Patent Publication Number: US-2010125328-A1

Title: Bioabsorbable stent

Description:
RELATED APPLICATIONS 
     This is a continuation application and claims the benefit of priority of co-pending U.S. patent application Ser. No. 11/213,817, filed Aug. 30, 2005, the entire specification of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     The disclosed invention relates generally to a medical device and more particularly to a bioabsorbable stent 
     Intraluminal stents are typically inserted or implanted into a body lumen, for example, a coronary artery, after a procedure such as percutaneous transluminal coronary angioplasty. Such stents are used to maintain the patency of a body lumen by supporting the walls of the lumen and preventing abrupt reclosure or collapse thereof. These stents can also be provided with one or more therapeutic agents adapted to be locally released from the stent at the site of implantation. In the case of a coronary stent, the stent can be adapted to provide release of, for example, an antithrombotic agent to inhibit clotting or an antiproliferative agent to inhibit smooth muscle cell proliferation, i.e., neointimal hyperplasia, which is believed to be a significant factor leading to re-narrowing or restenosis of the blood vessel after implantation of the stent. 
     Stents are commonly formed from biocompatible metals such as stainless steel, or metal alloys such as nickel-titanium alloys that are often employed because of their desirable shape-memory characteristics. Metallic materials are advantageously employed to construct stents because of the inherent rigidity of metallic materials and the consequent ability of the metallic stent to maintain patency of the lumen upon implantation of the stent. Metallic stents can also cause complications, however, such as thrombosis and neointimal hyperplasia. It is believed that prolonged contact of the metallic surfaces of the stent with the lumen may be a significant factor in these adverse events following implantation. 
     The above described potential adverse affects of metallic stents can be reduced by adapting the stent to provide localized release of a therapeutic agent. To provide the therapeutic agent, metallic stents are coated with a biodegradable or non-biodegradable material containing the therapeutic agent. The coating may also provide a more biocompatible surface directly in contact with the body lumen wall. 
     The use of such drug eluting stents has helped to reduce the occurrence of restenosis of the body lumen; however, physicians would prefer not to leave the stent permanently in the body lumen. When a body lumen requires multiple stenting procedures, complications for later surgeries can result, creating what is commonly referred to as a “full metal jacket.” Because of this, biodegradable stents constructed of a magnesium alloy or biodegradable polymers have recently been developed. These magnesium stents can be absorbed by the body over time. 
     Although the use of stents formed with magnesium alloys, iron alloys, or biodegradable polymers provides both functional and safety benefits over conventional non-absorbable stents, and the characteristic rate of bioabsorption of the materials used in such stents can lead to other problems. The absorption rate of the stent depends on a variety of factors including the material composition, the size and surface area of the stent, and physiology and biochemical interactions with the stent at the treatment site in the body lumen. Rapid bioabsorption of bioabsorbable stents may be undesirable for both health and functional reasons. For example, rapid bioabsorption of bioabsorbable stents may result in the stent being absorbed before the body lumen has sufficiently healed and become self-supporting. Further, high rates of absorption of the stents&#39; constituent materials may have undesirable physiological effects. Conversely, if the stent is absorbed too slowly, it may interfere with a subsequent stenting procedure. Thus, there is a need for a bioabsorbable stent that can be configured with a preselected and controllable rate of absorption. 
     SUMMARY OF THE INVENTION 
     A medical device includes a support structure formed of a metal that is absorbable by a mammalian body. A polymer is disposed on the support structure in at least partially overlying relationship. The polymer has a thickness and a rate of absorption by a mammalian body such that the polymer is substantially completely absorbed, exposing the underlying portion of the support structure, before the underlying portion of the support structure is absorbed. In another embodiment, the medical device includes a support structure formed of a first material, the first material being absorbable by a mammalian body. An absorption inhibitor disposed on the support structure in at least partially overlying relationship and formed of a second material different from the first material. The second material being absorbable by the mammalian body. The absorption inhibitor reducing a rate of absorption of the portion of the support structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. For example, item  20  is identical or functionally similar to item  120 . 
         FIG. 1  is an illustration of a stent according to an embodiment of the invention shown positioned in a cross-sectional view of a body lumen. 
         FIG. 2  is an end view of the stent illustrated in  FIG. 1 . 
         FIG. 3  is an alternative end view of the stent illustrated in  FIG. 1 . 
         FIG. 4  is a side perspective view of a stent according to an embodiment of the invention. 
         FIG. 5  is a side perspective view of a stent according to another embodiment of the invention shown positioned in a cut-away view of a portion of blood vessel. 
         FIG. 6  is a side perspective view of a stent according to another embodiment of the invention. 
         FIG. 7  is a side perspective view of a stent according to another embodiment of the invention. 
         FIG. 8  is a sectional view taken along line  8 - 8  in  FIG. 7 . 
         FIG. 9  is a sectional view taken along line  9 - 9  in  FIG. 7 . 
         FIG. 10  is a side perspective view of a stent according to another embodiment of the invention. 
         FIGS. 11 ,  12  and  13  are a side perspective views of a stent according to another embodiment of the invention shown positioned in a cross-sectional view of a body lumen at various stages of a bioabsorption process. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of a stent  20  embodying the principles of the invention. The stent  20  is configured to be placed or otherwise implanted into a natural body lumen L of a mammal (e.g., a blood vessel or ureter) to support the walls W of the body lumen L, such as after a medical procedure (e.g., a coronary angioplasty). A bodily fluid F may flow through lumen L. The stent  20  is constructed of materials that are configured to be absorbed by the mammalian body over a controlled or predetermined period of time. 
     Specifically, the stent  20  includes a support structure  22  and an absorption inhibitor layer  24 .  FIGS. 2 and 3  illustrate two alternative cross-sections of the stent  20  shown in  FIG. 1 . The support structure  22  includes an inner surface  28  and an outer surface  30 , and defines a lumen  26 .  FIG. 2  illustrates stent  20  having an absorption inhibitor layer  24  disposed on the outer surface  30  of stent  20 .  FIG. 3  illustrates stent  20  having an absorption inhibitor layer  24  disposed on the inner surface  28  of stent  20 . The support structure  22  may be constructed in a variety of different configurations, including any of the conventional configurations commonly used, such as a lattice or network of struts forming a framework having multiple apertures or open spaces, a perforated tube, or a coil configuration. 
     The support structure  22  is configured to provide the desired degree of structural support for the body lumen L. The desired degree of structural support decreases over time. For example, a stent disposed in a coronary artery after an angioplasty requires a certain degree of structural strength to resist relapse of the coronary artery immediately after the procedure. As the artery heals, less structural support is required to prevent relapse. Eventually no internal support is needed. After that point in time, it is desirable for the support structure to be absent from the artery, for the reasons described above. A support structure  22  formed of a bioabsorbable material can meet this time-varying support requirement, in that it can be configured such that before the onset of any biodegradation it provides the requisite initial structural support to the lumen L, and such that it is completely biodegraded after the time at which structural support to the body lumen L is no longer required and before the time by which it is desired that the artery be clear of the stent  20 . 
     The structural strength of a stent as a function of time after placement in the artery may not meet the desired profile. For example, if the stent immediately begins to biodegrade, its structural strength may drop below the required value. It is undesirable to add more material to the stent so that it remains at or above the required strength at all points in time because, for example, this would increase the total amount of material absorbed by the body and during delivery, for example, when the stent is mounted on a balloon catheter, make the stent bulkier and/or less flexible and maneuverable. Therefore, the biodegradation of the support structure  22  of the stent  20  can be modulated or controlled by the presence of the absorption inhibitor layer  24 . 
     The absorption inhibitor layer  24  can reduce the rate of absorption of the surface(s) of the support structure  22  that it overlies, and may reduce the rate of absorption to zero. If the absorption inhibitor layer  24  itself is absorbed, its effect on the rate of absorption of the underlying surface of the support structure  22  is eliminated once the absorption inhibitor layer  24  is completely absorbed. If the absorption inhibitor layer  24  is not absorbed, its effect persists until the underlying surface of the support structure  22  is absorbed (through the absorption inhibitor layer  24  at a reduced rate and/or from another direction). Thus, the duration of the absorption inhibitor layer&#39;s effect on the rate of absorption of the underlying surface of the support structure  22  depends on the rate of absorption of the absorption inhibitor layer  24  and/or its thickness. 
     The absorption inhibitor layer  24  may have a thickness and a rate of absorption such that the layer is substantially completely absorbed, exposing the underlying portion of the support structure  22 , before the underlying portion of the support structure  22  is absorbed. Alternatively, the respective thicknesses and absorption rates of the support structure  22  and the absorption inhibitor layer  24  may be such that the absorption of the support structure  22  starts after the complete absorption of the absorption inhibitor layer  24  starts, and in a further variation the absorption inhibitor layer  24  may be substantially completely absorbed before the absorption of the support structure  22  begins. 
     By varying the thickness of the absorption inhibitor layer  24  on a particular portion of support structure  22 , the absorption of some portions of the support structure  22  can be delayed longer than other portions. Thus, the stent  20  can be configured such that selected portions of the support structure  22  can remain in position in the body lumen for longer periods of time. Portions of the support structure  22  may have no absorption inhibitor layer  24 . These portions of support structure  22  will begin to bioabsorb immediately upon implantation into the body lumen. Alternatively, the thickness of the absorption inhibitor layer  24  may be constant. 
     The rate of absorption of the absorption inhibitor layer  24  also depends on its formulation and the nature of the tissue and/or bodily fluid with which the layer is in contact. For example, an absorption inhibitor layer  24  disposed on a radially outer surface of a cardiovascular stent would be in contact with the inner wall of the coronary artery, while an absorption inhibitor layer  24  disposed on the radially inner surface would be in contact with the blood flowing through the artery. The absorption rate can also be time dependent. For example, the physiochemical properties of the artery will change as the artery heals following an angioplasty procedure. Delaying the absorption of some or all of the support structure  22  allows time for the support structure  22  to be encapsulated by the body lumen intima before starting to bioabsorb, which in turn can lead to a different rate of absorption of the support structure  22 . 
     An absorption inhibitor layer  24  that is formulated to allow, at a reduced rate, absorption of the surface of the support structure  22  underlying the absorption inhibitor layer  24 , can also have a time-varying effect on the absorption rate. For example, the absorption mechanism by which the support structure  22  is absorbed through the absorption inhibitor layer  24  can depend on the thickness of the absorption inhibitor layer  24  as described above. The absorption rate of the support structure  22  could therefore increase as the absorption inhibitor layer  24  is absorbed (and thus reduced in thickness). There are, therefore, several variables affecting the time profile of the absorption rate of the surface of the support structure  22  underlying the absorption inhibitor layer  24 , including the absorption rate of the absorption inhibitor layer  24  (which can depend on the body tissue or fluid with which it is in contact), its thickness, and the degree to which it reduces the rate of absorption of the underlying support structure material, which in turn may vary with other factors such as thickness. 
     These factors provide significant flexibility in tailoring the time profile of the absorption rate of the surface of the support structure  22  underlying the absorption inhibitor layer  24 . In turn, the integral of the absorption rate over time yields the amount of support structure  22  absorbed as a function of time. In combination with the other factors, such as the structural properties of the support structure material, the initial local thickness, and support structure geometry/configuration, the amount of absorption over time determines the support structure&#39;s degree of support to the body lumen L, and the amount over time. 
     The spatial distribution of the absorption inhibitor layer  24  over the surfaces of the support structure  22 , including thickness and location, can also be varied to yield the desired time profile of support and quantity of support structure material remaining Thus, the absorption inhibitor layer  24  can be disposed on the entirety of the outer surface of a tubular support structure  22 , and/or the entirety of the inner surface, and/or one or both end surfaces, and/or on the sides of perforated struts, links, or other such surfaces of the support structure  22 . The absorption inhibitor layer  24  can also be placed on a portion, rather than the entirety, of any or all of the above surfaces. The portions of any surface can be a continuous portion (such as one-half of a surface), one or more discontinuous portions (in radial or circumferential strips (parallel and/or intersecting)), circular or polygonal patches or spots, etc. The thickness of the absorption inhibitor layer(s)  24  can be constant or can vary from surface to surface or along one surface. 
     The absorption inhibitor layer  24  can be homogeneous (i.e., uniform composition) or heterogeneous. Thus, different absorption inhibitor materials, with different properties, can be used in combination. The different materials can be used on different surfaces, on the same surfaces (spaced or abutting), or partially or completely overlapping. 
     Thus, the time by which the support structure  22  will absorb will depend on a number of factors identified above, including the shape and size of the support structure  22 , and the particular composition of the alloy. The material composition of the absorption inhibitor layer  24  may be formulated, and the layer is arranged with respect to the material of the support structure  22 , such that at least a portion of the support structure  22  is not absorbed until after the material of the absorption inhibitor layer  24  is substantially absorbed by the body. The material of the absorption inhibitor layer  24  may be configured and formulated such that it is substantially completely absorbed into the body in 1 to 30 days. The material of the support structure  22  may be configured and formulated such that the support structure  22  is substantially completely absorbed into the body in 14 to 56 days. The absorption of the support structure  22  may be delayed such that it is substantially completely absorbed by the body within 12 months after implantation into the body. The stent  20  may be configured to retain at least approximately 90% of its structural strength for at least 180 days. 
     Optionally, stent  20  can include a delivery layer  36 , disposed on some or all surfaces of support structure  22 , by which a therapeutic agent can be delivered to the body. Therapeutic agents are commonly used to help reduce restenosis and thrombosis of the body lumen. Delivery layer  36  can be of any conventional formulation or composition, to deliver any known therapeutic agent. Delivery layer  36  can be formulated to bioabsorb, delivering the therapeutic agent as it is absorbed, or not to bioabsorb, delivering the therapeutic agent by other mechanisms. In either case, absorption inhibitor layer  24  can be arranged to overlie delivery layer  36  to modulate the rate of relapse of the therapeutic agent in a manner similar to the way in which it can modulate the rate of absorption of the support structure  22 . In other embodiments, the function of delivery layer  36  and absorption inhibitor layer  24  can be combined, i.e., the absorption inhibitor layer  24  can be formulated to include a therapeutic agent. 
     As used herein, the term “therapeutic agent” includes, but is not limited to, any therapeutic agent or active material, such as drugs, genetic materials, and biological materials. Suitable genetic materials include, but are not limited to, DNA or RNA, such as, without limitation, DNA/RNA encoding a useful protein, DNA/RNA intended to be inserted into a human body including viral vectors and non-viral vectors, and RNAi (RNA interfering sequences). Suitable viral vectors include, for example, adenoviruses, gutted adenoviruses, adeno-associated viruses, retroviruses, alpha viruses (Semliki Forest, Sindbis, etc.), lentiviruses, herpes simplex viruses, ex vivo modified and unmodified cells (e.g., stem cells, fibroblasts, myoblasts, satellite cells, pericytes, cardiomyocytes, skeletal myocytes, macrophage), replication competent viruses (e.g., ONYX-015), and hybrid vectors. Suitable non-viral vectors include, for example, artificial chromosomes and mini-chromosomes, plasmid DNA vectors (e.g., pCOR), cationic polymers (e.g., polyethyleneimine, polyethyleneimine (PEI)) graft copolymers (e.g., polyether-PEI and polyethylene oxide-PEI), neutral polymers PVP, SP1017 (SUPRATEK), lipids or lipoplexes, nanoparticles and microparticles with and without targeting sequences such as the protein transduction domain (PTD). 
     Suitable biological materials include, but are not limited to, cells, yeasts, bacteria, proteins, peptides, cytokines, and hormones. Examples of suitable peptides and proteins include growth factors (e.g., FGF, FGF-1, FGF-2, VEGF, Endothelial Mitogenic Growth Factors, and epidermal growth factors, transforming growth factor α and β, platelet derived endothelial growth factor, platelet derived growth factor, tumor necrosis factor α, hepatocyte growth factor and insulin-like growth factor), transcription factors, proteinkinases, CDK inhibitors, thymidine kinase, and bone morphogenic proteins (BMP&#39;s), such as BMP-2, BMP-3, BMP-4, BMP-5, BMP-6 (Vgr-1), BMP-7 (OP-1), BMP-8, BMP-9, BMP-10, BMP-11, BMP-12, BMP-13, BMP-14, BMP-15, and BMP-16. These dimeric proteins can be provided as homodimers, heterodimers, or combinations thereof, alone or together with other molecules. Cells can be of human origin (autologous or allogeneic) or from an animal source (xenogeneic), genetically engineered, if desired, to deliver proteins of interest at a desired site. The delivery media can be formulated as needed to maintain cell function and viability. Cells include, for example, whole bone marrow, bone marrow derived mono-nuclear cells, progenitor cells (e.g., endothelial progentitor cells), stem cells (e.g., mesenchymal, hematopoietic, neuronal), pluripotent stem cells, fibroblasts, macrophage, and satellite cells. 
     The term “therapeutic agent” and similar terms also includes non-genetic agents, such as: anti-thrombogenic agents such as heparin, heparin derivatives, urokinase, and PPack (dextrophenylalanine proline arginine chloromethylketone); anti-proliferative agents such as enoxaprin, angiopeptin, or monoclonal antibodies capable of blocking smooth muscle cell proliferation, hirudin, and acetylsalicylic acid, amlodipine and doxazosin; anti-inflammatory agents such as glucocorticoids, betamethasone, dexamethasone, prednisolone, corticosterone, budesonide, estrogen, sulfasalazine, and mesalamine; antineoplastic/antiproliferative/anti-miotic agents such as paclitaxel, 5-fluorouracil, cisplatin, vinblastine, vincristine, epothilones, methotrexate, azathioprine, adriamycin and mutamycin; endostatin, angiostatin and thymidine kinase inhibitors, taxol and its analogs or derivatives; anesthetic agents such as lidocaine, bupivacaine, and ropivacaine; anti-coagulants such as D-Phe-Pro-Arg chloromethyl keton, an RGD peptide-containing compound, heparin, antithrombin compounds, platelet receptor antagonists, anti-thrombin antibodies, anti-platelet receptor antibodies, aspirin (aspirin is also classified as an analgesic, antipyretic and anti-inflammatory drug), dipyridamole, protamine, hirudin, prostaglandin inhibitors, platelet inhibitors and tick antiplatelet peptides; vascular cell growth promotors such as growth factors, Vascular Endothelial Growth Factors (VEGF, all types including VEGF-2), growth factor receptors, transcriptional activators, Insulin Growth Factor (IGF), Hepatocyte Growth Factor (HGF), and translational promotors; vascular cell growth inhibitors such as antiproliferative agents, growth factor inhibitors, growth factor receptor antagonists, transcriptional repressors, translational repressors, replication inhibitors, inhibitory antibodies, antibodies directed against growth factors, bifunctional molecules consisting of a growth factor and a cytotoxin, bifunctional molecules consisting of an antibody and a cytotoxin; cholesterol-lowering agents, vasodilating agents, and agents which interfere with endogenous vasoactive mechanisms; anti-oxidants, such as probucol; antibiotic agents, such as penicillin, cefoxitin, oxacillin, tobranycin; angiogenic substances, such as acidic and basic fibrobrast growth factors, estrogen including estradiol (E2), estriol (E3) and 17-Beta Estradiol; and drugs for heart failure, such as digoxin, beta-blockers, angiotensin-converting enzyme (ACE) inhibitors including captopril and enalopril. 
     Some therapeutic materials include anti-proliferative drugs such as steroids, vitamins, and restenosis-inhibiting agents such as cladribine. Restenosis-inhibiting agents include microtubule stabilizing agents such as Taxol, paclitaxel, paclitaxel analogues, derivatives, and mixtures thereof For example, derivatives suitable for use in the present invention include 2′-succinyl-taxol, 2′-succinyl-taxol triethanolamine, 2′-glutaryl-taxol, 2′-glutaryl-taxol triethanolamine salt, 2′-O-ester with N-(dimethylaminoethyl)glutamine, and 2′-O-ester with N-(dimethylaminoethyl)glutamide hydrochloride salt. Other therapeutic materials include nitroglycerin, nitrous oxides, antibiotics, aspirins, digitalis, and glycosides. 
     The support structure  22  may be formed from a variety of bioabsorbable materials, including metals, polymers, and bioactive glass. A bioabsorbable metal is preferred because of its greater structural strength. Suitable bioabsorbable metals include known magnesium alloys, including formulations such as the magnesium alloy disclosed in US Patent Publication No 2002/0004060 (the disclosure of which is incorporated herein by reference in its entirety), which includes approximately 50-98% magnesium, 0-40% lithium, 0-5% iron and less than 5% other metals. Other suitable formulations include a magnesium alloy having greater than 90% magnesium, 3.7%-5.5% yttrium, and 1.5%-4.4% rare earths, as disclosed in US Patent Publication No 2004/0098108 (the disclosure of which is incorporated herein by reference in its entirety). Alternatively, the support structure  22  can be formed of a suitable polymer material, such as polyactic acid, polyglycolic acid, collagen, polycaprolactone, hylauric acid, adhesive protein, co-polymers of these materials, as well as composites and combinations thereof Known bioabsorbable polymer stents, which also have drug eluding capabilities include the stents disclosed in U.S. Pat. Nos. 5,464,450; 6,387,124; and 5,500,013 (the disclosures of which are incorporated herein by reference in their entirety). 
     Absorption inhibitor layer  24  and delivery layer  36  can also be formed of a variety of biocompatible materials. As discussed above, such material may or may not be bioabsorbable, depending on whether it is desired to have some or all of the absorption inhibitor layer  24  and/or the delivery layer  36  remain in the body lumen after the support structure  22  has bioabsorbed. The material can be a polymer. Suitable polymers include those bioabsorbable polymers discussed above for the support structure  22 . A specific example of a biodegradable polymer material incorporating a drug is described in the U.S. Pat. No. 5,464,450 patent and includes a poly-L-lactide. 
     Having described above various general principles, several exemplary embodiments of these concepts are now described. These embodiments are only exemplary, and many other combinations and formulations of support structure  22 , absorption inhibitor layer  24 , and delivery layer  36  formulations, and configurations are possible, contemplated by the principles of the invention, and will be apparent to the artisan in view of the general principles described above and the exemplary embodiments. 
       FIG. 4  illustrates a side perspective view of a stent  120  according to an embodiment of the invention. The stent  120  includes a support structure  122  formed of a bioabsorbable metallic or polymer material in a coil configuration. An absorption inhibitor layer  124  is disposed on a portion of an outer surface  130  of the support structure  122 . In this embodiment, support structure  122  includes exposed portions  132  and  134 . Exposed portions  132  and  134  of the support structure  122  will begin to be absorbed by the body in which the stent  120  is implanted immediately upon implantation, while the covered portions of support structure  122  will have a delayed absorption based in part on the rate of absorption of the absorption inhibitor layer  124 . 
       FIG. 5  illustrates a stent  220  according to another embodiment of the invention shown within a blood vessel V. Stent  220  is substantially tubular and includes a support structure  222  (the detailed configuration of which is omitted for ease of illustration), constructed of a bioabsorbable polymer or metallic material and an absorption inhibitor layer  224  at least partially disposed on an inner surface of the support structure  222 . In this embodiment, an outer surface  230  of the support structure  222  will contact the blood vessel intimal layer I and have a first rate of absorption from contact with the intimal layer I, and the absorption inhibitor layer  24  will contact blood B flowing through the lumen of the blood vessel V and have a second rate of absorption from contact with the blood flow. The first rate of absorption associated with the support structure  222  can be different from the second rate of absorption associated with the absorption inhibitor layer  224 . In other embodiments, the absorption inhibitor layer  224  may be disposed on the outer surface  230  of the support structure  222 . In such an embodiment, the support structure  222  will contact the blood B flowing through the blood vessel V and the absorption inhibitor layer  224  will contact the intimal layer I of the blood vessel V. 
       FIG. 6  illustrates a stent  320  according to another embodiment of the invention. In this embodiment, an absorption inhibitor layer  324  is disposed on an outer surface of a support structure  322 . The absorption inhibitor layer  324  includes multiple pores  328 . The pores  328  provide for a faster rate of absorption of that portion of support structure  322  where support structure  322  is exposed to the body lumen and/or bodily fluid through the pores  228 . 
       FIGS. 7 ,  8  and  9  illustrate a stent  420  according to yet another embodiment of the invention. Stent  420  includes a support structure  422  defining a lumen  426 , and an absorption inhibitor layer  424  disposed on an outer surface of support structure  422 . In this embodiment, the absorption inhibitor layer  424  is substantially overlying the outer surface of the support structure  422 . Support structure  422  also includes a second absorption inhibitor layer  440  disposed on an inner surface of support structure  422 . The absorption inhibitor layer  424  disposed on the outer surface of support structure  422  has varying thicknesses along the length of the support structure  422 , as shown in the cross-sectional views of  FIGS. 8 and 9 . As stated previously, by varying the thickness of the absorption inhibitor layer  424 , the rate of absorption of support structure  422  can be varied along the length of the support structure  422 . In addition, because the support structure  422  includes both an absorption inhibitor layer  424  disposed on the outer surface, and an absorption inhibitor layer  440  disposed on the inner surface, the absorption of support structure  422  will be further delayed. 
       FIG. 10  illustrates a stent  520  according to another embodiment of the invention. The stent  520  includes a support structure  522  defining a lumen  526 , and an absorption inhibitor layer  524  disposed in radial strips at spaced distances along the length of support structure  522 . A delivery layer  536  is disposed in radial strips on an outer surface of two of the absorption inhibitor layers  524  in partially overlying relationship. The delivery layer  536  can contain a therapeutic agent configured to be released into the body as the delivery layer  536  is bioabsorbed into the body as previously described Alternatively, a therapeutic agent may be contained in the absorption inhibitor layer  524 . If the agent is contained in the absorption inhibitor layer  524 , the release of the agent into the body will be delayed for those portions of the absorption inhibitor layer  524  that are covered with the delivery layer  536 . To increase the rate of absorption in such an embodiment, the delivery layer  536  may be permeated, allowing the therapeutic agent to be released from the absorption inhibitor layer  524  and pass through the delivery layer  536  without waiting until the delivery layer  536  has begun to bioabsorb. 
       FIGS. 11 ,  12  and  13  illustrate a stent  620  according to another embodiment of the invention, shown positioned in a body lumen L at various stages of bioabsorption after insertion into the body lumen L. Stent  620  includes a support structure  622  defining a lumen  626 , and an absorption inhibitor layer  624  disposed on a portion of an outer surface  630  of support structure  622 . In this embodiment, support structure  622  is constructed with a metallic bioabsorbable material in a lattice framework configuration.  FIG. 11  illustrates the stent  620  immediately after implantation into the body lumen, with no visible bioabsorption.  FIG. 12  illustrates stent  620  with portions of support structure  622  bioabsorbed, while a substantial portion of the absorption inhibitor layer  624  still remains intact.  FIG. 13  illustrates the stent  620  at a later time in the bioabsorption process from that shown in  FIG. 12 , where the support structure  622  is further bioabsorbed by the body lumen L, and the absorption inhibitor layer  624  is partially absorbed, exposing portions of the underlying surface of the support structure  622 . 
     Conclusion 
     While various embodiments of the invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the invention should not be limited by any of the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents. 
     The previous description of the embodiments is provided to enable any person skilled in the art to make or use the invention. While the invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those skilled in art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, the various features of stent  20  ( 120 ,  220 ,  320 ,  420 ,  520 ,  620 ) may include other configurations, shapes and materials not specifically illustrated, while still remaining within the scope of the invention. 
     For example, the support structure may be constructed of a bioabsorbable metallic material, such as a magnesium alloy, or a bioabsorbable polymer. The absorption inhibitor layer and the delivery layer may be constructed with a variety of bioabsorbable polymers. In addition, the absorption inhibitor layer may be disposed on various portions of the support structure, including the inner surface and/or the outer surface, and the delivery layer may be disposed on various portions of the support structure and/or the absorption inhibitor layer. 
     Further, the stent  20  ( 120 ,  220 ,  320 ,  420 ,  520 ,  620 ) may include more than one absorption inhibitor layer and one or more delivery layer. In some embodiments, the stent may include a therapeutic agent contained in one or more of the support structure, the absorption inhibitor layer and the delivery layer. The absorption inhibitor layer and/or the delivery layer may include pores and may be permeable to the therapeutic agent. 
     Various aspects of the invention relating to the above are enumerated in the following paragraphs: 
     Aspect 1. A medical device, comprising: (a) a support structure formed of a metal that is absorbable by a mammalian body; and (b) a polymer disposed on the support structure in at least partially overlying relationship, the polymer having a thickness and a rate of absorption by a mammalian body such that the polymer is substantially completely absorbed when implanted within the mammalian body, exposing the underlying portion of the support structure, before the underlying portion of the support structure is absorbed within the mammalian body. 
     Aspect 2. The medical device of aspect 1, wherein the support structure is substantially tubular shaped. 
     Aspect 3. The medical device of aspect 2, wherein the support structure is configured to be insertable into a natural body lumen of a mammal. 
     Aspect 4. The medical device of aspect 1, wherein the support structure defines an internal lumen and includes an inner surface and an outer surface, the polymer is at least partially disposed on at least one of the inner surface or the outer surface of the support structure. 
     Aspect 5. The medical device of aspect 1, wherein the polymer includes one of a polyactic acid, a collagen, a polyglycolic acid, and a polycaprolactone. 
     Aspect 6. The medical device of aspect 1, further comprising: a therapeutic agent carried by the medical device, the therapeutic agent formulated to be released into the mammalian body when the medical device is disposed therein, the therapeutic agent being disposed on at least one of the support structure or the polymer. 
     Aspect 7. The medical device of aspect 1, wherein the metal is a first material, the polymer is a second material, the medical device further comprising: (a) a therapeutic agent formulated to be released into the mammalian body when the medical device is disposed therein; and (b) a third material disposed on at least one of the first material or the second material, the therapeutic agent being contained within the third material. 
     Aspect 8. The medical device of aspect 7, wherein the third material is a polymer. 
     Aspect 9. The medical device of aspect 7, wherein the third material is configured to be absorbed by a mammalian body. 
     Aspect 10. The medical device of aspect 7, wherein the third material is permeable. 
     Aspect 11. The medical device of aspect 1, wherein the polymer is permeable. 
     Aspect 12. The medical device of aspect 1, wherein the medical device is configured to be substantially completely absorbed by a mammalian body within 12 months after insertion into the mammalian body. 
     Aspect 13. The medical device of aspect 1, wherein the medical device is configured to retain at least approximately 90% of its structural strength for a least approximately 180 days. 
     Aspect 14. The medical device of aspect 1, wherein the medical device is configured to be inserted into a mammalian blood vessel that has been subjected to an angioplasty procedure and the medical device is configured to have structural properties sufficient to prevent relapse of the blood vessel for a sufficient time after insertion into the blood vessel for the blood vessel to heal sufficiently to be self-supporting. 
     Aspect 15. The medical device of aspect 1, wherein the support structure is tubular and includes an outer surface and an inner surface, the support structure configured to be disposed in a mammalian blood vessel with the outer surface in contact with an inner wall of the blood vessel, the metal is a first material and the polymer is a second material, the polymer disposed on the inner surface, the medical device further comprising: a third material disposed on the outer surface, the third material having a thickness and a rate of absorption by the mammalian body, and wherein the first and second materials are substantially completely absorbed before the third material is substantially completely absorbed. 
     Aspect 16. A medical device, comprising: (a) a support structure formed of a first material, the first material being absorbable by a mammalian body; and (b) an absorption inhibitor disposed on the support structure in at least partially overlying relationship, the absorption inhibitor being formed of a second material different from the first material, the second material being absorbable by the mammalian body, the absorption inhibitor reducing a rate of absorption of the portion of the support structure underlying the absorption inhibitor. 
     Aspect 17. The medical device of aspect 16, wherein the absorption inhibitor is configured, and the second material is formulated, such that the absorption inhibitor is substantially completely absorbable in a mammalian body in 1 to 30 days. 
     Aspect 18. The medical device of aspect 16, wherein the portion of the support structure underlying the absorption inhibitor is configured, and the first material is formulated, such that the portion of the support structure is substantially completely absorbable in a mammalian body in 14 to 56 days. 
     Aspect 19. The medical device of aspect 16, wherein the support structure is tubular shaped and configured to be disposed within a lumen of a blood vessel, the first material configured to contact the blood vessel intimal layer and has a first rate of absorption from contact with the intimal layer, the second material configured to contact blood flowing through the lumen of the blood vessel and has a second rate of absorption from contact with the blood flow, the first rate of absorption being different than the second rate of absorption. 
     Aspect 20. The medical device of aspect 19, wherein the first material has a thickness and the second material has a thickness, the first thickness and the second thickness each selected such that the absorption of the first material associated with contact with the blood vessel intimal layer starts after the absorption of the second material associated with the blood flow within the blood vessel. 
     Aspect 21. The medical device of aspect 19, wherein the first material has a first thickness and the second material has a second thickness, the first thickness and the second thickness each selected such that the absorption of the second material associated with the blood flow within the blood vessel is substantially completed before the absorption of the first material associated with contact with the blood vessel intimal layer begins. 
     Aspect 22. The medical device of aspect 16, wherein the absorption inhibitor includes a portion having a first thickness and a portion having a second thickness, the absorption inhibitor having a first rate of absorption associated with the portion having a first thickness and a second rate of absorption associated with the portion having a second thickness, the first rate of absorption being different than the second rate of absorption. 
     Aspect 23. The medical device of aspect 16, wherein the absorption inhibitor is permeable. 
     Aspect 24. The medical device of aspect 16, wherein the second material is a polymer. 
     Aspect 25. The medical device of aspect 16, wherein the first material is at least one of a polymer or a metal. 
     Aspect 26. The medical device of aspect 16, further including a therapeutic agent contained in at least one of the support structure or the absorption inhibitor, the therapeutic agent configured to be absorbed by the mammalian body. 
     Aspect 27. The medical device of aspect 16, wherein the support structure includes a coil configured to be disposed in a vascular lumen. 
     Aspect 28. The medical device of aspect 16, wherein the support structure is tubular and configured to be inserted into a natural body lumen of a mammal. 
     Aspect 29. A stent, comprising: (a) a support structure configured to be inserted into a mammalian body, the support structure being formed of a first material, the first material being a metal configured to be absorbed by the mammalian body; and (b) a layer substantially covering one of an outer surface and an inner surface of the support structure, the layer formed of a second material, the second material being absorbable by the mammalian body, the second material being different than the first material. 
     Aspect 30. The medical device of aspect 29, wherein the layer includes a portion having a first thickness and a portion having a second thickness, the first thickness associated with a first rate of absorption of the layer and the second thickness associated with a second rate of absorption of the layer. 
     Aspect 31. The medical device of aspect 30, wherein the support structure has a first rate of absorption associated with the portion of the layer having the first thickness and the support structure has a second rate of absorption associated with the portion of the layer having the second thickness, the first rate of absorption of the support structure being different than the second rate of absorption of the support structure. 
     Aspect 32. The medical device of aspect 29, wherein the second material is a polymer. 
     Aspect 33. The medical device of aspect 29, wherein the metal includes magnesium. 
     Aspect 34. The medical device of aspect 29, further comprising: a third material disposed on at least one of the support structure or the layer in at least partially overlying relationship, the third material being absorbable by the mammalian body. 
     Aspect 35. The medical device of aspect 34, further comprising: a therapeutic agent carried by at least one of the third material or the second material, the therapeutic agent configured to be released into the mammalian body when the medical device is disposed therein.