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 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 tetrahydrofuron ( thf ) hpcl grade ( aldrich or em ) was added . for a 37 . 5 % heparin coating , for example : w silicone = 5 g ; solid percent = 35 %; w hep = 5 × 0 . 35 × 0 . 375 /( 0 . 625 )= 1 . 05 g . 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 crosslinker may be any suitable and compatible agent including platinum and peroxide based materials . 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 calculated . 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 was 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 , 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 ). 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 . any appropriate method can be used 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 , 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 tie layer as at 18 and thereafter additional upper layers , which may be of a different composition with respect to bioactive material , the matrix polymeric materials and crosslinking agent , for example , are applied as the top layer as at 20 . the application of the top layer follows the same coating procedure as the tie layer with the number and thickness of layers being optional . of course , the thickness of any layer can be adjusted by modifying 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 pre - polymer 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 are exposed to gamma sterilization at 2 . 5 - 3 . 5 mrad . the radiation may be carried out with the stent in either the radially non - constrained status — or in the radially constrained status . with respect to the anticoagulant material heparin , the percentage in the tie layer is nominally from about 20 - 50 % and that of the top layer from about 0 - 30 % active material . the coating thickness ratio of the top layer to the tie layer varies from about 1 : 10 to 1 : 2 and is preferably in the range of from about 1 : 6 to 1 : 3 . 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 subcute thrombosis . 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 top layer as shown by the same drug loading and similar tie - coat thickness in fig6 . the effect of average particle size is depicted in the fig7 - 11 in which coating layers with an average coating thickness of about 25 microns ( μm ), prepared and sterilized as above , were provided with dispersed heparin particles ( to 37 . 5 % heparin ) of several different average particle sizes . fig7 shows plots of elution kinetics for four different sizes of embedded heparin particles . the release took place in phosphate buffer ( ph 7 . 4 ) at 37 ° c . the release rate using smaller , particularly 4 - 6 μm average sized particles noticeably reduces the initial rate or burst effect and thereafter the elution rate decreases more slowly with time . average particle sizes above about 15 μm result in initial release rates approaching bolus elution . this , of course , is less desirable , both from the standpoint of being an unnecessary initial excess and for prematurely depleting the coating of deserved drug material . in addition , as shown in the photomicrographs of fig8 - 11 , as the average particle size increases , the morphology of the coating surface also changes coatings containing larger particles ( fig9 - 11 ) have very rough and irregular surface characteristics . these surface irregularities may be more thrombogenic or exhibit an increased tendency to cause embolization when the corresponding stent is implanted in a blood vessel . accordingly , it has been found that the average particle size should generally be controlled below about 15 μm to reduce the burst effect and preferably should be ≦ about 10 μm for best results . the 4 - 6 μm size worked quite successfully in the laboratory . however , it should be noted that larger particle size can also be advantageously used , for instance , when the drug load is low , such as below 25 weight percent . elution kinetics can be adjusted by a combination of changing the particle size and changing the load or concentration of the dispersed drug material . 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 modified 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 anti - thrombotic drugs in the topcoat layer . in this manner , the anti - thrombotic drugs , for example , heparin , will elute first followed by antiproliferation drugs , e . g . dexamethasone , 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 method of coating implantable open lattice metallic stent prosthesis is disclosed which includes sequentially applying a plurality of relatively thin outer layers of a coating composition comprising a solvent mixture of uncured polymeric silicone material and crosslinker and finely divided biologically active species , possibly of controlled average particle size , to form a coating on each stent surface . the coatings are cured in situ and the coated , cured prosthesis are sterilized in a step that includes preferred pretreatment with argon gas plasma and exposure to gamma radiation electron beam , ethylene oxide , steam .