Abstract:
A device  1  for delivering a partially-ionised first stream of gas, comprises a generator  24  of non-thermal plasma having a plasma generating chamber  25  defining a flow path therethrough for the first stream of gas and communicating downstream of the chamber with at least one first outlet  30  from the device for the partially ionised first stream of gas. The device  1  additionally comprises at least one second outlet  34  from the device for a second stream of gas, the configuration of outlets  30  and  34  enabling the second stream to shield the first stream downstream of the said first outlet  30 . Interaction of the first stream of gas with the surrounding atmosphere is thus kept down.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a device for delivering a partially-ionised first stream of gas and a method of delivering a partially-ionised first stream of gas. 
         [0002]    There is currently much research interest in the use of non-thermal gaseous plasma in a number of therapeutic normal care applications. Suggested uses of a non-thermal gaseous plasma include the treatment of wounds, the cosmetic whitening of teeth, both to remove stains and to whiten tooth enamel, and the cleaning of teeth. See, for example, US-A-2009/004620 and EP-A-2 160 081. 
         [0003]    The non-thermal plasma is typically formed by striking an electric discharge between electrodes and a cell containing a helium atmosphere. Typically, the flow of helium passes through the cell and is then directed from the cell to a surface or substrate to be treated. The effect of the electric discharge is to ionise some of the helium atoms in the cell. Other helium atoms are excited by the electric discharge. That is to say, in each excited helium atom, an electron is raised to a quantum level above its ground state. Excited and ionised helium atoms are no longer inert and can directly or indirectly mediate, for example, the sterilisation, at least in part, or the cleaning of a substrate or surface. 
         [0004]    We have found that exposure of the gas stream downstream of the plasma cell to the atmosphere can profoundly influence its composition. This is as a result of reaction between components of the atmosphere and components of the gas stream. One important component of the atmosphere is water vapour. Since, for example, the concentration of water vapour in the atmosphere fluctuates, the effect on the composition of the gas stream can be variable and unpredictable. 
       SUMMARY OF THE INVENTION 
       [0005]    According to the present invention there is provided a device for delivering a partially-ionised first stream of gas, comprising a generator of non-thermal plasma having a plasma generating chamber defining a flow path therethrough and communicating downstream of the chamber of at least one first outlet from the device for the partially-ionised first stream of gas, wherein the device additionally comprises at least one second outlet from the device, for a second stream of gas, the configuration of outlets enabling the second stream to shield the first stream downstream of the said first outlet, and the device is hand-held and hand-operable and comprises a capsule for storing compressed gas, and a housing for housing the capsule and the generator, wherein the capsule is operable to supply the first and second streams of gas. 
         [0006]    The invention also provides a method of delivering a partially-ionised first stream of gas, comprising passing a first stream of gas through a plasma-generating chamber, generating a non-thermal gaseous plasma in the first stream of gas as it flows through the chamber, discharging the first stream of gas in partially-ionised state, and shielding the discharged first stream of gas with a second stream of gas, wherein the first and second streams of gas are supplied from a common source. 
         [0007]    Preferably, the second outlet is out of communication with the chamber. The second stream of gas therefore bypasses the plasma-generating chamber. By shielding the first stream of gas, the second stream of gas keeps down its interaction with the ambient atmosphere. 
         [0008]    The first stream of gas may comprise helium or argon or a mixture of helium and argon. Helium is preferred in view of its thermal properties. 
         [0009]    The first stream of gas preferably comprises (a) a noble gas selected from helium, argon and mixtures of helium and argon and (b) an additive gas selected from water vapour, air, oxygen, nitrogen, hydrogen, carbon dioxide, carbon monoxide, nitrous oxide and nitric oxide and any mixtures of any two of more thereof, the additive gas warming up to 1% by volume of the first stream of gas. 
         [0010]    The invention is based on a number of different findings in plasma chemistry. The non-thermal plasma may simply be discharged to the atmosphere from the plasma generator, or may travel from the plasma generator in a tube or similar applicator which tube discharges into the atmosphere. If desired, the discharge end of the tube can be inserted the oral cavity and pointed at the surface to be treated. Strictly speaking, once the flow of gas mixture leaves the plasma generator, it is no longer a plasma unless an electrical potential continues to be applied to it. It is simply a gas mixture containing ionic and excited species. The term “plasma” shall be reserved herein for the description of a partially-ionised gas or gas mixture to which an electric potential is applied. The plasma typically glows. The flow of the gas mixture along the applicator shall be referred to as an “afterglow”, and the flow of the gas mixture once it has exited the applicator shall be referred to as a “plume”. Our experimental findings are that if the gas supplied to the plasma generator is essentially pure helium, the resulting plume contains very few ions indeed. It is to be understood that the number of ions in the gas mixture is related to the number of other active species, namely free radicals and excited atoms and molecules. The number of ions may be diminished by recombination with electrons to reform ionised atoms and by reaction with gaseous species entrained from the surrounding atmosphere. 
         [0011]    We have further found that there is no simple linear relationship between the population of ionic species in the plume and the concentration of additive gas in the first gas stream. On the contrary, the maximum population is reached when the concentration of additive gas in the first gas stream is much less than 1% by volume. Under our experimental conditions, we were finding, depending on the choice of the additive gas, that the maximum population of ions was achieved at concentration levels of additive gas of less than 0.5% by volume but more than 0.01% by volume. 
         [0012]    The total concentration of additive gas in the first gas stream is preferably therefore in the range of 0.01% by volume to 0.5% by volume, more preferably in the range of 0.02% by volume to 0.25% by volume. 
         [0013]    We attribute these results partly to a tendency for the additive gas to quench the non-thermal plasma in the plasma generator. Once a maximum has been reached, the plasma-quenching effect reduces the total number of ions present in the plume. Further, in the example of helium as the noble gas, because it has particularly high ionisation energy, ions of the additive gas will, we believe, be formed preferentially in the discharge. 
         [0014]    The first stream of gas is preferably provided from a pressure vessel such as a gas cylinder in which it is preformed. 
         [0015]    The first stream of gas may be delivered to the oral cavity and may perform one of the following non-clinical or cosmetic oral treatments: 
         [0016]    the removal of stains from teeth; 
         [0017]    whitening of tooth enamel; 
         [0018]    the general cleaning of teeth to destroy harmful bacteria; 
         [0019]    the interdental cleaning of teeth; 
         [0020]    the freshening of breath; 
         [0021]    the treatment of halitosis; 
         [0022]    the treatment of gingivitis; 
         [0023]    the treatment of periodontal disease. 
         [0024]    In each of the above examples, the flow of the first gas stream downstream of the plasma generator may be directed at the tooth or teeth to be treated, or the area of gums to be treated, or in the case of breath freshening or the treatment of halitosis, at the back of the mouth for a sufficient period of time to have a desired effect. 
         [0025]    Normal treatment time periods for a typical treatment are from 10 seconds to 10 minutes. Treatment may be repeated daily or at shorter or longer intervals. 
         [0026]    Preferred additive gases are those that readily form hydroxyl radicals or active oxygen atoms or molecules. Air, oxygen and water vapour are thus preferred. 
         [0027]    The plasma generator preferably has a gas outlet temperature of from 10° C. to 40° C. Higher gas temperatures are generally unsuitable for oral treatments and may damage the mouth or teeth if sustained for too long a period. Temperatures lower than 10° C. may be found uncomfortable by the person undergoing the treatment and in any event are difficult to achieve without unnecessary cooling of the first gas stream. The plasma generator is preferably operated at atmospheric pressure or in the range 0.5 bar to 2.0 bar. 
         [0028]    A number of different configurations of first and second outlets may be employed. For example the said second outlet may be annular and may circumscribe the first outlet. In such examples, the first and second outlets are coaxial. In other examples, the second outlet comprises a ring of orifices, the ring circumscribing the first outlet. The ring is typically coaxial with the first outlet. 
         [0029]    Typically the said outlets are formed in a nozzle. If desired, the device can take the form of a toothbrush, the said outlets being dispersed in the head of the toothbrush. 
         [0030]    A device according to a typical embodiment comprises a control for selectively releasing gas from the capsule so as to form said first and second gas streams. 
         [0031]    The housing typically additionally houses a source of electrical energy and energising means electrically connected to the source of electrical energy for creating an electrical discharge within the first stream of gas in the generator. 
         [0032]    The gas capsule preferably has a maximum storage pressure of at least 50 bar, typically one in the range 100 bar to 200 bar. The gas capsule has a relatively small capacity, typically in the range 5 to 100 ml water capacity. 
         [0033]    The device according to the invention preferably additionally comprises a pressure reducer for reducing pressure of gas released from the capsule. The pressure reducer may comprise an expansion chamber or a pressure control valve. A device according to the invention may also include an on-off valve, for example a solenoid valve, downstream of the pressure reducer, the on-off valve being operable by means of a manual control on the exterior of the housing. 
         [0034]    A device according to a typical embodiment also comprises a gas distribution passage downstream of the on-off valve, the gas distribution passage communicating with the said first outlet and the said second outlet. 
         [0035]    Typically, the energising means includes a time-delay electrical circuit for delaying the creation of the electrical discharge for a chosen time after initiation of flow of the first gas stream through the generator. Such an arrangement enables impurities to be flushed from the chamber prior to striking the electrical discharge. 
         [0036]    If desired the said first outlet provides an outlet from the chamber. Alternatively, the chamber may have an outlet communicating with an applicator and the first outlet is provided at the distal end of the applicator. The distal end of the applicator may thus be of a size and shape that enables it to be inserted into the mouth and pointed, say, at a tooth to be treated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Methods and devices according to the invention will now be described by way of example with reference to the accompanying drawings, in which: 
           [0038]      FIG. 1  is a schematic diagram of a device according to the invention; 
           [0039]      FIG. 2  is an end view of a first form of applicator for use in the device shown in  FIG. 1 ; 
           [0040]      FIG. 3  is an end view of an alternative form of applicator for use in the device shown in  FIG. 1 ; and 
           [0041]      FIG. 4  is a schematic sectional side elevation of a third form of applicator for use in the device shown in  FIG. 1 . 
       
    
    
       [0042]    The drawings are not to scale. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0043]    Referring to  FIG. 1 , a device  1  is shown for generating a non-thermal plasma and to emit a plume comprising partially-ionised gas. The flow of gas plasma is generated and emitted from the device generally at atmospheric pressure. The device comprises a gas capsule  4 , or pressure vessel, for holding a gas or gas mixture under pressure and forming a flow of gas through a generator  24  of non-thermal plasma to an applicator  26 . Gas released from the gas capsule  4  is energised in the generator  24  to form the non-thermal gaseous plasma. 
         [0044]    All the components of the device save for the applicator nozzle  26  and control button(s) are contained within a housing  2 . The applicator nozzle  26  engages the housing  2 , as will be described below. The housing  2  is typically of a size and shape such that it can be held and operated in the hand. Similarly, the total weight of the device  1  is such that it can be readily held and operated in the hand. The housing  2  may be made of any suitable plastics material. The gas capsule  4  has a water capacity in the range 5 to 100 ml. Typically, its water capacity in the range 20 to 40 ml. The gas capsule  4  is formed with a neck  6  and a mouth  8 . A closure  10  is provided in or over the mouth  8 . The closure  10  may be a puncturable diaphragm or a valve such as a Schrader valve. 
         [0045]    The gas mixture is stored under pressure in the gas capsule  4 . The storage pressure is typically at least 50 bar. For example, it may be in the range 100 bar to 200 bar. The gas mixture may be any of those described herein above. For example, it can comprise 99.5% by volume of helium with a balance of one or more additive gases selected from water vapour, oxygen, nitrogen and air. 
         [0046]    The housing  2  is provided with an externally accessible compartment  13  in which the gas capsule  4  is received. The compartment  13  may be provided with a holder  12  with which the gas capsule  2  may be engaged. The act of engaging the gas capsule  2  with the holder  12  may actuate a mechanism to puncture or otherwise open the closure  10  so as to permit the flow of gas out of the gas capsule  4 . Alternatively, the holder may have a mechanism (not shown) which permits such opening of the gas capsule  2  by operation of an external actuator (not shown) at any time when it is desired to deliver gas from the capsule  4  to the generator  24  of non-thermal plasma. 
         [0047]    The housing  2  is internally configured so as to enable gas released from the capsule  4  to flow in a controlled manner to the generator  24  of non-thermal plasma. Immediately downstream of the holder  12  there is provided a pressure reducer  14 . The pressure reducer  14  may simply take the form of an expansion chamber or a pressure control valve. The pressure control valve may be of the kind disclosed in US-A-4 655 335. Such a pressure reducer  14  can be designed to give a downstream pressure which remains relatively constant even though the gas pressure in the capsule  4  falls as gas is drawn from it. If desired, the downstream pressure can be set at about 2 bar absolute. 
         [0048]    An on-off valve  16 , which may, for example, take the form of a solenoid valve is positioned within the housing  2  downstream of the pressure reducer  14 . The on-off valve  16  will normally be in a closed position. When it is desired to deliver gas, after having engaged the gas capsule  4  with the holder  12 , the on-off valve  16  may be opened by depression of a switch of push-button  48  located externally of the housing in a position such that it can be readily depressed by the user. The on-off valve  16  controls communication of the gas capsule  12  with a gas distribution passage  18 . The gas distribution passage  18  feeds gas drawn from the gas capsule  4  to a first conduit  20  and a second conduit  22 , the conduits  20  and  22  being in parallel. 
         [0049]    The first conduit  20  communicates with a gas inlet to the plasma generating chamber  25  of the generator  24  of non-thermal plasma. The chamber  25  has an outlet which communicates with an axial passage  28  through the applicator nozzle  26 . A first stream of partially ionised gas is thus able to issue from the nozzle  26 . The second conduit  22  by-passes the generator  24  and communicates with an outer passage or passages  32  formed through the nozzle  26  from which a second stream of gas issues as a shielding gas to limit reaction between the ambient atmosphere and the first stream of gas. The passage  28  has a circular outlet  30 . A single outer passage  32  may surround the passage  28  and be coaxial therewith. The passage  32  may have an annular outlet  34  as illustrated in  FIG. 2 . Alternatively there may be from, say, 4 to 6 passages  32 , each of circular cross-section surrounding the central gas passage  28 . Each of the passages  32  has a circular outlet  34 . The outlets  34  all lie on a notional ring which is coaxial with the passage  28 . Such an arrangement is shown in  FIG. 3 . In either case, the second conduit  22  communicates with a gas distributor  35 . In the embodiment shown in  FIG. 2 , the gas distributor  35  is located at the side of the applicator nozzle  26  and communicates with the outer gas passage  32  through suitable radial bores (not shown). In the embodiment shown in  FIG. 3 , the proximal ends of the passages  32  may terminate in the distributor  35 . 
         [0050]    Referring again to  FIG. 1 , the chamber  25  of the generator  24  typically has a plurality of electrodes  40  and  42  associated therewith. A signal generator  44  located within the housing  2  is adapted to provide a plasma generating voltage across the electrodes  40  and  42  such that a glow discharge can be created in the gas mixture flowing through the chamber  25 . The gas mixture is partially ionised by the voltage signal. Depending on the composition of the gas mixture, the partially ionised gas has the characteristic glow of a particular colour. Electrical energy is provided to the signal generator  44  by the battery or batteries  46  which are received in an externally accessible compartment  47  formed in the housing  2 . The battery of batteries  46  may be disposable or rechargeable. When the device  1  is not being used, the battery or batteries  46  are not in electrical circuit with the signal generator  44 . When it is desired to use the device  1  in order, say, to clean ones teeth the switch or push button  48  may be actuated to close the switch  50  and thus place the signal generator  44  in electrical circuit with the battery or batteries  46 . If desired, instead of using the same control that operates the on-off valve  16 , there may be a second push button (not shown) which is dedicated to opening and closing the switch  50 . The battery or batteries  46  may typically provide a DC voltage in the order of 12V. The signal generator  44  comprises electrical circuits that are able to transform the voltage into a relatively high frequency pulsed DC or AC signal of the required voltage for the generation of the non-thermal plasma in the generator  24 . The peak voltage generated by the signal generator  44  may be in the order of 1 to 6 kV. Each voltage peak may last for one millisecond and voltage peaks may be spaced by an interval of 5 to 10 milliseconds. Electrical circuits including one or more transformers for achieving such a transformation of the voltage are known in the art. Further information is contained in our co-pending application No. PCT/GB2010/000413. Typically, the discharge within the gas mixture flowing through the chamber  25  is through dielectric members (not shown) associated with the electrodes  40  and  42 . 
         [0051]    In order to operate the device  1  shown in  FIGS. 1 to 3 , the battery or batteries  46  and the gas capsule  4  may be fitted. The holder  12  is then actuated to open the gas capsule, and the push button or buttons  48  depressed in order to start the flow of gas to the first and second conduits  20  and  22  and to actuate the signal generator  44 . Typically, there is an arrangement (not shown) whereby no voltage is struck across the electrodes  40  and  42  until gas has passed for a chosen period of time (say 15 seconds) through the chamber  25 . This is to enable impurities to be flushed from the chamber  25  prior to the striking of the discharge therein. Creating an electrical discharge in the gas mixture flowing through the chamber  25  results in the formation of a non-gaseous plasma. This plasma has a temperature of less than 40° C. at the outlet from the chamber  25 . Depending on the composition of the gas mixture, the plasma contains a range of different ionic, excited and free radical species. Oxygen radicals and ions and hydroxyl radicals and ions are believed to be particularly effective in, for example, cleaning or whitening teeth. Other ionic and radical species, for example, nitrogen may mediate the formation of active oxygen and hydroxyl species at the surface of a tooth being cleaned or whitened. In order to use the device  1 , therefore, the applicator nozzle  26  is pointed, say, at a tooth or teeth to be cleaned and moved thereacross. 
         [0052]    The gas supplied to the outer passage or passages  32  of the applicator nozzle  26  shields from premature reaction with atmospheric gases the gas stream issuing from the central outlet  30  of the applicator nozzle  26 . As a result, more active species are available for reaction at the surface of the tooth or the surrounding gum. 
         [0053]    Typically the shielding gas is approximately equal to the flow of partially ionised gas. If the gas capsule  4  has a water capacity of 21 ml (which is a standard size for such gas capsules) and is charged with gas mixture to a pressure of 200 bar, and the delivery rate of partially ionised gas mixture is in the order of 0.5 to 1 litre per minute the gas capsule  2  typically gives a treatment time in the order of three and a half to 7 minutes and allows additional time for purging of the chamber  25  immediately prior to treatment. 
         [0054]    The treatment may be performed after conventional cleaning of teeth with a toothbrush and toothpaste. The treatment with the device  1  may be continued until the gas capsule  4  is approaching exhaustion. A full gas capsule may be substituted for an empty one after each treatment. If desired, the gas capsule  4  may be refillable. Alternatively, it may be disposable. 
         [0055]    An alternative embodiment of applicator is shown in  FIG. 4 . The applicator shown in  FIG. 4  may be generally the same as that shown in  FIGS. 1 and 2  or  FIGS. 1 and 3 , but is now in the form of a brush. That is to say the nozzle  26  is provided with an outer sheath  60 , the distal end of which is provided with a ring  62  of bristles. The device  1  may therefore be used to brush the teeth at the same time as the partially-ionised gas is directed at them. 
         [0056]    Referring again to  FIGS. 1 to 3 , it is desirable that the applicator nozzle  24  is designed so as to minimise radial gaps between the shielding gas and the central partially-ionised gas stream. Accordingly, the radial distance between the outlet or outlets  34  and the outlet  30  is preferably kept to a minimum. As a result, trapping of atmospheric air between the shielding gas stream and the central partially-ionised gas stream is kept down. The reduction of such impurities is aided by a natural tendency for the gas streams to diverge on issuing from the outlets  30  and  34 .