US 20050033411 A1
An implantable medical device, such as a stent or graft, having asperities on a designated region of its outer surface is disclosed. The asperities can serve to improve retention of one or more layers of a coating on the device and to increase the amount of coating that can be carried by the device. The asperities can be formed by using a stream of pressurized grit to roughen the surface. The asperities can also be formed by removing material from the outer surface, for example, by chemical etching with or without a patterned mask. Alternatively, the asperities can be formed by adding material to the outer surface, for example, by welding powder particles to the outer surface or sputtering.
22. An implantable medical device comprising a substrate having an outer surface, the outer surface including asperities on a designated region of the outer surface so as to provide for a roughness factor greater than 40 nm.
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34. A stent comprising a generally tubular structure having a surface with a roughness factor greater than 40 nm.
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52. An implantable medical device, comprising:
a surface including protrusions and indentations, the protrusions having a height; and
a coating material deposited on the surface so that the height of some of the protrusion is greater than the thickness of the material deposited on the surface.
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1. Field of the Invention
This invention relates to surface features of implantable medical devices, for example stents and grafts, and to methods for forming such surface features.
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 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 radially compress against and remodel the artery wall for dilating the lumen. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.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 procedure includes formation of intimal flaps or torn arterial linings that can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require 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 implantable device, an example of which includes an expandable stent, is implanted in the lumen to maintain the vascular patency. Stents are scaffoldings, usually cylindrical or tubular in shape, functioning to physically hold open, and if desired, to expand the wall of the passageway. Typically stents are compressed for insertion through small cavities via small catheters, and then expanded to a larger diameter once at the desired location. Examples in patent literature disclosing stents include U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
A problem encountered with intravascular stents is that, once implanted into the blood stream, platelets and other blood components tend to adhere to any portion of the stent surfaces having roughness or irregularity. Adhesion and aggregation of platelets and other blood components can lead to thrombosis and restenosis. Therefore, an important aspect of manufacturing and finishing stents is ensuring that all stent surfaces are made extremely smooth, without roughness and irregularities. This is accomplished by highly polishing the entire surface of the stent material, typically by electropolishing or by using an abrasive slurry, as described in U.S. Pat. No. 5,746,691 titled “Method for Polishing Surgical Stents” issued to Frantzen and U.S. Pat. No. 5,788,558 “Apparatus and method for Polishing Lumenal Prostheses” issued to Klein.
To further fight against thrombosis and restenosis, and in treating the damaged vascular tissue, therapeutic substances can be administered. For example, anticoagulants, antiplatelets and cytostatic agents are commonly used to prevent thrombosis of the coronary lumen, to inhibit development of restenosis, and to reduce post-angioplasty proliferation of the vascular tissue, respectively. To provide an efficacious concentration to the treated site, systemic administration of these drugs often produces adverse or toxic side effects for the patient. Local delivery is a highly suitable method of treatment in that smaller levels of medication, as compared to systemic dosages, are concentrated at a specific site. Local delivery therefore produces fewer side effects and achieves more effective results.
One commonly applied technique for the local delivery of therapeutic substances is through the use of medicated stents. A well-known method for medicating stents involves the use of a polymeric carrier coated onto the body of the stent, as disclosed in U.S. Pat. No. 5,464,650 issued to Berg et al., U.S. Pat. No. 5,605,696 issued to Eury et al., U.S. Pat. No. 5,865,814 issued to Tuch, and U.S. Pat. No. 5,700,286 issued to Tartaglia et al. The therapeutic substances are impregnated in, located on, or provided underneath the polymeric coating for release in situ once the stent has been implanted.
An obstacle often encountered with the use of stent coatings is poor adhesion of the polymeric coating to the surface of a stent. During stent delivery, a poorly adhering coating can be rubbed and peeled off of the stent if the coating contacts an arterial wall while the stent is being moved into position. Also, when a coated stent is expanded in situ, the distortion the stent undergoes as it expands can cause the coating to peel, crack, or tear, and disengage from the stent. Poor adhesion of the coating material can promote thrombosis and restenosis, by providing additional surfaces for platelets and other blood components to adhere. Additionally, poor adhesion and loss of the coating also leads to loss of a significant amount of the drugs to be delivered from the coating.
Another technical challenge in using stent coatings to deliver drugs is loading enough drug onto the stent, so that an effective amount of the drug or drug combination is delivered to the treatment site. The total amount of a drug that can be loaded onto a stent in a polymeric coating is limited by the amount of drug that can be mixed into the polymer (the concentration of the drug in the polymer), and the amount of polymer and drug mixture that can be coated onto the stent (the thickness of the coating on the stent for a given stent size). Therefore, a stent that carries more coating can deliver greater amounts of drugs. However, increasing the thickness of a stent coating can be difficult, particularly if the coating does not adhere well to the stent material.
When delivering drugs from a stent, it is also desirable to control the timing and rate of release of the drugs being delivered. Controlled release can be achieved by coating a stent with a number of layers. For instance, each layer can contain a different drug or be made of a polymer that releases drugs at different rates. However, additional layers tend to adhere poorly to underlying layers. The additional layers peel off either during application of the additional layer over an underlying layer, or, as described above, when the stent is delivered and expanded within the artery.
Another complication for drug delivery from a stent is that the arterial wall tissue the stent is compressed against can be tough and fibrotic, preventing medication released from the stent from penetrating the tissue in which the medication may be therapeutically beneficial.
An implantable medical device capable of delivering therapeutic substances from a coating is provided, which provides a high retention of one or more layers of coating material. The implantable device also allows a greater total amount of coating to be carried by the device, allowing for greater amounts of therapeutic substances to be delivered from the device. In some embodiments the implantable device can penetrate the arterial wall to enhance delivery of therapeutic substances into the arterial wall.
In one embodiment within the present invention, the implantable medical device has a generally tubular structure with an inner surface and an outer surface. The outer surface has asperities on designated regions that have roughness factors, Ra, of greater than 40 nm. The designated regions can be the entire outer surface, a middle section of the outer surface, or ends of the outer surface. Typically, the inner surface is smooth.
In various embodiments, the outer surface, or portion thereof, can be coated with a coating containing a therapeutic substance or substances, a polymer, or a combination of therapeutic substances and polymer. The coating can be made of one or more layers and the layers can hold different therapeutic substances, polymers, or combination of therapeutic substances and polymers.
The asperities may have surface protrusions and indentations of various shapes. In some embodiments, the asperities can have sharp tips, be rounded, be square, or a combination of these shapes.
Exemplary embodiments are made by polishing the inner surface of the implantable medical device and forming asperities on the designated region of the outer surface. In some embodiments, the inner surface is polished before the asperities are formed and the inner surface is protected while the asperities are being formed.
In some embodiments, the asperities are formed by projecting grit, which can be beads or sand, at the designated region of the outer surface.
In some embodiments, the asperities are formed by depositing material onto the designated region. In one method, the material is deposited by adding particles to the designated region and bonding the particles, for instance by sintering, to the designated region. In one method, the material is deposited by sputtering. In some embodiments, the material added to the outer surface is radio-opaque.
In some embodiments, the asperities are formed by applying a chemical etchant to the designated region and rinsing the chemical etchant off of the region after a predetermined period of time. The chemical etchant can by applied by sponging or spraying the etchant onto the outer surface. In some embodiments, a patterned mask, which has openings, is applied to the outer surface and then the chemical etchant is applied, allowing the chemical etchant to etch through the openings.
In some embodiments, the asperities are formed by machining or laser cutting.
These and other features and aspects of the various embodiments of the present invention may be better understood in view of the drawings and the following detailed description.
The present invention includes surface treatments for implantable medical devices, such as stents and grafts, often referred to as endoprostheses.
An exemplary graft 120 is illustrated in
In some embodiments, the implantable device may include a single depot, or a plurality of depots, formed in an outer surface thereof. The materials may be filled, fully or partially, with a material, such as a therapeutic substance or polymer, as discussed below.
In some embodiments, depots 130 contain material 132, as depicted in
The conventional practice of having highly polished implantable medical device surfaces contributes to the poor adhesion problem and other problems mentioned above that are associated with coatings used on implantable devices and drug delivery from such coatings. Therefore, in accordance with the present invention, asperities are created on a designated region or regions of the surface of the implantable devices. The asperities cause the designated region to have a roughness factor, Ra, defined below, greater than about 40 nm (nanometers), which is the upper limit of roughness factors typical of polished stent surfaces. Typically, the asperities so formed cause the designated region to have a roughness factor, Ra, greater than about 100 nm.
It is often desirable for the inner surface of the implantable medical device, which contacts the blood flowing through the vessel, to be smooth, in order to prevent aggregation of platelets from the blood on the stent. The outer surface of the implantable devices, which contacts the arterial wall, need not be smooth. Therefore, the designated region or regions having the asperities are typically on the outer surface of the implantable device.
A roughness factor, Ra, is used to quantify the surface roughness. In
The roughness factor, Ra, of asperities on an implantable medical device within the present invention may be measured, for example, by using a commercially available Veeco Metrology Group (Tucson, Ariz.) WYKO NT-2000 system, which is a non-contact optical profiler. VSI (vertical scanning interferometer) mode is used. For cylindrical implantable medical devices, such as stents, cylinder and tilt terms are removed so the stent surface appears flat. A low pass filter is used which removes the effects of high spatial frequency roughness, smoothing over features that are smaller than a nine pixel window. A 50× objective and 2.0× FOV converter sense are used to produce an effective magnification of 100×. This objective and converter combination profiles a 0.58 μm×0.44 μm evaluation area, at a spatial sampling interval of 159.13 nm. For statistical purposes, samples are measures at five separate locations. Equation 1 is used to calculate a value of Ra.
In general, outer surface asperities in accordance with the present invention can have a variety of shapes, some examples of which are illustrated in
As illustrated by the exemplary surfaces in
The asperities can also allow two or more layers of a coating to adhere to an implantable device, as illustrated in
The multiple coating layers of
If the implantable device contains depots 330 (as illustrated in
If the asperities are sharp, having, for instance, sharp protrusions as in 319,
In the discussion that follows, methods are described for forming asperities on the outer surface of a stent, as an example of an implantable medical device. It is to be understood that, in accordance with the invention, these methods can be applied to any implantable medical device, including grafts and other implantable devices.
In one method for manufacturing a stent, a thin-walled, small diameter, cylindrical tube is cut to produce the desired stent pattern. Cutting is typically accomplished through laser cutting, as described in U.S. Pat. No. 5,759,192 “Method and Apparatus for Direct Laser Cutting of Metal Stents” to Saunders, or chemical etching, as described in U.S. Pat. No. 6,056,776 “Expandable Stents and method of Making Same” to Lau et al. Other methods of cutting stent lattice patterns can be used. Instead of cutting the pattern into a tube, the stent lattice pattern can be cut into on a flat sheet of material, which is then roll and joined in a cylindrical configuration.
The tube or flat sheet used to form the stent may be made of any suitable biocompatible material such as a metallic material or an alloy, examples of which include, but not limited to, stainless steel, “MP35N,” “MP20N,” elastinite (Nitinol), tantalum, nickel-titanium alloy, platinum-iridium alloy, gold, magnesium, or combinations thereof. “MP35N” and “MP20N” are trade names for alloys of cobalt, nickel, chromium and molybdenum available from standard Press Steel Co., Jenkintown, Pa. “MP35N” consists of 35% cobalt, 35% nickel, 20% chromium, and 10% molybdenum. “MP20N” consists of 50% cobalt, 20% nickel, 20% chromium, and 10% molybdenum. The stent also may be made from bioabsorbable or biostable polymers.
Often, cutting the stent lattice pattern, especially by laser, leaves scrap that must be removed, in a process known as descaling, to reveal the lattice pattern of the stent. Descaling is typically accomplished by ultrasonically cleaning the stent in heated acid, as described in U.S. Pat. No. 5,759,192. The stent is also polished so the stent surfaces are made as smooth as possible. In conventional processes, a cleaning or rinsing step, usually with an alcohol solution, follows the polishing step to remove any of the polishing solution that may be left on the stent.
Another method that is used to manufacture stents is to mold the stent into the desired shape. This method is especially useful if the stent is made from a polymeric material. Polishing is sometimes done after molding to ensure smooth stent surfaces.
Depots 130, 330 may be formed as a laser trench on the implantable device by exposing the surface to an energy discharge from a laser, such as an excimer laser. Alternative methods of forming such depots 130, 330 include, but are not limited to, physical and chemical etching techniques. Techniques of laser fabrication or etching to form depots are well-known to one of ordinary skill in the art. Depots 130, 330 may be added before or after the asperities are formed
In general, the asperities may be added to the outer surface of the stent at any point in the stent manufacturing process, provided that subsequent processing does not remove the asperities desired in the completed stent.
For example, in one embodiment, the asperities on the stent outer surface are created after the stent has been polished. In this case the smooth inner surface is protected during creation of the asperities to preserve the smoothness of the inner surface. This may be done, for example, by inserting a mandrel or a hypo tube inside the stent or inserting and inflating a balloon inside the stent. The inner surface can also be masked with a temporary protective coating, which is removed after the asperities have been added to the outer surface of the stent. For example, a poly vinyl alcohol (PVA) solution (80% by weight dissolved in hot water at 100° C.) can be applied to the inner surface with a syringe and allowed to air dry, and can be removed by soaking in water. In another embodiment, the asperities are created before the stent pattern is cut. In these cases, the asperities need to be preserved when the stent is cut, descaled, and polished, and therefore this embodiment works well with asperities that are not likely to be destroyed in subsequent processing. If subsequent processing might destroy the asperities, a temporary protective coating can be applied to the outer surface of the stent to preserve the asperities.
If the stent contains depots, the asperities can be formed either before or after formation of the depots, depending on the methods used to form the depots and the asperities. In some methods, if the asperities are formed after the depots, the surface of the depots may also be roughened.
Methods of Treating the Surface
A. Mechanical Methods for Creating Asperities
A variety of methods can be used to create the asperities on the outer surface of the stent.
The roughness factor achieved using a pressurized grit source can be controlled by the size of the grit, e.g., the diameter of the beads, the pressure used, the distance between the grit source and the stent surface, and the length of time the grit is blasted at the surface. By way of example and not limitation, the grit can be beads having a diameter of between 10 μm and 50 μm. Pressures of 30 PSI (pounds per square inch) to 60 PSI can be used to project the beads from a distance of approximately 3-10 cm from the stent. The grit source is passed very quickly, in approximately 1-3 seconds, down and up over the stent outer surface. The stent is then rotated 120° and the process repeated twice so the entire stent is roughened.
Average roughness values achieved are typically in a range of 100 nm to 300 nm, and usefully in a range of 130 nm to 210 nm.
Note also, that the entire outer surface need not be roughened. For example, the grit may be projected at only a middle region 624 (
If the stent contains depots, the depots may be in regions where there are asperities, may be in regions where there are not asperities, or both.
In another embodiment, the asperities may be formed by grinding the tubular stent between two sanding plates or using sandpaper to scratch the outer surface. In this method, the roughness factor achieved depends on the grain size of the surface of the sanding plates. This method may be particularly useful with polymer stents or stents made from relatively soft materials. Typically, this method is used to create asperities after the stent pattern has been cut into the hypotube (or cut into a flat piece of material and the material rolled into a tube), and after the stent has been descaled, if descaling is necessary, and polished.
B. Creating Asperities by Removal of Stent Material
In another embodiment, chemical etching is used to create asperities on the outer surface of a stent. Chemical etching methods are known to those of ordinary skill in the art. The chemical etchant solution typically includes an acid that degrades and dissolves the material from which the stent is made. The chemical etchant is applied to the outer surface, and then rinsed off of the surface after a predetermined period of time. The length of time the etchant is left on the surface depends on the etch rate and the depth of etch desired.
The roughness factor produced by the chemical etchant depends on the method of application of the etchant. In one embodiment, the chemical etchant is applied to the stent outer surface with a fine sponge or by spraying a mist of the chemical etchant onto the stent. A temporary protective coating, e.g., poly vinyl alcohol dissolved in water, is applied to the inner surface of the stent to protect the inner surface from the etchant to keep the inner surface smooth. The chemical etchant is rinsed off of the stent, leaving a rough outer surface. The roughness factor will depend on the stent material and the chemical etchant used, the amount and length of time the etchant was applied, and how the etchant was applied.
In one embodiment, the stent is made of a composite material where one of the components of the composite material can be easily dissolved by the chemical etchant. For example, a metal alloy, such as stainless steel can be used to form the stent and nitric acid can be applied to the outer surface to create the asperities. A hydrochloric acid solution, 37% by volume in water, can also be used to create the asperities.
In another embodiment, a patterned mask that has openings is applied to the stent outer surface before the chemical etchant is applied. The etchant is then applied in a uniform manner and allowed to etch the stent material through the openings of the mask. The use of patterned masks with chemical etchants is known to those of ordinary skill in the art. Because it is usually easier to mask and etch a flat piece of material than a hypo-tube, the mask is typically applied to a flat piece of the stent material and the resulting outer surface pattern etched into the surface. Then, the flat piece of stent material is cut, rolled and joined into a stent. The flat piece of stent material may be polished before the mask and etch process, so that the asperities created by the mask and etch are not degraded in a subsequent polishing process. Additionally, a cutting process that does not leave scrap between the stent struts can be used to eliminate the descaling step, which may also damage the asperities. For example, the stent pattern can be cut using another chemical etching step where the second mask, which protects the asperities, is in the pattern of the stent and the etchant is allowed to dissolve all material not masked.
In this embodiment, the patterned mask can be used to make asperities that are composed of more regular and well-defined shapes. The shapes can be chosen depending on the eventual application of the stent. In one example, a patterned masking followed by etching is used to create asperities composed of relatively sharp tips (319 of
If, when creating the asperities by chemical etching, a significant amount of stent material will be removed, for instance to reduce the average thickness T of the stent strut by more than 10%, a thicker material can be used to form the stent.
Therefore, the finished stent has the desired strut thickness T after etching so that the structural integrity of the stent is not compromised by creating the asperities.
In other embodiments, asperities are formed by machining or laser cutting the outer surface. These methods can also be used to make asperities composed of more regular and well-defined shapes. For example, a laser discharge machine tool can be programmed to cut a desired pattern into the outer surface of a stent.
Because machining and laser cutting will typically remove material from the stent, a thicker material can be used to form the stent, resulting in a completed stent having the desired average strut thickness.
C. Creating Asperities by Adding Material
In another embodiment, additional material is deposited onto the outer surface of the stent to create the asperities. In one method, the additional material, which is typically a metal, is first made into a powder. The powder particles are then bonded to the outer surface to form the asperities. The powder can be made, for example, by grinding the metal to form particles of a predetermined size. The particles are then put onto the outer surface of the stent, for example, by rolling the stent in the powdered metal or spaying the powdered metal onto the stent. The stent can be electrostatically charged to a charge, typically negative, that is opposite that of the powdered metal, to improve the adhesion of the powder to the stent.
The stent coated with the particles is then heated to weld the particles to the stent by methods, for example sintering in a furnace or with a flame, known to those of skill in the art. Typically, the stent material, particle material, and sintering temperature are chosen so that melting temperatures of the materials allow the particles to be welded to the stent without distortion of the stent by the heat treatment. For example, the stent material may be 316L stainless steel and the particles may be gold, aluminum, or copper. The sintering temperature would have to be below the melting temperature of 316L stainless steel, 1388° C.
In this method, the inner surface can be protected by inserting a mandrel or applying a temporary coating to the inner surface that prevents the particles from contacting and staying on the inner surface when they are applied to the stent. The coating, typically a polymeric material, will burn off of the stent in the subsequent heat treatment. The temporary coating can also be used on the outer surface of the stent if it is desired to create asperities on only designated regions of the outer surface. For example, if it is desired to have the asperities adjacent to the ends of the stent (as illustrated in
The roughness factor, Ra, achieved using this method depends on the size of the particles in the powder and is in the range of 50 nm to 5 μm.
The additional material may also be added via physical deposition processes, for example, sputtering, which is known to those of ordinary skill in the art. In sputtering, an energy beam, for instance an ion beam, is directed at a target formed of the material which is to be deposited on the substrate. The energy beam dislodges atoms of the target material. The target material atoms are transported to the substrate, which in this instance is the stent on which asperities are being formed. Once at the substrate, the atoms form “islands”, or small nodules of the deposition material on the substrate.
Conventionally, sputtering is used to form a thin film of the deposition material over the substrate, and sputtering conditions are used such that the “islands” grow, spread, and condense on the substrate to form a thin film. To form the asperities, however, the sputtering process conditions are set so that instead of creating a uniform film over the substrate, the “islands” do not grow, spread, and condense, leaving a rough surface. Process conditions in which a lower pressure and shorter deposition time than is typically used for thin film deposition are used to form the asperities.
Typically, a cut stent is inserted into the sputtering chamber, with inner surface protected, and rotated while material is deposited on the outer surface. Alternatively, material can be deposited onto the hypo-tube before the stent pattern is cut into it, or flat piece of stent material before it is cut and rolled into stent. If the subsequent processing of the stent will destroy the added asperities, a temporary protective coating can be applied to protect them.
The sputtering, however, may only loosely deposit the material onto the outer surface of the stent. In such cases, to further enhance attachment of the deposited material to the substrate, a heat treatment, for instance sintering can be performed. As discussed above, the temperature ranges for heating will depend on the deposition material and the stent material. The roughness factor achieved in this embodiment will have a lower value of approximately 50 nm and an upper value of approximately 200 nm.
Typically, material is added to the outer surface after the stent is polished, although it can be done at any point in the process provided the subsequent processes do not remove the asperities. This will again depend on the material that is used to manufacture stent and the material deposited on the stent.
In one embodiment, radiopaque materials are deposited on the stent, to not only create asperities on the stent, but to allow visualization of the stent after implantation. For example, the radio-opaque material can be applied adjacent to the ends of the stent.
Coatings for Delivering Therapeutic Substances
Methods of coating an implantable medical device with a therapeutic substance or substance, or with a polymer containing one or more therapeutic substances are known to one of ordinary skill in the art. One or more therapeutic substances can be added to an implantable medical device by dissolving or mixing the therapeutic substances in a solvent and applying the therapeutic substance and solvent mixture to the implantable medical device. To cover the implantable device with a polymer containing the therapeutic substance or substance combination, a solution of the polymeric material and one or more therapeutic substances are mixed, often with a solvent, and the polymer mixture is applied to the implantable device. An implantable device can also be coated with a polymer which does not contain a therapeutic substance, for example, to form a sealant layer over an underlying layer, which does contain a therapeutic substance.
Methods of applying the therapeutic substance, polymer, or therapeutic substance and polymer mixture to the implantable medical device are known to those of skill in the art. Methods include, but are not limited to, immersion, spray-coating, sputtering, and gas-phase polymerization. Immersion, or dip-coating, entails submerging the entire implantable device, or an entire section of the implantable device, in the mixture. The implantable device is then dried, for instance in a vacuum or oven, to evaporate the solvent, leaving the therapeutic substance or therapeutic substance and polymer coating on the implantable device. Similarly, spray-coating requires enveloping the entire device, or an entire section of the device, in a large cloud of the mixture, and then allowing the solvent to evaporate, to leave the coating. Sputtering typically involves placing a polymeric coating material target in an environment, and applying energy to the target such that polymeric material is emitted from the target. The polymer emitted deposits onto the device, forming a coating. Similarly, gas phase polymerization typically entails applying energy to a monomer in the gas phase within a system set up such that the polymer formed is attracted to a stent, thereby creating a coating around the stent.
If the implantable device contains depots (130 of
Materials 132 may be deposited into depots using any suitable method. In an exemplary method of depositing material 132 into depots 130, material 132 is added to a first solvent. Material 132 is dispersed throughout the first solvent so that material 132 is in a true solution, saturated or supersaturated with the first solvent or suspended in fine particles in the first solvent. The first solvent can be virtually any solvent that is compatible with material 132. A high capillary permeation and a viscosity not greater than about ten centipoise allows the first solvent to penetrate into depots 130 more quickly, eliminating a requirement to apply the first solvent to the implantable device for a prolonged period of time. Examples of suitable first solvents include, but are not limited to, acetone, ethanol, methanol, isopropanol, tetrahydrofuran, and ethyl acetate. The first solvent containing material 132 is applied to the implantable device, for example by immersing or spraying the solvent using procedures that are well-known to one having ordinary skill in the art. The first solvent containing material 132 is applied for a predetermined period of time, the specific time depending on the capillary permeation and viscosity of the first solvent, the volume of depots 130, and the amount of material 132 to be deposited.
After applying the first solvent containing material 132 for a selected duration, the first solvent is removed from the implantable device using conventional techniques, such as evaporation in ambient pressure, room temperature and anhydrous atmosphere and/or by exposure to mild heat (e.g., 60° C.) under vacuum condition. The implantable device typically has a clustered or gross formation of material 132 gathered on the body surface. The cluster is generally removed by immersing the device in a non-solvent and agitating the implantable device via mechanical perturbation techniques, such as vortexing or vigorous shaking. The non-solvent can have a low capillary permeation or a contact angle greater than about 90° and a viscosity not less than about 0.5 centipoise so that the second fluid is not capable of significantly penetrating into depots 130 during the process of agitation. Examples of a suitable non-solvent include, but are not limited to, saturated hydrocarbons or alkanes, such as hexane, heptane, and octane.
The implantable device is rinsed in a second solvent to facilitate dissolution of material 132. The second solvent generally has a low capillary permeation and a viscosity of not less than about 1.0 centipoise and is therefore incapable of significantly penetrating into depots 130 during the rinsing stage. The rinsing is conducted rapidly for example in a range from 1 second to about 15 seconds, the exact duration depending on the solubility of material 132 in the second solvent. Extended duration of exposure of the implantable device to the second solvent may lead to the penetration of the second solvent into depots 130. The rinsing step is repeated, if desired, until all traces of material 132 are removed from the surface of the implantable device. The second solvent removes excess material 132 from the surface of the implantable device body. Useful examples of second solvents include, but are not limited to, dimethylsulfoxide (DMSO), water, DMSO in an aqueous solution, glyme, and glycerol. The second solvent is removed from the implantable device body using conventional techniques. The first and second solvents as well as the non-solvent are selected to not adversely affect the characteristics and composition of material 132. Although one method of depositing material 132 into depots 130 has been described here, depots 130 may be filled using any suitable method.
Once the depots have been filled, a coating can be applied to the outer surface of the implantable device, as described above.
The polymer used for coating the implantable device and filling depots is typically either bioabsorbable or biostable. A bioabsorbable polymer bio-degrades or breaks down in the body and is not present sufficiently long after implantation to cause an adverse local response. Bioabsorbable polymers are gradually absorbed or eliminated by the body by hydrolysis, metabolic process, bulk, or surface erosion. Examples of bioabsorbable, biodegradable materials include but are not limited to polycaprolactone (PCL), poly-D, L-lactic acid (DL-PLA), poly-L-lactic acid (L-PLA), poly(lactide-co-glycolide), poly(hydroxybutyrate), poly(hydroxybutyrate-co-valerate), polydioxanone, polyorthoester, polyanhydride, poly(glycolic acid), poly(glycolic acid-cotrimethylene carbonate), polyphosphoester, polyphosphoester urethane, poly (amino acids), cyanoacrylates, poly(trimethylene carbonate), poly(iminocarbonate), copoly(ether-esters), polyalkylene oxalates, polyphosphazenes, polyiminocarbonates, and aliphatic polycarbonates. Biomolecules such as heparin, fibrin, fibrinogen, cellulose, starch, and collagen are typically also suitable. Examples of biostable polymers include Parylene®, Parylast®, polyurethane (for example, segmented polyurethanes such as Biospan®), polyethylene, polyethlyene teraphthalate, ethylene vinyl acetate, silicone and polyethylene oxide.
Therapeutic substances can include, but are not limited to, antineoplastic, antimitotic, antiinflammatory, antiplatelet, anticoagulant, antifibrin, antithrombin, antiproliferative, antibiotic, antioxidant, and antiallergic substances as well as combinations thereof. Examples of such antineoplastics and/or antimitotics include paclitaxel (e.g., TAXOL® by Bristol-Myers Squibb Co., Stamford, Conn.), docetaxel (e.g., Taxoteree from Aventis S. A., Frankfurt, Germany) methotrexate, azathioprine, vincristine, vinblastine, fluorouracil, doxorubicin hydrochloride (e.g., Adriamycin® from Pharmacia & Upjohn, Peapack N.J.), and mitomycin (e.g., Mutamycin® from Bristol-Myers Squibb Co., Stamford, Conn.) Examples of such antiplatelets, anticoagulants, antifibrin, and antithrombins include sodium heparin, low molecular weight heparins, heparinoids, hirudin, argatroban, forskolin, vapiprost, prostacyclin and prostacyclin analogues, dextran, D-phe-pro-arg-chloromethylketone (synthetic antithrombin), dipyridamole, glycoprotein IIb/IIIa platelet membrane receptor antagonist antibody, recombinant hirudin, and thrombin inhibitors such as Angiomax™ (Biogen, Inc., Cambridge, Mass.) Examples of such cytostatic or antiproliferative agents include angiopeptin, angiotensin converting enzyme inhibitors such as captopril (e.g., Capoten® and Capozide® from Bristol-Myers Squibb Co., Stamford, Conn.), cilazapril or lisinopril (e.g., Prinivil® and Prinzide® from Merck & Co., Inc., Whitehouse Station, N.J.); calcium channel blockers (such as nifedipine), colchicine, fibroblast growth factor (FGF) antagonists, fish oil (omega 3-fatty acid), histamine antagonists, lovastatin (an inhibitor of HMG-CoA reductase, a cholesterol lowering drug, brand name Mevacor® from Merck & Co., Inc., Whitehouse Station, N.J.), monoclonal antibodies (such as those specific for Platelet-Derived Growth Factor (PDGF) receptors), nitroprusside, phosphodiesterase inhibitors, prostaglandin inhibitors, suranin, serotonin blockers, steroids, thioprotease inhibitors, triazolopyrimidine (a PDGF antagonist), and nitric oxide. An example of an antiallergic agent is permirolast potassium. Other therapeutic substances or agents that may be used include alpha-interferon, genetically engineered epithelial cells, and dexamethasone. In other examples, the therapeutic substance is a radioactive isotope for implantable device usage in radiotherapeutic procedures. Examples of radioactive isotopes include, but are not limited to, phosphoric acid (H3P32O4), palladium (Pd103), cesium (Cs131), and iodine (I125). While the preventative and treatment properties of the foregoing therapeutic substances or agents are well-known to those of ordinary skill in the art, the substances or agents are provided by way of example and are not meant to be limiting. Other therapeutic substances are equally applicable for use with the disclosed methods and compositions.
The following example illustrates a method for creating asperities on an outer surface of a stent, and then coating the stent. The results are compared to the result obtained for a stent coated in the same manner but without a roughened outer surface. The example is given by way of illustration and not limitation.
Stents # 1-5 were fabricated by cutting, descaling, and polishing a 316L stainless steel hypo-tube as described in U.S. Pat. No. 5,759,192. The inner surface of stents # 1-#4 were protected by inserting a mandrel, hypotube, or inflated balloon into the interior of the stent to protect the inner surface. The outer surface of each stent was bead blasted, using bead diameters in the range of 10 μm to 50 μm. Stents #1 and #2 were bead blasted at 60 PSI pressure and stents #3 and #4 were bead blasted at 30 PSI pressure, as indicated in Table 1. Bead blasting was done at a distance of approximately 5 cm from the stent. The beads were passed very quickly, in approximately 1-3 seconds, down and up over the stent outer surface. The stent was then rotated 120° and the process repeated twice so the entire stent was roughened. After bead blasting, stents # 1-# 4 were cleaned by immersion in an ultrasonic isopropyl alcohol bath for 20 minutes and allowed to dry. Stent #5 was also cleaned in an ultrasonic isoproply alcohol bath for 20 minutes and allowed to dry. Uncoated stents #1-#5 were weighed.
A coating solution was made by adding EVOH (ethylene vinyl alcohol copolymer) to DMSO (dimethysulfoxide) in a weight ratio of 1:4 EVOH:DMSO. Tetrahydrofuran (THF) was added to the EVOH:DMSO solution. Tetrahydrofuran constituted 20% by weight of the total weight of the solution. The solution was vortexed and placed in a tube. Stents #1-#5 were each dipped into the solution. The coated stents were passed over a hot plate, for about 3-5 seconds, with a temperature setting of about 60° C. The coated stents were cured in a convection over at 60° C. for one hour. The coating stents were then re-dipped into the solution, passed again over the hot plate as above and cured in the convention over at 60° C. for four hours. The coated stents were weighed and the weight difference between the coated and non-coated stent was calculated. The results are shown in Table 1 below. For the stents #1-#4 the amount of coating on the stent is greater than that of stent #5.
To test the adherence of the coating, stents # 1-# 5 were expanded on a balloon and examined visually with a microscope. No peeling or defects in the stent coatings were observed.
While particular embodiments of the present invention have been shown and described, it will be clear to those of ordinary skill in the art that changes and modifications can be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.