according to the present invention , the stent coatings incorporating biologically active materials for timed delivery in situ in a body lumen of interest are preferably sprayed in many thin layers from prepared coating solutions or suspensions . the steps of the process are illustrated generally in fig1 . the coating solutions or suspensions are prepared at 10 as will be described later . the desired amount of crosslinking agent ( if any ) is added to the suspension / solution as at 12 and material is then agitated or stirred to produce a homogenous coating composition at 14 which is thereafter transferred to an application container or device which may be a container for spray painting at 16 . typical exemplary preparations of coating solutions that were used for heparin and dexamethasone appear next . silicone was obtained as a polymer precursor in solvent ( xylene ) mixture . for example , a 35 % solid silicone weight content in xylene was procured from applied silicone , part # 40 , 000 . first , the silicone - xylene mixture was weighed . the solid silicone content was determined according to the vendor &# 39 ; s analysis . precalculated amounts of finely divided heparin ( 2 - 6 microns ) were added into the silicone , then tetrahydrofuran ( thf ) hpcl grade ( aldrich or em ) was added . for a 37 . 5 % heparin coating , having 35 % solids , w silicone = 5 g and w hep = 5 × 0 . 35 × 0 . 375 /( 0 . 625 )= 1 . 05 g were used . the amount of thf needed ( 44 ml ) in the coating solution was calculated by using the equation w silicone solid / v thf = 0 . 04 for a 37 . 5 % heparin coating solution . finally , the manufacturer crosslinker solution was added by using pasteur p - pipet . the amount of crosslinker added was formed to effect the release rate profile . typically , five drops of crosslinker solution were added for each five grams of silicone - xylene mixture . the solution was stirred by using the stirring rod until the suspension was homogenous and milk - like . the coating solution was then transferred into a paint jar in condition for application by air brush . silicone ( 35 % solution as above ) was weighed into a beaker on a metler balance . the weight of dexamethasone free alcohol or acetate form was calculated by silicone weight multiplied by 0 . 35 and the desired percentage of dexamethasone ( 1 to 40 %) and the required amount was then weighed . example : w silicone = 5 g ; for a 10 % dexamethasone coating , w dex = 5 × 0 . 35 × 0 . 1 / 0 . 9 = 0 . 194 g and thf needed in the coating solution and w silicone solid / v thf = 0 . 06 for a 10 % dexamethasone coating solution . example : w silicone = 5 g ; v thf = 5 × 0 . 35 / 0 . 06 ≈ 29 ml . the dexamethasone was weighed in a beaker on an analytical balance and half the total amount of thf wag added . the solution was stirred well to ensure full dissolution of the dexamethasone . the stirred dex - thf solution was then transferred to the silicone container . the beaker was washed with the remaining thf and this was transferred to the silicone container . the crosslinker was added by using a pasteur pipet . typically , five drops of crosslinker were used for five grams of silicone . the application of the coating material to the stent was quite similar for all of the materials and the same for the heparin and dexamethasone suspensions prepared as in the above examples . the suspension to be applied was transferred to an application device , at 16 in fig1 . typically a paint jar attached to an air brush , such as a badger model 150 , supplied with a source of pressurized air through a regulator ( norgren , 0 - 160 psi ) was used . once the brush hose was attached to the source of compressed air downstream of the regulator , the air was applied . the pressure was adjusted to approximately 15 - 25 psi and the nozzle condition checked by depressing the trigger . to secure the stent for spraying and rotating fixtures were utilized successfully in the laboratory . both ends of the relaxed stent were fastened to the fixture by two resilient retainers , commonly alligator clips , with the distance between the clips adjusted so that the stent remained in a relaxed , unstretched condition . the rotor was then energized and the spin speed adjusted to the desired coating speed , nominally about 40 rpm . with the stent rotating in a substantially horizontal plane , the spray nozzle was adjusted so that the distance from the nozzle to the stent was about 2 - 4 inches and the composition was sprayed substantially horizontally with the brush being directed along the stent from the distal end of the stent to the proximal end and then from the proximal end to the distal end in a sweeping motion at a speed such that one spray cycle occurred in about three stent rotations . typically a pause of less than one minute , normally about one - half minute , elapsed between layers . of course , the number of coating layers did and will vary with the particular application . for example , typical tie - layers as at 18 in fig1 for a coating level of 3 - 4 mg of heparin per cm 2 of projected area , 20 cycles of coating application are required and about 30 ml of solution will be consumed for a 3 . 5 mm diameter by 14 . 5 cm long stent . the rotation speed of the motor , of course , can be adjusted as can the viscosity of the composition and the flow rate of the spray nozzle as desired to modify the layered structure . generally , with the above mixes , the best results have been obtained at rotational speeds in the range of 30 - 50 rpm and with a spray nozzle flow rate in the range of 4 - 10 ml of coating composition per minute , depending on the stent size . it is contemplated that a more sophisticated , computer - controlled coating apparatus will successfully automate the process demonstrated as feasible in the laboratory . several applied layers make up what is called the undercoat as at 18 . in one process , additional upper undercoat layers , which may be of the same or different composition with respect to bioactive material , the matrix . polymeric materials and crosslinking agent , for example , may be applied as the top layer as at 20 . the application of the top layer follows the same coating procedure as the undercoat with the number and thickness of layers being optional . of course , the thickness of any layer can be adjusted by adjusting the speed of rotation of the stent and the spraying conditions . generally , the total coating thickness is controlled by the number of spraying cycles or thin coats which make up the total coat . as shown at 22 in fig1 the coated stent is thereafter subjected to a curing step in which the prepolymer and crosslinking agents cooperate to produce a cured polymer matrix containing the biologically active species . the curing process involves evaporation of the solvent xylene , thf , etc . and the curing and crosslinking of the polymer . certain silicone materials can be cured at relatively low temperatures , ( i . e ., rt - 50 ° c .) in what is known as a room temperature vulcanization ( rtv ) process . more typically , however , the curing process involves higher temperature curing materials and the coated stents are put into an oven at approximately 90 ° c . or higher for approximately 16 hours . the temperature may be raised to as high as 150 ° c . for dexamethasone containing coated stents . of course , the time and temperature may vary with particular silicones , crosslinkers and biologically active species . stents coated and cured in the manner described need to be sterilized prior to packaging for future implantation . for sterilization , gamma radiation is a preferred method particularly for heparin containing coatings ; however , it has been found that stents coated and cured according to the process of the invention subjected to gamma sterilization may be too slow to recover their original posture when delivered to a vascular or other lumen site using a catheter unless a pretreatment step as at 24 is first applied to the coated , cured stent . the pretreatment step involves an argon plasma treatment of the coated , cured stents in the unconstrained configuration . in accordance with this procedure , the stents are placed in a chamber of a plasma surface treatment system such as a plasma science 350 ( himont / plasma science , foster city , calif .). the system is equipped with a reactor chamber and rf solid - state generator operating at 13 . 56 mhz and from 0 - 500 watts power output and being equipped with a microprocessor controlled system and a complete vacuum pump package . the reaction chamber contains an unimpeded work volume of 16 . 75 inches ( 42 . 55 cm ) by 13 . 5 inches ( 34 . 3 cm ) by 17 . 5 inches ( 44 . 45 cm ) in depth . in the plasma process , unconstrained coated stents are placed in a reactor chamber and the system is purged with nitrogen and a vacuum applied to 20 - 50 mtorr . thereafter , inert gas ( argon , helium or mixture of them ) is admitted to the reaction chamber for the plasma treatment . a highly preferred method of operation consists of using argon gas , operating at a power range from 200 to 400 watts , a flow rate of 150 - 650 standard ml per minute , which is equivalent to 100 - 450 mtorr , and an exposure time from 30 seconds to about 5 minutes . the stents can be removed immediately after the plasma treatment or remain in the argon atmosphere for an additional period of time , typically five minutes . after this , as shown at 26 , the stents may be exposed to gamma sterilization at 2 . 5 - 3 . 5 mrad . the radiation may be carried out with the stent in either the radially unconstrained status or in the radially constrained status . preferably , however , the surface is modified prior to plasma treatment or just prior to sterilization by one of several additional processing methods of which some are described in relation to the following examples . the undercoat of a stent was coated as multiple applied layers as described above thereafter and cured as described at 22 . the heparin content of the undercoat was 37 . 5 % and the coating thickness was about 30 - 40 μ . fluorosilicone ( fsi ) spray solution was prepared at 30 from a fluorosilicone suspension ( applied silicone # 40032 ) by weighing an amount of fluorosilicone suspension and adding tetrahydrofuran ( thf ) according to the relation equation of v thf = 1 . 2 × the weight of fluorosilicone suspension . the solution was stirred very well and spray - coated on the stent at 32 using the technique of the application of the undercoat process at 18 and the coated stents were cured at 90 ° c . for 16 hours . the coated stents are argon plasma treated prior to gamma sterilization according to the procedures described above in accordance with steps 22 - 26 . fig7 is a plot of heparin release kinetics in phosphate buffer system with fluorosilicone topcoat and without any topcoat . the thickness of the topcoat is about 10 - 15 a . while it does not appear on the graph of fig7 it should be noted that the release rate for the coating without fsi is initially about 25 times higher than that with fsi , i . e ., during the first 2 hours . this is , of course , clearly off the scale of the graph . it is noteworthy , however , that the coating with the fsi top layer or diffusion barrier does show a depressed initial release rate combined with an enhanced elution rate after the first day and through the first week up until about the tenth day . in addition , the fluorosilicone ( fsi ) topcoat , by virtue of the high electronegativity of fluorination maintains non - thrombogenic surface qualities during and after the elusion of the biologically active heparin species . in addition , because of the negative charges off the heparin itself , the electro - negativity of the fluorosilicone topcoat may be , at least in part , responsible for the modified heparin release kinetic profile . fig8 compares a plot of fluorosilicone ( fsi ) top coating containing 16 . 7 % imbedded heparin with one containing fluorosilicone ( fsi ) only . an undercoating is identical to that utilized in fig7 containing about 37 . 5 % heparin to a thickness of about 30 - 40 microns . these elution kinetics are quite comparable with the heparin - free fsi top layer greatly reducing the initial burst of heparin release and otherwise the heparin in the fsi top layer imparts a slightly greater release over the period of the test . an undercoat was coated on a stent and cured at 22 as in example 1 . the stent was then treated by argon gas plasma as at 24 and ammonium gas plasma at 40 . the equipment and the process of argon gas plasma treatment was as has been described above . the ammonium plasma treatment was implemented immediately after the argon gas plasma treatment , to aminate the surface of the coating . the ammonium flow rate was in the range of 100 - 700 cubic centimeter per minute ( ccm ) in preferably in the range of 500 - 600 ccm . the power output of radio frequency plasma was in the range of 50 - 500 watts , preferably in ˜ 200 watts . the process time was in the range of 30 sec - 10 min , preferably ˜ 5 min . immediately after amination , the stents were immersed into electrophilically activated polyethylene glycol ( peg ) solution it 42 . peg is known to be an inhibitor of protein absorption . examples of electrophilically activated peg are peg nitrophenyl carbonates , peg trichlorophenyl carbonates , peg tresylate , peg glycidyl ether , peg isocyanate , etc ., optionally with one end terminated with methoxyl group . molecular weight of peg ranged from about 1000 - 6000 , and is preferable about 3000 . it has been observed that simple ammonium amination will not generate large quantities of primary and secondary amines on the elastomeric polymer surface ( for example silicone ). instead , imine (& gt ; c ═ n — h ), and other more oxidative nitro containing groups will dominate the surface . it is generally necessary to add reductive agent , such as nabh 3 cn into the reaction media so that the functional group on peg can react with imine and possibly other nitro - containing species on the surface , and therefore immobilize peg onto the surface . the typical concentration of nabh 3 cn is about 2 mg / ml . since peg and its derivatives dissolve in water and many polar and aromatic solvents , the solvent used in the coating must be a solvent for peg but not for the drug in the undercoat to prevent the possible loss of the drug through leaching . in the case of eluting - heparin coating , a mixed solvent of formamide and methyl ethyl ketone ( mek ) or a mixed solvent of formamide and acetone are preferred solvents , ( preferably at ratios of 30 formamide : 70 mek or acetone by volume ), since they will not dissolve heparin . the concentration of peg , the reaction time , the reaction temperature and the ph value depend on the kind of peg employed . in the case of eluting heparin coating , 5 % peg tresylate in ( 30 - 70 ) formamide / mek was used successfully . the reaction time was 3 hours at room temperature . peg was then covalently bound to the surface . gamma radiation was then used for sterilization of this embodiment as previously described . with respect to the anticoagulant material heparin , the percentage in the undercoat is nominally from about 30 - 50 % and that of the topcoat from about 0 - 30 % active material . the coating thickness ratio of the topcoat to the undercoat varies from about 1 : 10 to 1 : 2 and is preferably in the range of from about 1 : 6 to 1 : 3 . to produce a topcoat which is substantially free of pores , materials such as porosigens , which can be removed or leached out of the topcoat should not be included in the composition used to form the topcoat . one way of preparing a substantially non - porous topcoat is to apply a topcoat composition which comprises substantially pure polymeric materials . these materials preferably impart biocompatibility to the implanted device during and after the release of the biologically active material . a topcoat which only partially covers the undercoat can be formed in a number of ways . such methods include controlling the thickness of the topcoat so that it is less than the diameter of certain drug particles in the undercoat . for example , when a drug used in the undercoat has an average particle size of 5 μm , it is possible that 15 % of the particles will be greater than or equal to 8 μm . at the molecular level , the surface unevenness is at least more than 5 microns . by application of a topcoat of about 5 micron or less . the uneven surface will become smooth to a certain degree . but uncovered areas will still exist , which allows water to penetrate into the undercoat , swelling the drug particle , or dissolving the drug . due to the osmotic pressure , the drug will elute out through the uncovered areas . if the osmotic pressure is too high , cracks or voids may form in the topcoat which allows drugs to elute from the undercoat to the body . as illustrated in fig9 in a coating 102 which covers a surface of a prosthesis 101 , a thin topcoat 104 of the coating 102 only partially covers the undercoat 103 of the coating 102 . by having the topcoat 104 cover less than the entire surface of the undercoat 103 , parts of the undercoat 103 , including a number of drug particles 105 , are exposed to body fluids at the implantation site so that the drug 105 can be released . by using a topcoat 104 having a thickness which is about the average particle size of the drug 105 , certain larger sized drug particles 106 will not be covered by the topcoat 104 . other methods to form a partially covered topcoat include using a polymer which is incompatible to the undercoat elastomer as the biocompatible material of the topcoat . because of the incompatibility between the materials , a microphase separation will form in the topcoat which will leave the undercoat partially uncovered . persons skilled in the art are aware of suitable combinations of such incompatible materials . another method involves applying a poor solvent wash to the topcoat to force the biocompatible polymer to shrink and create uneven surfaces voids in the topcoat which forms a topcoat which partially covers the undercoat . in yet another method of making a topcoat which partly or fully covers the undercoat , topcoat and undercoat materials having different young &# 39 ; s moduli ( either before or after they have cured ) are used . for instance , a topcoat material having a higher young &# 39 ; s modulus compared to that of the undercoat material can be employed . when the coated device is mounted on the delivery device or during deployment of the coated device , the topcoat undergoes compression or stretching or other types of mechanical challenges . since the topcoat material has a higher young &# 39 ; s modulus , it will tend to crack and form voids in the topcoat to allow the drug of the undercoat to elute therefrom . another method for forming the topcoat involves using a bioabsorbable material in the topcoat , which can cover the entire undercoat or only a part of the undercoat . when the coated device comes into contact with body fluid , the topcoat begins to degrade either at the surface or throughout the bulk of the topcoat . the rate of degradation depends upon the type of bioabsorbable material used . once the topcoat has been partially absorbed , the undercoat is exposed to body tissue and the drug in the undercoat is released , but the burst release or effect is reduced . suppressing the burst effect also enables a reduction in the drug loading or in other words , allows a reduction in the coating thickness , since the physician will give a bolus injection of antiplatelet / anticoagulation drugs to the patient during the stenting process . as a result , the drug imbedded in the stent can be fully used without waste . tailoring the first day release , but maximizing second day and third day release at the thinnest possible coating configuration will reduce the acute or subacute thrombosis particularly if drugs such as heparin are incorporated . fig4 depicts the general effect of drug loading for coatings of similar thickness . the initial elution rate increases with the drug loading as shown in fig5 . the release rate also increases with the thickness of the coating at the same loading but tends to be inversely proportional to the thickness of the topcoat as shown by the same drug loading and similar undercoat thickness in fig6 . what is apparent from the data gathered to date , however , is that the process of the present invention enables the drug elution kinetics to be controlled in a manner desired to meet the needs of the particular stent application . in a similar manner , stent coatings can be prepared using a combination of two or more drugs and the drug release sequence and rate controlled . for example , antiproliferation drugs may be combined in the undercoat and antiplatelet drugs in the topcoat . in this manner , the antiplatelet drugs , for example , heparin , will elute first followed by antiproliferation drugs to better enable safe encapsulation of the implanted stent . the heparin concentration measurement were made utilizing a standard curve prepared by complexing azure a dye with dilute solutions of heparin . sixteen standards were used to compile the standard curve in a well - known manner . for the elution test , the stents were immersed in a phosphate buffer solution at ph 7 . 4 in an incubator at approximately 37 ° c . periodic samplings of the solution were processed to determine the amount of heparin eluted . after each sampling , each stent was placed in heparin - free buffer solution . as stated above , while the allowable loading of the elastomeric material with heparin may vary , in the case of silicone materials , heparin may exceed 60 % of the total weight of the layer . however , the loading generally most advantageously used is in the range from about 10 % to 45 % of the total weight of the layer . in the case of dexamethasone , the loading may be as high as 50 % or more of the total weight of the layer but is preferably in the range of about 0 . 4 % to 45 %. it will be appreciated that the mechanism of incorporation of the biologically active species into a thin surface coating structure applicable to a metal stent is an important aspect of the present invention . the need for relatively thick - walled polymer elution stents or any membrane overlayers associated with many prior drug elution devices is obviated , as is the need for utilizing biodegradable or reabsorbable vehicles for carrying the biologically active species . the technique clearly enables long - term delivery and minimizes interference with the independent mechanical or therapeutic benefits of the stent itself . coating materials are designed with a particular coating technique , coating / drug combination and drug infusion mechanism in mind . consideration of the particular form and mechanism of release of the biologically active species in the coating allow the technique to produce superior results . in this manner , delivery of the biologically active species from the coating structure can be tailored to accommodate a variety of applications . whereas the above examples depict coatings having two different drug loadings or percentages of biologically active material to be released , this is by no means limiting with respect to the invention and it is contemplated that any number of layers and combinations of loadings can be employed to achieve a desired release profile . for example , gradual grading and change in the loading of the layers can be utilized in which , for example , higher loadings are used in the inner layers . also layers can be used which have no drug loadings at all . for example , a pulsatile heparin release system may be achieved by a coating in which alternate layers containing heparin are sandwiched between unloaded layers of silicone or other materials for a portion of the coating . in other words , the invention allows untold numbers of combinations which result in a great deal of flexibility with respect to controlling the release of biologically active materials with regard to an implanted stent . each applied layer is typically from approximately 0 . 5 microns to 15 microns in thickness . the total number of sprayed layers , of course , can vary widely , from less than 10 to more than 50 layers ; commonly , 20 to 40 layers are included . the total thickness of the coating can also vary widely , but can generally be , from about 10 to 200 microns . whereas the polymer of the coating may be any compatible biostable elastomeric material capable of being adhered to the stent material as a thin layer , hydrophobic materials are preferred because it has been found that the release of the biologically active species can generally be more predictably controlled with such materials . preferred materials include silicone rubber elastomers and biostable polyurethanes specifically . this invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the example as required . however , it is to be understood that the invention can be carried out by specifically different devices and that various modifications can be accomplished without departing from the scope of the invention itself .
a coating and method for a coating an implantable device or prostheses are disclosed . the coating includes an undercoat of polymeric material containing an amount of biologically active material , particularly heparin , dispersed therein . the coating further includes a topcoat which covers less than the entire surface of the undercoat and wherein the topcoat comprises a polymeric material substantially free of pores and porosigens . the polymeric material of the topcoat can be a biostable , biocompatible material which provides long term non - thrombogenicity to the device portion during and after release of the biologically active material .