 |
Claims  |
|
|
What is claimed is:
1. A method for separating particulates from a colloidal suspension in
which plugging of a filter membrane is inhibited said method comprising:
inhibiting plugging of a membrane from a sample blood suspension, by
passing the suspension through the filter membrane, the filter membrane
selected to substantially block passage of the particulates; and
simultaneously inducing a relative oscillation between the filter membrane
and the suspension with a frequency and displacement selected to provide a
Reynold's number of at least about 1.
2. A method as in claim 1, wherein the filter membrane is oscillated while
the suspension remains substantially static.
3. A method as in claim 1, wherein the colloidal suspension is oscillated
while the filter membrane remains substantially static.
4. A method as in claim 1, wherein the suspension is passed through the
membrane filter by providing a negative pressure on a downstream side of
the membrane filter.
5. A method as in claim 1, wherein the suspension is passed through the
membrane filter by providing a positive pressure on an upstream side of
the membrane filter.
6. A method as in claim 1, wherein the frequency is in the range from about
25 to 250 Hz.
7. A method as in claim 6, wherein the displacement between the filter
membrane and the suspension is in the range from about 0.3 to 5 mm.
8. A method for separating plasma from blood samples with a filter
membrane, comprising: inhibiting plugging of the filter membrane, by
introducing a blood sample into a receptacle having the filter membrane in
contact with said blood sample;
providing a differential pressure across the membrane to cause the plasma
to flow therethrough;
simultaneously reciprocating the receptacle in a direction substantially
normal to the direction of blood plasma flow through the filter membrane
with a frequency and displacement sufficient to disperse a concentration
polarization layer at the membrane surface; and
collecting the blood plasma as it flows through the filter membrane.
9. A method as in claim 8, wherein the frequency and displacement of the
reciprocation are sufficient to provide a Reynold's number of at least
about 1.
10. A method as in claim 8, wherein the receptacle is an open capillary
tube and the blood sample is introduced by contacting a tip of the
capillary tube with the blood sample.
11. A method as in claim 10, wherein the blood sample is a drop of blood
drawn by lancing a patient's skin.
12. A method as in claim 10, wherein the filter membrane forms a portion of
a wall of the capillary and the differential pressure is provided by
drawing a negative pressure in a receiver jacket circumscribing the filter
membrane.
13. A method as in claim 8, wherein the receptacle is an enclosed bag
having flexible walls and wherein the filter membrane is mounted within
the bag and attached to a receiver having a port external to the bag, said
differential pressure being provided by drawing a negative pressure on the
port.
14. A method as in claim 8, wherein the receptacle is defined by a
cylindrical membrane filter and is mounted inside a rigid enclosure having
a pressure port, said differential pressure being applied by drawing a
negative pressure through said port.
15. A method as in claim 14, wherein the receptacle is rotationally
reciprocated about a cylindrical axis of the membrane filter.
16. A system for separating particulates from a colloidal blood suspension
in which plugging of a membrane filter by the particulated is inhibited,
said system comprising:
the filter membrane selected to block passage of the particulates in the
suspension;
means for passing the colloidal blood suspension through the membrane
filter;
means for collecting permeate after it has passed through the filter
membrane; and
means for inhibiting plugging of the membrane by the colloidal blood
suspension, including
means for inducing relative oscillation between the filter membrane and the
suspension with a frequency and displacement sufficient to provide a
Reynold's number of at least about 1.
17. A system as in claim 16, wherein the filter membrane has a molecular
weight cutoff in the range from about 0.1 to 1000 kilodaltons.
18. A system as in claim 16, wherein the means for passing the colloidal
suspension through the membrane filter is a differential pressure
generator connected across the filter membrane.
19. A system as in claim 16, wherein the differential pressure generator
applies a negative pressure on a downstream side of the membrane filter.
20. A system as in claim 19, wherein the means for collecting permeate
includes a receiver attached to a downstream side of the filter membrane
and wherein said receiver is attached to the negative pressure.
21. A system as in claim 16, wherein the differential pressure generator
applies a positive pressure on an upstream side of the membrane filter.
22. A system as in claim 16, wherein the means for inducing a relative
oscillation includes means for reciprocating the membrane filter in a
direction normal to the passage of the permeate therethrough.
23. A system as in claim 16, further comprising a receptacle for holding
the colloidal suspension on an upstream side of the membrane filter.
24. A system as in claim 23, wherein the means for inducing relative
oscillation includes means for applying an oscillatory pressure on the
colloidal suspension in the receptacle, whereby the colloidal suspension
is reciprocated past the filter membrane.
25. A system as in claim 16, wherein the means for inducing relative
oscillation operates at a frequency in the range from about 25 to 250 Hz
and imparts a displacement in the range from about 0.3 to 5 mm.
26. A system as in claim 25, wherein the means for inducing relative
oscillation operates with a frequency and displacement selected to provide
a Reynold's number greater than about 10.
27. An apparatus comprising: means for separating particles from a
colloidal blood suspension, including:
an assembly including a filter membrane having an upstream side and a
downstream side, a receiver attached to the downstream side to collect
fluid passing through the filter membrane, and means for connecting a
differential pressure across the membrane to induce fluid flow
thereacross; and
means for detachably mounting the assembly on an oscillator.
28. An apparatus as in claim 27, wherein the filter membrane is formed on
at least a portion of the inside surface of a capillary tube.
29. An apparatus as in claim 28, wherein the receiver is a rigid jacket
circumscribing the capillary tube.
30. An apparatus as in claim 29, wherein the means for connecting a
differential pressure is a port in the rigid jacket.
31. An apparatus as in claim 27, wherein the filter membrane is a flat
sheet.
32. An apparatus as in claim 31, wherein the filter membrane includes a
pair of sheets sealed over a collection manifold which defines the
receiver.
33. An apparatus as in claim 27, wherein the filter membrane is cylindrical
and the receiver is a rigid enclosure circumscribing the filter membrane.
34. An apparatus as in claim 33, wherein the filter membrane is a hollow
porous fiber. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
The present invention relates generally to the separation of particulates
from colloidal suspensions, and more particularly to methods and apparatus
for the separation of cellular components from blood to provide blood
plasma.
The separation of particulates from colloidal suspensions is usually
accomplished by either centrifugation or filtration. While centrifugation
is relatively time consuming and requires relatively costly equipment, it
is presently the only technology practical for the separation of small
sample sizes, typically 5 mL and below. Filtration is substantially faster
and more economic, but colloidal particles cause severe fouling and
plugging of the filter membrane. Although such plugging can be at least
partially overcome by utilizing a continuous shear flow over the membrane
or pulsatile flow through the membrane, such approaches are not suitable
for the-filtration of small volumes. Thus, it would be desirable to
provide improved methods for the filtration of colloidal suspensions. It
would be particularly desirable if such methods allowed the filtration of
relatively small sample sizes below about 5 mL without plugging.
Most medical diagnostic testing is performed on small samples of blood
plasma or serum. As blood is a colloidal suspension of cellular components
in plasma, whole blood samples (which are generally on the order of
several mL) are presently clotted and centrifuged in order to obtain blood
serum prior to testing. Such clotting and centrifugation generally takes
about 10 minutes or longer and requires the use of a relatively costly
centrifuge. The use of filtration to prepare plasma from such small blood
samples would be desirable as it is a more rapid and less costly
procedure, generally requiring fewer steps than centrifugal separation of
serum. The inability of present filtration technology to adequately
separate such small volume colloidal suspensions, however, renders
filtration impractical.
For these reasons, it would be desirable to provide improved filtration
methods capable of separating particles from colloidal suspensions without
suffering from deleterious plugging of the filter membrane. It would be
particularly desirable if such methods could handle very small sample
volumes, preferably 5 mL and below, and more preferably 100 .mu.L and
below. The filtration methods should be rapid and require only relatively
inexpensive equipment to be performed. The methods should further be
suitable for automated sample handling, requiring a minimum number of
manual steps. In the case of plasma preparation from whole blood, it would
be desirable if user exposure to the blood could be limited or avoided
entirely, and that the plasma obtained be free from hemolysis and other
degradation.
SUMMARY OF THE INVENTION
The present invention provides improved methods and apparatus for the
filtration of colloidal suspensions. By inducing a relative oscillation
between the surface of a filter membrane and the bulk of the colloidal
suspension, plugging of the filter membrane is inhibited even for very
small suspension volumes on the order of 5 mL and below. In particular,
relative oscillation at a frequency and displacement sufficient to provide
a Reynold's number of at least about 1 has been found to substantially
inhibit the particulates in the suspension from forming a concentration
polarization layer (which results in plugging of the filter membrane)
allowing rapid separation of the colloidal suspension. Preferably, the
relative motion between the bulk suspension and the membrane surface is at
a frequency in the range from about 25 to 250 Hz, more preferably between
about 60 and 200 Hz, and usually between about 120 and 180 Hz, and a
maximum displacement in the range from about 0.3 to 5 mm, more preferably
between 0.5 and 3 mm, and usually between about 1 and 2 mm.
Relative motion between the filter membrane and the bulk suspension is
induced in a direction generally parallel to the membrane surface and
normal to the flow of permeate (filtered fluid) therethrough. Depending on
the membrane configuration, the reciprocation may be linear or rotational,
with either the membrane or the bulk solution being reciprocated. The
membrane will usually be reciprocated mechanically, although
electromagnetic induction might also find use. The bulk solution may
conveniently be reciprocated by applying an alternating pressure on
opposite sides of the suspension volume. It is usually preferred to
reciprocate the filter membrane as this provides the strongest shear force
at the membrane surface, resulting in maximal inhibition of plugging. Flow
through the filter is induced simultaneously with the relative
reciprocation of the membrane and bulk suspension, conveniently by
providing a differential pressure across the membrane.
A number of specific embodiments are set forth herein which are intended
primarily for separating plasma from whole blood. The embodiments
generally include a filter cartridge which is utilized in combination with
a base unit having an oscillator for inducing relative motion between a
filter membrane in the cartridge and the colloidal suspension. In its
simplest form, the filter cartridge is a probe intended for immersion into
an open volume of the colloidal suspension. In that case, the cartridge
will include an exposed filter membrane which is connected to a negative
pressure (vacuum) source for drawing fluid through the membrane while the
cartridge is being oscillated. More usually, however, the filter cartridge
will include a closed or open receptacle for holding a discrete sample of
the colloidal suspension in contact with a membrane filter. A receiver
mounted on the opposite side of the membrane filter collects the permeate
passing through the filter, as the relative oscillation is induced
simultaneously with a differential pressure being applied across the
membrane.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of the operating principle of the
present invention.
FIG. 2 is a schematic representation of an alternate operating principle
for the present invention.
FIG. 3 illustrates a first embodiment of a filter cartridge constructed in
accordance with the principles of the present invention employing a
capillary tube receptacle.
FIG. 4 illustrates a first approach for inducing relative oscillation
between a filter membrane and bulk suspension in the capillary tube filter
cartridge of FIG. 3.
FIG. 5 illustrates a second approach (as with FIG. 4) the capillary tube
filter cartridge of FIG. 3.
FIG. 6 illustrates a second embodiment of a filter cartridge constructed in
accordance with the principles of the present invention employing an
internal filter membrane disposed inside a flexible bag receptacle.
FIG. 7 is a perspective view of the internal filter membrane of FIG. 6
probe showing its construction in greater detail.
FIG. 8 is an exploded view of a third embodiment of a filter cartridge
constructed in accordance with the principles of the present invention
employing a disk filter membrane which is rotationally oscillated about a
central axis normal to its plane.
FIG. 9 illustrates the disk filter cartridge of FIG. 8 mounted in a base
unit capable of inducing a rotary oscillation.
FIG. 10 illustrates a disk filter cartridge similar to that of FIG. 8
intended for use on relatively large liquid volumes.
FIG. 11 illustrates a filter cartridge constructed in accordance with the
principles of the present invention and intended primarily for drawing
blood samples and separating the blood sample into plasma within the
cartridge.
FIG. 12 is an exploded view of the filter cartridge of FIG. 11.
FIG. 13 illustrates the filter cartridge of FIG. 11 during the plasma
separation process.
FIG. 14 is a graph illustrating experimentally obtained permeation rates as
a function of Reynold's number.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
According to the present invention, methods and apparatus are provided for
the improved filtering of colloidal suspensions substantially free from
plugging of the filter membrane which has heretofore been a problem. By
providing a rigorous shear flow at the interface between the filter and
the suspension, the concentration polarization layer primarily responsible
for plugging of the filter membrane is removed. The shear flow is caused
by oscillating the filter membrane relative to the suspension fluid at a
frequency and displacement selected so that the relative motion has a
Reynold's number greater than about 1.0. Preferably, the oscillatory
frequency will be in the range from about 25 to 250 Hz, more preferably
between about 60 and 200 Hz, usually being between about 120 and 180 Hz,
and the maximum displacement will be in the range from about 0.3 to 5 mm,
usually being in the range from about 0.5 to 3 mm, and usually being in
the range from about 1 to 2 mm. While it has been found that oscillation
of the membrane is most effective (as it provides a maximal shear flow at
the boundary between the suspension fluid and the membrane), oscillation
of the liquid suspension phase can also be effective, particularly in very
small systems such as the capillary tube embodiment described hereinbelow.
In either case, the relative oscillation is performed simultaneously with
passage of the liquid suspension through the filter membrane, typically by
providing a differential pressure across the membrane, in order to effect
the desired filtering.
Apparatus of the present invention will usually include a filter cartridge
assembly and a separate base unit for operating the filter cartridge
assembly, although it would be possible to construct devices incorporating
all components of the invention in a single unit. The filter cartridge
assembly comprises a filter membrane having an upstream side and a
downstream side, and a receiving chamber which is fluidly coupled to the
downstream side of the filter membrane to collect permeate passing
therethrough. Usually, a receptacle is provided on the upstream side of
the membrane for holding a discrete volume of the colloidal suspension for
filtration, although it is also possible to utilize the filter cartridge
as a probe by immersion into an open volume of the colloidal suspension.
The base unit of the present invention will provide the oscillatory force
required for effecting the relative oscillation between the filter
membrane and the liquid suspension and, optionally, the differential
pressure necessary for driving the permeate through the filter, although
some filter cartridges will be initially evacuated below atmospheric
pressure so that flow may be induced by exposing the receptacle to
atmospheric pressure Typically, the oscillatory force will be provided by
a mechanical linkage to the filter cartridge, although it would also be
possible to utilize electromagnetic coupling. Differential pressure across
the membrane is usually provided by the base unit which applies a negative
pressure on the downstream side of the membrane (usually through a
connection to the receiving chamber), a positive pressure to the upstream
side of the membrane (usually through a connection to the receptacle), or
both. Alternatively, in certain embodiments, having both a receptacle for
holding the colloidal suspension and a receiver for the permeate, flow
across the membrane may be induced by providing the filter cartridge with
both the receptacle and receiver evacuated to subatmospheric pressure.
After introducing the suspension to the receptacle, transmembrane flow is
caused by venting the receptacle to the atmosphere so that the permeate
enters the receiver which is still below atmospheric pressure. In certain
large volume embodiments, it might be possible to rely on static pressure
of the fluid in order to pass permeate through the membrane, although such
unassisted flow will usually not be preferred. In the case of oscillations
caused by reversing flow of the liquid suspension, the base unit will
normally provide additional pressure connections on the receptacle in
order to effect the desired flow reversals. Each of these aspects of the
present invention will be described in greater detail in connection with
the specific embodiments set forth hereinbelow.
Colloidal suspensions which may be separated by the present invention
include a wide variety of particulates suspended in a liquid phase. The
particulates may vary widely in size, typically having dimensions in the
range from about 10.sup.-3 to 10 microns, with separation of particles in
the size range from about 0.002 to 0.02 microns being referred to as
ultrafiltration and separation of particles in the size range from about
0.02 to 10 microns being referred to as microfiltration. The liquid phase
of the suspension may be aqueous or organic, usually being aqueous. The
nature of the suspension is not critical, with separation of cellular
components from blood and conditioned media being of primary present
interest.
In the case of blood, the cellular components are typically sized in the
range from about 1 to 8 microns, including erythrocytes, leukocytes,
platelets, and cellular debris. The plasma fraction remaining after
separation of the cellular components retains the proteins and other
soluble factors characteristic of the whole blood sample. In particular,
it has been found that the filtration of the present invention does not
result in hemolysis which is unacceptable in plasma used for many
purposes, including diagnosis and transfusion. In the case of conditioned
media, the cellular components may be filtered, leaving the desired
protein products in the permeate. The separation of the present invention
will frequently be employed to assist in assaying the conditioned media
for the content of the protein product and other factors.
The membrane utilized in the filter cartridge may be constructed from a
wide variety of materials depending on the mechanical strength and
chemical resistance required by the particular application. The membranes
will generally be porous, with the pore size chosen to inhibit passage of
the particulates to be separated while allowing the liquid phase of the
suspension and certain soluble factors, such as proteins, to pass through
in the permeate. Suitable materials include natural substances, such as
cellulose and natural rubber; organic polymers, including non-polar
polymers such as polyethylene, polypropylene, polycarbonate, nylon, and
the like, and polar polymers, such as polyamides; and inorganic
substances, such as sintered glass and ceramics The molecular weight
cutoff of the filter membrane may vary widely depending on the desired
separation, usually being in the range from about 0.1 to 1000 kilodaltons
(kD), more usually being in the range from about 1 to 500 kD.
The filter membranes may assume a wide variety of geometries, including
planar geometries, tubular geometries, spiral-wound geometries, and the
like. Conveniently, hollow fibers composed of organic polymers with inside
diameters as small as 10 microns may be utilized in certain applications.
The membranes may also be formed as composites, with different membrane
materials and/or different pore size materials being layered in order to
provide for particular separation characteristics. The fabrication of
membranes from any of these materials in virtually any of the geometries
set forth is well known in the art and amply described in the patent and
scientific literature.
For the separation of plasma from blood, the desired membrane material will
be polycarbonate with a pore size in the range from about 0.1 to 0.6
.mu.m. A 10 .mu.m thick polycarbonate membrane can be formed by track
etching pores of suitable dimensions. Using of such membranes avoids the
need to add surfactants for wetting. The presence of surfactants in the
separated plasma is undesirable for most purposes.
The present invention relies on providing a sufficiently vigorous shear
force at the membrane surface in order to disrupt the concentration
polarization layer which can result in plugging of the membrane filter.
The magnitude of the shear force may be quantitated by determination of
the Reynold's number (N.sub.Re) of the relative oscillation between the
membrane surface and the bulk of the colloidal suspension. The Reynold's
number for the system of the present invention may be calculated as
follows:
N.sub.Re =d(.omega..rho./.mu.).sup.0.5, where
d=maximum displacement of membrane relative to bulk suspension (cm), which
is equal to r.theta. for rotary oscillation where r is the radius of a
curved membrane and .theta. is the maximum angular displacement in
radians;
.omega.=frequency of oscillation of membrane relative to bulk suspension
(sec.sup.-1);
.rho.=density of colloidal suspension (g/cm.sup.3); and
.mu.=viscosity of colloidal suspension (g/cm-sec).
Based on this calculation, the system Reynold's number should be at least
about 1, preferably being at least about 2, more preferably being at least
about 5, and usually being 10 or greater, for most applications. While
there is no maximum theoretical Reynold's number, the system Reynold's
number will usually not exceed 250, more usually being below about 100.
In addition to the limitation on Reynold's number, it has been found that
the frequency (.omega.) of relative oscillation between the filter
membrane and the bulk suspension should be maintained in the range from
about 25 to 250 Hz, usually being in the range from about 60 to 200 Hz,
and preferably in the range from about 120 to 180 Hz and that the maximum
relative displacement (d) should be maintained in the range from about 0.3
to 5 mm, usually being in the range f | | |
