Endoprosthesis having a fiber meshwork disposed thereon

[0001] This invention relates to medical devices, such as endoprostheses, and methods of making such devices.

[0002] The body includes various passageways including blood vessels such as arteries, and other body lumens. These passageways sometimes become occluded or weakened. For example, they can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is an artificial implant that is typically placed in a passageway or lumen in the body. Many endoprostheses are tubular members, examples of which include stents, stent-grafts, and covered stents.

[0003] Many endoprostheses can be delivered inside the body by a catheter. Typically the catheter supports a reduced-size or compacted form of the endoprosthesis as it is transported to a desired site in the body, for example the site of weakening or occlusion in a body lumen. Upon reaching the desired site the endoprosthesis is installed so that it can contact the walls of the lumen.

[0004] One method of installation involves expanding the endoprosthesis. The expansion mechanism used to install the endoprosthesis may include forcing it to expand radially. For example, the expansion can be achieved with a catheter that carries a balloon in conjunction with a balloon-expandable endoprosthesis reduced in size relative to its final form in the body. The balloon is inflated to deform and/or expand the endoprosthesis in order to fix it at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn.

[0005] In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded (e.g., elastically or through a reversible phase transition of its constituent material). Before and during introduction into the body until it reaches the desired implantation site, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired site, the restraint is removed, for example by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.

[0006] To support or keep a passageway open, endoprostheses are sometimes made of relatively strong materials, such as stainless steel or Nitinol (a nickel-titanium alloy), formed into struts or wires. The material from which an endoprosthesis is made can impact not only the way in which it is installed, but its lifetime and efficacy within the body.

[0007] A medical device, comprising: an underlying structure having a fiber meshwork disposed thereon, wherein the underlying structure is biodisintegrable. The medical device can further comprise a layer-by-layer coating situated on the fiber meshwork.

[0008] A medical device for implantation into an organism, comprising: an underlying structure; and a fiber meshwork disposed upon the structure, wherein the fiber meshwork is configured to ensure steady biodisintegration of the underlying structure over a period of time inside the organism.

[0009] A medical device comprising a fiber meshwork formed of a material that is deposited by FFESS on to an underlying structure.

[0010] A method of effecting controlled release of a pharmaceutically active agent from a medical device, wherein the medical device is implanted in an organism, comprising: depositing a fiber meshwork containing a therapeutic agent on an underlying structure, wherein the fiber meshwork degrades over a period of time inside the organism and simultaneously releases the therapeutic agent.

[0011] A method of achieving steady disintegration of a medical device in an organism, comprising: forming the device by depositing a fiber meshwork on to an underlying structure; and implanting the device into the organism; wherein the disintegration of the underlying structure occurs over a first period of time inside the organism.

[0012] A method of making a medical device comprising depositing a fiber meshwork by FFESS on to an underlying structure.

[0013] A method of using a medical device that comprises an underlying structure having a fiber meshwork disposed thereon, wherein the underlying structure is Biodisintegrable, the method comprising implanting the medical device in a body passageway of an organism.

[0014] The various details of one or more embodiments of the invention are set forth in the accompanying drawings and the description hereinbelow. Other aspects, features, and advantages of the invention will be apparent from the description and drawings, and from the claims.

[0021] Like reference symbols in the various drawings indicate like elements.

[0022] Although endoprostheses have been highly effective at removing restrictions in body passageways, a number of problems have emerged that arise from their long-term placement. Restenosis is one; another is that, over time, microbes and other material can build up on a structure such as a stent and cause their own obstruction to free passage of body fluids through the lumen. Restenosis has been addressed by coating a stent with a polymer containing a drug that inhibits growth of smooth muscle cells (see, e.g., “The Billion Dollar Stent,” Technology Review, 108(10), 41, (October 2005)). Accumulations of unwanted material have been deterred by loading the device structure with an anti-microbial agent, but with limited success. Accordingly, there has been a move towards making endoprostheses out of bio-absorbable materials, such as magnesium or iron alloys and biodegradable polymers, that ensure that the device structure naturally degrades over time. Such materials may, however, disintegrate too quickly thus requiring them to be made out of thicker elements than would be preferred. On the other hand, the presence of a coating on the device, such as is typically used to achieve local delivery of a therapeutic agent at the implantation site, can hinder degradation of the underlying device structure. For example, with coating patterns that are based on a closed polymer matrix, the coating impedes fluid access to the device surface, thereby significantly delaying—or even preventing inception of—the desired degradation process. In many instances, it would be preferable to have the underlying structure disappear (or at least start to disappear) before the coating has disappeared. This would also ensure a much longer delivery regime of a therapeutic agent absorbed in the coating than the timeframe over which the device structure disappears.

[0023] On the other hand, polymer coatings that disappear in a very short timeframe, thereby offering the exposure of the underlying device structure required to bring about its degradation, have several attendant drawbacks. First, the rapid decay of the coating leads to a massive initial drug release. The consequent inability to control the process of polymer disintegration, also entails the risk of a highly variable drug release profile during the process. Slight variations in various environmental parameters such as temperature, fluid flow-rate, and local concentrations of critical agents, can cause a huge difference in the degradation course of different regions of exposed surface area. Finally, if the polymer coating disintegrates too rapidly, the device structure itself may also. disintegrate in a non-uniform manner so that large particles loosen, break off and float into the bloodstream, causing boli and secondary blockages.

[0024] Accordingly, the devices herein address such issues by incorporating material that is strong enough to last longer than the underlying structure so as to prevent both its uncontrolled breakdown and non-uniform drug release, yet which is flexible enough to permit regulated access of bodily fluids to the underlying structure.

Definitions

[0025] A biocompatible material is a material that can be introduced into living tissue or a living system, and is non-toxic or non-injurious to the tissue or system, and does not cause an immunological reaction or rejection.

[0026] As used herein, a “biodisintegrable material” is a biocompatible material that undergoes at least one of dissolution, degradation, absorption, erosion, corrosion, resorption, chemical transformation, or other disintegration processes over the period that the device formed at least in part from the biodisintegrable material is designed to reside in an organism. Chemical transformation can include oxidation or other chemical reactions of the stent body material.

[0027] Specifically, a biodisintegrable material is a material that exhibits substantial mass or density reduction by one or more of dissolution, degradation, absorption, erosion, corrosion, resorption, decomposition, degeneration, chemical transformation and/or other disintegration processes after it is introduced into an organism. The disintegration occurs to a desirable extent in a timeframe that can provide a clinical benefit. Mass reduction of a biodisintegrable device can also occur, but in some cases does not occur, by fragmentation of the material. The disintegration can be the result of the chemical and biological interaction of the material with the physiological environment into which it is implanted and/or can be initiated by applying a suitable triggering influence, such as a chemical reactant or source of energy to the stent.

[0028] In some embodiments, a biodisintegrable material for use with the present invention exhibits substantial mass reduction after a period of time for which a function of the material, such as support of a lumen wall or delivery of a therapeutic agent in the immediate vicinity of the device, is no longer needed or desirable. By “a substantial reduction” is meant that the biodisintegrable material exhibits a mass reduction through biodisintegration of at least about 10%, at least about 20%, at least about 25%, at least about 30%, at least about 50%, at least about 75%, or at least about 90%, after a period of implantation. The period of implantation over which the mass reduction through biodisintegration takes place can be chosen to be one day or more, 14 days or more, 30 days or more, 60 days or more, 90 days or more, 180 days or more, 300 days or more, 600 days or more, or about 1,000 days or less. Thus, it would be understood that the level of biodisintegrability can be tailored to achieve a given level of mass reduction over a certain desired duration. For example, a medical device may be required to have reached a 75% reduction in mass in 30 days. In another embodiment, it may be required to have attained a 30% reduction in mass in 180 days. It would also be understood by one of ordinary skill in the art that a period of days, such as 300 days, as used herein, entails a level of imprecision such that periods of 3-5 days either shorter or longer than the period in question are also acceptable equivalent timescales for measuring levels of biodisintegrability.

[0029] In certain embodiments of the present invention, only portions of the device exhibit biodisintegrability. For example, an exterior layer or coating may be non-biodisintegrable, while an interior layer or body is biodisintegrable.

[0030] A degradable material is a material that can dissociate, depolymerize, or otherwise reduce in molecular weight from its starting molecular weight, such that a resulting compound is soluble in an aqueous medium such as water or, if insoluble, can be suspended in a body fluid and transported away from an implantation site without obstructing the flow of the body fluid. A biodegradable material is one that will degrade into biocompatible compounds as part of a biological process.

[0031] In some embodiments, a biodegradable material exhibits substantial mass reduction after a period of time for which a function of the material, such as support of a lumen wall or delivery of a therapeutic agent in the immediate vicinity of the device, is no longer needed or desirable. By “a substantial reduction” is meant that the biodegradable material exhibits a mass reduction through biodegradation of at least about 10%, at least about 20%, at least about 25%, at least about 30%, at least about 50%, at least about 75%, or at least about 90%, after a period of implantation. The period of implantation over which the mass reduction through biodegradation takes place can be chosen to be one day or more, 14 days or more, 30 days or more, 60 days or more, 90 days or more, 180 days or more, 300 days or more, 600 days or more, or about 1,000 days or less. Thus, it would be understood that the level of biodegradability can be tailored to achieve a given level of mass reduction over a certain desired duration. For example, a material may be required to have reached a 25% reduction in mass in 600 days. In another embodiment, it may be required to have attained a 30% reduction in mass in 300 days. It would also be understood by one of ordinary skill in the art that a period of days, such as 180 days, as used herein, entails a level of imprecision such that periods of 3-5 days either shorter or longer than the period in question are also acceptable equivalent timescales for measuring levels of biodegradability.

[0032] A resorbable material is a material that is soluble, biodisintegrable as defined herein, or is an aggregate of soluble and/or disintegrable material(s) with insoluble material(s) such that, with the resorption of the soluble and/or disintegrable materials, the residual insoluble materials are of sufficiently fine size that they can be suspended in a body fluid and transported away from the implantation site without obstructing the flow of the body fluid. Ultimately, the particles are eliminated from the body either by excretion in fluids such as perspiration, urine or feces, or are themselves dissolved, degraded, corroded or otherwise metabolized into soluble components that are then excreted from the body. A bioresorbable material is a resorbable material that is biocompatible.

[0033] The term “body fluid” as used herein refers to fluids in the body of an organism—especially a mammal—including, but not limited to, blood, urine, saliva, lymph, plasma, gastric, biliary, or intestinal fluids, seminal fluids, and mucosal fluids or humors.

[0034] The terms “therapeutic agent”, “pharmaceutically active agent”, “pharmaceutically active material”, “pharmaceutically active ingredient”, “drug” and other related terms may be used interchangeably herein and include, but are not limited to, small organic molecules, peptides, oligopeptides, proteins, nucleic acids, oligonucleotides, genetic therapeutic agents, non-genetic therapeutic agents, vectors for delivery of genetic therapeutic agents, cells, and therapeutic agents identified as candidates for vascular treatment regimens, for example, as agents targeting restenosis.

[0035] By small organic molecule is meant an organic molecule having 50 or fewer carbon atoms, and fewer than 100 non-hydrogen atoms in total.

[0036] As used herein, an “antimicrobial agent” is any agent that is harmful to microbes, especially pathogenic bacteria.

[0037] As used herein, “treatment” refers to the prevention of a disease or condition, the reduction or elimination of symptoms associated with a disease or condition, or the substantial or complete elimination of a disease or condition.

[0038] In certain embodiments, as further described herein, biostable materials, e.g., polyelectrolytes, may be utilized. As used herein, a “biostable material” is a material that does not undergo substantial dissolution, degradation, absorption, erosion, decomposition, corrosion, chemical transformation, resorption and/or other disintegration processes over the period that the material is designed to reside in an organism.

Overview

[0039] Medical devices having a fiber meshwork disposed upon an underlying structure such as a mechanical support, and methods of making the devices are disclosed. Two views of an exemplary device 1 are shown in FIGS. 1A and 1B, in which an underlying structure 10 is covered at least in part by a fiber meshwork 20 comprising one or more fibers (not individually depicted in FIGS. 1A or 1B) that may optionally contain or be coated with material containing at least one therapeutic agent such as a pharmaceutically active ingredient. Exemplary device 1 is generally tubular in shape and as depicted may be, e.g., a stent. As shown in the exploded view of FIG. 1B, the underlying structure 10 may optionally be coated by a multi-layer structure 30, in one or more layers of which is optionally contained a therapeutic agent. The fiber or fibers of which the fiber meshwork 20 is composed may also optionally be coated with a multi-layer structure 40 as shown in FIG. 1B. Multi-layer structure 40 may also optionally contain at least one therapeutic agent in one or more of its constituent layers.

[0040] In some embodiments, the underlying structure 10 is bio-disintegrable and is made from a different material than is the fiber meshwork 20. For example, the underlying structure can include (e.g., be formed of) a biodisintegrable metal or a biodisintegrable polymer, as described in Bolz, U.S. Pat. No. 6,287,332; Heublein, U.S. Patent Application Publication No. 2002/0004060 A1; U.S. Pat. No. 5,587,507; and U.S. Pat. No. 6,475,477. The fiber meshwork can wrap around the underlying structure without totally encapsulating it, thereby permitting fluid access to it. The fiber meshwork can be of a sufficient strength to retain its shape for a longer time than that of the underlying structure itself. If the mechanical support is bio-disintegrable, it is thereby allowed to degrade in a controlled manner, while the fiber meshwork retains its structure.

[0041] Fiber meshwork 20 can be formed from a material impregnated with a therapeutic agent that releases such an agent slowly over time. Alternatively, the fiber meshwork 20 is overcoated with a multilayer structure. The multilayer structure can be a layer-by-layer (LBL) coating wherein one or more layers contain a therapeutic agent. The fiber meshwork is thus able to ensure both a controlled release of a therapeutic agent absorbed within it or coated over it, as well as a controlled degradation of the underlying structure. A still further advantage of deploying a fiber meshwork impregnated with a drug or coated with layers containing a drug is that, when used in conjunction with an endoprosthesis whose underlying structure itself has a lattice or mesh structure, or similar, it can provide a more uniform release profile of the drug in the region of the stent than, e.g., a coating directly deposited on such a lattice or mesh structure. The latter may tend to release higher concentration of drug along the struts of the endoprosthesis than in the gaps in between such struts.

[0042] In alternative embodiments, the underlying structure 10 is also covered with a multilayer structure such as a LBL coating.

[0043] In an alternative embodiment, the fiber meshwork acts as an enclosing envelope to regulate release of a therapeutic agent from loosely packed particulate materials situated within it.

[0044] The fiber meshwork can be created with electro-spun nano-fibers or micro-fibers, and by flow-limited field-injection electrostatic spraying (FFESS), as further described herein.

Fiber Meshwork

[0045] The fiber meshwork can be amorphous, i.e., does not possess a regular structure. Thus, the fiber meshwork contains one or more fibers. Each individual fiber can be considered to consist of a number of contiguous segments. Each fiber wraps around the underlying structure in such a manner that one of its segments crosses either another segment of the same fiber or another fiber at least once. In this way, the fiber meshwork is composed of a number of fiber segments that cross one another, when viewed from a given direction.

[0046] In some embodiments, the fiber meshwork comprises a single fiber. However, equally satisfactory fiber meshworks may be utilized that comprise more than one fiber. The fiber meshwork can utilize 10 or fewer fibers, such as 2, 3, 5, or 7 fibers. From 10 to about 100 fibers can also be used, for example, 20, 50, or 80 fibers are effective. In such embodiments, not all of the fibers need to be made of the same material. Each of such fibers may be as long as several millimeters, or may be as long as about 1 centimeter, or as long as about 5 centimeters, or as long as about 10 centimeters, and may even be as long as about 20 centimeters. Where more than one fiber is present, it is not necessary that every fiber have the same or a similar length. Thus it is possible that several fibers are used, all having lengths of between 5 and 7 mm. It is also possible that several fibers are used, having lengths of 5 mm, 2 cm, 5 cm, and 10 cm.

[0047] A fiber meshwork can also be constructed based on a very large number of short lengths of fibers. Such fibers may be as short as about 10 micron, or may be from about 10 to about 100 μ, but the overall meshwork may consist of many thousands, and may even consist of many hundreds of thousands of such fibers.

[0048] It is not required that the fiber meshwork itself envelop the entirety of the exterior surface of the underlying support. Alternatively, the fiber meshwork, when suitably rigid, can be manufactured to extend beyond the dimensions of the underlying structure, thereby permitting delivery of a therapeutic agent contained therein to regions of the body lumen outside of those supported by the underlying structure.

[0049] The nature of the fiber meshwork can be such that there are minute gaps between the various segments of fiber so that body fluids can contact the underlying structure. The fiber meshwork thus has an effective porosity that results from its structure in a manner different from that of other materials, for example a polymer matrix. Advantageously, the benefits of the fiber meshwork arise from a combination of its porosity and durability. Regarding porosity, defined as the proportion of the non-solid volume to the total volume of material, the fiber meshwork is able to attain a porosity higher than that of a polymer matrix of comparable volume. In some embodiments, the porosity of the fiber meshwork is in the range 0.1-0.6, and in other embodiments it is in the range 0.2-0.4. By contrast with a polymer matrix, however, the fiber meshwork is harder and less flexible, thereby retaining more of its structure as an underlying structure degrades.

[0050] The fiber meshwork can retain a therapeutic agent and allow for its release over time. The nature of the fiber meshwork is such that it has a large surface area and thus is capable of releasing uniformly a therapeutic agent absorbed within it or in a layer coated on top of it.

[0051] The fiber meshwork may comprise a polymeric material such as a polymer, or a blend of polymers. A “polymer” is any macromolecule composed of two or more monomers, and includes dimers, trimers, tetramers, etc. A “monomer” is a polymerizable molecule. Typically, the polymeric materials comprise polymer molecules having a median number of monomers that numbers in the tens (10 to 99), in the hundreds (100 to 999), in the thousands (1,000 to 9,999), or in the tens of thousands (10,000 to 99,999) as well as a mixture of polymers having different median numbers of monomers. The polymeric materials can comprise polymer molecules having a median number of monomers that is 100,000 or more.

[0052] Such polymers may be crosslinked or uncrosslinked, linear or branched, natural or synthetic, thermoplastic or thermosetting, and may be biostable, biodegradable, bioabsorbable, biodisintegrable, or dissolvable.

[0053] In some embodiments, the polymers used to form the fiber meshwork are biodegradable. For example, the polymers can substantially degrade over a period of time that is different from the period of time over which the underlying structure substantially biodisintegrates. In such embodiments, the polymers of the fiber meshwork biodegrade over a longer period of time than the period of time for which the underlying structure biodisintegrates. In other embodiments, the polymers are biostable.

[0054] Polymers for use in the fiber meshwork can include any polymer that is biocompatible, and in particular a polymer that is compatible with the specific implantation environment in instances where local conditions are unusually hostile to non-native materials. An exemplary polymer is poly(D,L-lactide-co-glycolide) (‘PLG’). Other polymers are further described herein. Still other exemplary polymers include, but are not limited to: poly(lactic acid); poly(glycolic acid); poly(caprolactone); poly(hydroxybutyrate); poly(orthoester); poly(alkane anhydride); gelatin collagen; oxidized cellulose; and poly(phosphazene).

[0055] The fiber meshwork may also be constructed from other materials that are biodegradable, including metals. Accordingly, in certain embodiments the fiber meshwork is made from magnesium fibers. In other embodiments, the fiber meshwork is made from fibers of iron, or zinc. Additionally, fibers of metal alloys are contemplated.

[0056] Typically the fiber meshwork is coated onto the underlying support so that it forms from about 10% to about 50% by weight of the entire device. For example, the fiber meshwork forms about 20% to about 30% by weight of the entire device weight.

[0057] In certain embodiments, the polymers from which the fiber meshwork is formed lead to self-supporting fibers so that, as shown schematically in FIG. 2A, at such time as the underlying structure 10 has substantially biodisintegrated, the fiber meshwork 20 retains its original shape or a close approximation thereof. By so doing, the fiber meshwork does not collapse and obstruct free passage of bodily fluids through the lumen in which it is situated. Instead, the fiber meshwork retains a support structure that helps to maintain the structure of the lumen itself where previously the underlying structure of the endoprosthesis had provided support. It is also envisaged that, in certain environments, the fiber meshwork becomes embedded, either partially or completely, in layers of cells as may be found on the internal surface of a body passageway. In such an eventuality, the fiber meshwork is reinforced, even after the underlying structure has degraded, and has a reduced propensity to collapse. Furthermore, by retaining its original shape or a close approximation thereof, the fiber meshwork is able to continue release of a therapeutic agent contained within it, as further described herein. By the term “close approximation thereof” as used herein, it is understood that the fiber meshwork will naturally distort overtime but it is intended that it does so to only a small extent.

[0058] The structure of the fiber meshwork produced with a method such as FFESS is such that fluid access to the surface of the mechanical support is permitted. The fiber meshwork itself can be chosen to be biostable or very slowly degrading, to act as an enclosing envelope for the more rapidly disintegrating device structure, thereby addressing the problem of uneven device degradation, as shown in FIG. 2B for the example of a stent. Furthermore, as also shown in FIG. 2B, were an underlying structure 10 to disintegrate unevenly, to give large particles such as 12, 14, those particles may be captured by the fiber meshwork 20 before floating off into the body fluid. Biostable materials for forming the fiber meshwork include, for example, styrene-isobutylene-styrene (‘SIBS’), silicone elastomers; poly(ethylene-co-vinyl acetate); and Polyacrylates.

[0059] In another embodiment, the underlying structure comprises a first ring and a second ring, wherein the first ring and the second ring are connected to one another by the fiber meshwork. In another variant of such an embodiment, the first ring is made of a first material and the second ring is made of a second material. In still another variant, the first material and the second material are different from a material of which the fiber meshwork is comprised. In an exemplary embodiment, the first and second ring are arranged coaxially to one another. It is to be expected, however, that due to flexibility of the fiber meshwork, the first and second ring may not be always situated exactly coaxially with respect to one another, and may also not always be disposed so that their respective planes are parallel to one another. For example, the centers of the respective rings may be offset with respect to one another and/or the planes of the rings may be disposed at an angle other than 180° with respect to one another. Such variations are to be expected within both manufacture and operation of the embodiment.

[0060] In such embodiments in which the underlying structure comprises two rings separated from one another, and connected to one another, by the fiber meshwork, the overall device has a flexibility imparted by the fiber meshwork and thus can, e.g., bend if necessary in a curved or flexible section of a body lumen. Such an embodiment has still further application in body lumens where the support function may be unnecessary, but delivery of a therapeutic agent, such as may be absorbed within the fibers of the fiber meshwork, is desired.

[0061] The fiber meshwork may also itself degrade in a controllable manner and therefore be used to fine-tune the disintegration of the underlying structure. As the fiber meshwork itself degrades, the biodisintegrable material of the underlying structure can become more exposed to water in the bodily fluids that surround it. This increased exposure to water can cause the underlying structure to begin to disintegrate, or to disintegrate more rapidly. Eventually, the underlying structure may disintegrate entirely.

[0062] Furthermore, the fiber meshwork can be comprised of polymers to which a surface charge can readily be attached, or which are naturally charged such that a multi-layer structure can be disposed thereon, using a layer-by-layer method that builds up layers of alternating charges, as further described herein.

FFESS

[0063] The fiber meshwork may be deposited by a number of methods of creating and depositing extremely thin fibers, known to one of ordinary skill in the art. Such methods are typically related to electrospraying. In essence, in electrospraying, an electric field is used to drive charged particles to a grounded substrate. A charged solution exits from a spray nozzle in a conical configuration, referred to as the Taylor cone. If the Taylor cone doesn't break up, the solution forms fibers; otherwise the material becomes individual spray droplets. The two principal parameters that control behavior of the Taylor cone are the viscosity and voltage at the nozzle. Exemplary methods of creating ultra-thin fibers for use in creating a fiber meshwork involve electro-spinning. Electro-spinning methods are described in Shin, Hohman, Brenner, and Rutledge, “Experimental Characterization of electrospinning: the electrically forced jet and instabilities”, Polymer 42, 9955-9967, (2001), incorporated herein by reference in its entirety. Fibers that are micrometers in diameter can be created by melt spinning or gel spinning, i.e., they are formed out of a gel or a molten melt.

[0064] A particularly exemplary method of depositing the fiber meshwork, is to use a process referred to as flow-limited field-injection electrostatic spraying (FFESS). FFESS is a form of electrospraying which offers a very high degree of control over shape and flow regimes, and which allows spinning a fiber-meshwork on top of a medical device, such as an endoprosthesis, with a glass spray nozzle. The nozzle generates a charge at the liquid meniscus that enables successful electrospray. A schematic comparison of apparatuses used respectively for conventional electro-spraying and FFESS is shown in FIG. 3 in which a structure to be sprayed is denoted 201. The two principal differences are first that FFESS sprays a polymer/solvent solution 230 from a smooth glass capillary 200 whereas CES uses a metal hypodermic needle 210, and second that FFESS uses a sharpened tungsten needle 220 inside capillary 200, whereas CES has no analogous structure. The overall effect of the FFESS apparatus is to improve jet stability and uniformity of the polymer sprayed by FFESS relative to that from CES.

[0065] Using the FFESS method for electro-spinning creates a fiber meshwork in which the one or more fibers have a highly controlled fiber diameter. In particular, as would be understood by one of ordinary skill in the art, by controlling the voltage, flow-rate, concentration of polymer in the spray fluid, the viscosity of the spray fluid, and the distance of the nozzle from the surface of the underlying structure 10, the diameter of the fibers formed during the spinning process can be controlled. For exemplary descriptions of the various factors, see, e.g., “Electrostatic Spinning and Properties of Ultrafine Fibers”, Rutledge, et al., National Textile Center Annual Report, M01-D22, (November 2001), incorporated herein by reference. See also further description on the internet at www.che.vt.edu/Wilkes/electrospinning/electrspinning.html. It is also consistent with the fiber meshwork that the diameter of the fibers can be changed during deposition.

[0066] A further advantage of FFESS is thus that, because of the high degree of control of the fiber diameter, if the weight of the fiber meshwork as well as the density of the polymer material for a given fiber diameter are known, the total surface area of the meshwork can be precisely calculated. Thus, the surface area of a fiber of diameter d, and of length l, assuming a uniform perfectly cylindrical constant cross-section along its length, is πdl, ignoring contributions from the ends of the fibers. FFESS is further described in “Controlling surface nano-structure using flow-limited field-injection electrostatic spraying (FFESS) of poly(d,l-lactide-co-glycolide)”, Berkland, Pack, and Kim, Biomaterials, 25: 5649-5658, (2004) and U.S. Patent Application Publication No. 2004/0022939, both of which are incorporated herein by reference in their entirety.

[0067] The median diameter of the fiber or fibers in the fiber meshwork can be less than about 1 micron, for example, less than about 500 nm. In some embodiments, the fibers have a median diameter from about 5 nanometers to about 500 nanometers. For example, the fibers can have a median diameter of about 50 nm, about 100 nm, about 150 nm, about 200 nm, about 250 nm, about 300 nm, about 350 nm, about 400 nm, or about 450 nm. By use of the qualifier “about”, it would be understood by one of ordinary skill in the art that a reference to a median diameter of, for example, about 300 nm, would encompass other median diameters within +/−25 nm of 300 nm. It is to be understood that where a fiber is not perfectly circular in cross-section, the diameter of a particular fiber can refer to an average length of any two orthogonal lines that both pass through the geometric center of the fiber cross-section and have end points on the perimeter of the fiber, or to the length of any one such line. It is also to be understood that although the diameter of a fiber can vary along its length, the diameter is the mean diameter of the fiber.

[0068] Other morphologies could be utilized to the extent that they embody the properties desired of a polymer structure for placing on the surface of an underlying structure 10. As further discussed herein, such properties include at least an ability to substantially retain shape after the underlying structure has biodisintegrated, a porosity sufficient to permit access of body fluids to the underlying structure, and a sufficient surface area to give rise to sustained release of a therapeutic agent absorbed within the polymer structure or from layers coated upon it. Other morphologies of polymer such as particles, beads, porous sheets, webs, nets and smooth surfaces may be created with FFESS, as further illustrated in Berkland, et al., Biomaterials, 25: 5649-5658, (2004) and U.S. Pat. App. Pub. No. 2004/0022939. A web or net-like structure for the fiber meshwork is advantageous because it reduces the chance that short individual fibers can become disconnected from the underlying structure.

[0069] It is also consistent with the fiber meshwork used herein that the fiber is deposited as a mixture of fibers and beads. As further described herein, a fiber composed of a biodegradable material in which is embedded a therapeutic agent will release the agent at a rate that depends upon the diameter of the fiber. It can be advantageous to mix in amongst such fibers beads of material which, being thicker than the fibers, will release the therapeutic agent over a more prolonged period of time. One of ordinary skill in the art is capable of manipulating the electrospinning process to ensure production of beads at one time, fibers at one time, or a mixture of beads and fibers. See, e.g., Shenoy, S. L., et al., “Role of chain entanglements on fiber formation during electrospinning of polymer solutions: good solvent, non-specific polymer-polymer interaction limit”, Polymer, 46:3372-3384, (2005), in particular FIG. 1 therein, and Tan, et al., “Systematic parameter study for ultra-fine fiber fabrication via electrospinning process”, Polymer, 46:6128-6134, (2005), both of which are incorporated herein by reference.

[0070] There are various options, familiar to one of ordinary skill in the art, for introducing a therapeutic agent into the fiber meshwork. A first option is to place the therapeutic agent inside the fiber material as it is being sprayed by a process such as FFESS. Thus the drug may be dissolved in the polymer solution before it is sprayed. This approach is also advantageous because it is possible to spray different therapeutic agents from different fibers using different nozzles to achieve simultaneous delivery of a mixture of drug components, as shown in FIG. 4. For example, at the top of FIG. 4 are shown three embodiments of a stent having disposed thereon, from left to right, a fiber meshwork made from a single fiber, two fiber meshworks made from fibers of the same thickness, and two fiber meshworks made from fibers of different thicknesses wherein the thicker fibers are only disposed at the ends of the stent. The lower portion of FIG. 4 shows two embodiments of a stent, having fiber meshworks composed of fibers of different thicknesses disposed thereon and, below each, drug release profiles showing concentration of released drug (vertical axis) as a function of time (horizontal axis). On the left hand side, the same drug is loaded into fibers of different thicknesses, and on the right hand side, different drugs are loaded into fibers of different thicknesses.

[0071] The level of control of fiber diameter achievable with FFESS is also useful because, as further described herein, release of a therapeutic agent absorbed in such fibers is directly related to the diameter of the fibers. The porous structure of the fiber meshwork disposed on the underlying structure allows all of the fibers to release their drug content directly to the surrounding milieu at the same time. Furthermore, by ensuring a specific fiber diameter, a specific release profile can be achieved. In other embodiments, use of FFESS permits creation of a differential release profile across the medical device. For example, by spraying a thinner fiber-structure at the end sections of the device, one can obtain a higher drug release at the ends by virtue of a higher surface area to volume ratio there.

[0072] A second option for introducing a therapeutic agent into the fiber meshwork is to embed Magnetite nano-particles into the fibers. Such nano-particles are ferro-magnetic and would themselves permit binding of magnetic drug delivery particles to the fiber matrix by methods within the capability of one of ordinary skill in the art. For examples of magnetic drug delivery particles, see, e.g., “Functionalisation of magnetic nanoparticles for applications in biomedicine”, Berry, C. C., et al., J. Phys. D: Appl. Phys., 36 R198-R206, (2003), incorporated herein by reference in its entirety. In one such embodiment, as a pre-requisite to facilitating such a delivery mechanism, one would start with a dispersion of a desired nanoparticle in a polymer-solvent solution; the solution would be spun into fibers, such as with FFESS, such that the fibers themselves have the nanoparticles incorporated therein.

[0073] The fiber meshwork may also contain a nanocomposite instead of or in addition to a purely polymeric material. “Nanocomposite” refers to a composition comprising a polymeric material and relatively small amounts (generally less than about 10% by weight) of nanometer-sized (average size smaller than 1 micrometer) mineral, clay, or nanosized ceramic particles dispersed therein. Sometimes nanocomposites are referred to as “nanoclay” or “nanoceramic”. For example, nanocomposites are disclosed in International Publication No. WO 93/1014118, and U.S. Pat. Nos. 5,385,776, and 6,251,980 all of which are incorporated herein by reference in their entirety. These particles may themselves contain therapeutic agents that are released as the polymer of the meshwork degrades.

[0074] Another approach is to use liposomes, which are embedded in the fiber meshwork. Methods of embedding liposomes into thin films have been described elsewhere, and are generally known to one of ordinary skill in the art (see, e.g., Vermette et al., “Immobilized liposome layers for drug delivery applications: inhibition of angiogenesis”, J. Controlled Release, 80:179-195 (2002)). Liposomes are lipid-bilayer bound vesicles, typically less than 1 micron in diameter, for example, in the range 50-400 nm in diameter, that can be used as drug-delivery vehicles. Typically such liposomes are electrically charged: both cationic and anionic liposomes are known in the art as drug delivery vehicles. Therapeutic agents can be encapsulated into liposomes prior to implantation in an organism, and slowly leech out as the liposome degrades. An advantage of attaching capsules to the PEI fiber meshwork instead of attaching them to the stent surface (e.g., by means of coating the stent with self-assembled poly-electrolyte multi-layers), is that if the capsules are large enough compared to the porosity of the fibrous network, then the capsules can be trapped in between the stent and the vessel wall.

[0075] Charged liposomes can bind to a fiber meshwork as described herein by, for example, coating the fiber meshwork with a layer-by-layer composition (of alternating cationic and anionic layers) such that the outermost (last-deposited) layer has an opposite charge to that of the liposome. Thus after coating, if fibers having an anionic outer layer (for example) are dipped into a solution of cationic liposome particles, the liposomes become bound to the fiber meshwork. It is also consistent with the medical device described herein that liposomes can be bound by magnetic attraction to a fiber meshwork. For example, as described hereinabove, fibers of the fiber meshwork can comprise magnetic nanoparticles. Liposomes having magnetite particles (see, e.g., Matsuoka, F., et al., “Hyperthermia using magnetite cationic liposomes for hamster osteosarcoma”, BioMagnetic Research and Technology, Vol. 2, p. 3 et seq., (2004), incorporated herein by reference) can bound to such a fiber meshwork.

[0076] In still other embodiments, as an alternative to liposomes, polyelectrolyte multilayer capsules may be used as transporters of drugs that may be embedded in a fiber meshwork. Drug-filled polyelectrolyte capsules can be prepared by any one of several methods known to one of ordinary skill in the art, (see, e.g., Antipov, A. A., and Sukhorukov, G. B., “Polyelectrolyte multilayer capsules as vehicles with tunable permeability”, Adv. Colloid and Interface Sci., 111, 49-61,(2004)).

[0077] The use of drug-containing capsules such as liposomes and polyelectrolyte multi-layer capsules addresses one of the drawbacks of devices in the art such as drug-eluting stents. Such devices offer only a non-homogeneous drug delivery profile because the drug is supplied by the framework of the device, such as the struts of a stent. This mechanism leaves large areas in between the struts free of the drug. Although there is some movement of the drug from the area immediately adjacent to the struts to the open cell area, there is always a difference in concentration between such regions. Trapping drug filled capsules in between a fiber meshwork and the wall of the body lumen gives a much more homogeneous release profile. This is particularly important in applications such as abluminal drug release.

[0078] In an alternative embodiment, a therapeutic agent is introduced into a medical device via a layer-by-layer (LBL) coating process which overcoats the fiber-meshwork with one or more alternating single molecular layers of cationic and anionic materials, as further described herein. Many bioactive molecules, such as proteins, enzymes, DNA, are charged, and can be readily included in these types of coatings. A number of LBL coatings, as shown in FIGS. 5 and 6, and as further described herein, can be used. The LBL coating process can permit coating of highly complex 3D structures without webbing, and is able to penetrate inside complex structures. Webbing is an effect that occurs when a very viscous solution is applied to coat very fine wires. The coating will form a kind of webbing between one fiber and another, which can be undesirable. Since layer by layer technology uses low viscous water-based solutions, it can penetrate into all manner of geometries without the risk for webbing. An example is described in, e.g., Artyukhin, et al., Langmuir, 20, 1442-1448, (2004), and H. Kong, et al., “Polyelectrolyte-functionalized multiwalled carbon nanotubes: preparation, characterization and layer-by-layer self-assembly”, Polymer 46:2472-2485, (2005), both of which are incorporated by reference herein in their entirety, and which demonstrate overcoating of multiwall carbon nano-tubes with several layers of different molecules. This process is carried out in a batch process without the fibers sticking together, as shown in FIG. 5. A functionalized multi-wall carbon nanotube (MWNT) 300 such as MWNT-COOH, MWNT-PAA (where PAA is polyacrylic acid), or MWNT-PSS (where PSS is poly(sodium 4-styrenesulfonate)) can accept a number of layers of different charge, disposed thereon. For example, HPSA (hyperbranched polysulfone amine) and PDMAEMA (poly(2-(N,N-dimethylaminoethyl) methacrylate), both of which are positively charged, can be disposed on a MWNT functionalized with negatively charged groups such as carboxylates. Thereafter, alternating layers of PSS (negatively charged) and PDMAEMA or HPSA can be further built up. Each of HPSA, PSS or PDMAEMA can be disposed as a fiber meshwork. The formulae of PSS, HPSA, and PDMAEMA are shown in FIG. 5B.

[0079] In any of the embodiments herein, the fiber meshwork is able to elute a drug contained within it, or absorbed in a layer upon it, over a longer period of time than the lifetime of the mechanical support.

Layer-By-Layer Technology

[0080] The fiber-meshwork sprayed by a method such as FFESS can be overcoated with several layers, one or more of which contains a therapeutic agent, using layer-by-layer (LBL) technology. The layer-by-layer coating process produces a multi-layered structure such as 40, in FIG. 1B, on the fiber meshwork. Advantages of such an arrangement include a controlled release profile of the therapeutic agent. For example, if desired, a profile may be designed to include a first phase, characterized by a massive release of the agent from the top LBL coating by virtue of its large surface area, followed by a secondary release of the agent out of deeper layers of the LBL coating, or from the fiber meshwork itself as also discussed herein. Adjustment of concentrations of the therapeutic agent at various layers, taking into account their respective thicknesses, solubilities, and surface areas, can ensure a specific desired release profile. LBL coatings are also very effective in preventing degradation of the fiber meshwork, because they follow complex surfaces precisely, such as those of the fiber meshwork, and they are composed out of charged molecules which impede free motion of water molecules through the coating.

[0081] In some embodiments, the use of an LBL coating on the underlying structure 10, either instead of or as well as one on the fiber meshwork, allows control of the disintegration (e.g., corrosion) of the underlying structure over a desired timeframe. This may be important if it is wished to maintain the mechanical integrity of the device for such a defined timeframe. FIG. 6 shows an underlying structure 10 having a fiber meshwork 20 disposed thereon. The fibers of fiber meshwork can also have a layer-by-layer coating 40 disposed thereon, as shown in the various cutouts. The underlying structure 10 can also have a layer-by-layer structure 30 disposed thereon. The bottom of FIG. 6 shows a schematic of a device having multiple fiber meshworks 600 disposed on an underlying structure (denoted “support”). Each of the fiber meshworks 602-612 alternates in charge to facilitate deposition. Peptide molecules are denoted “P” and are attached to fibers of one of the fiber meshworks 612. It is to be understood that molecules other than peptides can be similarly attached to one or more of the fiber meshworks.

Layer by Layer Coatings

[0082] In some embodiments, at least one of the charged layers includes a polyelectrolyte. Polyelectrolytes are polymers having charged groups or groups that dissociate to form ions (also called polyions). The number of these groups in a polyelectrolyte can be so large that the polymer is soluble in polar solvents (including water) when in ionic form. One or more charged layers can include one type of polyelectrolyte or different types of polyelectrolytes.

[0083] Depending on the type of dissociable groups, polyelectrolytes can be classified as polyacids and polybases.

[0084] When dissociated, polyacids form polyanions, after protons have been released. Polyacids include inorganic and organic molecules, as well as biopolymers. Examples of polyacids are polyphosphoric acids, polyvinylsulfuric acids, polyvinylsulfonic acids, polyvinylphosphonic acids, and polyacrylic acids. Examples of the corresponding salts, which are called polysalts, are polyphosphates, polyvinylsulfates, polyvinylsulfonates, polyvinylphosphonates and polyacrylates, respectively.

[0085] Examples of anionic polyelectrolytes (polyanions) include poly(styrenesulfonate) polyanions (e.g., poly(sodium styrene sulfonate) (PSS)), polyacrylic acid polyanions, sodium alginate polyanions, eudragit polyanions, gelatin polyanions, hyaluronic acid polyanions, carrageenan polyanions, chondroitin sulfate polyanions, and carboxymethylcellulose polyanions.

[0086] Polybases contain groups that are capable of accepting protons, e.g., by reaction with an acid, to form a salt. By accepting protons, polybases form cationic polyelectrolytes (polycations).

[0087] Examples of polybases having dissociable groups within their backbone and/or side groups are polyallylamine, polyethylimine, polyvinylamine and polyvinylpyridine.

[0088] Still other examples of polyelectrolyte cations (polycations) include protamine sulfate polycations, poly(allylamine) polycations (e.g., poly(allylamine hydrochloride) (PAH)), polydiallyldimethylammonium polycations, polyethyleneimine polycations, chitosan polycations, gelatin polycations, spermidine polycations and albumin polycations

[0089] Some polyelectrolytes have both anionic and cationic groups, but nonetheless have a net positive or negative charge. An example of such a polyelectrolyte is gelatin. Some polyelectrolytes have combinations of both anionic and cationic groups such as to maintain electrical neutrality. Whether a polyelectrolyte having both anionic and cationic groups has a net positive or negative charge can depend, for example, on the pH of the environment surrounding the polyelectrolyte.

[0090] The polyelectrolytes for use with layer-by-layer coatings can include those based on biopolymers. Examples include alginic acid, gum arabicum, nucleic acids, pectins, proteins, chemically modified biopolymers such as carboxymethyl cellulose, and lignin sulfonates.

[0091] The polyelectrolytes for use with layer-by-layer coatings can also include those based on synthetic polymers, such as polymethacrylic acid, polyvinylsulfonic acid, polyvinylphosphonic acid and polyethylenimine.

[0092] Polyelectrolytes may be linear or branched, either or both of which can be used. Using branched polyelectrolytes can lead to less compact polyelectrolyte multilayers having a higher degree of wall porosity. In some embodiments, polyelectrolyte molecules can be crosslinked within and/or between the individual layers, to enhance stability, e.g., by crosslinking amino groups on one polymer molecule with aldehyde moieties on another.

[0093] Furthermore, in certain embodiments, polyelectrolytes can be amphiphilic. Amphiphilic substances can include any substance having hydrophilic and hydrophobic groups. Amphiphilic polyelectrolytes, e.g., amphiphilic block or random copolymers having partial polyelectrolyte character, can be used in some embodiments to affect permeability towards polar small molecules. In some embodiments, a layer including an amphiphilic polyelectrolyte may be more permeable to polar molecules than a layer including a polyelectrolyte that is not amphiphilic.

[0094] Amphiphilic polyelectrolytes can be used as ionic amphiphilic substances in some embodiments. For example, a polyelectrolyte comprising charged groups (which are hydrophilic) as well as hydrophobic groups, such as polyethylenimine (PEI) or poly(styrene sulfonate) (PSS), can be employed.

[0095] Cationic and anionic surfactants may also be used as amphiphilic substances in some embodiments of the layer by layer coatings used with the present invention. Cationic surfactants include quaternary ammonium salts (R₄N⁺X⁻), where R is an organic radical and where X⁻ is a counter-anion, e.g., a halide. Thus, examples of cationic surfactants include didodecyldimethylammonium bromide (DDDAB); alkyltrimethyl-ammonium bromides such as hexadecyltrimethylammonium bromide (HDTAB), dodecyltrimethylammonium bromide (DTMAB), myristyltrimethylammonium bromide (MTMAB), or palmityltrimethylammonium bromide; tertiary amines (R₃NH⁺X⁻), such as cholesteryl-3β-N-(dimethyl-aminoethyl)-carbamate; and N-alkylpyridinium salts; or mixtures thereof. Anionic surfactants include alkyl or olefin sulfates of general formula R—OSO₃ ⁻M⁺ where M⁺ is a metal counter-ion. Examples include a dodecyl sulfate such as sodium dodecyl sulfate (SDS), and a lauryl sulfate such as sodium lauryl sulfate (SLS). Anionic surfactants further include an alkyl or olefin sulfonate of general formula R—SO₃ ⁻M⁺, for example, sodium-n-dodecylbenzene sulfonate. Still further examples of anionic surfactants include fatty acids (of formula R—COO⁻M⁺, where R is an organic radical and M⁺ is a counter-cation), for example, a dodecanoic acid sodium salt, or other acids including phosphoric acids, cholic acids, fluoro-organic acids such as lithium-3-[2-(perfluoroalkyl)ethylthio]propionate, or mixtures thereof.

[0096] Exemplary polyelectrolytes for use with the multi-layer structures have molecular weights ranging from a few hundred Daltons (low-molecular weight polyelectrolytes) up to several million Daltons, as commonly found with macromolecular polyelectrolytes (e.g., polyelectrolytes of synthetic or biological origin).

[0097] When disposed on the underlying device structure, the polyelectrolyte layers can restrict or prevent water molecules and/or certain ions from accessing the biodisintegrable materials of the underlying device structure. In certain embodiments, polyelectrolytes can, in addition to the fiber meshwork, prevent the biodisintegrable materials from disintegrating prematurely (e.g., during delivery and/or deployment of the medical device to a target site). In still other embodiments, such layers include one or more therapeutic agents. In still other embodiments, one or more layers contain a radiopaque material, and/or one or more layers capable of enhancing the mechanical properties of the device structure.

[0098] In some embodiments, the layers of the multi-layer structure disposed on the underlying structure include biodegradable polyelectrolytes that degrade at a slower rate than the rate of disintegration of the biodisintegrable material of the underlying structure. As the underlying structure disintegrates, it may break into multiple pieces. Because the layers of polyelectrolytes degrade at a slower rate than the biodisintegrable material of the underlying structure, at least some of the layers may limit or prevent movement of these multiple pieces to other places in the body, causing the pieces to disintegrate and be resorbed instead at the implantation site within the body.

[0099] In some embodiments, a layer formed of a biodegradable polyelectrolyte can degrade over a period of at least about one second (e.g., at least about 10 seconds, at least about 30 seconds, at least about one minute, at least about 10 minutes, at least about one hour, at least about five hours, at least about 10 hours, at least about one day, at least about two days, at least about four days, at least about six days), and/or at most about one week (e.g., at most about six days, at most about four days, at most about two days, at most about one day, at most about 10 hours, at most about five hours, at most about one hour, at most about 10 minutes, at most about one minute, at most about 30 seconds, at most about 10 seconds).

[0100] In some embodiments, biodegradable polyelectrolytes can be used so that a therapeutic agent can be released into the subject at a rate that is dependent upon the rate of degradation of the polyelectrolyte layers. For example, by using polyelectrolytes that are biodegradable near the outer surface of the medical device, this rate can be initially highest. Biodegradable polyelectrolytes can also be used in embodiments in which the underlying structure, and/or a fiber meshwork disposed thereon, is itself biodisintegrable. As the biodegradable polyelectrolytes in layers of the coating disintegrate, they may provide less protection for the underlying device structure. As a result, the structure can begin to disintegrate or can disintegrate at a faster rate.

[0101] Examples of biodegradable polyelectrolytes include heparin, polyglycolic acid (PGA), polylactic acid (PLA), polyamides, poly-2-hydroxy-butyrate (PHB), polycaprolactone (PCL), poly(lactic-co-glycolic)acid (PLGA), protamine sulfate, polyallylamine, polydiallyldimethylammonium species (e.g., poly(diallyldimethyl-ammonium chloride) (PDADMA, available from Aldrich)), polyethyleneimine, chitosan, eudragit, gelatin, spermidine, albumin, polyacrylic acid, sodium alginate, poly(styrene sulfonate) (PSS, Scientific Polymer Products), hyaluronic acid, carrageenan, chondroitin sulfate, carboxymethylcellulose, polypeptides, proteins, DNA, and poly(N-octyl-4-vinyl pyridinium iodide) (PNOVP). Biodegradable polyelectrolytes are described, for example, in T. R. Farhat and J. B. Schlenoff, “Corrosion Control Using Polyelectrolyte Multilayers”, Electrochemical and Solid State Letters, 5(4) B13-B15 (2002), incorporated by reference herein.

[0102] In other embodiments, the layers are made from biostable polyelectrolyte materials, or the entire multi-layer structure is constructed so as to be biostable. Thus if, for example, various layers are cross-linked to each other so that the multi-layered structure is biostable, the underlying structure may disintegrate over a given period of time, while the multi-layer structure remains in the body of the subject. Accordingly, in certain embodiments, an endoprosthesis can include two or more polyelectrolyte layers that are cross-linked to each other. The cross-linked polyelectrolyte layers may be used, for example, to confine a biodisintegrable material in the underlying structure. In certain embodiments, this confinement of the biodisintegrable material may limit the likelihood that one or more pieces of the biodisintegrable material will break away from the endoprosthesis during use and move to a location other than the target site.

[0103] In some embodiments, a medical device can include a structure (e.g., a multi-layered structure) having a combination of cross-linked polyelectrolytes and biodegradable polyelectrolytes, to provide further tailoring of the disintegration of the device.

[0104] In certain embodiments, a biodegradable polyelectrolyte in one layer can be cross-linked (e.g., using heat and/or UV radiation) to another biodegradable polyelectrolyte in another layer. In some embodiments, cross-linking between polyelectrolytes in different layers can cause the polyelectrolytes to degrade at a slower rate than they would otherwise. In certain embodiments, a layer including a cross-linked polyelectrolyte can degrade over a period of at least about one week (e.g., at least about two weeks, at least about three weeks, at least about four weeks, at least about six weeks, at least about eight weeks, at least about 10 weeks, at least about 12 weeks, at least about 14 weeks, at least about 16 weeks, at least about 18 weeks, at least about 20 weeks, at least about 22 weeks), and/or at most about 24 weeks (e.g., at most about 22 weeks, at most about 20 weeks, at most about 18 weeks, at most about 16 weeks, at most about 14 weeks, at most about 12 weeks, at most about 10 weeks, at most about eight weeks, at most about six weeks, at most about four weeks, at most about three weeks, at most about two weeks).

[0105] As an example of cross-linking layers, a polyelectrolyte layer including diazonium cations may be covalently cross-linked to a polyelectrolyte layer including sulfonate groups or acrylic acid groups, using UV radiation or heat. As another example, a polyelectrolyte layer including a diazo resin may be cross-linked to a polyelectrolyte layer including polyoxometalates. As an additional example, ammonium groups in one polyelectrolyte layer may be covalently bonded to carboxylate groups in another polyelectrolyte layer. In certain embodiments, polyelectrolyte layers including poly(allylamine hydrochloride) (PAH) can be covalently bonded to polyelectrolyte layers including poly(acrylic acid) (PAA). Cross-linking of polyelectrolyte layers is described, for example, in Zhang et al., “Improving multilayer films endurance by photoinduced interaction. between Dawson-type polyoxometalate and diazo resin”, Materials Chemistry and Physics, 90 (2005), 47-52, and in Zhang et al, “Ways for fabricating stable layer-by-layer self-assemblies: combined ionic self-assembly and post chemical reaction”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 198-200, 439-442, (2002), both of which are incorporated herein by reference in their entirety.

[0106] In some embodiments, one or more of the top polyelectrolyte layers on a medical device can be cross-linked. This can, for example, limit or prevent degradation or dissolution of the multi-layer structure on the medical device in the body. For example, multiple layers of polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) can be deposited on a plurality of other polyelectrolyte layers. The entire multi-layered structure can then be heated at 130° C. for about an hour under a nitrogen atmosphere to crosslink the ammonium groups of the PAH and the carboxylic groups of the PAA to form amide bonds. A nylon-like top film that is impermeable to liquids can be created. In certain embodiments, this liquid-impermeable top film can later be removed (e.g., using excimer ablation).

[0107] In some embodiments, one or more regions of a polyelectrolyte layer may be cross-linked to one or more regions of another polyelectrolyte layer (e.g., by selectively irradiating certain regions of the polyelectrolyte layers), while the polyelectrolyte layers may not be cross-linked to each other in other regions.

[0108] In certain embodiments in which a multi-layered structure is biostable, the multiple layers may have one or more (e.g., two, three, four, five, 10, 15, 20, 25, 50, 100, 500, or 1,000) holes in them in order to facilitate biodisintegration of the device on which the structure is placed. In some embodiments, the one or more holes can be added to a layer using a laser. The holes can, for example, provide water and/or ions with limited access to the underlying structure, thereby helping to cause the underlying structure to disintegrate.

[0109] In some embodiments, the multi-layer structure comprises one or more non-polyelectrolyte layers. The non-polyelectrolyte layers can be biodisintegrable or biostable. Examples of non-polyelectrolyte biodisintegrable materials include polylactides such as polylactic acid (PLA), polyglycolides such as polyglycolic acid, poly(lactide-co-glycolides), polyanhydrides, and polyorthoesters. Accordingly, an endoprosthesis according to the present invention, can further include one or more non-polyelectrolyte layers, which can be added to it using, for example, a spraying technique. The non-polyelectrolyte layer(s) may be added on top of and/or underneath the multi-layered structure. In some embodiments, an endoprosthesis can include more than one multi-layered structure, and/or can include one or more non-polyelectrolyte layers located between the multi-layered structures. In some embodiments, the non-polyelectrolyte layer(s) may be thicker than one or more of the layers in the multi-layered structure(s). In certain embodiments, a non-polyelectrolyte layer may exhibit better adhesion to the surface of a tubular structure of an endoprosthesis than a polyelectrolyte layer.

[0110] In certain embodiments, a multi-layered structure may include at least two positively charged layers that are formed of different materials (e.g., different polyelectrolytes) and/or at least two negatively charged layers that are formed of different materials (e.g., different polyelectrolytes).

[0111] In some other embodiments, one portion of the underlying structure may be coated with a multi-layered structure, while another portion of the underlying structure may not have any coatings on it, or may be coated with just one layer.

[0112] In certain embodiments, when it is desired to increase the permeability of the medical device to magnetic fields, charged layers of a multi-layer structure containing various polyelectrolytes can be interleaved with other layers that contain magnetic clusters. Examples of incorporating inorganic materials such as polyoxometalates in a multi-layered structure using a layer-by-layer method are described, for example, in Caruso et al., Langmuir (1998), 14, 3462-3465, and also in pending U.S. patent application Ser. No. 10/985,242, both of which are incorporated herein by reference in their entirety.

[0113] A multi-layered structure in conjunction with a fiber meshwork can also be applied to the underlying structures of numerous medical devices. For example, they can be applied to grafts, filterwires, valves, filters (e.g., vena cava filters), aneurysm coils, distal protection devices, guidewires, and other implantable devices. In some embodiments, a multi-layered structure can be applied to a catheter (e.g., a renal or vascular catheter such as a balloon catheter). In certain embodiments, a multi-layered structure can be applied to a balloon. In some embodiments, a multi-layered structure such as multi-layered structure can be applied to a coil (e.g., an aneurysm coil). Coils are described, for example, in Twyford, Jr. et al., U.S. Pat. No. 5,304,195.

[0114] In some embodiments, the multi-layered structure can include nanoparticles. The nanoparticles can, for example, enhance the mechanical properties (e.g., strength) of the multi-layered structure. The nanoparticles can have at least one dimension (e.g., the thickness for a nanoplate, the diameter for a nanosphere, a nanocylinder and a nanotube) that is less than 1,000 nanometers (e.g., less than 100 nanometers). Nanoplates can have at least one dimension that is less than 1,000 nanometers; nanofibers can have at least two orthogonal dimensions (e.g., the diameter for a cylindrical nanofiber) that are less than 1,000 nanometers; and other nanoparticles can have three orthogonal dimensions (e.g., the diameter for nanospheres) that may be less than 1,000 nanometers.

Forming Layer-by-layer Structures

[0115] In certain embodiments, charged layers containing various polyelectrolytes can be interspersed with other layers to form a multi-layer structure using a method in which the layers electrostatically self-assemble. In this method, the so-called layer by layer method, a first layer comprising a first material having a first surface charge is deposited on an underlying structure or substrate, followed by a second layer comprising a second material having a second surface charge that is opposite in sign to the surface charge of the first layer. Thus, the charge on the outer layer is reversed upon deposition of each sequential layer. Additional layers, for example of first and second materials, can then be alternatingly deposited on the substrate to build a multi-layered structure to a predetermined or targeted thickness. This method allows a multi-layer structure to be formed on an underlying structure directly and/or, for example, on a flexible sleeve (e.g., a polymer sleeve) carried by the underlying structure, as well as on a fiber meshwork of the present invention. The layer-by-layer structure thus formed is capable of controlling the disintegration of the underlying structure, while allowing the device to remain flexible and adaptable to the vessel in which the device is implanted. Layer-by-layer self-assembly is described, for example, in Liu et al., Journal of Cluster Science, Vol. 14, No. 3, 405-419, (2003); and Caruso et al., Langmuir, 14, 3462-3465, (1998). The substrate can be the underlying structure, or the fiber meshwork.

[0116] Accordingly, once a layer having a preselected charge is provided on the underlying structure or fiber meshwork, the layer can be coated with a layer of an oppositely charged material. After each application of a successive layer, the device can be washed to remove excess material. A multi-layer structure can be formed by repeated treatment with alternating, oppositely charged, materials, e.g., a positive polyelectrolyte and a negative polyoxometalate, as described in pending U.S. patent application Ser. No. 11/127,968, incorporated by reference herein. The respective layers self-assemble by electrostatic layer-by-layer deposition, thus forming a multi-layered structure over the underlying structure or the fiber meshwork of the device.

[0117] In another embodiment, the multi-layer structure is formed by exposing a selected charged substrate (e.g., the underlying structure of a stent, or a fiber meshwork) to solutions or suspensions that contain species of alternating net charge, such as solutions or suspensions that optionally contain charged magnetic clusters, charged therapeutic agents and/or nanoparticles. The concentration of the charged species within these solutions and suspensions, which can be dependent on the types of species being deposited, can range, for example, from about 0.01 mg/ml to about 30 mg/ml. The pH of these suspensions and solutions can be such that polyelectrolytes, optional magnetic clusters, and therapeutic agents and/or nanoparticles to be included within the layers maintain their charge. Buffer systems can be used to maintain the pH and thereby the charges of the respective species.

[0118] The solutions and suspensions containing the charged species can be applied to the charged substrate surface using a variety of methods. Examples of such methods include spraying methods, dipping methods, roll and brush coating methods, ink jet methods, spin coating methods, web coating methods, microstamping methods, and methods involving coating via mechanical suspension such as air suspension, as well as combinations of these methods. Layers can be applied over an underlying structure by immersing the entire structure into a solution or suspension containing the charged species, or by immersing half of the structure into the solution or suspension, flipping the structure through 180°, and immersing the other half of the substrate into the solution or suspension to complete the coating. In some embodiments, the substrate is rinsed after application of each charged species layer, for example, using a washing solution with a pH that maintains the charge of the outer layer.

[0119] In some embodiments, the substrate is itself charged. A fiber meshwork can therefore be composed of charged polymer molecules or, at least, carries a surface charge to facilitate deposition of a first layer of a layer-by-layer structure upon it. Examples of such polymers include PEI, polyamides, polyacrylic acid, carboxymethyl cellulose, chitosan, PEI, polyamides, and polystyrene. The last of these, polystyrene, can be dissolved in an organic solvent such as THF after the layer-by-layer coating is assembled, thereby giving rise to a hollow fiber that can be filled with one or more therapeutic agents. The extent of surface charge on the surface of a given material can be deduced by measuring the Zeta-potential of the material, using methods known to one of ordinary skill in the art.

[0120] Using the methods described herein, multiple layers of alternating charge can thereby be applied to the fiber meshwork, or to an underlying structure. The number of layers varies according to application. In some embodiments, at least 10 layers (e.g., at least 20 layers, at least 30 layers, at least 50 layers, at least 100 layers, at least 200 layers) and/or at most 300 layers (e.g., at most 200 layers, at most 100 layers, at most 50 layers, at most 30 layers, at most 20 layers) can be applied over the substrate.

[0121] The total thickness of a multi-layered structure built up by a layer-by-layer process can be a function of the materials (e.g., polyelectrolytes) used, and can range, for example, from 5 nanometers to 1,500 nanometers. In some embodiments, the total thickness of the multi-layered structure can be at least five nanometers (e.g., at least 10 nanometers; at least 50 nanometers; at least 100 nanometers; at least 500 nanometers; at least 1,000 nanometers; at least 1,500 nanometers; at least 2,000 nanometers; at least 5,000 nanometers; at least 10,000 nanometers; at least 20,000 nanometers; at least 30,000 nanometers) and/or at most 40,000 nanometers (e.g., at most 30,000 nanometers; at most 20,000 nanometers; at most 10,000 nanometers; at most 5,000 nanometers; at most 2,000 nanometers; at most 1,500 nanometers; at most 1,000 nanometers, at most 500 nanometers, at most 100 nanometers, at most 50 nanometers, at most 10 nanometers).

[0122] The number of layers and/or the total thickness of a multi-layered structure deposited on a fiber meshwork, or an underlying structure, can be determined empirically and can be a function of, for example, the compositions of the layers and the type of medical device. For example, for a given medical device, the number of layers, their sequences and compositions, and/or the total thickness of multi-layered structure can be varied and the effectiveness of the multi-layered structure can be tested. After an effective combination is determined, the same combination can be repeatedly applied to other such devices.

[0123] In some embodiments, the multi-layered structure can be formed on a substrate, removed from the substrate, and subsequently applied (e.g., with an adhesive) to an underlying structure or a fiber meshwork of a medical device according to the present invention. When separating the multi-layered structure from the substrate, the substrate can be removed by destroying it, for example, by melting, sublimation, combustion, or dissolution, or a combination thereof, to free the multi-layered structure. For example, a removable substrate made of dental waxes (such as those available from MDL Dental Products, Inc., Seattle, Wash., USA) or polyvinyl alcohol can be used. These materials can respectively melt at moderately elevated temperatures (e.g., 60° C.) and dissolve in hot water. Other methods of using a removable substrate are described in Sukhorukov et al., “Comparative Analysis of Hollow and Filled Polyelectrolyte Microcapsules Templated on Melamine Formaldehyde and Carbonate Cores” Macromol. Chem. Phys., 205, 2004, 530-535; and U.S. patent application Ser. No. 10/849,742, both of which are incorporated herein by reference in their entirety.

[0124] Layers of a multi-layer structure may have the same thickness as one another or different thicknesses. In some embodiments, the thickness of a layer may depend on the molecular weight of the materials, such as polyelectrolyte(s), included in the layer, and/or the presence of other materials (e.g., nanoparticles) in the layer. For example, a layer comprising a relatively low molecular weight polyelectrolyte, such as low molecular weight heparin (e.g., heparin having a molecular weight of from about 1,000 Daltons to about 10,000 Daltons) may be relatively thin. In certain embodiments, the thickness of a layer may depend on the conditions (e.g., salt concentration and/or pH) during the deposition of the layer. In some embodiments, an individual layer and/or an individual layer may have a thickness of at least about 0.2 nanometer (e.g., at least about 0.5 nanometer, at least about 1.0 nanometer, at least about 5.0 nanometers, at least about 10 nanometers, at least about 50 nanometers, at least about 100 nanometers, at least about 300 nanometers), and/or at most about 500 nanometers (e.g., at most about 300 nanometers, at most about 100 nanometers, at most about 50 nanometers, at most about 10 nanometers, at most about five nanometers at most about one nanometer, or at most about 0.5 nanometer).

[0125] In certain embodiments different regions of the medical device are coated with multi-layer structures having different numbers of layers. In some embodiments, the biodisintegrable material in a region of the underlying structure that is coated with a relatively large number of layers may begin to disintegrate after, and/or more slowly than, the biodisintegrable material in a region that includes a relatively small number of polyelectrolyte layers. Thus, the polyelectrolyte layers on an underlying structure may be used to provide different disintegration rates of biodisintegrable material in different regions of the medical device. In some embodiments, an endoprosthesis can include an arrangement of polyelectrolyte layers that causes one or both of the ends of the endoprosthesis to start disintegrating before the middle of the endoprosthesis. This may limit the likelihood of the medical device breaking apart into two or more pieces during disintegration. Further examples of such devices are found in pending U.S. patent application Ser. No. 11/127,968.

[0126] Many embodiments of a medical device having different numbers of layers on one portion from another portion are possible. By “portion” is meant some non-vanishing part that is less than the whole. Thus, in some embodiments, one portion of a medical device includes a multi-layered structure with at least 10 layers (e.g., at least 20 layers, at least 30 layers, or at least 40 layers), and another portion of a medical device includes a multi-layered structure with at least 20 layers (e.g., at least 30 layers, at least 40 layers, or at least 50 layers). For example, one portion of a medical device may include a multi-layered structure with 10 layers and another portion of the medical device may include a multi-layered structure with 40 layers. In certain embodiments, a multi-layered structure on one portion of a medical device can include from five to 50 layers (e.g., from 10 to 30 layers) more than a multi-layered structure on another portion of the medical device.

[0127] In some embodiments, one or more portions of a medical device may not be coated with any layers at all.

[0128] In some embodiments, devices having portions with different numbers of layers on them can be formed by dipping one end (e.g., ⅔) of a device in one material, turning the device around, and dipping the other end (e.g., ⅔) of the device in another material, and repeating the process multiple times. The result is that the middle of the device (e.g., the middle ⅓ of it) receives more layers than either end of the device. In certain embodiments, devices having portions with different numbers of layers on them can be formed by other techniques, such as ink jet techniques, microstamping, spraying, roll coating, or brush coating.

[0129] The underlying structure or fiber meshwork can be pretreated prior to forming a multi-layer structure on it. For example, an underlying structure can be cleaned to remove surface contaminants, such as oil, that can affect the homogeneity of the multi-layered structure. The underlying structure or fiber meshwork can be cleaned, for example, in a solvent such as acetone, or in a mixture such as H₂O₂/HCl, HCl/HNO₃, H₂SO₄/K₂Cr₂O₇, H₂O₂/NH₃, and/or NaOH/NaOCl. The structure can also be pretreated with a solution including 10⁻² M SDS/0.12 N HCl for 15 minutes at 100° C.

[0130] An embodiment of a method of making a device using a layer-by-layer technique includes pretreating an underlying structure for layer-by-layer deposition. Next, a charged layer, e.g., containing a polyelectrolyte, is applied to the underlying structure. A layer of opposite charge is then applied to the previously applied charged layer. The previous two steps can then be repeated to build a multi-layered structure of a desired thickness. In some embodiments, as described herein, multi-layered structure can further include one or more layers that contain a therapeutic agent, one or more layers that contain a radio-opaque material, and/or one or more layers capable of enhancing the mechanical properties of the underlying structure. These additional layers can be applied between layers of different charges in any combination.

[0131] In another example, the material can be provided with a positive charge by covalently attaching functional groups having a positive charge (e.g., amine, imine or other basic groups) or functional groups having a negative charge (e.g., carboxylic, phosphonic, phosphoric, sulfuric, sulfonic, or other acid groups) to it.

[0132] In yet another example, a surface charge can be provided by exposing the device to a charged amphiphilic substance. In certain embodiments, the amphiphilic substance includes at least one electrically charged group to provide the device surface with a net electrical charge. Therefore, the amphiphilic substances that are used herein can also be referred to as ionic amphiphilic substances.

[0133] Thus, a surface charge can be provided on a material by adsorbing cations (e.g., protamine sulfate, polyallylamine, polydiallyldimethylammonium species, polyethyleneimine, chitosan, gelatin, spermidine, and/or albumin) or by adsorbing anions (e.g., polyacrylic acid, sodium alginate, polystyrene sulfonate, eudragit, gelatin (an amphiphilic polymer that fits in both categories depending how it is being prepared), hyaluronic acid, carrageenan, chondroitin sulfate, and/or carboxymethylcellulose) to the surface of the material as a first charged layer or to impart a surface charge. As an example, poly(ethylene imine) (PEI, Aldrich, MW ˜25 kD) can be dissolved in water in a concentration of about 0.5 g/L to apply a first coating. In some embodiments, more than one surface charge layer can be applied to provide complete coverage of the material. Application of surface charge layers is described in, e.g., “Multilayer on Solid Planar Substrates”, Multi-layer Thin Films, Sequential Assembly of Nanocomposite Materials, Wiley-VCH ISBN 3-527-30440-1, Chapter 14; and “Surface-chemistry Technology for Microfluidics” Hau, Winky L. W. et al., J. Micromech. Microeng., 13, 272-278, (2003).

[0134] The species for establishing a surface charge can be applied to the material by a variety of methods. Examples of such methods include spraying methods, dipping methods, roll and brush coating methods, methods involving coating via mechanical suspension such as air suspension, ink jet methods, spin coating methods, web coating methods and combinations of these processes. Dipping and spraying methods (without masking) can be employed, for example, to apply the species to an entire device. Roll coating, brush coating and ink jet printing can be employed, for example, to apply the species only to selected portions of the device (e.g., in the form of a pattern).

[0135] In an embodiment, the polymer from which the fiber meshwork is made is charged. Thus, an oppositely-charged layer can be easily deposited thereon.

Delivery of Therapeutic Agents

[0136] In embodiments, the multi-layered structure includes one or more layers that contain a therapeutic agent. As an example, one or more therapeutic agents can be disposed on or within the multi-layered structure giving the medical device a drug releasing function upon implantation. Therapeutic agents may be used singly or in combination.

[0137] Examples of therapeutic agents can be found at cols. 4-6 of U.S. Pat. No. 6,899,731 to Li et al., and at cols. 5-8 of U.S. Pat. No. 6,923,996 to Epstein et al., the disclosures of which are incorporated by reference in their entirety. It is to be understood that the therapeutic agents that can be used are not limited to those found herein.

[0138] Examples of therapeutic agents and methods of incorporating such agents into a multi-layer structure are described in U.S. patent application Ser. No. 10/849,742, filed May 20, 2004. U.S. Pat. No. 5,733,925, to Kunz et al., also provides general guidance for incorporating therapeutic agents into layers.

[0139] The fiber meshwork or a multi-layer structure may instead or additionally be used to deliver an antimicrobial agent, such as for the purpose of preventing or limiting local infection in the vicinity of the device. Exemplary antimicrobial agents have broad-spectrum activity and include triclosan, chlorhexidine, silver sulfadiazine, silver ions, benzalkonium chloride, and zinc pyrithione, as well as broad-spectrum antibiotics such as quinolones, fluoroquinolones, aminoglycosides and sulfonamides. Antiseptics such as iodine, methenamine, nitrofurantoin, validixic acid and other acidifying agents, including acids extracted from cranberry juice may also be used.

[0140] The therapeutic agent can be charged, either because it is itself a charged molecule or because it becomes charged upon, for example, a change in ambient pH or upon association with a charged species. Examples of charged therapeutic agents include small molecule and polymeric therapeutic agents containing ionically dissociable groups. In some embodiments in which the therapeutic agent does not possess one or more charged groups, it can nevertheless be provided with a charge, for example, through non-covalent association with a charged species. Examples of non-covalent associations include hydrogen bonding, electrostatic, van der Waals, and hydrophobic/lipophilic interactions. For instance, a therapeutic agent can be associated with an ionic amphiphilic substance.

[0141] In certain embodiments in which a charged therapeutic agent is used, one or more layers consisting of the charged therapeutic agent are deposited during the course of assembling the multi-layer structure. Thus, in some embodiments an entire layer may be composed of a therapeutic agent. For example, the therapeutic agent can itself be a polyelectrolyte (e.g., where the therapeutic agent is a polypeptide or a polynucleotide) and is thereby used to create one or more polyelectrolyte layers within the multi-layer structure. In other embodiments, the charged therapeutic agent is not a polyelectrolyte (e.g., it may be a charged small molecule drug), but one or more layers of the charged therapeutic agent can be substituted for one or more layers of the same charge (i.e., positive or negative) during the layer-by-layer assembly process.

[0142] A wide range of therapeutic agent loadings can be used. The amount of such loading can be readily determined by those of ordinary skill in the art, and will ultimately depend upon the condition to be treated, the nature of the therapeutic agent itself, the avenue by which the therapeutic-agent-loaded layer-by-layer structure or fiber meshwork is administered to the intended subject, and so forth. The loaded fiber meshwork, or multi-layered structure, may comprise, for example, from about 1 wt. % to about 70 wt. % therapeutic agent.

[0143] The amount of the therapeutic agent may be limited by the propensity of such agent to cause an undesirable localized or systemic toxic reaction and by the impairment of mechanical properties necessary for proper functioning of the device.

[0144] In still other embodiments, the therapeutic agent can be provided within charged nanocapsules, which are formed, for example, using methods such as those described in U.S. Patent Application Publication No. 2005-0129727, entitled “Localized Drug Delivery Using Drug-Loaded Nanocapsules”. In such embodiments, one or more layers of charged nanocapsules can be deposited during the course of the layer-by-layer assembly process.

[0145] In still other embodiments, the multi-layer structure is loaded with a therapeutic agent subsequent to its formation. For example, the porosity, and thus the permeability, of the multi-layer structure can be modified by adjusting the pH exposed to the structure, as described, for example, in Antipov, A. A., et al., “Polyelectrolyte multilayer capsule permeability control,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 198-200, 535-541, (2002). A porous layer can absorb a therapeutic agent after the layer is in place.

Device Materials

[0146] The underlying structure of the medical device of the present invention is, in some embodiments, formed of a biocompatible material, such as the materials described herein. Specific examples of biocompatible materials from which the underlying structure can be formed are described in U.S. patent application Ser. No. 10/440,063, filed May 15, 2003; and U.S. Patent Application Publication Nos. 2003-0018380, 2002-0144757, and 2003-0077200. Still further examples of biocompatible materials are described, for example, in Weber et al., U.S. Patent Application Publication No. 2004/0230290 A1, published on Nov. 18, 2004; Craig et al., U.S. Patent Application Publication No. 2003/0018380 A1, published on Jan. 23, 2003; Craig et al., U.S. Patent Application Publication No. US 2002/0144757 A1, published on Oct. 10, 2002; and Craig et al., U.S. Patent Application Publication No. 2003/0077200 A1, published on Apr. 24, 2003.

[0147] The biocompatible material can be suitable for use in, for example, a balloon-expandable stent, a self-expandable stent, or a combination of both (see e.g., U.S. Pat. No. 5,366,504). A self-expandable stent can be formed of a continuous solid mass of a relatively elastic biocompatible material, such as a superelastic or pseudo-elastic metal alloy, for example, a Nitinol (e.g., 55% nickel, 45% titanium). A self-expanding stent has a mechanical memory such that it can return to a preformed shape after it has been compressed or deformed. The stent is initially configured in its final desired shape and is then contracted by deforming or constraining it using any of several methods known in the art. It remains in a contracted state until it is delivered to the target site where it is allowed to expand to its initial state. Examples of materials that can be used for a balloon-expandable stent include noble metals, radiopaque materials, stainless steel, and alloys comprising stainless steel and one or more radiopaque materials.

[0148] The underlying structure can be formed of a biodisintegrable material, such as a biodisintegrable metal, a biodisintegrable metal alloy, or a biodisintegrable non-metal. Biodisintegrable materials are described, for example, in U.S. Pat. No. 6,287,332 to Bolz; U.S. Patent Application Publication No. US 2002/0004060 A1 to Heublein; U.S. Pat. Nos. 5,587,507 and 6,475,477 to Kohn et al. Examples of biodisintegrable metals for use with the underlying structure include alkali metals, alkaline earth metals (e.g., magnesium), iron, zinc, and aluminum. Examples of biodisintegrable metal alloys include alkali metal alloys, alkaline earth metal alloys (e.g., magnesium alloys), iron alloys (e.g., alloys including iron and up to seven percent carbon), zinc alloys, and aluminum alloys.

[0149] In some embodiments, a biodisintegrable material from which the underlying structure is formed, can include at least one metallic component and at least one non-metallic component, or at least two different metallic components. In some embodiments, a biodisintegrable material can include at least one of the following: manganese, cobalt, nickel, chromium, copper, cadmium, lead, tin, thorium, zirconium, silver, gold, palladium, platinum, rhenium, silicon, calcium, lithium, aluminum, zinc, iron, carbon, and sulfur. In certain embodiments, a biodisintegrable material can include at least two of the following metals in proportions by weight of greater than about 1%: magnesium, titanium, zirconium, niobium, tantalum, zinc, or silicon, and lithium, sodium, potassium, calcium, iron, or manganese. In certain embodiments, the biodisintegrable material can include a first component selected from the group consisting of: magnesium, titanium, zirconium, niobium, tantalum, zinc, silicon, and another, different, component selected from the group consisting of: lithium, sodium, potassium, calcium, iron, manganese.

[0150] Examples of biodisintegrable non-metals include biodisintegrable polymers, such as polyiminocarbonates, polycarbonates, polyarylates, polylactides, or polyglycolic esters. In some embodiments, a biodisintegrable metal or metal alloy containing metals such as magnesium, iron, or zinc, can be sintered. In some embodiments, the biodisintegrable material can be a polymer, such as, without limitation, those described at cols. 8-9 of U.S. Pat. No. 6,918,869 to Shaw et al.

[0151] In still other embodiments, the biodisintegrable material can be a ceramic such as an alumina-based ceramic, or a glass-based ceramic such as Macor®.

[0152] The properties of the underlying structure depend upon the material from which it is formed. Magnesium, for example, has a relatively low mass attenuation factor, and the CT visibility of the region (e.g., a body lumen) in which a magnesium structure is located can be relatively high.

[0153] The underlying structure of a medical device for use as described herein can be manufactured, or can be obtained commercially. Methods of making medical devices such as stents are described in, for example, U.S. Pat. No. 5,780,807, and U.S. Patent Application Publication No. 2004-0000046-A1, both of which are incorporated herein by reference. Stents are also available, for example, from Boston Scientific Corporation, Natick, Mass., USA, and Maple Grove, Minn., USA.

[0154] Metallic materials from which the underlying structure is made may be made into filaments and then woven so that the underlying structure forms a regular network of metal mesh. Polymer filaments may also be used together with the metallic filaments to form a network. If the network is made of metal, the intersection between different filaments may formed by welding, twisting, bending, gluing, tying (with suture), heat sealing, or by any other manner known in the art.

[0155] As another example, although a stent may include a tubular structure that is formed entirely of a biodisintegrable material, in some embodiments, the tubular structure of a stent can include one or more biostable materials in addition to including one or more biodisintegrable materials. One or more polymers may be used (as described herein) to control the disintegration of one or more of the biodisintegrable regions of the stent. The polymers may be in the form of layers over the biodisintegrable and/or biostable regions of the stent or a fiber meshwork similarly disposed. Examples of biostable materials include stainless steel, tantalum, nickel-chrome, cobalt-chromium alloys such as Elgiloy® and Phynox®, Nitinol (e.g., 55% nickel, 45% titanium), and other alloys based on titanium, including nickel titanium alloys, thermo-memory alloy materials. Stents including biostable and biodisintegrable regions are described, for example, in U.S. patent application Ser. No. 11/004,009, filed on Dec. 3, 2004, and entitled “Medical Devices and Methods of Making the Same”.

[0156] Devices, such as stents, may be formed from many known constructions such as cross-hatched or mesh filaments or interlocking loops.

Stents/Devices

[0157] The embodiments described herein may be used in conjunction with various medical devices, in particular endoprostheses. Exemplary medical devices are implantable or insertable medical devices, including catheters (for example, urinary catheters or vascular catheters such as balloon catheters), guide wires, balloons, filters (e.g., vena cava filters), stents of any desired shape and size (including coronary vascular stents, aortic stents, cerebral stents, urology stents such as urethral stents and ureteral stents, biliary stents, tracheal stents, gastrointestinal stents, peripheral vascular stents, neurology stents and esophageal stents), grafts such as stent grafts and vascular grafts, cerebral aneurysm filler coils (including GDC-Guglilmi detachable coils-and metal coils), filters, myocardial plugs, patches, pacemakers and pacemaker leads, heart valves, and biopsy devices. Indeed, embodiments herein can be suitably used with any underlying structure (which can be, for example, metallic, polymeric or ceramic, though typically metallic) which is coated with a fiber meshwork in accordance with methods herein and which is designed for use in a patient, either for procedural use or as an implant.

[0158] The medical devices may further include drug delivery medical devices for systemic treatment, or for treatment of any mammalian tissue or organ. Subjects can be mammalian subjects, such as human subjects. Non-limiting examples of tissues and organs for treatment include the heart, coronary or peripheral vascular system, lungs, trachea, esophagus, brain, liver, kidney, bladder, urethra and ureters, eye, intestines, stomach, colon, pancreas, ovary, prostate, gastrointestinal tract, biliary tract, urinary tract, skeletal muscle, smooth muscle, breast, cartilage, and bone.

[0159] In some embodiments, the medical device is used to temporarily treat a subject without permanently remaining in the body of the subject. For example, in some embodiments, the medical device can be used for a certain period of time (e.g., to support a lumen of a subject), and then can disintegrate after that period of time.

[0160] The underlying structure of the medical device can be generally tubular in shape and can be a part of a stent. Simple tubular structures having a single tube, or with complex structures, such as branched tubular structures, can be used.

[0161] Depending on specific application, stents can have a diameter of between, for example, 1 mm and 46 mm. In certain embodiments, a coronary stent can have an expanded diameter of from about 2 mm to about 6 mm. In some embodiments, a peripheral stent can have an expanded diameter of from about 4 mm to about 24 mm. In certain embodiments, a gastrointestinal and/or urology stent can have an expanded diameter of from about 6 mm to about 30 mm. In some embodiments, a neurology stent can have an expanded diameter of from about 1 mm to about 12 mm. An abdominal aortic aneurysm (AAA) stent and a thoracic aortic aneurysm (TAA) stent can have a diameter from about 20 mm to about 46 mm.

[0162] Stents can also be a part of a stent-graft or a covered stent. In other embodiments, stents can include and/or be attached to a biocompatible, non-porous or semi-porous polymer matrix made of polytetrafluoroethylene (PTFE), expanded PTFE, polyethylene, urethane, or polypropylene.

[0163] Stents can also be biodisintegrable, such as a biodisintegrable abdominal aortic aneurysm (AAA) stent, or a biodisintegrable vessel graft.

[0164] Collagen type I (Sigma-Aldrich (St. Louis, Mo.)) dissolves in 1,1,1,2,2,2-hexafluoro-2-propanol (HFIP). A solution was prepared making a 8% by weight solution of Collagen in HFIP. A stent (BSC, stainless steel, express stent, 16 mm length, 3.0 mm diameter), was crimped on a Teflon mandrel having diameter 3.0 mm, with a fine 10 micron gold wire (California Fine Wire, 338 So. Fourth Street, Grover Beach, Calif. 93433-0199) placed in between the stent and the mandrel such that there was electrical contact between the stent and the gold wire. The gold wire was grounded.

[0165] A nozzle with Syringe was placed at a distance of 7 cm from the stent surface and connected to a syringe pump (type SP101i, World Precision Instruments, Liegnitzer Str.15, D-10999 Berlin, Germany), and a high voltage supply (Type CS2091, High Voltage Power Solutions, Inc. (HVPSI), Dallas, Tex. 75370). The mandrel was rotated at 5 Hz during the spraying process and moved along the axis in a cyclic movement at 12 Hz with an amplitude of 2 mm. The spraying was done at the following settings (18 kV, 0.02 ml/min, 10 seconds on cycle). This resulted in a 90% porous fibrin structure being 10 micrometer thick covering the stent. The collagen nanofiber network spun in this way was crosslinked by glutaraldehyde vapor at room temperature for 12 hours (Sigma-Aldrich (St. Louis, Mo.)).

[0166] Polyetherimide (PEI) was purchased from Aldrich Co. (St. Louis, Mo.)), and Biopol™, polyhydroxybutyrate-valerate (PHBV) was purchased from Monsanto Company (800 North Lindbergh, St. Louis, Mo. 63167). Both components were mixed in chloroform making solutions having 23 wt. % PEI and 21 wt. % PHBV. These solutions were mixed to a ratio of 75/25 (PEI/PHBV)

[0167] A stent (Boston SciMed Corporation, stainless steel, express stent, 16 mm length, 3.0 mm diameter), was crimped on a Teflon mandrel diameter 3.0 mm with a fine 10 micron gold wire (California Fine Wire, 338 So. Fourth Street, Grover Beach, Calif. 93433-0199) placed in between the stent and the mandrel, such that there was electrical contact between the stent and the gold wire. The gold wire was grounded.

[0168] A nozzle with syringe was placed at a distance of 15 cm from the stent surface and connected to a syringe pump (type SP101i, World Precision Instruments, Liegnitzer Str.15, D-10999 Berlin, Germany), and a high voltage supply (Type CS2091, High Voltage Power Solutions, Inc. (HVPSI), High Voltage Power Solutions, Inc., Dallas, Tex. 75370). The mandrel was rotated at 5 Hz during the spraying process and moved along the axis in a cyclic movement at 12 Hz with an amplitude of 2 mm. The spraying was carried out at the following settings: 15 kV, 0.05 ml/min, 15 seconds on cycle. The stent sprayed in this way was thermally treated for 90 minutes at 210° C. in a nitrogen environment to decompose the PHBV component and leave behind a fiber meshwork made of porous PEI fibers.

[0169] The fibers created in this way were coated with a layer of Heparin via a polyelectrolyte self-assembly process. Heparin in sodium salt form, from Bioiberica (Barcelona, Spain) is negatively charged and binds readily to the positively charged PEI surface when the stent with fiber meshwork is dipped for 10 minutes in an aqueous(2 mg/ml) heparin solution followed drying with a stream of nitrogen.

[0170] The PEI fiber meshwork created in this way can also be used to deliver drug-filled liposome capsules (e.g., as described in Vermette et al., “Immobilized liposome layers for drug delivery applications: inhibition of angiogenesis”, J. Controlled Release, 80:179-195 (2002), incorporated herein by reference in its entirety) or other polyelectrolyte capsules filled with various drugs. Leaving the stent with the electro-spun network on the Teflon mandrel while attaching the capsules will assure that they only attach to the outside and that, in use, they are therefore all trapped in between the stent and the wall of the vessel or body lumen.

[0171] In another embodiment, instead of using PEI fibers as in Example 2, the collagen fibers as discussed in Example 1 can also be coated with additional polyelectrolyte layers, but using only non-polymeric biological material such as collagen and alternating hyaluronic acid or heparin elements (see e.g., Zhang, et al., “Natural polyelectrolyte films based on layer-by-layer deposition of collagen and hyaluronic acid”, Biomaterials, 26:3353-3361, (2005)). To those layers again, capsules are bound using polyelectrolyte constructions.

[0172] All non-patent literature publications, patent applications, patent application publications, and patents, referred to in the instant application are incorporated herein by reference in their entirety.

[0173] Other embodiments are to be found within the appended claims.