Patent Publication Number: US-2021186725-A1

Title: Implantable medical device constraint and deployment apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/639,699, filed Mar. 5, 2015, which claims the benefit of U.S. Provisional Application 61/949,100, filed Mar. 6, 2014, both of which are incorporated herein by reference in their entireties for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to apparatus and methods for the delivery and deployment of implantable medical devices. 
     Discussion of the Related Art 
     With the growing proliferation of sophisticated implantable medical devices and more advanced treatment apparatus, and particularly medical devices and treatments that are delivered and deployed through minimally invasive procedures such as stents, stent-grafts, balloons, filters, occluders, and the like, there has been a growing interest in finding improved devices and methods to effectively constrain, deliver, and/or deploy such devices and treatment apparatus. 
     Examples of previous devices and methods for delivering and deploying such devices include the use of everting sleeves to constrain the implantable device and then deploy the devices in a controlled manner in situ, such as the devices described in U.S. Pat. No. 7,285,130 to Austin, US Application 2008-0281398 to Koss et al., and PCT Application WO13025470 to Cully et al. The use of everting sleeves as described in these references has many advantages, but there are limitations on length, geometry, tensile strength, and other properties of the devices that can be effectively mounted in and deployed from many of these sleeves. Additionally, prior everting sleeves deployment apparatus have been challenged to balance adequate constraint of implantable devices prior to deployment against accurate and easy deployment of the implantable devices in the desired location in the body. 
     Another prior constraint and delivery apparatus that has been technically and commercially successful is described in U.S. Pat. No. 6,315,792 to Armstrong et al. This apparatus employs a knitted fiber cover that constrains the implantable device prior to deployment, which is then removed to allow the device to deploy in the desired location in the body. While this apparatus works very well, it also has limitations on the size and types of implantable devices that it can effectively constrain and deploy. One concern with this and other fiber constraints is that fibrous covers may not smoothly mount and/or deploy from certain device geometries or features (e.g., fibers may catch of barbs or other features on the implantable device). 
     Even more recent interest in providing implantable devices and treatment apparatus that are covered with drugs or other bioactive agent has further increased the challenge of effectively constraining and delivering such devices. The act of pulling a drug coated device through a funnel into a constraining apparatus runs a serious risk that bioactive agent may be removed or displaced during the loading and mounting process, which could compromise the effectiveness of the device once deployed. Similarly, interaction between the constraint and the implantable device during deployment also creates a risk that the bioactive agent may not remain properly applied to the device once fully deployed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a scalable delivery system that protects medical devices during delivery in a body while providing simple, accurate, and reliable device deployment. The delivery system is configured so that loading and deployment forces are not directly related to device diameter, length, or design, thus providing a more useful delivery system that can be employed across various delivered device configurations and product lines. 
     Among the advantages of the delivery system of the present invention are: more predictable deployment forces that facilitate smoother and more predictable device delivery (e.g., by reducing adverse interaction between the implantable device and the constraint, there is less risk of snagging and catheter displacement); ability to deploy devices with irregular features (e.g., scallops, barbs, anchors, apices, and other features that may interfere with smooth operation of deployment apparatus); ability to create devices with smaller device delivery profiles; ability to contain delivery lines within a sheath so as to reduce or eliminate “bow-stringing” of the line during deployment; and ability to protect an implantable device from shear forces during manufacture and delivery, which is particularly useful to shield various coatings (e.g., drugs or other bio-active materials) applied to the device from damage or premature release. 
     A further benefit of the present invention is that it imparts minimal stress to the delivered device. In the present invention the delivered device is encapsulated prior to loading and remains encapsulated until deployment. For drug delivery devices, this reduces drug loss and particulation. This may also eliminate contact between device and tooling during device mounting and isolates the device from surface shear during loading and deployment. The present invention also can eliminate tensioning of device during loading so as to allow for lower implantable device mass and lower profile. 
     In one embodiment of the present invention, a deployment system for an implantable medical device is provided that comprises an expandable medical device having a larger deployed diameter and a smaller compacted diameter for delivery; a sheath surrounding the compacted medical device, the sheath everted back over itself, wherein an outer portion of the sheath surrounds an inner portion of the sheath; and a filamentary constraining member, located between the inner and outer portions of the sheath, wherein the medical device is deployed to the larger diameter by the application of simultaneous actuation forces to the sheath and the filamentary constraining member. 
     In another embodiment of the present invention, a method for loading an implantable device on a deployment system is provided that comprises: providing an implantable device; placing the implantable device within a sheath element that includes a segment extending beyond the implantable device; providing a funnel and a constraining element; applying tension on the segment of the sheath element to pull the sheath element and the implantable device through the funnel so as to compact the implantable device within the sheath element and into the constraining element so that the implantable device is constrained in a compacted state; wherein the sheath element and constraining element are configured to be removed to deploy the implantable device in use. 
     In a further embodiment of the present invention directed to device deployment, a medical device deployment system is provided that comprises: a first sheath and a second sheath, wherein each of the first and second sheaths is non-proportionally actuated by the application of tension to tensile members connected to each of the first and second sheaths, wherein on deployment by the application of a single input force, the input force is variably distributed between said tensile members. 
     In an additional embodiment of the present invention, a medical device deployment system is provided that comprises: two or more pulleys that engage through a rotating planetary rolling element, wherein each of the pulleys is configured to accumulate a deployment line, wherein the deployment lines actuate deployment of the medical device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a side cross-section view of an implantable device constrained in a delivery and deployment apparatus of the present invention. 
         FIG. 2  is a side view of an implantable device prior to constraint into the delivery and deployment apparatus of the present invention. 
         FIG. 3  is a side view of a traction tube sheath element of the present invention surrounding the implantable device shown in  FIG. 2 . 
         FIG. 4  is a side view, shown partially in cross-section, of the sheath element and implantable device combination illustrated in  FIG. 3  being drawn through a funnel into a constraining member. 
         FIG. 5  is a side view, shown partially in cross-section, of the sheath element and implantable device combination illustrated in  FIG. 3  being radially compacted and then drawn into a constraining member. 
         FIG. 6  is a side view of the sheath element, implantable device, and constraining member combination created in the process illustrated by  FIG. 4  or  FIG. 5  shown prior to mounting on a delivery catheter. 
         FIG. 7  is a side view, shown partially in cross-section, of the sheath element, implantable device, and constraining member combination illustrated in  FIG. 6 , shown mounted on a delivery catheter and having traction lines formed to effectuate removal of the sheath element and constraining member during deployment of the implantable device. 
         FIG. 8  is a side view, shown partially in cross-section, of the implantable device and delivery apparatus shown in  FIG. 7 , with the implantable device shown partially deployed. 
         FIG. 9  through  FIG. 12  are schematic representations of various apparatus that may be used to effectuate simultaneous removal of multiple traction lines that may be employed with the present invention. 
         FIG. 13  is a top plan view, shown partially in cross-section, of a differential gear that may be used to effectuate simultaneous removal of multiple traction lines that may be employed with the present invention. 
         FIG. 14  is a cross-section view of another embodiment of a differential gear that may be used to effectuate simultaneous removal of multiple traction lines that may be employed with the present invention. 
         FIG. 15  is a side view, shown partially in cross-section, of the sheath element and implantable device combination illustrated in  FIG. 3  being drawn into a transfer funnel, with a portion of the sheath element everted over the transfer funnel. 
         FIG. 16  is a cross-section side view of a second sheath element everted over a transfer tube. 
         FIG. 17  is a side view, shown partially in cross-section, of the implantable device, sheath element, and transfer funnel combination shown in  FIG. 15  and the second sheath element and transfer tube combination shown in  FIG. 16  arranged in abutted orientation with each other and with the implantable device shown partially transferred from the transfer funnel to the transfer tube. 
         FIG. 18  is a side view, shown partially in cross-section, of the implantable device mounted within the second sheath following the transfer process illustrated in  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The present invention provides improved apparatus to constrain, deliver, and/or deploy a medical device. The invention may be used in conjunction with a wide variety of devices that may be temporarily or permanently deployed in a patient, including without limitation stents, stent-grafts, balloons, filters, traps, occluders, devices for delivering drugs or other therapeutic substances or treatments, and the like. As such, the terms “medical device” and “implantable device” in the present application are intended to be broadly construed to encompass any device that is temporarily or permanently placed in a body. 
     The apparatus of the present invention may be employed to deliver self-expanding devices, devices that are expandable by balloons or other means, self-expanding/expandable hybrid devices, and devices that are not intended to change dimensions in situ. 
     Particular embodiments of the present invention are described below by way of illustration. It should be understood by one of skill in the art that the present inventors do not intend to limit the scope of the present invention to these particular embodiments. 
       FIG. 1  shows an implantable device  20 , in this case a stent or stent-graft, mounted on a catheter  22  and constrained in a deployment apparatus  24  of the present invention. The deployment apparatus  24  comprises an everted traction tube sheath element  26  having an outer layer  28  and an inner layer  30 . Contained within the outer layer  28  and the inner layer  30  of the sheath element  26  is a constraining member  32 . The deployment apparatus  24  includes a first deployment line  34  operatively coupled to actuate the sheath  26  and a second deployment line  36  operatively coupled to actuate the constraining member  32 . 
     As is explained in greater detail below, the sheath element  26  is constructed from a thin, flexible material that is adapted to surround and protect the implantable device. The flexible material should have sufficient longitudinal tensile strength so that it can serve as a traction tube to help pull the implantable device  20  through compaction apparatus and into the constraining member  32  during the manufacturing process. Preferably the flexible material should also have sufficient coverage and structural integrity to protect any bioactive coating or other surface treatment on the implantable device until the device is ultimately deployed in vivo. It may further be desirable for the flexible sheath to be constructed from a lubricious material that can aid in the manufacturing process described below. 
     The sheath element  26  is not required to provide any significant constraint to the implantable device  20  as that function, if required, may be primarily provided by the constraining member  32 . As such, the sheath element  26  may be constructed from very thin and flexible material that exhibits some degree of radial compliance. In fact, it may be desirable for the sheath element  26  undergo necking when under longitudinal tension so as to aid in the compaction process during manufacturing. 
     The flexible material of the sheath element  26  may be formed from a variety of different materials, including without limitation: a continuous tube or sheet of material; a woven, knitted, or other fabric material; non-woven materials such as a felt; or a composite of two or more different materials. Suitable materials for use as a sheath  26  include tubes or sheets of material that may comprise but are not limited to: polytetrafluoroethylene (PTFE), expanded PTFE, polyester, polyethylene, nylon, rayon, polyimide, polyamide, polypropylene, and/or polyurethane. 
     The sheath element  26  may be formed from a radially distensible material and/or it may be constructed in a wide variety of configurations. For example, the material may be radially distensible, or radially necking, and/or have a wide range of strength or other properties. Additionally, it may be beneficial to construct the sheath element in the form of a pleated sheath or a helically pleated sheath so as to assist in radial compliance or release of the device. 
     As is shown in  FIG. 1 , in one embodiment of the present invention the sheath element  26  is constructed from a continuous tube that is split at its trailing end, at point  37  in  FIG. 1 , so as to form the first deployment line  34  from the same material as the sheath  26 . The deployment line  34  may be formed by, for example, spirally winding, heating, or otherwise manipulating the split tube into thread structure. Further, other materials, such as a thread of polyamide, polyimide, PTFE, ePTFE, polyester or similar material, may be used alone or added to the split tube to provide a more robust deployment line construct. 
     For deployment of a stent or stent-graft device, a suitable sheath element  26  may comprise a tubular sheath of expanded PTFE with a thickness of approximately 0.0015 to 0.15 mm, a longitudinal tensile strength of approximately 0.5 to 10 kgf. The sheath should have sufficient toughness to withstand any strains that may be applied by the constrained device (e.g., forces from stent apices, fins, anchors, etc.). As is explained below, for some applications it may be desirable for the sheath to have the ability to neck to an intermediate diameter when longitudinal tension is applied to the sheath. The sheath may be formed from any suitable base material, including without limitation a tube, sheet, and/or fibers (e.g., weave or braid of material). 
     As has been noted, the constraining member  32  serves to provide the effective constraint for the implantable device  20 . As such, the constraining member  32  should be formed from a relatively non-compliant material that will resist any expansion force delivered by the implantable device  20 . The constraining member  32  may be formed from a variety of different materials, including without limitation: a continuous tube or sheet of material; a woven, knitted, or other fabric material; non-woven materials such as a felt; or a composite of two or more different materials. Additionally, it may be beneficial to construct the constraining member  32  in the form of a pleated sheath or a helically pleated sheath, such as that disclosed in U.S. Pat. No. 8,845,712 to Irwin et al., so as to assist in radial compliance or release of the device. Suitable materials for use as a constraining member  32  include tubes, sheets, or fibers of material that may comprise but are not limited to: polytetrafluoroethylene (PTFE), expanded PTFE, polyester, polyethylene, nylon, rayon, polyimide, polyamide, polypropylene, and/or polyurethane. 
     The constraining member  32  may be effectively formed from a filamentary material, such that described in U.S. Pat. No. 6,315,792 to Armstrong et al. (“Armstrong et al. Patent”), incorporated in its entity by reference herein. The knitted constraining members described in that patent provide very effective device constraint yet easily unravel from the implantable device during deployment. As has been noted, the Armstrong et al. Patent&#39;s constraints have proven to be very accurate and effective in implantable device delivery and deployment. However, by combining the constraining member  32  of Armstrong et al. Patent with the everted sheath element  26  described above, significant benefits have been demonstrated. It has been determined that if the filamentary constraints described in the Armstrong et al. Patent are used alone, the fibers can snag on features of some implantable device constructions (e.g., certain forms of anchors, barbs, stent apices, etc.), which can create difficulties in mounting the constraint during manufacture and/or in releasing the constraint during deployment. By sandwiching the filamentary constraining member construct of the Armstrong et al. Patent within the everted sheath element  26 , the sheath element  26  serves to cover and isolate any problematic features on the implantable device  20  so that the constraining member  32  can be readily mounted on the implantable device  20  during manufacture and then readily removed from the implantable device  20  during deployment. This benefit greatly enhances the types of implantable devices that can now be successfully deployed using the apparatus of the Armstrong et al. Patent. 
     The second deployment line  36  may comprise the same material as the constraining member  26 , such as when the constraining member  26  is formed in accordance with certain embodiments of the Armstrong et al. Patent. Alternatively, other materials, such as a thread of polyamide, polyimide, PTFE, ePTFE, polyester or similar material, may be used alone or added to the deployment line  36  to provide a more robust construct. 
     The process for constructing the deployment apparatus  24  of the present invention is illustrated in  FIG. 2  through  FIG. 8 .  FIG. 2  illustrates an implantable device  20 , such as a stent or stent-graft, prior to mounting into the deployment apparatus  24  of the present invention. In  FIG. 3 , the implantable device  20  is placed within sheath element  26 . 
     In  FIG. 4  the sheath element  26  and implantable device  20  combination illustrated in  FIG. 3  is drawn through a funnel  38  into a constraining member  32 . As has been noted, the sheath element  26  is serving as a traction tube to provide a smooth and even surface to help ease the implantable device  20  through the funnel  38  and into the constraining member  32 . In this regard, any coating that may be applied to the implantable device  20  is thus protected from abrasion from the funnel  38  and constraining member  32 . Additionally, to whatever degree that the sheath element  26  undergoes any necking during the process of pull-down through the funnel  38 , the necking force further assists in the compaction of the implantable device  20 . 
     It should be appreciated that the sheath element  26  is also isolating the implantable device  20  from the forces necessary to pull the device  20  through the funnel  38  and into the constraining member  32 . In more conventional compaction processes, tether lines would typically be applied to one end of the implantable device  20  in order to pull it through a funnel into a constraint. As such, an implantable device must be constructed from materials and in a manner that allows it to withstand the substantial longitudinal forces necessary to compact it to its delivery dimensions (that is, if the implantable device is not sufficiently robust, it will be damaged under the forces of the tether lines during the compaction process). The compaction forces become significantly greater for longer implantable device constructs and when greater compaction ratios are undertaken. By using the sheath member of the present invention to apply traction forces along the entire length of the implantable device, it is possible to effectively compact implantable devices which would otherwise be too fragile to undergo compaction through conventional traction lines and/or to apply far greater compaction forces (and thus achieve far greater compaction ratios) than would previously be possible. In this regard the sheath element provides augmented axial strength to the implantable device during the compaction and loading processes. 
     An alternative compaction process is illustrated in  FIG. 5 . In this embodiment, the sheath element  26  and implantable device  20  combination illustrated in  FIG. 3  is compacted by a compression apparatus  40 , such as a radial crush device. In this embodiment the sheath element  26  facilitates the compacted implantable device to be more readily pulled out of the compression apparatus and into constraining member  32 , with all of the benefits previous noted. 
     Whether compacted by the process illustrated in  FIG. 4  or  FIG. 5  or through any other suitable means, as is shown in  FIG. 6  once the implantable device  20  is compacted into the constraining member  32  there is extra sheath element  26  extending from both sides of the compacted device  20 . As is shown in  FIG. 7 , the constrained device  20  may then be mounted on a delivery catheter  22  and the extra sheath element  26  may then be everted back over the constraining member  32  so as to sandwich the constraining member  32  within the inner layer  30  and outer layer  28  of the sheath element  26 . Deployment line  34  may then be attached to or formed out of the sheath element  26 . Likewise, deployment line  36  may then be attached to or formed from constraining member  32  (for example, if the constraining member  32  is constructed from a knitted material, such as that described in the Armstrong et al. Patent, the deployment line  36  may comprise an cord formed from the unraveled constraining member  32 ). 
     It should be appreciated that for some applications the sheath element  26  may also be employed as a single layer. Among the benefits of employing a single layer sheath element are the opportunity to provide reduced delivery profile and decreased length of deployment line. 
     Once constructed in the manner described herein, the implantable device  20  and deployment apparatus  24  can be delivered to a desired treatment site in a patient in a conventional manner. It should be appreciated, however, that the encapsulation of the implantable device  20  within the sheath element  26  provides additional protection to the implantable device  20  during the delivery process. As has been noted, when an implantable device  20  is provided with a drug or other bioactive coating, it is desirable that the coating is not exposed prior to reaching the intended deployment site. With certain constraint constructs, such as open mesh or open filament braids and the like, drug coatings will necessarily be exposed to blood and tissue long before reaching the intended deployment site, which can lead to possible abrasion of the coating from the device and unintended release of the bioactive materials in undesirable locations in the body. However, through use of the sheath member  26  of the present invention, the bioactive coating can be safeguarded against damage or premature release independent of how open the structure of the constraining devices may be. 
     Once the constrained device  20  is properly positioned in the body, the device  20  can be released by actuating the two deployment lines  34  and  36 , with each of the constraining member  32  and the sheath element  26  pulling away from the device upon actuation. This process is illustrated in  FIG. 8 , with implantable device  20  shown beginning to deploy from the deployment apparatus  24 . 
     While there are many noted benefits in providing a deployment apparatus that includes both a sheath element and a constraining member, it has been determined that there is one challenge when trying to remove two covers simultaneously from the constrained implantable device. Since the sheath element  26  and the constraining member  32  may be constructed of different materials and comprise different forms, they are unlikely to retract at the same rate or in the same manner. Additionally, depending on construction, the deployment lines may not actuate at linear rates, thus requiring some degree of modulation of the rate of actuation of each of the lines. This presents the clinician with an undesirable challenge of trying to actuate the two deployment lines  34  and  36  simultaneously but at different rates. 
     The present inventors have determined that this challenge can be fully addressed by employing one of a variety of differential mechanisms that allow the clinician to apply a single deployment force to the deployment apparatus  24  while the differential mechanism automatically modulates the rate of actuation of each of the deployment lines  34  and  36 . 
       FIG. 9  through  FIG. 12  illustrate various apparatus that may be used to effectuate simultaneous removal of multiple traction lines that may be employed with the present invention.  FIG. 9  illustrates the simplest form of differential whereby a the two deployment lines  34  and  36  are attached together at their terminal ends  42  and  44 , respectively, and a pulley  46  is employed so that varying retraction rates on each of the deployment lines  34 ,  36  are instantly accommodated. The clinician simply applies a single deployment force  48  to the pulley and the sheath element and constraining member will retract at the same rate while any rate differential between the two are corrected by the pulley  46 . 
       FIG. 10  illustrates another form of differential whereby any difference in rate of withdrawal is pre-calculated and different fixed ratio pulleys  50 ,  52  are used to withdraw each of the deployment lines  34 ,  36  at their predetermined rates. 
       FIG. 11  illustrates still another form of differential whereby a spring or other passive displacement mechanism  54  is associated with the deployment line  34  that is determined to require the most slack during deployment (that is, the deployment line that needs to be withdrawn slower). In this manner, the clinician can pull both lines at an even rate but the lines will effectively withdraw at their appropriate differential rates. 
       FIG. 12  illustrates yet another form of differential whereby a clutch drive  56  is employed so that a driven pulley  58  takes up one of the deployment lines  36  at a set rate and a friction interface  60  between the driven pulley  58  and a second pulley  62  provides a clutch mechanism so that deployment line  34  will accumulate at a slower rate of speed. 
     A more sophisticated differential mechanism is illustrated in  FIG. 13 . In this embodiment, two pulleys  64 ,  66  are provided, each associated with deployment lines  34 ,  36 , respectively. Pulley bevel gears  68   a ,  68   b  and planetary bevel gears  70   a ,  70   b  are mounted between the two pulleys  64 ,  66  so as to accommodate different rates of actuation of each of the pulleys  64 ,  66 . A clinician-actuated thumb wheel  72  is provided to actuate the planetary gears through a center shaft with thumb wheel fixed at one end, and similarly to the clutch mechanism illustrated in  FIG. 12 , the planetary gears  70   a ,  70   b  serve to accommodate different actuation rates between the two pulleys  64 ,  66 . The bevel gears may be chosen to actuate each of the pulleys at their appropriate rates. For example, the sum of rotation of pulleys  64  and  66  can be established to be, for instance, 2× the input rotation of the thumb wheel  72 . 
     A similar differential mechanism is illustrated in  FIG. 14 . In this embodiment, pulleys  74  and  76  are each associated with each of the deployment lines (not shown) and are attached, respectively, to ring gears  78 ,  80 . Planetary gears  82 ,  84  are each provided with a clinician-actuated thumb wheel  86  that engages directly with the planetary gears. 
     It should be understood that the various differential mechanisms described herein are by way of illustration only and that any of these or other mechanism may be employed within the scope of the present invention. Further various improvements or refinements to these mechanism are also within the scope of the present invention, including, for example, that a motor or other drive mechanism may be substituted for thumb wheels in the embodiments of  FIG. 13  and  FIG. 14 . 
     For some applications, it may be beneficial to transfer the implantable device to an intermediate compacted diameter prior to final compaction and mounting of the implantable device for delivery. For example, by compacting the implantable device in multiple steps, a thinner and less robust sheath may be used for final device compaction and delivery, which may allow for desirable smaller device delivery dimensions. One method of accomplishing is to modify the mounting procedure illustrated in  FIGS. 2 to 6 , above, with additional process steps as illustrated in  FIGS. 15 to 18 . 
     In  FIG. 15  the sheath element  26  and implantable device  20  combination illustrated in  FIG. 3  is drawn into a transfer funnel  88 , with a segment  90  of the sheath element  26  everted over the transfer funnel  88 . The compaction of the implantable device  20  into the transfer funnel  88  may be accomplished by applying tension  92  to the segment  90  so as to draw the sheath  26  and device  20  into the funnel  88 . Once the implantable device  20  is positioned at the tip  94  of the transfer funnel  80 , the device is ready for transfer. 
       FIG. 16  shows a second sheath element  96  that is everted over a transfer tube  98 . The second sheath element  96  is configured to be a diameter smaller than the diameter of sheath element  26 . The transfer tube  98  is constructed from an essentially non-compliant material, such as a metal or plastic hypotube. The transfer tube  98  has a diameter that is approximately equal to that of the tip  94  of the transfer funnel  88 . A portion  100  of the second sheath on the outside of the transfer tube  98  may be compacted or “scrunched” to make actuation of the second sheath easier in the transfer step described below. 
     In order to accomplish transfer of the implantable device  20  from the sheath element  26  to the second sheath element  98 , the implantable device  20 , sheath element  26 , and transfer funnel  88  combination shown in  FIG. 15  is placed in abutted orientation with the second sheath element  96  and transfer tube  98  combination shown in  FIG. 16 , as is shown in  FIG. 17 . By applying tension  92  to the sheath element  26  and simultaneously applying tension  102  to the second sheath element  98 , as shown in  FIG. 17 , the implantable device  20  will transfer from the transfer funnel  88  and sheath element  26  into the transfer tube  98  and second sheath element  96 . This transfer is shown partially completed in  FIG. 17 . 
     Once the implantable device  20  is fully transferred into the second sheath element  96 , the device  20  and second sheath  98  can be removed from the transfer tube, as is shown in  FIG. 18 . The implantable device  20  is now contained at a small diameter and/or contained in a sheath with thinner wall thickness than the device and sheath illustrated in  FIG. 3 . At this stage the combined device and sheath can be further processed as previously described with respect to  FIG. 4 through 6 . 
     It should be appreciated from this description of  FIGS. 15 to 18  that the present invention may be practiced through a number of ways whereby the implantable device may be partially compacted to one or more intermediate diameters during the manufacturing process. 
     As has been explained, the present invention provides many benefits over prior medical device deployment apparatus, including without limitation: 
     (1) The present invention provides a delivery system that protects medical device during delivery in a body while providing simple, accurate, and reliable device deployment that is scalable to work on a wide variety of implantable device forms and sizes. 
     (2) The delivery system is configured so that loading and deployment forces are not directly related to device diameter, length, or design, thus allowing a more universal delivery system across various delivered device configurations and product lines. In this regard, forces required to constrain implantable devices and deploy implantable devices can be decoupled from the length and other properties of the implantable devices. 
     (3) By employing the sheath element to present a more consistent surface on the outside of the implantable device, deployment forces may be smoother during delivery so as to minimize catheter movement (for example, increases in delivery force due to adverse interaction between the implantable device and the constraint can be avoided). Similarly, the present invention can accommodate deployment of devices with irregular features (e.g., scallops, barbs, anchors, apices, and other features that may otherwise interfere with smooth operation of deployment apparatus). 
     (4) The use of the sheath element may help reduce device delivery profiles, both by allowing compaction forces to be decoupled from device longitudinal tensile strength and by providing a smoother and possibly more lubricious surface on the outside of the implantable device to allow for easier compaction of the device. By eliminating direct tensioning of device during loading, the present invention also allows for lower implantable device mass and lower profile. 
     (5) The present invention can be configured so as to contain delivery lines within a sheath element in order to reduce or eliminate “bow-stringing” of the line during deployment. 
     (6) By encapsulating the implantable device prior to loading and keeping it encapsulate until deployment, the present invention imparts minimal stress to the implantable device. For drug delivery devices, this can reduce drug loss and particulation during handling of the device in both manufacture and use. In this regard, the encapsulation of the device can reduce or eliminate contact between device and tooling during device loading and mounting and also may isolate the device from surface shear and other damage during delivery and deployment. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.