Abstract:
A closure device includes a delivery member, a sealing material applicator, and an expandable member. The delivery member is insertable through a tissue tract to a vessel puncture. The sealing material applicator is configured to supply a volume of sealing material to the delivery member and includes first and second multi-chamber devices and a valve assembly. Each chamber of the first and second multi-chamber devices holds a component of the sealing material, and operating the valve assembly permits mixing of at least some of the components prior to connecting the sealing material applicator to the delivery member. The expandable member is positionable within the vessel to temporarily seal closed the vessel puncture from within the vessel. The closure device is operable to deliver the sealing material from the sealing material applicator, through the delivery member, and to the tissue tract to seal closed the vessel puncture from outside the vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of U.S. Provisional Application No. 61/334,051, filed 12 May 2010, and entitled BIOADHESIVE APPLICATOR, the disclosure of which is incorporated, in its entirety, by this reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a method and system for handling bioadhesive materials, and more particularly, to methods and systems for storage, mixing and dispensing bioadhesive material for use in sealing tissue punctures. 
     BACKGROUND 
     Various surgical procedures are routinely carried out intravascularly or intraluminally. For example, in the treatment of vascular disease, such as arteriosclerosis, it is a common practice to invade the artery and insert an instrument (e.g., a balloon or other type of catheter) to carry out a procedure within the artery. Such procedures usually involve the percutaneous puncture of the artery so that an insertion sheath may be placed in the artery and thereafter instruments (e.g., catheters) may pass through the sheath to an operative position within the artery. Intravascular and intraluminal procedures unavoidably present the problem of stopping the bleeding at the percutaneous puncture after the procedure has been completed and after the instruments (and any insertion sheaths used therewith) have been removed. Bleeding from puncture sites, particularly in the case of femoral arterial punctures, is typically stopped by utilizing vascular closure devices, such as those described in U.S. Pat. Nos. 6,179,963 and 6,090,130, which are hereby incorporated in their entireties by this reference. 
     While there are a variety of prior art devices and techniques for closing such punctures, one primary problem is ensuring a complete seal of the puncture. One technique includes the use of a bioadhesive material to seal the puncture. Some types of bioadhesive materials must be activated prior to use, and should be activated just prior to use in order to avoid premature activation of the bioadhesive material that would affect its performance. The handling and activation of bioadhesive materials in the context of vascular and other tissue puncture closure devices present a number of challenges. 
     SUMMARY 
     One aspect of the present disclosure relates to a closure device operable to seal closed a puncture in a vessel. The closure device includes a delivery member, a sealing material applicator, and an expandable member. The delivery member is insertable through a tissue tract to the vessel puncture. The sealing material applicator is configured to supply a volume of sealing material to the delivery member and includes first and second multi-chamber devices and a valve assembly. Each chamber of the first and second multi-chamber devices holds a component of the sealing material, and operating the valve assembly permits mixing of at least some of the components prior to connecting the sealing material applicator to the delivery member. The expandable member is positionable within the vessel to temporarily seal closed the vessel puncture from within the vessel. The closure device is operable to deliver the sealing material from the sealing material applicator, through the delivery member, and to the tissue tract to seal closed the vessel puncture from outside the vessel. 
     The sealing material applicator may maintain separation of at least some of the components of the sealing material prior to connecting the sealing material applicator to the delivery member. The delivery member may be configured to mix the components together during delivery of the sealing material to the vessel puncture. The delivery member may include a hub, a delivery tube, and a guide sheath, wherein the sealing material applicator is connectable to the hub, the expandable member and delivery tube are delivered through the guide sheath to the vessel puncture, and the sealing material is delivered to the vessel puncture through the delivery tube. The first and second multi-chamber devices may be syringes having at least two chambers. 
     Another aspect of the present disclosure relates to a method of sealing a vessel puncture in a vessel. The method includes providing a closure device that includes a delivery member, a sealing material applicator, and an expandable member. The sealing material applicator includes first and second dual chamber applicators and a valve assembly, wherein each chamber of the first and second dual chamber applicators may hold a component of the sealing material. The method also includes operating the valve assembly to provide flow communication between the first and second dual chamber applicators, mixing some of the components in the sealing material applicator while maintaining separation of other components in the sealing material applicator, storing the mixed components in the first dual chamber applicator, connecting the first dual chamber applicator to the delivery member, expanding the expandable member within the vessel to temporarily seal closed the vessel puncture, and delivering the sealing material through the delivery member to the vessel puncture to seal closed the vessel puncture from outside of the vessel. 
     The method may also include retracting the expandable member and withdrawing the expandable member through the sealing material. The first dual chamber applicator may include a first chamber having a first component, and a second chamber having a second component, and the second dual chamber applicator may include a third chamber having a third component and a fourth chamber having a fourth component, wherein mixing at least some of the components includes mixing the first and third components and mixing the second and fourth components. 
     The method may include mixing all of the components during delivery of the sealing material through the delivery member. The first and third components may include different materials, and the second and fourth components may include the same material. Alternatively, the second and fourth materials may include different materials. The method may include detaching the first dual chamber applicator from the valve assembly prior to connecting the first dual chamber applicator to the delivery member. The first component may include a first polymer in a liquid state, the second component may include a second polymer in a liquid state, the third component may include a third polymer in a solid state, and the fourth component may include a fourth polymer in a solid state. The solid polymer is miscible in the liquid polymer with which it is mixed. Alternatively, one of the liquid state polymers may be, instead, a buffer solution. Further, alternatively, one of the solid polymers may instead be a liquid polymer such that a first and a third component may be the same liquid component or different liquid components. For example, the first and third liquid components may be the same liquid polymer or the same buffer solution, if the system requires a buffer solution. It is clear that various permutations of liquid and/or solid components are possible in forming the sealing material, dependent upon the nature of the sealing material. 
     Another aspect of the present disclosure relates to a sealing assembly that includes a sealing material applicator and a delivery device. The sealing material applicator includes first and second dual chamber syringes and a valve assembly. The first dual chamber syringe is configured to store a first sealing material component in a first chamber and a second sealing material component in a second chamber. The second dual chamber syringe is configured to store a third sealing material component in a third chamber and a fourth sealing material component in a fourth chamber. The valve assembly is operable to control fluid flow between the first and third chambers and the second and fourth chambers. The sealing material applicator is operable to mix the first and third sealing material components to create a first mixture, mix the second and fourth sealing material components to create a second mixture, and store the first and second mixtures in the first dual chamber syringe with the first and second mixtures separated until connecting the first dual chamber syringe to the delivery device. The first and second mixtures are mixed in the delivery device and delivered by the delivery device to a tissue puncture to seal closed the tissue puncture. 
     The first dual chamber syringe may be detachable from the valve assembly and attachable to the delivery device. The first and third sealing material components may include different active ingredients, and the second and fourth sealing material components may include the same active ingredients. Alternatively, the first and third sealing material components may include different active ingredients, and the second and fourth sealing material components may include different active ingredients. The first and third sealing material components may be in liquid and solid states, respectively, and the second sealing material component may be in a liquid state. 
     A further aspect of the present disclosure relates to a method of sealing a tissue puncture. The method includes providing a sealing material applicator having a valve assembly, a first device carrying a first sealing material component in a first chamber and a second sealing component in a second chamber, and a second device carrying a third sealing material component in third chamber and a fourth sealing material component in a fourth chamber. The method includes operating the valve assembly to permit flow communication between the first and third chambers and the second and fourth chambers, mixing the first and third components to create a first mixture and mixing the second and fourth components to create a second mixture, connecting a portion of the sealing material applicator to a delivery device, and mixing the first and second mixtures while delivering the first and second mixtures to a tissue puncture with the delivery device to seal closed the tissue puncture. 
     The method may also include temporarily sealing closed the tissue puncture internally with an expandable member prior to delivering the mixed first and second mixtures to the tissue puncture. The method may include retracting the expandable member and withdrawing the expandable member through the mixed first and second mixtures that have sealed closed the tissue puncture. The method may include storing the first and second mixtures in the first device, and detaching the first device from the valve assembly, and connecting the sealing material applicator to the delivery device includes connecting the first device to the delivery device. 
     The foregoing and other features, utilities, and advantages of the invention will be apparent from the following detailed description of the invention with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate various embodiments of the present disclosure and are a part of the specification. The illustrated embodiments are merely examples of the present disclosure and do not limit the scope of the invention. 
         FIG. 1  illustrates an example tissue puncture closure device in accordance with the present disclosure. 
         FIG. 2  illustrates the tissue puncture closure device of  FIG. 1  with an expandable member expanded within the vessel to temporarily seal closed the tissue puncture. 
         FIG. 3  illustrates the tissue puncture closure device of  FIG. 1  delivering a sealing material to the vessel puncture. 
         FIG. 4  illustrates the tissue puncture closure device of  FIG. 1  with the sealing material cured and the device prepared for withdrawal from the tissue puncture. 
         FIG. 5  illustrates the sealed tissue puncture after removal of the tissue puncture closure device. 
         FIG. 6  illustrates an example sealing material applicator with valves in a closed position in accordance with the present disclosure. 
         FIG. 7  illustrates the sealing material applicator of  FIG. 6  with the valves in an open position. 
         FIG. 8  illustrates another example sealing material applicator with valves in a closed position in accordance with the present disclosure. 
         FIG. 9  illustrates the sealing material applicator of  FIG. 8  with the valves in an open position. 
         FIG. 10  illustrates one of the multi-chamber sealing material devices of  FIG. 7  carrying mixtures of components of the sealing material. 
         FIG. 11  illustrates another example sealing material applicator with valves in a closed position in accordance with the present disclosure. 
         FIG. 12  illustrates the sealing material applicator of  FIG. 11  with the valves in an open position. 
         FIG. 13  illustrates one of the multi-chamber sealing material devices of  FIG. 12  carrying mixtures of the sealing material components. 
     
    
    
     Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. 
     DETAILED DESCRIPTION 
     The systems disclosed herein may be used to close or seal percutaneous punctures made through the body tissue of a patient to gain access to a body cavity of a patient. Access through these percutaneous punctures allows a physician to carry out various procedures in or through the body cavity for examination, surgery, treatment and the like. While not meant to be limiting, the systems are illustrated being used to seal percutaneous punctures that provide access to blood vessels in patients for various procedures. It will be appreciated that the systems are applicable to other procedures requiring sealing of a puncture through body tissue into a cavity including, for example, laparoscopic surgery and other microscopic surgery techniques using a relatively small incision. 
     The terms proximal and distal are used herein to refer to the relative positions of the components of an exemplary tissue puncture closure device  10 . When used herein, proximal refers to a position relatively closer to the exterior of the body or closer to the surgeon. In contrast, distal refers to a position relatively further away from the surgeon or closer to the interior of the body. 
     As used in this specification and the appended claims, the terms “engage” and “engagable” are used broadly to mean interlock, mesh, or contact between two structures or devices. Likewise “disengage” or “disengagable” means to remove or capable of being removed from interlock, mesh, or contact. A “tube” is an elongated device with a passageway. The passageway may be enclosed or open (e.g., a trough). A “lumen” refers to any open space or cavity in a bodily organ, especially in a blood vessel. The words “including” and “having,” as well as their derivatives, as used in the specification, including the claims, have the same meaning as the word “comprising.” The terms “biomaterial” or “composition” refer to a material intended to interface with biological systems to preferably evaluate, treat, or seal any tissue, organ or function of the body. Biomaterial refers to the complete material (precursor molecules plus all additives, base or solvents and bioactive agents, if any) at and after having reached and passed its gel-point. Composition refers to the complete material before having reached its gel-point. “Cross-linking” as used herein means the formation of covalent linkages. However, it may also refer to the formation of non-covalent linkages, such as ionic bonds, or combinations of covalent and non-covalent linkages. The term “gel” refers to the state of matter between liquid and solid. As such, a gel has some of the properties of a liquid (i.e., the shape is resilient and deformable) and some of the properties of a solid (i.e., the shape is discrete enough to maintain three dimensions on a two dimensional surface). 
     The general structure and function of tissue closure devices used for sealing a tissue puncture in an internal tissue wall accessible through an incision in the skin are well known in the art. Applications of closure devices including those implementing principles described herein include closure of a percutaneous puncture or incision in tissue separating two internal portions of a living body, such as punctures or incisions in blood vessels, ducts or lumens, gall bladders, livers, hearts, etc. 
     An exemplary embodiment of the tissue puncture closure device  10  is illustrated in  FIGS. 1-4 , which is used to seal a percutaneous puncture  90  made through a tissue layer  92  and a vessel wall  96  of a vessel  94 . Typically, the vessel  94  is a femoral artery in the groin region with a relatively large vessel passage or lumen  98  to facilitate improved locating of the vessel  94  and permitting a sufficiently large puncture to be made through the vessel wall  96  to carry out the procedure. Medical procedures that are typically performed through such a puncture include, for example, angioplasty and other procedures that pass a catheter or other type of probe into and along the vessel lumen  98 . 
     When such a procedure is performed, an initial percutaneous puncture  90  with an appropriate needle is made through the tissue layer  92  and the vessel wall  96  and into the vessel lumen  98 . A guide wire is installed through the percutaneous puncture  90 , the needle is removed, and a guide sheath  12  of the tissue puncture closure device  10  is advanced over the guide wire and into the percutaneous puncture  90  to enlarge the puncture to permit easier access into the vessel  94 . Other features of the tissue puncture closure device  10  may be advanced over the guide wire. The guide wire may be removed prior to operation of the tissue puncture closure device  10  to seal closed the percutaneous puncture  90 . 
     The tissue puncture closure device  10  may include, in addition to the guide sheath  12 , a temporary sealing component  14 , a delivery tube  16 , a hub  18 , and a multi-chamber sealing material device  20  which carries a sealing material  24  for sealing closed the percutaneous puncture  90 . Initially, the guide sheath  12  may extend through the percutaneous puncture  90  and into the vessel lumen  98  to enlarge or dilate the percutaneous puncture  90 . The guide sheath  12  may be withdrawn until a distal end  30  is positioned at some location within the percutaneous puncture  90  or even withdrawn distally from the percutaneous puncture  90  after advancing other features of the tissue puncture closure device  10  into position (see  FIG. 1 ). Any one or a combination of the guide sheath  12 , delivery tube  16 , and hub  18  may be referenced as a delivery device, delivery member, or delivery system that assists in delivering the sealing material  24  to the percutaneous puncture  90 . 
     The temporary sealing component  14  may be inserted through the percutaneous puncture  90  until a portion is positioned within the vessel lumen  98 . The temporary sealing component  14  may include a distal tip  34  and an expandable portion  36 . The expandable portion contacts an inner surface of the vessel wall  96  when expanded (see  FIGS. 2-3 ). The expandable portion  36  may move between an expanded position (see  FIGS. 2 and 3 ) and a retracted position (see  FIGS. 1 and 4 ). The expandable portion  36  may be expanded by an inflation fluid. Alternatively, the expandable portion  36  may be expanded using a mechanical structure covered by a membrane. The temporary sealing component  14  may be used to temporarily seal closed the percutaneous puncture  90  from within the vessel  94 . In some arrangements, the distal tip  34  may be detachable within the sealing material  24  upon withdrawal of the temporary sealing component  14  to further seal closed percutaneous puncture  90  after removal of temporary sealing component  14 . 
     The delivery tube  16  may be positioned within the percutaneous puncture  90 . In some arrangements, the temporary sealing component  14  extends through the delivery tube  16 . In other arrangements, the temporary sealing component  14  and delivery tube  16  are arranged in parallel extending through the guide sheath  12 . The sealing material  24  carried by the multi-chamber sealing material device  20  may be delivered through the delivery tube  16  to the percutaneous puncture  90  (see  FIGS. 3-4 ). The sealing material  24  may change from a liquid state to a gel or solid state to help retain the sealing material  24  within the percutaneous puncture  90  after removal of the temporary sealing component  14 . The sealing material  24  may flow into and take a shape of the percutaneous puncture  90 , thereby providing an improved seal for the percutaneous puncture  90  that is less likely to leak blood. 
     In operation, the multi-chamber sealing material device  20  is connected to a delivery port  42  of the hub  18 , and the guide sheath  12 , temporary sealing component  14 , and delivery tube  16  are positioned relative to the percutaneous puncture as shown in  FIG. 1 . In some arrangements, the valve assembly  26  remains connected to the multi-chamber sealing material device  20  and is interposed between the hub  18  and the multi-chamber sealing material device  20  to control flow of the sealing material  24 . The expandable portion  36  of the temporary sealing component  14  is expanded to temporarily seal closed the percutaneous puncture  90  from within the vessel  94  as shown in  FIG. 2 . 
     The multi-chamber sealing material device  20  is operated to deliver the sealing material  24  through the delivery tube  16  to the percutaneous puncture  90  as shown in  FIG. 3 . The sealing material  24  is permitted to cure or at least partially cure so that the sealing material  24  does not flow into the vessel lumen  98  upon retracting the expandable portion  36  (see  FIG. 4 ). The temporary sealing component  14  may be removed through the sealing material  24 . The distal tip  34  may be lodged within the sealing material  24  in the empty cavity left behind upon removal of the temporary sealing component  14 . The delivery tube  16  and guide sheath  12  may also then be removed from the percutaneous puncture  90 . The sealing material  24  may continue to expand or flow within the percutaneous puncture  90  to fill any voids or cavities left behind upon removal of the guide sheath  12 , temporary sealing component  14 , and delivery tube  16  (see  FIG. 5 ). 
     Referring now to  FIGS. 6-7 , the multi-chamber sealing material device  20  may be part of a sealing material applicator  80 . The sealing material applicator  80  may include first and second multi-chamber sealing material devices  20 ,  22 , and a valve assembly  26 . Each of the first and second multi-chamber sealing material devices  20 ,  22  may include a housing  44  and a plunger assembly  46 . Each of the housings  44  may include at least first and second chambers  48 ,  50 , first and second outlet openings  52 ,  54 , and a flange  56 . The housings  44  may be integral with the first and second chambers  48 ,  50 . Alternatively, at least one of the first and second chambers  48 ,  50  may be removable from the housing  44 . In one example, the housing  44  acts as a bracket member that connects together multiple syringe-type devices that each carry a portion of the sealing material. 
     The plunger assembly  46  may include at least first and second plunger members  58 ,  60 , seal portions  62 ,  64  associated with the first and second plunger members  58 ,  60 , respectively, and a thumb applicator  66 . The first and second plunger members  58 ,  60  may be used to expel and draw in components of the sealing material  24  into the first and second chambers  48 ,  50 , respectively. The first and second plunger members  58 ,  60  may move in tandem upon application of an axial force to the thumb applicator  66 . 
     The valve assembly  26  includes first and second actuators  28 A,B and first and second sets of ports  29 A,B. The first and second multi-chamber sealing material devices  20 ,  22  are connected to the first and second sets of ports  29 A,B. The first and second actuators  28 A,B operate between open and closed positions to control fluid flow between the first chambers  48  and the second chambers  50  of the first and second multi-chamber sealing material devices  20 ,  22 . 
     The first and second actuators  28 A,B of the valve assembly  26  may be configured as ball valves or other valve structures that operate using application of a rotational force.  FIGS. 8-9  illustrate another valve assembly  126  having an actuator  128  that is actuated upon application of a linear force rather than a rotation force. The valve assembly  126  controls fluid flow through both sets of ports  129 A,B with a single linear actuation force in a lateral direction Y. In contrast, the valve assembly  26  includes separate first and second actuators  28 A,B which independently control fluid flow through each of the sets of ports  29 A,B. Many other types of actuators and valve assemblies may be used to provide controlled of fluid flow between the first and second multi-chamber sealing material devices  20 ,  22 . 
     Typically, when the valve assemblies  26 ,  126  are closed as shown in  FIGS. 6 and 8 , the plunger assemblies  46  are unable to move because of the fluid tight connection with the valve assembly  26 ,  126 . However, once the valve assemblies  26 ,  126  are open as shown in  FIGS. 7 and 9 , the plunger assemblies  46  can be operated in the X 1 , X 2  directions to mix the contents held in the first and second chambers  48 ,  50  of the first and second multi-chamber sealing material devices  20 ,  22 . 
     In one example, the first multi-chamber sealing material device  20  includes components A 1 , B 1  in the first and second chambers  48 ,  50  (see  FIG. 6 ), and the second multi-chamber sealing material device  22  includes components A 2 , B 2  in the first and second chambers  48 ,  50 . Actuating the plunger assemblies  46  as shown in  FIG. 7  mixes the components A 1 , A 2  together to form a first mixture, and mixes the components B 1 , B 2  to form a second mixture. The first and second mixtures may be stored in the first and second chambers  48 ,  50  of one of the first and second multi-chamber sealing material devices  20 ,  22  as shown in  FIG. 10 . The multi-chamber sealing material device carrying the first and second mixtures may then be connected to the tissue puncture closure device  10  to provide a supply of sealing material for delivery to the percutaneous puncture  90 . The first and second mixtures may remain separated until after the multi-chamber sealing material device is connected to the hub  18  of the tissue puncture closure device  10 . In some arrangements, the first and second mixtures held by the multi-chamber sealing material device may remain separated until delivery of the first and second mixtures, at which point the first and second mixtures are mixed together during delivery through the delivery tube  16  (or at another location within the tissue puncture closure device  10  such as the hub  18 ) to the percutaneous puncture  90 . 
     In one example, the component A 1  includes a first polymer component and the component A 2  contains a second polymer component, which when combined together create a cross-linked polymer. At least one of the components B 1 , B 2  may include an activator or buffer, which when combined with the mixture of A 1 , A 2  creates an activated polymer that begins to cure. Typically, the mixture A 1  A 2  is activated just prior to delivery of the sealing material to the percutaneous puncture  90 . Thus, there may be advantages to providing a multi-chamber sealing material device that maintains separation of the mixtures A 1 , A 2  and B 1 , B 2  until delivery of the sealing material through the delivery tube  16  to the percutaneous puncture  90 . 
     In some examples, the components A 1 , A 2  have a different physical state (e.g., A 1  includes a liquid or gel with an active ingredient dissolved therein, and A 2  is a solid that is dissolved in A 1  by mixing). The components B 1 , B 2  may also have different physical properties such as liquid, gel or solid. Preferably, at least one of the components in the mixture A 1 , A 2  and the mixture B 1 , B 2  is a liquid or gel to promote effective mixing and dissolving of the other component. 
     Referring now to  FIGS. 11-13 , another example sealing material applicator  280  includes first and second multi-chamber sealing material devices  20 ,  22  that include components A, C and B, C, respectively. The mixture is created by a sealing material applicator  280  that includes a mixture having components A, B and a mixture that includes component C alone (see  FIG. 13 ). In some arrangements, one of the first and second multi-chamber sealing material devices  20 ,  22  includes the component C and the other does not include component C (i.e., is left empty rather than also carrying the component C). In other arrangements, the components C in the first and second multi-chamber sealing material devices  20 ,  22  may be in different physical states (i.e., liquid and solid), but have the same active ingredient. Many other variations may be utilized with the number, physical state, and types of components handled by the sealing material applicators. In some arrangements, three or more chambers may be included in each of the first and second multi-chamber sealing material devices, and different variations of the type of component held in each chamber are possible. 
     In one example (see  FIGS. 6-7 ), the component A 1  may include a first polymer in a liquid state, the component B 1  may include a second polymer in a liquid state, the component A 2  may include a third polymer in a solid state, and the component B 2  may include a fourth polymer in a solid state. A solid polymer is miscible in the liquid polymer with which it is mixed. Alternatively, one of the liquid state polymers may be, instead, a buffer solution. Further, alternatively, one of the solid polymers may instead be a liquid polymer such that, for example, the components B 1 , B 2  may be the same liquid component or different liquid components. In another example, the components A 1 , A 2  or B 1 , B 2  may be the same liquid polymer or the same buffer solution, if the system requires a buffer solution. Various permutations of liquid and/or solid components are possible in forming the sealing material, dependent upon the nature of the sealing material. 
     The sealing materials and sealing material applicators disclosed herein may be used with other types of tissue puncture closure devices besides the tissue puncture closure device  10  shown with reference to  FIGS. 1-4 . The sealing materials may be used in other applications besides sealing closed a tissue puncture such as a puncture in a vessel wall or a puncture in the tissue leading to a vessel. The need to restrict flow of sealing material into the vessel provides some unique challenges in the context of sealing closed a percutaneous puncture in a vessel wall. 
     The preceding description has been presented only to illustrate and describe exemplary embodiments of the invention. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the following claims.