Liquid purification or separation – Casing divided by membrane into sections having inlet – Membrane movement during purification
Reexamination Certificate
2001-09-18
2002-12-10
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Casing divided by membrane into sections having inlet
Membrane movement during purification
C210S321680, C210S321870, C210S497010
Reexamination Certificate
active
06491819
ABSTRACT:
The present invention relates generally to the separation of particles suspended in solution based upon unique characteristics of the various particles such as the shape, size and/or deformability, and more particularly to the selective separation or filtration of cells, cell components or fragments thereof which have one or more various unique physical characteristics.
BACKGROUND OF THE INVENTION
Techniques for the separation of constituents of various medical/biological fluids, such as whole blood, are in wide use for many diagnostic, therapeutic, and other medically-related, applications. For example, centrifugal separation based upon the different densities and settling velocities of the constituent components to be separated is well known. The CS-3000 separator, sold by Baxter Healthcare Corporation of Deerfield, Ill., is one example of a centrifugal separator that has been very successfully used in separating whole blood into constituent components, such as red blood cells (RBC), white blood cells (WBC), platelets, and plasma for collection or depletion of the desired components from a donor or patient. While centrifugation has proven to be a generally satisfactory method of achieving separation, in certain applications the purity of the separated components is not as high as desired due to the very close and/or overlapping densities and settling velocities of the different suspended particles.
Mesh and aggregate structures and membranes are also used to remove particles from suspension. Typically such filters exhibit substantial surface area and/or roughness that can cause particle damage in biological suspensions, e.g., RBC hemolysis and platelet activation in blood.
Separation of biological fluids using a filter membrane with a nominal pore size is also common. For example, it is widely known that a filter membrane having 0.22 micron nominal pore size can be used to filter out assorted bacteria and the like from a liquid. Such membranes, also sometimes called capillary pore membranes, are available in polyester and polycarbonate material from, e.g., Nuclepore Corporation, and in polysulfone from Gelman Sciences, Inc. Such filter membranes have also been used to filter the cellular components of blood (sometimes called the “formed” components) from liquid plasma, i.e. “plasmapheresis.”
While these membranes have worked satisfactorily in certain applications, such filter membranes have only a nominal pore size, as distinguished from pores of precise and consistent size, shape, and relative spacing to one another. Indeed, it is not uncommon for such nominal pore size membranes to include “doublets” (i.e., overlapping, non-conforming pores) which would allow passage through the membrane of particles larger than the nominal pore size. To be useful in performing procedures in which particles in a solution are “cleansed” of undesirable particles, the undesirable particles being several times larger than the desired particles, filter membranes must exhibit virtually no doublets.
The occurrence of doublets in prior art filter membranes, due to their fabrication techniques, has forced a compromise in their design. Specifically, in order to keep the occurrence of doublets to an acceptable low level, the mean pore-to-pore spacing must be relatively large, which limits the porosity (i.e., the ratio of the total pore area to the total membrane area) of these prior art membranes to about 7% and less. Generally, a lower porosity results in a lower flow rate through the filter membrane. Thus, although a filter membrane having a nominal pore size is suitable for defining an average or nominal maximum particle size that passes through the filter membrane, such membranes are not precisely sized to permit selective filtration of particles of comparable size based on other unique characteristics such as shape or deformability, and have significant drawbacks that limit their application.
A further difficulty with membrane separation of biological and other fluids is impairment of flow through the membrane due to the fouling or clogging of the filter membrane. Such fouling or clogging generally results from the deposition on the surface of the filter membrane of particles too large to pass through the membrane and plugging of the pores. Various methods are known for reducing or preventing the clogging of such membranes. For example, U.S. Pat. No. 5,194,145 to Schoendorfer, herein incorporated by reference, discloses a “couette flow” filter system in which the extraction of filtrate is accomplished through a membrane mounted on a cylindrical rotor within a stationary cylindrical cell. The relative movement between the two concentric cylinders generates a surface velocity that establishes vigorous vortices at the surface of the rotor. These vortices, called Taylor vortices, constantly sweep the membrane surface to limit cell deposition, while continuously replenishing the medium to be filtered.
A different technique to reduce membrane fouling is disclosed in U.S. Pat. No. 4,735,726 to Duggins, herein incorporated by reference. This patent discloses a method and apparatus for carrying out plasmapheresis by conducting blood over the surface of a microporous membrane in reciprocatory pulsatile flow by a peristaltic oscillator or other suitable pump for causing reciprocatory pulsations.
More specifically, Duggins discloses a filter housing having a blood flow region between two plasma flow regions. A central blood inlet port is connected to the blood flow region of the housing, while a blood collection channel is connected to a plasma-depleted blood outlet port, and a plasma collection port is connected to a plasma outlet port. A pair of membranes is disposed between each plasma flow region so that there is a blood flow path between the membranes. Blood is conducted in a forward direction (i.e., away from its source) over the first surface of each filter membrane by, e.g., a rotary peristaltic pump, a piston or syringe-pump, or a plunger or hose pump. Blood flow is pulsed in a reciprocatory fashion by a peristaltic oscillator connected to the housing through ports connected to areas near the end of the flow path. As a result, blood can be conducted in the forward direction and in a reverse direction over a first surface of each membrane at a net positive transmembrane pressure, while reducing the transmembrane pressure during the forward and reverse conduction of the blood. The frequency and volume of the reciprocatory pulses are selected to maximize the flow of plasma through the membranes without causing extensive blood trauma. The plasma which passes through each membrane is collected, while the plasma depleted blood is recirculated to the blood flow region.
More recently, it has been possible to make microporous filter membranes with pores having precise size and shape through techniques such as those shown in U.S. application Ser. No. 08/320,199, entitled “Porous Microfabricated Polymer Membrane Structure”, filed Oct. 7, 1994, having the same assignee as the present invention and which is incorporated herein by reference. The aforesaid application generally discloses a process for microfabricating precise membranes using etchable polyimide film on a silicon substrate. A polymer film layer is made from a photoimageable polyimide material. The film is processed using negative photoresist techniques or etchable membrane fabrication technique to create a predefined geometric pattern of holes and intermediate spaces defining strands.
Alternatively, other processes, such as positive photoresist techniques, RIE (Reactive Ion Etching), LIGA (an abbreviation of the German for lithographic, galvanoformung, abformung, or in English, lithography, electroforming, and molding), may be used to create filter membranes with extremely small pore size (e.g., less than 10 microns) and having virtually zero doublets that are exceptionally uniform, with a high degree of consistency from one pore to the next. Further, electron beam and ion etch techniques also are possible means to produce precision, high porosity
Bellamy, Jr. David
Pekkarinen Michael O.
Prince Paul R.
Sternberg Shmuel
Baxter International Inc.
Drodge Joseph W.
Kolomayets Andrew G.
McFarron Barry W.
Price Bradford R. L.
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