Patent Application
20060266692
This invention relates to cross flow filters for concentrating or purifying liquid samples. In particular, this invention relates to multistage cross flow filters which are formed using photolithographic fabrication techniques.
Cross-flow or tangential-flow filtration is commonly used in applications where concentration or purification of fluid samples is desired. Through the use of this technique, a volume of fluid may be processed while reducing the clogging issues that are commonly associated with depth-type (dead-end) filtration, wherein the fluid of interest is forced to flow primary perpendicular to the plane of the filter. Most commonly such cross flow filters may rely on the use of commercially available filtration membranes or filter papers, assembled in a non-disposable structure. The non-disposable assembly typically comprises at least two parallel plates which sandwich the filter membrane or paper between the plates, and form the flow channel for the fluid.
In such a cross flow filter, a fluid to be filtered travels over the surface of the filter in a direction largely parallel to the plane of the filter. The filter may be, for example, a porous cellulose filter for filtering particulate matter or semi-permeable membrane sheets for filtering materials of various chemical compositions. An exemplary cross flow filter 100 arrangement is shown in
Multistage cross flow filters can be constructed by laminating a series of filter papers or membranes with intervening spacer layers or support members, to form a cross flow filter with multiple filtration stages. As one or more of the filtration stages becomes clogged, the unit may be disassembled and the filter papers or membranes replaced. Such a filter assembly is described in, for example, U.S. Pat. No. 5,593,580 incorporated by reference herein in its entirety.
The advantage of cross flow filters over filters wherein the flow is entirely perpendicular to the plane of the filter, is that the filter is less likely to become clogged through use, and that the cross flow filter can produce effluent streams with a concentrated or diluted proportion of particles of a given size, as described above with respect to
A number of disadvantages are associated with the cross flow filter structure 100 of
Because the pores in the particulate filters used in prior art cross flow filters may be made by mechanically stamping or puncturing a flexible sheet, or using the voids between fibers of a paper sheet as the pores, the prior art cross flow filters do not have very tight control of the particle diameters of the particles transmitted or blocked by the filters or membranes of the prior art cross flow filters.
Furthermore, the cross flow filter 100 of
Systems and methods are described here for fabrication of a cross flow filter using microelectromechanical systems (MEMS) batch processing techniques. The resulting filter structure may have pores with tolerances which are very tightly controlled, resulting in improved filter selectivity.
The microfabricated cross flow filter may include at least one flow channel photolithographically defined in a substrate between an input orifice and an output orifice, wherein the flow channel is substantially in a plane parallel to a top surface of the substrate, and at least one filter structure disposed in the flow channel including a plurality of photolithographically defined barriers defining a filter line and separated by photolithographically defined gaps between the barriers, wherein at least a portion of the flow in the flow channel is in a direction tangential to the filter line.
The microfabricated cross flow filter may also have multiple filtration stages, all on a unitary substrate. The multistage cross flow may include a second filter structure also including a plurality of barriers photolithographically defined in the substrate and separated by photolithographically defined gaps between the barriers. In various exemplary embodiments, because the barriers and gaps in the filter structure are formed photolithographically, they may have various unusual shapes, such as crescents or trapezoids, in addition to the usual parallel-wall surfaces. Such unusual shapes may be used to accomplish various purposes, such as reducing the tendency of the microfabricated cross flow filter to clog, or to reduce the shear forces acting on the particles in suspension. In various other exemplary embodiments, the gaps in the second filter structure may be of a different size than the gaps in the first filter structure.
The multistage cross flow filter may produce multiple effluent streams each having particles in a particular range of sizes. Since the filtration stages are formed photolithographically, the multistage cross flow filter may be batch fabricated very inexpensively and may be disposable.
The filter structures may also be made at an angle with respect to the central axis of the flow, thereby distributing the flow evenly across all portions of the cross flow filter. The microfabricated cross flow filter may therefore combine aspects of dead-end filtering with aspects of cross flow filtering.
These and other features and advantages are described in, or are apparent from, the following detailed description.
Various exemplary details are described with reference to the following figures, wherein:
In the systems and methods described herein, a cross flow filter is microfabricated on one or more substrates. In one exemplary embodiment, several filtration stages are created in the plane of the substrate, and the flow is substantially in the plane of the substrate, tangential to the filtration stages. In other exemplary embodiments, the filtration stages are created in separate substrates, which are then assembled into a filter structure.
Each filtration stage 250, 260 and 270 may be associated with an outlet port 255, 265 and 275, respectively. Each outlet port 255, 265, and 275 serves to output an effluent stream containing only particles smaller that the respective gaps 252, 262, and 272 of filtration stages 250, 260 and 270. Therefore, importantly, outlet port 255, for example, may produce a fluid stream which contains particles which cannot traverse the first filtration stage 260. The presence and functioning of this outlet port 255 may be important to reduce the clogging tendencies of filtration stage 260, as it may remove particles of a size which cannot traverse filtration stage 260, and would otherwise remain trapped in this chamber, eventually leading to the clogging of filtration stage 260.
Outlet port 265 produces an effluent stream with a preponderance of particles within a particular size range, in this case, in the size range smaller than the gaps 262 in filtration stage 260 and larger than gaps 272 in filtration stage 270. Similarly, outlet port 275 produces a fluid stream with a preponderance of particles smaller than the gaps 272 in filtration stage 270.
The cross flow filter 200 may have two or more input ports 248 and 258 to introduce an influent stream to the microfabricated cross flow filter 200. Input port 248 may introduce a solvent or dilutant, such as saline, and input port 258 may introduce a fluid stream containing the particulate matter in suspension, such as human blood. Furthermore, any of outlet ports 255, 265 or 275 may be coupled to input ports 248 or 258 to provide further filtering of the effluent streams produced at outlet ports 255, 265 or 275.
In one exemplary embodiment of cross flow filter 200, the gaps 252 and 272 are both about 3 μm in size, and gaps 262 are between about 10 μm and about 15 μm, and may be nominally about 13 μm in size. The width of the barriers 251, 261 and 271 may be, for example, about 20 μm in size. Accordingly, in this embodiment, outlet port 265 produces an effluent stream with a preponderance of particles smaller than 13 μm, and outlet port 275 produces an effluent stream with particles smaller than 1 μm.
As mentioned above, this embodiment may be particularly suited for the filtration of biological samples, such as human blood. In this application, saline may be injected into port 248, and a blood sample into port 258. The saline may serve to thin the blood and reduce clogging in microfabricated cross flow filter 200. The first filtration stage 250 may serve to remove any large particles or debris from the saline stream. Suspended in the blood sample injected into port 258 may be a large concentration of erythrocytes, such as red blood cells, which are a biconcave disk about 7 μm in diameter and 3 μm thick, leukocytes such as white blood cells which may be 12-15 μm in diameter, and platelets which may be 1-3 μm in diameter. Also suspended in the blood plasma may be a smaller concentration of hematopoietic stem cells (progenitor cells, capable of generating all types of blood cells) which may be 4-8 μm in diameter. Using microfabricated cross flow filter 200, the effluent stream produced by outlet port 265 may have an enhanced concentration of red blood cells and blood stem cells relative to the effluent streams produced by outlet port 275. Such an effluent stream with an enhanced concentration of blood stem cells may be useful for performing additional downstream manipulations or tests, such as cell lysis, binding with a fluorescent marker, or sorting the blood cells as will be discussed further below with respect to
When using cross flow filter 200 to filter samples which clog easily, for example, biological samples such as human blood, it may be advantageous to couple cross flow filter 200 to an acoustic modulator 290. Acoustic modulator 290 generates acoustic waves or pressure pulses, which can be coupled to the sample fluid in the cross flow filter 200 using any convenient means, such as a probe tip or wire 292, which may transmit acoustic energy from acoustic modulator 290 to cross flow filter 200. In particular, the acoustic modulation may be applied to the sample fluid as a pressure pulse by contacting the acoustic modulator 290, such as a speaker diaphragm, to the input port 258 of the microfabricated cross flow filter 200, as shown in
Each of filtration stages 250, 260 and 270 may be created photolithographically, by patterning the appropriate features in a substrate 210. The substrate 210 may be composed of any suitable material, for example, silicon or glass, which is compatible with the photolithographic processes used to form the features in the substrate. One particularly convenient substrate may be a silicon-on-insulator (SOI) substrate, which is a thick silicon wafer (the “handle” wafer), for example 675 μm thick, on which a thin insulating layer such as silicon dioxide (SiO2), 1 μm thick for example, is grown or deposited. A thinner upper layer of silicon (the “device” layer), from about 1 to over about 100 μm thick for example, is then deposited, bonded or otherwise secured to the top of the silicon dioxide layer. The filter barrier features 251, 261 and 271, may then be formed photolithographically in the thinner silicon device layer, down to the oxide by, for example, deep reactive ion etching (DRIE). The oxide layer may then form the etch stop for the DRIE process. Based on the above-mentioned thicknesses of the layers of the silicon-on-insulator substrate, the depth of the flow channels in microfabricated cross flow filter 200 may be anywhere from 1 to over 100 μm deep, as determined by the thickness of the silicon device layer, which was removed in the DRIE process to form the channels.
It should be understood that the exemplary upper bound of 100 μm for the device layer is exemplary only, and that any thickness of device layer may be chosen to achieve certain purposes. For example, the device layer may be chosen to be hundreds of microns thick, in order to increase the active filter area. However, the increased active filter area may be achieved at the price of reduced control of filter gap spacing, as the DRIE etch may not result in perfectly perpendicularly etched walls, as the aspect ratio of the etch may be inherently limited.
The photolithographic process used to form the features in the device layer may include depositing a layer of photoresistive material over the silicon-on-insulator substrate 210. The photoresistive material may then be illuminated through a mask which contains the pattern desired in the microfabricated cross flow filter, for example, the flow channels and the filtration stages 250, 260 and 270. For a positive photoresist, the illuminated portions of the photoresistive material may then be developed and removed. Alternatively, a negative photoresist may also be used, in which case the unexposed portions may be dissolved and removed. The etching process, for example, deep reactive ion etching (DRIE), may then be performed on the exposed areas of the substrate to form the flow channel and the gaps 252, 262 and 272 between the barriers 251, 261, and 271 of filtration stages 250, 260 and 270, respectively. Because the gaps between the barriers are created photolithographically, the tolerances of the gaps 252, 262 and 272 may be controlled very tightly, for example to within ±0.1 μm.
Using, for example, an SOI wafer as the substrate 210, and after patterning substrate 210 to form the flow channels and filtration stages, substrate 210 may be covered with a top cover 220, for example, a glass slide or another silicon wafer, to enclose the flow channel between inlet ports 248 and 258, and outlet ports 255, 265 and 275 through filtration stages 250, 260 and 270. The top cover 220 may be secured to substrate 210 by, for example, using epoxy or a photolithographically patterned bonding material.
Since the barriers 251, 261 and 271 are formed photolithographically by etching patterns exposed in a photoresist, any of a number of alternative shapes for barriers 251, 261 and 271 may be employed to achieve various purposes.
Although two particular examples of barrier shapes are shown in
A sample fluid may be input to microfabricated cross flow filter 300 via an input port 330. Clearance for input port 330 may be afforded by spacer layer 350, interposed between a top plate 310 and a first filtration stage 360. The spacer layer 350 also provides a flow channel between upper plate 310 and first filtration stage 360. The top plate 310 may be made of any suitable material, and may be transparent, for example. The sample fluid may contain at least three different sizes of particles, a relatively large sized particle 331, a medium sized particle 332, and a relatively small sized particle 333.
The individual substrate layers are microfabricated using photolithographic techniques similar to those described above with respect to
Filtration stage 370 may be similarly formed, including a plurality of barriers 371 separated by gaps 372. The gaps 372 in layer 370 may be narrower than gaps 362 in layer 360. Therefore, filtration stage 370 may only allow particles of a certain size, for example particles 333 to traverse the filter barrier, whereas particles 332 and 331 may not cross filtration stage 370. Therefore, the particles contained in the region below filtration stage 370 may only be the relatively small sized particles 333, whereas the particles contained in the region below filtration stage 360 may be particle sizes 332 and 333.
In order to provide outlet ports and flow channels for each effluent stream, the filtration stages 360 and 370 are separated by spacer layers 363 and 373, respectively. Spacer layer 373 may separate filtration stage 370 from the bottom substrate 320. Outlet ports 335, 365 and 375 may guide the effluent streams from the microfabricated cross flow filter 300. The effluent stream emerging from outlet port 335 may contain excess sample fluid and a reduced concentration of particles 331, 332 and 333. The effluent stream emerging from outlet port 365 may contain particles 332 as well as particles 333. The effluent stream emerging from outlet port 375 may only contain particles 333. Depending on the effluent flow rates from outlet ports 335, 365 and 375, the flow through microfabricated cross flow filter 300 may still be substantially parallel to the top plate 310, and at least a portion of the flow may be tangential to filtration stages 360 and 370.
Although not shown in
Microfabricated cross flow filter 300 may be made by first fabricating filtration stages 360 and 370, and then assembling them with spacer layers 363 and 373 using any convenient adhesive, such as epoxy, which is inert to the components of the fluid stream The upper plate 310 may then also be epoxied to the upper spacer layer 350 which is first epoxied to the first filtration stage 360. Spacer layer 350 provides clearance for the installation of inlet port 330 and outlet port 335.
Like microfabricated cross flow filter 200, microfabricated cross flow filter 300 may be coupled to an acoustic modulator 390, which may help microfabricated cross flow filter 300 avoid becoming clogged by particles 331, 332 and 333.
Microfabricated cross flow filter 300 of
A third embodiment of microfabricated cross flow filter 400 is shown in
Microfabricated cross flow filter 400 may also be provided with one or more additional input ports for introducing the sample fluid. For example, human blood may be introduced at inlet port 468, and various additional reagents such as binding agents which bind to certain blood constituents, may be introduced into the blood sample at multiple input ports 469. By providing a plurality of such input ports 469, the reagent may become more thoroughly mixed with the fluid sample before entering the filter structures 460 and 470.
The filter structures 460 and 470 may be angled with respect to the inlet port and outlet port to obtain improved filter efficiency. To improve filter efficiency, the flow rate through the filter may be more uniform from inlet to outlet. As fluid travels from the inlet 430 to the outlet 435, a portion of the fluid may pass through the filter stage, 460, meaning that as the fluid approaches 435, there is less of it. By designing filtration stage 460 at an angle, it is possible to obtain more nearly uniform pressure drop and hence flow through the filter at all points. Because fluid is both entering and leaving the central region of the filter between 460 and 470, 460 and 470 will be more nearly parallel. However, depending on the desired ratio of flow through 460 and 470, and the amount of fluid exiting through outlet port 465, the angle may not be parallel for the same reasons as mentioned above. Finally, in the region between 470 and 475, the angle of the filtration stage 470 with respect to the solid wall is again calculated to provide a nearly uniform back-pressure across the entire length of the filter structure.
The filter stages 460 and 470 may be fabricated using designs and techniques similar to those employed for filter stages 360 and 370 of microfabricated cross flow filter 300, and filter stages 250, 260 and 270 of microfabricated cross flow filter 200. In particular, filtration stages 460 and 470 may be formed in the same substrate 420, which may, for example, be silicon, silicon-on-insulator, or any other suitable material, compatible with the processes used to form the features of filter stages 460 and 470. To seal microfabricated cross flow filter 400, a plate or glass slide 410, for example, may be secured, for example using epoxy or a photolithographically patterned bonding material, to the top surface of the substrate 420. Any gap remaining between the plate or slide 410 and the substrate 410 should preferably be smaller than the gaps 462 and 472 between barriers 461 and 471 in each of filtration stages 460 and 470, respectively, in order to avoid having particles of diameter larger than the gaps 462 and 472 leak around filtration stages 460 and 470.
Although microfabricated cross flow filter 400 is depicted with the barriers 461 and 471 of filtration stages 460 and 470, respectively, arranged to form straight lines, it should be understood that this embodiment is exemplary only, and that barriers 461 and 471 may be arranged to form any of a number of shapes, such as curves, or portions of straight lines and portions of curves. Any shape which can be formed lithographically on a substrate surface can be used for the arrangement of barriers 461 and 471. For example, a complex shape such as a portion of an ellipse or parabola may be used for the arrangement of barriers 461 and 471, in order to make the back pressure across barriers 461 and 471 follow any prescribed function across the filtration stage 460 and 470, respectively.
As was noted above with respect to microfabricated cross flow filters 200 and 300, any of the effluent streams emerging from the outlet ports 435, 465 or 475 may be routed back to the inlet ports 430, 468 or 469, to perform additional filtering of the fluid stream.
Like microfabricated cross flow filters 200 and 300, microfabricated cross flow filter 400 may be coupled to an acoustic modulator 490, which may help microfabricated cross flow filter 400 avoid becoming clogged by particles.
It should be understood that any of microfabricated cross flow filters 200, 300 or 400 may be coupled to one or more additional microfabricated cross flow filters 200, 300 or 400, in a series-type or parallel-type arrangement. In such a series arrangement, the effluent stream from one microfabricated cross flow filter becomes the influent stream for a next microfabricated cross flow filter. Using a series arrangement of multiple microfabricated cross flow filters, effluent streams having enhanced purity or concentrations of a species of interest in the fluid sample may be obtained. A parallel arrangement may be used to increase the overall throughput of the microfabricated cross flow filter system Furthermore, any one of microfabricated cross flow filters 200, 300 or 400 may be combined with any other type of microfabricated cross flow filter 200, 300 or 400 in a series or parallel arrangement.
Any of microfabricated cross flow filters 200, 300 or 400 may also be used as an input stage for on a device which further manipulates the filtered sample. For example, microfabricated cross flow filter 200, 300 or 400 may be used as an input stage to a cell sorting chip, such as that described in U.S. Pat. No. 6,838,056, incorporated herein by reference in its entirety. An exemplary embodiment of such a system is shown in
In the cell sorting system of
Use of microfabricated cross flow filter 200, 300 or 400 may thereby concentrate human hematopoietic stem cells for sorting by the cell sorting chip, and also exclude other particles found in human blood from entering the chip and potentially clogging the chip or otherwise interfering with its operation. In particular, microfabricated cross flow filter 200 may improve the functioning of the cell sorting chip 500, by increasing the concentration of the species of interest, the human hematopoietic stem cells.
While various details have been described in conjunction with the exemplary implementations outlined above, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that are or may be presently unforeseen, may become apparent upon reviewing the foregoing disclosure. For example, while only two or three filtration stages are shown in the embodiments depicted in
