Patent Publication Number: US-8974409-B2

Title: Occlusion catheter having compliant balloon for use with complex vasculature

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/046,477, filed Mar. 11, 2011, which is a continuation of U.S. application Ser. No. 10/235,064, filed Sep. 4, 2002, which claims the benefit of U.S. provisional Application Ser. Nos. 60/317,232 and 60/318,215, filed Sep. 4, 2001, and Sep. 7, 2001, respectively, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to balloon catheters used in the treatment of aneurysms and other vascular diseases in a mammalian patient. 
     2. References 
     The following publications are cited in this application as superscript numbers: 
       1  Castaneda-Zuniga, et al.,  Interventional Radiology  , in Vascular Embolotherapy, Part 1, 1:9-32, Williams &amp; Wilkins, Publishers (1992) 
       2  Greff, et al.,  Compositions for Use in Embolizing Blood Vessels  , U.S. Pat. No. 5,667,767, issued Sep. 16, 1997 
       3  Evans, et al.,  Cellulose Diacetate Compositions for Use in Embolizing Blood Vessels  , U.S. Pat. No. 5,580,568, issued Dec. 3, 1996 
       4  Evans, et al.,  Novel Embolizing Compositions  , U.S. Pat. No. 5,695,480, issued Dec. 9, 1997 
       5  Jones, et al.,  Methods for Embolizing Vascular Sites with an Embolizing Composition Comprising Dimethylsulfoxide  , U.S. Pat. No. 5,830,178, issued Nov. 3, 1998 
       6  Whalen, et al.,  Novel Embolizing Compositions Comprising High Polymer Concentrations  , U.S. patent application Ser. No. 09/574,379, filed May 19, 2000 
       7  Evans, et al.,  Methods for Embolizing Blood Vessels  , U.S. Pat. No. 5,702,361, issued Dec. 30, 1997 
       8  Evans, et al.,  Methods for Embolizing Blood Vessels  , U.S. Pat. No. 6,017,977, issued Jan. 25, 2000 
       9  Wallace, et al.,  Intracranial Stent and Method of Use  , U.S. Pat. No. 6,007,573, issued Dec. 28, 1999 . 
       10  Racchini, et al.,  Porous Balloon For Selective Dilation and Drug Delivery  , U.S. Pat. No. 5,458,568, issued Oct. 17,1995 
       11  Whalen, et al.,  Novel High Viscosity Embolizing Compositions  , U.S. patent application Ser. No. 09/574,379, May 19,2000 
       12  Szikora, et al.,  Endovascular Treatment of Experimental Aneurysms with Liquid Polymers: The Protective Potential of Stents  , Neurosurgery, 38(2):339-347 (1996) 
       13  Kinugasa, et al.,  Direct Thrombosis of Aneurysms with Cellulose Acetate Polymer, Part II—Preliminary Clinical Experience , J. Neurosurg., 77:501-507 (1992) 
       14  Kinugasa, et al.,  Cellulose Acetate Polymer Thrombosis for the emergency Treatment of Aneurysms: Angiographic Finding, Clinical Experience, and Histopathological Study , Neurosurgery, 34:694-701 (1994) 
       15  Mandai, et al.,  Direct Thrombosis of Aneurysms with Cellulose Acetate Polymer.: Part I—Results of Thrombosis in Experimental Aneurysms , J. Neurosurg., 77:497-500 (1992) 
       16  Talia, et al.,  Bioabsorbable and Biodegradable Stents in Urology , J. Endourology, 11(6):391 (1997) 
       17  Wallace, et al.,  Intracranial Stent and Method of Use  ( Delivery System ), U.S. application Ser. No. 08/762,110 (pending application). 
     All of the above references are herein incorporated by reference in their entirety to the same extent as if each individual reference was specifically and individually indicated to be incorporated herein by reference in its entirety. 
     3. State of the Art 
     Aneurysms arise in mammalian subjects and, in particular, human subjects as a result of vascular disease wherein the arterial wall weakens and, under pressure due to blood flow, the arterial wall “balloons.” Continued growth and/or eventual rupture of the ballooned arterial wall is associated with high morbidity and mortality rates. Intracranial aneurysms are of particular concern because surgical procedures to treat these aneurysms before rupture are often not feasible and further because rupture of these aneurysms can have devastating results on the patient even if the patient survives rupture. Accordingly, treatment protocols for intracranial aneurysms may be prophylactic in nature, i.e., to inhibit rupture or rerupture of the aneurysm rather than to inhibit bleeding from the ruptured aneurysm. 
     Methods well documented in the art to inhibit intracranial aneurysmal rupture/bleeding include the delivery into the aneurysmal sac of non-particulate agents such as metal coils which are designed to induce thrombosis after delivery to the aneurysm, thereby inhibiting blood flow into the aneurysm 1 ; delivery of a fluid composition into the aneurysmal sac which composition solidifies in the sac to inhibit blood flow into the aneurysm 2-6 ; or a delivery of a combination of non-particulate agents and a fluidic composition into the aneurysmal sac to inhibit blood flow into the aneurysm. 7-8    
     In each case, the cranial aneurysm is treated by filling the aneurysmal sac in a manner which inhibits blood flow into the sac. This reduced blood flow correlates to reductions in aneurysmal stress and, hence, a reduction in the likelihood of rupture. However, care must be taken to ensure against migration of non-particulate agents or fluid composition beyond the aneurysmal sac (which can occur, for example, by overfilling of the sac) because this can result in parent artery or distal embolization which, in turn, has its own high level of morbidity associated therewith. 12    
     One method of containing the embolizing agent in the aneurysmal sac involves the use of a catheter having an occlusion or attenuation balloon. The catheter, and, specifically, the occlusion/attenuation balloon provided at the distal end thereof, performs the dual functions of blocking or impeding flow in the vessel during treatment, such that embolizing agent migration potential is significantly reduced, and providing a sealing wall or harrier against the neck of the aneurysmal sac, which aids in retaining the embolizing agent within the sac during its introduction. 
     Because the aneurysmal sac is usually associated with diseased tissue whose structural integrity is therefore compromised, it is important to minimize the exertion of pressure against the vessel. The occlusion balloon, therefore, must be designed to provide proper support against the vessel, conforming to the shape of the vessel and providing the necessary functionality associated with its use, without unduly stressing the tissue, In simple cases, wherein the aneurysmal sac is non-bifurcated and is located in a symmetrical, substantially constant-diameter vessel, this is readily accomplished with conventional balloon designs. However, in inure complex vasculature, for example bifurcated, multi-branched or varying-diameter vessels, conventional designs are confronted with challenges which they do not satisfactorily meet. Specifically, these conventional designs utilize balloons, which, when inflated to the limits of safe operation—that is, to levels which do not impart unsustainable tissue deformation—fail to fully conform to the complex vessel shape, and, accordingly, fail to provide the proper amount of flow impedance or occlusion and support against the outflow of embolizing agent during treatment. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the invention a microcatheter suitable for temporary vascular occlusion is provided. The microcatheter includes a substantially tubular structure with at least one lumen, and a balloon located at the distal portion of the tubular structure and in fluid communication with the lumen. The balloon is designed to deform at a pressure which is less than the pressure required to deform the vascular wall when placed into a blood vessel. 
     Further in accordance with the invention, a method for treating an aneurysm in a mammal is provided, wherein the method includes identifying an aneurysm in the mammal, and providing a catheter comprising a substantially tubular structure with at least one lumen and a vascular balloon located at the distal portion of the tubular structure and in fluid communication with the lumen, wherein the balloon deforms at a pressure less than the pressure required to deform the vascular wall in the vicinity of the aneurysm. The catheter is inserted into the blood vessel associated with the aneurysm, wherein the distal portion of said catheter is located at a position such that upon expansion of said balloon blood flow into the aneurysm is either attenuated and/or diverted prior to deformation of the vascular wall. An embolizing agent is delivered into the aneurysm to inhibits blood flow into said aneurysm. 
     Further in accordance with the invention, a catheter suitable for treating an aneurysm in a vascular wall is provided. The catheter includes a substantially tubular structure having at least one lumen, and an inflatable balloon located at a distal portion of the tubular structure and in fluid communication with the lumen, wherein at least a portion of the inflatable balloon comprises a material which deforms at a pressure which is less than the pressure required to deform the vascular wall containing the aneurysm. 
     Further in accordance with the invention, a method for treating an aneurysm formed in the vascular wall of a mammalian patient is provided, the aneurysm being formed in the vicinity of at least one lumen defined by a blood vessel associated with the aneurysm. The method includes identifying an aneurysm in the mammalian patient, introducing a balloon into the vicinity of the aneurysm, the balloon being provided on a catheter which comprises a substantially tubular structure having at least one lumen in communication with the balloon, at least a portion of the balloon comprising a material which deforms at a pressure which is less than the pressure required to deform the vascular wall in which the bifurcated aneurysm is formed, inflating the balloon such that blood flow into the aneurysm is attenuated and/or diverted prior to undesired compression of the vascular wall, and delivering into the aneurysm an embolizing agent which inhibits blood flow into said aneurysm. 
     Further in accordance with the invention, a catheter suitable for use in a complex vessel is provided. The catheter includes a substantially tubular structure having at least one lumen, and an inflatable balloon located at a distal portion of the tubular structure and in fluid communication with the lumen, wherein at least a portion of the inflatable balloon comprises a material which, when the balloon is inflated, conforms to the shape of the vessel for a sufficient length of the vessel to attenuate and/or occlude blood flow therein without significantly deforming the vessel. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a diagrammatical view of the use of the catheter in accordance with the invention during an embolization procedure; 
         FIG. 2  is a perspective view of a distal portion of a catheter in accordance with the invention; and 
         FIGS. 3A and 3B  are cross-sectional views of concentric ( FIG. 3A ) and non-concentric ( FIG. 3B ) lumen arrangements of catheters in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows an occlusion catheter  10  in use in the treatment of an aneurysmal sac  12  in accordance with the invention. Blood vessel  14  is complex in shape and structure and is shown as having a main portion A, and as bifurcating into two branches B and C. It will be appreciated that more complex vessel structures, including those having varying diameters, or those having more than two branches, originating from the same or different locations relative to the main portion, can be treated in accordance with the methods and devices of the invention without departure from. the spirit or scope of the invention. Blood vessel  14  is a neurological vessel, in accordance with the preferred application of the teachings of the invention. 
     Also shown in  FIG. 1  is an injection catheter  16 , depicted in position during treatment of aneurysmal sac  12 . Catheter  16  is a separate catheter from catheter  10  and is used to introduce an embolizing agent  18  into the aneurysmal sac  12 . The embolizing agent may be a one or more coils (not shown), or it may be a fluid composition, preferably comprising either a biocompatible polymer or a biocompatible prepolymer, but can also be a combination of coils with a fluid composition, as described by Evans 7-8 . When a biocompatible polymer is employed, the fluid composition preferably comprises a biocompatible polymer, and optionally a biocompatible contrast agent, and a biocompatible solvent which solubilizes the biocompatible polymer wherein sufficient amounts of the polymer are employed in the composition such that, upon delivery to the aneurysm, a polymer precipitate forms which fills at least a portion of the aneurysmal sac thereby inhibiting blood flow therein. Preferably, the viscosity of the polymer composition is at least about 150 cSt at 40° C. 
     Such polymer composition can comprise, for example, a biocompatible polymer at a concentration of from about 2 to 50 weight percent; a biocompatible contrast agent at a concentration of from about 10 to about 40 weight percent; and a biocompatible solvent from about 10 to 88 weight percent wherein the weight percent of the biocompatible polymer, contrast agent and biocompatible solvent is based on the total weight of the complete composition. 
     Preferably, in this particular composition, the concentration of the polymer ranges from 6 to 50 weight percent and more preferably 8 to 30 weight percent. 
     Preferably, the polymer composition has a viscosity of at least about 150, preferably at least about 200 and more preferably at least 500 cSt at 40° C. More preferably the viscosity ranges from about 200 to 40,000 cSt at 40° C., more preferably from about 500 to 40,000 cSt at 40° C. The viscosity can also range from about 500 to 5000 cSt at 40° C. 
     In another aspect of the invention, the biocompatible polymer can be replaced with a biocompatible prepolymer and, when so used, the presence of the biocompatible solvent becomes optional. In a further preferred embodiment, the biocompatible solvent is dimethylsulfoxide (DMSO), ethanol, ethyl lactate or acetone. 
     The term “biocompatible polymer” refers to polymers which, in the amounts employed, are non-toxic and substantially non-immunogenic when used internally in the patient and which are substantially insoluble in the body fluid of the mammal. 
     The biocompatible polymer is preferably non-biodegradable. Suitable non-biodegradable biocompatible polymers include, by way of example, cellulose acetates 2-6  (including cellulose diacetate), ethylene vinyl alcohol copolymers, hydrogels (e.g., acrylics), polyacrylonitrile, polyvinylacetate, cellulose acetate butyrate, nitrocellulose, copolymers of urethane/carbonate, copolymers of styrene/maleic acid, and mixtures thereof. 
     Preferably, the biocompatible polymer employed does not cause an adverse inflammatory reaction when employed in vivo. The particular biocompatible polymer employed is selected relative to the viscosity of the resulting polymer solution, the solubility of the biocompatible polymer in the biocompatible solvent, and the like. For example, the selected biocompatible polymer should be soluble in the amounts employed in the selected biocompatible solvent and the resulting composition should have a viscosity suitable for in vivo delivery by, e.g., injection, or by screw-assisted syringe delivery. Such factors are well within the skill of the art. 
     Preferred biocompatible polymers include cellulose diacetate and ethylene vinyl alcohol copolymer. 
     Cellulose diacetate polymers are either commercially available or can be prepared by art recognized procedures. 
     Ethylene vinyl alcohol copolymers comprise residues of both ethylene and vinyl alcohol monomers. Small amounts (e.g., less than 5 mole percent) of additional monomers can be included in the polymer structure or grafted thereon provided such additional monomers do not alter the properties of the composition. Such additional monomers include, by way of example only, maleic anhydride, styrene, propylene, acrylic acid, vinyl acetate and the like. 
     The term “contrast agent” refers to a biocompatible radiopaque material capable of being monitored during injection into a mammalian subject by, for example, radiography. The contrast agent can be either water soluble or water insoluble and preferably does not contain radioactivity above the native or endogenous amounts naturally occurring in the elements employed (i.e., are “non-radioactive”). 
     Examples of water soluble contrast agents include metrizamide, iopamidol, iothalamate sodium, iohexol, iodomide sodium, and meglumine. Examples of water insoluble contrast agents include tantalum, tantalum oxide, and barium sulfate, each of which is commercially available in the proper form for in vivo use including a preferred particle size of about 10 μm or less. Other water insoluble contrast agents include gold, tungsten, and platinum powders. 
     Preferably, the contrast agent is water insoluble (i.e., has a water solubility of less than 0.01 mg/ml at 20° C.). 
     The term “biocompatible solvent” refers to an organic material liquid at least at body temperature of the mammal in which the biocompatible polymer is soluble and, in the amounts used, is substantially non-toxic. Suitable biocompatible solvents include, by way of example, dimethylsulfoxide, analogues/homologues of dimethylsulfoxide, ethanol, acetone, ethyl. lactate, and the like. Aqueous mixtures with the biocompatible solvent can also be employed provided that the amount of water employed is sufficiently small that the dissolved polymer precipitates upon contact with the blood. Preferably, the biocompatible solvent is dimethylsulfoxide. 
     As seen from  FIG. 1 , catheter  10  is provided with a balloon  20  at a distal portion thereof. Balloon  20 , shown in an inflated state, occupies portions of the interior of main portion A, and branches B and C, of vessel  14 . In this mariner, balloon  20  occludes, or attenuates, flow in vessel  14 , such that embolizing agent  18  can be introduced into aneurysmal sac  12  with minimal migration and loss. The size and material of balloon  20 , and the inflation pressure applied thereto, are selected such that the balloon conforms to the irregular shape and structure of vessel  14 , and provides the necessary flow occlusion and/or attenuation to effectively apply embolizing agent  18 , without appreciably deforming or stressing the vessel. Further, by such appropriate selection, balloon  20  can provide an effective barrier against neck  22  of aneurysmal sac  12 , such that undesired outflow of embolizing agent  18  from the aneurysmal sac is reduced. All of these factors significantly contribute to the safety and efficacy of the embolization procedure. 
       FIG. 2  shows in greater detail the distal portion of catheter  10  of the invention. The distal portion includes balloon  20 , shown in a deflated state, and comprising a layer of flexible balloon material  26  defining a chamber  28 . Holes  30  provide fluid communication between chamber  28  and a central lumen  32  extending longitudinally within elongate, generally tubular body  34  of catheter  10 . 
     The distal portion of catheter  10  is also provided with radiopaque marker bands  36  and  38 , which are retained in position around body  34  by corresponding marker band retainers  36   a  and  38   a . Retainers  36   a  and  38   a  are preferably of a flexible material, such as plastic, which is heat shrunk over markers  36  and  38  during catheter fabrication. It will be appreciated that retainers  36   a  and  38   a  can alternatively be made of a single, unitary component extending over markers  36  and  38 , in which case it would be provided with holes in registry with holes  30  to facilitate communication between chamber  28  and central lumen  32 . Marker bands  36  and  38  may be of metallic or polymeric material, and may alternatively be adhesively bonded in position, rather than using retainers  36   a  and  38   a.    
     Central lumen  32  is in fluid communication with a fluid reservoir (not shown) at the proximal end of catheter  10 . Fluid (not shown) from the reservoir is selectively introduced into chamber  28 , via holes  30 , in order to inflate balloon  20  during operation. The fluid may be saline and/or a contrast agent injected by syringe (not shown). 
     In an alternative structure, the catheter is provided with more than one lumen, at least one of which is in communication with the exterior of catheter  10  at the distal portion of the catheter, such that material can be introduced into or removed from vessel  14 , or such that in situ measurements of various patient conditions, such as pressure, can be accurately performed. Moreover, additional lumens may be provided to accommodate a guidewire or a microcather in order to facilitate introduction of the catheter to the target portion of the patient&#39;s vasculature, in accordance with known catheterization techniques. Lumen  32  can be adapted for this purpose, such that lumen  32  can serve the multiple purposes of accommodating a guidewire and providing inflation fluid to balloon  20 . The multiple lumens can be concentric, or they can extend along different portions of the catheter, as seen from  FIGS. 3A and 3B , respectively showing cross-sectional views of multiple lumens  40 ,  42  and  46 ,  47  in concentric and non-concentric arrangements in catheters  44  and  48  in accordance with the invention. 
     Balloon material  26  is selected to be flexible, such that balloon  20 , when inflated, is very compliant. Preferably, material  26  is of a composition which is based on styrenic olefinic rubber and hydrogenated isoprene, such as that sold under the trade name ChronoPrene™, available from CT Biomaterials, a division of CardioTech International, Inc. ChronoPrene™ includes the additives polypropylene as a reinforcing agent, and mineral oil as a plasticizer and processing agent. Balloon material  26  is sterilizable and biocompatible, and is compatible with the materials used during the embolization process. In particular, it is compatible with solvents (for example dimethylsulfoxide (DMSO)), polymers, prepolymers and other materials involved in the embolization process. The contemplated thickness of the balloon material  26  is in the range of about 0.004 inches to about 0.006 inches, and is preferably about 0.005 inches. 
     The characteristics of balloon material  26 —including its material, shape, size and the manner in which it is formed and applied relative to tubular body  34 —are selected such that balloon  20 , when inflated, readily takes the path of least resistance within the blood vessel, and minimally impacts the shape and integrity of the compromised tissue. This reduces the threat of rupture, and of subsequent stenosis. At the same time, the functionally of the balloon is unhindered—that is, the balloon serves to effectively occlude or impede blood flow, or, in some situations, divert it from an existing flow pattern in a bifurcated, complex vessel in a controlled, safe and reliable manner. The balloon  20 , or at least a portion thereof, will thus be more deformable than the vascular wall in the vicinity of the aneurysmal sac, even when that vascular wall is diseased and is of compromised strength and stiffness. The balloon  20 , or portions thereof, however, will remain sufficiently rigid during operation so as not to appreciably penetrate into the opening, or neck, of the aneurysmal sac, even in situations where the neck of the aneurysmal sack is larger than the opening of a branch of the bifurcated vessel. 
     Factors to consider in providing the necessary balloon performance are modulus and tensile strength of balloon material  26 . A modulus at 300%, having a value less than about 300 psi and/or a tensile strength of about 600 psi, is preferred. ChronoPrene™ 15A can be used, which has a Hardness-Shore A ASTM D2240 (3 sec.) rating of about 15, a specific gravity of about 0.89, tensile strength of about 600 psi, and elongation of greater than about 1,000%. Alternatively, ChronoPrene™ 40A can be used, which has a Hardness-Shore A ASTM D2240 (3 sec.) rating of about 40, a specific gravity of about 0.90, tensile strength of about 700 psi, and elongation of about 500%. The material  26  may be pre-stretched or otherwise mechanically and/or thermally manipulated to improve performance. 
     In forming balloon  20 , material  26  can be shaped into a tubular form and subjected to pre-treatment, which may include stretching and heating and annealing. The tube can then be bonded to tubular body  34 . Bonding is preferably effected using adhesives, or using heat treatment, for example using laser, direct contact heat application, or forced air heating. Bonding can also be effected ultrasonically, or by RF (radio frequency) welding, or mechanically swaging or pinching, or by other techniques. In this manner material  26  is sealed against tubular body  34 , or against retainers  36   a  and  38   a , at regions  50  and  52 , such that fluid-tight chamber  28  is achieved. 
     Designing the balloon  20  to be relatively large in length will compensate for any shortcomings arising from its relative softness and compliancy, and its low impact design. Larger size, in terms of the diameter of the balloon  20  when inflated and in terms of the overall length of the balloon is contemplated. A preferred balloon length range is about 4 mm to about 30 mm, and a more preferred balloon length range is about 7 mm to about 20 mm. A preferred balloon diameter range is about 1.5 mm to about 10 mm, and a more preferred balloon diameter range is about 3 mm to about 7 mm. A preferred length-to-diameter ratio range is about 0.5:1 to about 5:1. In this manner the balloon will effectively occlude/impede blood flow due to its large size, and particularly, large surface area, and will not need to rely on forming a tight seal over a small contact area with the blood vessel. Thus minimal stress is exerted against the vessel. 
     Balloon  20  is constructed such that an inflation pressure of about 200 mmHg, or about 3/10 atmosphere, will provide the necessary volume and force for proper operation. Such pressure will thus be slightly greater than systolic pressure, and will be just sufficient to inflate balloon  20  without causing appreciable deformation of the vessel. Accordingly, damage to the vessel tissue is minimized. It is however contemplated that pressures to about 600 mmHg are possible. Application of embolizing agent can take place when the vessel and aneurysmal sac are in their true shape and form, uninfluenced by straightening or mis-shaping forces. This will ensure that the introduced material is retained in place, without imparting prolonged stress to the region. Acceptable performance using ChronoPrene™ 15A has been realized, although ChronoPrene™ in the range of 5A to 25A can also be used. 
     It is also contemplated that balloon  20  is predisposed to take on certain irregular shapes matching those of the vasculature in which it is to be introduced. The balloon thus can be provided with nodules, or pre-stretched or pre-oriented during fabrication to take an these irregular shapes, for example to bulge or self-position in a particular direction, as seen in  FIG. 1 , with balloon  20  bulging to the left, into branch B of vessel  14 . 
     The distal portion of catheter  10  is preferably relatively flexible in order to facilitate introduction of the catheter into tortuous vasculature. Other portions of catheter  10 , however, may be relatively stiffer, in order to increase column strength and enhance pushability, thereby further facilitating control of the catheter. Relative stiffness may be achieved in a variety of ways, including suitable material selection, reinforcement, and so forth. 
     While described in the context of embolization procedures, it will be appreciated that the catheter and method of the invention are generally useful for occluding and/or impeding flow in any vasculature whose structural integrity has been compromised, since the invention involves minimal pressure and exertion of force against the vascular tissue, and will therefore not exacerbate the condition of the vessel. 
     The above are exemplary modes of carrying out the invention and are not intended to be limiting. It will be apparent to those of ordinary skill in the art that modifications thereto can be made without departure from the spirit and scope of the invention as set forth in the following claims.