A thromboresistant coating for a medical device, method of coating and coated medical device, the coating including an in situ cross-linked co-polymer of a cross-linkable biomolecule, preferably an adsorbable biomolecule such as a heparin activity biomolecule with at least one prosthetic hydrophobic unit, and a multifunctional crosslinking agent, such as a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol, wherein the crosslinking is by means of covalent complexation through at least two functional groups of the multifunctional crosslinking agent.

Description of the Invention

BRIEF SUMMARY OF THE INVENTION

The invention provides a method of forming a cross-linked coating on a medical device, which method includes the steps of immersing the medical device in a first solution including an organic solvent and a multifunctional crosslinking agent, and immersing the medical device in a second solution including an organic solvent and a cross-linkable biomolecule. It is to be understood that in general either step may occur first. In the method, prior to immersing the medical device in the first solution or second solution as provided, the medical device can be immersed in a wetting solution. In a preferred embodiment, the first solution does not include water and the second solution includes from about 10 to 80 percent water by volume. In one embodiment of the method, immersion in the solution including the multifunctional crosslinking agent occurs prior to immersion in the solution including a cross-linkable biomolecule, and the method further includes the step of immersing the medical device in the first solution including an organic solvent and a multifunctional crosslinking agent subsequent to immersing the medical device in the second solution. The multifunctional crosslinking agent can include a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol. The cross-linkable biomolecule can be a cross-linkable adsorbable biomolecule, including a cross-linkable adsorbable heparin activity biomolecule.

In another embodiment, the invention provides a method of forming a thromboresistant coating on a porous surface of a medical device, which method includes the ordered steps of: (a) providing a medical device with a porous surface; (b) wetting the porous surface by immersion in a wetting solution; (c) immersing the porous surface in a first solution including a first organic solvent and a multifunctional crosslinking agent; (d) immersing the porous surface in a second solution including a second organic solvent and a cross-linkable biomolecule; and (e) immersing the porous surface in the first solution including the first organic solvent and the multifunctional crosslinking agent.

In the ordered method, the porous surface of the medical device can include expanded polytetrafluoroethylene. The wetting solution can include an organic solvent, such as acetone, isopropanol, acetonitrile, methanol, ethanol or a combination thereof. The multifunctional crosslinking agent can consist of a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol. In one embodiment, the multifunctional crosslinking agent is bis-(benzotriazole carbonate) polyethylene glycol. Where the multifunctional crosslinking agent is a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol, it is at a concentration between about 0.001 mg/mL and 500 mg/mL, more preferably between about 0.2 mg/mL and 10 mg/mL. The first organic solvent can be acetonitrile or acetone, and preferably the first solution does not include water. Thus in one embodiment the first solution does not include water and the second solution includes from about 10 to 80 percent water by volume. The cross-linkable biomolecule can be a cross-linkable adsorbable biomolecule, and in a preferred embodiment, a conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule. The conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule can have from 1 to about 30 hydrophobic silyl moieties conjugated to the heparin activity biomolecule. In this embodiment, the heparin activity molecule with from 1 to about 30 hydrophobic silyl moieties conjugated thereto is at a concentration in the second solution of from about 0.01% to about 10%, and more preferably from about 25% to about 1.5%. In a preferred embodiment, the conjugate of from 1 to 30 hydrophobic silyl moieties and the heparin activity biomolecule is benzyl-bis(dimethylsilylmethyl).sub.x-oxycarbamoyl-heparin. The second organic solvent can be the same as the first organic solvent. In a preferred embodiment, the second solution further includes from about 10 to 80 percent water by volume. In the method, immersing in each step can be for between about 5 minutes and two hours, preferably where immersing the porous surface in the first solution is in each step for between about 15 minutes and about one hour, and immersing the porous surface in the second solution is for between about 45 minutes and about 75 minutes.

The invention further provides a thromboresistant expanded polytetrafluoroethylene vascular graft including a tubular expanded polytetrafluoroethylene construct with an interior lumen and a cross-linked co-polymer coating on the surface of the interior lumen, the cross-linked co-polymer coating consisting essentially of a conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule cross-linked with a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol. In this graft, the conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule can be from 1 to 30 hydrophobic silyl moieties conjugated to the heparin activity biomolecule. The bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol can be bis-(benzotriazole carbonate) polyethylene glycol.

The invention further provides a medical device with a thromboresistant blood-contacting surface including at least one porous blood-contacting surface and a cross-linked co-polymer coating on the porous surface, the cross-linked co-polymer coating consisting essentially of a conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule cross-linked with a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol. In this medical device, the at least one porous blood-contacting surface can include expanded polytetrafluoroethylene, or alternatively can include a woven polymeric surface. In the medical device, the conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule can be from 1 to 30 hydrophobic silyl moieties conjugated to the heparin activity biomolecule. The bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol can be bis-(benzotriazole carbonate) polyethylene glycol.

In yet another embodiment, the invention provides a thromboresistant coating for a medical device, including an in situ cross-linked co-polymer consisting essentially of a conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule cross-linked with a bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol. Here too the conjugate of at least one prosthetic hydrophobic unit and a heparin activity biomolecule can be from 1 to 30 hydrophobic silyl moieties conjugated to the heparin activity biomolecule, and the bis-variant of polyethylene glycol, polyethylene oxide, or polyethylene glycol can be bis-(benzotriazole carbonate) polyethylene glycol.

A primary object of the present invention is to provide a coating composition for contacting surfaces of implantable medical devices, particularly porous medical devices such as ePTFE vascular grafts or sheets or woven materials, wherein the composition includes a cross-linked co-polymer including a biomolecule and a multifunctional crosslinking agent, which cross-linked co-polymer is preferably attached to the contacting surface by hydrophobic interaction.

A further object of the invention is to provide a coating composition and method wherein the attachment to a substrate can be varied, such that in one embodiment the invention provides a cross-linked silyl-heparin and multifunctional crosslinking agent wherein the number of silyl moieties per heparin molecule is varied, thereby varying the hydrophobicity of the resulting cross-linked co-polymer.

A further object is to provide a coating composition and method wherein the biomolecule is a heparin activity molecule, including heparin and heparin derivatives.

A further object is to provide a coating composition wherein the biomolecule is a crosslinkable bioactive molecule other than a heparin activity molecule, including serum albumin and collagen.

A further object of the present invention is to provide a cost effective and commercially feasible method for coating polymeric medical devices, including medical devices with a porous microstructure, with a coating including a thromboresistant bioactive molecule.

A further object of the present invention is to provide a cost effective and commercially feasible method for coating polymeric medical devices, including medical devices with a porous microstructure, with a thromboresistant coating including a cross-linked silyl-heparin composition.

A primary advantage of the present invention is that it provides for coating contacting surfaces of medical devices of complex geometries and surfaces with a durable and low-cost coating that promotes the desired biological or therapeutic effect.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for making a biocompatible, thromboresistant medical device, and preferably a blood-compatible medical device, by means of a cross-linked co-polymer coating. The invention is composed of a cross-linkable biomolecule, preferably an adsorbable cross-linkable biomolecule, and a bifunctional crosslinking agent reacted such that the resulting co-polymer is formed in situ on a medical device. The resulting cross-linked co-polymer coating preferably has a resident time on the device longer, and preferably substantially longer, than that of either the biomolecule alone or the biomolecule adsorbed by conjugation of a prosthetic unit.

The invention provides a covalently cross-linked co-polymer coating, formed by the in situ crosslinking of a cross-linkable biomolecule, preferably a cross-linkable and adsorbable biomolecule, with a multifunctional crosslinking agent, wherein the crosslinking is through at least two functional groups of the multifunctional crosslinking agent. A preferred cross-linkable biomolecule is a heparin activity biomolecule, such as heparin and derivative and related molecules, including heparan sulfate, hyaluronic acid, dextran, dextran sulfate, chondroitin sulfate, dermatan sulfate, or a molecule including a mixture of variably sulfated polysaccharide chains composed of repeating units of D-glucosamine and either l-iduronic or D-glucuronic acids, salts of any of the foregoing and derivatives of any of the foregoing. Other biomolecules that may be used include chondroitin sulfate A, chondroitin sulfate C, dermatan sulfate, heparin, keratan sulfate, keratosulfate, chitin, chitosan 1, chitosan 2, and mixtures or derivatives of these glycosaminoglycans. An adsorbable biomolecule can include any biomolecule, including heparin activity biomolecules, which contain a hydrophobic prosthetic unit, such as benzylated silyl groups or alkane chains. In one embodiment silyl-heparin is an adsorbable biomolecule. Biomolecules may be employed which are not heparin activity biomolecules, providing that such biomolecules may be covalently cross-linked by means of the multifunctional crosslinking agent and are biologically active. The multifunctional crosslinking agent is a multifunctional compound with at least two functional groups, and includes bis-variants of polyethylene glycol, polyethylene oxide, and polyethylene glycol compounds as well as other carbon-based units including linear sequences and branched chains. In a preferred embodiment the multifunctional crosslinking agent is PEG with benzotriazole carbonate moieties as the active functional groups, but the multifunctional crosslinking agent may also include bis-variants of polyethylene glycol, polyethylene oxide, and polyethylene glycol compounds where the functional groups are homo- or hetero-functional groups such as succinimidyl esters, nitrophenyl activated esters, azidophenyl groups, maleimido groups, imido esters, carbodiimides, benzotriazole carbonates, epoxide groups, or aldehdye groups.

The covalently cross-linked co-polymer coating is preferably present on a porous structure, such as a structure including ePTFE. The coating is made, in part, by means of sequential deposition, such as by immersion in a solution, first of one member, such as the bioactive molecule, and second and separately by a second member, such as the multifunctional crosslinking agent. It is hypothesized, without wishing to be bound thereby, that particularly with porous structures the structure surface creates an interface boundary, such that for example a bioactive molecule in a suitable solvent enters the matrix of the porous structure and subsequently a multifunctional crosslinking agent is utilized in a second different suitable solvent, such that there is diffusion out of the bioactive molecule and diffusion in of the multifunctional crosslinking agent resulting in reaction of the two groups at an interface boundary proximate the surface of the porous structure.

The invention further provides a method for in situ crosslinking of a cross-linkable biomolecule, preferably a cross-linkable and adsorbable biomolecule, with a multifunctional crosslinking agent, wherein the crosslinking is by means of covalent complexation through at least two functional groups of the multifunctional crosslinking agent.

In one embodiment, the structure to be coated is made of ePTFE, such as a vascular graft structure. Because in part of the microstructure of the structure, including the porosity thereof, it is desirable to wet the structure. This may be done by means of a suitable wetting solvent, preferably a suitable organic solvent. In the case of ePTFE, one solvent that may be employed is acetone. Following wetting of the structure by immersion in a suitable solvent such as acetone for a period sufficient to result in wetting of the entire structure, the structure is transferred to a solution including either the biomolecule, preferably an adsorbable biomolecule, or the multifunctional crosslinking agent. The biomolecule or multifunctional crosslinking agent, as the case may be, is dissolved in a first solution preferably including an organic solvent. In one embodiment, the organic solvent employed to wet the structure is miscible, preferably completely miscible, with the organic solvent utilized for the biomolecule or multifunctional crosslinking agent. Following immersion of the structure in a solution including the biomolecule or multifunctional crosslinking agent, the structure is then immersed in a second solution containing the member not present in the first solution. The member in the second solution is similarly dissolved in a solution preferably including an organic solvent, and preferably an organic solvent that is miscible, preferably completely miscible, with the solvent of the first solution. In one embodiment, the same organic solvent is employed in both the first solution and the second solution, but in varying concentrations. Thus it may be seen that the first solution can include dissolved therein a biomolecule, preferably an adsorbable biomolecule, and the second solution can include dissolved therein a multifunctional crosslinking agent. Conversely, the first solution can include dissolved therein a multifunctional crosslinking agent and the second solution can include dissolved therein a biomolecule, preferably an adsorbable biomolecule. It is further possible and contemplated that one or more applications can be repeated, such that for example the following schemes may be employed: (a) immersion in first solution including the biomolecule, which is preferably an adsorbable biomolecule; (b) thereafter immersion in second solution including a multifunctional crosslinking agent; and (c) thereafter immersion in a third solution that is identical to first solution and includes a biomolecule and preferable an adsorbable biomolecule. Alternatively the following scheme may be employed: (a) immersion in a first solution including a multifunctional crosslinking agent; (b) thereafter immersion in a second solution including a biomolecule and preferable an adsorbable biomolecule; and (c) thereafter immersion in a third solution that is identical to the first solution and includes a multifunctional crosslinking agent. In the immediately preceding scheme, it is also possible and contemplated that the multifunctional crosslinking agent in the third solution can be different from the multifunctional crosslinking agent in the first solution, or alternatively it can be present in a different concentration. In instances where the first and third solution includes a biomolecule, the biomolecule may be different or alternatively present in different concentrations. For example, the first solution may include an adsorbable biomolecule, such as a silyl-heparin, while the third solution may include a biomolecule, such as heparin, that is less adsorbable than the adsorbable biomolecule. It may also be seen that either of the foregoing schemes can be extended to a fourth or subsequent solution. In any event, following immersion in the last solution including either a biomolecule or multifunctional crosslinking agent, the structure may be rinsed in an appropriate solvent, such as an organic solvent, to remove unreacted biomolecules or multifunctional crosslinking agents and any breakdown or other products resulting from the crosslinking reaction.

In a preferred embodiment, an adsorbable biomolecule is employed, wherein the biomolecule intrinsically contains reactive amino groups (--NH.sub.2) for crosslinking, and further wherein one or more hydrophobic prosthetic units are conjugated to the biomolecule. Silyl-heparin is one example of an adsorbable biomolecule that may be so employed. In this embodiment, a preferred multifunctional crosslinking agent is a multifunctional compound with at least two functional groups, and includes bis-variants of polyethylene glycol, polyethylene oxide, and polyethylene glycol compounds as well as other carbon-based units including linear sequences and branched chains, and preferably PEG which contains benzotriazole carbonate moieties as the active functional groups. The method and coating is preferably employed with a substrate including matrices, such as an ePTFE structure.

For use with ePTFE structures, a preferred organic solvent for use as a wetting solution is acetone, with wetting by immersion of the ePTFE structure in acetone for a period between about ten minutes and one hour, preferably between about twenty minutes and forty minutes, at a temperature less than the boiling point of acetone, and preferably at a temperature between about 27.degree. C. and 40.degree. C., most preferably about 37.degree. C. Other wetting agents may be employed, such as isopropanol, acetonitrile, methanol, ethanol and the like. Following wetting, the ePTFE structure is transferred to a first solution consisting of an organic solvent, such as 100% acetonitrile, and a multifunctional crosslinking agent, such as a bis-variant of polyethylene glycol, preferable bis-benzotriazole carbonate(polyethylene glycol) dissolved in acetonitrile at a concentration between about 0.001 mg/mL and 500 mg/mL, preferably between about 0.2 mg/mL and 10 mg/mL. Other solvents may be employed, such as for example acetone. In one embodiment a bis-benzotriazole carbonate(polyethylene glycol) with a molecular weight of between about 3,400 and 10,000 daltons is employed. Immersion is for a sufficient time to allow the multifunctional crosslinking agent to enter the ePTFE structure by means of diffusion, typically between about 5 minutes and 2 hours or more, preferably about 30 minutes, with immersion at any suitable temperature, such as room temperature. The ePTFE structure is removed from the first solution and immersed in a second solution consisting of a second organic solvent, such as 60% acetonitrile, and an adsorbable biomolecule, such as silyl-heparin, preferably a benzyl-bis(dimethylsilylmethyl).sub.x-oxycarbamoyl-heparin, at a concentration between about 0.01% and 10%, preferably between about 0.25% and 1.5%. Immersion is for a sufficient time to allow the adsorbable biomolecule to enter the ePTFE structure, adsorb to the structure thereof and cross-link with the multifunctional crosslinking agent, typically between about 5 minutes and 2 hours or more, preferably about one hour, with immersion at any suitable temperature, such as room temperature. In a preferred embodiment, the ePTFE structure is then immersed in a solution containing the multifunctional crosslinking agent, such as the first solution, for a suitable period of time, such as about 30 minutes. Alternatively, the order of immersion may be reversed, with the first solution including the adsorbable biomolecule such as a benzyl-bis(dimethylsilylmethyl).sub.x-oxycarbamoyl-heparin in 60% acetonitrile but with immersion preferable for about 30 minutes, and the second solution including the multifunctional crosslinking agent such as bis-benzotriazole carbonate(polyethylene glycol) in 100% acetonitrile but with immersion preferable for about one hour. In either instance, a third or subsequent application of an alternating substance may be made. Following the final immersion, the ePTFE structure may be rinsed, such as by rinsing in four serial changes of acetonitrile using a 15 minute incubation at each rinse.

It may be seen that while the foregoing uses the term "immersion", any of a variety of application methods may be employed and are contemplated thereby. For example, if the objective is to coat the interior lumen of a structure such as a tube, immersion may be by pumping or otherwise passing the various solutions through the tubing.

As used herein a "medical device" is defined as any article or device that has surfaces that contact tissue, blood, or other bodily fluids in the course of their operation. This includes, for example, extracorporeal devices for use in surgery such as blood oxygenators, blood pumps, blood sensors, tubing used to carry blood and other devices which contact blood which is then returned to the patient. This also includes endoprostheses implanted in blood contact in a human or animal body such as vascular grafts, stents, pacemaker leads, heart valves, and other devices implanted in blood vessels or in the heart. This also includes devices for temporary intravascular use such as catheters, guide wires, and other devices placed into blood vessels or the heart for purposes such as of monitoring or repair. Of particular utility in the practice of the invention are vascular grafts composed of extended polytetrafluoroethylene (ePTFE).

The medical device surfaces that may be coated by the present process include homo- and co-polymers, for example polyolefins, such as polyethylene, polypropylene, polyisobutylene, polybutadiene, polyisoprene, naturally occurring rubbers and polyethylene-copropylene; halogen-containing polymers, such as polyvinyl chloride, polyvinylidene chloride, polychloroprene, polytetrafluorothylene and polyvinylidene fluoride; polymers and co-polymers of vinylaromatic monomers, such as polystyrene, polyvinyloluene, polystyrene-co-vinyltoluene, polystyrene-co-acrylonitrile and polystyrene-co-butadiene-co-acrylonitrile, polycondensates, for example polyesters, such as polyethylene terephthalate and polybutylene terephthalate; polyamides, such as polycaprolactam, polylaurolactam and the polycondensate of adipic acid and hexamethylenediamine; and polyurethanes, polyethers, polycarbonates, polysulfones, polyether ketones, polyester-amides and -imides, polyacrylonitrile, polyacrylates and polymethacrylates. Blends of two or more polymers or co-polymers can be used in medical device surfaces, as can combinations of various plastics that are joined to one another, such as by adhesive bonding, welding or fusion.

In a preferred embodiment, the medical device is an artificial vascular prosthesis used as a vascular graft, made from a porous material. One such material is ePTFE having a microstructure consisting of nodes interconnected by fibrils, such as fibrils from about 5 .mu.m length up to about 100 .mu.m length, and typically with fibrils from between about 20 and 40 .mu.m length. In another preferred embodiment, the medical device is made from a porous material, such as a matrixed polymeric material. In yet another preferred embodiment, the porous material is a woven material.

A "multifunctional crosslinking agent" is a multifunctional compound with at least two functional groups, constituting a bifunctional crosslinking agent if two functional groups are present, such as bis-variants of polyethylene glycol, polyethylene oxide, and polyethylene glycol compounds as well as other carbon-based units including linear sequences and branched chains. The molecular weight of the multifunctional crosslinking agent is preferably between 3,000 and 11,000 daltons, but may be between 100 daltons and 500,000 daltons. The multifunctional crosslinking agent preferably contains benzotriazole carbonate as the active functional group, but may alternatively include functional groups selected from any of a number of agents known to those skilled in the art, and in particular bis-variants of polyethylene glycol, polyethylene oxide, and polyethylene glycol compounds where the functional groups are composed of homo- or hetero-functional groups such as succinimidyl esters, nitrophenyl activated esters, azidophenyl groups, maleimido groups, imido esters, carbodiimides, benzotriazole carbonates, epoxide groups, or aldehdye groups. A number of these compounds are commercially available. Of particular utility are bis(benzotriazole)polyethylene glycol and succinimidyl esters of polyethylene glycol such as bis(succininymidyl propionate) polyethylene glycol and bis(succininymidyl butanoate) polyethylene glycol. Other crosslinking agents such as bis[2-(succinimidyloxycarbonyloxy)-ethyl]sulfone, bis(sulfosuccinimidyl)suberate, 1,5 difluoro-2,4-dinitrobenzene, dimethyl adipimidate, dimethyl pimelimidate, dimethyl suberimidate, disuccinimidyl glutarate, dithiobis(succinimidyl proprionate), disuccinimidyl suberate, ethylene glycol bis(succinimidylsuccinate) and others known to those skilled in the art may similarly be employed.

An "organic solvent" is a solvent containing at least one component including carbon atoms, such as acetone, acetonitrile, methylene chloride, dimethyl formamide, tetrahydrafuran, methanol, ethanol, isopropanol, dimethyl sulfoxide, or the like or mixtures or combinations thereof. An organic solvent may include any percentage of water, such as a 60% organic solvent solution which includes 40% water.

An "adsorbable biomolecule" is a biomolecule that adheres to the surface of a medical device by hydrophobic interaction, particularly where the biomolecule has been rendered adsorbable by conjugation with one or more hydrophobic prosthetic units. Prosthetic units containing benzylated silyl groups or alkane chains are of particular utility. One adsorbable biomolecule is silyl-heparin as described in U.S. Pat. No. 5,955,588 and other silyl-heparin variants. Another adsorbable biomolecule is dodecyldimethylsilylmethyl heparin carbonate. The adsorbable biomolecule may also be inherently adsorbable.

A "biomolecule" is a cross-linkable biologically active molecule. A biomolecule may, but need not, constitute an adsorbable biomolecule. One such biomolecule is heparin. Heparin inhibits the coagulation of blood by interacting with antithrombin III and thrombin to inhibit the conversion of fibrinogen to fibrin. Other biomolecules include extracellular matrix molecules such as collagen, gelatin, elastin, fibronectin, glycosaminoglycans, antibacterial and antimicrobial agents; anticoagulant and antithrombotic agents; platelet agents; anti-inflammatories; enzymes; catalysts; hormones; growth factors; drugs; vitamins; antibodies; antigens; nucleic acids; dyes (which act as biological ligands); DNA and RNA segments; and proteins and peptides. The biomolecules can be synthetically derived or naturally occurring. Biomolecules also include heparin, heparin fragments, heparin-mimetics, prostaglandin E.sub.1 (PGE.sub.1), ticlopidine, plasmin, urokinase, tissue plasminogen activator, hirudin, dextran sulfates, gelatin, albumin, and bioactive polypeptides. Ticlopidine and prostaglandin E.sub.1 inhibit the activation of platelets. Plasmin, urokinase, and TPA are serine proteases that lyse fibrin. Certain biomolecules contain reactive groups, such as amino groups (--NH.sub.2) present in heparin, which may be employed in crosslinking. It is to be understood that the reactive group on the biomolecule must be complementary to the active functional group of the multifunctional crosslinking agent, such that on crosslinking a covalent bond linkage is formed. For biomolecules not containing an appropriate reactive group, a reactive group may be introduced by means of chemical modification.

A "heparin activity biomolecule" is a biomolecule which includes heparin or derivative and related molecules, including heparan sulfate, hyaluronic acid, dextran, dextran sulfate, chondroitin sulfate, dermatan sulfate, or any molecule including a mixture of variably sulfated polysaccharide chains composed of repeating units of D-glucosamine and either l-iduronic or D-glucuronic acids, salts of any of the foregoing, derivatives of any of the foregoing and combinations of any of the foregoing. A heparin activity molecule may be an adsorbable biomolecule, and specifically an adsorbable heparin activity biomolecule, when it includes one or more hydrophobic prosthetic units.

A "biocompatible" material is one that does not generally cause significant adverse reactions, such as toxic or antigenic responses in the body, whether it degrades within the body, remains for extended periods of time, or is excreted whole. Ideally, a biocompatible material will not induce undesirable reactions in the body as a result of contact with bodily fluids or tissue, such as tissue death, tumor formation, allergic reaction, foreign body reaction or rejection or inflammatory reaction.

A "blood compatible" material is one that will not induce undesirable reactions in the body as a result of contact with blood, such as blood clotting. This can be demonstrated by reduced thrombin generation, for example.

As used herein, "silyl-heparin" is a family of adsorbent molecules based on benzyl-bis(dimethylsilylmethyl).sub.x-oxycarbamoyl-heparin, and which may be synthesized in accord with procedures described generally in U.S. Pat. No. 5,955,588. Silyl-heparin is amphipathic and is readily adsorbed onto hydrophobic surfaces. Silyl-heparins are generally applied to surfaces such as medical devices by "dip-coating", such as application by immersion. Silyl-heparins are easy and simple to apply as a coating. They require no special equipment and no special technical skills for use, and can be applied to most metals and synthetic polymers used in the construction of medical devices including polypropylene, polyethylene, polyurethane, polyvinyl chloride, poly tetrafluoroetheylene, polycaprolactone, and poly (lactide:co-glycolide), as well as stainless steel, titanium, and platinum.

In one embodiment, the medical device is a vascular graft. The vascular graft may be composed entirely or in part of ePTFE, DACRON.RTM. synthetic fibers, polyurethane, or other appropriate materials. Preferably, the vascular graft has a porous microstructure.

Preferably, a porous medical device such as a vascular graft is wetted by treatment with an organic solvent. The need for the use of a wetting agent is determined by the chemical nature and geometry of the medical device, and may, in some cases, not be required or desired.

The medical device is treated with a multifunctional crosslinking agent dissolved in an organic solvent such as acetonitrile. Ideally, the wetting agent, if used, should be completely miscible with the organic solvent employed with the multifunctional crosslinking agent.

The multifunctional crosslinking agent preferably includes a PEG. The functional groups of the multifunctional crosslinking agent are preferably composed of benzotriazole carbonate or succinimidyl groups. In one embodiment the molecular weight of the multifunctional crosslinking agent is preferably approximately 3,800 daltons.

In one embodiment, bis-benzotriazole carbonate(polyethylene glycol) (BTC-PEG) is dissolved in acetonitrile and the medical device immersed in the solution. The concentration of BTC-PEG can range from 0.001 mg/mL to 500 mg/mL, but preferably is between about 0.2 and 10 mg/mL. The length of time the medical device is immersed can range from about 5 minutes to 2 hours or more but is preferably about 30 minutes. In the case of a porous device such as a porous vascular graft, the BTC-PEG enters the wall of the vascular graft by diffusion.

The BTC-PEG-treated medical device is transferred to a solution, miscible with the BTC-PEG solution, which contains the adsorbable biomolecule. In one embodiment, the adsorbable biomolecule is a silyl-heparin dissolved in 60% acetonitrile. As the BTC-PEG diffuses out from the porous medical device such as a porous vascular graft and the silyl-heparin diffuses in and is adsorbed to the medical device surface, the functional groups of the BTC-PEG react with the silyl-heparin thereby resulting in an adsorbed, cross-linked co-polymer coating that is thromboresistant. Synthesis of the coating is carried out using a proportion of multifunctional crosslinking agent and adsorbable biomolecule to optimize the polymerization of the biomolecule, bioactivity, and removal of unreacted multifunctional crosslinking agent or its hydrolysis products. This allows for a simultaneous in situ polymerization of the biomolecule and coating of the medical device.

Unreacted BTC-PEG and breakdown products are removed by repeated rinsing in an appropriate solvent, typically acetonitrile. Acetonitrile is of particular utility when used with vascular grafts composed of ePTFE, as drying from this solvent does not result in shrinkage or foreshortening of the graft. Thereafter, the vascular graft is air-dried at a suitable temperature, such as 56.degree. C., although any number of drying conditions may be used.

The present invention provides a simple method for making a biocompatible, thromboresistant medical device, and preferably, a blood compatible medical device, through the use of a cross-linked co-polymer coating. The invention is composed of an adsorbable biomolecule and a multifunctional crosslinking agent reacted such that the resulting co-polymer is deposited in situ on a medical device. The following generally describes methods applicable to vascular grafts composed of ePTFE utilizing a silyl-heparin adsorbable biomolecule. The vascular graft is wetted by immersion in acetone and then transferred to a solution containing acetonitrile and BTC-PEG. The BTC-PEG enters the pores of the graft by diffusion. The BTC-PEG impregnated graft is transferred to a solution containing silyl-heparin dissolved in 60% acetonitrile. As silyl-heparin has low solubility in 100% acetonitrile, it tends to accumulate at the interface of the acetonitrile diffusion gradient that is moving to the outside of the graft. This interface moves interior to the wall of the graft as the acetonitrile diffuses out providing the BTC-PEG crosslinking agent the highest probability of reacting with target groups, here amines, in the silyl-heparin. As the silyl-heparin is diffusing into the graft it is also adsorbed onto the surface of the graft by means of the silyl prosthetic groups. The overall result of the silyl-heparin adsorption onto the graft surface and crosslinking in situ is a network of cross-linked silyl-heparin-PEG. The network of silyl-heparin-PEG benefits from the multiplicity of adsorption sites on the polymer with a consequent increase in resident time on the graft surface. The multiplicity of adsorption sites contributes synergistically to the resident time of the heparin molecules on the surface.

The use of 60% acetonitrile as a solvent for silyl-heparin has additional advantages relative to BTC-PEG. Benzotriazole hydrolysis products and PEG are soluble in 60% acetonitrile, thereby reducing the concentration of such products in the graft wall. Benzotriazole hydrolysis products are also soluble in the acetonitrile used in the wash steps, thereby further reducing the concentration of breakdown products.

The resulting co-polymer coating has a resident time on the device longer than that of either the biomolecule alone or an adsorbable biomolecule alone, such as biomolecule conjugated to an adsorbable prosthetic unit.

The order of introducing the components of the invention is variable. For example, the multifunctional crosslinking agent can be applied first to the medical device and followed by an adsorbable biomolecule. Alternatively, the adsorbable biomolecule can be applied first and followed by the multifunctional crosslinking agent. In yet another embodiment, the multifunctional crosslinking agent is applied first to the medical device, followed by an adsorbable biomolecule, and then followed by application with a multifunctional crosslinking agent. In the latter case the initial and subsequent multifunctional crosslinking agents may be the same or different, and may be at the same or different concentrations.

It has surprisingly been found that the residence time of the covalently cross-linked co-polymer coating, such as in vivo residence time after coating on a vascular graft composed of ePTFE, is longest when the multifunctional crosslinking agent is applied first, followed by application of the adsorbable biomolecule in an organic solvent including water, and then followed by a second application of the multifunctional crosslinking agent. For example, when the multifunctional crosslinking agent is BTC-PEG, and the adsorbable biomolecule is silyl-heparin, residence time is longer using this method than is the case where silyl-heparin is applied first, followed by BTC-PEG, and then followed by a second application of silyl-heparin. Thus while either order may be followed, in a preferred embodiment the multifunctional crosslinking agent, such as BTC-PEG, is applied first.

Experimental data further established that with medical devices such as a vascular graft composed of ePTFE, a coating formed by immersion of the medical device in a solution consisting of heparin came off or disassociated from the medical device very quickly, that a coating formed by immersion in a solution consisting of silyl-heparin came off or disassociated from the medical device less quickly, that a coating formed by sequential immersion in silyl-heparin followed by BTC-PEG came off or disassociated from the medical device still less quickly, and that a coating formed by sequential immersion in BTC-PEG followed by silyl-heparin came off or disassociated from the medical device the slowest of all coatings using a two-step immersion. The slowest disassociation rate was observed with a coating employing a three-step immersion, formed by sequential immersion in BTC-PEG followed by silyl-heparin and followed by BTC-PEG as a last immersion step. Thus in a preferred embodiment a three-step immersion is employed, utilizing a multifunctional crosslinking agent in the first step, an adsorbable biomolecule in the second step, and the multifunctional crosslinking agent in the third step.

In the case of heparin activity biomolecules, cross-linking with a multifunctional crosslinking agent such as BTC-PEG did not result in the desired residence time of the coating where the heparin did not contain one or more hydrophobic prosthetic units, such as a silyl moiety. Thus cross-linked silyl-heparin had a statistically relevant longer residence time than did cross-linked heparin.

The chemistry employed in the method of this invention advantageously may be simply and conveniently modified for specific applications. For example, the hydrophobicity of a heparin molecule may be varied by varying the number of silyl moieties per heparin molecule. The heparin load on a medical device may be varied by changing either the concentration of heparin or the incubation time, or both. For example, heparin concentrations, such as silyl-heparin concentrations, may vary from about 0.1% to about 1% or greater. Similarly, incubation time may vary from less than five minutes to two hours or more.

The method of this invention further provides a number of advantages over more complex chemistries. For example, many chemistries employ introduction of "tether" group to a substrate, such as introducing an amino group to a medical device surface. Sequential steps then add a thromboresistant component such as heparin, optionally followed by one or more crosslinking agents. However, any such coating including heparin will degrade over time, due to a variety of biological and mechanical factors. In the instance of introduced tether groups, a "nub" or part of the molecule is frequently left permanently attached to the medical device, which molecular structure may precipitate an antigenic, allergic or inflammatory reaction. By contrast, in the method of this invention the heparin is bound to the substrate by means of a hydrophobic prosthetic unit, such that the heparin molecule is adsorbable, and thus the entire heparin molecule is removed by biological or mechanical factors, without leaving a residual nub or part of the molecule.

In the prior art, at least one patent, U.S. Pat. No. 6,051,648, discloses mixing a first synthetic polymer and a second synthetic polymer, and applying a thin layer of the reaction mixture before substantial crosslinking has occurred between the nucleophilic groups on the first synthetic polymer and the electrophilic groups on the second synthetic polymer. However, this method has a number of substantial drawbacks. In the case of reagents such as silyl-heparin and BTC-PEG, if they are mixed in a solution crosslinking proceeds very quickly, with the cross-linked co-polymer precipitating out of solution. The rate of crosslinking is dependent on a variety of parameters, including temperature, concentration, pH, and the like, many of which are difficult to closely control. Additionally, the method does not permit sufficient deposition of cross-linked co-polymer in highly matrixed or porous materials, such as ePTFE.

It is frequently advantageous to have the solvent for the multifunctional crosslinking agent differ from the solvent for the adsorbable biomolecule for a number of reasons. If the solvents differ, such as a first solvent containing 100% organic solvent and a second solvent containing 60% of the same organic solvent and the balance water, there is an osmotic pressure difference between the first solvent and the second solvent. This is hypothesized to result in a solvent interface boundary, thereby facilitating crosslinking between the reagents. Additionally, with many multifunctional crosslinking agents the solvent can be selected such that crosslinking is enhanced. For example, the reactive groups on BTC-PEG are significantly more labile or reactive on exposure to water. Thus the BTC-PEG can be employed in a first solvent that is composed of 100% organic solvent, with silyl-heparin in a second solvent that includes 40% water. The water in the second solvent activates or increases the reactivity of the reactive groups on BTC-PEG, thereby increasing the rate and efficiency of crosslinking.
 

Claim 1 of 21 Claims

1. A method of forming a cross-linked coating on a medical device, comprising the steps of: (a) immersing the medical device in a first solution comprising an organic solvent and a multifunctional crosslinking agent selected from the group consisting of a bis-variant of polyethylene glycol or polyethylene oxide, and (b) immersing the medical device in a second solution wherein the second solution comprises an organic solvent and a cross-linkable biomolecule selected from the group consisting of chondroitin sulfate, heparan sulfate and heparin and rendered surface adsorbable by conjugation with a silyl moiety of formula I -- see Original Patent.

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