Scalable co-current tangential flow filtration

Abstract

A serpentine co-flow fluid filtration system includes two or more filter housings including filter elements, a pump to transfer fluid from a vessel to the filter housings, and a permeate recirculation pump to transfer permeate back to each filter housing. The system includes a control system programmed to transfer the fluid from the vessel to the first filter housing, selectively remove permeate from the first filter housing, and selectively recirculate permeate to a second filter housing. The permeate in each filter housing is provided at a flow rate and pressure such that a difference between a pressure of the fluid and the permeate at an entrance to the filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the filter housing. The system allows for additional filters to be connected in series.

Description

Docket No. 1580.00226WOSCALABLE CO-CURRENT TANGENTIAL FLOW FILTRATIONCROSS REFERENCE

[0001] This application claims priority to pending U.S. Provisional Patent Application Serial Number 63 / 712,722 filed October 28, 2024, the entirety of which is incorporated by reference herein.TECHNICAL FIELD

[0002] Embodiments of the disclosure relate generally to filtration systems, and more particularly to a filtration system that includes an arrangement for controlling a circulation flow of permeate through a series of filter housings during filtration operations.BACKGROUND

[0003] Filtration is often performed to separate, clarify, modify, and / or concentrate a fluid solution, mixture, or suspension. In the biotechnology, pharmaceutical, and medical industries, filtration is vital for the successful production, processing, and analysis of drugs, diagnostics, and chemicals as well as many other products. As examples, filtration may be used to sterilize fluids and to clarify a complex suspension into a filtered “clear” fraction and an unfiltered fraction. Similarly, constituents in a suspension may be concentrated by removing or “filtering out” the suspending medium. Further, with appropriate selection of filter material, filter pore size and / or other filter variables, many other specialized uses have been developed. These uses may involve selective isolation of constituents from various sources, including cultures of microorganisms, blood, as well as other fluids that may be solutions, mixtures, or suspensions.

[0004] Biologies manufacturing processes have advanced through substantial process intensification. Both eukaryotic and microbial cell culture to produce recombinant proteins, virus-like particles (VLP), gene therapy particles, and vaccines now include cell growth techniques that can achieve 100e6 cells / ml or higher. This is achieved using cell retention devices that remove metabolic waste products and refresh the culture with additional nutrients. One common means of cell retention is to perfuse a bioreactor culture using hollow fiber filtration using Alternating Tangential Flow (ATF).Docket No. 1580.00226WO

[0005] ATF is a filtration method where a cell culture flows through a bundle of hollow fibers in an alternating flow from both filter ends. The hollow fibers can have a variety of predetermined pore sizes, and filters with a specific pore size are often used for different applications. Tests demonstrate that the entire length of a hollow fiber filter is not uniformly utilized when such hollow fiber filters are used in conventional ATF systems.

[0006] Further, co-current filtrate flow has attempted to decouple transmembrane pressure from crossflow. Co-current flow attempts to reduce Starling recirculation. Low filtrate fluxes and high pressure drops across the length of the filter causes Starling recirculation, which is a reverse flow of the permeate back into the filter fibers. A co-current filtrate process provides recirculating co-current flow of permeate and retentate volumes through a hollow fiber filter housing. This recirculation of permeate provided an equal pressure at the inlet to the retentate and the inlet to the permeate. However, the pressures of the retentate and the permeate diverge along the length of the filter and as the retentate and the permeate exited the filter.

[0007] It would be desirable, therefore, to provide an improved pumping and filter arrangement that is scalable for higher flow and for larger vessels.BRIEF SUMMARY

[0008] Recirculating co-current flow of permeate and retentate volumes through a hollow fiber filter housing equalizes pressure in the permeate cavity of the hollow fiber filter through the entire fiber length. Pressure feedback in the permeate line can be used to control the flow rate of a permeate recirculation pump. Providing compatible pressures at permeate connection points to the filter housing can result in more uniform use of the entire hollow fiber length, which in turn can result in longer filter life. Concurrent flow in the retentate and permeate filter chambers of each of a series of filters equalizes pressure along the entire length of each filter, which helps with uniform filter use.

[0009] The permeate pump and / or a permeate valve control the flow rate and pressure of the permeate recirculation stream that enters at the filter entrance and exits at the filter exit. The retentate pump and / or a retentate valve control the flow rate and pressure of the retentate recirculation stream that enters at the filter entrance and exits at the filter exit. A processor for a control system managing the pumps and / or the valves controls the flow of the retentate and / or the permeate. The control system seeks to control the flow such that the transmembrane pressure ("TMP") is consistent along the length of the filter for each of the serpentine series of filters.Docket No. 1580.00226WOThe consistent TMP may be verified by observing that a difference between the pressure of the retentate at the filter entrance and the pressure of the permeate at the filter entrance is equal to the difference between the pressure of the retentate at the filter exit and the pressure of the permeate at the exit. The control system allows the system to be scaled up to higher filtration rates, such as a crossflow rate over 400mL / min, by keeping the pressure gradient across the filter consistent along the length of the filter.

[0010] In one example, the method includes first determining a starting shear rate. The method then gradually decreases the shear rate during the process as the viscosity of cell culture increases to maintain a constant pressure drop inside the filter. This method allows the system to match the pressures of the feed / retentate flow and the permeate flow when the process may have increasing viscosities. The method is useful in perfusion applications where these challenges persist.

[0011] Further, the system allows for scaling up to greater filtration rates by allowing multiple filters to be connected in series such that the retentate and the permeate flows are also in series. The filters are configured with ports that allow the permeate to be ported from the permeate side of one filter to the permeate side of the next filter in the series of filters. The ports provide balanced pressure drops to match the pressure drops of the recirculating retentate in each filter.

[0012] In an example, two filters are configured to be adjacent to one another. The permeate ports may be configured to be conjoined the two filters to minimize the tubing connections. The permeate ports may have a valve or other control element to manage the pressure of the permeate that passes to the subsequent filter. In another example, four filters are configured to be adjacent to one another in a square bundle. The permeate ports may conjoin the four filters to form a square arrangement. The permeate ports allow flow from each filter in the series of filters to the next without a need for additional tubing or connections.

[0013] A method of operating a fluid filtration system is disclosed. The method includes: transferring a fluid from a fluid storage vessel to a first filter housing; directing the fluid through a filter element disposed in the first filter housing; selectively removing permeate from the first filter housing; and selectively recirculating permeate through the first filter housing at a flow rate and pressure such that a difference between a pressure of the fluid and the permeate at an entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the first filter housing. The system directs a retentate from theDocket No. 1580.00226WO first filter housing to a retentate inlet of a second filter housing and directs the permeate from the first filter housing to a permeate inlet of a second filter housing.

[0014] In some embodiments, the method allows that a difference between the pressure of the fluid and the permeate at the entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at a point approximately in the center of the first filter housing. In some embodiments, the permeate from the second filter housing is recirculated to the first filter housing by a pump controlled by a control system. In some embodiments, the steps of selectively recirculating permeate through the first filter housing and directing the fluid through a filter element cause the permeate and the fluid to move concurrently.

[0015] In some embodiments, the method further includes: directing the permeate to a second filter housing through a port; restricting a flow of the permeate to the second filter housing such that a difference between a pressure of the fluid and the permeate at an entrance to the second filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the second filter housing. In some embodiments, the port comprises a flow restrictor, which may be a flow control valve or an orifice plate.

[0016] In some embodiments, the method further includes: directing the fluid to a third filter housing; directing the permeate to a third filter housing through a second port; restricting a flow of the permeate to the third filter housing such that a pressure of the fluid and the permeate at an entrance to the third filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the third filter housing. In some embodiments, the first filter housing, the second filter housing, and the third filter housing are disposed in a single plane, while in other embodiments, the filter housings are disposed in a bundle.

[0017] A fluid filtration system is disclosed, including a fluid storage vessel; a filter housing including a filter element therein; a pump to transfer fluid from the fluid storage vessel to the filter housing; a permeate recirculation pump to transfer permeate to the filter housing; a control system coupled to the first pump and the permeate pump, the control system including a processor programmed to execute instructions to: transfer the fluid from the fluid storage vessel to the filter housing; direct the fluid through the filter element disposed in the filter housing; selectively remove permeate from the first filter housing; direct a retentate from the first filter housing to a retentate inlet of a second filter housing; direct the permeate from the first filter housing to a permeate inlet of a second filter housing; and selectively recirculating permeate from the second filter housing to through the first filter housing at a flow rate and pressure such that a differenceDocket No. 1580.00226WO between a pressure of the fluid and the permeate at an entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the first filter housing. In some embodiments, a difference between the pressure of the fluid and the permeate at the entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at a point approximately in the center of the first filter housing.

[0018] In some embodiments, the fluid filtration system includes a permeate port to transfer permeate from the exit of the first filter housing to an entrance to the second filter housing. In some embodiments, the permeate port includes a restrictor that restricts a flow of the permeate to the second filter housing such that a difference between a pressure of the fluid and the permeate at an entrance to the second filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the second filter housing. In some embodiments, the restrictor is a flow control valve or an orifice plate.

[0019] In some embodiments, the fluid filtration system includes a third filter housing; and a second permeate port to transfer permeate from the exit of the second filter housing to an entrance to the third filter housing. In some embodiments, the permeate at the exit of the third filter housing is recirculated to the first filter housing by a pump controlled by the processor. In some embodiments, the pressure of the fluid and the permeate at the entrance to the first filter housing and the pressure of the fluid and the permeate at the exit of the first filter housing are each measured by a pressure transmitter.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0020] To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced. The accompanying drawings illustrate preferred embodiments of the disclosed method so far devised for the practical application of the principles thereof, and in which:

[0021] FIG. 1 is a schematic view of an example serpentine pump and filter system operating in series according to the present disclosure;

[0022] FIG. 2 is a schematic view of a single co-flow pump and filter system;

[0023] FIG. 3 is a graph of protein passage of a serpentine co-flow system, a single filter coflow system, and a TFF filter system; and

[0024] FIG. 4 is a graph of pressure profiles of a two filters in a serpentine co-flow system and a single filter co-flow system.Docket No. 1580.00226WODETAILED DESCRIPTION

[0025] A system is disclosed, including a fluid storage vessel, two or more pumps, and two or more filters. A first pump moves fluid from the fluid storage vessel, which may be a bioreactor, through a first filter. The filter may include a filter housing and an internal filter media, which in some embodiments is a hollow fiber filter. Such a system has applications in perfusion of cultured animal cells as well as other varied filtration applications. Retentate from the filter exits the filter housing and is directed to a second filter that operates in a similar manner as the first filter. Any number of filters may be used in the serpentine series of filters. The retentate from the last filter is returned to the fluid storage vessel. Permeate that is captured from the retentate exits the filter separately from the retentate and is directed to the permeate inlet of the next filter in the series. At least a portion of the permeate from the last filter is pumped in a recycle feed back to an entrance to the first filter. At least a portion of the permeate is harvested from the permeate lines by a harvest outlet.

[0026] The permeate pump is controlled to provide permeate at a desired pressure and flow rate to each filter to cause a pressure differential with the retentate at the feed to equal a pressure differential at the exit. That is, for each filter in a series, a control systems controls the flow such that a difference between the pressure of the retentate at the filter entrance and the pressure of the permeate at the filter entrance is equal to the difference between the pressure of the retentate at the filter exit and the pressure of the permeate at the exit. The control system may additionally or alternatively control a control valve or other flow control device to control the permeate recycle flow. The control system allows the system to be scaled up to higher filtration rates, such as a crossflow rate over 400mL / min, by keeping the pressure gradient across the filter consistent along the length of the filter. In some examples a single pump and valve systems controls the feed and retentate flows for all filters. In other examples, multiple pumps and valve systems may be used to provide the desired pressures and flow rates.

[0027] The filters in the series are configured with ports that allow the permeate to be ported from the permeate side of one filter to the permeate side of the next filter in the series of filters. The ports provide balanced pressure drops to match the pressure drops of the recirculating retentate in each filter.

[0028] In some embodiments, operational control of the various pumps and valves in the system can be based on an algorithm that can coordinate and / or synchronize recirculation of permeate through the one or more filter housings. These technologies allow the system to be scaled up toDocket No. 1580.00226WO industrial filtration rates by keeping the pressure gradient across the filter consistent along the length of the filter. By employing the filtration system of across multiple serpentine filters in series, a scaled-up filtration process may be used to filter larger vessels with greater filtration rates. Any suitable number of filters in series may be used, such as two, four, five, or six.

[0029] These and other advantages will be discussed below.

[0030] FIG. 1 is a schematic view of an example pump and filter system according to the present disclosure.

[0031] The process described herein allows for scaling up to greater filtration rates by allowing multiple filters, such as filter 502 and 504, to be connected in series such that the retentate and the permeate flows are also in series.

[0032] In the example, a fluid storage vessel 102 is provided that stores a fluid to be filtered. For example, the vessel 102 may be a bioreactor, a fermenter, or any other vessel 102, nonexclusively including vats, barrels, tanks, bottles, flasks, containers, and the like which can contain liquids. The vessel 102 may be composed of any suitable material such as plastic, metal such as stainless steel, glass, or the like. Each of the two or more filters, such as filter 502 and 504, include a filter housing that includes one or more hollow fiber filters to separate at least one material from the fluid in the vessel 102. In the example, the fiber media are 1.4mm ID, 65 cm long hollow fiber filters. The disclosed systems and methods have applications in perfusion of cultured animal cells as well as other varied filtration applications.

[0033] The retentate pump 108 moves the fluid in a retentate feed 118 from the vessel 102 into a feed port of the first filter 502. The series of filters are configured with permeate ports that allow the permeate output 124 to be ported from the permeate side of one filter to the permeate side of the next filter in the series of filters. The permeate ports may provide balanced pressure drops to match the pressure drops of the recirculating retentate in each filter. The arrangement of filters 104 eliminates a need to have multiple feed and permeate pumps and / or multiple filters 104 in parallel.

[0034] In the illustration, a retentate loop for the vessel 102 is provided with two filters - a first filter 502 and a second filter 504. Any number of filters may be used based on the flow rates, the pump speeds, the product being filtered, the filter media, or any other suitable factors. The filters are in series such that retentate is pumped by the retentate pump 108 into the first filter 502 at the retentate feed 118. The retentate output 520 exits the first filter 502. Instead of returning to the vessel 102, the retentate in the retentate output 520 is directed to the feed of theDocket No. 1580.00226WO second filter 504. The pumping of the retentate output 520 to the second filter 504 may be caused by the motive force provided by the retentate pump 108, by an additional pump, by gravity, or by any other suitable motive force. The retentate output 122 of the last filter (in this example, the second filter 504) may be returned to the vessel 102.

[0035] The permeate pump 106 located in the permeate loop provides a motive force to the permeate exiting the last filter housing, such as filter 504, in a permeate output 124 back to a permeate feed 126 into a feed port of the filter 502. The permeate pump 106 may be located after the final filter in the series, between the filters 502 and 504, or in any other location in the permeate loop. In other examples, multiple permeate pumps and / or control valves may be used to maintain a desired pressure at any location in the permeate loop.

[0036] A portion of the permeate may be harvested by a purge valve (not pictured) or other technology to generate a permeate harvest 125. In an example, a harvest pump (not pictured) may be used to extract permeate from the permeate loop.

[0037] In the illustration, the permeate is pumped by the permeate pump 106 into the first filter 502 at the permeate feed 126. The permeate exits the first filter 502 at permeate output 124. The pressure and flow imparted by the permeate pump 106 and / or the retentate pump 108 may be controlled such that the pressures of the permeate and the retentate are equal across the first filter 502 at the feed and exits of the first filter 502.

[0038] The second filter 504 operates in a similar fashion to the first filter 502 with a retentate output 122 being generated and directed to the vessel 102. If a third filter were in the system, the retentate from the second filter 504 would be directed to a feed of the third filter. The permeate output 124 of the second filter 504 would be pumped to a permeate feed of the third filter. The pressure and flow imparted by the permeate pump 106 and / or the retentate pump 108 may be controlled such that the pressures of the permeate and the retentate are equal across the second filter 504 at the feed and exits of the second filter 504 as described with respect to FIG. 1 herein.

[0039] In the illustration, the permeate exits the second filter 504 at permeate output 124. The permeate is directed by the force created by the permeate pump 106 back to the permeate inlet 126 of the first filter 502. The pressure and flow imparted by the permeate pump 106 and / or the retentate pump 108 may be controlled such that the pressures of the permeate and the retentate are equal across the third filter 506 at the feed and exits of the third filter 506 as described with respect to FIG. 1 herein.Docket No. 1580.00226WO

[0040] Pressure readings may be obtained at any point of the permeate loop, the retentate loop, or in the filters 502, 504. Illustrated herein are pressure readings from a pressure transmitter of retentate feed pressure 1 10 (1 psi), retentate exit pressure transmitter 1 16 (-0.3 psi), permeate feed pressure transmitter 112 (0.9 psi), and permeate exit pressure transmitter 114 (-0.4 psi). However, pressures across the filter medium at multiple points of the filter may be used. Pressures at other points of the retentate loop and the permeate loop may also be obtained.

[0041] The pressures may be determined by any suitable type of pressure transmitter or pressure gauge, such as a pressure transmitter that communicates a control signal back to the control system. The pressure may be obtained in absolute or gauge pressures. The pressure readings my be communicated to a control system to allow the control system to assess the permeate and retentate flows.

[0042] The control system may be any type of industrial, commercial, or personal computing system. The control system may receive inputs such as flow rates of the pumps, pressure readings from the transmitters, configured pressure requirements from operators, retentate compositions, temperatures, or any other suitable data that allows the control system to operate the filtration system. The control system may operate one or more algorithms that provide operational control of the various pumps and valves in the system to coordinate and / or synchronize recirculation of permeate through the filter housing.

[0043] The algorithm causes the control system to provide signals to functions such as the retentate pump 108 and the permeate pump 106 to modulate the flow rate of materials to the filters 502, 504. The filtration system may further employ valves to manage the flows. For example, a control valve may restrict the flow of retentate or permeate to the filter or exiting the filter 502 to create a desired pressure and flow for the feed and / or the permeate.

[0044] In an example, the control system manages the permeate flow and / or the retentate flow to cause the pressure gradient across the filter 502 to be consistent throughout the length of the filter and as the flows exit the filter housing. For example, the control system modulates the permeate pump 106 to provide a flow rate and pressure of permeate to the filter 104 such that the transmembrane pressure ("TMP") is consistent along the length of the filter. The consistent TMP may be evidenced by observing that the difference between the retentate feed pressure 110 and the permeate feed pressure 112 is equal to the difference between the retentate exit pressure 116 and the permeate exit pressure 114. If the difference between the retentate feed pressure 110 andDocket No. 1580.00226WO the permeate feed pressure 112 is 0.2 psi, then the difference between the retentate exit pressure 116 and the permeate exit pressure 114 is approximately 0.2 psi.

[0045] As illustrated, the pressure of the retentate feed at pressure transmitter 110 is approximately the same as the pressure of the permeate feed at pressure transmitter 112. Similarly, the pressure of the retentate exit at pressure transmitter 116 is approximately the same as the pressure of the permeate feed at pressure transmitter 114. The equalized pressure across the filter medium as the flows travel along the filter provides the filtration benefits described herein. In a similar manner, the pressures of the retentate and the permeate in the second filter 504 or any subsequent filter are equalized.

[0046] In one example, this process includes first determining a starting shear rate. The control system then gradually decreases the shear rate during the process as the viscosity of cell culture increases to maintain a constant pressure drop inside the filter. This process allows the control system to match the pressures of the feed / retentate flow and the permeate flow when the process may have increasing viscosities. This process is useful in perfusion applications where these challenges persist.

[0047] Other pressure readings along the length of the filter 502 may also be used. For example, pressure transmitters may be used to measure the TMP at a point halfway along the length of the filter 502. In the previous example, this difference would also be 0.2 psi.

[0048] The consistent pressure gradients allow the system to minimize Starling recirculation in the filter 104 at higher filtration rates. Conventional systems are unable to control the TMP when crossflow rates are over 400mL / min and are not usable for larger vessels 102 and / or larger filters 104. When operating the system as described herein, higher filtration rates at industrial scale are possible, such as for vessels 102 that are 500L to 2000L. Conventional processes filters may be able to process up to 1000-2000 L / m2. For perfusion rates of two vessel volumes per day and a 20-day perfusion process, the total perfused volume would be 20,000 L for a 500L reactor, and 10 or 20m2 of filter surface area would be needed. This volume would require two large-scale filters. However, a challenge of increasing the bioreactor volume to 2000L is that this volume would require up to 40 to 80m2. In a conventional system, this would require four filters operating in parallel with one change-out of filters using a total of eight filters. The large footprint and the complexity of a system required for a process using four filters next to the 2000L bioreactor becomes challenging. Operating only a single system with the filters in series instead of in parallel as described herein and balancing the pressure drop throughout the filtersDocket No. 1580.00226WG in series reduces the complexity and makes the described methods and devices more scalable compared to multiple systems in parallel.

[0049] FIG. 2 is a schematic view of a single co-flow pump and filter system.

[0050] The filter 104 operates in a similar manner as a single filter of the series of filters represented in FIG. 1. The filter 104, however, does not provide the retentate output to a subsequent filter, such as filter 504. Instead, the retentate output 122 of the filter 104 returns to the vessel 102. Similarly, the permeate output 124 of the filter 104 does not proceed to the permeate inlet of a subsequent filter but instead is recycled back to the permeate inlet of the filter 104.

[0051] The retentate pump 108 moves the fluid in a retentate feed 118 from the vessel 102 into a feed port of the filter 104. The permeate pump 106 moves the permeate exiting the filter housing in a permeate output 124 back to a permeate feed 126 into a feed port of the filter 104. A portion of the permeate may be harvested by a purge valve or other technology to generate a permeate harvest 125. Pressure transmitters, 110, 112, 114, and 116 may be used to manage the TMP in a similar manner as described in FIG. 1. As described in FIG. 1, a difference between the pressure of the retentate at the filter entrance and the pressure of the permeate at the filter entrance is equal to the difference between the pressure of the retentate at the filter exit and the pressure of the permeate at the exit.

[0052] The single filter 104 of FIG. 2 only provides a single filter housing with a filter medium to filter the product. As illustrated in the following charts, the serpentine process of FIG. 1 allows for higher filtration rates at industrial scale.

[0053] FIG. 3 is a graph of protein passage of a serpentine co-flow system, a single filter coflow system, and a conventional TFF filter system.

[0054] In the perfusion process, IgG passes from the fluid in the vessel 102 in three filter configurations. The percentage of IgG sieved is measured for each day for 22 days. In the serpentine co-flow system, the retentate pump 108 of the example pumped at a rate of 169 mL / min, and the permeate pump 106 of the example, pumped at a rate of 145mL / min. In the single filter co-flow system, the retentate pump 108 of the example pumped at a rate of 169 mL / min, and the permeate pump 106 of the example, pumped at a rate of 125 mL / min.

[0055] The graph illustrates a percentage of protein that passes through the filter media in the filter, such as filter 502, 504. The protein in the example is based on the total mAb filtered from bioreactor vessel 102 for a perfusion process.Docket No. 1580.00226WO

[0056] As illustrated, the protein passage for the single filter co-flow system (as illustrated in FIG. 2) begins with 100% protein passage. The serpentine co-flow system (as illustrated in FIG. 1 ) begins with a lower percentage protein passage. Over the course of 19 days, the protein passage of the serpentine system is slightly lower, but within 10% of the single co-flow system. However, in days 19-22, the serpentine system has a much closer protein passage percentage indicating that the systems have a similar performance for this parameter. This indicates that the protein passage over time is nearly equivalent. A system was also tested using a conventional TFF filtration system without the equalized pressure of the co-flow arrangements. As illustrated, the conventional TFF filtration system has significantly lower protein passage throughout the run.

[0057] FIG. 4 is a graph of pressure profiles of a two filters in a serpentine co-flow system and a single filter co-flow system.

[0058] In the example process, mAb passes from the fluid in the vessel 102 in a filter 104. The percentage of mAb sieved is measured for each day for a volumetric throughput of 1415 L / m2. The transmembrane pressure (TMP) for the single filter of the co-flow system was measured and the TMP for both filters of a serpentine co-flow system.

[0059] The TMP of the single co-flow filter rose steeply after approximately 400 L / m2from 0. 1 psi to over 1.1 psi. This steep increase in TMP is indicative of fouling of the filter. Conversely, the TMP of both filters of the serpentine co-flow system remained fairly constant at approximately 0.2-0.3 psi. This steady TMP indicates that the serpentine co-flow system allows for greater amounts of fluids to be filtered for longer periods of time without fouling of the filters. Accordingly, since the serpentine system provides a similar level or protein passage and can filter more fluids for a longer period of time without fouling, the serpentine system has improved performance over a single filter co-flow system and a conventional TFF filtration system.

[0060] The control system described in various examples herein may include a processor or microprocessor configured to run an operating system, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), etc., or combinations thereof. The control system may include memory which may include, but is not limited to, electronic, optical, magnetic, or any other storage or transmission device capable of providing a processor, ASIC, FPGA, etc. with program instructions. The memory may include a memory chip, flash memory, Electrically Erasable Programmable Read-Only Memory (EEPROM), erasable programmableDocket No. 1580.00226WO read only memory (EPROM), or any other suitable memory from which the controller can read instructions. The instructions may include code from any suitable programming language.

[0061] In some embodiments, the processor of the control system may execute instructions (e.g., a subroutine) to actuate the valves to control the flow and pressure of the retentate and the permeate. As will be appreciated, the control system may also control the speed of the retentate pump 108 and / or the permeate pump 106 to adjust flow through the system. A variety of set points and operating positions can be stored in controller memory and executed by the processing portion of the control system upon user command or automatically.

[0062] The control system can include a processor and associated memory for storing information regarding the retentate pump 108, the permeate pump 106, any associated valves, the filter element and / or other aspects of the system. The memory can include instructions executable by the processor for controlling operation of the pumps and the valves to thereby control flow of fluid between the vessel 102 and the filters 502, 504 and filter element, and to ensure concurrent flow of retentate and recirculated permeate in the desired manners described herein. The control system can also include a user interface for allowing a user to input information into the controller and / or operate the system in a desired manner.

[0063] In various embodiments, a user interface is provided where users of the disclosed systems can input and / or monitor various facets of the system and operation of associated pumps and valves. For example, the user interface may be programmed to display one or more graphical outputs of data received and analyzed by control system. The user interface may also display other data stored in the memory of control system, including type and size of filter, flow direction mode, permeate pump mode (normal, cleaning), TFF mode, ATF mode, system flow, system pressure, and system status (running, off). Further still, additional parameters that may be displayed to the user at the user interface include a flow rate and a cycle time for one or more process steps. User interface with SCADA can be exclusive to a control system or controller can be connected to a network where a portion or the entire factory is controlled from one center.

[0064] Further still, the user interface of certain exemplary embodiments permits the user to control starting or stopping of a control process carried out by the control system. In some embodiments, starting and stopping functions may be controlled via buttons provided on a touchscreen display, for example. The user interface also allows for input (entry) of specified control parameters.Docket No. 1580.00226WO

[0065] As will be appreciated by one of ordinary skill in the art, the filter 104 encloses a filter element (not shown), which in one non-limiting exemplary embodiment is a hollow fiber filter, although this is not critical and any of a variety of other filter elements can be used. The filter 104 can be made from plastic, metal, such as stainless steel, glass, and the like. In some implementations, the hollow fiber filter has a pore size of about 0.1 to 5.0 microns, e.g., 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 microns, or 1, 2, 3, or 4 microns, or about 500 to 1000 kD, e.g., 550, 600, 650, 700, 750, 800, 850, 900, or 950 kD.

[0066] To minimize the negative impact of pumping on the cells within the pumped fluid, a specialized pump type is desirable. In the illustrated embodiments the permeate pump 106 and the retentate pump 108 are appropriate low-shear pump types. In non-limiting examples, such pumps can include Levitronix pumps (www.levitronix.com), and Quattroflow pumps manufactured by Holland Applied Technologies (www.hollandapt.com). It will be appreciated that other specialty pumps, such as peristaltic pumps for shear sensitive liquids can be also used. Standard peristaltic pumps can be used for non-cell culture applications.

[0067] The vessel 102 may be any suitable container for housing a fluid to be filtered. For example, the fluid vessel may be a bioreactor, a fermentor or any other vessel, nonexclusively including vats, barrels, tanks, bottles, flasks, containers, and the like which can contain liquids. The vessel may be composed of any suitable material such as plastic, metal such as stainless steel, glass, or the like.

[0068] In some embodiments the filter element is a hollow fiber filter. In some embodiments the fluid storage vessel is a bioreactor. In some embodiments the fluid comprises cell cultures.

[0069] The disclosed systems and methods have applications in perfusion of cultured animal cells as well as other varied filtration applications. Cultured animal cells can mean mammalian cells suspended in a liquid culture medium. Cultured animal cells can have a cell density of greater than about O. l x lO6cells / mL (e g., greater than about l * 106cells / mL, greater than about 5 >< 106cells / mL, greater than about 10x l06cells / mL, greater than about 15* 106cells / mL, greater than about 20x l06cells / mL, greater than about 25x l06cells / mL, greater than about 30x l06cells / mL, greater than about 35x l06cells / mL, greater than about 40x l06cells / mL, greater than about 45x 06cells / mL, greater than about 50x 06cells / mL, greater than about 55x 106cells / mL, greater than about 60x l06cells / mL, greater than about 65x l06cells / mL, greater than about 70x 106cells / mL, greater than about 75 x 106cells / mL, greater than about 80x l06cells / mL, greaterDocket No. 1580.00226WO than about 85x 106 cells / mL, greater than about 90x l06cells / mL, greater than about 95x l06cells / mL, or greater than lOOx lO6cells / mL).

[0070] While the present invention has been disclosed with reference to certain embodiments, numerous modifications, alterations and changes to the described embodiments are possible without departing from the spirit and scope of the invention, as defined in the appended claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language of the following claims, and equivalents thereof.

Claims

Docket No. 1580.00226WOCLAIMSWhat is claimed is:

1. A method of operating a fluid filtration system, the method comprising: transferring a fluid from a fluid storage vessel to a first filter housing; directing the fluid through a filter element disposed in the first filter housing; selectively removing permeate from the first filter housing; directing a retentate from the first filter housing to a retentate inlet of a second filter housing; directing the permeate from the first filter housing to a permeate inlet of a second filter housing; and selectively recirculating a permeate from the second filter housing to the first filter housing at a flow rate and pressure such that a difference between a pressure of the fluid and the permeate at an entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the first filter housing.

2. The method of claim 1, wherein a difference between the pressure of the fluid and the permeate at the entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at a point approximately in the center of the first filter housing.

3. The method of claim 1 , wherein the permeate from the second filter housing is recirculated to the first filter housing by a pump controlled by a control system.

4. The method of claim 1, wherein the steps of selectively recirculating permeate through the first filter housing and directing the fluid through a filter element cause the permeate and the fluid to move concurrently.

5. The method of claim 1, further comprising: restricting a flow of the permeate to the second filter housing such that a difference between a pressure of the fluid and the permeate at an entrance to the second filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the second filter housing.Docket No. 1580.00226WO6. The method of claim 5, wherein the port adjoins the first filter housing and the second filter housing such that additional tubing for the permeate between the exit of the first filter housing and the entrance to the second filter housing is not required.

7. The method of claim 5, wherein the port comprises a flow restrictor.

8. The method of claim 7, wherein the flow restrictor in the port is a control valve.

9. The method of claim 5, further comprising: directing the permeate to a third filter housing through a second port; and restricting a flow of the permeate to the third filter housing such that a difference between a pressure of the fluid and the permeate at an entrance to the third filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the third filter housing.

10. The method of claim 9, wherein the first filter housing, the second filter housing, and the third filter housing are disposed in a single plane.

11. The method of claim 9, wherein the first filter housing, the second filter housing, and the third filter housing are disposed in a bundle.

12. A fluid filtration system, comprising: a first filter housing and a second filter housing, each including a filter element therein; a pump to transfer fluid from the fluid storage vessel to the first filter housing; a permeate recirculation pump to transfer permeate to the first filter housing; and a control system coupled to the first pump and the permeate pump, the control system including a processor programmed to execute instructions to: transfer the fluid from the fluid storage vessel to the first filter housing; direct the fluid through the filter element disposed in the first filter housing; selectively remove permeate from the first filter housing; direct a retentate from the first filter housing to a retentate inlet of a second filter housing; direct the permeate from the first filter housing to a permeate inlet of a second filter housing; and selectively recirculate a permeate from the second filter housing to the first filter housing at a flow rate and pressure such that a difference between a pressure of the fluidDocket No. 1580.00226WO and the permeate at an entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the first filter housing.

13. The fluid filtration system of claim 12, wherein a difference between the pressure of the fluid and the permeate at the entrance to the first filter housing is equal to a difference between a pressure of the fluid and the permeate at a point approximately in the center of the first filter housing.

14. The fluid filtration system of claim 12, further comprising: a permeate port to transfer permeate from the exit of the first filter housing to an entrance to the second filter housing.

15. The fluid filtration system of claim 14, wherein the permeate port comprises a restrictor that restricts a flow of the permeate to the second filter housing such that a difference between a pressure of the fluid and the permeate at an entrance to the second filter housing is equal to a difference between a pressure of the fluid and the permeate at an exit of the second filter housing.

16. The fluid filtration system of claim 14, wherein the restrictor is a flow control valve.

17. The fluid filtration system of claim 14, further comprising: a third filter housing; and a second permeate port to transfer permeate from the exit of the second filter housing to an entrance to the third filter housing.

18. The fluid filtration system of claim 17, wherein the permeate at the exit of the third filter housing is recirculated to the first filter housing by a pump controlled by the processor.

19. The fluid filtration system of claim 12, wherein the pressure of the fluid and the permeate at the entrance to the first filter housing and the pressure of the fluid and the permeate at the exit of the first filter housing are each measured by a pressure transmitter.

20. The fluid filtration system of claim 12, wherein the first filter housing and the second filter housing are disposed in a single plane.

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