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Elastomeric copolymer coatings for implantable medical devices

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Elastomeric copolymer coatings for implantable medical devices


Implantable medical devices with elastomeric copolymer coatings are disclosed.

Browse recent Abbott Cardiovascular Systems Inc. patents - Santa Clara, CA, US
Inventor: Yunbing Wang
USPTO Applicaton #: #20120330404 - Class: 623 138 (USPTO) - 12/27/12 - Class 623 
Prosthesis (i.e., Artificial Body Members), Parts Thereof, Or Aids And Accessories Therefor > Arterial Prosthesis (i.e., Blood Vessel) >Absorbable In Natural Tissue

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The Patent Description & Claims data below is from USPTO Patent Application 20120330404, Elastomeric copolymer coatings for implantable medical devices.

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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.

BACKGROUND 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.

SUMMARY

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

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.

DETAILED DESCRIPTION

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.

An implantable medical device can be made partially or completely from a biodegradable, bioabsorbable, biostable polymer, or a combination thereof. A polymer for use in fabricating an implantable medical device can be biostable, bioabsorbable, biodegradable or bioerodable. Biostable refers to polymers that are not biodegradable. The terms biodegradable, bioabsorbable, and bioerodable are used interchangeably and refer to polymers that are capable of being completely degraded and/or eroded when exposed to bodily fluids such as blood and can be gradually resorbed, absorbed, and/or eliminated by the body. The processes of breaking down and absorption of the polymer can be caused by, for example, hydrolysis and metabolic processes.

As indicated above, a medicated implantable medical device, such as a stent, may be fabricated by coating the surface of the device with a drug. For example, a stent can have a coating including a drug dispersed in a polymeric carrier disposed over a substrate of the stent. Such a coating layer may be formed by applying a coating material to a substrate of an implantable medical device, such as a stent. The coating material can be a polymer solution and a drug dispersed in the solution. The coating material may be applied to the stent by immersing the stent in the coating material, by spraying the material onto the stent, or by other methods known in the art. The solvent in the solution is then removed, for example, by evaporation, leaving on the stent surfaces a polymer coating impregnated with the drug.

Stents are typically subjected to stress during use. “Use” includes manufacturing, assembling (e.g., crimping a stent on balloon), delivery of a stent through a bodily lumen to a treatment site, deployment of a stent at a treatment site, and treatment after deployment. Both the underlying scaffolding or substrate and the coating experience stress that result in strain in the substrate and coating. In particular, localized portions of the stent\'s structure undergo substantial deformation. For example, the apex regions of bending elements 130, 140, and 150 in FIG. 1 experience relatively high stress and strain during crimping, expansion, and after expansion of the stent.

Furthermore, polymer substrates and polymer-based coatings may be particularly vulnerable to mechanical instability during use of a stent. Such mechanical instability for coatings can include fracture and detachment from a substrate, for exampling, peeling. Some polymers may be susceptible to such mechanical instability due to insufficient toughness at high deformations. Additionally, detachment of coatings may be due to poor adhesion of the polymer-based coating to the substrate or another polymer layer. Therefore, polymer-based coatings are highly susceptible to tearing or fracture, and/or detachment, especially at regions subjected to relatively high stress and strain. Thus, it is important for a polymer-based coating to (1) be tough and have a high resistance to cracking and (2) have good adhesion with an underlying layer or substrate and to have a high resistance to detachment in the range of deformations that occur during crimping, during deployment of a stent, and after deployment.

As indicated above, a device may be composed in whole or in part of materials that degrade, erode, or disintegrate through exposure to physiological conditions within the body until the treatment regimen is completed. The device may be configured to disintegrate and disappear from the region of implantation once treatment is completed. The device may disintegrate by one or more mechanisms including, but not limited to, dissolution and chemical breakdown. The duration of a treatment period depends on the bodily disorder that is being treated. For illustrative purposes only, in treatment of coronary heart disease involving use of stents in diseased vessels, the duration can be in a range from about a month to a few years. However, the duration is typically in a range from about six to twelve months. Thus, it is desirable for polymer-based coatings and substrates of an implantable medical device, such as a stent, to have a degradation time at or near the duration of treatment. Degradation time refers to the time for an implantable medical device to substantially or completely erode away from an implant site.

Embodiments of the present invention can include an elastomeric polymer coating disposed over a polymer surface of a device, such as a stent scaffolding. In certain embodiments, the coating can be disposed directly over the surface of a polymer substrate of a device. FIG. 2A depicts a cross-section of a stent surface with an elastomeric polymer coating layer 210 over a substrate 200. In the embodiment shown in FIG. 2A, elastomeric polymer coating layer 210 includes a drug 220 dispersed in an elastomeric polymer 230. The substrate can be composed of a bioabsorbable polymer.

In other embodiments of the present invention, the elastomeric polymer coating can be over a polymer coating layer that is disposed over a substrate. FIG. 2B depicts a cross-section of a substrate 240 of a stent with a polymeric layer 250 disposed over substrate 240. An elastomeric polymer coating layer 260 is disposed over polymeric layer 250. Coating layer 260 includes a drug 270 dispersed within an elastomeric polymer 280. Polymeric layer 250 can be a primer layer for improving the adhesion of drug-polymer layer 260 to substrate 240. In the embodiment of FIG. 2B, substrate 240 can be metallic, polymeric, ceramic, or other suitable material.

In certain embodiments of the present invention, the elastomeric polymer coating can include a block copolymer having an elastic block and an anchor block. In such embodiments, the elastic block is a homopolymer that exhibits elastomeric or rubbery behavior at physiological conditions. In addition, the anchor block is miscible with the surface polymer and enhances the adhesion of the block copolymer coating with the surface polymer. In some embodiments, the elastic block, the anchor block, or both can be bioabsorbable polymers. In certain embodiments, all or a majority of the coating may be the block copolymer. Additionally, the coating can be a therapeutic layer with an active agent or drug mixed or dispersed within the block copolymer.

As mentioned above, the block copolymer coating exhibits rubbery or elastomeric behavior at physiological conditions. An “elastomeric” or “rubbery” polymer refers to a polymer that exhibits elastic deformation through all or most of a range of deformation. Physiological conditions include, but are not limited to, human body temperature, approximately 37° C. The elastic block of the block copolymer is an elastomeric or rubbery polymer that allows or provides the elastomeric or rubbery properties of the coating. Such elastomeric properties provide the coating with a high fracture toughness during use of a device such as a stent.

In some embodiments, the elastic blocks can have a glass transition temperature (Tg) below body temperature. Additionally, the block copolymer may be completely or substantially amorphous. Exemplary biodegradable polymers that are elastomeric or rubbery at physiological conditions include, but are not limited to, polycaprolactone (PCL), poly(tetramethyl carbonate) (PTMC), poly(4-hydroxy butyrate) (PHB), and polydioxanone (PDO).

As discussed above, the anchor block of the block copolymer can be miscible with the surface polymer. In one embodiment, the anchor block can have the same chemical composition as the surface polymer. Alternatively, the anchor block can have a chemical composition different from the surface polymer, but similar enough so that the anchor block is miscible with the surface polymer. In an exemplary embodiment, the block copolymer can have a PLLA anchor block and be disposed over a PLLA surface, which can be the surface of a PLLA substrate. In another exemplary embodiment, the block copolymer can have a PLLA anchor block and be disposed over a poly(L-lactide-co-glycolide) (LPLG) surface, which can be the surface of an LPLG substrate.

In certain embodiments, the anchor block can be a random copolymer. In such embodiments, the composition of the anchor block copolymer of the block copolymer coating can be selected so that the anchor block is miscible with the surface polymer. In addition, the units of the copolymer can be selected to adjust the degradation rate of the block copolymer. In one embodiment, the anchor block can include units that are more hydrolytically active or hydrophilic than other units to increase the degradation rate of the coating. In an exemplary embodiment, the anchor block can be LPLG. In such an embodiment, the surface polymer can be an LPLG copolymer. The composition of LLA and GA in the anchor block can be adjusted so that the LPLG anchor block is miscible with the LPLG surface polymer. In some embodiments, the surface polymer can be a copolymer having a high percentage of LLA units, for example, at least 60 wt %, 70 wt %, or 80 wt % LLA units.

In additional embodiments, the block copolymer can additionally include a fast degrading block that is selected to increases the degradation rate of the block copolymer coating. In some embodiment, the fast degrading blocks can be glassy at physiological conditions or have a Tg above body temperature. Additionally or alternatively, the fast degrading blocks can be immiscible with the surface polymer.

In some embodiments, the fast degrading block may have a higher degradation rate than the anchor block, the elastic block, or both. The fast degrading block may be composed of units that are more hydrophilic or more hydrolytically active than the elastic block or the anchor block. Additionally, fast degrading block may have acidic and hydrophilic degradation products. Since the rate of the hydrolysis reaction tends to increase as the pH decreases, acidic degradation products can increase the degradation rate of the block copolymer coating. Glycolide (GA) units, for example, have acidic degradation products which can increase the degradation rate of the coating. Exemplary fast degrading blocks can include poly(glycolide) (PGA) and LPLG that may not be miscible with a surface polymer.

In some embodiments, the toughness of the block copolymer coating can be adjusted by increasing or decreasing the weight percent of elastic blocks. As the weight percent of elastic blocks increases, the block copolymer can become more flexible and tougher. For example, for a PCL-b-PLLA coating, as the weight percent of PCL increases, the block copolymer becomes more flexible and tougher. The composition of the elastic blocks of the block copolymer can be greater than 20 wt %, 30 wt %, 40 wt %, 50 wt %, 60 wt %, 70 wt %, 80 wt %, 90 wt %, or greater than 90 wt % of the block copolymer.

In exemplary embodiments, the molecular weight of the elastic blocks can be between 20 kg/mol and 150 kg/mol, or greater than 150 kg/mol. The molecular weight of the anchor blocks can be between 20 kg/mol and 150 kg/mol, or greater than 150 kg/mol. The relative weight percent of the elastic blocks and the anchor blocks can be between 1:5 and 10:1.

Additionally, in other embodiments, the degradation rate of the coating can be adjusted by increasing or decreasing the weight percent of fast degrading blocks. The degradation rate of the coating can be increased by increasing the weight percent of fast degrading blocks. For example, the weight percent of PGA in a PLLA-b-PGA-b-PDO block copolymer can be increased to increase the degradation rate of the polymer.

Embodiments of the block copolymer of the elastomeric coating can have two or more blocks. The block copolymer can be a diblock, triblock, tetrablock, pentablock, etc. copolymer. Diblock copolymers can include, for example, PLLA-b-PDO, PLLA-b-PCL, and PLLA-b-PTMC. Exemplary triblock copolymers include PLLA-b-PDO-b-PLLA, PLLA-b-PCL-PLLA, and PLLA-b-PTMC-b-PLLA, PLLA-b-PGA-b-PDO, etc. Such block copolymers may be suitable as coatings over a PLLA or LPLG surface.

In some embodiments, the block copolymer can be a branched polymer which corresponds to a polymer with “side chains.” Branched polymers include, for example, hyperbranched-like polymers, comb-like polymers, star polymers, dendrimer-like star polymers, and dendrimers. A star polymer refers to a polymer having at least three chains or arms radiating outward from a common center. A dendritic polymer is a branched polymer resembling a tree-like structure. A comb structure corresponds to a linear polymer segment or backbone having a plurality of side chains extending outward from a position along the linear segment. In such embodiments, a block copolymer can be a branched polymer with at least one branch that is an elastic block and at least one branch that is an anchor block. The branched block copolymer can further include at least one branch that is a fast degrading block.

In these embodiments, the block copolymer can be a star block copolymer having at least three arms or branches with at least one arm being an elastic block and at least one arm being an anchor block. The star block copolymer can further include at least one arm that is a fast degrading block.



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stats Patent Info
Application #
US 20120330404 A1
Publish Date
12/27/2012
Document #
13598465
File Date
08/29/2012
USPTO Class
623/138
Other USPTO Classes
International Class
61F2/82
Drawings
3



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