HK1084068A - Microfluidic devices with porous membranes for molecular sieving, metering, and separations - Google Patents

Description

Microfluidic device with porous membrane for molecular screening, measurement and separation

Technical Field

The present invention relates generally to microfluidic devices, and more particularly, but not exclusively, to microfluidic devices having integrated porous silicon (silicon) membranes for filtering molecular components of an injected stream to measure and/or separate chemical and/or biological molecules.

Background

As the range of microchip manufacturing technologies has expanded, a new technology related to micro-mechanics, called microfluidic devices, has emerged. Microfluidic devices typically include miniaturized reservoirs, pumps, valves, filters, mixers, reaction chambers, and capillary networks interconnecting the miniature components, which are being developed for use in a variety of configuration scenarios. For example, microfluidic devices can be designed to perform multiple reaction and analysis techniques in one micro-instrument by providing the ability to perform hundreds of operations (e.g., mixing, heating, separation) without human intervention. In some cases, microfluidic devices may be used as air toxin detectors, rapid DNA analyzers for crime scene investigators, and/or new drug testing devices to accelerate drug development.

While the application of such microfluidic devices is virtually unlimited, it limits the range of functions that can be achieved by a single device or a combination of devices, since it is technically difficult to integrate certain micro-components into a microfluidic system. In particular, current microfluidic systems have not adequately integrated a size-separating (or filtering) filter into a microfluidic chip. Therefore, separation is typically performed in an externally wrapped porous media or polymer-based nanopore (nanoporous) membrane, thereby increasing the risk of contamination and introducing additional complexity and human intervention in the performance of the analysis or other techniques.

Drawings

In the drawings, wherein like reference numerals represent like parts throughout the several views of non-limiting, non-exclusive embodiments of the present invention, and wherein:

FIGS. 1 a-1 e show various views of a microfluidic device according to one embodiment of the present invention, in which a single porous silicon membrane is integrally formed in a substrate;

FIGS. 2 a-2 e show various views of an embodiment of a microfluidic device using a stacked channel structure, in which a porous membrane is placed between the ends of an upper microfluidic channel and a lower microfluidic channel;

3 a-3 e show various views of an embodiment of a microfluidic device in which a plurality of monolithic porous silicon membranes are placed at various points along the microfluidic channel;

FIGS. 4 a-4 d illustrate various views of an embodiment of a microfluidic device having a stacked channel-like structure, wherein the platform substrate comprises a plurality of substrate layers;

FIGS. 5 a-5 e illustrate various views of an embodiment of a microfluidic device having a stacked channel-like structure, wherein the platform substrate includes an upper substrate member and a lower substrate member;

FIGS. 6 a-6 f show various cross-sectional views of a microfluidic channel in which MEMS hinge actuators are used to rotate a porous silicon membrane between a pass-through position and a filter position;

FIG. 7 is a flow chart depicting operations performed in accordance with one embodiment of the present invention to form a monolithic porous silicon membrane within a microfluidic channel;

FIG. 8 is a flow chart depicting operations that may be used to fabricate a porous membrane in accordance with one embodiment of the present invention; and

various processing stages corresponding to an operation for fabricating the MEMS hinge actuator and porous silicon membrane of fig. 6 a-6 f in accordance with one embodiment of the present invention are described with reference to fig. 9 a-9 f.

Detailed Description

Embodiments of microfluidic devices with integrated porous silicon membranes for molecular screening, measurement, and separation, and methods of making and using the same, are described in detail herein. In the following description, numerous specific details are provided, such as identification of various system components, to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of embodiments of the invention.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

In summary, embodiments of the present invention provide a microfluidic device having at least one integrated porous silicon membrane to screen, measure, and/or separate molecular components from an injection stream introduced into the microfluidic device. Other features of the exemplary embodiments will become apparent to the reader from the foregoing disclosure and appended claims, taken in conjunction with the accompanying drawings and the detailed description and discussion.

Referring now to the drawings, and in particular to FIGS. 1 a-1 e, an embodiment of a microfluidic device 100 is shown. The microfluidic device 100 uses an integrated nanoporous membrane 102 that is placed in a section of a microfluidic channel 104 formed in a substrate 106. The microfluidic channel 104 includes an input section 108 connected to an inflow reservoir 110 and an output section 112 connected to an outflow reservoir 114. In one embodiment, the device further includes an overlay (cover) 116. The cover layer 116 may generally include apertures (e.g., apertures 110A and 114A) through which fluid may be input and/or drawn. Alternatively, the cover layer may be made of a material that is easily perforated for the purpose of delivering or drawing fluid through a syringe or similar device.

As described herein, the porous membranes of various embodiments include a porous structure that can be used to filter, measure, and/or separate chemical and/or biological molecules. The porous membrane may generally be manufactured such that its porosity is greatest in a selected direction. In addition, the pore size can be adjusted from several nanometers to several micrometers by the following manufacturing process, thereby enabling screening, measurement, and separation of target chemical and biological molecules.

Fig. 2 a-2 e illustrate an embodiment of a "stacked channel" microfluidic device 200 using a porous membrane 202 placed between the ends of an upper channel 204 and a lower channel 206. In a specific implementation of the embodiment depicted in fig. 2 a-2 e, the influent fluid is input into the influent reservoir 210 and flows into the upper channel 204. A portion of the influent then passes through the porous membrane 202 and into the lower channel 206. The portion of the fluid that passes through the porous membrane (including the effluent fluid) may then be collected in an effluent reservoir 214.

In one embodiment, microfluidic device 200 comprises a three-piece assembly including an upper substrate member 220 and a lower substrate member 222 sandwiched around porous membrane 202. Generally, a recess may be formed in either the upper or lower substrate members to accommodate the porous membrane, such as recess 224 formed in upper substrate member 220. A portion of a given reservoir may be formed in the respective substrate members, depending on the configuration of the reservoir, such as depicted by portions 210A and 210B (corresponding to inflow reservoir 210) and portions 214A and 214B (corresponding to outflow reservoir 214). Similar to that described above for microfluidic device 100, microfluidic device 200 may also include a cover layer that includes reservoir wells or is made of a material suitable for use with a syringe or similar device (not shown).

The microfluidic device 300 shown in fig. 3 a-3 e uses a plurality of integrated porous membranes. In this exemplary embodiment, microfluidic device 300 includes a serpentine channel 304 formed in a substrate 306. The input end of the channel may generally be connected to a storage means or a supply source for the influent fluid, such as an influent reservoir (not shown) or an input port 330 via which the influent fluid is supplied. Similarly, the output end of the channel may be connected to a means for storing the effluent fluid (not shown), or may include an exhaust port 332 through which the effluent fluid may be collected by an effluent capture means.

Microfluidic device 300 uses a plurality of porous membranes, including porous membranes 302A, 302B, 302C, 302D, 302E, and 302F, placed at various locations along channel 304. In one embodiment, the pore size decreases with each porous membrane encountered by the fluid, thereby enabling the fluid under analysis to separate into various compounds. In another embodiment, the porosity of the porous membrane is substantially similar, thereby allowing more complete filtration of the target molecule.

Microfluidic device 300 may or may not generally employ a cover layer, such as cover layer 316. The cover layer may include holes for accessing the fluid contained in the channel segments, or may include a material that is easily penetrable to enable access to such channel segments by a syringe or similar device.

Fig. 4 a-4D illustrate a multilayer stacked channel microfluidic device 400 that employs a plurality of porous membranes 402A, 402B, 402C, and 402D disposed between respective substrate layers 406A, 406B, 406C, 406D, and 406E. A plurality of stacked microfluidic channels are defined in the substrate layer such that (portions of) the fluid being analyzed flows through the porous membrane in a stacked fashion, the channels including upper microfluidic channels 404A, 404B, 404C, and 404D, and lower microfluidic channels 406A, 406B, 406C, and 406D. The device further comprises a plurality of reservoirs 410A, 410B, 410C, 410D and 410E placed between the porous membranes, wherein the separated portions of the fluid can be stored and accessed. As described above, the microfluidic device 400 may also include a cover layer (not shown) that may be required if the amount of fluid is small. The porous membrane may be generally placed in a recess formed in a substrate layer connecting an upper microfluidic channel or a lower microfluidic channel, such as recesses 424A, 424B, 424C, and 424D. As noted above, the porous membranes may have different porosities (e.g., for finer and finer filtration) or similar porosities (for providing multiple filtration cycles within a single device).

Fig. 5 a-5 e illustrate a stacked channel microfluidic device 500 that uses upper and lower substrate members 520 and 522 instead of multiple substrate layers. It should be noted that the various components and features of the microfluidic devices 400 and 500 that have the same last two digits perform substantially similar functions in both embodiments.

Porous film fabrication and Properties

According to one aspect of the invention, the porous membranes used herein comprise porous structures that can be used for filtering, measuring and/or separating chemical and/or biological molecules. Porous membranes can generally be manufactured so that their porosity is greatest in a selected direction. Furthermore, the pore size can be adjusted from a few nanometers to a few micrometers via the manufacturing process described below, thereby enabling the screening, measurement, and isolation of target chemical and biological molecules.

Typically, the porous membranes used in the integrated embodiments (i.e., microfluidic devices 100 and 300) are made of the same material as the substrate. With the stacked channel embodiments (i.e., microfluidic devices 200, 400, and 500), a wide range of materials can be used to fabricate the porous membranes, in which nano-and micro-porous structures can be formed, regardless of the material used for the substrate layer or means. For example, such materials include, but are not limited to: monocrystalline Porous Silicon (PSi), Porous Polycrystalline Silicon (PPSi), porous silica, zeolites, photoresists, porous crystals/polymers, and the like.

In one embodiment, porous silicon is used for the porous membrane. Porous silicon is a well-behaved material made by an electromagnetic electrostatic, chemical or photochemical etching process in the presence of HF (hydrofluoric acid) (a.g. cullis, et al., j.appl.phys., 1997, 82, 909.). Porous silicon can be made into complex, anisotropic nanocrystalline structures (see http:// chemfaculty. ucsd. edu/sailor) in silicon layers, typically by electrochemical etching or dye etching, to form porous silicon. The size and orientation of the holes can be controlled by the etching conditions (e.g., current density, etc.) and the type of substrate and its electrical properties (R.L. Smith, et al, "ports silicon Formation mechanisms", J.Appl. Phys., 1992, 71, R1; P.M Fauchet, "Pitsand ports: Formation, and Significance for Advanced luminescence materials", P.Schmuki et al, eds. Pennington, NJ: electrochemical. Soc., 1997, 27). Typically, the centerline (mean) hole size ranges from about 50 angstroms to about 10 microns, and the holes in silicon maintain a high aspect ratio (about 250) over a distance of several millimeters.

Another class of porous silicon may be formed by spark erosion (R.E Hummel, et al, "on the earth of phosphor in spark-ignited (porous) silicon", appl. phys. lett., 1993, 63, 2771), forming a Si surface with varying sizes of pits and lands on the micrometer to nanometer scale. Si nanostructures can be created by oxidation after anisotropic etching (A.G. Nassiocoulos, et al, "Light emission for semiconductor nanostrucrures reduced by y coherent atomic and reactive etching techniques", Phys.Stat.Sol. (B), 1995, 1990, 91; S.H. Zaidi, et al, "Scalable polymerization and optical polymerization of nm Si structures", In Proc.Symp.Mater.Soc., 1995, 358, 957.). Although microcrystalline films deposited by chemical vapor deposition are oxidized, Si crystallites can be passivated by SiO to form nanocrystalline structures (h.tamura, et al, "Origin of the green/blue luminescence center from nanocrystalline silicon", appl.phys.lett., 1994, 65, 92).

Referring to the flow chart of fig. 7, the integrated porous membranes for the microfluidic devices 100 and 300 can be fabricated as follows. In block 700, microfluidic channel segments separated by one or more respective independent gaps (gaps) may be fabricated in a silicon substrate using standard microelectronic techniques. The silicon gap may then be etched by electrochemical etching or dye etching to form porous silicon in block 702. The size and direction of the holes can be controlled by appropriate etching conditions (e.g., current density, etc.) and the type of substrate and its resistivity.

Referring to the flow chart of fig. 8, the manufacturing process of the porous membranes (e.g., porous membranes 212, 402, and 502) of the stacked channel-type embodiment according to one embodiment of the present invention proceeds as follows. First, in block 800, porous silicon is etched in a silicon layer, typically about 0.01 to 50 microns thick, by electrochemical or dye etching to form porous silicon. In another embodiment, Porous Polysilicon (PPSi) is deposited by Low Pressure Chemical Vapor Deposition (LPCVD), according to block 802. The size and orientation of the pores, porosity, particle size, thickness, etc. can be controlled via appropriate etching conditions (e.g., current density, current time, etc.), deposition conditions (e.g., temperature, pressure, etc.), also including the type of substrate and its electrochemical properties, etc.

Next, in block 804, the PSi film (or PPSi film) may be physically separated from the PSi etched or PPSi deposited silicon by electropolishing "lift-off" and suspended in solution. Alternatively, the PPSi film may be formed when deposited directly on a substrate (e.g., silicon, quartz, etc.) and may be physically separated by various standard etching or micromachining techniques, or saved as part of the original structure for immediate use in further etching, micromachining, etc. Then, in block 806, the PSi or PPSi film is secured in a corresponding recess formed in the substrate approximately half way through the cross-channel region.

After manufacture, the porous membranes are assembled to be placed in respective separate recesses in the upper or lower substrate members or substrate layers. The substrate members and layers can generally be made of a variety of substrate materials including, but not limited to, crystalline substrates (e.g., silicon) and polymers. In one embodiment, the substrate material comprises Polydimethylsiloxane (PDMS).

Dynamic positioning of porous membranes using MEMS actuators

According to one aspect of the invention, embodiments can be made that use a porous membrane that is disposed within a microfluidic channel and rotatably coupled to the substrate. For example, fig. 6 a-6 f show details of channels corresponding to the described embodiments of microfluidic device 600. In this embodiment, the porous silicon membrane 602 is rotationally coupled to the substrate 606 in the base of the channel 604 via a microelectromechanical system (MEMS) hinge actuator 640. In one embodiment, the device further uses an optional position-locking MEMS actuator 642 formed on the underside of the cover layer 616.

Fig. 6a and 6b show the initial position of MEMS actuators 640 and 642. In fig. 6c and 6d, the MEMS hinge actuator 640 is electrically activated, causing the porous silicon membrane 602 to rotate to a vertical "filter" position, thereby blocking the channel 604. In this position, the porous silicon membrane provides a semi-permeable barrier in the manner described above with reference to porous membranes 102 and 302. In one embodiment, the porous membrane is rotated until it reaches a stop point extending downward from the underside of the cover layer 616 or outward from both sides of the channel (neither shown). In this exemplary embodiment, the position of the porous silicon membrane is locked in place by electrical activation of the position-locking MEMS actuator 642, as shown in fig. 6e and 6 f.

MEMS components generally include integrated electromechanical elements or systems having corresponding nanometer or micrometer scale dimensions. The MEMS components may be fabricated on a common platform, such as a silicon-based or equivalent substrate (e.g., Voldman, et al, Ann. Rev. biomed. Eng, 1: 401-425, 1999) using well-known microfabrication techniques. MEMS components may be fabricated using Integrated Circuit (IC) processes, such as Complementary Metal Oxide Semiconductor (CMOS), Bipolar or Bipolar CMOS (BICOMS) processes and the like. Which may be patterned using photolithographic and etching methods well known in computer chip manufacturing. The micromechanical components may be fabricated using compatible "micromachining" processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and/or electromechanical components. The basic technique in MEMS fabrication involves depositing a thin film of material on a substrate, applying a patterned mask on top of the film by photolithography imaging or other known photolithography, and selectively etching the film. The deposition techniques used may include chemical processes such as Chemical Vapor Deposition (CVD), Low Pressure Chemical Vapor Deposition (LPCVD), electrode deposition, epitaxy (epitax), and thermal oxidation, as well as physical processes such as Physical Vapor Deposition (PVD) and casting.

Referring to fig. 9 a-9 f, the fabrication process of the MEMS hinge/porous silicon membrane structure of the microfluidic device 600 according to one embodiment of the present invention proceeds as follows. Initially, a suitable microfluidic channel portion 604 is formed in a substrate 606, wherein a channel base 650 is depicted in fig. 9 a. Recesses 652 and 654 are then further formed in the substrate. The channels and recesses may be formed using one of a variety of well-known micromachining techniques, as described above. Next, as shown in fig. 9b, a layer of fill 656 is deposited in the recess 654. This layer should preferably be made of a material that can be easily etched with an etchant that will have no or minimal effect on the other structural layers.

After the filler material is applied, the MEMS hinge actuator is formed. In one embodiment, the MEMS hinge actuator can be fabricated using a piezoelectric ceramic bimorph sandwich structure. In short, piezo ceramic bimorph sandwich elements are known for use in actuators that cause bending or deformation of the element. The bimorph sandwich typically comprises alternating layers of conductor and piezoceramic material. Upon actuation of the potential across the interlayer, the piezoceramic layer(s) causes expansion or contraction while the conductive layer remains substantially unaffected. As a result, the actuator bends due to the change in length of the piezoceramic layer(s) in a manner similar to the bending of a bimetallic strip when exposed to various temperatures.

Referring to fig. 9c, a conductor layer 658 is placed proximate to the recesses 652 so as to overlap a portion of the filler 656 in a predetermined pattern. The conductor layer may generally comprise one of various metals, such as copper or aluminum. Next, a piezoceramic layer 660 is placed on the conductor layer 658, as shown in FIG. 9 d. Additional alternating conductors and piezoceramic layers (not shown) may be added in a similar manner depending on the particular characteristics of the bimorph element used. This results in a structure for the MEMS hinge driver 640.

At this time, the porous silicon thin film is formed. In one embodiment, the porous silicon membrane 602 is formed via deposition of polysilicon according to the method used to form the nanoporous polysilicon described above. Fig. 9e shows the result of this operation. The process is completed by etching away the filler 656, leaving a void 662 under the porous silicon membrane and a portion of the MEMS hinge actuator 640, as shown in fig. 9 f. This frees the MEMS hinge driver and a portion of the porous silicon membrane from the substrate 606, actuating the porous silicon membrane to rotate upon electrical actuation of the MEMS hinge driver.

Operation of the embodiments

The various embodiments described above are generally useful for filtering and separating biological and chemical molecules. For example, in microfluidic device 100 (fig. 1 a-1 e), an analyte comprising an influent fluid flows into input channel segment 108 where it encounters integrated porous membrane 102. As the analyte passes through the porous membrane, smaller molecules are able to pass through the matrix of pores faster, leaving larger molecules trapped for a longer period of time in the pores of the membrane. This produces filtered effluent fluid that flows into the output reservoir 114 where it can be collected.

Similar processing is performed using the microfluidic device 200 (fig. 2 a-2 e). In this case, the fluid to be analyzed is introduced into the upper microfluidic channel 204 and passes through the porous membrane 202. As described above, the smaller molecules are able to pass through the pore matrix faster, thereby allowing the larger molecules to be confined in the membrane pores for a longer period of time. The filtered effluent fluid may then be collected from the output reservoir 214.

In the stacked filter approach used in microfluidic devices 300, 400, and 500, the influent fluid passes through a plurality of porous membranes. As noted above, in certain embodiments, the porous membranes will be configured such that the nominal size of the pores of each porous membrane encountered by the fluid is small. This produces a separation effect in which fluid retained between successive porous membranes can be selectively filtered to limit a smaller range of molecular sizes. Further, such selectively filtered fluids may be drawn from various channel portions (e.g., for microfluidic device 300), or from reservoirs placed between the porous membranes (e.g., for microfluidic devices 400 and 500).

While the invention has been illustrated and described with respect to a limited number of embodiments, the invention may be embodied in various forms without departing from the spirit of the essential characteristics thereof. Accordingly, the embodiments described and illustrated herein, including what is described in the summary of the invention, are to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims (30)

1. A microfluidic device, comprising:

a substrate having a microfluidic channel formed therein; and

a porous membrane disposed in the microfluidic channel to form a semi-permeable barrier between an input section and an output section of the microfluidic channel, and having a plurality of pores to selectively filter an incoming fluid that may be introduced into the input section of the microfluidic channel, thereby producing filtered outgoing fluid at the output section of the microfluidic channel.

2. The microfluidic device of claim 1, further comprising an inflow reservoir formed in the substrate and connected in fluid communication with the input section of the microfluidic channel.

3. The microfluidic device of claim 1, further comprising an outflow reservoir formed in the substrate that is connected in fluid communication with the output section of the microfluidic channel.

4. The microfluidic device of claim 1, wherein the porous membrane is integrally formed from the substrate.

5. The microfluidic device of claim 1, further comprising a cover layer disposed over at least a portion of the microfluidic channel.

6. The microfluidic device of claim 1, wherein the porous membrane has a plurality of pores having a centerline diameter in the range of about 50 angstroms to about 10 microns.

7. A microfluidic device, comprising:

a substrate platform, comprising:

an upper substrate member having an upper microfluidic channel formed therein;

a lower substrate member having a lower microfluidic channel formed therein; and

a porous membrane disposed between the ends of the upper microfluidic channel and the lower microfluidic channel, the porous membrane comprising a semi-permeable barrier having a plurality of pores to selectively filter an influent fluid introduced into the upper microfluidic channel to produce a filtered effluent fluid in the lower microfluidic channel.

8. The microfluidic device of claim 7, wherein the porous membrane comprises porous nanocrystalline silicon.

9. The microfluidic device of claim 7, wherein the porous membrane comprises porous polysilicon.

10. The microfluidic device of claim 7, wherein the centerline thickness of the porous membrane is in a range of about 10 nanometers to about 50 micrometers.

11. The microfluidic device of claim 7, wherein the porous membrane has a plurality of pores having a centerline diameter in the range of about 50 angstroms to about 10 microns.

12. The microfluidic device of claim 7, wherein the substrate comprises a polydimethylsiloxane substrate.

13. The microfluidic device of claim 7, further comprising at least one respective independent reservoir formed in the platform substrate, the reservoir being connected in fluid communication with at least one of the upper and lower microfluidic channels.

14. A microfluidic device, comprising:

a substrate having a microfluidic channel formed therein; and

a plurality of porous silicon membranes disposed within the channel defining a plurality of channel segments, each porous silicon membrane forming a semi-permeable barrier between the respective channel segments on opposite sides of the porous membrane and having a plurality of pores through which an influent fluid provided at an input side of a porous silicon membrane is selectively filtered to produce a filtered effluent fluid at an output side of the porous silicon membrane.

15. The microfluidic device of claim 14, wherein each of the plurality of porous membranes is integrally formed from the substrate.

16. The microfluidic device of claim 14, further comprising a cover layer disposed over at least a portion of the microfluidic channel.

17. The microfluidic device of claim 14, wherein the microfluidic channel is substantially serpentine in structure.

18. The microfluidic device of claim 14, wherein the plurality of porous membranes are sequentially arranged along a flow path through the microfluidic channel such that each successive porous membrane encountered along the flow path has a nominal pore size smaller than the previous porous membrane.

19. The microfluidic device of claim 14, wherein the plurality of porous membranes have substantially similar nominal pore sizes.

20. A microfluidic device, comprising:

a substrate platform having a plurality of stacked microfluidic channels formed therein, each of the stacked microfluidic channels including a pair of upper and lower microfluidic channels; and

a plurality of porous membranes, each disposed at an end of a respective independent pair of upper and lower microfluidic channels, and comprising a semi-permeable barrier having a plurality of pores to selectively filter an influent fluid introducible into a microfluidic channel on an input side thereof, thereby producing a filtered effluent fluid in the lower microfluidic channel on an output side thereof.

21. The microfluidic device of claim 20, wherein the substrate platform comprises a plurality of substrate layers, each substrate layer having at least one of an upper microfluidic channel and a lower microfluidic channel formed therein.

22. The microfluidic device of claim 20, wherein the substrate platform comprises an upper substrate member and a lower substrate member that are sandwiched around the plurality of porous substrates when assembled.

23. The microfluidic device of claim 20, further comprising a plurality of reservoirs disposed along a flow path through the plurality of stacked microfluidic channels.

24. A microfluidic device, comprising:

a substrate having microfluidic channels formed therein; and

a porous membrane disposed within the microfluidic channel and rotationally coupled to the substrate to function as a semi-permeable barrier when the porous membrane is rotated to a filtering position, the semi-permeable barrier having a plurality of pores to selectively filter an influent fluid that may be introduced into the microfluidic channel at an input side of the porous membrane to produce a filtered effluent fluid at an output side of the porous membrane, and the fluid flowing through the microfluidic channel may freely bypass the porous membrane when the porous membrane is rotated to a passing position.

25. The microfluidic device of claim 24, wherein the porous membrane and the substrate are rotationally coupled via a first microelectromechanical system (MEMS) driver.

26. The microfluidic device of claim 24, further comprising a cover layer coupled to the substrate and disposed over at least a portion of the microfluidic channel.

27. The microfluidic device of claim 24, further comprising a second MEMS actuator for locking the porous membrane in the filtration position.

28. A method for fabricating a microfluidic device, comprising:

forming a microfluidic channel in a silicon substrate, the substrate comprising a plurality of channel segments separated by one or more silicon gaps; and

etching the one or more silicon gaps to form one or more corresponding porous membranes, the membranes being placed in microfluidic channels between the channel segments.

29. The microfluidic device of claim 28, wherein the one or more porous membranes comprise a plurality of porous membranes each having a porosity that decreases along a flow path of the microfluidic channel.

30. The microfluidic device of claim 28, wherein the one or more silicon gaps are etched using an electrochemical etch or a dye etch.