Patent Publication Number: US-11033378-B2

Title: Medical device

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/846,071, filed Dec. 18, 2017, which is continuation of U.S. patent application Ser. No. 15/251,959, filed Aug. 30, 2016, now U.S. Pat. No. 9,844,433, which is a continuation of U.S. patent application Ser. No. 14/586,686, filed Dec. 30, 2014, now U.S. Pat. No. 9,433,518, which is a continuation of U.S. patent application Ser. No. 13/959,617, filed Aug. 5, 2013, now U.S. Pat. No. 8,920,430, which is a continuation of U.S. patent application Ser. No. 11/586,899, filed Oct. 25, 2006, now U.S. Pat. No. 8,500,751, which is a continuation-in-part of U.S. patent application Ser. No. 10/580,139, filed May 19, 2006, now U.S. Pat. No. 9,585,668, under 35 U.S.C. § 371 as a U.S. National Stage Application of PCT International Patent Application No. PCT/SG2004/000407, filed Dec. 13, 2004, which claims priority to Singapore Patent Application No. SG200401735-6, filed Mar. 31, 2004; the contents of each of the aforementioned applications are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a medical device for insertion into a bodily vessel to treat an aneurysm. 
     BACKGROUND OF THE INVENTION 
     Vascular diseases include aneurysms causing hemorrhage, atherosclerosis causing the occlusion of blood vessels, vascular malformation and tumors. Vessel occlusion or rupture of an aneurysm within the brain causes of stroke. Aneurysms fed by intracranial arteries can grow within the brain to a point where their mass and size can cause a stroke or the symptoms of stroke, requiring surgery for removal of the aneurysms or other remedial intervention. 
     Occlusion of coronary arteries, for example, is a common cause of heart attack. Diseased and obstructed coronary arteries can restrict the flow of blood in the heart and cause tissue ischemia and necrosis. While the exact etiology of sclerotic cardiovascular disease is still in question, the treatment of narrowed coronary arteries is more defined. Surgical construction of coronary artery bypass grafts (CABG) is often the method of choice when there are several diseased segments in one or multiple arteries. Conventional open-heart surgery is, of course, very invasive and traumatic for patients undergoing such treatment. Therefore, alternative methods being less traumatic are highly desirable. 
     One of the alternative methods is balloon angioplasty that is a technique in which a folded balloon is inserted into a stenosis, which occludes or partially occludes an artery and is inflated to open the occluded artery. Another alternative method is atherectomy that is a technique in which occlusive atheromas are cut from the inner surface of the arteries. Both methods suffer from reocclusion with certain percentage of patients. 
     A recent preferred therapy for vascular occlusions is placement of an expandable metal wire-frame including a stent, within the occluded region of blood vessel to hold it open. The stent is delivered to the desired location within a vascular system by a delivery means, usually a catheter. Advantages of the stent placement method over conventional vascular surgery include obviating the need for surgically exposing, removing, replacing, or by-passing the defective blood vessel, including heart-lung by-pass, opening the chest, and general anaesthesia. 
     When inserted and deployed in a vessel, duct or tract (“vessel”) of the body, for example, a coronary artery after dilatation of the artery by balloon angioplasty, a stent acts as a prosthesis to maintain the vessel open. The stent usually has an open-ended tubular form with interconnected struts as its sidewall to enable its expansion from a first outside diameter which is sufficiently small to allow the stent to traverse the vessel to reach a site where it is to be deployed, to a second outside diameter sufficiently large to engage the inner lining of the vessel for retention at the site. A stent is typically delivered in an unexpanded state to a desired location in a body lumen and then expanded. The stent is expanded via the use of a mechanical device such as a balloon, or the stent is self-expanding. 
     Usually a suitable stent for successful interventional placement should possess features of relatively non-allergenic reaction, good radiopacity, freedom from distortion on magnetic resonance imaging (MRI), flexibility with suitable elasticity to be plastically deformable, strong resistance to vessel recoil, sufficient thinness to minimize obstruction to flow of blood (or other fluid or material in vessels other than the cardiovascular system), and biocompatibility to avoid of vessel re-occlusion. Selection of the material of which a stent is composed, as well as design of the stent, plays an important role in influencing these features. 
     Furthermore, implantable medical devices have been utilized for delivery of drugs or bioreagents for different biological applications. Typically, the drugs or bioreagents are coated onto the surfaces of the implantable medical devices or mixed within polymeric materials that are coated onto the surfaces of the implantable medical devices. However, all the current available methods suffer from one or more problems including uncontrollable release, form limitations of drugs, and bulky appearance. 
     Therefore, there is desire for an implantable medical device that is able to deliver drugs or reagents efficiently to the endovascular system, especially intracranial blood vessels. 
     A method for treating bifurcation and trifurcation aneurysms is disclosed in the previously filed cross-related application entitled “A Method for Treating Aneurysms”, the contents of which are herein incorporated by reference. 
     SUMMARY OF THE INVENTION 
     In a first preferred aspect, there is provided a method for treating a bifurcation or trifurcation aneurysm occurring on a first artery, the first artery and a second artery joining to a third artery, the method comprising: 
     inserting a medical device such that it is at least partially located in the first artery and is at least partially located in the third artery; 
     expanding the medical device from a first position to a second position, said medical device is expanded radially outwardly to the second position such that the exterior surface of said medical device engages with the inner surface of the first and third arteries so as to maintain a fluid pathway through said arteries; and 
     positioning the medical device such that a membrane of the medical device is located against an aneurysm neck of the aneurysm to obstruct blood circulation to the aneurysm when the medical device is expanded to the second position, and at least a portion of the membrane is secured to the medical device to maintain the position of the membrane relative to the medical device when expanded to the second position; 
     wherein the membrane is permeable and porous, the size of the pores of the membrane and the ratio of the material surface area of the membrane being such that blood supply to perforators and/or microscopic branches of main brain arteries is permitted to improve healing of the first artery but blood supply to the aneurysm is prevented. 
     The medical device may be inserted such that blood circulation to the second artery is unobstructed by the membrane. 
     The distance between adjacent pores may be from about 40 to 100 microns. 
     The membrane may be made of a biocompatible and elastomeric polymer. 
     The membrane may have a thickness of about 0.0005 to 0.005″. 
     The ratio of the material surface area of the membrane may be from about 25 to 75%. 
     The membrane may have pores between 20 to 100 microns in size. 
     The membrane may be made from polymeric material or biodegradable material. 
     The biodegradable material may form multiple sub-layers mixed with drugs or reagents. 
     The at least one reagent may be any one form selected from the group consisting of: solid tablet, liquid and powder. 
     The membrane may be capable of isotropic expansion. 
     The membrane may be disposed on the exterior surface of the device. 
     The membrane may circumferentially surround a portion of the device. 
     The membrane may cover a portion of the device. 
     The membrane may have fabricated pores between 20 to 100 microns in size. 
     The pores may be fabricated by laser drilling. 
     The distance between the pores may be less than 100 μm. 
     The membrane may comprise a plurality of polymeric strips secured to the medical device. 
     The strips may be less than 0.075 mm and the distance between adjacent strips is less than 100 μm. 
     The membrane may comprise a mesh secured to the medical device. 
     Spaces of the mesh may be less than 100 μm and the width of the meshing is between 0.025 to 0.050 mm. 
     The aneurysm may be any one from the group consisting of: a regular size, giant or wide neck aneurysm having an aneurysm neck greater than 4 millimeters or a dome to neck ratio greater than 2, berry aneurysm, CC fistula and fusiform aneurysm. 
     The medical device may comprise a generally tubular structure having an exterior surface defined by a plurality of interconnected struts having interstitial spaces therebetween. 
     The medical device may be self-expandable or balloon expandable. 
     The membrane may be supported by the generally tubular structure and is attached to at least one strut. 
     The medical device may be a stent. 
     The membrane may be tubular having a diameter similar to a nominal initial diameter of the stent; and wherein the membrane is disposed onto the outer surface of the stent or introduced by dip coating or spraying between the struts of the stent. 
     The membrane may be a segment of a tubular structure disposed onto a portion of the outer surface of the stent. 
     The membrane may substantially cover the entire circumferential surface of the medical device. 
     The permeability and porosity of the membrane may alter the hemodynamics of the aneurysm sac of the aneurysm to initiate intra-aneurysmal thrombosis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of the invention will now be described with reference to the accompanying drawings, in which: 
         FIGS. 1A and 1B  are two exemplary balloon expandable stents; 
         FIG. 2  shows a self-expanding stent; 
         FIG. 3A  is diagrammatic view of a stent disposed in the location of an aneurysm; 
         FIG. 3B  is diagrammatic view as  FIG. 3A  except that a port of the stent is formed of opened cells; 
         FIG. 4  shows a delivery system with a stent expanded onto the balloon; 
         FIG. 5  is diagrammatic view of a stent partially covered by a membrane with pockets; 
         FIG. 6  is a cross-sectional view of a sleeve as a membrane supported by two ring-like stents; 
         FIG. 7  is a diagrammatic view of a membrane joining two stents for treating a bifurcation aneurysm; 
         FIG. 8  is a diagrammatic view of an aneurysm covered with the membrane of a stent to obstruct blood circulation to the aneurysm; 
         FIG. 9  is a table of typical dimensions for the stent; 
         FIG. 10  is a diagrammatic view of a stent with a membrane having a pattern of 20 pores; 
         FIG. 11  is a diagrammatic view of a stent with a membrane having polymer strips; 
         FIG. 12  is a diagrammatic view of a stent with a membrane having a mesh; 
         FIG. 13  is a diagrammatic view of a membrane secured to the struts of a stent; 
         FIG. 14  is a diagrammatic view of a membrane before the stent is deployed; 
         FIGS. 15A-15C  are diagrammatic views of a stent with a membrane secured at three different positions and with three different sizes; 
         FIG. 16  is a diagrammatic view of a membrane flipping inside the vessel rather than staying close the vessel wall; 
         FIG. 17  is a diagrammatic view of a stent with a membrane being used to treat a bifurcation aneurysm in a first example; 
         FIG. 18  is a diagrammatic view of a stent with a membrane being used to treat a bifurcation aneurysm in a second example; and 
         FIG. 19  is a diagrammatic view of a stent with a membrane being used to treat a bifurcation aneurysm in a third example. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Implantable medical devices include physical structures for delivering drugs or reagents to desired sites within the endovascular system of a human body. Implantable medical devices may take up diversified shapes and configurations depending upon specific applications. Common implantable medical devices include stents, vena cava filters, grafts and aneurysm coils. While stents are described, it is noted that the disclosed structures and methods are applicable to all the other implantable medical devices. 
     The endovascular system of a human body includes blood vessels, cerebral circulation system, tracheo-bronchial system, the biliary hepatic system, the esophageal bowel system, and the urinary tract system. Although exemplary stents implantable  202  in blood vessels are described, they are applicable to the remaining endovascular system. 
     Stents  202  are expandable prostheses employed to maintain vascular and endoluminal ducts or tracts of the human body open and unoccluded, such as a portion of the lumen of a coronary artery after dilatation of the artery by balloon angioplasty. A typical stent  202  is a generally tubular structure having an exterior surface defined by a plurality of interconnected struts having interstitial spaces there between. The generally tubular structure is expandable from a first position, wherein the stent is sized for intravascular insertion, to a second position, wherein at least a portion of the exterior surface of the stent contacts the vessel wall. The expanding of the stent is accommodated by flexing and bending of the interconnected struts throughout the generally tubular structure. It is contemplated that many different stent designs can be produced. A myriad of strut patterns are known for achieving various design goals such as enhancing strength, maximizing the expansion ratio or coverage area, enhancing longitudinal flexibility or longitudinal stability upon expansion, etc. One pattern may be selected over another in an effort to optimize those parameters that are of particular importance for a particular application. 
     Referring to  FIGS. 1A and 1B , there are provided two exemplary balloon expandable stent designs.  FIG. 1A  shows a tubular balloon expandable stent  100  with end markers  103  to increase visibility of the stent  100 . The stent  100  is composed of stent struts of a ring  101 , ring connectors  102 , and end markers  103 . 
     Referring to  FIG. 1A , the stents  100  are made of multiple circumstantial rings  101 , where the ring connectors  102  connect two or three adjacent rings  101  to hold the rings in place. For the end markers  103 ,  FIG. 1A  shows a “disc” shaped marker. Actually, the shape is not critical so long that the marker can be used to increase further visibility to the stents  100 .  FIG. 1B  shows a tubular balloon expandable stent  104  which is similar to the stent  100  as shown in  FIG. 1A  except that the stent  104  comprises of center markers  105 ,  106 . The center markers  105 ,  106  help to locate an aneurysm opening during an implantation operation. The center markers  105 ,  106  can be of the same material and shape as the end markers  103 . 
     Referring to  FIG. 2 , there is provided a self-expanding stent  107  that is made of wires/ribbons. While a self-expanding stent may have many designs,  FIG. 2  shows the stent  107  having a typical braided pattern  108  with welded ends  109 . The stent  107  is so designed that is relatively flexible along its longitudinal axis to facilitate delivery through tortuous body lumens, but that is stiff and stable enough radially in an expanded condition to maintain the patency of a body lumen, such as an artery when implanted therein. 
     Turning to  FIG. 4 , it is shown an expanded tubular stent  112 . When the tubular stent  112  is fully expanded to its deployed diameter, the latticework of struts takes on a shape in which adjacent crests undergo wide separation, and portions of the struts take on a transverse, almost fully lateral orientation relative to the longitudinal axis of the stent. Such lateral orientation of a plurality of the struts enables each fully opened cell to contribute to the firm mechanical support offered by the stent in its fully deployed condition, to assure a rigid structure which is highly resistant to recoil of the vessel wall following stent deployment. 
     While a stent  112  may be deployed by radial expansion under outwardly directed radial pressure exerted, for example, by active inflation of a balloon of a balloon catheter on which the stent is mounted, the stent  112  may be self-expandable. In some instances, passive spring characteristics of a preformed elastic (i.e., self-opening) stent serve the purpose. The stent is thus expanded to engage the inner lining or inwardly facing surface of the vessel wall with sufficient resilience to allow some contraction but also with sufficient stiffness to largely resist the natural recoil of the vessel wall. 
     In one embodiment, the implantable medical devices are intracranial stents  202  and delivery systems for stenotic lesions and aneurysms  201 . Due to the characteristics of intracranial blood vessels, the intracranial stents  202  are designed to be very flexible, low profile (0.033″-0.034″ or even less as crimped onto delivery catheter) and thin wall (0.0027″-0.0028″). The intracranial stents  202  do not necessarily have the highest possible radial strength because there is no need of high strength for intracranial applications. The radiopacity of the intracranial stents may be provided by either including radiopaque markers  205  made from gold or platinum or making the stents  202  from platinum/iridium/tungsten alloys. Stents  202  for treating aneurysms  201  have a special type of platinum “star markers”  204  in the middle of their bodies to assist in precise indication and alignment of the stents  202  over the aneurysm neck  201  and allow further operation with aneurysms  201 . 
     As shown in  FIG. 3A , the intracranial stent  202  is disposed in the location of an aneurysm  201 . The membrane  203  partially covers the stent  202  and is positioned to seal the neck of the aneurysm  201 . The radiopaque markers  204  are located in the middle of the stent  202  to provide visibility of the stent  202  during operation and post-operation inspection. Referring to  FIG. 3B , a portion of the stent  202  is formed of opened cells  205 . This design avoids blocking perforators. The perforators refer to small capillary vessels that have important and distinctive blood supply functions. It is possible that tubular stents can block perforators and inhibit important functions. 
     Referring to  FIG. 4 , the delivery system includes a guide wire lumen  110 , a balloon inflating lumen  111 , a connector  116 , a balloon catheter shaft  113 , and platinum marker bands  115  on the catheter shaft  113 . The guide wire lumen  110  is used for introducing a guide wire in a balloon catheter, and the balloon inflating lumen  111  for inflating the balloon after the stent to be placed reaches its targeted location. The connector  116  is used for separating the guide wire lumen  110  and the balloon inflating lumen  111 . The balloon catheter shaft  113  carries the guide wire lumen  110  and the balloon inflating lumen  111  separately, with a typical length of about 135-170 cm. The ring markers  115  on the catheter shaft  113  are used for showing the start of balloon tapers and the edges of the stent. In  FIG. 3 , an expanded stent  112  is shown being mounted onto an expanded balloon. The delivery catheter can be essentially a conventional balloon dilatation catheter used for angioplasty procedures. The balloon may be formed of suitable materials such as irradiated polyethylene, polyethylene terephthalate, polyvinylchloride, nylon, and copolymer nylons such as Pebax™. Other polymers may also be used. In order for the stent to remain in place on the balloon during delivery to the desired site within an artery, the stent is crimped onto the balloon. 
     In a preferred embodiment, the delivery of the stent is accomplished in the following manner. The stent is first mounted onto the inflatable balloon on the distal extremity of the delivery catheter. Stent is mechanically crimped onto the exterior of the folded balloon. The catheter/stent assembly is introduced within vasculature through a guiding catheter. A guide wire is disposed across the diseased arterial section and then the catheter/stent assembly is advanced over a guide wire within the artery until the stent is directly under the diseased lining. The balloon of the catheter is expanded, expanding the stent against the artery. The expanded stent serves to hold open the artery after the catheter is withdrawn. Due to the formation of the stent from an elongated tube, the undulating component of the cylindrical elements of the stent is relatively flat in transverse cross-section, so that when the stent is expanded, the cylindrical elements are pressed into the wall of the artery and as a result do not interfere with the blood flow through the artery. The cylindrical elements of the stent which are pressed into the wall of the artery will eventually be covered with endothelial cell layer which further minimizes blood flow interference. Furthermore, the closely spaced cylindrical elements at regular intervals provide uniform support for the wall of the artery, and consequently are well adopted to tack up and hold in place small flaps or dissections in the wall of the artery. 
     For resilient or self-expanding prostheses, they can be deployed without dilation balloons. Self-expanding stents can be pre-selected according to the diameter of the blood vessel or other intended fixation site. While their deployment requires skill in stent positioning, such deployment does not require the additional skill of carefully dilating the balloon to plastically expand the prosthesis to the appropriate diameter. Further, the self-expanding stent remains at least slightly elastically compressed after fixation, and thus has a restoring force which facilitates acute fixation. By contrast, a plastically expanded stent must rely on the restoring force of deformed tissue, or on hooks, barbs, or other independent fixation elements. 
     The presence of a stent in a vessel tends to promote thrombus formation as blood flows through the vessel, which results in an acute blockage. In addition, as the outward facing surface of the stent in contact or engagement with the inner lining of the vessel, tissue irritation can exacerbate restenosis attributable to hyperplasia. Moreover, it is desirable to deliver drugs or reagents into the aneurysms to enhance the blockage of blood flow into the aneurysms. Finally, implantable medical devices have been used as vehicles to deliver drugs or reagents to specific locations within the vascular system of a human body. 
     In one example, an intracranial stent  202  is specially designed for low pressure deployment. The stent  202  has adequate radial strength for targeting a specific environment of fragile intracranial vessel. The stent  202  is designed to allow for delivering high stent performance and absolutely conforming longitudinal flexibility. 
     Low pressure deployment of a stent is defined as a pressure equal to or below 4 atm. This level of pressure enables the stent  202  to be fully deployed to support a stenosed intracranial vessel or aneurysm neck  201  without introducing trauma or rapture of a target vessel. The stent  202  can be deployed using balloon techniques or be self-expandable. 
     The stent  202  comprises structural elements that restrict potential over expansion, matching the inner diameter of the vessel and to make deployment extremely precise. This feature of the structural elements in combination with low pressure deployment potentially reduces vessel injury, rupture or restenosis. 
     The stent  202  also has longitudinal flexibility equal to or better than what is provided by a delivery catheter. This means that the stent does not add increased rigidity to the device. The trackability of the stent  202  depends on the mechanical properties of the catheter and is not restricted by stent  202  alone. The longitudinal flexibility of the stent  202  can be measured by force in grams to deflect the stent from neutral line. This force brings stent deflection to 1 mm for less than 8 grams. 
     Existing catheters can provide 20-22 grams per 1 mm deflection. This condition is also extremely important when creating stent compliance to particular vessels and saves the vessel from possible traumatic reaction. 
     The structure of the stent  202  is designed to provide a normalized radial force of 18-19 grams/mm of length and may reach values close to the ones found in existing coronary stents. Stent structural support provides 3-4% of deflection of the stent structure together with intracranial vessel wall natural pulsing. This leads to greater stent conformity and a reduced vessel injury score. 
     The intracranial stent  202  has profile in compressed delivery mode 0.020″. 
     The intracranial stent  202  is designed to be compressed onto delivery catheter with a profile as low 0.014″-0.016″ having stent profile 0.020″-0.022″. 
     The intracranial stent  202  has even material distribution and wall coverage, creating needed vessel support. The material ratio is in the range of 10-17% depending on deployment diameter. 
     The intracranial stent  202  has a strut thickness and width not larger than 0.0028″. 
     Strut dimensions are selected which make the least intrusive stent material volume and to reduce the vessel injury score. 
     The stent surface to length ratio is set to be 1.1-1.3 mm2/mm to provide minimal vessel injury score. 
     At least one membrane  203  is disposed onto the outer surface of a stent  202 . The membrane  203  comprises pockets which serve as receptacles for drugs or reagents to deliver the drugs or reagents into vascular systems. The membrane  203  covers a part of a stent  202  as shown in  FIGS. 3A and 3B , wherein the size of the membrane  203  is variable depending on application. In one example, the membrane  203  covers the whole outer surface of a stent  202 . Thus, the membrane  203  may be in any shape or size. 
     In certain embodiments, the membrane  203  comprises a first layer attached to the outer surface of an implantable medical device such as a stent  202 . An intermediate layer is attached to the first layer wherein the intermediate layer comprises at least two circumferential strips being separated from each other and a second layer covering the first layer and the intermediate layer. The spaces surrounded by the first layer, the circumferential strips and the second layer form the pockets that serve as receptacles for drugs or reagents. In other embodiments, the intermediate layer includes at least one opening so that the pockets can be formed within the openings. The shapes and sizes of the openings may vary in accordance with specific applications. As shown in  FIG. 5 , a stent  202  is partially covered by a membrane  203  that comprises a first layer  206  and a second layer  207 .  FIG. 5  also shows the drug releasing pores  208 . 
     Many polymeric materials are suitable for making the layers of the membrane  203 . Typically, one first layer is disposed onto the outer surface of a stent. The first layer has a thickness of 0.002″-0.005″ with pore sizes of 20-30 microns and similar to nominal initial diameter. 
     In certain embodiments, the first layer serves as an independent membrane  203  to mechanically cover and seal aneurysms  201 . In certain embodiments, the first and/or second layers can be comprised of biodegradable material as a drug or reagent carrier for sustained release. 
     It is desirable that the intermediate layer be formed of a material which can fuse to the first and second layers or attached to the first layer in a different manner. In certain embodiments, the intermediate layer may be merged with the first layer to form a single layer with recessions within the outer surface of the merged layer. 
     The second and intermediate layers can be made of biodegradable material that contains drugs or reagents for immediate or sustained controlled release. After biodegradable material is gone through the degradation process, the membrane  203  is still in tact providing vessel support. 
     The second layer may be composed of a polymeric material. In preferred embodiments, the second layer has a preferable thickness of about 0.001″ with pore sizes of about 70-100 microns. 
     The polymeric layers may also be formed from a material selected from the group consisting of fluoropolymers, polyimides, silicones, polyurethanes, polyurethanes ethers, polyurethane esters, polyurethaneureas and mixtures and copolymers thereof. Biodegradable polymeric materials can also be used. 
     The fusible polymeric layers may be bonded by adhering, laminating, or suturing. The fusion of the polymeric layers may be achieved by various techniques such as heat-sealing, solvent bonding, adhesive bonding or use of coatings. 
     Types of drugs or reagents that may prove beneficial include substances that reduce the thrombogenic, inflammatory or smooth muscle cell proliferative response of the vessel to the implantable medical devices. For example, cell inhibitors can be delivered in order to inhibit smooth muscle cells proliferation. In intracranial or some other applications fibrin sealants can be used and delivered to seal aneurysm neck and provide fibroblasts and endothelial cells growth. Specific examples of drugs or reagents may include heparin, phosporylcholine, albumin, dexamethasone, paclitaxel and vascular endothelial growth factor (VEGF). 
     The drug or reagents can be incorporated into the implantable medical devices in various ways. For example the drug or reagent can be injected in the form of a gel, liquid or powder into receptacles of the pockets. Alternatively the drug or reagent can be supplied in a powder which has been formed into a solid tablet positioned in the receptacles. 
     Another prerequisite of a successful treatment of these extremely small diameter vessels is that the stent delivery system is highly flexible to allow it to be advanced along the anatomy of the cerebral circulation. In addition, the total stent delivery system must be of extremely small profile, to treat diseased intra-cranial arteries generally ranging from 1.5 mm to 5 mm. 
     Referring to  FIG. 6 , in certain embodiments a membrane  203  is embodied as a sleeve  301  supported by two ring-like short stents  302  at both ends of a device so that the membrane  203  covers the whole area of the device  302 . There is no scaffold support in the middle of the device  302 . Radiopaque markers  303  are located at both ends of the stent  302 . Depending on applications, the rings are balloon expandable and made from stainless steel or self-expandable made from NiTi (memory shaped nickel-titanium alloy). 
     The membrane  203  is part of a hemorrhagic stent structure designed to effectively occlude aneurysm neck and “recanalize” the vessel. It&#39;ll allow rebuilding vessel and essentially eliminating aneurysm. No need of expensive (and extra-traumatic, sometimes too massive) coiling is expected. 
     This device is a preferable solution to treat: giant and wide neck aneurysms, bifurcation and trifurcation aneurysms. It is also a preferred treatment solution for cc fistula ruptured in cavernous sinus, pseudoaneurysms, saccular aneurysms. 
     The membrane  203  is elastic to allow its own expansion five to six times without disintegration and detachment from the stent structure. The thickness of the membrane  203  is expected to be not more than 0.002″ in crimped position and 0.001″ in expanded form. The mechanical properties do not introduce extra rigidity to the intracranial stent  202  and have no resistance to stent expansion. The membrane material also allows an expanded membrane  203  to endure normal blood pressure. 
     The membrane  203  is not solid, but is formed as strips between stent struts, or with a series of holes or ovals. The membrane  203  therefore could be porous, or woven mesh. The membrane  203  could also be designed and structured in a way such that there is a system of holes to allow blood penetration into the system of perforators and not allow it into the aneurysm  201 . 
     For upper brain arteries above Siphon, a porous and permeable membrane  203  is ideal. Such a membrane  203  treat an aneurysm neck  201  without blocking microvessels (perforators). It is expected that interventional neuroradiologists (INRs) to be more willing to use the membrane  203  than other known techniques for dealing with aneurysm necks  201 . The permeable membrane  203  has a system of holes or pores with borders between them not larger than 100 microns. The holes or pores may range between 50 to 100 microns. The membrane  203  is able to significantly improve hemodynamics around the aneurysm  201 , since it has a lower delivery profile and is more flexible compared to a stent  202  with a solid membrane. 
     The membrane  203  is attached to the stent struts. The membrane  203  may be attached using spraying, a dipping technique or heat bonding to the intermediate polymeric layer. The stent  202  is placed on a mandrel (hard PTFE or metal), or hung on a hook and the PU solution is sprayed and solidified with a quick drying process. Alternatively, the stent  202  is placed on the mandrel or on the hook and submerged into a PU solution. 
     A biodegradable membrane  203  enables drug delivery and is later dissolved. There are applications where there is no need for a membrane  203  to exist after exceeding 15 to 20 days after placement and thus the membrane  203  could be dissolved. 
     The membrane  203  may be made from PU, Silicon, or any other elastomeric medical grade polymer. 
     Referring to  FIG. 7 , a membrane  203  for bifurcational stents  202  to treat a bifurcation or trifurcation aneurysm  201  is provided. At least 30 to 35% of aneurysms are located at bifurcation sites of intracranial vessels. This membrane  203  is one-sided and non-circumferential. The bifurcation stents  202  are joined by a membrane  203  to cover the aneurysm neck  201 . The same pattern can be applicable to self-expandable (super-elastic) or balloon expandable (stainless steel, CoCr, PtIr alloys) stents  202 . 
     Referring to  FIG. 8 , an aneurysm  201  is covered with the membrane  203  of an intracranial stent  202  to treat and prevent ischemic and hemorrhagic stroke. The intracranial stent  202  coupled with a membrane  203  acts as a scaffold to open clogged arteries, and as a cover to prevent blood circulation to the aneurysm  201 . Obstructing blood supply to the aneurysm  201  isolates the aneurysm  201  from normal blood circulation, and thereby eventually causes it to dry out. Complete obstruction to the aneurysm  201  may not be necessary. 
       FIG. 9  provides a table with typical dimensions for the intracranial stent  202  for use with the membrane  203 . The material for the membrane  203  is biocompatible, has good adhesion to stent struts made from stainless steel 316L, and is formed by a stable film. In other embodiments, the film is blood “permeable” rather than being a solid film. The covered sections, that is, the borders between pores or holes do not exceed 75 μm so as to prevent any part of the stent  202  or the membrane  203  from blocking perforators. Several options can be undertaken to achieve this. The membrane  203  is made from a thin film that does not exceed 0.001″ in width. The film has good expandability, and can expand up to 400% at a low force. The membrane  203  also has a shelf life or chemical stability at ambient conditions and is stable in sterilization conditions (Eto). 
     In one example, polyurethane is used to make the membrane  203 . Specifically, solution grade aromatic, polycarbonate based polyurethane is used. The physical properties are: durometer (Shore) is 75A, tensile strength is 7500 psi and elongation to 500%. 
     Referring to  FIG. 10 , to make a permeable membrane  203 , holes are drilled into a solid film to form pores. The pore size is between 0.025 to 0.050 mm, while the distance between pores is less than 100 μm. 
     Referring to  FIG. 11 , threading strips  203  of a polymer are wrapped laterally around the stent  202 . The strips are interlaced above and below the struts of the stent. The width of the strips is less than 0.075 mm and distance between adjacent strips is less than 100 μm. 
     Referring to  FIG. 12 , a sheet of weaved material  203  is wrapped around the stent  202 . The mesh size of the sheet is around 0.025-0.050 mm, while the width of the polymer is less than 100 μm. 
     Referring to  FIG. 13 , the film  203  completely surrounds the stent strut and is a stable film between the struts of the stent. The film between struts is either within the struts or on the outer struts. The polymeric film stays as close to vessel wall as possible. This is to minimize the film “flipping” inside of vessel as shown in  FIG. 16 . 
     Referring to  FIG. 14 , the membrane  203  is secured onto the struts, and is difficult to dislodge or be torn from the stent  202 . The thickness of the membrane  203  does not add any significant profile to the crimped assembly, that is, it contributes to less than 0.001″ of the crimped stent profile. The membrane  203  also has uniform shrinkability. 
     Referring to  FIGS. 15A-15C , the membrane  203  may completely cover the stent  202  ( FIG. 15A ), cover the mid-section of the stent  202  ( FIG. 15B ), or cover a radial section of the stent  202  ( FIG. 15C ). The membrane  203  expands with the stent  202  and does not restrict or alter the 25 expansion characteristics of the stent  202 . The membrane  203  is easily expandable up to 400%. The membrane  203  has a minimum effect on the mechanical properties of the stent  202  such as flexibility, trackability, expandability, recoil and shortening. The membrane  203  is also stable in normal shelf life conditions and stable in sterilization conditions (Eto). The properties of the polymer 30 film are preserved and not changed after sterilization. The membrane  203  is prevented from sticking to the balloon material (Nylon) after crimping. The membrane  203  is able to tolerate temperature variations (of up to 60 C). The edges of the membrane  203  are aesthetically acceptable, and have smooth, not rough edges. 
     Referring to  FIGS. 17 to 19 , the stent  202  is used to treat a bifurcation or trifurcation aneurysm  201 . The stent  202  is implanted to be partially located in a main artery extending to be partially located in a subordinate artery. For example, in  FIG. 17 , two vertebral arteries join to the basilar artery. The stent  202  is deployed such that it is located in the basilar artery and in a vertebral artery (right side) where the aneurysm  201  is formed. On the other vertebral artery (left side), blood continues to flow to the basilar artery without any obstruction since the membrane  203  is permeable to blood flow. Preferably, the membrane  203  covers the whole stent  202 , and the permeability of the membrane  203  allows blood flow through the left vertebral artery (left side). 
     In  FIG. 18 , the middle cerebral artery divides into the superior trunk and the inferior trunk. The stent  202  is deployed such that it is located in the middle cerebral artery and in the inferior trunk. Again, the struts of the stent  202  do not inhibit blood flow to the superior trunk, and blood flows through the stent  202  to the inferior trunk. 
     In  FIG. 19 , the stent  202  is deployed in the vertebral artery. As the aneurysm  201  in this example is located in a middle portion of the vertebral artery, there is no need for the stent  202  to be located in more than one artery. 
     When implanted, the stent  202  diverts blood flow away from the aneurysm  201 . This leads to occlusion of the aneurysm  201  and keeps the arterial branches and the perforators patent. The stent  202  does not require precise positioning because preferably, it is uniformly covered with the permeable membrane  203 . In other words, most of the circumferential surface of the stent  202  is covered by the membrane  203 . Due to the particular porosity and dimensions of the membrane  203 , blood circulation to the aneurysm  201  is obstructed while blood supply to perforators and microscopic branches of main brain arteries as well as larger arteries is permitted. As described earlier, obstructing blood supply to the aneurysm  201  isolates the aneurysm  201  from normal blood circulation, and thereby eventually causes it to dry out. The stent  202  and membrane  203  treats the aneurysm  201  by causing an alteration in the hemodynamics in the aneurysm sac such that intra-aneurysmal thrombosis is initiated. At the same, blood flow into the arteries (branch, main, big or small) are not significantly affected by the implantation of the stent  202  or the membrane  203  due to the special porosity of the membrane  203 . 
     Although a bifurcation aneurysm has been described, it is envisaged that the stent  202  may be used to treat a trifurcation aneurysm in a similar manner. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope or spirit of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects illustrative and not restrictive.