in an illustrative method of making embodiment of the invention , as shown in fig2 , fumed silica 11 ( cab - o - sil eh - 5 , surface area 380 m 2 / g , cabot , tuscola , ill .) was suspended in toluene and refluxed under stirring . fumed silica has reactive silanol groups 12 on its surface . in this specific embodiment , there are about 2 . 5 to 4 . 5 silanol groups / nm 2 on the surface . an equivalent amount of an amino silane coupling agent 13 in toluene was added to the fumed silica suspension . the mixture was further refluxed and stirred overnight to produce derivatized fumed silica 14 . in this particular embodiment , the amino silane coupling agent 13 has the formula : ( ch 3 o ) 3 si ( ch 2 ) 3 ) nhr where r may be h , ch 3 , ( ch 2 ) 2 nh 2 , and ( ch 2 ) 6 nh 2 . of course , in the embodiment where r is h , the amino silane is a primary amine . as will be seen from the discussion hereinbelow , particularly preferred coupling agents are the latter three , or n - methylaminopropyl - trimethoxysilane , n -( 2 - aminoethyl )- aminopropyl - trimethoxysilane and n -( 6 - aminohexyl ) aminopropyl - trimethoxysilane . all of these agents are available commercially from gelest , tullytown , pa . the derivatized fumed silica 14 was centrifuged at 3000 rpm , rinsed with toluene three times , and then oven - dried at about 120 ° c . overnight . chn elemental analysis indicated that about 50 - 80 % of the amino silane was coupled onto fumed silica ( sil ) to form the derivatized fumed silica , or aminated - silica ( siln ) 14 . the composition of the resulting aminated - silica compound , of course , depends on the composition of the amino silane coupling agent . for the specific illustrative amino silane coupling agents covered by the formula ( ch 3 o ) 3 si ( ch 2 ) 3 ) nhr , where r is as indicated , the aminated - silica compounds are designated herein by the following nomenclature : the aminated - silica 14 was dispersed in a solvent , illustratively 10 % 0 . 2n sodium methoxide / methanol in n , n ′- dimethylformamide ( dmf ) or tetrahydrofuran ( thf ) ( dried over 4 å molecular sieves ), purged with argon , and charged with no to 80 psi under stirring for 10 - 168 hours , preferably about 24 hours , to obtain diazeniumdiolated fumed silica ( siln 2 o 2 ) 15 . a chemiluminescence no analyzer , chn elemental analysis , and the photometric greiss assay ( see , schmidt , et al ., in methods of nitric oxide research , feelisch , et al ., eds ., page 41ff , john wiley , new york , ( 1996 )) indicated that 20 - 50 % of the amino groups were diazeniumdiolated . a typical no loading for siln 2 o 2 is about 600 μmol / g . when dissolved in a 1 n naoh solution , siln 2 o 2 shows the characteristic diazeniumdiolate absorbance band with a maximum absorbance at 246 nm for secondary amines and 252 nm for primary amines . this band disappears when the siln 2 o 2 is decomposed by water . referring to fig2 a , the characteristic uv absorbance of the aminated - silica sil - 2n [ 6 ] and the diazeniumdiolated fumed silica sil - 2n [ 6 ]- n 2 o 2 na , in a 1m naoh solution , are shown as a function of wavelength in nm . studies were conducted that demonstrated that no loading of fumed silica derivatized with n -( 6 - aminohexyl ) aminopropyl - trimethoxysilane was affected by the solution in which the particles were dispersed during no charging . low loading was achieved in plain organic solvents ( 0 . 2 - 2 . 0 mmol / g no ) or medium bases ( 0 . 1 - 0 . 2 mmol / g no ), such as sodium acetoxide or sodium phenoxide , whereas high loading was achieved with strong bases , such as sodium methoxide or sodium trimethylsiloxide . in addition to the foregoing , studies were conducted to ascertain whether the counterion in the diazeniumdiolate zwitterion affected no loading and / or decomposition . to vary the counter cations ( m + ), sodium methoxide , potassium methoxide and lithium methoxide were used as the solvent during no charging . tables 1 and 2 below demonstrate that the counter cations na + , k + , or li + have minimal effect on the formation of the diazeniumdiolate in terms of no loading as measured using a chemiluminescence no analyzer to measure no using the chemiluminescence reaction between no and ozone ( table 1 ), or by measuring the molar absorptivity at 246 nm for secondary amines or 252 nm for primary amines ( ε max , mm − 1 . cm − 1 ) ( table 2 ). furthermore , the counter cation had minimal effect on the rate of diazeniumdiolate dissociation as shown in table 3 . the half - life ( t 1 / 2 , min .) of the diazeniumdiolates in phosphate - buffered saline ( pbs , ph = 7 . 4 ) at 37 ° c . was measured with a chemiluminescence no analyzer . as is known , the release rate of no from a given diazeniumdiolate , incorporated into a polymer , is proportional to the half - life of its release rate in buffer . advantageously , derivatized fumed silica particles can be pre - loaded with no and used at a later time in any desired polymer . the siln 2 o 2 particles are relatively stable when stored at freezing temperatures , but slowly decompose at room temperature . tables 4 and 5 shows the half - life of siln 2 o 2 na particles at various ph and temperatures . the diazeniumdiolate derivatized fumed silica ( siln 2 o 2 ) is incorporated into a polymer by dispersing between 5 - 40 wt %, and preferably between about 10 - 20 wt %, in a solution of the polymer and then solvent casting or dip - coating to form thin films of no - releasing polymer on a surface . in one specific illustrative embodiment , polyurethane ( medical grade carbothane pc 357a ( 135 mg ) was dissolved in thf ( 1 ml ). a fine dispersion of siln 2 o 2 ( 15 mg ) in thf ( 1 ml ) was obtained by sonication , added to the polymer solution , and vortexed . the mixture was cast on a teflon plate / glass well and cured in air overnight . in another specific illustrative embodiment , low molecular weight pvc ( fluka , 9 mg ) was dissolved in thf ( 1 ml ). a fine dispersion of siln 2 o 2 ( 15 mg ) in thf ( 1 ml ) was obtained by sonication for 10 minutes , added to the polymer , and vortexed . the mixture was cast on a teflon plate and cured in air overnight . in yet another illustrative example , diazeniumdiolated fumed silica was used as a reinforcing filler to prepare silicone rubber polymer films using the following technique : hydroxy - terminated polydimethylsiloxane ( viscosity 20 , 000 cst , 320 mg ), methyl - trimethoxysilane ( 10 . 2 mg ), and dibutyltin dilaurate ( 6 . 4 mg ) were dissolved in toluene ( 1 . 9 ml ). a fine dispersion of diazeniumdiolated fumed silica ( 18 mg ) in toluene ( 1 ml ) was added to the polymer solution , vortexed , and sonicated for 10 minutes . the mixture was cast on a teflon plate and cured in air for 2 days to form a diazeniumdiolated fumed silica silicone rubber film . no - releasing polymers of the type disclosed hereinabove are then incorporated as an inner layer in a multilayer structure of the type shown schematically in fig3 . referring to fig3 , no - releasing polymer layer 33 , having the diazeniumdiolated fumed silica ( e . g ., particles 35 ) dispersed therein , is sandwiched between at least one polymer top coat 32 and a bottom substrate 34 . in the embodiment shown , there are two polymeric top coats 32 and 31 , coat 31 having the blood - contacting surface 36 . layers 32 and 31 are plain base polymers and do not contain diazeniumdiolated fumed silica . however , the top coats do not interfere with the desirable flux rate of no at the polymer - blood contacting surface . the bottom substrate 34 may be another layer of plain polymer or it may be a surface of another device , such as pvc tubing of the type used in extracorporeal circuits or a metallic stent or electrode lead . for an actual image of a multilayer structure of the type shown in fig3 , see fig7 which is a scanning electron micrograph of coated pvc tubing . of course , the multilayer structure of fig3 is purely illustrative , and it is to be understood than many alternate embodiments can be devised within the spirit and scope of the invention . for example , the multilayer structure can incorporate more than one interior layer of a no - releasing polymer . the following specific examples were fabricated with diazeniumdiolated fumed silica and used in the experiments reported hereinbelow : polyurethane monolayer films , about 16 mg per disc ( 1 cm ), were formed by solvent casting to have the following compositions by weight percent : 45 % polyurethane ; 45 % 2 - nitrophenyl octyl ether ( npoe ; fluka , switzerland ); and 10 % loading with diazeniumdiolated fumed silica ( specifically , sil - 2n [ 6 ] n 2 o 2 na , as shown on fig2 ). a second polyurethane film having twice the siln 2 o 2 loading was formed to have the following composition by weight percent : 40 % polyurethane ; 40 % npoe ; and 20 % loading with sil - 2n [ 6 ]- n 2 o 2 na . in this specific illustrative embodiment , a tri - layer structure was formed to have the overall composition 30 % polyurethane ; 60 % npoe ; and 10 % siln 2 o 2 in a multilayer structure where the top and bottom layers were plasticized polyurethane ( 1 : 2 by wt pu / npoe ) and the middle layer was 1 : 2 : 1 by wt pu / npoe / sil - 2n [ 6 ] n 2 o 2 na . a multilayer coating was formed on the inner wall surface of ¼ ″ pvc tubing . the four layer structure comprised : a polyurethane bottom coat adjacent the tubing surface ; a polyurethane / siln 2 o 2 layer , a polyurethane top coat on the no - releasing layer , and a second top coat of pvc as the blood - contacting surface . the total thickness of this multilayer coating was about 250 micrometers . films # 1 -# 3 were cut into 1 cm diameter disks and soaked in 5 ml aliquots of phosphate buffered saline ( pbs ; ph 7 . 4 ; sigma , st . louis , mo .) placed in a shaker incubator at 37 ° c . no levels in the pbs were determined indirectly by measuring nitrite levels using the greiss assay . the results are shown on fig4 , which is a graphical representation of the no release profiles of the polymer films , shown as the total no released ( measured in μmol ) as a function of time in hours . although most siln 2 o 2 particles have a half - life of within an hour in pbs buffer at 37 ° c ., they release no for a prolonged period of time when incorporated into a polymer matrix . as shown in fig4 , polyurethane films having varying siln 2 o 2 loading , bathed in pbs at 37 ° c ., continuously emit no over a three day period . the amount of no generated was nearly proportional to the siln 2 o 2 loading for similar film compositions . in film # 3 , siln 2 o 2 was only incorporated into the middle layer of a tri - layer film in order to minimize leaching of siln 2 o 2 . surprisingly , the tri - layer configuration exhibited a slightly faster no generation rate than the monolayer compositions ( films # 1 ) with the same siln 2 o 2 loading . however , this particular tri - layer film ( film # 3 ) contained 15 % more plasticizer than the monolayer film ( film # 1 ). higher plasticizer content may result in a faster water uptake rate and , thus , higher no flux , from the siln 2 o 2 - loaded films . fig5 is a graphical representation of the mean no flux ( 10 − 10 mole . cm − 2 . min − 1 ) over time in hours for the tri - layer structure ( film # 3 ). the solid line indicates the no production rate from endothelial cells as estimated by vaughn , et al ., am . j . physiol . , vol . 274 ( heart circ . physiol . , vol . 43 ) page h2163 ( 1998 ). the dashed and dotted lines indicate the no flux from bradykinin stimulated and unstimulated endothelial cells , respectively , as estimated based on data published by radomski , et al , s . proc . natl . acad . sci . usa , vol . 87 , page 5193 ( 1990 ). as shown in fig5 , the no fluxes from the siln 2 o 2 - loaded polyurethane film # 3 were comparable to those produced by endothelial cells up to 30 hours , and were very high during the first two hours . this film is expected to have good blood compatibility since the observed levels of no fluxes should effectively prevent platelet activation and aggregation on the film &# 39 ; s surfaces . fig6 is a graphical representation of the no flux from the multilayered film # 4 on pvc tubing in phosphate buffered saline at ph 7 . 4 at 37 ° c . the no flux , at about 4 × 10 − 10 mole . cm − 2 . min − 1 is comparable to that produced by the endothelial cells . fig7 is a scanning electron micrograph image of a cross - section of the multilayer - coated pvc tubing ( film # 4 ) used in this experiment . referring to fig7 , a polyurethane bottom coat is seen adjacent the inner surface of ¼ ″ pvc tubing , followed by the no - releasing layer of polyurethane with diazeniumdiolated fumed silica . a polyurethane top coat is adjacent to the no - releasing layer followed by a second top coat of pvc . an in vivo experiment was conducted using the multilayer - coated pvc tubing ( film # 4 ) in an extracorporeal circuit . the tubing was connected to rabbits through cannulas for venovenous circulation via a roller pump . blood was circulated through the tubing for 4 hours at a rate of about 100 ml / min . fig8 is a graphical representation of the platelet count expressed as a percentage of initial platelet count versus time in hours . in a study of ten rabbits , 5 with the no - releasing pvc tubing in the circulation circuit and 5 with plain pvc tubing as controls , there was less platelet - count drop observed for the no - releasing tubing as compared to the control over the 4 hours of circulation . scanning electron micrograph images were made from a cross - section of the pvc tubing ( control ) and no - releasing pvc tubing used in the extracorporeal circulation experiments . the results are shown fig9 and 10 , respectively . fig1 shows much less clotting on the surface of no - releasing tubing indicating improved thromboresistance . as is evident from the data presented herein , no - releasing fumed silica is a good source of no that may be readily incorporated into different polymer matrices to fabricate multilayer biomaterials having improved blood compatibility . such biomaterials could be used in a variety of biomedical applications , specifically including , but not limited to , the fabrication of extracorporeal devices and intravascular sensors . illustratively , the no - releasing biocompatible polymer coatings disclosed herein may be used on the blood - contacting surfaces of existing medical or diagnostic devices , such as in - dwelling catheters , plastic extracorporeal tubing sets ( including kidney dialysis , open - heart surgery heart - lung machines ), and the like . it is to be understood that the no - releasing polymer coatings of the present invention may comprise a membrane , film , matrix , tubing , or any other device that is likely to come into contact with tissue or blood . of course , the no - releasing polymer itself may be cast , molded , or otherwise formed into any desired configuration and subsequently coated with one or more top coats . of course , the novel polymers would be useful for fabricating chemical sensors that can continuously monitor or measure physiologically important ions ( e . g . h + , k + , na + ) and gases ( co 2 and o 2 ) in the blood for intraarterial or extracorporeal applications . in addition to the foregoing , nitric oxide releasing polymers may also be able to inhibit bacterial adhesion and growth on the surface of in - dwelling polymeric devices , such as urinary catheters . therefore , the term “ blood - contacting surface ” can refer to any liquid , or body fluid , contacting surface . although the invention has been described in terms of specific embodiments and applications , persons skilled in the art can , in light of this teaching , generate additional embodiments without exceeding the scope or departing from the spirit of the invention described herein . accordingly it is to be understood that the drawing and description in this disclosure are proffered to facilitate comprehension of the invention , and should not be construed to limit the scope thereof .
biocompatible polymeric materials capable of providing in situ release of nitric oxide included diazeniumdiolated fumed silica as a filler in a multilayer polymer structure to release no upon contact with water . the blood - contacting polymer surface is preferably multi - layered so that the no - releasing layer , containing the diazeniumdiolated fumed silica , is shielded from blood contact by one or more top coats . when in contact with blood , the no released at the surface of the polymer prevents platelet activation and adhesion to the surface , thereby reducing platelet consumption , risk of thrombus formation and other clinical complications associated with interactions between blood and foreign materials .