in describing preferred embodiments of the present invention , specific terminology is employed for the sake of clarity . however , the invention is not intended to be limited to the specific terminology so selected , and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose . thin bond or tie coat layers according to the invention may be applied to difficult - to - bond - to substrates in order that other layers which cannot normally be bonded to such substrates may be satisfactorily bonded . the polymers of the invention are sufficiently resistant to degradation by solvents in succeeding layers that the coating does not lose adhesiveness when soaked in water and is impervious to water diffusion from the surface . classes of polymers which may be employed include acrylic polymers and copolymers based on monomers such as methylmethacrylate , butylmethacrylate , isobutylmethacrylate , ethylmethacrylate , methylacrylate , acrylic acid , styrene methacrylate , styrene acrylate , and others ; vinyl polymers and copolymers such as polyvinylpyrrolidone , vinylpyrrolidone - vinylacetate copolymers , ethylene acrylic acid copolymers , epoxy polymers , and others . exemplary commercial products that may be used in the invention include acrylics such as aryloid r ( rohm & amp ; haas ) at - 63 , at - 51 , at - 81 , wr - 97 ; polyvinylpyrrolidone polyvinyl acetate copolymers such as pvp / va ( gaf ) e - 335 , e - 635 ; ethylene acrylic acid copolymers such as primacor ™ ( dow ) 5989 , 5990 ; melamine resins such as cymel ( cytec industries ) 303 , 370 , 380 ; epoxies such as epon ( shell ) 1001 . other appropriate polymers having the requisite characteristics will be apparent to persons of ordinary skill . the polymers preferably , but not necessarily , contain reactive groups or points of reactivity such as hydroxyls , mono -, di - and tertiary amines , acids such as carboxyl , amides , or other groups which represent points of chemical reactivity . the polymers and points of chemical reactivity are able to form attractive forces such as hydrogen bonding toward the medical device surface , and also toward the coating layers to be applied over them . such bonds are very strong , and prevent penetration of the top coat layer and water without requiring covalent or other ionic links between the device surfaces and the thin polymer tie coatings . polymers with reactive groups are preferred to help bond with substrates like metals . however , polymers lacking such groups such as acrylic or styrene polymers may also be used . the reactive groups can also react to form a cross - linked matrix or help to form a cross - linked matrix . if desired , cross - linkers such as urea resins , melamines , isocyanates , phenolics , and others may be incorporated to cross - link the polymers of the invention with themselves , by reacting with the points of chemical reactivity on the polymer chains . alternatively , cross - linkers may react with themselves to form a cross - linked matrix in which the tie coat polymers are enmeshed , resulting in a solvent - resistant layer . cross - linking within the thin polymeric tie coats ( either between the principal polymers or around them ) is useful in promoting effective adhesion by ensuring that the solvents used in succeeding coating layers do not attack and degrade the tie coat polymer layer excessively and by resisting water penetration . when the tic coat layers are subjected to excessive solvent attack the polymer tie coat layer may be diluted by the succeeding coating layer thereby degrading the adhesive bond between the tie coat layer and the medical device surface . excessive water penetration can also degrade adhesion . coatings according to the invention may be prepared with polymers that lack points of reactivity , such as acrylic or styrene polymers or copolymers . likewise , coatings may be made without cross - linking . however , with such coatings a greater tie coat thickness may be required or desirable than with layers made of polymers with points of reactivity and layers with cross - linking , in order to achieve a high degree of adhesion of succeeding layers according to the invention . for example , cross - linked coatings with polymers having reactive groups may be about two to about ten microns thick , in contrast with a coating as in example 1 , where a water - borne acrylic styrene copolymer is applied to metal , with a hydrogel layer on top , and a total thickness of about 30 - 40 microns . the tie coat layers of the present invention are extremely durable , even when immersed in water for prolonged periods . as will be shown in examples , coatings on stainless steel can be soaked in water for months without losing adhesion , even when hydrogel layers are applied to the samples . hydrogel layers typically absorb several times their weight in water and serve as a pathway for water diffusion into the layer ( s ) between the hydrogel layer and the medical device surface . such exposure to water , especially for extended periods represents a considerable challenge to the tie coats of the present invention and the fact that they are able to endure such challenges without adhesive failure is a surprising result . the tie coat layers of the present invention are so thin , typically less than 5 microns , that the adhesiveness is all the more remarkable . the coatings of the invention may be thin , on the order of 0 . 0002 ″- 0 . 0005 ″ ( 5 - 12 microns ), although it may be as thick as is desirable . preferably , the coating is in the range of about 2 to about 100 microns , more preferably less than about 80 microns , or 60 microns , and particularly preferred embodiments are less than about 15 microns thick . bond coats of about 2 to about 10 microns are generally quite adequate . if the coating is thicker , it may cause other problems in certain applications where thinness is important . a coating according to the invention may include a bond coat of about 5 microns and a two - layer hydrogel comprising a 5 micron base coat and a 5 micron top coat , with a total thickness of about 15 microns . examples of substrates and bond coat formulations that are effective with them are listed below . many other combinations will be apparent to a person of ordinary skill following the teachings of the invention . the coatings are coherent in that they form a continuous surface layer . when coated with a top coat , the resulting coatings are resistant to removal on prolonged soaking in aqueous fluids , and are adherent to a wide variety of substrates . there are several useful tests of adhesion of coatings comprising the bond coat of the invention . two of them are the dry adhesion tape test and the wet rub test . uncovered tie coat coatings generally adhere well to a substrate , as do tie coat coatings with a base coat such as a cellulose ester layer , but problems frequently arise when a surface coating is applied , such as a hydrogel . completed coatings according to the invention are able to endure immersion in water for at least an hour and remain adhesive and resistant to removal by abrasion as indicated by the wet rub test , and , after drying , the tape test . this sets them apart from the prior art . in the wet rub test , parallel cuts are made through the coating with a razor or knife . the coating is immersed in water for a predetermined period , such as an hour . a finger is then rubbed briskly across the cuts . peel - back of the coating constitutes coating failure . in the dry adhesion test , adhesive tape is pressed firmly onto the coating , then peeled off briskly . removal of the coating constitutes failure . the coatings according to the invention may be applied to the surface of a biomedical device or other device with sufficient thickness and permanence to retain the coating &# 39 ; s desirable qualities throughout the useful life of the coated device . they have sufficient thinness to be useful in many applications inappropriate for prior art coatings . the coatings of the invention are nonreactive with living tissue and are non - thrombogenic in blood . the coatings may be applied by various techniques such as dip , spray , brush , wipe , or other methods known to those skilled in the art . the coating solutions have low viscosities , typically less than 100 cps , and have good spreading properties . the coatings are baked at elevated temperatures , typically 50 ° c . to 100 ° c ., to drive off the organic solvents . gas plasma treatment may be done according to conventional methods . a vacuum is drawn , a gas such as oxygen or ammonia is allowed in , it is excited with rf , and the surface is allowed to stay in contact with the resulting plasma for a sufficient time , such as 20 minutes , to put functional groups on the surface . oxygen produces hydroxyl surface groups , and ammonia produces amine groups covalently bound to the polymer surface . over time the groups tend to fold into the surface and become less reactive , so plasma - treated surfaces are best used fresh . the coating systems described herein produce coatings that remain bonded in aqueous fluids on surfaces such as polyethylene , polypropylene , polyamide , polyester , silicone and metals such as stainless steel , platinum , gold , nickel , titanium , nickel - titanium alloys , chrome and other surfaces that are generally considered as presenting adherence problems . it may be necessary to treat some surfaces with gas plasma or other ionizing treatment to promote adhesion to the substrates . the following examples show some embodiments of how the invention can be used . the coatings of the present invention are designed to remain coherent and adhesive under torturous conditions . in one embodiment , a guide catheter is inserted into a femoral artery at the leg up to the aortic arch . this is a fairly straight and direct route . into the guide catheter , a guide wire is inserted . a device catheter is then inserted into the guide catheter and over the guide wire . the device catheter may be used for , for example , angioplasty , stenting , drug delivery aneurysm repair or other purposes . the device catheter typically has a gradual curve at the last about 1 cm of the end being inserted over the guide wire and may generate a great deal of abrasion between the guide wire and the device catheter during use as described below . the device catheter and guide wire are then further inserted into the vasculature beyond the end of the guide catheter at the aortic arch and into the cardiac arteries or the brain . in continuing into these further channels in the body , the guide wire and device catheter must be capable of traversing numerous low radius turns encountered in the vasculature . when a bifurcation is encountered in a vessel , the curvature at the tip of the device catheter is turned in the desired direction , directing the guide wire . the guide wire and the device catheter are moved stepwise through the vasculature , i . e . the guide wire is inserted a short distance and then the device catheter is inserted over the guide wire . for example , the guide wire may be inserted 2 cm while holding the device catheter in place . the device catheter is then inserted over the guide wire while the guide wire is held in place . this produces even further abrasion between the guide wire and the device catheter . in addition , there is abrasion between the guide wire and tissue and between the device catheter and tissue . as mentioned above , numerous turns may be encountered in the vasculature while inserting the catheter and guide wire . for example , there may be numerous turns on the order of 1 - 3 cm in diameter . in addition , there may be turns with a diameter of less than or equal to 1 cm . under these extreme conditions , coatings present in the prior art fail by losing adhesion , losing lubricity or cracking . such failure can be catastrophic in the intravascular environment ( e . g . failure could cause an embolism ) and must be avoided . accordingly , the coatings of the invention remain adherent when subjected to bending through a small radius . typically , a small radius means less than about 3 cm . unlike prior art coatings , the coatings of the present invention are able to withstand these extreme uses . use of the present coatings on the guide wire , on the inside and / or outside of the device catheter and / or on the inside and outside of the guide catheter sufficiently prevent abrasion of the coatings while maintaining lubricity . this allows the catheters and guide wires to be inserted without damaging tissue , the guide wire or the catheter , even in demanding applications that require a great deal of bending of the device catheter and guide wire and that require repeated motion between the device catheter and guide wire . in u . s . pat . no . 5 , 620 , 738 to fan et al ., inventors at union carbide focused their disclosure on a particular class of copolymers that may be used with hydrophilic polymers to form lubricious coatings . the fan et al . copolymers are formed of at least one vinyl moiety and at least one carboxylic acid moiety , in the same monomer or in different monomers of the copolymer . the vinyl monomer is preferably vinyl chloride or another vinyl halide and is present as a major component of the polymer . having carboxylic acid moieties together with the vinyl moieties in copolymers is critical to the coatings described by fan et al . the particular binder copolymers disclosed by fan et al . each include a vinyl chloride monomer , or a vinyl acetate or butadiene monomer , copolymerized with maleic acid or acrylic acid . in contrast , in accordance with the present invention , there is no requirement for a copolymer of a vinyl monomer with a carboxylic acid monomer . in most of the examples in fan et al ., the substrate is polyvinylchloride , and the binding polymer is vinyl chloride , and it is preferred to have the same monomer in the binder as the substrate . for example , fan et al . coated polyvinyl chloride tubes with vinyl ether - maleic anhydride copolymer and converted the anhydride to maleic acid . in other examples , a vinyl chloride - vinyl acetate - maleic acid copolymer was used to coat pvc tubes . alternatively , the copolymer had - acrylate monomer instead of - maleic acid . however , pvc is easy to coat because it is susceptible to solvent attack , leading to an inter - penetrated surface . although fan et al . mention substrates other than pvc , and have three examples applied to polyethylene tubes , the fan et al . coatings are preferably penetrated into the substrate . this teaches away from use of such coatings on metals and inert non - penetrable surfaces . indeed , in contrast with examples using pvc substrates , fan et al . contains no statement that the coatings applied to polyethylene tubes were abrasion resistant , suggesting that the fan et al . coating lacked the superior abrasion - resistance of the present invention . in contrast to coatings such as in fan et al ., the present invention is effective on inert substrates without the need for penetration . there may be some incidental or superficial penetration on certain polymer substrates , but not significant penetration as would be found , for example , with polyvinylchloride substrates . in this context , the phrase “ not penetrated ” embraces incidental or superficial penetration as will be understood by a person of skill in the art . thus , the benefits of the invention are met without penetration , but the presence of some penetration does not eliminate the advantages of the invention . the coatings of fan et al . may be applied as a one step or two step coating , and fan et al . does not distinguish in the end result arising from use of one or the other approach . in contrast , with the present coatings , the outer layer is applied to the bond coat layer . this permits the bonding polymer components to bond to the substrate and to each other before application of the surface coating material . fan et al . teach away from selecting a two step process in which the bonding layer is formed first . fan et al . do not teach any coating that would adhere to stainless steel , silicone , polyamide , or nylon , in contrast to the claimed invention here , which achieves that extremely difficult standard . the composite coatings of the present invention achieve unexpectedly superior performance on such substrates . prior art coatings comprising non - cross linked chlorinated polyolefins suffer from other disadvantages . for example , such coatings do not adequately bond to metals . in addition , if plasma pretreatment of substrates is used , polyolefin coatings must be applied within 24 hours of pretreatment to prevent failure of the coatings . in contrast , the coatings of the present invention may be applied more than 24 hours after pretreatment , and the coatings of the present invention are sufficiently adhesive to metals to withstand the severe abrasion and numerous turns of the vasculature which may be encountered in use . the present invention is an improvement over coatings such as chlorinated polyolefin coatings on polyethylene tubing . the present coatings have unexpectedly superior adhesion and abrasion resistance as compared to the prior art coatings . thus , while the prior art chlorinated polyolefin coating is suitable for less stringent applications such as on polyurethanes and polyesters that do not require high specific adhesion , the improved coatings of the present invention allow for use in a much broader range of applications . these additional applications include use on substrates such as metals , including nickel titanium alloy , nickel , gold , chrome , platinum and stainless steel , and plastics , including polyolefins such as polyethylene and polypropylene , silicones , latex rubbers , polyisocyanates , and others , for which the prior art coatings are not sufficiently adherent nor sufficiently abrasion resistant . a stainless steel surface was brush coated with the following solution , and dried for 30 minutes at 85 ° c . add in order , stir until dissolved . the coating was tested for adhesion by cutting lines through it with a knife and then rubbing briskly across the cuts with a finger after the coating was immersed in water . no failure of adhesion ( i . e . peel back ) occurred after the wet rub test . next , the coating dry adhesion was tested by pressing universal tape 83436 tape ( united stationers supply , co .) firmly onto the coating and peeling the tape off briskly . no coating should be removed by this test . this sample showed no adhesion failure on the tape test . a styrene acrylic aqueous dispersion polymer ( 55 % solids ) was brush coated on a stainless steel surface , and dried for 30 minutes at 85 ° c . this coating showed excellent adhesion when tested according to example 1 . a sample as per example 2 was overcoated with a hydrogel composition consisting of : the coating was dried for 25 hours at 85 ° c . the coating passed the wet and dry adhesion tests according to example 1 . the following solution was brush coated on a stainless steel surface , and dried at 85 ° c . for 2 hours . this coating was dried for 15 hours at 85 ° c . the adhesion of the coating was tested according to example 1 , and had good adhesion under both wet and dry conditions . a sample as per example 4 was overcoated with the following solution and dried 2 hours at 85 ° c . next the sample was overcoated with the following hydrogel solution and dried for four hours at 85 ° c . the adhesion of the coatings was tested according to example 1 and had good adhesion under both wet and dry conditions . the sample had good wet lubricity . if the first coating was omitted the adhesion failed under the test condition . the following solution wag dip coated on a stainless steel wire and dried for 2 hours at 85 ° c . next the sample was overcoated with the same two overcoating solutions per example 5 . the adhesion was good when tested according to example 1 under wet and dry conditions . the sample continued to show good adhesion after soaking in water for more than 130 days . the coating had good wet lubricity . polyethylene tubing was exposed to oxygen plasma treatment . the pe tube was then coated with the same coatings as per example 6 . the adhesion was good when tested according to example 1 under wet and dry conditions . the sample had good wet lubricity . polyethylene tubing was treated as in example 7 , except that the middle coating just underneath the hydrogel consisted of : the sample had good adhesion when tested according to example 1 under both wet and dry conditions , and had good wet lubricity . silicone tubing was treated as in example 8 . the coating had good adhesion when tested according to example 1 under wet and dry conditions , and the coating had good wet lubricity . silicone tubing was exposed to oxygen plasma treatment by placing in an evacuated vessel and subjecting to alternate cycles of adding oxygen and cycling rf power . initially , oxygen is fed in at 550 ± 50 mtorr for 0 . 25 minutes . the oxygen is turned off , and the rf power is turned on , with 450 ± 50 watts forward and ≦ 50 watts reverse , for 2 minutes . these two steps are repeated five times , with the remaining oxygen cycles lasting 2 minutes . the tie coat is typically applied to the plasma treated surface before degradation of the plasma treatment , within a day or two . next , the treated tubing was dip coated with the following solutions and dried one hour at 85 ° c . this coating was overcoated with the same hydrogel as used in example 3 . the coated sample had good adhesion when tested according to example 1 under both wet and dry conditions , and had good wet lubricity . stainless steel was coated with the following solution and dried 60 minutes at 85 ° c . next , the sample was overcoated with the last two coatings that were used to overcoat the first coating in example 5 . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and the sample had good lubricity . a sample of polyester tubing was treated as per example 8 . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and the sample had good wet lubricity . a stainless steel surface was dip coated with the following tie coat solution and dried 2 hours at 85 ° c . next , the sample was overcoated with the same hydrogel coating as per example 3 , and dried for 2 hours at 85 ° c . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and had good lubricity . a sample of stainless steel was dip coated with the same tie coat solution as used in example 1 , and was then dried for 2 hours at 85 ° c . next , the sample was overcoated with the last two coatings of example 5 . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and the sample had good lubricity when wet . a sample of stainless steel was dip coated with the following tie coat composition , and was dried for 2 hours at 85 ° c . next , the sample was overcoated with the last two coatings of example 5 . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and the sample had good lubricity when wet . a sample of pebax polyamide tubing was treated according to example 10 . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and had good wet lubricity . a sample of nylon 12 tubing was treated as in example 16 , except that no oxygen plasma treatment was used . the sample had good adhesion when tested according to example 1 under wet and dry conditions , and had good wet lubricity . the embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention . nothing in this specification should be considered as limiting the scope of the present invention . modifications and variations of the above - described embodiments of the invention are possible without departing from the invention , as appreciated by those skilled in the art in light of the above teachings . it is therefore to be understood that , within the scope of the claims and their equivalents , the invention may be practiced otherwise than as specifically described .