the term “ assay ” as used herein refers to the analytic procedure for qualitatively assessing or quantitatively measuring the presence or amount or the functional activity of a target entity ( the analyte ). the term “ chandler loop assay ” as used herein refers to a system of rotating tubes that simulates the circulation of blood . this assay is suitable for testing the hemocompatibility of medical devices placed into the blood stream . the term “ coagulation ” or “ blood clotting ” as used herein refers to the process by which blood changes from a liquid to a gel . it potentially results in hemostasis , the cessation of blood loss from a damaged vessel , followed by repair . the term “ coating ” or “ coated ” as used herein refers to the functional layer of material that is applied to the surface of an object , usually referred to as the substrate . the term “ covalent bonding ” as used herein refers to the chemical bond that is formed as a result of the stable balance of attractive and repulsive forces between atoms when they share electrons . the term “ endothelial cells ” as used herein refers to the cells that line the blood vessels . the term “ enzyme ” as used herein refers to a biological catalyst that facilitates a metabolic process . the term “ fibrin ” as used herein refers to a fibrous , non - globular protein involved in the clotting of blood . it is formed by the action of the protease thrombin on fibrinogen , which causes the latter to polymerize . the polymerized fibrin together with platelets forms a hemostatic plug or clot over the wound site . the term “ fibrinolysis ” as used herein refers to the degradation of fibrin . the term “ fibrinolytic ” as used herein refers to the ability of a substance to degrade fibrin and hence prevent blood clots from growing and becoming problematic . the term “ freeze - dried ” or “ lyophilized ” as used herein refers to materials that are dehydrated for the purpose of preservation . the term “ hemocompatible ” refers to a set of properties that allow contact with flowing blood without causing adverse reactions such as thrombosis , hemolysis , complement activation , or inflammation . the term “ immobilization ” as used herein refers to the attachment of a substance to an inert , insoluble material , allowing for increased resistance to changes in conditions such as pi or temperature . in particular , it allows enzymes to be held in place throughout a reaction , thus facilitating their reuse . the term “ mediator ” as used herein refers to an agent that mediates a physical , chemical , or biological process , such as a coating that facilitates the immobilization of enzymes to a substrate . the term “ non - thrombogenic ” as used herein refers to the tendency of a material in contact with the blood to prevent the formation of a thrombus , or clot . the term “ plasma - activated coating ” or “ pac ” as used herein refers to a process of immobilizing plasmin on stainless steel substrates using a plasma - activated coating mediator to create a surface that substantially attenuates thrombus formation . the term “ plasmin ” as used herein refers to the enzyme present in blood that degrades many blood plasma proteins , including fibrin clots . the term “ plasminogen ” as used herein refers to the blood circulating glycoprotein which is the precursor of plasmin . the term “ platelets ” or “ thrombocytes ” as used herein refers to blood cells whose function is to stop bleeding . platelets have no nucleus , they are fragments of cytoplasm which are derived from the megakaryocytes of the bone marrow , and then enter the circulation . the term “ reduced ” as used herein refers to having been made smaller or less in amount , degree , or size . the term “ stent ” as used herein refers to a mesh tube that is inserted into a natural passage or conduit in the body to prevent or counteract a disease - induced , localized flow constriction . the term “ streptokinase ” as used herein refers to the enzyme secreted by several species of streptococci that can bind and activate human plasminogen . the term “ substrate ” as used herein refers to the material that underlies the mediator and non - thrombogenic protein . the term “ thrombin ” as used herein refers to the serine protease that converts soluble fibrinogen into insoluble strands of fibrin , and that catalyzes many other coagulation - related reactions . the term “ thrombogenicity ” as used herein refers to the tendency of a material in contact with the blood to produce a thrombus , or clot . the term “ thrombosis ” as used herein refers to the formation of a blood clot inside a blood vessel that obstructs the flow of blood through the circulatory system . the term “ thrombus ” or “ blood clot ” as used herein refers to a solid or semi - solid mass formed from the constituents of blood within the vascular system that is the product of blood coagulation . there are two components to a thrombus , aggregated platelets that form a platelet plug , and a mesh of cross - linked fibrin protein . the term “ tissue plasminogen activator ” or “ tpa ” as used herein refers to a protein involved in the breakdown of blood clots . it is a serine protease found on endothelial cells . as an enzyme , it catalyzes the conversion of plasminogen to plasmin , the major enzyme responsible for clot breakdown . the term “ urokinase ” as used herein refers to the serine protease that is present in the bloodstream and acts on plasminogen . the term “ valve ” as used herein refers to a device that controls the passage of fluid through a pipe or duct , allowing movement in one direction only . the term “ vain ” as used herein refers to the plasmin substrate d - val - leu - lys - p - nitroanilide . the term “ valy activity assay ” as used herein refers to an assay that measures the cleavage of valy by plasmin into p - nitroanilide and d - val - leu - lys . the term “ zymogen ” as used herein refers to an inactive enzyme precursor that requires biochemical change to become an active enzyme . it has been discovered that plasma polymerization of a substrate can greatly enhance biocompatibility , which is further enhanced by coupling of enzymes and other anti - thrombotics such as heparin that inhibit clotting and platelet activation . these are collectively referred to herein as “ anti - thrombotics ”. the anti - thrombotic coating is robust enough to withstand deployment and blood flow and presents the anti - thrombotic in a biologically active conformation , thereby decreasing thrombogenicity . the enzyme is covalently bound to metal substrates via a polymer intermediary such as acetlyene ( ethylene ). the acetylene layer is blended with the metal surface using plasma polymerisation , converting the inert metal surface into a reactive polymer surface . the composition of the polymer layer can be widely varied and conditions for optimal anti - thrombotic binding varied . similar results can be achieved using other carbon chains ( such as hexane ) or different plasma conditions . fig1 is a schematic showing the steps or stages of plasmin immobilization . in the absence of modification ( step 1 ), stainless steel recruits platelets and red blood cells , and activates fibrin ( step 2 ), leading to clot formation ( step 3 ). following surface activation with the pac process ( step 4 ), plasmin can be covalently retained ( step 5 ) and it can prevent the formation of fibrin networks , resisting clot formation ( step 6 ). given the current problems with regards to late stent thrombosis in drug eluting stents , many groups are exploring the use of biodegradable coatings for drug release . in such instances , a biodegradable drug release coating may be applied over a biocompatible coating such as enzyme covalently bound by plasma polymerization . this would allow local elution of a drug , leaving behind a stent with a biocompatible coating . stents can also be manufactured from degradable materials as alternatives to permanent metallic scaffolds . these bioresorbable stents have commonly been manufacted from polymers such as poly - lactic acid and poly - glycolic acid , which remain in the body for 6 - 24 months ( zilberman and eberhard , ann . rev . biomed . eng ., 8 : 153 - 180 ( 2006 )). bioresorbable stents can also be made from metal alloys such as magnesium . these are completely resorbed within 2 months and have shown promising clinical outcomes ( erbel , di mario , et al ., lancet , 369 : 1869 - 75 ( 2007 )). plasma polymerisation and / or coating with enzyme is also relevant to the improvement of the short term biocompatibility of these temporary scaffolds and could easily be adapted for their modification . materials which can be plasma polymerized include metals , polymers , carbon , and ceramic . the anti - thrombotic , can be applied to , crosslinked with , tethered to , blended with , or laminated as part of , one or more materials to form a surface , component , or device . in the preferred embodiment , a graded polymer such as acetylene layer is deposited on the surface of a metal , such that the initial deposition is metal , with increasing polymer , finishing with 100 % polymer . the effect of this graded layer is that there is no defined metal / polymer interface and no resultant peeling off of the coating . the polymer layer is chemically activated using treatment with gas plasma , pre - disposing it to form covalent bonds with anti - thrombotics . immersion of the plasma polymerised surface in a anti - thrombotic solution is sufficient for covalent attachment , with no separate cross - linking agent required . importantly , bioactivity is retained . typical metals include stainless steel and titanium . in one embodiment , the material is or includes one or more biodegradable or non - biodegradable synthetic polymers such as polylactides , polyglycolic acids , polycaprolactones , polycaprolactams , polyhexamethylene adipamide , polycarbonates , polyamides , polyanhydrides , polyamino acids , polyesters , polyacetals , polycyanoacrylates , polyvinyl alcohols , polyvinyl chlorides , polyethylenes , polyurethanes , polypropylenes , polyacrylates , polystyrenes , polyvinyl oxides , polyvinyl fluorides , poly ( vinyl imidazoles ), polyethylene oxides , polytetrafluoroethylenes , silicone polymers and copolymers and combinations thereof . in another embodiment , the material is or includes one or more natural materials such as a protein , sugar or polysaccharide , or combination thereof . representative examples include collagen , preferably type 1 and / or type 3 , fibrin , gelatin , vitronectin , fibronectin , hyaluronic acid , glycosaminoglycans , their derivatives and mixtures thereof . preferred glycosaminoglycans include chondroitin sulfate , dermatan sulfate , keratan sulfate , heparan sulfate , heparin and hyaluronan . the application will determine the selection and design of the mechanical properties . the material can be applied as a part of a variety of clinical vascular applications including a vascular conduit , a stent , a stent - graft , a surgically or percutaneously implantable heart valve , a vascutarlseptal occlusion device , avascular closure device , endovascular implant , stent graft , graft , pacemaker lead vascular occluder , left atrial appendage occlusion device , endovascular valve , vascular closure devices including atrial septal and patent foramen ovale closure , or vena caval filters , or as a surface coating for a vascular device / application . the protein can also be used to form coatings on materials such as microchips , which may be formed of a material such as a silicon chip , which may be used as sensors , electrodes , or for drug delivery , or a device such as an implantable pump . other useful materials are matrices for tissue engineering and / or drug delivery , bone implants and prosthetics including pins , rivets , screws and rods , as well as artificial knees and other joints , especially at the surfaces where the metal , ceramic or bone interfaces with the host tissue . in the majority of these cases , the critical role of the enzyme is to increase the biocompatihility of the implant or matrix , promoting cell attachment or diminishing the formation of scar tissue , abnormal proliferation of cells ( i . e ., restenosis or scarring ), and integration of the implant into the host . preferred enzymes include streptokinase , urokinase , tissue plasminogen activator ( tpa ) including alteplase , reteplase , tenecteplase and desmoteplase , and plasmin . other anti - thronogenic proteins such as direct thrombin inhibitors ( e . g . bivalirudin etc .) and anti - platelet agents can also or alternatively be immobilized on the substrates . other materials such as heparin and heparin fragment can also be immobilized on metal or polymeric substrates . a plasma - activated coating ( pac ) process covalently binds biomolecules in their bioactive state , has low thrombogenicity and can be robustly applied to medical devices , resisting delamination when deployed in vivo ( yin et al ., biomaterials , 30 : 1675 ( 2009 ); waterhouse et al ., biomaterials , 31 : 8332 ( 2010 ); waterhouse et al ., biomaterials , 33 : 7984 ( 2012 )). the substrate material is modified to create reactive surface groups which facilitate covalent interaction . in the case of inert polymeric materials like eptfe , the surface requires activation . both ‘ classical ’ plasma processes ( bilek et al . ( 2004 ) in smart materials iii , vol . 5648 ( ed , wilson , a . r .) spie , pp . 62 - 67 ) and higher energy plasma immersion ion implantation ( bilek , et al . surface and coatings technology , 156 : 136 - 142 ( 2002 )) ( piii ) can be used . in a preferred embodiment , the enzyme is covalently tethered to the polymer when a solution of the protein is incubated with the activated surface . piii has recently been shown to increase the functional lifetime of attached proteins and may be preferred ( nosworthy , et al . acta biomater , 3 : 695 - 704 ( 2007 )). metallic substrates can be also be functionalized by applying a modified plasma process to the substrate while it is immersed in a carbon containing plasma or in a vapor of the monomer used to deposit the plasma polymer layer or by codeposition of a graded substrate / polymer layer which terminates in the polymer ( yin , et al ., surf . coat . technol ., 203 : 1310 - 1316 ( 2009 )). a range of short chain carbon - based polymers including hexane and acetylene can be used to form the basis of the plasma polymer layer . the plasma chamber also contains a background carrier gas , examples of which include oxygen , hydrogen , argon , nitrogen and combinations thereof this plasma mixture is essential to efficacy . in a preferred embodiment acetylene is injected into the plasma chamber and activated together with a combination of nitrogen and argon background gas , subsequently condensing to form polymerized surfaces . this technique can be used to bind enzyme to a range of metals including stainless steel , as demonstrated by yin , et al ., biomaterials , 30 : 1675 - 1681 ( 2009 ). the present invention will be further understood by reference to the following non - limiting examples . reagents : all reagents were purchased from sigma - aldrich , st louis and used without further purification unless otherwise noted . human umbilical vein endothelial cells ( huvecs ) were harvested enzymatically from umbilical cords . endothelial cells from passages 2 - 4 were used . sample preparation : the substrates were 316l , stainless steel foil ( ss ) 25 μm thick ( brown metals ), or 3 . 0 × 10mm 316lvm stainless steel stents ( laserage , calif ., usa ). plasma - activated coating on 316l stainless steel ( pac ) surfaces were generated from acetylene in , argon mixed with nitrogen . stainless steel stents were imaged with a zeiss evo 50 scanning electron microscope . samples were incubated with increasing concentrations of plasmin ( 0 . 1 , 1 . 0 and 10 μg ) in pbs at 37 ° c . overnight and washed in pbs prior to use . surface characterization : the contact angle between pac and de - ionized water was measured using a kruss contact angle analyzer ds10 employing the sessile drop method . x - ray photoelectron spectroscopy ( xps , specs - xps , mode xp - 50 high performance twin anode with focus 500 ellipsoidal crystal monochromator and promos 150 mcd - 9 analyser ) was utilized to provide data on the elemental composition of pac variants over time . casa xps was used to calculate areas of elemental peaks with the concentration of each element expressed as an atomic percentage . as shown in fig2 a , the relative percentage of nitrogen in the surface decreased from 32 . 3 ± 1 . 0 % on day 1 , to 24 . 2 ± 0 . 5 % on day 23 . this corresponded to a small increase in oxygen from 7 . 1 ± 0 . 5 % up to 8 . 2 ± 0 . 3 % and in the relative carbon content from 60 . 6 ± 1 . 7 % to 67 . 6 ± 1 . 1 % from day 1 to 23 , respectively . the starting water contact angle of the pac was 42 . 9 ± 2 . 4 ° 30 minutes after treatment , increasing to 52 . 9 ± 1 . 0 ° after 2 hours ( fig2 b ). surface chemistry appeared to have stabilized by day 7 , when the water contact angle was observed to be 61 . 6 ± 0 . 4 °. only minor changes were observed from this time , out to 24 days . spectra of pac and ss surfaces after incubation with plasmin contained characteristic peaks associated with the internal protein vibrations and confirmed the presence of a cross - linked polymeric layer containing predominantly carbon and nitrogen , with hydrogen and oxygen terminations ( fig2 c ). bond vibrations attributed to both saturated and unsaturated c — c and c — n bonds are observed . c — h , o — h , and n — h absorptions indicate that hydrogen terminations are present and that the surface has been oxidized by exposure to atmosphere . the relative intensities of characteristic amide a , i , and ii ftir peaks for plasmin were compared before and after washing with detergent ( fig2 )). after detergent washing , surfaces displayed only covalently attached plasmin , with retention of 54 . 2 ± 3 . 8 % of originally bound plasmin on pac , but complete removal from stainless steel . covalent attachment : samples were washed with water to remove salt and dried prior to accumulation of spectra using a digilab fts7000 ftir spectrometer fitted with an attenuated total reflection ( atr ) accessory with a trapezium germanium crystal at incidence angle of 45 °. to obtain sufficient signal / noise ratio and resolution of spectral bands , 500 scans with a resolution of 1 cm − 1 were taken . difference spectra were used to detect changes associated with the presence of plasmin , and analysis carried out . unbound protein was removed by aspiration and the surfaces were washed with pbs . non - covalently bound protein was removed by sds - washing . samples were treated with 5 % ( w / v ) sds for 1 hat 80 ° c . following the sds treatment , samples were washed with pbs and distilled water . bioactivity assay : the enzymatic activity of plasmin was monitored using a commercially available kit . one unit of activity is defined as the production of one micromole of p - nitroartilide from d - val - leu - lys - p - nitroanilide ( valy ) at ph 7 . 5 at 37 ° c . activity was monitored over time , up to 210 mins , and compared free plasmin in solution to plasmin immobilized on pac and pac alone as a negative control . measuring the color change that occurs as valy is converted to p - nitroanilide at 405 nm was used to monitor the enzymatic activity of plasmin . both fresh plasmin solution and plasmin immobilized on pac were able to convert the substrate , showing an increased absorbance over the time course , up to 290 minutes ( fig3 a ). pac alone did not produce p - nitroanilide . endothelial cell interactions : for proliferation assays , huvecs ( 20 , 000 cells / ml ) were plated in 24 - well plates for 3 and 5 days . attachment and proliferation of cells to and on plasmin - coated wells was analyzed in comparison to tissue culture plastic alone and to wells coated with fibronectin ( 10 μg / well ). cells were quantified at 3 and 5 days post - seeding using the mtt ( 3 [ 4 , 5 - dimethylthiazol - 2 - yl ]- 2 , 5 diphenyl tetrazolium bromide ) assay according to manufacturer &# 39 ; s instructions . dimethyl sulfoxide ( dmso ) was used to dissolve insoluble formazan crystals , and the absorbance at 540 nm was measured using a spectrophotometer ( biorad ). after 3 days of incubation , cell numbers on tcp , plasmin and fibronectin ( fn ) were not significantly different ( fig3 b ). at day 5 , cell proliferation on plasmin was 56 . 40 ± 3 . 2 % higher than tcp alone ( p & lt ; 0 . 001 ), but remained statically less than then fn positive control , which was a further 22 . 12 ± 1 . 8 % higher than plasmin ( p & lt ; 0 . 01 ). when immobilized on pac , there was again no significant difference between the conditions on day 3 ( fig3 c ). by day 5 , pac and pac + plasmin showed a 20 . 47 ± 1 . 6 % and 31 . 16 ± 2 . 4 % increase over stainless steel ( ss ) respectively , though this did not reach statistical significance . pac + fn increased huvec proliferation 53 . 49 ± 2 . 8 % ( p & lt ; 0 . 01 ) over stainless steel , but only 17 . 02 ± 1 . 2 % more than pac + plasmin ( p = ns ). example 4 , thrombogenicity in vitro . thrombogenicity assessment : whole blood was obtained from healthy , non - smoker , male volunteers with informed consent in accordance with the declaration of helsinki , who had not taken aspirin two weeks prior to donation . approval for this work was granted by the university of sydney , human research ethics committee ( protocol 05 - 2009 / 11668 ). experiments were conducted at least three times with different donors &# 39 ; blood . samples of ss , pac or pac + plasmin were incubated with heparinized whole blood ( 0 . 3 u / ml ) for 30 min at 37 ° c . whilst rocking . concentrations of plasmin increased from 0 . 1 - 10 u were used initially to determine an optimal coating density . thrombogenicity under flow conditions was investigated using a modified chandler loop . briefly , samples were balloon expanded into 28 cm lengths of tygon s - 50 - ht tubing ( sdr , australia ), connected into loops using 1 cm silicone connectors and filled with heparinized whole blood ( 0 . 5 u / ml , 2 . 5 ml ). the loops were rotated at 34 rpm at 37 ° c . for 60 min . the thrombus and steel from each loop was removed for imaging and weighing . the blood from each loop was combined with 10 % ( v / v acid citrate dextrose ( acd ) and centrifuged at 1000 rpm for 15 min to obtain serum . soluble p - selectin was detected via an elisa ( r & amp ; d systems , usa ). for stent evaluation , 0 . 3 u / ml heparin , 90 mins , was evaluated . the relative thrombogenicity of stainless steel , pac alone , and plasmin covalently bound to pac was studied using a whole blood adhesion assay ( fig4 ). increasing concentrations of plasmin , 0 . 1 u , 1 . 0 u , and 10 u , immobilized on pac demonstrated a dramatic reduction of thrombus weight in a dose - dependent manner , compared to stainless steel controls . pac alone reduced thrombus weight by 45 . 4 ± 9 . 1 %, but further reductions were observed for 0 . 1 u ( 62 . 3 ± 6 . 4 %), 1 u ( 78 . 3 ± 6 . 4 %) and 10 u ( 90 . 5 ± 1 . 3 %) plasmin , relative to stainless steel ( p & lt ; 0 . 001 ). the reductions in thrombus weight are also demonstrated in representative images of the samples . surface fibrinolysis was also demonstrated by incubation with whole blood containing fluorescently labeled fibrinogen . a complete interconnected fibrin network was observed on stainless steel after 30 minutes , while on pac only this network was also present but notably less dense . on plasmin coated pac only the rudiments of interconnected fibrin were observed . to more directly assess the contribution of surface - bound plasmin , the enzyme was denatured prior to incubation with pac . following repeated freeze - thaw cycles , plasmin was confirmed to be inactive using the valy conversion described above ( fig5 a ). denatured plasmin - bound surfaces continued to show superiority to stainless steel , but were statistically equivalent to pac only surfaces and had significantly higher clot weights than fresh plasmin on pac ( fig5 b ). considering the potential to store plasmin coated pac surfaces , samples were freeze - dried prior to rehydration and re - tested with whole blood . immediately following freeze - drying ( fig5 c ) and up to 14 weeks later ( fig5 d ), clot weights of freshly prepared and stored plasmin on pac were equivalent . under flow conditions in a modified chandler loop ( fig6 a ), stainless steel samples generated substantial thrombus formation ( 61 . 8 ± 8 . 3 mg ) ( fig6 b ). in contrast , the thrombogenicity of pac alone was reduced significantly to 15 . 8 ± 1 . 1 mg ( p & lt ; 0 . 001 ), while immobilization of 10 u plasmin on pac further reduced clot weight to 1 . 4 ± 0 . 4 mg ( p & lt ; 0 . 001 ), a 97 . 7 ± 1 . 3 % reduction relative to stainless steel controls . these differences are well demonstrated in the representative images , which show a clear contrast between the clotted stainless steel samples , and the 10 u plasmin samples , which are largely thrombus free . this striking thrombus reduction was driven by a significant decrease in the amount of sp - selectin detected in the samples ( fig6 d ). stainless steel controls , activating platelets generated 119 . 7 ± 4 . 8 ng / ml of sp - selectin , reduced to 88 . 1 ± 0 . 9 ng / ml in the presence of pac only . addition of plasmin to pac resulted in a further reduction to 57 . 16 ± 3 . 5 ng / ml , significantly lower than both stainless steel ( p & lt ; 0 . 001 ) and pac alone ( p & lt ; 0 . 01 ), and not significantly different from the no implant control which represents the baseline level of activation in this assay . stainless steel stents ( 3 mm × 10 mm , 316 lvm ) were laser cut and electropolished to remove any surface contaminants . pac treated stents were macroscopically darker than untreated stainless steel stents . under scanning electron microscopy ( 50 × magnification ), pac coated stents had a smooth , contiguous appearance , free from cracking or delamination . the blood compatibility of stainless steel , pac only and pac + plasmin steins was demonstrated by incubation with whole blood containing fluorescently labeled fibrinogen in a chandler loop . after 15 minutes , only faint fluorescence was observed for all conditions . in contrast , after 30 minutes , significant fibrin deposition was observed for stainless steel , while little was seen on pac + plasmin . fibrin fluorescence on pac only was intermediate between these two conditions .
a plasma - activated coating process covalently binds enzymes in their bioactive state , has low thrombogenicity and can be robustly applied to medical devices , resisting delamination when deployed in vivo . applying this process to attachment of proteins such as enzymes that inhibit thrombosis and anticoagulants such as heparin or heparin fragments , one can produce medical devices and other materials for use in vascular applications having a number of benefits including covalent attachment , not requiring intermediate linkers or chemistry ; substrate independent works on polymers , metals , ceramics , 3d shapes like stents , valves , etc . ; bioactivity is retained ; surface may retain greater bioactivity over time in vivo ; simultaneously supports endothelialisation ; can be stored for long periods , following freeze drying , and retains effectiveness when rehydrated and ; surface is able to bind many fibrinolytic enzymes such as streptokinase , urokinase , tpa , plasmin ).