes the effects of insertion loss (cable loss) and phase variations associated with a microwave cable assembly, which is a drawback for systems with the tuning network housed with the generator electronics. By incorporating the tuning mechanism in the hand-piece it should be possible to match the plasma energy into any load impedance. The current invention may also enable the dynamic tuning mechanism to react quicker than if the tuning mechanism was located inside the generator or some other remote location away from the applicator.

BACKGROUND

Bacteria are single-celled organisms that are found almost everywhere, exist in large numbers and are capable of dividing and multiplying rapidly. Most bacteria are harmless, but there are three harmful groups; namely: cocci, spirilla, and bacilla. The cocci bacteria are round cells, the spirilla bacteria are coil-shaped cells, and the bacilli bacteria are rod-shaped. The harmful bacteria cause diseases such as tetanus and typhoid. The current invention may be used to reduce or kill these bacteria.

Viruses can only live and multiply by taking over other cells, i.e. they cannot survive on their own. Viruses cause diseases such as colds, flue, mumps and AIDS. The current invention may be used to treat or kill viruses.

Fungal spores and tiny organisms called protozoa can cause illness. The current invention may be used to treat or kill viruses.

For practical use inside or on the surface of the body, i.e. for wound sterilisation to kill bacteria within the wound or bacteria that resides on the surface of the wound, the reduction of bacteria contained within natural orifices inside the body, and other surfaces that require sterilisation where it is undesirable for the temperature to rise excessively, e.g. for beds, curtains, pillows and certain plastics, the temperature at the surface or at the treatment site (the biological tissue or external environment) produced by the plasma should not exceed normal human body temperature. It may be desirable to consider the maximum temperature at the surface produced by the plasma jet to be limited to a maximum of io°C above room temperature, i.e. Tr �= T �= (Tr + io°C), where Tr is the room temperature (°C), and Tt is the treatment temperature (°C). A nominal temperature for non-thermal plasma is 37°C. The length of the plasma and the temperature produced at the tissue (or other) surface may be found using the energy balance equation, i.e. electron-induced heating of heavy particles versus energy losses by thermal conduction. The length of the plasma jet (L) may be calculated using equation 1 given below: L= / m3KT (i) mCVINDKTE m Atomic mass K1 = Thermal Conductivity = Effective electron-atom Collision frequency ND = Electron density = Temperature Change TE = Energy Level

ARGON CARBON-HELIUM NITROGE

DIOXIDE N

Thermal 16.2 14.5 146.10 24.3 Conductivity Wm-1 K-' Effective Electron-6.3117*107 3.22*109 2.27*108 1.497*108 Atom Collision Frequency Atomic Mass 39.948 44.01 4.002602 14.0067 Kg Table 1: Parameters for Calculation of Plasma Length, T= 300K and Pressure 1 Torr

PARAMETER VALUE

Electron Density 3.22*10 rn-3 Electron Mass 9.1O9*1031 Kg Boltzmanne 1.380622*1023 Constant Energy Level 3 eV Temperature ioK Change Table 2: Other parameters used in the Calculation

ARGON CARBON-HELIUM NITROGEN

DIOXIDE

Plasma Length (m) 5.35*104 7.438*105 8.478*1o4 4.337*10.4 Table: Calculated Plasma Length at T = 300K and Pressure = 1 Torr Tables 1 to 3 shown on the previous page provide values for the parameters associated with equation 1, and give a set of calculated plasma lengths for the gases that have been identified as being of interest for this work.

The non-thermal plasma can be used to create highly reactive molecules called free radicals that can be used to break down contaminants. Free radicals are atoms or molecules that have unpaired electrons.

The non-thermal plasma may use the high electric fields to create large quantities of highly reactive free radicals.

Cell Inactivation factors: Ultraviolet photons affect the cells of bacteria by inducing the formation of thymine dimers in the DNA. This inhibits the ability of the bacteria to replicate properly -this affect may be particularly useful in the application of treating sexually transmitted diseases where it may be desirable to reduce the level of bacteria, but not totally destroy it.

It has been recognised that reactive species play an important role. It has been shown that discharges containing oxygen have a strong germicidal effect.

The invention may be used to address the following applications: hospital ward sterilisation, wound bed sterilisation, hospital equipment (beds, curtains, instruments, clip-boards, pens) sterilisation, internal tissue or biological insert sterilisation, treatment of sexually transmitted diseases, treatment of ulcers or bed sores to kill bacteria, treatment of water, treatment of airborne germs or viruses, treatment of athletes foot, or treatment of alopecia aerate. This invention is not limited to these applications.

ESSENTIAL FEATURES

The plasma sterilisation system described here may be used to kill or reduce bacteria, viruses, parasites and fungal spores.

The current invention makes use of a controllable microwave energy source that may be modulated, a single or mixture of inert gases, and an applicator or antenna with a tuning mechanism contained within to create atmospheric plasma that can be used to treat or destroy bacteria, viruses or infections. The tuning mechanism contained within the applicator may be set up to provide one specific impedance or may be set up to provide automatic impedance adjustment to enable the plasma energy to be matched with the impedance of the tissue (or other surfaces or regions that maybe of interest).

In this invention, the power level is adjustable in a controlled manner, and the microwave energy can be modulated in a controlled manner using at least one modulator or means of modulation.

The invention also draws upon the availability of moisture either through the environment where the plasma is being generated (applications within the body) or by introducing the moisture into the applicator through external means, e.g. steam produced by vaporising a vessel of water. The introduction of moisture maybe used to enable hydroxyl radicals to be produced, which are known to be effective for killing bacteria or fungi.

In order to be able to produce useful hydroxyl radicals, it is necessary to have three components available: ozone, ultraviolet (UV) germicidal irradiation, and moisture (humidity). When ozone, UV and moisture are combined, powerful oxidants, which are significantly more powerful than ozone alone, are produced.

This invention may enable these three components to be produced and combined to generate useful hydroxyl radicals.

The current invention makes use of controlled microwave energy to enable controlled and focussed UV to be generated, which, in turn, may be used to create the desired hydroxyl radicals (OH).

As such, a first aspect of the current invention is a controllable microwave energy source and an applicator or antenna structure with a tuning network contained therein, and a single or mixture of gases to enable active plasma to be produced that is useful for killing bacteria or fungi or viruses.

A second aspect of the current invention is an arrangement that enables said tuning network to be automatically adjusted to ensure the plasma can be efficiently struck, maintained, and matched or coupled to the treatment surface (biological tissue or external surface) or the treatment site when a change in impedance takes place due to the physical manipulation of the applicator with respect to the tissue or changes in the characteristics of the site occurs due to the plasma treatment process. This tuning mechanism may consist of an arrangement of mechanical tuning stubs contained inside the applicator or antenna, an electromechanical actuator to move the stubs, a means of measuring the impedance or the forward and reflected energy inside the applicator, and a controller that takes the sampled forward and reflected signals and provides the necessary control signals to the actuator necessary to allow the tuning stubs to be moved into a position to create the necessary impedance to strike, sustain or match the plasma. The tuning mechanism may consist of an arrangement of mechanical rods (or stubs), semiconductor power varactor diodes or PIN diodes.

In the case of the diodes, a control voltage is used to change the characteristic of the diode, which, in turn, creates or sets up the necessary resonant or tuned condition.

A third aspect of the current invention is a system that introduces microwave energy, a single or mixture of gases, and moisture inside the applicator or antenna in such a manner that the plasma produced contains hydroxyl radicals and these radicals are used to effectively treat the surface or site and provide a solution with the necessary level of efficacy to enable the solution to be adopted by the NHS and the medical community throughout the world.

Solutions are presented for the automatic tuning mechanism contained within the hand-piece. One solution contains an analogue signal processor designed using operational amplifiers, and a second solution uses a digital microprocessor to process the information obtained from the sampled forward and reflected power signals to control an electromechanical actuator.

The analogue processing solution presented in this specification may be integrated into the hand-piece along with the tuning stub (s) and actuator (s) to produce a self contained unit.

The method of providing automatic tuning introduced here enables simple waveguide applicator structures to be used to form the antennae since the high impedance condition necessary to strike the plasma, the low impedance condition necessary to maintain the plasma, and a variant condition needed to maximise the match into the tissue, are all set up by automatically varying tuning elements contained within a waveguide cavity. This may overcome the need to use complex applicator or antenna structures. The preferred applicator structures are cylindrical or rectangular waveguide, but this inventioon is not limited to using these shapes. The waveguide may be loaded with a dielectric or magnetic, or dielectric and magnetic materials in order to reduce the size of the structure.

Co-axial applicator structures may also be considered, where the automatic tuning mechanism is used to vary the diameter or length of the structure to provide the tuned condition. It may also be possible to introduce tuning stubs inside the co-axial structure by introducing them through the wall of the outer conductor into the dielectric material that separates the inner and outer conductors. It may also be desirable to introduce the tuning stubs into the outer wall of an applicator design that contains a single or plurality of resonator or impedance transformation sections, i.e. quarter wave impedance transformers, and vary the lengths of the stubs inside the co-axial structure to vary the EM field set-up within the structure to assist in obtaining the desired condition necessary to create optimal plasma for sterilisation.

The applicator may take the form of a plurality of waveguide or co-axial arrangements fed from a single microwave source via a power splitter or, individually fed, to form a useful applicator that can be used to treat large surface areas The applicator(s) may be arranged to emit plasma (ionising radiation) and/or microwave (non-ionising) radiation. The same microwave source may be used to create both the plasma and microwave energy or different sources may be used.

The same automatic tuning mechanism introduced here may be used to match both the microwave energy and the plasma into the treatment surface or biological tissue. This arrangement may be useful for sterilisation of mattresses and pillows in hospital wards where the bacteria may manifest beneath the surface as well as on the surface. In this instance, plasma may be used to destroy the bacteria on the surface and the microwave energy may be used to destroy the bacteria beneath the surface, for example, 2mm to 2omm beneath the surface.

In the physical implementation of this invention, the jets of plasma may be emitted through a plurality of nozzles disposed along a tube containing the necessary impedance transformations and an automatic tuning mechanism. This arrangement may enable a plasma comb or brush to be realised, which could be useful for treating dermatitis or alopecia.

DETAILED DESCRIPTION

Features of the invention are now explained in the detailed description of examples of the invention given below with reference to the accompanying drawings, in which: Figure i shows a complete system for generating non-thermal plasma whereby an automatic tuning mechanism is contained within in the applicator; Figure 2 shows an arrangement where one stub is used to vary the impedance within the waveguide applicator to enable plasma to be struck and maintained.

The control of the stub is based on the signal obtained from a loop sensor which is also contained within the applicator; Figure 3 gives a circuit for the analogue controller used to detect the signal produced by the sensor contained within the waveguide and enable the stub position to be changed in accordance with this signal; Figure 4 shows a mechanical tuning arrangement, whereby three fixed tuning stubs are used to create the desired impedance within the waveguide applicator to enable plasma to be created.

Quartz tubes or quartz slices may be used inside the waveguide structure for the purpose of intensifying or modifying the electric field. It is preferable to use a low loss quartz material. It has also been shown that the plasma may be struck and maintained without the use of a quartz tube (the quartz tube is not shown in these figures).

A separate supply of steam or moisture may be introduced inside the waveguide cavity to help assist in the generation of hydroxyl radicals (OH). The moisture may also be introduced by external means, e.g. through moist tissue or an external generator located in close proximity to the distal end of the waveguide applicator where the plasma is emitted (the inclusion of an additional tube to introduce the moisture into the antenna is also not shown in the figures given here).

As already identified, it may be desirable, and indeed beneficial, in terms of system performance to incorporate the tuning mechanism inside the hand-piece for implementation of the proposed microwave plasma generation system for sterilisation for treating or killing a range of bacteria, viruses or fungi.

One advantage of locating the tuning elements inside the hand-piece is that a high Q' resonant cavity can be formed using the tuning network and a waveguide cavity, and this high Q' network is particularly well suited for generating the high electric field required to strike' the plasma. It can then be maintained and adapted by varying the tuning elements in accordance with the forward and/or reflected power measurement signals that can be used to determine the state of the plasma plume and the treatment site.

The current invention overcomes problems associated with locating the tuning elements inside a generator, where the microwave energy is delivered to the antenna or applicator using a cable assembly. In such an arrangement, it is necessary to generate high fields inside the cable and the attenuation or loss associated with the cable limits the ability of the tuning network in terms of being able to match into any impedance load.

It is known that the Q' of a resonant cavity is defined as the ratio of the energy stored in the cavity to the energy dissipated or lost within the cavity (energy stored/energy dissipated). It is desirable to minimise the energy dissipated in order to maximise the Q' of the cavity, hence maximise the electric field.

Dissipated energy manifests itself in the form of losses in the microwave structure, for example, cable loss, waveguide cavity loss and connector loss. Any loss due to the waveguide cavity may be attributed to the length of the cavity and the conductor loss, hence it is desirable to plate the inner walls of a waveguide with a low loss conductor to ensure best performance, e.g. the walls of the cavity may be silver platted.

In the instance where tuning stubs or tuning rods are used, it is desirable to minimise the conductor loss associated with these elements, thus it is preferable to manufacture the tuning rods from materials that exhibit a high conductivity, for example, copper, brass or silver. It is also preferable to minimise the conductor loss of the waveguide cavity. In a particular embodiment, it may be preferable to silver plate both the tuning rods and the inner walls of the waveguide cavity.

A system for performing automatic tuning to strike, maintain and match the plasma using three tuning stubs and a means of adjusting the tuning stubs contained within an applicator (or hand-piece) is shown in figure 1. In the arrangement given, an electromechanical actuator 240 is used to move the position of the three tuning stubs 250, 260, 270 within waveguide cavity 280.

The length of the three stubs is determined by control signals produced by microprocessor unit no contained within the generator and control system 2000. The control signal sent to electromechanical actuator 240 is based on the manipulation of signals measured at the coupled ports of forward and reflected powers couplers 8o and 90 respectively. In practice, it may be necessary to measure the reflected power only in order to establish the condition required to produce the high electric field within the cavity that is necessary to strike the plasma, to determine the condition to sustain the plasma, and to match it to the varying state of the surface or tissue of which the plasma is coupled. The signals from reflected power coupler 90 and forward power coupler 8o are fed into a detector (or receiver) ioo whose function is to convert the microwave signal into a format that is acceptable for the microprocessor no to use. This signal may be a DC voltage, or a lower frequency signal that contains phase and magnitude information. The DC voltage or the phase and magnitude signals are processed using the microprocessor unit 110 to determine the signals that need to be sent to the electromechanical actuator 240 to move the three tuning stubs 250, 260, 270 to the position necessary to strike or maintain the plasma. The detector ioo may take the form of a diode detector with a low pass filter (for example, a tunnel diode, or a Schottky diode and a simple single pole C-R filter), or a heterodyne detector (or a homodyne detector) using a microwave frequency mixer and local oscillator signal. It may be preferable to implement the heterodyne detector (or a homodyne detector) using more than one frequency mixing down stage, i.e. a double IF heterodyne receiver may be employed that uses two microwave frequency mixers and two local oscillators.

The microwave power level produced by the amplifier chain A1 (40) and A2 (50) is controlled using a PIN diode attenuator A1 (30), and the system may be switched on and off (or modulated) using PIN switch S1 (20). The power level controller 30 may be a PIN diode attenuator that may be a reflective or absorptive type. The low power input signal is produced by source oscillator 10, which is preferably a low power microwave source oscillator, i.e. a device capable of producing power levels of greater than -2OdBm to less than 2OdBm. The oscillator io should produce a well controlled single frequency signal that can be adjusted over a narrow band of frequencies, i.e. it may have a centre frequency of 900MHz that can be adjusted between 850MHZ and 950 MHz. The source oscillator to may be a voltage controlled oscillator (VCO), a dielectric resonator oscillator (DRO), a gunn diode oscillator or a similar device that is capable of producing a controllable low power microwave signal. A frequency synthesiser that comprises of a plurality of VCOs or DROs may also be used.

The power amplifier comprising of first and second stages 40, 50 respectively may use the following semiconductor devices: high frequency bipolar junction transistors (BJTs), heterostructure bipolar transistors (HBTs), metal oxide semiconductor field effect transistors (MOSFETs), or metal semiconductor transistors (MESFETs). In terms of the semiconductor materials that may be used, of particular interest is gallium arsenide (GaAs) and gallium nitride (GaN).

For example, a recently reported single-ended amplifier using small packaged GaN FETs has demonstrated a record output power at 2.14GHz. The amplifier was composed of paralleled 48 mm gate periphery FET die, delivering a peak saturated output power of 371 W with a linear gain of 11.2 dB at a drain voltage of V. The output power density (output power/package size) of 1.1 W/mm2is said to be twice as high as that of existing GaAs PET amplifiers.

GaN FETs offer a higher DC power to microwave power efficiency over GaAs FETs. This feature is of particular interest when developing a plasma system that is capable of providing high power plasma energy since heating effects caused by the DC power loss are reduced, which increases the portability of the system and simplifies the thermal design issues that need to be overcome when developing the system.

For applications of the system that relate to hospital ward sterilisation or other applications where a patient is not directly involved with the plasma treatment, it may be required to create plasma that can cover a large surface area, hence a plurality of, e.g. 500, jets may be required to implement a clinically useful system. For example, it may be necessary to cover a section of the floor of a hospital ward, or to sterilise a mattress contained within a hospital bed, that may be infected with the MRSA virus. In such embodiments of the invention, it may be desirable to use an array of plasma plumes generated using arrangements comprising of a plurality of co-axial transformer based resonators similar to those identified elsewhere by MicroOncology for treating the other clinical applications identified above, but the source of microwave power may, this time, be derived from a higher power microwave energy generating device such as a magnetron or a klystron, travelling wave tube (TWT), twystron (hybrid combination of a klystron driver and TWT output section in tandem in the same envelope), or a gyrotron. It is more difficult to control the level of power produced by these devices than it is when using semiconductor devices, but this may not be a problem when the plasma produced by the device is not in direct contact with patient tissue. For example, pulsed power levels in excess of 10 mega watts (MW) have been obtained using the twystron and multicavity klystrons and this may be acceptable for treating a MRSA or VRSA infected hospital ward floor.

For implementation of a semiconductor based system, it is desirable to be able to switch the main device power supplies (drain supply in PETs and the collector supply in BJTs) off during periods when it is not required to produce microwave power, i.e. when the switch contact of first modulator switch 20 is in the off position. A second modulator may be employed to perform this function (not shown here). Said second modulator may comprise a plurality of lower frequency power MOSFET or BJT switches that enable the DC power supplies to be connected to the high frequency power BJTs or FETs when it is required to generate microwave power to produce the plasma. The operation of the lower frequency power devices that form the second modulator can be controlled by varying the gate voltage or base current of the power FETS or power BJTs respectively. The control signals would be provided by microprocessor no and the signals used to control the operation of the second modulator may be synchronised to the control signal used to control the operation of first modulator 20. The second modulator would have a slower response time than that of first modulator 20, therefore, it may be desirable to modulate or pulse using first modulator 20 inside a window of time when the second modulator is enabled or switched on. For example, the second modulator may be switched on for a time slot of iooms and off for a time slot of 1 second; during the on period, first modulator 20 may produce 50 pulses with an on time of ims and an off time of ims. First modulator 20 and the second modulator (not shown here) will enable the energy produced by the plasma to be controlled to ensure that the temperature of the plasma and the plasma energy is controlled to ensure that non-thermal plasma is generated at the distal end of the applicator to enable optimal clinical effects to be achieved in terms of killing or reducing bacteria and/or viruses, and/or fungi.

The plasma may be delivered into the tissue under footswitch control, whereby a jet of plasma is produced when a user depresses a footswitch pedal connected to the instrument. The footswitch forms a part of user interface 120 and is not shown as a separate item here.

In the embodiment shown in figure 1, a microprocessor unit 110 is used to control the operation of the system. This invention is not limited to using a single microprocessor, for example, a microprocessor and a digital signal processing (DSP) unit may be used, two microprocessors may be used, or the control functions may be implemented using a microcontroller or PlC device.

The sampled signals produced by forward and reflected power couplers 80, 90 may also be used to ensure that potentially high levels of microwave power are not radiated from the distal end of the waveguide applicator in the instance where a plume or jet of plasma has not been struck due to the gas supply having run out or it has been turned off. A safety sequence may involve shutting off the microwave generator if the impedance of the waveguide cavity has not reduced from the high impedance strike state to a lower impedance conducting gas state within 10 milliseconds or ioomilliseconds after the microwave energy has been applied. The capability of being able to continuously measure the impedance of the waveguide cavity may also be used to shut-off the microwave source in a timely manner when the gas cylinder becomes empty.

It is desirable for the three tuning stubs 250, 260, 270 to be set to an initial state where it is guaranteed that a resonant cavity will be set-up in order to produce a high enough electric field to strike the plasma as soon as the microwave energy source 10, 20, 30, 40, 50 is switched on. Once the plasma has been initiated, the three tuning stubs 250, 260, 270 will be moved to a position to enable the microwave energy to be matched to the impedance of the waveguide cavity 280 containing plasma 300, hence a null or a minima should be detected at the coupled port of the reflected power coupler 90.

It should be noted that a PID controller could be used between microprocessor no and electromechanical actuator 240 to control the adjustment of stubs 250, 260, 270. Alternatively, the PID control functions may be handled by microprocessor iio. A further alternative is to replace the mechanical tuning system with a power PIN or varactor diode arrangement, whereby the bias voltage applied to the diodes is used to adjust the depletion layer within the diodes to produce a capacitance variation.

The power transistors used in the output stage of microwave power amplifier 50 are protected from damage caused by excessive levels of reflected power going back into the amplifier, caused by either an impedance mismatch at the applicator where the plasma is generated, damage to microwave cable assembly 170, or the applicator or cable assembly becoming disconnected, using microwave circulator 60 and power dump load 70. In the event that microwave power is returned to the generator, for whatever reason, the reflected power will enter protection circulator 60 at the second (output) port and be diverted into dump load 70, via port three of circulator 60, where the reflected microwave power is absorbed and turned into heat. Dump load 70 should be attached to an appropriate heatsink and/or have a fan connected to it to ensure that the temperature of the load doesn't exceed its maximum specified working temperature and become damaged.

The microprocessor unit no also controls an electrically controlled valve 130, which is opened to allow gas to enter the waveguide cavIty 280. It is preferable to ensure that the gas enters the cavity before the microwave energy is applied or input into the cavity in order to ensure that non-ionised microwave radiation is not emitted from the distal end of the waveguide into the skin or other biological tissue.

It may also be desirable to control the rate of gas flow using an electrically controlled flow meter (not shown here). By knowing the initial volume of gas contained within gas cylinder 190 and the flow rate and time, it is possible to determine the volume of gas left in the cylinder at any one time. This information may be used to ensure that the microwave energy source is turned off prior to the gas cylinder becoming empty.

The arrangement shown in figure i shows three inputs entering the applicator (or hand-piece), namely: the control signal lines to the electromechanical actuator i6o, the microwave cable assembly that connects the microwave energy generator to the applicator 160, and the tube to carry the gas supply from gas cylinder into the waveguide cavity i6o. It may be desirable to house the three inputs inside a single jacket in order to facilitate ease of use or manipulation of the applicator.

The gas 190 enters the waveguide applicator through a opening in the wall 230 of waveguide 290.

The input microwave connector 210 shown here uses an H-field probe 220 to couple the microwave power into waveguide 290. This invention is not limited to this arrangement, i.e. an E-field probe may be used to couple the microwave energy into the structure.

The user interface 120 provides an interlace between the user (clinician or surgeon) and the treatment system (man-machine-interface). For example, it maybe required to enter the type of gas (or types of gases) 190 used (He, C02, Ar, Ne, 02 etc), the duration of the treatment and the power level, the flow rate. The dosage of plasma energy can be calculated from this information and may be displayed. User interface 120 will also indicate error or fault conditions. The user interface may take the form of a LED/LCD display and a keypad, a touch screen display, or the like.

A power mains voltage to DC voltage power supply 140 is used to provide the voltage/current required by the electrical components within the system. It is preferable for power supply 140 to be a switched mode power supply in order to obtain optimal AC mains to DC voltage efficiency. Power factor correction may be included within the unit to optimise this efficiency.

Figure 2 shows a solution where the complete automatic tuning system is contained within the applicator (or hand-piece). The arrangement shown uses only one tuning stub 250 for convenience, but it may be preferable to use two, three, or more stubs in practice.

The automatic tuning mechanism works by setting the distance stub 250 protrudes inside the cavity 290 to a length L1 determined by drive signals V1/i1 (700) and V2/i2 (800) at the input to the electromechanical actuator 240 used to move the tuning stub 250 inside waveguide cavity 290. A single pole two throw (SP2T) switch 400 is used to route the particular drive signal 700, 8oo to the actuator 240 and the switch position (or pole position) S1 / S2 is determined by control line signal C1 and reset signal 410. It may be preferable to use a MOSFET device or a relay to implement the SP2T switch 400. In the instance where SP2T switch 400 has only one control signal input, it may also be necessary to provide the reset and control functions 410 C1 using a logic gate arrangement, for example, a D-type flip flop or an arrangement of logic gates. Figure 2 shows the switch as a block with two inputs and does not show the additional glue logic that may be required.

In order to sense the magnitude of the electric field set-up inside the waveguide cavity, an H-field ioop coupler 330 is included and is shown located near the distal end of the applicator 340. The connector arrangement used to enable the output signal from the H-loop coupler to be connected to the rest of the circuit 350 may be an SMA or N-type connector arrangement. In order to be able to successfully detect a portion of either the forward going or reflected signal, it will be necessary to include a non-coupled port within the arrangement (this is not shown here). It may be preferable to use an E-field probe to sense the magnitude of the electric field. Coupler 330 senses a portion of the field set-up inside the waveguide cavity 280. The coupled signal is fed into detector 600, which may be a magnitude detector, a phase and magnitude detector, or a phase detector.

Detector 600 produces a DC or low frequency AC voltage signal which is fed into the input of threshold comparator 500, whose function is to provide a control signal to SP2T switch 400 to change the pole position in accordance with the value of the electric field set-up inside the waveguide cavity 280 and, and determine whether or not the microwave source is switched on (this is determined by the status of the reset signal).

Figure 2 gives an arrangement where stub 250 is set to a position to produce a maximum electric field inside the waveguide cavity 280 in order to enable the plasma to be struck when a suitable gas is supplied to the waveguide cavity and the microwave source is switched on. Once the plasma has been struck, the electric field 320 will be reduced and this will be detected by a change in the voltage Vu picked up using H-field sensing coupler 330. The change in the magnitude of the electric field 320 may be used to change the state of the output of the threshold comparator 500 to cause the switch position to change to S1 to enable the signal V1/i1 700 to be seen at the input to the electromechanical actuator 240 to cause the length L1 of stub 250 protruding inside the waveguide cavity 280 to change. The new condition will enable the microwave energy to be impedance matched into the waveguide cavity 280 to sustain the plasma and provide efficient energy delivery with a minimum level of reflected microwave energy being returned back to the microwave source.

For the practical realisation of this arrangement, it may be desirable to use the high voltage detected when the high electric field 320 is present to trigger the threshold comparator 500 to move the stub 250 to the second position necessary to sustain the plasma. If it is assumed that plasma will definitely be struck once a high enough electric field 320 has been established then the threshold comparator 500 may be triggered at a predetermined time after the high electric field 320 has been detected (or has been established) using H-field coupler 330.

A time delay may be introduced into the system using a retrigerable monostable circuit or a L-C, C-R delay circuit to enable this sequence of events to occur.

As a practical embodiment, it may also be preferable to move the physical position of the tuning stub 250 closer to the distal end of the waveguide applicator (hand-piece) and it may also be preferable to move the position of the field sensing coupler 330 to another location within the waveguide cavity 280.

Figure 3 shows a specific embodiment for the arrangement described above and given in figure 2 to enable the plasma to be struck and maintained where the stub 250, the electromechanical actuator 240, the field sensor 330, and the control circuitry 400, 500, 600, 700, 800 may all be contained within the applicator (or hand-piece).

The implementation shown here uses analogue signal processing for speed of operation, ease of implementation, and simplicity. It saves the need to implement a PlC or microprocessor and associated peripheral components. The two actuator drive signals V1/i1 (700) and V2/i2 (8oo) are derived using operational amplifiers A (710) and A2 (810) respectively, configured or set-up as non-inverting amplifiers. The voltage/current applied to the electromechanical actuator M (240) to cause the stub 250 to be moved to enable it to protrude inside the cavity 280 to a desired length L1 to enable a high electric field 320 to be set-up to initiate or strike the plasma V1 is given by equation 2 below.

V1 = V, (1 + R1/R2) (2) Where: V is the voltage applied to the non-inverting input terminal of operational amplifier A1 (710) [V]; R1 (720) is the feedback resistor connected between the output of A1 (710) and the inverting input to A1 (710) [f']; R2 (730) is a resistor connected between the inverting input to A1 (710) and ground [f].

The voltage applied to cause the stub 250 to be moved by the electromechanical actuator 240 to enable it to protrude inside the cavity 280 to a length L1 to enable the plasma to be maintained V is given by equation 3 below: V2 V(i.+R3/R4) () Where: V is the voltage applied to the non-inverting input terminal of operational amplifier A2 (810) [V]; R3 (820) is the feedback resistor connected between the output of A2 (8io) and the inverting input to A2 (810) [fl]; R4 (830) is a resistor connected between the inverting input to A2 (8io) and ground En].

Operational amplifiers A1 (710) and A2 (8io) may be contained in a single packaged integrated circuit and may come in the form of a small surface mount device.

The detector 600 comprises of RF or microwave diode D1 (6io), filter capacitor C1 (620) and zener clamp diode D2 (630). The input signal to detector 600 is the voltage picked up from H-field coupler 330 contained within the waveguide cavity applicator 280 and is denoted here as Vu. Diode D1 (6io) may take the form of a zero bias Shottky diode or a tunnel diode, Ci (620) is preferably a low loss capacitor, for example a o.ipF COG, and D2 (630) may be a 4.7V zener diode.

Diode D2 (630) is used to ensure that the input voltage going into the non-inverting terminal of buffer amplifier A3 (510) does not exceed 4.7V, thus this component protects the rest of the circuit following detector unit 600.

The threshold comparator 500 comprises of two operational amplifiers A3 (510) and A4 (560), where A3 (510) is configured as a unity gain buffer and A4 (560) is configured as a voltage comparator. The buffered signal produced at the output of the buffer amplifier VA is delayed using a single pole low-pass filter arrangement comprising of series connected resistor R5 (520) and shunt connected capacitor C2 (530). The voltage at the non-inverting input to A4 (560) V0 is given by equation 4 shown below: V0 = V (i -et/T) () Where: T is the time constant of the circuit = R5 (520) x C2 (530) Es]; The voltage applied to the inverting input of operational amplifier A4 (560) S given by the output from the potential divider formed by the series connected resistor chain R6 (540) and R7 (550). The reference voltage applied to the inverting input terminal is thus determined by equation 5 given below: +V (R7/ (R6 + R7)) (5) Where: Ro (540) is a resistor connected to a +5V supply and the non-inverting input terminal of A4 (560) [ci]; R7 (550) is a resistor connected to the non-inverting input terminal of A4 (560) -and ground [ci].

Once the voltage applied to the non-inverting terminal of A4 (560) reaches the threshold voltage determined equation 5 then the output from A4 (560) will change the pole position of MOSFET switch 400 from S1 to S2 to enable the stub 250 to be moved into a second position to enable a low impedance condition to be set-up inside the waveguide cavity 280 to sustain the plasma. When a reset signal (410) 15 present, the pole position will move back to S1. The electromechanical actuator 240 is shown here as a motor and the stub 250 is not shown. The electromechanical actuator 240 could also take the form of a linear motor, or a linear actuator, for example, a magnetostrictive material based linear actuator arrangement.

A sequence of events representing the operation of the system may be as follows: i. Reset system using the reset signal 410 to ensure that the SP2T switch 400 is in position 1, and move stub 250 to a position that will create a high electric field 320 within the waveguide cavity 280 to enable the plasma to be struck; 2. Turn on gas supply 190 using regulator 200 and valve 130 (controlled by microprocessor ho) and ensure that gas has entered waveguide cavity 280; 3. After a predetermined delay (to ensure that the cavity 280 is filled with the gas 190) turn on microwave energy source 10, 20, 30, 40, 50 using control signals produced by microprocessor no; 4. The high electric field 320 set-up in waveguide cavity 280 filled with appropriate gas 190 should now enable the plasma to be initiated or struck; 5. After a short time delay given by: V (i -et/U5.C2)) switch pole position of SP2T switch 400 to S2 to enable the plasma to be maintained by creating a low impedance condition inside waveguide cavity 280 to enable the output power from microwave energy source 10, 20, 30, 40, 50 to be impedance matched with the conducting gas (the plasma) to enable clinically useful plasma to be set up and maintained.

Practical implementation of the circuit given in figure 3 may be carried out using surface mount components, for example 0201 and 0603 devices, in order to keep the physical size of the circuits to a minimum to enable a compact hand-piece design to be manufactured. Both active and passive devices are now available in these small packages and so it is feasible to implement the circuits in this manner to enable the circuit to be contained within the hand-piece in a non-obtrusive manner.

It may be preferable to spring load the tuning stubs 250, 260, 270 and use a ratchet mechanism to enable the three stubs to be set in two positions only. The first position will enable the plasma to be struck and a second position will enable the plasma to be maintained (level of reflection is minimised) as described above.

In this particular arrangement an automated tuning mechanism may not be required. The distance between the centres of the three stubs is preferably a quarter or three quarters of the guide wavelength (more details are provided on this particular aspect below), but the invention is not limited to using this spacing, i.e. one eighth or half wavelength may also be used.

In another embodiment, it may be possible to initially set tuning stubs 250, 260, 270 to one fixed position to enable the plasma to be struck when the gas 190 and microwave energy emanating from the output of the second stage of the power amplifier o is present, and use a suitable sensor, for example, a directional coupler 80, 90 and a detector, or a voltage measuring device 100, to sense this condition, and once this condition has been established change the position of the three stubs 250, 260, 270 to a second position where the microwave energy source is impedance matched to a low impedance state caused by the conductive gas or plasma formation. Using this method of control, the lengths of the three stubs inside the cavity may also be varied in accordance with the changing impedance of the surface or tissue that the plasma is being coupled into, i.e. the magnitude and/or phase of the signal produced by reflected power coupler 90 will change in accordance with the impedance match between plasma plume and the surface, and this signal can be used to vary the position of the stubs to minimise the change or set up a conjugate match condition. It may be preferable to use the high voltage state to trigger a latch or mechanical actuator that can be used to move the stubs to a new pre-defined position. In this instance, it may not be necessary to measure reflected power level and use an associated optimisation routine to ensure that the position of the tuning stubs coincide with a null or minima in the reflected signal.

It should be noted that the microwave source is synchronised with the gas supply using an electrically operated valve or switch 130, which is controlled using digital signal processor or a microprocessor unit 110.

A particular advantage of the implementation of the main aspect of the current invention is that the resonant cavity may not suffer from the reduction in Q caused by the insertion loss of the cable assembly inserted between the generator and the applicator; this reduction in Q may cause the electric field generated inside the cavity to be reduced, which could limit the ability of the system to sustain the plasma.

Figure 4 shows a waveguide applicator consisting of a waveguide cavity 280 and three tuning stubs 250, 260, 270 protruding inside the waveguide cavity with preset lengths to enable a high electric field 320 to be set-up inside the cavity to enable plasma to be struck when waveguide cavity 280 is filled with an appropriate inert gas. The figure shows a gas inlet tube 170 inserted inside the waveguide cavity wall 290 and located on the opposite wall to where the tuning stubs 250, 260, 270 are located. The gas inlet tube 170 could be inserted at other locations inside the cavity. The microwave feed shown here is a H' -field ioop 220 and the energy is coupled into the cavity along the direction of the cavity. It may be desirable to use an E' -field probe to couple the energy into the cavity with the launch probe positioned perpendicular to the direction of the cavity. The stub spacing shown here is a quarter of the guided wavelength measured between the centres of adjacent stubs. The guide wavelength can be calculated using equation 6 given below: Ag = J( (1/A02) -(1/X2)) (6) Where: Ag is the guide wavelength (m); A0 is the wavelength at the frequency of interest (m); A is the cut-off wavelength of the waveguide structure -this is a function of the geometry of the waveguide (m).

This invention is not limited to using a quarter guide wavelength spacing; for example, it may be preferable to use an odd multiple of a quarter guide wavelength spacing. An eighth of the guide wavelength may also be considered in the instance where lower frequency microwave energy is used in order to enable an acceptable physical geometry to be fabricated. Other stub spacing may also be considered. It may be preferable to arrange the stubs on the opposite face of the waveguide to that shown in figure 4. It may be preferable to alternate the position of the three stubs between the two faces or walls in order to produce a more practical solution. In the arrangement shown in figure 4, the distances the three stubs 250, 260, 270 protrude within the waveguide cavity are controlled by the manual adjustment of the stubs by placing a screw driver into the slots within screw heads 251, 261, 271 respectively and turning the screws clockwise or anticlockwise.

This arrangement is not limited to using three tuning stubs, i.e. it may suffice to use one or two stubs. It may also be desirable to use more than three tuning stubs.

In the arrangement shown in figure 4, the input microwave energy is fed from a microwave source 2000 with an output impedance of 5oS1 (i8o) and a maximum power level of iooW. Assuming that the waveguide cavity is lossless, then it may be assumed that the output power is the same as the input power, thus the voltage generated at the distal end of the waveguide structure may be calculated as the square root of the product of the input power (P) and the impedance seen at the end of the waveguide structure (Z0t), for example, if the output impedance is iooKI), then the voltage available to strike the plasma is given by: v"( P1, x and, in this particular instance, will be: -J (ioo x ioooo) 3162V.

The adjustment of the stub positions is not limited to tuning the heads of threaded screws held in the waveguide wall. The stub positions may be set-up using semi-automatic mechanical arrangements, e.g. a spring and ratchet mechanism or arrangement, automated sensing and adjustment circuitry contained within the hand-piece, or an arrangement using an electromechanical actuator contained within the hand-piece where the control and sensing circuitry is contained within the generator, i.e. housed in a separate location.

It is preferable to insert a single or plurality of waveguide chokes into the wall of the waveguide where the screws or rods move in and out of the cavity in order to minimise the amount of microwave field radiating out of the holes or apertures produced to guide the stub. A choking arrangement also ensures good electrical contact between the stubs and the waveguide walls. It must also be ensured that the stubs are not set up in such a manner that they act as aerials and radiate energy out of the waveguide, i.e. half wavelength or quarter wavelength dipoles or monopoles respectively.

Claims (1)

1. CLAIMS1. Apparatus for producing controllable non-thermal plasma for killing or destroying a range of bacteria and/or viruses and/or fungi contained on the surface of or inside human or animal tissue structures or other external structures, the apparatus comprising: a controllable microwave energy source; an antenna to direct the microwave energy into the bacteria; a means of automatically matching the microwave energy with the antenna structure, to enable: the plasma to be struck, and/or the plasma to be maintained, and/or the plasma to be efficiently coupled or matched into the treatment surface and/or tissue, and/or to enable efficient microwave energy to plasma energy conversion; a means of connecting or channelling the microwave energy from the microwave energy source to the antenna containing the automatic tuning network; a signal processing and control unit to enable the plasma energy or dosage launched into the treatment site to be controlled in terms of microwave power level and delivery time, and a user interface to enable the user to input information into the system and the system to output useful information to the user.

   2. Apparatus according to claim 1, to produce active plasma for sterilisation, and the plasma contains one of more of the following: UV radiation, 1-Jydroxyl radicals, Ozone, Ions.

   2. Apparatus according to claims 1 or 2, whereby ozone, UV radiation and moisture are combined to produce Hydroxyl radicals that can be used for sterilisation of biological tissue or external surfaces.

   4. Apparatus according to claim 3, whereby the moisture is introduced into the plasma applicator with the gas and microwave power or where moisture is introduced externally or by external means.

   5. Apparatus according to any one of the above claims whereby the automated tuning mechanism uses a single or a plurality of mechanical stubs inserted into one or more walls of the antenna.

   6. Apparatus according to the aforementioned claims whereby the automated tuning mechanism uses varactor or PIN diodes.

   7. Apparatus to produce controllable non-thermal plasma for sterilisation using a controllable microwave source, an antenna and an automatic tuning mechanism contained within the antenna, whereby the control system is also integrated into the antenna or the hand-piece and is implemented using a combination of the following analogue components: a voltage or signal comparator; a voltage or signal detector; a signal conditioning circuit using operational amplifiers or discrete transistors; a voltage signal controlled switch; a sensor to pick up a portion of the electric or magnetic field generated inside the antenna; an actuator or actuators to control the movement of the tuning stub(s); a single or a plurality of tuning stub(s).

   8. An arrangement according to claim 7 where the field sensor is arranged as a directional coupler to sense the magnitude of the reflected signal.

   9. An arrangement according to claim 8 where the field sensor is an H-field loop or an E-fleld probe.io. Apparatus according to the aforementioned claims whereby the antenna is a cylindrical or rectangular waveguide cavity.ii. An antenna according to claim 10, whereby the cylindrical or rectangular waveguide is loaded with a dielectric and/or magnetic material to reduce the size of the antenna structure and enhance the match between the microwave generator and the treatment site.12. Apparatus according to the aforementioned claims whereby the antenna is a coaxial structure comprising of an inner and outer conductor and the microwave field set up between the two conductors is adjusted by introducing metallic or dielectric tuning stubs into the coaxial structure trough the outer wall of the structure between the inner and outer conductors to change the microwave fieldor the field pattern set up within the structure.13. An antenna arrangement according claim 12, whereby the length and/or the diameter of the coaxial structure is automatically adjusted to enable the desired impedance to be set up to enable plasma to be struck, and/or maintained, and/or adapted in accordance with a change in the environment.14. Apparatus in accordance with the aforementioned claims to be used in the following applications: hospital ward sterilisation, wound bed sterilisation, hospital equipment (beds, curtains, instruments, clip-boards, pens) sterilisation, internal tissue or biological insert sterilisation, treatment of sexually transmitted diseases, destruction of bacteria contained within natural orifices, treatment of ulcers or bed sores to kill bacteria, treatment of athletes foot, or the treatment of alopecia.Amended claims have been filed as follows:- 1. A hand held plasma applicator for a plasma sterilisation system, the applicator comprising: an enclosed plasma generating region having a gas inlet for receiving gas from a gas feed, an energy inlet for receiving microwave energy, and an outlet for delivering plasma out of the plasma generating region towards matter to be sterilised; and an impedance adjustor arranged to control the impedance at the plasma generating region when gas and microwave energy are delivered thereto.2. A hand held plasma applicator according to claim 1, wherein the impedance adjustor is adjustable to create a high impedance condition suitable for striking a non-thermal plasma in the plasma generating region when gas and microwave energy are delivered thereto.3. A hand held plasma applicator according to claim 1 or 2, wherein the impedance adjustor includes a dynamic tuning mechanism arranged to maintain an impedance match between a non-thermal plasma delivered from the outlet and the matter to be sterilised.4. A hand held plasma applicator according to any preceding claim including a controller arranged to detect microwave power propagating through the plasma generating region and provide control signals to the impedance adjustor based on the detected power. S.. : 5. A hand held plasma applicator according to claim 4, wherein the controller includes: S...an H-field or E-field coupler arranged to sense the electric field in the plasma generating region; and SI.an analogue signal processor arranged to receive an S..output from the H-field or E-field coupler, compare it with a :.". threshold value, and generate a control signal based on the * result of the comparison.SI S** *.0 6. A hand held plasma applicator according to claim 4, wherein the controller includes: a forward power coupler and a reflected power coupler arranged to sample forward and reflected power signals respectively in the plasma generating region; and a digital microprocessor arranged to receive the sampled signals from the detectors and to generate the control signals.7. A hand held plasma applicator according to any preceding claim, wherein the impedance adjustor includes: one or more mechanically movable tuning stubs that are insertable into the plasma generating region; and an electromechanical actuator arranged to move the stubs.8. A hand held applicator according to any one of claims 1 to 6, wherein the impedance adjustor comprises a variable capacitance connected in shunt to the plasma generating region relative to a microwave energy propagation path from the energy inlet to the outlet.9. A hand held applicator according to claim 8, wherein the variable capacitance comprises a plurality of semiconductor power varactor diodes or PIN diodes, the control signal for the impedance adjustor being a bias voltage applied to the diodes to adjust their depletion layers.10. A hand held applicator according to any preceding claim, wherein the enclosed plasma generating region is formed in a waveguide cavity.11. A hand held applicator according to any one of claims 1 to 9, wherein the enclosed plasma generating region is formed between the inner and outer conductors of a coaxial structure. S..* 12. A method of automatically tuning a hand held plasma applicator in a plasma sterilisation system, the method * . S * comprising: *:..0 delivering gas into a plasma generating region formed within the applicator; * 38 delivering microwave energy into the plasma generating region; controlling an impedance adjustor in the applicator to create a high impedance condition inside the plasma generating region suitable for striking a non-thermal plasma for delivery out of the plasma generating region towards matter to be sterilised; and after the plasma is struck, detecting microwave power propagating through the plasma generating region; and controlling the impedance adjustor based on the detected power to create a low impedance condition suitable for maintaining the plasma.13. A method according to claim 12, wherein the impedance adjustor is dynamically controlled to maintain an impedance match between a non-thermal plasma delivered from the outlet and the matter to be sterilised.14. A hand held applicator for a plasma sterilisation system substantially as herein described with reference to, and as illustrated in, the accompanying drawings. * * * * * a... * *. . -S..SS * S. * S a a. 5 S *J