US20110319987A1 - Medical implant - Google Patents

Abstract

Disclosed is a self-expanding medical implant for placement within a lumen of a patient. The implant comprises a woven or non-woven structure having a substantially tubular configuration, and is designed to be low-profile such that it is deliverable with a small diameter catheter. The implant has a high recoverability and desired mechanical properties.

Description

• This application is a continuation-in-part of, and claims the benefit of priority to, U.S. patent application Ser. No. 12/783,261, filed May 19, 2010, by inventors Lee Core et al., entitled “Medical Implant,” and further claims the benefit of priority to U.S. Patent Application 61/179,834, filed May 20, 2009, by inventors Lee Core et al., entitled “Medical Implant,” to U.S. Patent Application 61/227,308, filed Jul. 21, 2009, by inventors Lee Core et al., entitled “Medical Implant,” and to U.S. Patent Application 61/251,984, filed Oct. 15, 2009, by inventors Lee Core et al.
FIELD OF THE INVENTION
• The present invention relates to medical implants, and more specifically, to self-expanding medical implants that are intended for placement within a lumen or cavity of a patient.
BACKGROUND
• A variety of medical conditions are treatable by the implantation of tubular devices into natural body lumens. For example, it is commonplace to implant metallic stents into the coronary arteries of patients with heart disease following balloon angioplasty to minimize the risk that the arteries will undergo restenosis. Recently, commercial stents have included drug-eluting polymer coatings that are designed to further decrease the risk of restenosis. Other examples of conventional tubular medical implants include woven grafts and stent-grafts that are used to span vascular aneurysms, polymeric tubes and catheters that are used to bypass strictures in the ureter and urethra, and stents that are used in the peripheral vasculature, prostate, and esophagus.
• Despite the evolution of metallic stents, they continue to have limitations such as the possibility of causing thrombosis and vascular remodeling. While the use of biodegradable and biostable polymeric materials for stents and other implantable devices has been proposed to eliminate the possible long-term effects of permanent implants, the use of such materials has been hindered by relatively poor expandability and mechanical properties. For example, the expansion characteristics and radial strength of prototype stents made from biodegradable and biostable polymeric materials has been significantly lower than that of metallic stents. This is particularly the case where such stents are low profile and make use of small diameter fibers or thin walled struts that comprise the stent body. Furthermore, the degradation rate and the manner in which such devices degrade in the body has been difficult to control. Finally, where such devices are used as a drug delivery vehicle, the drug elution rate has been difficult to reproducibly characterize.
• There is therefore a need for low-profile, self-expanding implantable tubular devices that have sufficient expansion characteristics, strength and other mechanical and drug release properties that are necessary to effectively treat the medical conditions for which they are used.
SUMMARY
• In one aspect, the present invention includes an implantable medical device for placement within a lumen or cavity of a patient. In another aspect, the present invention includes a method of loading the medical device into a delivery catheter just prior to being implanted into a patient. In another aspect, the present invention includes a method of treating a patient by delivering the medical device to a target location within the patient. In yet another aspect, the present invention includes a kit that comprises the implantable medical device.
• The implantable medical devices of the present invention are generally tubular, self-expanding devices. The devices have a combination of structure, composition, and/or strengthening means that provide them with exceptional expandability and mechanical properties when compared with conventional self-expanding devices.
• In one embodiment, the implantable medical device comprises a self-expanding tubular structure that comprises at least one strand. The at least one strand comprises a first polymer characterized by a modulus of elasticity greater than about 1 GPa. The implantable medical device further includes a strengthening means comprising a second polymer at least partially coating the strand. The second polymer is characterized by a percent elongation to break that is greater than about 150 percent at body temperature (37° C.). The second polymer increases the mass of the self-expanding tubular structure by at least about 20 percent.
• In another embodiment, the implantable medical device comprises a self-expanding tubular structure that comprises at least one strand comprising a first polymer. The implantable medical device further includes a strengthening means comprising a second polymer at least partially coating the strand. The second polymer comprises a first elastic component having a Tg less than about 37° C. and a second component that is harder than the first component. The second polymer increases the mass of the self-expanding tubular structure by at least about 20 percent.
• In other embodiments, the implantable medical device comprises a self-expanding tubular structure comprising a unitary framework rather than at least one strand. In certain embodiments, the implantable medical device comprises a therapeutic agent for delivery into a patient's body.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a side view of an implantable braided medical device, in accordance with an embodiment of the present invention.
• FIG. 2 is a side view of an implantable unitary framework medical device, in accordance with an embodiment of the present invention.
• FIG. 3 is a cross-sectional view of a strand of an implantable medical device in accordance with an embodiment of the present invention that includes a support coating.
• FIG. 4 is a side view of a strand of an implantable medical device in accordance with an embodiment of the present invention that includes discrete areas of therapeutic agent thereon.
• FIG. 5 is a cross-sectional view of a strand of an implantable medical device in accordance with an embodiment of the present invention that includes a therapeutic agent coating and a topcoat.
• FIG. 6 is an end view of an implantable medical device in accordance with an embodiment of the present invention.
• FIG. 7 is a graph of diameter recovery as a function of support coating weight, for certain embodiments of the present invention.
• FIG. 8 is a graph of diameter recovery as a function of gel content within a support coating, for certain embodiments of the present invention.
• FIG. 9 is a graph of COF values of certain embodiments of the present invention, as well as for known commercial self-expanding stents.
DESCRIPTION OF PREFERRED EMBODIMENTS
• The present invention provides for self-expanding medical implants that have expansion characteristics and mechanical properties that render them suitable for a broad range of applications involving placement within bodily lumens or cavities. As used herein, “device” and “implant” are used synonymously. Also as used herein, “self-expanding” is intended to include devices that are crimped to a reduced configuration for delivery into a bodily lumen or cavity, and thereafter tend to expand to a larger suitable configuration once released from the delivery configuration, either without the aid of any additional expansion devices or with the partial aid of balloon-assisted or similarly-assisted expansion. When compared with conventional self-expanding medical implants, the implants of the present invention recover to an exceptionally high percentage of their manufactured diameter after being crimped and held in a small diameter for delivery into a bodily lumen. Moreover, when compared with conventional self-expanding implants and particularly polymeric implants, the implants of the present invention are characterized by much improved strength and other desired mechanical properties. As used herein, “strength” and “stiffness” are used synonymously to mean the resistance of the medical implants of the present invention to deformation by radial forces. Examples of strength and stiffness measurements, as used to characterize the medical implants of the present invention, include radial resistive force and chronic outward force, as further defined herein.
• In one embodiment shown inFIG. 1, theimplant100 preferably comprises at least one strand woven together to form a substantially tubular configuration having alongitudinal dimension130, aradial dimension131, and first andsecond ends132,133 along the longitudinal dimension. As used herein, “woven” is used synonymously with “braided.” For example, the tubular configuration may be woven to form a tubular structure comprising two sets ofstrands110 and120, with each set extending in an opposed helix configuration along the longitudinal dimension of the implant. The sets ofstrands110 and120 cross each other at abraid angle140, that may be constant or may change along the longitudinal dimension of the implant. Preferably, there are between about 16 and about 96 strands used in the implants of the present invention, and thebraid angle140 is within the range of about 90 degrees to about 135 degrees throughout the implant. The strands are woven together using methods known in the art, using known weave patterns such as Regular pattern “1 wire, 2-over/2-under”, Diamond half load pattern “1 wire, 1-over/1-under”, or Diamond pattern “2 wire, 1-over/1-under”.
• Although the strands may be made from biostable polymeric or metallic materials, they are preferably made from at least one biodegradable polymer that is preferably fully absorbed within about two years of placement within a patient, and more preferably within about one year of placement within a patient. In some embodiments, the strands are fully absorbed within about six or fewer months of placement within a patient. The first and second strand sets110,120 may be made from the same or different biodegradable polymer. Non-limiting examples of biodegradable polymers that are useful in the at least one strand of the present invention include poly lactic acid (PLA), poly glycolic acid (PGA), poly trimethylene carbonate (PTMC), poly caprolactone (PCL), poly dioxanone (PDO), and copolymers thereof. Preferred polymers are poly(lactic acid co-glycolic acid) (PLGA) having a weight percentage of up to about 20% lactic acid, or greater than about 75% lactic acid (preferably PLGA 85:15), with the former being stronger but degrading in the body faster. The composition of PLGA polymers within these ranges may be optimized to meet the mechanical property and degradation requirements of the specific application for which the implant is used. For desired expansion and mechanical property characteristics, the materials used for the strands preferably have an elastic modulus within the range of about 1 to about 10 GPa, and more preferably within the range of about 6-10 GPa.
• To facilitate the low-profile aspects of the present invention (e.g., the delivery of the implants into small diameter bodily lumens or cavities), the strands used in theimplant100 preferably have a diameter in the range of from about 125 microns to about 225 microns, and are more preferably less than about 150 microns in diameter. The use of small diameter strands results in an implant with minimal wall thickness and the preferred ability to collapse (i.e., to be crimped) within low diameter catheter delivery systems. Where multiple strands are used, they may be of substantially equal diameters within this range, or first strand set110 may be of a different general diameter than second strand set120. In either event, the diameters of strands are chosen so as to render theimplant100 preferably deliverable from a 10 French delivery catheter (i.e., 3.3 mm diameter) or smaller, and more preferably from a 7 French delivery catheter (i.e., 2.3 mm diameter) or smaller. The ability to place the implant of the present invention into small diameter delivery catheters allows for its implantation into small diameter bodily lumens and cavities, such as those found in the vascular, biliary, uro-genital, iliac, and tracheal-bronchial anatomy. Exemplary vascular applications include coronary as well as peripheral vascular placement, such as in the superficial femoral artery (SFA). It should be appreciated, however, that the implants of the present invention are equally applicable to implantation into larger bodily lumens, such as those found in the gastrointestinal tract.
• In another embodiment of the present invention, the implant is a non-woven, self-expanding structure, such as a unitary polymeric framework. As shown inFIG. 2, thenon-woven implant100 is preferably characterized by a regular, repeating pattern such as a lattice structure. The use of a unitary framework may provide a reduced profile when compared to the use of woven strands, which yield a minimum profile that is the sum of the widths of overlapping strands. In addition, a unitary framework eliminates the possible change in length of the implant associated with crimping and subsequent expansion, known as foreshortening, which is common in braided stents. When theimplant100 is a unitary framework, it is fabricated using any suitable technique, such as by laser cutting a pattern into a solid polymer tube. In a preferred embodiment, when theimplant100 is a unitary framework, it is formed by laser cutting and includes a wall thickness of between about 75 and about 100 microns. It should be recognized that while the present invention is described primarily with reference to woven strand configurations, aspects of the present invention are equally applicable to non-woven, self-expanding structures unless necessarily or expressly limited to woven configurations.
• There are a variety of strengthening means that are useful in the present invention to help provide the expansion and mechanical properties that are needed to render theimplant100 effective for its intended purpose. Two measures of such mechanical properties that are used herein are “radial resistive force” (“RRF”) and “chronic outward force” (“COF”). RRF is the force that the implant applies in reaction to a crimping force, and COF is the force that the implant applies against a static abutting surface. Without wishing to be bound by theory, the inventors believe that the self-expanding implants of the present invention should preferably recover to a high percentage of their as-manufactured configuration after being crimped for insertion into the body, and the thus expanded implant should have a relatively high RRF to be able to hold open bodily lumens and the like, yet have a relatively low COF so as to avoid applying possibly injurious forces against the walls of bodily lumens or the like. For example, the implants of the present invention preferably expand to at least 90% of their as manufactured configuration after being crimped, have an RRF of at least about 200 mm Hg, have an acute COF (at the time of delivery into a bodily lumen or cavity) of about 40-200 mm Hg, and a COF of less than about 10 mm Hg after about 28 days in vivo.
• In one embodiment, the strengthening means is asupport coating410 on at least one of the strands of theimplant100. Although referred to herein as a “coating,” thesupport coating410 does not necessarily coat theentire implant100, and may not form a discrete layer over the stands or unitary framework of theimplant100; rather, thesupport coating410 and underlying strands or unitary framework may be considered as a composite structure. Thesupport coating410 is made from an elastomeric polymer that, due to its elastic nature when compressed or elongated, applies a force to implant100 that acts in favor of radial expansion and axial contraction, thus enhancing radial strength. The polymer of thesupport coating410 is preferably biodegradable. Alternatively, thesupport coating410 is made from a shape memory polymer or a polymer that otherwise contracts upon heating to body temperature. The inventors have surprisingly found that the use of support coatings on the polymeric implants of the present invention can result in the recovery of more than 90% of implant diameter post-crimping, and yield significantly higher radial forces when compared with uncoated implants or even with self-expanding metallic stents. Thesupport coating410 may be applied as a conformal coating (as shown in cross-section of an individual strand inFIG. 3), may be partially applied to one or more individual strands such that thesupport coating410 is applied to only a portion of the implant along its longitudinal dimension, or may be applied to only the inner or outer diameter of one or more individual strands. Also, thesupport coating410 may optionally vary in weight along the length of the implant; for example, the ends of the implant may be coated with a thicker layer than the mid-section to provide added support to the ends of the implant. In addition, the support coating may accumulate at the crossover points or “nodes” of the woven device, which has the effect of aiding in diameter recovery and the achievement of preferred COF and RRF values.
• Examples of polymer materials used for thesupport coating410 include suitable thermoplastic or thermoset elastomeric materials that yield the elongation, mechanical strength and low permanent deformation properties when combined with the implant strand(s). The inventors have found examples of suitable polymers to include certain random copolymers such as poly(lactic acid-co-caprolactone) (PLCL), poly(glycolide-co-caprolactone) (PGCL), and poly(lactic acid-co-dioxanone) (PLDO), certain homopolymers such as poly trimethylene carbonate (PTMC), and copolymers and terpolymers thereof. Such polymers are optionally crosslinked with a crosslinker that is bi- or multi-functional, polymeric, or small molecule to yield a thermoset polymer having a glass transition temperature (Tg) that is preferably lower than body temperature (37° C.), more preferably lower than room temperature (25° C.), and most preferably lower than about 5° C. The thermoset elastomers provide a high elongation to break with low permanent deformation under cyclic mechanical testing.
• In one preferred embodiment, the polymer material used for thesupport coating410 is a biodegradable thermoset elastomer synthesized from a four arm PGCL polymer having a weight ratio of approximately 50:50 GA:CL that is crosslinked with hexamethylene diisocyanate (HDI) to give a polyester with urethane crosslinks. Without wishing to be bound by theory, the inventors believe that the combination of the elastic segment (polyester portion) and the interactions (such as hydrogen bonding, allophanate or biuret formation) between the urethane segments of such polymers, in addition to a certain crosslinking density, yields preferred properties such as a high degree of elastic recovery under cyclic mechanical strain and high overall elasticity.
• In other preferred embodiments, the support coating comprises PLCL having a weight ratio of approximately 50:50 PL:CL. In yet another preferred embodiment, a PLCL 50:50 crosslinked with hexamethylene diisocyanate support coating is applied to a PLGA 75:25 braided implant.
• The polymer material used forsupport coating410 may be optimized for desired mechanical properties. For example, the inventors have found that the molecular weight of such polymers may be manipulated to enhance coating performance. As an example, when PLCL 50:50 crosslinked with hexamethylene diisocyanate is used as the support coating of the present invention, the inventors have found that a molecular weight (Mn) between about 30 kDa and 100 kDa, preferably from 33 k to 65 k, results in a lower modulus of elasticity and a higher strain to fracture, thus making the coating better able to adhere to a PLGA braided implant during crimping and post-crimping expansion and therefore less likely to fracture during deployment. Similarly, the inventors have found that when PGCL 50:50 crosslinked with hexamethylene diisocyanate is used as the support coating of the present invention, a molecular weight (Mn) from 8 kDa to 20 kDa does not yield an appreciable change in properties, but that a further increase in molecular weight to 50 kDa results in a four-fold increase in the strain at failure of the coating material. As such, a preferred range of molecular weight (Mn) for PGCL used in the implants of the present invention is about 23 kDa to about 50 kDa. Additionally, the inventors have found that the viscosity of the spray coating solution, the weight percent of crosslinker used in the spray coating solution, and the temperature and duration of the support coating crosslinking process can be optimized to provide preferred coating morphologies and radial forces.
• Thesupport coating410 is coated onto the surface of theimplant100 using any suitable method, such as spraying, dipping, electrospraying, rolling, and the like. Ifimplant100 is a woven structure, thesupport coating410 may be applied to individual strands prior to forming the woven structure, or to the woven structure after the formation thereof. In this case, owing to surface tension, the coating preferably collects at intersection points between strands. Ifimplant100 is a non-woven structure, thesupport coating410 may be applied, for example, to a solid polymer tube either before or after the removal of material such as by laser cutting to form a patterned, non-woven structure.
• The amount ofsupport coating410 applied to theimplant100 has been identified as one of the factors that contribute to the expansion characteristics and mechanical strength of the implant. Preferably, the application of thesupport coating410 increases the weight of theuncoated implant100 by about 20% to about 100%, more preferably, by about 24% to about 70%, and most preferably by about 30% to about 60%.
• In another embodiment, the strengthening means includes the attachment of strand sets110,120 at one or more intersections of the strands along the longitudinal dimension of theimplant100. Such attachment may be achieved by use of an adhesive, or by fusing the strands at predetermined intersections such as by heat, laser, or ultrasound. In this embodiment, the strands comprise materials having sufficient elasticity such that the local strains induced by the adhesive/welded joints would not cause plastic deformation thereof.
• In yet another embodiment, the strengthening means includes the incorporation of additives into one or more of the strands. In one example, such additives are neutralizing agents such as calcium salts (e.g., calcium carbonate or calcium phosphate) or other salts such as barium salts that increase the mechanical strength of the strands into which they are incorporated, and further act to neutralize any acidic byproducts resulting from the degradation of the strand material(s). In another example, such additives are plasticizers such as polyethylene glycol (PEG) that dissolve from the strand(s) in vivo, thus increasing the flexibility of the strand(s) and the implant over time.
• In one embodiment, theimplant100 delivers one or more therapeutic agents at the site of implantation. The therapeutic agent(s) may be applied to one or more strands for delivery therefrom in a number of ways. In one example, the therapeutic agent(s) are embedded within aconformal polymer coating210 that adheres to one or more individual strands of theimplant100. Such acoating210 is preferably made from a biodegradable polymer admixed with the therapeutic agent(s) such that the agent is eluted from the polymer over time, or is released from the coating as it degrades in vivo. In another example as shown inFIG. 4, one or more therapeutic agents are applied indiscrete areas220 on one or more individual strands (shown as length of individual strand). Like coating210,discrete areas220 are preferably made from a biodegradable polymer admixed with the therapeutic agent(s) and eluted from the polymer over time, or are released from the coating as it degrades in vivo. In either ofcoating210 ordiscrete areas220, the biodegradable polymer may be the same as or different from the biodegradable polymer(s) used in the strands of the implant. In yet another example, the therapeutic agent(s) are admixed within the strand(s) of theimplant100 such that the agent(s) are eluted from the one or more strands over time, or are released from the one or more strands as the strand(s) degrade in vivo. In yet another example, the therapeutic agent(s) are admixed within a support coating, as described herein. Likewise, in embodiments in which theimplant100 is a non-woven structure, the therapeutic agent(s) may be admixed with the polymer used to fabricate theimplant100.
• The therapeutic agent(s) used in the present invention are any suitable agents having desired biological effects. For example, where the implant of the present invention is used to combat restenosis, the therapeutic agent is selected from anti-thrombogenic agents such as heparin, heparin derivatives, urokinase, and PPack (dextrophenylalanine proline arginine chloromethylketone), enoxaparin, hirudin; anti-proliferative agents such as angiopeptin, or monoclonal antibodies capable of blocking smooth muscle cell proliferation, acetylsalicylic acid, paclitaxel, sirolimus, tacrolimus, everolimus, zotarolimus, vincristine, sprycel, amlodipine and doxazosin; anti-inflammatory agents such as glucocorticoids, betamethasone, dexamethasone, prednisolone, corticosterone, budesonide, estrogen, sulfasalazine, rosiglitazone, mycophenolic acid, and mesalamine; immunosuppressants such as sirolimus, tacrolimus, everolimus, zotarolimus, and dexamethasone; antineoplastic/antiproliferative/anti-mitotic agents such as paclitaxel, 5-fluorouracil, cisplatin, vinblastine, cladribine, vincristine, epothilones, methotrexate, azathioprine, halofuginone, adriamycin, actinomycin and mutamycin; endostatin, angiostatin and thymidine kinase inhibitors, and its analogs or derivatives; anesthetic agents such as lidocaine, bupivacaine, and ropivacaine; anti-coagulants such as D-Phe-Pro-Arg chloromethyl ketone, an RGD peptide-containing compound, heparin, antithrombin compounds, platelet receptor antagonists, anti-thrombin antibodies, anti-platelet receptor antibodies, aspirin (aspirin is also classified as an analgesic, antipyretic and anti-inflammatory drug), dipyridamole, hirudin, prostaglandin inhibitors, platelet inhibitors and antiplatelet agents such as trapidil or liprostin, tick antiplatelet peptides; DNA demethylating drugs such as 5-azacytidine, which is also categorized as a RNA or DNA metabolite that inhibit cell growth and induce apoptosis in certain cancer cells; vascular cell growth promotors such as growth factors, Vascular Endothelial Growth Factors (VEGF, all types including VEGF-2), growth factor receptors, transcriptional activators, and translational promotors; vascular cell growth inhibitors such as antiproliferative agents, growth factor inhibitors, growth factor receptor antagonists, transcriptional repressors, translational repressors, replication inhibitors, inhibitory antibodies, antibodies directed against growth factors, bifunctional molecules consisting of a growth factor and a cytotoxin, bifunctional molecules consisting of an antibody and a cytotoxin; cholesterol-lowering agents; vasodilating agents; and agents which interfere with endogenous vasoactive mechanisms; anti-oxidants, such as probucol; antibiotic agents, such as penicillin, cefoxitin, oxacillin, tobranycin; angiogenic substances, such as acidic and basic fibrobrast growth factors, estrogen including estradiol (E2), estriol (E3) and 17-Beta Estradiol; drugs for heart failure, such as digoxin, beta-blockers, angiotensin-converting enzyme (ACE) inhibitors including captopril and enalopril, statins and related compounds; and macrolides such as sirolimus and everolimus. Preferred therapeutic agents used in the present invention to treat restenosis and similar medical conditions include sirolimus, everolimus, zotarolimus, vincristine, sprycel, dexamethasone, and paclitaxel. Also preferred is the use of agents that have a primary mechanism of action of inhibiting extracellular matrix remodeling, and a secondary mechanism of action of inhibiting cell proliferation. Such agents include 5-fluorouracil, valsartan, doxycyclin, carvedilol, curcumin, and tranilast.
• Coating210 orareas220 containing one or more therapeutic agents are applied to implant100 by any suitable method, such as spraying, electrospraying, rolling, dipping, chemical vapor deposition, and potting. As an alternate embodiment, coating210 orareas220 is further coated with a biodegradable topcoat as shown inFIG. 5 (individual strand shown in cross-section), that acts to regulate the delivery of the therapeutic agent from coating210 orareas220 into bodily tissue. In one embodiment, thetopcoat211 acts as a diffusion barrier such that the rate of delivery of the therapeutic agent(s) are limited by the rate of its diffusion through thetopcoat211.
• In another embodiment, the therapeutic agent(s) cannot diffuse through thetopcoat211 such that delivery thereof is simply delayed until the degradation of thetopcoat211 is complete. Thetopcoat211 preferably comprises a biodegradable polymer that is the same as or different from that of thecoating210 or the strands. Ifimplant100 is a woven structure,coatings210,220, or211 may be applied to individual strands prior to forming into the woven structure, or to the woven structure after the formation thereof. Ifimplant100 is a non-woven structure,coatings210,220, or211 may be applied, for example, to a solid polymer tube either before or after the removal of material such as by laser cutting to form a patterned, non-woven structure. In embodiments that includesupport coating410, thecoatings210,220, and/or211 are preferably applied oversuch support coating410, or thesupport coating410 itself may include the therapeutic agent(s).
• Theimplant100 of the present invention is self-expanding in that it is manufactured at a first diameter, is subsequently reduced or “crimped” to a second, reduced diameter for placement within a delivery catheter, and self-expands towards the first diameter when extruded from the delivery catheter at an implantation site. The first diameter is preferably at least 20% larger than the diameter of the bodily lumen into which it is implanted. Theimplant100 is preferably designed to recover at least about 80% and up to about 100% of its manufactured, first diameter. The inventors have found that implants in accordance with the present invention have a recovery diameter greater than 80%, and preferably greater than 90%, of the manufactured, first diameter after being crimped into exemplary delivery catheters of either 1.8 mm or 2.5 mm inner diameter and held for one hour at either room temperature (25° C.) or body temperature (37° C.). Such implants a braided structure comprising 32 strands ranging in size from 0.122 mm to 0.178 mm, braided with an inner diameter ranging from 5 to 6 mm. In a preferred embodiment, theimplant100 has a manufactured, first diameter (inner) of about 4.0 mm to about 10.0 mm.
• Various factors contribute to the radial strength ofimplant100. For woven implant structures, these factors include the diameter(s) of the strands, thebraid angle140, the strand material(s), the number of strands used, and the use of strengthening means. The inventors have found that it is preferred that the woven implants of the present invention have abraid angle140 within a range of about 90° to 135°, more preferably within a range of about 110° to 135°, and most preferably within a range of about 115° to 130°. The inventors have confirmed through experimentation that braid angle may affect certain mechanical properties of the implants of the present invention. For example, the inventors have found that while a braid angle of 110° for 6 mm PGCL coated braided implants made from PLGA 10:90 copolymer strands yields post-crimp RRF and COF values at 4.5 mm of 370 mm and 147 mm Hg, respectively, the same coated implants having a 127° braid angle are characterized by post-crimp RRF and COF values at 4.5 mm of 900 mm and 170 mm Hg, respectively. RRF and COF values for 7 mm PGCL coated PLGA 10:90 copolymer implants having a 140° braid angle were not obtainable because the samples buckled in the test equipment, thus demonstrating that a high braid angle may result in the inability of the implant to uniformly collapse during crimping into a catheter. Braid angle was also found to have an effect upon the ability of the implants of the present invention to recover to their as-manufactured diameters. For example, 7 mm PGCL coated braided implants made from PLGA 10:90 copolymer and having braid angles of 133°, 127°, and 110° were found to recover to about 96%, 94%, and 91% of their as-manufactured configurations, respectively, after being held at 37° C. for one hour.
• In another embodiment specific to woven structures, the ends of strands are fused in patterns on the outside and/or inside of theimplant100, as shown inFIG. 6. Fusing the strands in this manner causes theends132,133 to flare outwards instead of curling inward, thus addressing a phenomenon in which the ends of the implant may naturally curl inward when the implant is crimped into a reduced configuration for insertion into a bodily lumen or cavity. In one embodiment, the ends are fused in patterns such that their final positions are axially staggered. Strands are fused, for example, by using heat, chemicals, adhesives, crimp swages, coatings (elastomeric or non-elastomeric), or other suitable fusing or joining techniques.
• The implants of the present invention are preferably radiopaque such that they are visible using conventional fluoroscopic techniques. In one embodiment, radiopaque additives are included within the polymer material of one or more strands ofimplant100. Examples of suitable radiopaque additives include particles comprising iodine, bromine, barium sulfate, and chelates of gadolinium or other paramagnetic metals such as iron, manganese, or tungsten. In another embodiment, the radiopaque groups, such as iodine, are introduced onto the polymer backbone. In yet another embodiment, one or more biostable or biodegradable radiopaque markers, preferably comprising platinum, iridium, tantalum, and/or palladium are produced in the form of a tube, coil, sphere, or disk, which is then slid over one or more strands of fiber to attach to the ends ofimplant100 or at other predetermined locations thereon. When the marker is in the form of a tube or coil, it has a preferable wall thickness of about 0.050 to 0.075 mm and a length of about 0.3 to 1.3 mm. The tube is formed by extrusion or other methods known in the art. The coil is formed by winding a wire around a mandrel of desired diameter and setting the coil with heat or other methods known in the art.
• To facilitate delivery, theimplant100 is preferably loaded into a delivery catheter just prior to being implanted into a patient. Loading theimplant100 in close temporal proximity to implantation avoids the possibility that the polymer of theimplant100 will relax during shipping, storage, and the like within the delivery catheter and therefore cannot fully expand to a working configuration. As such, one aspect of the invention includes a method of delivering an implant of the invention that comprises the step of loading the implant into a delivery catheter within a short period of time, and preferably within one hour, before implantation into a body lumen. It should be noted, however, that it is not required that the implants of the present invention are loaded into delivery catheters just prior to being implanted. In fact, one advantage of the present invention is that it provides self-expanding implantable medical devices with preferred expansion characteristics and mechanical properties even after being loaded in a delivery catheter for prolonged periods.
• The present invention is further described with reference to the following non-limiting examples.
Example 1
• Braided implants were manufactured using a PLGA 10:90 copolymer by spooling fiber spun monofilaments onto individual bobbins. Each bobbin was placed on a braiding machine, strung through rollers and eyelets and wrapped around a mandrel. The braiding tension of the machine was set for the size of the monofilament (i.e., 70 g min, 215 g max for 0.005″ fiber). The pix/inch was set to obtain an ideal angle to maximize radial strength but still allow the braid to be removed from the mandrel (i.e., 110 to 135 degrees for a 6 mm mandrel). The braid pattern was selected and the monofilaments were braided off the spool onto the mandrel by the braiding machine. Tiewraps were used on the end of each mandrel to keep the tension on the filaments, which can be important for heat annealing and obtaining high modulus properties. The braided polymer was heat annealed on the mandrel, and then cut into desired lengths with a blade and removed from the mandrel. A representative implant was measured to have an outer diameter of about 6.0 mm and a length of about 20 mm.
• Implants were coated with a support coating made from poly(glycolide-co-caprolactone) (PGCL) cured with hexamethylene diisocyanate. The PGCL 50:50 copolymer was prepared as follows. A 100 mL round-bottom flask was dried in oven at 110° C. and then cooled to room temperature under a nitrogen atmosphere. The flask was charged with Sn(Oct)₂(15 mg), pentaerythritol (68 mg), glycolide (10.0 g), and s-caprolactone (10.0 g), respectively. Subsequently, the flask was equipped with a magnetic stir bar and a three-way valve connected to a nitrogen balloon. The flask was thoroughly degassed under reduced pressure and flushed with nitrogen. The flask was then placed into an oil bath which was preheated to 170° C. The reaction was stirred at 170° C. for 24 h under a nitrogen atmosphere. After cooling to room temperature, the solid obtained was dissolved in dichloromethane and precipitated from anhydrous diethyl ether. The solution was decanted and the residual sticky solid was washed thoroughly with diethyl ether and dried in vacuum. Typically, around 18 g of polymer was recovered through the purification. GPC characterization revealed a number average molecular weight (Mn) of 39,900 and a polydispersity index (PDI) of 1.23.
• The four-arm PGCL 50:50 (1.0 g) and HDI (375 μL) were dissolved in 20 mL dichloromethane to make a stock solution for spray-coating. A steel mandrel of 2 mm in diameter was mounted vertically onto a mechanical stirrer, and the braided implant was placed over the mandrel. A spray gun (Badger150) was arranged perpendicular to the mandrel, and connected to a nitrogen cylinder and a reservoir containing the stock solution. The mechanical stirrer was turned on to spin the mandrel and the solution was sprayed onto the braid by applying the nitrogen flow. The coating weight could be controlled by the spray time. After spray coating, devices were dried in air for 1 h and then cured at 100° C. for 16 h. A catalyst such as tin octanoate or zinc octanoate may also be used in the curing process to reduce the curing time and/or temperature.
• Coated devices were placed in an MSI radial force tester to obtain RRF and COF values, both measured in mm Hg, at time points prior to and subsequent to crimping the device to 2.5 mm.
• The inventors surprisingly found that whereas uncoated implants were able to recover to only about 50% of their original diameter after being crimped at 37° C. for one hour, the application of the PGCL/HDI coating resulted in a recovery of up to about 95% of the device original diameter. This effect was found to be at least partially dependent upon the coating weight on the implant. As illustrated inFIG. 7, diameter recovery was found to be related to the mass increase attributable to the application of the coating (i.e., coating weight), with an appreciable increase in the diameter recovery ability beginning when the increase in implant mass due to the application of the coating is about 15%. The effect of coating weight levels off when the increase in implant mass due to the application of the coating is at least about 20%, at which point the device recovery diameter is at least about 90%.
• Implant diameter recovery was also affected by the percent gel content in the coating, as shown inFIG. 8. Gel content is defined as the quantity of coating that is sufficiently crosslinked so that it is no longer soluble in a solvent. The calculation of gel content in the PGCL/HDI coating is described in Example 3. The inventors have found in this Example that a coating gel content of greater than about 80% is preferred to achieve a diameter recovery of at least about 90%.
• Table I shows the percent recovery, radial resistive force (RRF) at a post-crimp diameter of 4.5 mm, and chronic outward force (COF) at a post-crimp diameter of 4.5 mm, for coated implants with varying coating weight. As can be seen from the data in Table I, the support coating increased the post-crimp recovery ability, RRF, and COF of the implants. Moreover, in addition to increasing recoverability of the implant acutely (i.e., after being crimped one hour in a 37° C. water bath), the support coating is able to increase recoverability chronically (i.e., after being crimped for five days at room temperature in air).

• TABLE I
  Percent recovery, RRF, and COF for coated implant samples that
  were crimped to 2.5 mm and held in water bath at 37° C. The
  percent increase in implant weight due to the coating is shown in
  the Sample Description column.

  		Water	% Recovery	RRF at	COF at
  Sample	Sample	Bath	after 2.5 mm	4.5 mm	4.5 mm
  No.	Description	Treatment	Crimp	(mmHg)	(mmHg)

  1	10:90 PLGA	1 hour	48	83.4	11.6
  	uncoated
  2	10:90 PLGA	1 hour	93.5	—	—
  	coated 45%
  3	10:90 PLGA	1 hour	94.5	645.0	245.0
  	coated 41%
  4	10:90 PLGA	1 hour	94.6	—	—
  	coated 33%
  5	10:90PLGA	5 days at	94.4	—	—
  	coated 33%	room T in
  		air (no
  		water
  		bath)
  6	10:90 PLGA	1 hour	91.5	1030.0	188.0
  	coated 24%
  7	10:90 PLGA	1 hour	77.9	897.0	84.0
  	coated 22%
  8	10:90 PLGA	1 hour	65.6	559.0	24.4
  	coated 17%
  9	10:90 PLGA	1 hour	58.2	—	—
  	coated 15%
  10	10:90 PLGA	1 hour	51.4	393.0	−2.6
  	coated 13%
  11	PDO	1 hour	79.0	53.4	0
  	uncoated
  12	PDO coated	1 hour	83.6	863.0	115.0
  	37%

Example 2
• Implants and coating solutions were manufactured as described in Example 1. Instead of being applied uniformly to the implants, support coatings were applied to just the ends of the implants (i.e., approximately 4-5 mm at each end). Whereas the uncoated implant was able to recover only about 48% of its original diameter after being crimped to 2.5 mm and held in a water bath for one hour at 37° C., the implant coated at its ends was able to recover, under the same conditions, to about 82% of its original diameter. Moreover, for these same implants, the RRF and COF was increased from 83.4 mmHg and 11.6 mmHg to 595.0 mmHg and 55.0 mmHg, respectively.
Example 3
• The gel content of a coated implant (such as the implant described in Example 1) was measured via extraction. The PGCL/HDI coated device was placed in 5 mL of dichlormethane and shaken at room temperature for about 1 hour. The solvent was removed, and the device was rinsed thoroughly with dichloromethane and subsequently allowed to air dry for about 10 minutes. The device was placed in a convection oven at 100° C. to remove any residual solvent. The gel content in the coating was then determined using the following equation: % gel content in coating=((mass of coated device after extraction−mass of uncoated device)/(mass of coated device before extraction−mass of uncoated device))×100. A control experiment on the uncoated woven PLGA 10:90 structure showed no appreciable mass loss in a similar experiment.
Example 4
• A pattern was laser cut into a tubular base material to produce self-expanding medical implants. Some of the implants were coated with a support coating made from PGCL cured with HDI, as described in Example 1. Similar to the previous examples, coated implants demonstrated higher RRF and COF properties as compared with uncoated implants.
Example 5
• Braided implants having an as-manufactured diameter of 6 mm were manufactured using a PLGA 75:25 copolymer using a manufacturing process similar to that of Example 1. The implants were coated with a support coating made from PLCL 50:50 prepared as follows. A 250 mL round-bottom flask was dried in an oven at 110° C. and cooled to room temperature in a nitrogen atmosphere. The flask was charged with Sn(Oct)₂(11.5 mg), pentaerythritol (204 mg), lactide (30.0 g), and ε-caprolactone (30.0 g), respectively. Subsequently, the flask was equipped with a magnetic stir bar and a three-way valve connected to a nitrogen balloon. The flask was thoroughly degassed under reduced pressure and flushed with nitrogen. The flask was then placed into an oil bath which was preheated to 170° C. The reaction was stirred at 170° C. for 48 h under a nitrogen atmosphere. After cooling to room temperature, the highly viscous liquid obtained was dissolved in approximately 200 mL dichloromethane and precipitated from 1200 mL anhydrous diethyl ether. The solution was decanted and the residual sticky polymer was washed thoroughly with diethyl ether and dried under vacuum. Typically, around 48 g polymer was recovered through the purification. GPC characterization revealed a number average molecular weight (Mn) of 52,500 and a polydispersity index (PDI) of 1.2.
• Coated devices were crimped to 1.85 mm in a MSI radial force tester (Model# RX550-100) to obtain RRF and COF values, both measured in mm Hg, at a diameter of 4.5 mm for 6 mm device and at a diameter of 5.5 mm for 7 mm device. Like the implants coated with PGCL described in Example 1, both RRF and COF were found to be directly proportional to the coating weight. The inventors note that both RRF and COF for the coated devices are significantly higher than for uncoated devices. For example, a 7 mm implant having an increase in weight due to the PLCL/HDI coating of about 45% resulted in RRF of about 450 mm Hg and COF of 90 mm Hg, measured at 4.5 mm, while the uncoated device resulted in RRF and COF of 80 mm and 0 mm Hg, respectively.
Example 6
• Braided implants having an as-manufactured diameter of 6 mm were manufactured using a PLGA 75:25 copolymer using a manufacturing process similar to that of Example 1. The implants were coated with a support coating made from poly trimethylene carbonate (PTMC) and hexamethylenediisocyante. The PTMC three arm polymer was prepared as follows. A 100 mL round-bottom flask, previously dried under heat and vacuum, was charged with Sn(Oct)₂(20 mg), triethanolamine (298.4 mg) and trimethylene carbonate (30 g) respectively. Subsequently, the flask was equipped with a magnetic stir bar and a three-way valve connected to a nitrogen balloon. The flask was thoroughly degassed under reduced pressure and flushed with nitrogen and then placed into an oil bath which was preheated to 70° C. The oil bath temperature was then increased to 100° C. over 15 minutes. The reaction was stirred at 100° C. for 23 h under a nitrogen atmosphere. After cooling to room temperature, the viscous liquid obtained was dissolved overnight in approximately 50 mL dichloromethane and subsequently precipitated from 550 mL ethanol. The precipitated polymer was stirred for one hour after which the ethanol was decanted. The process of dissolving the polymer in dichloromethane and precipitating in ethanol was repeated. The polymer was then dissolved in dichloromethane, precipitated into 550 mL diethyl ether and stirred for one hour after which time the diethyl ether was decanted. The polymer was then dried under vacuum @ 70° C. for a period of 72 hours. Typically 24 g of polymer was recovered using above process. GPC characterization of the final polymer revealed a number average molecular weight (Mn) of 29 kDa and a polydispersity index (PDI) of 2.0.
• An MSI radial force tester (Model# RX550-100) was used to obtain radial resistive force (“RRF”) and chronic outward force (“COF”), at a post-crimp diameter of 4.5 mm for PTMC/HDI coated devices. For example, a 6 mm implant having an increase in weight due to the PTMC/HDI support coating of about 50% resulted in RRF of 490 mm Hg and COF of 83 mm Hg, measured at 4.5 mm. PTMC is considered to be a surface-degrading material which does not generate acidic byproducts upon degradation.
Example 7
• A support coating solution was prepared by dissolving 1.0 g of PLCL 50:50 copolymer (Mn=67 kDa) in 19.875 mL of methylene chloride. Subsequently, 0.125 mL of hexamethylene diisocyanate is added to the solution, which was then transferred to a 60 mL polypropylene syringe using a 14 gauge needle. No catalyst was added to the support coating solution at any time.
• The support coating solution was sprayed onto PLGA 85:15 braided implants. After spray coating, the implants were allowed to dry for up to 60 minutes in a nitrogen atmosphere. The implants were then transferred to a curing oven for a two-step curing process consisting of a first step at 75° C. for 16 hours, followed by a second step at 100° C. for a minimum of 96 hours.
Comparative Example 1
• Implants of the present invention were compared to several commercially available, biostable self-expanding implants, namely the VIABAHN® Endoprosthesis (a self-expanding nitinol stent-graft from W.L. Gore & Associates, Inc.), the S.M.A.R.T.® stent (a self-expanding nitinol stent from Cordis Corp.), and the WALLSTENT® Endoprosthesis (a self-expanding steel stent from Boston Scientific Corp.). Specifically, the RRF and COF values of these commercially available implants were measured and compared with 6 mm (having a 127° braid angle) and 7 mm (having braid angles of 127° and 110°) diameter implants made from PLGA 75:25. The PLGA implants were made using a manufacturing process similar to that of Example 1, and coated with a support coating made from PLCL 50:50 copolymer and hexamethylenediisocyanate as described in Example 5 with a coating weight of between 44% and 50% for the 7 mm devices and a weight of 47% to 57% for the 6 mm devices. RRF and COF values were measured at post-crimp diameters corresponding to the range of intended target vessel diameters. The results demonstrated that the implants of the present invention are characterized by mechanical properties such as RRF and COF that are comparable to their commercially available metallic counterparts.
Comparative Example 2
• Braided implants having an as-manufactured diameter of 6 mm were manufactured using a PLGA 75:25 copolymer using a manufacturing process similar to that of Example 1. One set of implants were coated with a support coating comprising PGCL 50:50 and hexamethylenediisocyanate, and another set of implants were coated with a support coating comprising PLCL 50:50 and hexamethylenediisocyanate. The implants were deployed into vessel-compliant tubing and maintained at 37° C. under simulated flow conditions for 0, 7, 14, and 28 days. At those time points, the implants were explanted from the tubing, dried overnight, and the COF of the implants was measured at a post-crimp diameter of 4.5 mm. The results are shown inFIG. 9, which demonstrate that the COF for both sets of implants decrease over time, with COF going to zero for the PGCL coated implants as of 28 days. Also shown inFIG. 9 are the COF values measured for the S.M.A.R.T. stent and WALLSTENT Endoprosthesis as described in Comparative Example 1, which are constant because those devices are metallic and do not degrade or relax over time. The inventors believe that the significant decrease in COF over time for the implants of the present invention is an advantage over their metallic stent counterparts because a continuous force applied by implants against surrounding tissue over time may result in chronic irritation and inflammation leading to restenosis.
• The present invention provides woven and non-woven self-expanding medical implants for placement within a bodily lumen that have sufficient strength and other mechanical properties that are necessary to effectively treat a variety of medical conditions. While aspects of the invention have been described with reference to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention.

Claims (25)

1. A polymer coating composition prepared by a process comprising the steps of:

dissolving poly (lactic acid-co-caprolactone) in a solvent to form a solution;

adding a diisocyanate crosslinker to the solution; and

curing said solution;

wherein said solution does not comprise a catalyst.

2. The polymer coating composition ofclaim 1, wherein said diisocyanate crosslinker is hexamethylene diisocyanate.

3. The polymer coating composition ofclaim 1, wherein said poly (lactic acid-co-caprolactone) comprises about 50 weight percent of lactic acid and about 50 weight percent of caprolactone.

4. The polymer coating composition ofclaim 1, wherein said solution comprises about 0.125 mL of said diisocyanate crosslinker to every 1.0 grams of poly (lactic acid-co-caprolactone).

5. The polymer coating composition ofclaim 4, wherein said diisocyanate crosslinker is hexamethylene diisocyanate.

6. The polymer coating composition ofclaim 4, wherein said solution comprises about 19.875 mL of a solvent to every 1.0 grams of poly (lactic acid-co-caprolactone).

7. The polymer coating composition ofclaim 6, wherein said solvent comprises methylene chloride.

8. A medical implant comprising a coating, said coating prepared by a process comprising the steps of:

dissolving poly (lactic acid-co-caprolactone) in a solvent to form a solution;

adding a diisocyanate crosslinker to the solution; and

curing said solution;

wherein said solution does not comprise a catalyst.

9. The medical implant ofclaim 8, wherein said diisocyanate crosslinker is hexamethylene diisocyanate.

10. The medical implant ofclaim 8, wherein said poly (lactic acid-co-caprolactone) comprises about 50 weight percent of lactic acid and about 50 weight percent of caprolactone.

11. The medical implant ofclaim 8, wherein said solution comprises about 0.125 mL of said diisocyanate crosslinker to every 1.0 grams of poly (lactic acid-co-caprolactone).

12. The medical implant ofclaim 11, wherein said diisocyanate crosslinker is hexamethylene diisocyanate.

13. The medical implant ofclaim 11, wherein said solution comprises about 19.875 mL of a solvent to every 1.0 grams of poly (lactic acid-co-caprolactone).

14. The medical implant ofclaim 13, wherein said solvent comprises methylene chloride.

15. The medical implant ofclaim 8, wherein said implant comprises polymeric strands coated with said coating.

16. The medical implant ofclaim 15, wherein said strands comprise poly(lactic acid co-glycolic acid).

17. The medical implant ofclaim 16, wherein said poly(lactic acid co-glycolic acid) comprises about 85 weight percent of lactic acid and about 15 weight percent of glycolic acid.

18. A medical implant comprising a coating, said implant prepared by a process comprising the steps of:

providing an implant comprising braided polymeric strands;

dissolving poly (lactic acid-co-caprolactone) in a solvent to form a solution;

adding a diisocyanate crosslinker to the solution;

coating said solution onto said polymeric strands; and

curing said solution;

wherein said solution does not comprise a catalyst.

19. The medical implant ofclaim 18, wherein said polymeric strands are biodegradable.

20. The medical implant ofclaim 19, wherein said strands comprise poly(lactic acid co-glycolic acid).

21. The medical implant ofclaim 20, wherein said poly(lactic acid co-glycolic acid) comprises about 85 weight percent of lactic acid and about 15 weight percent of glycolic acid.

22. The medical implant ofclaim 18, wherein said diisocyanate crosslinker is hexamethylene diisocyanate.

23. The medical implant ofclaim 18, wherein said poly (lactic acid-co-caprolactone) comprises about 50 weight percent of lactic acid and about 50 weight percent of caprolactone.

24. The medical implant ofclaim 18, wherein said solution comprises about 0.125 mL of said diisocyanate crosslinker to every 1.0 grams of poly (lactic acid-co-caprolactone).

25. The medical implant ofclaim 24, wherein said diisocyanate crosslinker is hexamethylene diisocyanate.

Images