AU2019100277A4

AU2019100277A4 - A plasma treatment apparatus for plasma immersion ion implantation of powders and structured materials

Info

Publication number: AU2019100277A4
Application number: AU2019100277A
Authority: AU
Inventors: David Mckenzie; Clara Tran
Original assignee: Individual
Current assignee: Individual
Priority date: 2018-03-26
Filing date: 2019-03-15
Publication date: 2019-05-16
Anticipated expiration: 2027-03-15

Abstract

Abstract A plasma treatment apparatus for plasma immersion ion implantation of powders and structured materials having a treatment chamber with walls that are at least partly electrically insulating, a first electrode surrounding part of the insulating wall of the treatment chamber, a second electrode, a gas reservoir for containing gas for admission to the treatment chamber through a gas flow controller or adjustable valve and a vacuum pump for creating a vacuum in the treatment chamber and in the gas reservoir, wherein the power supply is connected to create a time varying potential difference between the first electrode and the second electrode which in turn forms an ionized gas or plasma from the gas present within the treatment chamber.

Description

There are 11 pages of the description only .

2019100277 17 Apr 2019

Description

Title of Invention

A plasma treatment apparatus for plasma immersion ion implantation of powders and structured materials

Field of invention

The present invention relates generally to an apparatus that can activate the surface of two dimensional and three dimensional objects by exposing their surfaces to the bombardment of ions from an ionized gas or plasma. More particularly, the invention relates to plasma deposition of polymers and plasma immersion ion implantation treatment of deposited and pre-existing polymer surfaces for many applications. The applications include medical applications such as the production of scaffolds for promoting the growth of bone replacements and the production of therapeutic medicines by the immobilisation of therapeutic molecules on surfaces that may be the surfaces of nanometre or micrometre dimensioned particles; chemical or biochemical purification processes that involve the extraction of wanted species from a mixture; chemical engineering processes that use immobilised enzymes on the surfaces of polymer sheets, tubes or grains; and detection processes for medical or other diagnostic investigations that require the immobilisation of a diagnostic probe molecule on the walls of receptacles, on flat surfaces that may form a detection array or on particles of micrometre and nanometre dimensions or larger.

Background of the Invention

Plasma immersion ion implantation (PHI) of a polymer surface is a method to treat or activate a surface by subjecting it to the bombardment of ionised species while the surface is immersed in an ionised gas or plasma. The surface is electrically biased relative to the plasma using an electrical potential difference or voltage applied to an electrode in the vicinity of the surface. If the surface is a polymer surface, the applied voltage attracts ionised species from the plasma to bombard the polymer surface, causing bonds to break in the polymer and thereby creating a reservoir of radical groups that contain unpaired electrons in the surface and subsurface of the polymer. The useful effects of PHI treatment of polymers have been explored and published [1,2]. In brief, they are an increase in the wettability of the treated surface which is observed from a decrease in the water contact angle and the imparting of an ability to form covalent bonds with biomolecules adsorbed to

2019100277 17 Apr 2019 the surface without the need for the prior provision of a special chemical linker group by a wet chemical process. Another benefit of the PHI treatment of a surface is to sterilize an object by the destructive action of the bombarding ions on microbes that may be present. Such as sterilization process is especially useful where the object to be sterilized cannot be subjected to the temperatures or pressures normally applicable in an autoclave sterilizer.

A form of the PHI treatment that may provide the above benefits of PHI for materials other than polymers is known. For example, it is possible to coat the surface of any object and simultaneously treat it by means of a variant of the PHI process. This variant process is known as plasma immersion ion implantation and deposition (PIII&D) in which the plasma contains a precursor gas that is broken down in the plasma by impacts with electrons in the plasma to form a coating on the surface from fragments of the precursor molecules. The coating may be subjected to the bombardment of ions derived from the plasma during its growth, creating radicals in its surface and subsurface, much as in the PHI process described above.

There is a need to adapt the PHI and PIII&D processes to treat three dimensional objects such as micro- and nano- dimensioned particles and the surfaces of structured porous scaffolds where the surfaces are present in the interior of the material forming the scaffold. We describe here our invention of a plasma treatment system intended for treating surfaces by either the PHI or the PIII&D process. This invention relates to the plasma treatment of surfaces, including insulating surfaces of ceramics and polymers by ion modification using ions derived from an ionised gas or plasma. It is especially adapted to the treatment of powders. It is also adapted to the treatment of the interior surfaces of porous materials especially those that contain connected pores.

The apparatus consists of an insulating vessel, made of a material such as glass, that is connected to a source of gases or gas mixtures and a vacuum pump for creating a partial vacuum within the insulating vessel. The apparatus is shown in Figure 1. The plasma required for operation is created by an electrical discharge of a type known as an electrical glow discharge. The operation of the electrical glow discharge known as a dielectric barrier discharge created within a cylindrical electrically insulating vessel with a hollow cylindrical electrode surrounding it has been described by Janssen [3]. In a dielectric barrier discharge, an electric field is created in a gas by applying a potential difference between two electrodes. At least one of the electrodes is separated from the gas by a layer of an insulating or dielectric material. The electric field in the gas causes the creation of ions and electrons from the atoms of the gas. The ions are drawn towards the negatively charged electrode and encounter the surface to be treated if it intercedes between the plasma and the

2019100277 17 Apr 2019 electrode. A feature common to dielectric barrier discharges as used in this invention is a capacitive coupling between the surface to be treated and the hollow cathode electrode. In addition to the PHI treatment which is based on ion bombardment of a surface, the invention can perform PHI combined with deposition onto surfaces (PIII&D). Therefore we identify two different functions of the invention: PHI treatment for carbon based polymeric substrates and PIII&D coating for both non carbon-based polymeric and non-polymeric substrates that may include among others, metals, glasses and ceramics.

For PHI treatment, it is assumed that a polymeric surface is capable of plasma activation. This is generally the case if the polymeric surface contains carbon based or carbon derived polymer that contains carbon within its chemical formula. In that case, the activation is associated with the presence of carbon-centred radical groups that contain an unpaired electron. The surface may be the surface of particles suspended in the plasma or the surface of pores in a insulating polymer material. An insulating polymer material with internal pores or spaces may be made up for example from a collection of polymer tubes packed together, or may be an additively manufactured ( 3D printed) object constmcted to contain the pores or spaces. In the case where the surfaces are the internal surfaces of pores or spaces with an otherwise solid polymer material, that material may from a substantive part of the of the space inside the treatment chamber. The polymeric particles and interior surfaces that have been modified by ion impacts from the plasma become more hydrophilic. The hydrophilic surface facilitates the dispersion of particles in solution or the drawing of water or water solutions into the interior of the pores of a porous polymer medium, dispersion in solution. The treated surfaces also have the ability to covalently bind, without any wet chemistry steps, many types of molecules, including biologically derived or biologically relevant molecules. Such molecules may include, but not exclusively, single stranded DNA or RNA oligonuleotides [4], double stranded DNA or messenger RNA (mRNA), small interfering RNA (siRNA), microRNA (miRNA), antibodies [5], proteins [6], peptides, enzymes [7], soluble factors such as metabolites, interlukins, cytokines, chemokines, hormones and drugs. In the case of drugs, the drugs may be chemotherapy drugs intended for the treatment of cancer. Cells such as bacteria and yeast may also be immobilized on the surface. Such immobilization may occur via the surface of the membrane of the attaching cell. The attached cells may be free to carry out a processing function by acting on substances in solution.

A particular motivation for this invention is the need to tether a single stranded DNA or RNA nucleotide sequence, termed a probe sequence or a recognition sequence onto the surface of a

2019100277 17 Apr 2019 particle. Such a sequence may form part of the DNA or RNA of an agent such as a toxin, virus, fungus, bacterium or other organism. In a specific example, a particle that has one or more magnetic cores surrounded by a polymeric coating can be activated in the apparatus and subsequently provided with a covalently bound probe sequence by exposing them in solution to the sequence. The immobilized probe sequence will hybridise or bind to its complementary sequence (termed a target sequence) if the target sequence is present in a clinical sample. The magnetic particles, together with the target sequences, may be extracted from the solution by a magnetic field before detection and quantification in a process known as quantitative polymerase chain reaction (qPCR). Another motivation of this invention is to modify polymer coated magnetic particles into useful tools for DNA and RNA collection from a solution in order to concetrate or purify it. PHI treated magnetic particles may have a surface charge that may vary in a wide range from negative to positive in conditions of different acidity or pH, as achieved by their presence in a buffer solution. The charged particles may attract various charged biomolecules from the solution. For example, we have found PHI treated polystyrene coated magnetic particles to have a positive zeta potential within 30-40mV in pH4 low salt acetate buffer which allows them to attract negatively charged oligonucleotides in solution. Apart from a mono-layer of oligonucleotides that covalently attach to the treated particle surface, a dense cloud of oligonucleotides are associated with the strongly charged particles and may also be separated out from the solution by magnetic field. This cloud can be released into a new solution by changing the solution pH.

For particles without a polymeric coating, they can be coated with a thin layer of plasma polymer from a precusor gas such as acetylene. The coating contains radicals in its structure which provide the particles with similar properties as in the PHI treatment, since bombardment by ions from solution also takes place. The thickness and surface roughness of the coating can be adjusted from the operation parameters of gas composition, pressure and applied voltage on the electrodes.

A further motivation for this invention is to functionalize the surface of up-converting nanoparticles (UCNPs) that are used for many applications such as for bioimaging detection because of their strong and/or characteristic fluorescence [8]. UCNPs contain rare earth elements and cannot be easily functionalized by PHI treatment. By using acetylene gas incorporated into the gas used to generate the plasma, we are able to coat the UCNP with a thin film of plasma polymer which enables the UCNP to covalently bind to biomolecules such as oligonucleotides and antibodies. In use, the attached molecules can bind to the target molecules immobilized on a surface in a

2019100277 17 Apr 2019 microarray and the fluorescence signal of the UCNPs on the array can be used for regconizing the target molecules.

For treating a porous scaffold, either carbon-based polymeric or non polymeric material, can be functionalized in this system by placing the object inside the electrode, which is hollow and surounds the object to be treated. Ions from plasma penetrate inside the pore structure of the object and periodically accelerated towards the pore wall during the pulse of applied voltage. Depending on the type of gas used to form the plasma, the ions can either bombard or neutral or ionized chemical species may attach to the wall, changing the surface property.

A further motivation for this invention is to functionalize a poly(ether-ether-ketone), (PEEK) scaffold which is printed using additive manufacturing to achieve a reproduction of an object that may replace some of the function of a patient’s bone that has been lost. The PEEK surface is hydrophobic and does not support the growth of cells when implanted into human body. By treatment with PHI, the PEEK scaffold both the outside and inside of its structure is modified, increasing its wettability and allowing a layer of protein to be covalently attached to it. In vivo, this protein layer helps prevent the scaffold from being regconized as a foreign object which would trigger the immune system. In addition, the protein layer will facilitate the growth of bone cells into the scaffold structure, turning it into a part of the body’s skeletal system.

Summary of Invention

There are three technical problems that this invention overcomes.

Firstly, in the PHI and PIII&D processes, it is required to impose, for at least a portion of the time, a negative potential difference between the surface to be treated and the plasma that is a source of the ions. This potential difference is required to furnish sufficient energy to ions present in the plasma to enable them to implant into the surface. It is required to find a means of supplying this potential difference to surfaces that may be relatively distant from the first electrode of the system or may be obscured from it by their presence inside a material which may be an electrically insulating material or being in the form of a powder. Our invention states the design requirement for this to be achieved is as follows: that the mutual capacitance between the surface to be treated and the first electrode is larger and preferably much larger than the mutual capacitance between the surface to be treated and the second electrode. In this way, the potential of the surface to be treated is closer to that of the first electrode than to that of the second electrode when a potential difference is applied between the

2019100277 17 Apr 2019 second electrode and the first electrode. This follows because the potential of a glow discharge plasma is generally closer to that of the second or normally positive electrode and has been referred to as a “positive column”. An example of the application of these principles in order to optimize the surface treatment is given in our published work [9].

Secondly, an isolation between the plasma treatment compartment which may contain powder and the pump system has been designed to protect the pumps from ingesting or drawing in the powder from the treatment chamber. The ingestion of powder may cause damage to the vacuum pump. A vacuum pumping system is normally used to create and maintain a reduced pressure in the treatment chamber during a plasma implantation and plasma deposition process. Depending on the desired pressure, a backing pump or a combination of a backing pump and a turbo molecular pump or diffusion pump can be used. A technical problem of treating the dry powder is that particles can be easily drawn into the pump system, causing the deposition of abrasive powder inside the pumps which causes damage that subsequently reduces their operational life-time. In the case of a turbomolecular pump, collision of the fast moving blades with small objects can cause failure of the pump. In our invented plasma system, the pumps and the treatment compartment are separate and connected by a valve which can isolate them when needed, as when powder is present in the treatment chamber.

Thirdly, the design of this invented plasma system allows the powder to be agitated under external forces to assist in the suspension of the powder in the plasma. Otherwise the powder may adhere to the walls of the treatment chamber. If the powder is adherent to the walls, it may not present all of its surfaces to the plasma and may not receive treatment on those obscured surfaces. Agitation of the powder is achieved by means of vibrating the walls by placing them in contact with a mechanical vibrator. Powder of micro- and nano-metre dimensions frequently aggregates and the individual grains are frequently difficult to separate, even if they are not adherent to the walls. This causes a difficulty for treatment of all of the particle surfaces evenly. Some particles may not be treated and others may only be partially treated. In our invention, we solve this problem by the mixing the powder with larger or higher density particles or beads to assist in the breaking up of aggregates. The larger or denser beads tend to collide with the aggregates of the powder of smaller particles, breaking up the aggregates. For example we have used steel beads (50 pm diameter) or glass beads (200 pm diameter) mixed with powder of smaller average dimensions. The larger or denser beads are readily set into motion by the vibration from the external mechanical vibrator or

2019100277 17 Apr 2019 drive. The agitation of these larger or denser beads induces the circulation of the grains of powder, helping them to expose otherwise hidden sides to the plasma.

Advantageous Effects of the Invention

The invention can be implemented as design with a relatively small footprint to enable the treatment apparatus to fit comfortably alongside other equipment in a laboratory or in hospital is and this is considered an advantage in some cases. We have made two further advances to enable the smaller footprint. The first further advance is a hollow cathode treatment chamber that is scalable to fit the size of the object to be treated, ranging in size from less than a centimetre to sizes of order 10 centimetres. We use a close fitting, hollow electrode that surrounds a substantial portion of the treatment chamber and the treatment chamber may be sized to fit closely around the object to be treated.

A second further advance is the use of a small step-up transformer instead of a large pulsed power supply. The large pulsed power supply delivers square pulses of chosen pulse length. Small step-up transformers are well known as automotive ignition coils. Such a step up transformer may be driven by a waveform generator or simply by an intermptor device that periodically interrupts current flowing in the primary coil. The high voltage output from the step up transformer drives the hollow electrode. The output consists of pulses that alternate in sign from positive to negative. This is termed bipolar operation. The ion implantation occurs during the negative pulses and the resulting charge on the insulating object caused by implanted ions is neutralised during the positive pulses.

The above advances increase the flexibility of the invented system to treat objects with different shapes and sizes effectively. For example the hollow cathode electrode may also be fitted around the base of a conical glass flask, enabling a large area of flat electrode to be available.

2019100277 17 Apr 2019

Description of Embodiments

Some examples of the invention are now discussed.

Plasma immersion ion implantation of polystyrene coated magnetic particles.

Magnetic particles (MPs) were obtained from Spherotech Inc (catalog number PMS-20-10) with a nominal size of 2-2.9 gm. Ten milligrams of the particles are mixed with 250mg of glass beads (diameter 200 gm) and placed in a borosilicate glass test tube acting as the treatment chamber. The tube is connected to the system by a vacuum fitting and the whole system is pumped to a pressure of 1.3x10-7 bar. The bottom of the tube is covered by a copper electrode connected to a pulsed powder supply while the vacuum fitting is grounded. Nitrogen gas is introduced into the treatment chamber and the reservoir and the treatment chamber is isolated from the pump by closing the valve in between them. The pressure inside the treatment chamber is adjusted to 0.4 mbar before commencing plasma treatment. The glass tube is vibrated by a mechanical vibrator at a frequency of 30Hz during the treatment. Negative pulses of square waveform from a RUP6 pulse generator (GBS Electronik GmbH, Dresden, Germany) are applied to the hollow cathode electrode with a pulse length of 20 gs at a frequency of 3000 Hz and a negative voltage of -10 kV. The pulses are delivered by a switch mode power supply. The light emission expected from a nitrogen plasma can be observed inside the tube, an emission which is especially strong within the hollow cathode electrode volume where many of the particles are suspended. Pressure inside the treatment chamber increases slowly during operation due to outgassing and sputtering of materials inside the glass. The treatment is conducted for 2 minutes and then the treatment chamber is removed by evacuation and refreshed with nitrogen for the next 2 minute period of plasma treatment. The total treatment time is 20 minutes. The colour of the MPs after the treatment is darker than the untreated particles as shown in Fig.3, right panels. The attachment of horseradish peroxidase (HRP) on untreated (UT) and PHI treated magnetic particles are compared in Fig3, left panel. More HRP molecules attached to the PHI treated MPs than UT MPs and most of them retain of the PHI treated MPs after triton detergent wash (1% v/v) while those on UT MPs are removed by the triton detergent wash. More details can be found in our published paper [10].

2019100277 17 Apr 2019

Plasma immersion ion implantation of a PEEK scaffold.

A scaffold with a porous structure was printed by a 3D printer using PEEK. The scaffold is placed at the bottom of a glass tube and a copper electrode covers the outside. The tube acting as treatment chamber is mounted to the system by means of sealed fitting and the whole system is evacuated to a base pressure of 1.3x1 O'⁷ bar. Nitrogen gas is introduced to the system and adjusted to a pressure of 0.4 mbar. Plasma is generated inside the treatment chamber with a pulse length of 40 gs at a frequency of 3000 Hz and a negative voltage of -10 kV from RUP6 pulse generator (GBS Electronik GmbH, Dresden, Germany). The treatment is conducted for 10 minutes under dynamic pump and continuous gas flow during which the connecting valve and the flow controller are used to control the flow. Plasma can be observed inside the scaffold stmcture even when the pore size is of millimetre dimensions. The internal surfaces can be observed when the scaffold is opened and they take the darker colour caused by the PHI treatment.

Plasma immersion ion implantation treatment of the internal and external surfaces of a urinary catheter.

Catheters (18 fr) made from PVC were cut into 5 cm long sections and fitted into a glass tube closed at one end (internal diameter x mm, 1mm wall thickness) and held vertically in a Teflon holder as shown in Fig 4. A copper electrode surrounds the part of the glass tube containing the sections of the catheter to be treated. This copper electrode was connected to RUP6 pulse generator (GBS Electronik GmbH, Dresden, Germany) to apply a negative voltage of -10 kV at a frequency of 500 Hz with a pulse length of 40 gs for 20 minutes. Nitrogen gas was maintained at a pressure (300 700mtorr) during the plasma treatment. A piece of polypropylene thin film was positioned inside the tube section (in the middle of the section length) and removed after the treatment to evaluate the effect of ion bombardment. Figure 5 compares the transmission of polypropylene films obtained from different plasma pressures, showing the increasing absorbance near UV range when nitrogen gas pressure increases due to the appearance of carbonized layer on the films. The forming of carbonized layers is an indication of ion implantation occurring on the inside wall of the tube.

Plasma coating on non-polymer substrates for biomolecule attachment

2019100277 17 Apr 2019

The surface of a silicon wafer was modified by covering it with a thin carbon coating rich in radicals for biomolecule attachment. Silicon wafer was cut into strips (1x4 cm ) and cleaned before plasma deposition. They were positioned at the bottom of a 100 ml conical flask. The outside of the flask bottom was covered by a copper electrode which connected to RUP6 pulse generator (GBS Electronik GmbH, Dresden, Germany). A mixture of acetylene and nitrogen with 1:1 ratio (by volume) was introduced into the flask and regulated at 250 mtorr. During the plasma deposition, negative pulses of -10 kV from RUP6 generator were applied to the copper electrode at a frequency of 3000 Hz with a pulse length of 40 qs for 8 minutes to obtain a carbon coating of approximately 200 nm thick. This carbon coating is rich in radicals was formed from an acetylene and nitrogen plasma which enables the coating to form covalent bonds with proteins [9]. The covalent attachment was tested by washing with detergent (Triton 1% and sodium dodecyl sulphate (SDS) 2% at 70°C). Figure 6A compares nitrogen signals detected on silicon wafer incubated with horseradish peroxidase (HRP) with and without SDS wash to detect this protein attached to silicon surface. The nitrogen content was higher after the incubation with HRP but the nitrogen signal characteristic of the HRP was completely removed from the silicon surface by SDS. On contrary, nitrogen could not be removed by SDS when HRP was immobilized on the plasma coating (Figure 6B), indicating covalent attachment of HRP molecules on the coating is occurring.

Industrial Applicability

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The invention may be implemented in at least two ways. The first way is to use it with a large treatment chamber so that many objects can be treated simultaneously. In this case, the treated objects are simultaneously sterilized and their surfaces activated. The ability to treat many objects simultaneously is advantageous to give a large processing throughput as part of a manufacturing facility.

The second way is to use the apparatus to treat small objects or powders near to the point of use in a hospital or research facility. This small scale operation of the invention is convenient where the objects need to be customised for individual needs of patients.

Citation list

1. Kondyurin, A. and M. Bilek, Ion beam treatment of polymers. 2008, Amsterdam: Elsevier.

2. Bilek, M. and D. McKenzie, Plasma modified surfaces for covalent immobilization of functional biomolecules in the absence of chemical linkers: towards better biosensors and a new generation of medical implants. Biophysical Reviews, 2010. 2(2): p. 55-65.

3. Jansen, B., H. Steinhauser, and W. Prohaska, Plasma treatment of the inner surface of polymer tubes for the improvement of their anticoagulant properties. Makromolekulare Chemie. Macromolecular Symposia, 1986. 5(1): p. 237-244.

4. Tran, C.T.H., et al., Covalent linker-free immobilization of conjugatable oligonucleotides on polypropylene surfaces. RSC Advances, 2016. 6(86): p. 83328-83336.

5. Kosobrodova, E., et al., Cluster of differentiation antibody microarrays on plasma immersion ion implanted polycarbonate. Materials Science and Engineering: C, 2014. 35: p. 434-440.

6. MacDonald, C., et al., Covalent attachment of functional protein to polymer surfaces: a novel onestep dry process. Journal of The Royal Society Interface, 2008. 5(23): p. 663-669.

7. Kondyurin, A.V., et al., Mechanisms for covalent immobilization of horseradish peroxi-dase on ion beam treated polyethylene. arXiv:1110.3125 2011.

8. Chen, G., et al., Upconversion Nanoparticles: Design, Nanochemistry, and Applications in Theranostics. Chemical Reviews, 2014. 114(10): p. 5161-5214.

9. Tran, C.T., R. Ganesan, and D.R. McKenzie, Quantifying plasma immersion ion implantation of insulating surfaces in a dielectric barrier discharge: how to control the dose. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science, 2018. 474(2215).

10. Tran, C.T.H., et al., A plasma ion bombardment process enabling reagent-free covalent binding of multiple functional molecules onto magnetic particles. Materials Science and Engineering: C, 2019. 98: p. 118-124.

Claims

What is claimed is listed below:

Claim 1. A plasma treatment apparatus consisting of a power supply, a treatment chamber with walls that are at least partly electrically insulating, a first electrode surrounding part of the insulating wall of the treatment chamber, a second electrode, a gas reservoir for containing gas for admission to the treatment chamber through a gas flow controller or adjustable valve and a vacuum pump for creating a vacuum in the treatment chamber and in the gas reservoir. The power supply is connected to create a time varying potential difference between the first electrode and the second electrode. The time varying potential difference creates a time varying electric field which in turn forms an ionized gas or plasma from the gas present within the treatment chamber, termed the background gas. A surface to be treated is present inside the treatment chamber. The arrangement of the electrodes and the surface to be treated allows for the mutual capacitance between a small test conductor placed on the surface to be treated and the first electrode to be greater and preferably substantially greater than the mutual capacitance of the small test conductor with the second electrode.
Claim 2. A plasma treatment apparatus as in Claim 1 wherein the potential difference applied between the first electrode and the second electrode takes the form of periodic pulses where, for a time interval, the first electrode has negative potential with respect to the second electrode followed by a time interval where it has a potential equal to that of the second electrode.
Claim 3. A plasma treatment apparatus as in Claim 1 wherein the potential difference applied between the first electrode and the second electrode takes the form of periodic pulses where, for a time interval, the first electrode has negative potential with respect to the second electrode followed by a time interval where it has a potential positive with respect to that of the second electrode. The positive potential may have an average magnitude equal to, greater than, or less than the average magnitude of the negative potential.
Claim 4. A plasma treatment apparatus as in Claim 1 wherein the power supply connected between the first electrode and the second electrode consists of a step-up transformer of which the primary circuit is connected to a current generator capable of supplying current that varies periodically in time with a waveform in which the current flows for a time interval and then is interrupted for a time interval. This arrangement may create voltage pulses in the secondary circuit that is connected between the first and second electrodes that are of the type described in Claim 3.
Claim 5. A plasma treatment apparatus as in Claim 1 wherein the power supply consists of a step-up transformer of which the primary circuit is connected to a current generator capable of supplying current that varies periodically with a triangular waveform in which the current rises from zero to a maximum during a time interval and at the end of the time interval it decreases more or less rapidly to zero. This arrangement may create pulses in the secondary circuit that is connected between the first and second electrodes that are of the type in Claim 2.
Claim 6. A plasma treatment apparatus as in Claim 1 wherein the background gas from which the plasma is created contains among other gases, gaseous elements or compounds selected from

2019100277 17 Apr 2019 one of the following: hydrogen, nitrogen, oxygen, argon, helium, neon, krypton, ammonia, methane, acetylene, propane, butane, ethane, ethylene, ethanol, water vapour or mixtures thereof.
Claim 7. A plasma treatment apparatus as in Claim 1 wherein the plasma is formed from a background gas or vapour containing the element carbon as a constituent either alone or combined with any of the elements or compounds specified in claim 6. Examples of such gases or vapours include acrylic acid, allylamine and acetic acid.
Claim 8. A plasma treatment apparatus as in Claim 1 in which the frequency of the pulses applied to the electrodes lies in the range 50Hz to 50kHz and more preferably in the range 500Hz to 5kHz.
Claim 9. A plasma treatment apparatus as in Claim 1 where the apparatus has a valve between the gas reservoir and the vacuum pump so that gas may be removed from the gas reservoir and then the gas reservoir may be sealed off from the vacuum pump. The valve between the gas reservoir and the treatment chamber may be sealed off from the vacuum pump by closing the valve whilst the pulses are being applied to the first and second electrodes.
Claim 10. A plasma treatment apparatus as in Claim 1 that has a pressure gauge fitted to the gas reservoir and a means of admitting gas to the gas reservoir to enable the pressure in the reservoir to be set in the range 0.1 mbar to 100 mbar and more preferably in the range lmbar to 10 mbar.
Claim 11. A plasma treatment apparatus as in Claim 1 that has a mechanical vibrator fitted to or adjacent to or in contact with the treatment chamber that applies a mechanical vibration to the walls of the treatment chamber so that powder held within the treatment chamber is agitated and suspended within the gas inside the treatment chamber. Preferably the surfaces to be treated are the surfaces of powder or grains suspended within the volume of the treatment chamber enclosed by the first electrode.
Claim 12. A plasma treatment chamber as in Claim 1 that is fitted with a mechanical vibrator as in Claim 11 where the mechanical vibrator applies periodic forces that induce movements of the wall of the treatment chamber that have a frequency in the range 1 Hz to 100 kHz and more preferably in the range 20Hz to 1 kHz.
Claim 13. A plasma treatment chamber as in Claim 1 and fitted with a mechanical vibrator as in Claim 11 and fitted in a manner as in either or both of Claims 11-12 so that powder held within the treatment chamber is suspended as in Claim 11 and the powder consisting of grains or particles of a certain mean diameter is mixed with grains or particles of more dense material and/or of larger mean diameter so that during agitation the larger or denser grains or particles break up by impact any clumps or clusters of grains or particles of smaller diameter or consisting of less dense material. In this way, the smaller or less dense grains or particles are able to be more effectively dispersed within the gas in the treatment chamber and their surfaces are more effectively treated by ions bombarding the surfaces of the grains or particles.