CA2100970A1

CA2100970A1 - Lubricous polyer network

Info

Publication number: CA2100970A1
Application number: CA002100970A
Authority: CA
Inventors: Paul J. Buscemi; Paul C. Slaikeu
Original assignee: Individual
Current assignee: Boston Scientific Scimed Inc
Priority date: 1991-12-18
Filing date: 1992-12-17
Publication date: 1993-06-19
Also published as: EP0572624A1; US5693034A; EP0572624A4; WO1993011751A1; JPH06506019A

Abstract

A polymer network useful as a lubricous coating, the polymer network comprising a reaction product of a vinyl prepolymer and an un uncrosslinked hydrogel retained within the reaction product such that the network exhibits a greater lubricity when wet.

Description

WO93/11751 21 O ~ 9 7 0 PCTtUS92/11021 LUBR1COUS POLY~ER NEn~ORK
BACKGROUND OF THE INVENTION
The present invention relates to a compositicn for forming a polymer network, and in particular t it relates to a pol~er network useful as a lubricous coating.
Lubricity of ~edical devices, such as catheters that are insertable i~to the body is ian important feature. One method of ~ncreasing lubricity is to 2pply a lubri~ous ooating to .he surface of ~he cztheter.
S licone has been used as a coating for man~f cle~in.znd ~.et~llic ~,edical aevices. Howe~Jer, sil~cene is hydr~phobic, and a~hough imparting so~.e lu~rici~y against cer~ain surfaces, siliconels coefficient of f.-ic~ion increases dramatically in the pr~sence of : watar, plasma, or blood.
Hydro~el poIymers have also been used in cc,~tln~ ydroaels are c~aracte~ized by an init'al 2~ tac~y quali~y rcllowed by lubricity upor, hydration.
: Many hydrogel compositions hydrate virtually instantaneously, while others require considera~'y more t~me. ~ ~ ~
The Lambert U.S.-Patent 4,45g,3l~ desorib2s a process for coating a polymer sur~ace w~th ~ hydrophilic coatlng ~with iow friction in a weti~condition.;' The process includes applying to; the polymer:surface 2 solution containing be~weén 0.05 to 40 percent`:of the compound, which comprises :at least two~:unreacted isocyanate groups per:molecule, evapo`rat~ns the.solven~, :~appl~ring`i a solut1on:contai~ing between 0.:05-'to ~50 ~percent~of~ olyethy}ene~oxide A to ~the~treated~polymer .

WO93/11751 ~ 1 0 U 9 7 0 PCT/US92/11021 .
surface and then evaporating the solvent. The coating is then cured at an elevated temperature.
The Johannson et al U.S. Patent 4,906,237 describes a hydrophilic coating that is made by applying a solution of an osmolality increasing compound to a non-reactive hydrophilic polymer surface layer and then evaporating the solvent of the solution.
The Bae et al U.S. Patent 4,931,287 describes a heterogeneous interpenetrating polymer network for use in the controlled release of drugs. The network is a heterogeneous matrix in one instance using polyethylene oxide crosslinked with for example a triisocyanate, as a hydrophilic component with styrene, an alkyl methacryla~e, or a polytetramethylene ether glycol as the hydrophobic componen~.
SUM~RY OF THE I~VENTION
The present invention includes a polymer network comprising a vinyl polymer and an uncrosslinked hydrogel retained within the vinyl polymer. The present invention also includes a method of makins such a pol~mer network including a vinyl polymer and an uncrosslinked hydrogel.
The present invention also includes a lubricous coating secured to the surface of a device insertable within the living body and to a process of ,applying such ~ coating to ~the device. ~ The process includ~s applying to~the sur~ace a solution contalning ~an uncrosslinked hydrogel and a vinyl prepolymer along with~a polymerization initiator, and then polymerizing the vinyl prepolymer such~that the hydrogel ls retained 'within the polymerized,vinyl~polymer. ~Preferabl~, the -~surface o~ the devlce is treated prior to application of the coatlng solution. ~
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: ' WV93/11751 PCT/US92~11021 The present invention also includes a drug delivery system comprising a coating secured to a device insertable into a living body wherein the coating comprises an uncrosslinked hydrogel secured to the device by a vinyl polymer. The drug can be permanently entrapped in the coating or can be leachable from the coating into the living body upon hydration.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention includes a polymer network comprising a vinyl polymer and an uncrosslinked hydrogel retained within the vinyl polymer that is useful as a high lubricity coating. The high lubricity coating is useful on devices that are insertable within living tissue.
By "polymer ne~work" is meant two polymers synthesized such that the polymer chains are intertwined within each other. There is no apparent carbon-to-carbon chemical bonding between the polymers except perhaps only accidental covalent bonding. The -`in~ertwining of the polymers,is of a permanent nature rendering the polymers physically inseparable from each - other. - ~
The polymer network of the present invention - is formed as a lubricous coating by application of a mixture of -an; uncrosslinked ,hydrogel,~ a vinyl;
prepolymer, and a free radical initia~or in à solven~ to an active, activated or, "primed" substrate. By prepolymer i~'meant monomers-or oligomers or both used , ;~as 'reactants to form a polymer.,, The three primary 30 constituents~of the-mixture are applled pr2ferablyjas a '~
'~' single component~and cure*;as a single system to,form -i thé''~polymer 'netwo'rk of~ the present,invention. , The ~ reaction o~ the vinyl prepolymer on the substratc in the . ~ ~
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WO93/1~7~1 2 1 O '~ O P~cr/usg2/l 1~21 presence of the uncrosslinked hydrogel produces the polymer network.
By hydrogel is meant a substance that when exposed to water and used as a coating is c~aracterized by a decrease in its coefficient o~ friction or an increase in its lubricity. Suitable hydrogels for use in the present invention are uncrosslinked hydrogels, and include polyethylene oxide, polyacrylic acid, polyacrylamide',polyf~sodium4-styrenesulfonate~,poly(3-hydroxybutyric acid), pcflyvinylpyrrolidone, and 2~hydroxyethyl methacrylate.
The hydrogel of the present invention is preferably a high-molecular weight hydrogel. The high molecular weight of the hydrogel provides at least two advantages. First, it ensures sufficient entanglement with the vinyl polymer such that the hydrogel does not leach out of the polymer network. Second, although the high molecular weight ensures that the hydrogel is unextractable ~rom the polymer network, smaller molecules'such as drugs ensnared within the hydrogel can leach out. A crosslinked hydrogel would, on the other hand, entrap a drug and prevent leaching. The extent of : ~ entrapment of the hydrogel is also dependent on the crosslink density of the vinyl polymer. In the case o~
25 ~~polyethylene::~fcffxide, molecular weights in the range of ~! ;;50,000 to'lO0,000 and abova are most.suitable.
' By vinyl::polymer .is.meant those polymers produced by ''chain reaction :polymerization~ Suitable vinyl polymers for use ~n the present invention.include methyl methacrylate and other:mono-functional acrylates, diàcrylates, glyceryl propoxy triacrylate and other~tri~
~ unctional;àcrylàte~, styrene and other vinyl.monomers, including divinyl benzenel~and other divinyl polymers.

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WO93/11751 ~ 210 0 9 7 0 PCT/US92/11021 One example of a diacrylate suitable i~ the present invention is neopentyl glycol diacrylate (NPG). Other diacrylates suitable in the present in~ention include ethylene glycol di(meth)-acrylate, 1,3-propylene glycol di(meth)acrylate, l,4-butanediol di(meth~acrylate, l,6-hexanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and neopentyl glycol di(meth)-acrylate.
In preparing the solution mixture of thepresent invention, the hydrogel is mixed with the vinyl prepolymer in a sol~ent. In one preferred embodiment, isopropyl alcohol in combination with water acts as a suita~le solvent. Water alone has also been used as a solvent. In one embodiment, a preferred ratio of polyethylena oxide to a diacrylatë prepolymer is l0:l.
Lower ratios ~f polyethylene oxide to prepolymer have also been found to be suitable.
~ Crosslin]cing is~Cacilitated by a small amount of a free radical initiator added to the mixture. When a diacrylate is used, the mole ratio of free radical initiator to the diacryIate is 10-4:l. To further `
faciIitate crosslinking,~oxygen should be degassed from thei solution since~ oxygen -inhibits frëe - radical ~reactions. One preferred method of degassing the oxygen :~is through the use of nitrogen bubbling. :---~ Next, the mixture is applied to a substrate.
A variety o~ sur~aces act as suitable substrates. For example/ the mixture may be ~applied to wood, metal, polymers or~theilike.~ If thè mixture is applied to a t~polyethylene sur~ace such~las^on a cath`eter/- preferably th- surface` o~ the^~catheter `is glow discharge~plasma :
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W093/~17~1 2 ~ ~ 0 9 7 ~ PCT/US92/11021 ., , `

treated. Other polymeric substrates, such a~ polyimides containing diaromatic ketones and polyethylene terephthalate, have also been found to be suitable .-substrates even when not plasma treated. Polyurethanes and nylons are primed with a vinyl functional isocyanate. Metals~ such as stainless steel and gold, require a primer such as a vinyl or acrylate functional silane for best adhesion.
The coated surface is then cured. The coated film is exposed to heat or W light for a short period of time. The heat or W light triggers the polymerization and crosslinking of the prepolymer.
Pre~erably, the mixture is cured using a high intensity ~;.
ultraviolet lamp. The precise amount of time needed to cure the sur~ace is dependent on the source of energy, the relative amounts of constituents in the composition, the thickness of the coating desired, and other ~actors.
Generally, the amount of time required ~or t~ermal cure is from about l to 30 minutes. W curing requires less :time and is generally in .the range of less than one minute. . .
. A great advantaga of the vinyl polymer of the present .invention is its. ability to adhere to a substrate ~hat can sup~ort free radicais, or can support 25~.~. other species--which can form free radicals, ~uch as . peroxides. The strong adherence of the vinyl polymer to the substrate!-aids .~in .the prevention of unwanted material breaking.off fro~ the coating and being left in .. ~he body. . . : .............................. . .
30 j.. ....., ~.:.:The present coating.has a:variety of uses in ithe .~edical~aevice market.-~. Onej.apparent use is the -~applicati~n !``, of :~the.,coating~. on ;various devices.used .within the human body.:~In the.preferred embodiment, the .

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coating is applied to catheters such as angioplasty catheters.
Applying the polymer network of the present invention as a coating to catheters or other medical devices has a number of advantages. First, the coating of the present invention is highly lubricous when wet.
In the dry state t the coating is virtually indistinguishable from the substrate. In contrast, silicone, which is widely used in devices such as catheters, and when acting as the coating is very noticeable in the dry state and often is more tacky when wet Second, as mentioned previously, the coating of the present invention can be applied to a variety of different substrates with strong adherence. Thust the polymer network of the present invention provides a lubricous, as well as an adherent and durable coating.
Visorcus rubbing and long-term hydration do not reduce the coating's lubricity, demonstrating the strong adhesion of the coating..
Third, the polymer network of the present invention is useful:as a drug.delivery system. By varying such parameters as hydrogel molecular weight and crosslink density of the.vinyl polymer,.an additional 25 .constituent,..such as:.a drug,l.can be incorporated into :.the present polymer: networ~. . In one;:;.preferred .embodiment, heparin :is~usedias;tha drug. .:The drug is entrapped...in the:polymer.network:and leaches out o~ the .coating.when ~he coating.is..~et delivering the heparin .to ..immediately.~adjacent:.areas:-of..:.the body. The ;advantages.of:::incorporating~a..drug~which is~released from....the ::coating~ on:~medical.~ devices..is apparent.
Effects of thro~bu~ formation, restenosis, infections, : ':

W093/~1751 - PCT/US92/~1021 ~lOOg70 ~

and even disease transmission could be minimized or eliminated through the use of the coating of this invention.
Fourth, as mentioned previously, the present invention includes a polymer network in which the hydrogel, such as potyethylene oxide, is virtually entrapped within the system. Entrapment prevents unwanted material from leaving the coating and entering the body. Coatings which remain intact and do not deposit undesired materials are generally preferred to erodible coatings, such as silicone coetings, which might have the capacity to induce a response from the body. The coating of the present invention leaves no unwanted foreign material within the body.
The following examples are illustrative only and are not intended to limit the present invention.
The examples are submitted in order to demonskrate more explicitly the process and composition of the present invention.
EXAMPLE 1 i 20 grams of a 5 percent solution of an uncrosslinked polyethylene oxide (PE0) from Aldrich ~hemical Co., (a hydrogel) having an average molecular weight of 900,000 was mixed with 0.10 grams of neopentyl glycol _ diacrylate i!;(NPG) from ;i Sartomer Co., of : Pennsylvania in a solution containing l8 grams of water and;71 grams of isopropyl alcohol (IPA). 0.8~grams o~
,a 0.001 percent solution o~ azobisisobutronitrile (AI~) in~lisopropyl alcohol Was added ~to the solu~ion. The 3 mole ratio o~ azobisisobutronitrile to neopentyl glycol diacrylate was-10~4~ Oxygen was-~removed-from~the ~solution by bubbling nitrogen through the~solution. l .. , ... .~ ~

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WO93/11751 2 ~ ~ ~0 9 7 0 PCT/US92/11021 _g_ The solution was then applied to a film of plasma treated polyethylene (PE). The wet coated film was then exposed to a 1500 watt ultraviolet tw) source for ~0 seconds at a distance of nine inches. The film was rinsed with running water. The resulting surface was highly lubricous and dramatically different than'the uncoated surface. Vigorous rubbing did not reduce the coating lubricity, nor was th~re noticeable residue on the fingers. Dry or wet storage did not reduce the wet lubricity.
EX~MPLE 2 A solution of PEO and NPG in IPA and water was prepared as in Example l. l0 mg of heparin was dissolved in l ml water~ 1 ml of IPA was added to the heparin water mixture, with resulting cloudiness indicating heparin precipitation. Severa71 drops of l~
lecithin (a surfactant) in chlorofo~-m was added to the heparin mixture until the solution was clarified. The heparin mixture was then added to the PEO/NPGIAIBN
solution such that the ratio of PE0:NPG:heparin was 50:5:1. The solution was applied to a plasma treated PE
film and cured as in Example l. The result ng coating had similar characteristics as the coating,o~ Example l.
-A detectable ~quantity of heparin was found using :infrared spectroscopy, in residue resulting after 15 ''~' minutes of;rinsing of,the coating.
EXAMP~E_3 , , A solution of PEO;and NPG in IPA and water containing heparin was ! prepared as in Example 2. The solution was applied,to approxi~ately,nine inches of the ~;distal end o~ a SKI~NY angioplasty catheter manu~ac~ured ; by.SciMed ~ife Sys~ems,~3nc.^,~or~,Maple,-Grove,~MinnesOta.
The catheter, prior to application of,the solution,~was .' ' , ~: .

: ~' W093/11751 ~ l ~ 0 ~ 7 0 PCT/U~92/11021 ~:
plasma treated. The catheiter was cured using the procedure of Example 2 but with manual turning of the catheter to insure direct exposure to W of all sur~aces of the coating. The resulting catheter had similar characteristics as the coating of Example 2.

The procedure of Example 1 was followed except that tripropylene glycol diacrylate from Sartomer Co., of Pennsylvania was used in place of the neopentyl glycol diacrylate. A similar highly lubricous coatir.g was produced. The coating withstood vigorous rubbing and did not leave a noticeable residue on the fingers.
EXA~PLE 5 The procedure of Example 1 was followed except that tri~unctional triacrylate ester from Sartomer Co., of Pennsylvania was substituted for neopentyl glycol diacrylate. A similar highly lubricous coating was produced. T~le coating withstood vigorous rubbing ~nd did not leave a noticeable residue on the fingers.
~ EXAMPLE 5 The procedure of Example 1 was followed except that-polyethylene~lycol 200 ~iacrylate *rom Sartomer Co., of Pennsylvania was substituted for neopentyl glycol diacrylate. -~ similar highly lubricous coating ~was produced.- The coating withs~ood vigorousirubbing and did not leave any noticeable residue on the ~.ingers.
~EX~MPLE'7 ;i: , The procedure o~ Example 1 was ~ollowed except that divinyl:benzene from Dow Chemical o~ Michigan was :substituted for neopentyl-glycol diacrylate.iA similar highly lubricous~coating~was produced;; Vigorous rubbing did ~ not reduce thé:~coating~lubricity, a~d~the coating did not leave;any noticeable residue'on the ~ingers.~
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~` 2laos70 The procedure of Example 1 was followed except that methylmethacrylate from Fisher Scientific of New Jersey was substituted for neopentyl glycol diacrylate.
A similar highly lubricous coating was produced.
Vigorous rubbing did not reduce the coating lubricity, and no noticeable residue was left on the fingers.

The procedure of Example 1 was followed except that 8 parts of methylmethacrylate (Fisher Scientific of New Jersey) to 2 parts of divinyl benzine (Dow Che~ical of Michigan) was substituted for the neopentyl glycol diacrylate. A similarly highly lubricous coating was produced. Vigorous ru~bing did not reduce the coating lubricity and no noticeable residue was left on the fingers.

The procedure of Example 1 was followed except that 1 gram of polyacrylic acid from Aldrich Chemical Co., of Wiscons.in having an average molecular weight of 250,000 was used as the hydrogel. 2 grAms of 5 percent neopentyl glycol ~iacrylate solution in isopropyl alcohol was mixed with`the hydrogel. 0.80 grams of 0.001 percent azobisisobutronitrile solution was then added.
~ he resulting!surface was highly lubricous.
Vigorous rubbing did not reduce the coating lubricity ~`
nor was there noticeable residue on the fingers.

~ The procedure of Example l was ~ollowed except that 56.0 grams of ~po}y(sodium 4-styrene~;sulfonate) haYing -~an--avera~e mole`cular :weight~of 70jO00 ~rom Aldrich Chemical Co.,! of Wisconsin~was-mixed with 2 ` . ':

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W0931~1751 2 l 0 a9 7~ PCT/US92/1102 grams of a 5 percent solution of neopentyl glycol diacrylate made by sartomer Co., of Pennsylvania in isopropyl alcohol. 0.8 grams of a o.oo1 percent solution of azobisisobutronitrile was added to the solutio~.
The resulting coating was highly lubricous and dramatically different than the uncoated surface.
Vigorous rubbing did not reduce the coating lubricity nor was there noticeable residue on the fingers.
EXAMP_,~E 12 The procedure of Example 1 was followed except that 1 gram of polyvinyl pyrolidone made by BASF Corp., of New Jersey was used as the hydrogel. The polyvinyi pyrolidone was mixed with 2 grams of 5 percent neopentyl 15 glyool diacrylate from Sartomer Co., of Pennsylvania in an isopropyi alcohol solution. 0.~0 grams of a 0.001 percent axobisisobutronitrile initiator was added to the solution.
The resulting coating was highly lubricous and drama~ically~;.differ2nt.~.than the uncoated s~r~race.
Vigorous rubbing did not reduce the coating lubricity nor was there noticeable-residue on the fingers.
EXAMP~E_~3 ~
;. . The procedure of Example 12 was followed except that the solvent used was an isopropyl alcohol/
,toluene solvent inskead:,o~ ~he water/i~opropyl alcohol solvent. .~,A .similarly..~ihighly,lubricous,coating,was produced. ,. , , ,, .."r ,~
Although the, present invention has been-described....with.~.referenoe .to preferred~embodiments,~workers~skilled in-the art will recognize,.~hat.ch~nges may,be,.made in form':and detail~;without departing from the ~pirit~and scope~of~,the invention.'i.~

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Claims

WHAT IS CLAIMED IS:

1. A polymer network comprising:
a polymer formed by the reaction product of a vinyl prepolymer; and an uncrosslinked hydrogel retained within the reaction product.

2. The polymer network of claim 1 wherein the vinyl prepolymer is an acrylate.

3. The polymer network of claim 1 wherein the hydrogel is an uncrosslinked polyethylene oxide.

4. The polymer network of claim 2 wherein the diacrylate is neopentyl glycol diacrylate.

5. The polymer network of claim 1 and further including:
a drug retained within the reaction product.

6. The polymer network of claim 5 wherein the drug is heparin.

7. The network of claim 6 wherein a surfactant is added to the heparin.

8. A method of making a polymer network comprising:
combining a vinyl prepolymer with an uncrosslinked hydrogel in a solvent solution containing a polymerization initiator;
applying the solution to a substrate; and permitting the solution to cure on the substrate.

9. The method of claim 8 and further including adding a drug to the solution.

10. The method of claim 8 wherein the prepolymer includes diacrylate monomers and oligomers.

11. The method of claim 10 wherein the diacrylate prepolymer is neopentyl glycol diacrylate.

12. The method of claim 8 wherein the vinyl prepolymer is an acrylate.

13. The method of claim 8 wherein the vinyl prepolymer is divinyl benzene.

14. The method of claim 8 wherein the hydrogel is an uncrosslinked polyethylene oxide.

15. A coating composition for securing to a substrate, the composition comprising:
a vinyl prepolymer;
a polymerization initiator;
an uncrosslinked hydrogel; and a solvent.

16. The coating composition of claim 15 wherein the prepolymer includes diacrylate monomers and oligomers.

17. The coating composition of claim 15 wherein the hydrogel is polyethylene oxide.

18. The coating composition of claim 16 wherein the diacrylate prepolymer is neopentyl glycol diacrylate.

19. The coating composition of claim 15 and further including a drug.

20. The coating composition of claim 19 wherein the drug is heparin.

21. The coating of claim 15 wherein the vinyl prepolymer is an acrylate.

22. The method of making a surface of a device insertable in living tissue more lubricous when wet, the method comprising:
applying a composition to the surface, the composition including a vinyl prepolymer, a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to cure sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.

23. The method of claim 22 wherein the prepolymer includes diacrylate monomers and oligomers.

24. The method of claim 22 wherein the hydrogel is polyethylene oxide.

25. The method of claim 23 wherein the diacrylate is neopentyl glycol diacrylate.

26. The method of claim 22 wherein the composition further includes a drug.

27. The method of claim 26 wherein the drug is heparin.

28. The method of claim 22 wherein the composition is cured using a UV energy source.

29. The method of claim 22 wherein the vinyl prepolymer is an acrylate.

30. The method of claim 22 wherein the surface of the device can support free radicals.

31. The method of claim 22 wherein the surface of the device can support species that form free radicals.

32. The method of claim 22 wherein the surface of the device includes polyethylene.

33. The method of claim 22 wherein the device is an intravascular device.

34. The method of claim 22 wherein the device is an intravascular catheter.

35. A method of making a surface of a device insertable in living tissue more lubricous when wet, the method comprising:
applying a composition to the surface, the composition including a vinyl prepolymer a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to by exposure to ultraviolet radiation sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.

36. A method of making a surface supporting free radicals of a device insertable in living tissue more lubricous when wet, the method comprising:
applying a composition to the surface, the composition including a vinyl prepolymer, a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to cure sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.

37. A method of making a surface that can support species that form free radicals of a device insertable in living tissue more lubricous when wet, the method comprising:
applying a composition to the surface, the composition including a vinyl prepolymer, a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to cure sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.

38. A method of making a surface that includes polyethylene of a device insertable in living tissue more lubricous when wet, the method comprising:

applying a composition to the surface, the composition including a vinyl prepolymer, a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to cure sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.

39. A method of making a surface of an intravascular device more lubricous when wet, the method comprising:
applying a composition to the surface, the composition including a vinyl prepolymer, a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to cure sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.

40. A method of making a surface of an intravascular catheter more lubricous when wet, the method comprising:
applying a composition to the surface, the composition including a vinyl prepolymer, a polymerization initiator, and uncrosslinked hydrogel; and permitting the composition to cure sufficiently such that the vinyl prepolymer adheres to the surface and retains the uncrosslinked hydrogel.