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STENT WITH DRUG COATING
CROSS REFERENCE TO RELATED
 This is a divisional application of U.S. Ser. No. 10/293,658, which was filed on Nov. 12, 2002.
BACKGROUND OF THE INVENTION  1. Field of the Invention
 This invention is directed to an implantable device, such as a stent, having a polymeric drug coating, and method of forming the same.
 2. Description of the Background
 Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to remodel the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
 A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings, which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may necessitate another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, an expandable, intraluminal prosthesis, also known as a stent, is implanted in the lumen to maintain the vascular patency.
 Stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically, stents are capable of being compressed so that they can be inserted through small lumens via catheters and then expanded to a larger diameter once they are at the desired location. Mechanical intervention via stents has reduced the rate of restenosis as compared to balloon angioplasty. Yet, restenosis is still a significant clinical problem with rates ranging from 20-40%. When restenosis does occur in the stented segment, its treatment can be challenging, as clinical options are more limited as compared to lesions that were treated solely with a balloon.
 Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a drug at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or even toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are
concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results.
 One proposed method of medicating stents involves the use of a polymeric carrier coated onto the surface of the stent. A composition including a solvent, a polymer dissolved in the solvent, and a drug dispersed in the blend is applied to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent surfaces a coating of the polymer and the drug impregnated in the polymer.
 Apotential shortcoming of conventional medicated stents is that there can be an unequal release of the drug to different areas of the treatment site. For instance, in conventional stents, the concentration of the drug on the stent is essentially constant along the length of the stent. In such drug delivery stents, after the stent is implanted in the biological lumen and the drug is released from the polymeric matrix, the concentration of the drug that is applied to the tissues along the length of the stent will not be constant along the length of the stent. In particular, the drug concentration in the blood stream is higher in the distal region of the biological lumen than the proximal region.
 Referring to FIG. 1, a stent 10 with a polymeric drug coating is implanted into a biological lumen 12, which has a proximal region 14 and a distal region 16. The blood in biological lumen 12 flows from proximal region 14 to distal region 16 as the drug is released from the polymeric coating. If the quantity and release rate of the drug are constant over the length of stent 10, when stent 10 is first implanted into biological lumen 12, the drug concentration in the blood will be constant along the length of stent 10 as graphically illustrated in FIG. 2A. As shown in FIG. 2B, however, over time more drug is released into the blood stream and the drug concentration in the blood in distal region 16 becomes significantly higher as compared to the drug concentration in proximal region 14. As a result, depending on the biological needs of the tissue in the respective regions, the tissue in distal region 16 can receive too much drug whereas the tissue in proximal region 14 may not receive enough drug.
 Another example of a related shortcoming of conventional medicated stents is that there can be an unequal release of the drug to the tissues adjacent to the points of contact between the stent and the tissues. Referring to FIG. 1, stent 10 can have a tubular body of structural members including struts 18 and connecting elements 20 that form a network structure. Struts 18 are radially expandable and interconnected by connecting elements 20 that are disposed between adjacent struts 18. Both struts 18 and connecting elements 20 have an outer (or lumen contacting) surface and an inner surface.
 In conventional stents, the concentration of drugs on the stent is essentially constant along the length of struts 18 and connecting elements 20, including any curved or bent segments. Referring to FIG. 3, when stent 10 is inserted into a biological lumen, stent 10 forms multiple contact points 30 with the tissue as shown with lines A-A and B-B. As the drug in the polymer is released from multiple contact points 30 to the tissue, delivery zones 32 are formed. If the quantity of the drug is the same along lines A-A and B-B, then some of delivery zones 32, for example delivery zone 32A, overlap.
As a result, the tissue area adjacent to the overlapping delivery zones receives a greater quantity of drug than other tissue areas. Therefore, some tissue adjacent to contact points 30 may receive too much drug.
 Accordingly, what is needed is a coating for a stent that addresses the aforementioned drawbacks.
SUMMARY OF THE INVENTION
 In accordance with one aspect of the invention, a stent is disclosed including a body having a first end and a second end and carrying a coating containing a drug. The concentration of the drug in the coating increases along the length of the body of the stent from the first end to the second end.
 In accordance with another aspect of the invention, a stent is disclosed including a body with a first end and a second end and carrying a coating containing a drug. The release rate of the drug from the coating increases along the length of the body from the first end to the second end.
 In a further aspect, a stent is disclosed that includes a body having a coating carrying a drug, where the concentration of the drug in the coating increases along at least a circumference of the body of the stent.
 In accordance with yet aspect of the invention, a strut for a radially expandable stent is disclosed having generally linear segments interrupted by a curved or bent segment. The strut also has a coating containing a drug disposed on the strut, where the concentration of the drug in the coating is greater in at least a portion of the curved or bent segment as compared to the linear segments.
 In another aspect, a method of coating a stent is disclosed, the stent having a first end and a second end. The method includes applying a composition including a drug and a polymer to the stent, where the amount of drug applied to the stent gradually increases along the length of the stent from the first end to the second end.
BRIEF DESCRIPTION OF THE FIGURES
 FIG. 1 is a side view of a conventional stent inserted into a biological lumen;
 FIGS. 2A and 2B are graphs of drug concentrations in blood along the length of the stent illustrated in FIG. 1;
 FIG. 3 is an enlarged view of a portion of the stent illustrated in FIG. 1;
 FIG. 4 is a side view of one embodiment of a stent of the present invention inserted into a biological lumen;
 FIG. 5 is a graph of the drug concentration in a stent coating along the length of the stent in accordance with one embodiment of the invention;
 FIGS. 6A and 6B are graphs of drug concentrations in blood along the length of the stent illustrated in FIG. 4;
 FIG. 7 is a cross-section along line 7-7 in FIG. 4; and
 FIGS. 8 and 9 are an enlarged views of a portion of the stent illustrated in FIG. 4 in accordance with embodiments of the invention.
Variable Drug Concentration or Release Rate Along the Length of the Stent
 The present invention is directed to a stent with a polymeric drug coating having a variable drug concentration or release rate along the length of the stent. Referring to FIG.
4, for example, a stent 40 coated with a polymeric drug coating can have a first segment 42, a second segment 44 and a third segment 46 along the length of stent 40, disposed between a first end 48 and a second end 50. In an embodiment of the present invention, the drug concentration in the coating gradually or incrementally increases along the length of stent 40 from first end 48 to second end 50. FIG.
5, for example, graphically illustrates a drug concentration in a coating that gradually increases along the length of a stent from the distal end of stent 40 to the proximal end of stent 40. The increase of the drug concentration can also be graphically illustrated to be linear or step-wise/incremental. In another embodiment of the present invention, the release rate of the drug from the coating gradually or incrementally increases along the length of stent 40 from first end 48 to second end 50.
 By varying the drug concentration in the polymeric coating, or the release rate of the drug from the polymeric coating, the present invention advantageously provides a coating that provides uniform drug delivery to the target site along the length of stent 40. The distribution of the drug in the blood stream along the length of the stented portion of the biological lumen depends on the Peclet number which is defined as
where V is the velocity of the blood stream, L is the length of the stented portion of the lumen, and D is the diffusivity of the drug in the blood stream. In essence, the Peclet number is the ratio of the convection to diffusion. As can be understood by the above equation, if the velocity of the blood stream is relatively high in comparison to the diffusivity, then much of the drug released from the stent will be carried away by the blood flow from the treatment area adjacent to the stent. However, in a typical blood vessel, the Peclet number is relatively low and diffusion dominated. As a result, the drug released from the stent will remain present in the blood stream at the local treatment site and will be available for absorption by the arterial wall. This is especially advantageous for portions of the arterial walls that are not being contacted by the stent but are being contacted by the blood. However, the relatively low Peclet number in a typical blood vessel results in a higher concentration of the drug in the blood stream in the distal region of the biological lumen as compared to the proximal region if the drug concentration is uniform along the length of the stent.
 A stent coated in accordance with the various embodiments of the present invention can provide a drug concentration in the blood that is initially high in the proximal region of lumen 12, as graphically illustrated in FIG. 6A. However, as shown in FIG. 6B, over time more drug is released into the blood stream and the drug concen