Abstract:
Disclosed is an expandable transluminal sheath, for introduction into the body while in a first, low cross-sectional area configuration, and subsequent expansion of at least a part of the distal end of the sheath to a second, enlarged cross-sectional configuration. The distal end of the sheath is maintained in the first, low cross-sectional configuration and expanded using a radial dilatation device. In an exemplary application, the sheath is utilized to provide access for diagnostic or therapeutic procedures such as ureteroscopy, cardiac electrophysiology, gastroenterology, and spinal access.

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. patent application Ser. No. 11/223,897, filed on Sep. 9, 2005, titled “Expandable Transluminal Sheath,” which claims priority to U.S. Provisional Application No. 60/608,355, filed on Sep. 9, 2004, titled “Expandable Transluminal Sheath.” Each of the foregoing applications is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to medical devices and, more particularly, to methods and devices for accessing the mammalian urinary tract. In one application, the present invention relates to methods and devices for providing access to the ureter and kidney. 
     2. Description of the Related Art 
     A wide variety of diagnostic or therapeutic procedures involves the introduction of a device through a natural access pathway. A general objective of access systems, which have been developed for this purpose, is to minimize the cross-sectional area of the access lumen, while maximizing the available space for the diagnostic or therapeutic instrument. These procedures are especially suited for the urinary tract of the human or other mammal. The urinary tract is relatively short and substantially free from the tortuosity found in many endovascular applications. 
     Ureteroscopy is an example of one type of therapeutic interventional procedure that relies on a natural access pathway. Ureteroscopy is a minimally invasive procedure that can be used to provide access to the upper urinary tract. Ureteroscopy is utilized for procedures such as stone extraction, stricture treatment, or stent placement. 
     To perform a procedure in the ureter, a cystoscope is placed into the bladder through the urethra. A guidewire is next placed, through the working channel of the cystoscope and under direct visual guidance, into the target ureter. Once guidewire control is established, the cystoscope is removed and the guidewire is left in place. A ureteral sheath or catheter is next advanced through the urethra over the guidewire, through the bladder and on into the ureter. The guidewire may now be removed to permit instrumentation of the ureteral sheath or catheter. 
     Current techniques involve advancing a flexible, 10 to 18 French, ureteral catheter with integral flexible, tapered obturator over the guidewire. Because axial pressure is required to advance and place each catheter, care must be taken to avoid kinking the tapered catheter during advancement so as not to compromise the working lumen of the catheter through which instrumentation, such as ureteroscopes and stone extractors, must now be placed. 
     One of the issues that arise during ureteroscopy is the presence of an obstruction or stenosis in the ureter, which is sometimes called a stricture, that prohibits a catheter with a large enough working channel from being able to be advanced into the ureter. Such conditions may preclude the minimally invasive approach and require more invasive surgical procedures in order to complete the task. Urologists may be required to use catheters with sub optimal central lumen size because they are the largest catheters that can be advanced to the proximal end of the ureter. Alternatively, urologists may start with a larger catheter and then need to downsize to a smaller catheter, a technique that results in a waste of time and expenditure. Finally, a urologist may need to dilate the ureter with a dilation system before placing the current devices, again a waste of time and a need for multiple devices to perform the procedure. In most cases, it is necessary for the urologist to perform fluoroscopic evaluation of the ureter to determine the presence or absence of strictures and what size catheter would work for a given patient. 
     Additional information regarding ureteroscopy can be found in Su, L, and Sosa, R. E.,  Ureteroscopy and Retrograde Ureteral Access, Campbell&#39;s Urology,  8th ed, vol. 4, pp. 3306-3319 (2002), Chapter 97. Philadelphia, Saunders, and Moran, M. E., editor,  Advances in Ureteroscopy, Urologic Clinics of North America , vol. 31, No. 1 (February 2004). 
     A need therefore remains for improved access technology, which allows a device to be transluminally passed through a relatively small diameter duct, while accommodating the introduction of relatively large diameter instruments. It would be beneficial if a urologist did not need to inventory and use a range of catheter diameters. It would be far more useful if one catheter diameter could fit the majority of patients. Ideally, the catheter would be able to enter a vessel or body lumen with a diameter of 6 to 10 French or smaller, and be able to pass instruments through a central lumen that was 12 to 18 French. These requirements appear to be contradictory but can be resolved by the invention described below. 
     SUMMARY OF THE INVENTION 
     Accordingly, one embodiment of the present invention comprises a device adapted for providing therapeutic or diagnostic access to a kidney through the ureter comprising a sheath having a non-expandable proximal end and a radially expandable distal end that can be expanded from an outer diameter of about 10 French or smaller to an outer diameter of greater than about 12 French. 
     Another embodiment of the present invention comprises an apparatus adapted for instrumenting a body lumen. The device includes a sheath having a proximal end and a distal end and means for radially collapsing the distal end of a sheath and maintaining said radially collapsed configuration. Means are also provided for introducing the sheath into the body lumen while the distal end is in its radially collapsed configuration and for dilating the radially collapsed distal end of the sheath. The sheath also includes means maintaining the dilated configuration of the distal end of the sheath, following dilation by the dilator, means for performing instrumentation, material introduction or withdrawal from the body lumen through the sheath and means for removal of the sheath from the body lumen. 
     Another embodiment of the present invention comprises a method of instrumenting a body lumen. A dilator is inserted into a sheath that has a radially collapsible distal end. The distal end of the sheath is collapsed radially inward around the dilator such that the distal end of the sheath is smaller in diameter than the proximal end of the sheath. The sheath and dilator are inserted into the body lumen and advanced to a target treatment site. The dilator is expanded and causes the distal end of the sheath to expand to a diameter substantially the same as that of the proximal end of the sheath. The dilator is removed and the central lumen that remains is substantially the same diameter moving from the proximal end to the distal end of the sheath. Instruments or catheters are inserted through the central lumen of the sheath for a therapeutic or diagnostic procedure. 
     Another embodiment of the present invention comprises an expandable medical access sheath for providing minimally invasive access to body lumens or cavities, length of proximal sheath tubing, said proximal sheath tubing being non-expandable. The sheath includes a length of distal sheath tubing. The distal sheath tubing is collapsed to a diameter smaller than that of the proximal sheath tubing. A transition zone exists between the distal sheath tubing and the proximal sheath tubing. The transition zone tapers between the larger diameter proximal sheath tubing and the smaller diameter distal sheath tubing. A dilator is disposed within both the proximal sheath tubing and the distal sheath tubing. Radial expansion of the dilator causes the distal sheath tubing to expand diametrically to substantially the same inner diameter as that of the proximal sheath tubing. 
     A transluminal radially expanding access sheath is provided according to an embodiment of the invention. In one embodiment, the radially expanding access sheath is used to provide access to the ureter, kidney, or bladder. In an embodiment, the sheath would have an introduction outside diameter that ranged from 4 to 12 French with a preferred range of 5 to 10 French. The inside diameter of the sheath would be expandable to permit instruments ranging from 10 French to 60 French to pass therethrough, with a preferred range of between 12 and 20 French. The ability to pass the large instruments through a catheter or sheath introduced with a small outside diameter is derived from the ability to expand the distal end of the catheter to create a larger through lumen. The proximal end of the catheter is generally larger to provide for pushability, control, and the ability to pass large diameter instruments therethrough. 
     Another embodiment of the invention comprises a transluminal access system for providing minimally invasive access to anatomically proximal structures. The system includes an access sheath comprising an axially elongate tubular body that defines a lumen, at least a portion of the distal end of the elongate tubular body being expandable from a first, smaller cross-sectional profile to a second, greater cross-sectional profile. In an embodiment, the first, smaller cross-sectional profile is created by making axially oriented folds in the sheath material. These folds may be in only one circumferential position on the sheath, or there may be a plurality of such folds or longitudinally oriented crimps in the sheath. The folds or crimps may be made permanent or semi-permanent by heat-setting the structure, once folded. In an embodiment, a releasable jacket is carried by the access sheath to restrain at least a portion of the elongate tubular structure in the first, smaller cross-sectional profile. The elongate tubular body is sufficiently pliable to allow the passage of objects having a maximum cross-sectional diameter larger than an inner diameter of the elongate tubular body in the second, greater cross-sectional profile. The adaptability to objects of larger dimension is accomplished by re-shaping of the cross-section to the larger dimension in one direction accompanied by a reduction in dimension in a lateral direction. The adaptability may also be generated through the use of malleable or elastomerically deformable sheath material. 
     In another embodiment of the invention, a transluminal access system for providing minimally invasive access includes an access sheath comprising an elongate tubular body having a proximal end and a distal end and defining an axial lumen. At least a portion of the distal end of the elongate tubular body is expandable from a first, folded, smaller cross-sectional profile to a second, greater cross-sectional profile. The sheath wall is axially crimped or folded to form the first smaller cross-sectional profile. The sheath wall is cut or transected laterally to form a plurality of axially aligned segments. The transaction may be generated using laser cutting, a blade, or other plastic cutting technology. The plurality of sheath segments is connected using a plurality of thin pliable connector links. Alternatively, the sheath segments may be connected by an outer or inner sheath membrane with elastomeric or axially deformable properties. The sheath segments may also be created by continuously cutting a spiral in the tube thus creating flexibility and linear attachments between sheath elements since the sheath, or a layer of the sheath is continuous from the proximal to the distal end. In this way, the axial flexibility of the access sheath is increased. The outer or inner sheath engages the plurality of sheath segments by mechanical friction or by adherence. In an embodiment, a releasable jacket is carried by the access sheath to restrain at least a portion of the elongate tubular member in the first, smaller cross-sectional profile. The sheath may also be spiral or ribbed cut only partially through the thickness of the wall, for example, only through the outer portion of the wall but not through to the inner lumen of the sheath. This configuration can be used on tubes, especially those of elastomeric nature, to provide kink resistance and bendability while retaining hoop strength. 
     In another embodiment of the invention, a transluminal access sheath assembly for providing minimally invasive access comprises a sheath that includes an elongate tubular member having a proximal end and a distal end and defining a working inner lumen. At least a portion of the distal end of the elongate tubular member is expandable from a first, smaller cross-sectional profile to a second, greater cross-sectional profile by plastic yield. Thin, plastically deformable materials such as polyethylene are suitable for this application. In another embodiment, the plastically deformable tubular member is replaced by a folded or creased sheath that is expanded by a dilatation balloon or axially translating dilator. An inner member, in an embodiment, generates the force to expand the sheath. The inner member, which can be a dilatation balloon, is removable to permit subsequent instrument passage through the sheath. Longitudinal runners may be disposed within the sheath to serve as tracks for instrumentation and minimize friction while minimizing the risk of catching the instrument on the expandable plastic. Such longitudinal runners are preferably circumferentially affixed within the sheath so as not to shift out of alignment. 
     Another embodiment of the invention comprises a transluminal access system, for providing minimally invasive access that includes an elongate tubular body that defines a lumen, at least a portion of the distal end of the elongate tubular body being expandable from a first, smaller cross-sectional profile to a second, greater cross-sectional profile. Optionally, a releasable jacket can be carried by the access sheath to restrain at least a portion of the elongate tubular structure in the first, smaller cross-sectional profile. The axially elongate tubular structure is further reinforced by a stent or stent-like support structure. The stent-like support structure is expandable and may be either elastomeric and self-expanding or malleable and require an active dilator for its expansion. The stent-like support structure may also be a coil that is unwound to increase its diameter. The stent-like support structure preferably has very little foreshortening when it is expanded radially. The stent-like support structure is affixed interior to or embedded within at least a part of the wall of the axially elongate tubular structure. An expandable dilating member is positioned within the elongate tubular body and configured to expand the elongate tubular body from the first, smaller cross-sectional profile to the second, greater cross-sectional profile. 
     Another embodiment of the invention comprises a transluminal access assembly that includes an elongate tubular body that defines an internal lumen. At least a portion of the distal end of the elongate tubular structure is expandable from a first, axially folded smaller cross-sectional profile to a second, greater cross-sectional profile. The elongate tubular structure may be creased or it may be elastomerically or malleably expandable. A releasable jacket is optionally carried by the access sheath to restrain at least a portion of the elongate tubular body in the first, smaller cross-sectional profile. A reinforcing structure fabricated from nickel-titanium alloy such as nitinol, provides the wall support to prevent re-collapse following dilatation. The nitinol can be severely distorted or pinched to form the folds or furls, and still be restored to its initial set-shape. In the first, folded, smaller-cross-sectional profile, the elongate tubular body of one embodiment, includes two creased outer sections that generally face each other. 
     Another embodiment of the invention comprises a transluminal access sheath system that includes an elongate tubular structure that defines an lumen, at least a portion of the distal end of the elongate tubular structure being expandable from a first, folded, smaller cross-sectional profile to a second, greater cross-sectional profile. The expandable portion of the sheath relies on unfurling longitudinal folds or creases, elastomerically expandable tubes, or a malleably yielding tubular structure surrounding, at least on the outside, a plurality of laterally disposed hoops that can be rotated on edge to form a low profile sheath cross-section or rotated fully laterally to maximize the through lumen of the sheath. 
     Another embodiment of the invention comprises a transluminal access assembly or access sheath that includes an elongate tubular body that defines an internal lumen. At least a portion of the distal end of the elongate tubular structure is expandable from a first, smaller cross-sectional profile to a second, greater cross-sectional profile. The elongate tubular structure may be creased or folded axially or it may be elastomerically or malleable expandable. A releasable jacket is optionally carried by the access sheath to restrain at least a portion of the elongate tubular body in the first, smaller cross-sectional profile. A reinforcing structure such as a stent, braid, or spiral reinforcement may optionally be incorporated into the wall of the elongate tubular body at the proximal end of the catheter. In this embodiment, the elongate tubular structure comprises, at least along part of its length, a double wall balloon that may be inflated to form a rigid or semi-rigid structure with an internal axial lumen therethrough, a pressurized annular lumen, and an exterior wall that provides at least some level of outward force to be exerted on surrounding tissue. The double wall balloon, located at the distal end of the sheath or catheter, is inflated from the proximal end of the access assembly. 
     Another embodiment of the invention comprises a transluminal access assembly that includes an elongate tubular body that defines an internal lumen. At least a portion of the distal end of the elongate tubular structure is expandable from a first, smaller cross-sectional profile to a second, greater cross-sectional profile. The elongate tubular structure may be creased or it may be elastomerically or malleable expandable. A releasable jacket is optionally carried by the access sheath to restrain at least a portion of the elongate tubular body in the first, smaller cross-sectional profile. In this embodiment, the elongate tubular structure comprises, at least along part of its length, a braided or counter-wound coil-like structure that may be radially expanded by forced axial compression. The braided or counter-wound coil-like structure can be made to form a rigid or semi-rigid expanded structure with an axial lumen therethrough and an exterior wall that provides at least some level of outward force to be exerted on surrounding tissue. The axial compression, in an embodiment, can be accomplished by pull-wires slidably affixed within, or adjacent to, the wall of the sheath and operated from manipulators located at or near the proximal end of the sheath. 
     In some embodiments, the proximal end of the access assembly, apparatus, or device is preferably fabricated as a structure that is flexible, resistant to kinking, and further retains both column strength and torqueability. Such structures include tubes fabricated with coils or braided reinforcements and preferably comprise inner walls that prevent the reinforcing structures from being exposed to the inner lumen of the access apparatus. Such proximal end configurations may be single lumen, or multi-lumen designs, with a main lumen suitable for instrument or obturator passage and additional lumens being suitable for control and operational functions such as balloon inflation. Such proximal tube assemblies can be affixed to the proximal end of the distal expandable segments described heretofore. The preferred configuration for the proximal end of the catheter includes an inner layer of thin polymeric material, an outer layer of polymeric material, and a central region comprising a coil, braid, stent or other reinforcement. It is beneficial to create a bond between the outer and inner layers at a plurality of points, most preferably at the interstices or perforations in the reinforcement structure, which is generally fenestrated. The polymeric materials used for the outer wall of the jacket are preferably elastomeric to maximize flexibility of the catheter. The polymeric materials used in the composite catheter inner wall may be the same materials as those used for the outer wall, or they may be different. In another embodiment, a composite tubular structure can be co-extruded by extruding a polymeric compound with a braid or coil structure embedded therein. 
     In an embodiment of the invention, it is beneficial that the catheter comprise a radiopaque marker or markers. The radiopaque markers may be affixed to the non-expandable portion or they may be affixed to the expandable portion. Markers affixed to the radially expandable portion preferably do not restrain the sheath or catheter from radial expansion or collapse. Markers affixed to the non-expandable portion, such as the catheter shaft of a balloon dilator may be simple rings that are not radially expandable. Radiopaque markers include shapes fabricated from malleable material such as gold, platinum, tantalum, platinum iridium, and the like. Radiopacity can also be increased by vapor deposition coating or plating metal parts of the catheter with metals or alloys of gold, platinum, tantalum, platinum-iridium, and the like. Expandable rings may be fabricated as undulated or wavy rings, or other structures such as are found commonly on stents used for implantation in the body. Expandable structures may also include dots or other incomplete surround shapes affixed to the surface of a sleeve or other expandable shape. Non-expandable structures include circular rings or other structures that completely surround the catheter circumferentially and are strong enough to resist expansion. In another embodiment, the polymeric materials of the catheter or sheath may be loaded with radiopaque filler materials such as, but not limited to, barium sulfate or bismuth salts, at percentages from 5% to 50% by weight, in order to increase radiopacity. 
     In one embodiment of the invention, in order to enable radial or circumferential expansive translation of the reinforcement, it can be beneficial not to completely bond the inner and outer layers together, thus allowing for some motion of the reinforcement in translation as well as the normal circumferential expansion. Regions of non-bonding may be created by selective bonding between the two layers or by creating non-bonding regions using a slip layer fabricated from polymers such as PTFE, ceramics or metals. Radial expansion capabilities are important because the proximal end needs to transition to the distal expansive end and, to minimize manufacturing costs, the same catheter may be employed at both the proximal and distal end, with the expansive distal end undergoing secondary operations to permit radial or diametric expansion. 
     In another embodiment, the distal end of the catheter is fabricated using an inner tubular layer, which is thin and lubricious. This inner layer is fabricated from materials such as FEP, PTFE, polyimide, polyamide, polyethylene, polypropylene, and the like. The reinforcement layer comprises a coil, stent, or plurality of expandable or collapsible rings, which are generally malleable and maintain their shape once deformed. The outer layer comprises a tube that is split longitudinally, like a collet, and is capable of collapsing inward radially until the gaps between the tubing are reduced to zero or a small size. This structure is fused or bonded to create a composite unitary structure. The structure is crimped radially inward to a reduced cross-sectional area. A balloon or translation dilator is capable of forced expansion of the reinforcement layer, which provides sufficient strength necessary to overcome any forces imparted by the polymeric tubing. The translation dilator may have a smooth distal end, a tapered distal end, a soft pliable distal end, a distal end that is slit longitudinally to permit inward tapering of the elements, or a combination of any of the aforementioned. The distal end of the translation dilator preferably does not catch or hang up on the interior of the expandable tube because of the modified distal tip configuration. 
     Another embodiment of the invention comprises an axially elongate sheath with a proximal end, a distal end, and a defined internal lumen. At least a portion of the distal end of the sheath is expandable from a first radially collapsed configuration to a second, radially expanded configuration. The expandable portion of the sheath is comprised of a braid, a plurality of longitudinal runners, a tube with staggered “brickwork” slits running longitudinally, or other radially expandable pattern. The sheath may further comprise an elastomeric outer sleeve about its expandable region fabricated from materials such as, but not limited to, polyurethane, silicone elastomer, C-Flex, latex rubber, or the like. An internal translation dilator is operated from the proximal end of the sheath and is advanced proximally to expand the distal end of the sheath. The outer diameter of the translation dilator is slightly smaller than the inside diameter of the proximal end of the sheath. The translation dilator is slightly rounded or tapered or slit to minimize strain and catching on the interior surfaces of the sheath. Once the translation dilator is advanced into the expandable portion of the sheath, the expandable portion of the sheath expands. The inner lumen of the translation dilator becomes the inner lumen of the sheath along its entire length. If a braided material is used to create the expandable portion of the sheath, it may be beneficial to affix stabilization wires between the distal end of the braid and some portion of the non-expandable portion of the sheath. These stabilization wires may prevent constriction of the braid when the translation dilator is being advanced therethrough. 
     Another embodiment of the invention comprises a method of providing transluminal access. The method comprises inserting a cystoscope into a patient transurethrally into the bladder. Under direct optical visualization, fluoroscopy, MRI, or the like, a guidewire is passed through the instrument channel of the cystoscope and into the bladder. The guidewire is manipulated, under the visual control described above, into the ureter through its exit into the bladder. The guidewire is next advanced to the appropriate location within the ureter. The cystoscope is next removed, leaving the guidewire in place. The ureteral access sheath is next advanced over the guidewire transurethrally so that its distal tip is now resident in the ureter. The position of the guidewire is maintained carefully so that it does not come out of the ureter and fall into the bladder. A removable obturator with a guidewire lumen can be used to assist with placement of the access sheath into the urinary lumens. The obturator is removed from the access sheath following correct placement. Expansion of the distal end of the access sheath from a first smaller diameter cross-section to a second larger diameter cross-section is next performed, optionally with an expandable member, to permit passage of instruments that would not normally have been able to be inserted into the ureter due to the presence of strictures, stones, or other stenoses. The method further optionally involves releasing the elongate tubular body from a constraining tubular jacket, removing the expandable member from the elongate tubular body; inserting appropriate instrumentation, and performing the therapeutic or diagnostic procedure. Finally, the procedure involves optionally collapsing the elongate tubular body to a cross-sectional profile smaller than the second, greater cross-sectional profile, and removing the elongate tubular body from the patient. Such final reduction in cross-sectional size prior to removal from the patient is optional but may be important in certain patients. 
     In one embodiment, where the transluminal access sheath is used to provide access to the upper urinary tract, the access sheath may be used to provide access by tools adapted to perform biopsy, urinary diversion, stone extraction, antegrade endopyelotomy, and resection of transitional cell carcinoma and other diagnostic or therapeutic procedures of the upper urinary tract or bladder. Other applications of the transluminal access sheath include a variety of diagnostic or therapeutic clinical situations, which require access to the inside of the body, through either an artificially created, percutaneous access, or through another natural body lumen. 
     For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention are described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. The means described herein for accomplishing the procedure can comprise some or all of the elements presented. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. These and other objects and advantages of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. Throughout the drawings, reference numbers are re-used to indicate correspondence between referenced elements. 
         FIG. 1  is a front view schematic representation of the urethra, bladder and ureter; 
         FIG. 2  is a front view schematic representation of the urethra, bladder and ureter with a catheter passed into the ureter by way of the urethra; 
         FIG. 3A  is a cross-sectional illustration of a radially expandable transluminal catheter or sheath comprising a tube that is folded, at its distal end in longitudinal creases, and a balloon dilator, both of which are in their radially collapsed configuration, according to an embodiment of the invention; 
         FIG. 3B  is a cross-sectional illustration of the radially expandable transluminal sheath of  FIG. 3A , wherein the sheath and the dilator are in their radially expanded configuration, according to an embodiment of the invention. 
         FIG. 3C  illustrates a cross-section of the radially expanded transluminal sheath of  FIG. 3B , wherein the dilator has been removed, according to an embodiment of the invention; 
         FIG. 4A  is an illustration of a radially expandable transluminal sheath comprising a longitudinally folded sheath and a balloon dilator, wherein the distal portion of the sheath further comprises segmentation to increase flexibility, according to an embodiment of the invention; 
         FIG. 4B  illustrates the radially expandable transluminal sheath of  FIG. 4A  wherein the segmented distal portion has been expanded by the dilator, according to an embodiment of the invention; 
         FIG. 5A  is an illustration of a radially expandable transluminal sheath further comprising a plurality of longitudinally disposed runners, an irreversibly stretchable outer wall, and a balloon dilator, according to an embodiment of the invention; 
         FIG. 5B  illustrates the transluminal sheath of  FIG. 5A  wherein the distal section has been expanded by a balloon dilator, according to an embodiment of the invention; 
         FIG. 5C  illustrates a lateral cross-section of the distal portion of the transluminal sheath of  FIG. 5A  wherein the sheath covering comprises flutes or longitudinally disposed lines of increased thickness, according to an embodiment of the invention; 
         FIG. 6A  illustrates a radially expandable transluminal sheath comprising a coil reinforcement that is disposed within an outer sleeve layer, wherein the coil is unwound to cause the sheath to expand, according to an embodiment of the invention; 
         FIG. 6B  illustrates the radially expandable transluminal sheath of  FIG. 6A  wherein the coil has been unwound to a larger diameter, according to an embodiment of the invention; 
         FIG. 7A  illustrates a radially expandable transluminal sheath comprising a malleable expandable stent-like reinforcement, a balloon dilator, and an expandable sleeve, according to an embodiment of the invention; 
         FIG. 7B  illustrates the radially expandable transluminal sheath of  FIG. 7A  wherein the distal section has been expanded by the balloon dilator, according to an embodiment of the invention; 
         FIG. 8A  illustrates a side view of a radially expandable transluminal sheath in its small diameter configuration, comprising an unfurling unsupported sleeve through which instruments may be passed, according to an embodiment of the invention; 
         FIG. 8B  illustrates a lateral cross-section of the distal end of the sheath illustrated in  8 A, showing the sheath furled about the obturator, according to an embodiment of the invention; 
         FIG. 9  illustrates a lateral cross-section of the distal end of a radially expandable transluminal sheath in its small diameter configuration, comprising thin wall unfurling sleeve over longitudinally disposed runners, through which instruments may be passed, according to an embodiment of the invention; 
         FIG. 10  illustrates a radially expandable transluminal sheath comprising hoop reinforcements that are rotated laterally to create a large diameter cross-section, and an exterior expandable sleeve, according to an embodiment of the invention; 
         FIG. 11A  illustrates a radially expandable transluminal sheath comprising a reinforcing coil or other winding embedded within a sleeve that further comprises one or more longitudinal folds, and a balloon dilator, according to an embodiment of the invention. The distal end of the sheath, comprising this structure, is crimped or compressed radially inward for delivery to the patient; 
         FIG. 11B  illustrates the expandable sheath of  FIG. 11A  wherein a balloon dilator has been used to expand the distal end of the sheath radially outward; 
         FIG. 12  illustrates a radially expandable transluminal sheath comprising a double wall and an internal annular inflation cavity that is pressurized to expand the sheath and provide some resistance against sheath wall collapse, according to an embodiment of the invention. The distal end of the sheath is shown in longitudinal cross-section; 
         FIG. 13A  illustrates a radially expandable transluminal sheath comprising an expandable distal outer wall, one or more wall reinforcements, and a hollow axially advanceable dilator, according to an embodiment of the invention; 
         FIG. 13B  illustrates the radially expandable transluminal sheath of  FIG. 13A  wherein the dilator has been translated or advanced forward to dilate the distal end of the sheath, according to an embodiment of the invention; 
         FIG. 13C  illustrates a lateral cross-section of the distal portion of the transluminal sheath of  FIG. 13A  wherein the sheath covering comprises one or more longitudinally disposed thin areas of the wall, which are incorporated to facilitate folding of the sheath, according to an embodiment of the invention; 
         FIG. 13D  illustrates the lateral cross-section with longitudinally disposed thin areas of  FIG. 13C , which has been folded or creased, according to an embodiment of the invention; 
         FIG. 14A  illustrates a radially expandable transluminal sheath comprising a self-expanding metallic reinforcing structure and an unfurling or elastomeric sleeve, according to an embodiment of the invention; 
         FIG. 14B  illustrates the expandable sheath of  FIG. 14A  wherein the self expanding stent has expanded the distal portion of the sheath, according to an embodiment of the invention; 
         FIG. 15A  illustrates a radially expandable transluminal sheath comprising an unfurling or elastomeric sleeve and an axially compressible braid at the distal end of the sheath, which is expanded axially and has a small diameter, according to an embodiment of the invention; 
         FIG. 15B  illustrates the expandable sheath of  FIG. 15A  wherein the braid has been axially compressed forcing it and the distal end of the sheath to become larger in diameter, according to an embodiment of the invention; 
         FIG. 16A  illustrates a radially expandable transluminal sheath comprising an elastomeric, unfurling, or plastically deformable sleeve, a series of split ring reinforcements, and a balloon dilator, according to an embodiment of the invention; 
         FIG. 16B  illustrates the expandable sheath of  FIG. 16A  wherein the split rings have been expanded by the balloon dilator, according to an embodiment of the invention; 
         FIG. 17A  illustrates a radially expandable transluminal sheath comprising a dilatation balloon, a malleable reinforced distal end and a spring coil or braid reinforced proximal end, the reinforcements coupled to an expandable sleeve, according to an embodiment of the invention; and 
         FIG. 17B  illustrates the expandable sheath of  FIG. 17A  wherein the dilatation balloon has expanded the distal end of the sheath, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     As set forth herein, in the description of the invention the terms catheter or a sheath will be used and can be described as being an axially elongate hollow tubular structure having a proximal end and a distal end. The tubular structure does not necessarily have a circular cross-section. The axially elongate structure further has a longitudinal axis and has an internal through lumen that extends from the proximal end to the distal end for the passage of instruments, fluids, tissue, or other materials. As is commonly used in the art of medical devices, the proximal end of the device is that end that is closest to the user, typically a surgeon or interventionalist. The distal end of the device is that end is closest to the patient or is first inserted into the patient. A direction being described as being proximal to a certain landmark will be closer to the surgeon, along the longitudinal axis, and further from the patient than the specified landmark. The diameter of a catheter is often measured in “French Size” which is 3 times the diameter in millimeters (mm). For example, a 15 French catheter is 5 mm in diameter. The French size is designed to correspond to the circumference of the catheter in mm and is often useful for catheters that have non-circular cross-sectional configurations. 
       FIG. 1  is a schematic frontal illustration of a urinary system  100  of the human comprising a urethra  102 , a bladder  104 , a plurality of ureters  106 , a plurality of kidneys  110  and a plurality of entrances  114  to the ureter from the bladder. In this illustration, the left anatomical side of the body is toward the right of the illustration. 
     Referring to  FIG. 1 , the urethra  102  is lined on its interior by urothelium. Generally, the internal surfaces of the urethra  102 , the bladder  104 , and ureters  106  are considered mucosal tissue. The urethra  102  is relatively short in women and may be long in men since it runs through the entire length of the penis. The circumference of the unstretched urethra  102  is generally in the range of pi times 8 mm, or 24 mm, although the urethra  102  generally approximates the cross-sectional shape of a slit when no fluid or instrumentation is resident therein. The bladder  104  has the capability of holding between 100 and 300 cc of urine or more. The volume of the bladder  104  increases and decreases with the amount of urine that fills therein. During a urological procedure, saline is often infused into the urethra  102  and bladder  104  thus filling the bladder  104 . The general shape of the bladder  104  is that of a funnel with a dome shaped top. Nervous sensors detect muscle stretching around the bladder  104  and a person generally empties their bladder  104 , when it feels full, by voluntarily relaxing the sphincter muscles that surround the urethra  102 . The ureters  106  operably connect the kidneys  110  to the bladder  104  and permit drainage of urine that is removed from the blood by the kidneys  110  into the bladder  104 . The diameter of the ureters  106  in their unstretched configuration approximates a round tube with a 4 mm diameter, although their unstressed configuration may range from round to slit-shaped. The ureters  106  and the urethra  102  are capable of some expansion with the application of internal forces such as a dilator, etc. The entrance  114  to each of the normally two ureters  106  is located on the wall of the bladder  104  in the lower region of the bladder  104 . 
       FIG. 2  is a schematic frontal illustration, looking in the posterior direction from the anterior direction, of the urinary system  100  comprising the urethra  102 , the bladder  104 , a plurality of ureters  106  having entrances  114 , a plurality of kidneys  110 , a stricture  202  in the left ureter, and further comprising a catheter  204  extending from the urethra  102  into the right kidney  110 . In this illustration, the left anatomical side of the body is toward the right of the illustration. 
     Referring to  FIG. 2 , the stricture  202  may be the result of a pathological condition such as an infection. The stricture may also be the result of iatrogenic injury such as that attributed to a surgical instrument or catheter that caused damage to the wall of the ureter  106 . The stricture  202  may be surrounded by fibrous tissue and may prevent the passage of instrumentation that would normally have passed through a ureter  106 . The catheter  204  is exemplary of the type used to access the ureter  106  and the kidney  110 , having been passed transurethrally into the bladder  104  and on into the ureter  106 . A catheter routed from the urethra  102  into one of the ureters  106  may turn a sharp radius within the open unsupported volume of the bladder  104 . The radius of curvature necessary for a catheter to turn from the urethra  102  into the ureter  106  may be between 1 cm and 10 cm and in most cases between 1.5 cm and 5 cm. The catheter is generally first routed into the ureter  106  along a guidewire that is placed using a rigid cystoscope. The rigid cystoscope, once it is introduced, straightens out the urethra  102  and is aimed close to the entrance  114  to the ureter  106  to facilitate guidewire placement through the working lumen of the cystoscope. 
       FIG. 3A  illustrates a longitudinal view of an expandable transluminal sheath  300  adapted for use in the urinary system  100  of  FIGS. 1 and 2 . The front section of the sheath  300  is depicted in exterior view and not in cross-section. The proximal region  302  and the central region are shown in longitudinal cross-section. The transluminal catheter  300  comprises a proximal end  302  and a distal end  304 . The proximal end  302  further comprises a proximal sheath tube  306 , a sheath hub  308 , a sleeve  310 , a sleeve grip  312 , an inner catheter shaft  318 , an outer catheter shaft  324 , and a catheter hub  316 . The distal end  304  further comprises a distal sheath tube  322 , which is folded longitudinally into one or more flutes or creases  328 , the inner catheter shaft  318 , and a balloon  320 . 
     Referring to  FIG. 3A , the proximal end  302  generally comprises the proximal sheath tube  306  that is permanently affixed to the sheath hub  308 . The sleeve  310  is tightly wrapped around the proximal sheath tube  306  and is generally able to be split lengthwise and be removed or disabled as a restraint by pulling on the sleeve grip  312  that is affixed to the sleeve  310 . The sleeve  310  is generally fabricated from transparent material and is shown so in  FIGS. 3A and 3B . The proximal end further comprises the inner catheter shaft  318 , the outer catheter shaft  324 , and the catheter hub  316 . The catheter hub  316  allows for gripping the dilator and it allows for expansion of the dilatation balloon  320  by pressurizing an annulus between the inner catheter shaft  318  and the outer catheter shaft  324 , said annulus having openings into the interior of the balloon  320 . The balloon  320  is bonded, at its distal end, either adhesively or by fusion, using heat or ultrasonic energy, to the inner catheter shaft  318 . The proximal end of the balloon  320  is bonded or welded to the outer catheter shaft  324 . Alternatively, pressurization of the balloon  320  can be accomplished by pressurizing a lumen in the inner or outer catheter shafts  318  or  324 , respectively, said lumen being operably connected to the interior of the balloon  320  by openings or scythes in the catheter tubing. The distal end  304  generally comprises the distal sheath tube  322  which is folded into flutes or creases  328  running along the longitudinal axis and which permit the area so folded to be smaller in diameter than the sheath tube  306 . The inner catheter shaft  318  comprises a guidewire lumen that may be accessed from the catheter hub  316  and passes completely through to the distal tip of the catheter shaft  318 . The guidewire lumen is preferably able to contain guidewires up to and including 0.038-inch diameter devices. 
     The distal end  304  further comprises the catheter shaft  318  and the dilatation balloon  320 . The catheter hub  316  may removably lock onto the sheath hub  308  to provide increased integrity to the system and maintain longitudinal relative position between the catheter shaft  318  and the sheath tubing  322  and  306 . The catheter shaft  318  and the balloon  320  are slidably received within the proximal sheath tube  306 . The catheter shaft  318  and balloon  320  are slidably received within the distal sheath tube  322  when the distal sheath tube  322  is radially expanded but are frictionally locked within the distal sheath tube  322  when the tube  322  is radially collapsed. The outside diameter of the distal sheath tube  322  ranges from 4 French to 16 French in the radially collapsed configuration with a preferred size range of 5 French to 10 French. The outside diameter is critical for introduction of the device. Once expanded, the distal sheath tube  322  has an inside diameter ranging from 8 French to 20 French. The inside diameter is more critical than the outside diameter once the device has been expanded. The wall thickness of the sheath tubes  306  and  322  ranges from 0.002 to 0.030 inches with a preferred thickness range of 0.005 to 0.020 inches. 
       FIG. 3B  illustrates a cross-sectional view of the sheath  300  of  FIG. 3A  wherein the balloon  320  has been inflated causing the sheath tube  322  at the distal end  304  to expand and unfold the flutes  328 . The distal sheath tube  322  has the properties of being able to bend or yield, especially at crease lines, and maintain its configuration once the forces causing the bending or yielding are removed. The proximal sheath tube  306  is affixed to the sheath hub  308  by insert molding, bonding with adhesives, welding, or the like. The balloon  320  has been inflated by pressurizing the annulus between the inner tubing  318  and the outer tubing  324  by application of an inflation device at the inflation port  330  on the catheter hub  316 . The pressurization annulus empties into the balloon  320  at the distal end of the outer tubing  324 . Exemplary materials for use in fabrication of the distal sheath tube  322  include, but are not limited to, polytetrafluoroethylene (PTFE), fluorinated ethylene polymer (FEP), polyethylene, polypropylene, polyethylene terephthalate (PET), and the like. A wall thickness of 0.008 to 0.012 inches is generally suitable for a device with a 16 French OD while a wall thickness of 0.019 inches is appropriate for a device in the range of 36 French OD. The resulting through lumen of the sheath  300  is generally constant in French size going from the proximal end  302  to the distal end  304 . The balloon  320  is fabricated by techniques such as stretch blow molding from materials such as PET, nylon, irradiated polyethylene, and the like. 
       FIG. 3C  illustrates a cross-sectional view of the sheath  300  of  FIG. 3B  wherein the catheter shaft  318 , the balloon  320 , and the catheter hub  316  have been withdrawn and removed leaving the proximal end  302  and the distal end  304  with a large central lumen capable of holding instrumentation. The sleeve  310  and the sleeve grip  312  have also been removed from the sheath  300 . The shape of the distal sheath tube  322  may not be entirely circular in cross-section, following expansion, but it is capable of carrying instrumentation the same size as the round proximal tube  306 . Because it is somewhat flexible and further is able to deform, the sheath  300  can hold noncircular objects where one dimension is even larger than the round inner diameter of the sheath  300 . The balloon  320  is preferably deflated prior to removing the catheter shaft  318 , balloon  320  and the catheter hub  316  from the sheath  300 . 
       FIG. 4A  illustrates a side view of a transluminal expandable sheath  400  comprising a proximal end  402  and a distal end  404 . The sheath  400  further comprises a dilator hub  316 , a dilator tube  318 , a dilator balloon  320 , a sheath tube  306 , a sheath hub  308 , an external sleeve  310 , a sleeve grip  312 , a proximal guidewire lumen connector  408 , and a plurality of slots, disconnections, or perforations  406 . The diametrically compressed sheath tube  306  comprises one or more longitudinal folds  328 . 
     Referring to  FIG. 4A , the sheath  400  comprises slots  406  that permit bending and flexing of the sheath tube  306 . The slots  406  may exist on the proximal part of the sheath tube  306 , the distal part of the sheath tube  306 , or both. The slots  406  may totally separate the sheath tube  306  adjacent segments or bendable members may interconnect the sheath tube segments. The bendable members may be disposed in one circumferential location, as a backbone, or they may be distributed around the circumference of the sheath, one or two per slot so as to allow bending in a given plane. Bendable members connecting two segments may be located at a different circumferential location relative to those connecting other segments to allow for bending in more than one plane. The segments and their separating slots  406  are configured so that the segments still retain the one or more longitudinal folds or wings  328  which permit the diametric expansion of the sheath  400  upon inflation of the balloon  320 . In the illustrated embodiment, the longitudinal fold  328  is shown aligned on the backbone connecting adjacent slots  406 . In another embodiment, the segments, which are present to increase flexibility and bendability of the sheath  400 , are replaced by a spiral pattern that is cut completely through the wall of the sheath. The spiral pattern can run continuously from the proximal end of the sheath to the distal end of the sheath  400  or from an intermediate location to the distal end of the sheath  400 . In yet another embodiment, the spiral pattern may be incompletely, or only partially, cut through the outer wall of the sheath tube  306 . This configuration increases the flexibility of the sheath tube  306  and the thicker areas serve to minimize kinking. The sheath tube  306  is fabricated preferably from materials with some elasticity. Candidate materials include, but are not limited to, Hytrel, silicone elastomer, polyurethane, and the like. 
       FIG. 4B  illustrates the sheath  400  of  FIG. 4A  wherein the balloon  320  has been inflated and has expanded the sheath tube  306  at the distal end  404  of the sheath  400 . The proximal end  402  of the sheath appears unchanged in this illustration. Note that the slots  406  have unfolded with the tube  306  but have not expanded longitudinally. In the illustrated embodiment, the slots  406  incompletely circumscribe the sheath tube  306  leaving a backbone or flexible connection between rings of the sheath tube  306 . Expansion of the balloon  320  is accomplished by attaching an inflation device (not shown) to the balloon inflation port  330  on the dilator hub  316 . The balloon inflation port  330  is operably connected and sealed to the annulus between an inner dilator tube  318  and an outer dilator tube  324  (not shown) so that fluid input into the balloon inflation port  330  flows distally and empties into the balloon  320 , whose proximal seal is affixed to the outside of the outer dilator tube (not shown). Alternatively, inflation pressure is carried by a central lumen on a valved catheter, to seal the guidewire at the distal tip, or by one of a plurality of lumens in a central catheter shaft and is transmitted into the balloon through scythes in the catheter shaft to open the pressure lumen to the balloon interior. This balloon dilator construction is appropriate for most of the embodiments of the invention that rely on a balloon dilator. The slots  406  permit bendability and flexibility of the catheter in the radially expanded configuration as well as the radially contracted configuration of  FIG. 4A . In another embodiment, the slots  406  are fabricated to cut completely through the wall of one tube while a concentrically affixed composite tubular structure provides the interconnectivity that keeps the sheath tube  306  from separating under tension. In this embodiment, the slots are cut through a material with little elasticity, such as PET. The composite structure may use an inner, or outer, tube fabricated from Pebax, Hytrel, silicone elastomer, C-Flex, polyurethane, or other elastomeric material. 
       FIG. 5A  illustrates a side view of a sheath  500  comprising a proximal section  502  and a distal section  504 . The distal section  504  comprises a length of dilator tubing  318 , a dilator balloon  320 , and an outer stretchable layer  510 . The distal section  504  further may comprise optional longitudinal runners  506  separated by longitudinal slits or slots  508 . The proximal section  502  comprises a proximal sheath cover  512 , a sheath hub  308  and a dilator hub  316 , further comprising a guidewire port  408  and a balloon inflation connector  330 . 
     Referring to  FIG. 5A , the outer stretchable layer  510  is disposed similarly as the wall  306  of the sheath  300  of  FIG. 3A . The stretchable layer  510  can be affixed, at its proximal end, to the distal end of the proximal sheath cover  512 , which is non-expansible and surrounds the sheath  500  at its proximal end. The internal lumen of the stretchable layer  510  is operably connected to the inner working lumen of the proximal sheath cover  512 . The outer stretchable layer  510  may be constructed from materials that are plastically deformable such that the circumference is irreversibly increased by expansion of the dilator balloon  320  and the outward forces created thereby. The wall thickness of the outer stretchable layer  510  will generally decrease as the outer stretchable layer  510  is dilated. The outer stretchable layer  510 , once dilated, will generally provide little hoop strength against collapse and serves as the liner of a potential space lumen. The optional longitudinal runners  506 , separated by the slits  508 , provide a reduced friction track for the passage of instrumentation within the outer stretchable layer  510 . The runners  506  can be fabricated from materials such as, but not limited to, PTFE, FEP, PET, stainless steel, cobalt nickel alloys, nitinol, titanium, polyamide, polyimide, polyethylene, polypropylene, and the like. The runners  506  may further provide column strength against collapse or buckling of the outer stretchable layer  510  when materials such as calcific stones or other debris is withdrawn proximally through the sheath  500 . The runners  506  may be free and unattached or they may be affixed to the interior of the stretchable layer  510  using adhesives, welding, or the like. The guidewire port  408  is generally configured as a Luer lock connector or other threaded or bayonet mount and the guidewire is inserted therethrough into the guidewire lumen of the dilator tubing  318  to which the guidewire port  408  is operably connected. The guidewire port  408  is preferably integrally fabricated with the dilator hub  316  but may be a separately fabricated item that is affixed to the dilator hub  316 . A Tuohy-Borst or other valved fitting is easily attached to such connectors to provide for protection against loss of fluids, even when the guidewire is inserted. 
       FIG. 5B  illustrates the sheath  500  of  FIG. 5A  wherein the balloon  320  has dilated the distal section  504  diametrically. The proximal sheath cover  512  is unchanged but the longitudinal runners  506  have become expanded and the longitudinal slits or slots  508  have become wider. The outer stretchable layer  510  of the distal section  504  has stretched permanently to increase the distance circumferentially between the longitudinal slits or slots  508 . The resultant distal section  504  is relatively unsupported by the thin wall of the sheath tubing  510  but may be suitable for instrument passage. 
       FIG. 5C  illustrates a lateral cross-section of the distal end  504  of another embodiment of the sheath  500 . In this embodiment, the outer stretchable layer  510  covering the distal section  504  is fabricated with flutes  520  on the interior, or exterior, surface. Interior flutes  520  are the preferred embodiment in this case. The flutes  520  represent longitudinally running increases in wall thickness of the stretchable layer  510  which are separated by longitudinally running regions of decreased wall thickness  522 . The flutes  520  are generally integral to the outer stretchable layer  510 . The flutes  520  are generally created by fabricating an extrusion die with slots that permit the polymer to extrude with ridges thereon. The flutes  520  may facilitate folding and minimize damage to optical scopes, such as ureteroscopes, angioscopes, and the like, when inserted therethrough, due to debris scratching the lens when the scope is advanced or retracted. When the stretchable layer  510  is dilated, the region of decreased wall thickness  522  between the flutes  520  will preferentially stretch because of the increased strength of the flutes  520 . In this embodiment, more circumferentially uniform stretching of the stretchable layer  510  is possible. It is important to minimize circumferential unevenness of the stretching since balloon dilators will generally impart unevenness in an object being dilated, such as a stent, unless steps are taken to even this process out. 
       FIG. 6A  illustrates a side view of an expandable sheath  600  comprising a proximal end  602  and a distal end  604 . The sheath  600  further comprises an outer covering  606 , a central torque rod  608 , a coil support  610 , an engagement catch  612 , a sheath hub  614 , a torque handle  616 , a lock  618 , a guidewire port  620 , and a fluid infusion port  622 . The distal end  604  has a reduced diameter relative to that of the proximal end  602 . In this embodiment, the outer covering  606  is either an elastomer or a furled substantially non-distensible material. 
     Referring to  FIG. 6A , the coil support  610  is disposed interior to the outer covering  606  primarily at the distal end  604  of the sheath  600 . The engagement catch  612  is affixed to the distal end of the coil support  610  and engages features affixed to the distal end of the central torque rod  608 . The torque handle  616  is affixed to the proximal end of the central torque rod  608 . The torque handle  616  and the torque rod  608  rotate within the outer covering  606 . The torque handle  616  can have friction, ratcheting, or other locking mechanisms  618  to maintain rotational position of the torque handle  616  relative to the sheath hub  614 , in which it rotates concentrically. Rotation of the torque handle  616  and attached torque rod  608  relative to the sheath hub  614  is performed manually or with the use of electric, hydraulic, or pneumatic drive systems. The sheath hub  614  is affixed to the proximal end of the outer covering  606 . The fluid infusion port  622  is operably connected to the inner lumen of the central torque rod  608 , which is hollow, thus allowing fluid to be injected or withdrawn into or from the distal end of the central torque rod  608 . 
       FIG. 6B  illustrates the sheath  600  of  FIG. 6A  wherein the coil support  610  has been unwound and expanded diametrically, thus expanding the outer covering  606  at the distal end  604  to a diameter substantially similar to that of the proximal end  602 . The lock  618  has been engaged to maintain relative position between the torque handle  616  and the sheath hub  614 . In another embodiment, the function of the hub  618  can be replaced by a high friction or ratchet mechanism between the coil support  610  and the outer covering  606 . In this latter embodiment, the central torque rod  608 , the torque handle  616 , and the engagement catch  612  can be removed from the sheath  600 , by withdrawing proximally, to reveal a large central lumen in the outer covering  606 , through which instrumentation (not shown) may be passed. The material of the coil support  610  is preferably spring-tempered metal such as, but not limited to, nitinol, titanium, stainless steel, Elgiloy, other cobalt-nickel alloys, and the like. The coil may be round or flat wire with a primary diametric dimension of 0.002 to 0.050 inches and preferably between 0.003 and 0.020 inches. The material of the torque rod  608  is the same as that for the coil support  610  or it may be a polymer such as PET, nylon, or the like. By counter-rotating the distal end of the coil support  610  relative to the proximal end, the coil support  610  can enlarge, or decrease, in diameter. An advantage of this embodiment is that the distal end  604  of the sheath  600  can be selectively dilated diametrically and then be recompressed prior to removal from the patient. A lubricant or slip layer may further be disposed between the coil support  610  and the outer covering  606 . The outer covering  606  may be an elastomer like C-Flex, silastic, polyurethane, or the like, or it may be inelastic materials such as PET, polyethylene, or the like, which are furled or folded into a smaller diameter. 
       FIG. 7A  illustrates a side view of an expandable sheath  700  comprising a proximal end  702  and a distal end  704 . The sheath  700  further comprises an outer covering  706 , a dilator shaft  708 , a support frame  710 , a dilatation balloon  712 , a sheath hub  714 , a dilator hub  716 , a guidewire port  720 , and a balloon inflation port  722 . The distal end  704  has a reduced diameter relative to that of the proximal end  702 . 
     Referring to  FIG. 7A , the support frame  710  is a structure very similar to a stent such as that used for treating stenoses in the coronary arteries. The support frame  710  is embedded within, or resides interior to and against, the inner diameter of the outer covering  706 . The support frame may be fabricated from stainless steel, titanium, martensitic nitinol, gold, platinum, tantalum, or other materials commonly used to fabricate cardiovascular stents. The support frame may be fabricated from wire, it may be laser cut from a tube or sheet of metal, or it may be photo-etched, mechanically machined, or machined using electron discharge methodology. The support frame, in an embodiment, is malleable and remains in the state to which it is dilated by the dilatation balloon  712 . The support frame is preferably radiopaque under the circumstances in which it is used in vivo and may be fabricated from, alloyed with, or coated with materials such as gold, platinum, platinum iridium, or tantalum. The support frame wall thickness can range from 0.002 to 0.025 inches and preferably be between 0.003 and 0.012 inches. The support frame preferably comprises structures that permit flexibility. Such flexibility enhancing structures include disconnected “Z” or diamond-shaped ring segments, ring segments connected by a backbone or alternating backbone of wire, continuous undulating spirals, and the like. The outer covering is either unfurling, malleably expansible, or elastomeric. An exemplary expansible outer covering  706  comprises a low-density polyethylene disposed so that it embeds the stent. Another expansible outer covering  706  comprises a polyurethane, silastic or thermoplastic elastomer sleeve disposed around and frictionally covering the support frame  710 . The outer covering  706  may further comprise an inner layer that is relatively low in sliding friction such as, but not limited to, high density polyethylene, FEP, PTFE, or the like. A furled outer covering  706  may be fabricated from stretch blow-molded PET. The outer covering  706  may be coated on its interior, exterior, or both, by silicone slip agents, hydrophilic hydrogels, or the like to minimize friction in passing the catheter through the body lumen as well as passage of instruments therein. 
       FIG. 7B  illustrates the sheath  700  of  FIG. 7A  wherein the support frame  710  has become expanded by the dilatation balloon  712  having been pressurized by fluid injected into the inflation port  722  on the dilator hub  716  and transmitted to the balloon  712  through the dilator shaft  708 . The support frame  710 , at the distal end  704 , has malleably expanded and holds the outer covering  706  in its radially expanded configuration. 
     Referring to  FIG. 7B , the support frame  710  is affixed to the distal end of the proximal portion  702  of the sheath  700 . The support frame  710  may be fully expanded at this proximal end even prior to expansion, as in  FIG. 7A , and then neck down in the distal portion  704 . Once expanded, the support frame  710  and the outer covering  706  have a generally continuous diameter and through lumen passing all the way from the proximal most portion of the sheath  700  to the distal end thereof. The outer covering  706  in the distal portion  704  will have stretched or unfurled to take on its larger diameter configuration. The recovery strength of the outer covering  706  is preferably such that it does not impart restorative forces greater than the resistive forces generated by the malleably expanded support frame  710 . The distal region  704  remains dilated once the dilatation balloon  712 , the dilator shaft  708 , the dilator hub  716 , and the inflation port  722  have been removed from the sheath  700 . Thus, a large central lumen is generated within the sheath  700 . 
       FIG. 8A  illustrates a side view of an expandable sheath  800  comprising a proximal end  802  and a distal end  804 . The sheath  800  further comprises a distal sheath covering  806 , a proximal sheath covering  824 , an obturator shaft  812 , a distal shroud  810 , a sheath hub  814 , an obturator hub  816 , and a guidewire port  818 . The distal end  804  has a reduced diameter relative to that of the proximal end  802  because the distal sheath covering  806  has been folded or furled to form one or more longitudinal creases or pleats  820 . The distal shroud  810  maintains the distal end  804  in its folded, small diameter configuration until it is removed by pushing it forward or pulling it backward. The transition region  822  connects the proximal sheath covering  824  and the distal sheath covering  806 . 
     Referring to  FIG. 8A , the distal sheath covering  806  begins at the transition region  822 . The distal sheath covering  806  is thin-walled material that is folded into a plurality of pleats  820 . The distal sheath covering  806  is fabricated from materials such as, but not limited to, PET, polyethylene, polypropylene, Hytrel, Pebax, polyimide, polyamide, HDPE, and the like. The wall thickness of the distal sheath covering  806  ranges from 0.001 to 0.020 inches. The distal sheath covering  806  may be heat set, or crosslinked by irradiation (e.g. gamma radiation or electron beam radiation), to sustain the pleats, creases, or folds  820 . The distal sheath covering  806  is affixed to the distal end of the proximal sheath covering  824  by welding or adhesive bonding. The distal shroud  810  is permanently affixed to the exterior of the obturator shaft  812 . The distal shroud  810  may be rigid or it may be flexible or elastomeric. The distal shroud  810  covers the distal sheath covering  806  and holds the distal end of said covering  806  compressed against the obturator shaft  812 . The distal shroud  810  may be fabricated from C-Flex, polyurethane, silicone elastomer, and the like. The distal shroud  810  may be injection molded or cut from an extrusion or dip-coated structure. The distal shroud  810  may further comprise an inner spacer to prevent inadvertent withdrawal of the obturator and shroud until the shroud is advanced distally to release the sheath covering  806  so that it can expand. The inner spacer can further comprise, on its proximal end, a taper to facilitate proximal withdrawal into the sheath. The inner spacer may further comprise an undercut or relief on its distal end, which allows the shroud to maintain a low profile following eversion prior to withdrawal proximally. The obturator shaft  812  has a central through lumen, generally 0.020 to 0.050 inches in diameter that is operably connected to the guidewire port  818  on the obturator hub  816 . The obturator shaft  812  is affixed to the obturator hub  816  by insert molding, welding, adhesive bonding, or the like. The obturator hub  816  is advanced forward causing the obturator shaft  812  to advance relative to the distal sheath covering  806 . The shroud  810  pulls forward therewith and no longer surrounds the exterior of the sheath covering  806 . The sheath covering  806  is now free to expand by unfurling the pleats  820  and serve as potential space for instrumentation once the obturator shaft  812  and its associated components are withdrawn from the sheath  800 . In another embodiment, the shroud  810  is evertable and the obturator handle  816  is withdrawn proximally to simply pull the shroud  810  off the sheath covering  806  and out through the central lumen of the sheath  800 . The shroud  810  may be fabricated from metals such as stainless steel or from polymers such as C-Flex, polypropylene, polyethylene, polyurethane, silicone elastomer, and the like. The proximal sheath covering  824  may be a unitary polymer tube fabricated from Hytrel, Pebax, polyethylene, FEP, PTFE, or the like. The proximal sheath covering  824  may further be a composite reinforced structure with an internal coil or braid reinforcement surrounded by polymers. The polymer on the interior may preferentially be a different polymer than that disposed on the exterior of the proximal sheath covering  824 . 
       FIG. 8B  illustrates a lateral cross section of the distal end  804  of the sheath  800  of  FIG. 8A . In the illustrated embodiment, the distal sheath covering  806  has been folded to form four longitudinal creases, furls, or pleats  820 . The obturator shaft  812  remains in place in the center of the sheath covering  806  and provides internal support to prevent buckling or kinking. When unconstrained, the sheath covering  806  is capable of expanding when a device is placed therethrough but there is no other support for the distal sheath covering  806 , which is a relatively thin-wall un-reinforced structure. 
       FIG. 9  illustrates a lateral cross-sectional view of the distal end  904  of another embodiment of the sheath  900 . The sheath  900  is the same as the sheath  800  of  FIG. 8A  with the addition of runners  908  displaced within the sheath covering  806 . The obturator shaft  812  is the same as the obturator shaft  812  of  FIG. 8A . The runners  908  provide some column strength to prevent longitudinal collapse of the distal end  904 , but also define a track through which instrumentation may be passed with minimal friction. The runners  908  expand freely in the radial direction when the pleats  820  unfold. The runners  908  may be affixed to the interior of the sheath covering  806  and further may be integral to or affixed to the non-expandable proximal portion of the sheath  900 . The runners  908  may be affixed to the proximal sheath covering  824 , referring to  FIG. 8A , using adhesives, welding, or similar processes. The runners  908  may be extensions of the proximal sheath covering  824  that are slit to form circumferentially spaced-apart longitudinally disposed rods. The runners  908  may be integrally formed to the sheath covering  806  by being extruded or co-extruded into the sheath covering  806  when the latter is being fabricated. The runners  908  may be fabricated from metals such as stainless steel, titanium, cobalt nickel alloys, nitinol, or the like, or they may be fabricated from polymers such as but not limited to, polyethylene, polypropylene, PTFE, FEP, polyester, polyimide, polyamide, and the like. 
       FIG. 10  illustrates an oblique view of another embodiment of a sheath  1000 . The sheath  1000  comprises a proximal end  1002  and a distal end  1004 . The proximal end  1002  has a fixed diameter and is terminated, at its most proximal point, with a hub  1012 . The hub  1012  further comprises a through port  1028 , a control lever  1020 , and a lock  1022 . The distal end comprises an expandable sheath covering  1014 , a plurality of rotating hoops  1024 , a control rod  1018 , and a stabilizer rod  1016 . Axial movement of the control lever  1020  moves the control rod  1018  and swivels or rotates the rotating hoops  1024  about the stabilizer rod  1016  into a more or less perpendicular orientation, relative to the longitudinal axis of the sheath  1000 . The lock  1022  maintains the position of the control lever  1020 . The control lever  1020  is retracted proximally to rotate the hoops  1024  back into a substantially longitudinal orientation. The sheath covering  1014  is disposed external to the hoops  1024  and is forced to expand when the hoops rotate into the perpendicular plane. The sheath covering  1014  may be an unfurling non-distendable membrane or an elastomeric membrane. The sheath covering  1014  is preferably fluid impermeable. The control rod  1018  and the stabilizer rod  1016  are preferably flexible and allow for bending of the array of hoops  1024 . 
       FIG. 11A  illustrates a side cutaway view of an expandable transluminal sheath  1100  comprising a fixed diameter proximal end  1102  and a radially expandable distal end  1104 . The sheath further comprises a proximal sheath outer layer  1124 , a distal sheath outer layer  1106 , a coil reinforcement  1116 , a dilator shaft  318 , a dilatation balloon  320 , one or more longitudinal creases  1110  formed in the outer layer  1106 , a sheath hub  308 , and a dilator hub  316 . 
     Referring to  FIG. 11A , the distal sheath layer  1106  has incorporated therein a coil reinforcement  1116 . The coil reinforcement  1116  is, in an embodiment, a wire fabricated from annealed metals such as, but not limited to, gold, stainless steel, titanium, tantalum, nickel titanium alloy, cobalt nickel alloys, and the like. The wire may be round wire or flat wire. Round wire diameters of 0.001 to 0.025 inches are appropriate for this embodiment, with preferable diameters in the range of 0.002 to 0.010 inches. Flat wires with widths of 0.005 to 0.040 inches and thickness ranging from 0.001 to 0.020 inches are suitable for this application. The wires of the coil reinforcement  1116  may be advantageously coated with materials that have increased radiopacity to allow for improved visibility under fluoroscopy or X-ray visualization. The radiopaque coatings for the coil reinforcement  1116  may be gold, platinum, tantalum, platinum iridium, and the like. The coatings may be imparted on the wire using vapor deposition processes, electroplating, dip coating, and the like. The coil reinforcement  1116  is preferably sandwiched between two layers of polymer, an inner and an outer layer. The inner layer may be a fluoropolymer such as fluorinated ethylene propylene, polytetrafluoroethylene, or the like. The inner layer may also be polyethylene, polyester, polyimide, polyamide, or other material that can be coated with lubricious coatings such as silicone oil or hydrophilic hydrogel. The outer layer may be fabricated from materials such as, but not limited to, polyethylene, Hytrel, PEBAX, polyurethane, C-Flex, or the like. The layers are preferably fused together with the coil sandwiched therebetween. Heat and pressure are applied to cause the fusing of the inner and outer layers. The wall is kept intentionally thin so that the entire distal tube structure comprising the coil reinforcement  1116  and the distal covering  1106  may be folded longitudinally to form flutes  1110 . The mechanical strength of the coil maintains the shape without the need for external sleeves, although an external sleeve is a viable option. The strength of the distal polymer layer  1106  is not enough to overcome the forces generated by the coil reinforcement  1116 . The external sleeve (not shown) can be either elastomeric or it may be a peel-away structure. Balloon dilatation occurs in the same way as that of the device in  FIGS. 3A and 3B . The distal polymer layer  1106  at the distal end  1104  of the sheath  1100  may be the same as that of the layer  1124  at the proximal end  1102  of the sheath  1100 . The exception is that the coil reinforcement in that area, if any, would be preferably spring temper. If the layers  1106  and  1124  are different, they are preferably fused together at their intersection. 
       FIG. 11B  illustrates the sheath  1100  of  FIG. 11A  wherein the coil reinforcement  1116  and the distal outer layer  1106  have become expanded by the dilatation balloon  320  located on the distal end  1104 . The coil reinforcement  1116 , at the distal end  1104 , has malleably unfolded and holds the outer covering  1106  in its radially expanded configuration. The crease line  1110  of  FIG. 11A  has been unfolded to permit full diameter opening of the sheath outer layer  1106 . The dilator including the balloon  320 , the dilator shaft  318 , and the dilator hub  316  are next removed by withdrawing them proximally relative to the sheath  1100 . The sheath  1100  is now able to form a path of substantially uniform internal size all the way from the proximal end to the distal end and to the exterior environment of the sheath  1100  at both ends. Through this path, instrumentation may be passed, material withdrawn from a patient, or both. 
       FIG. 12  illustrates a radially expandable transluminal sheath  1200  comprising a proximal end  1202  and a distal end  1204 . The proximal end  1202  comprises an outer tube  1206 , an intermediate tube  1212 , a sheath hub  1208 , an inflation port  1224 , and an obturator hub  1210 . The distal end  1204  comprises a balloon inner layer  1214 , a balloon outer layer  1216 , the outer tube  1206 , the intermediate tube  1212 , the obturator shaft  1218 , the inner balloon bond  1220 , and the outer balloon bond  1222 . Referring to  FIG. 3A , the sheath  1200  may further comprise an optional tear away sleeve  310  and sleeve grip  312  to maintain the balloon inner layer  1214  and the balloon outer layer  1216 , closely furled around the obturator shaft  1218 . 
     Referring to  FIG. 12 , the inflation port  1224  is integral to the sheath hub  1208  and is operably connected to the annulus between the outer tube  1206  and the intermediate tube  1212 , which opens into the interior of the annular balloon formed by the balloon outer layer  1216  and the balloon inner layer  1214 . The balloon inner layer  1214  is bonded to the intermediate tube  1212  by the inner balloon bond  1220 . The balloon outer layer  1216  is bonded to the outer tube  1206  by the outer balloon bond  1222 . The balloon bonds  1220  and  1222  are either heat welds or adhesive welds, depending on the materials used for fabrication of the balloon  1214  and  1216  and the tubing. The balloon layers  1214  and  1216  are generally the same piece of material and are formed integrally, although they could be separate pieces alternatively fused together at the distal end. The balloon layers  1214  and  1216  may, one or both, be further reinforced with longitudinal strengthening members (not shown), similar to battens on a sailboat sail, that maintain column strength and prevent the balloon from compressing longitudinally when it is pressurized by fluid injected into the inflation port  1224 . The battens are preferable to a tubular structure because they can dilate radially by separating but maintain longitudinal force. The sheath  1200  would have the advantage of being able to exert some modest pressure against tissue surrounding it and would still allow for instrument passage therethrough, as opposed to a sheath that had a collapsing potential space type of distal section. The obturator shaft  1218  and obturator hub  1210  are removed to permit passage of instruments through the sheath  1200 . In yet another embodiment, the outer wall  1206  has a balloon inflation lumen (not shown) that would allow elimination of the intermediate tube  1212  and maximization of the through lumen of the sheath  1200 . 
       FIG. 13A  illustrates a side cutaway view of an expandable transluminal sheath  1300  comprising a fixed diameter proximal end  1302  and a radially expandable distal end  1304 . The sheath further comprises a proximal outer layer  1306 , a distal outer layer  1308 , an outer layer transition zone  1324 , and a sheath hub  1314 . The sheath  1300  also comprises a translation dilator  1320  further comprising a dilator shaft  1310 , a dilator hub  1312 , and a dilator tip  1322 . The dilator hub  1312  comprises an instrumentation port  1326  and an infusion port  1328 . 
     Referring to  FIG. 13A , the construction of the distal end  1304  comprises a distal outer layer  1308  that is either very thin walled and furled, or it is elastomeric and resilient. The thin walled and furled construction involves a distal outer layer  1308  that is approximately 0.0005 to 0.010 inches thick and preferably from 0.001 to 0.005 inches thick. The distal outer layer is fabricated from materials such as, but not limited to, polyethylene, polypropylene, polyurethane, polyvinyl chloride, Pebax, Hytrel, PET, FEP, PTFE, or the like. The distal outer layer  1308  is flexible and may be folded into longitudinal pleats that can be wrapped tightly circumferentially to create a small diameter axially elongate structure. The distal outer layer  1308  may further comprise flutes or runners integral to or affixed to the inner surface of the distal outer layer. The flutes or runners reduce friction between the distal outer layer  1308  and the dilator shaft  1310  as it is passed therethrough. They also may enhance folding and drainage. The distal outer layer  1308  may be further heat set, or radiation crosslinked, to cause it to be biased in the tightly wrapped furled configuration. The distal outer layer  1308  may comprise yet another layer exterior thereto, which is elastomeric and helps restore the distal outer layer  1308  to its small diameter configuration when the dilator shaft  1310  is retracted therefrom. The elastomeric layer may be fabricated from materials such as, but not limited to, C-flex, other thermoplastic elastomer, silicone elastomer, polyurethane, latex rubber, or the like. The elastomeric layer wall thickness is between 0.001 and 0.010 inches. The elastomeric layer may be affixed, using adhesives or welding, to the proximal tubing  1306  or it may be affixed to the distal end of the layer  1308 , or both. The attachment of the elastomeric layer is preferably not complete circumferentially such that the attachment does not impede diametric expansion or contraction of the distal outer layer  1308 . Radiopaque markers preferably delineate the proximal end and the distal end of the distal outer layer  1308 , these radiopaque markers being able to move with the expanding and contracting sheath and not restricting its radial size change. The distal outer layer  1308  is able to bend through a radius as small as approximately 1 cm, or less, without kinking. 
     The proximal outer layer  1306  comprises construction elements that permit flexibility, kink resistance, column strength, thin walls, torqueability, lack of interior bumps or roughness, and lubricity. The proximal outer layer  1306  is affixed to the distal outer layer  1308  at the transition zone  1324 , or the distal outer layer  1308  can be the continuation of the proximal layer  1306 . The proximal end of the proximal outer layer  1306  is further permanently affixed to the distal end of the sheath hub  1314  so as not to obstruct the through lumen of the sheath  1300 . The preferred construction of the proximal outer layer is a composite structure with a coil or braid of metal or high strength polymer surrounded by inner and outer layers of polymers that form the impermeable wall of the proximal outer layer  1306 . The braid or coil is fabricated preferably from spring-temper metals such as, stainless steel, titanium, nitinol, or the like. The braid or coil may also be fabricated from polymers such as, but not limited to, PET, PEN, polyimide, polyamide, polyether-ether-ketone, or the like. The proximal outer layer  1306  is, in an embodiment, able to bend through a bend radius of 1 to 3 cm without kinking. The proximal outer layer  1306  walls are fabricated from materials such as, but not limited to, polyethylene, polypropylene, polyurethane, polyvinyl chloride, Pebax, Hytrel, PET, FEP, PTFE, or the like. 
     The translation dilator  1320  comprising the hub  1312 , the shaft  1310 , and the tip  1322  are flexible, kink resistant, possess column strength, torqueability and a through lumen with smooth walls free of roughness. The translation dilator  1320  has very thin walls, between 0.001 and 0.025 inches, so as to maximize the instrument carrying capacity of the system. The translation dilator is preferably fabricated from a coil or braid reinforced polymer and has construction similar to that of the proximal outer layer  1306  with the same materials applying to its construction. The tip  1322  is formed by radio frequency or induction heating or it is a separate piece of material welded or bonded to the distal end of the dilator shaft  1310 . The tip  1322  is tapered on its exterior to facilitate passage through the diametrically compressed distal layer  1308 . The tip  1322  is either hard or, in a preferred embodiment, soft and resilient. The translation dilator  1320 , the proximal tube  1306 , the distal tube  1308 , all or any thereof may be coated with hydrophilic hydrogel or silicone oil to reduce friction and maximize pushability. The translation dilator is advanced distally by manual force on the dilator hub  1312  relative to the sheath hub  1314 . In another embodiment, the translation dilator  1320  comprises an obturator (not shown) which projects beyond the distal end of the distal outer layer  1308  and facilitates introduction of the sheath  1300 . The obturator may further comprise a guidewire lumen so that the sheath  1300  may be tracked over said guidewire into its target site. 
       FIG. 13B  illustrates the sheath  1300  of  FIG. 13A  wherein the outer layer  1306  has become expanded by the translation dilator  1320  which has been advanced distally to radially dilate the distal outer layer  1308 . The distal outer layer  1308 , has unfolded or elastically expanded and surrounds and is supported by the outer diameter of the hollow translation dilator  1320  to form its radially dilated configuration. The translation dilator  1320  further comprises an internal through lumen, accessed by the instrumentation port  1326 , suitable for passage of instruments or the withdrawal or infusion of materials from (or into) the body. The translation dilator  1320  may be secured in its fully advanced position by engaging a lock or snap with the sheath hub  1314 . Instrumentation passed through the sheath  1300  is passed through the instrumentation port  1326 , which is operably connected to the through lumen of the dilator shaft  1310 . Fluids may be injected or withdrawn through the fluid injection port  1328 , which is operably connected to the through lumen in the dilator shaft  1310 . The length of the sheath  1300 , with the translation dilator  1320  fully advanced, is between 5 cm and 200 cm with a preferred length of between 25 and 75 cm. 
       FIG. 13C  illustrates a lateral cross-section of another embodiment of the distal tubing  1308 . The distal tubing, in this embodiment, is extruded with thin areas  1332  and normal wall  1330 . The illustrated embodiment shows two thin areas  1332  prior to folding. The spacing and magnitude of the thick and thin areas do not necessarily have to be uniformly placed or equally sized. The thin areas can be used to enhance the ability to form tight folds for diameter reduction. 
       FIG. 13D  illustrates the distal tubing  1308  of  FIG. 13C  after it has been folded longitudinally. Other folds, including Napster™-type styles, star shapes, clover-leafs, and the like, are also possible. Such profiling would be performed on tubing fabricated from materials such as, but not limited to, polyethylene, PTFE, polyurethane, polyimide, polyamide, polypropylene, FEP, Pebax, Hytrel, and the like, at the time of extrusion. The liner would then be used, as-is, or it would be built up onto a mandrel with other layers as part of a composite tube. The composite tube can include coil, braid, or stent reinforcement. The thin areas  1332  facilitate tight folding of the layer  1308  and minimize the buildup of stresses and strains in the material that might prevent it from fully recovering to a round shape following unfolding. This type of sheath construction is suitable for the sheath embodiments shown in  FIGS. 3 ,  4 ,  5 ,  11 ,  13 ,  15 ,  16 , and  17 . 
       FIG. 14A  illustrates a side cutaway view of an expandable transluminal sheath  1400  comprising a fixed diameter proximal end  1402  and a radially expandable distal end  1404 . The sheath  1400  further comprises a proximal outer layer  1406 , a distal outer layer  1408 , a self-expanding support structure  1410 , and a sheath hub  1414 . The sheath  1400  also comprises an optional releaseable sleeve or restraint (not shown), similar to that shown in  FIG. 3A . 
     Referring to  FIG. 14A , the self-expanding support structure  1410  underlies the distal outer layer  1408  or is embedded therein. Preferably, pockets exist in the distal outer layer  1408  to hold the self-expanding support structure  1410  so the transverse elements of the self-expanding support structure  1410  do not lock up on the outer layer  1408  when the elements are rotating or changing length during expansion, should the support structure  1410  be embedded within the outer layer  1408 . The self-expanding support structure  1410  is fabricated similarly to a cardiovascular stent and comprises materials such as, but not limited to, superelastic nitinol, shape-memory nitinol, Elgiloy, titanium, stainless steel, and the like. The self-expanding support structure has element thicknesses of between 0.001 and 0.025 inches, and preferably between 0.002 and 0.015 inches. The configuration of the self-expanding support structure may include, but not be limited to, that of a serpentine spiral, a series of disconnected “Z” or diamond rings, a series of connected Z or diamond rings, brickwork slits, or the like. The distal outer layer  1408  is preferably a thin-wall unfurling structure or an elastomeric structure, both of which are similar to those appropriate for the distal tubing  1308  of  FIGS. 13A and 13B . The proximal outer tubing  1406  is constructed similarly to that of the proximal tubing  1306  of  FIGS. 13A and 13B . 
       FIG. 14B  illustrates the sheath  1400  of  FIG. 14A  wherein the distal outer layer  1408  has become expanded by the self-expanding support structure  1410 . The dilation of the support structure  1410  was initiated by the removal of the peel away sleeve or other restraint, or by phase change from martensitic to austenitic in the case of nitinol, either by exposure to body temperature or by Ohmic heating. The self-expanding support structure  1410  may further have dilated as a result of elevated temperature exposure in the body that caused shape-memory materials such as nitinol to assume austenitic phase. The distal outer layer  1408 , has unfolded or elastically expanded and surrounds and is supported by the outer diameter of the self-expanding support structure  1410  to form its radially dilated configuration. The sheath  1400  in its expanded configuration further comprises an internal through lumen, suitable for passage of instruments or the withdrawal or infusion of materials from (or into) the body. 
       FIG. 15A  illustrates a side cutaway view of an expandable transluminal sheath  1500  comprising a fixed diameter proximal end  1502  and a radially expandable distal end  1504 . The sheath  1500  further comprises a proximal outer layer  1506 , a distal outer layer  1508 , a radially expanding braided support structure  1510 , and a sheath hub  1514 . The sheath  1500  further comprises a pusher tube  1516 , a pusher hub  1512 , a plurality of tensioning wires  1518 , and a hub lock  1520 . 
     Referring to  FIG. 15A , the braid  1510  is affixed at the distal end to the distal end of the distal outer layer  1508 . The distal outer layer  1508  is fabricated from a furled thin-walled polymeric material or an elastomeric material much the same as the sheath distal outer layer  1308  in  FIGS. 13A and 13B . The crease or flute of the furled sheath  1518  is disposed longitudinally along the axis of the sheath  1500 . The proximal end of the braid  1510  is affixed to the distal end of the pusher tube  1516 . The pusher tube  1516  is affixed at its proximal end to the pusher hub  1512 . The distal end  1504  of the sheath  1500  further comprises the plurality of tensioning wires  1518  which are affixed to the braided support structure  1510  at their distal end and affixed to the proximal outer layer  1506  at their proximal end. Axially distal movement of the pusher tube  1516  relative to the sheath hub  1514  causes the braided support structure  1510  to compress axially, thus expanding radially to is design diameter. Axially proximal movement of the pusher tube  1516  relative to the sheath hub  1514  causes the braid and surrounding distal sleeve to contract diametrically. The sheath  1500  may track through the lumen over a guidewire routed through its central lumen and it may further comprise an obturator within its inner lumen to aid in positioning. 
       FIG. 15B  illustrates the sheath  1500  of  FIG. 15A  wherein the distal outer layer  1508  has become expanded by the advancing the pusher tube  1516  distally to compress the braid  1510  against the tension wires  1518 . In another embodiment, the braid  1510  may have dilated as a result of elevated temperature exposure in the body that caused shape-memory materials such as nitinol, comprising the braid, to assume austenitic phase. The distal outer layer  1508 , has unfolded or elastically expanded and surrounds and is supported by the outer diameter of the braid  1510  to form its radially dilated configuration. The sheath  1500  in its radially expanded configuration further comprises an internal through lumen suitable for passage of instruments or the withdrawal or infusion of materials from (or into) the body. The pusher hub  1512  may be temporarily, and reversibly affixed to the sheath hub  1514  by way of the hub lock  1520 . 
       FIG. 16A  illustrates a side view of an expandable sheath  1600  comprising a proximal end  1602  and a distal end  1604 . The sheath  1600  further comprises an outer covering  1606 , a dilator shaft  1608 , a split-ring support frame  1610 , a dilatation balloon  1612 , a sheath hub  1614 , a dilator hub  1616 , a guidewire port  1620 , and a balloon inflation port  1622 . The distal end  1604  has a reduced diameter relative to that of the proximal end  1602 . 
     Referring to  FIG. 16A , the split-ring support frame  1610  is a malleable structure that can be dilated by forces exerted by the inflated balloon  1612 . The dilation is the same as that generated by the sheath  700  of  FIGS. 7A and 7B . The split-ring support frame can be fabricated from wire or from flat sheets of metal or from tubes of metal. The preferred metal is selected from materials such as, but not limited to, titanium, tantalum, annealed stainless steels such as 316L, 304, and the like. The split ring support frame  1610  is disposed inside the inner diameter of the distal sheath tubing  1606 . The split ring support frame  1610  has the advantage of being inexpensive to fabricate relative to other stent-like support designs. The split ring support frame  1610  can be configured as a series of ribs and a backbone, or as a series of staggered backbones to facilitate flexibility along more than one axis. Alternatively, in another embodiment, the split ring support frame  1610  can be self-expanding. The preferred configuration for the distal sleeve  1606  is a thin wall polymer that is furled into longitudinal flutes. 
       FIG. 16B  illustrates the sheath  1600  of  FIG. 16A  wherein the support frame  1610  has become expanded by the dilatation balloon  1612  having been pressurized by fluid injected into the inflation port  1622  on the dilator hub  1616  and transmitted to the balloon  1612  through the annulus between the outer and inner tubes comprising the dilator shaft  1608 . The split-ring support frame  1610 , at the distal end  1604 , has malleably expanded and holds the outer covering  1606  in its radially expanded configuration. The through lumen of the distal end  1604  is substantially similar to that of the proximal end  1602 . 
       FIG. 17A  illustrates a side view of an expandable sheath  1700  comprising a proximal end  1702  and a distal end  1704 . The sheath  1700  further comprises an outer covering  1706 , a dilator shaft  1708 , a malleable support coil  1710 , a dilatation balloon  1712 , a sheath hub  1714 , a dilator hub  1716 , a guidewire port  1720 , a longitudinal crease  1726  in the distal outer covering  1706 , and a balloon inflation port  1722 . The distal end  1704  has a reduced diameter relative to that of the proximal end  1702 . The proximal end of the sheath  1702  further comprises a spring temper reinforcing coil or braid  1724  to support the outer covering  1706 . 
     Referring to  FIG. 17A , the support coil for the proximal end  1702  is fabricated from spring-temper materials and is of constant diameter. The preferred spacing for the coils is roughly equivalent to the width of the wire to minimize any bumpiness on the interior of the sheath  1700 . The wire winding is preferably flat wire to minimize bumpiness. Acceptable flat wire dimensions range in thickness from 0.001 to 0.025 inches and in width from 0.003 to 0.040 inches. The sheath cover  1706  preferably comprises some elasticity or malleability to maximize flexibility by stretching between the coil segments. The distal support  1710  is the same as that illustrated in  FIGS. 11A and 11B  and the distal sheath cover  1706  is also similar, although any of the distal segments disclosed in this document are suitable for use with this construction. Note that the pitch of the winding  1724  does not have to be the same as that for the winding  1710  because they have different functionality in the sheath  1700 . 
       FIG. 17B  illustrates the sheath  1700  of  FIG. 17A  wherein the malleable support coil  1710  has become expanded by the dilatation balloon  1712  having been pressurized by fluid injected into the inflation port  1722  on the dilator hub  1716  and transmitted to the balloon  1712  through the annulus between the outer and inner tubes comprising the dilator shaft  1708 . The malleable support coil  1710 , at the distal end  1704 , has malleably expanded and holds the outer covering  1706  in its radially expanded configuration. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. For example, the sheath may include instruments affixed integrally to the interior central lumen of the mesh, rather than being separately inserted, for performing therapeutic or diagnostic functions. The hub may comprise tie downs or configuration changes to permit attachment the hub to the skin of the patient. The embodiments described herein further are suitable for fabricating very small diameter catheters, microcatheters, or sheaths suitable for cardiovascular or cerebrovascular access. These devices may have collapsed diameters less than 3 French (1 mm) and expanded diameters of 4 to 8 French. Larger devices with collapsed diameters of 16 French and expanded diameters of 60 French or larger are also possible. Such large devices may have orthopedic or spinal access applications, for example. Other devices of intermediate size, can have application in cardiovascular access using appropriate hemostatic valves and seals at the proximal end of the device. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Although this invention has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been shown and described in detail, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or subcombinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combine with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.