U.S. patent application number 11/479908 was filed with the patent office on 2008-01-03 for ultrafiltration membranes and methods of making.
This patent application is currently assigned to Millipore Corporation. Invention is credited to Philip Goddard, Willem Kools, Nitin Satav, Gabriel Tkacik.
Application Number | 20080004205 11/479908 |
Document ID | / |
Family ID | 38877432 |
Filed Date | 2008-01-03 |
United States Patent
Application |
20080004205 |
Kind Code |
A1 |
Tkacik; Gabriel ; et
al. |
January 3, 2008 |
Ultrafiltration membranes and methods of making
Abstract
The present invention is an integral multilayered composite
membrane having at least one ultrafiltration layer made by
cocasting or sequentially casting a plurality of polymer solutions
onto a support to form a multilayered liquid sheet and immersing
the sheet into a liquid coagulation bath to effect phase separation
and form a multilayered composite membrane having at least one
ultrafiltration layer.
Inventors: |
Tkacik; Gabriel; (Bedford,
MA) ; Goddard; Philip; (Nashua, NH) ; Kools;
Willem; (Reading, MA) ; Satav; Nitin;
(Chelmsford, MA) |
Correspondence
Address: |
MILLIPORE CORPORATION
290 CONCORD ROAD
BILLERICA
MA
01821
US
|
Assignee: |
Millipore Corporation
Billerica
MA
|
Family ID: |
38877432 |
Appl. No.: |
11/479908 |
Filed: |
June 30, 2006 |
Current U.S.
Class: |
210/500.21 ;
210/335; 514/1.1; 530/416 |
Current CPC
Class: |
B01D 2325/022 20130101;
B01D 61/145 20130101; C07K 1/34 20130101; B01D 67/0013 20130101;
B01D 67/0018 20130101; B01D 69/12 20130101; B01D 71/68 20130101;
B01D 2323/12 20130101; B01D 71/34 20130101; B01D 2323/42 20130101;
B01D 67/0016 20130101 |
Class at
Publication: |
514/2 ; 530/416;
210/335 |
International
Class: |
A61K 38/16 20060101
A61K038/16; C07K 14/47 20060101 C07K014/47; B01D 29/00 20060101
B01D029/00 |
Claims
1.) An unsupported integral multilayer composite ultrafiltration
membrane comprised of at least a first porous membrane layer having
a first side and an equivalent second side, and at least a second
porous membrane layer having an equivalent first side and a second
side, the junction of the first layer and the second layer being
integral and having a transition zone of porosity from said
equivalent first side of said second layer through said equivalent
second side of said first layer, wherein at least one of said
layers is an asymmetric ultrafiltration membrane.
2.) The membrane of claim 1, wherein the first layer is a skinned
asymmetric ultrafiltration membrane layer and the second layer is a
microporous membrane layer.
3.) The membrane of claim 1, wherein the first layer is a skinned
asymmetric ultrafiltration membrane layer and the second layer is
an asymmetric ultrafiltration membrane layer.
4.) The membrane of claim 1, wherein the first layer is a thin
microporous ultrafiltration membrane layer and the second layer is
asymmetric ultrafiltration membrane layer.
5.) The membrane of claim 1, wherein the first layer is a skinned
asymmetric ultrafiltration membrane layer and the second layer is
an asymmetric ultrafiltration membrane layer, and wherein the first
ultrafiltration membrane layer has a MWCO higher than the second
ultrafiltration membrane layer.
6.) A process for forming an integral multilayered composite
ultrafiltration membrane compromising the steps of operatingly
positioning a polymer solution applying apparatus having at least
two dispensing outlets relative to a moving carrier surface, and;
supplying each dispensing outlet with a different polymer solution,
and; applying said solutions onto said moving carrier surface so as
to create a multiple layer coating on said carrier, and wherein;
said multiple layers are dispensed with essentially no time
interval between successive layers being applied, and; subjecting
said multiple liquid layers to a phase separation process so as to
form a wet multilayer ultrafiltration membrane.
7.) A process for forming an integral multilayered composite
ultrafiltration membrane compromising the steps of operatingly
positioning a polymer solution applying apparatus having at least
two dispensing outlets relative to a moving carrier surface, and;
supplying each dispensing outlet with a different polymer solution,
and; applying said solutions onto said moving carrier surface so as
to create a multiple layer coating on said carrier, and wherein;
said multiple layers are dispensed sequentially and the previous
layer is only partially phase separated before each successive
layer is applied, and; subjecting said multiple liquid layers to a
phase separation process to complete phase separation and to form a
wet multilayer ultrafiltration membrane.
8.) The process of claim 6 wherein the membrane is then dried.
9.) The process of claim 6 wherein the membrane is then washed.
10.) The process of claim 6 wherein the membrane is then washed and
dried.
11.) The process of claim 7 wherein the membrane is then dried.
12.) The process of claim 7 wherein the membrane is then
washed.
13.) The process of claim 7 wherein the membrane is then washed and
dried.
14.) A virus removal methodology, suitable for conducting a
high-flux fluid separation of a virus from a protein in the course
of biopharmaceutical manufacture, the methodology comprising the
steps of: (a) providing a filtration device comprising a housing
having a fluid inlet and a filtrate outlet, and containing at least
one multilayered composite membrane having at least one
ultrafiltration layer produced from at least two polymer solutions
membranes, wherein: (i) said composite membranes are each
substantially hydrophilic, (ii) said composite membranes are each
capable of substantially preventing the passage therethrough of
said virus and substantially permitting the passage therethrough of
said protein, (iii) said composite membranes have each a tight-side
and an open-side, the average surface pore size of said tight-side
being less than the average surface pore size of said open-side,
and (iv) the foremost composite membrane is oriented such that
fluid introduced into said housing through the fluid inlet
commences passage through said foremost asymmetric membrane through
its open-side; (b) providing a manufactured protein-containing
solution, wherein the predominant solute in said solution is said
protein, and wherein the solution is prone to contamination by said
virus; and (c) flowing said manufactured protein-containing
solution through said filtration device under conditions sufficient
to effect substantial passage of said protein through each of said
composite membranes and out of said housing through said filtrate
outlet, whereby any of said virus contaminating said manufactured
protein-containing solution, being substantially prevented from
passage through said asymmetric membranes, is substantially removed
therefrom.
15.) The virus removal methodology of claim 14 wherein the
composite membranes form a pleated tube.
16.) The virus removal methodology of claim 14 wherein each of said
composite membranes are substantially identical in their
composition and porosity, and wherein the porosity of each of said
asymmetric membranes is defined to enable performance of the virus
removal methodology, yielding a log reduction value (LRV) greater
than 6 and a protein passage greater than 98%.
17.) A filtration capsule, suitable for use in virus removal
methodologies, comprising a tubular housing though which a fluid
process stream can be conducted, the housing having a fluid inlet
and a filtrate outlet, and containing a pleated tube composed of
one, two or three interfacially-abutting multilayered composite
membranes having at least one ultrafiltration layer produced from
at least two polymer solutions membranes, wherein the pleated tube
is positioned within said process stream between said fluid inlet
and said filtrate outlet, and wherein each of said composite
membranes is: (a) substantially hydrophilic, (b) capable of
substantially preventing the passage therethrough of viruses, (c)
provided with a tight-side and an open-side, the average pore size
of said tight-side being less than the average pore size of said
open-side, and (d) oriented such that fluid introduced into said
housing through the fluid inlet commences passage through each
asymmetric membrane through its open-side.
18.) The virus removal methodology of claim 14 wherein the
filtration device employed in said methodology is the filtration
cartridge of claim 19.
19.) The filtration capsule of claim 17 wherein the asymmetric
membranes are each capable of substantially permitting the passage
therethrough of protein.
20.) The filtration capsule of claim 17 wherein each asymmetric
membrane is composed of polyethersulfone.
Description
CROSS-REFERENCED TO RELATED APPLICATIONS
[0001] The present application relates to U.S. Provisional
Application No. 60/686,363, filed on Jun. 1, 2005 and to U.S.
Provisional Application No. 60/583,209, filed on Jun. 25, 2004 the
entire contents of which are incorporated herewith.
[0002] This invention provides for multilayered composite membranes
having at least one ultrafiltration layer produced from at least
two polymer solutions, and a novel method of manufacturing such
membranes. The membranes are particularly suited for use in
dead-end ultrafiltration.
BACKGROUND OF INVENTION
[0003] Ultrafiltration and microporous membranes are used in
pressure-driven filtration processes. Practitioners in the field of
separation processes by membranes easily differentiate between
microporous and ultrafiltration membranes and generally distinguish
between them based on their application and aspects of their
structure. Microporous and ultrafiltration membranes are made, sold
and used as separate and distinct products. Despite some overlap in
nomenclature, they are separate entities, and treated as such in
the commercial world.
[0004] Ultrafiltration membranes are primarily used to concentrate
or diafilter soluble macromolecules such as proteins, DNA, starches
and natural or synthetic polymers. In the majority of uses,
ultrafiltration is accomplished in the tangential flow filtration
(TFF) mode, where the feed liquid is passed across the membrane
surface and those molecules smaller than the pore size of the
membrane pass through (filtrate) and the rest (retentate) remains
on the first side of the membrane. As fluid also passes through
there is a need to recycle or add to the retentate flow in order to
maintain an efficient TFF operation. One advantage of using a TFF
approach is that as the fluid constantly sweeps across the face of
the membrane it tends to reduce fouling and polarization of the
solutes at and near the membrane surface leading to longer life of
the membrane.
[0005] Microporous membranes are primarily used to remove
particles, such as solids, bacteria, and gels, from a liquid or gas
stream in dead-end filtration mode. Dead-end filtration refers to
filtration where the entire fluid stream being filtered goes
through the filter with no recycle or retentate flow. Whatever
material doesn't pass through the filter is left on its upper
surface.
[0006] Ultrafiltration membranes are generally skinned asymmetric
membranes, made for the most part on a support which remains a
permanent part of the membrane structure. The support can be a
non-woven or woven fabric, or a preformed membrane.
[0007] Microporous membranes are produced in supported or
unsupported form. Usually, the support has the membrane or a
portion of the membrane formed in the support, rather than on the
support, as in ultrafiltration membranes.
[0008] The early cellulosic, nylon and polyvinylidene fluoride
microporous membranes were symmetric and for the most part,
unskinned. Presently, some asymmetric microporous membranes are
produced, and some of these are skinned.
[0009] While it would seem that the two types of membrane could be
differentiated by pore size, this is not the case, as will be
discussed below. The reasons for this are that they are used in
different applications, requiring different characterization
methods. None of the methods usually used give an absolute pore
size measure, and different methods cannot be directly
compared.
[0010] Despite the similarities between microporous membranes and
ultrafiltration membranes, the history of their development is
quite different. It is therefore not surprising that there is more
than one accepted demarcation between them.
[0011] Microporous membranes were commercially developed from the
work of Zsigmondy by Sartorius Werke (Germany) in 1929. These were
what are now call "air cast" membranes made by evaporating a thin
layer of a polymer solution in a humid atmosphere. These membranes
were and still are symmetric and generally unskinned. Since they
were used to remove or hold bacteria, they were rated by the
bacteria size that would be retained. This method resulted in pore
size ratings in microns.
[0012] A common method used to rate microporous membranes is the
bubble point test. In this method, the microporous membrane is
placed in a holder and saturated with a test liquid. Gas pressure
is applied to one side of the membrane and the pressure is
increased at a fixed rate. The appearance of the first stream of
bubbles from the downstream side is a measure of the largest pore.
At a higher pressure where the liquid is forced out of the majority
of the pores, the foam all over point (FAOP) is reached. These are
described in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved
1976).
[0013] Ultrafiltration membranes (UF) are a spin-off of the reverse
osmosis membrane development research of Leob and Sourirajan. Alan
Michaels fixed 1965 as the time when the first rudimentary UF
membranes and devices first appeared on the market. UF membranes
are made by immersion casting methods and are skinned and
asymmetric. The initial commercial applications were related to
protein concentration and the membranes were rated by the molecular
weight of the protein that they would retain, i.e., the molecular
weight cutoff rating of the membrane (MWCO).
[0014] While membrane ratings based on testing with proteins is
still done, a common method uses non-protein macromolecules having
a narrow molecular weight distribution, such as polysaccharides
(Dextrans) or polyethylene glycols. See for example, A rejection
profile test for ultrafiltration membranes and devices,
BIOTECHNOLOGY 9 (1991) 941-943.
[0015] As membrane applications were developed in the 1960's and
1970's, UF membranes expanded to larger pore sizes and microporous
membranes (MF) to smaller pores sizes. As this occurred,
practitioners began to differentiate between the two types of
membranes. It is interesting from a historical perspective that the
earliest literature referred only to ultrafiltration. Both Kesting
Synthetic Polymer Membranes A Structural Perspective, Robert E.
Kesting, John Wiley & Sons 1985 and Lonsdale "The Growth of
Membrane Technology". K. Lonsdale, J. Membrane Sci. 10 (1982) 81
cite to Ferry's major review of 1936 in which ultrafiltration
refers to both ultrafiltration and microfiltration membranes.
Kesting states "The term ultrafiltration has changed its meaning
over the years." In fact, even in a 1982 review Pusch Synthetic
Membranes--Preparation, Structure, and Application, W. Pusch and A.
Walch Angew. Chem. Int. Ed. Engl. 21 (1982) 660 uses
ultrafiltration to denote sieving membranes of from 0.005.mu. to
1.mu.. Kesting, in table 2.9 (pg 45) has UF as 10-1000 Angstroms,
0.01-0.1 microns, and MF as 1000-100.000 Angstroms, 0.1-10
microns.
[0016] A 1969 chart from Dorr-Oliver has microporous pore size
ranging from 0.03.mu. to over 10.mu., and UF ranging from 0.002.mu.
to 10.mu.. A recent handbook chapter. Handbook of Separation
techniques for Chemical Engineers--Third Edition, Section 2.1
Membrane Filtration, M. C. Porter, McGraw-Hill 1996 claims this
"reflects confusion in the literature among MF, UF and RO." In 1975
Porter Selecting the Right Membrane, M. C. Porter, Chem. Eng. Sci.
71 (1975) 55 proposed that UF cover the range from 0.001 to 0.02
microns, and MF from 0.02 to 10 microns. Lonsdale referred to this
in Reference 2 and Porter uses this definition again in reference
4.
[0017] Cheryan Ultrafiltration Handbook, M. Cheryan, Technomic
Publishing Co. Chapter 26--Introduction and Definitions
(Ultrafiltration) S. S. Kulkarni et al Chapter 31--Definitions
(Microfiltration) R. H. Davis 1986 has both Porter's ranges for UF
and MF (uncited) and a chart that appears to be from the
Dorr-Oliver chart. In Membrane Handbook, Davis, Van Nostrand and
Reinhold DATE Davis gives MF as 0.02-10 microns, and Kulkarni et al
describe UF as 10 to 1000 Angstroms, 0.001-0.1 microns. Another
example of pore size ranges is from the Encyclopedia of Polymer
Science and Engineering, Volume 9 pg 512, John Wiley and Sons 1987
which has UF as from 0.01 to 0.1 microns and MF as from 0.1 to 10
microns. Zeman Microfiltration and Ultrafiltration, L. Zeman and A.
Zydney, Marcel Dekker, Inc 1996, p 13 has a chart in which UF
ranges from 0.001 to 0.1 micron and MF from about 0.02 to 10
microns.
[0018] With respect to the present invention, we will define
ultrafiltration membranes as compared to microporous membranes
based on the definitions of the International Union of Pure and
Applied Chemistry (IUPAC), "Terminology for membranes and membrane
processes" published in Pure Appl. Chem., 1996, 68, 1479.
[0019] "72. microfiltration: pressure-driven membrane-based
separation process in which particles and dissolved macromolecules
larger than 0.1 .mu.m are rejected"
[0020] "75. ultrafiltration: pressure-driven membrane-based
separation process in which particles and dissolved macromolecules
smaller than 0.1 .mu.m and larger than about 2 nm are
rejected."
[0021] The definition for ultrafiltration membranes will be based
on what they do, and how they do it. Ultrafiltration membranes are
capable of concentrating or diafiltering soluble macromolecules
that have a size in solution of less than about 0.1 micron and
operating continuously in a tangential flow mode for extended
periods of time, usually more than 4 hours and for up to 24 hours.
Microporous membranes are capable of removing particles larger than
0.1 micron and being used in dead-end filtration applications.
Microporous membranes generally allow soluble macromolecules to
pass through the membrane.
[0022] Ultrafiltration membrane production methods by immersion
casting are well known. A concise discussion is given in
Microfiltration and Ultrafiltration: Principles and Applications
Marcel Dekker (1996); L. J. Zeman and A. J. Zydney eds. These
preparations are generally described to consist of the following
steps: a) preparation of a specific and well controlled preparation
of a polymer solution, b) casting the polymer solution in the form
of a thin film onto a substrate, c) coagulating the resulting film
of the polymer solution in a nonsolvent and d) optionally drying
the ultrafiltration membrane.
[0023] The common form of ultrafiltration membranes is the
asymmetric membrane, where the pore size of the membrane varies as
a function of location within the thickness of the membrane. The
most common asymmetric membrane has a gradient structure, in which
pore size increases from one surface to the other. Asymmetric
membranes are more prone to damage, since their retention
characteristic is concentrated in a thin surface region or skin. A
membrane skin is a thin dense surface penetrated by surface pores.
It has been found, however, that increased productivity results
from having the feed stream to be filtered contacting the larger
pore surface, which acts to prefilter the stream and reduce
membrane plugging.
[0024] Practitioners in the art of making ultrafiltration
membranes, particularly asymmetric membranes, have found that
membranes which contain large (relative to membrane pore size)
hollow cavernous structures have inferior properties compared to
membranes made without such hollow structures. These hollow
structures are sometimes called "macrovoids", although other terms
are used in the art. Practitioners striving for membranes of very
high retention efficiency prefer to make membranes without such
hollow structures.
[0025] Perhaps the most direct variation of the single layer
structure is a multilayered unbonded laminate. While laminates can
be made from layers of the same or different membranes, they have
drawbacks. Each layer has to be made in a separate manufacturing
process, increasing cost and reducing manufacturing efficiency. It
is difficult to manufacture and handle very thin membranes, less
than say 20 microns, because they deform and wrinkle easily. This
adds to the inefficiency of producing a final product with thin
layers. Unbonded laminates can also come apart during fabrication
into a final filter device, such as a pleated filter, which will
cause flow and concentration non-uniformities. Other methods of
forming multilayered porous membrane structures are known. U.S.
Pat. No. 4,824,568 describes a composite ultrafiltration membrane
made by casting a thin ultrafiltration membrane onto a preformed
microporous membrane. U.S. Pat. No. 5,228,994 describes a method
for coating a microporous substrate with a second microporous layer
thereby forming a two layer composite microporous membrane. These
processes require two separate membrane forming steps, forming one
on top of the other preformed membrane and are restricted by the
viscosities of the polymer solutions that can be used in the
process to prevent excessive penetration of casting solution into
the pores of the preformed substrate.
[0026] In U.S. Pat. No. 5,620,790, a method of making a microporous
membrane is described wherein the membrane is made by pouring out a
first layer on a support of polymeric material onto a substrate and
subsequently pouring out one or more further layers of a solution
of polymeric material onto the first layer prior to the occurrence
of turbidity in each successively immediate preceding layer, the
viscosity of each immediately successive layer of a solution of
polymeric material having been the same or less than that of the
preceding layer. US Patent Application 20030217965, directed to
microporous membranes, provides for a method of producing an
integral multilayered porous membrane by simultaneously co-casting
a plurality of polymer solutions onto a support to form a
multilayered liquid sheet and immersing the sheet into a liquid
coagulation bath to effect phase separation and form a porous
membrane. U.S. Pat. No. 6,706,184 discloses a process for forming a
continuous, unsupported, multizone phase inversion microporous
membrane having at least two zones comprised of the acts of:
operatively positioning at least one dope applying apparatus,
having at least two polymer dope feed slots, relative to a
continuous moving coating surface; applying polymer dopes from each
of the dope feed slots onto the continuously moving coating surface
so as to create a multiple layer polymer dope coating on the
coating surface; subjecting the multiple dope zone layer to contact
with a phase inversion producing environment so as to form a wet
multizone phase inversion microporous membrane; and then washing
and drying the membrane. In these structures, each layer or zone is
a microporous membrane. US Patent Application 20040023017 describes
a multilayer microporous membrane containing a thermoplastic resin,
comprising a coarse structure layer with a higher open pore ratio
and a fine structure layer with a lower open pore ratio, wherein
said coarse structure layer is present at least in one membrane
surface having a thickness of not less than 5.0.mu., a thickness of
said fine structure layer is not less than 50% of the whole
membrane thickness, and said coarse structure layer and said fine
structure layer are formed in one-piece. The fine structure is not
skinned. This structure is formed from a single solution.
BRIEF DESCRIPTION OF FIGURES
[0027] FIG. 1 illustrates a co-casting coating head
[0028] FIGS. 2a and 2b illustrate the position of the layers for
two layer membranes
[0029] FIG. 3 shows results from filtration of fluorescent
beads
[0030] FIGS. 4a-4d show scanning electron micrograph images of the
membranes of Examples 1 and 2.
[0031] FIG. 5 shows scanning electron micrograph images of the
membranes of Example 3.
SUMMARY OF THE INVENTION
[0032] This invention comprises an integral multilayer flat sheet
membrane made from more than one polymer solution, wherein at least
one layer is an ultrafiltration membrane. The method of making the
membrane is included in the present invention
[0033] In an embodiment, the invention comprises a skinned
asymmetric ultrafiltration membrane layer joined to a microporous
membrane layer, where the junction has a gradient of pore sizes,
transitioning from the pore size of the microporous layer in the
vicinity of the junction to the pore size of the ultrafiltration
layer in the vicinity of the junction.
[0034] In an embodiment, the invention comprises a microporous
membrane layer joined to the tight pore side of an ultrafiltration
layer, where the junction has a gradient of pore sizes,
transitioning from the pore size of the microporous layer in the
vicinity of the junction to the pore size of the ultrafiltration
layer in the vicinity of the junction.
[0035] In an embodiment, the invention comprises a skinned
asymmetric ultrafiltration membrane layer joined to a second
asymmetric ultrafiltration membrane layer, the second
ultrafiltration membrane having an average retentive pore larger
than that of the skinned asymmetric first layer, where the junction
has a gradient of pore sizes, transitioning from the pore size of
the second ultrafiltration layer in the vicinity of the junction to
the pore size of the first ultrafiltration layer in the vicinity of
the junction.
[0036] In an embodiment, the invention comprises a process for
forming an integral multilayered composite ultrafiltration membrane
compromising the steps of operatingly positioning a polymer
solution applying apparatus having at least two dispensing outlets
relative to a moving carrier surface, and; supplying each
dispensing outlet with a different polymer solution, and; applying
said solutions onto said moving carrier surface so as to create a
multiple layer coating on said carrier, and wherein; said multiple
layers are dispensed with essentially no time interval between
successive layers being applied, and; subjecting said multiple
liquid layers to a phase separation process so as to form a wet
multilayer ultrafiltration membrane.
[0037] The invention further embodies the use of the membranes of
the present invention in a process to remove viral particles from a
manufactured protein-containing solution, made in the course of
producing biotech derived pharmaceuticals, wherein the membranes is
each capable of substantially preventing the passage therethrough
of the virus particles and substantially permitting the passage
therethrough of the protein,
DETAILED DESCRIPTION OF THE INVENTION
[0038] The present invention is an integral multilayered composite
membrane having at least one ultrafiltration layer made by
cocasting a plurality of polymer solutions onto a support to form a
multilayered liquid sheet and immersing the sheet into a liquid
coagulation bath to effect phase separation and form a porous
ultrafiltration membrane. After formation, the porous membrane is
washed free of solvent and other soluble materials. It can then be
further extracted to reduce fugitive materials to a low level and
then optionally be dried.
[0039] In order to reduce the present invention to practice, the
inventors had to overcome the fundamental problem of making an
integral multilayered structure from markedly dissimilar
membrane-forming solutions. The inventors faced practical
difficulties associated with the differences in structure and
formation of the dissimilar membrane layers. While the prior art
describes methods of making multilayered microporous membranes, in
that art the membrane formulation solutions and mechanisms of
membrane formation and membrane structure of each layer are quite
similar.
[0040] In the present invention, pore size differences between the
ultrafiltration (UF) layer and the microporous (MF) layer can
differ by an order of magnitude. Also, the rate of formation of UF
and MF membranes is different, with UF forming significantly faster
in the coagulation bath. Potential and real problems that can arise
with the fabrication of membranes of the present invention
include:
[0041] If the last solution coated is a UF-forming solution, it may
form too fast and too densely to permit the coagulation liquid to
penetrating to the microporous forming layer at a rate necessary to
form a satisfactory membrane. If the coagulant diffuses through the
top layer too slowly, formation of the underlying microporous layer
will be hindered, and pore size will be uncontrollable. As well, it
may even occur that the underlying layer may not solidify before
the coated sheet exits from the coagulant bath.
[0042] If the MF-forming solution is coated over the UF forming
layer, the formation of the microporous layer will greatly affect
the formation of the UF layer. It will prevent the formation of the
skin surface, and change the pore size that would have resulted if
the UF layer were cast separately.
[0043] Other problems can result from the disparity between
viscosity ranges for the two types of membranes. UF membrane
forming solutions are usually of much higher viscosity than those
for MF membranes. Coating multilayers with viscosity discrepancies
only exacerbates the problems of making porous membranes
[0044] The inventors have found that within certain limited ranges
on key variables, integral multilayer cocast composite
ultrafiltration membranes with practical properties can be
made.
[0045] For simplicity, the process of making a multilayer cocast
composite ultrafiltration membrane will be described for a two
layer example. Although three or more layers may also be made by
the same process. The preferred process comprises the steps of
making two polymer solutions, one for each layer. Solutions for
making porous membranes by immersion casting usually comprise a
polymer, solvent and additives to modify and control the final pore
size and porous nature (i.e., percent porosity, pore size
distribution, etc.) of the membrane. Other additives are sometimes
used to modify physical properties such as hydrophilicity,
elongation, modulus, etc.
[0046] Preferred polymers include but are not limited to
polyvinylidene fluoride (PVDF), nylons such as Nylon 66,
polyamides, polyimides, polyetherimides, polyethersulfones,
polysulfones, polyarylsulfones, polyphenylsulphones,
polyvinylchlorides (PVC), polycarbonates, cellulose, regenerated
cellulose, cellulose esters such as cellulose acetate or cellulose
nitrate, polystyrenes, acrylic polymers, methacrylic polymers,
copolymers of acrylic or methacrylic polymers, or blends of any of
the above and the like.
[0047] Solvents used include but are not limited to such examples
as dimethyl formamide, N,N-dimethylacetamide, N-methylpyrrolidone,
tetramethylurea, acetone, dimethylsulfoxide and triethyl
phosphate.
[0048] Examples of the many porogens have been used in the art,
include but are not limited to compounds such as formamide, various
alcohols and polyhydric compounds, water, various polyethylene
glycols, polyvinyl pyrrolidone and various salts, such as calcium
chloride and lithium chloride.
[0049] Examples of other additives include surfactants to improve
wettability, and polymers compatible with the primary membrane
polymer used to modify mechanical properties of the final
membrane.
[0050] After the solutions are made, they are applied to a moving
carrier. For an unsupported membrane, which does not have a web
attached to the final membrane, the carrier is usually a plastic
film, such as polyethylene terephtalate, or a polyethylene coated
paper, or similar smooth continuous web that can be easily removed
from the formed membrane.
[0051] Application can be done by any standard method. The object
is to coat a first solution onto the carrier and a second solution
upon the first solution. A highly preferred method is co-casting,
in which the two layers are coated with essentially no time between
coatings. This can be done with a double knife over roll apparatus
a pressurized dual slot coating head or any other pre or
postmetering coating device as is known in the industry. Co-casting
means that the individual layers are cast essentially
simultaneously with each other with substantially no time interval
between one cast layer and the next cast layer. This method is
described in detail in US published Patent Application 20030217965.
Co-casting is an important aspect of the invention because it
allows for formation of controlled pore size regions at the
junctions of layers. In the prior art, a well-defined demarcation
line is formed between the sequentially cast layers. A drastic
change in pore size going from a more open to a more tight
structure can lead to undesirable fast accumulation of retained
solute at the interface and consequently a drastic flux decline.
Possibly due to partial mixing of adjacent co-cast lacquers or due
to high shear forces at the interface between two adjacent co-cast
lacquers, a sharp interface can be replaced by a more subtle change
in pore size between two adjacent layers with a cocast process.
Such an interfacial zone is beneficial for the retentive behavior
of the overall structure of the multilayered membrane and is
therefore preferable in some applications. However even with the
cocast technique one can, if desired, form a sharp or well-defined
demarcation line between layers with the proper selection of
materials and application methodologies.
[0052] The membranes of the present invention are preferably
produced using a premetered coating process. Premetered coatings
are those in which the exact amount of coating solution to be
deposited is fed to the coating head. The height of the layers are
set by the deposition rather than by some post application means
such as a doctor blade which sets the thickness of the structure
after metering of the layers (commonly referred to as "post
metering process"). The premetered term is applied to die coating,
slide and curtain coating among other methods of forming the
structure. The present invention preferably uses a double knife box
or double slot die. Post metered applications can also be used if
desired.
[0053] FIG. 1 illustrates a multiple layer forming apparatus 10 for
casting multilayered membranes. As shown, the apparatus is designed
to produce a two-layered liquid film and has two chambers 50 and 60
containing the solutions 14 and 16, one for each layer, to be cast.
If desired, additional chambers may be added to form additional
co-cast layers. The apparatus comprises a front wall 20 and a back
wall 40 with a separating wall 30 between the front and back walls.
The separating wall defines the volumes of the two chambers. Two
side walls, not shown, complete the apparatus. In operation, the
apparatus is fastened onto a typical membrane casting machine, and
a support web 18 is moved or passed under the stationary apparatus
and the two solutions are dispensed through gaps or outlets 80 and
90. The thickness of the two layers is controlled by the distance
(gap) set between the moving web and the outlet, illustrated by gap
settings 80 and 90. The final liquid layer thickness is a function
of gap distance, solution viscosities, and web speed. The back wall
of the apparatus usually is held a small distance above the support
to prevent wrinkling or marring the support. Back wall gap, support
speed and solution viscosity are adjusted in practice to prevent
solution from leaking out through the back wall gap. The apparatus
can be fitted with heating or cooling means for each chamber
separately, or for the apparatus as a whole, if necessitated by the
solution characteristics, or to further control final membrane
properties.
[0054] A slot die consists of an enclosed reservoir with an exit
slot having a smaller cross-section. An extruder or positive
displacement pump, or in some cases a pressurized vessel feeds the
coating into the reservoir at a uniform rate, and all of the fluid
that goes into the die is forced out from a reservoir through a
slot by pressure, and transferred to a moving carrier web. The slot
is positioned perpendicular to the moving carrier web. Multiple
layer coatings require a die with individual reservoirs, and
associated feed method, and exit slots for each layer.
[0055] After the layers are coated onto the moving carrier, the
carrier with the liquid sheet is immersed into a liquid that is a
nonsolvent for the polymer, and miscible with the solvent and
porogens. This will cause phase separation and the formation of a
porous membrane.
[0056] The formed composite membrane is then usually separated from
the carrier and washed to remove residual solvent and other
material. The membrane can then be dried. Ultrafiltration membranes
are usually dried with a humectant, such as glycerine, by first
immersing the washed membrane in an aqueous glycerine solution, of
from 5% to 25% concentration by weight, and removing excess liquid,
before proceeding through the drying step. Drying is done in a
manner to remove the majority of the water and to leave sufficient
glycerine to prevent pore collapse.
[0057] In the coagulation of a multilayered liquid sheet,
coagulation occurs from the liquid film surface that first contacts
the coagulation bath and then through the subsequent layers of the
multilayered liquid sheet. Each layer dilutes and changes the
coagulant as the coagulant diffuses through the layers. Such
changes to the nature of the coagulant affect the membrane
formation of each layer and of the final multilayer membrane. Layer
thickness, composition and location of each layer relative to the
other layers will affect membrane structure and properties. Each
layer forms differently than it would if it were to be made from a
single layer solution or from laminates of single layers.
[0058] In another embodiment, the two or more layers are
sequentially cast successively on to the prior cast layer with some
time between each cast so that some phase separation may occur in
the earlier cast layer. All other steps of the process are the same
as for those described with the cocast embodiment. This embodiment
of sequential casting allows one to form UF containing structures
similar to embodiments using only cocasting methods.
[0059] Reference is made to FIGS. 2a and 2b as an aid in the
description of these multilayered membranes. It is common
convention of those skilled in the art to denote as the "top
surface" of an asymmetric membrane the facial surface having the
smallest pore size. We will use this convention as a basis. For the
case of multilayered membranes having an skinned asymmetric
ultrafiltration membrane layer with no other layers contacting the
skinned surface, the skinned asymmetric ultrafiltration membrane
layer will be the first layer, or top layer, and subsequent layers
numbered two, three, etc. This is illustrated in FIG. 2a. To be
consistent, for the case where a microporous membrane is the top
layer over an asymmetric ultrafiltration layer, the microporous
layer will be denoted as the first layer, the ultrafiltration layer
the second layer and so on. This is illustrated in FIG. 2b. Another
manner of equivalently describing the nomenclature is to denote
that the first side will be the top layer, i.e., the last solution
coated onto the carrier, of the multilayered liquid sheet that has
been cast.
[0060] The multilayered membrane of the present invention is not
the same as an additive series of equivalently made single layer
membranes. Due to the integral joining of the layers, there is a
region where the pore size transitions from one layer to the next.
To describe the structures, we will use the following device, with
a two layer membrane as an example. A single layer membrane
consists of a first side, a second side, and a porous structure
between. Similarly, a laminate of two membranes would consist of a
first layer with a first side, a second side, and a porous
structure between, and a second layer with a first side, a second
side, and a porous structure between. For a two layer membrane of
the present invention, the first layer has a first side, and a
second equivalent side. The second equivalent side would be a
second side if this layer was a single layer membrane, but here it
is part of the integral joining of the two layers. Likewise, the
second layer has an equivalent first side and a second side and a
porous structure between. The two equivalent sides are conjoined to
form the conjoined thickness, that is, the transition zone between
the two layers.
[0061] Asymmetric ultrafiltration membranes are sometimes used in
dead-end filtration with the open or large pore surface at the
upstream or high pressure side. An important application for such
use is in removal of viral particles from process solutions in the
manufacture of biotech therapeutic drugs. This is described in U.S.
patent application Ser. No. 10/145,939.
[0062] The advantage of dead-end filtration lies in its simplicity.
The pressurized feed stream is contacted with one side of the
membrane and the fluid passes through while the material to be
removed is retained by the membrane. In comparison, in tangential
flow filtration (TFF), the pressurized feed stream is directed
tangentially across the membrane face, and a portion of the feed
stream passes through the membrane, while the remainder, the
retentate, is usually recycled with added make-up feed, or returned
to the feed tank. TFF requires extra pumping equipment, and more
controllers to maintain the proper ratios of flows and pressure.
However, dead-end filtration with ultrafiltration membranes has not
been commonly used because the membranes tended to lose permeation
properties too quickly to be useful.
[0063] The inventors have found that the multilayer ultrafiltration
membranes of the present invention have greatly improved properties
over prior art ultrafiltration membranes. The apparent reason for
this improvement lies in the structure of the membranes, although
no limitation should be put on the scope of the invention by the
following discussion. It appears to the inventors that the pore
size transition in the conjoining region plays a key role in the
improved properties.
[0064] This is illustrated in FIG. 3 in which an example of a
membrane of the present invention, a skinned asymmetric
ultrafiltration membrane conjoined to a microporous layer, is
compared to a two layer polyvinylidene fluoride (PVDF)
ultrafiltration membrane made by casting an ultrafiltration layer
on a pre-formed microporous membrane, (Viresolve.RTM. membrane
available from Millipore Corporation of Billerica, Mass.).
[0065] FIG. 3 shows crossectional views of the membranes after
having been used to filter fluorescent polystyrene beads from the
open pore side in a dead-end mode. Three tests were done with bead
sizes of 31 nm, 60 nm, and 170 nm.
[0066] For testing done with the 31 nm particles with the membrane
of the present invention, the filtered particles are distributed
throughout the thickness of the ultrafiltration layer. However, for
the two layer PVDF, the particles are concentrated just under the
small pore surface of the membranes. Since this surface provides
the limiting pore size for flow, the concentrated particle layer
will be more likely to plug these pores and will have a more
deleterious effect on permeation.
[0067] For testing with the 60 nm particles, the particles are held
away from the ultrafiltration layer and are diffusely distributed
in the conjoining region. For the two layer PVDF, the particles
form a concentrated layer near or at the junction of the
microporous substrate and the ultrafiltration layer for the PVDF
membrane.
[0068] Similar results are seen for the 170 nm particle testing.
The membrane of the present invention retains the particles in a
diffuse layer away from the skin. The two layer PVDF again traps
the particles in a dense layer at the junction of the two
layers.
[0069] In all these cases, the membrane of the present invention
traps particles in a diffuse manner, which spread out the effects
of pore plugging and increase filter flow and lifetime.
[0070] In an embodiment of the present invention, we use the
teachings of U.S. Pat. No. 5,444,097 ('097) in a novel manner. The
'097 patent teaches the use a polymeric solution exhibiting a lower
critical solution temperature (LCST) to make microporous membranes.
Heating an LCST solution above the LCST causes phase separation.
This step is incorporated in the process of the present invention
after the multilayered liquid film is formed to further vary and
control the structures of the resulting membrane layers. One or
several of the solutions of the present invention would be a LCST
solution. It has been found that the temperature to which the
solution is raised above the LCST, and the time the solution is
held above LCST, controls the final pore size of the membrane
layer. Furthermore, if there is a temperature gradient in a liquid
layer, then there will be a corresponding pore size gradient.
[0071] In the present invention, the use of LCST solutions is used
to produce a variety of structures.
[0072] For an bilayer composite ultrafiltration membrane made using
LCST solutions having a first layer of a skinned asymmetric layer
on a second microporous layer, a preferred solution for the first
layer will have a polymer content of from about 15% to about 30%
polymer solids, with a more preferred range of from about 20% to
about 25% polymer solids. All percentages related to solutions are
by % weight of the solution. For the microporous layer, the polymer
solution will have a polymer content of from about 10% to about 20%
polymer solids, with a more preferred range of from about 15% to
about 18% polymer solids by weight of the solution. The LCST of the
first layer solution is preferably from about 700 to about
150.degree. C. For the second layer, the LCST range is preferably
from about 40.degree. to about 60.degree. C. The thickness of the
first layer is from about 2 microns to about 100 microns,
preferably 2 microns to about 50 microns, with a more preferred
range from about 2 microns to about 25 microns. The microporous
second layer has a thickness range of from about 50 microns to
about 200 microns, with a preferred thickness of from about 80
microns to about 150 microns, with a more preferred range of from
about 100 microns to about 125 microns. It is preferable that the
total thickness of the cocast composite membrane be in the range of
from about 52 microns to about 300 microns, preferably from about
75 microns to about 200 microns, with a more preferred range of 90
microns to about 120 microns. If the pore size is determined by the
temperature to which the LCST solution is raised above LCST, and
the time maintained above LCST, the practitioner will determine by
routine trial and error the proper conditions for operating their
particular process equipment. Heating the solution can be done by
several methods. The support coated with the polymer solution
layers can be conveyed over a heated surface, such as a flat plate,
a block, or a rod. A preferable method is to use a rotating heated
drum. Heating can also be done by non-contact methods such as for
example, infrared heating or microwave energy. If a heated drum is
used to raise temperature of the coated web, the thickness and
thermal insulating properties of the carrier web, and thickness of
the polymer solution will be germane to obtaining a desired pore
size. The temperature of the drum and the speed of the process will
then be determined and controlled to produce the desired membrane.
The temperature of the heated surface is determined by the
equipment and the manufacturing process conditions as described
above.
[0073] For the case of a microporous first layer and a second
ultrafiltration layer, a preferred solution for the first layer
will have a polymer content of from about 10% to about 20% polymer
solids, with a more preferred range of from about 12% to about 16%
polymer solids. All percentages related to solutions are by %
weight of the solution. For the second ultrafiltration layer, the
polymer solution will have a polymer content of from about 15% to
about 30% polymer solids, with a more preferred range of from about
20% to about 25% polymer solids. The LCST of the first layer
solution is preferably from about 40.degree. to about 60.degree. C.
For the second layer, the LCST range is preferably from about
70.degree. to about 120.degree. C. The thickness of the first layer
is from about 2 microns to about 50 microns, with a more preferred
range from about 5 microns to about 25 microns. The ultrafiltration
membrane second layer has a preferred thickness of from about 80
microns to about 150 microns, with a more preferred range of from
about 100 microns to about 125 microns. It is preferable that the
total thickness be in the range of from about 90 microns to about
120 microns. Similar to the above case, the practitioner will
determine by routine trial and error the proper conditions for
operating their equipment.
[0074] If it is desired to make two layers from ultrafiltration
forming solutions, the parameters above will serve as guides to the
individual layer compositions and process parameters.
[0075] In an embodiment, the first layer is formed from a solution
and under conditions that would give a skinned asymmetric
ultrafiltration membrane if cast as one layer. The second layer
would give a microporous membrane if cast as one layer. The
resulting structure is a skinned asymmetric ultrafiltration
membrane on a microporous layer, with an integral transition zone
between them. In a preferred method, both solutions from which the
layers are cast have a LCST, with the ultrafiltration layer having
a higher LCST. The cast multilayered liquid sheet is heated to a
preplanned temperature above the LCST of the second (microporous)
layer but below the LCST of the first (ultrafiltration) layer
before immersion into the precipitation bath. This has been found
to result in an ultrafiltration layer over a microporous layer with
a transition zone between.
[0076] A preferred version of this embodiment can be done where the
ultrafiltration layer does not have a LCST, or does not have a
measurable LCST, while the microporous layer solution has a LCST,
and the same general structure will result.
[0077] It is also possible to use two solutions, neither of which
have a LCST, but which individually would make the combination of
ultrafiltration and microporous layers required.
[0078] In an embodiment illustrated by Example 3, the membrane is
formed from a top layer ultrafiltration membrane made from a
solution with an LCST higher than the drum temperature used to heat
the formed solution layers before immersion, and a bottom layer
made from a solution having an LCST lower than the drum
temperature. In this example, the LCST of the UF layer solution is
assumed to be greater than 150.degree. C. because it could not be
measured due to limitations of the test equipment. Surprisingly, as
the drum temperature approximately equals the LCST of the
microporous layer solution, the gradient between the two layers
becomes less observable. However, the composite membrane so-formed
out-performs a two layered membrane made by casting an
ultrafiltration layer on a preformed microporous membrane
(Viresolve, Millipore Corporation). Without being limited by the
following, it is the inventors present theory that the Viresolve
membrane-making process results in interpenetration of the top
layer into the bottom layer, which gives the type of results
discussed in relation to FIG. 3. However, the membrane of Example
3, because it is formed in a single step, does not have the same
type of "bottleneck" at the interface of the two layers. In fact,
it has a gradient, which, albeit sharp, still functions as
described herein as a membrane of the present invention.
[0079] In an embodiment, the first layer is a microporous layer
preferably thin, that is, between 5 to 30 microns thick, and the
second layer is made from a solution and under conditions that
would produce an ultrafiltration layer. In a highly preferred
embodiment, the microporous and ultrafiltration layers are produced
from solutions having a LCST, with the LCST of the ultrafiltration
layer solution being higher. When heated above the LCST of the
microporous layer solution, but below that of the ultrafiltration
layer solution, the microporous layer will phase separate.
Subsequent immersion will fix the microporous structure and cause
phase separation of the ultrafiltration solution to form the
multilayered membrane. A preferred version of this embodiment can
be done where the ultrafiltration layer does not have a LCST, while
the microporous layer solution has a LCST, and the same general
structure will result. It is also possible to use two solutions,
neither of which have a LCST, but which individually would make the
combination of ultrafiltration and microporous layers required.
[0080] In an embodiment, the multilayered ultrafiltration membrane
is made of two layers of ultrafiltration membrane-making solutions
that would, if cast as single layers, produce skinned asymmetric
ultrafiltration membranes.
[0081] In a similar manner, solutions with an upper critical
solution temperature (UCST) which phase separate when cooled below
the UCST can be used to make the inventive membranes, are formed
into a multilayered liquid film in a heated state and cooled to
obtain phase separation. In both the LCST and UCST embodiments,
further phase separation can be provided by immersion into a
coagulant, as described previously.
[0082] Control of the transition zone or region is important for
the present invention. In order to get a useful transition zone,
the inventors have found that it is desirable to control the
thickness of each layer, in particular the first layer, as well as
the relative viscosities of the two solutions, so that the
viscosity difference is not too great, and the relative time of
formation, that is, solidification, of the layers. The variables
above will serve as a guideline for other practitioners, but it
must be appreciated that for each set of solutions and the
particular equipment used, there may be differences from those
stated within the present description.
[0083] The present invention provides a high-resolution
membrane-based method for removing a virus from a manufactured
protein-containing solution, the method being particularly
characterized by its capacity to be performed quickly (i.e., as
measure by flux) and efficiently (i.e., as measured by log
reduction value, LRV).
[0084] Conduct of the methodology involves flowing a manufactured
protein-containing solution through a filtration device containing
the composite ultrafiltration membranes of the present invention
under conditions sufficient to effect passage of said protein
through said composite membranes, and whereby any
specifically-targeted virus contaminating said protein-containing
solution, is substantially prevented from passing through said
asymmetric membranes, is thereby substantially removed from the
solution.
[0085] A "manufactured protein-containing solution" as used herein
is a term of specific definition. In contrast to a solution having
naturally-occurring protein content (e.g., water having
naturally-occurring microbial content), the protein content in a
"manufactured" solution will be enriched, as a result of human
intervention and possible conduct of other solution refinement
processes, such that the predominant solute in said solution is
said protein.
[0086] In respect of the composite membranes, several criteria need
to be present to perform the inventive methodology. First, each
must be substantially hydrophilic. Secondly, the composite
membranes must be capable of substantially preventing the passage
therethrough of the targeted virus, whilst substantially permitting
the passage therethrough of the bio-manufactured protein
[0087] Aside from, but relevant to, the virus removal methodology,
the present invention also provides a filtration capsule comprising
a pleated tube formed of one, two or three interfacially-contiguous
composite ultrafiltration membranes. Although perhaps having
applicability elsewhere, this product configuration has been found
quite effective in the conduct of the inventive virus removal
methodology, in respect of its durability, reliability, cost, and
ease of use and replacement.
[0088] In light of the above, it is an objective of the present
invention to provide a methodology for removing at a high
resolution a virus from a manufactured protein-containing solution,
and particularly, one capable of being performed effectively at a
log reduction value of greater than 6 for a comparatively large
virus (e.g., murine leukemia virus) or from a comparatively smaller
virus (e.g., parvo virus).
[0089] It is another objective of the present invention to provide
a filtration capsule useful for conducting said virus removal
methodology.
[0090] It is another object of the present invention to provide a
device for removing a virus from a solution, the device comprising
a housing suitable for containing a filtration material and further
characterized by an inlet for receiving fluid to be filtered and an
outlet for removing filtrate, the filtration material comprising
one, two or three composite void-free membranes, the upstream layer
oriented such that its "tightest" side faces downstream.
[0091] In general, it has been found that by incorporation of
multiple asymmetric ultrafiltration membranes, arranged in a
pleated configuration with the membranes in "tight side down
stream" orientation, the resulting filter capsule will have good
viral retention capabilities, yet maintain good flux. Although
these may not be as high without using all the teaching underlying
the inventive methodology, such high degree of accomplishment
(particularly with respect to viral retention) is not always
required in all circumstances. For example, for certain
non-pharmaceutical purification applications, log viral reduction
values need not approach a value greater than 2.
[0092] As to its preferred structure, the filtration capsule
comprises a tubular housing and a pleated filtration tube
substantially co-axially enclosed within said housing. The tubular
housing of the filtration capsule is constructed to contain and
channel a fluid process stream conducted there through--and
accordingly is provided with a fluid inlet and a filtrate outlet.
The fluid process stream u, upstream of the pleated filtration
tube, is introduced into the filtration capsule through the fluid
inlet. Downstream of the pleated filtration tube, the fluid process
stream d is released from the filtration capsule through filtrate
outlet.
[0093] The materials used for the tubular housing will depend
largely on its intended application. Injection-moldable
thermoplastic materials such as polyethylene polypropylene and the
like are the most likely candidates. However, the use of metals,
glass, and ceramics are also contemplated. If sought for use in
viral clearance of biopharmaceutical protein products, the material
selected should be compatible with the fluids (e.g., solvents) and
environmental parameters (e.g., temperature and pressure) involved
therein, and should have low protein-binding characteristics. A
preferred material in this regard is polypropylene.
[0094] Because filtration devices, in general, often need to
satisfy several structural and functional criteria in the course of
most filtration protocols, it is unlikely that its overall
construction, including its housing and any internal components,
will be simple. Although a single continuous and unitary structure
is possible, in all likelihood the tubular housing will comprise
several cooperating assembled parts which typically include a
tubular housing that comprises an upper shell and one or two end
caps.
[0095] The pleated filter tube is positioned within the tubular
housing such that it will divide, in operation, the fluid process
stream that flows between the fluid inlet and the filtrate outlet.
The pleated filter tube is composed of at least one layer of the
asymmetric membranes of the present invention. Preferably, the one
or more layers are all oriented such that fluid introduced into
said housing through the fluid inlet commences passage through each
respective asymmetric membrane through its open-side.
[0096] The pleats of the filter tube can be configured in a
corrugated shape or spirally positioned and can have a loop-shaped
cross section or a folded cross-section, such as a W-shaped
cross-section. As used herein, the term "pleat" or "pleated" is
intended to include all such cross-sectional shapes. Relative to
occupied volume, the pleated structure presents to an incoming
fluid process flow more surface area than that which would be
presented by use of flat sheet. This is of particular advantage in
consideration of the desire to maximize flux, especially when
dealing with high-resolution viral clearance protocols.
[0097] The pleated filter tube is packaged within a replaceable
cartridge. While it is possible, at least conceptually, to place
pleated filter tube within the filter capsule without the agency of
a cartridge, replaceable or otherwise, in practice, commercial and
environmental advantages are realized by allowing the possibility
of easily replacing a spent pleated filter tube, without having to
undergo burdensome and/or cumbersome dismantling procedures, and/or
requiring disposal of an entire filter capsule. The replacement is
performed by unscrewing end cap from upper shell, unplugging a
spent filter cartridge from the filtrate outlet to which it is
frictionally mated, plugging therein a fresh cartridge, and
screwing the cap back on.
[0098] The one or more tubular pleated sheets are maintained in a
relatively fixed tubular conformation within the filter capsule by
use of the external and internal supports that together form the
replaceable cartridge. These supports are made of rigid material
and provided with uniformly dispersed holes to allow the inward
flow i of fluid from regions peripheral to the pleated filter tube,
through the membranes thereof into tube's core, and then ultimately
out of filter capsule.
[0099] For further details regarding the construction and functions
of a replaceable filter cartridge, reference can be made to U.S.
Pat. No. 5,736,044, issued to S. Proulx et al. on Apr. 7, 1998.
Among other subject matter, the patent describes a composite filter
cartridge that includes both sheet membranes and depth filters.
Aspects of such composite filter can be imported into the
construction of the present filter capsule, without departing from
the spirit and scope of the invention as defined herein.
[0100] To remove virus from a protein solution, a solution
containing protein(s) of interest and one or more types of viruses
and subjecting the solution to a filtration step utilizing one or
more ultrafiltration membranes which can be conducted either in the
TFF mode or the NFF mode. In either mode, the filtration is
conducted under conditions to retain the virus generally having a
20 to 100 nanometer (nm) diameter on the membrane while permitting
passage of protein(s) through the membrane. In addition, when
filtration of the solution is completed, the membrane is flushed
with water or an aqueous buffer solution to remove any retained
proteins. The use of the flushing step permits obtaining high yield
of protein solution substantially free of virus.
EXAMPLES
[0101] Cloud Point
[0102] The visual cloudpoint temperature is used to approximate the
lower critical solution temperature for a polymeric solution of a
given composition. This is the temperature at which a polymeric
solution phase separates from one phase into two phases upon
heating.
[0103] The procedure involves heating a small lacquer sample
enclosed in a transparent container in a heating bath and observing
the temperature at which the solution begins to turn cloudy. The
procedure is performed slowly enough to ensure that the temperature
indicated by a thermometer in the bath is the same as that in the
lacquer sample.
[0104] Auto Ramp Bubble Point
[0105] The ABP Tester is an automated pressure-ramping device used
for measuring bubble points on ultrafiltration and microporous
membranes. The ABP bubble point is the "foam-all-over" pressure,
visually observed by the operator.
[0106] Vmax
[0107] Vmax is a measure of the amount of solution membrane can
filter before being plugged so that the flux is reduced to
approximately zero flow. Vmax is measured by filtering a solution
at a predetermined pressure and recording the volume filtered as a
function of time. Time divided by volume is plotted versus volume.
The inverse of the slope is Vmax.
[0108] Viral Particle Retention Testing
[0109] Testing was carried out using a single 47 mm disk in a
stainless steel holder (Millipore, Billerica, Mass.) cat # XX44 047
00) at a constant pressure of 30 psid, and data was collected
automatically through a computer data acquisition package.
Membranes were wet out with Milli-RO water (Millipore Corporation,
Billerica, Mass.). All trials began with a buffer flush for 2-5
minutes to equilibrate the membrane and determine permeability.
Membranes were run with their open pore side to the feed pressure.
All candidates were tested with a solution containing 1 mg/mL human
plasma IgG (Bayer, lot # 648U035) and 10.sup.7 pfu/mL Phi-X174
(Promega, cat # I1041 m lot # 7731801) in 10 mM acetate buffer, pH
5.0. Challenge particles of bacterial phage, Phi X 174 were assayed
by a plaque assay using their host bacteria. A dilution series was
generated to determine concentration. LRV was calculated as the
negative logarithm of the ratio of permeate concentration to feed
concentration.
Example 1
[0110] In the examples, solution preparation was done as
follows.
[0111] Polyethersulfone (PES) membranes were cast from a solution
consisting of the polymer, Polyethersulfone (PES), Radel A200 resin
(Solvay) solvent, N-methylpyrrolidone (NMP) and non solvent,
triethylene glycol (TEG). The solution was homogeneous at room
temperature but phase separates when heated. The temperature at
which the solution starts to phase separate called the cloud point
temperature and was a function of the composition of the solution
and extremely sensitive to the concentration of water. It is
important to minimize the exposure of the raw materials, especially
TEG, and the final solution to the atmosphere. The polymer was
predried at for example 150.degree. C. for three hours.
[0112] The mix was made in 2 steps. First, the polymer was added to
a mixture of all the NMP and only part of the TEG. This portion was
mixed while heated to between about 50.degree. C. and 80.degree. C.
until the solution is clear. The temperature was lowered to between
about 30.degree. C. and about 50.degree. C. The remaining TEG was
then added to form the final solution.
[0113] A first polymer solution was prepared by dissolving 17% PES
(Radel A200) in 29.2% NMP and 53.8% TEG. The resulting cloud point
was 50.2.degree. C.
[0114] A second polymer solution was prepared by dissolving 22% PES
(Radel A200) in 28.1% NMP and 49.9% TEG. Additional NMP was added
(6.3% of final solution) to arrive at a cloud point of 83.6.degree.
C.
[0115] The two solutions were co-cast as described in WO 01/89673
(Kools), using a slot die coater. The cast thickness of the first
solution was adjusted to give a final layer thickness of 145 .mu.m.
The cast thickness of the second solution was adjusted to give a
final layer thickness of 15 .mu.m or about 10% of the overall
membrane thickness.
[0116] The formation conditions are selected so that the first
solution was quickly heated on the casting drum, a temperature
above its cloud point, before the point of immersion into an
aqueous immersion bath at 55.degree. C. At the same time the second
solution does not reach a temperature of its cloud point. As a
result, a formation of a microporous layer resulted from the first
polymer solution and a formation of an ultrafiltration layer
resulted from the second polymer solution. The final membrane
characteristics were varied by adjusting the drum temperature, and
the thickness of the second layer was minimized in order to avoid
undesirable macrovoid formation.
[0117] The resulting structures and properties are shown below. The
retentive nature is indicated by the high bubble point, while the
void-free UF layer and dense UF surface can be clearly seen in the
scanning electron microscope images (FIG. 4a. and FIG. 4b) (Drum
temp 45.degree. C. is shown). The transition from UF to MP is less
clear, which could be advantageous for maximizing throughput.
TABLE-US-00001 Drum Bubble Phi-X 174 Sample Temp (.degree. C.)
Point (psi) Retention (LRV) 1 58 102 2 56 109 0.4 3 55 108 0.5 4 50
111 1.6 5 45 112 2.8
[0118] The data in the Table show increased virus retention with
decreasing drum temperature. This effect of drum temperature is
unexpected because the retentive UF layer has a LCST much above any
of the drum temperatures used, and it is not expected that heating
the UF solution to this degree would have any effect on membrane
formation. However, as can be seen form the data, reducing the drum
temperature from 58.degree. C. to 45.degree. C. increased virus
retention by more than two orders of magnitude.
[0119] Sample 5 was compared to a two layer membrane made by
casting an ultrafiltration layer onto a preformed microporous
membrane to form a composite membrane with two distinct layers
(Millipore PPVG membrane). The results below show that for similar
permeability and BAP bubble points, the membrane of the present
invention (Sample 5) had greatly improved Vmax properties and
better virus removal.
TABLE-US-00002 Flux ABP liters/sq. Phi-X Membrane Bubble point
meter/hr/psi LRV Vmax Sample 5 112 58 2.8 7717 PPVG 128 45.6 2.3
437
Example 2
[0120] A first polymer solution was prepared by dissolving 22% PES
(Radel A200 resin) in 28.1% NMP and 49.9% TEG. The resulting cloud
point was 48.6.degree. C.
[0121] A second polymer solution was prepared by dissolving 14% PES
(Radel A200 resin) in 29.2% NMP and 56.8% TEG. An additional 3% of
NMP was added to arrive at a cloud point of 59.3.degree. C.
[0122] The two solutions were co-cast as described in Example 1.
The cast thickness of the first solution was adjusted to give a
final layer thickness of 140 .mu.m. The cast thickness of the
second solution was adjusted to give a final layer thickness of 13
.mu.m or 8% of the overall membrane thickness.
[0123] The formation conditions were selected so that the first
solution was quickly exposed to the heated drum at 55.degree. C.,
which was above its cloud point, before the point of immersion into
an aqueous immersion bath at 45.degree. C., which was below its
cloud point. As a result, a formation of a macrovoid-free UF layer
resulted from the first polymer solution and a formation of a thin
microporous layer resulted from the second polymer solution.
[0124] The resulting bubble point was relatively high, and it is
assumed that higher levels can be attained by additional variances
in process conditions. The resulting structures are shown below,
where the void-free UF layer and open MP surface can be clearly
seen in the scanning electron microscope images (FIGS. 4c and
4d).
[0125] It was very surprising to the inventors that these
conditions gave a membrane with no macrovoids and a very porous
microporous surface. The drum temperature was above the LCST of the
ultrafiltration layer, but below the LCST of the microporous layer.
(Drum temperatures below the LCST of the ultrafiltration layer gave
an ultrafiltration layer with voids.) However, the surface of the
membrane of Example 2 showed a very high surface porosity.
Example 3
[0126] A first polymer solution was prepared by dissolving 17% PES
(Radel A200) in 29.2% NMP and 53.8% TEG. The resulting cloud point
was 43.degree. C. This would be the bottom or support microporous
layer.
[0127] A second polymer solution was prepared by dissolving 21% PES
(Radel A200) in 37% NMP and 42% TEG. The cloud point could not be
measured, being above 150.degree. C. This would be the top
ultrafiltration layer.
[0128] The two solutions were co-cast as described in WO 01/89673
(Kools). The second solution layered on the first solution. The
cast thickness of the first solution was adjusted to give a final
microporous membrane layer thickness of 160 .mu.m. The cast
thickness of the second solution was approximately 30 .mu.m or
about 20% of the overall membrane thickness.
[0129] The formation conditions were selected so that the layered
solutions were quickly heated on the casting drum, at a range of
temperatures around its cloud point, before the point of immersion
into an aqueous immersion bath at 55.degree. C. The second solution
did not reach a temperature of its cloud point. As a result, a
formation of a microporous layer resulted from the first polymer
solution and a formation of an ultrafiltration layer resulted from
the second polymer solution. The final membrane characteristics
were varied by adjusting the drum temperature, and the thickness of
the second layer was minimized in order to avoid undesirable
macrovoid formation.
[0130] The resulting structure and properties are shown below. The
retentive nature is indicated by the high retention, while the
void-free UF layer and dense UF surface can be clearly seen in the
scanning electron microscope image (Drum 50.degree. C. is shown).
The transition from UF to MP is more observable than in example 1,
yet the resulting throughput was unaffected.
TABLE-US-00003 Parvovirus Drum Temp (.degree. C.) Retention (LRV)
Vmax 60 2.8 4110 55 3.7 2269 50 5.0 1119 45 5.1 228 40 5.5 204 35
2.7 75
[0131] As in Example 1, lowering the drum temperature increased
virus retention. The LRV at 35.degree. C. does not agree with the
trend seen in other experiments. These conditions were repeated and
the membrane produced had LRV of 5, and Vmax of .about.20.
Example 4
[0132] A first polymer solution was prepared by dissolving 18% PES
(Radel A200) in 30.2% NMP and 51.8% TEG. The resulting cloud point
was 56.degree. C. This would be the bottom or support microporous
layer.
[0133] A second polymer solution was prepared by dissolving 23% PES
(Radel A200) in 37% NMP and 42% TEG. The cloud point could not be
measured, being above 150.degree. C. This would be the top or
ultrafiltration layer.
[0134] The two solutions were co-cast as described in WO 01/89673
(Kools). The second solution layered on the first solution. The
cast thickness of the first solution was adjusted to give a final
microporous membrane layer thickness of 155 .mu.m. The cast
thickness of the second solution was approximately 10 .mu.m or
about 6% of the overall membrane thickness.
[0135] The formation conditions were selected so that the layered
solutions were quickly heated on the heated surface, at a range of
temperatures around its cloud point, before the point of immersion
into an aqueous immersion bath at 55.degree. C. The second solution
did not reach a temperature of its cloud point. As a result, a
formation of a microporous layer resulted from the first polymer
solution and a formation of an ultrafiltration layer resulted from
the second polymer solution. The final membrane characteristics
were varied by adjusting the surface temperature, and the thickness
of the second layer was minimized in order to avoid undesirable
macrovoid formation. The resulting properties were displayed below,
showing both relatively high retention and high Vmax at the higher
surface temperatures.
TABLE-US-00004 Parvovirus Surface Temp (.degree. C.) Retention
(LRV) Vmax 60 4.6 1774 55 4.5 1339 45 5.5 260
* * * * *