JP2003504190A - Polyvinylidene difluoride films and methods for making such films - Google Patents

Abstract

SUMMARY A porous membrane (10) comprising polyvinylidene difluoride and a method for making such a membrane are disclosed. The membrane (10) is not cross-linked and is not contacted with a strong alkaline solution during manufacture and is wettable by aqueous solutions even after repeated wetting and drying. This membrane has a support (18) and a surface (11) treated with a hydrophilic coating. Separately, such membranes are also useful in an apparatus (60) for separating a biological fluid into two or more components.

Description

Detailed Description of the Invention

The present invention relates generally to porous polyvinylidene difluoride (PVDF) membranes and methods for making the membranes. More specifically, the present invention is a PVDF that is hydrophilic and retains its hydrophilic properties even after repeated wetting and drying.
It concerns membranes (and methods for making them).

BACKGROUND The use of PVDF membranes for liquid filtration is well known. However, PVDF is hydrophobic in nature (lacks an affinity for water). Therefore, in order to use a PVDF membrane for filtering aqueous fluids, first of all, the membrane must be rendered hydrophilic (ie easily wetted by water). PVDF is typically treated with wetting agents or surfactants to impart hydrophilicity to the membrane.

One drawback of treated PVDF membranes is the potential for extraction or leaching of reagents or surfactants from the membranes into the filtrate during membrane use. This is particularly, but not exclusively, possible when PVDF membranes are used for the treatment of biological fluids (eg blood) intended for human administration. Extraction of surfactants or wetting agents also results in a lack of hydrophilicity and reduces the effectiveness or utility of the membrane as a filter for filtering aqueous solutions. Lack of hydrophilicity can also prevent reuse of the membrane. Reuse is less of a consideration when the membrane is part of a disposable fluid treatment device (and which is only intended for single use) (eg, in a filter used in a medical or medical procedure). Not, and reusable membranes may be desired in other settings.

Methods for treating PVDF membranes have been developed to ensure that surfactants or wetting agents remain on the surface of the membrane and / or are substantially not extracted during use. For example, US Pat. No. 4,618,533 discloses composite PVDF membranes. The PVDF membrane can be made in part by placing the membrane in a reagent bath containing free radically polymerizable monomers, a polymerization initiator and a crosslinker. A polymerization reaction is carried out, which results in a porous membrane onto which a permanent hydrophilic coating has been grafted and / or deposited.

US Pat. No. 5,032,331 discloses a process of impregnating the pores of a hydrophobic PVDF membrane with a solution that renders the pores of the PVDF membrane hydrophilic. After this impregnation process,
This PVDF membrane is subjected to chemical treatment in a strong base solution containing an oxidizing agent. According to this patent, the membrane so treated "shows semi-permanent hydrophilicity and can be repeatedly dried and used." As can be seen from the above, hydrophilic, rewettable PVDF Existing methods of making membranes typically include additional chemical and / or crosslinking steps. Therefore, a porous hydrophilic PVDF membrane (and method for making this membrane) that does not require such a step, can be wetted multiple times, and does not produce high levels of extractables. It is desirable to provide

SUMMARY The present invention generally embodies rewettable, porous, hydrophilic PVDF membranes and methods for making such membranes. According to a first described aspect of the present invention there is provided a porous membrane comprising: (1) polyvinylidene difluoride (P
VDF) porous layer and (2) polyvinyl alcohol coating on the PVDF layer, which is sufficient to render the porous membrane hydrophilic. The membrane, after drying, is rewettable by the aqueous liquid and is not cross-linked or contacted with a strong base solution. The membrane remains substantially rewettable, even after repeated wetting and drying.

In one aspect of the invention, such a membrane is exposed even after being exposed to or contacted with an aqueous liquid as hot as 40 ° C., and possibly as hot as about 100 ° C. Maintains hydrophilicity.

In one embodiment, the polyvinyl alcohol coating solution is about 1-2 wt% dissolved in a solution of water and alcohol (eg, isopropyl alcohol).
Including polyvinyl alcohol. This solution contains 20-80% by weight of water and 20
-80% by weight of isopropyl alcohol may be included. Further, the polyvinyl alcohol is derived from polyvinyl acetate having a degree of saponification of between about 80-100%.

In another aspect, the invention features a porous, non-crosslinked, rewettable, hydrophilic PV.
A method for making a DF membrane. The method involves providing a porous, dry, hydrophobic PVDF membrane and contacting the membrane with a coating solution. This coating solution is made from polyvinyl alcohol dissolved in a solution of water and alcohol.

In one embodiment, the coating solution is provided by dissolving polyvinyl alcohol in a solution of water and alcohol (eg, isopropyl alcohol). This solution may contain 20-80 wt% water and 20-80 wt% isopropyl alcohol. The membrane can be coated by dipping the PVDF membrane in a bath of polyvinyl alcohol coating solution.

[0011] In another aspect, the invention features an apparatus for separating a biological fluid into two or more components. The device comprises a housing including an interior wall defining a fluid chamber, a fluid inlet in communication with the chamber and at least one fluid outlet. The device also comprises a rotor within the chamber and a porous membrane disposed on either one of the inner walls or the rotor. The membrane is a porous layer of polyvinylidene difluoride and sufficient to render the polyvinylidene difluoride layer hydrophilic.
Made from a vinyl alcohol coating on it.

DETAILED DESCRIPTION The membrane of the present invention may be a flat sheet-like, porous membrane. One embodiment of a membrane 10 embodying the invention is generally shown in FIG. Membrane 10 generally comprises one or more layers of polymeric material including polyvinylidene difluoride (PVDF). This layer can be made entirely of PVDF, or it can be made of a blend of PVDF and other suitable polymers and copolymers. Accordingly, as used herein, the terms "PVDF membrane" or "PVDF layer" are all PVD.
Includes membranes or layers made from F and / or blends of PVDF with other polymers and / or copolymers. The surface 11 of the PVDF membrane is typically treated with a hydrophilic coating. In one particular and preferred embodiment, the PVDF membrane is coated with a wetting agent such as polyvinyl alcohol.

Alternatively, and more preferably, as shown in FIG. 1A, the membrane 10 can be used with an inner support 18, which is made of a fibrous, porous polymeric material such as polyester mesh. Typically made and on top of it one of PVDF
One or more layers are applied to form a PVDF film.

In one example, the membrane (with internal support) is about 4-7 mils, or about 0.
． It may have a thickness of 004 to 0.007 inches. Typically, the membrane is substantially isotropic, but can also be anisotropic (ie, where the pore size is different on one surface of the membrane and the other). Membranes made in accordance with the present invention have
Can be a "porous" membrane, which is less than about 100 microns, typically about 0.01
It has an apparent pore size between -10 microns. Further, the membranes made in accordance with the present invention may be "ultrafiltration" membranes, which have an apparent pore size of less than about 0.01 microns, and / or for filtration of dissolved proteins, for example. Is appropriate for In one example, the porous membrane can have an apparent pore size of about 0.2-1.2 microns. More typically, the apparent pore size of the membrane can be about 0.8 microns or greater.

PVDF is based on Elf Atochem (Philadelphia, Penn
available from many different sources such as Sylvania). According to a preferred embodiment, PVDF is typically dissolved in a suitable solvent prior to forming the membrane. A suitable solvent for PVDF is dimethylacetamide (DMAC). In one embodiment, about 18-22% by weight PVDF is dissolved in DMAC to provide a PVDF solution. Preferably, the PVDF solution is maintained ("cured") between 28-35 [deg.] C, more preferably about 31 [deg.] C for about 18 hours before forming the membrane. Of course, other polymeric materials may be used here, and different solvents, different cure times and temperatures may be used.

Membranes of the type described above can be made by solution casting or extrusion forming. Figure 2
Illustrates one method and associated apparatus for making the membranes of the present invention. The method shown in FIG. 2 is known as the solution casting method, where the membrane is continuously formed on a moving support surface such as a web or belt made of a suitable material. However, in its broader aspect, it is understood that the invention is not limited to the particular method used in making the membrane, or the presence or absence of a supporting surface. For example, a PVDF membrane without a support as shown in FIG.
It can be made by applying VDF to a drum and then peeling the film from the surface of the drum. Alternatively, PVDF membranes are prepared by casting a PVDF solution onto a support of the type described above, as described in US Pat. No. 4,203,848, which is incorporated herein by reference. Can be made by forming a membrane and then separating the membrane from the support.

As shown in FIG. 2, the web or support 18 is fed from a feed roll 22 into a V-shaped groove or chamber 24 filled with a cured PVDF solution 25 (described above). As the support 18 passes through the chamber 24, the PVDF solution becomes
It is applied to the outer surface of the carrier web 18. (It is understood that, if desired, only one side of the support can be coated with the PVDF solution.) The support has PV
The DF solution has been applied and exits the chamber through an opening in the bottom of chamber 24. As shown in FIG. 2, the device includes a series of rollers 36 over which the support weaves, as generally shown. Rotation of roller 36 moves support 18 from dispenser 22 through a series of bath and drying devices. This is described in detail below. The rate of movement of the support 18 may depend on the required or desired residence time of the support (with the membrane coated thereon) in the bath and drying device. In one embodiment, the speed of movement may be between about 1-5 feet / minute, more typically about 3 feet / minute.

The coated support 18 then passes through a first flocculation bath 26. Typically,
This first flocculation bath 26 is a non-solvent for the polymer (eg, PVDF) portion of the PVDF solution, but is not a solvent portion of the PVDF solution (eg, DMAC).
Retain a liquid or solution that is freely miscible with Contact with the liquid in the flocculation bath 26 causes the polymer solids (PVDF) to flocculate and the solvent portion (ie DMA
C) is extracted from the applied layer of PVDF, thus forming a porous PVDF membrane on the support. This “solvent / non-solvent” method of film formation is well known to those of skill in the art and is described, for example, in US Pat. No. 3,642,668, which is incorporated herein by reference.

The support 18 having a PVDF membrane on its surface is then sent from the coagulation bath 26 to one or more extraction baths 28. Typically, the extraction bath (s) will contain any residual solvent (eg, DMAC) used to dissolve the polymer from the membrane.
It contains a liquid to extract. In a preferred embodiment, this liquid can be water. Depending on the type and strength of solvent used, the membrane may undergo a series of washing steps in one or more extraction baths 28, each bath further washing the solvent from the membrane, Then remove it. For purposes of illustration only, three extraction baths 28 are shown in FIG.

Once the solvent is substantially extracted from the membrane, the membrane is dried. Various drying techniques can be used. For example, after the final extraction bath, the membrane can be introduced into the drying oven 40 shown in FIG. Alternatively, and perhaps more preferably, the membrane can be dried by contacting the membrane sheet with one or more heated drums, which drums dry the membrane in a manner well understood by those skilled in the art.

In one embodiment, if a drying oven 40 is used, a drying temperature of about 65 ° C. or less may be sufficient to completely dry the membrane. Alternatively, if a series of heated drums is used, the temperature of the first drum may be higher than the temperature of the last drum in the series and substantially all of the water may be evaporated from the wet film. The final drum set at a lower temperature (eg, but not limited to 50-60 ° C) ensures that the membrane is completely dry. Regardless of the drying device used, excess water can also be removed from the membrane by passing the membrane between wipers 41 prior to drying, as generally shown in FIG.

After the membrane has dried, it should be further treated by coating the surface of the membrane with a wetting agent to render it hydrophilic. After drying, as further shown in FIG. 2, the membrane can be introduced into another bath 44 containing the hydrophilic coating solution. The method of coating is not limited to dipping the film in the coating solution, but also includes spraying or otherwise applying a wetting agent to the film.

In a preferred embodiment, the wetting agent is polyvinyl alcohol dissolved in a solution of water and isopropyl alcohol. Polyvinyl alcohol is derived from saponified polyvinyl acetate. (Saponification refers to the process of hydrolyzing an ester). Polyvinyl alcohol is available in various grades based on the degree of saponification. Polyvinyl alcohol having a degree of saponification of at least about 40% may be used in the PVDF membrane of the present invention. However, the high degree of saponification,
For example, polyvinyl alcohol having a saponification degree higher than 80% is preferable.

In dip coating, a polyvinyl alcohol coating solution can be prepared as follows. Highly saponified polyvinyl alcohol is converted into hot water (for example, about 9
Water) having a temperature of 0 ° C.). The dissolved polyvinyl alcohol is then further mixed with another alcohol to provide the coating solution. In one embodiment, the coating solution comprises about 20-80% water and 20-80% other alcohol, more preferably 40-60% water and 40-60% other alcohol. One particularly useful alcohol for the coating solution is isopropyl alcohol. Other simple alcohols (eg methanol) are also
May be appropriate. In a more preferred embodiment, the coating solution may include about 50% water and 50% isopropyl alcohol. Polyvinyl alcohol
A coating solution is provided that has about 0.1% to 10% by weight polyvinyl alcohol dissolved in the solution. Preferably, the concentration of polyvinyl alcohol in the coating solution is about 1-10% by weight, more preferably about 1-2% by weight.

In another embodiment, lower saponified polyvinyl alcohol may be used (eg, polyvinyl alcohol having a degree of saponification of at least about 40% and less than about 80%). In this embodiment, the polyvinyl alcohol may first be dissolved in an alcohol such as isopropyl alcohol. Water is the above water /
It can be added until the alcohol ratio is reached. Alternatively, the lower saponified polyvinyl alcohol can be dissolved in a mixture of alcohol and water.

Now returning to the method of making the membrane shown in FIG. 2, the dry membrane is then immersed in a coating solution containing polyvinyl alcohol. As generally shown in FIG.
Where it enters the coating solution bath 44, the membrane preferably does not contact the rollers. That is, the level of liquid 45 in coating solution bath 44 is preferably such that the film enters bath 44 where the film does not contact roller 36. This allows for better removal of air by the coating solution, which results in improved and / or more uniform wetting of the membrane.

The membrane can then be further dried in another drying device such as an oven 48,
The oven 48 dries the membrane as described above for the PVDF membrane before wetting. After drying, the film can be cut (to its desired width) and accumulated on take-up roll 50. This membrane is available in shorter lengths or widths,
It can be further cut.

Membranes made in accordance with the present invention can have many different applications. For example, the membranes of the present invention can be used whenever a component or particle has to be separated from the liquid in which it is suspended. In one particular embodiment, membranes made in accordance with the present invention can be used in a disposable processing set for separating biological fluids such as blood into its components. For example, whole blood can be passed through the membrane to separate it into plasma on the one hand and concentrated blood cells (such as red blood cells and white blood cells) on the other hand. Examples of separation devices or separators that can be used with the membranes of the invention are described in further detail below. However, this separation device is only one example of a possible use for this membrane, and none of the following description should be construed as limiting the invention for use with such devices. Indeed, the membranes made according to the invention may be used in a laboratory or industrial environment, where the membrane is not limited to single use but rather may be used repeatedly.

A separator 60 including a PVDF membrane made in accordance with the present invention is shown in FIG. Separator 60 is typically a device for separating whole blood into plasma and concentrated blood cells (
For example, part of a disposable fluid processor used in conjunction with the Autopheresis-C® plasmapheresis device marketed by Baxter Healthcare Corporation. The construction and operation of this plasmapheresis device (including separator 60) is described in US Pat. No. 5,194,145.
No., which is hereby incorporated by reference and the detailed description is not repeated here.

Briefly, however, as shown in FIG. 3, the separator 60 comprises a housing 66 that defines a generally cylindrical inner surface 70. The housing includes a fluid inlet 72,
It has a first outlet 74 and a second outlet 76. A rotor 78, having a generally cylindrical outer surface, is rotatably mounted within the housing, the outer surface of the rotor being spaced from the inner surface of the housing to define a small gap 82 therebetween. The membrane 10 of the present invention is installed on this rotor, where it is the rotor 78 and the housing 6.
Facing the gap 82 located between the two. This membrane rests on top of a series of spaced support ribs 86 on the surface of the rotor. These raised support ribs support the membrane and form channels for collecting the filtrate passing through the membrane 10.

The membrane is shown on the surface of the rotor in FIG. 3, but alternatively the membrane is
It can be mounted on the substantially cylindrical inner surface of the housing. In this case, the surface of the housing may also have raised ribs to support the filtration membrane and collect the filtrate that passes through the membrane.

In the separator 60 shown in FIG. 3, a fluid such as biological suspension or blood is
It is introduced through inlet 72 and flows through a gap 82 between the outer surface of rotor 78 and the inner surface of housing 66. During the passage through this gap, the high speed rotation of the rotor causes turbulence in the form of Taylor vortices, which clears the membrane and eliminates coagulated cells or debris. Assisted by the appreciable transmembrane pressure generated by the flow control pump of the plasmapheresis device, plasma from the blood passes through the membrane 10 and is collected in the channel defined between the spaced apart raised ribs. It The plasma flows through the channel into the collection manifold and passes through the first outlet 74. The remaining portion of the fluid or suspension (eg, concentrated blood cells) is collected from the housing through the second outlet 76.

Membranes made according to the present invention, ie PVDF membranes rendered hydrophilic by coating the PVDF membranes with polyvinyl alcohol, are those membranes that are biocompatible with biological fluids (such as blood). In terms of sex, substantially as described above,
Useful in treating blood. The membranes of the invention are particularly well suited for sonic welding to rotors of the type described above. As is now understood, PVDF can form alloys during sonic welding with the acrylic materials typically used to make such rotors. This provides improved adhesion of the membrane to the rotor resulting in improved quality and reliability of the separator and reduced scrap rate.

The membranes of the present invention can be sterilized by radiation sterilization such as gamma or electron beam radiation. A further advantage of PVDF membranes made in accordance with the present invention is that complement and platelet activity is relatively minimal, especially when used to separate plasma from whole blood.

Complement is a naturally occurring soluble protein found in plasma that provides immune protection. They circulate throughout the body and are activated upon exposure to antigen and / or in response to tissue damage. However, in some cases, complement is
It can be activated in the absence of activated antigen (eg, by contact with equipment used in processing blood). Since these complements are essential for the immune response in humans, it is desirable that complement activation during blood processing be maintained to a minimum.

Platelets are essential in the repair of damaged body tissue in that they help clot formation and prevent excessive bleeding. However, platelets can also be activated by contact with equipment used in the processing of blood, including the membranes used to separate blood components. Therefore, it is desirable that platelet activation also be kept to a minimum.

A blood separator of substantially the type described above (Plasmacell-C® separation chamber used with Autopheresis C®, both Baxter Healthcare Corporation, D
eerfield, available from IL), whole blood was collected from the donor,
Placed in two different anticoagulants (sodium citrate and ACD-A). This blood was separated into plasma and concentrated blood cells. Samples of blood from this donor were taken before and after the separation procedure. Collected plasma and concentrated blood cell (CCC) samples were also collected. These samples were assayed for complement activation and platelet activation.

[0038]   The results of these assays are shown in Tables 1 and 2 below.

[0039]

[Table 1] As shown in Table 1, although some complement was activated as a result of the treatment of blood as described above, the level of complement activation was generally earlier in other membranes and / or materials. It was lower than the level observed.

The results of the platelet activation assay are shown in Table 2 below. The presence of CD62 on the surface of platelets, and the presence of β-thromboglobulin and platelet factor 4 in plasma is evidence of platelet activation.

[0041]

[Table 2] As shown in Table 2, blood treatment using the membranes of the present invention resulted in levels of platelet activation in plasma components comparable to pre- and post-procedure levels, although some platelets were activated. However, this indicates that no further platelet activation was triggered by this device and / or procedure.

Although the membranes of the present invention have been described above as part thereof in terms of a single-use, disposable, treatment set for blood and blood components, the membranes of the present invention are also reusable. It can also be used in other environments where it is desired to be possible. For example, it was observed that membranes made in accordance with the present invention remained hydrophilic even after repeated use. In other words, the membranes of the present invention can be "rewet" after such use and after drying. As used herein, "rewet" or "rewettable" refers to about 1% of the aqueous liquid with which the previously moistened and dried membrane comes into contact.
It represents the ability to absorb substantially (ie, in the absence of any dry spots) in less than a minute.

Furthermore, the membranes of the present invention remain hydrophilic, even if the membranes are “non-crosslinked”. Although it is possible that a small amount of cross-linking may occur during the manufacture of the membrane, such minimal and incidental cross-linking is when the term "non-cross-linked" is used herein. It is understood to be within the meaning of.

Membranes made in accordance with the present invention were wetted with an aqueous liquid, dried and rewet to demonstrate the rewetability of such membranes. These tests and their results
It is described below.

Example 1 A PVDF membrane treated with 1% PVOH in the manner described above was placed in a Gelman filter funnel and about 600 ml of room temperature water was passed through the membrane. Then, this membrane is
It was dried at 0 ° C. for about 5 minutes and then contacted with 0.9% saline. It was observed that the membrane rewet immediately. The same membrane was then placed in the funnel again and about 800 ml of 40 ° C. water was filtered through the membrane. The membrane was dried and after drying contacted with 0.9% saline. It was observed that the membrane rewet immediately.

Example 2 A sample of a membrane made substantially as described above was placed in a beaker filled with about 800 ml of water. These beakers were placed on a hot plate and heated to bring the water to a boil. A magnetic stir bar was placed in this water with four membrane samples. After boiling for 1.5 hours, the membranes were dried. After drying, room temperature water was passed through the membrane. The membranes were then dried again at 50 ° C. for about 1 hour and then again contacted with room temperature water. Boiled and dried film is about 5
It was observed to rewet within seconds.

Thus, even after such membranes were boiled in hot water and dried, these membranes retained their hydrophilic properties and were reusable as membranes. This is in contrast to PVDF membranes prepared by the same process described above but treated with other known wetting agents such as polyethylene glycol (PEG). PVDF membranes made hydrophilic with 5% PEG coating solution (dissolved in water and isopropyl alcohol) wet initially but not rewet after drying.

The invention has been described with reference to the preferred embodiments. However, it is understood that various modifications of the membranes and the methods of making these membranes are possible without departing from the scope of the invention as set forth in the appended claims.

[Brief description of drawings]

[Figure 1]   FIG. 1 is a cross-sectional view of a membrane without an internal support embodying the present invention.

FIG. 1A   FIG. 1A is a cross-sectional view of another membrane embodying the invention and having an internal support.

[Fig. 2]   FIG. 2 is a diagram showing a method for producing a PVDF membrane according to the present invention.

FIG. 3 is a perspective view of a separation device incorporating a PVDF membrane, embodying the present invention, with a portion of the device cut away to show the interior of the device.

─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Boggs, Daniel Earl.             United States Illinois 60044, Les             Iku Bluff, N. Walkan             Road number 118 30039 (72) Inventor Pauly, Robin G.             United States Illinois 60046, Les             Ikubira, W. ground             Avenue 24866 (72) Inventor Moi, Bonnie             United States Illinois 60030, Gu             Rays Lake, Country Drive             1908, apartment 204 F-term (reference) 4C077 AA12 BB02 EE01 HH03 JJ03                       KK11 LL02                 4D006 GA06 HA21 KC21 MA03 MA09                       MA10 MA22 MA25 MB09 MB19                       MC29X MC33X NA10 NA46                       NA64 PB09 PB42 PB43

Claims (22)

[Claims] 1. A porous membrane comprising: a porous layer of polyvinylidene difluoride; and on a porous layer of polyvinylidene difluoride sufficient to render the polyvinylidene difluoride hydrophilic. A polyvinyl alcohol coating, wherein the membrane is rewettable by an aqueous liquid after drying and is not crosslinked and has not been contacted with a strong alkaline solution. 2. The membrane of claim 1, wherein the membrane is rewettable after contact with an aqueous liquid having a temperature of at least about 40 ° C. 3. The membrane of claim 1, wherein the membrane is rewettable after contact with an aqueous liquid having a temperature of about 100 ° C. 4. The membrane according to claim 1, wherein the aqueous liquid is water. 5. The membrane of claim 1, wherein the polyvinyl alcohol coating is made from a solution of 1-2% by weight polyvinyl alcohol dissolved in a mixture of water and alcohol. 6. The membrane of claim 1, wherein the polyvinyl alcohol is derived from polyvinyl acetate having a degree of saponification of at least about 40%. 7. The membrane of claim 6, wherein the degree of saponification is about 80-100%. 8. The alcohol in which the polyvinyl alcohol is dissolved is:
The membrane according to claim 5, comprising isopropyl alcohol. 9. The coating solution comprises about 20-80% by weight water and about 20%.
9. The membrane of claim 8 containing .about.80 wt% isopropyl alcohol. 10. The membrane of claim 1, wherein the apparent pore size of the membrane is about 0.8 μm or greater. 11. The membrane of claim 1, wherein the layer of polyvinylidene difluoride is supported by a layer of porous material. 12. The porous material comprises a polyester mesh.
The membrane according to 1. 13. A method for making a porous, non-crosslinked, rewettable, hydrophilic polyvinylidene difluoride (PVDF) membrane, comprising: a) porous, dried. Providing a hydrophobic PVDF membrane; and b) contacting the membrane with a coating solution of 0.1-10 wt% polyvinyl alcohol dissolved in a solution of water and alcohol. 14. The method of claim 13 including the step of providing the coating solution by dissolving polyvinyl alcohol in water and then adding alcohol to the polyvinyl alcohol dissolved in the water. 15. The method of claim 13, comprising dissolving the polyvinyl alcohol in a solution containing about 20-80% water and about 20-80% isopropyl alcohol. 16. The method of claim 13, wherein the coating solution contains about 1% by weight polyvinyl alcohol. 17. The method of claim 13 comprising providing polyvinyl alcohol having a degree of saponification of at least 80%, and dissolving the polyvinyl alcohol in an aqueous solution. 18. The method of claim 17, wherein the temperature of the aqueous solution is about 90 ° C. 19. The method of claim 13, wherein the contacting step is performed by passing the membrane through a bath of the coating solution. 20. A porous membrane made according to the method of claim 13. 21. An apparatus for separating a biological fluid into two or more components comprising: a housing having an inner wall defining a fluid chamber, and a fluid inlet and at least one fluid outlet. A housing having the inlet and the outlet in communication with the chamber; a rotor disposed within the chamber; and a porous membrane disposed either on the inner wall or on the rotor, The porous membrane comprises a porous layer of polyvinylidene difluoride and a polyvinyl alcohol coating on the porous layer, the polyvinyl alcohol coating being sufficient to render the polyvinylidene fluoride layer hydrophilic. A device comprising a porous membrane. 22. The apparatus of claim 21, wherein the membrane is sonic welded to the rotor.