CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of U.S. application Ser. No. 11/810,652, filed on Jun. 5, 2007, and published as U.S. Patent Application Publication No. 2008-0306592 A1, on Dec. 11, 2008, which is incorporated by reference in its entirety, including any drawings, herein.
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OF THE INVENTION
1. Field of the Invention
This invention relates to elastomeric coatings for implantable medical devices.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel.
A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region, such as a lesion, in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen.
In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a constraining member such as a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn which allows the stent to self-expand.
The stent must be able to satisfy a number of mechanical requirements. First, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, is due to strength and rigidity around a circumferential direction of the stent. Radial strength and rigidity, therefore, may also be described as, hoop or circumferential strength and rigidity.
Once expanded, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. For example, a radially directed force may tend to cause a stent to recoil inward. Generally, it is desirable to minimize recoil. In addition, the stent must possess sufficient flexibility to allow for crimping, expansion, and cyclic loading. Longitudinal flexibility is important to allow the stent to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
The structure of a stent is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements often referred to in the art as struts or bar arms. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. The scaffolding is designed so that the stent can be radially compressed (to allow crimping) and radially expanded (to allow deployment). A conventional stent is allowed to expand and contract through movement of individual structural elements of a pattern with respect to each other.
Furthermore, it may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from biodegradable, bioabsorbable, and/or bioerodable materials such as bioabsorbable polymers should be configured to completely erode only after the clinical need for them has ended.
Additionally, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active or bioactive agent or drug. Polymeric scaffolding may also serve as a carrier of an active agent or drug. Potential problems with therapeutic coatings for polymeric implantable medical devices, such as stents, include insufficient toughness, slow degradation rate, and poor adhesion.
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OF THE INVENTION
Certain embodiments of the present invention include an implantable medical device comprising a coating above a polymer surface of the device, the coating comprising: a block copolymer including an elastic block and an anchor block, the elastic block being a homopolymer and elastomeric at physiological conditions, the anchor block being miscible with the surface polymer.
Further embodiments of the present invention include an implantable medical device comprising a coating above a polymer surface of the device, the coating comprising: a elastomeric copolymer including elastic units and anchor units, the elastic units providing elastomeric properties to the copolymer at physiological conditions, wherein the anchor units enhance adhesion of the coating with the surface polymer, wherein the copolymer is a star block copolymer having at least three arms, the arms comprising the elastic units and the anchor units.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 depicts a view of a stent.
FIG. 2A depicts a cross-section of a stent surface with a block copolymer coating layer over a substrate.
FIG. 2B depicts a cross-section of a stent surface with a block copolymer coating layer over a polymeric layer disposed over a substrate of the stent.
FIG. 3 depicts a cross-section of a stent surface with the block-copolymer coating layer over a substrate of the stent showing an interfacial region.
FIG. 4 depicts a cross-section of a stent showing a coating material layer over a swollen surface polymer layer.
FIG. 5 depicts a polymer surface pretreated with a solvent.
FIG. 6 depicts the cross-section of a stent surface with a drug-polymer layer over a block copolymer primer layer disposed over a substrate of the stent.
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OF THE INVENTION
Various embodiments of the present invention include an implantable medical device with a coating having an elastomeric polymer above a polymeric surface of the device. The polymeric surface may be a surface of a polymer coating disposed above a substrate that can be composed of metal, polymer, ceramic, or other suitable material. Alternatively, the polymeric surface may be a surface of a polymeric substrate or body. “Above” a surface is defined as higher than or over a surface measured along an axis normal to the surface, but not necessarily in contact with the surface.
The present invention may be applied to implantable medical devices including, but not limited to, self-expandable stents, balloon-expandable stents, stent-grafts, and grafts (e.g., aortic grafts), and generally expandable tubular devices for various bodily lumen or orifices. A stent can have a scaffolding or a substrate that includes a pattern of a plurality of interconnecting structural elements or struts. FIG. 1 depicts a view of an exemplary stent 100. Stent 100 includes a pattern with a number of interconnecting structural elements or struts 110. In general, a stent pattern is designed so that the stent can be radially compressed (crimped) and radially expanded (to allow deployment). The stresses involved during compression and expansion are generally distributed throughout various structural elements of the stent pattern. The variations in stent patterns are virtually unlimited.
In some embodiments, a stent may be fabricated by laser cutting a pattern on a tube or a sheet rolled into a tube. Representative examples of lasers that may be used include, but are not limited to, excimer, carbon dioxide, and YAG. In other embodiments, chemical etching may be used to form a pattern on a tube.