referring to the drawings , fig1 is a side view of a medical device delivery system 100 for treating vascular lesions , according to the present invention . the medical device delivery system comprises a catheter having a distal end cap 10 , with an elongated inner member 14 attached to end cap 10 . also attached to distal end cap 10 is flexible outer member 12 . in one embodiment , both inner member 14 and outer member 12 are attached to end cap 10 using adhesives . in another embodiment , both inner member 14 and outer member 12 are attached to end cap 10 using fusion bonding . in one embodiment to the invention , a radiopaque marker 18 is embedded in the distal end portion 10 to facilitate placement of the distal tip of the catheter at the desired location in the vascular system . the distal end portion 10 is comprised of silicone , or a biocompatible polymeric material such as polyurethane , polyethylene or polytetrafluroethylene ( ptfe ). the use of silicone in certain embodiments may reduce the incidence of vascular tissue damage due to the lubricious surface provided by silicone . the distal portion of end cap 10 is shaped to facilitate passage of the catheter through the vascular system . in various embodiments of the invention , end cap 10 is rounded , tapered or bullet - shaped , among other appropriate shapes . in one embodiment of the invention , the end cap 10 has a lumen 28 that can accommodate a guide wire running longitudinally through the end cap 10 . the guide wire is , in one embodiment , of metallic construction , and is inserted into the femoral artery and threaded through the vascular system to the target site . the end cap 10 is then slipped over the guide wire and the catheter is guided along the vascular route , until both the guide wire and catheter are at their desired target locations . the guide wire is then withdrawn from the body . using fluoroscopy , the radiopaque marker 18 can be observed during the placement procedure , and thereby facilitate placement of the distal tip of the catheter at a desired location . the inner member 14 comprises a rod or shaft attached at its distal end to end cap 10 and extends the entire length of the catheter . inner member 14 must be sufficiently flexible so that the catheter can be threaded through the vascular system , but must also have sufficient longitudinal stiffness so that it does not kink during placement of the catheter . inner member 14 may be made of a metallic material such as stainless steel , titanium or nitinol , or a biocompatible polymeric material such as polyurethane , polyethylene , nylon , ptfe or combinations of these or similar materials . the diameter of inner member 14 is selected to give it the needed longitudinal stiffness and lateral flexibility , and will depend on the properties of the material ( s ) of which it is composed . generally , the diameter of inner member 14 will be in the range of 0 . 2 to 2 . 0 mm . attached to end cap 10 is at least one outer member 12 . the primary purpose of outer member 12 is to transport the stent during transit through the vascular system and to deploy the stent at the target site . in one embodiment , outer member 12 is a flexible sheath . outer member 12 may comprise a flexible , biocompatible , polymeric material such as polyurethane , polyethylene , nylon , or ptfe . in other embodiments , outer member 12 is a mesh of flexible wires comprising nylon , polyether - block co - polyamide polymers such as pebax ® resins , a metallic material such as braided stainless steel or polymer - coated , braided stainless steel . the polymer coating comprises nylon , polyether - block co - polyamide polymers such as pebax ® resins , or any other appropriate polymeric material . a distal portion 26 of outer member 12 holds the stent in place as it is transported through the vascular system . in one embodiment , a stent or other tubular prosthesis 16 is slipped over the distal end of the catheter , positioned over a distal portion 26 of outer member 12 and crimped to a reduced diameter so that the stent 16 is held firmly in place . the diameter of the distal portion 26 of outer member 12 is selected so that the outer diameter of the crimped stent is no greater than the diameter of end cap 10 , giving the catheter a smooth outer surface and facilitating its passage through the vascular system . two radiopaque circumferential bands 15 and 17 are located on inner member 14 at each end of the stent 16 , and serve as radiopaque markers so that the precise location of the stent within the vascular system can be observed using fluoroscopy . the radiopaque markers 15 , 17 , and 18 comprise gold or any other appropriate material . in other embodiments , a plurality of radiopaque circumferential bands 15 are located on the inner member 14 . a hub 20 is attached to the proximal end of outer member 12 , and a second hub 22 is attached to the proximal end of inner member 14 . in one embodiment , the two hubs are locked to each other as shown in fig1 . when the two hubs are locked , the outer member 12 is maintained in a fully extended configuration and held taut between the distal end cap 20 and the locked hubs ( 20 and 22 ). this configuration prevents outer member 12 from flexing inward toward the center of the catheter during its passage through the vascular system and releasing the stent prematurely . when the distal portion of the catheter is placed adjacent to the target site , the hubs 20 and 22 are unlocked from each other , allowing inner member 14 and outer member 12 to move in relation to each other . inner member 14 is partially retracted as shown in fig2 . as inner member 14 is retracted , it draws end cap 10 in a proximal direction , causing the distal portion 26 of outer member 12 to pass over end cap 10 , and fold longitudinally ( fig2 ), allowing end cap 10 to move in a proximal direction and pass through the lumen of the stent 16 , surrounded by a double layer of outer member 12 . the diameter of the combination of end cap 10 and the folded outer member 12 causes the stent 16 to expand and to be deployed from the catheter to the target site . the diameter of end cap 10 and the thickness of the folded outer member 12 are selected so that the combination causes the stent 16 to expand to a predetermined diameter , beginning at the distal end of the stent 16 , and progressing to its proximal end . the rate of stent deployment is controlled by the rate at which the operator retracts inner member 14 ; consequently , the stent may be deployed slowly if so desired . after the stent is deployed , inner member 14 is advanced in a distal direction causing outer member 12 to unfold . when outer member 12 is fully extended , the hubs 20 and 22 will be aligned with each other , and can once again be locked to each other . the catheter will have regained its smooth outer surface and low profile , and may be removed from the patient . in one embodiment of the invention , the distal portion of outer member 12 is coated with a gel that includes or encapsulates a drug or therapeutic agent . the therapeutic agent or agents may be dispersed within or encased by a polymeric coating , and are eluted at the target site within the vascular system as the stent is deployed . a therapeutic agent is capable of producing a beneficial effect against one or more conditions including coronary restenosis , cardiovascular restenosis , angiographic restenosis , arteriosclerosis , hyperplasia , and other diseases and conditions . the therapeutic agent may comprise , for example an antirestenotic drug , an antisense agent , an antineoplastic agent , an antiproliferative agent , an antithrombogenic agent , an anticoagulant , an antiplatelet agent , an antibiotic , an anti - inflammatory agent , a steroid , a gene therapy agent , an organic drug , a pharmaceutical compound , a recombinant dna product , a recombinant rna product , a collagen a collagenic derivative , a protein , a protein analog , a saccharide , a saccharide derivative , a bioactive agent , a pharmaceutical drug , a therapeutic substance , or combinations thereof . in one embodiment of the invention , the device can be used to deliver a drug or therapeutic agent into the vascular system . in this embodiment , inner hub 22 includes a port and a means to accommodate a syringe such as a luer fitting , or any other appropriate fitting . inner member 14 includes a lumen connected to the port at hub 22 and to lumen 28 connecting to an orifice in the distal end cap 10 for delivery of the drug or therapeutic agent . the drug or therapeutic agent may be any of those described above . fig3 is a flowchart illustrating a method 200 for treating a target site within the vascular system by delivering a stent or other tubular prosthesis in accordance with the present invention . the method begins at block 30 wherein a target site is selected . next , as indicated at block 32 , a guide wire may be inserted into the femoral vein or at another site , advanced to the target site , and positioned so that the distal tip of the guide wire is adjacent to the target site . the end cap of the delivery system may be slipped over the guide wire and the catheter threaded through the vascular system . the distal tip of the catheter is placed adjacent to the target site , as indicated in block 34 . next , the hubs at the proximal ends of the inner member and the outer member are unlocked from each other as indicated in block 36 . with the hubs unlocked , the inner member and the outer member can be moved in relation to each other . as indicated in block 38 , the inner member is retracted while the outer member remains where it was placed . this causes the end cap to be retracted and drawn into the lumen of the stent , and the distal portion 28 of the outer member to fold longitudinally and pass over the exterior of the end cap 10 , as shown in fig2 . the diameter of the end cap 10 with the folded outer member surrounding it is larger than the inner diameter of the stent 16 , and therefore , causes the stent to expand and be deployed from the catheter . once the stent is deployed , the inner member may be advanced to its original , distal position , so that the outer member is fully extended , as indicated in block 40 . the hubs may then be locked to each other , as in block 42 , so that the catheter will have a smooth exterior surface and a constant diameter . finally , as indicated in block 44 , the catheter may be safely removed from the body . because the system of the present invention does not require a balloon or other complex apparatus to deploy the stent , it is simple and inexpensive to manufacture compared to previously disclosed stent delivery systems . further , its low profile makes it comparatively easy to maneuver through the vascular system , and reduces the potential for damage to the vascular tissues . in addition , the delivery system of the present invention can be used with a variety of stent designs and other tubular prostheses , and the rate of delivery may be controlled by the operator . while the invention has been described with reference to particular embodiments , it will be understood by one skilled in the art that variations and modifications may be made in form and detail without departing from the spirit and scope of the invention .
a delivery system for a stent includes a catheter having an elongated inner member and at least one flexible , longitudinally oriented outer member that is operably attached to the distal portion of the inner member . a stent is placed concentrically around the flexible outer member , near the distal end of the catheter . when the elongated inner member is retracted , the flexible outer member folds longitudinally adjacent to the lumen of the stent causing the stent to be deployed . another aspect according to the invention provides a method for delivering a stent to a treatment site within the vascular system .