Coating processes – Medical or dental purpose product; parts; subcombinations;... – Analysis – diagnosis – measuring – or testing product
Reexamination Certificate
2000-05-16
2002-05-28
Parker, Fred J. (Department: 1762)
Coating processes
Medical or dental purpose product; parts; subcombinations;...
Analysis, diagnosis, measuring, or testing product
C427S407100, C427S002240, C427S002300, C427S002310, C427S243000, C427S244000, C427S245000, C427S282000
Reexamination Certificate
active
06395325
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the fields of organic chemistry, polymer chemistry, materials science, medicine and medical device technology. More specifically, it relates to a method for producing porous membranes, in particular hydrophilic porous membranes, to the membranes per se and to uses for them.
BACKGROUND OF THE INVENTION
Porous membranes are well-known and have considerable utility. They find use, for instance, in osmosis, reverse osmosis, chromatography, filtration, ion exchange and dialysis. They also are useful as components of medical devices such as stents, catheters and timed-release drug delivery systems.
Presently, porous membranes are made by taking a solution of a polymer or polymers, casting a thin film of the solution on a support base such as a glass plate, allowing partial evaporation of the solvent from the cast film for a pre-determined period of time and then contacting the partially evaporated cast film with a leaching liquid to remove remaining solvent and render the film porous.
When the milieu in which a porous membrane to be used is aqueous, such as in water filtration devices and dialysis, it is desirable that the porous membrane itself be hydrophilic, that is, that it have an affinity for water and be freely wettable. Furthermore, a hydrophilic membrane is preferred when the intended use is in vivo because ion transport occurs much more readily through such a membrane than through a hydrophobic membrane.
It is possible to prepare a hydrophilic membrane by the above-described procedure using one or more polymers which are inherently hydrophilic. However, there are relatively few such polymers known in the art and membranes made from them have limited utility because they lack mechanical strength, chemical resistance and aging and thermal stability. Thus, while polymers such as regenerated cellulose, some polyamides (nylons), polyvinyl alcohols, polyacrylic acid, polyethylene glycol, polyvinylpyrrolidone, polyvinylamine and the like can be formed into hydrophilic membranes, they all lack one or more of these highly desirable characteristics. Heat stability is particularly important in relation to ablation catheters which cause lesions in tissue by heating the tissue where it comes in contact with the catheter tip.
One approach presently employed to overcome the shortcomings of hydrophilic membranes made from hydrophilic polymers is to prepare a membrane using a hydrophobic polymer which results in a membrane having generally superior mechanical strength, heat stability and chemical resistance, and then physically or chemically modifying the membrane surface to render it hydrophilic. Means for accomplishing this include, among others, corona discharge treatment of the membrane surface and the grafting of hydrophilic polymers onto to the surface to render the membrane hydrophilic.
An alternate approach to the above problem is to use a combination of polymers, one or more which is hydrophobic, to give mechanical strength, heat stability and chemical resistance to the resulting membrane and one or more of which is hydrophilic, a sufficient amount of which is included to render the membrane hydrophilic while retaining the beneficial properties conferred by the hydrophobic polymer.
Each of the above techniques suffers from drawbacks which limit their utility. For example, some are expensive, either due to equipment requirements or raw material costs, some require the use of hazardous materials, some are difficult to control with regard to such parameters as degree of cross-linking, leaching of hydrophilic polymer from the membrane, homogeneity of hydrophilicity, control of pore size and the nature of the surfaces to which the membrane can be applied. What is needed is an inexpensive, safe and controllable method for forming porous membranes, in particular, hydrophilic porous membranes that have desirable physical and chemical characteristics and that can be formed on virtually any surface.
SUMMARY
The present invention provides the needed inexpensive, controllable method for forming a porous membrane, in particular a hydrophilic porous membrane which is mechanically strong, chemical resistant, heat stable and which can be formed on virtually any surface.
Thus, one aspect of this invention is a method for forming a porous membrane, comprising: dissolving one or more polymers in one or more solvents at a temperature at which a true solution of the polymer or polymers forms; applying the solution to a surface; and, drying the applied solution at a temperature that is above the flash point of each of the solvents and below the softening point of each of the polymers used until the solvent or solvents have been essentially completely evaporated.
A second aspect of this invention is a method of forming a porous membrane as set forth above using one polymer.
It is an aspect of this invention that the one polymer used is a hydrophobic polymer.
It is an aspect of this invention that the one polymer used is a hydrophilic polymer.
A further aspect of this invention is use of the method set forth above using a plurality of polymers.
A still further aspect of this invention is use of the method set forth above using a plurality of polymers where one or more of the plurality of polymers used is a hydrophobic polymer and one or more of the plurality of polymers used is a hydrophilic polymer.
A presently preferred aspect of this invention is use of the method set forth above in which the porous membrane formed is hydrophilic.
Another aspect of this invention is use of the method set forth above where the hydrophobic polymer or polymers are dissolved in the solvent or solvents before the hydrophilic polymer or polymers are added.
It is a presently preferred aspect of this invention that the hydrophobic polymer or polymers used in the method set forth above each has a softening point above 100° C.
It is likewise a presently preferred embodiment of this invention that the hydrophilic polymer or polymers used in the method set forth above each has a softening point above 100° C.
It is an aspect of this invention that two polymers, one hydrophobic and the other hydrophilic, are used in the method set forth above.
A further presently preferred embodiment if this invention is the use of two polymers in the method set forth above where one of the polymers is poly(vinylidene fluoride), which is hydrophobic, and the other polymer is poly(N-vinylpyrrolidone), which is hydrophilic.
It is an aspect of this invention that, when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention, the weight/weight ratio of poly(vinylidene fluoride) to poly(N-vinylpyrrolidone) is from about 1:0.5 to about 1:2.
A still further aspect of this invention is that when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention, the weight/weight ratio of poly(vinylidene fluoride) to poly(N-vinylpyrrolidone) used is about 1:1.
Another aspect of this invention is that, when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention, the solvent used comprises N, N-dimethylacetamide.
In a presently preferred aspect of this invention that, when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention, the solvent also contains an acid.
It is an aspect of this invention that, when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention, the acid used is an organic acid.
It is a presently preferred aspect of this invention that, when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention, the acid used is glacial acetic acid.
A further aspect of this invention is that, when poly(vinylidene fluoride) and poly(N-vinylpyrrolidone) are used in the method of this invention and the solvent is N,N-dimethylacetamide, the temperature at which the poly(vinylidene fluoride) is dissolved in N,N-dimethylacetamide is from about 30° C. to about 50° C.
In
Hedge Anant V.
Swanson David K.
Kolb Michener Jennifer
Lyon & Lyon LLP
Parker Fred J.
Sci-Med Life Systems, Inc.
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