for this invention , there has been suggested the following drug / polymer combination for the stent coating : drug : “ rapamycin ”, c 51 h 79 no 13 , an immunosuppressant polymer : med3 - 6605 rtv silicone , supplied as a 20 % solution in 1 , 1 , 1 - trichloroethane solvent , from nusil silicone technology , carpinteria , calif . 93013 . the stent to be coated were ps153 model stents ( cordis corporation , miami lakes fla .) and the rapamycin drug . the med3 - 6605 solution and sp - 1 silicone adhesion primer were acquired from nusil . in this project , it was desired to focus on dip coating methods , as opposed to spraying . two approaches were considered : dip and dry — dip the stent in the coating solution , let the solution drain and then dry to form coat ; coat , move on a mandrel and dry — dip stent in coating solution , move onto a mandrel , push along the mandrel to remove excess coating solution and then dry to form coat . it was subsequently found that the “ dip and dry ” method did not work well , as the solution dried too quickly leaving a blob of silicone on the stent . thereafter the coating method was focused upon . stent mandrels were prepared from long septum needles ( popper & amp ; sons , inc .) with outside diameters of 0 . 7 mm ( 22 gauge ), 1 . 05 mm ( 19 gauge ), 1 . 27 mm ( 18 gauge ) and 1 . 48 mm ( 17 gauge ). septum needles have hardened , smooth surfaces which are particularly suitable for use in this application . the needles were cut from a luer syringe attachment and the ends were polished to remove any sharp edges . dip stent into med3 - 6605 solution , then transfer to 0 . 9 mm diameter mandrel . shake off the excess solution and move forward on mandrel , dry , and push off into collection vial ( procedure a ). again , it was found that that the polymer solution dried too quickly . using scanning electron micrography (“ sem ”), the coating was found to be much too thick ( see fig1 ). it was apparent from the previous trials that the nusil med3 - 6605 20 % silicone solution was too viscous to obtain the desired 10 to 15 μm layer coating thickness . therefore , dilution was necessary . to dilute the solution , ethyl acetate solvent ( omnisolv , bdh , toronto , ontario spectroscopic grade ) was used . this solvent was chosen based on previous experience with this type of silicone solution in a spray coating application . it has comparable solvency for silicones and has a boiling point and evaporation rate similar to 1 , 1 , 1 - trichloroethane . solutions of 9 . 8 %, 5 . 1 %, and 2 . 3 % silicone were prepared as shown below : the previous experiment also indicated that too much solution remained even after shaking to remove the excess . therefore , the method was changed to include a rotational spin , which removed the excess solution , by centrifugal force ( procedure b ). the stents coated using this procedure are shown below : visual examination of the coated stents showed that all had blockage of at least some of the stent openings ( see fig2 ). the most dilute coating had minimal blockage , but the coating thickness was relatively thin (& lt ; 1 μm ) ( fig3 and 4 ). nevertheless , it was apparent that this method held the most promise for further development . from the foregoing , it was suspected that the solvent evaporation rate would have a significant effect on the coating procedure . to test this theory , two solvents were used in place of ethyl acetate — toluene ( fisher scientific , toronto , ontario , certified acs grade ) and diethyl ether ( fisher scientific , toronto , ontario , certified acs grade ). toluene has an evaporation rate roughly ⅓ that of trichloroethane , whereas diethyl ether is about 3 times faster . it was decided to position the stent on a 0 . 9 mm mandrel close to one end . thereafter , solution was applied to the stent , rotated with fingers . the mandrel was shaken to remove excess solution . slowly move stent forward on mandrel towards far end , drying as the stent moves forward . push off into collection vial and let cure overnight at ambient conditions . ( this was procedure c .) solutions were made up as shown below starting with a 10 % solid mixture : the solutions were used to prepare the stents as shown below with coating procedures as listed . it was found that the diethyl ether solutions evaporated too quickly and coatings were poorly formed on the stents , whereas the toluene solvent gave much more evenly coated stents . still , problems with blockage of stent openings remained . it had previously been noted that that a relatively close - fitting mandrel gave the best coating results . in lieu of this and taking into consideration the above results , the following changes were made . we decided to use of a 1 . 5 mm mandrel and the solvent xylene ( fisher scientific , certified acs grade ) solvent , which has even a lower evaporation rate than toluene . solutions of about 10 % solids content were made as shown below : the solutions were used to prepare the stents as shown below using coating procedures as listed : note : for procedure d , we used nusil sp - 120 coating on stents prior to placing the stents on mandrel , and thereafter followed the steps of procedure c . it was found that the close fitting mandrel did reduce the amount of stent opening blockage and that the coating seemed more uniform on the stents ( see fig5 ). however , it was difficult to remove the stent without damaging it as it essentially was bonded to the mandrel by the coating . examination under a microscope also showed that coating material “ piled up ” at one end of the stent openings as the stent was pushed forward along the mandrel . silicone alone had been used in previous experiments , but in this series of experiments , a drug ( rapamycin ) was added . for this series of runs the diameter of the mandrel was varied along with details of the preparation method , while only one coating solution was used as shown below : this solution was used to prepare the stents as shown below using coating procedures as listed : procedure f : the stents were cleaned by sonication in dichloromethane ( dcm ) for 1 minute . this removed any dirt or fibers adhering to the stent . the stents were left in the dcm solvent until use . a stent was placed on a 1 . 55 mm diameter mandrel and the mandrel was spun rapidly between the fingers for about 20 seconds to remove dcm . the stent was dried under a stream of dry nitrogen gas . the coating solution was delivered to the stent using a 1 . 0 ml syringe . the mandrel was spun rapidly for about 10 seconds to remove excess solution . the stent was pushed forward on the mandrel at least one stent length and the spinning operation repeated for another 10 seconds . the stent was moved forward again on the mandrel , rotated , and dried with nitrogen gas . procedure g : this was the same as procedure f , except to spin the 1 . 55 mm mandrel between the fingers for 20 seconds . procedure h : this was the same as procedure g , except spin for 1 minute . procedure i : this was the same as procedure g , except to use a 1 . 27 mm mandrel and to spin for 20 seconds . procedure j : this was the same as procedure g , except to spin for 10 seconds on a 0 . 9 mm mandrel . procedure k : this was the same as procedure j , except to spin for 30 seconds . from these experiments it was found that the 1 . 27 mm diameter mandrel gave very good results ( see fig6 ), as the coating material did not “ bunch up ” at the end of the openings as with the 1 . 5 mm mandrel experiments . conversely , the coating was too thick and of poor quality with the 0 . 9 mm diameter mandrel . overall , long spin times had no observable effect on the coating quality ; about 20 seconds was deemed sufficient . an apparatus 5 ( see fig2 ) was assembled to mechanize some of the steps derived from the previous experience to improve the reproducibility of the coating procedure . the setup consists of a mandrel 10 driven by a high - speed motor 20 with momentary switches 30 and a movable stage 40 for bearing support of the mandrel 10 during the coating operation . the stents [ palmaz — schatz ® crown stents cordis , miami lakes fla .) were prepared using the solutions listed below : procedure l : first , clean stents using sonication for one minute in dcm . insert a stent on a 1 . 27 mm diameter mandrel and dry with nitrogen gas . fix the mandrel 10 into the apparatus and coat stent with solution . immediately spin at 2500 rpm for about 5 seconds . move the stent forward at least one stent length and repeat the spin procedure . move the mandrel forward one additional stent length and spin . if stent moves freely on mandrel , dry with a nitrogen gas flow for about 10 seconds and then transfer stent from mandrel into glass vial . procedure m : this was the same as procedure l , except to spin the mandrel at 1200 rpm for 5 seconds . procedure n : this was the same as procedure l , except to spin the mandrel at 2500 rpm for 10 seconds , using a 1 . 50 mm mandrel . procedure o : this was the same as procedure l , except to spin the mandrel at 4000 rpm for 15 seconds . procedure p : this was the same as procedure l , except to spin the mandrel at 4000 rpm for 30 seconds . it was found that the apparatus significantly improved the problem of slot clogging of the coating on the stents . more than likely , this was attributed to the higher speeds “ flinging away ” excess solution at the very beginning of the coating process . it was observed that the best procedure was that with 4000 - rpm spin for 15 to 30 seconds . a model of the coating process is shown in fig2 . in step 1 the stent is covered ( in excess ) with the coating solution . as the mandrel 10 and stent 100 are brought up to high revolutions , excess solution is thrown away from the stent on its outer side , although more solution is retained in the stent / mandrel gap 25 ( step 2 ). however , as the stent 100 is moved forward ( using carriage motor 20 ) on the mandrel the inner coating solution is left behind on the mandrel 10 . eventually the stent 100 is no longer in contact with the solution on its underside , while contacting the mandrel 10 . ( steps 3 & amp ; 4 .) finally , the wet solution remaining only on the stent 100 is dried with applied nitrogen gas ( step 5 ). for evaluation purposes , a series of rapamycin / silicone - coated and silicone - only stents were prepared . the solutions used for the coating preparation are shown below : again , the stents were palmaz schatz ® crown stents of varying lengths as described below . eight each of rapamycin / silicone and silicone - only coated stents were prepared : the stents marked with an asterisk (*) on the chart above were also pre - coated with sp1 silicone adhesion primer ( manufacturer : nusil ). a scanning electron micrograph of a typical silicone - only stent surface is seen in fig7 ( for 99 - 05 - 8071 - 67 - 10 ) which shows a fairly smooth , even coating . the thickness for the coat ( see fig8 ) is somewhat thin at about 3 μm to 10 μm . the rapamycin / silicone - coated surface ( see 99 - 05 - 8071 - 67 - 3 in fig9 ) has particles distributed over it , rather than dissolved into the solution . this result is related to the solvency of the added xylene solvent . experiments have shown that solvents that can maintain rapamycin in solution ( e . g . chloroform , chcl3 ) gave a coating surface essentially devoid of these particles ( see 99 - 05 - 8071 - 52 - 2 in fig1 ). unfortunately , these solvents did not have the slow evaporation rate required for this process . the coating thickness for the 99 - 05 - 8071 - 67 - 3 stent was about 1 to 8 μm ( see fig1 ). four cured , coated stent samples were then analyzed for volatility , as described below : the volatility of each of the stents was quite low ; the silicone - only coatings are particularly clean from residuals of 1 , 1 , 1 - trichloroethane , ethyl benzene , xylene and acetic acid ( a residue from the curing process ). it then had to be determined if the stents coated in such a manner would inflate as to nominal dimensions . inflation was carried out on the following four coated stents : the inflated surface of the 99 - 058071 - 67 - 1 silicone / rapamycin - coated stent is shown in fig1 . no obvious cracks are seen where the stent crossover t is seen in fig1 . however , there may be some detachment of the film , which might be due to handling damage . it may be that the silicone is relatively poorly bonded to the stainless steel surface . some improvement comes from using the sp - 1 adhesion promoter ( see fig1 ). the inflated silicone - only coated 99 - 058071 - 67 - 8 stent is shown in fig1 and 16 . there are no evident cracks or other problems . when the above described processes were used with other stents ( crossflex ® and bx - velocity ® stents , again produced by cordis ), it was found that a 10 . 9 % silicone solution concentration was the highest that could be used without causing blockage in open areas of the stents . sem photographs of the inflated - coated crossflex ( fig1 and 18 ) and bx - velocity ( fig1 and 20 ) show that the coating did not crack on inflation . it is important to realize that the polymer coating used to accomplish the objects of this invention is unimportant , so long as the stent is coated properly with a therapeutic amount of drug which continues to cling to the stent after removal from the mandrel 10 . for instance , it has been determined that a copolymer of n - butyl methacrylate and n - hexyl methacrylate with rapamycin contained therein works well as a coating for the crossflex ® stents described above . similarly , the use of n - butyl methacrylate can be substituted with 2 - ethylbutyl methacrylate , again to coat the stent with rapamycin . this mixture also works well . other combinations are certainly foreseeable too . the results of the stent coating experiments can be summarized as follows . a simple stent dip , followed by insertion over a mandrel to remove the excess coating solution , did not work well . the solution dried too quickly and in an uncontrolled manner , so that the thickness of solution on the stent varied considerably from stent to stent . a coat , spin , move and dry procedure was the best method . the ability to turn the mandrel to “ spin off ” the excess drug / polymer solution is an important part of creating an even coat on the stent . the excess solution would otherwise contribute to a thick , uneven coating and blockage of stent slots . initially done by hand , the process is better served by an apparatus that can turn the mandrel at high speeds in a reproducible manner . also , the solvent used in the process must have several properties : it must solvate the polymer ( in this case the rtv silicone ). the solvent should dry slowly enough that it is possible to form the coat into its final configuration before drying changes its viscosity appreciably . also , the solvent should solvate the drug , to avoid particulates forming on the stent surface . from the foregoing examples , we see that xylene acts as a suitable solvent for the silicone polymer and had a relatively slow evaporation rate , but rapamycin was not soluble in it . the solution viscosity is important to facilitate flow in “ spin - off ” and re - arrangement of the film to an even coating . viscosity is dependent on solids type / concentration and solvent type . as the solution dries , its viscosity rises , making it more difficult for the solution to move or be spun off ( hence the requirement for slow drying solvents in the coating mix ). silicone solids in the range of 10 % to 16 % in solution was determined to result in coating thickness within the range of the desired 10 μm . higher percentage solid coatings would give thicker coatings , but with increased likelihood of blockage of stent slots . the size of the mandrel 10 is important . it should be thick enough in diameter to prevent excessive retention of solution but small enough that the stent does not stick to the mandrel during stent removal . the stent is placed at one end of the mandrel and slowly moved forward in a series of steps , each of which slowly whittles down the interior layer thickness . in this work , a 1 . 27 - mm diameter mandrel gave good results with a 1 . 5 - mm inner diameter stent . 1 . put stents to be cleaned in a 100 - ml beaker and add about 20 ml of reagent grade dichloromethane ( dcm ) solvent . 5 . clean 1 . 27 mm diameter mandrel by wiping with a kimwipe ®, soaked with acetone . set aside to dry on a clean surface . 6 . place stent on mandrel by using end of mandrel to insert into one end of the stent and slowly pushing onto the mandrel from the dcm solution . 7 . by tapping the mandrel from the opposite end , allow the stent to slide over the mandrel to the opposite end of the mandrel . you can also use a scalpel to slowly push the stent down ; it should move freely . 9 . once on the apparatus , turn on the mandrel motor and rotate at high speed for a few seconds while blowing the stent with dry , clean nitrogen gas . this serves to dry off the stent of any residual dcm solvent 10 . weigh out , to 5 decimal places , the silicone polymer solution into a 2 - ml vial . 11 . weigh out the rapamycin drug ( if required ) and put into vial . 12 . add solvent ( e . g . xylene ) to vial . record weights of all components . 13 . seal the vial with a screw top ( note : vial top should be compatible with the solvents in use ) and then shake up the vial to mix the components . sonicate the mixture for about 5 minutes . 14 . using a plastic 1 ml syringe , transfer a small amount ( about 0 . 5 ml ) of the solution onto the stent by dropping the solution over the stent on the mandrel . be sure to thoroughly coat the stent . 15 . immediately turn on the mandrel motor and rotate at 4000 rpm . this serves to throw off any excess solution from the stent and provide the proper distribution of the solution on the stent surface . use the push button to turn the motor on and off in pulses of about 1 sec . this process serves to constantly accelerate / decelerate the stent and keeps it moving relative to the mandrel to avoid sticking . 16 . after about 15 seconds , turn off the motor and move the stent about one stent length down the mandrel onto a clean section . this step is commonly known as “ indexing ” the stent . again turn on the mandrel motor and pulse the motor for about 10 seconds . repeat this step 16 two ( or more ) times . 17 . turn off the motor and move the stent about 2 stent lengths down the mandrel to a clean section . turn the motor to full speed while blowing the stent with clean , dry nitrogen for about 20 seconds . 18 . turn off the motor , and slowly move the stent forward using a scalpel or other instrument , and push off into a receiving vial . 19 . remove the mandrel from the apparatus , clean , and prepare for the next run . 20 . the polymer is an rtv silicone , which means that it cures at room temperature in about 24 hours . moisture is required for the cure . therefore , place the sample in a very moist environment for 1 day . one easy way to do this is to use a forced air oven and place a container of water at the bottom of the oven . maintain the oven temperate at ambient or slightly above . the forced air will pick up the moisture by evaporation from the container and cure the polymer on the stent .
a stent is positioned on an undersized mandrel and the stent is coated with an excess of a polymer and drug solution . the stent is rotated to spin off the excess of the coating . the stent is then moved into a new , clean position on the mandrel . the process is repeated a few times , after which time the coating is already dry and non - sticky . this process forms a conforming coating . various important polymer solution parameters include viscosity , solvent evaporation rate and several others . the actual type of coating polymer is not as important as how the surface of the stent is treated , according to the steps described herein .