Abstract:
An apparatus and method for improved control of low viscosity fluid flow during electrohydrodynamic spray deposition of the fluid to coat small targets, such as medical devices like stents. The apparatus includes a target holder which applies a first electrical potential to a target, a coating fluid transporter such as a wick, a siphon tube or a siphon tube with a wick therein along which the coating fluid flows from a reservoir to a dispensing end of the transporter, and an electrode which applied a second electrical potential to the coating fluid sufficient to cause the coating fluid to be attracted from the dispensing end of the transporter toward the target. This provides a target coating apparatus with highly self-regulating coating fluid flow characteristics despite the low viscosity of the coating fluid, while producing highly consistent and uniform target coatings.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally regards an apparatus and method for application of coatings to small devices, such as the application of therapeutic and protective coatings to stents. More specifically, the present invention pertains to an apparatus and method for improved control of very low viscosity coating fluid flow during electrohydrodynamic spray deposition of coatings onto targets such as medical stents.  
         BACKGROUND  
         [0002]    Medical implants are used for innumerable medical purposes, including the reinforcement of recently re-enlarged lumens, the replacement of ruptured vessels, and the treatment of disease such as vascular disease by local pharmacotherapy, i.e., delivering therapeutic drug doses to target tissues while minimizing systemic side effects. Such localized delivery of therapeutic agents has been proposed or achieved using medical implants which both support a lumen within a patient&#39;s body and place appropriate coatings containing absorbable therapeutic agents at the implant location. Examples of such medical devices include catheters, guide wires, balloons, filters (e.g., vena cava filters), stents, stent grafts, vascular grafts, intraluminal paving systems, implants and other devices used in connection with drug-loaded polymer coatings. Such medical devices are implanted or otherwise utilized in body lumina and organs such as the coronary vasculature, esophagus, trachea, colon, biliary tract, urinary tract, prostate, brain, and the like.  
           [0003]    The term “therapeutic agent” as used herein includes one or more “therapeutic agents” or “drugs”. The terms “therapeutic agents” and “drugs” are used interchangeably herein and include pharmaceutically active compounds, nucleic acids with and without carrier vectors such as lipids, compacting agents (such as histones), virus (such as adenovirus, andenoassociated virus, retrovirus, lentivirus and α-virus), polymers, hyaluronic acid, proteins, cells and the like, with or without targeting sequences.  
           [0004]    Specific examples of therapeutic agents used in conjunction with the present invention include, for example, pharmaceutically active compounds, proteins, cells, oligonucleotides, ribozymes, anti-sense oligonucleotides, DNA compacting agents, gene/vector systems (i.e., any vehicle that allows for the uptake and expression of nucleic acids), nucleic acids (including, for example, recombinant nucleic acids; naked DNA, cDNA, RNA; genomic DNA, cDNA or RNA in a non-infectious vector or in a viral vector and which further may have attached peptide targeting sequences; antisense nucleic acid (RNA or DNA); and DNA chimeras which include gene sequences and encoding for ferry proteins such as membrane translocating sequences (“MTS”) and herpes simplex virus-1 (“VP22”)), and viral, liposomes and cationic and anionic polymers and neutral polymers that are selected from a number of types depending on the desired application. Non-limiting examples of virus vectors or vectors derived from viral sources include adenoviral vectors, herpes simplex vectors, papilloma vectors, adeno-associated vectors, retroviral vectors, and the like. Non-limiting examples of biologically active solutes include anti-thrombogenic agents such as heparin, heparin derivatives, urokinase, and PPACK (dextrophenylalanine proline arginine chloromethylketone); antioxidants such as probucol and retinoic acid; angiogenic and anti-angiogenic agents and factors; anti-proliferative agents such as enoxaprin, angiopeptin, rapamycin, angiopeptin, monoclonal antibodies capable of blocking smooth muscle cell proliferation, hirudin, and acetylsalicylic acid; anti-inflammatory agents such as dexamethasone, prednisolone, corticosterone, budesonide, estrogen, sulfasalazine, acetyl salicylic acid, and mesalamine; calcium entry blockers such as verapamil, diltiazem and nifedipine; antineoplastic/antiproliferative/anti-mitotic agents such as paclitaxel, 5-fluorouracil, methotrexate, doxorubicin, daunorubicin, cyclosporine, cisplatin, vinblastine, vincristine, epothilones, endostatin, angiostatin and thymidine kinase inhibitors; antimicrobials such as triclosan, cephalosporins, aminoglycosides, and nitorfurantoin; anesthetic agents such as lidocaine, bupivacaine, and ropivacaine; nitric oxide (NO) donors such as lisidomine, molsidomine, L-arginine, NO-protein adducts, NO-carbohydrate adducts, polymeric or oligomeric NO adducts; anti-coagulants such as D-Phe-Pro-Arg chloromethyl ketone, an RGD peptide-containing compound, heparin, antithrombin compounds, platelet receptor antagonists, anti-thrombin antibodies, anti-platelet receptor antibodies, enoxaparin, hirudin, Warafin sodium, Dicumarol, aspirin, prostaglandin inhibitors, platelet inhibitors and tick antiplatelet factors; vascular cell growth promotors such as growth factors, growth factor receptor antagonists, transcriptional activators, and translational promotors; vascular cell growth inhibitors such as growth factor inhibitors, growth factor receptor antagonists, transcriptional repressors, translational repressors, replication inhibitors, inhibitory antibodies, antibodies directed against growth factors, bifunctional molecules consisting of a growth factor and a cytotoxin, bifunctional molecules consisting of an antibody and a cytotoxin; cholesterol-lowering agents; vasodilating agents; agents which interfere with endogeneus vascoactive mechanisms; survival genes which protect against cell death, such as anti-apoptotic Bcl-2 family factors and Akt kinase; and combinations thereof. Cells can be of human origin (autologous or allogenic) or from an animal source (xenogeneic), genetically engineered if desired to deliver proteins of interest at the insertion site. Any modifications are routinely made by one skilled in the art.  
           [0005]    Polynucleotide sequences useful in practice of the invention include DNA or RNA sequences having a therapeutic effect after being taken up by a cell. Examples of therapeutic polynucleotides include anti-sense DNA and RNA; DNA coding for an anti-sense RNA; or DNA coding for tRNA or rRNA to replace defective or deficient endogenous molecules. The polynucleotides can also code for therapeutic proteins or polypeptides. A polypeptide is understood to be any translation product of a polynucleotide regardless of size, and whether glycosylated or not. Therapeutic proteins and polypeptides include as a primary example, those proteins or polypeptides that can compensate for defective or deficient species in an animal, or those that act through toxic effects to limit or remove harmful cells from the body. In addition, the polypeptides or proteins that can be injected, or whose DNA can be incorporated, include without limitation, angiogenic factors and other molecules competent to induce angiogenesis, including acidic and basic fibroblast growth factors, vascular endothelial growth factor, hif-1, epidermal growth factor, transforming growth factor α and β, platelet-derived endothelial growth factor, platelet-derived growth factor, tumor necrosis factor α, hepatocyte growth factor and insulin like growth factor; growth factors; cell cycle inhibitors including CDK inhibitors; anti-restenosis agents, including p15, p16, p18, p19, p21, p27, p53, p57, Rb, nFkB and E2F decoys, thymidine kinase (“TK”) and combinations thereof and other agents useful for interfering with cell proliferation, including agents for treating malignancies; and combinations thereof. Still other useful factors, which can be provided as polypeptides or as DNA encoding these polypeptides, include monocyte chemoattractant protein (“MCP-1”), and the family of bone morphogenic proteins (“BMP&#39;s”). The known proteins include BMP-2, BMP-3, BMP-4, BMP-5, BMP-6 (Vgr-1), BMP-7 (OP-1), BMP-8, BMP-9, BMP-10, BMP-11, BMP-12, BMP-13, BMP-14, BMP-15, and BMP-16. Currently preferred BMP&#39;s are any of BMP-2, BMP-3, BMP-4, BMP-5, BMP-6 and BMP-7. These dimeric proteins can be provided as homodimers, heterodimers, or combinations thereof, alone or together with other molecules. Alternatively or, in addition, molecules capable of inducing an upstream or downstream effect of a BMP can be provided. Such molecules include any of the “hedgehog” proteins, or the DNA&#39;s encoding them.  
           [0006]    Coatings used with the present invention may comprise a polymeric material/drug agent matrix formed, for example, by admixing a drug agent with a liquid polymer, in the absence of a solvent, to form a liquid polymer/drug agent mixture. Curing of the mixture typically occurs in-situ. To facilitate curing, a cross-linking or curing agent may be added to the mixture prior to application thereof. Addition of the cross-linking or curing agent to the polymer/drug agent liquid mixture must not occur too far in advance of the application of the mixture in order to avoid over-curing of the mixture prior to application thereof. Curing may also occur in-situ by exposing the polymer/drug agent mixture, after application to the luminal surface, to radiation such as ultraviolet radiation or laser light, heat, or by contact with metabolic fluids such as water at the site where the mixture has been applied to the luminal surface. In coating systems employed in conjunction with the present invention, the polymeric material may be either bioabsorbable or biostable. Any of the polymers described herein that may be formulated as a liquid may be used to form the polymer/drug agent mixture.  
           [0007]    In a preferred embodiment, the polymer used to coat the medical device is provided in the form of a coating on an expandable portion of a medical device. After applying the drug solution to the polymer and evaporating the volatile solvent from the polymer, the medical device is inserted into a body lumen where it is positioned to a target location. In the case of a balloon catheter, the expandable portion of the catheter is subsequently expanded to bring the drug-impregnated polymer coating into contact with the lumen wall. The drug is released from the polymer as it slowly dissolves into the aqueous bodily fluids and diffuses out of the polymer. This enables administration of the drug to be site-specific, limiting the exposure of the rest of the body to the drug.  
           [0008]    The polymer used in the present invention is preferably capable of absorbing a substantial amount of drug solution. When applied as a coating on a medical device in accordance with the present invention, the dry polymer is typically on the order of from about 1 to about 50 microns thick. In the case of a balloon catheter, the thickness is preferably about 1 to 10 microns thick, and more preferably about 2 to 5 microns. Very thin polymer coatings, e.g., of about 0.2-0.3 microns and much thicker coatings, e.g., more than 10 microns, are also possible. It is also within the scope of the present invention to apply multiple layers of polymer coating onto a medical device. Such multiple layers are of the same or different polymer materials.  
           [0009]    The polymer of the present invention may be hydrophilic or hydrophobic, and may be selected from the group consisting of polycarboxylic acids, cellulosic polymers, including cellulose acetate and cellulose nitrate, gelatin, polyvinylpyrrolidone, cross-linked polyvinylpyrrolidone, polyanhydrides including maleic anhydride polymers, polyamides, polyvinyl alcohols, copolymers of vinyl monomers such as EVA, polyvinyl ethers, polyvinyl aromatics, polyethylene oxides, glycosaminoglycans, polysaccharides, polyesters including polyethylene terephthalate, polyacrylamides, polyethers, polyether sulfone, polycarbonate, polyalkylenes including polypropylene, polyethylene and high molecular weight polyethylene, halogenated polyalkylenes including polytetrafluoroethylene, polyurethanes, polyorthoesters, proteins, polypeptides, silicones, siloxane polymers, polylactic acid, polyglycolic acid, polycaprolactone, polyhydroxybutyrate valerate and blends and copolymers thereof as well as other biodegradable, bioabsorbable and biostable polymers and copolymers. Coatings from polymer dispersions such as polyurethane dispersions (BAYHDROL®, etc.) and acrylic latex dispersions are also within the scope of the present invention. The polymer may be a protein polymer, fibrin, collage and derivatives thereof, polysaccharides such as celluloses, starches, dextrans, alginates and derivatives of these polysaccharides, an extracellular matrix component, hyaluronic acid, or another biologic agent or a suitable mixture of any of these, for example. In one embodiment of the invention, the preferred polymer is polyacrylic acid, available as HYDROPLUS® (Boston Scientific Corporation, Natick, Mass.), and described in U.S. Pat. No. 5,091,205, the disclosure of which is hereby incorporated herein by reference. U.S. Pat. No. 5,091,205 describes medical devices coated with one or more polyisocyanates such that the devices become instantly lubricious when exposed to body fluids. In another preferred embodiment of the invention, the polymer is a copolymer of polylactic acid and polycaprolactone.  
           [0010]    The delivery of expandable stents is a specific example of a medical procedure that may involve the deployment of coated implants. Expandable stents are tube-like medical devices, typically made from stainless steel, Tantalum, Platinum or Nitinol alloys, designed to be placed within the inner walls of a lumen within the body of a patient. These stents are typically maneuvered to a desired location within a lumen of the patient&#39;s body and then expanded to provide internal support for the lumen. The stents may be self-expanding or, alternatively, may require external forces to expand them, such as by inflating a balloon attached to the distal end of the stent delivery catheter.  
           [0011]    Where a stent or other medical device is to be coated, care must be taken during its manufacture to ensure the coating is properly applied and firmly adherent. When the amount of coating is insufficient or is depleted through stripping of poorly adherent coating during manufacture or deployment within the patient&#39;s body, the device&#39;s effectiveness may be compromised and additional risks may be inured into the procedure. For example, when the coating of the device includes a therapeutic, if some of the coating were removed during deployment, the therapeutic may no longer be able to be administered to the target site in a uniform and homogenous manner. Thus, some areas of the target site may receive high quantities of therapeutic while others may receive low quantities of therapeutic. Similarly, if the therapeutic is ripped from the device it can reduce or slow down the blood flowing past it, thereby increasing the threat of thrombosis or, if it becomes dislodged, the risk of embolisms. In certain circumstances, the removal and reinsertion of the device through a second medical procedure may be required where the coatings have been damaged or are defective.  
           [0012]    The mechanical process of applying a coating onto a stent or other medical device may be accomplished in a variety of ways, including, for example, spraying the coating substance onto the device, so-called spin-dipping, i.e., dipping a spinning device into a coating solution to achieve the desired coating, and electrohydrodynamic fluid deposition, i.e., applying an electrical potential difference between a coating fluid and a target to cause the coating fluid to be discharged from the dispensing point and drawn toward the target.  
           [0013]    Common to these processes is the need to apply the coating in a manner to ensure that an intact, robust coating of the desired thickness is formed on the stent. Obtaining a uniform coating often becomes difficult when working with coating fluids with low viscosities. In the case of electrohydrodynamic coating, when viscosity drops to a low viscosity, for example in the vicinity of one centipoise, it is difficult to control how much coating fluid is exposed to the electrohydrodynamic conditions. Non-uniform coating application is also a problem at very low coating fluid flow rates, where difficulty controlling conventional pumping means can result in erratic fluid flow from the coating dispenser. Lack of uniformity can be a problem as well where the coating fluid flow is applied to individual target work pieces in a start-stop fashion if the low viscosity coating fluid is given enough time to retreat from the coating fluid discharge point.  
           [0014]    Further, obtaining a coating of the desired thickness requires precise coordination of the operating elements of the coating dispensing apparatus. In the case, for example, of an electrohydrodynamic coating system that employs a positive displacement pump to power the coating fluid dispenser, the electrohydrodynamic conditions must be closely matched to the pump in order to consistently achieve the desired uniform target coating. For example, if the electrohydrodynamic impulse is not applied as quickly as the pump is able to supply coating fluid to the dispensing head, excess fluid drops may form on the dispensing head and randomly alter the coating fluid deposition rate and spray pattern. Conversely, if the electrohydrodynamic impulse is faster than the pump can deliver coating fluid to the dispensing head, insufficient coating may result when the amount of coating material at the dispensing head is depleted by the electrohydrodynamic pulse before the pump can supply an adequate volume of additional coating fluid to the dispensing head. Under these conditions, it is difficult to achieve and sustain the required pump and electrohydrodynamic impulse coordination over the course of stent coating production runs.  
           [0015]    Thus, there is a need for an apparatus and method for electrohydrodynamically applying a low viscosity coating fluid to a target such as a stent in a manner that results in a high quality coating of desired thickness, and preferably accomplishes this objective at high production rates.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention is directed to an apparatus and a method for overcoming the foregoing disadvantages. Specifically, there is a provided an apparatus in which a target is held at a first electrical potential, and a coating fluid transporter configured to provide consistent flow of the coating material to a discharge point. When a second electrical potential is applied to the coating fluid, it is uniformly dispensed from the discharge point toward the target.  
           [0017]    More specifically, the present invention includes a coating fluid transporter device, such as a wick, a siphon tube or a siphon tube containing a wick, that is sized to provide a stable fluid flow to the coating fluid spray dispenser when responding to flow demands generated by the application of the electrical potential, and to prevent the coating fluid from draining out of the transporter while the electrical potential is not being applied, such that the transporter remains primed and ready to apply the coating fluid to a subsequent target when the electrical potential is next applied.  
           [0018]    In the case of a wick transporter, the coating fluid moves by capillary action from a reservoir end of the wick to a discharge end, where application of an electrical potential between the coating fluid and the target will cause the coating fluid to be discharged from the wick tip toward the target. Similarly, a siphon tube transporter of appropriate inner diameter for the viscosity of the coating fluid to be applied may be utilized to transport the coating fluid from the reservoir to the discharge point. A further embodiment features the combination of an appropriately-sized siphon tube with a wick within the tube&#39;s inner lumen along its interior to provide further enhanced coating fluid flow stability, particularly during intermittent coating fluid application.  
           [0019]    The present invention thus provides the desired uniform coating fluid application to a target in a manner well suited to high volume, efficient coating of devices such as stents. In addition, due to the stability of coating fluid flow along an appropriately-sized wick or siphon tube, the coating fluid flow is self-regulating, with the supply being determined by the electrohydrodynamic conditions. The invention thereby may be used without additional flow control devices, such as pumps or flow control valves. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic cross-section view of an electrohydrodynamic coating fluid delivery apparatus in accordance with a first embodiment of the present invention.  
         [0021]    [0021]FIG. 2 is a schematic cross-section view of an electrohydrodynamic coating fluid delivery apparatus in accordance with a second embodiment of the present invention.  
         [0022]    [0022]FIG. 3 is a schematic cross-section view of an electrohydrodynamic coating fluid delivery apparatus in accordance with a third embodiment of the present invention.  
         [0023]    [0023]FIG. 4 is a schematic cross-section view of an electrohydrodynamic coating fluid delivery apparatus in accordance with a fourth third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]    A first embodiment of the present invention is illustrated in FIG. 1. In this embodiment, a target  1  to be coated with a coating fluid is held by target holder  2 . Target  1  in this instance is a stent that is to be coated with a therapeutic material. In addition to holding stent  1  in a position suitable for coating application, stent holder  2  functions as a electrode, and is maintained at a first electrical potential. Stent holder  2  may hold stent  1  by any number of means, such as by the stent holders described in U.S. patent application Ser. No. 10/198,094 (the disclosure of which is hereby expressly incorporated by reference herein), and may be adapted for use with high-speed automated stent handling apparatus.  
         [0025]    Directly beneath stent  1  is a coating fluid spray dispensing device  3 , schematically illustrated in FIG. 1 as comprising a coating fluid reservoir  4  holding a quantity of coating fluid  5 , a solution of a therapeutic material in either an organic solvent or water with low viscosity (preferably below 100 centipoise), to be applied to the target stent. An electrode  6  is provided in contact with coating fluid  5  for establishing an electrical potential in the coating fluid. A wick  7  is provided in the reservoir with a first device end comprising a first wick surface  8  in contact with coating fluid  5 . Wick  7  is supported in dispensing device  3  by a reservoir cover  9 , which maintains wick  7  at a position directly below target stent  1 . Wick  7  may be any of a number of well-known non-conductive fibrous or porous materials suitable for transporting liquid solutions via capillary action and resistant to degradation by the solution, e.g., and organic solvent-resistant engineered plastic. In this stent coating embodiment, wick  7  is cylindrical in shape. The diameter of wick  7  may be between approximately 0.5 mm and 3 mm, varied as desired to obtain a desired coating fluid flow rate. The distance between the dispensing end of the wick and the target may be maintained over a broad range, as the voltage difference that drives the electrohydrodynamic discharge of coating fluid toward the target may be readily adjusted to ensure the coating fluid reaches the target with a desired coating efficiency. Typical separation distances may be approximately 50-150 mm from the target. The wick  7  may alternatively be constructed of one or more tubes of very small diameter sufficient to transport the fluid via capillary action.  
         [0026]    In operation, coating fluid  5  climbs along the structure of wick  7  via capillary action to reach the second or spray dispensing end  10  of wick  7 . In the absence of the application of an electrical potential, surface tension forces between the coating fluid and the wick ensure that the coating fluid that reaches dispensing end  10  remains on wick  7 . When coating material is to be applied to stent  1 , an electrical potential is applied to reservoir electrode  6 . Because coating fluid  5  is in direct contact with electrode  6 , the coating fluid, including the coating fluid at wick dispensing end  10 , is subjected to the same electrical potential. By maintaining the electrical potential on electrode  6  higher than the electrical potential on stent holder/electrode  2  (at a ground potential relative to electrode  6  in this embodiment), the coating fluid at wick dispensing end  10  is attracted to the lower potential at target stent  1 . When the potential difference between wick end  10  and stent  1  becomes large enough, typically on the order of between 6,000-20,000 volts, a portion of coating fluid  5  leaves dispensing end  10  and travels toward stent  1  to apply the desired coating. In th present embodiment, a preferred voltage of 12,000 volts is maintained. As the coating fluid at wick end  10  is dispensed toward stent  1 , additional coating fluid flows onto wick  7  at first surface  8  and moves toward wick end  10  to provide consistent fluid replenishment flow to end  10 . The presence of reservoir cover  9  prevents bulk movement of coating fluid  5  in reservoir  4  directly toward stent  1 .  
         [0027]    The amount of coating fluid  5  delivered toward stent  1  may be adjusted by altering the length of time the electrical potential is applied to electrode  6 , adjusting the potential difference between the first and second electrodes, and/or by adjusting the flow rate of the coating fluid up wick  7 , for example by increasing or decreasing the cross-sectional area of the wick or changing the amount of wick surface area in contact with the coating fluid. This latter adjustment may be made by inserting wick  7  deeper into coating fluid  5  to increase the area of first surface  8  in contact with the fluid, or by altering the level of coating fluid  5  in reservoir  4 .  
         [0028]    After a predetermined time has lapsed, the electrical potential applied to electrode  6  is removed, eliminating the attraction of coating fluid  5  to stent  1 , and thereby halting the coating fluid spray from dispensing end  10 . When the electrical potential is removed from electrode  6 , the momentum of the coating fluid flowing up the wick is immediately arrested by the surface tension of the coating fluid along the entire length of the wick  7  structure. As a result, there is no tendency for excess coating fluid to accumulate at, or leak from, wick dispensing end  10 . The fluid surface tension also ensures that the coating fluid does not immediately retreat back toward first wick surface  8  and return to reservoir  4 . Wick  7  therefore remains primed with coating fluid, ready for the next target coating operation, even where the fluid has a very low viscosity. The precise control of coating fluid by the present invention provides the additional benefit of permitting spray dispensing device  3  to be rapidly cycled in automated, high-speed target coating processes while still maintaining the desired precise and uniform coating application.  
         [0029]    The present embodiment is not limited to target and spray device arrangements in which the target is held above the spray dispensing head, as the interaction of the surface tension forces between the wick and the coating fluid effectively eliminate undesired release of coating fluid when the electrical potential is not being applied to the electrode associated with the coating fluid supply. Thus, alternative orientations may be utilized, such as locating the coating fluid dispensing device above or alongside the target.  
         [0030]    A second embodiment of the present invention is illustrated in FIG. 2. In this embodiment, the coating fluid dispensing element of the first embodiment, wick  7 , is replaced with a siphon tube  11 . The inner diameter of siphon tube  11  must be sized sufficiently small to ensure that the surface tension forces in the fluid are not overcome, thereby avoiding formation of voids in the tube. The inner diameter must also be sufficiently small that the surface tension forces forming meniscus  16  at dispensing end  14  of siphon tube  11  are sufficient to preclude coating fluid leakage from the siphon tube when the apparatus is inactive.  
         [0031]    In the present embodiment, the necessary inner diameter required to avoid loss of siphon will vary with the specific material used for tube  11  and the viscosity of the coating fluid. An example embodiment employing a non-metallic material such as a ceramic or plastic tube with an inner diameter of sufficient to prevent loss of siphon with a coating fluid with a viscosity on the order of one centipoise.  
         [0032]    In order for the siphon tube to function properly, the level  12  of coating fluid in reservoir  13  should be maintained higher than the coating fluid dispensing end  14  of siphon tube  11 . Inlet end  15  of siphon tube  11  is submerged into the coating fluid in reservoir  13 . The height  12  of the coating fluid should be maintained above inlet end  15  to preclude loss of siphon from the reservoir end of siphon tube  11 , and to ensure the meniscus formed at the opening of spray dispensing end  14  does not recede back into siphon tube  11 .  
         [0033]    As with the first embodiment, the flow rate of the coating fluid through the siphon tube fluid transporter may be adjusted by changing the height of coating fluid level  12  relative to siphon tube dispensing end  14 . In addition, the flow rate can be adjusted by changing the height of siphon tube dispensing end  14 . Care should be taken to ensure that dispensing end  14  is not so far below fluid level  12  that the surface tension forces meniscus  16  are overcome, such that the fluid begins to flow freely from dispensing end  14 , or that conversely, dispensing end  14  is so high that surface tension forces between the fluid and the siphon tube surfaces are overcome, permitting the formation of siphon-breaking voids.  
         [0034]    [0034]FIG. 2 also shows an alternative electrode arrangement, in which the electrode  17  that applies an electrical potential to the coating fluid is incorporated into dispensing end  14  in a manner that permits the coating fluid to be in direct contact with the electrode. As with the first embodiment, target  18  is held by target holder  19  at a lower potential than electrode  17 . After the siphon tube has been initially primed with the coating fluid, an electrical potential is applied to electrode  17 . This potential in turn is applied to the coating fluid immediately adjacent to dispensing end  14 , resulting in the coating fluid being discharged from dispensing end  14  toward the target  18 . As the coating fluid is being discharged from end  14 , additional coating fluid is drawn into siphon inlet end  15  and through the siphon tube fluid transporter to sustain the coating flow from dispensing end  14  until the electrical potential is removed from electrode  17 . When the electrical potential is removed from electrode  17 , the surface tension forces between the small inner diameter tube and the coating fluid, including the surface tension forces at meniscus  16 , prevent further discharge of coating fluid from dispensing end  14 . The pressure differential and surface tension forces also resist retreat of the coating fluid back towards reservoir  13 , thereby maintaining the siphon in siphon tube  11 .  
         [0035]    While the embodiment illustrated in FIG. 2 shows coating fluid dispensing end  14  arranged above target  18 , alternative arrangements may be utilized, such as coating fluid dispensing toward an alongside target, so long as fluid level  12  is maintained above the fluid dispensing end  14 .  
         [0036]    As a further embodiment of the present invention, FIG. 3 illustrates a modification wherein wick material  20  is included within the lumen through siphon tube  11 . The addition of wick  20  further enhances the stable, self-regulating flow properties of the siphon tube by providing significantly greater surface area for surface tension interaction between the low viscosity coating fluid and its transporter. This greater surface area enhances capillary flow through the tube during coating flow dispensing operations, enhances the arresting of fluid momentum when coating spray is halted, and discourages retreat of the coating fluid back to the reservoir when the apparatus is idle. Moreover, because the presence of wick material  20  decreases the widths of gaps between adjacent surfaces within siphon tube  11 , it improves the siphon tube&#39;s resistance to depriming. This latter point is of particular usefulness, as it permits the use of a siphon tube with a larger inner diameter than would otherwise be possible with a low viscosity fluids in a hollow siphon tube. This in turn permits greater coating fluid flow rates during each target coating operation.  
         [0037]    In a fourth embodiment of the present invention illustrated in FIG. 4, siphon tube  11  is coupled to a float  21  that rests upon the surface of the coating fluid in reservoir  13 , and the electrical connection to the siphon tube electrode (not shown) is sufficiently flexible to permit siphon tube  11  to move vertically as the coating fluid level in reservoir  13  changes. If necessary, a counterweight  22  may be placed on or in float  21  to help counteract any tendency for siphon tube  11  to tip the float over.  
         [0038]    With this embodiment, float  21  maintains siphon tube inlet end  15  at a constant depth relative to the surface of the coating fluid in reservoir  13 , and accordingly maintains a constant height difference between the level of the coating fluid in reservoir  13  and siphon tube dispensing end  14 . This latter fixed height difference results in the hydrostatic pressure at dispensing end  14  remaining constant as the depth of the coating fluid within the reservoir decreases, thereby improving the consistency of coating fluid discharge. Further, if siphon tube  11  is slidably mounted in float  21 , the flow rate of coating fluid from discharge end  14  may be readily adjusted by sliding siphon tube inlet end  15  within float  21  to change its depth below the coating fluid surface. In a further variation of this embodiment, a mechanism may be provided to maintain a desired distance between the target and dispensing end  14  as coating fluid level in the reservoir decreases.  
         [0039]    While the present invention has been described with reference to what are presently considered to be preferred embodiments thereof, it is to be understood that the present invention is not limited to the disclosed embodiments or constructions. On the contrary, the present invention is intended to cover various modifications and equivalent arrangements. For example, a stopcock may be placed in the siphon assembly to prevent the fluid flow to be turned on or off as desired, or, rather than float-mounting the siphon tube to maintain a constant hydrostatic pressure at dispensing end  14 , a mechanism may be provided which automatically refills reservoir  13  to maintain a fixed coating fluid level in the reservoir. In addition, while the various elements of the disclosed invention are described and/or shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single embodiment, are also within the spirit and scope of the present invention.