ENHANCED BIOCOMPATIBILITY COATINGS FOR MEDICAL IMPLANTS

Abstract: Enhanced biocompatibility coatings for medical implants and acute exposure medical devices are disclosed in conjunction with methods for their production and use. The biocompatible coatings are particularly advantageous when used in conjunction with high surface area materials such as porous resins, fabrics and textiles and comprise ionically bound complexes of at least one anticlotting compound and at least one quaternary ammonium compound having greater than 0.15 and less than 40 IU of anticlotting compound per CM<2>. The coatings are produced by forming a solution of the complex having a concentration greater than 0.005 % and less than 1.5 % w/v which is applied to the surface and allowed to dry. The coatings reduce the propensity for thrombus deposition without impairing healing and tissue ingrowth into the device.

Description: ENHANCED BIOCOMPATIBILITY COATINGS FOR MEDICAL IMPLANTS FIELD OF THE INVENTION The present invention is broadly directed to the field of medical implants, specifically implants or acute exposure medical devices having fabric or textile coatings or structures manufactured from high surface area materials. More particularly, the present invention is directed to improved coatings for such devices which enhance the tissue biocompatibility and long term stability of the devices and which remain active after repeat sterilization. BACKGROUND OF THE INVENTION With the increasing sophistication of developing medical technologies has come a corresponding increase in the sophistication and variety of medical implants. Early medical implants were simple structures developed as prosthetic devices intended to function in place of damaged or diseased body parts. As the technology progressed, medical implants were developed with more sophisticated designs and functions. Over the years, extensive efforts have been devoted to designing medical implants that more closely mimic the functions of the normal body structures they are intended to replace. Now, even complex body structures are being replaced or augmented with increasingly more complex medical implants. Concurrently with the evolving functional and structural development of medical devices has been a corresponding development in technologies aimed at increasing the functional duration or lifetime of medical implants. The human body has incredibly complex natural systems for protecting itself against invasive foreign bodies. These natural protective systems are unable to distinguish between beneficial medical implants designed to prolong life and improve the implant recipient's quality and scope of life and harmful invasive foreign bodies. Thus, much like an oyster turns a grain of sand into a pearl to isolate its tissues from the sharp damaging edges of the grain, the wound healing response of the human body effectively isolates irritating materials including the surfaces of implantable medical devices. However, in contrast to the rigid, inert layers of a pearl, the fibrous capsule formed by the human body to cover the surfaces of an implanted medical device is soft, living tissue. As the understanding of these natural protective processes has increased, efforts have been made to modify or control their progress and development. Left unchecked, this fibrous material formed by scar tissue fibroblasts may proceed to the point that the device may become inoperative over time. This is particularly true with cardiothoracic and cardiovascular implants designed to augment or replace damaged portions of the heart or blood vessels. Blood is a very reactive fluid. When blood contacts the surfaces of the implanted devices, the immediate response is protein adsorption, platelet adhesion and thrombus formation. Thrombus is the result of clotting on the artificial surfaces of the device. Over time, these thrombotic surface deposits are covered over and replaced by soft tissue overgrowth resulting in an endothelialized surface over regenerated tissue known as "pannus" or "neointima". Early efforts at controlling these natural healing mechanisms generally focused on two alternative, yet complimentary, approaches. Generally speaking, the first approach involved modifying or reducing the natural propensity for thrombus deposition onto the surfaces of the implant by reducing the thrombogenicity of the implant surfaces. The second approach involved attempting to encourage or direct soft tissue ingrowth into specific surfaces of the implant while discouraging the formation of thick or uneven thrombotic deposits. Deposits can break free or flake off forming thromboemboli which can lead to blockage of blood vessels and severe complications. Thick thrombotic deposits also can grow to the point where they block passageways or discourage the formation of smooth tissue ingrowth and the diserable formation of a thin, even pannus. Similarly, even though pannus formation at the interface of the implant and the surrounding tissue is normally a self-limiting phenomenon, undesirable fibrous tissue can thicken the endothelial overgrowth, possibly impacting the function of the implant or causing an obstruction as well. After extensive research, it was found that most of the various metals, plastics, ceramics, and the like utilized to construct implants do not have optimal thromboresistance, meaning they can induce blood platelet aggregation and thrombotic deposition in some applications. Thus, more successful approaches at reducing the thrombogenicity of implant surfaces have involved the utilization of intravenous or oral antithrombogenic, antiplatelet, anticoagulant or thrombolytic compounds. For the purposes of the present invention, all of these compounds will be referred to as "anticlotting" compounds. Of these, known anticoagulant compounds include heparin, Coumadin (warfarin), and hirudin as well as others. Heparin is by far the most common antithrombogenic compound in use. The earliest and most simple form of antithrombogenic treatment was to add anticoagulants to the patient's blood. Later, techniques were developed to deliver anticoagulant solutions directly to the implantation site. Though effective, such techniques utilize relative large amounts of circulating anticoagulant which may have the undesirable side effect of discouraging clot formation in other parts of the patient's body where clotting may be desirable in response to injury. More advanced anticlotting technologies have involved surface treatments and coatings designed to bind or attach heparin or other anticoagulants to the blood or tissue contacting implant surfaces. Because these coating technologies utilize static quantities of anticoagulant, they have focused on attaching enough anticoagulant to the surfaces, in its active form, to provide an effective antithrombogenic or reduced thrombogenicity surface for at least the initial period following implantation. Compounding matters, even though the antithrombogenic mechanisms of anticoagulant compounds such as heparin are not well understood, it is known that the mechanism of attachment or immobilization of the compound is critical to ensuring its stability and bioactivity. Conventional implant sterilization procedures such as steam autoclaving or ethylene oxide gas sterilization techniques may displace compounds such as loosely bound heparin from the coated implant surfaces or may degrade the compounds. Thus, the most effective antithrombogenic coatings may be difficult to sterilize and may be impossible to resterilize, if resterilization becomes necessary. As noted above, the control of pannus tissue layer/neointima formation does not occur independently of the control of thrombus formation. It is believed that the initial step in the mechanism of pannus/neointima formation is the deposition of blood proteins followed by components including platelets and fibrin from the blood onto the surface of the implant. This initial deposit is gradually replaced by a layer of fibrous tissue with a coating of normal endothelium (cells that line the blood vessels and other body cavities). Thus, if the initial fibrin deposition propagates to the point of forming a harmful thrombosis or if adhesion between the fibrin layer and the implant surface is poor, the deposit may come loose from the implant or prevent the formation of a smooth endothelial layer. This problem can be aggravated where the implant is subjected to movement or fluid flow as occurs with heart valves and other cardiovascular implants. Because it has been found that smooth surfaces are especially prone to sloughing off the initially deposited fibrin layers, implants have been developed with rougher surfaces which stimulate the endothelial ingrowth. These surfaces include porous polymers and fabrics such as velours and textiles. Fabrics and textiles provide the added benefit of being an easily suturable surface which facilitates surgical attachment of the implants to surrounding tissue. Unfortunately, it is difficult to coat these porous surfaces with anticlotting compounds. Coating the porous material or fabric prior to manufacturing the implant may make the material stiff and difficult to work with or manipulate. Similarly, coating the porous materials after manufacture may also render the fabrics unacceptably stiff for implantation purposes. Additionally, manipulating the coated fabric and shaping it around the implant may expose untreated and hence thrombogenic areas. It also may be difficult to completely impregnate the gaps and interstices in the porous materials with antithrombogenic compounds. Factoring in the additional complexities of producing antithrombogenic high surface area porous or woven materials that can be sterilized, or resterilized, and the difficulty of simultaneously addressing all of these countervailing design objectives can be appreciated. Accordingly, it is a primary object of the present invention to provide enhanced biocompatibility coatings for medical implants and related medical devices having porous or high surface area structures intended to function in contact with blood and tissue for extended periods of time. It is an additional object of the present invention to provide enhanced biocompatibility coatings for such devices which inhibit or reduce the propensity for thrombus deposition without impairing desirable tissue ingrowth. It is still a further object of the present invention to provide enhanced biocompatible coatings for such devices which remain biocompatibly active following sterilization. SUMMARY OF THE INVENTION These and other objectives are achieved by the novel, enhanced biocompatibility coatings of the present invention which provide a previously unavailable combination of properties. In accordance with the teachings of the present invention it is now possible to balance the known interactions at the blood, tissue, and implant material interfaces to provide medical implants and related devices having fabric or porous, high surface area structures with the unprecedented ability to reduce undesirable thrombus formation without impairing soft tissue compatibility and healing. Moreover, the enhanced biocompatibility coatings of the present invention remain effective after single and multiple combinations of sterilization procedures, are easy to apply, and do not require sophisticated or special procedures for their manufacture or use in association with known medical devices. Before proceeding further, it should be appreciated by those skilled in the art that the unique features of the present invention come in complete contrast to the teachings of the prior art. Prior to the present invention, the simple surface coating of ionically bound heparin onto the surfaces of medical devices was widely believed to produce safe, low toxicity, antithrombogenic coatings. This was understandable as the safe and beneficial antithrombogenic properties of heparin had been observed in the circulating blood of patients for years. Consistent with this understanding, early efforts at reducing thrombosis associated with high surface area, cloth covered implants focused on the delivery of large quantities of heparin directly to the implantation site for extended periods of time. For example, M. Schwartz et al., "Local Anticoagulation of Prosthethic Heart Valves", Supplement III to Circulation. Vols. XLVII and XLVIII, July 1973, disclosed the direct infusion of 15,000 units of heparin every 24 hours for four weeks to discourage thrombus formation and to allow rapid pannus formation with cloth covered prosthetic heart valves. Schwartz et al. utilized a manifold underneath the cloth coating of the valve to provide a much higher concentration of heparin in the fabric interstices and immediately adjacent areas then could be achieved by orally administering heparin. Thus, it was a completely unexpected result when the present inventors determined that elevated concentrations of ionically bound heparin applied to high surface area fabrics intended to function as sleeves, suture rings or covers for implantable prosthetic devices such as heart valves and annuloplasty rings not only discouraged endothelialization and pannus formation but were actually detrimental to soft tissue. Compounding matters, traditional thinking in the art has taught that common sterilization methods including steam autoclaving and ethylene oxide gas sterilization significantly reduce or even completely eliminate heparin activity. See, for example, NHLBI, "Guidelines for Blood- Material Interactions" NIH Publication No. 85-2185, Revised September 1985. Accordingly, it was believed by those skilled in the art, including the present inventors, that it would be impossible to produce a biocompatible coating having reduced quantities of ionically bound anticlotting compound such as heparin that would continue to function effectively on medical implants having high surface area structures. Further, it was a completely unexpected result when the present inventors were able to produce and demonstrate enhanced biocompatibility coatings for medical implants and related devices that not only were effective at inhibiting thrombus deposition and formation but also were advantageously compatible with soft tissue. As a result, the enhanced biocompatibility coatings of the present invention are directly applicable to improving the functional performance of fully implantable medical devices and medical devices intended for use in long term contact with blood and tissue where tissue compatibility in conjunction with antithrombogenicity are important to the long term operability of the device. In a broad aspect, the enhanced biocompatibility coatings of the present invention generally comprise an ionically bound complex of surfactant and anticlotting compound such as heparin that is applied to the surfaces of an implant or medical device as a dipped, sprayed or other solvent-applied coating in a weight to volume concentration of anticlotting complex to solvent ranging from greater than 0.005% to less than 1.5%. An exemplary preferred solution concentration will range from 0.01 % to 0.2% weight to volume. As those skilled in the art will appreciate, adsorption of the enhanced biocompatibility coatings from such solution concentrations will vary depending upon the surface area and other properties of the materials being coated. Accordingly, the enhanced biocompatibility coatings of the present invention can also be understood as an ionically bound complex of surfactant and anticlotting compound such as heparin which produces a coating having from 0.15 IU to 40 IU anticlotting compound per cm . Preferably, in accordance with the teachings of the present invention an exemplary anticlotting compound will be heparin and the surfactant component of the complex will be a quaternary amine such as benzalkonium chloride (BC), di-dodecylmethyl ammonium chloride (DiMAC), or tri-dodecylmefhyl ammonium chloride (TDMAC) or others including proprietary heparin-quaternary ammonium compounds such as Duraflo® available from Baxter Healthcare Corporation. In accordance with the teachings of the present invention, these unique, enhanced biocompatibility coatings remain effective at reducing thrombogenicity following steam or ethylene oxide gas sterilization or repeat sterilization yet are safely biocompatible with soft tissue. Because the coatings of the present invention do not require exotic surface pre-treatments or chemistry, they can be applied easily to a wide variety of high surface area materials including porous resins, woven and non-woven fabrics, textiles, knits, and velours manufactured from a wide variety of synthetic materials including PTFE, polyesters, polyurethanes, polyacrylates, polysiloxanes, and other synthetic materials commonly known in the medical arts. For example, the coatings can be applied through simple spraying or immersion application techniques. A further understanding of the features and advantages of the present invention will be afforded to those skilled in the art from the following detailed description of exemplary embodiments thereof. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to enhanced biocompatibility coatings for medical implants and related devices, to methods for producing medical implants with enhanced biocompatibility coatings, and to medical implants so produced. To understand the scope of the present invention it should be appreciated that the present invention is broadly applicable to virtually the entire class of implantable medical devices and to what are known as acute exposure medical devices. In essence, all such devices are intended to function in contact with blood and tissue for extended periods of time where concerns for reducing thrombus deposition without impairing tissue healing are relevant. As known in the art, such devices are formed from one or more of a wide variety of substrates including polymers or plastics, metals, ceramics, glasses, fabrics, textiles and the like. While it is contemplated as being within the scope of the present invention to coat any and all of the materials and structures forming such medical implants and devices, it is preferred that the present invention be practiced in conjunction with implantable or acute exposure medical structures having relatively high surface areas. These structures emphasize the particular features and advantages of the present invention as the high surface areas make it possible to impregnate or load the high surface area materials with correspondingly high, active concentrations of ionically bound anticlotting compositions. For purposes of illustration and explanation, the present invention will be discussed in the context of exemplary annuloplasty ring implants such as the Carpentier-Edwards Physio® Annuloplasty Ring, Cosgrove-Edwards® Annuloplasty System, and the Carpentier-Edwards™ Classic Annuloplasty Ring available from Baxter Healthcare Corporation. Annuloplasty rings and annuloplasty systems are intended to augment the function of heart valves by providing added dimensional stability to the valve annulus. As such, they typically are formed as fabric covered rings made of flexible polymers or metals and dimensioned to be sutured in place around the base of a heart valve in order to stabilize the valve against distortion and resultant fluid leakage. Annuloplasty rings are excellent examples for illustrating the principles of the present invention because they are almost entirely covered with high surface area textile materials and are intended to remain implanted for ten or more years in the dynamic environment of a beating human heart. Thus, it would be particularly advantageous to provide annuloplasty rings and annuloplasty systems that encourage stabilizing tissue ingrowth into the porous high surface area coverings in order to assist in maintaining the implanted ring in position while discouraging the growth and development of thrombotic obstacles to blood flow through the implant augmented heart valve. With this understanding, it should again be emphasized that the present invention is not limited to the production of biocompatibly enhanced annuloplasty rings or to the treatment of textile covered implants alone. It is contemplated as being within the scope of the present invention to apply the teachings of the invention to virtually any implantable or acute exposure blood or tissue contacting medical device including heart valves, suture rings, stents, vascular grafts, left-ventricular assist devices, artificial hearts, acute exposure catheters, shunts, drains, and other devices. Similarly, the application of the present invention is not limited to any particular implant structure or material and the exemplary textile materials discussed herein are not intended to limit the scope of the present invention. Thus, it should be appreciated that any implant or medical device surface intended to be used in circumstances where clotting or thrombosis can occur or where tissue ingrowth into the surfaces of the device is either expected, desirable, or critical for the long term stability or function of the device, is an appropriate candidate for the enhanced biocompatibility coatings of the present invention. Additionally, the present invention is not intended to be limited to the use of heparin as the anticlotting compound or to Duraflo® as the exemplary heparin-quaternary ammonium compound. Though preferred, these compositions are exemplary of the teachings of the present invention and are not intended to limit the scope of the invention. Thus, it is contemplated as being within the scope of the present invention to form the enhanced biocompatibility coatings from heparin, protamine, hirudin, aspirin, prostaglandin, tPA, urokinase, streptokinase, prourokinase or other anticlotting compounds, which for the purposes of the present invention, generically include antithrombogenic, antiplatelet, and thrombolytic compounds. Nonetheless, heparin is the preferred anticlotting antithrombogenic material for practicing the present invention. Similarly, Duraflo® is the preferred heparin-quaternary ammonium complex for providing an enhanced biocompatibility ionically bound surfactant and heparin coating in accordance with the teachings of the present invention. It will be appreciated that a wide variety of quaternary ammonium compounds can be reacted with heparin to form this complex including benzalkonium chloride, or tri-dodecylmethylammonium chloride, benzyldimethylstearylammonium chloride, cetylpyrrdinium chloride, benzylacetyldimethylammonium chloride, and the like. Duraflo® is preferred in that it has demonstrated superior binding characteristics and better stability when exposed to blood. All exemplary surfactant-heparin coating complexes work in a similar manner. The surfactant components are detergent-like molecules having a hydrophobic or "water hating" portion and a hydrophilic or "water loving" portion. The surfactants adsorb to the artificial surfaces of the medical device with the hydrophobic portion attached to the surface and the hydrophilic portion extending outwardly and providing an anchor to bind the heparin molecule. When exposed to blood the heparin binds to a circulating blood component known as antithrombin III. This causes a conformational change in the antithrombin which increases its affinity for binding to thrombin. As antithrombin III binds with thrombin it releases the heparin molecule which is able to bind to another antithrombin III molecule and repeat the process. This results in a highly localized antithrombogenic effect. Within approximately 24 hours of continual exposure to blood or similar solutions, the majority of the heparin will have leached away from the coating leaving a residual quaternary amine coating. Forming and applying the enhanced biocompatibility coatings in accordance with the teachings of the present invention facilitates the subsequent desirable tissue ingrowth by eliminating any of the unexpected residual effects of the remaining quaternary amine surfactant. Thus, the ability of the present invention to encourage or control healing and to avoid undesirable thrombosis is most acute during and immediately after implantation or initial contact with blood or body tissues. Because excessive or thick thrombus deposition discourages subsequent positive tissue ingrowth thrombus deposition is reduced, the present invention sets the foundation for healthy pannus formation. These features and benefits are particularly advantageous in the context of modern minimally invasive surgical implantation techniques. These recently developed techniques utilize small and sometimes unconventional surgical access pathways to the implantation target site. This restricted access may complicate the undesirable inflammatory responses and thrombus formation normally associated with medical implantation procedures because of the close proximity of cut tissues that may contact the implant during implantation surgery. Cut tissues exude a variety of highly thrombogenic components known as "thromboplastins". Contacting cut tissues with the implanted device as it is passed through the minimally invasive incision may coat portions of the device surfaces with thromboplastins which may stick to the high surface area fabrics and porous materials. As a result, the thromboplastins may induce immediate and enhanced thrombogenisis at the implant site. However, as those skilled in the art will appreciate, treating the implant in accordance with the teachings of the present invention to provide the implant surfaces with enhanced biocompatibility coatings significantly reduces these effects and the potential complications of minimally invasive implant surgery. With this broad based understanding of the scope of the present invention in mind, a more detailed explanation of the exemplary embodiments thereof will be provided by the following, non-limiting examples. One of the primary benefits of the present invention is its ability to produce medical implant surfaces which exhibit long term soft tissue biocompatibility. As discussed above, the prior art failed to appreciate the potentially negative impact of large quantities of anticlotting-quaternary ammonium compounds on medical implant surfaces. This prior art misconception was compounded by the corresponding belief in the art that smaller quantities of heparin-quaternary ammonium compounds would be ineffective following sterilization. The ability of the present inventors to overcome this delicate balance of conflicting misinformation is demonstrated as follows: Example I Exemplary anticlotting-quaternary ammonium compounds were prepared utilizing conventional techniques. For example, Duraflo®, a proprietary heparin-surfactant compound developed and marketed by Baxter Healthcare Corporation was dissolved as directed in appropriate solvents (volatile chloroflourocarbon and alcohol) to a nominal concentration. Alternatively, a heparin-benzalkonium chloride compound can be prepared by mixing approximately equal volumes of 10% (by weight) aqueous sodium heparin with 17% (by weight) benzalkonium chloride solution as known in the art. After the exemplary solution was prepared, it was assayed to ensure compliance with concentration specifications as required by the manufacturer. Following assembly of the exemplary annuloplasty rings, an identification tag was affixed to each utilizing a single suture. While suspended by the identification tag, each annuloplasty ring specimen was dipped in the Duraflo® solution. After approximately five seconds in this solution, the rings were removed and air dried in a fume hood. As known in the art, most heparin- quaternary amine compounds formed in such solutions have a somewhat waxy consistency and, after evaporation of the organic carrier solvent produce elastic, tenaciously attached coatings. Solutions of varying concentrations were prepared in order to test the efficacy and tissue compatibility of the resultant coatings. The concentrations ranged from greater than 0.005% (weight/volume of heparin-surfactant compound in solvent) to 1.5%. The treated rings so produced were cut into small segments and utilized as test articles in the following exemplary procedures. Example 2 The activity of the heparin coating was determined utilizing a two step procedure. First, the heparin was leached from the test article into a solution. The second step involved assaying the solution for heparin activity in order to calculate the amount of heparin. Because the heparin and surfactant material are combined in a known ratio it was possible to quantify both the amount of heparin adsorbed onto the surface of the test article or medical device as well as the total quantity of surfactant. In order to leach the heparin from the device surface into solution, the coated test article was soaked in 4.5% human albumin solution for two hours in a silicone coated beaker. Previous time dependency studies in this solution had shown that two hours is sufficient time to remove almost all leachable heparin from the article surface. The amount of heparin in the solution was determined by assaying the solution with a commercial kit, Coatest® Heparin available from Chromogenix AV. This commercial kit measures the amount of unbound factor Xa (FXa) in solution as follows. First, antithrombin (AT) was added to the solution in excess to bind up all of the heparin. FXa was then added to excess in a known quantity. The heparin- AT complex bound to the added FXa and the remaining, unbound FXa reacted with a color changing indicator that could be read photometrically. The results of the test were compared to standardized curves from which the amount of heparin in each solution was calculated. Utilizing this procedure, the test articles were determined to have adsorbed 0.15 IU to 5.3 IU of heparin per square cm from the different dipping solution concentrations. Additionally, it was determined that differences in the fabric coatings and configurations of the test articles resulted in differing levels of coating adsorption within these ranges. Example 3 In vitro cell culture studies were conducted to identify or evaluate any cytotoxic effects associated with the various treatment solution concentrations. Generally, these procedures involved the evaluation of cytotoxic effects of a material growth medium extract on a human fibroblast monolayer. Utilizing this protocol, a cell monolayer was grown to confluency in cell growth medium and incubated for 24 hours with growth medium that had been mixed previously with an equal volume of test article normal saline extract. The incubated cells were then examined for evidence of damage or lysis. A sample was adjudged to be non-cytotoxic if lysis was not greater than that observed with the negative control. Typically, the negative control had a lysis score of zero. Test articles treated with 1.5%, 0.5%> and 0.1 % solutions were found to be 100%> cytotoxic utilizing this procedure. Variations in cytotoxcity were observed between the various test articles apparently due to differences in polyester cloth (standard cloth versus double velour) and configuration (silicone tubes containing multiple components versus a silicone band). These results were completely unexpected in view of the then available understanding of the prior art which suggested that the coatings would be harmless. Appreciating that the in vitro testing protocol produced a static environment with prolonged direct contact at the cellular monolayer it was determined that further in vivo testing was in order. Example 4 In vivo implantation studies were undertaken to confirm the previously observed in vitro testing results. This testing method involved placing sections (typically slivers approximately 1 mm by 1 cm) of the treated test articles intramuscularly or subcutaneously at a minimum of four sites in the back of a minimum of two test animals. Implant durations were either 7 or 30 days. After the predetermined period of implantation, the sites were examined for gross visible evidence of tissue damage. Gross visible evidence of tissue damage could include obvious inflammation, encapsulation, hemorrhage or necrosis adjacent to the implanted test article sample. Utilizing these procedures it was surprisingly determined that biocompatible coatings produced from solutions having concentrations greater than 0.005%> and less than 0.3%> were biocompatible. An initial implant study utilizing coated compositions obtained from 1.5% and 0.5%> solutions induced a host response greater than the negative control but acceptable for purposes of the present invention. At both treatment concentrations, the host response was considerably more marked at the seven day implant duration than at the 30 day interval. By 30 days, the marked necrosis had been reduced to minimal, but the treated samples continued to induce an increase in vascularity, number of hetrophils, lympocytes, fibrin and hemorrhage. Subsequent studies evaluated coatings produced from 0.1% and 0.05%) solutions. These reduced concentrations induced a tissue response similar to the negative control at seven days. At the 30 day interval, both samples were found to be slightly more reactive than the control but well within acceptable limits for implantation. Utilizing this procedure a material was determined to be biocompatible if there was no gross visible evidence of tissue damage. In sum, utilizing established testing procedures it was determined that the teachings of the prior art were incorrect and that more heparin-quaternary ammonium compound was not necessarily better than less. Thus, developing coating techniques intended to maximize the loading of anticlotting materials into the porous surfaces of medical devices was not an appropriate objective. Having recognized a previously unexpected upper limit to the practical utility and operability of such coatings in the context of medical implants and acute exposure medical devices, efforts were undertaken by the present inventor to identify tissue biocompatible coatings that were effective at modifying and reducing platelet-fibrin coagulum (thrombus) upon contact with blood. Once again, prior art teachings strongly suggested that this search would not be fruitful because sterilization effects would inactivate or reduce the anticlotting compounds to useless levels. In spite of this, a search for a minimum efficacious coating concentration was conducted utilizing the following protocol. Example 5 Previous studies performed at Baxter Healthcare had demonstrated that steam autoclaving effectively reduced heparin activity to undetectable levels. As discussed above, prior art publications also confirmed that steam sterilization reduced heparin activity by 18%. Differences in assay techniques and difference in substrate materials tested may account for these differences in effect. Similarly, literature studies had indicated that ethylene oxide sterilization was believed to eliminate heparin activity by replacing heparin with ethylene oxide. Moreover, standard enzymatic assays had shown that there was no enzymatic activity following sterilization. Accordingly, it was decided to measure complement activation to identify the inflammatory response generated by a given material and the hoped for reduction in complement activation generated by materials treated in accordance with the present invention. The complement system involves various plasma proteins which are involved in the neutralization of infection and the generation of an inflammatory response once activated by specific antibodies. Generally speaking, following activation of the complement system, after blood-implant contact, a series of detectable biological activities occurs. The level of detectable complement activation is roughly proportional to the inflammatory response. A commercial kit known as the Quidel C3a Enzyme Immunoassay™, available from Quidel was utilized to quantitatively determine complement C3a activation utilizing the enzyme-linked immunosorbent assay (ELISA) or sandwich technique. Generally speaking, a plastic support coated with specific anti-human C3a monoclonal antibodies captures the C3a to be measured. Then, anti-C3a antibody coupled with peroxidase is bound to the remaining free antigenic determinants of the C3a to form the "sandwich". The quantity of bound enzyme peroxidase is determined by its activity following incubation for a predetermined period of time with the treated substrate. After the incubated reaction is stopped with a strong acid, the intensity of color produced by the bound enzyme peroxidase is measured and directly related to the original C3a concentration. As before, test articles having enhanced biocompatibility coatings produced from varying solution concentrations in accordance with the teachings of the present invention were tested utilizing this standardized protocol. Surprisingly, even though it was anticipated that no reduction in complement activity would be observed at these low concentrations, the enhanced biocompatibility coatings of the present invention did, in fact, reduce complement activation. Of equal importance, the effectiveness of the coating at reducing complement activation and hence reducing thrombus deposition appeared to correspond to the concentration or strength of the coated composition. For example, an enhanced biocompatibility coating produced from a solution of 0.05% Duraflo® was more effective than a coating prepared from a 0.01%) solution. Additional studies were conducted to demonstrate and quantitate the ability of the enhanced biocompatibility coatings of the present invention to remain efficacious following sterilization procedures as follows. Example 6 The complement C3a activation assay of Example 5 was repeated with a test article having an enhanced biocompatibility coating produced from a 0.01% Duraflo® solution. However, prior to the assay the test article was gas sterilized with a 120 minute 100% ethylene oxide sterilization process. In contrast to the teachings of the prior art, a significant reduction in complement activation was measured indicating that the biocompatible coating of the present invention remains antithrombogenic after gas sterilization. Example 7 The in vivo subcutaneous implantation assays of Example 4 were repeated on sterilized test articles following exposure to two ethylene oxide gas sterilization cycles. The test articles had enhanced biocompatibility coatings produced from Duraflo® solutions having concentrations of 0.1% to 0.5%. Following a seven day implantation interval the implantation sites were examined for gross physical evidence of tissue damage. The host response to the various implants was similar in nature, but differed in the magnitude or severity of the response. An elevation in the severity of the response was noted when concentrations rose above 0.2%. The added exposure of the test article to flash autoclave cycles (132°C for 15 minutes) in addition to the ethylene oxide gas sterilization resulted in a mild increase in the host response to the implanted material. However, implants treated in accordance with the teachings of the present invention continued to demonstrate acceptable levels of host response indicating continued antithrombogenicity in conjunction with soft tissue biocompatibility following multiple gas sterilizations and multiple gas sterilizations in conjunction with flash autoclave sterilization. Example 8 A final assay was conducted to evaluate and demonstrate the functional ability of the enhanced biocompatible coatings of the present invention to reduce thrombogenicity as evidenced by their impact on blood platelet deposition utilizing an ex vivo shunt model on test animals. In accordance with accepted humane protocol for such testing, test shunts were utilized incorporating polyester fabric lined shunts on their interior surfaces, with and without the enhanced biocompatibility coatings of the present invention formed from a solution having a Duraflo® concentration of 0.08%). Each of the test samples was exposed to single or multiple ethylene oxide gas sterilization procedures or a single ethylene oxide sterilization procedure followed by flash autoclave sterilization. The test shunts were evaluated using a variety of techniques to determine the impact of the enhanced biocompatibility coatings of the present invention on their thrombogenicity. Blood flow through the shunt was adjusted to maintain a flow rate of 100 mL/min for a period of 180 minutes or until the shunt occluded and zero flow was measured. Counts of In -labeled platelets were measured using a Gamma camera over two minute intervals. As shown in Table I, platelet activity at 60 minutes indicated that platelet aggregation of the treated groups were all significantly less than that of the untreated control group. Moreover, all sterilized and repeat sterilized samples exhibited significantly lower platelet aggregation as well, confirming that the enhanced biocompatibility coatings of the present invention remain effective after sterilization. Table 1 Analysis of Platelet Accumulation at 60 Minutes In 11 '-Platelet Counts (cpm) at T=60 Minutes Subject 1 Subject 2 Subject 3 Subject 4 Average Std. Dev Control 555 403 64 212 309 186 lxETO 41 24 34 55 39 11 3xETO 128 77 39 31 69 38 ETO+Flasl l 95 169 57 53 94 47 Example 9 The same ex vivo shunt protocol was also used to measure the overall thrombogenicity of the shunts through a shunt time-to-time occlusion analysis. The formation of blood clots consists of both platelet aggregation and the polymerization of fibrin deposits. Thus, measuring platelet aggregation alone may not accurately quantify thrombogenicity. Accordingly, the time to occlusion was selected as a relevant measure of overall thrombogenicity. For each shunt, the duration for which the shunt remained open was recorded. Shunts that remained open at the end of the study were recorded as 180 minutes. As shown in Table 2, the occlusion times were shortest for the untreated control group and longest for the gas sterilized treated shunts. Moreover, no statistically significant difference could be shown between the single and multiple gas sterilized shunts. Additionally, treated shunts that were gas sterilized followed by flash autoclaving also demonstrated longer occlusion times confirming the operability and effectiveness of the present invention. Table 2 Duration of Shunt Studies Time to Occlusion or Study Termination In Minutes Subject 1 Subject 2 Subject 3 Subject 4 Average Std. Dev Control 70 60 50 62 61 7 lxETO 180 180 180 118 165 27 3xETO 150 96 148 180 144 30 ETO+Flash 82 100 104 84 93 10 The foregoing examples indicate that regardless of subsequent sterilization treatment, producing medical implants with enhanced biocompatibility surfaces in accordance with the teachings of the present invention will effectively reduce the thrombogenicity of the material surfaces without impairing soft tissue biocompatibility. In conclusion, those skilled in the art will appreciate that the present invention provides safe and efficacious enhanced biocompatibility coatings for medical implants and acute exposure medical devices demonstrating clear reductions in the propensity of such devices to induce thrombus deposition in conjunction with soft tissue biocompatibility that facilitates healing and tissue ingrowth. This previously unobtainable and unexpected combination of properties enhances the long term stability and functionality of the treated devices. The enhanced biocompatibility coatings of the present invention are readily applicable to existing manufacturing techniques and do not significantly alter the character, flexibility, stiffness or appearance of the treated medical implants. As such, existing manufacturing and implantation protocols need not be modified to incorporate the teachings of the present invention in order to produce improved medical implants and acute exposure medical devices. The present invention is particularly advantageous when used in conjunction with implants and medical devices having structures formed of high surface area materials. However, this is not essential to the practice of the present invention. Additionally, the enhanced biocompatibility coatings of the present invention remain functional after single and multiple sterilization procedures, particularly gas sterilization and steam autoclave sterilization procedures. However, alternative sterilization procedures are contemplated as being within the scope of the present invention such as conventional steam sterilization or gamma radiation. Accordingly, the present invention is limited only by the appended claims.

Claims: Claims 1. An enhanced biocompatibility coating for medical implants and acute exposure medical devices, said coating comprising an ionically bound complex of at least one anticlotting compound and at least one quaternary amine having from 0.15 to 40 IU of anticlotting compound per cm . 2. The enhanced biocompatibility coating of claim 1 having from 0.15 to 5.3 IU of anticlotting compound per cm . 3. The enhanced biocompatibility coating of claim 1 wherein said anticlotting compound is selected from the group consisting of antithrombogenic compounds, antiplatelet compounds, and thrombolytic compounds. 4. The enhanced biocompatibility coating of claim 3 wherein said antithrombogenic compound is selected from the group consisting of heparin, protamine, and hirudin. 5. The enhanced biocompatibility coating of claim 4 wherein said antithrombogenic compound is heparin. 6. A method for the production of an enhanced biocompatibility coating on the surface of a medical implant or acute exposure medical device, said method comprising the steps of: forming a solution having a concentration of at least one anticlotting compound-quaternary amine complex having a concentration greater than 0.005% w/v and less than 1.5% w/v; applying said solution to said surface; and drying said applied solution. 7. The method of claim 6 wherein said solution has a concentration ranging from 0.01 % w/v to 0.02% w/v. 8. The method of claim 6 wherein said anticlotting compound is selected from the group consisting of antithrombogenic compounds, antiplatelet compounds, and thrombolytic compounds. 9. The method of claim 8 wherein said antithrombogenic compound is selected from the group consisting of heparin, protamine, and hirudin. 10. The method of claim 9 wherein said antithrombogenic compound is heparin. 11. The method of claim 6 wherein said surface has a relatively high surface area. 12. The method of claim 11 wherein said surface is a fabric. 13. The method of claim 11 wherein said surface is a textile coating. 14. The method of claim 13 wherein said medical implant is an annuloplasty ring. 15. The method of claim 13 wherein said medical implant is a heart valve. 16. A medical implant or acute exposure medical device having an enhanced biocompatibility coating produced in accordance with the method of claim 6. 1 17. A medical implant having an enhanced biocompatibility coating 2 comprising an ionically bound complex of at least one anticlotting compound and 3 at least one quaternary amine having from 0.15 to 40 IU of anticlotting compound 4 . per cm 2. 1 18. The medical implant of claim 17 having from 0.15 to 5.3 IU of 2 anticlotting compound per cm . 1 19. The medical implant of claim 17 wherein said anticlotting 2 compound is selected from the group consisting of antithrombogenic compounds, 3 antiplatelet compounds, and thrombolytic compounds. 1 20. The medical implant of claim 19 wherein said antithrombogenic 2 compound is selected from the group consisting of heparin, coumadin, protamine, 3 and hirudin. 1 21. The medical implant of claim 20 wherein said antithrombogenic 2 compound is heparin.

Inventors: HELMUS MICHAEL, SCOTT MICHAEL J, HELMUS, MICHAEL, SCOTT, MICHAEL, J.
Applicants: BAXTER INT, HELMUS MICHAEL, SCOTT MICHAEL J, BAXTER INTERNATIONAL INC., HELMUS, MICHAEL, SCOTT, MICHAEL, J.
Application year: 1999
Main classification: A61L33
IPC codes: A61L33